How power meter works! (and other bike sensors)

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If you ever wondered how your bike sensors work and what is the difference between them, you are in the right location. This article will tell you everything you need to know about power meters, cadence sensors, and speed sensors for your bike.

Before starting about how a power meter works, let’s first go through how speed sensors and cadence sensors work. They are easier to understand and would help to have a better grasp of power meters.

Difference between speed sensor and cadence sensor

A cadence sensor measures the rotational speed of your pedals (in RPM). A speed sensor measures the speed of your bike (in mph or km/h). Both sensors measure angular velocity but at different locations on the bike: the wheel rotation for speed; the crank rotation for cadence.

In short, both sensors have similar hardware. Nowadays, it mainly consists of accelerometers. The real difference is the metric in which they send the data to your device. Their purpose is different, but their core sensor is similar.

A speed sensor is placed on the wheel whereas the cadence sensor is placed on the crank. This means that the speed sensors use the angular velocity times the wheel radius to determine speed. The wheel radius (or diameter) can be specified within your GPS watch or bike computer. If you don’t specify it, the app will automatically calculate it. This is done using wheel diameter as a variable until the speed of the sensors matches the speed from the GPS. After this calibration, the speed of the sensor will work even when the GPS signal is bad.

Cadence sensors are usually located on the crank or pedals to assess the rotation of the pedals and provide a value of rotations per minute. In this case, the measure of RPM is made directly thanks to accelerometers.

Cadence sensors can also be used inside power meters. In this case, they can be located in different areas. When not located on the crank or pedals, the cadence cannot be measured directly. In this case, the RPMs are determined thanks to the fluctuation in the power measurements.

The concept is that no one has a perfectly steady cadence, there are fluctuations when pedals are at the top position for instance. By measuring the periodicity of these fluctuations, the RPM can be determined even if the measurement is not made at the crank or pedals.

Otherwise, power meters located on the crank or pedals will measure the cadence exactly as dedicated cadence sensors do.

Does a power meter measure speed and cadence?

Most power meters installed on the crank or pedals will measure cadence but not speed. On the other hand, a power meter installed on the hub can measure both speed and cadence. However, while its speed measure is accurate its cadence isn’t. Crank or pedal-based ones have better cadence accuracy.

As most people use GPS anyways to measure speed, a power meter in the crank or pedals is recommended to have all the metrics without the need to buy all bike sensors. If your power meter is hub-based, then you may need to buy an additional cadence sensor to measure RPM.

If your power meter is a crank or pedal one, investing in a speed sensor can make sense for pros or people wishing to compete. Indeed, GPS speed accuracy is dependant on the GPS signal quality. A speed sensor does not have this kind of problem and is more accurate in remote areas. GPS bike computer or watches tends to smoothen the values when the signal is not great.

Thus it is a question of scale. A speed sensor will give a better reading on the micro-scale. It can be great to track your speed on a very specific short interval, like a short but steep hill. But looking at the overall ride, the data will be smoothened and you won’t notice much difference between GPS speed data and speed sensor data.

Is a power meter the same as a cadence sensor?

A power meter is different than a cadence sensor. Power meters are embedded with strain gauges to measure the force your legs apply. This metric plus the angular velocity are used by power meters to determine power. A cadence sensor does not have strain gauges and can only provide a measure of RPMs.

Many power meters do provide a measure of RPM, it is something usual on the latest power meters for crank or pedals. So, some power meters can be used as cadence sensors. The opposite is however not true. Cadence sensors cannot measure power since they are not equipped with strain gauges.

Please note that power meters located on the wheel hub should not be able to measure cadence. Some manufacturers say they do, but it is a derived value from the fluctuation in power. It does work, but this is less accurate than a power meter located on the crank or on the pedals, where the cadence is really happening. It is simply physics!

Do you need a cadence sensor with a power meter?

You don’t need to buy a cadence sensor if you own a crank or pedal-based power meter. Most of these power meters send two signals by Bluetooth or ANT+: one for the cadence and one for the power. You can use both with your bike computer or home trainer. Most hub-based ones don’t measure cadence.

This is due to the fact that power meters need a measure of rotational speed in order to calculate power. Thus if they measure rotational speed near the pedals they do know the cadence or RPMs. I have a dedicated article explaining this topic here.

For instance, power meters from Favero, Stages, 4iiii, and Garmin can display the cadence on your bike computer without the need for an extra cadence sensor. I am not sure about other brands though.

Do you need a speed sensor with a power meter?

A speed sensor is not necessary if you already own a power meter. Power meters only measure power and sometimes cadence. But power meters need a GPS bike computer or GPS watch to operate. As these latter already measure speed, owning a speed sensor might be redundant.

A reason to own a speed sensor would be to have more accurate speed measurements. Indeed, the speed given by GPS is dependent on the quality of the signal. In areas where GPS signal is bad, buying a speed sensor on top of a power meter can make sense. This is particularly true if you ride off-road, with a gravel bike for instance.

If you are a pro, or plan to train seriously, a speed sensor is something you may need. Indeed, GPS tends to smoothen speed values. Whereas it is nothing to worry about on the macro scale (1 km or so), a speed sensor will give a better reading on the micro-scale. It can be great to track your speed on a very specific short interval, like a short but steep hill.

In short, a GPS speed would be very accurate when looking at the overall bike ride but may not be as accurate when zooming on a specific hill. If you require accurate data at the microscale in your training, then a speed sensor is needed. This is even more true if the GPS strength is bad where you train.

How does smart trainers measure cadence and speed?

How does the Wahoo Kickr measure cadence?

The Wahoo Kickr does not measure cadence directly but derives it from speed measurements. It determines it from the small variation of speed which is minimum at the top of your pedal stroke. The time between two minimum values equals half a revolution, allowing to determine RPM.

As there are two pedals, the minimum is achieved two times per revolution. Therefore 1 RPM is achieved when the speed reaches a minimum twice.

How does the Wahoo Kickr measure speed?

The Wahoo Kickr measures speed thanks to an optical sensor. The sensor will send an electric signal once per revolution, allowing to determine speed. Therefore it does not require an additional speed sensor.

Please note that has the sensor is located on the flywheel and not the “wheel” itself. A reduction factor is applied to consider the change of speed due to the belt.

How does the Tacx trainers measure cadence?

Most Tacx trainers do not measure cadence directly but derive it from speed. It determines it from small variations in the speed measurement. However the Tacx Neo 2 measures it directly thanks to a capacitive sensor.

The capacitive sensor of the Tacx Neo 2 is activated when the magnetic field around it is changed. This happens when a pedal moves around it. Therefore it may happen that it is not activated properly if your cranks are too short. There is a way to fix this problem explained here.

For all the other Tacx trainers, the way cadence is determined is similar to Wahoo trainers, i.e. from speed. The time between two minimum values equals half a revolution, allowing to determine RPM. As there are two pedals, the minimum is achieved two times per revolution. Therefore 1 RPM is achieved when the speed reaches a minimum twice.

How does the Tacx Neo measure speed?

The Tacx Neo measures speed directly from the cassette. The measure of the speed is direct and does not need any additional computing.

The Tacx Neo is a much simple design compared to the Wahoo Kickr. There is no belt and no flywheel. Therefore speed is directly measured thanks to a dedicated sensor and without compromise.