I rode an old converted road bike with a track wheel on it for a while in the city. it killed my knees, probably because I was pushing a gear that was too low and spinny, but I had to deal with some uphills as well, so a harder gear would have been more difficult. after a few fast downhills in the low gear, I needed physical therapy because I could barely walk. my knees have not been the same since, and that was ten years ago. I've had it examined by several doctors and no one can diagnose a real injury. i switched it to a freewheel and have not looked back.