Sufferlandrians using Xert to track fitness?

For everyone using xert or other tracking app, how do you record record accurate power?

I have a left crank pm and I am dual recording as there can be approx 10w difference between that and my H3 smart trainer. It’s a bit cumbersome as I have to record the crank via head unit and then upload to Strava/xert. I am using trainer to drive erg etc in suf. etc which it is excellent at doing.

I could just record and track using my trainer power but it’s could be inaccurate compared to outdoors.


The problem with left only cranks is, that if you have a (significant) L/R imbalance, the reported value is off.

Likewise, I don’t know how accurate your H3 is supposed to be.

Bottom line: either can be correct or off and the ‘truth’ might really be in the middle. For consistency, you’re better off using the same power source for all rides, indoors and outdoors.

Depending on your setup, you can use the Ant+ channel to send your H3 or your Stages crank power stream to both recorders (Xert/SUF).

If you’re BLE only (phone and desktop or tablet), you’ll have to pick one (device) to record on Xert and one to record on SUF.

Obviously, neither can control your Stages, so I’d work with the H3. If that’s BLE and Ant+, you’re good to go and if your desktop is lacking Ant+, buy a cheap adapter for it. Then send BLE to your phone and Ant+ to your desktop, or the other way around, if you have an Android phone with Ant+ and a BLE only desktop.

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Yeah, thats a good topic…
I’m in the exact same boat…comparing my stages left only with my Flux S, the stages reports about 6-8% more power. Which one is right I have no idea. As when I go outside, in general I don’t follow interval training and it is more to enjoy.
I have decided to do the Full Frontal test with my Flux and use the Flux Power for all the sessions. Anyhow, obviously I’m aware that once I go outside this might mess up slightly my stats and I hate that :smiley:
But I believe ERG mode etc works better when regulated completely by the trainer.

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topic might shift from ‘using xert to track fitness’ when we get in to power meters inside/outside.
Beware TL:DR coming up …

There are so many approaches to this.

  1. For a person who rides one bike only and doesn’t use power from a trainer (or doesn’t care about other bikes for power)
  • then use it for everything - it will be the closest one can get to consistent power info. Still means that outdoors/indoors is different though (heat/ cooling/ natural changes in terrain/psychological/even quality of air?)
  1. With two sources of power (one being a trainer inside), one being something on a bike (pedals/cranks…)
  • the numbers will just be different. More factors come in to play - as well as indoors vs outdoors often resulting in different numbers, the power meters will be different … and if single sided then the difference may be exaggerated.

Options are to do an indoor comparison a few times running whatever the trainer is vs whatever the bike has (using a head unit to record it precisely same start/finish times) and check the difference.
Then we know what the difference is between both the bike device is and the trainer is indoors. Can be used to estimate tss if you use that. Still doesn’t account for outdoors though.

  1. Might have a MTB with a crank, road bike with power pedals AND a trainer.
    To even this lot out it’s the same exercise to measure how they all stack up against each other be a trainer.

All in all, for complete consistency either the training always has to be inside on the same device, or outside on the same device.
Everything else requires either dual measurement like mentioned above. Or just measure once and apply a ‘factor’ to the effort for measuring fitness / tss type stuff.

Fwiw - having done some of this over the years, I now treat indoor training as the number generator for how many power looks … it’s consistent and easy to relate to.

(I then keep a note of how the power curves look like for the outdoor riding, but I keep that completely separate and don’t directly compare it to the indoor stuff)

And when riding outdoors a lot, for tss guesstimating only I simply take 10% off the tss when it uploads to trainingpeaks to account for a) the 2-3% difference between cranks and Neo and the rest is for outside / inside.

There so many variations I found it was important to pick ‘a way’ and then use it.

I think the “inaccuracy” complaints about left sided PMs is well overblown. At the end of the day it doesn’t matter if the readings are consistent and accurate. Everyone talks about the left right imbalance, but you don’t really do anything about it. If it’s because of an injury, it’ll be pretty apparent during rehab too.

The more likely reason for your crank based pm to not match your trainer is because they’re both measuring at different locations. The differences have been pretty well documented by DCR and GPLlama. There’s really no reason to rehash this conversation ad nauseum.

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But they are not, if you use a left sided crank PM and a smart trainer.

I have Favero Assioma Duo’s and compared to my Neo 2T - they were ~5 Watts off (lower in my case) and the Favero app has the option to adjust that, so I did. Stages doesn’t offer that possibility.

Even if my Neo is not 100% accurate, at least both sources are within acceptable parameters and since it’s dual sided, my (natural) 48/52, 47/53 - it varies abit - imbalance doesn’t matter much.

It would on a left only crank, as on average, it would be ~10 to 12 Watts.

