PM readings - Duo vs left sided

Hello good people! hope everyone is well.

I just bought the assioma duo and I have a technical question regarding power reading on dual vs single sided PM. Nerd stuff - so I hope I can count on you guys to understand it better! haha

over the last years I have been using crank based single sided power meters (4iiiiiiii) and they used to track power very closely across all zones, However I noticed some relevant variances when I upgraded to the favero duo.

On lower power intervals, difference can be as big as 14% (favero always shows lower power), while on intervals close to threshold (current ftp around 285w) the difference gets % smaller (around 5-6%).

I also noticed L/R balance changes following power difference ( around 54/46 on intervals close to ftp , but big differences on lower zones)

as far I am aware, I don’t have any injuries and I do off bike strength work regularly, but I know we’re humans thus not perfectly balanced.

do you have any data / knowledge on this topic that could help me to understand better these differences and if this is something noticed on a broader user base? Also, do you know if there is any difference on reading due to the method the PM uses? (crank vs pedal vs axle etc)

I know TR has huge amount of power data, maybe you can share some thought on this. As I’ve been using left sided cranks for so long, my reference is biased towards the numbers (and effort level references) I used to read on them, so the change to me seems more complicated than adjusting my ftp down 5%

thanks (and sorry for the huge topic)!

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Sounds like exactly the issue I have. On my Duos am at 5050 above threshold and 58/42 at lower intensities. Over a ride it averages out at about 55/45. I too have no injuries. Bizarrely I actually have an arthritic knee on my stronger leg. In my case I appear to start the power phase earlier and finish it later on my left leg. My non expert thoughts are that it may me something to do with ankle mobility, slight leg length differences, seat height & set back. I’ve stopped worrying about it now. It is what it is. The only thing I’ve done is adjust the power scaling factor on my single sided 4iiii by 10%. Not ideal but it’s the best I can think of.

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Left and right balances shift from one zone to the other. I don’t remember in which direction in which zones, but everything within 5% is nothing to worry about.

If you have more then 5% differences (over all the power zones) you might start thinking about strength work.

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It’s very common to have leg asymmetry that varies with power – in fact, it’s kind of uncommon to have constant asymmetry that doesn’t. That asymmetry can shift not only with power but also cadence and crank torque. This is why I’m not a huge fan of single-sided power meters – the manufacturers act like asymmetry is constant over power and over time, but it’s not.

To clarify, asymmetry itself isn’t particularly important: asymmetry is common and standard. What’s important is that you get good data, and asymmetry combined with a single-sided power meter means that you won’t get good data.

Some people say “oh, I’ve run single-sided and dual-sided and my averages are only 3% apart so it doesn’t matter.” That’s myopic. That asymmetry isn’t constant means that you can’t just inflate or deflate power by 3% to get the equivalent of dual-sides.


I’ve often wondered if my left leg is fuelling differently to my right😀

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I used to have a stages single sided, then I had assioma uno before I upgraded to the duo. It was only when I upgraded to the duo that I realized I had an imbalance, but its not consistent between zones.

I am left leg strong, at lower power zones I am normally 55/45 but as I go further up to my threshold and above it starts to even out a bit more to around 52/48; then at anaerobic and all out efforts I am very left leg dependent and can record numbers like 58/42. For me once I saw this data I realized that I really needed dual sided to be accurate as my left only had been over inflating my power.

I also had a quarq dzero, which claims to read left right balance, but I was never any more than 51 or 52 / 49 or 48 on that; as it is not a true dual sided power meter. When I put both the quarq and duo on the bike they were overall extremely accurate to each other, but I could see that the left right balance on the quarq was garbage.

My take away was that if you really want accuracy, then a proper dual sided power meter is needed. If you just want to track your ftp for training and only use one power meter and don’t care if its 100% accurate … then a left only does the job.

Personally I have never bothered trying to balance out my left / right discrepancy as I don’t think it really matters.

My right leg is my strong leg, it used to be visibly so before my leg length imbalance was sorted out. So I tell my self after a disappointing FTP test that it would have been higher if I had a doubled sided PM :face_with_raised_eyebrow:

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