Re: torsens and slip

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

>>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. 

>Hmmm ... I must have missed this one.  ;^) 

mmm, i postulated 4 scenarios for cornering, based on various outcomes.  its in
the archives, guess you didn't read it...

my point here is that this is a very difficult situation to re-create, and is
on the ragged edge.  how do you get rear lateral slip and minimum longitudional
movement?  lift off oversteer is the obvious way.  by definition the torsen is
*not* acting at this point, and the vehicle is in the process of spinning. 
when you come back on the power, the rear's adhesion [lateral] is still being
exceeded, making it relatively easy for longitudional traction to be exceeded
also (by the application of power).  presto, the car transitions from the
corner with opp-lock 4-wheel drift.  btdt many times.

the torsen works here, and very well indeed..

>>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.
>>

[snip] 
 
>Be careful here ... "can be supported" is a much better choice of words than
>"needed."  (I also wonder whether wheel lift is truly possible with a Torsen
>center diff but that's a topic for another discussion...) 

yes, sure is.  done on both my '85 and my '90 ur-quattros. a particular tight
(45 degree) lh uphill corner with a high entry speed and a heavy camber and the
lf lifts every time (with enough entry speed).  stiff car helps (it's harder in
the rs2) ;-)

>>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. > 

>Whoa ... before you start adding more variables into the equation, let's go
>back and look at what happens when the Torsen center diff starts shuffling
>torque between the front and rear of the car.  More torque sent to one end 
>of the car also means the slip angle MUST increase to maintain the existing
>front-to-rear balance; the end of the car that lost torque likewise
>experiences a decrease in slip angle.  This point is non-negotiable; it's
>easily derived from the Torsen paper's description of how it operates and 
>was confirmed by the paper's author. > 
>The fact that a change in slip angle _doesn't_ necessarily result in a 
>change in grip at the tire -- as you've pointed out further on, lateral grip
>v. slip angle plateaus over some range -- means you can have several slip
>angles yielding the same amount of grip.  The Torsen, however, doesn't know
>this ... being "dumb," it responds only to the _delta_ in resistance to
>rotation between the two driveshafts and it has no way of telling whether 
>one driveshaft's resistance is increasing or the other's decreasing; for
>traction purposes (i.e., longitudinal acceleration), it doesn't matter as 
>the two scenarios are effectively one and the same.  The same doesn't hold
>true for lateral grip (i.e., lateral acceleration), however, as the delta in
>driveshaft resistance to rotation might also be caused by a relative
>difference in slip angles between the front and rear ends of the car.  This
>is precisely how the Torsen can be -- dare I say, IS? -- "fooled" into
>incorrectly distributing torque between them whenever a car equipped with 
>one in the center diff position goes around a corner... 

not sure i follow you here.  if, as you agree, "you can have several slip
angles yielding the same amount of grip", then *how* is the torsen involved? 
all it sees is the grip as it's a "dumb" device.  right?

my point is that the vast majority of slip angles encountered makes absolutely
*no* difference to the torsen.  because each yeilds the same coefficient of
friction.  you imply that you accept this.  at the limits of the slip envelope,
then other factors come into play, and the picture becomes much less clear.

so seen by myself, phil, and a number of road testers (see separate post).  i
can safely play at 7/10th's in my wagons with *no* worries what-soever.  as can
many other people it seems.  it exactly matches what i have postulated.  now, i
have an explaination of that, based on the characteristics of the torsen, and
the way it treats slip.

do you?

>>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).

>Agreed ... except that the Torsen will automatically reallocate 
>torque based on changes in slip angle without any regard to the amount of
>grip that is or isn't available at the front or rear tires.  THIS IS THE
>PROBLEM!  

no this is not the problem, the torsen *won't* do this if the change in slip
angle yields *no* change in the co-efficient of friciton.  and, as you've said,
this is true over the vast range of slip angles.

the "problem" will only come into play when the tyre loses adhesion (cof to the
tyre decreases), and at that point a lot of other factors also come into play. 
this is the ragged edge.  you're welcome to it.

[snip]

>>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). > 

>I disagree ... in my book, "the vehicle will track accurately and 
>faithfully" can only mean neutral-steer and _not_ understeer or oversteer.  

no, actually to corner fastest, you want a vehicle which will understeer (entry
and mid-corner) in the corner, and oversteer at other times (exit).  oversteer
on entry is not desirable, nor is understeer on exit.  neutral handling means
you're not trying hard enough...

>>an awd vehicle operating in these conditions will track accurately and be
>>able to be balanced on the throttle. 

>Again, I disagree ... modulating the throttle in a Torsen-equipped car 
>alters the front-to-rear torque split, which in turn alters a whole host of
>other parameters.  This is how a minor problem can quickly become a much
>larger problem ... everything the driver does to correct is either amplified
>or quashed and the vicious cycle of correction/counter-correction -- aka
>"spider bite" -- begins.  

no, this is not true.  your point is, surely, that the torsen car *cannot* be
modulated, it lacks control, it's not doing what you want it to do.  that is
quite different from a car which *is* modulated by the drivers input.  my
torsen cars are throttle steerable when cornering.  i control it.  easy, peasy.
 i've documented deliberately applying full throttle *4* times *during* a wet
corner in the rs2, with full throttle lift between times, *without* un-toward
behaviour.  2nd gear, 315hp, scott will tell you the torque being transmitted
in 2nd gear to the road through the tranmission.  that is throttle controlable
in my book...

>>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. > 

>While this is true, bringing it up at this point only complicates the
>discussion unnecessarily.  The Torsen is fully capable of perceiving
>differences when they exist.  It's also fully capable of perceiving them 
>when they don't -- it's DUMB, remember? -- and this is how problems arise
>when it's used as a center diff in an AWD car. >

no, i disagree.  this point needs to be understood and accepted.  slip angles
are not the bogey you make then out to be.  at least not at 70% effort on a dry
road.  reference carrol smith.  you create a straw man for the purposes of your
argument, and then try to deny disection of said flammable being....

mmmm, all centre diff technology, short of the active centres used in rally
cars and the german itc class 1 racers, are dumb.  the dumbest of all is the
locked centre.  sometimes it works, most of the time it gives you a very heavy
dose of understeer.  not for me thanks...

>>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. > 

>True enough but these factors really determine only when the "bite" occurs
>and the extent of it.  I think you might find a "band-aid" for the problem 
>by playing around in this regard but not an out-and-out cure.  I also think
>this is probably a good time to remind everybody that the Torsen DOES have
>some GOOD qualities although I'm intentionally not bringing them up here
>because I don't want to confuse anyone ... further.  :^) >

good point.  as is probably apparent, i object strongly to the statement that
"70% effort on a dry road and the spider bite (uncontrollable torsen-derived
vehicle dynamics) occurs".  this is misleading, mischevious and just plain
wrong.  i am *not* saying that problems at the edge of the envelope might not
exist.  at 10/10th's driving *any* car, your vehicle is going to do things
quickly and you're going to have to beat it, in order to master it.  corner a
911 fast in the wet, same with the m3, provoke a number of other cars to fast
oversteer, any you pay the piper.

i also stongly disagree that the vehicle, and its dynamics, and its driver are
not factors here as well...

having said that, playing with slip angles should be a rewarding experience in
any car.  it certainly is in my ur-quattro; more so that in the rs2, due to
greater 'feel' from the chassis, and greater involvement with the process.

[snip]

dave 
'95 rs2 
'90 ur-q