||Audi Quattro pioneered 4WD technology in 1980 .... not only dominated rally series, its designer also claimed, "One day, 4WD will become as popular as 4-wheel disc brake in today."....|
||Today, Audi builds 4WD versions for every of models. This S4 is the high-performance version of A4 sedan ....|
||But 4WD still links strongly to rally. Rally brings us some superb machines like this Subaru Impreza WRX Ver IV STi.|
Advantages and Disadvantages
Traction and Grip :
We all know the objective of differential. During cornering, the outside wheels have to travel faster than the inside wheels, therefore we need a differential to distribute different torque to the wheels. For a 4WD car, we in addition need the Centre Differential because the front wheels have to travel faster than the rear wheels. The following diagram illustrates this :
This is the LSD used by Subaru Impreza WRX Ver IV STi
However, just the 3-differential layout alone cannot cope with the basic requirement for 4WD - provides superior grip in the worst roads. In real world driving, for instance, when pushing the car over its limit in corner, or running on slippery surface, tyre slip is inevitable. When a wheel loses traction, a normal differential will transfer nearly all the driving torque to that wheel. As a result, the spinning wheel will spin even wilder, but the wheel that having traction will never share driving torque, therefore the car will be difficult to get out of the trouble. This problem occurs in all kinds of car, no matter 2-wheel drive or 4WD, but it is relatively more important to 4WD because 4WD cars are designed to run in worse roads or cornering harder.
Therefore 4WD cars (or even many latest 2WD sports cars) need Limited Slip Differential (LSD). A LSD lock up both drive shafts whenever tyre slip occurs, thus helps the car get out of trouble quickly. The result is enhanced stability and even higher cornering limit.
In fact, LSD is the core of 4WD technology. There are several types of LSD: Torsen LSD, Viscious Coupling LSD, VC differential lock and Active LSD. They have different effectiveness, characteristic and cost so that car makers choose them according to their needs. Now we are ready to look deeper inside these 4 types of LSD.