Strain gauges in each crank arm give this system unique capabilities.
Rotor is throwing their hat into the increasingly crowded ring of power meter producers. They hired Italian measurement engineering firm Studio A.I.P. to help design the system and the engineers embedded four strain gauges in the holes drilled into all of Rotor’s 3D+ cranks.
The system will use ANT+ transmission, cost $2,000 and measure power to within roughly the same linearity (a measure of accuracy, more on this later) as other options. Accuracy testing is not yet complete, but preliminary tests conducted by Rotor are showing the system to be accurate within +/- one percent.
A power meter located in the crank with these is fundamentally very similar to other options, especially SRAM’s outstanding new Quarq power meter, but Rotor’s system has a few features that subtly differentiate it from others.
Instead of locating all the strain gauges in the spider, Rotor Power’s gauges are imbedded in both crank arms themselves. With four gauges in each arm, Rotor Power is effectively two independent power meters. This allows the crank to measure true right- and left-leg power. Garmin 500 and Garmin 800 computers are already capable of displaying the ratio of power produced by both legs, a metric intended to help a rider create power evenly.
Rotor Power is also quite light. Weighing just 556 grams, it’s only 30g beefier than the standard MAS (Multi-Adjustable Spider) 3D+ crank.
Both legs have to be independently zeroed, but Rotor says once they are set, the system doesn’t have to be re-zeroed until crank arms or pedals are swapped, regardless of riding conditions.
The battery is housed behind a screw-on cap that requires no special tool to open, allowing riders to swap the battery at home without mailing the crank in for service as SRMs require. Rotor claims 300 hours of battery life and both crank arms use a common CR2477 battery.
Power meters don’t actually measure constant power output, they take a lot of individual instantaneous recordings of the power being produced and average them. Rotor Power measures 500 times a second, creating a very detailed picture of the rider’s power. Antonio Brivio, Studio A.I.P. engineer, says this high measurement rate makes the data taken by his system extremely repeatable.
Recording independent leg data this quickly also lets Rotor Power draw a chart of the power produced through a complete pedal stroke. Rotor uses this data to take the ratio of positive to negative power (negative power takes place briefly during the upstroke), to create a metric called Torque Efficiency. This score is a tool for measuring the amount of additional power the rider has to create beyond the power directed to the rear wheel.
A metric called pedal smoothness will also be available, generating info about the “roundness” of the rider’s pedal stroke. It is similar to Computrainer’s Spin Scan.
Rotor hopes to have Power available by the end of the year. Garmin and other computer manufacturers will have to update in order to display Rotor Power’s new metrics, so it might be longer until the system is fully functional. The $2,000 price point does not include crank arms or a computer.