From: rick@posms.cactus.org (Rick Kirchhof)
Subject: Re: Mech ABS retrofit (ABS-TRAX)
Organization: Department of Redundancy Dept., Austin
Date: Sun, 22 Nov 1992 17:45:25 GMT

In article <53060095@hpsgm2.sgp.hp.com> leonglaw@hpsgm2.sgp.hp.com (Lawrence LEONG) writes:
>I've just seen a mechanical ABS retrofit.  Its called "ABS-TRAX" and is
>supposed to be derived from aircraft systems.  Have you heard of it, and if
>so, any data-points or anecdotes?
>
>The system consists of a small pressurised chamber which fastens (one per 
>brake circuit) onto the master cylinder output, somewhat like a reserviour.
>The chamber contains a pressurised Nitrogen chamber and diagphram; when the
>brake fluid hits a pre-set pressure (supposed to be within 5-10% of lockup),
>a valve will actuate to relieve the pressure and keep the wheels on the 
>threshold of skidding (somewhat similar to threshold braking).  Under normal
>braking, it is supposed to not activate.
>
>There is no speed sensor like the electronic versions.  Since this device 
>works on a principle similar to threshold braking (I think), it appears to
>be better than electronic ABS, no?
>
>The sales guy could not tell me how/what was used to determine the threshold 
>nor the method of pressure relief.
>
>This system is supposed to in use on aircraft, and is supposed to reduce 
>braking distances by up to 30%.  Its also supposed to be DOT certified (for
>what its worth).  The company claims to have sold 15000 units in US alone.
>
>
>Does anyone have any experience with this?  Does this system sound viable? 
>Is this really used in aircraft?  Should I part with good money for this?
>
>(price is approx US$620, incl installation).
>
>comments/suggestions welcome.
>lawrence
>leonglaw@hpsgm2.sgp.hp.com
>
>

I have seen the tech package that is sent to potential installers of this
product.  The active principle here is that the brake rotor usually
contains high spots.  When a high spot is within the caliper, the
braking action is increased.  They claim the high spots are the main
cause of brake lockup.   The system provides an adjustable volume
for the brake fluid to displace, eliminating (they say) lockup.


Three problems here:

	Many, if not most cars have very little thickness variation on
	the rotors.  This is what you usually feel as a pulsing brake
	pedal.  It is very common to have some lateral runout, but this
	won't effect brake performance unless it is pronounced or the	
	caliper is not floating properly.  Brake repair specifications
	on almost any car will allow a fair amount of runout, but very
	little thickness variation.


	Problem two is that brake fluid _should_ create a positive
	displacement in the brake system.  Otherwise, a soft brake pedal
	results.  If this is such a great idea, why do manufacturers set
	up brake bleeding procedures.  If you displace fluid in one area
	it should create the transfer of force to another area like the
	caliper or wheel cylinder in a predictible manner.

	Problem three is that your insurance company may not consider it 
	to be an anti-lock brake system.

Think about it.

-- 
Rick Kirchhof   Austin, Texas                   | Experience is what you
Domain: rick@posms.cactus.org                   | get when you don't
Bang path: ...!cs.utexas.edu!peyote!posms!rick  | get what you want.            
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Newsgroups: rec.autos