Re: Chirps

["Graydon D. Stuckey" <graydon@apollo.gmi.edu>]

On Sat, 15 Jun 1996, Andy Poling wrote:

> That may be because some other non-braking load was present (i.e.  attmpting
> to generate side force - AKA turning).  The ABS cannot remove this load to
> prevent loss of traction - it can only reduce a conflicting load from the
> brake system in an attempt to get you back within your traction budget.

I was talking about a straight ahead brake test, with no weord forces 
present, but your point is valid.

> One factor is how many channels your ABS system has.  It seems that many
> modern cars have three channel systems - meaning the ABS can reduce
> hydraulic pressure to any or all of three different "zones" at any given
> moment.  The rear wheels seem to share a channel on most of these systems. 


> An even bigger factor is the cycle time of the ABS system.  This has been
> improving over the years.  A newer, shorter cycle system will reduce the
> time that the tire spends outside the traction budget.

This is a good point.  I read a couple years ago when Benneton was 
testing ABS on their F-1 car.  Thay were testing on a runway in Germany, 
and had 150-30mph times in the region of 3-4 seconds, in the wet.  They 
had a very custom ABS system I am sure.

> 1) The human can't observe the individual wheels to sense impending lock-up
> (or even actual lock-up).  The poor creature has to guess from feedback in
> the steering wheel, audible hints and past experience to guess when a wheel
> is locking up due to excessive brake pressure.

Usually, a good driver in a car that communiates well _can_ do this.

> 2) The human only has one brake pedal.  This makes the human a single
> channel brake pressure controller.  If the brake bias is not exactly
> correct, the human must reduce pressure to all wheels (three of which may
> still be able to handle more pressure) because one wheel seems to be sliding.

This is very true, and this is the one area that even a crude ABS system 
has the potential to perform admirably.


> That said... I autocross with the ABS enabled because I don't want flat-spots
> on my expensive R1's.  Besides, ABS really doesn't really come into play all
> that often when I'm autocrossing.

Agreed, I ran a '92 Corvette a year ago in an autocross, and at the 
owner's direction (a nationally ranked Detroit Region driver) I left the 
ABS on, and only a couple times I got a couple pulses from the ABS.  
Seems it was calibrated to start pulsing only when the normal limits of 
the tires were FAR exceeded.

Later,								
Graydon D. Stuckey								
graydon@apollo.gmi.edu								
Flint, Michigan   USA
'86 Audi 5000 CS Turbo Quattro, GDS Racing Stage II