Re: <e40><all>2.3,torsen's,and traction control Oh My

[Brian Brown <bpbrown@xxxxxxxxxxxx>]

Jeffrey Lloyd wrote:

> does anyone know If BMW modified their traction control systems for
> the new Z3 line to use the torsen LSD The way I pictured it The Torsen
> would give more torque to the wheel that the traction control is
> trying to slow down Is this correct? I drove a 2.3 roadster and it was
> eqquipped with "ASC+Torsen" In my limited knoledge of mechanical
> operations this combination seems impossable?  what am I missing
>
> Jeff Lloyd

TORSEN is a concatenation of 'Torque Sensing'.   The original Torsen
design by Gleason dates back to the late fifties.  More recently, a
'Torsen II' version has been appearing in some production cars.  The
Torsen II has an internal layout that is markedly similar to the Quaife
ATB differential (which was a conceptual takeoff of the original
Torsen).  BMW is starting to use the Torsen II in some of its cars
instead of a conventional clutch disk type LSD.

The Torsen design uses a characteristic of worm and ring gears.  A worm
gear is usually a long thin helical gear that mates with the edge of a
large diameter ring gear.  When the worm gear is turned, it causes the
ring gear to rotate.  Tried the other way:  attempting to turn the ring
gear directly will just cause everything to lock solid.  Motion is
allowed in one direction only.  This is partly due to the typical high
gear ratio involved with a conventional worm and ring gear pair, but a
major factor is the interaction of the shape of the gear teeth.

The actual gears inside the Torsen don't have the appearance of
conventional worm and ring gears, but they still are classified as worms
and rings.  In this case, the worm gears are significantly larger in
diameter than the ring gears.

Each output shaft passes into the differential housing, where it is
driven by an output gear.  This output gear is a worm.

In the original Torsen, the differential housing has several pairs (3-5)
of ring gears (they don't actually look like rings) that are mounted in
cutouts in the housing.  Each pair of ring gears are connected to each
other by conventional gears that act as synchronizers.  One ring gear of
the pair meshes with the right output worm gear.  The other ring gear of
the pair meshes with the left output worm gear.  The reason several
pairs (3-5) of ring gears are used is to increase load capacity.

The Torsen II has a simpler arrangement, using long helical toothed
planet shafts in place of ring gear pairs with synchronizer teeth.  The
principles of operation remain the same.

When the differential housing rotates, the ring gears are rotated around
the output worm gears.  Thus, force is being applied from a ring gear to
a worm gear.  As described above, no relative motion can occur between
these two gears because they lock up solid.  This means that full force
is being applied from the differential housing to the output shaft.
This occurs regardless of whether or not the other output shaft has any
load (traction).

When the car goes around a corner and one wheel needs to go faster, the
force from the faster outer wheel goes *into* the differential through
the output gear.  Now we have a situation where a force is being applied
from a worm gear to a ring gear.  Relative motion between these two
gears is allowed when the force is in this direction.

To summarize the two main characteristics in a different way:  Forces
between the housing and an output shaft (engine power to a wheel) are
directly coupled.  Forces between two output shafts (differences in
speed between the two wheels) allow the internal gears to rotate.

The real beauty of this design is that these two characteristics are
autonomous.  Both things can be happening at the same time.  Full power
can be applied while going around a corner.  The wheels are allowed to
turn at independent speeds.  Full torque can be applied to a wheel even
if the other has lost traction.  Changes in the situation are
automatically adjusted for instantly by the inherent nature of the
design.  Everything operates in a precise balance.

There is no need to choose a trade off between maximum traction, and the
ability to go around corners.

It's also important to note that while this design relies on the
friction characteristics of the gear teeth to control its behavior, it
*doesn't* use friction to transfer power (like a Clutch Plate LSD).
This design doesn't have any more wear than a conventional differential.

Another advantage the Torsen has over a conventional LSD is its
*smoothness*.  When a clutch plate LSD starts to lock, it applies a jerk
to the wheel with better traction, increasing the possibility that both
wheels will spin and lose traction and cause the car to fish tail.  For
this reason, the ASC + T traction control system has a more difficult
time keeping the rear end in line if a conventional LSD is present.
This was particularly a problem on the shorter M roadster and M coupe,
with their lower polar inertia.  In some of their literature, BMW
actually cites the Torsen's smoothness as being an advantage for the ASC
+ T traction control system.

Basically, from the ASC + T system's perspective, the presence of a
Torsen differential just makes it seem like the car has better traction
to begin with, so less electronic intervention is necessary.  The Torsen
and ASC + T system compliment each other well.

Brian Brown.
bpbrown@domain.elided
'96 318tiS
BMW CCA #130878

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