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ABS questions (and NOT about when to use it!)
The recent thread about ABS got me thinking about exactly how it works ... I
know it senses the individual wheels' rotational speeds by counting the
teeth on the hubs with a sensor but how much tolerance does it allow? How
much of a speed differential can there be between the tires before the ABS
decides something's up and kicks in? Five percent? Ten percent?? Twenty
For instance, if you install a pair of brand new front tires on a car with
worn-out rear tires, the difference in the diameter could be as much as 1/2"
or about 2% ... does this cause the ABS to kick in just a smidgen sooner
than it might if the tires have equal diameters?
Also, when you're turning, the inside tires will turn slower than the
outside tires, the exact difference being determined by the corner radii ...
if it's a tight corner and you're left-foot braking, does this mean the ABS
might be triggered prematurely?
Does the ABS brain give less weight to a side-to-side wheel speed
differential (or diagonal differential) than it does to a front-to-back
speed differential? Does it behave the same way at 30mph that it does at
60mph? Or 90mph?
Since the ABS system is self-contained, it appears the only two factors it
takes into account directly are wheel speed and line pressure. As such, I
assume it's programmed to use a "look-up table" or map to figure out what it
should do at any given moment ... if this is true, might it be possible to
modify this program for improved braking performance during "spirited"
driving versus everyday commuting the same way that the ECU can be hacked to
increase boost or alter ignition timing? Has anyone done this or ever heard
of it being done? Does this mean that an ABS brain for a, say, Porsche 928
might be programmed differently than one for an Audi 200q? Are they
Obviously, I've got something in mind here (as usual!) and I know that some
or all of these questions have been answered in a white paper somewhere but
a quick check of the Engineering Library at ASU didn't turn up anything ...
any comments and/or pointers to appropriate reference material will be
P.S.: I'm not talking so don't ask... ;^)
/ | _| o | \ _| o Jeffrey Goggin
/__| | | / | | __ | | | | / | | firstname.lastname@example.org
/ | |_| \_| | |_/ |_| \_| | http://people.delphi.com/audidudi