With the prevalence and affordability of cycling power meters, more athletes are training with power than ever before. As with any training tool, power meters come with their own array questions – including the oft discussed topic of calibration. We asked PowerTap Engineer, BJ, to weigh in on some of these burning calibration FAQs.
1. WHAT DOES CALIBRATION MEAN?
In most cases, calibration is simply zeroing the power meter. If a bike power meter is designed well, there should be a good linear correlation between the force measurement output (Y-axis below) and input (x-axis below). The same input can generate very different outputs when the power meter does not have the right zero point. While the slope of that response is very consistent regardless of conditions, the zero-load point can drift based on temperature and time.
Figure 1 below is an example of what the output would look like, with good (zeroed) power meter data in blue, and bad (not zeroed) power meter data in red.
Figure 1. Strain Gage Readout
2. HOW OFTEN SHOULD I CALIBRATE MY POWER METER?
In general, we suggest zeroing your power meter before each ride. This will help ensure your power meter is providing accurate, consistent data. However, if you consider yourself a more casual rider this every-ride calibration may be less of a concern for you and your training.
The P1 Pedals in a factory calibration stand here at our headquarters in Madison, Wisconsin.
3. IS CALIBRATION PROTOCOL DIFFERENT FOR THE DIFFERENT TYPES OF CYCLING POWER METERS (I.E., HUBS VERSUS PEDALS)?
The type of power meter does affect the benefits of zeroing. The unique design of our pedals, which measure force on the pedal body, cancels out most of the error from non-zeroed force offsets. Because the pedal body stays in the same orientation all of the time, if there is a force offset pointed down, then that adds to your power on the front side of the pedal stroke but subtracts from your power on the back side of the pedal stroke, effectively canceling itself out.
Other pedal-based power meters measure on the spindle instead of the pedal body, and therefore the errors do not cancel out. If there is a non-zero force offset in any power meter other than our pedals, that error just keeps adding up as the system rotates around.
Additionally, the PowerTap hub-based power meters generally do not drift from zero without a major change, like a large temperature swing.
Our G3 hub power meter under goes a wheel build.
4. WHY SHOULD I CALIBRATE MY POWER METER?
If you’re tracking training data, then zeroing regularly is important to get data that is comparable week-to-week. A power meter can easily drift by 2% in a few days if it is not being zeroed regularly, so to track long term training improvements zeroing often is important.