[torsen] RE: Torsens in the new milenium...

Robert Pastore rpastore at animalfeeds.com
Wed Jun 29 18:20:57 EDT 2005

<Big Snip>


>>>>> You really should talk with Keith Maddock about this.  He is a
traction control engineer, working with EDL every day, and with plenty of
torsen and AUDI knowledge.  He was/(is?) on this list and the s-car list,
and is very helpful.   Here is a link to one of his very informative posts
on the s-car list:  
http://tinyurl.com/43467   Another person well-versed in this is Scott
"Torsen Boy" Justusson

The bigger problem, as I explained in my post, is that because the Torsen
center diff sends a multiple of the torque produced at the wheels with the
least amount of resistance to rotation to the wheels with the greatest
amount of resistance to rotation (the Torsen perceiving resistance to
rotation as a proxy for the amount of traction available) and anything
multiplied by zero is zero, it behaves exactly like an open diff under these
circumstances and allocates what little torque the engine can develop absent
any load equally to the front and rear wheels.
For all practical purposes, the net result is that AWD effectively becomes
ZWD and the car behaves exactly as it would if it was coasting.  Anyone
who's ever tried to corner hard or accelerate out of a turn with the car in
neutral will immediately understand how devastating this can be to a car's
lap times.

>>>>>> Basically true, but technically it really isn't "zero", because the
mass & friction of turning the wheel has some rotational resistance and this
(albeit small) resistance will be interpreted by the torsen as traction
available.   With a bias ratio of 4.0, the result will be some usable
torque. But this theory stuff won't help much in accelerating the car
quickly out of the turn.

In an attempt to band-aid this design characteristic of the Torsen (read:
flaw), Audi uses EDL to brake the spinning wheel and thus allow the
non-spinning wheel to support more torque.  While this works in theory, in
practice, EDL is too slow to respond, works by soaking up power that could
otherwise be used to accelerate the car, wears out brake pads and rotors
quickly, and cuts out above a certain speed.

>>>>>>  AFAIK, Audi never combined Torsen and EDL on the same axle.  With a
torsen center diff and rear EDL, a lifter rear wheel will result in most of
the torque getting shifted to the front axle.   Putting a rear torsen from
an old V8 into a 95.5 s6 (OEM was torsen center, open rear & EDL), and you
get the best total traction, as described in Keith's post referenced above.
While you might feel EDL is slow to respond, I really can't agree that you
would experience ZWD if the wheel of a torsen & EDL equipped car lifted.
Maybe what you experienced (or theorized?) is actually blowing through the
EDL's cut-out speed....but really I am not sure if this is measured at the
spinning or tractive wheel.   Keith?

A better (but more costly) solution is install a limited-slip rear diff,
which will prevent the lesser loaded wheel from spinning in the first place.
>>>>I think you would ruin your handling balance, adding more understeer,
with a LSD (or a locker) as opposed to a torsen.  

Anyway, you get the idea...

>>>>>>>>>>>>>>one of Scott's Torsen posts on the s-car list is pasted below:

TB will jump in here.

Depends on the S car of which you speak.  Since you apparently already have 
an early S car with a locker rear, the answer might be different than
with a later S car with EDL....  It also depends on your driving a bit too 

3 Channel ABS CAR Torsen rear vs Locker rear
IMO, for most drivers, the torsen *rear* will provide the better traction 
advantage over the locker.  IME (91 v8 auto with torsen rear), the torsen is

seemless and subtle in it's torque allocation.   A locker on the other hand,
quite noticeable in "engagement" from both a tractive standpoint and a
one.  Put another way, you drive the torsen like you do an open diff car,
drive a locker like a locked diff car (read lockers add massive understeer
attempted wheel speed differences in cornering that must be compensated

I personally find the torsen rear the easiest to drive in all track 
conditions.  I find the rear locker to be faster in snow, and potentially
faster on the 
dry track, but to make this claim takes a LOT of experience and practice 
(btdt).  Now, if one hasn't already done the blue white interrupt on the
locker disable, and you don't regularly mess with the switch, the torsen
rear will 
give you a massive advantage over what you have.

4 Channel ABS CAR Torsen rear vs Locker rear
Pretty much a no brainer.  Torsens and EDL are great complements in terms of

toolage, EDL and Lockers will fight to the death (read:  You will only use
or da other).  

Mark, I presume you to have the earlier car given the switch hack comment.  
This swap to a torsen is a bolt in mod.  IMO/E *given* a center torsen, a
torsen is an excellent upgrade.  Given a center locker, IMO a rear torsen is

desireable 85%+ of the time (unless you are a die hard locker fan like me), 
including track.  If you never mess with that rear locker unless you are
go with the torsen rear, cuz 99.9% of the time it's better than a rear
in the unlocked position.


Scott "TB" Justusson
'91 v8 locker center, torsen rear
'84 RS2URQ locker center,  torsen rear 
'83 Urq locker center, locker rear

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