RE: yaw'n about diffs

[QSHIPQ@aol.com]

>as i said before, you either get a passive diff to shift torque (speed or
>torque sensing), but better to get a computer to do it for you, to make the
>feedback loop tighter, and the control more precise.  we can all go back to
>the understeering locked centre technology because it's safe and it works,
>but we could use the same argument to dispense with the motronic, or the abs
>computer...

You miss the point.  Get computers and brakes good enough, you don't need a 
locking differential at all (audi partially proved that with the edl system). 
 Now all we have to do is convince the safety pinions about not making road 
cars inherently understeer at the limit, that's a corporate liability safety 
device.  Makes for interesting tcs programming.

In your original post you claim:

>>while generation 1 awd systems (audi locked centre, early subaru) where
>>all-or-nothing devices (100% torque shift front to rear when locked), the
>>next generation of awd systems (generation 2 shall we say) were passive
>>based with automated torque dispensation depending on either torque sensing
>>or speed sensing across the centre differential. the emphasis on generation
>>3 systems (audi's current technology) is to provide a feedback loop to the
>>passive awd system using the wheel speed sensors (abs) and the engine
>>(traction control system, tcs) to modify vehicle behaviour while cornering.
>>the new generation systems (generation 4) are increasingly dispensing with
>>the basic passive centre differential, and moving to technology which allows
>>more direct control of vehicle dynamics (yaw control) by directly
>>controlling the torque shift.

Not quite right above.  The first generation locker doesn't *shift* any 
torque when locked, it is a fixed split 50/50.  It can't *shift* any torque 
by definition.  When it is open, it can *shift* 100-0-100.  Let's think about 
this in terms of active traction control.  The tcs systems from audi controls 
side to side traction independently, and subsequent elimination of the rear 
torsen accompianies that system.  Get the system smart enough (front to rear, 
side to side, diagonally), you don't need the center torsen, or any other LSD 
for that matter.  Why would you?  If the computer and sensors determines that 
100% of the traction available is at 1 wheel, wouldn't you want an open diff 
that is able to transfer 100% of torque to that wheel.  By design, no LSD 
differential can give that kind of bias.  For a truly computerized braking 
tcs, open diffs rule.  Could you use limited slip locking to control 
oversteer and understeer?  Sure.  Could you use tcs-open to do the same 
thing? Sure, apply braking to the inside (oversteer) or outside (understeer) 
wheel/s.  

>what will audi do?  no-one has any idea yet.  certainly none of us do.  the
>s6 sounds like it's either the next generation, or simply that the getrag
>box requires a different solution.  time will tell.  i'm perfectly confident
>that when it debuts, it will be good, and it will be better than what has
>gone before.  just as generation 3 (torsen with edl and esp) was better than
>generation 2 (torsen) was better than generation 1.  progress through
>technology.  bring it on.

We could debate what the definition of "better" is and why.  In absolute 
traction terms in a straight line, you are correct in the systems available 
(but what about a 4wheel dynamic tcs system comparo, no LSD's?).  The 
dynamics of turning dictate that the above might be the simplist conclusion, 
not necessarily the correct one for all circumstances.  Until you have a 
totally dynamic four wheel tcs system, LSD are necessary.  When the x 
generation computers get "active" enough, the LSD will become a thing of the 
past, and the open diff awd will rule.  Progress/regress through technology.

We just aren't there yet.  That's a lot of programming.  On who's profile 
(Walter, Blomqvist, maybe mouton for the women?) do we base the programming 
on?  What do we do with the "understeer pinions" at corporate?  The concept 
of a totally dynamic tcs system has it's merits.  In the realm of 2wd (fwd or 
rwd) tcs systems have been tested and really equalized the "quattro 
advantage" in a variety of testing methods and venues (even the latest issue 
of C&D indicated that tyres make a bigger difference than hardware - comparo 
fwd/rwd tcs with awd gen x).  My thinking is that when you get a 4 wheel 
dynamic awd tcs system with open diffs, "quattro" can rule again, by 
definition it commands 100% of torque and traction at all 4 wheels vs 
controlling 2.  Getting there seems to be the tough part.  Any time I see 
"another" LSD in a "new" quattro system, I'm convinced audi isn't quite there 
yet either.  

My .02 arbitraged thru the peso

Scott Justusson