new torsen paper (no flames)

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

much a i hate to disturb the raging peace which has broken out in the torsen
front, but....

i have received a new 1995 zexel paper about the torsen diff.  rather than the
general introduction provided by the paper at jeff's website, this paper deals
with the applications of the torsen.  The title is "traction and handling
safety synergy of combined torsen differential and electronic traction
control".  it develops an understanding of the behaviour of the diff in various
applications, and then presents the case for the diff being a major advantage
when used in conjunction with a tcs, as opposed to a tcs on it's own.

the paper begins by discussing axle/wheel kinematic conditions and elastic
conditions to differentiate different wheel/axle behaviour at different speeds.
it then defines the behaviour of a solid axle, an open diff, and then a torsen
in a single axle configuration.  it then deals (3 pages) with the application
of the torsen in a centre diff application using the same model, and disucsses
drive, oversteer, understeer and coast modes of operation.

the specific modes covered are:
1) normal drive
2) power oversteering
3) normal coast
4) coast with drop throttle oversteer

for example, the paper states that, in drive mode with the centre torsen, in a
slow speed corner, the higher driving torque is sent to the axle which tends to
turn slower ie. more torque to the rear and less understeer, until as corner
speed rises and elastic conditions catchup kinematic conditions, the torsen
operates as a rigid connection.  at higher speed, either oversteer occurs (rear
axle slip ratio increases) and surplus torque is sent to the front, or
understeer occurs (front axle slip ratio increases), where the torsen sends
torque to the rear, correcting the understeer.

the paper also makes the point that, in coast mode, the centre torsen is still
operating and doing good things, although the other way around (higher braking
torque is sent to the axle which tends to turn *faster* than the other one).  

anyway, in the summary to the discussion of the torsen in a centre diff
application, the conclusion is pertinant to the discussion that we have been
having about the relevance or not of chassis dynamics: 

"the ideal centre diff torque bias ratio layout in the 4 operating modes is a
function of vehicle dimensions (wheel base, track width, centre of gravity
height, ...), suspension elastio-kinematic design (stiffness front/rear,
angular variations, ...) and engine torque characteristics for given road
conditions." 

the reason that the famed spider bite hasn't been seen in an ur-q or rs2 (or
s4?), but has been seen in a type 44 is given above.  chassis dynamics.

dave
'95 rs2
'90 ur-q

ps.
i am endeavouring to digitise the paper but, due to the 30 or so graphs/charts
in it, this will take some time to do properly.  anyone who wants a copy, drop
me a line and i'll oblige when i've finished (expect 2-4 weeks).

alternatively, i'll scan the centre diff section and post the text only to the
list...