Re: TORSEN CENTER - OPEN FRONT!!

[ejfluhr@austin.ibm.com (Fluhr)]

Regarding this Torsen thread, Sarge says:
>Go hot into the corner, don't hit the brakes until you begin turn-in.
>This will set-up mild trail-braking oversteer. The rear wheels are now
>rotating less RPM's than the front. Reach apex, apply full-throttle.

>PAY ATTENTION HERE !!!
>Just before you applied full throttle, the rears were turning slower
>(b/c of the slide), thereby *fooling* the Torsen (upon power
>application) into ERRONEOUSLY deciding the rear wheels had more traction
>than the fronts.


Here is a question that has been bugging me for a while.  I have seen this
explanation several times, now, and don't follow the logic.  It seems that
the group that believe the torsen to have this unpredictable behavior all
support this model of a car driving through a turn at maximum capability.
On the other hand, the group that does not believe in the unpredictable
behavior of the torsen seems to overlook (ignore as irrelevant?) this model
of the car's behavior in a turn.

Note that I don't have lots of track time and consider myself to be just an 
average driver.  Also, my only significant driving experience in a quattro 
has been in my father's '97 A4 1.8tqm (torsen center, open diffs front & 
rear with EDL, as I understand it), in which I have not reached the limits
of the car's performance.  I am trying to follow this [quite interesting]
Torsen thread, but my understanding is still rather poor at this point.


How are the rear wheels turning slower than the fronts in this model
of cornering?


The only explanation I can see is that the brakes are causing the rear
wheels to run slower than the pavement is turning them, but has not yet
caused one or both wheels to lock up.  So, this action only occurs during
trail-braking into the turn.  Now, when you transition from brakes to
throttle, is there not a [short] period of time in which the rear brakes
are off, which allows the pavement to spin the rear wheels back up to the
same speed as the front?  Then when you apply throttle, the torsen does
not see the rear diff as spinning differently than the front, hence the
power is distributed fairly evenly between the front and back wheels, and
the driver does not experience the proposed unpredictability of the torsen
center diff.


Now, if we ASSUME that the above model (of the car's behavior) is correct,
then this whole arguement about the torsen boils down to HOW the car is
being driven.  IF the pro-torsen guys are not trail-braking hard enough
(or at all) into the turn to cause this rear-wheel slowdown, OR IF the
car is set up differently such that the rear wheels do NOT slow down at
this time (less body roll, e.g. the ur-quattro vs. type 44 cars?), OR IF
they have a longer pause between off-the-brakes and on-the-throttle
(which "resets" the rear wheels to approximately the same speed as the
front), it does not appear that they will experience the Torsen behavior
that the anti-torsen group is proposing. 

It does seem to appear that the anti-torsen group is mainly talking about
the type 44 (?) cars, while the pro-torsen group is referring to entirely
different cars.  I don't know if this is a factor, but perhaps it is a
variable that needs to be looked at.

Later,
Eric
'85 Coupe GT
---
Eric J. Fluhr                                Email:  ejfluhr@austin.ibm.com
630FP Logic/Circuit Design                   Phone:  (512) 838-7589
IBM Server Group                             Austin, TX