Elapsed time different on Zwift to Wahoo (trying to dual record!)

Hi there, first time posting!

In an effort to be more transparent/check my smart trainer isn’t wildly out, I’ve started to try and dual record my rides, using my elite direto to Zwift, and 4iiii PM to my Wahoo for the second recording.

When trying to line up the graphs to match the data, I have found something a bit odd!

It seems that the data from the smart trainer and the data from the PM slowly start to diverge on the timeline. For example, I have cropped to adjust for the fact I started my Wahoo 9s late and at the start of the gran fondo on Saturday, the power numbers match up. However the further I go into the ride, they slowly started to diverge and by the end, the 4iiii-Wahoo combo shows my ‘final sprint’ finishing at 2h28min8s, but the corresponding spike on the direto-zwift is almost 45 seconds earlier at 2h27min26s! I seem to have lost 42s of data somewhere compared to the powermeter! The 4iiii data corresponds much better with my ZP finishing time for the event. I can get the data to match up if I change the offset for the direto but then it means it is then not matched up at the begginign of the ride! Somehow it has compressed the data to seem like I have completed it nearly 45s sooner?

Interestingly, I have noticed that I also ‘lose’ about 15-20s an hour when doing structured workouts, in that despite setting a workout for say 1 hour exactly, I complete it in 59mins45s and I start noticing this from about 10 minutes in that the ‘time completed’ and time left’ starts to mismatch.

I am on an apple TV 4th gen set up. I had a look through the forum and it seems the workout discrepancy has been raised before but can’t see any answers.

Any thoughts?

Yes, there is another thread with the same issue.
I can’t find it not, but if you already found it perhaps ask if there is anything similar in your setups (ATV? Trainer?) - could be a good start to find what the reason is.

Just FYI (I know it doesn’t help you, sorry) - I did double recording with Zwift (powermeter, P2M NGEco, connection with Companion App) and Jepster Android App (Tacx Neo, connection ANT+ on my cell phone) and had never any difference.

Two things. The zwift fit file always starts 10 to 20 seconds after the start of the race when you cross the line coming out of the pens, so it will always looks shorter than the powermeter file. Also if you stop in-game the time does not continue to run (unless you are racing I think), but the fit file should show a gap where you stopped. (I don’t do structured workouts, so no idea what is going on there.)

For comparison, upload your dual to the Zwiftpower analysis tab. It is rather good for aligning and cropping files if you want it to look pretty. I have never seen divergence of files happen with the tool.


So I picked this up when uploading the dual record to zwiftpower.

Perhaps there are some zeros in the fit file where it stops the timer and cumulatively this adds up but as I was doing the Fondo, riding hard, I never stopped so this shouldn’t be the case!

I have definitely lined up the start of the 2 files with the crop and offset but by an hour in the 2 files are out if sync by 10-15s and by the end almost 45s. I can post some screenshots when I’m back on my PC. I couldn’t work out why it’s diverging!

I wasn’t sure if you were using the ZwiftPower analysis since it wasn’t on your public profile.

If it is deviating, maybe there is something weird going on with certain zwift clients as noted in the other post. I have certainly never seen that happen with zwift on either the iPad or PC with a Garmin comparison fit file.

Yeah, I have seen this as well in a teammate’s recordings, she was using an Apple TV as well. The individual data points are tagged with the actual time and date (and not elapsed time since start), so I guess it just means that the real time clock in ATV is rubbish.

I presume it is not possible to configure the device to update the clock with NTP every five minutes or something…

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It is the result of bad coding on slower devices (older ATV, older IPADs, Android devices, etc, i reported it 1.5years ago with Android , they couldnt fix it yet, as You still dont have main menu in Zwift app … probably it will be solved 10years later.), If You wanna accurate timings, record your Direto’s power with an additional head unit (or with Wahoo Fitness app but it works via ANT+ if your ATV is using the BT, or record the Direto’s power with your Wahoo head unit via ANT+, and the 4iiii’s power with Wahoo Fitness app via BT ) and the 4iiii’s power with Wahoo head unit, and You will have 3 FIT files, the 2 head units 's files will match without any time length issue.