I'm pretty handy, but it has been a LONG time since I took a bike apart. My son's bike is changing gears and the pedals turn, but it is not turning the wheel. What's my problem?
The freewheel or cassette/freehub pawls, which are what clicks when one coasts and drive the wheel when pedaling, are not engaging. If it never catches at all it's likely that dried up grease, dirt, etc. is keeping them from engaging. You need to first Google "freewheel or cassette" if you don't know which you have, then search for "clean cassette" (or freewheel). If cleaning does not fix the problem the pawl springs may be broken or the pawls damaged, and you will need a new freehub or freewheel.
