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

C1J1Miller at aol.com C1J1Miller at aol.com
Tue Mar 18 13:20:52 EST 2003


Keith:  the auto tranny v8 center diff locks the diff if wheel speed differences are found between front/rear axle, so it's like the old lockers (but only up to 62 mph, and not when decellerating or braking, or cruising).  So, torque in a locker goes to the wheels with the least grip; if that's the rear axle, then the torsen at the rear tries to give the wheel with the best traction more grip, until it reaches the bias ratio...

Chris




In a message dated 3/18/2003 2:18:08 PM Eastern Standard Time, Keith.Maddock at trw.com writes:

> 
> 
> 
> 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.
> 
> Cheers,
> Keith
> 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,
> 
> Dan
> 1990 V8q
> 
> We had a pretty sizable snowstorm a couple weeks ago and like a lot of giddy
> Q-
> dudes, I had decided to go for a little cruise.  I found myself pulling into
> a
> 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
> AVS
> 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,
> further
> fortifying my snow-capped postition with 10" of rear wheel wedging heavy
> snow.
> Then after a couple quick back-and-forth sessions, I leave the car in
> reverse
> (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
> all).
> 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
> all
> four as expected).
> 
> I thought I might share this, in case anyone has had similar moments or
> thoughts on how this can happen.
> 
> Mike
> 
> 
> 
> 
> ------------------------------------
> Andres Kroonmaa <andre at online.ee>
> CTO, Microlink Data AS
> Tel: 6501 731, Fax: 6501 725
> Pärnu mnt. 158, Tallinn
> 11317 Estonia
> _______________________________________________
> Audifans torsen mailing list
> Send posts to: mailto:torsen at audifans.com
> Unsub/Manage your list options: http://www.audifans.com/mailman/listinfo/torsen
> _______________________________________________
> Audifans torsen mailing list
> Send posts to: mailto:torsen at audifans.com
> Unsub/Manage your list options: 
> http://www.audifans.com/mailman/listinfo/torsen 



More information about the Torsen mailing list