I just installed a small, basic wireless cycle computer on my commuter bike today. It has a sensor attached to the fork that detects a small magnet attached to one of the spokes. The computer has a setting for various wheel sizes, which got me to thinking: there's no specific instruction for where on the spoke the magnet, and therefore the sensor, are supposed to go. I imagine different makes of bike are going to have the sensor and magnet at varying distances from the middle of the wheel; so does the wheel size really make any difference, as far as the computer is concerned? And if the magnet could be pretty much anywhere along the length of the spoke, how can the computer have any hope of accurately measuring speed and distance? Are these things just a waste of money? I mean, yes, they're a waste in the sense that they're unnecessary for most purposes other than idle curiosity, but do they even work???