Anybody build their own 5th wheel?

[Bruce Bowling]

~ 
~ I'm building myself a race analysis package for an upcoming "run for fun"
~ drag race at Milan dragway in Michigan on July 27th.  Here is what I've come
~ up with so far, as well as my questions concerning the acquisition of speed and
~ distance.
~ 
~ I'm using a GM-designed ProDAS model 6 data acquisition system to record my
~ data.  It has inputs for analog, digital, and differential/single-ended
~ frequency signals.  In addition it can "count" the pulses from the frequency
~ input. (Also known as the trigger.)
~         I will be recording these signals:
~                 RPM
~                 forward and rearward acceleration in G's 
~                 Distance traveled in feet
~                 Speed in MPH.
~ 
~ The first two signals come from the ignition, and an Analog Devices 1 axis
~ accelerometer.  The second two are going to come from a 5th wheel that I want
~ to make from 10 speed bicycle tire.  Has anyone had any experience with
~ making one of these?  I am going to make a light aluminum frame for the
~ wheel that attaches to the rear bumper of the car.  The frame will have a
~ photo diode and an LED mounted on it so as to do one of two things:  
~         Illuminate the Photodiode continuously, such that the light beam is
~ broken         by the spokes.  
~         
~         Reflect the light off the aluminum rim, which has been painted with
~ flat black paint, except for radial dashes every fifteen degrees. 
~ 
~         Does anyone have any suggestions or comments on how I can improve
~ this design?   Any suggestions for an LED/Photo-receptor package that is not
~ influenced by stray light?   
~ 
~ --- Dan.
~ 
~                 ws6transam at voyager.net
~ 
~ 
The nice thing about this setup is that RPM, position, and acceleration
can be cross-checked between the sensors, in that acceleration can be
integrated to get velocity which should correspond with the 5th wheel.
Also, slippage in the drivetrain/wheels will show up in correlations
between RPM and 5th wheel.

If you are going to use the data to obtain engine power or torque, be
sure to correct for the current operating conditions (temp, barometer,
humidity) to a "standard" condition so you can compare with
other runs in the future.

Playing with some of the vehicle motion simulators written by Al Grippo,
I have found that 20 samples a second is a good number, in that
adding more samples/second did not greatly change simulation output,
but having less than 20 Hz did affect results. This result should apply 
to your measurement, since you will be doing the same computations.

The simulations discussed above determine acceleration, and then compute position and 
velocity by numeric integration.  Since numeric integration is 
generally less error-prone than numeric differentiation 
(i.e. going from position to velocity and acceleration) I would convert the 
accelerometer readings to velocity and position, and then perform the correlations.

Your experiment is of great interest to me (and I am sure others), so
please post your results!

- Bruce


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               Bruce A. Bowling
  Staff Scientist - Instrumentation and Controls
 The Continuous Electron Beam Accelerator Facility
    12000 Jefferson Ave - Newport News, VA 23602
                 (804) 249-7240
                bowling at cebaf.gov  
        http://devserve.cebaf.gov/~bowling
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