Re: torsens and slip

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

thanks jeff for your efforts.  light is much better than heat, any day :-)

as i said more than a month ago, i can certainly conceptualise the torsen
inappropriately allocating torque to a [virtual] axle which is sliding (has
lost longditudinal speed).  in this case, both the viscous and torsen diffs
will be fooled.  as will a clutch-derived system, unless sensors for detecting
vehicle rotational dynamics are used.  in this situation the locked centre,
because it insists on equal rotational speeds will be better.  by deninition,
this is cornering on the ragged edge.  my understanding of the theory is offset
somewhat by my experience of oversteer in low cf conditions in my cars where
control is easier than the above would seem to suggest, particulalry as the
torsen will correct when longitudional speed is applied to the slower [sliding]
axle (ie. accelerate).  also as i have shown, the other 'ragged edge' scenario,
inside wheel lift, is better with the torsen and vcd, than the locked centre as
more torque goes to the end where it is needed, while the unlocked centre will
be much worse because it has no bias ratio to limit the damage (this is the
famed genration 1 quattro spider bite, btdt).  

anyway, the point of this note is that i've been thinking about the slip angle
thing, and keep coming back to the importance of wheels/tyres and chassis
design to this equation.

for street tyres and applications (by definition), a *wide* spread of slip
angle will occur with *very similar* tyre co-efficients of friction (cof) . 
[picture the cof on the y-axis and the degree of slip on the x-axis.  the line
will rise steeply until a certain cof and then plateau over a spread of slip
angles until tailing off, as tracton is lost.] this will mean that the front
and rear *can* exhibit *different* slip angles *without* any meaningful change
in the tyres perception of grip (their co-efficient of friction).  in this case
the vehicle will track accurately and faithfully *despite* adopting understeer
(front slip angle greater than the rear), or oversteer (rear slip angle greater
than the front).  an awd vehicle operating in these conditions will track
accurately and be able to be balanced on the throttle.

this is precisely my experience of my two cars.

if, however, due to the choice of tyres, i select tyres which have a much
higher co-efficient of friction (aka racing tyres), the relationship of cof and
slip angle is different in that the cof/slip angle graph will rise steeply,
plateau over a relatively narrow slip angle range, and then tail off sharply. 
in this situation, the cof will change significantly over a narrow range of
slip and the vehicle dynamics will be affected.  a torsen in these
circumstances may well perceive different cof's from the front and rear axles,
and treat this as traction.  as will a vcd.

factors affecting this would clearly be the types of tyres used (their cof/slip
relationship - the use of racing tyres with much more sensitive cof/slip angle
reltionships), the suspension of the car (and chassis dynamics) which clearly
affect the *actual* cof (rather than the theoretical), and the way in which the
car is being driven (ie. the driver operating at the edge of the cof envelope).
 other factors which could also "push the envelope" here would be the use of
different wheel/tyre sizes front/rear.

for a better discription of this affect, carrol smith's book "tune to win" has
been very useful for me.

comments anyone?

certainly one further question i'd ask mr chocholek (chocolate?) would be about
section 5.2 which seems to imply that for the vehicle to turn, split at the
bias ratio is taking place.  "a torque division between drive axles at the bias
ratio is a precondition for differentiation under all circumstances of
operation".  in my experience, this is simply not the case, so his
undertanding/clarification of the words would be useful.

thanks again.

dave
'95 rs2
'90 ur-q

>------------------------------
>
>Date: Sat, 2 May 1998 13:49:43 -0400 (EDT)
>From: audidudi@mindspring.com (Jeffrey J. Goggin)
>Subject: S.E. Chocholeck talks Torsens...
>
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