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Home > Forum > Triathlon Forum > Wwhy do fit files recorded on zwift vs Head units differ?

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Wwhy do fit files recorded on zwift vs Head units differ?
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davidalone
Dec 30, 20 9:57
Post #1 of 9 (817 views)
I am doing some power testing of an Xcadey Xpower S versus my elite direto and I noticed something strange.

I have the Xcadey reporting to my fenix, the elite direto reporting to my garmin 820 head unit and also to zwift on my laptop.

When analysing the fit files from zwift and the head unit, the values are mostly the same... Except that zwift reports higher power in the sprints (short intervals) than the head unit. Anyone have any idea why!?
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Re: Wwhy do fit files recorded on zwift vs Head units differ? [davidalone] [ In reply to ]
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gregf83
Dec 30, 20 12:13
Post #2 of 9 (783 views)
Not sure how Xcadey works but certainly with SRM the power recorded by zwift is different than Garmin as the power is calculated by the head unit (or zwift) and uses torque, cadence and zero offset. With an SRM, the zero offset used by zwift is not the same as the zero offset used by the Garmin head unit. Zwift uses auto-zero with SRM which isn't the same as a manual, unclipped zero-offset.
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Re: Wwhy do fit files recorded on zwift vs Head units differ? [davidalone] [ In reply to ]
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rrutis
Dec 30, 20 17:13
Post #3 of 9 (746 views)
DC Rainmaker and GPLama have done lots of testing on all manner of power meters and trainers, that power mismatch shows up not infrequently. A lot of times it seems to be tied to how the cadence is measured...if the cadence registers low on the XCadey then the power will be low.
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Re: Wwhy do fit files recorded on zwift vs Head units differ? [gregf83] [ In reply to ]
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davidalone
Dec 30, 20 21:14
Post #4 of 9 (719 views)
For clarification this has nothing to do with the Xcadey-- it is the elite direto reporting to the garmin 820 head unit versus the elite direto reporting to my laptop on zwift.

The elite direto has no zero offset, so surely that isn't an issue?

The accuracy of the Xcadey is a different matter.
Last edited by: davidalone: Dec 30, 20 21:17
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Re: Wwhy do fit files recorded on zwift vs Head units differ? [rrutis] [ In reply to ]
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davidalone
Dec 30, 20 21:16
Post #5 of 9 (718 views)
Again, this has nothing to do with the Xcadey. It is the elite direto reporting to the garmin head unit versus the elite direto reporting to zwift on my laptop.
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Re: Wwhy do fit files recorded on zwift vs Head units differ? [davidalone] [ In reply to ]
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gregf83
Dec 30, 20 22:34
Post #6 of 9 (703 views)
Try taking the zwift log files and dropping them on zwiftalizer.com. That will tell you if you’re getting dropouts. It wouldn’t explain a constant bias but if the results are randomly different it could be due to a poor signal to the head unit or the laptop.
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Re: Wwhy do fit files recorded on zwift vs Head units differ? [davidalone] [ In reply to ]
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RChung
Dec 31, 20 15:40
Post #7 of 9 (617 views)
Could this be part of the issue?

https://zwiftinsider.com/sticky-watts/
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Re: Wwhy do fit files recorded on zwift vs Head units differ? [RChung] [ In reply to ]
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Rappstar
Jan 1, 21 9:08
Post #8 of 9 (568 views)
RChung wrote:
Could this be part of the issue?

https://zwiftinsider.com/sticky-watts/

it's almost certainly this. For what I think should be obvious reasons, Zwift handles dropouts very differently than a head unit does. If your headunit records a dropout as a "zero," it doesn't affect anything (other than your records), but if Zwift were to do the same, you'd stop moving (or at least slow down).
"Non est ad astra mollis e terris via." - Seneca | rappstar.com | FB - Rappstar Racing | IG - @jordanrapp
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Re: Wwhy do fit files recorded on zwift vs Head units differ? [Rappstar] [ In reply to ]
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trail
Jan 1, 21 10:13
Post #9 of 9 (553 views)
Rappstar wrote:
RChung wrote:
Could this be part of the issue?

https://zwiftinsider.com/sticky-watts/


it's almost certainly this. For what I think should be obvious reasons, Zwift handles dropouts very differently than a head unit does.

Sticky watts has nothing to do with dropouts, though. (Unless you were bringing up a new point with the 2nd sentence).
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