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Originally Posted by Zappo In doing the above the ABS unit (the ECU) also ensures that the car does not go into a skid because of the locked wheels. Now in a loose gravel, snow etc kind of situation where the friction offered by the surface to the rubber is significantly different from the standard surface (the reference point for most of these lookup tables must be based on the tar or concrete roads I would assume) one may find that though the car wheels are not exactly locking in and thus the car does not skid the car also does not stop as quickly either.
On the other hand, a non ABS car on such surfaces will have its wheels locked and may start fishtailing but still it will probably stop much quicker as the wheels would have completely locked (in emergency braking). Remember the manual says that the braking distance for an ABS car will be more on such surfaces in comparison to a non-ABS car but it does not say that in doing so the non-ABS car will not do any theatrics. It may stop but you may find yourself facing the way you came (particularly on loose sheet ice, snow etc). And in extreme cases where you are driving a non-ABS barge (say a tall SUV kind of vehicle) you may even have a scope of the fishtailing vehicle topling over sideways during the skid. |
There is no reference point for ABS systems. Like you pointed out earlier, ABS ECUs just keep a check on the differential speed of all the wheels and when one wheel's speed is significantly out, it cuts in and un-brakes the wheel. Another point that is often overlooked is that, once this un-braked wheel spins up again,and it's speed is again greater than the other wheels, ABS again kicks in and brakes this wheel. That's why the pumping sensation.
A non ABS car doesn't stop because the tyres are locked in. It is important to understand that a skidding vehicle will always have a greater stop distance compared to a correctly braked vehicle.
An explanation to this is that, in a non ABS system, the tyres, because of getting jammed, dig into the underlying soft surface (dirt/gravel/snow) and hence the stop distance is lesser. ABS prevents this from occurring since the tyres never lock up. Some ABS calibrations reduce this problem by slowing the cycling time, thus letting the wheels repeatedly briefly lock and unlock. Some vehicle manufacturer provides an "off-road" button to turn ABS function off. The primary benefit of ABS on such surfaces is to increase the ability of the driver to maintain control of the car rather than go into a skid — though loss of control remains more likely on soft surfaces like gravel or slippery surfaces like snow or ice. On a very slippery surface such as sheet ice or gravel, it is possible to lock multiple wheels at once, and this can defeat ABS (which relies on comparing all four wheels, and detecting individual wheels skidding). Availability of ABS relieves most drivers from learning threshold braking.
( wikipedia + allied info)
Last edited by neel385 : 1st June 2009 at 20:59.
Reason: placed reply in quote
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