From: crash@hal.COM (Chris Craddock)
Newsgroups: rec.autos
Subject: Re: Anti-Lock Brakes makes stopping SAFER?
Date: 30 Dec 92 17:50:55 GMT
Organization: HAL Computer Systems, Inc.
NNTP-Posting-Host: sydney.hal.com

In article <CARL.92Dec30105349@atlantis.Cayman.COM>, carl@Cayman.COM (Carl Heinzl) writes:
|> 
|> >  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!

|> 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?

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.

BOSCH (and TEVES and probably others) have accelerometers in each wheel,
but they also have brake pressure sensors, and 3D inertial sensors too.

The reality is that modern multi-channel ABS systems react to average
deltas in rotational speeds between corners as well as absolute rotational
speed per wheel and also accelerations around the cars 3d axes. Thus, they 
really *can* sense "impending lockup" well before the wheel has actually
stopped turning.

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. 

If the ABS decides the wheel is still "locked" after the pulse, it pulses 
again - and again - and again (you get the idea). However, if the ABS decides that the wheel speed is "within range" it ceases pulsing. If you have ever
watched ABS equipped cars braking hard on a race track you may have even
seen this phenomenon in action.

Fortunately the description of Traction Control above is correct.

Chris

BTW - its is politically correct to add that ABS doesn't suspend any of
the laws of physics. If there isn't enough braking room before the obstacle
you may very well hit it - but at least the ABS gives you a fighting chance
of steering around it.
--
"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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