From: mdennie@beryllium.mc.xerox.com (Matt Dennie)
Newsgroups: rec.autos
Subject: Re: Anti-Lock Brakes makes stopping SAFER?
Date: 30 Dec 92 17:48:39 GMT
Reply-To: mdennie@beryllium.mc.xerox.com
Organization: Xerox Corporation

In article 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.
>
>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.  It virtually
>immediately (for this discussion) reapplies it, up to the point of
>locking up, releases it again, etc.  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...
>
>Now, ABS systems pulse at different rates, some pulse 10 times per
>second, some pulse at 100 (or several hundred) times per second).
>
>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-
>--

Your lack of knowledge (of) how ABS true<-ly works is showing through here.

At least some ABSs I have read about DO detect "impending lockup" by 
measuring the "slip ratio."  Whether this is "lockup" according to anyone's
definition is up for grabs.  Historically, lockup has meant that the wheel
STOPS TURNING.  Impending lockup is detected when a wheel is not turning
at the same speed as another wheel, indicating that, although the wheel is
turning, it is losing traction - time to reduce the braking force until
it has full traction again.  Note that it is NOT necessary to TOTALLY
release pressure, only to reduce it to the point the the wheel is spinning
at full speed again.

By varying the "slip ratio" at which the pressure is bled off the brake line,
the point at which ABS kicks in can be varied.  Obviously there are other
conditions for activating the ABS, because, for example, in cornering, all
wheels are not rotating at the same speed.

BTW, this is also the system used for TCS (Traction Control Sys), but the brakes
are APPLIED to the offending corner (or the ignition retarded, or the fuel
supply cut).

I admit the most of this information comes from the study of motorcycle ABS
and TCS, but the principles are the same.

---
--Matt Dennie                              Internet: mmd.wbst207v@xerox.com
Xerox Corporation, Rochester, NY (USA)
"Reaching consensus in a group often
 is confused with finding the right answer."  -- Norman Maier