torsen and en and en

[David Eaton <david.eaton@computerland.co.nz>]

scott, when the (rear for example) is slipping as you describe, then it
is describing an arc (by definition).  this is a pretty radical
cornering attitude, by definiton (aka handbrake turn)...

another thing which i've gone on about is that the torsen is responsible
in the ur-q for a major change in the balance of the car in the 'mb' and
'rr' models, towards one which is much more neutral in it's balance.
this is, as you descibe, because of the fact that in 'normal' cornering,
the torque is biased towards the rear, due to the slip angle thing
happening at the front.  this improves front-end steering 'bite' and of
course, raises the understeering limits of the chassis.  a good thing.
big time.  makes the car more chuckable and more nimble.

the torsen helps traction again, no difficulties there, better than the
locker.

the torsen's behaviour in the 'wheel lift' scenario is also more
beneficial again than the locked centre diff.

this leaves the case you (and jeff) describe of what happens when the
torsen is faced with excessive slip angles, and the choices it makes in
these situations.

i'm going to have to think about this some more to add any meaningful
comments, but my ur-q on gravel is not the handful which i would expect
given your description of the problems that the torsen has with high
slip angles.  you have high slip angles on gravel by default.  the
quattro is nicely balanced even when slipping around on gravel (fat
tyres), under power, under braking, through corners.  it's a hoot.

jeff's descriptions of this problem occur with a type-44 on low-speed
right angles turns, with a lot of power being applied (autocross).  as
he says, the type-44 in these conditions is very different animal to the
ur-quattro and the rs2 (my data base).  both these cars handle the
gravel 'j' [aka handbrake] turn very well indeed.  again with your
scenario, i would expect this to cause big problems.

has anyone out there with a torsen ur-quattro, have any access to a
large gravel surface and the ability to do some tests for us?

btw, this 7/10's thing is a complete misnomer.  as we've seen the torsen
handles wheel lift better than the locker.  so what do you call wheel
lift - 5/10's?  i prefer to think of your case as the 0.1% case...

dave
'95 rs2
'90 ur-q



Date: Sat, 7 Mar 1998 09:34:31 EST
From: QSHIPQ <QSHIPQ@aol.com>
Subject: Re: Torsen Tunnel Visions

In a message dated 98-03-07 00:18:12 EST, you write:

<< but you've just said that the 'cf' *isn't* constant, because you've
got
slip
> front or rear...
 
 >orin's point is that 50:50 does not occur with slip and a locked
centre.
  >>
For KISS, let's not get into the braking part.

Again, Dave, you are missing the key to the whole issue.  In your
scenario, a
wider tire travelling sideways in a turn, can't have more "traction"
than a
narrow tire in a turn, or vice versa.  Specifically, doesn't a wider
tire with
a higher cf have more 'traction' (forward rotation ability) and a narrow
tire
have with a lower cf (snow FI) have more 'traction' (forward rotation
ability), EVEN if they are sliding sideways?  What does that tell you
about
the difference between Slip Angle and Traction?  IN a straight line you
have
absolute traction.  IN a turn you have two forces, Slip Angle and
Traction.
The very definition of a turn.  If they are interpreted as one force (as
a
Torsen Center does), and exactly the opposite to reality, don't you see
a
problem with that in terms of chassis dynamics?

Dave, take your car on a wet road, 50mph throw it sideways, steer with
the
skid, clutch in.  The back tires will stop rotatating, while the fronts
still
do...  Why?  Slip Angle rear is higher than Slip Angle front.  Get that
Slip
Angle high enough, you can have the rears traveling backwards and the
fronts
stopped.  

Now, since you are correctly speaking of Torque Sensing, understand that
we
don't need Slip Angle to be that high.  Why?  Because a Center Diff
reacts to
Slip Angle, before the driveshafts actually spin at different speeds.
So,
extrapolating that, the Slip Angle Center Torsens will "react to" on dry
pavement is "somewhere" between straight ahead, and 90 degrees.  Add to
that,
your point that the 'significance' depends on how "tight" a turn might
be,
another variable....

>no, in this case, if you're steering with a slide around the apex
>oversteering), the front is travelling *less* distance around the corner than
>the rear, by definition.

>because it is travelling less, then the torsen sees a better torque reaction
>and feeds the power to the *correct* [front] axle.  this is what you want.

UP TO a 'significant' slip angle.  So, initially the fronts will travel
slower
around a turn, mild throttle, we have mild understeer.  Why?  Because of
the
above.  What does that mean to the torsen?  T
hat the rears are "spinning"
(since they are travelling faster), so torque to front.  So initially
the
fronts get the torque.  As Slip Angle increases, you get the rears
slowing in
relation to the fronts.  Why?  Because you can steer.  Torsen:  Rears
slower
than fronts, fronts "spinning", rears get torque.  Torsen Centers don't
care
what the actual chassis dynamics are Dave.  That's not it's job.  It's
dumb,
it controls ALL chassis dynamics in an absolute traction world.  Either
the
front and rear driveshaft spin at the same speed, or distribute torque.
Don't
make the mistake of making it smarter than it is.  In the above
scenario, it
has things backwards to reality.  The backs travel "faster" around a
corner
than the fronts.  That is an absolute traction equation to the torsen,
specifically, the rears are "spinning" in relation to the fronts.  A
Torsen
center can't interpret inputs ANY other way.

When Slip Angle is a constant, fwd/rwd, what happens between tires can
be
measured in an absolute traction world.  That is the design.  You put
that
torsen in the center, a whole 'nother set of variables comes aboard, and
can
ONLY be looked at thru that same absolute traction window.  It's beauty
to
some, can be costly to others.  My exact point.  A torsen should behave
consistently in the real world in which we drive.  And interpret
"variables"
correctly, so that the driver can predict outcomes of inputs.  When it
is an
absolute traction device all the time, that interpretation of actual
events,
can become exactly opposite Torsen design, and your expectations.  My
argument
is, that is exactly why it can bite the best of drivers.

The most often heard quote by those bitten (including myself):  "I
wasn't even
driving that hard!!"  By the definition of a Center Torsen.  At the very
edge
(wheel up) it works correctly, per Dave E.'s and Jeff and my posts.  So,
drive
up to 7/10ths OR drive 10/10>  and watch your cf?  Ok.... 

Scott Justusson
QSHIPQ@aol.com