Trailing axle

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Trailing axle on a coach
Truck with raised trailing lift axle when moving swap bodies

In heavy, mostly three-axle commercial vehicles ( trucks or buses ), a trailing axle is a non-driven axle that serves to reduce weight and is located downstream of the drive axle. As a rule, the trailing axle, in contrast to the twin-tyred drive axle, only has single tires , but in Scandinavian countries there are also twin-tyred trailing axles. The principle goes back to a Büssing construction from the 1920s.

Trailing axles are used wherever high loads have to be transported and vehicles with two axles ( gross vehicle weight (GVW) up to 18 t) are no longer sufficient. A vehicle with an additional axle can take more load, with a third axle with single tires up to 25 t.

Trailing axles can be rigid, liftable (as a lift axle ) , positively steered or trailing steered . Steered trailing axles have the advantage of less shearing movement on tires and road surface, less rolling noise and a reduction in the turning circle . However, trailing axles are disadvantageous when cornering faster - especially when it is wet and slippery, as the vehicle has a larger overhang and the drive axle is relieved of its lateral guidance. As a result, the vehicle tends to break away more quickly.

Trailing axles are mostly used in trucks with swap bodies , as well as in three-axle, extra-long solo buses (> 12 meters). In articulated buses with a drive in the front end (so-called puller ), the non-driven axle in the rear carriage, i.e. the trailer of the articulated bus, is also referred to as the trailing axle. This center-axle drive was very widespread until the 1980s, as Mercedes-Benz was the only bus manufacturer to hold the patent rights for the necessary articulation angle control for articulated pusher buses developed in 1977 . All competitors such as MAN or Setra were initially still forced to build articulated buses with a drive in the front end. These vehicles were usually shorter than articulated pusher buses and also more manoeuvrable thanks to the trailer steering. The disadvantage was that the motor could only be accommodated in the rear for reasons of space, which meant that a complex power transmission through the joint to the second axle was necessary. The lack of stabilization led to rolling movements at higher speeds. Furthermore, when the vehicle was empty, the drive axle in the center of the vehicle was not sufficiently loaded, which meant that the wheels tended to spin. After the patent protection was lifted, MAN and Setra built (almost) exclusively only articulated pusher buses. In some articulated buses from foreign manufacturers (e.g. Berkhof, Van Hool ) you can still find this follow-up steering due to the center-axle drive available there. Instead of underfloor motors, tower motors are used there in the middle of the car .

The self-steering systems on these articulated buses are self-steering systems, as they are also used on some trailers. These steer depending on the angle of the joint, in which a steering kinematics, usually located at the connection to the front end, follows the vehicle movement of the front end and actuates the follower steering as a function of this. This active principle of this forced steering can also be found in many trailers or semi-trailers.

In motorized vehicles, self-steering trailing axles usually actively steer via the steering mechanism, i.e. H. The wheels on the trailing axle are moved by a hydraulic cylinder when the steering is operated, even when the vehicle is stationary, while in the case of semi-trailers and trailers, a hydraulic system on the coupling operates the steering depending on the cornering of the towing vehicle. Purely trailing-steered axles run loosely on the road, i.e. are passively steered along with the vehicle. When reversing, the latter must therefore be locked or relieved, otherwise they would twist in an uncontrolled manner. Likewise, the steering of most of the trailing axles is usually locked for reasons of driving stability from certain speeds (usually from 40 to 60 km / h). There is not always a mechanism that locks these positively steered axles on trailers (including the corresponding puller articulated bus types) when reversing, so that these axles have to be "oversteered" in the opposite direction due to the given steering impulse, in order to then adjust the steering angle and follow curves.

However, if the center of gravity of the vehicle is in the rear, as is the case with rear-loading garbage trucks or roll-off dump trucks , axle arrangements with leading axles are more suitable, as the rear axle with twin tires can take more load and increase driving stability. However, this axle arrangement increases the turning circle, which limits the usability of these vehicles in narrow road conditions. For reasons of driving stability, leading axles are also used in vehicles with short wheelbases, such as tractor units.

See also

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