4iiii and Kickr power discrepancy

Hey all, like the title suggests, I’m having a significant discrepancy between my 4iiii PM (left only) and my Wahoo Kickr, and I’m not sure which reading to trust. Normally I wouldn’t care too much, but I do a weekly zwift race with my team and don’t want to cheat. Also I’ve been counting all my calories, so the more accuracy I can get the better. I’m reading about 20 watts more from my powermeter on average.

The 4iiii just came back from the factory after I had sent it in for issues with connectivity, and has seen a fair bit of use - at least 12k miles over the last 3 years. The Kickr is an older model (2018), but I’ve only started using it heavily since this last November and so it’s only seen maybe 3k ‘miles’.

I suppose the real test would be to borrow a friend’s kickr and see if the discrepancy holds, but because of lockdown that seems like it would be complicated.

I guess my question is - which power reading would you all trust more?

Here is the results of a head-to-head test of the two power meters using DC Rainmaker’s analyzer tool.

I’m in a very similar position with a 4iiii left crank and a kickr core.
The reality is that there’s no way to determine which is “right” short of serious lab testing.

It’s Segal’s Law - “A man with one watch always knows what time it is. A man with two watches is never sure.”

My thinking is that since I use my 4iiii on the road and for testing so I’m going to just powermatch the kickr to the 4iiii and stop worrying about power discrepancies.

You can keep testing and comparing all you like but there’ll always be differences and you’ll likely not ever know which is “right” only which are closer

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I also have 10% difference between my power meter and trainer, but that difference is constant, stable, so it doesn’t really matter as long as I have reliable point of reference.
There is no real answer really for zwift racing because truth may be in the middle, and as for calories it really isn’t that big of a difference, nutrition labels are less precise than this, you don’t know your exact efficiency anyway etc (and I’m not dismissing counting calories, I’m also trying to do it as precise as possible but knowing if I spent 500 or 550 kcal on a bike ride isn’t really make much impact)

I’ve got a similar experience with my left only 4iiii and my Tacx Neo 2T, the 4iiii generally reads a good bit higher, sometimes more than 10%.

A while back I had checked my Neo 2T against another of my bikes with a left side Stages, they had been pretty close, only 1-2% difference. I might revisit this again to see how close they are, then try to tweak the 4iiii’s scaling factor to bring it in line with the Neo 2T and Stages.

Thanks for the replies everyone - I’m actually glad to hear that a ~10% discrepancy isn’t too uncommon.
I raced with the more generous power meter tonight and I definitely did not win the race, so I suppose I shouldn’t worry too much and just use the PM that I’ll be using outdoors.

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Time to get a true left/right pm :stuck_out_tongue_winking_eye:

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10% is a huge gap. I have selled my 4iiii because of the difference and not always the same. Low power 20% difference and around FTP 5%…I cant work with that.

With my Quarq and P2M the difference is between 0.5 and 2% with my kicker.

Probably neither. You could have a imbalance in your leg strength and to complicate it that can vary over a ride. Turbos are notorious for reading/estimating wrong too. All that really matters is that you have something consistent you can use for training. I’d use the power meter and power match as you can use that inside and out.

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This - the two are probably within 2-3% of each other, then you add in the difference in where they are measuring the power, probably another 2-3%, then you add in left/right power imbalances that could comfortably be 5%. If you care about the difference then use power match so you stick with one power source. Otherwise just don’t worry.

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My 105 4iii generally reads lower than my Original Hammer, but within margin error (when you factor in both devices margin of error, different recording locations).

Zwift accepts either as “a source of truth”, so it’s only really if you’re competing where you need dual recording. In which case, I would say the simplest way would be to adjust the 4iiii in the app?

In the realms of zwift, I wouldn’t consider you picking the higher “cheating” as you have no way of knowing. Unlike all the people racing with “aspirational” weights…

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My 4iiii reads 6,67% higher than my Tacx Neo @ 300 watts. I’d normally use 0.97 scale factor to correct this, but I’ve had issues with the app so I have to leave it at 1.00.

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My Kickr seems to read 5-10 watts higher than my Stages.

The best solution is to use a single power meter inside or out, so I record power from the Stages for all rides.

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If you’re using them both together, then use the stages, since you can compare that data with your outdoor data too.

You could have 10 PMs and trainers and you still wouldn’t know which reading (if any) was “correct” so just choose one source and stick with it

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Just use the 4iiii as your source of truth and ignore what the Kickr says. I have both a 4iiii left only and the Kickr Core varies depending on how long it’s been able to warm up as well as the last time I’ve done a spin down, but the 4iiii has been very precise (meaning it has a repeatedly consistent reading) while the kick can be all over the place in my experience.

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My 4iiii reads 8-10% lower than my KICKR. Its frustrating as all hell.

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4iiii sucks. Thats why i sell my 4iiii. I have now quarq on racebike, p2m on my winterbike and kickr core for indoor. All between a range of max 2%

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I always wonder how GPLama and DCR have readings that are so close together, most of the time.

Anyhow, I would always ignore a trainer and use a PM. Trainers seem to be a real crap shoot.

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I would expect drive train losses to be 10W to 12W. But if you’ve got a dirty, stretched chain or a crunchy bottom bracket or a gummed up jockey wheel…who knows how bad it could be?

Anyhow, I suspect drive train losses first. In any event, power at the pedal vs power at the rear hub should be ~10W difference normally.

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Here’s some data that Jason/Velonews cranked out comparing 1x vs 2x drive train losses.

“We simulated a rider output of 250 watts pedaling at a cadence of 95RPM with both setups, and used the same total gear range.”
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“The average friction within the 1X drivetrain was 12.24 watts. This was computed as the sum of the drivetrain power losses in each of the 11 gears divided by 11. The average friction of the 2X drivetrain was 9.45 watts, computed as the sum of the power losses in each of the 15 optimal gears divided by 15.”

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I have exactly the same setup and the 4iiii (left only) reads about 3-4% higher than the Kickr, especially at higher wattages. I’m going to try powermatching or possibly lowering the increment on the 4iiii.

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