[torsen] Re: [V8] differential/torque sensing question

Andres Kroonmaa andre at online.ee
Wed Mar 19 04:20:31 EST 2003

 uh, I'm sorry to suspect an error, but Torsen design is specifically
 made so that bias ratio CAN be different for onpower vs coasting. Its
 done via different shape of gear toothing and gear friction with washers.
 Forces change direction for different scenarios.
 Whether that feature is implemented is question about specific torsen
 unit. Given that abs is sensitive to torque shuffling, I believe
 they really do implement it. If you know that this is not true, maybe
 you can quote some authority?

 "Torsen is at most capable of transmitting 80% of torque to the axle with 
  most traction."
 This alone is misleading. Torsen is able to transfer 4 times more
 torque to axle with most traction with reference to axle with least
 traction. This means that if slipping axle is able to support 2% of
 engine torque, best axle will receive no more than 8% of engine torque,
 rest goes into wheelspin. In noslip condition this will be 20/80 split
 and normal differentiation.

 I don't know about older audis or V8s, but newer one don't have that bad
 weight distribution. 60/40 about max. But would need to backup that too.
 But anyway, if you assume that reverse bias is 1.5:1 instead of 4:1, then
 your explanation seems even more correct.

 Interesting thing about torsen is that when stuck in such scenario as
 described, applying little brake causes immediately very notable amount
 of torque to be available at other axle. Specially handbrake. This causes
 torsen to sense traction in rears, and after multiplicating it by bias
 ratio, apply to fronts. I imagine that if Mike had applied mild slipping
 handbrake, he'd get out of situation with style.

On 18 Mar 2003, at 18:18, Keith Maddock <Keith.Maddock at trw.com> wrote:

Torsen works the same in the forwards direction and in the reverse direction. 
During engine braking, it distributes engine torque according to the bias ratio. 
I can forward on a couple of technical papers to anyone who wants a good technical 
read  that will explain this and more..(or a give you a splitting headache depending 
on how much you like engineering-nerd stuff)

Torsen is at most capable of transmitting 80% of torque to the axle with 
most traction. In some points in time, Audi used a unit with a lower bias 
ratio that was only capable of 75% at most.

*If* your V8Q has a Torsen center differential, your parking lot situation  is still
probably a pretty plausible situation to explain(if it has a automatic clutch type
AWD system, then this is happening for some other reason that I can't explain without
knowing more about the AWD system it has).

First remember that V8's and most Audi Quattro's are very front heavy.  I don't know 
the exact weight distribution, but I'd be surprised if it was any better than 60/40, 
my guess is more like 65/35. to 70/30

So already, consider that the front wheels at rest have about twice the traction 
capability due to higher normal force (weight pushing straight down)    
(Traction = NormalForce   * FrictionCoefficient)

So even at the heaviest torque split your static traction distribution is pretty close 
to the maximum traction distribution of the Torsen.  
So its reasonable to consider that with only idle torque,  that 25% of the engine 
torque was able to spin the rear wheels, and 75% of the engine was not spinning the 
fronts.  Consider in a dynamic situation where initially the torsen was in normal 
50/50 mode, and this would obviously initially cause the rears to go into slip as 
the torsen started change the torque transfer.  Once in slip, the wheels have even 
less traction than the static wheel.  So its easy consider that the front wheels 
have more than 3x the traction of the rear wheels, and since the Torsen is incapable 
of sending more than 3x the torque to one axle than the other, then the rears are 
only spinning.

Should you have gotten on the gas, you would have spun all 4 wheels up, but this 
doesnt give you any more traction (now the front wheels also have less traction), 
so you still wouldn't have gone anywhere...

Note this gets even worse when your going uphill in reverse with a torsen.  Now 
your weight distribution is more like 4-6x as much on the front as the rears 
depending on the slope of the hill.  So no doubt, the rears are going to spin 
while the fronts don't, this certainly doesnt mean that the fronts aren't getting 
a healthy dose (up to 75-80%) of the engine torque.

93 S4    Torsen Center (#2)  (soon also Torsen Rear (#5))
95 968CS    Torsen Rear (#4)

Keith Maddock, TRW Automotive,  Koblenz, Germany
Slip Control Systems, Systems Design, Traction Control
+49 (0)261/ 895 2474    -   -   keith.maddock at trw.com

>>> "Andres Kroonmaa" <andre at online.ee> 13:37:21 18.03.2003 >>>

 Not to be considered authoritive opinion, but torsen should have different
 bias ratios for forward applied torque vs reverse. Reverse is much like
 engine braking. I think its designed in to avoid interfering with abs.
 Thus in reverse, torsen behaves much more like open diff.

On 17 Mar 2003, at 10:55, Daniel Heatley <danielheatley700 at hotmail.com> wrote:

Hi Mike and all,

My V8q does the same, I had just assumed until now that the 4WD only worked
forwards and that some kind of one way clutch was fitted. My car will
happily spin the rear wheels on any lose stuff in reverse. I have a very
steep concrete driveway that it is very difficult to start reversing up when
it is slick, without breaking traction.

I too, would like some input on all this, just in case it is a sign of
upcoming disaster.

All the best,

1990 V8q

We had a pretty sizable snowstorm a couple weeks ago and like a lot of giddy
dudes, I had decided to go for a little cruise.  I found myself pulling into
halfway plowed parking space that was covered with about 6-8" of snow.  When
leaving this spot I had tried to pull out (in reverse) but got STUCK, yes I
thought my wheels must have been off the ground!  Anyway, I have those Yoko
tires (I know, where are my snows??) so I had stepped outside to assess the
situation which to my surprise a plow had gone by the rear of the car,
fortifying my snow-capped postition with 10" of rear wheel wedging heavy
Then after a couple quick back-and-forth sessions, I leave the car in
(idling) and step out to see tht both rear wheels are spinning freely on the
ice/snow and the front wheels were taking a leisurely break (no spin, at
I thought "torsen" sent the power to the wheels with the most grip (i.e. the
front).  So there I was looking at my car, that looked liked either a posi-
traction marvel or someone had welded the spider gears?? A few more minutes
proved successful and going forward the car seemed unaffected (pulling with
four as expected).

I thought I might share this, in case anyone has had similar moments or
thoughts on how this can happen.


 Andres Kroonmaa <andre at online.ee>
 CTO, Microlink Data AS
 Tel: 6501 731, Fax: 6501 725
 Pärnu mnt. 158, Tallinn
 11317 Estonia

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