The wireless bicycle computers use small sensors that attach to the wheels and forks. Each of these sensors records the frequency of each wheel revolution. Typically you’ll find a tiny magnet that’s attached to a spoke on the front wheel and a sensor to one of the fork legs. As riders, you need to calibrate your computers with the specific wheel size initially. This way, when the magnet passes the sensor each time, the computer recognises the distance travelled. Some can also measure speed, heart rate, and so on.