From: mchaffee@ux4.cso.uiuc.edu (REAL LIFE?!?!  HA!!)
Subject: Re: Mech ABS retrofit (ABS-TRAX)
Organization: University of Illinois at Urbana
Date: Sat, 21 Nov 1992 18:16:54 GMT

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?

NO! NO!!! NO!!!!!
Think about it!  Every car has a different brake system.  The diameters of 
hoses, pistons, wheel cylinders, etc. make the effective line pressure for a
given level of deceleration different in every situation.  With the difference
in brakes, tires, car weight, etc. there is no way to predict the appropriate
maximum line pressure to prevent lockup.  Therefore, the effectiveness of this
system in any condition is questionable.  Additionally, it cannot adapt.  If
the system is calibrated to give you threshold braking on dry pavement, do you
know what's going to happen the first time you hit the brakes in the rain or 
snow?  Yep, your dry-weather-threshold pressure will be high enough your wheels
will lock.  Electronic ABS keeps each wheel individually at threshold
and adapts to changing road surfaces etc.

There's two other things that would bug me.  First, I don't like the concept
of the thing from a mechanical point of view.  It's just that many more valves
and fittings that could let go in a system that needs to be fail-safe.
Second, I wouldn't trust the thing to give me actual threshold braking.  Sup-
pose the thing was miscalibrated and it would only let me have 75% of my avail-
able braking power.  99% of the time you wouldn't notice, but that one time you
needed true threshold braking, you'd really wish you had it.

>The sales guy could not tell me how/what was used to determine the threshold 
>nor the method of pressure relief.

If this was a shop that specialized in the things, I would be wary if nobody
was willing to tell you how the things work.  My suspicion is they don't want
to tell you because the design is primitive or something.

>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.

I don't know what type of aircraft they are referring to.  Last time I checked,
airplanes were using real ABS like cars, not this system.  Another point:  If
ABS through limited pressure works, why hasn't it been implemented on cars?
The idea of limiting pressure is so much simpler than the systems that cars
use now; if it worked you'd be seeing it on new cars.

>Does this system sound viable? 
>Is this really used in aircraft?  Should I part with good money for this?

I don't think the system is a good idea.  It removes an element of control
with little appreciable gain, and may introduce a feeling of false security
in bad conditions.  I _DOUBT_ this is used in aircraft as such; the pressure-
regulator design may be the same as the pressure regulators that aircraft
use for other purposes, but I'm pretty sure that large aircraft, at least, use
electronic speed-sensing ABS.
As for parting with good money, for $600 you can go to a drivers' school and
be taught by professionals how best to threshold-brake for yourself.  That
would be a far better investment in safety, IMHO.
If, however, you're dead-set on getting this thing, check it out first.  Go to
the shop that wants to install it and ask to test-drive a car or two with the
system installed.  If they even have a demo car there, try to take a test drive
in different conditions too, especially things like rain and gravel.  Ask for
references - If there are 15,000 cars with this thing on them, you should be
able to get a few phone numbers to check with prior customers.

But I really recommend passing the thing up and working instead on your own
braking skills.  I think in the long run you'll find that more beneficial by 
far.

Michael T. Chaffee        |******BOYCOTT COLORADO******|  ______    
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