[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]

Torsens: The Trojan Horse...

Dave E. writes:
>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
>vehicle rotational dynamics are used. 

Be careful with tossing the viscous and clutch systems in with Torsens.  VC's,
and clutch systems are not the same as torsens.  A torsen is NOT a
freewheeling diff.  A VC, clutch, and magnet plate are.  What does that mean?
Well, 'fooling' the diff, doesn't necessarily create a different chassis
character.  FI, look at a 3kgtVr4, 64r/36f VC,  on normal travel, it is 100%
rear driver with oversteer characteristics.  When slip increases or traction
is reduced, the VC locks.  At 64r/36f, that is still an oversteer condition at
WOT Dave.  An audi with a VC that is normally fwd, then locks at 50/50, is
always an understeering chassis at WOT thru a turn.  Better or worse than a U-
O-U Torsen at WOT?  You can ONLY make the 'Torsen is better'  (TIB -<tm> -
Dave E.) argument if the compared device changes the inherent chassis
dynamics.  So we could look to a VC as BETTER than a torsen and an open in
terms of slip angle.  Again, you are interchanging slip, traction and diffs
with chassis dynamics.  You have argued to date regarding chassis dynamics,
and make incorrect assumptions there too.  This is the first time you have
indicated that a torsen can be fooled.  Be careful tossing in the other
'fooled' devices, and summarizing "TIB".

> 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.

NO, a VC can be equal or better.  OVERSTEER  on throttle all the time.  NOT so
with the locker, that's UNDERSTEER on throttle all the time.
As the turn decreases in radius, the 'ragged edge' is NOT where you are
cornering, but the Torsen reacts to a tighter turn (relative slip angle) as
well as slip angle.  We need to get the 'ragged edge' out of the description
Dave.  A wheel lifted is cornering harder than 4 wheels on the ground.  For
the bite, 4 wheels are on the ground. (This basic goes back a long way). How
can a wheel lift with a torsen?

>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
>axle (ie. accelerate).  also as i have shown, the other 'ragged edge'
>inside wheel lift, is better with the torsen and vcd, than the locked centre
>more torque goes to the end where it is needed, while the unlocked centre
>be much worse because it has no bias ratio to limit the damage (this is the
>famed genration 1 quattro spider bite, btdt).  

Wait, slow down dave.  You are taking wheel lift, and appying it to a locked
center and an open center.  "TIB" than an open center.  A locked center?
Well, what is at the diffs, a torsen or a locker or an open?  With wheel lift,
Trg to the ground is reduced in a vc, torsen or locker.  In an unlocked center
100% of torque goes to the spinning wheel, it just costs more in tires  The
net effect is the same, Trg to the ground is reduced, claiming one to be
'better' is ignoring the effect of wheel lift on any of the diffs.  Reread
section 2.5 of the paper.  Again you want to compare to the VC.  Not there
with you either.  A torsen with U-O-U vs a VC with O-O-O.  Not sure the
advantage goes to torsen. Are we talking of the absolute switch or the
absolute switch in terms of chassis dynamics.   Again you use chassis dynamics
AND traction device arguments as one.  They are two separate arguments.  And
where you are confused.  Given the device, what are the chassis dynamics.
Not:  Given the device, what is the absolute traction of the device.  The
latter is ONLY a TIB 'win' in a straight line, or in a turn comparing other
diffs with a U-O-U character.  That's a pretty narrow scope sir.  Look at the
VC applications to a given chassis.  Do you really want to make this a blanket
statement?  We can make blanket U-O-U with an audi, cuz a center torsen is
ONLY used in an audi.

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

Not with you on this...  You are overloading the supercray again.  The
variable is CF, not the tires relation to cf.  You stepped too far given the

>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
>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
>in the tyres perception of grip (their co-efficient of friction). 

"Grip" and perception of it has nothing to do with driveshaft 'rotation' in
regards to slip angle. "Dumb, &*$#! gear jamming friction device", isn't
smart, by definition.   We could say for a given cf, a given tire has a given
'grip'.  This 'grip' given cf affects the 'significance' chassis dynamics of
slip angle.  You must identify the cf first. Given 4 tires of the same make
and size, really not necessary. Making this too complicated Dave.  Driveshaft
rotation and slip angle don't care what your 'grip' is.  The bite happens
sooner or later.  Tires irrelevent.  You want to say :"At a high enough grip,
no torque shift forward occurs".  Wrong.  Regardless of tire, you still have
U-O-U in a turn in a audi 78/22/22/78 center torsen.  

