c) Permanent AWD
In contrast to the shiftable variants, all-wheel drive can also have a permanent effect. In this case however, the associated stresses and slip have to be compensated by speed compensation between the axles. The various concepts which have been realized so far, differ according to the method of torque distribution and the differential locks used. Using a transfer case, the driving torque can be distributed to the front and rear axles in a definite ratio. Bevel-gear differentials are employed for symmetric torque ratios while spur-gear planetary differentials for an unequal distribution of the torques to the axles. Speed compensation at the differentials can be realised or blocked, as the case may be, using locking concepts already discussed in Chapter 2.5.3.