Power meter inaccuracies

I thought it would be fun to have a power meter on the bike to pace myself better on the climbs and do some of the workouts outdoors.

Initially I did the “control w/ Ant+ power” on my kickr core, did a half monty and was a happy bunny. FTP and MAP numbers didn’t really change and I assumed that the two were in agreement.

Fast forward to Butter and the intervals were not reaching the targets (too low) and the rest power did not go all the way down… Despite being in erg mode.

As a test, i did the Giro time trial in Level mode and disconnected the power meter from the kickr. During the warm up the two are pretty much spot on. Then they slowly diverge up to a point where they are 13% apart, but the difference then reduces to about 5% at the cool down…

So one of m is being funny. Any suggestions what I should do?

Done the spin down on the Kickr and the offset thing on the Stages.

The Stages measures lower than the Kickr, so it’s unlikely to be mechanical losses.

If you’re looking for advice Sir Erik @TrapMeSuf my 2 cents is to use the Kickr power when training indoors and don’t even bother with the Stages.


Yes, I would agree. But that’s admitting that me getting a power meter is a fail. Im not ready for that yet.

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Hmmm. You ride the bike with the pm outside though right?

Is the Stages power meter 2-sided or 1-sided? If it is only measuring your weak leg and doubling, it would make sense that it reads lower than the KICKR, which is measuring the power from both legs. But 13 percent is a lot of difference.

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It’s only the left side. I don’t think one leg is significantly stronger than the other, but yes, it is a possibility…

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But Sir Erik, regardless of how close the 2 are, the single sided PM will still allow you to pace your climbs and do some workouts outdoors. The most important thing about the PM is consistency. You might not match the Kickr numbers but if your PM numbers are consistent that’s what you’ll be pacing yourself to up the climbs. Next to my Varia radar, an outdoor PM was one of my best investments and like you, I’ve just got the single sided one. I’ve used 4iiii and Stages and they’ve both been great.

You won’t regret the purchase.

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And there lies the issue. The discrepancy is not constant over time. Assuming the kickr is correct, the stages is somewhere between 0 and 13% out.

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Ok. I get it. So there’s a bunch of tests you could do then.

Eg. try a test where you do a couple solid minutes in Erg mode using the Kickr at a number of different intensities.

Eg 1 minute at
100 watts
110 watts
120 watts
130 watts etc

At the same time, simultaneously record your Stages using a head unit and compare the two recordings in Golden Cheetah or some other app that allows you to overlay the workouts for comparison


Ride a straightforward workout in SYSTM using the Kickr as controller while simultaneously recording the power output from your Stages on RGT or Zwift then compare those files.

Smoothing can have an effect on what you see too so you might want to make sure you have smoothing off.

Fwiw, I tested similarly my single sided 4iiii and Stages against a Kickr Snap and was able to get within 1-2% of each other.


Probably not helpful for you but I had similar trouble between my kickr core power and a single sided 4iiii power meter.

I tried a bunch of tests, dial recording and editing the offset on the 4iiii. Had anything ranging from down 11% to 2% variation.

At the end of the day i settled on “consistency” so use the 4iiii for power via the kickr control with ant+. i’m then using the same measurement source indoor and out. I then have another 4iiii on another bike which i figure is closer between two 4iii left side PMs than between a single side and kickr.

Think my left leg is a bit weaker from a previous broken ankle


I had a similar issue but using dual Wahoo Powr pedals, and a Kickr Snap. I contacted support, and after a “factory spin down” both have stayed within 2 or 3%. Also don’t forget to zero offset your power meter.


I should also add that you should be doing the Zero offset of the Stages prior to riding each time, same as 4iiii


I have Assioma Duo power pedals. Put them on the bike on the trainer, confirmed they agreed pretty closely to what the trainer said, and use the trainer for power on the trainer and the pedals for power outdoors. I did try using the pedals for power with the trainer, but trainer erg mode response and stability wasn’t as good as using the trainer’s power, which seems reasonable as the trainer directly controls its resistance with its own PM rather than having to loop out through the app to get power from the external PM. FWIW: I have a single sided PM on the mtb, put that on the trainer, and did the same test with similar results.


A man with one power meter…

I’ve been down this road, started with a SS Stages, compared to a Kickr v1. I hated the Kickr because it was reading low. Wouldn’t use ERG. Sold my Kickr, tried rollers (not good in a condo with neighbors), bought Garmin Vector dual-sided power pedals, bought a STAC Zero, finally back to a Kickr and using ERG mode.

So in my rabbit trail of pursuing power readings (and significant expense), I discovered I’m left leg dominant, so my Stages was reading high. My v1 Kickr wasn’t all that accurate either, but closer than the Stages. My Garmin pedals read higher than the STAC Zero (it was great but I had to replace the Zero when my next bike came with a through axle) and also a bit higher than the Kickr 2018.

All that to say, I only found peace with this by choosing a “source of truth” for indoor training. Consistency beats accuracy every day. Multiple sources only drove me batty.


I found that my power meter was way off ehat the calculated power was. Indoors I use calculated power, outside the meter.

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What is calculated power? Just curious

I think you’re making a great point here. No need for me to go through that same experience (when I can learn from you)

I’m going to ignore the Stages whilst indoors.

Outdoors I’ll try and find a balance between perceived effort and the readings for workouts. Pacing-wise, it still works, the actual value doesn’t really matter.


Ibmay do this in erg mode to see how they combine. In Level mode, the difference is not consistent and ranges between 0% and 13%.

When I control the power with the Stages power meter I’ve got a problem where the interval targets cannot be reached (in erg mode) it just sits 20-30W too low, but then the rest sections end up too high.

This is the Kickr in Level mode and power smoothing off. Stages measuring independently:

It starts off pretty much identical. Then diverges to 13% difference, to settle down to 5% difference at the end again.

This is Butter in Erg mode, with the Stages used to control the power:

Workouts don’t reach the target, rest periods take ages to settle down to the correct level. It’s very frustrating to train like this. Hence, I cannot use the Stages to control the power on the Kickr.

Reading the responses (thanks all!), I will not try to actually understand and solve this - it seems like a rabbit hole. I will disconnect the Stages for the indoor rides and use it as a guide only for outdoor rides.

(The graphs are generated in compare-the-watts.com )