External power meter readings jumps up and down with consistent power


Bought an external powermeter (4iii Precision 3). I suddenly gained 40w+ compared to using my tacx flux s. I’m ok with this ofc as long as it’s consistent readings. Using my external powermeter now as main source for both inside and outside.

Anyways I do think my power readings are varying too much / jumps up and down, even though I try to maintain consistant power. Here is a comparison between my tacx flux s and my 4iii. It’s a 20 min FTP test so shouldnt vary that much in power atleast in the 20 min?

Unable to insert link to it but it’s: zwiftpower address + /analysis.php?set_id=386375

Do others have the same issue? Trying to think if it’s related to specific 4iii or also other power sources?


I would say it is normal, even when you think you are holding a (near) constant power for it to fluctuate, at least within reason, because we just can’t be that consistent. With the graph you included, though, we can’t tell if your fluctuations are 3 watts or 100 watts, as there is no scale provided.


The key shows average figures for both trainers so is a good indication of scale.

Hi Nigel,

Thanks and apologies for the poor screenshot. I was trying to “type” the URL but not able to post links. if you can replace DOT with . here and check? https :// zwiftpowerDOTcom/analysis.php?set_id=386375 :slight_smile:
Anyways it jumps 50-60 watts and compared to the tacx flux s power readings below it looks “wrong”?

That seems normal. Did a ERG workout yesterday with a section that averaged at 193W and and the power file shows constant fluctuation between about 185w and 200w. We’re not machines. My guess is that your Tacx does some form of power smoothing before it sends out the data. My Kickr Core is also capable of that. With it on its a pure flat line in ERG mode. With it of it give a much better indication of how power is actually always fluctuating,

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Thanks. I understand it displays raw data and changing between 185w and 200w is still pretty “stable” in my opinion.

Mine changes 50-60w but maybe thats also ok. Just weird to see it changing without feeling any further change in resistance. But if rest of you is also seeing something similar I guess the power meter is correct.

50W does seem a bit on the high side. Haven’t done a free ride steady effort in some time. Guess that would have a bit more fluctuation then ERG mode. Not sure it 50W tho.

Is your 4iiii a left only PM? That could also affect the fluctuation a bit.

I also just realized that almost 70w average difference between a crack PM and a driver drive trainer is fairly high. You did calibrate both devices before this effort?

It’s a left side pm only. I actually didn’t calibrate my Tacx before the ride, but it has been pretty consistent with the somewhat “low” wattage compared to to 4iii. Ofc I wonder if it’s correct but most importantly is if the fluctuation is ok.

Thanks again

Both your power meter and your trainer are showing fluctuating poqery, which is normal.

However you can’t really rely on a one-sided power meter to give you fully accurate power figures because most people have a L/R imbalance. So given your power meter is reading a fair bit higher than the trainer, it might be that your power on the PM side is higher than the other side, hence it’s being over-generous when doubling the PM side figure for its power reporting.