Cadence is the number of revolutions the crank arm and pedals on your bicycle make per minute. Cyclists want to keep that number roughly stable while they cycle, whether you are going up or downhill, slow or fast. Many people recommend keeping your cadence at around 90 rpm (revolotions per minute), because that’s when you cycle most efficiently. You can adjust your gear in order to keep the cadence constant.
Measuring cadence is the primary function of the cadence sensor, but it also measures the revolutions of the rear wheel.
These two measurements are done by detecting the movement of two small magnets: one mounted on the crank arm, which measures cadence, and the other on a rear wheel spoke, which helps measure speed. There is a small sensor that needs to be attached to the chain stay on your bicycle that detects the movement of the magnets, and transmits the information wirelessly using Bluetooth® Smart to the Multi-Sports GPS watch. To measure speed accurately from the sensor, the watch also needs to know the circumference of the rear wheel in millimetres, which you can set in the settings for a Cycle activity. As the watch then knows the number of revolutions the wheel makes, from the sensor, and the circumference of the wheel, which is the distance travelled in one revolution, it can calculate the speed.
Your watch uses the GPS signal as the primary method for calculating speed, but it uses the information from the Cadence Sensor as extra input, especially if GPS reception is lost, for example in tunnels or other situations. Using both the GPS signal and the Cadence Sensor makes the overall measurement of speed more accurate.
You can use the TomTom Runner GPS watch on a bicycle, but the Runner does not provide the same information or support for cycling: