>I remember reading an article in the early 80's on the Audi
>Quattro Sport Group B rally car. Which did have a switch to
>turn off ABS.
>
>The switch was because braking distances in gravel or dirt
>shorten if you lock up the wheels because you build a wedge
>of debris in front of the wheel that slows you down much
>faster. If you don't lock the wheels you don't get the wedge.

Also the tire is more likely to bite down through the loose stuff to
the hard-packed dirt below. Which is why rally cars are fitted with
narrow tires for dirt sections, as these don't "gravelplane" as much
as wide tires.

>That said, I don't think that ABS shorten's braking distances
>at all. You stop pretty fast when you lock up the wheels...
>you just can't steer your vehicle and are dragging (and flat
>spotting) four erasers down the road. If your attitude wasn't
>perfectly straight when you locked you can also expect that
>the vehicle will swerve when you lock.

a good ABS system can hold the wheels just on the shy side of lockup,
giving max retardation without danger of actually slowing the wheel
rotation below roadspeed, resulting in ideal stopping distances.
Less-effective systems may well give less retardation, thereby
increasing stopping distances. Of course steerability is vitally
important, and any ABS system should provide this.

>My mid 80's Chevrolet Caprice Classic Wagon (complete with
>fake wood panneling) used to lock its rear wheels all the
>time. It forced me to get pretty damn good at threshold
>braking, that is braking at the limit right before lockup.

My diesel Skyline was atrocious for locking its FRONT brakes, despite
the heavy engine. Nearly rear-ended other vehicles several times!
--
Regards
Andrew