RE: A new look at torsens

["Dave Eaton" <Dave.Eaton@clear.net.nz>]

scott seems puzzled by oversteer in a locked centre quattro.  once again
scott, you are quick with claims, and quick to ask for proof, but you are
very very slow to do any work yourself.  when asked to backup your own
claims, you usually dodge for cover.

anyway, to oversteer in a "locked centre" quattro.

1) pressing on around a slippery bend, locked centre.

2) with the locked centre, torque follows tractive force with the result
that more is at the front wheels (audi weight distribution).  which are also
trying to steer.  an understeer condition.

3) once tractive force begins to be exceeded at the front, (e.g. too much
steering input) the front slip exceeds traction and the fronts lose
sideforce.  result heavy understeer.  torque is now going to the rear.

4) lift throttle and one of 2 things will happen depending upon rear slip:
(a) either an oversteer twitch as torque goes rearwards through the locked
centre.  this overcomes rear tractive forces and you have an oversteer
moment.  or
(b) lifted throttle restores sideforce to the fronts.  however, you've just
also reduced rear slip angles  but, as the rear slip angles weren't
critical, they will reduce even *more* than the fronts (which were
critical).  hence understeer remains or becomes stronger.

5) back on the throttle and steering input and you're back to understeer,
albeit at a less critical cornering speed.

so we have u-o-u, or u-u-u, depending upon slip angle differences front and
rear. quite easy to make a case for u-o-u-o-u depending upon slip angles,
speed and cf.  is the locked centre fooled by slip angles?  of course it is.
it's just a dumb device.

you need to remember scott (you've got this wrong before), that with the
locked centre, up to 100% of torque will go to the axle with tractive force.
a locked diff does not, indeed cannot, distribute torque evenly to each
driveshaft, but will allocate torque according to available traction.  for
example, with a lifted front wheel, 100% (near enough) torque will go
rearwards (that's big oversteer).  drop the front wheel and you're back to
normal (understeer - torque to the front).

what should be clear in this is that driver skill at the limit is important
if the hedge isn't going to be rustled.  steering input, and throttle
control are part and parcel of handling a big, heavy car at the limit of
adhesion.

btw, the locked centre does control axle speed.  which, imho, is a win
whenever ultimate traction is required, and only 1 locking diff is an
option.  if you can afford 3 torsens, then that is clearly the way to go for
ultimate traction.  or else, you use a tcs.  either way you don't have to
put up with the understeer inherent in the gen 1 quattro.  and you have abs.

hth,
dave
'95 rs2
'90 ur-q
'88 mb 2.3-16


-----Original Message-----
Date: Mon, 18 Oct 1999 07:55:35 EDT
From: QSHIPQ@aol.com
Subject: RE:  A new look at torsens


>also, you continue to misunderstand that the *fixed* centre diff (not just
>the torsen) continually varies torque front and rear through a corner.  so
>your statement "a torsen with torque applied to it, ALWAYS varies the BR
>when you aren't in a straight line", could equally well be "a locked centre
>diff with torque applied to it, ALWAYS varies the BR when you aren't in a
>straight line".

Your conclusion.  Ok,  for sake of argument let's let you take that position
for now.  Explain how a locked center oversteers?  Or per your intrinsic
argument, explain how a locked center gets U-O-U in the same turn ala
torsen.
Lots of us locker owners would love to know.  Buffum included.