G
Greg Reed
!!bogus said:
Anti-lock brake systems work by employing wheel speed sensors that can tell
when a wheel locks up under braking. When this condition is detected, the
ABS system modulates brake pressure to the affected wheel (or sometimes to
multiple wheels, depending on how cheap an ABS system you have). This has
the effect of "pulsing" the brakes very rapidly, so that the wheel
alternately locks and is realeased very quickly. The point of this is to
retain steering control under heavy braking and, according to the
literature, reduce stopping distance. When the front wheels are locked
during braking, you have zero steering control. And when the rears lock,
your car is extremely prone to oversteer. For these reasons, I don't
contest that ABS achieves the first claimed benefit (of retaining steering
control). It is worth noting, however, that one can retain steering control
under hard braking *without* ABS by simply paying attention and modulating
brake pressure manually to prevent wheel lockup.
This leads me to the second claim about ABS -- namely, that it reduces
stopping distance. A car will stop faster with ABS than with the wheels
locked up, this I don't doubt. And so for a severely unskilled driver --
one who's incapable of anything more than simply standing on the brake pedal
when something unexpected happens -- ABS may well result in a shorter
stopping distance. But because the brakes are alternately engaged and
released with ABS, stopping distance is actually longer than if continuous
brake pressure is applied just below the point of lockup. (This is called
"threshold braking.") The benefits of threshold braking as compared to
ABS-controlled braking seems by my experience to be related directly to
available road adhesion. On dry pavement, the benefit of threshold braking
over ABS might only be 3 or 4 feet in a stop from 70 mph. But as roads
deteriorate to the snow-covered conditions we often experience in northern
Michigan's winter months, the advantage of threshold braking over ABS is
significant. It can literally mean the difference between stopping before
an intersection or continuing into the path of that big truck. I therefore
prefer vehicles whose ABS systems can be manually disengaged by the driver
from within the cockpit (as my Audi has). If you are driving an
ABS-equipped car in the winter months, I would strongly advise you to allow
*lots* of stopping distance. And if you see me and my Audi out on a snowy
road, don't attempt to keep up or to wait as long as I do to stop unless you
know what you're doing. You'll likely end up in a ditch or connecting with
another vehicle.
It is worth noting that any ABS system will only make itself known when it
senses wheel lockup. As long as you never brake hard enough to lock a
wheel, you'll never know that your car has ABS. ABS is nearly standard
equipment any more -- only the very cheapest of cars still don't have it.
You can probably take this to mean that it's not somthing you should worry
about. But as with any aspect of your car's handling characteristics, it's
important to know both the benefits and the limitations of ABS, and drive
accordingly.
Perhaps the best advice I can give regarding ABS or any other aspect of
driving is to not be afraid to find out what your car's limitations are and
what it does when you exceed them (under appropriate conditions, of course).
The experience will help you to avoid exceeding those limitations in
conditions when such shenanigans are not appropriate. But more
importantly, it will familiarize you with just what your car will do when
those limitations are exceeded. Such familiarity will help to remove the
"panic" feeling that so often paralyzes drivers in emergency situations,
thereby allowing you to do more than just mash the brake pedal to the floor.
It will make you a better -- and safer -- driver.
- Greg Reed
--
1976 Cadillac Fleetwood 75 9-Pass sedan
(FS: http://www.dataspire.com/caddy)
1989 Audi 200 Turbo Quattro 5-Speed sedan
2000 Oldsmobile Intrigue
2001 Chevy Astro AWD (wife's)
2005 Subaru Legacy GT Wagon (when available in U.S.)