Power meters on 2 bikes seem off vs. trainer ... which to rely on?

I made the mistake of comparing 2 separate 4iiii crank power meters to my Kickr Core :rofl: I understand that even similar power meters/trainers can read differently, but I wanted to get a sense whether my numbers were comparable on my road vs gravel bikes.

  • Road bike: Specialized Roubaix Expert with Ultegra R8000 left crank Precision 2 power meter
  • Gravel bike (and primary trainer bike): Salsa Cutthroat with XT8000 left crank Precision 2 power meter

For comparison purposes, I put the Roubaix on my Kickr Core and warmed up the trainer for 10 mins. I calibrated the trainer and PM, then did a 15 minute workout with varying intensities recording to TR and my Garmin. I then removed the R8000 crank arm and installed the XT8000 crank arm on the Roubaix’s R8000 crankset, zeroed that, and then did the same 15 min workout. Trainer should still have been warmed up and this is all using the same bike/cassette/chain etc. I’m going to look the the DC Rainmaker comparison tool, but overall stats look out of whack.

PM Compare

The XT crank was consistently 10% high, which I could chalk up to PM differences, drivetrain wear etc. if I didn’t have another reference. The R8000 crank arm average power was lower, and NP higher. I have no idea how to tell which one is “right” but I’ve been primarily using the XT PM in the offseason.

If you were in my shoes, would you:

  1. Live with the differences
  2. Scale each PM to the Kickr and never look back (which would mean a drop in FTP :grimacing:)
  3. Invest in pedal power meter (likely Assioma Duo-Shi with SPD cleats) and use those as “source of truth” across bikes for training and important rides
  4. Other (ANY advice is appreciated!)

I just use the one I’m going to race/group ride with as the “truth” because that is where pacing matters the most.


The problem is that use both for that! Have a bunch of gravel and road events on the calendar this year.

Do you know if you have L/R power imbalance.
Be aware though that madness will result in trying to reconcile different power meters.


This is a good question and I have no idea. In principle I like the idea of recording total power (pedal or spider PM’s) so I wouldn’t have to worry about that, but that gets expensive quickly. I figured the same model left PM, from the same manufacturer, would at least lead to being consistent (if not 100% accurate).

Guess that’s not always the case! :rofl:

Yes, just live with the differences. Focus on times. Your data has is very low quality and the solution is either expensive or time consuming and cumbersome.

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Maybe a stupid question but are the cranks the same length and if so have you adjusted the length on your head unit?

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What would be a solution? Not being argumentative…just curious!

Average power for these sample rides differs by ~13%…sweetspot on one becomes threshold on the other.

Yup! Both 175mm. I’m not even sure how/if you could change that for these.

Youre not going to like the answer but you need a different ftp for each PM. Or stick to using a certain bike for certain interval work

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That’s a big difference between the two Stages, and that’s annoying. Power meters are (still) expensive, and we buy them to resolve questions, not raise new ones. You did the right thing by mounting them on the same bike (with the same chain and same chainring and same cassette). That makes the discrepancy more annoying.

There are ways to tell which is closer (though with only single-sided, you can’t ever be sure) but they’re a pain in the butt to do so hardly anyone does them–whether you want to depends on how much this difference annoys you.

First, however, re-install the R8000 arm, warm up and re-calibrate, and try the same test again to see if you get the same (or different) results. If you get different results, that’s interesting; if you get the same results, that’s interesting too.

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But then do I need a different indoor/outdoor FTP for each… so I’d have 4 FTP’s? /s. Sorry…maybe that spilled over from another thread.

This sounds like a nightmare for tracking progress long term frankly. I appreciate the input though.

This is clearly the most cost effective next step!! I’ll do this for sure just to see if maybe the first test was a fluke or something. Thanks!!

FTP’s for each bike / outdoor / indoor or

SRM powermeter$$$$

For reference, I have a kicker for winter, a gravel bike (Stages) and a MTB (quarq)

They are all probably a bit different, but it doesn’t disrupt my training bcs. Outside I mostly care about times and winning, Indoor for structure during the winter. And the nature of the efforts is different in each bike.

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I’d suggest you get some more data on the PM that has the discrepancy between average and normalized power - that one seems the most problematic to me

Regardless of differences in values between the trainer and each power meter - the significant variance in average and normalized could be a larger problem. Figure that out by getting more data on that one - if it doesn’t come out as noise in a larger data set then you might want to discard that PM entirely

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If you have the opportunity, check results on a dual pm.
I recently bought a wahoo Kickr and noticed it was 13% higher than my single sided pioneer. A while later I installed some dual favero assiomas and found out my L/R balance was 55/45. (13% lower on one side)

Also, average pw can be misleading, some pm react slower than others

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This will disrupt him even more at this point. Getting a one side power meter without knowing if you have an imbalance is a terrible decision from a data integrity point of view.

Maybe you misunderstood? My suggestion was not to get a one sided power meter, but if the opportunity was available try a Dual Sided solution.

The difference in Power could simply be attributed to a L/R imbalance - as I also experienced

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I that case, assuming the cranks are torqued up to same setting, I’d adjust each of them to the Kikr value using the facility on the 4iiii app. Thats what I do with a couple of single sided PM’s except I align them with my main Assioma Duo pedals. The reason mine are out is due to a large L/R imbalance. My trainer, which is a cheap Elite Zumo, reads consistently 10% lower than the Assiomas so I use power match.

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i don`t know why nobody has taken you up on your suggestion # 3.

If you can afford it and you dont want the hustle of multiple ftps, crank arm validations and everything…

then yes… i would just get a pair of power meter pedals and use them across bikes as one source for everything!

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Can’t justify a new bike with the wife :rofl::rofl: so there’s some money in the budget for Assiomas. I think I might end up going this route. At least that way I can see if I have a significant L/R imbalance or whether any of my current devices are in the ballpark.

Thanks for the input!!