Multiple power meters

Does anyone have any advice on how to better deal with using multiple power meters? For all my outdoor rides, I use my Vector pedals, but most of my indoor rides are in a gym, which has Stages bikes with left only power meters… on the one occasion that I’ve done an FTP test and recorded the results from both my vector pedals on one head unit and the stages power meter on another head unit, I found that my 20 min power was about 13% lower from my dual sided meter (which is consistent with a leg strength discrepancy and accuracy tolerances)…

Is there any way to tell Xert (or Training Peaks for that matter) to reduce my power output from the Stages bikes by 13% to bring it in line with the vectors?

Does Strava / Garmin connect record the ANT+ ID or device type used to record power? It would be brilliant if I could automatically adjust any left-only power sources.

Hi Ashley, very interesting post and like your questions. Try to address herewith some ideas I have myself. Adaption of a large number of different brands is a fact and also trainers bring wattage numbers since FE-C protocol. There is a lot of documented experience about spread one can find on the web. You found a average offset (13%) and I’ve heard from my LBS that same kind of numbers. I compared yesterday my Neo on one ride with my Vector pedals and find on 60 minute mmp that the Tacx Neo is minus 4%. The questions however is how Xert respond on the data because I do not think that average offset is important but the hole picture. See attached:
Further I use also a Garmin and the fit file includes the name of this device.


I certainly agree that L/R balance will probably vary with time and even in a particular ride, so any compensation is going to be approximate… However, I know that currently, on average my L/R balance is such that I can expect a left only power meter to read roughly 13% too high (compared with a dual sided meter)… On any given ride, it might be 10% or 15%, but I know it’s roughly in that ballpark…

In an ideal world, I would only use power meters that properly measure total power, but unfortunately that isn’t an option for me - a large number of my rides are with stages left-only meters. In this situation, unfortunately, I don’t have enough data to get the whole picture, so the best I could do is scale all the power readings down by 13%.

In terms of feature requests, I guess I’d like Xert to have the option to adjust individual rides by a certain factor…

Ideally, if the device ID is recorded in the data file, I’d like the same compensation factor applied to all rides with that device automatically.

Even better (again, if the data is available) would be for xert to ‘learn’ from my rides with dual sided power and automatically apply a compensation to any left-only rides… To begin with, just some kind of moving average of the L/R balance from the last 10 dual-sided rides to automatically apply to left-only rides… You could obviously get even more sophisticated and learn patterns about how L/R balance changed with power output and tiredness, etc. to produce a more sophisticated compensation for left-only rides…

Ashley, there are really four main features that are in our current pipeline that will help you to better manage your power data:

  1. Altitude compensation - Although this doesn’t affect the power data directly, it will affect your analysis.
  2. Power scaling - As you point out, some power meters / trainers have very different power data at times. Allowing you to select which power source and to have the ability to scale it is a feature we will be adding.
  3. Activity flagging - sometimes the data quality is so poor that it’s throwing things off. The ability to flag an activity so that you can keep it but it doesn’t get included in your progression is another important feature. The best option at the moment for these activities, is to just delete them.
  4. Improved power data correction - some power meters have data errors that, on the surface, don’t really mean much. Other software is generally unaffected by these errors. In Xert, these errors can accumulate and throw your signature off during events like crits and final sprints. We have plans to reduce/elimi nate these errors.

Now keep in mind that using the Signature Save/Lock feature is an easy way to address these issues directly yourself. The benefit we’ll bring is to make managing these things a lot easier and done automatically, where possible.

Brilliant - the power scaling in particular looks like it would be really helpful - what’s the rough ETA for it? Let me know if you need a beta tester… :wink:

Thanks Ashley. We’ve got a lot of pokers in the fire so all depends. Smart Workouts are due up next and all hands are on deck for that. Activity/workout scheduling is another important feature we need to add. These 4 are right up there too. …along with HR analytics, multi-sport, slow-recovery fatigue modeling, etc. etc. … lots to do!!!

Any news on when power scaling might become available?

