The invention relates to all-wheel drive systems for vehicles and, more particularly, to a new and improved all-wheel drive system having a controllable coupling between the front and rear wheels of the vehicle.
All-wheel drive systems for vehicles can be roughly subdivided into two groups, namely, one in which the four wheels are driven permanently, i.e., continuously, independent of road conditions (e.g., the Audi Quattro and Audi 80 Quattro), and one in which the wheels of one axle are driven continuously and the wheels of the other axle are driven only at certain times such as during cross country driving. In the latter case, the connection of the second axle drive occurs either automatically by means of a mechanical overrunning device when a predetermined slippage of the steadily driven axle is exceeded (e.g., German Pat. No. 892,275; DE-AS No. 1,077,537; DE-OS No. 2,413 288; DE-OS No. 2,928,351), or it is initiated manually by the operator whenever necessary by actuation of a corresponding control lever.
A driving arrangement (e.g., EP OS No. 68,309) whereby the wheels of one axle only are continuously driven by way of a conventional axle drive while the drive for the wheels of the second axle is obtained automatically by means of a viscosity coupling arranged in the driving train between the front and the rear axles corresponds closely to a permanent all-wheel drive system.
Such viscosity couplings transmit only very small torques at small speed differences but transmit very large torques at higher speed differences. Depending on the point of installation within the driving train, such viscosity couplings can therefore assume either the tasks of only a center differential or the tasks of both a center differential and an axle differential. Especially under low traction conditions, which occur when roads are wet, icy, snow-covered or sandy, an all-wheel drive with a viscosity coupling acts, depending on the point of installation, either as a permanent all-wheel drive with a blocked center differential or as permanent all-wheel drive with a blocked center differential and a blocked axle differential.
In general, such a rigid coupling between the driven front and rear wheels provides certain advantages on braking, including shortening of the braking distance. When the front wheels of a vehicle are overbraked, resulting in locking, however, coupling of the two axles also causes the rear wheels to be over-braked and locked through the driving train between the front wheels and the rear wheels. This occurs even though the braking system of a vehicle such as an automobile is usually arranged so that the braking effect is larger on the front axle than on the rear axle, assuring that the front wheels will lock before the maximum possible braking forces are attained on the rear wheels. It is known that locking of the rear wheels leads to loss of driving stability.
Accordingly, it is an object of the present invention to provide an improved all-wheel drive system for vehicles having driving wheels which are connected permanently and, in particular, to provide such a system in which the driving stability of the vehicle is not impaired even on overbraking of the front wheels.