Comparing Power Meters using a Spreadsheet

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My Stages SB20 Smart Bike has crank power meters.  I routinely compare them with readings from my Garmin Rally Pedals.  Here's how I do it.

Add Gamin Pedals







Mount Garmin pedals on the SB20 Smart Bike.

This means two independent power meter systems are available to simultaneously measure the same activity.





Prepare an Activity

You can use Zwift to do a free ride, race or structured workout (or consider using a advanced structured workout app like TrainerDay). 


Configure Zwift to use the Stages Bike for Resistance (formerly Controllable).

Use the Stages PM as the Power Source and for Cadence.

Tip: Also use an HRM. Heart rate data helps you synchronize metrics in a later step.
   


Get a Second Opinion

Use another app or device to simultaneously record your Zwift Activity.  I use a Garmin computer to record activities on the Stages Bike.  It is configured to use the Garmin Pedals for Power and Cadence. It also gets heart rate data from the same HRM used in the previous step.

Test Scenario

The overall scenario looks like this:

Summary

Zwift will:

1.  Get Power and Cadence data from the Stages PM.

2.  Control the SB20 Smart Bike.

3.  Get Heart Rate data from a specific HRM. 

Garmin Computer will:

4.  Get Power and Cadence data from the Garmin pedals.

2.  Gets simulation data SB20 Smart Bike that is controlled by Zwift.

3.  Get Heart Rate data from the same HRM as #3 above. 

Doing Test Ride

You can now start a free ride, race or structured workout in Zwift:
  • Zwift Event: Start the activity in Zwift.
  • Garmin Computer: Start the Garmin Computer when the Zwift Event starts.
When the event ends:
  • Zwift Event: Save the event. You can optionally configure Zwift transfer the results to connect.garmin.com to make the next steps easier.
  • Garmin Computer: Stop the Garmin Computer when the Zwift Event ends. Transfer the workout to connect.garmin.com.
You now have two views on the same event. If you configured Zwift to transfer results to connect.garmin.com, both workouts will listed there as Activities. If you didn't set this up, just download the .FIT file for this event from Zwift. 

Zwift and Garmin computer files measure the same power produced by a rider.  Speed and distance do not match.  Read why it doesn't matter: Smart Bikes and Bike Computers are WRONG. 

Getting .FIT files from connect.garmin.com





Download the .FIT file(s) for the test:

✓ Open the Activity
✓ Click ⛭ utility icon in the top right corner
✓ Export Original menu item.


 


You now have two .FIT files: 

1.  From Zwift with SB20 power readings.
2.  From Garmin Computer with Garmin pedal power readings.
 

Convert .FIT files to .csv 



Use website utility: https://gotoes.org/strava/convert_fit_files_to_csv.php to convert your .FIT files to .csv

Reload this page twice to create .csv files for each of your .FIT files.





Load each .csv into a Spreadsheet

Use your spreadsheet tool of choice. Excel and Google Sheets are fine. I used Apple Numbers.


  Notes:

  1. I normally take just the heart rate, cadence and power columns from each .csv file. I rename SB20 columns to SB20 HR, SB20 cadence and SB20 power. I repeat this pattern for the Garmin columns.
  2. Samples usually do not align. Timestamps don't help much. Instead, I the HRM to align patterns in the data. In this example, I push the SB20 data up by one row.
  3. (optional) I add calculations for averages of the different cadence and power columns. This is helpful for quick comparisions.

Building Charts

There are lots of analysis options, depending on your spreadsheet skills. 

To get started, I normally select the different power columns and create a line chart:


If you got this far, you're now on your own.  Good luck and good testing 🚴🏼‍♂️.

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