Welcome to buses! I got interested in buses a while back and found the world far more complicated than I had imagined. I was very happy to have people help me learn stuff. This document is my attempt to summarize what I have learned so I can help other people have fun with fewer suprises than I had.

I undoubtedly have made mistakes and omissions, and there is probably information that is there but poorly organized and hard to find. If you find something here which seems wrong, it probably is. If you have questions, suggestions, corrections, or whatever, please drop me a note at <bus-0035@pardo.net>. Happy bus!

Why bus?

I expect most readers are in one of a few broad groups:

I am interested in all of those things, but I am not equally knowledgeable in these areas, and I am not very knowledgeable in any of these areas. So don't take my word for it --- think about whether it makes sense, talk with other folks, and so on. I do hope that despite errors I make, the information will be at least largely correct and will help you to understand the issues.

Some of the information here is of special interest to one category of reader and less interesting to another. Some of that is handled just by the organization, but some stuff is mixed in. For example, RV enthusiasts may want to know if there is room for a generator set, while seated enthusiasts probably are interested in storage decisions without need to know if it works for a generator set, and folks interested in transporation will be curious about how storage sizes and layout changed to reflect changes in how buses were used. So while some things are separated out, others are necessarily or accidentally mixed in. I don't know a better way to organize it, so please bear with me, and I hope to have an index at some point.

Finally, some people prefer the term ``coach'', but since I am including school buses and some other commercial passenger vehicles built on truck chasis, ``bus'' seems more appropriate in many places. This document does not distinguish between ``buses'' and ``coaches'', so please do not be upset if I call your pride and joy a ``bus''!

A Brief History of Buses

Buses as we know them today first appeared about 1900 and in some ways have changed substantially and in other ways have changed little. This brief history of buses is wildly incomplete but should serve as a good starting place for following discussions.

The term ``bus'' probably comes from ``omnibus'' meaning ``many things at once'' or ``for all''.

The first regular bus service was started in Paris in the 1660's. It's invention is credited to Blaise Pascal, a famous mathemetician and scientist. Buses were distinguished from taxis and other coaches because they ran on fixed routes, on a regular schedule, and with lower fares. The system was successful during it's year-long trial, but was abandoned on Pascal's death.

In about 1900, most public transit was horse-drawn cabs and coaches, horse- electric- or coal-powered rail vehicles, or water craft. As gasoline motors became practical, companies started using multi-passenger vehicles to provide transportation where rails and water transportation were not available. Buses became more and more popular in part because of their speed compared to horses and their flexibility compared to rail vehicles. They were also popular for their relatively low cost. High-volume rail service is cheaper to operate in the long term, but the starting cost of track and heavy-duty rail vehicles is too high to pay back on low-volume routes. Even where long-term payback is possible, the relatively low cost of entry in to the bus market is highly attractive. Roadways are built and maintained at public expense, while rails are largely built and maintained at private expense, so bus service avoids some costs through taxation ``sleight of hand''. Finally, in United States in the 1940's, companies selling buses, rubber and oil offered special deals to transportation companies to help get them as long-term customers, further reducing the cost of entry. Today (circa 2000), rail is returning and airplanes are by far the most popular way to travel long distance. However, buses maintain low cost of entry and route flexibility and they are a standard part of public transportation both within cities and between cities. Probably the biggest challenge to bus transportation today is relatively slow speed compared to cars.

The earliest buses were built by adpating other vehicles, for example by putting benches on a truck bed, or extending a car body to hold more passengers. As bus sales increased, buses were usually built with the body or body and frame made by a body builder and the mechanical components bought from other suppliers. As production volumes increased further, it became more common for high-volume buses to have most parts of a bus built all by one company, notably Ford, GMC and Mack. By about 1960, Ford and Mack exited the market and the U.S. government started antitrust investigations against GMC, and there was been a return in the U.S. to bus companies which build bodies and frames and purchase drivetrain and suspension items from other companies. In Europe, M.A.N., Mercedes and Volvo still build most parts of the bus.

Dates below are for entry in to production rather than demonstration or protoype, unless noted otherwise.

Early buses were stretched cars or truck frames with special bodies built by independent coachworks.

More notes

Yellow Coach (later GMC) is typically credited as first in the U.S. with a diesel-powered bus. A Twin Coach brochure from 1936(?) lists a Hercules diesel as one option, and a photo dated 1935 (reproduced below) shows the same. I mentioned this "who was first" iss to Steve Russell, who follows "firsts" in trains; he said (paraphrased) "There are at least three 'first's: first to offer, first to ship, and first to succeed."

[1935 Twin Coach diesel engine by Hercules]
Print 1159-1, Caption: 'The Hercules Diesel engine installation as built for the Twin Coach Corpration by Hercules Motors Corporation. This engine is interchangeable in mounting dimensions with the Hercules gasoline engine.'

Kinds of Buses

There are as many ways to describe buses as there are ways to slice bread. Here are some broad categories. These categories are not preceise, because things always spill over categories and because different parts of the world have different needs.

Loosely, a ``bus'' is a motor-driven road vehicle for carrying many passengers and designed for commercial use several to many hours a day with an expected life in this kind of service of more than ten years. Many buses also carry some amount of freight, and several percent of buses are sold new without seats to be used as RVs, since the same many of the attributes which make them desirable for commercial service make them desirable for RV use.

A close cousin of the bus is called the ``trackless trolley'' or ``trolley coach''. It is an electric bus powered by overhead wires. These share almost all components with buses except the drivetrain, so this document typically includes them under the term ``bus'' except where drivetrain is concerned.

Broad Categories

Intended Use

Buses are built differently, depending on their intended use. some major categories include


Buses entered the mainstream of transporation in the 1920's. Mechanically, buses have evolved slowly and continually. The bodies have gone through (roughly) four major eras:

1927 Flxible

[Low-roof, engine-forward]   Early buses often used wood in their construction, were built on truck chassis, had low roofs --- making it necessary to stoop when walking inside, and sometimes had one door per row of passengers. These buses were built typically in the 1920's and early 1930's.

1950's Mack
Elmer Lindquist's Mack (2000)

[Old Look]   Many buses built from the 1930's through the 1950's -- and some in the 1960's -- were built with one or two entrance doors and were tall enough to walk in standing upright. They were usually aluminum-bodied and many were all-aluminum construction. They were boxy with rounded corners, were fully painted, and were built with relatively small windows made of small pieces of rectangular pieces of flat glass. Transits typically had a higher second row of ``standee'' windows. Mechanically, old-look buses introduced rear-engine and transverse-engine designs, air conditioning in the 1930's, and in the 1940's, mid-engine buses and city buses with automatic transmissions, typically 2-speeds.

[New Look]
Pacific Bus Museum; former MUNI #3270. (C)PBM.

[New Look] From the 1950's through the 1970's -- and some in the early 1980's -- many buses were made using much larger pieces of glass, with curved glass for the front and rear windows, and non-rectangular glass for the side windows. The large curved front windows on new-look transit buses gave them the nickname ``fishbowls''. Many buses were less boxy, with more pronounced features, and many buses were partly painted, with anodized aluminum showing through elsewhere. Bodies also got wider and longer -- up to 265cm (104") instead of the more common 245cm (96"), and a greater proportion of buses were 12m (40') instead of 10.5m (35'). Mechanically, ``new look'' buses also introduced air-bag springs instead of steel springs and power steering as an option. The new look era also saw the widespread use of ``stepped'' highway buses with a low floor in the front and high floor in the rear.

[Late Century]
Pacific Transit's Orion-I, Roster D14; circa 1999.

[ADB]   From the 1970's through today, some bus designs transitioned through a rounded design that was less space-efficient, then to a less-rouned boxy form that was more space efficient than the New-Look buses. Many buses switched to either polished stainless or fully-painted bodies, with some of them being fiber panels on a conventional, rather than monocoque, frame. Frames and sometimes bodies started to be made of stainless steel for corrosion resistance better than either plain steel or aluminum. Bodies continued to get larger -- 265cm (104in.) width beceame fairly standard in the U.S., and an lengs of 14m (45') became common for highway buses and articulated city buses of 18m (60') appeared and became common. Mechanically, city transit buses got many-speed transmissions that would carry them more readily to highway speeds, wheelchair lifts, and articulated transit buses became common. Highway buses got automatic transmissions -- in 1980, most highway buses sold in the USA had manual transmissions, by 1999, a survey of 6,000 buses showed that only 12 of them were delivered with manual transmissions ([missing link]). By 2000, new highway buses were often fitted with wheelchair lifts.

Body/Frame Construction

There are many choices for construction:

Engine/Transmission Layout

Other Broad Differences

Until the 1950's, most buses used leaf spring suspensions. Leaf springs are simple but have fairly high friction and give a harsh ride. In the 1950's, rubber-in-shear and airbag suspensions were introduced. All three systems are in widespread use today; see below for more details [link].

Early buses used mechanical, then hydraulic brakes. In the 1930's, air brakes were introduced on heavy buses. Hydraulic brakes are common today on lighter-duty buses; air on heavier units. In the 19??'s, anti-skid brakes were introduced. See below [link] for more about brakes.

Buses are heavy and can be difficult to steer at low speeds. Power steering was introduced for transit use in the 19XXs. Steering is easier at speed, so power steering was not widespread among highway buses until 19XX.

Early buses had limited interior lighting. Gradually, generator sizes increased and were then replaced by alternators. Increasing available electric power was used in part to drive more and brighter lamps. Then, transit incandescents were replaced by fluorescents, which dramatically increased the interior brightness. For transit buses, the front lamps are artificially dimmed to reduce glare for the driver.

Destination signs were originally simple placards. The desire to keep signs ordered led to the introduction of ``roll'' signs -- painted or printed cloth signs wrapped around and stretched between two spools; by rolling the spools, the appropriate destination could be displayed. Rolls were then driven by electric motors to allow rapid scrolling through many destinations. Next, electronic destination signs were introduced, allowing every bus in a fleet to carry all destinations and allowing moving or multipart displays for showing complex route information. Early electronic signs used a magnetized flap with color on one side and black on the other; an array of electromagnets flipped the flaps to show routes. More recent electronic signs use bright LEDs (light emitting diodes) for improved reliability.

Early transit buses were operated in cold weather with the assumption that passengers were dressed for the cold. Heating was minimal. Over time, interior heating systems have been improved.

Air conditioning was offered as a standard option starting with the 1936 Yellow Coach 743. Then, and for many years, the air conditioning was driving with a separate gasoline motor because the main motor was not powerful enough to drive both air conditioner and the bus. As more powerful V-configuration engines became available, it became standard to drive the air conditioning from the main engine; when the air conditioner load was low, the extra power was available to propel the bus.

So You Want To Get A Bus

This section is for enthusiasts who wish to own and operate a bus, either as a ``seated coach'', or as a motorhome conversion.

This section is also a useful introduction to many of the buse life-cycle issues which are discussed in detail later in the book. For example, imagine you run a small bus business and are considering purchase of another bus. There are many new and used units available, what should you get?


For most people, the first consideration is financial:

If you are terrifically rich, you might be purchasing a new unit. In that case most of the following does not apply. However, a new medium-duty bus with no interior costs well over US$100,000 and so that is not an option for most folks. In addition, there are tradeoffs between available new units, so some of the following does apply.

A used bus costs much less, but it costs less exactly because it has problems that make it less desirable to the transportation company, agency, department or individual that originally purchased it. For example:

Although many of these problems can be overcome by a private owner, they are not to be dismissed lightly:

Most of these issues can be overcome or worked around, but these issues do mean that it is highly important to use care when purchasing.

A heavy-duty bus will typically see 500 to 1,500 kilometers of service every day. That translates in to 3,000 to 10,000 kilometers per week, and 150,000 to 500,000 kilometers per year. Buses typically see ten years of regular service before being retired. Although buses are often out of service for repairs and are often rotated to light-duty routes, it is common for buses to go over 1,000,000km before the first owner sells them; and such buses are often sold in to continued light duty service before reaching a price an individual buyer can afford.

It is common for people to say ``I will not be driving it that much, so I do not need to worry about service.'' However, after millions of km of use, many parts are fatigued and can break in light use. Sometimes, even new parts can and do sometimes break in light service. For example, starting the engine, especially a cold one, causes some kinds of wear worse than running it. And, as many buyers find out, when a heavy-duty part breaks in light service, it can be very expensive to repair.

For example, a set of tires for an RV might be US$200 instead of US$2,000 for a bus. A rebuilt motor for an RV might be US$2,000 instead of US$10,000 for a bus. So while a bus may ``seem'' like it should last forever, it can actually be quite expensive to keep on the road. In the best case, everything lasts. But even a drivable bus may need substantial repairs to be roadworthy and reliable -- if you have to get new tires and fix the brakes, engine, transmission, and so on, you can easily spend several times the original purchase price and several times the cost of an adequate RV.

As a bus approaches retirement, it is common to save effort and costs by omitting normal periodic maintainance, replacement and repair beyond what is required for safe operation. Areas of reduced maintenance include:

Often, buses are sold either because something is wrong or because the bus is aging and the maintenance costs are beginning to rise. And, if a seller has several similar buses, the best parts may be swapped off the bus being sold in order to keep the others on the road. Thus, it is common for a bus to leave service needing repairs or having worn and tired items which will soon need repairs. Therefore, expect needed service and expenses in all areas. Common issues include:

In addition, it is rare for a vehicle to leave service and get sold immediately. Often, the seller is getting rid of it exactly because it is not in use. When any vehicle sits, various things go wrong. For example,

If the bus is already converted for motorhome operation, there are a large number of additional items which may need service, repair or replacement:

You should also consider what it will cost you to modify and own a bus. For example, many transit buses through the 1980's are unable to travel at highway speeds. Drivetrain changes for high-speed operation can easily run thousands of dollars, and hill-climbing may be slower as a result of those changes. Similarly, the normal service and repair costs on diesel engines are much higher than on gasoline engines. For commercial use, those costs are spread over hundreds of thousands of kilometers, but for a bus which is used only occasionally, the costs can be much higher per kilometer traveled. Normal wear-and-tear items to consider include

Note that the costs may vary widely with the age and make of the bus. It might seem, for example, like old parts should be cheap. And, indeed, there are some parts which have been used on trucks and buses for decades and which are available at junkyards all over. On the other hand, some parts may be difficult to find or easy to find but very expensive. For example, a used distributor cap for a 1950's Hall-Scott engine may cost about US$500 -- just for the cap. Although most parts are not so expensive, it may be hard to find or adapt parts for a bus that is 50 years old. Include the possible costs of parts when deliberating the cost of a bus, and shop around for parts suppliers before you leave on a trip, so there is less risk that a breakdown will strand you and your bus for weeks while you are finding parts.

Finally, many buses are sold ``as-is, where-is'' and may be non-running or otherwise non-drivable. Trucking or towing a bus often costs in the range of US$2/km. Since a non-operational bus will cost something to make it run, the added costs of moving it may make a cheap or free bus be a very poor bargain indeed.

That is all pretty scary. And it should be, because operating a bus can be a very expensive proposition. That said, many people buy and operate buses with modeset expense, and many others ultimately have large unexpected expenses but are prepared to deal with the problem and so have, overall, a good experience and the feeling their money has been well-spent.

The important lesson here is it can be very expensive, so make sure you do your homework to avoid or anticipate expenses, and so that if an overwhelming expense does occur, you can get rid of the bus before you run out of money, rather than after you run out.

A good rule of thumb when looking is: there will always be another bus that will do. Do not buy a bus because ``it might work out''. Instead, buy a bus because you are reasonably well-informed about what you want, what is generally available, and because you have thoroughly investigated the bus in question and it you have fair confidence that it can work out, and that if it does not work out you will be merely disappointed rather than broke.

Intended Use

The next major consideration is: how do you intend to use the bus? As a seated coach? As an occasional motorhome? As a full-time motorhome? A truck? A restoration project? Something else? How you use is a very important part of deciding what to buy.

If you want a motorhome, a bus that needs lots of work and then is not really suited to use as a motorhome is no bargain, even if it is free. Conversely, if you want to restore a vintage bus, there is no point in spending big money to get a ten-year-old bus in good shape.

If you want a bus, note that many states license buses based on gross vehicle weight, and that can be over US$2,000 per year. In addition, insurance for a commercial vehichle can be quite high. If you are considering an older bus, check vintage vehicle registration in your area. Although it may restrict you to (say) less than 5,000 kilometers per year, the reduction in registration and insurance fees can be tremendous. Similarly, some areas allow private buses to be licensed as cars. Any time a bus is licensed for non-commerical use, it must never be used commercially, and, typically, it must also not be used to transport passengers.

Whether your goal is motorhome, seated, or something else, you should also consider what level of quality will make you happy. Some people are happy with frequent breakdowns as long as they are never too costly and as long as the vehicle is safe. Other people are looking for a bus which repsents the best in its class, with no visible or hidden blemishes, no breakdowns, and so on. You should be honest with yourself about what you want so that you get a bus which you are fundamentally happy to own, or you will be happy with the decision to not get a bus because the combination of model, quality, and price you demand are not currently available..

Another consideration is what will you be happy driving? For example, some folks will want a shorter and narrower bus, while others are willing to drive something larger. The smaller bus will be easier to drive and park and is allowed more places, but a smaller vehicle has less inside space and less storage space. In addition, specific makes and models of buses are only be available in specific sizes.

Storage is an issue, too. Crica 2000, one yard said that 10m spaces were US$60/month and available at the time I called; 11m spaces were US$70/month and had a several-month waiting period, 12m spaces were US$80/month and were a yet longer wait, and 13m spaces were US$90/month and were probably unavailable until the next year. Best if you have your own space! Parking while traveling can be similarly constraining. For example, many camp grounds have a 10m maximum length, municipalities often limit parking of long or tall vehicles, and some roads have vehicle length limits.

Driving a vehicle which is clearly expensive can be a benefit while traveling on the road. For example, some travelers do unpleasant things like dump feces in the street (I have seen it), panhandle or steal; and poorly-maintained vehicles are more likely to have breakdowns that become a hassle for individuals or the community. Many areas have laws that regulate whether you can sleep overnight in your vehicle; how long you can park in one place; and so on. If you appear ``undesirable'', you may be unwelcome, may be harassed, or may be subject to stricter enforcement of those laws -- either because the police are being extra-careful or because residents call to complain. If you have a bus which is obviously well-maintained, it shows you have money and helps community members feel at ease.

Conversely, if you have a super-expensive vehicle, you may be harassed or targeted for theft while traveling through poorer areas. This is probably less of a problem, but still a consideration.

There are many types of buses. Aside from age and condition, some common issues, especially for mothorhome conversion, are:

The condition of the basic bus ... how much work are you willing to do just to get it on the road, safe, and sufficiently reliable? You can hire out any work you will not or cannot do, but often at great cost. Best to become familiar with rates before you buy.

How much work to make it ready for your intended use? Conversion is a time-consuming process, with many hidden detours such as insulation, plumbing and wiring; working around the existing bus layout and limits; fitting stuff to the inside; etc. Similarly, authentic restoration often requires a great deal of tracking down parts. Thus, the more you need to change or repair, the more you care about details of the final result, and the higher the quality of fitting you require, the greater the time and/or cost.

To summarize: there are no ``right'' answers, but think a lot about what you want, go look at what is available (including looking at a few in person!), and then re-think what you want based on the realities of what is available.

Bus Type Tradeoffs

Here are a few more things to think about:

I'm actually pretty new to buses myself, and I have spent a lot of time learning about them. I have had a lot of fun, but have been super-glad to have folks to talk to. I'd love to write a little ``welcome to the world of buses''. If you are still interested in buses after reading all of the above, ask more questions and I'll see if I can answer them and start writing my ``welcome to buses'' thingy (hey, I am starting already!). If you are a little put off, I don't blame you a bit. Have fun -- that's what really counts!

Some Details

There is always a tradeoff between complexity and ease-of-use. The simpler system is typically cheaper and more reliable; but the complex system may require less effort, less time in service, or may be easier --- less distracting --- to use. A complex system is also necessarily less durable: doubling the number of parts requires that each part is four times as reliable in order to achieve the same level of reliability as the simpler system.

Similarly, there is always a ``packing'' problem: the goal of a bus is to get as many people and as much cargo from A to B, but in a reasonable time and with reasonable comfort. Tighter packing can increase the revenue per trip, but may also require more expensive construction or compromises in operating efficiency, service, and so on.

Finally, buses have only a finite service life: parts break, buses are in accidents, and the rules of operation change. In addition, both private and government operators benefit greatly from a lower initial bus price. Thus, while durability is a big goal, there gets to be a point where improved durability comes at a high enough price that buyers would rather buy cheaper and less durable buses and replace and repair them more often.

This section discusses some features of buses with an eye towards issues such as complexity, efficiency, cost.

Driver Controls

Accellerator Pedal

Early buses used a simple mechanical linkage to the motor mounted in front of or next to the driver. When rear-engine buses appeared, they continued to use a mechanical linkage, typically a long wire or cable.

Cables stretch. Mechanical parts need lubrication and wear nonetheless. Wear and forgotten lubrication makes the pedal hard to push and leads to stronger return spring, which makes the pedal yet harder to push. Getting a straight run to run a cable the length of the bus can be difficult. The linkage needs to be immune to small motor motions that occur as the motor moves under load, or the movement may change the throttle. And so on.

For these reasons, many newer buses use an air throttle. The pedal is an air valve, like an air brake. The motor has a diphragm like a small version of a brake pot. An air line runs from pedal to motor.

Diesel buses fitted with air throttles are also often fitted with a ``throttle delay'' mechanism. The throttle delay allows the throttle to open only slowly. When you push gently on the pedal, the throttle opens precisely. When you push hard --- open the throttle suddenly --- the throttle delay limits the rate at which the throttle opens. Driving with a throttle delay is most noticible leaving a stop: you press hard on the accellerator and the engine revs up only slowly. Most other times the delay is not significant because you are changing the throttle slowly or changing it suddenly but only a small amount; and because you feel less seat-of-the-pants ``kick'' when underway than you do (or don't) when leaving a stop.

The throttle delay exists for several reasons. First, opening the throttle suddenly can cause diesel engines to smoke a lot, which is annoying to people at the curb, and which also produces a lot of air pollutin. A throttle delay reduces this problem. Second, the problem is particularly noticible with turbocharged diesels, because the maximum fuel that can burn depends in part on the speed the turbocharger is spinning. At low speeds, the governor may inject too much fuel, making the smoke problem extremely bad. When the engine smokes a lot, it is also dumping more waste in to the oil, which is bad for the engine life. A throttle delay brings up the engine speed and turbocharger speed together, reducing the amount of smoking.

Some regions have a throttle ``snap test'' in which an idling diesel engine is suddenly set to full throttle and the emissions are measured. Typically, the goal is to measure opacity. It may be measured using instrumentation, or officers may somteimes do it by eye and issue a ``get it checked out''. The basic idea is that at normal operating temperatures, black smoke indicates too much fuel and not enough air; if the problem is bad enough to show on sudden accelleration then it is both occuring commonly (accelleration is common) and may be a problem other times but not so noticible.


Early buses used mechanical brakes, where linkages ran from the driver to the wheel. As you might imagine, getting high leverage was important to be able to stop, but at the same time it was important to have minimum slop and stretch in the linkage so that the driver's energy was not all used up before it got to the brakes.

Hydraulic brakes perform the same function as mechanical brakes, but the oil in the system can go directly to the wheels with very little slop or stretch. Hydraulic brakes appeared in cars the early 1920's (attributed to Malcolm Lougheed, who changed his name later to Lockheed). Hydraulics are improvement over mechanicals because hydraulics lose less energy between driver and brake. They also have disadvantages: hydraulics can leak; and when hot, the fluid can boil. Good hydraulic fluids also tend to absorb water, so brake lines are prone to rust.

Power assist is a further improvement because the driver does not need to supply all the energy needed to stop the vehicle. The idea of power assist is that the says what to do, and the assist amplifies the driver's instructions. Power assist first appeared in passenger cars in 1928. One disadvantage is that if the power source goes away --- for example, the engine stalls --- then the brakes suddenly get much worse. Thus, power brakes typically have a power resivoir that helps with at least a few brake applications after power goes away.

