Most bike sensors are just a reed-switch and a magnet. Those components are generally really cheap. Making sure the magnet is strong enough/close enough to trigger the switch is probably the only pain you'd have.
You could overcome the directional problem (ie. are they peddaling backwards) if it matters to you by using two reed-switches. It'd be a pretty cheap and simple project to knock together if you can use one of those old style DSUB 15 gameports rather than a USB port. I'm not entirely sure how up to it the gameport is in terms of how many samples per second you could get.
You could overcome the directional problem (ie. are they peddaling backwards) if it matters to you by using two reed-switches. It'd be a pretty cheap and simple project to knock together if you can use one of those old style DSUB 15 gameports rather than a USB port. I'm not entirely sure how up to it the gameport is in terms of how many samples per second you could get.