Re: Why did tires 'chirp' in panic stop?

[Chris Newbold <cnewbold@transarc.com>]

Brett Dikeman wrote:

> Then, of course, there's the highly unlikely(at speed) probability
> that you would lock all 4 wheels at exactly the same time.  If this
> happens, ABS will simply "think" the car has screeched to a halt
> thanks to Super Wonder Brakes.

I've also heard this about earlier ABS systems. There are two things that
lead me to think that newer systems cannot be fooled this way...

(I do not know the following to be fact, but I believe them to be true. If
anyone has hard, accurate technical data one way or the other, please
jump in...)


* Newer ABS systems do more than simply compare wheel speeds; they also
  compute the _rate_ of deceleration of each wheel ( delta V ). This is used,
  along with the data from other wheels, in some sort of a threshholding
  operation which recognizes impending lockup not only by absolute V, but
  by "excessive" delta V.

  Namely, it knows that a wheel "cannot" go from 60 MPH to 0 MPH
  instantaneously.

  I have some empirical evidence to back this up: I've played with the
  ABS on my 1993 90 in icy parking lots. Get up to about 30 MPH and
  stomp on the brakes--this would produce instant lockup of all four
  wheels. However, the ABS does catch this and cycle fiendishly trying to
  keep the wheels rotating.

* The most modern ABS systems also incorporate accelerometers to directly
  measure deceleration independently of wheel rotation. Combined with
  a vehicle speed signal, this can recognize that you must still be moving
  even though the wheels are not.

  (Last speed reading was 30 MPH, current speed reading is 0 MPH, but the
   accelerometer did not register the massive deceleration which would have
   been required to get from 30-0 MPH in a fraction of a second.)

-Chris
1993 90CS 5spd 71k miles