I guess most of the time when we emergency brake the front and left
wheels will be on surfaces with similar grip; if one wheel is on (say)
an oil patch it will soon move off it and onto tarmac with similar grip
to the other wheel.
If one wheel was on a more slippery surface, the car would tend to yaw
towards that side, because (ABS or not) the wheels could not provide
such a large braking force. For the car to yaw significantly, the rear
wheels would have to lose grip, but the ABS should prevent this by
releasing the rear wheels sufficiently that they continue to turn, and
grip the road, and act kind of like a rudder to keep the car
straight-ish. However, I'd still expect the car to yaw to some extent,
and I'd also expect the steering wheel to pull towards whichever front
wheel had more grip, assuming the tyre contact patch was outboard of the
trunnion axle (i.e. excluding the Citroen mentioned earlier). This
steering pull, unless counteracted by the driver, would only make things
worse. This assumes that the ABS works on each wheel individually; if
the ABS released BOTH front brakes when EITHER front started to lose
grip, the braking force should be pretty much equal on both sides,
regardless of a difference in surface - maybe that's what they do?
EBD should help this by detecting the yaw towards the slippery side, and
increasing the braking force on that side (or reducing it on the grippy
side) so the braking force is equal on both sides and the car pulls up
straight.
Now I want to go and pour old engine oil all down one side of the road
past my house and do a bit of experimentation ;) Oh, but none of my
cars have ABS - damn :( I used to have an Audi 100 Quattro with ABS -
one time the roads turned to sheet ice around here, and the ABS
basically refused to slow the car at all, in order to keep the wheels
turning. Turning the ABS off wasn't any better, the wheels locked, I
didn't slow down any more effectively, but I lost what steering I had.
With the Quattro I was still doing a lot better than most of the cars
around me at the time!
Richard & Sammy ('73 Black Tulip BGT)
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