From: rhaar@albert.cs.gmr.com (Bob Haar)
Newsgroups: rec.autos.tech
Subject: Re: More on anti-lock brakes
Date: 18 Nov 92 13:44:12 GMT
Reply-To: rhaar@albert.cs.gmr.com
Organization: G.M. Research and Environmental Staff

In article 16714@CSD-NewsHost.Stanford.EDU, sankar@Xenon.Stanford.EDU (Sriram Sankar) writes:
|>
|>Ok, here is one more posting from me on anti-lock braking.  I've included
|>some text from previous postings in response to my original posting for
|>context.  I learned a couple of new things yesterday to add to this
|>discussion.  The details follow the included portion.
|>
|>> What you have described is called a three channel system. It treats
|>> the two back wheels as a single entity and cannot independently
|>> control the braking of the rear wheels. If one starts to slip, braking
|>> force is reduced at both wheels and you lose some braking ability.
|>> 
|>> Most passenger car systems are four channel - they have separate wheel
|>> speed sensors for each wheel and can modulate the hydraulic pressure
|>> at each wheel.  These systems are quite good at handle the split-mu
|>> (high traction on one side, low on the other) situation that you
|>> described.  A three channel system will allow you to maintain
|>> stability because the front (steering) wheels are not skidding. But
|>> you won't get quite as much braking.
|>> 
|>> |>2. Can the pumping be isolated to just one wheel and leave the
|>> |>   braking effect on other wheels as is even if they are on the
|>> |>   same hydraulic circuit?
|>> 
|>> Only in a fully independent four-channel system.
|>> 
|>> |>4. Do cars differ in the number of wheels on which anti-lock is
|>> |>   supported?
|>> 
|>> Yes. The system in your Saab is somewhat behind the times, but it is much
|>> better than no ABS at all.
|>> 
|>> 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.
|>
|>My car is a 1992 Saab 9000 with three hydraulic circuits for the brakes.
|>I learned yesterday that the 1993 Saab 9000's come in two forms - one
|>with three hydraulic circuits (just like the 1992), and the other with
|>just two hydraulic circuits (one for each front wheel and the diagonally
|>opposite rear wheel).  The three circuit version is used with models with
|>traction control.  Also, the 1992 Honda Accord user manual says that
|>it has two hydraulic circuits for the brakes.
|>
|>So, if the responses above are true, then:
|>
|>1. The Saab 9000 ABS which someone stated is behind the times is going
|>   further behind the times.  I can't believe that Saab would do this
|>   especially since they already have it and one of their main selling
|>   points is safety.
|>
|>2. If the Honda info is correct (I'm positive about the Saab info, but
|>   I don't trust a user manual to give accurate technical facts - so I'm
|>   not sure about Honda), then the Honda ABS is very primitive.
|>

I believe there is some confusion here between the base braking
systems and the ABS channels of control.

For many years, pasenger cars in the US have had dual braking subsytems
that amount to two independent hydraulic circuits. At the master cylinder,
there are two separate cylinders, that provide hydraulic pressure. One
circuit actuates the right front and left rear brakes; the other circuit
actuates the remaining two brakes. This criss-cross connection provides
the maximum vehicle stability if either one of the sub-systems is lost.
This is nothing more than a mechanical fail-soft system.

A four channel ASB system is superimposed on top of this basic two
circuit braking system. The ABS actuators can modulate the hydraulic
pressure to one of the wheels (say the left-front) without affecting
the other one on the same hydraulic circuit (the right rear).

---
	Robert Haar               InterNet : rhaar@gmr.com 
	Computer Science Dept., G.M. Research and Environmental Staff
DISCLAIMER: Unless indicated otherwise, everything in this note is
personal opinion, not an official statement of General Motors Corp.


From: jefftb@hpmwtd.sr.hp.com (Jeff Brown)
Date: Tue, 17 Nov 1992 21:18:45 GMT
Subject: Re: advice on engine manual
Organization: HP Santa Rosa Site (SRSD MWTD)