So, it does matter…

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Still not a convincing reason why it matters.
Use the power on the trainer for power tests indoors and then record using the crank based PM. So the trainer will spit out whatever it needs to on erg mode (that you tested with) and your power readings will be consistent between the road and indoors.

Also of course they’re going to differ. They’re measuring power at a different place. It’s the exact reason why crank based, hub based, and pedal based don’t really read the same. They can be offset to match, but they’re all going to ‘see’ a different power output regardless.

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Okay :ok_hand:

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I still <3 you.
your brother in suffering.

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What I was asking was more related to xert, if I didn’t use xert or any similar tracking software then wouldn’t care, we all know each power source is accurate to itself and not B to B anything else. But when you are pushing for a breakthrough on xert 5% difference in power is not small. I wondered if people let it slide instead of dual recording.

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I’m not sure how to answer that. Exactly because I don’t want differences between my in- and outdoor rides, I have the setup I described.

Assuming your Xert signature is based on all activities, in- and outdoor, you should be to able achieve a BT in either scenario.

If your Xert TP/MPA is too high for your indoor setup, it will be difficult.

I wouldn’t mind much - I know XATA will start to warn you that your signature is stale as a result of lacking BT’s, I’ve ignored that every time.

A BT will eventually happen anyway, if you keep training, following SUF or XATA workouts, in- or outdoors.

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Yes thanks, I agree best to keep all one source so I am going to continue to dual record, my setup is iPad with suf and H3, then head unit recording from crank.

I don’t upload the suf workout to Strava, just the one from head unit automatically goes to Strava which in turn syncs with xert. Only downside is you don’t get any of the nice graphics and info from suf in strava, wahoo bolt just sends time and power which is all xert needs

Xert actually only uses the Elapsed time which in this case was 1hr, but Strava just shows moving time.


After using SUF and xert now since good 3 weeks, I m pretty surprised how close those 2 are. The training recommendations on xert are pretty much fitting my SUF trainingplan in regards to XSS suggestions.
And more interesting is the Time to total exhaustion and time to total recovery from xert (using the garmin app). Having this running in parallel on my edge 530 during the SUF sessions it was really cool to see that the intervals are spot on here. During my « The shovel » session at 100%, xert showed me 3 seconds left in the time to exhaustion before going to recovery. And the recovery part of the SUF workout almost cleared the time to total recovery to 100%. I could see the same on other sessions.
Is it just coincidence or did others have similar observations?
The science both are based on seems to be the same or almost at least.

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interestingly, I’ve just been listening to some of the casts from the Endurance conference, and one of them (still going through it) makes this point, that once you know someones critical power output, you can then workout what time it is to failure across a range of power output. With this in mind, I looked at Xert today (haven’t used it for 4 years!) and may also track both to see.

Hi everybody,
I also use Xert and dread to take yet another FF. So my question at @ZenTurtle and @NicoAttacker is: Both of you seem to use Xert and put in manually 5sec, 1min, 5min, TP Power from the xert curve (when you feel you have an accurate signature) in the 4dp settings. Do you feel the numbers work for you?
Will try during the next week for myself, just curious if it works for you guys.

Hi @fgmow,
If you follow the process of FF before plan and Half Monty in the middle of the plan, you don’t need to adjust manually the values … Xert is a good way to track progress, and fitness on the long term (automatic calculation of XSS is more reliable than TSS (manual FTP value) ). I only use Xert PDC to target goals before FF with improved metrics of latest fitness signature .
It’s great to use Xert when you are racing on Zwift … it helps to pace yourself and strategy.

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Thanks for your reply @NicoAttacker !
I guess my real question is: How much do your 4dp values differ from your Xert pdc (assuming you had a breakthrough during ff or short before)? And (hypothetical) if there is little difference to the xert pdc, why bother putting me through a FF? I mean c’mon … executed properly this is madness.
For my part, I put in the xert values some days ago, did Team Scream and Cobbler and both felt just about right (I’ll admit that both workouts target upper Tempozone so probably just ftp is right and MAP is off, but I doubt that and will confirm via The Chores later today.)
Edit: The Chores felt just about right, too.

I am expected to do FF next Sunday per plan, and my PDC values match with 4DP excepted with AC (my strength).
Last hard workout was Half is Easy in level mode and I went deep. Result of new fitness signature is an AC value of 500 W … my personnal best is 430 W, current value is 350 W.
So I will try to beat my AC PB on Sunday … It’s all about freshness and mental at the end of the test.

I am starting to explore Xert.

Do you use the Xert workout designer with MPA to figure out the optimal pacing strategy for FF?

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What a good idea @Heretic … I never tried this way

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