Re: Chirps

[Andy Poling <andy@realbig.com>]

"Graydon D. Stuckey" <graydon@apollo.gmi.edu> sez:
> On Fri, 14 Jun 1996, Al Powell wrote:
> > My experience is that if you really nail the ABS, you do get 
> > intermittent "chirps from the tires as they NEAR lock-up before 
> > releasing, so I would expect that you might see some small black 
> > marks here and there through the braking path, as it's possible that 
> > the tires would leave a small mark as they "chirp".  No black LINES, 
> > though.  A solid line would indicate lock-up
> Yep,
> 
> 	I have seen lots of pictures of ABS-equipped skid marks, and they 
> aften leave dotted lines, instead of the customary solid lines.  

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.

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.

> Therein 
> lies the secret to why most topend race cars don't use ABS.  That space 
> between the dark lines indicates that there was a discrete amount of 
> time in which the brakes were not braking at their ultimate capability.  
> The dark line, if the ABS system is calibrated well, indicates thath teh 
> tire was exceeding the 4-5% slip that is optimum (Not sure about the 
> number, but I think it should be around 5%.)  If you were to add the time 
> spent just a little over the optimum braking point, and the time spent 
> well below the optimum brakign point, you would find that the total 
> braking experience was less effective than a well modulated manual brake 
> system with a sensitive foot on the pedal.  IN this case, you would see a 
> light steady gray line along the entire distance.

This is all well and good, and I won't argue that a human isn't a vastly
superior learning machine.  The human, however, is handicapped by two big
problems:

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.

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.

Thus, on paper, the ABS should win every time.  I performed tests with
electric eyes and measured marks at an autocross schoool with my
ABS-equipped Miata two years ago.  I won over the ABS almost every time.  I
think, though, that I won because I knew that I had racing rubber on and it
has better sliding friction properties than most street rubber.  I
definitely had a harder time preventing extended lock-up of one or more
wheels.

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.

-Andy

PS - If you've ever hefted an ABS pump, you know what I consider the real
reason racing teams shun ABS.

'91 Miata: "VROOM"       '96 A4 Quattro: "SWOOSH"       '84 Yam RZ350: "ZING"  
                        '88 Bronco II 4WD: "GRUNT"