Paul -
Aha! You've discovered ABS's weak point. Allow me to ask a question:
How does ABS know that a wheel is locked? Think about this for a second
before reading the next line.
Ready for the answer?
By comparing rotional speeds of the four wheels.
Now for the next question: How can an ABS system tell the difference
between these two states (a) all four wheels locked and sliding, and (b)
stopped?
Answer? It can't. And, until they come up with a sensor to pick up on
the abruptly rising adrenaline level in the driver's bloodstream, no
amount of software tweaking will help. (Okay, a fuzzy logic system that
compared pre-braking wheel rotation speeds to post-braking wheel
rotation speeds, AND from that figure out that the indicated rate of
deceration was impossible, AND conclude correctly that you were indeed
sliding, COULD do it, but don't count on that getting implemented for a
while.)
Herein lies the problem: ABS (current technology) helps a bunch when
you're in a slightly tricky situation (one or more wheels sliding),
particularly if its the front wheels sliding and you need to steer. BUT
it can engender a false sense of confidence in the machine's ability to
take care of you, which will fail you totally when you are in a
situation where you really need all the help you can get (all 4 wheels
sliding).
In effect, it raises the threshold at which one is aware one is in
trouble to the level at which uyou are in REALLY deep doo-doo. Some
people prefer a car with high adhesion at the limit, even if it gives
little warning of an impending slide, others will trade a little
adhesion for a little warning.
ABS is okay, so long as you are fully aware of its limits. I happen to
think that too few people are. I'm still not convinced that it's better
FOR ME, than a close awareness of the capability of my car's braking
systems, unenhanced by a computer.
Chris Kotting
ckotting@iwaynet.net
> Date: Thu, 20 Jan 2000 19:46:00 -0500
> From: cgmoog@ibm.net
> Subject: Re: ABS experience (long)
>
> My limited experience with ABS leads me to believe that its is good in
> most but not all situations. About 5 years ago I was on a business trip
> to Rochester and Buffalo. Left Rochester in a rental GM vehicle
> (A-body) with ABS. The NYS Thruway was closed east of Rochester due to
> heavy snow.
>
> The ABS allowed me to brake much more aggressively then I would have
> thought possible (I'd apply the brakes until I felt the ABS cycle and
> then ease up slightly).
>
> Once in Buffalo with snow still falling I drove around the city
> completing my work. At one intersection (which I was approaching at ~10
> mph) I braked normally to stop for a stationary snowplow. Applying the
> brakes did not slow the vehicle at all. I kept pressing the brakes
> harder and harder while rolling toward the plow. I was about to through
> the transmission into park when the pedal suddenly sunk and inch or so
> all four wheels locked up and I slid to a halt inches from the plow.
>
> Further experimenting showed that in very low friction driving (at low
> speeds) I could get the ABS to malfunction in one of ten stops.
>
> So while I liked it in most situations I would rather have a switch to
> disable it (or be convinced that the software has improved since the
> early 90's) before I'll purchase a vehicle with it.
>
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