It is not if you use only single sided pm for training and riding outside - so one source of truth. Then your power will be āinflatedā by the imbalance but it will be consistent and it is only important thing in power.
Not to mention that leg imbalance can change (my is from 53:49 to 49:51 - depending on zone I am in, cleat position position on a bike etc).
Yeah, Iām not sure itās a problem. All I want from my power meter is that I can determine the right zone Iām in, consistantly. Iāll always use my left foot on the left side of the bike, so if the power is different to the right side, I donāt care that much.
i think the drifting from left to right and back to left over 1 ride is the āproblemā.
If itās simply a averaging to 52% left or even 58% left, that can still be consistent.
When you record this ride with a single Left pm, the first 15 minutes would show more then 10% more power then from minute 25 to minute 70 ā¦
I donāt think many people will have a pattern like this though, but yes, for some a single sided pm is not very accurate.
I havenāt seen a drift like the picture above, but one thing I do notice from my dual sided power meter is that my imbalance grows larger as I ride harder. Iām about 47/53 L/R hanging out in low Z2, but closer to 43/57 when I start pushing sweetspot and above. Then it evens out again for sprints or super hard surges where my cadence gets high.
I use my single sided stages on my road bike as directional and just try to compare like for like efforts, rather than setting training zones. For example, Iāll compare pull on one ride vs past rides to see how power/duration looked, and whether I fell off at all during the ride. But I set my zones and do my structured workouts on my quarq on the TT bike. I did set my scale factor to 1.05 or 1.06 and have found those to line up much closer than the quarq, but thereās still some inconsistency/variability you donāt have with dual.
Iām the opposite I balance out almost linearly up to 5/6 minute power where it appears to be 50/50!. For some obscure reason my stronger leg is the one with the arthritic knee.
Iāve seen drift like that before. I assumed one of the pedals was cookedā¦ 
Based on OPās picture, if using L side only, power reported would start at 130% of āactualā power and drop to 106% over the course of the ride. Thatās not consistent, which is presumably OPās point.
You could be in low tempo and your computer is saying threshold - āMy TTE is amazing!ā
Thank you. Exactly. Or if I was right leg dominant, my computer would be saying Sweet Spot and I would be well into Threshold and saying I āI canāt do Sweet Spotā
Consistant ādriftā is also odd. I think it comes from one leg tiring earlier, or from a gradual shift in saddle position.
Shifting your weight on the bike can have a massive influence on the power distribution - on rollers, taking my hand off the bars for a drink can change power by 40W.
I donāt know, I still think as long as you are aware of things like that, you can easily live with it.
Yes this is quite odd. As imbalances in different positions and power levels are normal I have never seen the drift like this during the effort - usually it changes almost instantly with change of power and position on the bike.
Itās also not just drift, because it goes back to the level at the beginning for the last minutes.
I donāt know what type of ride the OP did, but I could see something like that happening in a long FTP interval or so, where you donāt really stopp pedalling or shift position, and your legs are tiring over the duration. If there were multiple short intervalls, I think youād reset (sit square on the saddle, etc), and you wouldnāt have as much of an effect.
Looking at the graph in more detail, I actually think there was a 15min warmup, where there is no strong trend. Then a 1h ride with ādriftā, and then a couple of minutes cooldown, where it has returned to normal.
Also wondering if this shouldnāt just be considered as a correct representation of the power profile - you tire, and you get weaker (or, the effort gets harder). If you have a leg imbalance, the stronger leg would gradually take more load to keep the power up, and youād tire sooner.
Evidently, you didnāt notice in the graph that the imbalance is not consistent. Iāve seen a lot of data from many riders, and not only is L/R balance not constant over time, itās not constant with power, or with cadence. Whatās often (but not always) true is that the inconsistency isnāt large but this myth that consistency is the only thing that matters in power is myopic. We do lots of things with power, and while some of them donāt rely much on accuracy, some do.
My imbalance reduces not only as I fatigue, My left leg is at sweet spot but my right is only at Tempo LOL but also with intensityā¦ I appear to have a longer power phase in my left for some reason. Maybe ankle flexibility, leg length discrepancy or something. Itās correct though that as I know whatās happening itās not a massive issue, but I was a bit confuse before I knew I had this " problem"
The drift shown above is in all likelihood an extreme and rare situation. Most riders will never see such drift and variability in their power balance.
But the point is validā¦.the problem is not just that there may be an imbalance, it is when and to what degree an imbalance may occur. They are not necessarily consistentā¦in degree or in time.
But I wouldnāt rule out using a single-sided PM altogetherā¦.they are an affordable way to get into power measurement for many people and can be a valuable tool. It is just that a dual-sided meter is a better, more precise tool.
It also depends how you use your PM. For me, Iām not chasing a specific number (outside of ftp test, lol), but I more use it to verify what Iām feeling, and to work out how sustainable something is. Power is only part of the picture anyway, for example offroad rides are often a lot harder than power alone will tell. I donāt really need it to be that accurate for that.
If youāre doing cdA testing on the track, yeah maybe you need to measure more accurately.
Could you elaborate? Trying to think what there could be, all I can come up with the where you use the force balance (aero drag, rolling resistance etc.), so CdA/Crr testing and pacing.
CdA/Crr testing is one, but others include nutrition/calorie replacement, Wā or FRC modeling, and (of course) Zwift racing. Training FTP is perhaps one of the least demanding things you can do with power data (which is why you can train FTP pretty well with HR or with just a wristwatch, but you canāt do those other things with just a wristwatch).
Also, if you ride in a power eco system it is very nice to have accurate power meters so your data is consistent across all sources. Data portability if you move from one PM to another.
How many threads exist on the internet where a user has two powermeters and is frustrated that they donāt match or correlate well. I often wonder if people would accept these PM issues with heart rate monitors.
As is total power.
Oh, no doubtā¦I personally donāt put a whole lot into the L/R balance thing. I focus on my overall / total power, not what each individual leg is doing.