John--
Re outlawing of rear wheel sensors-- What's a rear wheel sensor? If I catch
vibration frequencies somewhere else in the car is that a rear wheel sensor?
Yesterday I commented on the possible future use of FFT vibration analysis to
sense wheel speed. Kind of blue sky stuff. The original bench top
instruments from HP 25 years ago sold for upward of $40K. Now all it takes is
an accelerometer, signal processor and a PC. This could become real race
hardware in the future. If you're not familiar with it what an FFT processor
does is gather a complex electrical signal out of something like an
accelerometer over a few millisecond time period and convert it into a
snapshot of the various frequencies present and how strong each one is. The
display would be a video screen sort of like an oscilloscope. The vertical
axis of the graph would be a dB voltage representative of the strength of the
vibration from the rotating member (which should be constant through a run as
long as you don't throw a wheel weight of something like that. The horizontal
display axis is frequency ( in a real product this could be theoretical ground
speed by a simple tire diameter entry.)
Let's say you're going 200mph with 27 inch tires all around, 10% slip on the
rear and 2% slip on the front. Engine is turning 7000 RPM. The display would
show a strong peak at about 41 Hz (revs/second) for the fronts and another at
about 44 Hz for the rears. There'd be one at about 117 Hz for the motor and a
bunch of others for suspension and frame flex vibrations. There might be a
couple more strong peaks at around 85 Hz representing the first harmonics of
the wheels. What the driver would watch for if a spreading between the rear
and front peaks or a spreading and dropping of the rear peaks into two (for
cars with differentials).
There are some big questions in my mind about bandwidth at the low frequency
end. Could a fractional second sampling time plus the computation and display
delay provide decent data in time for the driver to do anything useful with
it? Too short a sampling time would degrade the data. Also, practical cost
electronics might be too slow. I'd like to ask anyone who has earned one of
those little spin pins about how long in seconds was it between the first time
you sensed that something wasn't right and the time the spin started.
By the way, you could build your own accelerometer that would work well below
500 Hz. No need for expensive tiny accelerometers.
Ed Weldon
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