4x4 Explained

Introduction to All Wheel Drive systems - By Eliot Lim

Traction Control

For all its technological glitz in the 1980s, all wheel drive cars eventually turned out to be a commercial failure with the exception of the niche brands like Audi and Subaru. In the late 1980s there were all wheel drive offerings from every major manufacturer as it was the latest fad. Many of these manufacturers have since dropped their all wheel drive models in favor of highly profitable truck based sport utility vehicles or SUVs in short. For the cars a more simple and inexpensive alternative was there waiting to be exploited.

Anti-lock braking systems have speed sensors on two to four wheels to detect differences in speed between wheels so that the computer could intervene and "pump" the brake on the locking wheel. By a few simple extensions to the system, it could be made to brake a spinning wheel, thus effectively transferring power to the one with grip. More sophisticated systems would reduce engine power to further slow the wheelspin, but generally speaking, traction control is merely an optimization of two wheel drive using ABS technology.

Current versions of Audi quattros (dubbed quattro IV) use all wheel drive in conjunction with 4 wheel traction control. Under no slip conditions, power is delivered 50-50 to the front and rear via a center torsen differential, which would take care of limiting slip between the axles. The traction control system would take care of limiting slip between each wheel of a given axle. Thus for the first time, the quattros have to lose traction on all 4 wheels before they become immobile.

The prior generation of quattros had center torsen differentials (except A8/V8) and manually lockable rear diffs which locked it solid. This featured automatic unlock at speeds exceeding 15 mph to aid the forgetful driver. The V8 quattro had a torsen diff in the rear and either a computer controlled clutch in the center (for automatic transmission) or a torsen (manual transmission).

The new Mercedes ML320 (and ML430) SUV uses a relatively simple implementation of open differentials and four wheel traction control. This implementation has been criticised from several sources as being inadequate. The main disadvantage of the M class AWD implementation is that excessive demands are placed on the brakes under extreme conditions and the system can also be prone to hunting. Zexel engineers found that if a torsen center differential is added to this system which would act before the onset of wheelspin, TCS brake activation would be reduced by over 50%. The presence of this data suggests that Mercedes might have gone a little too far in cutting costs by eliminating limited slip or torque sensing capability in its center differential.

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