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Subject: RE: The Definition of Spider Bite (tm -SJ) in a TorsenCenter Diff
I know the torsen thread is not very popular, but I feel understanding it is
a key to understanding the later generations of quattro.
I think best when I write things out, so excuse the bandwidth or delete.
In a turn, more torque is sent to the rear axle by the torsen due to the
front wheels travelling a different distance than the rear (more wheel
revolutions at front; slip angle). At this point the car is neutral or very
small understeer due to the chassis setup.
Add more power.
The power to the rear can overpower the traction available at the rear, and
the rear end _begins_ to go out in an oversteer (wheel revolutions at the
rear approach or equal wheel revolutions up front, removing the slip angle
differences). As this begins to happen, power is shifted forward towards a
50/50 split, which should move the car from oversteer to neutral again (or
slight understeer); then the slip angle differences will move the power back
to a rear split, but only after the rear has regained traction.
If the torque shift forward overpowers the traction at the front wheels, then
the car begins to understeer, and power is again shifted towards the rear,
moving the car towards neutral.
If there's too much power for the traction, this can result in all wheels
slipping (front is understeering, rear is oversteering, i.e., a powerslide).
So, SJ's argument (as I understand it) is that the torsen "overshoots" the
neutral or slight understeer "normal" attitude of the chassis; alternatively
gaining and losing traction at the front and rear of the car.
I could see that occuring if the torsen was merely a front/rear two-phase
switch, but the limits to torque transfer should minimize this. It could
also happen if there was a significant lag in response from the torsen, which
is reportedly not the case. I suppose it could also happen if the tires used
go suddenly from grip to major slip, instead of progressively (think of
hitting black ice, for example), which would accentuate any lag in torque
transfer in the torsen diff. I'd think going from understeer to oversteer
and back rapidly is more related to improper driver input (lift throttle or
braking, thus weight transfer, steering angle change, or braking in a
full-throttle at the limit turn, and improper control of the resulting slip)
than the differential giving unpredictable responses. The torsen is really
trying to make both ends slide...not one at a time.
chris miller, windham nh, firstname.lastname@example.org