© 1997-2008 H. Pietschmann
Why ABS on a dirt road could be a disaster
ABS was designed to keep wheels from locking up. ABS was designed to make driving safer.
Locked up wheels are not steerable. For example, you would not be able to steer around an obstacle with locked wheels - with wheels locked you would slide straight into it. "Not steerable" means that even though you are able to turn your steering wheel and the tires are actually turned as well - the vehicle does not follow this command.
Most ABS computers also achieve maximum friction between tire and ground for short stopping distances. *
Sand and gravel under your tires act like tiny ball bearings when you step on the brakes. As a result there will be very little friction/traction between tires and ground.
Therefore all four wheels want to lock up immediately.
ABS keeps them from doing so.
So you roll and roll, and roll, and roll. You are possibly rolling too long and your stopping distance is going to be very long. Too long to be safe.
To deal with this known shortcoming of ABS, some manufacturers had installed a kill switch for ABS in their four wheel drive vehicles (for example Mercedes-Benz G-Class) but recent changes in the law prevents them from providing this option any longer. Seasoned four wheelers have either installed their own kill switch on the dash board or simply pull the ABS fuse before hitting long stretches of dirt road.
Here is what they get with ABS off: When you hit the brake good and hard the wheels will lock up (this time locking wheels are good for you) and start sliding - but the tires will immediately begin pushing sand and gravel - this builds a little berm in front of each tire which very effectively slows down the vehicle.
In fact, stopping without ABS on sand or gravel is so rapid that you better have your seatbelt on - otherwise you will end up hitting your nose against the windshield.
*Some ABS systems prevent locking wheels but not really for shorter stopping distances. In fact some create substantially longer braking distances.
By the way:
Aside from ensuring that you can steer your vehicle even under heavy braking ABS will keep your vehicle from fishtailing.
Another safe way to avoid fishtailing is: "Slow down!!"