200tq ABS problem?

[mark schofield]

Brendan wrote:

> 5 mph down a snow-covered hill. ABS on. Hit brakes.
> crackle-crackle-crunch-crunch, car still sliding.
> I bump it up against the curb to get it to stop, disengage ABS.
Depress
> brake under speed. No more crunchies.
> I'm assuming something in the chain ain't workin' or ABS wasn't
designed to
> handle that situation.
> Anyone?
>
> Brendan

Owners handbook suggests that you may be better off without ABS under
the sort of road conditions you describe. Audi claim that this is
because ABS-off and wheels locked causes a wedge of snow to build up in
front of the tyre abd hence the car slows more quickly than with ABS
activated. Maybe, but I personally think the nub of the problem is that
the ABS is configured to cover the sort of road conditions that most
users encounter most of the time i.e. between bone dry through soaking
wet with the treachoursly slippery /greasy neither one thing nor the
other in between. Iced, slippery snow offers way less adhesion than a
road surface that is just wet or  greasy but only such conditions only
rarely ocurs during actual usage for the majority.

There's only one control system in the ABS i.e. they don't give us a
general / snow option and the designers have optimised it for the
majority user. It's my guess that they could have improved it's
performance in snowy conditions but only at the expense of less than
optimum performance elsewhere. Like most control systems, the smaller
the operating range the more accurately you can control perfomance
within that range. Or, for a given control system, it's a lot easier to
optimise retardation within say the range 1g - 0.8g than 1g - 0.1g.
(These decelerations just for illustration, I don't claim any knowledge
of what real world retardations are!). The analogy is a little crook
anyway as the link between retardation and change in wheel speed which
is what the ABS is actually measuring.

Just my =A30.02 worth and I now lay myself open to rubbishing by all of
you out there who really understand ABS!

Mark Schofield
'87 200 Quattro