ARTICULATED TRANSIT BUS
STEERING CONSIDERATIONS
With respect to steering behavior, articulated buses may be grouped into two general categories--those with steered rear axles and those with non-steered rear axles. Whether or not an articulated bus is equipped with a steered rear axle, steering is usually a function of engine and drive axle placement, plus other technical considerations.
At least for purposes of this discussion, the axles of the articulated bus may be designated as follows:
Axle: Front axle, steered by driver.
Axle: Middle axle, driven or non-driven, non-steered.
Axle: Rear axle, driven or non-driven, steered by articulation angle or non-steered.
Although exceptions exist, the traditional propulsion system arrangement for articulated buses employing a steered rear axle consists of the A-axle and the B-axle located in the front body, with the powerplant mounted underfloor between the A-axle and the B-axle, and with the powerplant driving the B-axle. This arrangement is depicted schematically inFigure 1 . Because the front body pulls the rear body, such buses are typically referred to as "pullers" with the front body being referred to as the tractor and the rear body being referred to as the trailer.
With the typical puller articulated bus, the C-axle's primary function is merely to support the trailer's weight and load. It is therefore relatively uncomplicated and inexpensive to utilize a steered rear axle as opposed to a non-steered trailer-type axle, and to actuate the steering automatically by mechanical linkage connected to the articulation joint or "bogie". With such a system, the C-axle usually steers mildly outward, in the direction opposite to the direction that the A-axle is steered by the driver. Such automatic steering linkages typically steer the rear wheels an amount proportional to the bogie angle.
Although exceptions exist as well, the traditional propulsion package arrangement for articulated buses not equipped with rear axle steering consists of propulsion system placement behind the C-axle, with the C-axle driven. This arrangement is depicted schematically inFigure 2 .
Because the rear body pushes the front body, such buses are typically called "pushers". Inasmuch as the front body does not pull the rear body, the terms "tractor" and "trailer" often used when discussing puller articulated buses are inappropriate when discussing pushers.
With the pusher, since the C-axle is actually propelling the bus in addition to supporting the weight of the rear body and its load, it is both complicated and expensive to steer as well as drive the rear wheels. Thus the typical pusher articulated bus employs an A-axle steered by the driver, a non-steered B-axle near the rear of the front body and a non-steered driven C-axle in the rear body. In such applications the B-axle may consist of a beam-type truck trailer axle or a driven axle housing similar to the C-axle with drive gears and axle shafts deleted.
Each of the two thousand odd articulated transit buses currently operating in North America consist of one or the other of the aforementioned traditional puller or pusher arrangements.
With a clear understanding of the fundamental differences between typical pullers with rear steering, versus typical pushers without rear steering, it is now appropriate to examine their turning behaviors.
A very important but widely misunderstood concept pertaining to vehicles steering behavior is that of "turn corridor". This term describes the corridor traversed by the bus when turning. Its outside limit is defined by the arc generated by the outermost point of the bus . . . usually the outside corner of the front bumper. Its inside limit is defined by the arc generated by the inner-most point of the bus . . . usually the body sidewall near the middle or rear axle. The area or lane between these two arcs is the turn corridor. For a given bus, the corridor will widen as the turn becomes tighter. The turn corridor for a typical non-articulated bus is depicted inFigure 3 .
It is with respect to the turn corridor and to the lateral displacement of the rear of the bus that the steering and turning behaviors of puller and pusher articulated buses differ most significantly. With pushers employing a non-steered driven rear axle, the rear end of the rear body will progressively encroach inward toward the center point of the turn, increasingly as the turn becomes tighter. This phenomenon, known as rear body intrusion, is similar to the behavior of truck tractors pulling large, single trailers.
Naturally, rear body intrusion widens the turn corridor (other factors remaining equal) by reducing the radius of the arc generated by the inside of the body. A turn diagram for a typical pusher articulated bus is depicted inFigure 4 .
Without rear steering, the intrusion of the rear body toward the focal point of the turn cannot be eliminated. It may be reduced, however, by employing a relatively short rear body, and by placing a pivot point of the bogie well behind the B-axle. This latter design technique causes the bogie to lead the front end of the rear body outward, thus "steering" the rear body slightly outward without actually changing the C-axle wheel angles.