My current fitness signature kind of makes sense for my indoor rides on a stages left-only meter, but is very misleading for any outdoor rides or home trainer sessions (where I use full L-R power pedals) and my power is measured 10 - 15% lower than the stages…

As I said before, in an ideal world, Xert would use rides with L-R balance to get an idea of what my average L-R offset is and then apply an adjustment to any data with left-only… Is it possible from the raw data from Strava to know what variety of power meter has been used? In a less ideal world, it would be useful to be able to apply a manual scale factor to rides so that I can correct left-only rides to make them ballpark accurate.

As mentioned, it is on our roadmap but we don’t have a set date for it at the moment.

Just following up to see if xert is able to detect/correct multiple power meters now. Thanks!

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Would it not be possible to just bring your pedals to the indoor bike at the gym and install them on that bike? Easy as turning a wrench, granted the pedals on the indoor bike aren’t seized due to sweat. :man_shrugging:t6: just a possible option.

That is exactly what I do with my Favero pedals, but that is in my own gym. There is a problem with the crank length and the dynamics of the spinning bike’s fly wheel though. It’s not simply ‘power is power’, as I first expected. So, the discrepancy with the pre-installed crank PM is very likely not only due to it being one-sided, simply doubling the power and your left leg being stronger or weaker.

+1 on this
Some platforms offer the option to look at your power curve indoor vs outdoor or per power meter.
Power read from a smart trainer at the rear axle can differ quite significantly from what is read at the crank or pedal by a proper meter.
With enough data it may be possible detect systematic discrepancies from multiple pose meters.

The cheaper option could be just an offset value for power, it should be enough to have good consistency across diffent sources.

Easiest option is to allow you to assign a bias to the data but this can hide what’s really going on. If the bias is the data itself, for example a different power meter, then there’s no issue. But if the bias is due to bike position such as moving from a road bike to tt bike then it becomes harder to represent and interpret. When is a drop in TP for example from a loss of fitness or from moving from outdoor to indoor? Do you bias the indoor power numbers higher to prevent interpreting the data incorrectly. Unsure how best to handle. Unlike other systems, fluctuations in your fitness numbers is what the system is detecting and helping you understand. Normalizing this process across various power sources and situations remains non trivial.

I’ve just picked on this post and have the same problems with different power meters.
As I see it there appear to be 2 main areas of concern.

  1. The effect an different power meter has on your fitness signature (particularly if a breakthrough is involved), and
  2. The effect on your Training Load and Recovery Loads.
    Would it be possible to have a user selectable scaling factor available on individual rides which would alter the power by whatever percentage you entered. In order for it not to affect you fitness signature the action of doing this could automatically flag that ride so it would not adversely affect your fitness progression but would still impact your TL & RL?
    It would be even better if someone could write a Garmin App which scale the power output that is displayed AND write that power value to the .fit file. I’d even pay for that facility!
    Just a thought

Thanks for the feedback.
Many people used to outdoor+power meters now are training indoor with smart trainers, so it would be nice to have an easy way to adjust the data across different sources.
This can be done as long as the user understands how to the adjustments affect the predictions, which of course is not trivial at all.
For the tt/road bike/position difference, one could argue that the power that matters is the one that can be transferred to the pedals: an athlete could have multiple profiles, one for each “hardware” configuration, able to predict the performances of each case.
Easy when you don’t have to develop and validate it, so I’ll probably keep my mouth shut for while.

Thanks again!

All great feedback.

In this respect, if an athlete gets a breakthrough from an indoor profile with, say, a lower signature than a outdoor profile, should the outdoor profile receive the same changes?

What if the only thing in common between the two profiles were the training load, weighted by the correlation between the different fitness signatures? For example, a breakthrough indoor doesn’t correspond to one outdoor, it could be just a better adaptation to the indoor conditions, even with the same power source, rather than a fitness improvement. What do you think?

On the Ios App you have a facility to scale the reported power output by a percentage. I would have thought that would be the simple thing to do on the activity page. On you can do that plus you can put in an offset as well.