Rear-Wheel Drive
Until the 30s, the rear-wheel drive layout was almost exclusively used. The concept of front-wheel steering almost inevitably led to a driven rear axle since the torque transfer to the steered wheels caused problems. Today only a third of the vehicles produced in Germany are equipped with rear-wheel drive. Fig. 3-31 shows the possible engine layouts.
The most widespread RWD type, the standard drive, consists of the engine being installed in front of the front axle (front engine). The torque transfer between transmission and driven axle (in the transaxle design, between engine and transmission located in front of the rear axle) takes place via the cardan shaft. This results in a negligibly higher weight and in passenger cars requires a tunnel in the passenger compartment.
Further engine layouts associated with RWDs include the centrally mounted engine located between front and rear axles and the rear engine layout behind the rear axle.
The centrally mounted engine layout highly impairs the amount of space available in the passenger compartment and also reduces the transporting capacity of a vehicle. In passenger cars, this layout is almost exclusively used in sports cars and in racing. In commercial vehicles the centrally mounted engine is applied in under floor layout.
Particularly in passenger cars, the rear engine layout also creates limitations in the maximum pay-load carrying capacity since the trunk is displaced to the area above the front axle. The necessity for larger wheel houses in this area (steering angle) reduces the available space. However, this layout is used widely in busses where the standard drive becomes unfavourable due to long wheel bases involved.
The RWD in passenger cars proves to be a disadvantage from the point of view of economy of space in each of its designs. The standard drive, which is the most frequently used design, is however advantageous in average and large vehicles as a result of the body stiffening effect produced by the tunnel for the cardan shaft.
The inherently oversteering tendency of rear-wheel driven vehicles in the absence of appropriate measures (refer AE-II) can largely be optimised for drivability specific to safety using modern chassis construction.
The main advantage of rear-wheel driven vehicles lies in the total absence of drivetrain influences on steering. In addition, modern rear-wheel driven vehicles have improved traction since in addition to their often excellent axle load distribution (50:50), the rear axle is additionally loaded during acceleration or by a pay-load (Chapter 3.4).
In commercial vehicles, which are primarily designed for high pay-loads and the high rear-axle loads associated with it, RWD is used since it provides sufficient traction also when fully loaded. In this case, the rear axle of heavy commercial vehicles is often replaced by a twin axle. In case of the latter, either one or also both axles are driven.