Calibrating trainer against reference power meter: an app idea, request for comments

Yesterday I compared my Tacx NEO 2T against Favero Assioma DUO power meter pedals.

The difference was larger than expected since both are considered good sources of power.

Overall, the pedals power value made more sense as the trainer under-reporting could be correlated with drivetrain losses - although more research is needed.
Also, the pedals react faster which is desirable for Zwift racing.

I would prefer not have to install the pedals on the bike on the trainer every time I’m Zwifting, yet still get consistent figures whenever the pedals or trainer are used as source of power data.

So I’m thinking about making an app to relay corrected power data from the trainer to Zwift.
The calibration process would be done by recording a few rides with two power meter sources at various intensities, then the app would create a correction profile automatically.
I believe it’s a preferable approach to setting one or several multipliers manually since:

  • it would make this app a very convenient way to cheat realistically
  • an algorithm relying on speed, cadence to match both power sources has more chance to make an accurate calibration profile

I have the skills and knowledge to make an Android app for myself quickly, but would anybody else be interested using it?

Potential other features I can think of for later:

  • Virtual chainring to apply a resistance offset - useful for MTBs in sim mode currently running out of gears.
  • Override sim mode to ERG from a toggle in the app.
  • Power + Cadence + Heart Rate bridge in a single Bluetooth LE connection for Apple TV (probably not useful anymore as the companion app already provides a solution for that)

Let me know :slightly_smiling_face:

There are quite a few reports of Neo 2T underreporting power against any number of power meters. This is my experience as well, and I’m using a waxed chain so definitely not buying the drivetrain losses explanation.

I’m still naïvely hoping that this will finally be fixed in a future firmware update, but until it is, I’ll stick to my Assiomata as well thank you very much.

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Thanks for confirming the significant power discrepancy.
In your case, do you always install the power meter pedals on your indoor bike?

I’m thinking that for owners of pedals, this calibration and matching app might not be that useful due to the ease of swapping pedals between bikes.

However if you have a crank / spider / hub based power meter one one bike and not another, then matching everything to what you decide is the reference might be a neat tool for consistent training and racing.

Likewise, the owner of a (consistently) inaccurate trainer could borrow a friend’s power meter and build a correction profile instead of having to make a costly upgrade for more accurate indoor training.

Yeah, I just keep the pedals on my fair-weather road bike that I use indoors. Perceived exertion is enough for me outdoors but somehow Zwift doesn’t provide an interface for that :slight_smile:

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I really would like something like this for two reasons:

1.My NEO 2T reads a lot lower than my Vector 3 pedals.
Because of this I had to record multiple rides. IE record the vectors via Zwift and the Neo via the Garmin head unit. The. I calculated the difference in percentages and changed my FTP according to that.
My FTP in real life was 314. In Zwift I had to downtune it to 301 to get the targeted power numbers when doing a workout. But still this is not perfect yet. Also my TSS scores will be off.
2. Because power match is not as smooth as using the NEO for workouts. The NEO responds quicker to changes in workouts or hills etc when riding in simulation mode. Power match seems to react slower. Especially when doing 5 to 10 second sprint workouts.

I very much would like this functionality. I realize that it has a risk of being abused, but there are other and easier ways to cheat. I also have a Tacx Neo 2T which consistently measures 5% lower than my NG Eco spider-based power meter on my XC bike. To get power targets to match indoors and out I have been using the power meter as my power source on Zwift. However, I would like to use my road bike on the trainer and this doesn’t have a power meter. It would be great if I could make this adjustment directly in Zwift, but if you could make a program that would be awesome too! Hopefully, it would work on my mac or with the zwift companion app on ios.

You should not be expecting these figures to match exactly. The Assiomas can have a 1% margin of error. So can the Neo 2T.

So, with the drivetrain losses added in as well, that can easily add up to the 5% that you’re seeing.

Interesting that with the trainer season coming up in the Northern atmosphere, the threads is picking up interest!
I don’t think I need the app anymore after realizing how easy it is to install and remove power meter pedals, and yes there’s the potential for abuse.

The potential for abuse could be mitigated by not giving the user a basic % correction factor slider but only allowing to calibrate in realtime one power source against another.
Otherwise I think that such an app might be detrimental to the community instead of helpful overall.

Let me know if more of you would need such app, then I (or maybe someone else) might build it :slightly_smiling_face:

Interesting, I’ve mounted my ‘old’ Favero BePro pedals on my Tacx Neo (later build of the first serie). The Favero BePro reads a higher power than the Neo. I’ve no clue why, the Neo cannot not be calibrated as far as I know (it doesn’t need it according to manual). The BePro indoor-values match the outdoor-values. So the power-loss may be the drivetrain-loss - though I think the gap is too big for that; or just a software thingy… that can be corrected by a FW update of the NEO(s) or a smart-trainer-specific correction in Zwift …?

Why does it seem people freq report the trainer as under reporting instead of the pedals as over reporting?
How do you know which is the correct one.
The truth is probably Both are as correct as possible.
Neither system is really reporting power.
They are reporting strain applied to a piece of metal in a pedal and the resistance of fluid in the trainer.

Both are used to approximate , very closely, how much power you are producing but both will have a built in measurement error.
I looked at the graph and thought both systems are very good and you can use either.

I recommend you go with the lower one as an incentive to work harder.

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Tests of power meters normally show that pedals and cranks all report values very close to one another, and unless they’re all wrong to the same degree, the most likely explanation is that they’re all very accurate. Similarly trainers seem to vary more from those power meters when tested side by side, indicating that the most likely explanation is that the trainers are less accurate (except for the most expensive ones, but even then there can be differences with trainers reading fractionally lower because of small drivetrain losses).

As for the power meter only measuring the strain applied to a bit of metal, well, yes, but you realise that Power = (Force x Distance) / Time, right? So the pedals or crank power meter are indeed measuring power applied to the pedal.

interesting, I don’t use power data from my Tacx, as I can pedal to vomit, and still get no higher than 58 watts, so I just use my Assioma pedals and swap when I cycle outside.
No amount of dicking about has ever fixed the Tacx, and it is easier and faster to use the pedals, so the Tacx is simply an expensive way of simulating hills for me.
Therefore, I reckon I would not use such an app.