SB20 - Compensating for Power Meter Inaccuracy

Are my Power Meters Accurate?

One of the most common complaints on cycling social media is: "My power meters are inaccurate."

Reasons include:

1. Riders perform better in different environments. Whether it's outdoor or indoor, many cyclists see higher power in specific scenarios.

2. You’ve done a dual meter comparison using something like ZwiftPower (Comparing Power Meters with ZwiftPower).

3. The power meters are actually bad (opinions vary so please keep reading)

 

Here's the problem: Comparisons can be misleading. Indoor and outdoor riding environments differ significantly. The resistance and acceleration vary greatly, causing many power meters to struggle with accuracy in one environment versus the other. Riding style is also a factor; smooth riders often see different average power measurements compared to those who vary their cadence and acceleration.

With dual power meter comparisons it is impossible to know which power meter is correct when only two are used. One could be better than the other Or both could be marginally inaccurate. A particular riding style can affect one power meter but not others. This means that two additional power meters in agreement does not guarantee accuracy. Finally, crank and spider power meters measure power differently than more complex power meter pedals.

Don't forget to check your batteries.  Just beware.  Low batteries don't necessarily cause low power readings.  See Batteries and SB20 Power Meters.

Conclusive Testing

The best way to test most power meters is with a torque test. This involves using a static weight to measure the deflection detected the power meter's sensor. Here's a complete description for measuring Stages crank-based power meters: Stages SB20 - Measuring Torque Against Factory Specifications.

As mentioned previously, you can compare your Stages PM against other sets of power meters.  This method is easier - it's just not as accurate.  Here's how to do it:  Comparing Power Meters with ZwiftPower and Comparing Power Meters using a Spreadsheet 

What if my SB20 Power Meters are Inaccurate?

If you've done a torque test or dual power meter comparisons with multiple power meters and found that your Stages power meters are inaccurate, here's what you can do:

• For errors less than 10%, you can tweak your declared crank length using the Stages Cycling or StagePower apps. Increasing the declared crank length over the actual length raises power readings, while decreasing it lowers them. Check out the "Making Minor Changes" section in Stages SB20 - Measuring Torque Against Factory Specifications.
  
  
• For errors greater than 10%, you're pretty much out of luck unless you can get new power meters from Stages or maybe on eBay. There is some good news: you can use a third party set of power meters alongside your Stages power meters and still maintain full functionality.

Using a Third Set of Power Meters with Stages Cranks

You can use both third-party and Stages power meters together. You'll need to use them differently in specific scenarios. Here's how:

Erg Mode

The Stages power meters are always used in SB20 Erg Mode in apps like Zwift, TrainingPeaks Virtual, or TrainerDay.

Use the +/- bias control to raise or lower the target power to correct for errors found in testing. For example, if the Stages power meters measure 20% low, you should lower the bias in your Erg mode workout by 20% so that you hit the desired target (you lower the bias - otherwise you would be working too hard in Erg mode).  Conversely, if your power meters measure high, you should increase Erg mode bias accordingly.

Note: Even when you connect to 3rd party power pedals, like shown in the next section, Erg mode in any app will always use the Stages power meters. I know it's counterintuitive. Think of it this way: Erg mode always uses the Stages power meters regardless of your workout app Power Source setting. 

Free Rides and Racing (Non-Erg Mode Sessions)

For free rides and races, use the third-party power meters that you know to be accurate. For instance, if you have power meter pedals, connect to these as your power source in Zwift.

Power reading will be provided by the 3rd party pedal so long as you stay in free ride / sim mode.

📣 Note: There are other limitations when using third-party power meters, particularly if your SB20 power meters have failed altogether. For more details, see SB20 - My Power Meter Cranks Failed - Now What?.