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Fwd: Torsen Tunnel Visions

Eric R.,
In the same flavour, this follows to the list...

Scott Justusson

In a message dated 98-03-09 12:33:29 EST, you write:

 Scott, Dave,
 I'm taking this off-list since there seems to be waning interest from 
 the masses... but not from me.
 >What does that mean to the torsen?  That the rears are "spinning"
 >(since they are travelling faster), so torque to front.  So initially the
 >fronts get the torque.  As Slip Angle increases, you get the rears 
 >slowing in relation to the fronts.  Why?  Because you can steer.
 But in your scenario, you're talking about on-throttle oversteer. Even 
 if the torsen has shifted torque to the front, the rears are still getting 
 at least 30% of the total torque. If the rears are already in a slide (slip 
 angle), it shouldn't take much torque to get them spinning (forward 
 rotation), right? If this is the case, then the torque shouldn't be shifted 
 back to the rear until everything is lined up right again (barring some 
 really stupid steering inputs).
>>>>>  No so sure this is correct.  On initial turn in, the torsen shifts
power to front - understeer (since backs traveling faster), then as you power
up, you get shift to rear  (oversteer), then as power breaks from traction in
the rear, tshift front (understeer).  U-O-U in the same turn
 >Torsen:  Rears slower than fronts, fronts "spinning", rears get torque.  
 >Torsen Centers don't care what the actual chassis dynamics are Dave.  
 >That's not it's job.  It's dumb, it controls ALL chassis dynamics in an 
 >absolute traction world.  Either the front and rear driveshaft spin at 
 >the same speed, or distribute torque.  Don't make the mistake of 
 >making it smarter than it is.

> Your interpretation sounds more applicable to VC. You say that as the 
> rears slow, torsen interprets this to be fronts spinning, so torque is 
 >sent rearward. There is no "spin-up" (speed difference) required to 
 >shift the torque, only a difference in supportable torque. The transfer 
 >is instantaneous. However, it IS true that if a loss of traction on either 
 >end occurs, then that end will spin faster than the end that can support 
 >torque as a consequence of the torque shifting mechanisms.
Wheel lift scenario, no argument.

 >Therefore, if the fronts are getting the torque, the rears must be (or 
> will soon be) spinning faster. If, as you say, the rears start to slow 
 >down because of slip angle, the torque is instantly and gradually 
 >transferred back towards equilibrium (50:50) according to how much 
 >torque the rear can support. The torsen does not look at the shaft 
 >speeds and at some point say "OK, rears are moving slower now, let's 
 >dump 70% torque back there in one single event (shades of VC). It's 
 >much too dumb for that.
Before the rears ACTUALLY slow, the torque has already shifted.  You are
correct with "supportable" torque.  Slip Angle MAKES for a slower turning rear
axle.  Before that actually happens, the max tshift is already there.  Let's
not lose sight of the numbers.  More torque in the rear is oversteer, more
torque in the front is understeer.  I suppose we could mu all the equations,
doesn't change the physics of the Torsen or of the chassis characteristics.
 >We're looking at this from opposite viewpoints. You say that torsen 
> "reads" shaft speed to determine torque split. I say shaft speed is a 
 >RESULT of torque split. What it "reads" is the shafts' resitance to 
 >applied torque.
Same thing, you are more "technically correct".  I argue tho, that you can
actually have the rear tires spinning and NOT have any torque shift.  Hence my
"speed" argument.  However, in terms of chassis dynamics these tshifts are
occuring before any "spin" is observed.  Up to maximum torque supported by the
wheel with maximum torque.  But that can be exceeded, and you can indeed have
a spin differential.
> I could very well be wrong on this, but that's the conclusion I draw 
 >from visualizing the forces at work in fig. 1.
I have no problem with your presentation. Understand Eric, this doesn't change
the character of the chassis.   The bottom line is that Tshift max occurs
before wheel lift.  Your presentation just tells me, it's probably a lot
sooner than Dave E considers in his 'radical' definition.  And true in my
experience as well.  Appreciate the input.  Clarifying tshift, doesn't change
the physics, or the outcomes.