Re: Torsen Tunnel Visions

["Eric Renneisen" <renneie@hlthsrc.com>]

[me:]
>>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).

[QSHIPQ:]
>  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

[me again:]
The "since backs traveling faster" statement doesn't apply. Only 
if the fronts present more resistance to torque will the power 
shift forward. For a brief time at initial turn-in, I will accept that 
the slip angle of the fronts will cause a _slight_ shift of power.

The oversteer I can accept, but if power breaks traction from the 
rear, and the power is sent up front, this will not neccesarily 
cause understeer. The rears are already out, so power up front 
should only serve to pull the rear end back in line, right? 
(presuming you are steering with the turn.) 

Also, the amount of torque sent up front depends upon how much 
torque the rear can support. If the rear is free spinning (low cf 
situation) the torque to the front will actually be *less* than 
before the back broke loose. Since we're talking about dry 
pavement here (right?) we can assume that the rear can support 
at least some significant torque. I think the term "shift" is a bit 
inappropriate, since at the limits of adhesion the "combined 
traction of the drive wheels" [section 5.1] will not support as 
much torque as straight line driving. Therefore, as torque is 
being shifted, the total torque transferred may be decreasing.

>>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.

My comments mention nothing of lift. They apply to the torsen in 
all cases. If torque shift occurs, the wheel receiving less torque 
will be forced to spin faster as a consequence of the shift.
 
>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.

Slip angle will make the rear turn slower when coasting, but not 
necessarily under power (especially if, as you say, the tshift is 
already back there).

>Let's not lose sight of the numbers.  More torque in the rear is 
>oversteer, more torque in the front is understeer.

Hmmm... let's see, back end comes around, torsen throws power 
up front (but just 2x what rear will support)... this is understeer? 
I thought understeer was when the fronts have higher slip angle than 
the rear.

>I argue tho, that you can actually have the rear tires spinning and 
>NOT have any torque shift.

I agree, as long as the rear can support 50% of the engine torque.

>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.

You *will* have a speed differential *if* torque is shifted. Your 
comments on wheel spin are what dragged me into this, now I'm in 
deep. I think that before you exceed the maximum supportable 
torque at the wheel with the MOST traction (using 70% engine 
torque), you'll exceed the max. supportable torque at the wheel with 
the LEAST traction (using 30% engine torque). When that happens, 
total torque delivered reduces.

>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.

I agree with you on this one.

>Clarifying tshift, doesn't change the physics, or the outcomes.

I think you're looking at tshift without considering the possibility 
of a reduction in total torque delivered. I'm certainly not out to get 
you, even though I have a vested interest in vindicating the torsen. 
I also have a vested interest in knowing how and why the thing 
works. I don't doubt that your 'bite' occurs, but since it seems to 
afflict the 44 more prominently (with a few 80/90 reports 
sprinkled in -- hence my interest) there may be more at work here 
than just the ol' torsen tshift.

I just had an idea for analyzing this effect (maybe it's already been 
mentioned, but I don't remember it). Could we tap into the ABS 
wheel speed sensors with a laptop to see exactly what the wheels 
are doing in relation to each other? It wouldn't answer all the 
questions, but it would go a long way.

Eric "what the h*ll am i doing!?!?!?" Renneisen
'90 CQ 20V  -  my 'racing-iron'  ;^)
Chattanooga, TN