4iiii versus kickr core Power meter accuracy

Hi so I just want another opinion on how to interpret this. I just did a ramp test with my left sided 4iiii crank PM running alongside my core as I wanted to try to get an ‘outdoor’ ftp estimate alongside my indoor. Both freshly calibrated.

Comparison 1: The whole ramp test
4iiii avg power 239
Core 248

Comparison 2: Last minute of ramp test
4iiii 338 W
Core 354

I also tested this on some 5 min VO2s and theres a 6-8ish W difference there.
Comparison 3: 5 min intervals at 284 and 292 W
4iiii 278 and 284 W
Core 284 and 292 W (ERG mode)

Should I just go with ~7 W decrease in FTP for my outdoor zones or should I err higher? My guess is I should go with the overall average because the last minute of hanging on at lower cadence I would assume I would have a lot more stochasticity on the crank PM.

Does anyone else have such differences like this between these two PMs? I would have assumed the crank PM would have a higher reading as it ignores drivetrain losses.

For some reason cores track higher. Definitely trust the crank power meter over the trainer.


Yes, I have (or had) the same two devices and saw the same thing. The Core was consistently higher on an FTP test by similar margins. I used Power Match so as to use the 4iiii power both indoors and outdoors. (I then also realised that I prefer the flexibility of level mode so ended up moving the Core on in favour of my old dumb trainer)

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I have both the core and 4iiii left only and agree - the 4iiii should be your source of truth.

What is “lower cadence” for you?

This is the third time I heard about this practice now. I have seen people grinding at 50 RPM and do not understand why one would mess with their ramp test result by grinding through the last minute (not implying the thread starter does). In general, I don’t think ERG and power measurement works well in really low cadences, so the number you get there is likely not accurate if you would really go that low.

Good observation but not a problem for me in this case. I consider low cadence for me between 70-85, as I spin at high 90s to 110 when trying to put out power. I went back and looked and I was actually still above 95 when I said I was ‘grinding’ so I wasn’t actually.

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What everyone said, use the same power meter as outside for your inside. The numbers are what they are, it’s about being precise across your workouts. Who knows what the “true watts” are, without testing in a lab, it doesn’t matter. Just needs to be repeatable.

If you are interested in Zwift racing, then go with the Kickr. :rofl:

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