Conversely, with the typical puller articulated bus employing a steered C-axle, the rear steering may easily be designed such that the rear body is steered outward . . . usually the amount necessary to cause the C-axle to follow the path of the B-axle in any forward direction constant-radius turn. This amount or "proportion" of trailer steering has been popular, as it enables the driver to accurately predict the path of the trailer tires.
With this design, the trailer will not intrude toward the center of the turn circle, and will not significantly widen the turn corridor.
Therefore, it may be stated--perhaps categorically--that with other factors being equal, rear steered articulated buses will exhibit narrower turn corridors than comparable articulated buses without rear steering. This is an obvious advantage in certain circumstances such as operation in confined terminal areas and on unusually narrow streets.
As with seemingly everything technical, there is seldom if ever a significant advantage without an equally significant disadvantage. The turning behavior of articulated buses is no exception to this axiom. Because the puller's C-axle steers outward to reduce intrusion of the rear body inward while turning, the rear corner of the trailer must necessarily be displaced outward beyond the arc generated by the outer sidewall of the tractor.
This phenomena is known as "rear corner excursion". It is a behavioral characteristic which may be tolerated by using the appropriate driving technique. However, such driving technique is quite different, perhaps even opposite, from techniques routinely taught to drivers of non-articulated buses. Therefore, compensating for rear corner excursion using driver technique may be practical only for operators able to dedicate specific drivers only to articulated buses.
Additionally, rear corner excursion is particularly difficult to deal with because, owing to the bend angle of the bogie, the excursion of the rear body outward will never be visible to the driver through his or her outward exterior mirror. This lack of visibility of the lateral displacement of the rear body is not the case with intrusion of the rear body inward with the typical pusher. With the pusher, the inward displacement of the rear body is visible to the driver at all times via the inward, exterior mirror.
A turn diagram for a typical puller articulated bus with C-axle steering is depicted inFigure 5 .
Certainly turn corridor width and lateral displacement of the rear body are very important issues which require careful consideration within the context of a given operator's traffic and roadway conditions, driver training, application of drivers, terminal facilities, etc. However, there are additional advantages and disadvantages pertaining to articulated buses with and without rear-axle steering that must be mentioned. These additional pros and cons are only indirectly related to the issue of steering, but are substantial, and therefore should be considered.
It has only been during the last two decades that the economic advantages of articulated buses have been utilized by North American transit operators. Prior to this time, the standard U.S. transit bus was a two-axle non-articulated unit with rear engine and rear-axle drive.
Given the forgoing reality, notwithstanding the present use of some underfloor engined articulated buses, the vast majority of the approximately 70,000 transit buses in North American service are rear engined. Thus rear engined articulated buses will typically be more familiar to maintenance personnel than articulated buses with underfloor powerplants.
Additionally, maintenance facilities and equipment typically in use by North American operators will obviously be appropriate for pusher buses, and may easily be to some extent less appropriate for buses with underfloor engines.
Finally, pusher applications lend themselves easily to upright engines which are currently popular throughout North American transit operators. Reciprocally, the underfloor engined puller must utilize a horizontal-type in-line engine, which is presently far less popular and less readily available.
In summary, the pros and cons of rear-steered versus non-rear-steered articulated buses, we may generalize as follows:
Pushers without rear steering will have wider turn corridors than pullers, and will thus be less maneuverable than pullers under confined conditions.
Pullers with rear steering will have narrower turn corridors owing to automatic outward steering of their trailer axles, and will therefore be very maneuverable in confined conditions.
The trailer axle steering of the typical puller results in outward excursion of the trailer's rear corner. This phenomena is invisible to the driver, and is a substantial disadvantage when pulling sharply away from curbs, when executing sharp right-hand turns, etc.
The special driving techniques necessary to mitigate the adverse effects of the puller's rear corner excursion typically tend to contradict the techniques traditionally taught to drivers or non-articulated buses.
Although only indirectly a steering issue, the pusher design, with its rear-mounted propulsion system, is familiar to most North American maintenance personnel, compatible with their facilities, and lends itself to the use of the currently popular upright engines.
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