In a message dated 01/08/2000 6:48:33 PM Pacific Standard Time,
cdsorkin@ix.netcom.com writes:
> Unrelated question about ABS systems: On normal dry road surfaces, with
> properly functioning brakes, doesn't an antilock braking system cause the
> car to require a LONGER stopping distance?
>
Decreased stopping distance is not design function of ABS. What ABS is
designed to do is to prevent the wheels from locking (this is why they call
it ABS not shorter stopping thingy:-) The second design criteria is that the
system work at all speeds and going around corners as well as in a straight
line. (it does disconnect at a couple of MPH where it is immaterial) Lastly
the system must be automatic and not require any special driver intervention.
Stopping distance is a function of traction of the tire against the road
surface. As you step on the brake the tire starts to slip on the road
surface (slip is fairly constant, for instance when going 60 mph on a dry
flat road your drive wheels are going about 61 to overcome drag) As slip
increases stopping force increases, and stopping distance decreases. To a
point. When slip exceeds about 15% braking force starts to decrease this is
commonly called a skid. The further slip goes above the 15% the longer the
stopping distance becomes. The greatest stopping force is generated just
before the tires start to skid (keeping the wheel slip in the 12-15% range)
Now can a non-ABS car out stop an ABS car?
Well, that depends on a couple of factors, first would be the car's ABS
system. Volvo (the guys that pay my salary) make a brake system that will
stop on a dime and give change. I have driven rental cars, on the other
hand, that when put into ABS (a guy cut me off) made me think that I could
drag my feet and do better. The second item is just how good is the driver
of the non-ABS car? I know that everyone thinks that they are superb
drivers, and in truth this list's drivers are, I'm sure, better than most,
but, and this is a big but, I have to ask this question. When presented with
an emergency can you absoulty and positively guarantee that you can brake the
car to the 12-15% slip range and modulate the brake and hold it there without
going above 15% slip and going into a skid? I know race drivers can, in a
race, but can you when presented with an idiot who pulls a U-turn directly in
front of you (like a melon head did to me tonight)
One more point a race driver can modulate a brake pedal about 5-6 times a
second, pretty good huh? A modern ABS system can modulate each of 4 wheels
indepently up to 15 times a second.
So in answer to your question I would say that most of the time (99.9%) given
a good ABS system, the ABS car will out stop a non-ABS car. This answer
applies to pavement only (dry or wet) snow, gravel, and sand ABS will give a
longer stopping distance. The only advantage of ABS in snow, gravel, or sand
is control. To get the shortest stopping distance in snow you must lock the
brakes and build a pile of snow in front of each wheel. However when the
wheels are locked you have no steering control. ABS may take longer to stop,
but you maintain control.
Now for a trivia question what does ABS stand for?
Sorry for being so long winded, hope this explains a little,
Rick Ewald
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