A cycling power meter is a device that measures a cyclist's power output while riding a bicycle. These devices work in different ways according to the type of cycling power meter being used, but the general idea is to measure power output in watts by using a specially designed hub mounted to the rear wheel, or by using sensors placed strategically on the bicycle. The information collected by the cycling power meter is transmitted either through wires or wirelessly to a computer unit mounted on the handlebars of the bicycle. These units are popular among professional cyclists who want to streamline their training programs.
The most common style of cycling power meter is one that is mounted to the chainrings or crank arms, which are the rings and arms to which the pedals are mounted. The sensor is a specific type of gauge known as a strain gauge that essentially measures the torque being placed on that particular part of the bicycle. The strain gauge measurement is combined with other measurements within the cycling power meter system, and the information is sent to the computer with a calculation of the overall power being produced by the cyclist's pedal stroke. Other systems may have a sensor mounted on the bottom bracket shell — the lowest part of the bicycle frame through which the bottom bracket bearings are mounted — rather than on the crank arms or chainrings, and still others have sensors mounted inside the freehub body, on which the rear gear cluster known as a cassette is mounted.
These units are not to be confused with regular cycling computers, which are also mounted on the handlebars and feature sensors and magnets at different locations throughout the bicycle. These computers measure the cyclist's current speed, average speed, maximum speed, odometer readings, and so on. They do not usually measure power output, though some computers combine the two tasks: they will measure speed as well as power output. These units are likely to be quite expensive. Regular cycling computers are more likely to only feature speed readings and a few other basic features, such as cadence readings. Such a reading will require a second sensor mounted on the frame near the crank arm rotation.
The cost of a cycling power meter will fluctuate based on which design and model is being purchased. Some power meters are built directly into the rear wheel, which means the cost of such a unit will rise because a professional will need to build the wheel around it. Other units that mount to different parts of the bike may be less expensive, depending on what additional features they include.