From: carl@Cayman.COM (Carl Heinzl)
Newsgroups: rec.autos
Subject: Re: Anti-Lock Brakes makes stopping SAFER?
Date: 30 Dec 92 19:34:12 GMT
	<CARL.92Dec30105349@atlantis.Cayman.COM> <1hsnhvINN4sn@hal.com>
Organization: Cayman Systems Inc., Cambridge, MA
In-reply-to: crash@hal.COM's message of 30 Dec 92 17:50:55 GMT
bcc: carl


>|> 
>|> >  ABS moves keeping the wheels from locking, a skill most drivers are not
>|> >  very good at, to the car/computer.  The computer detects when the wheels
>|> >  are ABOUT to lock, most drivers aren't going to notice anything is wrong
>|> >  until they are alreay starting to skid.  Think about it, why should you,
>|> >  the driver, have to brake any differently just because the road is wet?
>|> >  Isn't this something the car should deal with?  I think so.
>|> 
>|> Your lack of knowledge how ABS truely works is showing through here.

>Be careful! Someone may start making analogies about pots and kettles!

no need to evey reply to this.  

>|> ABS waits until the whell *DOES* lock up and then, by detecting the
>|> difference of rotational speeds of the wheels releases the brake fluid
>|> pressure on the wheel that has stopped rotating.  
				  ^^^^^^^ -note(1)
>|>It virtually
>|> immediately (for this discussion) reapplies it, up to the point of
>|> locking up, releases it again, etc.  
	       ^^^^^^^^^^^^^^^^^ -note(2)	
>|>There is no simple way that ABS
>|> could tell that the whell is *ABOUT* to lock up (short of measuring
>|> the coefficient of friction of the road at the exact contact point of
>|> the tires and calculating the acceleration of the vehicle at each
>|> wheel, etc...
 		^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ -note(3)
>|> Now, ABS systems pulse at different rates, some pulse 10 times per
>|> second, some pulse at 100 (or several hundred) times per second).
	        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ -note(4) 
>|> This is also how many cars implement traction control.  They sense the
>|> rotational speed of the wheels and when one (or more) wheels suddenly
>|> accelerate (and the others don't) the brakes are applied to those
>|> wheels to bring the rotational speed back to normal.
>|> 
>|> -Carl-


>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?

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?

You've made an awful big statement here with no justification
whatsoever...  I was going to put a very sarcastic statement here but
rather than be obnoxious I really would rather have an intelligent
discussion.

Can you tell us just *which* cars have BOSCH and TEVES systems???
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).

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 have used the ABS (mostly in testing) on dry
surfaces, mixed traction surfaces (one side of the car had traction,
the other didn't), and in some snowy/icy conditions.

Oh, my system *is* a multi-channel system with each wheel being
independantly monitored...

>So (1) and (3) aren't true. Now, given the previous discussion we can
>assert that (2) & (4) aren't *necessarily* true either, although they often
>are. The "pulse" is a momentary *release* of brake line pressure - not a 
>momentary application of pressure. 

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.

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

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.

-Carl-
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