TORSEN CENTER - OPEN FRONT!!

[Sargent Schutt <sargent@novagate.com>]

I CAN'T HELP MYSELF ANY LONGER. So here goes.

I've already stated this example, but it was never fully comprehended by
many. I will be as brief as possible.

Conditions: Rain soaked track
Corner: Abrupt 90 degree 2nd gear turn
Car: type 44 chassis, turbo motor, Torsen center diff, open front diff,
open (but lockable) rear diff.

Do the following (HERE IS YOUR REPEATABLE TEST).

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.

Now the rears get a greater share of the torque (as much as 70%). They
quickly accelerate and cause further oversteer. Yee-ha!

Torsen realizes it's MISTAKE only after the rears have lost longitudinal
traction (and outpaced the front) and a comensurate amount of lateral
traction.

At this point Torsen says 'whoops' to itself and sends the FRONT OPEN
DIFF the Lion's share of the torque. Since you are accelerating, the
front wheels have less relative weight over them vs. the rear. You are
still cornering, so the outside front suspension is max'ed out, fully
loaded. THE INSIDE FRONT HAS THE *LEAST* TRACTION OF ALL FOUR WHEELS AT
THIS POINT. YET BECUASE THE FRONT DIFF IS OPEN OPEN OPEN, IT SENDS THE
VAST MAJORITY OF TORQUE TO THAT INSIDE FRONT WHEEL.

So you have the inside front wheel all lit up and are back in UNDERSTEER
MODE, though you had NOTHING to do with that change. You were
oversteering controllably, the next second TORSEN SENDS UP TO 70% OF THE
POWER TO THE INSIDE FRONT WHEEL!!!! THE ONE WITH THE LEAST TRACTION.
WHAT IS SO DARNED GREAT ABOUT THAT?!?!?

 Tell me where I'm so completely mistaken, LINE BY LINE. I'LL GIVE YOU A
HINT - I'M *NOT*!!! NOR IS SCOTT OR JEFF. NONE OF US IS IMAGINING THIS!
IT IS REAL!!

Torsen doesn't hunt all the time (perceptibly), and it doesn't manifest
itself in evil ways, EXCEPT IN THE RIGHT CONDITIONS. DO THE ABOVE TEST
AND YOU WILL EXPERIENCE THE TORSEN ACHILLE'S HEEL. I can make it happen
over and over again. I spent many laps in the rain at Thunderhill on the
above-described corner in an attempt to generate prolonged, controlled
oversteer. Not because it's the fastest way around the track, but b/c
it's a critical skill to have in your bag of car-control tricks. The
conclusion of my 'research': Torsen center diff plus open front diff
precludes prolonged, controlled throttle-induced oversteer in the rain
on that 2nd to 3rd gear corner in  a made for USA 91 200q TAP.

The mechanical explanation for this conclusion suggests that the same
behavior could manifest itself in other locations and other conditions
in this car. My further experiences support that, and there is no
MECHANICAL EXPLANATION that would support any theory to the contrary.

Dave wrote:
>scott: you have not explained how a torsen can transfer 70% of torque
to one
>axle and then switch the same to the other axle in the same turn.  you
>misunderstand that the torsen is *designed* to only transfer *enough*
torque
>(within the bias ratio) to ensure torque equilibrium.  it is not an
>all-or-nothing device.

Yes, he has. You just haven't understood him. He's laboriously explained
it, much more patiently than I, at least a dozen times now. Torsen sends
more power to the diff that spins slowest. Given OPEN front and rear
diffs, Torsen is free to make fallous conclusions as to which set of
wheels has more traction. EXPLAIN TO ME WHY THE OPEN FRONT DIFF IS
IRRELEVANT. Again, not only is it relavent, it is CONTROLLING!. By
virtue of the open front and rear diffs,  torsen's system of logic is
insufficient to make correct judgements as to which end of the car
really has more traction. It is especially prone to mistakes at the
track.

AND FOR GOD'S SAKE: For the purposes of this debate, I don't care if you
are occasionally in the presence of a much more knowledgeable driver
than yourself. I don't give one teensy weensy little bit who you drive
with or what he says about newer or older cars or cars of different
brands.  None of your instructors were referring specifically to the
set-up that exists in our US torsen center, open front, lockable rear
diff cars from what I read. I don't care how bad-ass you think you are
behind the wheel. I don't care how many miles you and/or your friends
have put on their Torsen cars. No logically sound or closely pertinent
reasoning in any of that. Nor does any of that address the specific
mechanical operation of the torsen IN THE LEAST.  Who you talk to and
who you attend track safety seminars with has NOTHING NADA ZIPPO ZILCH
to do with the mechanical explanation of the undesirable effects of
torsen center with an open front diff as described above.

By request, I have given an infinitely repeatable TEST MODEL; So tell us
all how this whole 'bitten' thing is so implausible. Think hard. Read it
CAREFULLY.

Regards,

Sarge

P.S. Please excuse my decidedly brash tone. I harbor no ill-will or
personal offense toward any of the parties involved in this discussion.
I take none either. I am just extremely frustrated at the vast rift
which has manifest over a relatively simple mechanical operation.

P.P.S. I personally don't like the 'bitten' charactarization that's been
given to this phenomenon. Bitten conveys a harsh surprise. To me it's no
surprise, it's just uncontrollable. I'd call it an achille's heel. The
Torsen system is seemingly sound and has advantages over the lockable
diff for the average driver in daily use. Hence Audi's use of it.
However, given the right set of circumstances, it can cause the car to
become unmanageable. I don't like it b/c it precludes predictable,
prolonged power-induced oversteer, which is sometimes a very important
aid to recovery. From what I can tell, the new EDL system is proabably
cheaper to produce, as it relies mainly on computer chips and brake
pads. It also should work well in tandem with future 'stability-control'
systems and the like.  All geared to the average driver (lowest common
denominator), of course. :-(