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How could I make a speedometer for a gas-powered go-kart? Answered

I was wondering how I could make a speedometer for my go-kart. Could I use a regular cyclocomputer from a bicycle?


Sure, why not? You'd have to adjust the result for the fact that the go-kart wheel is smaller and turns more times per mile, of course.

All a spedometer really is, is a frequency counter measuring the interval between rotations and inverting that to rotations per interval. So if you want one that produces the right number directly, all it takes is the reed-switch (or other) sensor to detect the wheel's motion and a bit of analog or digital circuitry. (These days, I'd probably go digital -- just plug it into an Arduino or similar and calibrate in software -- but analog circuits are easy to find.)

Or there's the low-tech solution: Stick a playing card in the spokes, listen for the note, match the note on a pitch pipe, and do the math from number-of-spokes-per-second to rotations-per-second to distance-per-second. (I've known folks who use that trick as a way of quickly double-checking the speed of a drill press, using the teeth of the chuck; they've learned which notes correspond to which speed setting.)

Thanks! Actually, I found my cyclocomputer and you can select the wheel curcumference in millimeters. All I had to do was: Multiply the wheel size of my go-kart (6 inches) by Pi (3.14), then multiply the resulting number by 25.4, and enter the number in my cyclocomputer wheel size selection (4 digits for millimeters) I havn't tried it out yet because of bad weather, but I can't wait to see how it works!

If it will accept that small a circumference, great -- you're all set.

Many cycle speedo's use the wheel diameter in mm to calculate.

Yes, you could, but you would have to take into account the difference in wheel diameter.

Off the shelf bicycle speedometers that I have used (on bicycles) allowed the user to select the wheel size, but the range of selectable sizes was of couse bicycle wheel sizes of 20 inches or greater. I suspect that your go kart would likely use smaller wheels than that. If you used the speedometer on a wheel that was smaller than the unit was configured for, it would read too fast.

Off the top of my head, I have a couple ideas to get around the size requirements. You could have a secondary shaft that is driven by the wheel axel. The secondary shaft would be geared down by means of a chain, belt, or gears, in the proper ratio to make the display unit see the correct number of sensor pulses per revolution. The second method would be to use an electronic circuit to divide down the number of pulses from the sensor before input to the display unit.