To lock or not to lock...

[audidudi@mindspring.com (Jeffrey J. Goggin)]

Being a self-pronounced "expert" on autocrossing Audis, I can't resist
adding my two cents here... 

>     Unfortunatley your downfall was probably the fact that you had the center
>diff locked.  With the center diff locked on a paved surface, i.e. where the
>wheels cannot slip this causes the drive train to bind or store torque in it's
>workings.

The tires can slip plenty with the center diff locked ... in fact, they're
always doing it to some extent as the wheels move up-and-down (and
forward-and-backward) due to suspension travel.  Also, the fact that it was
raining means that the coefficient of friction of the road was low enough
that tire slippage should not have been a problem.

>When you put on the brakes weight was transferred from the rear of
>the car to the front, causing the rear end of the vehicle to rise.  This
>sudden loss of weight on the rear tires casued them to "unbind" i.e. slipping
>at the road surface, once that occurred the back end went it's own way.

This is partly true ... the problem, though, is that once you lose it with
an AWD car, it's very difficult to recover, period.  With the diff locked,
without it locked, it doesn't matter ... check out the March '88 issue of
the UK's Performance Car for an article that explains this phenomenon in
great detail. 

>How do I know this, during an autocross I accidentally had my center diff
locked
>and the first time I nailed on the brakes and truned slightly, the ass end of
>my 4000q went it's own direction.

This is precisely the reason why I *always* lock the center diff on my Ur-Q
when autocrossing!

>This occured on several occasions the same day until I realized what
happened, the stupid diff lock lights >didn't work..

Both myself and my co-driver have tested the car with the diff locked v.
unlocked and it is consistently .5 to .6 seconds quicker on a 50-60 second
course with the diff locked.  In fact, since my Ur-Q will lift the inside
rear tire 2" off the ground using Hoosier bias-ply tires, you end up with a
one-wheel drive car if you don't!
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   / |      _| o    | \       _| o   Jeffrey Goggin
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