Air brakes also perform the same function as mechanical brakes, and work in much the same way as hydraulics but use air instead of hydraulic fluid. Air brakes first appeared in trains; George Westinghouse invented them in 1868. Car air brakes first appeared on the 1903 Tincher. Air brakes have the advantage that strong brakes can be controlled using a simple valve. In addition, no special fluid is reqired, so trailer brakes can be connected and disconnected easily.

Although air brakes are similar to hydraulics, there are a few key differences. One difference is that hydraulic brakes push directly on the shoes, while air brakes run at lower pressures and so the air pushes on a lever that moves a cam to apply the brakes. The extra linkage is a source of some maintainance problems and failures compared to the direct application of hydraulics. A second difference is that hydraulic fluid sits in the hydraulic system all the time, whereas air must be pumped (compressed) in to the air system. As a result, the hydraulic system still works if power goes away, though it may work worse with the power assist gone. However, when power goes away from an air system, the compressor stops pumping air in and the brakes stop working entirely. Although air brakes are equipped with a compressed air resivoir, the brakes stop working entirely soon after the compressor stops working.

A third difference is that the driver must learn a special set of procedures for using air brakes. The driver typically has both an air pressure gague and a warning drop flag [image] or buzzer to indicate low air. The driver must wait for air pressure to build before it is safe to move the bus. On loss of air pressure, the warning will drop or sound and the driver must be prepared to react before the brakes stop working. Many regions require special licensing for commercial operation of air-braked vehicles. The license requirements include knowledge of a daily air brake inspection and testing procedure.

Early air-braked buses used a separate mechanical brake for parking and emergencies. The most common arrangement is a brake mounted on the transmissions and activated by a long lever next to the driver. The transmission brake has the advantage of leverage through the differential (typically 2:1 to 4:1 leverage) and the long hand lever gives both a long travel and good leverage. Such brakes are typically called ``Johnson Bar'' brakes, I do not know why.

Two disadvantages of the manual parking/emergency brake are (a) it does not self-apply if the air pressure drops; and (b) strong application depends on the operator pulling hard. Thus, buses after about 1965 are typically equipped with a brake which self-applies if pressure drops too far, and which can be applied with an on/off switch for parking. The two major types are ``spring'' brakes and ``DD3'' brakes. From the driver's perspective they are similar; see [link] for mechanical differences. Automatic brakes are typically used only on the rear wheels, so if air pressure suddenly drops, the front wheels can be used to steer even though the rear wheels are likely skidding. Automatic brakes also require that air pressure builds up to release the brakes before the vehicle can be moved.


Most buses with manual transmissions --- all except the [Mack clutchless] --- have a clutch pedal. As with the accelerator and brake, there is a desire for simplicity, but the simplest clutch may be difficult to operate.

Clutch force was a relatively minor problem with older buses, because the engines were relatively small --- often around 150 KW. However, greater engine torque requires stronger springs to keep the clutch engaged, and modern buses often have engines with twice the torque of the older engines. As a result, disengaging the clutch can take a lot of strength. And while bus engines may be smaller than many car engines, bus engines often produce their power at low RPMs and high torque compared to a car engine, so torque may be four times that of a car engine with similar total horsepower.

The simplest clutch mechanism is a mechanical link from pedal to clutch. As with an accelerator pedal, wear, forgotten lubrication, the labor of remembered lubrication, getting a straight run for the linkage, and so on are all issues. An additional issue is that manual transmissions are difficult to shift when stopped, so it is common to sit at a light with the clutch depressed. That can lead to a tired leg in a hurry.

For these reasons, some buses are equipped with a hydraulic, assisted, or powered clutch. A hydraulic clutch still uses just the driver's power to operate the clutch, but a hydraulic mechanism may have less slop and stretch than a mechanical linkage. On heavy-duty buses, the most common aid is an assisted clutch: when the driver pushes on the clutch, an air cylinder helps out. Some buses have an air clutch, where the clutch pedal is like an air brake or air throttle pedal, and air pressure runs a diaphragm at the clutch. An air clutch has the disadvantage that it does not work until there is air pressure.

For reasons I do not understand, all are fairly rare, even though drivers frequently complain of fatigue from operating the clutch on heavy-duty buses --- a problem which is made more severe by the relative rarity of synchromesh on manual-transmission buses.

Gear Shift

A gear shift seems like a simple item, but there are several ``suprising'' gear shift arrangements on buses.


Buses from the 1940's through the 1970's often used buses where reverse is selected by a reverse switch. From neutral, activate the reverse switch and move the shift lever to a position which is normally a specific forward gear, and the bus is now in reverse. Sometimes, the reverse switch is on the gear shift lever [[image Flxible Visicoach]]. Other times, the switch is somewhere on the instrument panel [[image GMC1, GMC2]].

Some buses select a low-low gear or select between speeds on a multi-speed differential or transfer case using a switch.

Shift Patterns

Even though many buses use just a few forward speeds, the shift pattern may be odd. For example, [[image Flxible Visicoach]] uses an ``H'' pattern. The usual starting gear on flat terrain is 2nd gear, with low-low first and reverse selected by solenoids. Note that the fourth gear is unlike car transmissions. This pattern is used for the convenience of the transmission builder, but leads to confusion when a single operator uses several types of buses in one fleet.

	2 R     5
	 |      |
	 |      |
	1 3     4

Shift Lever Location

Most buses place the shift lever on the floor so that the shift linkage can go as directly as possible back, along the floor line, to the transmission. A direct path minimizes slop and stretch.

The GMC PD-3751 ``Silversides'' (1948-??) is probably the only bus with a manual transmission shifted from the steering column. Drivinng a Silversides reputed to be especially difficult because the shift lever has a rather rubbery connection to the transmission.


Turn the steering wheel, steer the bus. Simple, right?

Buses are heavy, which makes it hard to turn the steering wheel. Buses use large steering wheels for better leverage, but if there is too much leverage, your hands have to move very fast for normal steering.

In addition, the long wheelbase of buses means you need to turn the wheels sharper than a car to take the same corner.

The faster you go, the easier it is to turn the steering wheel, but at low speeds it can be quite tiring.

Transit buses spend a lot of time at low speeds, and pulling in and out of bus stops. Some routes are on winding streets, and some routes may be over an hour from end to end, with little or no break if traffic has been heavy. Thus, driving in transit service can be quite taxing. As the joke goes, ``Power steering by Armstrong.''

Transit buses were often equipped with power steering as early as the 1950's, shortly after it was offered for cars, and before it was common on school or highway buses. Some buses use air-assist power steering; it is rumored to be helpful, but far short of ``power'' steering. With hydraulic power steering, many transit buses use smaller-diameter steering wheels and/or quicker ratios, reducing the driver effort to get in and out of bus stops.


Instruments tell a driver key things: Is the bus working right? Is the bus moving at a safe speed? Is the bus moving at a legal speed?

For example, buses with air brakes need enough air pressure to work the brakes, so an air gague and an low air pressure signal tell the driver if things are working right. Similarly, it is common to have a temperature guage or signal: bus engines work hard, and can be damaged if run hot. Oil pressure and battery charging instruments are also common.

The speedometer helps with safety and in legal driving. Many roadways are marked with safe speeds. The safe speed may depend on visibility, curves, hills, road conditions, and so on. For example, bus brakes can overheat easily going down hills, so signs will give safe speeds for heavy vehicles. Similarly, maximum speed may be governed by law for non-safety reasons, such as noise, uniformity of traffic flow, and so.

Some older buses use the tachometer to indicate both engine speed and road speed. The tachometer is marked with several bands, one per gear, indicating road speed based on the engine speed and known ratios between engine and road speed.

Using the tachometer this way is simple, cheap, and reliable. Unfortunately, it also requires more of the drivers attention, since the tachometer is complicated and thus more difficult to read [[image]]; and because the driver must remember or discover the current gear in order to determine the proper part to read. Such complications are unfortunate for at least two reasons. First, they are error-prone, so the driver may get the wrong number due to a slight error. Second, they distract the driver from paying attention to road hazards. There are also mechanical complications: changing any of tire size, differential ratio, or transmission renders the speedometer markings incorrect; and, an such markings are impractical with an automatic transmission.

Strip-of-indicators in Flxible transits [[image]].

Visibility vs. driver position [[image: RTS speedometer]].

Accessory Controls

Driver controls are more complex than in cars because there are many more things to control. [[images of some driver control areas: 743, GM, Flxible clipper/transit, ...; describe]]


Combined Switches

E.g., on/off day/night on GMC transits


Transit buses have door controls [[images; describe]].

Highway buses --- external release

Modern key switch

Drivetrain --- Engine Location

Engine Location

Buses are built in several styles: with engines at the front under a separate hood, in the front, next to the driver, in the middle under the floor, and at the rear. Engine placement is a suprisingly comlicated issue.

Let us work backwards from the wheels. It is simplest to transmit power to wheels which do not steer. Rear-wheel steering is unstable, so the front wheels are steered. As a result, all large buses, and most other buses, are rear-wheel drive.

Buses of the 1920's and early 1930's typically placed the engine at the front of the bus out in front of the main body. This is known as a ``conventional'' layout. Conventional placement gives easy motor access, separates passengers from engine noise, and gives plenty of space to lay out a simple drivetrain. In addition, many parts can be shared with truck designs, which helps to reduce the bus purchase price. However, there are several disadvantages. The space in front is ``wasted'' since it does not carry passengers or cargo. It may be tricky to fit the engine, suspension, and steered front wheels in to the same space, leading to design compromises. Heavy engines are over the front wheels, which tends to overload the wheels, but steering pairs of wheels is difficult [link]. Finally, the forward engine must go to the back axle, making it more difficult to build a low-floor bus. For light-duty buses, the advantages

Starting in the 1930's, some buses were made with the engine in front, and with the passenger compartment built around it. [[diagram]] This ``cab forward'' layout allows the bus to seat more passengers. However, the motor housing usually intrudes in to the passenger compartment [[picture]], and the motor is still in front so cab-forward does not solve the other problems of a ``conventional'' layout. The engine also intrudes into the passenger compartment, reducing some of the potential space benefits, while increasing the amount of heat and noise in the passenger compartment.

From the 1930's to the 1990's, some mid-engine buses were built with the engine under the floor, between the front and rear axles. Mid-engine designs use a flat or ``pancake'' design so the engine does not intrude in to the passenger compartment. The mid-engine design can thus seat many passengers. The engine does not overlap the front axle, reducing steering/suspension compromises. The engine weight is shared with the rear axle, reducing the front axle loading. There are at least two major drawbacks to the mid-engine design: a mid-mounted engine is difficult to service, and and the mid-mounted engine interferes with under-floor storage and low-floor designs. Examples of mid-engine designs include some ACF-Brill, Crown, and White buses.

Also starting in the mid 1930's, buses were built with the motor mounted behind the rear axle. This is the dominant design for heavy-duty buses today. A rear engine puts the engine weight on the rear axle. Since the rear wheels do not steer, it is convenient to use dual wheels or even multiple axles to support the weight. A rear engine counter-balances over the rear axle and thus removes weight from the front axle [[diagram]]. A rear engine also intrudes little in to the passenger compartment and allows larger under-floor baggage bays, or low-floor construction. A rear engine is relatively easy to service compared to a mid engine. It is also relatively easy to isolate drivetrain heat and noise from the passenger compartment.

Probably the biggest problem with rear engines is that the engine, transmission, and differential must all fit in a short space. Thus large engines and multi-speed transmissions are difficult to fit. Even where the bus design allows a large rear overhang, the power train must still be short: placing the engine too far aft may remove too much weight from the front axle, making the bus unstable [[diagram]].

How crowded is it in a rear-engine bus? Consider, for example, the length from the rear axle to the rear of the bus. A Yellow Coach 743 is [[distance]] from the center of the axle to the back bumper. The engine is [[length]], the clutch is [[length]], and the differential is [[length]] from the center of the axle to the U-joint yoke. In addition, the bus requires a telescoping driveshaft, so the axle can go up and down relative to the bus body. For 15cm of travel, the minimum driveshaft length is about 30cm. The overall length of engine plus clutch, plus driveshaft, plus differential is [[length]] --- which is longer than the back of the bus! Clearly some innovative packing is needed to make it all fit!

[[A rear engine is near the drive wheels, reducing frame and drivetrain windup.]]

Rear-Engine Designs

Straight In

The simplest design places the engine, transmission, and drive shaft all directly behind the differential. This layout is called ``T-drive''. [[Diagram.]]

[[Example: Flxible. Note engine length, axle-bumper, ...]]

T-drive has the advantage that it uses standardized parts. However, in the 1930's heavy-duty engines were all inline engines. That made the drivetrain too long to fit in buses with a short rear overhang. In addition, inline engines are relatively tall, so they bulged in to the passenger area. These problems grew worse with increasing engine sizes.

Some buses worked around these problem by placing a baggage compartment at the rear of the bus. Thus, the engine only crowded the baggage space. Examples of this type include the some Becks, the Flxible Clipper and its successors, and medium-duty GMC highway buses such as the GMC PGA-3301. Other designs ``annexed'' the rear of the bus. Although there was no engine bulge in the passenger area, the last 1-2m of the bus also held not passengers or luggage. Examples of this type include the GMC PD-3302 [link PBM ODT #1945].

In the late 1950's, V-6 and V-8 engines were developed which were neither as long or as tall as inline engines. At the same time, bus floors were relatively high, especially for inter-city busses with large baggage bays. Thse changes made it practical to build heavy-duty T-drive buses.

T-drive is arguably a better engineering choice than V-drive. However, V-drive was the most common configuration for transit buses well in to the 1980s. At any given time, transit properties would have many ``prior-generation'' V-drive buses in service, and staying with V-drive simplified inventory and training. V-drive finally started waning because [[?? GM was forced to leave the transit business? Because fuel economy demanded better T-drive transmissions? ...?]].

Drop Box

One limit to overall drivetrain size is the driveshaft must be a minimum length. Some buses use a ``drop box'' which allows the driveshaft to to the forward side of the differential, and then gears ``drop'' the power to the differential input. The drop box moves the driveshaft mount point about 20cm [[??]] forward. Examples include Flxible VL-100 and MCI [[XX]].

Disadvantages of the drop box include increased unsprung weight, complexity, nonstandard parts, more friction, and more to break. Historically, VL-100 drop boxes came loose. Solution: remove studs and replace with high-grade bolts, make 'em tight.

90-Degree Drive

One way to get a large engine at the back of the bus is to turn it ``sideways'' so it is parallel to the rear axle. Bevel gears turn the power 90 degrees and send it forward to the differential, where more bevel gears drive the axle. This arrangement allows a short rear overhang and uses a conventional rear axle.

A disadvantage of the system is that motor space is still constrained. Mack designs such as the 1934 CQ model centered the differential. Thus, the engine could be no more than half the width of the bus ([Mack Arch], pg. 71). Since engines are raltively heavy, side-to-side balance may have been hurt by the offset engine. Ford offset the differential on the axle, which allowed a slightly longer power pack. However, there is relatively little space between the rear wheels, especially with wide brake drums, so the gains were only modest.

Since the motor width is constrained, it is not convenient to mount the fan, radiator, and other accessories with the engine. Instead, extra machinery is required to mount them on the other side of the bus.

Another disadvantage is that the drive power goes through two sets of 90-degree bevel gears, and bevel gears are less efficient than gears between parallel shafts.

The Mack CQ and CM also use a non-standard transmission, The Ford uses a standard transmission, but doing so constrains the engine size.

[[Diagram of Mack and Ford transmissions.]]


The V-drive is like a 90-degree drive where the engine is so wide that both ends of the motor are behind rear wheels. Instead of sending the drive shaft straight forward, the drive shaft exits the transmission at an angle and enters the differential at an angle. This angle layout is called V-drive. [[Diagram. Show both rotations.]] V-drive was patented by Dwight Austin of Pickwick; he was subsequently hired by Yellow Coach and the first V-drive appeared in 1934 as the Yellow Coach model 718, type 41.

Compared to a 90-degree drive, V-drive allows a longer engine, better side-to-side weight balance, and simpler mechanical layout. Driveline efficiency may be slightly better: although drive goes through angle gears, the angle is not as steep as a 90-degree arrangement. The driveshaft is also slightly longer, allowing it to operate with less vibration at the extremes of suspension travel.

Disadvantages of this design include the need for both a special transmission and a special differential. The system is sensitive to driveline angle, including offset and mismatched angles [[diagram.]] For example, a 19XX GMC [[???]] uses an [[XX]]-degree driveshaft, while most newer GMC buses use an [[YY]]-degree driveshaft. Thus, it is not possible to switch only the power pack or only the rear differential. (As an added complication, the rear axle of the [[???]] is not bolt-compatible with newer axles, so it is not even easy to replace all components as a unit.)

RJ Long says (paraphrase) 4104 ring & pinion pumpkin offset to driver's side of axle centerline. 4106 ring & pinion pumpkin offset to curb side. (Viewed from above lookingforward from the engine compartment.) Dunno about angle, etc.

An ``accidental'' disadvantage of the V-drive layout is that GMC buses fitted with Detroit Diesel motors put the transmission on the right, which requires the engine to rotate in the opposite direction from normal. Detroit Diesels are easy to reverse (by changing the camshaft), but there is often a large cost difference between V-drive bus engines and otherwise identical engines with normal rotation. Mack used a V-drive with a normal-rotation engine, for example on CM and CO bus models ([Mack Arch], pg. 74), but GMC, the largest volume maker of buses, set the standard. Curiously, some small GMC buses, such as the TGH-3102, used a conventional-rotation engine and the transmission on the left, thus introducing inconsistencies in the product line.

The V-drive layout was important when inline engines were common. When shorter V-configuration engines became available, it was possible to use T-drive, but many transit properties have continued to specify V-drive configuration simply because they already have a shop full of V-drive equipment, and mixing units would mean duplicate spares and varying maintainance and repair procedures.


Another choice is to use an integrated motor, transmission and differential. A transaxle arrangement is short because the differential is rigid with the rest of the power pack and the driveshaft is eliminated. Instead, there are axle shafts leading from the differential to each wheel. A disadvantage of the transaxle is that it requires a non-standard rear axle, and the rear axle must clear the area where the differential is located so it cannot be a simple axle through the centers of the wheels.

Transaxle layout is used widely, for example, on older Tatra, Porsche, and VW cars. A transaxle layout was used in the Yellow Coach model 700 type 40, which may also have been the first rear engine production bus. Yellow Coach soon switched to V-drive; I do not know of any other buses to use a transaxle layout, nor do I know why it is unpopular. I do note the engine compartment did intrude in to the passenger compartment ([YC Arc], pg 42), but it is not clear the intrusion was any more than the pushed-forward seats required by a similarly-tall transverse engine. It is also possible that a laid-over ``pancake'' engine would have fit well.


Remove need to co-locate engine, transmission, differential. Used widely since the 1930's for locomotives, but in buses historically expensive, inefficient, and heavy. Even modern electric-drive systems use a few ranges of straight-through gearing for highest efficiency.

Historical Development

The longer the bus, the larger rear overhang can be tolerated, because there the forward weight has more leverage to counter-balance the overhung engine weight. When rear-engine designs appeared in the 1930's, most buses were 10m (30ft.) and under. Today (circa 2000), most rear-engine buses are 12m (35ft.) and over.

Consider, for example, the AC Transit cut-down Dial-A-Ride buses which were reduced from about 12m to about 10m by removing a window section and the associated bodywork. The bus was fitted with a concrete-filled ballast tank near the front axle to keep enough weight on the front wheels that the bus would handle well.

Allison V-Drive Automatic Transmissions

Allison V-drive automatic transmissions were used widely in buses from 1947 to 1990. There were probably between 70,000 and 100,000 Allison V-drive automatic transmissions made, spanning many models. This section provides a brief introduction and overview.

ModelSpeedsFeatures & Notes
VH2Introduced 1947
VS2-63For DD 6v71-class engine; 0.80:1 highest ratio
VS2-8.13For DD 8v71-class engine; 0.60:1 highest ratio
VS2-8.23For DD 8v71-class engine; 0.72:1 highest ratio
V73030.85:1; introduced 1979.


There are five VH versions: VH4, VH5, VH6, VH7, VH9.


There are three VS2 versions: the VS2-6; and two VS2-8. Detaled info is in the Allison Service Manual (SA 1239G) on the VH and VS transmissions.

In each of the VS2 transmissions, there is a bevel gear ratio and a splitter ratio. The ``splitter'' is a planetary gear set that is on the input shaft. It provides the overdrive function. The overall ratio is the overall ratio from the input shaft to the output shaft in top gear.

In all VS models, the torque converter ratio is 3.75:1 at stall. ``Stall'' is when power is on but the wheels are not turning. The torque converter ratio is typically only important at low speeds, since once you are moving at normal road speeds you have a geared connection.

The VS2-6 has a bevel gear ratio of 1.04:1. The splitter ratio is 0.69:1 and the highest overall ratio is 0.80:1. The highest ratio is obtained by multiplying the bevel gear ratio and the splitter ratio. This is the same ratio that the Spicer four speed had in the 4106. With the 4 1/8 (4.125:1) rear end, it normally gave a top speed of about 120 kph (75 mph).

The VS2-8 had two versions. Version 1 used a bevel gear ratio of 0.87:1 and a splitter ratio of 0.69: giving a highest overall ratio of 0.60:1. Version 2 used a bevel gear ratio of 1.04:1 and a splitter of 0.69:1, giving a highest ratio of 0.72:1.

V700 Series

A typical V730 had a 15% overdrive giving a 0.875:1 highest overall ratio. Some were built with different bevel gear ratios, giving a 1.04:1 overhall highest ratio. The lower-ratio units were mostly used with smaller (e.g, DD 6v71) engines and in steeper hills. By 1990 the sales brochure only showed the overdrive unit. See the assembly number found on the transmission nameplate. Look up the assembly number in the Allison parts manual, which will list it as an A or B group code. Group code A is the 0.875:1 ratio and group code B is the 1.04:1 ratio.

There were several changes to the V730 during its production run. For example, V730d reflects the fourth version.

The VS2 can be overhauled ``in frame'' whereas the V730 must be removed to the bench. One person reports his VS2 gave flawless service for over 80,000 km, and he bought it was used so did not know how many miles it had before he installed it in the bus. The V730 he installed was overhauled twice in 50,000 km --- at great cost.


Each transmission was manufactured with a ``data plate'' that often provides enough information that an Allison dealer can tell you how the transmission was originally built. Note that many parts are interchangeable between transmission models. Therefore, a transmission today may not match the original build.

There were various other sub-models of Allison V-drive automatic transmissions. There was an air-operated Allison(?). It used compressed air internally to shift. Later,there was a hydraulic shit version. The first hydraulic version was called a ``dip shift'' because the transmission would ``dip'' the engine speed while it was shifting to high, in order to avoid a jerk. Later, there was the ``power shift'' which could shift without needing to dip the engine.

See: ``Allison Automatic Transmission V700 series parts Bevel Gear Section Compiled by Gary Carter.''

Spicer V-Drive Automatic Transmission

There was a Spicer 2-speed. Some early Spicers were repudedly troublesome ([Mack Arch], pg. 92). Today, it is difficult to get parts for Spicer V-drive transmissions, and seals are a notable problem, though it may be possible to remanufacture other seals to fit (Coach Maintenance, 2002).


The driveshaft takes power from the transmission and delivers it to the differential. In nearly all buses, the transmission is mounted to the bus frame, while the differential is mounted ot the axle. Since the axle is sprung, it moves; the driveshaft must deliver power while the axle moves up and down. Also, when power is applied to the wheels, there is an oppositereaction force that tries to twist the axle. Although springs and links tend to resist the force, they are not perfectly stiff, and so the axle rotates some under load. Thus, the driveshaft must also allow for that misalignment. [[diagram]]

By far the most common kind of driveshaft is a splined telescoping unit with Cardan-style universal joints, also called ``U-joints''. The spline allows the driveshaft to both transmit drive torque and also get longer and shorter. When the spline is loaded, there is friction between the two parts of the driveshaft and it gets harder to make the drive shaft grow and shrink. Thus, hitting a bump with the power on can cause the drive shaft to pull on the U-joints and other drivetrain and suspension parts. It is thus vital to keep the driveshaft lubricated.

