Da diffs

[QSHIPQ@aol.com]

Jeff G writes:
>Of course, if you have a Torsen center diff, then this isn't a problem as it 
>was *designed* to rotate continuously ... in a way, it's too bad my 200q is 
>an '89.5 version and doesn't have flared fenders since running different 
size 
>tires front and rear is probably the best after-its-left-the-factory cure 
for 
>"spider bites."  

I thought we weren't going to share the potential  cure :)....  My thinking 
is that on a torsen you really don't need to go plus 1 or 2 to get benefits.  
A wider tyre with a slightly increased overall diam (plus 0 concept) is most 
likely enough.  Given the definition of "torque sensing" I doubt it really 
takes much.  By the time you have rotational speed difference due to 
different tyre diam, you already have shifted all the torque to the maximum 
BR.  The problem I see with this Jeff, is that we have roughly defined that 
equal size tyres in a torsen center has straight line traction advantages.  
Somewhere between that straight line and a given turn (and given cf) up to 
the torsen locking BR, you have the advantage to the different tyre sizes.  
Is the matrix figuring that out, another supercray concept or, straight math? 
 The tyre size chosen would really depend on the 'condition' you are trying 
to improve.  Again, in a sideways slide, with countersteer making all sorts 
of f/r torque splits, you are only preloading the rear, not optimizing it 
necessarily.

Still, an interesting avenue to explore.  If a static tyre size Bias Ratio 
differential (biased to the rear > larger tyres in the rear), makes for 
rotational differential in the front and rear axles, by torsen definition you 
just made a car with a 75r/25f torque split just about all the time.  
Optimal?  Not so sure, the race and rally cars (audi and all the other awd 
boys) pick 55-65r/45-35f as the optimal for awd chassis characteristics.  And 
you still have the problem that once those rear tyres are overloaded (lift 
throttle oversteer, or WOT oversteer + it will happen sooner) your torsen 
will be fighting you with the instant tendency for understeer.  Most likely 
better than the equal tyre size torsen in a turn, all else being equal.  But 
maybe not, going from full BR rear to anything front biased (understeer) can 
create a more radical torque induced chassis dynamic.  Then, back to the 
straight line....

Some experimenting with this is certainly in order.  My own thinking is that 
the brake bar used on the 2wd Paris-Dakar winner is probably the best 
"active" device to use on the torsen car, and really indicates where fully 
"active" systems will go in the future.  It obviously works well, even in the 
crude but effective driver application of the concept in 2wd form.   Many of 
the offroad boys have gone with torsen diffs front and rear (locked center, 
un/locking front hubs), and find the autoboxes are the best for optimizing 
torsen traction, since applying the brakes while seeking optimal traction, 
biases torsen torque to the other side of the car.  Unfortunately, they have 
also found that in high HP applications torsens fail regularly.  

I'm all for fully active systems, and making computers smarter.  I'm not sure 
you will cover every dynamic, but I'd bet you'd get close enough to claim an 
improvement to the 7/10ths world audi seems to be concerned with marketing to.

That's just not me, however :)

Scott Justusson
QSHIPQ@oal.com
'87 5ktqwRS2 -10vt
'84 URqRS2-20vt
'87 4Runner turbo A/T