[s-cars] torsen rear diff; first impressions

[Keith Maddock]

Kirby wrote:
>I had a torsen rear diff installed last week.  I obtained it from Bob
>Pastore who recently informed the list that he had a new shipment.  I
>believe these were acquired in Germany by Keith Maddock and come from
>V8s.

Yup.  And there *might* be chance for a last batch before I leave Germany 
this summer, if enough people let Bob know they'd be interested.

>Normally in slippery conditions with open diffs at each end of the car
>and a torsen at the center, one tire at each end of the car will receive
>power.

Ack. Ack!  No!

Open differentials always apply equal amounts of torque to both outputs!
Otherwise stated, both wheels on a open-diff equipped axle always receive 
the same amount of torque.   This means that both wheels can only receive 
as much torque as the wheel with the least traction.

So even  if you have a triple open-diff AWD setup, you are still sending 
equal torque to all 4 wheels.    Total traction of the vehicle is then 
limited to four times the torque that the lowest-traction-wheel has.  (note 
some center open diffs are planetary types that allow a static torque split 
other than 50/50, such as 38/62 on X5 and some GM's, however this torque 
split is fixed and not variable)

With Front/Rear open diffs, and a center torsen,  each axle has the same 
torque on both wheels, but the torsen then has the ability to allow more 
torque at one axle vs. the other (up to the limit of the bias ratio).  Each 
axle is limited to receiving twice the torque that the 
lowest-traction-wheel on that axle can handle.  Total torque becomes 
limited when the lowest traction axle has less than 1/BR of the traction of 
the highest traction axle (BR being Bias Ratio, 3.5 in our cars I beleive!)

The rear axle of the S-Car, in anything short of full-boost acceleration, 
has less weight distribution (normal force) than the front axle, therefore 
it tends to be the limiting factor in total vehicle traction.  By 
installing a torsen (in any URS, EDL equipped or not), you're increasing 
the total traction capacity of that axle, which increases total traction 
capacity of the vehicle.

>   With a
>torsen rear diff, torque is applied to both rear tires similarly to
>their being locked, but with proportioning according to slip.  And this
>behavior is available at all speeds.

Ack again.  The Torsen LSD biases torque according to torque (traction) 
difference, not slip.  This means Torsen reacts faster and "pre-emptively" 
when compared to a clutch type  or viscous type LSD.

>Anyway, I had noticed before this transplant that I could get a rear
>tire to slip under various conditions.  Last night I followed a newish
>(and very clean) Corvette into Derry.  It seemed to have its wide wheels
>on (?!) and was tiptoeing thru town more than would normally be required
>by pedestrian risk and police supervision.  At a light where he was
>intending to turn left I took the r.h. lane.  When the light changed I
>accelerated at a level that he presumably could beat in dry conditions,
>but surely knew he couldn't manage on the snow.  Obviously we weren't
>racing as he was going a different way.  However, for me, besides
>demonstrating to him the vast superiority of a quattro in the snow, I
>wanted to check out wheel slip.  I didn't detect any.  Perhaps in dry
>conditions I could have put more power to the pavement, but I was
>impressed.  Little steering correction was needed.

Good to hear that feedback, I'm really looking forward to having my own 
Torsen rear diff installed in my S4 (along with a 6-speed + LWFW + Spec 
?*!? clutch) this spring.

Nice also to hear that a bit of dirty (quite literally) work on my current 
side of the ocean has been worth it!


Cheers,
Keith

keith at maddock.com    http://keith.maddock.com/

93 S4 (mit RS2 + BR upgrades) Portland OR
95 968CS (Ring Tool + Daily Driver) Koblenz, DE
1/8 89 GTI  16V (Spare Ring Toy)  Nürburg, DE