BMW M3 - The Variable M Differential Lock

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 high.
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 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 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.

Please contact:

Friedbert Holz, BMW AG
011 (49) 89 382-24360 / friedbert.holz@bmw.de
e-mail: presse@bmw.de