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Variable M Differential Lock (archive)

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Posted by daanesh on July 25, 2000 at 15:36:15:

Munich, Germany, June 21, 2000...BMW M Cars have always been powerful and
dynamic - and the new BMW M3 is no exception. That is why this
high-performance model already features a limited slip differential,
ensuring both superior driving stability and optimum traction, particularly
when accelerating out of a corner.
Up until now, 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, of course,
because the two drive wheels of a car cover distances different in length in
a corner, the inner wheel not traveling as far as the outer wheel. This
fundamental difference is offset by the axle differential.
The limited slip differential builds up a certain locking force whenever
necessary - for example when one of the two drive wheels threatens to lock
on a slippery surface. Particularly enthusiastic drivers appreciate the
limited slip differential since it helps them to capitalize on the positive
characteristics of rear-wheel drive - above all with a sporting style of
driving and on roads with a frictional coefficient ranging from average to
A torque-sensing limited slip differential adjusts the power delivery to
each rear wheel based each wheel's ability to deliver power to the road. In
very low friction conditions, for example on snow, gravel or particularly
smooth ice, the benefits that this conventional limited slip differential is
able to offer in terms of traction are limited by the so-called slip forces
beyond which the wheel loses its grip.
The 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 very demanding driving situations, for example when 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 a standard of driving excellence and
handling in winter previously regarded as quite impossible with a rear
wheel-drive sports car.

Maintaining Drive Power and Thrust In Every Situation

A further advantage of the Variable M Differential Lock is the increase in
locking forces parallel to any increase in differential speed between the
drive wheels. As a result, a wheel suddenly relieved of its load and drive
force - for example the inner wheel in a corner while driving fast on a
mountain pass - 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 whenever one of the drive wheels loses grip, either because it has
become unloaded or hit a slippery surface, 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
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 is reduced accordingly and locking action decreases. This
self-controlled pump system is maintenance free and is filled with highly
viscous silicon oil.
The big advantage for the driver of the BMW M3 is the ability to accelerate
far better on surfaces with dramatically different levels of grip between
the two drive wheels, since there is now more traction available. In
addition the Variable M Differential Lock noticeably improves both handling
and driving stability, thus elevating safety and driving pleasure to a new,
unprecedented level of excellence.

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