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Re: ABS questions (and NOT about when to use it!)
Jeffrey J. Goggin wrote:
> 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
> greatly appreciated.
> P.S.: I'm not talking so don't ask... ;^)
> _ _
> / | _| o | \ _| o Jeffrey Goggin
> /__| | | / | | __ | | | | / | | firstname.lastname@example.org
> / | |_| \_| | |_/ |_| \_| | http://people.delphi.com/audidudi
> ________________________________________________________________________Jeff:ABS measures independently the deceleration rate of any wheel. That is, given any
rotational speed (as measured by the time periods between teeth in the wheel sensors)any
excessive deceleration rate ( pulse rate from the wheel sensor decreasing rapidly)_is
used to anticipate a wheel lock. ABS the removes hydraulic pressure to that wheel's
cylinder and back to the brake system.. etc
Therefore it is very important that the sensor does not loose and teeth pulse from a
given speed set, because...the system says...we are decelerating wheel xxx to
quickly, therefore.... back goes the hydraulic juice...