[s-cars] torsen rear diff; first impressions

[Kirby Smith]

Thanks Keith for clearing up my misconceptions (and acquiring the
differentials).  One would think after all these years of torsen wars
that the concept would be indelibly engraved in my brain, but (I'll
claim its due to my cold) this was surely a mental lapse.

kirby


Keith Maddock wrote:
> 
> 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