Well, actually, Fred, GM quality control issues aside, you do NOT stop faster
when you
lock up the wheels. The kinetic coefficient of friction at the interface of any
two
materials you wish to name, but in this case, rubber and pavement, is ALWAYS
less than
either static coefficient of friction. When you are braking hard, just before
the point
of lockup, you are imposing the highest friction force (f =mu x F, where f =
friction
force, mu (pronounce mew) is the coefficient of friction and F is the normal
force,
essentially the weight of the vehicle) at the interface between the tire and
the pavement.
Once a tire locks up the lower mu value for kinetic friction comes in to play
and the
value of f is decreased accordingly. Not only that, but the tire is scrubbing
off little
roller bearing particles of rubber and/or generating a slippery layer of melted
rubber,
further decreasing the friction at the interface.
The whole idea of ABS is to allow the brakes to operate just at the edge of
lockup, where
braking is most efficient. The other benefit of ABS is the maintenance of equal
braking at
each wheel. If one wheel locks up before another, especially if it is a rear
wheel, you
are going to find your vehicle stopping, not only long, but in other than a
straight-ahead
attitude. Pickup trucks are notorious for wanting to swap ends under hard
braking because
the nose-heavy weight distribution (with no load in the bed) means that the
rear wheels
tend to lock up under heavy braking, resulting in a rear axle that wants to
travel faster
than the front. The result of this is usually a quick move on the steering
wheel to avoid
a spin ('steering into the skid' is what they called it in Driver's Ed lo these
many years
ago - panic-reflex accident avoidance is what it really is!).
The only place(s) where ABS is less effective than standard brakes is in snow
or gravel,
where the plowed up material in front of a locked up wheel aids braking.
Gary McCormick
San Jose, CA
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Fred_Katz@ci.sf.ca.us wrote:
> Hi Peter,
>
> [snip]
>
> Incidentally, I hate ABS. One time I was driving a company GM van with
> ABS and the brakes failed on a panic stop. I think GM may have resolved
> the problem but I don't trust ABS. There's nothing like locking up the
> wheels and screeching to a stop in a short distance, which I don't think
> ABS does well at all.
>
> Fred
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