Steve Shoyer wrote:
> The way it's been explained to me, if you lock up the wheels when stopping
> on snow/sand/gravel, a wedge of snow/sand/gravel is created in front of the
> tire from being pushed, which helps to stop the car. With ABS, because the
> wheel never locks up, it continues to roll over the snow/sand/gravel, the
> build-up doesn't happen, so the car doesn't have that additional friction to
> slow it down. Something like that.
>
> --Steve
Not completely true, Steve. What normally happens when the brakes of a
non-ABS vehicle lock is that the rear wheels lock first and the vehicle tends to
swap ends-dry pavement or ice-it doesn't matter. When that happens the benefit
of added friction is negligible and there was very little friction to begin with
since the tire is now riding on a thin layer of melted rubber. Figuring dry,
worn concrete has a coeffiecient of friction of 60-75 % on a good day, adding
sand, gravel, or snow only reduces the coeffiecient of friction. Conversely,
adding sand or cinders to a roadway covered w/ glaze ice (COF of 10-15%) can
only increase the friction because there was precious little to begin with.
Now if the sand or gravel were deep enough like a truck runaway ramp in the
mountains, then whomever told you about sand and gravel would be right. The
vehicle sinks into the aggragate far deeper than sliding along w/ the material
only building a little and then coming out from around the tire.
You are right about the ABS vehicle not building up the additional
friction. The plan is to maximize the braking power w/out locking up the tires,
as you note. Some folks think the ABS works like "pumping the brakes" (am I
dating myself?) but it does not let the brakes ever lock but releases pressure
on the brake just short of locking.
I had a '71 Lincoln Continental Mk III in the early 70's. It was equipped
w/ a Kelsey-Hayes setup that was a fairly primative anti lock brake system for
the rear tires. It made one heck of a racket when it activated but one could
not lock the rear brakes. The idea is not as new as some might think.
Obligatory MG content: tomorrow is supposed to be 60 and fair around here.
It might be a good day to spend some time on the V8 project!
--
Larry Dickstein
Lone Jack, MO
|