Re: Tshift

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

>------------------------------
>
>Date: Fri, 6 Mar 1998 06:53:57 EST
>From: QSHIPQ <QSHIPQ@aol.com>
>
>Dave E. writes:
>>btw, it is still not accepted by me that you can get t(shist)=t(max) by
>>a torsen without losing traction as you describe.  i simply cannot see
>>how this can possibly occur.
>
>Put your car on gravel road, narrow grippy M&S tires.  Get the thing sideways.
>You didn't lift a wheel, and the forces of gravity in relation to slip angle
>still apply.  By your own argument you have to agree that you have a Tshift
>max will occur.  So, by extension, your same argument is that cf HAS to be
>low.  Ok, 'cept I've btdt on a dry track 110degree right, WOT,  no wheel lift.

yes, but you are describing a loss of traction.  hence my point.  how can
tshift(max) occur without a loss of traction?  traction *has* to be lost for
this to happen...

>
>>central to this point is whether the torsen uses *rotational* shaft
>>speed differences as the determinant of the torque bias to apply, or
>>whether it uses forces *from* the driveshafts *into* the diff to do this
>>(ie. effective traction, or torque reaction).  I am convinced that it is
>>the later.
>>it is called a "torque sensing" differential after all.
>>i am sure this is the basis of your misunderstanding.
>
>To the Center Diff, what's the difference Dave?  What "other" forces are
>there?   In a Center diff that can be traction OR slip angle, by definition of
>both.  They ultimately tell the center torsen the same thing.  The torsen is a
>dumb device, it doesn't know the difference between the two.  What's the
>difference between a Center and a fwd/rwd?  Slip Angle relative to the drive
>wheels is a constant in the fwd/rwd, a variable in the Center.
>

well, you are contradicting yourself here scott. the torque reaction [the
ability of the wheels to provide a reaction to the torque being transmitted] of
the wheels sets up the centre diff and the amount of differentiation which it
allows (which means before slip occurs, as stated in the paper).

with the torsen, if slip angles are large at the rear (normal oversteer), then
differentiation occurs to the *front* due to the lower torque reaction of the
rears (slip is occuring).  if slip angles are large at the front (understeer),
then differentiation occurs to the *rear* due to the lower torque reaction of
the front.  this is *exactly* what you want, torque shift to the axle with the
highest torque reaction (effective traction).  the torsen does this better than
the open diff, and the locked centre diff, and is, i believe the reason why the
'mb' and 'rr' ur-quattro's understeer much less in hard cornering than the 'mb'
(reference any number of independent road tests).

the case you mention i'm still struggling with.  it has the car suffering so
much oversteer that the rear wheels are *not turning* relative to the front, so
that the torsen gets fooled into sending torque to the rear.  as i've shown in
this case, the torsen is still working (thanks to the bias ratio) in sending
some torque to the front.  it is just not as good in this instance (the 0.1%
case imho), than the centre diff.



>As you well pointed out, wheel lift creates an absolute traction issue.
>Torsen is a very effective absolute traction device.   No wheel lift, on the
>other hand, doesn't necessarily create a traction problem.  The key to
>understanding this whole issue is how Slip Angle is percieved by the torsen.  
>  

i understand what you are saying about slip angles, i just have no information
about how they would affect the operation of the torsen.  by definiton they
affect the torque reaction of the wheels, and so would affect torque
distribution.  but by how much?  on dry roads, not much at all (relative f:r
split), although on snow, it could be more.  the same factor affects the open
or locked centre diff...

it will be interesting to hear the results of jeff's discussions with the
gleason folk on this issue..

>
>HTH
>
>Scott Justusson
>QSHIPQ@aol.com