Re: 4000 Quattro Differentials

[ti@bazooka.amb.org (Ti Kan)]

>What kind of differentials does the 4000 Quattro and do they function
>differently than the Torsen differentials?

The 4000 quattro, like all quattros, has three differentials.
The front is a conventional type, the same as what's found in
other front-drive Audis.  The center and rear differential on the
4000Q and other earlier quattro models are also conventional type,
except that they have a vacuum-operated lock feature that allows
the driver to lock them in case the traction gets too slippery.
The torque split on these early quattro models is straight 25% to
each wheel, except when the differentials are not locked and a
wheel loses traction, then most of the torque goes to the spinning
(non-tractive) wheel.  Locking the differentials will force the
torque split to even out between the respective axles.

It is worth noting that when the differentials are locked, ABS
brakes cannot function because all wheels are forced to rotate
at the same speed, and ABS relies on wheel speed difference to
detect a locking wheel.

The TORSEN differential is used in later quattros in place of
the vacuum-lockable units.  Most models use the TORSEN only for
the center differential, the rear continues to use the conventional
type (with speed-sensitive auto lock-disengage).  The V8 automatic
used the TORSEN in the rear, but has an electronic multi-plate
locking clutch center differential.

The TORSEN works like a conventional differential (even torque split
under normal conditions) except in the event a wheel loses traction,
the torque is directed away from the spinning wheel (opposite of
conventional differential), thereby increasing traction.  TORSEN
requires no manual locks, and is therefore fully compatible with ABS.

Although the TORSEN is a relatively simple and compact design, it
improves the dynamics of the quattro system by leaps and bounds.
Together with ABS, it offers much more than the (more costly) traction
control systems found on other 2WD cars, and works just as well as
the (more complicated) electronic differentials as found on the
Mercedes-Benz 4-matic system.

-Ti
--
    ///  Ti Kan                vorsprung durch technik
   ///   AMB Research Laboratories, Sunnyvale, CA. USA
  ///    ti@amb.org
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