From: sankar@Xenon.Stanford.EDU (Sriram Sankar)
Subject: re.  How anti-lock brakes work (may start an interesting discussion)
Originator: sankar@Xenon.Stanford.EDU
Organization: Computer Science Department, Stanford University.
Date: Mon, 16 Nov 1992 20:10:47 GMT


The following is a reply to my original posting.  He could not get it on the newsgroup,
so I'm posting it on his behalf.

Sriram.
=====================================

Date: Mon, 16 Nov 92 14:37:23 EST
From: biman@eleceng.ee.queensu.ca (Aapoolcoyuz)
To: sankar@CS.Stanford.EDU
Subject: How ABS works

I would post this to the newsgroup directly but my @#!$% news poster
is down.  So here is a personal email to answer your question.

The ABS system on most cars usually have 3 - 4 hydraulic circuits.  Each
front wheel gets one, and depending on the car, the rear wheel gets either
one per wheel or one total.  Each circuit is equipped with a separate
actuator which can release hydraulic pressure in just that circuit.

When you apply the brake, sensors read the wheel speed every 10-15 ms.
If the change in wheel speed exceeds an amount corresponding to 1.2g it
means that the wheel has locked and is skidding. (In non motor-sport cars
the wheel cannot decelerate at more than 1.1-1.2g while still rotating). 
The ABS then releases pressure in that circuit until the wheel starts 
turning again.  Then it  takes speed measurements again and releases 
pressure in which ever circuit requires it.  SInce all 3 (or 4) circuits
work independently, you ABS will work fine even if one wheel is on ice.
All that would happen is that since ice cannot support a large stopping
friction force, almost no stopping force will be present on the "ice" wheel
and most of the stopping will occur on the other wheels.  Mind you this
will lead to a large yaw moment on the car, and steering effort will be
required to counter this.  If one of the rear-wheels is on ice and
the ABS only has 1 circuit for both rear wheels, then both wheels will 
have brake pressure modulation equal to that required by the wheel with
the least friction  In answer to your question one circuit cannot modulate
different pressures to differnt wheels.  Both wheels get treated equal.  So
if one wheel is on ice, both are treated as if they are on ice.

Yes drum brakes support ABS, the drum brakes are generally connected to
one circuit since they are usually found at the rear.  (Back in the mid
to early 70's -- yes seventies --when ford first introduced "Anti-skid"
braking it was on a car with four drum brakes so is is possible).  

Nearly all cars which have ABS have it on all 4 wheels.  THe only 
exception is some full-size pick-up trucks.  These have it only
on the rear wheels, since under heavy braking some trucks have a 
tendency to let the rear end fish-tail. 

I have never heard of a system with only 2 circuits.  This would mean that
the front wheels would work with 1 ckt only.  Hmmm..this is not a good 
idea especially when trying to steer.  So that ice/ashpalt scenario you
describe is not likely to happen.

The main function of ABS is to provide steering control while fully braking.
Race drivers know how to threshold brake (so do certain road drivers), so 
as not to lock the wheels and still provide some steering.  It is the 99%
of other drivers who tend to slam the brakes (lock wheels) and still expect
to steer around an obstacle that ABS helps.  Another advantage to ABS is that
Dry and Wet stopping distances is minimized since the brakes keep the wheels
just on the edge of locking up.  This is where the wheel genreates most of
the stopping power. (Mu static is always greater thatn mu dynamic.  Mu is
the coefficient of friction.)    

Well that is my input on the matter...I'm sure that in the newsgroup
your posting is sure to start a good volley of ideas.


A. Biman
biman@eleceng.ee.queensu.ca
Newsgroups: rec.autos.tech