Re: Torsen Tunnel - absolute traction

[QSHIPQ <QSHIPQ@aol.com>]

In a message dated 98-03-10 13:10:54 EST, you write:

<< 
> [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.
let's not forget that a audi chassis understeers.  As you add power, that
"can" become oversteer, but not always.  And it can be both in the same turn,
as with a center torsen.
 
 >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.) 
NOT necessarily.  We could just understeer MORE
 
>>> 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.

No sure I agree.  Slip Angle and Traction in terms of Trg (T1 + T2 = Trg =
engine output max) are two different entities.  I think "shift" is
appropriate.  Only if you raise a wheel can you "assume" a Center Torsen to
reduce Trg on dry pavement.  On a lower cf, you are correct, tho I say the
significance of that Trg is higher in terms of how much tshift is occuring.
Slip Angle messes up that "combined traction" argument when you do a center
torsen in a turn.
  

>> 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).
Semantics I suppose.  Tshift happens because Slip Angle is interpreted by
Torsen as a slipping front driveshaft (specifically, traction lost front).
The rear WANTS to turn slower, the Torsen doesn't want it to.  Under power,
the torsen interprets the slower rear to mean that exactly the fronts are
"spinning".  Whether they are or not, is irrelevent to the operation of the
Torsen.  Now, let's understand that Trg can be max, because although the
Torsen assumes that 'differential' to mean slipping or spinning, in fact we
have none, so we haven't exceeded the combined traction ability of the four
wheels (see your argument above).  So Trg is MAX, so then is Tshift.

 
> >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.
3 X actually.  You go from a rwd to a fwd car in the same turn.  Does that
help?  So, entering turn you have 234lb/ft r 64lb/ft front.  At some point
that changes, up to 64r 234 front.  At some point (and since I drive a locker,
and know that a 44 chassis car, and my Urq both have) 50/50 torque
distribution you have understeer.  More than that is more understeer.  So a
torsen, we could argue, can understeer MORE than a locker and  more oversteer
than a locker.  All in the same turn.
  
 >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.
Nope, I present that that isn't true UNLESS you look at a torsen as an
absolute traction device.  Slip Angle is NOT an absolute traction variable.
Specifically, relative slip angle isn't.  I can have Tshift max to the rear,
and all four tires on the ground, NOT spinning.  Max Trg.  I can tshift max to
the front, and all four tires on the ground, with the rears at the right Slip
Angle.  No wheel has lost traction yet, so Trg is maximum, so is Tshift.

What is the difference between Slip Angle to a rwd/fwd and Slip Angle to a
Center Torsen?  
 
> 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.
Reduction in torque delivered, may or may NOT affect the understeer oversteer
understeer character of a Torsen.  The best hope is that Trg is reduced to the
combined traction of all four wheels before hunt.  That is an absolute
traction argument.  Straight line OR wheel lift argument.  Turning changes
things.   My first experience with the Tshift WAS in a 88 90q.  And it was the
torsen.  And I don't see a reduction in Trg as a good thing when trying to re-
induce oversteer, or understeer or something to avoid the tree I was sliding
sideways at.

 >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.
  >>
I suppose.  I also think the EXACT answer to the 'bite' is in the 5.2
"unlikely" section.  To rwd and fwd, that scenario works.  To a Torsen,
"unlikely" can be very likely, by the very same presentation.  Relative Slip
Angle to traction in a dual axle car is a constant to the operation of the
Torsen.  Relative Slip Angle to traction in a Center Torsen 4 axle car is a
VARIABLE to the operation of the Torsen.  What has changed to accomodate that
variable?  Not a thing.  So Relative Slip Angle is a constant to a Center
Torsen in terms of absolute traction.  I have a problem with that jump.

HTH

Scott Justusson