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Re: Torsen stuff

In a message dated 96-04-03 16:01:03 EST, kingjer@Eng.Auburn.EDU (Jeremy R
King) writes:

>An open differential sends more torque to a spinning wheel 
>once it starts spinning.

Actually, an open diff sends equal torque to both output sides, all the time,
no matter what. In practice, a spinning wheel is transmitting essentially
zero torque; so is the stationary wheel on the other side of the diff.  An
open diff differentiates rotational speed, not torque.

On the other hand, a pure locked diff allows zero difference in rotational
speed, but will allow a zero to 100% torque split (theoretically). A Torsen
behaves like an open diff until the delta in rotational speeds goes beyond a
certain point - adjustable in the design within limits. When it "senses" a
high delta in rotational speed it semi- locks, but unlike a pure locked diff,
the Torsen splits torque in a variable ratio. On the Audi, this is from 75:25
to 25:75 (was 78:22 to 22:78 on the earlier cars).

>So if you have one side (say left front and rear) tires on ice, 
>the Torsen sends equal torque to front and rear.

Right, because it doesn't sense any difference in rotational speed. Tthis is
how you can get stuck in a quattro with a Torsen center diff, if you do not
lock the rear diff (or have Electronic Differential Locking as the latest
quattros have). In a situation with one side of the car on a zero traction
surface, it will spin those wheels uselessly and go nowhere. In practice,
there is usually some minor traction which permits the car to get moving, or
you lock the rear diff. It should work at home by jacking up one side of the
car off the ground - air is the ultimate low coefficient surface. Don't try
this if you have EDL, a Torsen rear diff, or if you lock the rear diff.

>the V8 had a center and rear Torsen with 
>and open diff in front for a year or so?

As far as I know, the V8 quattros with automatic transmission had an
electrically engaged center differential (essentially a switchable
multi-plate clutch) and a Torsen rear.  The V8 quattros with sticks had
center and rear Torsens. The electric center diff, which the A8 also uses in
some of its forms, can lock to transmit torque in a ratio from 0:100 or
100:0. By varying the duty cycle, you can also achieve theoretical ratio
splits in between. Is one better than the other? Probably in some arcane
condition, but not for the customers who buy these cars (new).