I made the mistake of comparing 2 separate 4iiii crank power meters to my Kickr Core 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.
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:
Live with the differences
Scale each PM to the Kickr and never look back (which would mean a drop in FTP )
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
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).
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.
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.
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.
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
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
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.
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.
Can’t justify a new bike with the wife 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.
my solution is to have dual sided always but the budget friendly method. Changing the Shimano left crank is stupid easy and takes less than 5 minutes, so I have 2 4iiii right sides, one on my road bike, and one on the gravel bike and I swap the left side. This also anchors the two right side meters because if I look at balance I can see the right side looks similar both bikes. I tend to use my gravel bike on the trainer and the reading I get from the Garmin from the dual sided line up very well to the wattage reported by my Computrainer… so it makes me happy all the way around. I report from my 4iiii to all software for fitness tracking etc so same all year round.
With your situation it is really hard to tell where the problem lies. I assume you torque your left arm to the same torque etc when changing them? but if there is a difference between 2 left side only PM’s vs a constant trainer reading then one is not telling the truth. If that is the case I would talk to 4iiii, their customer service is fantastic.
I found this to be my case. My 4iiii left crank was reading a bit higher than my Assioma. When adjusted for the L/R difference, they’re just about spot each other now. 4iiii have the calculation on their website.
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.
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