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Average Power Discrepancy in Garmin and Strava

4419820
over 3 years ago

I just got a Stages Power Meter for my MTB. I did my second ride yesterday and there is a discrepancy in the average power between Garmin and Strava. I do have auto-pause enabled on the 530. Garmin shows 186 for average power and Strava shows 173. There is also a discrepancy in the speed, on my road bike Strava is always - .1 MPH lower. I went back and looked at the first ride when I used the power meter and there is a similar discrepancy. I have the Vector 3 on my road bike and the power numbers between Garmin and Strava are usually exact. So I am trying to figure out why there is such a discrepancy on the MTB. Thank you.

https://drive.google.com/open?id=1RtBjXIM-QStIAfDzR8npjHuCLVuPU3nn

https://www.strava.com/activities/3447179522

  • 0 over 3 years ago

    It sounds like Strava is averaging the power over the elapsed time and not the activity time as the Edge is with auto-pause enabled.

    Let me have a look at the activity file for you.

  • 0 over 3 years ago in reply to aweatherall

    The value of 173 is the same value I get if I sum and average the individual power records over the activity time.

    The 186 value is computed using the accumulated power value reported by the power meter.

    The power meter sends a running value of the power produced as well the individual messages.

    I'm not sure why the difference is so large. You can see that the sum of the power records and the reported accumulated power values drift part throughout the ride.

  • 0 over 3 years ago in reply to aweatherall

    Thanks for looking into this. That is what I don't comprehend, when I am on my road bike the elapsed time is always greater than the moving time, yet the power numbers between Garmin and Strava are consistent. Not sure why there is a difference with the Stages Power Meter on my MTB.

  • 0 over 3 years ago in reply to 4419820

    I suspect that the issue is down to the different riding styles from road to mountain and how the Stages cadence algorithm works on that first revolution after a coasting period.

    You will see from the data plot on your MTB ride that the cadence is frequently going to zero. Power is computed on each complete crank revolution determined by the power meter. I'm guessing it might be that each time you start pedaling it is taking more than 1 crank revolution from the position you are starting to  pedal in to generate a complete crank revolution.

    A new power record is sent to the Edge on each complete crank cycle. I'm guessing that the power generated on the incomplete crank revolutions is getting added to the accumulated power value that the power meter is tracking and sending to the Edge, but that data is not making into an individual power record.

    You can test this by riding a fire road or gravel path where you can maintain your cadence. I suspect that you will find that with fewer starts and stops of cadence that the values will match again.

  • 0 over 3 years ago in reply to aweatherall

    Thank you, I will try that out. I don't understand why Strava doesn't just use the data from Garmin. I read a bunch of threads on this issue.