the spider bite described - peace in our time - finally!!!!!

[Dave Eaton <dave.eaton@minedu.govt.nz>]

mmmm, it seems as though private mail has found its way to the list.  again.

actually, i'm not too upset, because, after 3 months, we finally have someone
who is prepared to put in writing what they think the spider bite is.  and, no
surprise, its not scott (despite the mbytes he's written on the subject).  i
cannot tell you what a relief this is, to finally have facts and not
bullsh*t...

i would recommend that anyone still reading this torsen sh*t, reads what jeff
has posted, and my response, and then makes up their own mind.  after this
post, i intend to retire from the thread.

so thanks jeff for the effort, if not for posting my private mail to the
list....

anyway, to address the spider bite description...

dave
'95 rs2
'90 ur-q

[some snipage (do i hear cheers?)]

>Let's now approach the tight corner at speed and start braking hard ...
>weight transfers forward, increasing grip at the front wheels, and away from
>the back end, decreasing grip at the rear wheels.  At the same time, the
>slip angle's increasing up front and transfering through the chassis to the
>rear wheels, which increases similarly, only more slowly and not quite as
>much.  The Torsen begins to sense a change in the resistance to rotation of
>the rear driveshaft and starts transferring torque from the front wheels to
>the rear.  Grip will increase further at the front and decrease further at
>the rear.  By now, you've completed the first third of the corner and have
>trailed off the brakes completely.  The car turned-in very crisply and you
>feel it begin rotating into the corner, its inherent understeer somewhat
>tamed ... so far, so good.

data point (not really pertinent to your conclusions, but to your description):
1) when braking (overrun), the torsen is not operating (no engine torque to
distribute).  by definition.  from the torsen paper...

"In braking situations where little or no torque is being conveyed by the
differential, a four to one apportionment of torque between drive axles amounts
to little or no torque difference between drive axles. Thus, the Torsen
differential will not support any appreciable torque 'wind-up' between drive
axles during braking and so does not interfere with the operation of anti-lock
braking systems."
 
2) when you get back on the power, the torsen is operating and in a steady
state condition (given relative slip angles).

>
>With the apex in sight, you begin to roll back on the throttle ... weight
>starts to transfer rearward again, increasing grip at the back and
>decreasing grip at the front.  Because the back end has drifted out a little
>bit, you make a slight correction with the steering wheel ... on a good day,
>that's it.  You clear the apex, put your foot to the floor and rocket
>through the rest of the corner, smiling.  On a bad day...

yup, you're correct (caveat about braking noted)...

>
>Thanks to turbo lag and Audi's funky gearing, however, you give it a smidgen
>too much throttle, a little too soon.  Because the car's still turning-in,
>the slip angle of the front wheels is several degrees larger than the rear
>wheels and more torque is being transferred to the rear wheels ... because
>they're also turning and have relatively less grip available than the front
>wheels, not to mention momentum from the initial rotation into the turn,
>their cornering force peaks as the boost kicks in and they start losing grip
>at a faster rate ... the back end starts sliding slightly and the Torsen
>decides that the decreased resistance to rotation of the driveshaft means
>there's more grip available there then it originally thought and transfers
>even more of the now increasing amounts of torque rearward ... your butt
>starts to send you a signal that the turn-in you found so satisfying a few
>moments ago has maybe started to go too far.

yup, no problem.  this behaviour is governed by the bias ratio.  in the audi
torsen, this is 25% to 75% torque split...

>
>The rear tires have started to slide and relative to the front driveshaft,
>the rear driveshaft slows further still, sending a signal to the Torsen that
>is interpreted to mean there's LOTs of grip to be had at the rear wheels and
>to which it responds by sending even *more* torque back there, just like
>it's supposed to do.  At this point, the back end finally lets go completely
>and snaps sideways ... you dial-in a steering correction and if you don't go
>any further than pointing the steering wheel straight ahead, the slip angle
>at the front wheels will decrease to zero and the Torsen will transfer
>torque forward, helping the rear tires to recover.  There's a good chance
>you'll save it and end up driving over the apex once the rear tires hook up
>again.
>

you don't mention the application of power in this case, which is a worry
(oversight?).  after rearwards slide (btdt many times btw, especially in the
wet), application of *power* will cause longitudinal acceleration to the rear
wheels, quickly overcoming whats left of the adhesion (friction circle) back
there, and producing wheel spin (not slide).  at that point the torsen shifts
torque to the front in order to equalise driveshaft rotation (torque reaction)
and presto, you have transitioned to understeer and are pulling through the
corner.

huge grin factor, hey, bad corner entry, but you've got a 4-wheel power slide
setup on exit.  same vehicle dynamics occur as a result of the "scandanavian
flick" (tm hannu mikkola)

i think that we agree on this behaviour....

