From: rhaar@albert.cs.gmr.com (Bob Haar)
Newsgroups: rec.autos.tech
Subject: Re: How anti-lock brakes work (may start an in
Date: 16 Nov 92 19:55:48 GMT
Reply-To: rhaar@albert.cs.gmr.com
Organization: G.M. Research and Environmental Staff

In article 24876@CSD-NewsHost.Stanford.EDU, sankar@Xenon.Stanford.EDU (Sriram Sankar) writes:
|>
|>I recently browsed through the discussion on wanting to disable anti-lock brakes, but
|>did not pay as much attention as I should have.  But then I got thinking about how
|>anti-lock brakes really work, and realized I have this whole bunch of questions.
|>

You have asked for a lot. If you want a good overview of ABS systems, I
suggest reading "Understanding Automotive Electronics" by Wm. Ribbens,
published by Howard W. Sams & Co.

(Also as a general suggestion, your message had a number of long lines. 
This makes it difficult to read or reply to on some systems. Try keeping
you lines to less than 80 characters.)

|>I'll use my car as an example - a Saab.  Its got anti-lock brakes, I believe on all
|>four wheels.  Its got three hydraulic circuits - one for each front wheel and one for
|>the rear wheels.  The anti-lock brake algorithm gets input on the speed of each wheel
|>about 12 times per second and uses this information to determine if a wheel is skidding.
|>At some opportune moment (don't know exactly when), it decides to start pumping the
|>brakes on the wheel that is skidding.  Now suppose one of the rear wheels is skidding,
|>say I'd driving on an icy road and one rear wheel is on ice and I apply the brakes.  So
|>that wheel skids.  Suppose the computer decides to pump the brakes on that wheel.  How
|>does this affect the braking of the other rear wheel which is on the same hydraulic
|>circuit? (and possibly on asphalt, so far from skidding).

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.

There are also one and two channel systems, mainly used on pickup trucks
where (when unloaded) the back end is much more likely to lock up.
Of course, a two channel system is sufficient for a motorcycle.  And there
are schemes for tractor-trailer setups that use more channels.

|>
|>So now I have the following questions:
|>
|>1. At what point does the brake pumping start?

The exact decision depends on the implementation, but it is generally
done at the point where lockup starts to occur. This is determined
by detection a rapid change in wheel rotational speed. Note that
the maximum braking effect is achieved when there is some slip ( 5 to
20 percent depending on conditions), so ABS controllers are designed
to work with a small amount of tire slip, but to reserve some traction
for steering.

|>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.

|>
|>Now lets go to another car, say an Accord EX, Civic EX, Integra GS, etc. that have anti-
|>lock brakes.  Some of these cars have drum brakes on the rear wheels, some have all four
|>disk brakes.
|>
|>So questions:
|>
|>3. Do drum brakes have the anti-lock feature?

Sure. As long as they are hydraulically actuated, there is really no difference.

|>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.

|>
|>Suppose these cars have only two hydraulic circuits and the answer to question 2 above
|>is NO.  Suppose one hydraulic circuit controls the front right and the rear left wheels
|>while the other controls the front left and rear right wheels (like many cars do).
|>Which leads to a question:
|>
|>5. How many hydraulic circuits do these mid-range cars have?

GM's low-cost ABS VI system uses the four channel approach. I believe 
that is the same for most car manufacturers today.

|>
|>To continue, suppose one of these cars is driving on an icy road and its right wheels go
|>into the shoulder which is icy, and the brakes are applied.  Both the right wheels skid
|>and the other two wheels are still firmly gripped in asphalt.  What if the right wheel
|>brakes are pumped by the anti-locking feature?  This effectively causes all four wheel
|>brakes to be pumped (note I assumed answer to 2 was NO).  My guess is that this will
|>severely decrease the braking effect as compared to not pumping the brakes at all.

No. In fact, this kind of split-mu situation is one where ABS can out-brake
the best driver because it will get the maximum braking from all four
wheels at the same time as allowing the driver to maintain control. Without ABS,
the driver would have to choose between locking up the slippery side or braking
with less than maximum capability on the dry side.

|>So in conclusion, I hope experts in this area start posting informative messages!

There has been much discussion of how ABS works and its advantages/disadvantages
in this group in the past couple of months. I am sure that you will see 
posts saying that an expert driver can brake faster without ABS.  This
may be true in very limited situations. But what ABS systems are designed
to do is maintain control and stability while get near-maximum use of the
braking potential.

---
	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.


Organization: Sophomore, Physics, Carnegie Mellon, Pittsburgh, PA