Re: What's the big deal about Torsen?

[Jeff W <hjr@buffnet.net>]

Part of the difficulty of "what's better" on this list is that there are
two distinct camps at work here: "Racers(and wannabees!)", and "daily
drivers", both groups which appreciate the quattro for its excellence in
technology (and whatever).

And I think that most people are overblowing the Torsen to be some
mystical beast that thinks for itself and "kicks in".  It doesn't.  It
is simply a differential that smoothly applies a ratio of torque bias to
both outputs.

The purpose of any differential, simplified,  is to allow wheels,
whether side to side or front to back, to turn at different speeds, as
is the case in turning (inboard wheels turn slower than outboard) or due
to slightly different wheel diameters. (main reason for center diff).
 
There are three general levels of torque biasing:

1. Conventional (no torque biasing; full differential action)

2. Limited Slip (some torque biasing; partial differential action)

3. Locking (total torque biasing; no differential action once locked)

Caveat: some locking diffs automatically allow diff action to a point;
then "Lock-up" either thru electronic control or mechanically as in the
"Detroit Lockers")

Early Audis have conventional fronts, centers and rears, with mechanical
locking provision for center and rear.  Later models ('88 on?)use a
Torsen center. 

A conventional differential "favors" the output with the least path of
resistance, hence when you remove the traction contact from one wheel,
it is the only wheel that spins; no torque is applied to the other
wheel(s).

Various limited-slip schemes have been used in automotive applications
thru the years with varying success to apply torque to the low traction
side.  Early "clutch pack" types were prone to erratic action and
wear-out.  More current schemes such as Torsen and viscous are more
consistent.  The viscous types bias some torque to the "lower traction"
output thru a coupling device similar to a autotrans torque converter.

Which brings us to the Torsen.  BTW, The Gleason Works (AKA DK Gleason)
sold the business; they are now called Zexel Torsen, Inc. (I think now a
division of MascoTech?)  The Torsen is unique in that it uses a
planetary gear arrangement to bias torque to the "low traction" output;
yet allows differential action.  It performs this action smoothly; not
like the "kick-in" attributed to electronic traction/locking devices.

Which is better?  Obviously depends on the situation.  

For the racer/wannabee; When you "get on it" with a Torsen, you will
have the lowest traction axle spinning faster than the other.  (Usually
the rear)  You won't have this with a locked center.  From this
perspective the control will not be as defined with a Torsen. (Yet
control always has its limits)

For me, the casual daily driver that deals with nasty snow and ice, I
much prefer the Torsen.  It's always there; and it is more predictable
under normal driving circumstances. (Although I would like a locker when
in about to get stuck or already there)

A classic example of control limits in differentials is the case of the
RWD Detroit muscle cars of the late 60's/early 70's when Posi-Traction
was the rage.  A stiff torque bias in the clutch packs made for awesome
traction and performance on dry, and arguably some help if stuck in a
snowbank, but if you ever drove one in low traction conditions;
rain/snow/ice, you had better have a clean pair of underwear in the
glovebox!  Holy S**t!! I can still remember some of the most memorable
intantaneous 360's.

BTW, Zexel makes two Torsen models the "T-1" and "T-2". For pictures of
Torsens and other what not check out their brief homepage at:
 
http://slpeng.com/zg.html
  

Jeff

'88 80q