From: crash@hal.COM (Chris Craddock)
Newsgroups: rec.autos
Subject: Re: Anti-Lock Brakes makes stopping SAFER?
Date: 30 Dec 92 23:09:32 GMT
Organization: HAL Computer Systems, Inc.
NNTP-Posting-Host: sydney.hal.com

In article <CARL.92Dec30143412@atlantis.Cayman.COM>, carl@Cayman.COM (Carl Heinzl) writes:
|> 
|> [original deleted]
|> 
    [I wrote...]
|> >We have heard how ABS kicks in "only when the brake locks" - see (1) above.
|> >And that it cycles furiously until you've come to a complete smoking halt
|> >- see (2) & (4) above. And that it can't tell of impending lockup -see (3).
|> 
|> >But does it?

   [in reply, Carl sez...] 
|> Yes, it does!!!
|> 
|> >If that was all there was to it, most "real" ABS systems (i.e. not the 
|> >simple minded rear brake proportioning valve that Chev and others claim
|> >as "rear wheel ABS") simply could not perform the feats they do.
|> 
|> WHY?

I went on to explain why. You deleted the explanation in your "reply"

|> You've made an awful big statement here with no justification
|> whatsoever...

Actually, I gave a fairly complete set of reasons.

|>I was going to put a very sarcastic statement here but
|> rather than be obnoxious I really would rather have an intelligent
|> discussion.

Good! Your original post criticized someone else for not understanding ABS.

|> Can you tell us just *which* cars have BOSCH and TEVES systems???

Bosch systems are standard equipment on Mercedes-Benz, BMW and many
other high performance cars. The TEVES system is used in the Corvette. 
I don't know of any others, but since they stay in business, there must 
be some. (I think they use a modified TEVES in some european trucks).

|>What are their stopping distances compared to mine (Acura Legend)? Of
|> course we should really compare their stopping distances on ice too
|> (which we can't do).

Stopping distances depend on many factors - raw data isn't very useful,
even when available. In any case, the cars mentioned above are respected
for their braking abilities too :O) Also, I made reference to two specific
systems (which I am intimately familiar with). Yours may well be different.

|> I just called my Acura dealer (Acura has their own ABS) and the system
|> on my car (Legend) does indeed "lock up/release brake pressure", at 10
|> times/second and I am able to perform the feats that you said
|> "couldn't be done"...  

I did *NOT* say it couldn't be done. I said your explanation of ABS
behavior is wrong - because it *is* wrong. I was also making the point that
the two specific ABS systems I mentioned are a lot more complex than the 
people in this thread seem to think. They get their good results *because* 
of this extra sophistication - not by just blindly cycling on and off!

As I said before...

|> >are. The "pulse" is a momentary *release* of brake line pressure - not a 
|> >momentary application of pressure. 

The difference is subtle, but think about it. There is a difference
between "grab, grab, grab" (what you're saying) and "release, release..."
which is actually what happens. The ABS isn't supplying the brake pressure,
YOU ARE. All the ABS does is momentarily defeat you by releasing pressure
at the wheel which is "out of range".

After the "release" the ABS checks to see if the wheel speed is within the 
range indicated by its other sensor inputs. If its not, it releases again 
- and it may do this many times per second. However, if it *is* within range 
it doesn't release at all. Then you're getting whatever braking effort you're
applying through the pedal.

|> Gee, if note(2) (the pusating action of releasing the brake pressure)
|> isn't happening then the ABS isn't really being active then is it
|> (other than monitoring the situation and getting ready to act), so I
|> fail to see that (2) isn't necessarily true and all your note(4) had
|> to do with was the *rate* at which the pusation happens.

NO. I did not say the pulsing isn't happening - after all that would be
pretty stupid. I was trying to explain that the two systems I mentioned
don't just cycle blindly until you take your foot off the brake. AND
that they decide what to do based on more information than just "is the
wheel stopped?". 

When the brakes are not applied, the ABS system is "passive" - its just
sampling its sensors. Once the brakes have been applied the system notices
and keeps checking its sensors. If any of the inputs get out of shape, it 
can take corrective action *and go back to passive mode* even if you've
still got the brake pedal mashed against the firewall.

Furthermore, the wheel doesn't have to come to a complete stop before
the ABS figures out something's wrong. You said "it can't sense impending
lockup" which is bullshit that's exactly what they do. They work by sensing 
impending lockup. They can't necessarily prevent it, but they can certainly
sense it.

|> So, on my car, which performs fine under all conditions everything
|> that I have said is true, including (1), (2), (3), and (4)...

I am not finding fault with your car's performance - just your explanation.

|> One other point.  The ABS systems that large trucks have on them most
|> certainly DO lock up the wheels and release them because even when
|> they work, they leave skid marks which would NOT happen if they
|> recognized impending lockup before it occurred.

This is quite silly - the systems are not necessarily the same, and in
any case, the observed results are not necessarily inconsistent. If the
re-application of braking pressure causes the wheel to lock in less time
than the cycle time of the system you will see exactly the phenomena you
described.

Feel better now?

Chris
--
"sometimes I think that the surest sign that intelligent life exists
elsewhere in the universe is that none of it has tried to contact us"
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