The U-joint allows angular misalignment between the two sides of the joint. If one side turns at a constant speed, the other side runs on average at the same speed, but it speeds up and slows down throughout each revolution. It is as if you were pressing and releasing the accelerator pedal twice for each revolution of the U-joint: on average you are pushing half-way on the throttle, but at any given moment you may be more or less than half-way. The speed may be said to ``flutter''. The larger the misalignment of the U-joint, the greater the flutter. [[Graph showing for input angle N, output is +/-.]]

If both ends of the drive shaft are misaligned by the same amount, then the first U-joint drives the driveshaft at a flutter, but the second one un-does the flutter. In effect, the second U-joint cancels the flutter of the first one. Although equal alignment should be smooth, in practice there is some vibration from constantly changing the speed of the driveshaft. Beware, also, that improper assembly can cause the driveshaft to double the flutter instead of canceling it.[[diagram]]

When the driveshaft ends are misaligned by different amounts, the speed variations do not cancel out. The motor and transmission are a large flywheel attempting to go a constant speed. Similarly, the bus is a heavy thing attempting to go a constant speed. However, the non-canceled flutter is constantly trying speed up and slow down the bus. The result is great vibration and high forces on the drivetrain, leading to quick wear.

For these reasons, drivetrain alignment is important. For simple parts replacement, things will probably be good. However, when installing a new power pack or axle, or when changing the suspension or typical bus loading, it may be necessary to examine the alignment. Note that a shorter driveshaft has to go through a more extreme angle when the axle moves, so alignment is more critical with short driveshafts. [[diagram]]

The usual goal is to have the output of the transmission in line with the differential when the bus is at its neutral or ``nominal'' loading so the axle is in the position it will be in most of the time when cruising down the road. Alignment means that the ends are in a straight line, but it also means that the shafts are in a straight line. [[diagram]]

V-drive is the same idea, it just happens the drive shaft runs at an angle. Note, though, that there is are extra things to consider in setting up a V-drive bus: the differential and transmission shafts can move side-to-side, just like on a T-drive bus, but making the transmission and differential closer or further apart also changes the alignment, which does not happen with T-drive. Also, all T-drive differentials and transmissions are 90deg, but V-drive differentials and transmission may have different angles. [[for example: ???]]. Although the alignment does not need to be perfect, it does need to be very close or the drivetrain will wear quickly.

Chain Drive

A few old buses (c. 1920's and earlier) used chain drive. In these designs, the motor drove the sprockets mounted on the frame, and the sprockets drove a chain that in turn turned the rear wheels. The chain tension under load was opposed by an [[??]] arm with pivots at both sprockets.

Chain drive is relatively simple and rugged. It also mounts the differential on the bus, rather than the axle, improving ground clearance. The drive chain is exposed, increasing service needs and risk of contamination. Chain drives cannot be used with high-speed buses because high centrifugal forces limit peak chain speeds.


The differential takes power from the driveshaft and delivers it to the axles.

Differentials come in many ratios. The axle often has a tag saying the ratio. Example: RTS were built with ratios of 5-3/8, 5 1/8, 4 5/8 and 4 5/9. At least four other gearings fit: 5 6/7, 4 1/8, 4 1/9, and 4 1/10. All are hard to find used. 4 1/8 and 4 1/9 were maybe available in GMC Suburbans. 4 1/10 is available from Meritor at about $1600.00 just for the two gears. It is reputedly harder and more expensive to set up. The expense of rebuilding the carrier could be another $1000. 5 6/7 == 50 mph 4 5/8 (4 5/9) == 65 4 1/8, 4 1/9, and 4 1/10 == 72mph A complete gear set/carrier is sometimes called a chunk or pumpkin. Finding a high speed set is difficult because transit axles often use a 14 bolt carrier instead of the 12 bolt that goes in the parlor coaches. The 14 axles have a physically larger ring and pinion than the 12 bolt. I have found a 4 5/8 NOS at a bargain basement price of $200, and I cannot afford to not use it. If I change to slightly larger tires, I can nudge 70 MPH part of the time. BTW, the governor on my 6V92 TA is already turned up to 2200 RPM, and all the Detroit experts to whom I have spoken, say that at 2300 a 6V92 risks throwing a rod and self destruction. The only other issue is fuel economy, and at 2200 I get about 6.5 MPG. With the 4 5/8 I will still get only 6.5, because I will still be going flat out at 2200 on the highway. With a 4 1/10 I could back off to 65 or 70 at 2000 RPMs and get about 8 or 9 MPG. This would be nice, but it actually only saves about $40 per 1000 miles in fuel. So it is a trade-off I can live with. Some equipment codes at

http://www.angelfire.com/ca/TORONTO/VINcode.html Very useful!


It may be desirable to put newer equipment in an older bus. Some reasons include:


ACF Brill

ACF Brill was the company formed when ACF, ``American Car and Foundry'' purchased J. G. Brill.

See the Pacific Bus Museum roster for a good summary of ACF Brill highway buses. ACF Brill also made transits.


Built a glasstop.


Resold Flyer products --- bus & trolley bus. AMG history 1908 John Willys bought Overland automative. Built Willys-Knight and the "Whippet". Reorganized in bankruptsy in 1936 became Willys-Overland Motors, Inc. Build 350,00 Jeeps for WWII. 150,000 vehicles for Korean war. Bought in 1953 by Henry J. Kaiser interests and renamed Willys Motors Inc. Started building USPS delivery vans in 1960; over 14,000 made. 1963 renamed Kaiser Jeep Corporation. 1964 bought Studebaker Corporation manufacturing facilities. 1967 started building USPS DJ-5 Dispatchers; biult over 150,000. 1970 bought by American Motors. 1971 renamed AM General Corporation. 1970's (?) entered bus field; exited 1978, over 5,000 buses produced at Mishawaka facility. Bus was a modified Flyer design. Both diesel and trolley buses. ([[Also articulated troelley buses?]]) 1979 started work on HMMWV (Hummer); 43,000 made by 2000. 1992 bought by Renco Group, Inc. of New York City. 1992 start producing civilian Hummer.


[picture] (Is this a model of ACF or Brill?)


Early body builder.




[Vern Rainey's 1940's Beck]

Beck built a DH-100 Scenicruiser lookalike. Worked with White. Bought by Mack about 1955.


Bender body company.


Front-engine and rear-engine steel-body medium-duty transit and school buses. In business circa 2000.




Built mostly heavy-duty school buses also built some highway buses. Mostly built mid-engine buses. It operated in the Los Angeles area of Southern California from the early 1900's to the early 1990's.

See the Buskid's web page for more on Crown.



Among other things, Dina bought rights to produce Flxible Hi-Liner and is one of only a few N. American bus companies operating circa 2000. [[Hi-liners reportedly redesigned for rougher Mexican roads and service.]] [[Now owned by MCI?]]


Divco bus.



Body builders. Twin engined 4WD Twin Coach 40 with Eckland body.


Bought by Twin Coach to form Twin-Fageol.


Muskegon Michegan Several Thousand built End 1957 Fitzjohn Falcon Jr., late 1930's Built some bodies for Ford, Reo, Studabaker.


Started as a motorcycle sidecar company. Built buses from XX to YY. Built the well-known ``Clipper'' models. Also built heavy-duty highway coaches -- VL-100 and Highliner, New Look and ADB transits. At least two of Elvis Presley's buses were Flxibles. Elvis was a truck driver and reportedly enjoyed driving. Flxible bought the Twin Coach bus business in the 1950's. The ``B-9'' model was raised 7 inches between floor and windows to provide a flat floor; not seated, used for showrooms, motorhomes, etc.


Some bodies by Fitzjohn


Garwood. Another Garwood.


Gillig made mostly school buses for a long time; currently (circa 2000) they produce transits almost exclusively. Manufacturing plant in Hayward, California.

See the Buskid's web page and Gillig's web page for more on Gillig.


General Motors Corporation. Purchased Yellow Coach. Built the well-known Silversides and "Scenicruiser" models. In the 1940's through the 1960's, built more American transit buses than all other American makers combined. Also built the PD-4104, which at [[4,500]] built, is the largest production run of any single bus model [[What about MCI's?]]

Highway Products

Built buses, HPOs, and ...? Used the old Twin Coach plant in Kent Ohio starting as early as 1960 (e.g., Terminal Island CA Transit Co. had a 1960 Highway Products HB-35, S/N HP-102). Built later-model 1960's and 1970's Twin Coach transits such as the TC-25 and TC-31 models, and briefly owned Divco (more here).


Indiana Mostly trucks but made some buses. (four-tire circa 1932)


In 1946, auto manufacturer Kaiser produced a prototype 63-passenger articulated highway bus that was all-aluminum [Kaiser bus]. [Kaiser Artic]


Spokane, Washington Built buses from ?? to ?? including a 1939 model designed to resemble the YC-743. MEHVA currently has shells of three Kenworth suburbans.

Lang: an early body builder ([YC Arch], pg. 17).



Started as a bus company, first Mack bus built in 1900, generally credited as first motor bus. Mack ceased bus production in ?? (still selling buses in 1985, mostly sold overseas) still in the truck business as of 2003. Bought Beck about 1955.



``Motor Coach Industries'' Bought by Greyhound in ?? after Scenicruiser problems.




Formerly Fithugh?


Canadian, operated an assembly plant in New York.


Body builder.


Early body [[and more?]] builder; created double-level sleeper coaches with restrooms in the 1920's, as well as a variety of other innovative designs.



Canadian. Entered U.S. market sometime after 1968.


Some bodies by Fitzjohn



Some bodies by Fitzjohn


Mexican builder. Built a split-deck model like a Scenicruiser, but with twin front steered axles. Museum of Bus Transportation.


Thomason 1980-present body builder. Mostly school buses.

Thompson 1930's body builder [YC Archive].

Twin Coach

Bought Fageol to make "Fageol-Twin".

Yellow Coach

Bought by GMC.


White. built bus chasis and buses. WC-18B, front-engine transity 1950's Worked with Beck.


Bought by Greyound ??. Built the first buses Greyhound built and the first purpose-built intercity buses.

Niagra Frontier


Coachworks 1954 9218 ACF Brill Formed of ACF and Brill. ACF made front-engine buses in the 1930's. ACF-Brill made mid-engine highway buses in the 1950's.

Noted Models


The GMC and TMC ``RTS'' model was one of the most distinctive-looking late-century ``ADB'' buses. It had wheelchair lifts and independent front suspension, and a rounded shape unlike other buses. Design features included:

Unfortunately it also had a large number of ``teething'' pains and a variety of design disadvantages compared to earlier buses, including

In addition to the above problems, GMC exited the bus business in [???], so the remaining models were all built by TMC, which was a Canadian subsidiary that was made a separate company when GMC exited the bus business. Although the TMC buses were built well, ordering from outside the U.S. was more complicated for some properties buying the buses.

Despite these problems, RTS buses were ordered in large numbers and many saw service lives over ten years. This was due to a variety of factors including


Built by GMC for Greyhound. One of the most recognizable buses built, it had a low deck in front and a high deck in back with extra storage underneath, a general design theme which was copied by many other makers for decades to come, although the basic form appeared at leastd as early in the 1920's [pic, pic]. The Scenicruiser is also rumored to have a first in toilets somehow though toilets had appeared as early as the 1920's on some limited-production and special purpose buses [1927 Pickwick sleeper]. and were in common use by the late 1930's, for examnple the Yellow Coach 743. There were 1001 Scenicruisers built, all for Greyhound.

The original Scenicruiser design gave Greyound many problems. There were reports of metal buckling under the high passenger windows. The drivetrain used a pair of Detroit Diesel inline 4-cylinder diesel engines, connected to each other by a viscous coupling. The engines and fluid couplings were a source of problems and Greyound eventually sued GMC, who subsequently replaced the engines with newer V-8 diesel engines, which gave reliable service. However, the episode strained relationships between GMC and Greyhound, leading Greyhound to purchase MCI and eventually stop buying buses from GMC.


The PD-4106 was built by GMC from ?? to ??. The predecessor, the PD-4104 was first offered in ?? and incorporated many modern features including air suspension, toilet, unit-body aluminum construction, ... The 4106 was a refined version of the 4104. Although the 4106 was produced in smaller numbers, it is regarded by many drivers as the pinacle of highway bus design -- powerful, available with automatic transmission, good handling, very reliable operation, and so on.

Although more recent buses do not sacrifice power, air ride, etc., and tend to be more comfortable and have more convenient transmissions, more luxury for passengers and the driver, etc., they also tend to have more complicated electronics and corresponding failures; many have been built with part-steel construction that leads to eventual rust problems; some have had problems with engine or transmission access; etc.


Motros from many makers were used in many buses.

Detroit Diesel in Yellow Coach/GMC, Crown and many bus brands after 1980. The 6-71 inline was offered in a horizontal configuration suitible for mid-bus underfloor installation. Detroit Diesel was divested from GMC in ??

Hall-Scott in Crown and Flxible. Hall-Scott offered their engines in a horizontal configuration suitible for mid-bus underfloor installation.

Hercules in Fitzjohn and Flxible International in Beck and Crown. Crown Gillig

[PBMa 01] Web page on the Pacific Bus Museum's Trailways ((Virginia Stage Lines) #705, From http://www.pacbus.org/pbm705picpage.htm, 2001/07/16.

The only thing I know to watch out for on Flxible buses is the windshields. They made that body style from the late 30's or early 40's (not sure which) until 1967. There were several body styles, all similar -- Clipper, Visicoach, X-Ray, Post Office, Starliner, Airliner, and I do not know what else. Here is a picture of a slightly newer Visicoach: http://www.pacbus.org/pbm77picpage.htm For the remainder of this note, I'll just use ``Flxible'' to mean one of the Clipper-esque models. They also made other highway coaches such as the VL-100, Flxliner, and Hi-Liner, and starting in the late 50's transit buses until they finally went out of business in 1997 or so. Never mind all those, let's just talk about the Clipper-like ones. From about 1946-1967 Flxible used the curved windshields. You can still find them, but you need to be sure to factor in the greater price. And if you do get a Flxible you might want to carry insurance in case the windshield does get busted and needs to be replaced in a hurry at great expense. Other than that, most of the parts on Flxibles seem to be pretty easy to find or replace with something else. Oh yeah -- the head badge (center, just under the windshield) is probably worth a couple hundred $. The Flxible buses are a medium-duty bus, with a steel body on a steel frame. This lends them to rust. Somebody recently gave me a Flxible with no engine or transmission; I am having one put in by somebody who (a) is a professional bus mechanic; and (b) has two Flxibles, one to restore and one as a parts bus. All three (his two and my one) have serious body rust. The worst possible rust is if the curved panels are rusty, as they have compound curves and cannot be made in a sheet metal brake, etc. That is why my friend has the parts bus -- the one to restore you can stick you finger through some of the curved body panels. On the other hand, there are plenty of Flxibles with just fine bodies, and rust can be treated and fiberglassed over if the panel still has it's shape. It's just something you need to know about. Aluminum buses corrode, too, but usually a lot slower. ``Medium-duty'' means you might not want to do a lot of cruising at high speed, according to a friend of mine. Their lower weight makes them better for windy roads, but a little more likely to blow around, according to one of my bus-driver friends. Nowhere near as much blowing around as a motorhome, though -- the Flxibles tip the scales around 16,000 lbs. These Flxibles have an unusal configuration: there is no storage under the passengers, instead there is a baggage bay at the rear, built around the engine. This means that a 33' bus (typical for the Flx) has less interior space than a similar-size ``another bus'', but it is also visually smaller and has better aerodynamics and should give slightly better fuel milage (all other things being equal). The Flxible has a dropped center aisle and somebody about 6' tall can walk down the center aisle, but not the sides. There were a few made with a raised roof, e.g., for post offices, portable X-ray buses, etc., which have 1' more height and can have a flat floor and still have plenty of clearance inside. Otherwise, if you are tall, plan on standing only in the middle. Flxibles sometimes have engine cooling problems. This is something to be aware of and maybe something to work around, but not serious if you take any of several remedial steps, and so is more a ``be aware'' than a ``be scared''. Again, every bus model has its private pitfalls. Some vintages of Flxibles (but I do not recall the exact years) do not use a conventional spring system, but instead use linkages and a pair of concentric tubes with rubber in between them, called ``torsion bar'' suspension (though different than a VW beetle). According to the guy who remanfuactures them (Dick Lender, contact info on the Flxible web site, see below) they tend to last about a million miles, but can be expensive when they go. ``I service about one set every five years'' he told me. First find out if the bus has conventional springs or torsion bars. If springs, you're done. If torsion bars, find out if they are at their adjustment limit. I _think_ 1952 uses springs, but I'd ask for sure. Flxibles are very popular for motorhome conversions. See the ``Flxible Owners International'' web site (let me know if you can't find it easily) for many pictures of many conversions. You might want to ask if the engine siezed in use -- which might indicate some cooling work is needed -- or siezed while sitting. You might want to find out if it is gas or diesel. The power pack that is in it might still be worth a few $ even though siezed. That one has roof vents as if it might have a 3-way fridge (good if it still works), the A/C would be a nice boon (if it still works). The body looks nicer (okay, it's a grainy picture) than the one I just got. Then there is the minor matter of getting it from Iowa to your home. I'm afraid that wasn't very well organized, but let me know if I can answer any questions, I'll try.


Water leads to rust; freezing water can pry things apart and lead to rapid destruction. Sunlight degrades rubber, plastic, cloth, leather, paint, ... just about everything other than metal. Outdoors, branches can fall, people shoot BB's and small guns, people spray grafitti, and sometimes folks will break in to the main area or storage areas just for fun. (A friend of mine damaged his bus after driving it without coolant, after somebody reached through locked grillwork and took the radiator cap.)

So indoor storage is good if you can arrange it. Covered outdoor storage is next. Beware of just draping plastic over it, though, as you can accidentally create a greenhouse and the heat can damage things.

Also protect the mechanical equipment. From Fast Fred:

Protect the systems, too:

More on old buses

Other notes:

Back to your original question: if it was a Crown, my guess is that it had dual rear wheels. If it was a Crown with slanted windows, my guess is that it was indeed a highway bus. I don't know how old it was or when you saw it, but 5 million miles seems like a lot just because that is a lot of hours of driving, even on a highway bus. If it wasn't a crown, I am still curious what it was, and either way I just learned a bunch about Crowns!

The Flxible Clipper/Visi-Coach/etc. is a medium-duty bus. GVW is about 7,000kg vs. about 10,000kg for a heavy-duty bus. They were relatively inexpensive in their day compared to a ACF Brill, Yellow/GMC, etc. At least some of the movie studio buses were used by the studios to give people tours -- the fellow who runs "sellabus.com" is selling his personal Flxible; if I recall correctly, it was originally that kind of bus. Flxibles were less durable and had less storage, so they were not typically used for all-day every-day service.

I do not know how many of this style were made. ``www.flxible.net'' has a history section which says "5,000" but I do not know if that is Clippers only or all Clipper-like Flxibles. Flxible made made Clipper-like buses from about 1940 to about 1970. For comparison, GMC made about 5,000 of their heavy-duty PD-4104 model over about 7 years. I think that is the most that any bus company has ever made of a single highway bus model. GMC was probably the biggest highway and transit bus maker from about 1940 to about 1980. The beginning of the end for GMC was when they built a run of buses for Greyhound which had severe mechanical problems and Greyhound bought their own bus company, MCI, and largely stopped ordering GMC buses.

Flxibles have steel bodies, which reduced manufacturing cost but means you have to be careful about rust today. They have less underneath storage which makes them less attractive for RV conversions. They were typically built with a low ceiling - 6'4" *maybe*. They use a dropped center aisle, so if you try to make a flat floor you lose 4" out of that. There were a smaller number of buses built (from the factory) with raised roofs. See the purple bus at ``www.sellabus.com

On the one hand they were medium duty and so less durable; on the other hand, they were usually used on side lines and so got less use and an RVer typically will not add a lot of miles to an existing bus (e.g., 10,000 miles a year for 20 years is a fair bit of driving but "only" 200,000 miles). Mechanically they are good buses -- nothing exceptional, but nothing bad I know of. They use a "T"-drivie configuration -- the engine crank is in line with the length of the bus -- and were originally equipped with a (fairly long) Buick straight-8. Therefore, you can put a newer power pack in them; many people do that. Most Flxible parts are relatively easy to find.

Probably the biggest problems are (a) limited headroom -- I am about 2m tall and I can walk the center aisle but just barely; (b) steel body and corresponding rust; (c) the front glass is hard to find and expect to pay on the order of US$500 for each pane for a new windshield -- so make sure you have the right insurance!

Maybe the owner ran the milage up?

It is certainly possible -- and the more proud the owner, I suppose, the more likely. My guess is that if it had slanted windows it may well have been a highway bus. Let's say it were a 1955 model and was retired in 1985, 5 million miles would be under 200,000 miles a year. That's in keeping with the miles run by a fleet operator, but fleet operators tend to have a lot of one kind of bus and get rid of their remaining few when it no longer makes sense to keep parts and tools for just a few of those buses. So it could go either way.

I wonder what is the most miles on one bus?

I have no idea. I'll ask one of my friends if folks even try to keep track of that.

> I have a clearer picture of what to look for, if I were
> going to look: Substantial headroom, diesel engine, Eaton
> 10-speed transmission....

And rust. Note that Mr. Sharkey's also had fatigue along the window pillars.

If you are talking specifically about Crown buses, yes. If you are talking about buses in general, no. For example, the only big problem with the Hall-Scott is parts. Plenty of other good gas engines are available if you don't need the engine to lay on its side. If you are looking outside of Crowns, there are lots of other good/bad things to look for.

My impression is that, among the older diesels, Cummins were very reliable, Fords were powerful, and GMs are maintenance headaches and to be avoided. How were the Caterpillars and John Deeres in your opinion? Did I leave any out?

If you can find more info on "GMs are maintenance headaches" I would like to know more. The things I have heard include (a) the design was basically unchanged from 1930 to 1990; (b) they were very reliable if not over-revved; (c) they leaked oils like a sieve until relatively recently -- rebuilds done after 1980 probably won't leak; (d) you can't get the exhaust as clean as a 4-stroke diesel; and (e) big rig drivers sometimes called them a "two-stroke joke" because they do not have the torque of a heavier engine. That said, I don't work on them, so if you have additional information I would like to know!

Note that Detroit Diesels are used widely in boats, which often have higher standards of reliability than wheeled vehicles.

I know essentially nothing about Ford and John Deere diesels. My impression of Cummins engines is they are heavier per horsepower than Detroit Diesels, but also have more torque and they are reliable. My impression of the Caterpillar engines is the old ones were even heavier per horsepower, some recent lighter ones were trouble but others were reliable. "Trouble" is relative to industrial use; I don't know if an occasional user would notice.

Is spring ride good or bad?


Spring ride uses a metal spring and it gives a pretty uniform spring rate across the entire travel. An air bag spring -- literally a rubberized bag full of air! -- gives a soft spring rate in the "middle" position but a stiffer spring when compressed. So you can get a soft spring in the middle without having it bottom easily at the ends of the stroke. In addition, a leaf spring (the usual kind of metal spring for high-load applications like buses) has internal friction that you have to overcome before you can move up and down, so the metal spring has more jolt over small bumps than an air spring over the same bump.

There is a third kind in common use in the bus industry, called a "Torsilastic". It has two tubes, one inside the other and rubber filling the space between them. One tube is attached via links to the wheel, the other to the frame, and wheel motion twists one tube inside the other. This avoids the friction problems with metal springs and has some other properties I don't undersdtand.

Springs are simple and reliable and it is easy to get parts. Air bags give a more comfortable ride but need to be inspected and replaced periodicalloy as they wear out (so do metal springs) but when they go they fail with a "bang!" and may cause the bus to jerk suddenly to one side. Air bags also allow the bus to "kneel", useful for transits. Torsilastics apparently have many of the advantages of air springs without the sudden failure problem. They were used in some Flxibles and Eagles, and the Eagles have a reputation for an especially nice ride. They Torsilastics have good durability -- in the million-mile range -- and fail by slowly sagging and losing travel. But getting a rebuild may be a problem. (Dick Lender in Michigan does it.)

