Issue with Power Data Recorded in Zwift .fit File


(Eric Oshlo) #1

I have my Quarq power meter paired both to Zwift as the power source (my Kickr is paired as a controllable trainer only) and to my Garmin, so the data being received by Zwift and the Garmin is identical. However, Zwift reports higher average and norm power and lower coasting time than that recorded by my Garmin when I “just ride”; particularly, when I freewheel (stop pedaling) repeatedly on downhills or following a hard sprint/KOM effort. The the average power and norm power reported by Zwift and my Garmin agree within a couple watts when I do an interval workout where I’m pedaling steadily (no coasting).

The data for my “just ride” session on Friday with some harder efforts (https://www.strava.com/activities/514227124) was as follows: Zwift 181/198/4%; Garmin 172/192/8%, average power, norm power and % of ride time at <10w (not pedaling)

Looking at the respective .fit files, there appears to be some form of response lag, smoothing or over-run of power data recorded by Zwift when I stop pedaling briefly compared to the data recorded by my Garmin Edge 1000. The screenshot below shows a 22 second segment of Friday’s ride consisting of the last  13 seconds of a sprint, a 3-4 second zero power freewheel at the finish and then the start of a bit of soft pedal. 

The 2 columns on the far right are the cadence and power data recorded from the Quarq by the Garmin Edge. The 2 right columns on the overlapping image are the cadence and power data recorded from the Quarq by Zwift. Note in particular the difference at between them at timestamp 10:41:29, :30 & :31 where the Garmin recorded 0 power and Zwift repeated the 10:41:28 power and cadence 3 additional times (The garmin also has a bug where it repeats the cadence an additional 2 times when the power is zero). The issue can also be seen at 10:46:54 in the fit file where Zwift again repeats the final power & cadence values 3 additional times before then properly reporting zeros.

 

 


(. TomH..) #2

Hi Eric, 

This is well known issue and we don’t recommend pairing power meters to more then one device/software to prevent this. 

Thank you for your feedback and Ride On! 


(Eric Oshlo) #3

Hey Tom,

Thanks for the response; however, I’d appreciate a little more insight.

What does pairing to only one device/software prevent? Will Zwift not “triple repeat” the last non-zero power data point when I stop pedaling if I don’t pair my garmin with the Quarq at the same time? 

It’s been my understanding that having multiple services reading the same ANT+ transmitted signal simultaneously is acceptable under the ANT+ standard, but I may be wrong about that.

Thanks!


(. TomH..) #4

Hi Eric, 

Yes, officially ANT+ signal can be read from multiple devices but we don’t recommend it as we know it may cause some problems including inaccurate readings. It could also be due to different ways the data are processed by your Garmin device.

Does that make sense?

 


(Eric Oshlo) #5

Thanks Tom,

Without a doubt Zwift and Garmin are processing & recording the data differently. As such though, It’s my sense that the Garmin is more accurate in apparently not recording 3 extra instances of the same power number (i.e. 3 seconds’ worth) in the .fit file when I start freewheeling.

I feel that is a Zwift anomaly since virtually nowhere else in the Garmin or Zwift file is there a 3+ second repeat of an identical power number (516w in this case) other than in Zwift when freewheeling starts.

Notwithstanding the ANT+ spec, it certainly is possible that signal timing with Zwift is causing problems when my Quarq is paired to both Zwift and Garmin. I’ll try a session with only Zwift/Quarq pairing (no garmin or other device) and see what is then recorded in the Zwift .fit file when I freewheel.

Thanks again for the response.


(. TomH..) #6

No problem Eric, that’s what we’re here for :) 

In order to prevent this problem I’d also run “only” Zwift on the PC - especially if you have a weak PC so it can fully focus on processing the Zwift/Trainer data. Let us know how it went! 

Good luck and Ride On!