How does it work?
The 20 gram power measurement unit contains strain guages (like in a bathroom scale), an accelerometer and a temperature sensor. The unit is bonded to a pre-determined location on the left side crank and the strain guages measure any flex or deflection on the crank during power loads. Aluminum cranks bend consistently (however slight) and this can be measured accurately by strain guages. The crank and the meter are calibrated with a power curve that correlates crank flex and power output and that is stored in the power meter. The crank is calibrated as well under different temperature readings so the behavior of the crank is calibrated as well under different temperatures. Finally, these are integrated with the accelerometer so the power meter knows when the crank is rotating and what direction the crank is in at all times.
Left Side Crank Only?
Perhaps the greatest decision of the Stages team was to measure the output on the left side crank only. This greatly reduced the complexity and the cost of the system as it simplified power measurement with less parts and more space in the left side to clear the frame. Stages assumes the left side and right side power of the rider is about equal and they base their power outputs on that.
So this can be seen as a weakness of the Stages Power Meter compared to the SRM, Quarc and Powertap systems. All those other systems can measure left and right side power as they are either hub based or crank drive side measurement systems. But according to Stages, most riders are within 1% on their left and right side power output. And when there is a big discrepancy in the left/right output rider, such as in a hip injury or deformity, there is little that can be done to correct or equalize this issue anyway. So by simplifying the problem, Stages is able to deliver an elegant and cost-effective solution.
Automatic Temperature Compensation
Materials flex differently depending on temperture. In general, the colder it gets, the stiffer a material becomes. Thus, all power meters are subject to error when the temperature changes significantly. Power meter manufacturers request users to ‘Zero Offset’ their device on every ride and when the temperature changes noticeably (+- 10 degrees Farenheit). ‘Zero Offset’ is performed by coasting or backpedaling and pressing a sequence of buttons on the head unit during a ride. Most riders never do this and thus the data is prone to error in areas where there are big temperature variations.
Stages recognized this problem and solved it by installing a temperature sensor in their power meter and automatically adjusting the power readings as the temperature changes. The result is their meter is more accurate than the competition in conditions where the temperature changes. This can be up to 50 watts of erroneous reading during high mountain rides with big shifts in temperature.