Low headroom ruins it for me.

I suggest you try it for yourself -- there are different models and maybe some or even many have lots of headroom. It should not be too hard to find some. Even if you plan to never own a bus, it might be fun to get "up close and personal" with a Crown and its owner. Where are you located?

Beware aluminum <-> steel joint problems.

I would just generally worry about it everywhere on every bus with mixed aluminum/steel construction. It is sometimes clearly a problem, but also clearly not a problem sometimes.

I don't expect to own one.

You can still have fun with 'em! I think that relatively few bus fans own buses -- even fewer train fans own trains, but that does not seem to interfere with anybody having fun with them.

One of the first bus that Raymond Lowey designed and I think it is pretty neat, too. This site has lots of other neat bus pictures, too.

If you like that sort of thing, you might also enjoy Fairfax and Sweepers.

Flxible clippers were less durable and had less storage, so they were used mostly on side-line service.

I believe -- though I could be wrong -- that the above "problems" were all fixable, but Flxible understood their market and decided to not build them as a heavier bus. At the time they introduced the Clipper, the country was on the heels of the Great Depression, and Yellow Coach was already a big player. There was a big market for well-built mid-priced buses. Later, Flxible had a sweetheart deal with GMC, which enabled Flxible to get Detroit Diesel (GMC engines and stay in business. Most other bus makers who did not manufacture their own engines exited the bus business. GMC built few medium-duty buses, so Flxible could stay in that market without biting the hand that fed it. Later, when the U.S. government started making anti-monopoly motions at GMC, Detroit Diesels became more widely available and Flxible could and did enter the heavy-duty transit and highway bus markets

I could be wildly wrong, too.

Steel, limited storage, limited headroom, center aisle -- that explains why there are so few Flxible conversions.

The Clipper family is one of the most-converted buses. Although the above things make them less desirable for convers, those things also tend to drive down the price and their other good attributes -- good looks, can put in newer drivetrains of all sorts, ready availability of most parts -- make them otherwise popular.

See ``www.flxible.net for some examples.

I think the conversion market can be divided in to "bands" of price with correction factors for age. Among older buses, the top-end conversions are heavy-duty flat-floor highway coaches with big storage bays. The GMC, MCI, Eagle, etc. buses are all examples. The big thing that drives down the price of these is deterioration and parts problems -- A GMC PD-4014 takes a reverse-turning Detroit Diesel 6-71 (inline) and 4-speed Spicer manual, and it is a problem to put just about any other power pack in it. PD-4104 parts are getting scarce, and ... But aside from those issues, the conversions tend to go for a lot. Note that in later years Flxible made a series of heavy-duty highway buses and the later ones were good for conversions but not a lot were made. The GMC PD-4501 "Scenicruiser" is also high-doller despite the floor because it is one of the most recognizable buses ever.

The next category is heavy-duty transits and medium-duty highway buses. I think the big examples here are GMC and Flxible new-look transits, which were produced in huge numbers and for which parts are still available, and Flxible Clipper-family buses. This class very rarely sells for more than US$50,000 whereas a swank highway bus conversion of the same vintage rarely sells for over US$200,000. (Late-model examples can sell for US$1,000,000.) Typical examples of the class are probably US$15,000, while typical examples of the highway bus class are probably US$30,000. Only part of this has to do with the bus itself: why put a US$100,000 interior in a US$10,000 bus, when you could put it in a US$20,000 bus and have a much nicer package for only a small percentage more. What gets put in at the high end of the market in turn affects what "trickles down" to the more affordable part of the market. The GMC PD-4104 buses are now "falling in" to this category. Older heavy-duty highway buses like the GMC PD-4103 have a center aisle and less headroom, less storage, and fewer parts. So they too are in this class, albeit in smaller numbers.

The third category is buses with yet less interesting looks, yet less headroom, yet less available parts, or other mechanical "challenges". Major examples in this class are school buses ("skoolies") and older transits. School buses often suffer from limited headroom, steel construction, medium-duty or light-duty construction, and because they have the engine out in front you get less usable floor space for the overall bus size. That drives down the price. The low price has made them popular for "budget" and "hippie" conversions, which in turn gives them a bad reputation and by extrapolation means if you are driving a school bus conversion, makes you an undesirable person. Which drives the price down further. Old heavy-duty transit buses very often have a top speed around 50mph and poor hill-climbing to boot, and are a parts problem both in terms of general maintainance and in terms of changing the power pack to get a higher top speed. This class rarely very sells for more than US$20,000 and typical examples of the class are probably US$5,000.

The same rules often apply to the price of a seated bus, though beyond the collector value and condition have a lot more to do with the price. For example, a seated GMC PD-4501 "Scenicruiser" will probably sell for more than a GMC PD-4102 of similar condition, even though the PD-4102 is rare. Why? The Scenicruiser is far more distinct and represents a big change in the bus industry.

As with most rules of thumb, there are gray areas and exceptions. Many. Recent (1950-2000) Crowns and Gilligs are more durable construction than typical school buses, have more floor space, etc. They fetch more. Elvis's Flxible VL-100 will undoubtedly fetch more than a flat-floor highway bus of similar quality.

Among newer buses, the prices are higher but the price differences are even greater. A brand new heavy-duty empty highway bus shell costs over $300,000 and according to [? I forget which magazine ?] in 1999, 10% of the highway buses made in North America (Mexico, U.S., Canada) were sold directly to RV conversion companies. Heavy-duty transit bus shells are slightly cheaper but lack under-floor storage. I am not aware of any that were sold to RV conversion companies (though almost certainly _some_ were). Among twenty-year-old conversions, highway buses go for far more than transits. Again, part of the issue is why put a lot of money in to a nice interior for a not-so-nice bus.

(All prices approximate; prices circa 2000; and so on.)

The only big problem with the Hall-Scott is parts? Didn't I read about the terrible gas mieage of the Hall-Scott? What was it, as little as 2 mpg?

See the writeup of the ACF Brill with a Hall-Scott engine, described under the Roster of the Pacific Bus Museum.

Poor fuel economy is aa problem, but it's not a big problem -- call it 3mpg and $1.50/gal. for fuel. (Gasoline is far too cheap in the U.S., since the price does not at all reflect the actual costs of consumption.) That is $0.50/mile for fuel, but nothing when you are stationary. An efficient bus will get more like 10mpg which is $0.15/mile for fuel. Other random costs will be in the neighborhood of $0.30/mile. The efficient bus or engine will also cost more to purchase and insure (since it drives up the value of the vehicle). How many miles will you drive it? If you drive 10,000 miles, the price difference is $8,000 vs. $4,500, at which point, if you have the 3mpg thing, you decide you really do like driving around and have also learned a lot about what you like and don't like in a vehicle, so you go get a different one. In the meanwhile you got in cheaper and got to play and learn and find out you really did like it. And 10,000 miles is a couple of loops around the U.S., which will take a long while if you take the time to stop and see the sights.

By comparison, if you have to buy more than a few expensive parts for your Hall-Scott, you can quickly blow through $3,500, and you didn't get any driving in for your money. That is a big problem.

For more about the costs of motorhoming, see ``http://bart.ccis.com/home/mnemeth/index.htm.'' This guy is great.

Detroit Diesels are 2-stroke? In diesels, is there an inherent advantage of one type over the other (aside from torque)?

Heavy-duty Detroit Diesels are 2-stroke up to about 1990 when they switched to a 4-stroke design for better emissions.

The diesel 2-stroke design is not as dirty as a gasoline 2-stroke because the diesel does "scavenging" -- cleans out the exhaust gases from the last power stroke -- with air, while a gasoline engine does scavenging with an air/fuel mixture, so you are always fighting between leaving useless exhaust in the cylinder and spewing unburned fuel out the tail pipe. In addition, most 2-stroke gasoline engines rely on fuel mixed with oil to lubricate the engine, which makes them quite a bit dirtier. In contrast, the 2-stroke diesel uses a supercharger to push air in ports at the bottom of the cylinder bore and then out conventional exhaust valves in the head, and fuel gets squirted in to the cylinder later on, at the top of the stroke. And lubrication is done conventionally, too. So the 2-stroke diesel is relatively clean compared to 2-stroke gasoline. 4-stroke diesel is cleaner than 2-stroke becuase you don't try to do as many things at once.

The diesel 2-stroke design was invented by Winton which was bought by GMC and turned in to Detroit Diesel. The initial big markets were boats and locomotives -- engines that weigh as much as a whole bus -- but smaller versions were put to use in many other applications. By 1939 or so Yellow Coach produced the 743D, an all-metal, rear-engined, diesel bus.

2-stroke engines produce power once for each up/down motion of the cylinder. 4-stroke engines produce power once for each two up/down motions of the cylinder. So all other things being equal, a two-stroke should produce twice as much power for the same size of engine, thus giving them a big power-to-weight advantage. In practice, 2-strokes have some inefficiencies, and there are a lot of design tradeoffs that make two otherwise-similar engines have differfent weights.

See, for exmaple, ``http://www.adieselengine.com/new_page_1.htm''. The -92, -71 and -53 DD's are 2-stroke. (Series-50 and -60 DDs are 4-stroke.) Here is some of the info:

4600 lbs => 7.3 lbs/HP, 4800 lbs => 6.6 lbs/HP, 2870 lbs => 8.5 lbs/HP 2870 lbs => 6.4 lbs/HP 3725 lbs => 7.1 lbs/HP 3725 lbs => 6.2 lbs/HP 2080 lbs. => 6.5 lbs/HP 2080 lbs. => 4.9 lbs/HP 3240 lbs. => 8.5 lbs/HP 3705 lbs. => 6.3 lbs/HP
DD 8v71240 KW (318 HP)1050 kgf (2310 lbs) 226 W/kgf (7.3 lbs/HP)
DD 8v71T270 KW (362 HP)1134 kgf (2496 lbs) 238 W/kgf (6.9 lbs/HP)
DD 16v71474 KW (635 HP)
DD 16v71T541 KW (725 HP)
Cummins NTA-855250 KW (335 HP)
Cummins NTA-855336 KW (450 HP)
Cummins KTA19392 KW (525 HP)
Cummins KTA19448 KW (600 HP)
Caterpillar 3208T239 KW (320 HP)
Caterpillar 3208TA317 KW (425 HP)
Caterpillar 3406BT300 KW (402 HP)
Caterpillar 3406BTA436 KW (585 HP)

250-ish KW motors are bus-sized, 500-ish KW motors are for heavy equipment and one of several engines in locomotives. Engines in the many thousands of KW are boat-class, mining and stationary.

The higher power/weight ratios are for engines which have more "gongs and whistles" like turbochargers, superchargers, intercoolers, and so on. The engine stresses are higher and the engine lifetimes are typically less.

A friend of mine told the story of going on board a diesel-powered tugboat built in the early 1900's. In normal operation, the engine turned around 100 RPM and presumably produced pretty low horsepower. (There was some detail about how cool the engine ran, indicating it was very low horsepower.) The engine room was outfitted with the original molds for a bunch of the engine parts so you could make replcaements as needed. He asked the operator about it, the operator said as far as he knew the engine had never had anything other than oil changes and adjustment. In something like 80 years of regular service. Good reliability, but there's not much market today for bus-weight engines that put out a few tens of kilowatts!

I believe torque is a function largely of the inertia of the moving engine components, and that Detroit Diesels have relatively more mass in the non-rotating parts.

I hear diesel injector parts are easily damaged by touching them.

I recall somebody said that through the 1970's, diesel injectors were the highest-precision moving part in mass-produced vehicles. It might still be true, I don't know. A piece of paper is about 5 thousandths of an inch thick; if I recall correctly, injectors need to be sized to a specific dimension plus or minus about 1/100 of that.

The ``Torsilastic'' suspension reminds me of torsion bar suspensions fron Volkswagens, Porsches, and Army tanks. I wonder why they never caught on in a big way. The technology was around since at least the '30s.

The basic issue with springs is how much energy can you store in how few pounds of material. With steel, if you exceed that limit, the spring will yield (bend and not spring back) and the vehicle sags. A friend of mine who worked at Porsche when they still used the VW front end said that both brands of cars were noted for sagging.

In a little bit more detail: for a given material, the big thing you can do to the spring design is to change the shape of the spring element. A square bar (as used in the VW/Porsche) has about 2/3 of the steel operating at relatively low stresses compared to the other 1/3, while a coil or leaf spring has more like 1/2 of the steel operating at relatively low stresses. So you can put more energy in to a pound of steel used for a coil or leaf spring than a pound of steel used for a VW torsion bar. A friend of mine who worked at Porsche in the 60's said that VWs and Porches of the day were known for sagging front ends.

This is not an intrinsic issue with torsion bars, but changing the VW/Porsche design increases the cost and may change the spring dimensions.

I would guess Torsilastics would break down gradually or snap--but you say they have great longevity.

The rubber does take a "set" after a while and the bus begins to sag, at which point you wind up the outer tube a bit and raise the front of the bus again. You keep doing that and eventually reach the limit of adjustability, at which point it is time to replace the rubber. Dick Lender said that takes about a million miles.

It sounds like airbags are the spendier system, in terms of maintenance over time. The Torsilastic system is a new one on me; but it sounds like a good one. So if rebuilds are a problem, aren't there stocks of new ones in standard sizes, like fanbelts?

An air bag is pretty cheap -- it is basically fabric and rubber, and an air bag that can hold up a bus weighs less than ten pounds. It will last long enough (decade) and is easy enough to replace that in regular service you probably have the bus apart for something anyway and just replace it. For RVing and on a budget, it is an extra thing to worry about.

Torsilastics were used mainly on Flxible and Eagle buses. Flxible made perhaps 3,000 buses with Torsilastics ending in about 1975, and Eagle similarly. (My numbers here are probably wrong.) Not very many. Yes, they are around, but in nowhere near the numbers of fanbelts.

The articles ``Power Options: Gas Versus Diesel'' [PascoeI] and ``Power Options: Gas Versus Diesel Part II'' [PascoeIII] describe marine use of gasoline and diesel engines of roughly the same size class as used in buses. Although not all rules of thumb apply for both boats and buses, there are some highly important points and the articles are worth reading.

Some issues looking back from about year 2000:

The bottom line is to be realistic about your needs. For a bus collector or RVer, 500,000 km between engine rebuilds sounds like a great thing but gasoline engines that get the same quality of maintainance can go 250,000 km and may cost a lot less to buy, insure, and service. In practice, most private individuals will not need to rebuild a good-condition gasoline engine during the time they own it, and a gasoline rebuild is usually cheaper than a diesel rebuild. For a fleet operator, the lower fuel prices of a diesel usually win against the other considerations.

* I spoke with the head mechanic at a property which in my estimation had very good service policies, detailed records, and other indications of good operaton. They had increased the maximum governed speed of Detroit Diesel 6v53T to take the peak bus speed from 55mph to about 60mph because they felt that it improved the passing safety of the buses which operated in mixed city/rural operation on 2-lane roads with slower-moving trucks and heavy equipment. They experienced typical engine rebuild intervals of about 250,000km compared to about twice that for similar buses and engines in service at other properties.

** For years, railroads kept their diesel-electric locomotives running when they were not in service because they believed the corrosion that occured with the engine off for 24 hours combined with wear on restart was more costly than the wear and fuel use that occured during the same time. It is not clear the above belief was based on fact, and today, diesel locomotives are switched off when not in use, but that may have been motivated by noise and pollution complaints rather than money savings.

Looking forward from circa 2000:

General Motors made a "Toroflow" line of diesels built by converting a truck gasoline engine to diesel. They were known as "Horroflow" and among other things had a reputation for head gasket failures more often than every 100,000 km. I understand that part of the settlement of a lawsuit by the U.S. Army against GMC included that GMC had to pay to have the Toroflow engines replaced; they were replaced with Cummins diesels. (GMC also turbocharged that batch of Toroflows, further reducing the service life.) GMC sold medium-duty buses with Toroflow engines and some properties replaced them with other engines despite the cost of a repower. Many of the GMC medium-duty buses used gasoline engines.

Why are diesels okay for cars but not boats?

This is a guess: My understanding is that most carbon collects when the engine runs relatively cool. Short idle periods do not let the engine cool down very much, so frequent idling is not a problem, but extended idling is.

One diesel efficiency is that at idle and low loads, the diesel can run with mostly air and a tiny amount of fuel injected just to keep things running. In contrast, a conventional gasoline engine must draw a cylinder full of fuel/air and if the mixture is too lean it will not ignite. So there is a "floor" below which the gasoline engine just burns too much fuel. As a result, the diesel runs cooler at idle.

Diesel and gasoline engines also produce different wastes. The diesel exhaust is much higher in particulates. The particulates are large enough that a relatively simple water (or, I believe, centrifugal) trap is moderately effective at reducing emissions -- even though water traps are relatively poor at collecting small particles (gases go through the water in bubbles with small particles suspended in the air).

I have been in diesel boats that idled quite a while before they got going. Some people do that with their cars, but it is pretty rare. And typical diesel cars run "pedal to the metal" from every stoplight, which helps to bring the engine temperatures back up. Lightweight auto engine construction helps too; but even heavy diesels for buses in transit service -- which stop and idle briefly very often -- do not seem to get carbon problems from normal use.

Good info for anybody considering a powerplant swap?

The usual rule of thumb is: don't swap. Engine swaps usually involve a lot of re-plumbing, making engine mounts, shoehorning the (usually-larger) engine in to a small bay, and so on.

In practical use, coil springs tend to oscillate unless damped. Leaf springs have damping. ``Back when'' shock absorbers were poor, leaf-spring cars would behave themselves while coil-sprung cars would bounce around long after a bump.

At high engine speeds, a simple coil valve spring can resonate so that both ends lift off from their rests at the same time. Once engine designers figured that out, an early workaround was to use a pair of springs one inside the other and slightly touching in order to provide damping. Today they are wound of taper gague wire in a taper diameter coil, and that avoids simple resonance problems.

Do torsion bars bounce like that?

A simple torsion bar will if you use it in the right frequency range, but it may be the normal frequency for a VW/Porsche front end is enough different from the resonant frequency of the torsion bar that it just isn't a problem.

The Torsilastics do not resonate because the rubber in shear to support the load also serves as a damper.

What about Lear steam buses?

A quick web search suggests that Lear abandoned it because it was not as efficient as a conventional engine. See http://lear-archives.com/learbiography02.htm.

The roster http://www.geocities.com/MotorCity/8249/sf_bus.txt suggests that it ran service at San Francisco Muni. Apparently Alameda County Transity had another one which was not satisfactory enough in testing to reach regular service. Apparently both are now in Carson City (NV?).

Thermodynamics predicts that a steam engine will have low efficiency. The basic problem is that it takes a lot of energy to boil water. Technically, it is called the "heat of vaporization". You toss lots of energy in to the water and eventually it reaches boiling energy. You then expand the steam and get back a little bit of energy, but then you either throw away the cooled steam and throw away all the energy you didn't already use, or you cool the steam back to water, which requires throwing away most of the energy but not quite all.

The basic problem behind that is that most of the energy goes to changing the temperature by very little. So, for example, you want to take 1g of wet steam at 101 degC and extract 600 calories from it while cooling it to 100 degC. That is not a problem if the thing you are heating is at 50 degC, but in practice you want to use the 600 calories to heat 1g of 99 degC water up to 100 degC. When the temperature difference is small, it is very hard to get energy to go from one thing to the other. That is where the inefficiency comes in.

You can work around that issue by using a different fluid, but the ones with good thermodynamic properties seem like they have other bad properties -- like they are corrosive and poisonous.

You can try to compress the wet (low-energy) steam to get it to re-condense without removing energy, but that takes energy -- enough that it doesn't seem to be a win, at least as we undersdtand how to do it so far. I suspect a similar argument applies to condensing the steam at higher pressure to heat lower-pressure water on the other side of the heat exchanger.

You can try to cool the wet steam, condense it, then once it is liquid re-heat it with the energy you got from cooling it. A problem here is that the temperature difference is small between wet steam and water, so if you try to cool 101 degC steam to 99 degC using 98 degC water, it takes a huge heat exchanger and a long wait.

A little bit more background: heating liquid water 1 degC takes one calorie (1/1000 of a food calorie). So heating water from room temperature (say, 20 degC) to boiling (100 degC) takes 80 calories. Going from 100 degC water to 101 degC steam takes 600 calories.

Water will boil at a different temperature if you change the pressure. Standard practice in steam engines is to pressurize the boiler and superheat the water (to much higher than 100 degC) then release the pressure and it turns to very hot "dry" steam. You can extract some energy from the steam and it is still steam, but cooler "wet" steam.

The boiling temperature of water depends on the pressure. Water boils at a lower temperature in Denver (1 mile altitude) than it does in Miami, so you have to cook food longer in Denver. You can try to play games, like heat the water to 99 degC and then drop the pressure to cause the water to turn to steam. But thermodynamics does not let you cheat here -- it is the energy of turning water to steam, not the temperature.

Steam has lots of great properties, like you can use just about any fuel, including clean ones, instead of the limited choice of fuels for internal combustion engines. However, high efficiency does not seem to be in the cards for steam. Maybe somebody will figure out a way, but it is a tough nut to crack.

Hydrogen fuels?

The basic rule with a lot of stuff is: you have to get the energsy from someplace. Where do you get the energy to make Hydrogen fuel? Separating it from water takes a lot of energy. Where do you get the energy -- burning oil? You can separate it from oil, but that takes some energy and uses oil.

A big win of Hydrogen is that it burns cleanly where it burns. However, cleaning up city air at the expense of increased global greenhouse gases or increased radioactive waste products is probably a long term loss. However, it may be possible to pick a relatively clean fuel used carefully at a central plant to produce hydrogen for use in lots of "little" internal combustion engines.

Rule of thumb: solar energy is 300 watts per square metre. A bus probably takes ballpark 30,000 watts average, so 10m x 10m minimum to power a bus. In practice, it takes energy to convert solar to hydrogen, and it takes more energy to move water to the desert (where the sunshine is good) and hydrogen from the desert to the bus, so in reality it probably takes about 100 times that much land -- a few city blocks per bus.

(There is a lesson in here somewhere about how many people the earth can sustain: our ultimate source of all energy is the sun; if we burn stored fuels at a greater rate than the global solar insolation then we ultimately on a path to run out of fuel. The details are very complicated.)

Finally, there are a bunch of "practical details" about making hydrogen work as a fuel. It is a very small molecule and can leak through seals which will not leak natural gas, water, carbon monoxide, etc. And to be stored compactly, it needs to be stored in a very high-pressure fuel tank, which is heavy to haul; or trapped in metal hydrydes which are not so easy to fill quickly.

So hydrogen does not "solve" the energy problem. The good news is that it does have advantages -- like it burns clean -- which may make it a useful tool despite its problems. People are working on this but I'd say wide-scale demos are a decade away and wide-spread production another decade or two beyond that.

Fuel cells

Fuel cells are widely regarded as "the next great thing" and many companies are working hard to make that regard in to reality. Some of the problems with fuel cells right now are: cost and energy density. Many people are working on these problems. There are currently prototype buses which have fuel cell power, but production vechicles are probably a decade or two away.

Advantages of fuel cells include better energy efficiency and reduced pollution. Ultimately, you are still running on fossil fuels, so (a) they still lead to wars, etc.; and (b) they still create pollution. However, any step in the right direction is a good step!

Near-Term Fuel Technologies

Bio-Diesel and other ``botique'' fuels: Bio-Diesel is made from vegetable oils and the exhaust is much lower in smog and carcinogenic compounds -- enough so that the National Parks approved it as "clean" fuel along with CNG. The fuel costs are currently higher than CNG. However, farmers like it, and unlike gasahol, an acre of soybeans produces enough fuel to grown and water the acre of soybeans and still have fuel left over. In addition, essentially all existing diesel vehicles can be converted easily to Bio-diesel at relatively low cost -- basically you only need to replace fuel rubber/plastic fuel lines and filter components. So while there are diesel buses in service with another 10-20 years of service life, even they can be cleaned up considerably. There is also work underway to produce diesel fuels from other sources that have similarly good properties. Cummins has been sponsoring a fuel which is part water plus a stablizer and in testing so far it has good properties. Other groups are looking at diesel additives or manufacturing diesel from other sources -- including natural and/or petroleum gas.

