Re: Stopping distances on snow/ice: ABS or not?

[Phil and Judy Rose <pjrose@servtech.com>]

I recall reading explanations of ABS in the terms that Paul has used. And
it is a very plausible theory--for so-called "dry friction". However when
slick (lubricated) surfaces are involved--even gravel-lubricated ones--I
suspect that the differences between static and kinetic (dynamic)
frictional coefficients become much less significant. Thus with
lubrication, some other factors such as the non-ideal ABS "implementation"
that Paul mentions could become dominant and may help to put ABS braking at
a relative disadvantage. Like Paul, I also wonder if Audi, or any other
manufacturer for that matter, makes use of "better than average" ABS
technology?

Phil Rose


>Date: Tue, 9 Dec 1997 20:46:12 -0500
>From: Paul Holzworth <hph@fns.com>
>Subject: Re: Stopping distances on snow/ice: ABS or not? (longish)
>
>Bringing up some old college physics:  The idea is that the coefficient of=
> friction (COF) of a locked up tire dragging across the surface is less than=
> the COF of a non-locked tire (dynamic vs static COF).  The stopping force=
> is maximally the weight x COF.  The static COF for materials is always=
> (almost always?) greater than the dynamic COF.  For maximum braking=
> ability, you want to brake to the point that the tire is just beginning to=
> lock up and then let the pressure off for just enough time to make sure the=
> tire doesn't lock and then apply it again.  The duty cycle is not really=
> relevant because you have reached the maximum braking force at the instant=
> the tires start to lock.  Any further braking pressure will put you into=
> the realm of dynamic COF where the friction (braking action) will be less. =
> This is the theory.  Now of course the implementation may not be effective.=
>  For instance, if you can't modulate the pressure fast enough, you may=
> allow the brakes to lock temporarily or you may not reassert braking=
> pressure quick enough.  I guess the conclusion is that ABS may or may not=
> be more effective depending on the implementation.  In theory, it certainly=
> can be more effective to use ABS.   Does anyone know more about Audi's ABS=
> such as modulation rate, sensor mechanism, etc?=20
>Paul Holzworth

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