What is a Cadence Sensor?
A cadence sensor on an e-bike is a device that measures the number of pedal rotations per minute (RPM) while the rider is pedaling. It is usually a small sensor that attaches to the bike’s frame and connects to the e-bike’s motor controller system.
The cadence sensor provides important information to the e-bike’s motor controller about how fast the rider is pedaling. This information can be used by the e-bike’s computer to adjust the amount of electric assistance provided to the rider, based on their desired level of effort. For example, if the rider is pedaling slowly, the e-bike’s motor system can provide less electric assistance and vice versa.
Having a cadence sensor on any bike can also help riders optimize their pedaling efficiency and technique. By monitoring their cadence, riders can adjust their pedaling speed to match what they are most comfortable with. This is typically also the most efficient.
Your ideal cadence is largely dependent on two things: your main muscle fiber type and your level of cardiovascular cycling fitness. The balance of those factors (which, of course, can change over time) determines your ideal cadence. Comfortable cadence rates are 60-90 RPM for most people. You can read a lot more about this online (“Does the Ideal Cycling Cadence Really Exist?“).
A primitive cadence sensor is typically designed using a ring of magnets with a hall sensor to detect when the magnets pass. In order to have a safe detection of cadence, a minimum of two magnets will typically need to pass the hall effect sensor element. This means you need 360 magnets in the magnet ring if you want an “angle-to-start” of about 2 degrees.
More modern sensors are getting rid of magnets (as engineers know, rule number one of using magnets in a product is: don’t). Some e-bikes even use an absolute angle sensor to measure the crank angle. From this sensor, you can derive the cadence simply by looking at how fast the angle changes. This sensor principle is the most responsive we see today.