Diesel exhaust "scrubbers": The exhaust is still rich in carbon monoxide/dioxide, but scrubbers remove some of the most carcinogenic compounds and some of the smog compounds. A disadvantage is increased exhaust back pressure, which hurts engine efficiency and makes them essentially impossible for use with 2-stroke (Detroit Diesel) engines. However, relatively few properties are still running 2-stroke DD's and even fewer will be running by the time scrubbers are ready.

Natural and/or Petroleum gas: 4-stroke gasoline or diesel engine can be adapted to run well on CNG and the emissions are much lower than gasoline or diesel. In addition, CNG is relatively available from U.S. production, improving our economy and helping reduce the political and economic damage we cause overseas. CNG buses have higher up-front fuel costs than diesels, but reduced air pollution leads to reduced medical costs, improved building durability, and other things which may make the real lifetime costs lower. Many urban transit properties are buying only CNG vehicles today -- no gasoline, no diesel. Circa 2000, roughly 20% of all new transit bus orders in the U.S. are for CNG-powered buses.

None of the near-term techniques -- scrubbers, bio-diesel, or CNG -- promises as low emissions as the longer-term techniques like fuel cells and hydrogen, but the long-term techniques seem to need more technological development and/or infrastructure. So both the near-term techniques and the long-term techniques are important.

By at least some measures, methanol-powered buses have been more expensive all around; they are not popular for new transit orders.

Bio-diesel from Soy is easy to use as a ``straight-across'' substitute for diesel. With some conversion headaches, other choices might work better.

Jake Brakes on buses?

Cars are not as cheap as they were.

The statistics I have seen indicate that by most measures cars are cheaper in inflation-adjusted cost today than they were in decades past, with improved reliability, less-frequent service intervals, and more luxury items, to boot. For example, http://www.dallasfed.org/htm/pubs/annual/arpt97.html.

The usual comparison is the number of hours somebody had to work to buy something now, vs. the number of hours somebody had to work at some other date. One significant factor is inflation: if it is uniformly 5% per year, then over (say) 25 years, both income and prices are 3.4 times higher, but with no real change in income.

A second important factor is the change in distribution of wealth. Although Americans have on average gotten richer, it is largely the very rich who have gotten much richer; they have gotten so rich that the average wealth has gone up, even though a much larger number of people have gotten poorer. So a fairer comparison uses the median income -- the one where half the people earn more, half the people earn less. A third measure is to look and see how the view is to people nearer the poor end of society, since poor people are hit the hardest by price changes. Looking at low income effects is usually done by looking at (say) the buying power of the salary where 90% of people earn more than that salary.

The numbers I have seen suggest that on average cars are getting a lot cheaper; on median they are getting somewhat cheaper; and for the 10th percentile -- people where 90% of people have higher salaries -- cars are getting more expensive.

Why not use CNG?

For a biased answer ("nothing") see: http://www.iangv.org/.

Conversion to CNG costs money. Factory manufacture is less, and volume manufacture reduces it even further, but the needed high-pressure tanks are still heavy and expensive.

Per-mile, CNG fuel costs are less.

Per-mile, CNG emissions are mostly lower, but CO (carbon monoxide) emissions are higher.

It is harder to find CNG filling stations. There are companies which professionally retrofit engines to CNG and the conversion cost is substantial but not astronomical. Large properties can afford CNG refuling stations and regular fuel deliveries, but long-haul companies have a much larger investment to set up for CNG.

CNG refuling can take a while.

If there were widespread adoption of CNG, the cost of a CNG vehicle would drop, and filling stations would be more convenient, but the price of CNG would go up.

Don't rock the boat as long as the customer is willing to buy.

While it is certainly true that it is safer to stay in a known business than try a new one, it is not just the industry owners themselves who resist change. It is also the economic system. And even if individual shareholders largely approve, lawyers looking for a good case use the legal definition of corporation to successfully sue.

If you and I are both in the car business, and you decide to try and get people to buy CNG vehicles, your costs will be higher, your short-term profits and thus stock price will be lower, and shareholders may sue.

There is not a single point of conspiratorial whatever, it's just that we currently drive things through the economy. In reality, the economy is not life and happiness. However, the economy is our main driver.

Why are 2-strokes sold for boats and manufactured overseas?

The U.S. regulations on marine (maritime) emissions are less stringent than for road and industrial vehicles. The 2-stroke diesel is simpler and has proven reliability; reliability concerns are greater for boats than for road and industrial vehicles. "Dirty" 2-stroke diesels are easier to manufacture without advanced manufacturing technologies -- one of the reasons the 2-stroke diesel has been around nearly unchanged for 70 years or so (I am simplifying) and competing 4-strokes have been relatively recent on the scene.

By the way, if you double the number of parts in a system, then to get the same reliability you need to quadruple the reliability of each individual part.

Why are diesels even considered for auto use, given the good points from the gas vs. diesel article?

Autos do not get used continually at constant load, but they do tend to get used regularly -- most cars are used at least several days a week. In addition, most cars are not intimate with salt water. So between those things, corrosion is much less of an issue. See also my earlier note about bus diesels -- one that has low hours over many years is often okay.

Diesels work "okay" for stop-and-go use and they are more efficient. Gasoline engines are actually pretty poor in stop-and-go use; I have heard (but not heard substantiated) that for a typical car, most of the important engine wear happens when the engine is cold: poorly-atomized cold gasoline spews in to the cylinders, washes away lubricant and rapid wear ensues until the oil pressure comes up enough (and the oil warms up enough) to relubricate the cylinder walls and valve guides. The thing the boat guy mentions as the biggest problem for diesels is extended idling at low internal temperatures. Careful engine controls (reduced cooling when idling; artificially shortened idle times) help that but may reduce engine reliability -- but that is okay for cars (as long as the reliability reduction is not great). Autos also are rarely idled for a long time. It is not frequent idling that clogs; it is idling for a long time.

One of the reasons that diesels are expensive in boats (and buses and so on) is they are built for long and reliable service. For auto use, lower reliability and shorter service life are acceptable.

Another reason gasoline engines are cheaper is they tend to have larger manufacturing and sales volumes. Again, put a diesel in volume production and it will drive some of the costs down. Boat buyers cannot affect that equation, but a car maker can.

For the most part, diesels in cars are not substantially worse, they just are no better. The places where they are better are improved efficiency (fuel economy), reduced CO emissions, and reduced fuel prices in places with much higher taxes on gasoline than on diesel fuel.

Why is the Wankel rotary largely ignored? Mazda fixed the oil-consumption problem. It is a simple engine with few moving parts and a high power/weight ratio.


One of the advantages of a piston engine is that each time you draw in a fresh charge of air or air/fuel, the hot spots in the engine get cooled. In a continuous-operation engine (like a turbine) or a staged intermittent engine (like a Wankel rotary), various parts get hot and don't get cooled. The cooling problems are solvable, but typically at some expense. In most engines, exhaust valves are the big probem.

In an ideal engine, all heat energy is converted to mechanical motion. Most gasoline engines are under 20% efficient and diesels around 30% efficient. One of the issues is that if you expose the flame to a cylinder wall, you take the fire's energy away as heat rather than mechanical energy. So the best shape for a combustion chamber would be a sphere, as it has the least wall space for the enclosed volume. But it is hard to make a metal sphere which expands and contracts. The conventional cylinder is a fair approximation. The combustion chamber in a Wankel is a worse approximation, so the efficiency is lower. The problem is also solvable by designing and building a surface which can run at high temperatures, but if you can do it for a Wankel you can do it for a conventional piston engine, too, and since the piston is thermodynamically "better", the piston will still win.

The science word is "adiabatic" meaning "no heat loss". Note, however, that with "no heat loss" you are in the same cooling race, you have to control hot spots.

A tiny bit of further reading:

Some of the conjecture presented is this engine had at least:

Note "the old one" suggests that there may have been problems and guessing rather than measurement for exhaust quality. Inventors are more likely to go in to the successes than the problems.

As long as we are at it, there have been a lot of "milage boost" systems. I don't know much about them, but one general theme is:

If a conventional engine operates at 10-20% efficiency and gives 15mpg in a particular car, then 75-150mpg is the best you could possibly do. Claims of 200+ mpg must therefore be false, and the real world being what it is, even approaching 100% should be tremendously difficult, so any very-high-milage claims are suspect.

Extracting most of the heat is difficult because the lower the temperature difference, the harder it is to get out energy. Getting power from a 1,000 degC difference is relatively easy, but getting power from a 1 degC difference is hard, even if you have a lot of whatever. The energy density is simply low, so it takes a lot of motor to extract energy from a lot of something with poor energy density.

If I had invented a 50 km/l carburetor, I would simply patent it and quietly hang out a shingle and start installing them. There are enough kooks to keep me going for the first month and that would give me enough money to offer commercial users in my home town a 1 month free trial, and so on. Given the apparently large number of high-efficiency carbs that were supposed to have existed, it is hard to believe that even a large conspiracy could have kept it down.

"If something sounds too good to be true, it probably is." A simple idea is good, but there are usually a lot of details in making something work, even a simple something.

Why are Jake brakes less common?

I believe they are installed on more vehicles than ever but city noise abatement laws prohibit their use in populated areas. They are used often in the mountains. There are even integrated with new engines delivered from many manufacturers.

They work on 2-cyle and 4-cycle diesels. See www.jakebrake.com

Diesel carbon clogging.

I am not a diesel engine designer, nor do I play one on TV. If this sounds wrong to you, it probably is:

My understanding is that most carbon collects when the engine runs relatively cool. Short idle periods do not let the engine cool down very much, so frequent idling is not a problem, but extended idling is.

One diesel efficiency is that at idle and low loads, the diesel can run with mostly air and a tiny amount of fuel injected just to keep things running. In contrast, a conventional gasoline engine must draw a cylinder full of fuel/air and if the mixture is too lean it will not ignite. So there is a "floor" below which the gasoline engine just burns too much fuel. As a result, the diesel runs cooler at idle.

Diesel and gasoline engines also produce different wastes. The diesel exhaust is much higher in particulates. The particulates are large enough that a relatively simple water (or, I believe, centrifugal) trap is moderately effective at reducing emissions -- even though water traps are relatively poor at collecting small particles (gases go through the water in bubbles with small particles suspended in the air).

I have been in diesel boats that idled quite a while before they got going. Some people do that with their cars, but it is pretty rare. And typical diesel cars run "pedal to the metal" from every stoplight, which helps to bring the engine temperatures back up. Lightweight auto engine construction helps too; but even heavy diesels for buses in transit service -- which stop and idle briefly very often -- do not seem to get carbon problems from normal use.

Cars are not as cheap as they were.

The stats I have seen indicate that by most measures cars are cheaper in inflation-adjusted cost today than they were in decades past, with improved reliability, less-frequent service intervals, and more luxury items, to boot.


Why do big rig operators leave the engines going?

Some leave them on high idle to avoid that problem. Many buses also have a "fast idle" switch. The fuel consumption is somewhat higher at fast idle, but it works.

They are also about to head out and drive for hours and hours at highway speeds (elevated engine temperatures) which will help to burn off what carbon has accumulated.

Restarting the engine also causes wear because (a) oil is not well-distributed in the engine; and (b) the cold cylinder walls collect fuel which dissolves remaining lubrication.

There is also lore and habit.

Do most diesel particulates drop to the ground before they reach us?

Diesel is high in very small particulates which are highly carcinogenic and which stay suspended in air for a long time. See: http://www.afscme.org/health/faq-dies.htm and http://www.osha-slc.gov/SLTC/dieselexhaust/. Note that in the second page, external links seem to be of the form ``http://www.foo.org/a/b/c.htm?'' and you need to remove the trailing "?" to reach the page you want..

Drive gently while cold, but do not warm up.

That is what I have heard since the 1970's oil embargo for passenger cars. Buses typically need to build up air pressure for the brakes so unless the bus has been run recently you will need to run the engine for a while before you can move. It is usual to put the bus on "high idle" while the air pressure builds up. If the air pressure is still high enoiugh, I observe the bus is usually driven off immediately.

Diesels have poor accelleration?

Diesel engines tend to be heavy per horsepower, even in "lightweight" auto applications. That alone limits the power/weight ratio of the car. In addition, engines tend to get substituted on a pound-for-pound basis in order to avoid redesigning the front end of the car, so that further limits the peak engine power and thus power/weight ratio.

With diesel cars is when you want to go you floor it and leave it there for a while. Essentially all heavy vehicles are diesel, rather than gasoline, and essentially all heavy vehicles have low power-to-weight ratios compared to cars.


Almost all heavy vehicles have air brakes, while lighter vehicles usually have hydraulic brakes.

Air brakes operate using an air pressure regulator at your foot, and air-filled pistons at the wheels. The harder you push on the pedal, the higher the pressure in the air lines, the wheel cylinders, and the stronger the brake application.

All systems can fail; brakes are especially critical because if the brakes fail you cannot stop. Early air brake systems used a backup brake operated by pulling a lever connected by rod to the rear wheels. The most common arrangement is called a ``Johnson bar'' brake. These are notoriously ineffective. Modern systems use a ``fail-safe'' system where losing air pressure causes the air brakes to apply and stop and hold the vehicle. The fail-safe also prevents a standing vehicle from rolling away as air pressure slowly leaks from the air system. There is also a brake called the "ICC brake" (?). I don't know what it is, but it has a poor reputation.

The ``fail-safe'' brakes are divided in to two main categories: spring brakes and DD3 brakes. The spring brake has a spring which is always trying to apply the brake. The parking brake is released by adding air pressure to a cylinder which overcomes the spring. The main ``service'' brake is applied by adding air pressure to a second cylinder which overcomes the air pressure that holds off the service brake. The parking brake is engaged by removing air pressure from the first cylinder; accidental air pressure loss also applies the brake. This system is fairly simple and cheap to maintain and repair. The major complaint against it is that the springs can weaken with use, and vehicles with aged spring brakes may roll away on hills. Servincing spring brakes also requires care as the spring is always under load and disassembling a unit without first ```caging'' the spring can cause parts to fly all over your work space. Flying metal leaves scars.

The other major type of air brake is the DD3. The DD3 uses air pressure to apply the parking brake and a mechanical latch to hold it applied. The brake is released by applying air pressure to a latch release. With the latch released, the service brake is activated just same way as a spring or old-style brake, by adding air pressure to a service brake cylinder. The parking brake is applied by removing pressure from the mechanical latch and adding pressure to the parking brake cylinder. The DD3 relies on a separate air pressure resivoir and lines to keep pressure and apply the emergency brakes if the main air pressure is lost. The main complaint about DD3 brakes is they are more expensive to service and maintain.

Because air pressure is needed to release the brakes, vehicles with air brakes need to run for a while to build up air pressure before they can drive away. In addition, low air pressure, including pressure loss caused by repeated brake application, can cause the parking brake to engage, leading to sudden and non-removable brake application. This means special care -- and thus special licensing -- is needed to drive a vehicle with air brakes.

Brakes wear, and are fitted with ``slack adjusters'' to compensate for minor wear. New shoe linings, and sometimes new drums, are required after extensive wear. Many older buses are fitted with manual slack adjusters. The adjusters must be tested and adjusted by hand, which typically involves choking the wheels to keep the bus from rolling, releasing the brakes, and crawling under the vehicle. Brakes can wear fast enough to require e.g., weekly adjustment.

For a proper adjustment procedure, see elsewhere. The general idea is to see how far the adjusters move during a brake application. This can be done, for example, by releasing the brakes and then applying them manually with a tool, while measuring travel. [[Cite a procedure.]]

Automatic adjusters can eliminate frequent service, but periodic inspection is needed to ensure the shoes are not worn out, and to check for failures. For example, a broken return spring may increase brake shoe wear rates and at the same time prevent the slack adjuster from working; springs may deteriorate in salt as fast as one year [NTSB02].

Note that broken springs can make the parking/emergency brake ineffective or inoperative. Broken springs appear to be a fairly common problem: an investigation of eleven tractor-trailer trucks, not in accidents, at a large firm with a good record, revealed an average of more than one broken spring on each unit [NTSB02]. A Canadian study found 15% of vehicles with manual slack adjusters, and over 8% of vehicles with automatic units, had the brakes misadjusted [CS].

Brakes require additional application stroke as the brake heats up. For example, about 5mm per 100degC. As a result, over about 350degC, even a properly-adjusted brake can run out of stroke before good braking is achieved. In addition, above about 300degC, the lining friction drops rapidly, resulting in loss of brake performance even when the shoes contact the drum at high force. In a test by Rockwell, brakes starting at 65degC needed an additional 30% distance to stop if they were not adjusted properly; at 200degC they required an additional 75%. For more information see [NTSB02].

For more on automatic brake adjusters (aka ``automatic slack adjusters''), see the NTSB safety letter of 2001/03/27.

``If your engine stops working, well, there you are. If your brakes stop working -- now where are you?''

Bus conversions http://www.busconversions.com/newsboard/articles/3981.html: suggested reading, brief reviews.

Transit is dangerous. Visit bus plunge.

Engine swaps expensive? Choose the right combo in the first place.

Or choose whatever, use it a while, and then bite the emotional overhead of selling the thing that isn't quite right and buying something that is closer to right. Of course a $20,000 engine swap makes a lot more sense on a $100,000 vehicle than on a $10,000 vehicle. The $10,000 vehicle might be worth $15,000 after the swap while the more expensive one $110,000.

Jaundiced eye towards rebuilt stuff.

Yes, quality goods are usually expensive, no matter how you get there. Still, a 300,000+ km "old" car may be nicer than a 150,000 km "newer" car.

From an RV group:

Would like to take this oppurtunity to post a warning. I have just replaced the main valve and the 10% valve and the gage (that's another story) on my propane tank. It had a leak around the stem where you turn it on/off. I always check all the fittings with soap bubbles before every trip so found the leaking valve. I worked at a dealership yrs ago and a fellow turned on the propane, went inside and lit the stove and blew the whole side out of a NEW camper. Luckily nobody was hurt but I have always checked the fittings with soap before each trip. Even though the coaches are smooth there are still vibes that can loosen fittings. Safety first, please! --Harold

You can check the fittings using a bowl with some soapy water plus something -- an old paintbrush or toothbrush, for example -- to "paint" things which might leak. If it is leaking, it will start to bubble the soapy water.

Note that old lines can leak either because of chafing or because vibration causes the line to develop fatigue cracks.

The ``warning stink'' they put in gases goes away if you let them sit around for a while. So while the warning stink is useful, lack of stink does not mean everything is safe.

In the ideal world, moving a bus is just go there and drive it where you want. THat's kind of the point of one in the first palce. In practice, when moving an old vehicle you want to consider the risk of breakdowns and the top speed of the bus -- for older transits, that may be as low as 40 mph. Trailering cars is relatively easy -- drive it on to a car carrier -- and is relatively inexpensive. A bus weighs much more and if it goes on a high-bed trailer it is too tall to meet bridge clearances. Medium-hight trailers typically cost US$1/km while the low-bed removable gooseneck (RGN) trailers cost US$2/km. Loading on to a RGN is just drive the bus on, but loading a medium height trailer requires a ramp which can handle the weight. Rail tends to be more expensive. Few places have the ramp needed for a medium-bed trailer, and fewer still will let you use it -- for liability reasons. It appears the best thing to do is find your local farm equipment dealer and ask them.

Peak RPMs for diesels is low.

One rule of thumb is that the larger the per-cylinder displacement of an engine, the lower the peak RPMs. Another rule of thumb is that diesels, which much withstand much larger internal loads than gasoline engines, must be built heavier and so must turn slowly in order to avoid creating their own speed-related internal loads.

Big diesels -- with pistons that weigh as much as a truck-sized Detroit Diesel -- turn much slower than 2,000 RPM; while automotive diesels turn faster, though not the high speeds typical of gasoline auto engines.

I believe the low engine speed does not play a role in suggish accelleration. Power is power no matter what the RPMs, and you can convert a slow-reving engine to high speeds by means of a gear train.

It is the case that some diesels have a very narrow power band, say from 1,800 to 2,100 RPMs, or less than 20% faster at the top than at the bottom. You may have heard trucks pulling away from a stop doing something that sounds almost like the drivers are tapping, jabbing, or "blipping" the throttle. What they are actually doing is shifting very quickly through a large number of close-spaced gears. If you spend a lot of time shifting, you spend less time accelerating.

Other diesels have a wider power band and so can use much wider steps between gears, similar to an auto engine. Mack used to offer two versions of their engine; the more expensive turbo-supercharged Maxidyne needed only a 5-speed transmission while the similar non-Maxidyne version needed nine or ten or so.

Many highway buses, have just four forward speeds. I don't know if that is different engines, better power/weight ratios, or something else.

Air vs. Cable Throttles

Good news: fine throttle control, no strong spring to fight, no sticky cable to break. An air line is easy to route.

Bad news: Less feel, more throttle lag and thus less "feel". No throttle control until you have air up. The King cruise control will fix that.

An air throttle seems to be a worse choice for a manual transmission, though some people are happy with them.

US Coach has spare throttle cables. Suggestion: clean out the old housing thorougly by flushing with diesel or other mile solvent, blow out with plenty of compressed air, install the new cable with plenty of lightweight Graphite/Molly or PTFE grease, make sure the cable ends are "booted" with a rubber bellows to keep new grit from getting in. The cable should feel good and and stay that way for many more miles than a typical bus will go.

On the one hand, cables and linkages have worked fine for millions of miles. On the other hand, like automatic transmissions and power steering, those who have changed don't usually go back.


Manuals are highly useful.

Many manual reprints are available through ``Coach Information Network''.

Serial Numbers

GMC info.

GMC 4106

Very little clearance to retrofit safety brakes. Major choices:

Problem: not room around the axle to fit all kinds of spring brakes, which have larger pots than traditional non-spring brakes. According to busnut #1638, (a) Tom Caffrey had it done at a shop in Oregon; (b) Bob Gallo got a kit from Transquip Industries in roughly 1990. P/N 64-02-3290, listed price $577, phone number on the literature was 713-641-2300.

More info about brakes under World of Recovery.


Tow, or super-small and put it in a luggage bay.

Satellite dishes

One-way: receive cable and internet.

Two-way: Dustyfoot info about the XXX two-way dish.

Bus Tradeoffs

What are the available 35 foot buses that are good for conversions?

A 35 foot bus can go more places than 40' or 45'. But it has less space inside and less space underneath.

Most newer highway buses are 40' or 45'. Most older highway buses have stick shift. Most older transits have automatic transmissions but may have trouble going highway speeds. Smaller buses and stick shift will typically get better fuel economy.

For a heavy-duty bus, a big diesel engine, like a DD 8v71, will give good fuel economy and will have better hill climbing. A smaller engine, like a DD 6-71 or DD 6v71, will mean you go slower on hills. A transit bus will likely have lower gears, giving better climbing but poorer economy.

Every coach make has points in its favor. However, all used mixed metals (steel and aluminum) in their construction, so weathering and road salt damage should influence your choice of a coach.

Some coaches have a lot of use, so look for cracking bulkheads and worn parts.

If you need an automatic in a highway coach, you will likely find it less expensive to go with Eagle or MCI, because they use T-drive, wherease GM uses V-drive, which limits the choice of automatics and rear end ratios.

One theory: the best platform in a coach is the one you find in the best shape for the money that you are willing to spend, and don't worry too much about the brand.

Curved glass is harder to find.

Thirty five foot intercity coaches are not common. The last GM coach was built in 1980. There were very few Eagles or Prevosts built in 35'. MCI built the MC5, 5A, 5B, & 5C models several years ago, but not in great quantity. There are a couple of new units such as the MCI F series, but these are out of the price range for most of us.

