Stages SB20 - Comparing Stages Power Meters with Garmin Rally Pedals

"Calibration by Comparison" Procedure

Stages doesn't officially support end-user calibration. The following procedure can be used to validate accuracy. It confirms that your PMs are working properly in real riding scenarios. 

Here's a summary (sorry - I wish it were simpler 😔):

1. Calibrate another power meter. In my case, Garmin Rally pedals.

2. Run tests to compare the Stages PM against the calibrated PM in an actual ride or event.

3. If Stages PM readings are incorrect, change the crank length to compensate (more on this later).

4. Repeat Steps 2 and 3 until the Stages PM gives readings that match the calibrated PM.

Calibrating Reference Power Meter - a Garmin PM  

Garmin Rally pedals support true "calibration": You position the cranks horizontally and then directly measure the twisting force created by hanging a known weight from the pedal. If a pedal is reading high or low, you can adjust the "offset" so it measures the expected amount of torque.

A Garmin computer shows static torque in Nm during its zero reset operation. The expected value is:

Torque (Nm) = Mass (kg) x gravity (9.81) x crank length (meters). 

During my calibration procedure, torque measured correctly (after adjusting offsets) as:

19.2 Nm = 11.34  * 9.81 * 0.1725

To calibrate SB20 Power Meters, see  Measuring Torque Against Factory Specifications 

Garmin versus Stages

I tested the calibrated Garmin pedals against my Stages PM. I used a structured workout with fixed and variable cadence along with erg and slope modes. Most Zwift rides will also work.

To create a chart like the one above, see my blog post on Comparing Power Meters using a Spreadsheet.

You can also get a copy of one of my comparison spreadsheets SB20 Garmin Compare.csv.

Analyzing Results

In this test, the SB20 readings were virtually identical to the Garmin pedals. 

But what if they were different?

Using Crank Length to Adjust Offsets

As mentioned earlier, an offset parameter is normally provided to adjust power meter readings that are consistently too high or low. Garmin and Favero have this feature. Stages does not officially support it. As a workaround, we need to change the crank length attribute.

 Using the Stages Cycling app:

• You can raise power readings by increasing configured crank length. For example: if your actual crank length is 172.5 mm, you can try setting 175.0 mm.  
• You can lower power readings by decreasing configured crank length. For example: if the your actual crank length is 172.5 mm, you can try setting 170.0 mm or lower.

Depending on the crank length you physically use, most people will be able to adjust their Stages PM readings by about -/+ 5% using this method.

You must then re-run your tests to ensure you are getting accurate Stages PM readings versus the calibrated PM.  

Repeat with different crank length settings using the Stages Cycling app until your readings match.

Can Stages Help?

Yes. Stages can do something here. 

Some SB20 owners have reported that the StagesPower app can indeed apply offsets to power readings using an undocumented procedure. Until these steps are known, end-user adjustments are restricted to the minor crank length changes described above.

Reaching out to Stages is your best option if you are convinced your power meters are inaccurate.

One more thing: Is this cheating?

If you change crank length to make your Stages PM more accurate, you are not cheating. If you use this method to "improve" your Zwift results, you are cheating.  

You must decide and do the right thing ⚖️.