Re: Torsen Test

[ScharfR@aol.com]

In a message dated 96-04-08 08:28:59 EDT, stricker@ll.mit.edu (Michael
Stricker) writes:

> Is my test conclusive in anyway?

The Torsen can effect a 3:1 or 1:3 torque split, or anythng between.  How
much torque do you suppose your front wheels were transmitting while spinning
in the air? Lets assume close to zero.  ...and how much is three times that?
Also close to zero.

The test isn't conclusive because in the example of your Coupe quattro, there
was nearly no torque to split.  Assume that you were probably dealing with a
few lb./ft of torque at the front axle. Three times that might be around 10
lb./ft.  That isn't going to be sufficient to move a stationary tire (the one
you left on the ground), especially with hundreds of pounds of weight over
the tire, a reasonable coefficient of friction with your garage floor, and
plenty of inertia.

Here is what I think was happening. With an unlocked rear diff, your airborn
rear wheel must have been spinning. So the Torsen was working, distributing
some inconsequential amount of torque front-to-rear, in a ratio that is
unimportant because the values are so low.  All of it was being expended
spinning three wheels uselessly in the air.  The wheel on the ground didn't
spin because, with an unlocked rear diff,  the insignificant available torque
was being directed elsewhere.

Locking the rear differential would have had the effect of forcing the rear
wheels to spin at the same rate.  That rate would be zero - resulting in
stationary rear wheels with the front pair still spinning.  How is that
possible?  You simply did not have enough torque delivered to the rear axle
to overcome the inertia and friction of the rear tire on the ground.
 Remember, we are only dealing with a few lb.ft.

The slight surge you felt was the Torsen hooking up, and you can be sure it
was transmitting approximately three times the front wheel's torque to the
rear.  The error was in thinking that this would be enought to move your car.
 It wasn't.  Ever try to wrench off a siezed bolt?  Plenty of torque, no
rotation. Same thing was going on in your car.  There was a torque on the
output shaft of the Torsen, just one too small to move anything.

This diagnosis would convince me that your Torsen is working fine.  Your rear
diff lock is also working fine.  But the experiment didn't produce a
sufficient level of torque to generate the observational data you expected.

Here is an interesting further experiment to conduct with that Coupe quattro
if it still has three wheels up in the air.

(Actually, just think about this one, I don't want to read about the results
in the tabloids)

Anyway, in your imagination, lock the rear diff and gradually apply the
brakes as you kept engine rpms up. With the front brake bias of most cars,
you would have begun to retard the front wheels - more than the rear wheels -
while applying more torque. The Torsen would split this torque, sending more
to the rear.  Perhaps eventually you would have enough to overcome the
inertia of the rear wheels, before the rear brakes generated sufficient power
to stop the car.  The sight of your car leaping or creaping off your jacks
should be sufficient persuasion of the function of the Torsen.  Just a
theory.

Anyway, there isn't anything wrong with your car.