ABS Brakes are simpler than friction, so I'll take a stab at it. First very
few cars have 4 wheel anti-lock brakes. Most have only rear anti-lock brakes
know as RABS. The advantages of front anti-lock brakes are marginal at best
and for a skilled driver may even be a disadvantage. RABS will almost always
be a help, even for the best driver. Here's why. When a car brakes the
weight shifts to the front wheels. The harder the stop the greater the shift.
A conventional brake system uses proportioning valves matched to piston diamete
rs to balance the front/rear braking for some average kind of stop. If you
stop faster than this the weight shifts more onto the front wheels and the
rear brakes lock up. This is bad. If you adjust the proportioning valve so
there is less pressure to the rear, the front wheels have to do more of the
braking, but in a moderate stop where they experience less downward loading
they will lock up. Bad again. To compound the problem we even want our brakes
to work with or without passengers, stuff in the trunk, etc. One solution to
this is to employ an adjustable proportioning valve that can sense the load
and weight shift and adjust the rear brake pressure accordingly. These were
widely used on pick up trucks and vans before RABS became common, but I've
never seen them on cars. I believe Ford still uses this system on their
super duty trucks. The mechanism is simply two rods with a hinge in the
middle connected between the real axle and a frame mounted valve body. As
the body moves relative to the axle, the motion of the rods adjusts the
proportioning valve. However, there are still a lot of factors such as
road conditions, lean of the car, etc. for which there is no adjustment. What
adjustment is offered is crude at best, adjustment of the mechanism is
quite critical, and it is quite exposed. RABS uses electronic sensors to feed
rotational speed of the wheels to a computer. If the computer discovers
that the wheels are not turning fast enough it cuts back the pressure to the
rear wheels until they catch up. The adjustment is very fast so slip is kept
to a minimum and control is maintained, yet at least in theory, something close
to optimal rear brake pressure is also maintained. In my experience, there is
a noticable loss of deceleration when the RABS kicks in, so there is room for
some refinement in the system. One nice feature of most RABS is a self test
system. Since most of the braking is done by the front wheels one can easily
be driving a conventional system with no rear brakes and never no it. RABS
checks itself out on start up just like a pc. If it isn't happy with itself
a dash light stays on. To find out what's wrong you ground a test lead and
count the number of times the light blinks. Then look in a book and it tells
you what to replace. If you want more details I can give them to you for Ford
products as I have ready access to shop manuals for all late model Fords. I
can only assume that other mfgrs. use similar systems.