here's a really good video about turning a regular bike speedometer into a cadence meter:
https://www.youtube.com/watch?v=qrL476g662I
it involves using a less than 10$ bike speedometer, improvising a magnet on the crank then enter a wheel diameter so the speed shows as X.Y revs per minute, so you multiply by 10 to get RPMs. The problem is i have both a wired and wireless computer but the wire isn't long enough to mount the sensor at the crank and put the display on the handle bar and the wireless doesn't have the range. The display pretty much needs to be on the handle bar to keep it from getting knocked around and because the decimal place on the display is rather small. I could buy one of the cheaper cateyes, but i need three of them for my bikes, plus i have a bunch of other people interested in biking that are watching what i do so the cheaper the better. Also, the cateyes for some reason make the speed number really big and cadence small, so its not really easy to see. It makes no sense to me at all, i grew up on a farm, all the tractors have tachometers because rpms matter, who cares about speed unless your breaking the speed limit. Tachs are critical for anything that has manual gearing. do the people who make bike parts actually ride bikes?