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What Bob said... Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Mon, 5 Feb 2001 09:11:25
In Reply to: Stupid (non-saab) ABS question?, NewSaab, Mon, 5 Feb 2001 01:48:23 Members do not see ads below this line. - Help Keep This Site Online - Signup |
is correct, as I understand it.
In theory, a 'utility' vehicle is designed to carry a range of loads. Yes, most of the time it just carrys the driver and a Big Gulp, but on occasion folks load them up, just like in the commercials. If the back end is empty, under breaking it is much more prone to lift, and when it lifts, it unweights the rear wheels. With less weight on the rear wheels, they're more prone to lock up under breaking, and once they lock up, the rear can skid and the vehicle spin.
Rear-only ABS is a way to minimize this, and is a lot cheaper than four wheel systems. This trick has been done for years - many cars dating back to the 70's (that I know of anyway) had a 'rear proportioning valve' - basically, a lever ran from the rear axle to a valve on the chassis. The valve controlled pressure to the rear brakes. If the distance between the axle and the chassis increased (as in hard braking, and the rear lifts), the lever would move, and the valve would reduce pressure to the rear wheels. Not true ABS in that it didn't detect a wheel locking, but it helped some.
The coefficent of sliding friction is much less than that of static friction. To stop a car, you're transferring forward 'energy' into heat in the brakes, but it requires gripping the ground through traction. If the fronts lock, they slide. The braking is reduced. Bummer. Don't lock up your brakes. But if the fronts slide, the car will tend to understeer. If the rears slide, the car will tend to oversteer (spin). In the case of a tall, tippy SUV, that spin could become a rollover, and pretty soon you see "DateLine NBC" in your shorts.
The ideal is 4 wheel ABS, but rear-wheels ABS is better than nothing, and much cheaper.
, Mon, 5 Feb 2001 09:11:25 <-- Viewing This Message
, Mon, 5 Feb 2001 06:30:45
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