There are some late model transit buses available with road gears. Less underneath space, but cheaper and often more common parts.

It is also possible -- at some expense -- to cut down a bus.

Considerations: Flat floor for no hassle conversion. Does it have a toilet to remove? Does it run on 12v, negative ground? If so, most RV and marine DC chargers and lights/toys/etc. work. If not, conversion to 12v/negative ground can be a problem.

A big alternator can run a small A/C.

GM's have a majority of the body/frame (monocoque) alluminum , so its about the lightest 35Ft and there is only a small bit of steel to keep painted. The 4106 is quick, due to the smaller frontal area and light weight. The V-drive hurts efficiency some.

[[Keep ALL the windows for good veiw.]]

4106 parts are mostly avilable with one or two phone calls. Transits share many parts and are more often scrapped. That means a "modern" 4 stroke engine, automatic transmission, bigger brakes and power steering front axle, will be avilable from a $2000 or so donor.

4104/4106 suspension is all air bags, about $75 each. The leveling valves need occasional replacement, about $35ea, 3 needed.

Check windshield prices and front side windows if they are curved. Other curved glass can be filled in or replaced with plastic, but not front glass. And it may be expnsive.


A ``buffalo'' is a GMC 4107, 4108, or 4905. The roof line is like a highly-stylized Scenicruiser with a roofline step but it is very near the front.


RV conversions often include insulation. Choices include sprayed-in foam and thermal paint.

One of the most common is sprayed-in foam. You can get a kit and do it yourself, or higher it. Expect to pay US$1,000 or more. A stiff foam can help firm up a flexible body, but gives relatively worse insulation. A flexible foam gives better insulation but is delicate. Foam must be sprayed in at the right temperature, etc., for good results. Do-it-yourself foam also often costs more than expected because of variations in spraying.

Another choice is rigid board cut and glue in place.

Essentially all foams except Ethafoam(tm) will outgas toxins after installation and are pretty toxic in a fire.

Compacting home fiberglass will lower the insulation value, so it is not typically a good choice.

You will want a vapor barrier, else the R value will gradually decrease as the foam fills with moisture.

Ceramic bead paints on the roof can dramatically reduce solar heating. May not have much effect on condution loads or radiant cooling to a cold night sky, though. There is some controversy whether removing such paint poses a health hazard, as the beads can break to form a fine sharp-edged dust. Such dusts are typically dangerous, e.g., asbestos.


Owens 1925. From Peerless page.


List of makers fan belts. (Graf & Stift, LAG, MCR, National Coach, ...)

Aerocoaches were built during and after W.W.II. They probably had steel bodies. The corporate name of Aerocoach was General American Aerocoach. They may have evolved into General American Transportation Co. (GATX) known for leasing railroad cars, mostly tank cars and piggyback cars.

I think all total there were probably over 100 or so Beck deck and a half buses built in 4 different models either 3 axle or 2 axle built between 1951-56. The biggest buyer was Carolina Coach, a Trailways affiliate. They were steel also. Both Aerocoachs and Becks were powered by either Continental or International Harvester 6 cylinder gas engines. Later models were available with Cummins diesels.


Some Gilligs in the 1950's had air clutches. Rumored to be tricky to operate.


Some brands have pictures at Intercoach Motor Coach Museum. Brands with pictures include Kassbohrer (became Eagle, Setra?), Scania, Van Hool.


Some makes and dates of manufacture.

More About Brakes

What is the ``alcohol evaporator'' or ``alcohol injector'' on my bus?

I am not an expert on brakes, but here is what I think is happening. When you compress air it gets hot. When you release the pressure, it gets cool. If you compress the air, cool it while it is still compressed, and then release the pressure, it winds up cooler than when you started. (This is how a conventional air conditioner works, though they use some referigerant like Freon(tm) rather than air.)

Now here's the problem: there is a little bit of moisture in the air. When you drop the temperature of the air, the moisture tends to ``fall out''. You get puddles of water collecting here and there in the brake system.

Mostly, a little water in the air system is okay, but more than a little is a problem. Water does not compress and spring back like air, so if water accumulates in the air storage tanks, it effectively reduces the size of the tank. As more and more water accumulates, your brake capacity goes down and down. At first you won't notice, but if you need to apply the brakes several times in a row (faster than the compressor can recharge) in traffic or going down a hill, you will suddenly find your brakes have gone from normal to very weak, and you have to wait a while before the brakes work again. This is very bad if you have already hit something by the time "wait a while" has gone by.

So rule #1 -- and this is important in all weather -- is to drain the air tanks regularly. Most buses newer than about 1960 have automatic drains on them, but if you drive a long time, park on a clean surface, restart, and you do not see a puddle of water appear under the bus, you may have manual drains or may have a plug-up. Another thing to look for is to perform the standard air brake test -- pump the brakes with air pressure up and the engine off -- and see if the air pressure drops faster than allowed. See your state's drivers guide and look at the air brake section. When the bus was new, the air tanks were sized big enough to pass the brake test. If pressure goes down too fast, it is likely because the tanks are smaller than they used to be, because they are partly full of water. When in doubt, visit a mechanic. Air brakes are pretty simple, so it should be pretty cheap to get the basics going, and at any rate a few dollars of brakes is better than crushing your bus or driving over a child.

Most air compressors also leak a little bit of oil in to the air, so what drains out of the air tanks is often oily.

Drain the tanks regularly -- once each day of use -- even if you have one of the other things which are described below. Note that the tanks may accumulate little water one day, and accumulate quite a bit on another day, depending on how you drive, the weather, and so on. So even if little water came out last time, keep draining the tanks regularly.

Another way that water is different than air, is that water can freeze at sensible temperatures, while air stays a gas at all temperatures seen by all buses. (At least to date -- maybe somebody will throw a bus in to deep space as an art project, and then the bus will get that cold. But I digress.) If you have a little puddle of water at the bottom of your air tanks -- which is the usual place that it collects -- then you have no problem. It can freeze, but even though water expands when it freezes, it won't expand by much and there is plenty of room for it to expand in the air tank because the tank does not have parallel sides.

However, frozen water doesn't drain from an air tank very well, so it is as if your air tank drain valve was stuck closed. More water will then fall out of the air and collect and freeze on top of the ice in your tank, which could fill it up as if you wre not draining it. On top of that, if you have water standing in a valve, or water in a hose or air line, it may be able to move back and forth when liquid but it may impede the flow of air when solid, or it may expand a break something. I do not know much about air brakes, so I am not sure which of these problems is the most common or the biggest deal, but judging by everything I have seen it is sometimes a problem.

One way to deal with the problem is an air dryer. I do not know much about how they work, but they seem to be installed just after the air compressor. On truck trailers, air dryers are also installed just after the air hookup. The general idea seems to be to get the water out as soon as possible so there is less to deal with later on. And since you are constantly pumping air through the system, dry air helps to evaporate whatever water has collected. Note, by the way, that if the air is sufficiently dry, then even ice will evaporate. It's called ``sublimation'' when ice evaporates without first becoming water. A disadvantage of air dryers is that the drying cartridge needs to be replaced periodically -- every few years seems to be the norm, but some newer units claim ten years between changes.

Another thing you can do is get things hot once in a while. For example, if you sometimes park in a heated garage, it should melt the ice to water and then you can drain it. I don't know how often you have to get things hot in order to solve the problem, but I am convinced that if your garage is located in Guadalajara, that will do it.

Another thing you can do is to mix in something which causes the water to stay liquid down to lower temperatures, and that is where the "alcohol evaporator" or "alcohol injector" comes in. Alcohol and water together stays liquid to a much lower temperature than just water. According to http://www.powerserviceproducts.com/tech06.html the right mixture of alcohol with water stays liquid down to -40deg. Conveniently, -40degC is the same temperature as -40degF, so they should be able to sell their product outside of the United States without changing the label. I look forward to the day the U.S. switches to metric.

According to the same page, one kind of antifreeze is 99% methanol, which is mixed in slowly with the air. The water/alcohol stays liquid when water alone would freeze. Since the mixture is liquid, the regular drains work even though it is below freezing. The page http://www.ezoil.com/airbrake.html lists two kinds of air brake anti-freeze, one of Methanol and one of Ethanol.

Methanol is moderately dangerous to people (relatively small amounts can make you permanently blind; a little more can kill you), and it is also quite flamable, and it burns with an invisible flame, so you can feel it burn but cannot see it -- not being able to see fire makes it a little more dangerous. However, methanol is otherwise pretty environmentalliy friendly and is not subject to all the weird regulations required for humnan-safe alcohols. The web page http://www.gunk.ca/msds-eng/MH1,%204,%2020.doc has some good information including physical, fire and explosion, reactivity, toxicity, and first aid.

The EZoil web page mentions that Methanol ``the negative effects of drying.'' The other type they mention is isopropyl alcohol, which ``does not cause drying but it is less effective at reducing freezing and is expensive.'' I believe that isopropyl alcohol is somewhat less dangerous than Methanol, but I am no expert here. There may be some other chemistry or cost reasons why, say, ehtanol is not also used, I do not know.

The EZoil web page mentions the addative of ``amino salts'' to reduce the effects of rusting, etc; and to provide lubrication and anti-wear. I cannot tell what is marketing and what is chemistry, but it does suggest that not all compounds are created equal. The EZoil FAQ at http://www.ezoil.com/faq.html suggests that methanol alone reduces the life of valves and diaphragms.

The Powers Srvice Products web page (above) says several interesting things including

Most driver's license booklets have a good introduction to air brakes. For example,

Other reviews of brakes, sales brochures, etc.:

As with most things, remember that almost everybody is trying to sell you something (``follow the money''), so be careful about reading too much in to any one description in the above.

http://www.busconversions.com/newsboard/articles/29887.html Posted by Scott Whitney on February 06, 19103 at 09:38:29 PST: In Reply to: "Newbie" Question posted by Curtis on February 06, 19103 at 01:03:46 PST: Hi Curtis, As others have suggested we can give you more helpful info if you refine your question something like, "Given that I have these parameters, a, b, c, etc. . ., what then might best fit my needs?" So far your only parameter was a shell under $15,000. In today's market that will buy you anything from a van-style school bus to a completely finished coach conversion (with some luck and bargain hunting) Having said that, I'll give you some brief positive points of some common shells, in my opinion: Prevost: High resale value, status, mfr still in biz, easily modified, mfr supports rv conversion Eagle: Great style, easily modified, versatile drive train MCI: Good value, mfr still in biz, modern drive train, mfr supports RV conversion on newe$t model$ GMC: Good value, vintage/classic style, light weight Early Flxible coaches: Vintage style RTS: Good value, stainless & fiberglass, many after mkt parts, roof raise not needed, 102 wide Crown: Good value, Wicked drive train, built like a tank, classic look Other transits: Good value, roof raise not needed, 102 wide Misc. Skoolies: Great value, easily modified, off-road capable So, that is in rough order of average pricing I have seen in the past. I am sure I have forgotten a brand or two and have ignored European brands. And of course, these are just my opinions. . . Given your 15K price parameter, it will probably exclude a Prevo unless you find a bargain, older seated coach. In general, busses like vintage Flxibles will only be found as full or partial conversions since they have been around, and out of revenue service, so long. RTS, on the other hand, can be had quite readily as a seated shell straight from a city transit authority's auction. (BTW, RTS is a GM product but is sort of a class of its own.) And with every shell type, there is a huge range of what you may find. If one of these brands sounds appealing, then maybe ask some specifics about it and the people who own(ed) one can probably answer all the specifics. It is a buyer's market right now. Shop until you find the perfect coach. Bargains abound. Scott


Older buses use engines powered by diesel, gasoline and LPG/LNG -- liquified petrolium or natural gas. Trolley buses and some very old (circa 1900) and very new (circa 2000) buses are powered by electricity. Some old (circa 1900) buses were run on steam, and a few prototype buses were built to run on steam. During WWII, some european buses were run on wood gas, made by ``cooking'' wood to release burnable hydrocarbons. Some buses run on combinations, for example diesel and LPG/LNG may be run together at the same time; and some hybrid buses run on diesel and/or electric depending on the situation. For the purpose of this discussion, diesel, bio-diesel, and vegetable oil are all considered diesel.

By far the most common choices are diesel, gasoline, and LPG/LNG, especially for people interested in operating private buses. All three types of engines are broadly similar but differ in numerous details. Some considerations include

The exact tradeoffs depend on many factors, including the age of the engine, the model of the engine, the physical location of the bus, typical usage, and so on. For example, Detroit Diesel 2-stroke diesel engines may turn either clockwise or counter-clockwise, depending on the cam installed. When a large number of 6-71 bus engines were being removed from commercial service, most were counter-clockwise [[? -- check, I forget]] and so they were cheaper than clockwise [[ditto]] engines used widely for trucks and boats. However, most 6-71 bus engines have now been removed from commercial service, and overall more clockwise [[ibid]] 6-71 engines were produced. Therefore, there was a time when bus 6-71 engines were cheaper, but now they are more expensive. Although a clockwise [[same]] engine may be converted to counter-clockwise [[again]], doing so means finding the right cam and partially disassembling the engine --- thus, conversion is relatively expensive.

For the private operator, ``collectability'' or predisposition is often an overriding consideration. For example, many operators want a diesel or even a specific kind of diesel for the simple reason that is what they want --- even though that engine may be worse in many respects --- more expensive to buy, service, and operate --- than other engine choices.

The earliest buses ran on gasoline, steam, and electricity either from batteries or overhead wires (for trolley buses). Gasoline engines were by far the most common. Diesel engines first became available for buses in the late 1930's, and by the late 1950's, most American-made highway and transit buses were powered by diesel engines, though gasoline engines have continued to be popular for lighter-duty buses. During the 1940's and 1950's, Twin Coach delivered a large number of LPG-powered buses; they continued to deliver a smaller number in to the 1970's. About 1990, mounting emissions considerations led to the reintroduction of LPG.

Diesel became most popular for large buses for several reasons. First, diesels are more efficient and so fuel costs are lower. In the 1940's, gasoline engines were far less efficient than today, and the difference in fuel costs was huge. Second, in the 1940's era, frequent service intervals on gasoline engines kept the buses off the road a greater fraction of the time --- and ``off the road'' means the bus is costing money not making it. Economic downsides of diesel include higher initial purchase price, higher parts costs, and worse power-to-weight ratio, making buses more sluggish and thus harder to keep on schedule. Another downside is diesels are typically much noisier.

Today, diesels have a reputation for economy and durability which makes many busy buyers want a diesel. But reputation, the state of technology in the 1950's, and economic tradeoffs which make sense for bus operators do not necessarily make economic sense today for a private operator. For example, while diesels often go longer between service, when they do need something they can be much more expensive to repair. You can buy a rebuilt modern gasoline engine for the price of some common diesel repairs. Also, note that gasoline engine service intervals and fuel economy have improved dramatically. Diesels still fare better if you travel long distances, but for occasional use the lower purchase and service price of a gasoline engine may win. Finally, note that buses operated on planned service routes and could count on diesel fuel being available, whereas diesel may be harder to find in America on trips away from main highways.

The fuel economy of an old gasoline engine is typically much worse than that of an old diesel, but newer gasoline engines with electronic ignition and fuel injection are much better. Newer diesels are often available with improved efficiency and power-to-weight ratios, but at the expense of higher purchase and service costs, and shorter service lifetimes compared to lower power ratings. Newer diesels are still more noisy than their gasoline counterparts. Recent LPG engines typically have higher fuel costs than recent diesels, but lower service costs due to reduced oil contamination and other wear factors.

For the private buyer, engine choices are often intertwinsed with bus choices. There are several issues. First, what is available among used buses is whatever is available. That, in turn, is influenced by what commercial operators bought when the buses were new. You can buy a bus and convert, but at higher expense. Buses with transverse engines typically run counter-clockwise [[check]] and so switching to a different engine may be hard.

Similarly, engine compartment configuration may limit choices. For example, many older transverse-engine buses (1930's-1950's) were delivered with inline-six engines. The engine compartment is narrow and will not fit a newer V-configuration engine. Changing the compartment requires structural modifications to the bus. Some buses have the engine located in the middle of the bus under the floor, for example some White and Crown buses; the amidship location requires an engine which is laid on its side, called a ``pancake'' configuration. The most common pancake configurations in 1940's to 1960's designs are Cummins 220 diesel, Detroit Diesel 6-71, and Hall-Scott gasoline. Hall-Scott gasoline engines are relatively infefficient and expensive to service. Some 1980's underfloor engines are horizontally-opposed. A third space-constrained example is Flxible Clippers, which use a standard T-drive configuration and standard clockwise [[whatever is standard, CW or CCW]] rotation. However, the engine bay was designed for an 8-cylinder inline gasoline engine, and larger V-configuration engines may not fit. For example, a Detroit Diesel 6v53 will barely fit, but the 6v71 model will not. An additional complication is that radiator cooling air must flow past the engine, so a cramped engine compartment may lead to cooling problems (and Clipper-style Flxibles are already noted for cooling problems). Later Clipper-style Visicoach and Starline models have a wider engine compartment.

Engine lifetime (durability) is related to the speed it usually turns: slower turning typically means longer life. Slower turning also means lower power-to-weight ratio. Thus, heavy-duty long-life engines tend to turn at lower RPMs. The lowered power-to-weight ratio is typically made even worse by more materials in larger bearing surfaces, etc.

From http://www.rv-coach.com/Forum.21191/current_category.199/forum_thread.html: The large heavy duty diesels like the DD -71, -92, and 60 series, the Cummins N-series, and the big Cats all govern out around 2100 RPM. The medium duty diesel engines like the DD -53 series and 8.2L, the IHC DT466, the Cat 1160/3208, and the Cummins 'B' and 'C' series engines govern out at 2600 RPM. The light duty diesels like the IHC/Ford 6.9L-7.3L-Powerstroke and the GM 6.2L-6.5L will govern out even higher.

Looking elsewhere, I see that a 1993 Detroit Diesel 6.5 litre (not sure the series) has a peak speed of about 4,000 RPM.

Note that the overal power pack determines the top bus speed and hill climbing abilities. For example, diesels typically have a top governed speed and they won't drive themselves any faster than that top speed. (Though you can overdrive one by going down a hill.) The top RPM times highest transmission gears times differential rear end ratio determines top speed.

Hill climbing depends on being able to run the engine at the engine speed which gives near peak output while at the same time running the bus at the peak speed that power will drive the bus up the hill. The higher the power, the faster you can go; the higher the total bus weight and the steeper the grade, the slower you have to go.

However, gears shift in steps. If fourth gear is too high, the engine cannot drive the bus up the hill that fast, and the bus slows down. Since the engine is driving the bus, the engine slows down. As the engine slows, the power output drops off: peak power is usually achieved near peak RPMs. With reduced power, the bus slows further, the engine slows, power output drops, and so on in a vicious circle. So pretty soon you have to downshift. If third gear is too low, the speed will pick up until the engine hits maximum goverend speed and then you go no faster, even though the engine has power to spare. But if you upshift, it will drag down again.

This can be pronounced in buses, which often have just a few transmission speeds. For example, the GMC PG-3302 has a top speed in 3rd gear of 55kph and a top speed in 4th gear of about 95kph. Thus, in order to shift, the engine speed must change by nearly a factor of two. However, in cutting the engine speed, power drops substantially.


http://groups.yahoo.com/group/flxowners/message/3637 From: "Rosique Family" Date: Wed Feb 12, 2003 4:00 am Subject: RE: [flxowners] not an orange, nor an apple, but more like a kiwi FYI... I have run 9.00 X 20 tires on that same exact rim. The tires were a little wide for the rims (as I remember-- but I have slept since then) but I don't remember any problems... you could also convert your axles to 10 lug Budd which would give you a much better/stronger rear end, and larger brakes. I believe your rear axle is the type that bolts together in the middle, rather than a drop out third member (pretty big project) Optimistic, Dave Rosique :) -----Original Message----- From: Dennis Toeppen [mailto:flx@6...] Sent: Tuesday, February 11, 2003 6:43 PM To: flxowners@yahoogroups.com Subject: [flxowners] not an orange, nor an apple, but more like a kiwi Ok, here's the deal on the wheels on the Boulder Clipper. They are six hole wheels, about 22 inches in diameter measured at the outermost edge of the rim, which makes them 20 inch wheels (I measured some 22.5 inch wheels and they're about 23.5 inches od. The wheels are stamped Tonka, er uh, Firestone. The tires are 37 inch OD and the treads are about 6 inches wide. So I'm guessing I won't find any polised aluminum wheels for this? Does anyone know if I can get tubeless steel wheels? Can anyone tell me what 8.5 x 20 on the tire means? Confused, Dennis Toeppen http://groups.yahoo.com/group/flxowners/message/3633 The LARGE axle Clippers used -- 10 hole Budds, and the SMALL axle Clippers used a 5 or 6 hole Budd, and they will not interchange!! http://groups.yahoo.com/group/flxowners/message/3623 what is referred to as the small hubs or the small axle with 8.25XX20 tires and a larger tire will not clear in the body cutout. I have a "small" Clipper that had the Chev 6, and a larger tire just won't go on! http://groups.yahoo.com/group/flxowners/message/3629 9xr20 about $250/ea. Les Schwab in Utah http://groups.yahoo.com/group/flxowners/message/3627 From: Ronald F Wickware Date: Mon Feb 10, 2003 10:54 pm Subject: Re: [flxowners] Re: clipper tires, cont'd Dennis, I just run stock wheels and tires on all my Flx's. I do all of my own tire work. My small Chevy Clipper has the same Budd wheels and tires as my 1973 Ford Schoobus. I run tube tires on all the rolling stock I own! I have no fear of what many today call "split rims"!! Many people driving buses around today have never even SEEN A TRUE "split rim". I have two vehicles, a '62 Chev school bus and a '62 GMC Express Van that have "split rims". In split rims the rim is split in two equal pieces, and lock together in the center of the wheel (without a lock ring). THEY truely can be dangerous. As far as safety etc. of tube tires is concerned, consider this: I just this fall had an interesting conversation with the local Chevrolet Dealer in my hometown in Northern Wisconsin, He owns and operates several fleets of truck and schoolbuses. His words, - - " 8 out of 10, thats 80%, of the vehicles I sell are ordered with tube tires". All of the school buses in that area run tube tires. If tube tires are fit for the MODERN schoolbuses, they are good enough for any vehicle I own and drive. IMHO..... Keep On Flx'n. Ron http://groups.yahoo.com/group/flxowners/message/3591 From: "KNNU21 " Date: Tue Feb 4, 2003 1:22 pm Subject: Re: Alcoa Wheels on a Clipper Approved narrowest rim width for the 10R 22.5 is 6.75", and for the 11R 22.5 it is 7.50". Problem with too narrow a rim is it causes the tread of the radial to wrap rounded, and stresses the sidewalls. The side walls will flex excessively. It also tends to make the vehicle feel like it is a Jello mold in crosswinds and sudden seering corrrections, rather uncomfortable. Best to go with widest rim that can fit and within recommendations. Michelin does carry their 10R 22.5 in their XZE series, rated for 65 mph, but closest size in the 75 mph XZA2 series is the 11R 22.5 . Dont know much about the Toyo tires. Kenneth


http://groups.yahoo.com/group/flxowners/message/3635 From: Robert Adame Date: Wed Feb 12, 2003 2:20 am Subject: Re: [flxowners] 427 conversion ... One thing for sure, convert your radiator to a pressurized system with the correct cap and coolant recovery bottle. That will help some.

Towing insurance on a bus can be very important. Towing is often US$100/hour or US$2/km, and is calculated from when the wrecker leaves the yard to when it returns.

RV towing insurance is often offered through RV organizations. For example, AAA has "RV Plus" insurance; FMCA associates with "coach.net"; Escapees with Foremost; Good Sam; and so on. (List above does not constitute endorsement.)

For buses, you may be on your own, but check around. Some places will give an overall discount for historical/antique vehicles because time on the road tends to be limited.

Tires are typically measured using three numbers

For example, a 12/60R15 is a 12" wide, 60% profile, R(adial), 15" rim.

