[torsen] RE: Snow Driving

QSHIPQ at aol.com QSHIPQ at aol.com
Fri Dec 13 07:50:13 EST 2002

[ Picked text/plain from multipart/alternative ]
In a message dated 12/13/02 4:15:11 AM Central Standard Time,
dave.eaton at clear.net.nz writes:

>>no, trg cannot change.  as you have stated, trg=t1+t2 (the 2 output
>>all that changes is the mix between t1 & t2 - this is, by definition, the
>>doing its thing.  if the torque can't be supported by one input shaft, it
>>sent by the spur gears to the other side.  trg stays the same.

Dave, speaking of when a wheel spins at the max BR, trg is reduced (for audi
awd we can call it Ttos - Torque Trans output shaft).  According to all the
SAE papers, T1 and T2 are the traction abilities of the front axle and the
rear axle.  The sum of these two are referred to as Trg.  Think of it as
Brake specific torque.  Without the brake (traction) Trg is reduced.  The
only way to have Trg = Teng + gearing is if T1 +T2 (however allocated) = no

>straight line (as was the example), and the effect on the torsen won't
>change.  As weight shift rear occurs, the fronts will slip first in that
>straight line.  As such locking the rear won't affect the conclusion of the

>characteristics in a straight line.  EDL *might* have an effect, but that
>requires wheel spin first (since EDL is based on wheel speed, not attempted

>wheel slip differences), at which point the torsen has already shifted to

>rear.  OR, you could propose a start on snow (not what the lister presented,

>but...), in which case there still is no difference, because the fronts will

>spin first, then the rears, but AT max TBR of 78r/22f.  That won't change
>with or without the rear diff locked.  T2 might be higher with rear diff
>locked, but the behavior of the torsen is the same, the torque allocation is

>still max rear.

>>perhaps a personal example of this would help to explain.  in my old house
>>had a steep drive with an up-hill, off-camber driveway - the lhs was a lot
>>than the rhs.  on entry (from the left) the front of the car would follow
>>camber, but once it came to the rear, the car nose was rising, but weight
>>still at the front.  the car would therefore lift the rhs rear tyre.  if
>>were slow enough the car would be stuck because the torsen (while it was >>
>>torque) would not be supporting enough torque to power the front (which had
>>the traction) up the hill.
>>the only way to progress was to lock the rear diff.

Getting further from the claim of the original post Dave.  The original post
was wrt a straight line flat surface.  Regardless, you don't need to lock the
rear diff, only apply the brake with the accelerator.  In *your* case ocking
the rear diff accomplished the same thing, which reduced the wheel spin of
the rear axle.  ANYTHING you can do to reduced the wheel spin will increase T
to that axle.  However, in a straight line, up to wheel spin, the torsen and
the locker will behave the same, either one locked or not.

>once this was done, the torsen "saw" the traction available at the rear, and
>supported torque distribution (both front and rear) enough to progress up
>hill.  this had nothing to do with the steering angle on the car (there
>any anyway), simply the traction available to allow the torsen to
>enough torque.

Torsen can't see a darn thing, Dave.  It only reacts to attempted speed
differentiation.  Lock the rear or brake the rear, both result in the same
action by the device.

>the torsen is very much aware of the traction available at either end of the
>car, and therefore (given the presence of open diffs at either end). it is
>correct to say that the torsen is aware of the traction of the *least
>wheel at either end of the car.  the only way to vary this behaviour in the
>early (generation 2) cars is using the rear diff lock.  modern torsen cars
>edl working across the front & rear diffs, and esp working across the centre
>diff, so have much less need for driver involvement...

Just to be clear, that *doesn't* include audi awd yet.  The torsen isn't
aware of anything, on snow or ice, or with spinning tires, it gets confused,
and it will continually hunt.

>in the scenario above, with an unlocked centre diff, the car would hang in
>the same fashion as with the torsen.  locking the centre would allow 100% of
>the torque to the front of the car (none is able to be supported by the
>in either scenario driver intervention would be needed.

Further from the original post.  The lister was referencing straight line
acceleration torsen vs non, then torsen vs + rear locked.  As posted, a
misunderstanding of how things work.  Your scenario doesn't change that, it
certainly provides outstanding anecdotes about your driveway.

Thanks Dave.  Since this is now way out from the original post, if you want
to carry on, please respond to the torsen list.


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