Re: Questions on AWD, ABS, and locking the rear differentia

["Mike White" <MIKE.WHITE@law.utah.edu>]

> the diffs are unlocked.  They say that allowing the wheels to lock 
> can improve stopping ability in snow.  (That's one thing I like 
> about Audi, they figure that the driver is able to decide when it 
> is appropriate to  disable ABS :)

Has someone proven my theory?

I have noticed that under the following conditions, that ABS actually 
hinder braking performance when compared to locking up the tire 
completely.

A) Washboard dirt roads downhill.
B) Washboard ice downhill.

A) In my '86 5KTQ, when making and emergency stop downhill on I-15 in 
a storm with the ABS on, I had little to no stopping power when I hit 
a patch of washboard ice.  The ABS didn't seem to be able to keep up 
with the rapid increase and decrease in traction as I bumped over 
this stuff.  I was not able to hit the 'ABS off' button because I was 
quickly approaching a car's rear end.  Knowing my brakes weren't going 
to stop me, I cranked the wheel and gunned it.  Thanks to Quattro, I 
was able to get beside the car in front of me, clearing only by a 
couple of inches.

To try to validate my theory, I went to a freshly oil sealed parking 
lot and tried braking with and without the ABS.  Overall I came out 
with the impression that I got better stopping power in uneven snowy 
conditions with the ABS off.  

I still recommend driving in snow with ABS on, since washboard 
situations are quite rare.

B)  After wrecking the Audi, and replacing it with the poor man's 
Quattro ('95 Urabus Legacy (w/warranty <grin>)), I noted similar 
results when hitting the brakes hard on a dirt road going downhill.
The brakes pulsated but I didn't feel that they were keeping up with 
the rapid increase and decrease in traction.  Unfortunately there is 
no way to disable the ABS on this vehicle, so I was unable to test it 
out otherwise.  However since I was on a corner it was advantageous 
to keep rolling and not skid off the road.

Thus are my feelings.

- Mike White 

[Mail: mike.white@law.utah.edu   -   mwhite@eng.utah.edu]
Visit my '57 23-Window VW BUS at: [http://fpc99.law.utah.edu/]