Left only power meter, inconsistent leg balance?

So, this is my first week with TR and I’m relatively new to indoor training (damn COVID). I recently bought a 4iiii left side power meter for my training and I noticed something: at the tempo zone my left leg starts outputting significantly less power (about 20 watts of difference) at times when compared to my right leg and this only appears to happen at this zone. Also, since this imbalance isn’t constant I really cannot just adjust the settings of my power meter.

Do you guys think this will be a problem? Thanks for your input!

How do you know it is 20 less watts if you have a left only power meter? What trainer are you using?


I noticed that my power dropped by that amount while the speed, cadence and gears remained constant. Plus the power went back up after I consciously started to balance the power output. I have a dumb magnetic trainer.

How long have you been riding when you see this? 10 minutes plus? I assume you are holding the same cadence at the same resistance setting.

I ask as you’re tire could ‘speed up’ after a few minutes so if you are holding the same cadence at the same resistance level, your actual power needed to do that will drop…

That could just be natural left/right imbalance in your pedalling. If when you’re not concentrating you favour your right leg, or you shift position slightly, then with a 55/45 split that would give you the 20W difference at 200W.
With a left only power meter I’ve noticed that the value drifts down when I’m tired and favour my right leg, also, changes in position can make quite a big difference.

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I also notice this. I have always had a left/right imbalance (right leg stronger), which I’ve verified comparing stages data to powertap on the same rides. It’s definitely more pronounced at higher power output and when I’m tired. A few years on stages has not noticeably corrected it, but if I detect my wattage dropping mid interval despite being sure I’m not decreasing output, I usually focus on smoothing my pedal stroke and it corrects. Funny enough if I focus on favoring the left leg, my power spikes then drops, but if I just focus on pedaling smoothly it rises up and sustainably stays there. I used to really want to fix it but after years I’ve come to accept it’s just how my body is and to just take coming up a little short wattage-wise on tired intervals with a grain of salt.

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So, a quick update:

In the end, I opted to upgrade my power meter to a an spider based one (rotor in-spider) and I couldn’t be happier with the results. Not only the described behaviour is gone, but now is much easier to maintain the required power. Here are the plots of two workouts:

On top you have the workout with Rotor inspider and on the bottom with the 4iiii left only source. The difference in power stability is huge!

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Yep - I’d like to get a dual side meter for this very reason. Most of the time the single sided doesn’t bother me but when you’re really pushing long intervals it takes extra effort to stay in that pocket. a lot of people disregard this as not a big deal but if it’s extra brain power when you’re already smoked I see it as a pretty big deal. That said, I have a single sided power meter on every bike so it will be a long way off before I fix this myself!

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