Bicycle cadence meters are useful for measuring the speed of your pedal stroke in RPM (rotations per minute). Many cyclists use this measurement to keep their efforts in check, know when to shift, and perfect their cycling technique. Get the most out of your pedal stroke by converting your old bicycle speedometer into a cadence meter.
Remove the zip ties that hold the stationary magnet to the bicycle fork and any zip ties holding the cable in place.
Relocate the stationary magnet to the bicycle's down tube, seat tube or chain stay. Be sure that the magnet is within range of the bike's crank arms.
Remove the rotational magnet from the wheel of the bicycle and zip tie it to the bike's crank arm so that it passes next to the stationary magnet.
Use your speedometer's owners manual to set the "wheel size" setting according to the length of your cranks. This measurement is in millimetres, and is equivalent to twice the length of your crank multiplied by pi (or 3.14159). For example, for a 175mm crank, use the equasion 175 x 2 x 3.14159, which comes out to 1099mm.
Because bicycle speedometers read out in mph, not rpm, reading it will require some mental math. For the crank length on most adult bicycles, the ideal cadence (around 90rpm for most cyclists) will read out as 3 - 4 miles per hour.
Only a cadence-specific bicycle computer will read out in rpm. If you need more than a general idea of your pedal speed, a cadence computer may be a necessary investment.