Power2max, the one and only!
https://www.power2max.com/en/product/ng-rotor-aldhu-r-24-power-meter-set-road/
Strong agree from personal experience. Get something that measures total power. That can mean true dual sided, or it can be spider based. Otherwise as soon as your L/R balance changes, your ‘power’ will change. In my case, that was extremely obvious following a leg injury, but even when healthy it changes within a ride.
I would see my osteo to check any alignment issues, and ramp up my off bike strength, activation and conditioning work including plenty of single leg exercises. For me that’s been a proven approach for getting back to somewhere near a 50:50 balance, having seen L:R numbers as extreme as 40:60 when something has been off. When they’re that far off there is normally a good reason and I’m aware of it e.g. When recovering from an injury or if I’m getting knee pain on one side. But I’ve also seen left contributions in the 44-45% range when everything has felt normal. Normal range for me is 47-50 when I’m healthy and everything is moving as it should.
As @The_Cog said above, the main issue is variability and that any deviation from 50% is doubled with a single sided PM. So when everything can feel normal but my Left contribution could be anywhere between 44 and 50%, that means my reported power could vary by up to 12%. That’s huge. If I knew my left leg was always contributing 44-46% that wouldn’t be so much of a problem (apart from for my ego and reported FTP) as it would at least be fairly consistent.
And the other benefit of 2 sided power is that if my left contribution has drifted down to 45% it’s usually a sign that something isn’t quite right, so seeing those numbers allows me to be proactive about getting it sorted before it becomes a problem. I may be a bit of an outlier in terms of the variation I see - I’m late 40s, I’ve had a few crashes over the years, and both my physio and my osteo say I’m pretty malleable which means I can get out of alignment fairly easily (but also can be adjusted easily and seem to heal fast). But I guess I only know about the variability because I have two sided PMs on all but one of my bikes so can keep an eye on it (and the one bike that doesn’t have it is a gravel bike that I mostly use for recovery and endurance rides so accurate power isn’t quite so critical, I’m using RPE more).
I may be missing something, but I’m not seeing how a left-right imbalance results in that difference being doubled with a single-side power meter. My liberal arts math is showing a much lower difference.
Example: If power meter side is showing 250 watts, but in actuality, the non power meter side is only producing 240 watts, the 250 watts gets applied (incorrectly) to both sides. So, (250x2)/2=250. A double-sided power meter would show (250+240)/2=245. The error only exists on one side, not both. How is the error being doubled?
The difference between 250 watts (single sided) and 245 watts (double sided) is 2% in this example. What am I missing?
I spent a very frustrating winter trying to match up the reported power on my single sided power meter with that reported from my turbotrainer. I’d never even heard of a power imbalance. Turns out it is a 54/46 @ endurance/normal cadence changing to almost 50/50 @ FTB and above. It also evens out as my cadence decreases or I fatigue.
If you can afford it I’d say get a dual sided one. I wish I had.
Your mistake is that you are dividing by two.
If your bike computer is showing 250W, that’s 125W for each leg with a 50L/50R split.
In this example, if your split was 135W L/115 W R, the single sided crank PM would read 270W vs. 250W for the dual sided PM.
Simpler maths example. Your total power is 100W. If your left leg is contributing 52W and your right leg is contributing 48W, you might think you’re only 2% away from a nice even 50:50 balance. But your power meter is doubling your left leg power to give a total power reading of 104W which is actually 4% out.
So at 695 euro for the duo, I’d be getting a solid dual sided setup , Look style pedals, and the flexibility to change to others bikes if desired. Looking very appealing!
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Meant to say in response to funkinslick and the Favero Assioma duos.
Dual or Total Power is obviously preferable. Personally, I have two left sided 4iii’s on my road and gravel bike. N+1 I’ll aim for total power, but funds will dictate.
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I don’t have a running list, but here we go from memory:
- Any dual-sided pedals (Favero, Garmin, Wahoo) since they sidestep the entire crank issue.
- 4iiii dual crank upgrade (which I think gives decent data on the redesigned drive-side crank?)
- Power2Max crank swap with Shim rings option.
- Rotor spider/crank upgrade
I am sure I am missing some options like the Chinese versions that may have a compatible bolt pattern too, but there are a mix out there.
In the words of Homer Simpson, Doh!
Thanks guys 
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At the moment, every single-sided power meter simply doubles whatever that single-side measures. Two things:
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There are ways to do a better estimate than simply doubling, but I don’t know of any manufacturer that does them.
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A lot of issues with single-sided would be handled better if manufacturers just didn’t double and instead just reported the truth of what that single-side said. That’d be just as consistent, for people who claim that consistency is all that matters. You could train just as effectively.
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I’ve used both, but never tested them side-by-side. I have a Quarq spider-based meter on one bike and a SRAM/Quarq left-sided meter on another. I’ve used a Stages (left side) in the past, as well as a hub-based meter (PowerTap). For most users, I think a single-sided PM is fine - they’re just interested in “is my power increasing?” and a single reading will answer that just fine.
For elite athletes, or athletes with known (and unusual) asymmetry, the extra accuracy and additional data points from a dual-sided PM might be worth it.
That said, with the price of Sigeyi PMs where they are, you aren’t saving much money going with a 1-sided PM. And the Quarq/SRAM aren’t far behind in price these days.
bUt tHe nuMbeR iS sMaLLer…
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Fair point. Maybe they should triple the number. Training would be just as effective.
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Yeah, but doesn’t that only really work well for someone with a single power data source?
It may be a minority, but we see plenty of people on the forum with at least a power meter and a smart trainer. Even in a “simple” case of 2x devices (assuming it’s not possible or practical to use the same single-sided PM for all riding), it would seem problematic to have that 1-side at the “lower” level while the trainer is the “higher” one.
I know that the one-sided aspect is part of the factors that lead into the common “my devices don’t match” issue, but swapping one of them to a different functional scale seems to introduce a similar level of complexity if there are more than one power device in play.
Chad no doubt knows this, but for those who may not, TR can use the power from your bike’s powermeter to control the trainer. That way you’re using the same powermeter for outside and inside rides. To do this, you add your bike’s power meter as a “device.”
Sure, PowerMatch might be a solution in that narrow instance, but I alluded to that not always being the case. For one thing, some people never put their “outside” bike with the power meter on the smart trainer so they will have 2 separate data streams in that instance.
Then there’s people like me with more power meters & smart trainers than some people have bikes… and you get a rather interesting mix of data. If one of those was a “derated 1-side” solution (of which I have nearly 50% in my mix), how would that factor in for stuff like tracking TSS and any power related data?
My only real point is that some of these “solutions” are narrow in scope and fall well short once you look at the wide range of use cases we see here on a semi-regular basis. The idea of pushing data from 1-side PM’s into a different realm works only if you have that single instance, or at the very least multiples that follow the same function. The moment you introduce a total power (smart trainer or spider PM) or 2-side power (pedals, cranks) you get something that I see as rather messy in a new way.
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