A crank arm power meter is a power meter where the power sensors and electronics are bonded onto the inside of the non-driveside crank arm. As you ride, the sensors can measure your force (power) as the cranks flex ever so slightly under load. Since the power meter is only measuring the power in your left leg, it works by doubling this amount in order to determine your total power (both legs). Note that this method of calculation assumes that both legs produce the same force. Stages Cycling and 4iiii Innovations both make crank arm power meters (please see our Stages vs. 4iiii comparison).
There are several advantages to a crank arm power meter:
- More affordable than most power meter types since you are just buying the left side crank arm.
- Lightweight as these power meters only add about 10-20 grams to the weight of the crank arm.
- This design allows for easy installation and swapping of the power meter between bikes (assuming both bikes have compatible cranksets).
However, as with any power meter type, there are also some things you should consider.
- You should always be sure your bike frame is compatible before buying a power meter. This is especially true with crank arm-based power meters as there can be clearance issues with the frame and the crank arm. Specifically, you need approximately 10 mm between the left crank arm and the chainstay in order for the sensor to clear the frame.
- If for some reason (perhaps due to a previous injury), you produce more power with one leg versus the other, these left only power meters can lead to less accurate results. For the majority of riders however, this isn’t much of a concern.