The Traffic Accident Reconstruction Origin -ARnews-

Re: ABS adjustment factor

Bill Wright (bwright@taro.com)
Wed, 24 Jul 1996 09:26:52 -0400 (EDT)

My experience with ABS cars and skid testing indicates
most systems are very easy to defeat. Removing the fuse
or disconnecting a wheel sender (plugs usually conveniently
located on a fender in the engine compartment) are simple
ways to render an ABS car as a conventional skid car.
The ABS system sees the fault the car can be skidded and
friction coefficients found in the normal fashion.

>From a testing standpoint this ability to enable/disable the
system is particularly nice for comparing the different
stopping rates on the same surface.

A more interesting question might be correlating the ABS
stop with acceleration rates and driver responses. As is
apparent in the g-Analyst files accompanying the ABS
scuffmark article Pedal take-up (that is the time required
to invoke ABS) and pedal force throughout the stop can
vary widely. It is also possible and commonly observed
that full ABS stops are not always produced by all drivers.
Just as dry pavement skidding is a dramatic control input
invoking ABS requires great effort applied to and maintained
on the brake pedal. It was ‘work' to be sure all tires were in an
ABS mode and not (due to pedal input proportioning or weight
x-fer variables) working at a sub-ABS rate.

If you have the benefit of a bumper gun backed with an
accelerometer you could instruct a variety of drivers to go do
an ABS stop. The variability in the time between brake
application and the appearance of scuff marks would be interesting.
Additionally with minimal emphasis on the pedal effort required to
obtain full ABS I expect you will see a wide variety of stopping
rates produced.

Having looked at accelerometer data of skids and ABS stops I find
that though the rates are usually greater with ABS they are also
much less consistent throughout the stop. The data usually requires
some interpreatation for choosing an average stopping rate. This
of course is what we are interested in when applying a drag factor
to skid marks: What was the average stopping rate for the measured
unknown stopping distance.

Looming still on the horizon are calculations that take vehicle path
into account to conceed friction forces lost to dramatic steering.
Bill Wright
bwright@taro.com

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