Powermeter accuracy

Hi all,

Probably asked before, but I’m struggling with my power readings at Zwift. I use a Wahoo Kickr for Zwift and always connect Zwift using the Wahoo powermeter. I however also have a Stages ultegra (left Crank only) powermeter on my bike. I realized that the readings differ a bit, since I measure higher values outside when the Stages powermeter.
To investigate further, I did an analysis yesterday by measuring both power outputs and made a comparison using Zwift power. The results are shown below; as you can see, they perfectly overlap in terms of power peaks, but the Stages is consistently about 8-10% higher. This makes it difficult to do my trainings inside and outside. One explanation is the fact that I’m left handed and probably have a slightly stronger left leg, which is being read by the Stages PM.
Does anyone have a suggestion how to resolve this? Is my Wahoo output too low? I recalibrate about weekly. I wil obviously be flagged on ZP when I change to my Stages. Furthermore, I think my stages output is really a bit too high but I can also underestimate my efforts…
Roy
Wahoo. Stages
1 Second 688 watts 709 watts(+2.96%)
5 Seconds 623 watts 648 watts(+3.86%)
15 Seconds 562 watts 576 watts(+2.43%)
30 Seconds 518 watts 529 watts(+2.08%)
1 Minutes 470 watts 477 watts(+1.47%)
5 Minutes 373 watts 408 watts(+8.58%)
10 Minutes 355 watts 386 watts(+8.03%)
20 Minutes 325 watts 357 watts(+8.96%)

Hi Roy,

The Stages power meter only measures power on the left side and then multiplies by 2 assuming your left and right legs put out equal power. They aren’t equal (although in general they are close) and there is no way to correct this. The second difference is Stages measures power at the crank while the Wahoo measures power after going through the drive line. There are power losses in the drive line that are real that will make the actual power at those 2 points different (likely at least a few % and a dirty drive line will lose more than a clean well oiled chain). The third difference is simply measurement accuracy. No power meter is 100% accurate. The combination of all of these factors explains the differences you’re seeing. There is no way to get them to match exactly and the correlation is reasonable so I don’t think you have any issues.

Ride on.

Kevin

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Hi Roy,

I have observed the same differences in power readings between my Stages left side crank arm only on a FSA crankset versus my Wahoo Kickr. As mentioned previously, these differences can be attributed to the fact that the Stages is measuring one side only and doubling, there are about 3-5% drivetrain losses for the Kickr, and there are the tolerances of about 1-2% for each of the powermeters. All of that can easily be 8-10% in difference. By the way, I’m right side dominate and my differences between the Stages and the Wahoo are about 5-8%, with the Stages being the highest reading at all wattage ranges. All the best in your training and riding!

which model version of the kickr are you using?

Thanks, this indeed makes sense. I’ll have to check if I can somehow lower the stages readings to have a better idea of my inside and outside performances.
The question is of course which output is reflecting the “true” power since it’s a significant difference when looking at my FTP ~320 (wahoo) vs ~345 (Stages).

I bought my Kickr Core in September 2018 right after the launch. Not sure if this is considered the 2019 version or 2018.

I completely understand the question as to which is the “true” power. What I do is to be consistent with which powermeter I use both inside and outside. Since the Stages is the only one I have that “works” both in and outside, I just always it to base my training. When I do an ERG mode workout on zwift, I use the Wahoo and lower my FTP about 5-10%. I’ve been doing this for about three seasons now and seems to work for me anyway.

I’ve gone back & forth with this. I have a pair of Favero Asioma Duo pedals which I use on my road bike during the season, and on my trainer bike during the off=season. I was gettingt~4% higher power from those over my Kickr18. I did a static weight test & several calibrations, and got the pedals & Kickr to read within 1-2 watts of each other. I don’t care which is higher or lower, I just want consistency for training. I tend to have balance at 51%/49%, frequently it’s 52%/48%, but I strive for 50%/50%. I’ve also got an Assoma Uno left pedal, but because of my L/R imbalance, it overestimates my power and it’s pretty much useless.
I train for log hill rides at specific power numbers, so reproducible power is really important to me.
“True power?”
It doesn’t matter what the actual number is, only that it’s reproducible.

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