BMW M3 - The Variable M Differential Lock

BMW M Cars have always been powerful and dynamic - and the new BMW M3 is
no exception. Precisely this is why this extra-powerful model already
features a differential lock on the rear axle ensuring both superior
driving stability and optimum traction particularly when accelerating
out of a bend. So far BMW M Cars have come with a torque-sensing
self-locking differential with locking action of up to 25 per cent and
a consistent basic locking force. This can be explained by the simple
fact that the two drive wheels of a car cover distances different in
length in a bend, the inner wheel not travelling as far as the outer
wheel. This fundamental difference is set off by the axle differential.
The differential lock builds up a certain locking force whenever
necessary - for example when one of the two drive wheels threatens to
lock, say, on a slippery surface. Particularly sporting and dynamic
drivers appreciate the differential lock since it helps them to
capitalise on the positive characteristics of rear-wheel drive above
all with a sporting style of motoring and on roads with a frictional
coefficient ranging from average to high. A torque-sensing differential
lock adjusts the total drive force the wheels are able to convey to the
force which the wheel with the lower frictional coefficient is
currently able to get on to the road. Wherever the frictional
coefficient is very low indeed, however, for example on snow, gravel or
particularly smooth ice, the benefits this conventional differential
lock concept is able to offer in terms of traction are limited by the
so-called slip forces beyond which the wheel loses its grip. Precisely
this is why 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 is in a
position to offer 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 sportscar. 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 bend
while driving fast and dynamically 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 clear
and straightforward principle: The differential speed building up
between the drive wheels whenever a drive wheel is relieved of its load
and forces or runs on a very slippery surface, generates pressure
spontaneously in an integrated shear pump. This pressure is then
conveyed via a piston to a multiple-plate clutch, in the process
conveying drive forces to the wheel with a better grip, these forces
being geared in all cases to the difference in wheel rotation speed. In
an extreme case the entire drive power of the engine may be transmitted
to the wheel on the road with the better frictional coefficient
(provided the frictional coefficient is sufficient for this purpose).
Once the difference in wheel rotation speed between the two wheels
becomes smaller, pump pressure is reduced accordingly and locking
action decreases. This self-controlling pump system is free of
maintenance and is filled with highly viscous silicon oil. The big
advantage for the driver of the BMW M3 is that he can set off and
accelerate far better on surfaces with frictional coefficients varying
most significantly on the two drive wheels, since he now has more
traction working to his advantage. A further point is that the Variable
M Differential Lock noticeably improves both handling and driving
stability, thus enhancing safety and driving pleasure to a new,
unprecedented level of excellence.