Re: ABS Stuff

[DeWitt Harrison <de@aztek-eng.com>]

On Sat, 15 May 1999 18:09:37 -0600, Al Powel wrote:

>A fair question.  i have been reflecting on this for a while.  The fact 
>is that using ABS allows you to stop a car with steering ability and 
>directional control, BUT....not NECESSARILY as quickly as if you 
>locked the wheels.
>
>Granted, once a tire stops turning and slides, the friction generated 
>IS less than that of a tire turning, but right at the verge of locking 
>up.  (Bondurant and others will gladly teach you this "threshold 
>braking" technique for non-ABS brakes...for $$.)
>[ ... ]

I'm not as confident as you are that a locked up tire produces less
friction than a "threshold" rolling tire. I admit that I have seen graphs
of cornering force vs. slip angle which show clearly that when you
push a tire too hard in a turn, it "lets go." But can we extrapolate
that to straight line braking? I don't know. The dangers of flat spoting
the rubber and losing control may be the overriding motivations for
threshold braking skill.

A couple somewhat related observations from experience: ABS is
great on the track (I don't know about autocross.) It largely removes
the need for driver adjustable front/rear proportioning so you can
track your daily driver and still get reasonable performance. ABS
keeps the rear inside wheel rotating during trail braking, etc. In
general, it reduces the skill factor while increasing the fun factor.
This may get a howl out of the purists but, hey, the technology
is there. Should I disable the quattro too?

Stopping distances on icy streets increase very noticably with
ABS active. The first time you glide majestically through a stop
sign at 2 mph going "dugga, dugga, dugga, ....." the fact that the
car remained straight will seem less wonderful and you will
find yourself punching off the goddamn ABS shortly thereafter.

DeWitt Harrison
Boulder, CO
88 5kcstq