Brifely, the ``profile'' is the ratio of sidewall height to tread width. A 60% profile means the sidewall height is 60% the width of the tread. For example, a 10"-wide tire with a 70% profile would have 7" tall sidewalls. No number usually means 70% (?) for auto tires.

According to Tejas Coach http://www.tejascoach.com/tires.html, as of 2003/04: Notes:

I saw an MC-5 being repaired. Non-structural aluminal panels facing the engine had beenrivted on; the rivets had self-destructed, o the panels were not being held by much. General comment: rivets fail, especially in dissimilarmetals. (I think this was rivets holding aluminum to a steel frame member.)

Same MCI: lower frame members were stainless steel; upper were mild steel. My guess: cost saving measure. It worked well in the sense that the coach lasted well past when it was retired from service, but note it was rusty in places. Can be an issue.


The basic job of the transmission is to take power coming out of the engine and change the speed up or down so it is a good match for the current road speed.


Transmissions are broadly of two types: manual, and automatic. The big difference is the manual requires you to do something every time the gearing is wrong, while the automatic does it all for you. An additional difference is the automatic does not require you to operate a clutch each time you start from a standstill. The big difference here is that a manual transmission vehicle will tend to roll backwards when you are trying to start up a hill.

There are at least two kinds of manual transmission and two kinds of automatic transmission that are in common use today (2003):

In addition to the above, there are several other transmission types. Depending on the bus you are looking at, it may be important to know about these.

Ratios, Efficiency, and Road Speed

It is desirable to move a bus as fast as possible using as little fuel as possible. On the flats, this usually means running the engine relatively slowly in order to minimize pumping losses inside the engine. Climbing hills, it is usually desirable to go as fast as possible, which means keeping the engine running at high speed, in order to get peak power, even as the road speed changes. On especially steep hills, it is necessary to have low gears. For tight manuevering, it is desirable to have a low gear so that the bus moves slowly.

The transmission is but one part of the overall drivetrain: Some engines have a power band of 1,000 to 2,000 RPM, while other engines produce power from 2,500 to 5,000 RPM. The output of the transmission goes to a differential which may have ratios ranging from about 2:1 to over 4:1. Finally, the outside diameter of the tire is important: the larger the tire, the higher the ``gear''. In addition, some systems have two-speed differentials, geared drive shafts, and so on.

To determine, say, top speed, you need to consider all four major components: engine, transmission, differential, tires. For example, 2100 RPM through a 0.86:1 top gear, through a 3:1 differential, through tires with an outside diameter of 1.05m (an 11R22.5 tire), the speed is:

  1500 revs motor   60 minute   1 rev dreiveshaft         1 rev axle
  --------------- x --------- x ----------------- x --------------------
     minute          1 hour     0.823 revs motor    3.11 revs driveshaft

            1.05m * pi metres forward      1 km forward
         x  ------------------------- x -------------------  = 115 kph
                   1 rev axle           1000 metres forward

Note that the units ``match'' above and below. For example, ``revs motor'' in the first part is above the line and it matches ``revs motor below the line in the third part. The only things that do not match up are ``km forward'' in the top and ``hour'' in the bottom, and the result is kilometres forward per hour. So you should be able to take this and plug in your own numbers, and if your tire sizes are inches or feet or whatever, just be sure the units agree above and below the line.

The above is enough to get started, but you need to know the maximum power output of a motor changes with the speed it is turning. For example, the following shows power (red line) vs engine speed for a 6.5l GM (General Motors) diesel 1993 vintage.

Note that the outpuat is about 120 KW (160 Horsepower) at 3,000 RPM, and about 65 KW (90 Horsepower) at 1,500 RPM. Suppose it takes 80 KW to drive the bus at 115 kph. With the gearing above, the motor turns at 1,500 RPM for 115kph, but the peak power at that engine speed is 65 KW, not 80 KW. Therefore, the motor cannot drive the bus at this speed, and the bus will slow down. If the bus is downshifted to, say, a 1:1 gear (instead of 1:0.823), the motor must turn at 1750 RPM to go 115kph. At that speed, the engine puts out about 75 KW, which is still not enough to drive the bus, so another downshift is required. If the next gear is 1.81:1, then the motor must turn 3300 RPM to keep the bus going 115kph. At 3300 RPM, the motor can put out up to about 110 KW, so there is plenty of power.

It is worth noting that at 3300 RPM, the motor is turning pretty fast. Still only about 80% of the maximum speed, but the motor puts out enough power (80 KW or about 105 Hp) above 1,900 RPM, so the motor is turning faster than it ``needs'' too. Turning ``too fast'' often means both increased noise and fuel consumption. In this case, if 80 KW is normal for 115kph --- instead of, say, the result of a headwind --- then you will rarely get to switch to fourth gear. You may only reach fifth gear on a downhill with a tailwind. Thus, replacing the transmission with one that has lower ratios, or switching the differential to one with a lower ratio, might be a good idea, since you can more often run at reduced engine speed, and you will have lower low gears.

Power consumption also goes up when climbing a hill. At a given speed, the steeper the grade, the more power you need. Suppose you are driving on the flats and you get to a grade. To maintain the same speed, the motor has to work harder and harder. Eventually, it reaches the max it can put out at that speed, and it starts to slow down. As it slows, the maximum power output also tends to fall off, so you slow down even faster. Soon, you need to downshift.

As the bus slows down, less power goes to overcoming air drag, so more power is left over to climb the hill. In addition, the power it takes to climb the hill is less the slower you are going. So if you go a little bit slower, it takes less power.

If all goes well, you have a gear which is a little bit lower and which allows you to go up the hill almost as fast as you were on the flats. However, with wide steps between gears, you may need to shift to a much lower gear. For example, the GMC PD-3302 [PD-3302] has a maximum governed speed in 4th gear of 105kph and a maximum governed speed in 3rd gear of about 60kph. Therefore, the step between gears is 1.75:1 (105/60), which is a big step. Suppose the bus reaches a hill at 100kph and the hill is enough that the bus starts to slow down. To keep going, it will be necessary to downshift, but the bus goes no faster than 60kph in 3rd gear. So even if the engine has enough power to get the bus up the hill at 80kph, it is still necessary to do it at 60kph.

All of which is to say that it is desirable to have many gears which are close together. Failing that, an engine which is much too large is a big help, though an engine which is much too large will tend to give worse fuel economy.

Ratios and Economy II

Which brings me to a point which has always confused me: why do American highway buses so often have just a few forward speeds? Historically, some buses were fitted with many-speed transmissions or transmissions with a few speeds and 2-speed differentials. But most American highway buses were made by GM and were equipped with just a 4-speed, and many of the other buses were fitted with a 4-speed or a 5-speed.

For example, in the case of the GMC PD-3302 [PD-3302], a ratio between 3rd and 4th would have allowed the bus to climb hills at much higher speeds. And the jump here is so large that it also would have provided better accelleration on the flats. Although power would have been applied to the wheels less time (no power to the wheels during a shift), the power output is very low at speeds just after up-shifting to 4th gear, and so accelleration is very poor. Switching to an intermediate gear would have made it possible to keep the engine at a higher power output.

The problem can be even worse with some automatic transmissions, which are 3-speed or even 2-speed units. Although it is possible to gear the such buses to run at speeds over 120kph, doing so makes the bus sluggish to accelerate and sluggish to climb hills, and at the same time the engine is spinning faster than it needs to in order to power the bus at 120kph, so fuel economy suffers.

The mystery for me is that an extra few ratios in the transmission would cost relatively little, but should provide big savings to the commercial operator in both schedule time and fuel economy. And, indeed, modern highway buses are often fitted with 5-speed or 6-speed automatic transmissions, and thanks to the torque converter, automatics typically need fewer ratios than a manual transmission. Perhaps somebody can explain why older manual-transmission buses were not fitted with six speeds or more.

Unfortunately, some buses are not easy to fit with modern transmissions. Notably, there are few choices of transmissions for V-drive powertrain layout, in either manual or automatic transmissions. I am not aware of any automatic transmissions with more than 3 forward speeds, and not aware of any manual transmission with more than four forward speeds, excepting XXX (GM planetary ``doubler''), which has a reputation of being fragile and it is hard to find parts for it.


A few buses, such as some by M.A.N., use geared hubs. In these, the drivetrain spins at higher speed and lower torque, and then the hubs gear down to the final wheel speed. Advantages include lower loads on many components, so they can be smaller, lighter, and cheaper, all without sacrificing durability. Disadvantages include one more source of friction and one more thing to break.


Automatic transmissions are typically built [...]. Climbing hills, automatic transmissions can spend a lot of time using the torque converter to gear down. All that slipping generates a lot of heat, which is both lost power and can damage the transmission if the transmission gets to hot. You can spend less time in fluid by using a lower low gear. That may mean a transmission with more gears, with wider gears, or a driveline change which lowers all the gears. A disadvantage of using lower overall gears is that the engine turns faster at highway speeds, and the top speed of the bus may be lower.

Typical automatic transmission working temperatures are in the range 60degC minimum, 70degC to 105degC typical, 120degC intermittent.


GM ``hydrashift'' -- 2-speed planetary near clutch, reputation for fragile.


Buses use springs between the wheels and the body of the bus. Springs increase ride comfort and keep the bus from being shaken apart. Essentially all American buses use rigid axles both front and rear. Some GM RTS buses were provided initially with independent front suspensions, but they developed a reputation for poor handling and rapid tire wear. Many were retrofit with rigid axles, and later models used rigid axles.

There are three major spring types used in buses:

All three systems have been used since the 1950's, and all are in common use today. Leaf springs are mostly used on less-expensive medimum-duty buses, where initial purchase price is a major concern.

Leaf springs are the oldest type. A leaf spring is a collection of metal strips or ``leaves'' stacked together. Leaf springs are simple and reliable but when the leaves bend under load they also move relative to each other. There is relatively high friction between the leaves, and so leaf springs do not respond well to small road variations and thus give a relatively harsh ride.

Another design is a piece of rubber in shear. ``Shear'' means that one part is held while the other part tries to move parallel to the ``hold''. For buses, the most common system is the Torsilastic. It is used, for example, on some Eagle and many Flxible highway buses made after about 1955. Torsilastic has a small metal tube sitting inside of, and attached to, a larger rubber tube. The rubber tube, in turn, sits inside of and is attached to a larger metal tube. The suspension uses a linkage to twist the inner metal tube, while the outer metal tube is attached to the bus. The rubber attached to both tubes tends to ``wind up'' in shear and supports the bus. Torsilastics can effectively be placed further outboard than leaf springs, for reduced sway in turns and crosswinds without switching to a stiffer vertical spring. Torsilastic advocates point out that leaf spring suspensions are rigid (metal on metal) side-to-side, while the Torsilastic also allow some lateral shock.

From promotional literature by www.rvcalifornia.com and www.lordmpd.com, as of 2003, and where VRSS is Torsilastic:

Torsilastics have a reputation for good durability, but high replacement cost.

The third common design is an ``air bag''. The air bag is made of material similar to the sidewall of a tire. The bag is inflated with compressed air and held between two plates, one on the bus and one on the suspension. Without weight on the suspension, air pressure tends to inflate the air bag to a ``resting'' shape. As weight is applied, the air bag deforms and the change in shape supports the added weight. [[diagram: how weight on a tire is supported by change in shape rather than change in air pressure.]] Like Torsilastics, air springs may be mounted further outboard for readuced sway, but at the expense of increased unsprung weight. Some torsilastic advocates claim air suspensions have poor lateral springing, but I have seen no evidence to support that claim, and air suspensions typically use control arms with large rubber bushings -- which should give good lateral springing. Air bags have the advantage that the ride height can be adjusted in service. This is used commonly to ``kneel'' or drop the front of transit buses for easier passenger on/off access, and is sometimes used to level an RV bus conversion when parked.

Some leaf spring systems have grease ports to inject grease and thus reduce the friction between leaves and improve ride. Leaf springs are durable, but may wear due to leaves rubbing against each other over many years. Also, the metal is highly stressed and over many years of use the spring may sag or may fatigue. The spring may be processed to remove sag, or new leaves can typically be made.

Rubber in shear is also reliable provided a good design. The ``Moride'' system has a reputation for poor reliability when used on heavy vehicles. Torsilastics have a reputation of 1-2 million kilometres between major overhauls.

Torsilastics also require periodic minor service. As the rubber deteriorates, the bus ``sags''. Eagle specified a ride height check every 30 days of service. If a Torsilastic unit had sagged, the suspension was adjusted to raise the bus to its previous level. Older units used a removable link on a star; newer units use shims. When the adjuster runs out of travel it is time for a major overhaul to replace the rubber.

Torsilastic major service may be much more expensive than either leaf springs or air bags. The Torsilastic ``core'' may cost in the range of US$1,000 to US$2,000 per wheel, and can be a high labor cost to install, especially in older designs. In contrast, replacement leaf springs may be manufactured at less cost and are relatively ``in the open'' and thus less expensive to install. The primary wear parts in an air bag suspension are the bags and the leveling valves, both of which are inexpensive --- roughly US$100 per piece per wheel --- and both of which are relatively low labor to replace.

Torsilastics are specialized so may have limited parts. However, as of 2003, they are still in current production and are used in school and highway buses. Torsilastics can sometimes be retrofit to a leaf spring design.

Air bags have far less friction than leaf springs and so give less shock. Air bags adjust to load changes, so the bus stays level as the total weight and weight distribution change. Air bags tend to be mounted inboard compared to Torsilastics, so air bag suspensions lean more in turns.

Air bag suspensions are more complex than either leaf springs or Torsilastics, but the individual pieces are fairly inexpensive and relatively durable. The suspension parts include: air bags and ``plates'' to attach the air bags; rubber-bushed control arms which ensure the axle only goes up and down; control valves which maintain the ride height; and more air tanks and air lines to maintain the suspension. The parts which fail most often are typically the air bags and the ride hight control valves. Air bags deteriorate under use and exposure to pollution, and typically deteriorate to failure somewhat faster than leaf springs or Torsilastics --- one estimate of typical air bag working life is about 20 years. [[Inspection: frays and cords; preventative replacement.]] There are rumors that air bags explode and lead to loss-of-control accidents. I have found no data to support that; it may be a rumor started by somebody wondering if such a thing were possible. Ride height control valves often become sticky over time causing the bus to ride wrong or to settle wrong as air bleeds away after shut-off. Air bags can sometimes be used to retrofit a leaf spring design (Reyco Granning).

More on Air Bags

Air bag suspensions can quickly drop (``bottom out'') on a loss of air pressure. While this is rare driving down the road, various servicing accidents can cause sudden loss of pressure. The bus can drop suddenly. If you are underneath, you could be killed. Therefore, block up the body before crawling around underneath. Be sure to use a support with a large resting area, as buses weigh 5,000kg to 20,000kg and if dropped on a conventional ``jack stand'' it can simply drive the support in to asphault, etc. instead of supporting the bus. Consider pre-deflating the air system to drop the bus before service.

Early air bag suspensions often used a relatively small air bag which was mounted so air could go in to a frame member. This increased the effective volume of the air bag without increasing the amount displaced when the axle moved, thus lowering the effective spring constant. For various reasons the ``air beam'' can become damaged and it is both possible and relatively common to repair the beam.

Shock Absorbers

A spring without a shock abosrber gives a poor ride: the vehicle bounces and bounces long after the bump is gone. A ``shock absorber'', also called a ``damper'', is a controlled way of bleeding off the energy that is making things bounce. A stiff shock absorber can give a quick end to bouncing, but may give a harsh ride; a soft absorber gives a gentler ride, but may allow the bus to wallow and bounce long after the bump is gone.

Shock absorbers are not just comfort items: poor shock absorbers mean the bus can bounce around a lot and get hard to control.

Leaf springs have internal friction and therefore act somewhat as shock absrobers. Older buses with leaf springs are often fitted with so-called lever shock abosrbers. Lever shocks arefilled with oil; loss off oil means loss of function. [[find out more about lever shock absorber service]]. Lever absorbers have a poor reputation, though it is not clear how much is poor original performance and how much is poor upkeep.

Note: do not disassemble a lever shock absorber without proper tools and instructions. It has a very strong inernal spring and can ``explode'' pieces on disassembly, hurting you and preventing re-assembly.

Modern vehicles use telescoping shock absorbers.

Shock absorbers can have a big influence on the handling of a vehicle. Shocks can be tuned so the damping varies with the wheel position, speed of wheel travel, rate of change of wheel travel, wheel load, and whether the wheel is leaving it's normal (``rest'') position or returning.

Modern (circa 2000) bus suspensions resist bus lean in corners and respond to many of the above position and load variations.

Although shock absorbers are a topic of great discussion for automobiles, they are much less often discussed for buses. I do not know why -- but I speculate that private coaches are often already set up reasonably well by the builder or original owner, so there is not a lot left to tinker.

www.boltscience.com --- lessons here about how fasteners work.

cut-down buses 4106, Munic Mack, AC Fishbowl. Pike's peak glasstop.

Emissions: some buses e.g., older Crown buses may only be sold for use outside the state of California.

Articulated: front section is like a 35' bus wheelbase some the rear section reverse-steers (dating as far back as the Kaiser [Kaiser bus]) so the rear wheels follow and do not get pulled over the kerb. Many drivers feel a 60' articulated bus is easier to drive in traffic than a 40' rigid bus. Two major layouts: mid pancake or rear. Mid pancake gives better control in poor traction but uses parts not compatible with rest of fleet, engine service is more difficult, and low-floor is harder. Rear handling problems can be compensated to some degree by an anti-jackknife pivot, but is not practical to steer trailer wheels so handling is worse. As of 2003, most articulated transits in US are made by New Flyer and use a rear engine.

Early articulated buses were a tractor-trailer configuration with a driver in the tractor, passengers in the trailer. Good for fixed service such as taking workers to remote areas, but not for watching over fare collections, answering passenger questions, etc. Example: Mack ([Mack Arch.], pg. 29).

Seattle-area early articualted bus had a tractor-trailer layout but driver sat in trailer. Some controls went through holes in the cab and thus moved as the tractor section turned. Was reputedly the cause of some early accidents. (Picture in Motor Coach Age?).

Note: the following is an overview; see your state's commercial drivers handbook for exact procedures.

Braking on long grades

Braking is tricky because bus brakes fail far more easily than auto brakes. Thus, bus drivers must pay special attention to drive in a way that is safe. The biggest problem is going down hills: you get a lot of energy from the hill, and that makes the bus go too fast so it crashes in to other vehicles or goes off the road. However, if you just step on the brakes, they get hot. When bus brakes get hot, the brake shoes do not press as hard, and at the same time the brake pad material gets less grabby [NHST02]. The result is the brakes ``fade'' --- that is, they get weak. In some cases, the brakes catch on fire or can ignite other parts of the bus.

The question, then, is how can you slow the bus to the speed you want without running out of brakes. One thing you should know is that the faster you go, the more of your energy goes to wind drag. And going a little bit faster takes a lot more power: going 25% faster dumps twice as much power in to the wind. So staying near the top speed you want to go means a greater fraction of your power is going in to the wind, and less in to your brakes.

On the other hand, the faster you go down a hill, the faster you have to dump the energy. The total energy you get out of going down a hill is the weight of the bus multiplied by the height of the hill, and that's true whether you go down at 1kph or 100kph. What does change is if you go down fast, you have to get rid of that energy fast.

Your brakes convert mechanical energy to heat. The heat is then dumped in to the air passing the brakes. The hotter the brake, the faster it dumps heat. When you use the brakes gently, they do not need to dump energy very fast, so they just get warm. When you use them hard, they have to dump the energy fast, so they get hot.

Thus, going down a hill fast may dump a greater fraction of the power in to air drag. But at the same time, the hill energy has to be dumped faster because you're going down the hill faster. Thus, the brakes can get too hot.

Now suppose you were to go down the hill very very slowly, for example, 1kph. Almost no energy would go to to air drag, and so almost all of the hill energy would go to your brakes. But at 1kph, it would take you 100 times longer to go down the hill than at 100kph. Remember that the hill provides the same energy whether you go down fast or slow. At 1kph, more total energy would go in to the brakes, but with 100 times as long to dissipate it, the brakes would not get as hot. You might not want to go down the hill at 1kph, but the same general idea works, for instance going from 100kph to 50kph.

Fade Is Worse With Mis-Adjusted Brakes

Brakes fade faster and further when they are out of adjustment. [[written up elsewhere in this document; reorganize.]]

Engine Braking

You can --- and should --- also use your engine for braking. If you try to ``idle'' your engine going down a hill, the hill will drive the engine up to a higher speed. The energy which goes to the engine is energy which does not go to heating up the brakes. Engine braking tends to heat up the engine, but the engine is otherwise idle and can dump a lot of heat through the radiator and exhaust. Some engines are fitted with a compression brake; a popular maker is ``Jacobs'', so compression brakes are sometimes called ``Jake brakes''. Compression brakes are very effective but can be loud, so they often need to be disabled in urban areas.

Engine braking often requires that you shift to a lower gear, since in high gears, the engine turns slowly and there is little braking effect. With a non-synchromesh manual transmission, it can be hard to downshift while going down a hill, as you must double-clutch (rev the engine) to go to a lower gear, at the same time you are braking and operating the clutch. Thus, it is often important to downshift before the hill begins in earnest. Many steep roads are marked with a ``grade'' symbol. When you see the symbol, that is a good time to downshift.

grade symbol] [American grade symbol]
USA only

Engine braking puts power in to the engine and can force it to spin too fast, which can damage it. Even a ``governed'' engine can spin too fast, as the governor simply controls how much fuel is going in to the engine --- and even if the governor shuts off all fuel, the motor may still be spun too fast by the grade. Therefore, when you use engine braking, be sure to pay attention to the tachometer. If the speed goes up, use your brakes to slow down.

As with regular brakes, the slower you go, the more energy you can dump through engine braking. A very steep grade should be taken at low speed. When in doubt, slow down too much and use too low a gear. Braking and shifting to a higher gear is easier than braking and shifting to a lower gear. If the engine is not spinning fast, you may be safe in using a higher gear.

Jake brake can ususally be used indefinitely. It may not give you as much stop as you want, but it's hard to overload except by over-reving the engine. Rule of thumb: brake horsepower meets or beats engine drive power.

Exhaust Brake

Cruder and less-powerful version of Jake brake. Rumor: do not use with Detroit Diesel 2-stroke our you will damage the supercharger.

Transmission Braking

Some transmissions have built-in brakes, also called ``retarders''. Advantage compared to Jake brake: higher peak braking, up to 60KW on the HD4060, braking does not depend on the gear, braking is smooth/variable where a Jake is more on/off, and transmission brakes are quiet. Transmission brakes heat the transmission oil significantly and so need to be used carefully.

Electric Braking

Some buses --- transits, mostly --- are built with an electric retarder, also by Jacobs. It can generate several hundred horsepower of braking but uses a lot of electricity. Not sure where the heat goes.

Braking Uses Air Pressure

[[``Service'' brakes.]]

Also note that rapid brake application can drain the air and also cause the brakes to fade or fail. This is especially noticible when the engine is turning slowly, as the engine drives the compressor which builds up air pressure. Try to drive so that you apply the brakes and hold a given level of braking, as holding the brakes does not use air, but releasing them does.

In slick conditions ... see your drivers manual: apply to skid, release.

Smooth Braking

Braking is not just about safety, it is also about comfort. Tape a cup of water and try to brake so it sloshes as little as possible, just like shifting (see shifting by RJ Long). The idea is that you want to slow down hard but you want a smooth transition in to hard braking and a smooth transition to a stop. To do that, apply the brake gently at first then harder and harder; and as you approach a stop you let up on the brake so that when you finally stop the brake is on only gently. Sometimes, new drivers jab on the brake suddenly, which jerks around the bus and passengers. Often, new drivers leave the brake on hard until they stop, which also jerks around the passengers. Or, they slow until they don't need the brakes any more, then release them suddenly. A little practice is worth a lot!

Fanning the Brakes

On buses before about 1960, it was common to ``fan'' air brakes to attempt to stop faster. ``Fanning'' is when you apply and partly release the brakes. I have never heard an explanation of why this should help. I can imagine:

Whatever the theory it appears that the practice was either ineffective or stopped being effective by the 1960's.