> in this case
>the vehicle will track accurately and faithfully *despite* adopting
>(front slip angle greater than the rear), or oversteer (rear slip angle
>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.

No.  Front slip angle greater than the rear on throttle may be balanced on
throttle, may not (bite).  Your scenario is a point of time, a specific turn
and cf only, Dave.  A wider tire will grip better in the dry, will it in the
wet?  No.  Will a wider tire grip better in snow?  No.  Gravel? No.  Will a
decreasing radius turn, or a tighter one affect slip?  Yes.  The supercray is
smoking.  How bout, given tires, and vary only cf.  Varying tires AND cf, is
unecessary for the matrix don't you think?

>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
>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
>and treat this as traction.  as will a vcd.
No. You are making the "different cf" argument again dave.  Assume cf to be a
constant on ALL 4 tires.  You are starting to confuse 'grip' and 'traction'
and 'slip' and 'driveshaft rotation'.  The torsen doesn't care what you put on
for tires.  Sure, a given tire can affect slip and grip on a given cf.  Change
cf, the given tire is in the same scenario as all the others, specifically
exactly the wrong one for cf. And the torsen is switching torque to the wrong
end of the car.  Changing tires, the phenomenon doesn't go away, just affects
when the phenomenon happens on a given cf.  You aren't doing anything to
'fool' the torsen into making all the right switches here.  Just delaying the

>factors affecting this would clearly be the types of tyres used (their
>relationship - the use of racing tyres with much more sensitive cof/slip
>reltionships), the suspension of the car (and chassis dynamics) which clearly
>affect the *actual* cof (rather than the theoretical), and the way in which
>car is being driven (ie. the driver operating at the edge of the cof
> other factors which could also "push the envelope" here would be the use of
>different wheel/tyre sizes front/rear.
A torsen shift is NOT at the edge of the envelope.  A raised wheel is at the
edge of the envelope.  We aren't there yet.  A different sized tire f/r will
fool the torsen.  Specifically, a larger rear tire will get more torque.
Again, a great argument for a narrow cf range.  However, with a 78/22 torsen,
I'm not at all convinced that you could get a small enough front, big enough
rear to make this claim, race or street.  

>certainly one further question i'd ask mr chocholek (chocolate?) would be
>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
>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.

If you read a few lines further, your answer is already there:
"The Torsen differential resists transfers of torque between drive wheels in
proportion to the torque applied to the differential housing, AND THIS RESULTS

FYI, let's translate that to a center diff
'The Torsen center resists transfers of torque between driveshafts in
proportion to the torque applied to the center diff (Trg), and this results in
a larger proportion of the applied torque being deliverd to the slower
rotating driveshaft.'

Look at the above, and slide your car sideways, where does torque go?

Dave, I look to the above and see more confusion.  A torsen is an absolute
traction device.  That by definition can be fooled in a turn.  You are making
two different arguments.  One can look to ALL your documented posts, and see
nowhere did you post an absolute traction device can be fooled in a turn.
What we can see, is a claim that your non-events means that an absolute
traction device can't fool the chassis dynamics in a turn.  This is a
different argument.

Your and Phil's argument to date has been, no bite experienced, no bite can
exist, the theory of torsen must be wrong.  That is a chassis dynamics
argument.  'Given Tshift, no effects on chassis dynamics while turning'.  Now
you want to claim that tires affects those chassis dynamics while turning.   I
believe Jeff to be right, it's more basic than that.  The above still
indicates a basic misunderstanding of chassis dynamics, while turning.  You
make this whole bite thing too complicated.  Given the 'dumb gear jamming
device' (tm) <if only he'd said switch, I woulda sued :)>  it isn't
necessarily smarter or better than other absolute traction devices in a turn.
'TIB' absolute traction device ONLY in a straight line.  By definition of the
device, AND chassis dynamics while turning.

Confusing the two, only creates more confusion.

Scott Justusson