The 2001 BMW M3 uses traction technology with the new Variable M Differential Lock
Whitby, Ontario...BMW M Cars have always been powerful and dynamic - and the
new BMW M3 is no exception. That is why this high-performance model features a
new and highly specialized limited slip differential, ensuring both superior
stability and optimum traction, particularly when accelerating out of a corner.
Up until the new M3, BMW M Cars have come with a torque-sensing self-locking
limited slip differential with locking action of up to 25 per cent and a
consistent basic locking force. Differentials are essential for performance
because the two drive wheels of a car cover different distances in a corner-
the inner wheel not traveling as far as the outer wheel. This fundamental
difference is offset by the use of an axle differential.
The limited slip differential builds up a certain locking force whenever
necessary: when one of the two drive wheels threaten to lock on a slippery
surface for example. Particularly, enthusiastic drivers appreciate the limited
slip differential since it helps them to capitalize on the positive
characteristics of rear-wheel drive -with a sporting style of driving on roads
with a frictional coefficient ranging from average to high.
A torque-sensing limited slip differential adjusts the power to each rear wheel
based on the wheel's ability to deliver power to the road. In very low friction
conditions like snow, gravel or ice, the benefits a conventional limited slip
differential is able to offer in terms of traction are limited by slip forces
beyond its control.
Improving on the limited slip differential principle
Engineers at BMW M, teaming up with specialists at GKN Viscodrive GmbH, have
developed an all-new differential system for the new BMW M3 - the Variable M
Differential Lock. This lock offers decisive advantages in traction even in
demanding driving situations where there is an extremely large difference in
frictional coefficients on the drive wheels. Accordingly, the Variable M
Differential Lock enables the new BMW M3, in combination with the fine-tuned
DSC system and the car's perfect front-to-rear balance, to offer winter driving
capabilities previously regarded as quite impossible with a rear wheel-drive
sports car.
Maintaining engine power and stability
A further advantage of the Variable M Differential Lock is the increase in
locking forces parallel to increases in differential speed between the drive
wheels. As a result, a wheel suddenly relieved of drive force can no longer
make drive forces collapse entirely. Instead, drive power and the car's forward
thrust are always maintained at an appropriate level.
The Variable M Differential Lock functions according to a simple principle: The
difference in rotational speed that builds up between the two drive wheels
generates pressure spontaneously in an integrated shear pump. This pressure is
then conveyed via a piston to a multiple-plate clutch, thereby channeling power
to the wheel with better grip.
In an extreme case, the entire drive power of the engine may be transmitted to
the wheel with better grip- provided that enough grip exists. Once the
difference in wheel rotation speed between the two wheels becomes smaller, pump
pressure and subsequent locking action is reduced. The self-controlled pump
system is maintenance free and filled with highly viscous silicon oil.
For the driver of a BMW M3, the main advantage is the ability to accelerate far
better on surfaces with dramatically different levels of grip between the two
drive wheels. In addition, the Variable M Differential Lock noticeably improves
both handling and driving stability, thus elevating the level safety and
driving pleasure.
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