>On the other hand, if you have to add opposite lock, the slip angle will
>stay the same (except its sign will reverse) and you'll either get out of
>the throttle completely, which spins the car, or mash your foot to the
>floor, which will cause the rear tires to start spinning and signal the
>Torsen to shift torque forwards.  As this happens and the car recovers, the
>front end hooks up again and you have to unwind all the opposite lock you
>dialed in ... because this causes the slip angle at the front wheels to
>decrease, the Torsen will direct even more torque forward.  At this point,
>you're approximately two-thirds the way through the corner and the torque
>split is around 50/50, which means the car has started understeering again
>... with luck, you keep it under control and off the grass as you exit the
>corner.
>

mmmm, again i have no difficulty with this behaviour.  the important thing at
this point is throttle control to recover from your cornering...btw, so you've
also screwed up the corner.  that slide you describe and we experience costs
lots of time, because you have lost adhesion with your tyres.  btw, you call
this 7/10th's?.  seriously???

once again, the loss of adhesion to the rear you describe means that the wheels
will (longitudionally) spin easily (by definition).  then the torsen sends
torque to the front away from the (now faster spinning) rear axle until a
torque equilibrium is established.  controlled by the throttle.  easy peasy. 
its the torsen 4-wheel drift that phil, myself, and others have described.  the
torsen is in a steady state, controlled by the throttle input.

again, sorry to say this, but i've can't see a bite here sorry, no
uncontrolled, unanticipated vehicle dynamics....
 
>The critical factor here is how much of a margin you have in the rear tires
>... at speeds under some unknown threshold -- let's call it 7/10 -- there's

i like how you slipped in the obligatory 7/10th's bit ;-)

>enough grip in reserve to keep the rear tires from peaking.  Once you're
>past that point, however, all you have to do is step over the line even
>slightly and the spider rears its ugly head.  Few people, even professional
>racers, can walk a line this fine without stepping over it.  I think chassis
>tuning can definitely help ameliorate the consequences of doing so but I
>don't think they will have any significant effect on the basic mechanism
>involved.  Plenty of band-aids but no cures...
>

i couldn't really car less about the rears.  i've cornered the ur-quattro hard
in the wet with non-wof rear tyres (sorry officer) and, the vehicle exhibits
(no surprise) more oversteer.  but, unless i'm really determined to let the vehicle go, it is *still* easy to transition power to the front by the
application of power after the apex.  o/steer before the apex is more fun, but
then i'm no scandnavian...

if i overstep the line, that is 10/10th's by definition; not 7/10th's, 8/10th's
or 9/10th's.  where that 10/10th's is compared to your buddy in the other car
is, of course, a separate issue...

>As for why you haven't been bit, I can only speculate ... perhaps your 20v
>motor doesn't have as much lag so you can better modulate the throttle?
>Perhaps its torque characteristics aren't as peaky so the tires don't get
>hit quite so hard?  Perhaps you aren't driving hard enough to use up the
>grip margin of the rear tires?  Frankly, I just don't know at this point ...
>but rest assured that I will figure it out eventually.  (I will leave it up
>to you to determine how an open and/or locked center diff will behave in the
>above scenario as I need to get some sleep ... been up for almost 21 hours
>now!)

good to see that you accept vehicle dynamics as a major input into this
equation...

with the torsen you, as you suggest, get much less understeer in normal
cornering.  it is quite possible, even easy, to set them up through a corner
such that they are balanced primarily on the throttle, as you suggest.  btdt
(see post from post grand prix session in my ur-quattro).

your steering input is also important.  this part is much more enjoyable in the
20v ur-q, than in the rs2, because of the 20v ur-quattro's great steering
(smaller wheel also).

i accept your point about engine response.  my 20v ur-q has excellent engine
response, much better than the rs2, which is better than my old wr.  so i
accept than in the wr (albeit without the torsen!), this sort of thing can be
more difficult, which is, as you suggest, where gearing comes into play.  the
rs2 has better gearing than the ur-quattro which negates the engine response
thing a lot (except in slow corners where in the rs2, you're forced into 1st).

overall then:-

1) i find no evidence in the description you post of uncontrolled, vindictive
vehicle behaviour (aka spider bite).  i see *driver-controlled and modulated*
oversteer on entry and understeer on exit.  not great lines and driving, but
predictable and you can live with it.  hey try cornering a rwd 911 or a fwd
pugueot 205 gti fast and you'll soon discover lift-off corner-entry oversteer
and corner-exit understeer.   hey watch the next f1 race at barcelona, and
you'll see the same thing...

2) i also see driving behaviour in ignorance of vehicle/tyre/driving limits.
this cornering, while fun, is not necessarily fast, nor particularly safe, but
you may need to do it to discover the extra 5%.  but, if you screw up the
corner at 10/10th's, you deal with the consequences.  this state of vehicle
dynamics you describe is most catagorically not 7/10th's driving.

3) know your car, the conditions, and your own skills.  chose the right gear
for the corner so that you have throttle control with any turbo car.

4) learn to setup your car for the corner, and to balance throttle, steering
and vehicle dynamics, against your speed and the conditions.

5) learn to left foot brake if you wish to further control your vehicle
dynamics and to more effectively deal with turbo lag.

6) get to a track, play and have some fun.

7) throttle control, throttle control and more throttle control.....