For example, the operator's manual for the GMC TDH-3301 (GMC part #X-7027) says in part:

Do Not ``Fan'' The Brake Treadle

This practice causes poor brake performance, wastes air pressure, and causes excessive wear on brake operating units and brake lining. ``Fanning'' does not increase brake line pressure, but decreases both reservoir and line pressure.

Some kind of brake pedal pumping is normally used in at least two situations. First, fanning is commonly used to drop the system air pressure as a part of the daily air brake inspection ([CaCDL 2003], pg. 66). Second, emergency stops without ABS (anti-skid braking) use ``stab'' braking, where the brakes are applied fully until a skid, then the brakes are released fully until the wheels start to roll, and then the brakes are fully reapplied ([CaCDL 2003], pg. 68). With an anti-skid brake system, stab braking is not used.

Air Brake Lag

Hydraulic brakes work nearly instantly. Air brakes take time to ``charge'' or ``pressurize'' the system, and so there can be a delay betweeen when your foot presses on the pedal.

Air Brakes in Cold Weather

As noted elsewhere in this document, water collects in the compressed air lines of buses. Tanks should be drained. In cold weather, automatic drain systems may freeze open making it impossible to build up. At least two things to do. When it happens, heat the drain valve. Many drain valves have electric heaters. May take 5 minutes to heat enough valve starts working. When risk of happening, ``fan'' the brake until pressure is under 90psi so the automatic purge valve will drain the tank and then close again. (Hm, seems like it might freeze closed...). More info..

Manual transmission how it works.

Profitability of bus transit decreased as auto became relatively cheaper. Fares dropped as %age of total earnings. Private operators went out of business, replaced or bought by subsidized public operators. Subsidies increased; tranist progressively larger losses.

Subsidized transit at first seems like a lose all around. However, serves several goals:

Consider that a 15-metre bus can carry 50 people comfortably. Suppose that the average speed of the bus is 20kph, and that the average safe following distance is 20 metres. Thus, in one hour, the bus carries 1,000 passenger-kms per 35m of roadway.

In contrast, suppose that a 4-metre automobile carries one person at an average speed of 30kph, with an average safe following distance of 25m. Thus, in one hour, the auto carries 30 passenger-kms per 29m of occupied roadway.

As a result, the bus carries about 25 times as many people per unit of roadway. Obviously more complex: cars are narrower and allow narrower lanes; public transit does not need to be parked and so can open up more space, but some space needs to be dedicated to parking. And so on. However, the basic result is that buses can carry more people per unit of roadway, so when more people ride buses, it frees space for other people to continue driving their cars.

People can be ``bought'' out of cars by offering transportation which is cheaper, faster, more convenient, and which allows people to relax and do something they want to do instead of being forced to focus on driving.

Amount of subsidy depends on demand for new roadways cost of new roadways and relative benefits of cars and public transit.

Secondary benefits of these policies include


Rule of thumb: 1/3 time passengers, 1/3 time traffic, 1/3 time moving.

Cut-down buses include AC Transit Dial-A-Ride service, cut down to manuever in winding narrow residential streets around Oakland, California [[Photos]]; GMC 4106 [[Pike's Peak?]]; and Mack buses operated by San Francisco's Muni [[Photo]]. Rear-engine design simplifies driveline changes. Wiring harnass ``looped up''. Mechanical controls must be cut and rejoined.


Door options include before front wheel, behind front wheel, mid, behind rear wheel. Urban transit routes typically get multiple doors for rapid passenger on/off, even though extra doors cut down on seating. Suburban and highway buses typically have a single door to maximize seating. Door sizes and types also vary. The simplest type is a single panel with a forward pivot. Although simple and easy to seal against the elements, not good for transit use where passengers are pressed in on the bus and there may be obstacles (posts, parking meters, benches, ...) which could be struck by an outward-swinging door. Early transits often used four-panel doors with a sliding track. Difficult to seal and poor visibility. Later, linkages to reduce to two panels: better sealing, improved visibility. TMC Citycruiser single-panel door swing in to protected area: good sealing and best visibility, but incompatible with handicapped access. Rear doors typically bi-fold or two parts swing out. Fewer people standing around so less risk of hitting somebody.

[[Diagrams of each open/close/motion.]]

Fare Boxes

[[Learn about!]]


Double-deck buses have the potential to carry twice as many passengers per bus. In practice, double-deckers carry a lot of passengers but not quite twice as many. Although no driver position or motor intrusions take space upstairs, steps do take space from both levels. Another issue with double-decker buses is that on/off times are substantially longer for upstairs passengers.

It is difficult to build a double-deck bus due to overall height restrictions. Minimum headroom for standing passengers is about 2m. Additional space is required for the thickness of the roof, two floors, and for ground clearance. Even if the roof and each floor is just 10cm thick, and even if ground clearance is just 20cm, the overall height is 4.5 metres (14-3/4 ft.). In contrast, most American roadway overpasses are only built to clear 4.1 metres (13-1/2 ft.).

A variety of approaches have been used to build double-deck buses including open-top and semi-open top, where the passenger is expected to stay seated any time there is a risk of low clearance; reduced inside clearance, so passengers are expected to duck their heads when walking about; staggered floors, so the upstairs aisle is over the downstairs seating; and overheight construction combined with restricted operation to avoid low obstacles.

		+--  --+	+------+	+------+	|      |
|      |	|      |	|      |	|      |	|      |
|SS  SS|	|SS  SS|	|SS  SS|	| SSS  |	|SS  SS|
+------+	+------+	+------+	| +--+ +	+------+
|      |	|      |	|      |	+-+  +-+	|      |
|      |	|      |	|      |	|      |	|      |
|SS  SS|	|SS  SS|	|SS  SS|	|SS  SS|	|SS  SS|
+------+	+------+	+------+	+------+	+------+

  open		semi-open	low inside	staggered	over-tall

Note that the staggered configuration typically has fewer total seats because all upstairs seats must be accessed by side aisles. Some buses use a single offset aisle. Doing so makes it difficult to access the seats farthest from the aisle. Quick access is relatively important for transit use, less so for suburban or highway use. Some staggered-aisle configurations use two narrow side aisles. However, even though the aisles are narrow, they use more floor space than a single aisle, leaving less space for seating.

[[Typical seating capacities?]]

Probably the most famous double-decker buses are those in London. Part of the reason for their continued usage is their fame and tourism value. However, they also serve a need: they have primarily been 10-metre units for good traffic negotiation. For a long time they ran front-engine units. More recently (19??), rear-engine and longer 12-metre units have become common. Although double-decker buses have been built in both high-floor and low-floor configurations, there has always been a greater pressure to keep the floors low in double-deck buses, due to their overall height.

It may appear that tipping is a common problem. However, most double-deck buses are remarkably stable. The drivetrain and suspension are heavy, low, and are a big portion of the overall weight. A typical motor, transmission, and rear axle is 2 tonnes, all of it near the ground. For example, 30 passengers per deck at 75kg per passenger is 2.25 tonnes per deck. Thus, 2.25 tonnes of upper deck passengers is held down in part by double the weight of lower-deck passengers and machinery. Overall, gross fully-loaded vehicle weight may approach 14 tonnes. Also, while a double-deck bus may look tall and tippy, the typical wheel track is about 2.5 metres compared to a height usually less than 4.5 metres.

Multiple Rear Axles

Historically, most bus types before 1950 used a single rear axle. In about 1950, dual rear axles became common for highway use. A greater number of axles allows greater total bus weight without exceeding weight-per-axle laws. Howeer, using another axle typically increases the bus price substantially, and tires on multiple axles wear faster due to ``scrubbing'' as the two axles do not track exactly the same line around corners.

Some buses drive both rear axles. More common is a single rear drive axle plus a lower-capacity non-driven ``tag'' axle. The tag axle allows greater weight-carrying capacity while avoiding the complexity of twin driven axles. A ``tag'' axle may also be placed to the rear of the driven axle, so that the driveline may be relatively long.

At least one bus (Eagle [[??]]) placed the tag axle in front of the driven axle. Rumored to have the worst turning radius of any bus built.

[[Steered tag axles?]]

[[Photos of tag axles -- alone, and mounted.]]

Production Runs

Tiny, compared to cars. ``Every bus was produced in such small numbers to make it an instant classic.'' -- Ralph Cantos.


Artillery, detatchable rim, disc, stamped steel, aluminum.

Aluminum is lighter and is unsprung weight, so cutting it reduces road shock and improves handlng. However, some reports of aluminum disintigrating under certain conditions [[get details -- urban legend?]].

Jump Seats

A ``jump seat'' is an extra fold-out seat which allows passengers to sit in the aisle. Standing is typically not a problem for transit service and clear aisles are important to quick service. However, for long journeys, standing is a problem. Jump seats can increase passenger capacity by 20% by converting 4-wide seating to 5-wide seating. Jump seats were outlawed in the 1940's due to safety concerns ([YC Archive], pg. 33).

Dual Power

Dual-power bus with both a gasoline engine nd trolley poles: Yellow Coach model 729. Used in regular service until 1948 [YC Arch]. Dual-power articulated buses appeared in Seattle in about 1990. Electric operation was used in the newly-built bus tunnel, in order to minimize pollution problems. Diesel was used outside. The trolley poles are on automatic lift and retract, and the wires at key locations have guides. Thus, the driver can pull in to a station at the end of the tunnel and switch from diesel to electric at the same time passengers get on and off. Similarly, leaving the station the driver may retract the poles and switch to diesel while passengers get on and off. (Though for some reason, the south end of the tunnel uses a separate staging area where all buses stop, thus forcing passengers to wait while the power switch is made.) Note that Seattle Metro runs many trolley buses, but they do not typically [[ever?]] run in the tunnel.


Long is good for a smoother ride. Moving the axle to the very rear, called a ``bobtail'', can improve the ride. However, a long bus cannot have a long wheelbase as it will bottom over relatively small things [[diagram]]. Example ``bobtail'' -- 1924 Yellow Coach ([YC Arch], pg. 20).


A low floor makes passenger on/off easier, especially for reduced-mobility passengers. Since about 1/3 of the total bus transit time is often taken with passenger on/off, reduced loading times can dramatically improve average bus speeds. Low-floor buses appeared as early as 1938 with the Yellow Coach model 738 ([YC Arch], page 73). They were then absent for many years but reappeared in the 1980's following ADA.

Low-floor buses also simplify the design of wheelchair lifts. With a low floor, the front suspension can be dropped to get the front floor within a few inches of the ground. Then, the front entrance floor can be dropped a small additional distance. The front floor also extends from the bus so the bus can load from a kerb.

Low-floor buses often have reduced seating because it is difficult to seat passengers over the wheel wells. In addition, rear axle components tend to intrude. For example, the differential appears in the middle of the aisle. [[picture or diagram]].

Low-floor buses often use smaller-diameter tires to reduce intrusion in to the passenger compartment, but small tires have limited load ratings and tend to give a harsher ride. Many American low-floor buses use a ``step up'' section in the rear to clear the back axle and power pack. European designs sometimes have sufficient design heroism they can hvae a low floor all the way to the back. [[Renault has a low-floor bus which is flat all the way to the back including a rear door that exits behind the rear axle. I had no way to look underneath so I have no idea how they avoided the usual axle and differential intrusion. I suspect it uses something like a drop axle and hydraulic motors, but that is just a guess.]]

Air Shocks

In the mid 1920's through the mid 1930's, it was common to fit air shocks on the front suspension of buses. They are clearly visible in pictures of buses of the vintage (e.g, 1924 Yellow Coach [YC Arch], pg. 21; 1924 Mack [Mack Arch], pg. 15, 19)]. [[Find out more about them: how they worked, how they were attached, when they appeared and disappeared, and what replaced them.]]

Luggage on Roof

In the 1920's and 1930's, it was common to put passenger luggage on the roof of the bus. Intercity buses were commonly built with ladders and luggage racks. Luggage was then covered with a tarp to protect it from the weather. Buses also had interior parcel racks.

In 1936, the Yellow Coach 719 introduced a raised floor with compartments underneath for passenger baggage.

Express Package Service

Profitable sideline. Since passengers have to be on a schedule anyway, offer express package service: drop it at the station and it will arrive at the destination station on the same schedule as passengers. Package service spurred development of huge baggage bins in some buses (e.g., GMC PD-4107 [[Picture]]). Profit of service went down with arrival of services such as UPS, Federal Express, and so on; but Americans carry more luggage than ever thus keeping up demand for large storage bays.

Axle Placement

The placement of the front and rear axles is also complicated. Ideally, the axles are far apart for good stability and rider comfort [[figures showing body displacement over a bump]]. Ideally, the axles are also close together for good ground clearance and tight turning [[figures showing ground clearance and turning]]. Also, if the rear of the bus hangs out a long way behind the rear axle, steering the bus one way makes the back of the bus to swing out the other way. Such ``swing'' sometimes causes accidents.

Dual Front Wheels

Several bus designs would benefit from having more carrying capacity on the front axle. The rear axle is commonly built with dual tires. Unfortunately, it is not practical to steer dual wheels, since it is difficult to build a pivot inside the wheel, and since wider wheels and larger steering offsets lead to ``bump steer''.

Many European trucks, and a few European buses, use twin front axles, each with a single steered wheel. [[picture]].

Mack C45GJ3017 is a former Rose City (Portland Oregon) transit. It is gasoline-powered and has an automatic transmission. It uses a V-drive configuration but with the transmission on the left. The interior has been removed. It has not been run in about 20 years. It currently sits in Vancouver Washington. It is owned by Elmer Lindquist, and is for sale. For more info, contact bus-0035 /at/ pardo /dot/ net.

Propane ``fumigation''.

Driving Tips

A bus is big, both long and wide. That means you often have to steer late so the back of the bus does not run over things, and so the middle and back do not hit things. At the same time, you have to steer sharply because the wheelbase is so long. And visibility is worse than in a car. And you go slower then other traffic in tight manuvering, climbing hills, and just about all the time: other people may get impatient and ``go around'' just where you are trying to go. Bus brakes are not as strong or reliable as car brakes, so other drivers may ``cut you off'' thinking you have plenty of time to stop. Bus steering is slower than car steering, so it takes longer to avoid obstacles, change lanes, etc.

Thus, there is a lot of potential to hit things. You can learn how to avoid them, but it takes perception, patience, and practice.

It is especially good to practice good driving habits while you are learning. When driving gets ``busy'', we often fall back on our habits. If you have good habits, they will help you in those situations where the risk is greatest. You can always intentinally do something else whenever you desire, but having the safe habit helps you out when things get tough.

See a state handbook for getting a commercial driver's license. There are lots of really good tips for staying out of accidents. Some other notes are here.


When you are getting set up for a turn, keep checking your rear view mirrors. Especially on a right-hand turn (U.S. traffic goes on the right-hand side of the road) you want to swing wide to avoid hitting things on the inside of the turn. At the same time, cars, bicycles, and pedestrians may come up on the right side.

In some situations you can ``crowd'' the right-hand side to keep auto traffic from coming up. Crowding also reduces the risk of cyclists coming up, pedestrians and some cyclists are still a risk. It can be helpful to note whether there are pedestrians or cyclists in the area as you approach the turn, but beware that cyclists can come up quickly and pedestrians can come from parked cars, buildings, and so on.

``Crowding'' the turn increases the risk you will hit something on the inside of the turn. It may be useful to get close to the curb before the turn, then pull out a little just before you start the turn. The rear end helps run interference for you, but then you get away from things a little while turning to reduce the risk of hitting things. Note this means you must check your left side while making a right turn, as cars and cycles may come up on your left while you are crowding right.

Be sure to pull well in to the intersection before you start to turn. For example, a good rule of thumb is to pull out far enough you can look down the line of parked cars where you want to be, then start to turn [[diagram]].

As you corer, keep checking your rear view mirrors. The part of the bus between the tires on the inside of the turn is most likely to hit things, with the greatest risk often just before the rear wheel. You want to keep checking your mirrors in slow turns because you can hit things at the end of the turn which you could not see in the mirrors at the start of the turn. You also want to keep checking because things may move in to where you are turning.

In slow turns, you also want to check the mirror on the outside of the turn since parts of the bus behind the rear wheel can hit things. [[diagram]]. And, again, you can hit things which were not visible in your mirrors when you started the turn.


If you are driving a manual transmission, you need to shift as you drive. Bus transmissions often lack synchromesh, so they require a special technique even while driving straight [[see lnk]].

There are at several other situations where shifting is different than cars. One is when you come to a stop. It is tempting to stop, wait for the light to change, then put the bus in gear and go. However, buses often shift poorly when stopped. Thus, you usually want to shift in to low gear just as the bus rolls to a stop. Then, leave your foot on the clutch while stopped. Do not worry about the throwout bearing while you are stopped, bus transmissions are built to handle it!

Another special situation is cornering. Everything except gradual turns are ``busy'', and downshifting is ``busy'', so you usually want to downshift either before a turn or after a turn, but not during the turn.

Engine braking is important for buses, and once a bus is out of gear it may be hard to shift it in to gear. Thus, you tend to want to keep the bus in gear at all times.

Shifting is both tricky, and it can slow you down since no power is applied while you are shifting. Often, you need to speed up for a little bit then slow down again, for example when you are coming out of one corner and approaching another. It is tempting to shift up to another gear but then you will almost immediately have to shift down. Diesel bus engines have a governor, so you can ``floor it'' and the engine will not spin too fast and get hurt. Therefore, on diesel buses, you can safely ``floor it'' and just leave it there until you get up to the next corner. The engine may be a bit loud because it is spinning fast, but this may be the easiest and the fastest way to get where you are going.


A bus is big and has no center rear view mirror. Thus, there is a very large ``blind spot'' where you cannot see. So step #1 is to make sure the space where you want to back is clear.

When backing up, warn anybody who may be walking/wandering in to your path. If you have a back-up beeper, great. If not, toot the horn a couple of times. This is an ``easy to forget'' item, so just always toot the horn even if nobody is around.

Scan back and forth between both mirrors while backing. If the bus has a back-up camera, scan them anyway, as you may have a clear path back but then hit things on the side of the bus.

Beware of tall items such as roof overhangs and tree branches. In a car, most things will clear people walking around, but buses are tall enough to hit things over a person's head.

Crossing Tracks

Law requires you stop fully and put on 4-way blinkers. In vintage coaches with no 4-ways, use right turn signal if available. Start again and leave blinkers on until fully cleared the tracks. If driving a manual shift, leave it in low gear until fully across the tracks, do not shift.

London double-decker buses are often less than 10m long. Routemasters as short as 8.4m (27.5 ft.).

Engine cooling fans: direct/belt/miter box, coupled (e.g., thermal), electric, hydraulic. For transits, noise has historically been an issue. Don't drive all the time saves power and noise. Miter box historically rare part for some buses (e.g., Scenicruiser). Electric has limited cooling capacity -- 1kw @ 12v is 100A. Just for the cooling fan! Hydraulic needs careful design, else when you let off on the throttle the fan will continue to rotate (inertia) and the fan motor will cavitate. Hydraulics are common, not sure how they're done: freewheel clutch or bypass valve or...?

If milky when drain air tank, that means the air compressor is passing oil. Keep an eye on the volume. Passing oil could be a sign of compressor failure, but some pass oil for years with no increase in volume.

Oil can damage rubber parts in the brake system: treadle O-rings, hoses, brake diaphragms.

MCI 5A motorhome conversions cover up engine access plate, then air compressor replace goes from minor -- unbolt and replace -- to major -- remove cylinder head.

Conversions and LPG buses: propane is heavier than air and tends to ``pool'' like water --- then explode when there is a spark, such as opening the access panel. Thus, tank must be mounted where leaking LPG cannot ``pool''. Outside is okay, but the tank will rust faster and filling stations typically refuse to refill if they see rust. In a baggage bay, etc., there need to be ``flow through'' vents so falling LPG can drain out and new air can come in to replace it. The area with the propane must be well-sealed from any other bays, etc., so leaking propane cannot pool in those bays.

Tank baffles: A low pressure pressure can add up to tremendous forces that can rupture a tank. For example if you have a rectangular tank with one side that is 30 x 60 inches, that is 1800 square inches. If you pressurize the tank at only 3 pounds per square inch, that would exert a force on the wall totalling 3 x 1800 = 5400 pounds. That's nearly 3 TONS! One reason for baffles inside a tank is to strengthen the sides against hydrostatic pressure. If the water in a tank is 24 inches deep, the pressure at the bottom will be nearly 0.9 pounds per square inch. Thus, pressures add up very quickly. Another reason is that liquids sloshing back and forth while underway can create their own pressure -- the weight and balance of the tank can change suddenly, which is bad; and a pile of sloshing fluid can act like a battering ram and damage/rupture the tank.

USCG requires 5psi pressure test. Fill with water and use a hose full of water with the water line 4m higher than the tank.

Commercial drivers have a pre-trip checklist. Private operators should, too. See, e.g., North Carolina pre-trip inspection.

Thicker oil to deal with auto transmission problems discussion.

Paint job can easily run US$1,000 for paint, US$10,000 for a fancy paint job. Some ``epoxy'' paints are quite durable but do not have the nice polish of a more expensive paint. The original Imron has been taken off the market in the U.S. due to environmental concerns --- although it is still used in other countries, and some people go there for paint jobs, it still poisons the planet and the people, so better to stick to what is available in the U.S.

Urethane is expensive. CHeaper is acrylic enamel, 5-7 years durability. Cheaper yet is synthetic ename, 3-4 year finish.

For a cheap job consider Alkyd enamel. If you ask for mis-tints, you may be able to get bargain US$2/gal. You can mix mistints to get a color. Disadvantage of mistints: harder to find a match for repairs. Alkyd covers about 6m^2 per litre (300 ft^2 per gallon).

Brake light switch is often run by air pressure. Some have a bleed hole and over time moisture enters and corrodes the electrical contacts. Consider relocating to save it from moisture.

Run out of fuel: suck crud from tank --- clog filters; lose prime -- can't restart. Change filters reuires vacuum relief. Re-prime requires pressurize tank or priming pump.

Repairs: J.B. Weld can do a god job, but will not bond to oil, etc. So clean, clean, clean. If repair cracked fuel tank, remove [[WARNING: if gasoline, drain and fill with water before removing, else tinys sparks cause explosions. Varsol dissolves gasoline residue.]] clean, clean again, clean with acetone, repair. Clean inside --- Dawn dishwashing liquid. Get out that old sediment. Consider drilling cracks to stress relieve ends, but do it with tank full of water (sparks === explosions).

tag axles. They retract (sometimes). Lower drag where higher axle loads are allowed. Air springs down, air spring retract.


[Kaiser bus] 1946: Articulated 60-foot aluminum Kaiser/Permanente. Hemmings copy of 1946 NYT article.

[PD-3302] Pacific Bus Museum web site, 2003/04/13 http://pacbus.org/PBM1945picpage.htm.

[MBT001] The Museum of Bus Transportation web site, 2002/01/17 http://www.busmuseum.org/series30/mbus4.jpg.

[NRDCa] ``Life After Diesel: The Alternatives'' from Chapter 6 of ``Exhausted by Diesel, How America's Dependence on Diesel Engines Threatens Our Health''. From http://www.nrdc.org/air/transportation/ebd/chap6.asp, 2002/01.

[Pascoe] ``Power Options: Gas-vs-Diesel'', David Pascoe. From http://www.yachtsurvey.com/GasDiesel.htm, 2002/01.

[PascoeII] ``Power Options: Gas-vs-Diesel Part II'', David Pascoe. From http://www.yachtsurvey.com/GasNdiesel.htm 2002/01.

[NTSB02] ``Collision Between Truck-Tractor Semitrailer and School Bus Near Mountainburg, Arkasnsas on May 31, 2001'', NTSB.

1954: Twin Coach/Flxible introduces predecessor of ``new look'' windshield

``DD-3 brakes were removed in 1974 after a fire on bus 1088'' SCRTD 1973 T8H-5307A. here.

[YC Archive] ``Yellow Coach Buses, 1923-1943 Photo Achive'', William A. Luke.

Copyright (C) 2002, 2003. All rights reserved. _