Reviewed - Axle based Power Meters
To self fit these Axle based power meters you must be confident at removing your crankset from your bottom bracket (or pay your LBS to do it for you). These units are very robust as the power sensors are contained within the axle housed inside Bottom Bracket assembly. This makes them ideal for MTB and XC riders.
The power is usually measured only from one side. They provide averaged total power by multiplying the power measured in one leg (like the Crank Arm power meters). The Rotor InPower devices are very popular as they use regular AA batteries giving 300 hours+. They have been designed with VERY EASY battery change in mind.
NOTE > there are a few units like the Rotor 2INPower that have TWO sets of sensors, one in the BB/axle (left leg) and the other in the crankarm (right leg). These bi-lateral power meters provide TRUE left & right power balance. The total power output is also a TRUE value as it's measured from both legs, not estimated. If power phase and power balance are parameters that interest you please consider some of these dual sided power meters.