Why Does The Back Of The Bus Bounce More?

A few persons have met us, because of their curiosity around trying to know the exact reason why the back of a bus bounces — and that’s why we’ve written this article, to quench your curiosity. Ready to get an answer to “Why Does The Back Of The Bus Bounce More?

Let’s proceed.

Why Does The Back Of The Bus Bounce More?

The front of the bus stays level, and the point halfway between the front and back wheels bounces up by three inches, the point right over the back wheels bounces up by six inches, and the point way at the back can bounce up by as much as ten or so inches, depending on how far out it’s sticking past the back wheels.

Front Wheels: It’s quite obvious that the front wheels are used to steer, in order to maintain directional stability, and this is the reason why four tires cannot be used. If eventually used, it becomes even more difficult to decide the angle of Ackerman.

Rear Wheels: The available rear wheels are connected to the live axle at the rear — and they work in propelling the vehicle. Since a bus is regarded as a heavy-duty vehicle, traction is very much important. If up to four wheels are used, there’s an increment in the contact patch area, which results in “increasing the traction.” There are brakes suitable for every one of the four wheels which increase the braking control at the rear.

You may be wondering, “what’s the angle of Ackerman?

What Is Ackerman’s Angle?

This Ackerman’s angle comes into play in the concept of Ackerman steering. You know what happens to all the tires when you go around a corner, right? They turn along the circle with a common center point. There’s an intention for Ackerman geometry: it’s to completely avoid the need for tires to slip sideways when respecting the path around a curve. The angles b and a can be calculated using the formulas below:

a =

b =

Articulated Transit Bus Steering Considerations

Words from Bob Woods found on the Forum, Bob Is The Oil Guy:

As regards the behavior of steering, articulated buses may be group into up to two general categories namely: those that have steered rear axles and those with non-steered rear axles. Whether an articulated bus comes with a steered rear axle or not, steering is usually a function of the placement of the engine and drive axle, in addition to other technical considerations. For the purpose of this section of this article, the articulated bus’ axles may just 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 there are series of exceptions, the traditional propulsion system arrangement meant for articulated bus, employing the use of a steered rear axle comprises of B-axle and A-axle — and you can find them at the front body, with the powerplant mounted underfloor between the A-axle and the B-axle, and with the powerplant driving the B-axle. Because the front body takes the stress of pulling the rear body, such buses are categorized as “pullers” with the front body being known as the tractor and the rear body being referred to as the trailer.

Related Questions:

Looking at the typical puller articulated bus, the primary function of the C-axle is basically to support the load and weight of the trailer. It’s relative how utilizing a steered rear axle can be uncomplicated and inexpensive — as compared to a non-steered trailer-type axle, and to automatically actuate the steering by using the mechanical linkage that’s connected to articulation “boggie” or joint. With such a kind of system, the C-axle — most times — steers mildly on the outside, in the direction that’s opposite to the direction which the A-axle is steered, steered by the driver. This type of automatic steering linkages works optimally in steering the rear wheel an amount which is proportional to the bogie angle.

Although exceptions as well exist, the traditional propulsion package arrangement that’s meant for articulated buses which isn’t equipped with rear axle steering comes with propulsion system placement behind the C-axle, with the C-axle driven. Because the rear body exerts pressure that pushes the front body, such buses are typically called “pushers.” Inasmuch as the front body doesn’t pull the rear body, the terms “trailer” and “tractor” often used when talking about puller articulated buses are inappropriate when pushers are concerned. With the pusher, since the C-axle is carrying out the job of propelling the bus, without leaving behind supporting the rear body’s weight and its load. It’s both expensive and complicated to steer as well as drive the rear wheels.

Thus the typical pusher articulated bus employs the use of an A-axle steered by the driver, a non-steered B-axle close to the rear of the front body and a non-steered driven C-axle right in the rear body. In such applications, the B-axle may comprise a beam-type truck trailer axle or a driven axle housing that’s similar to the C-axle with axle shafts and drive gears deleted. Every one of the two thousand odd articulated transit buses which operates in North America has one or the other of the aforementioned traditional puller or pusher arrangements.

With a crystal clear understanding of the fundamental differences between typical pullers with rear steering, versus typical pushers without rear steering, it’s now appropriate to examine their turning behaviors. A very essential but a concept that’s widely misunderstood as regards vehicles steering behavior is that of “turn corridor.” This term vastly describes the corridor traversed by the bus when turning.

The arc generated defines its outside limit by the outermost point of the bus . . . usually the front bumper’s outside corner. 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.

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. 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 phenomenon 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.

Frequently Asked Questions (FAQs)

Why Is The Back Of The Bus So Bumpy?

Length and angles, too: the back of the bus is quite farther from the front of the bus. Any significant angle changes between the contact points on the road will be exaggerated. So if the back sticks out much further from the back tires, it’ll exaggerate any movement (think of a see-saw with one end jutting out farther).

Why Does The Back Of The Bus Bounce More – Conclusion

As a recap of the response we gave to the question — Why Does The Back Of The Bus Bounce More? — we stated that:

The front of the bus stays level, and the point halfway between the front and back wheels bounces up by three inches, the point right over the back wheels bounces up by six inches, and the point way at the back can bounce up by as much as ten or so inches, depending on how far out it’s sticking past the back wheels.

Thanks for reading.

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