I have one. Why can’t you train with it?
Very good points.
This was why I originally went dual-sided. I have about two years of L/R data now, and while its interesting, nothing for me is actionable, and I’m to the point where until today I hadn’t even bothered to look at it for probably six months. Hence my stance for the “99 percent” (which is probably overstated!).
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No regrets going single sided here. I guess the only downside would be if you are trying to do single leg drills on the trainer in erg mode with powermatch it could get interesting. I don’t do single leg drills as I think they are useless so it never bothered me.
I did buy a Power2Max so I could asses L/R balance after learning about some physical asymmetries (leg length, lateral pelvic tilt) during a several month long bike fitting process. After looking at the L/R data for 2-3 weeks and seeing there were no huge differences I haven’t seen any benefit from having dual sided power.
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Most likely, yeah…there have not been any studies saying that a 50/50 balance is beneficial or provides any benefit. As noted, it varies day-to-day and even within a given ride.
Just to clear up my points above, I was not referring to being able to even out your balance, just noting that w/ a single-sided PM, your FTP, or your training zones on a given day, could be off by as much as 4% (if not more). That can make a big difference at higher intensities.
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Thanks to this thread, I went back and looked through my power files for the last few months… worst day was 53.3/46.7, and on my most recent 2+ hour outdoor ride I was 50.1/49.9. There’s no rhyme or reason for it that I could discern; some easy rides I’m right near 50/50, but some hard rides I’m less than 0.5% different… The other thing I’d point out is that one wacky reading or power spike can render these numbers meaningless, and unless you go in and cut out specific spikes to make your numbers proper if that happens, you could be operating under false info.
This is just my opinion as a coach and long-time athlete, and there will be dissenting opinions out there, but unless you are recovering from a serious injury or have a life-long issue with different leg lengths or strength mismatches, you’re probably wasting your time focusing on trying to get 50/50. Even if you know you have a marked difference, there’s not much you’re going to be able to do about it on the bike.
I dislocated my right foot playing basketball a number of years ago, and I had a strength mismatch between legs for about 12 months. That was resolved through PT and weight lifting focused on single leg/calf strength exercises. If you’re convinced you have a strength mismatch, that’s the way I would go (Pistol squats, single leg calf raises/extensions, single-leg presses, etc.).
As mentioned, when I first got my dual-sided Vectors, I looked at it all the time. Now, I never look at it, and the most value it provides me any more is to remind me when I’ve forgotten to link my right pedal to the left after I change the batteries! 
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Won’t it be need a road PM though (105) because of the different q factor on the MTB (XT) cranks?
FWIW I’ve got 105 Stages single sided with DA cranks. Have never, ever worried about L/R imbalances, because what I am going to about correcting the imbalance?
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Since I made the original thread, I just wanted to provide some updates. I went with a Stages L only. The reason I got a Stages L instead of a pair of Assioma Duo pedals was that I got a great deal on the Stages. I used the 20% Clever VIP coupon on their website and it came out to $280 USD for an Ultregra L crank. Decided to save some money as the lockdowns were ramping up in my province and I wasn’t sure if I would still be employed. Good news is that I’m still employed, bad news is that the stages L and my Kickr Core is quite off ( ~10% on avg). See the comparisons below.
Andrews + 1
Part of Holt Hill + 1
Part of Juneau - 1
I’m still using the Kickr Core power reading to keep consistency until my next ramp test where I’ll be using stages L instead with powermatch. Yes, I’ve done spindowns and zero offsets on both devices.
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I haven’t drilled into that level of detail, but just on this morning’s ride, I had TR paired to my Kickr Core and Zwift running ANT+ to my Vectors, and the readings were about 5% different (Core was ~5% higher). Eventually paired the Vectors to TR and carried on smartly.
Lesson learned: do your Zwift races with your smart trainer providing power! 
(Why didn’t I think of this before now??!!)
I had previously trained with a stages left only power meter without issue. I think as long as that is the only power meter you use, they are consistent and that is all that matters. However, I upgraded my crank and got a Quarq Dzero, and noticed consistently lower power than my stages, which was a little frustrating.
I have also just purchased Assioma Uno pedals for my new TT bike, and also, this is reading around 10% higher than my Quarq Dzero. In my case, I am convinced that it is because I am left leg dominant. I have a stronger left leg than my right, and although according to my Quarq, my power balance is pretty even, the Quarq does not really read both sides, it just estimates based on the area of the pedal stroke that the power is applied. There is a youtube video in which a quarq ‘dual sided’ meter is compared to power pedals and it is clear that the quarq reads power on both pedals, even when only one pedal is clipped in and the power pedals are reading 100 / 0 on the balance.
Due to this discrepancy, I have just ordered the upgrade pedal from Assioma, and hopefully this will confirm my theory that my left leg is more dominant and this will even up the mismatch between my pedals and Quarq. (even though I would love to have an FTP of 330 rather than 300, I don’t buy it).
So in answer to the thread, do I regret having single sided power, yes, but only because I also have another power meter on another bike that does not compare accurately. I am leaning towards using my Assiomas on all my bikes and selling the Quarq to have consistent power between bikes / indoor trainers.
As others have said, if you only have one power meter and it is single sided, it is perfectly fine and I was happy with my stages for 3 years when I just had one bike.
Doesn’t any spider-based power meter (Quarq, SRM, etc.) measures the power of both legs?
They give power from both legs, but the way they measure is not by taking a measurement from both sides. It takes the power applied from different parts of the pedal circle and estimates from which leg it comes. It measures total power, but it is only Pseudo dual sided power not true figures from both sides. I have been skeptical of my quarq as it pretty much always has me at 50 / 50 or 51 / 49, when my left leg is stronger.
There is a DC rainmaker video in which he compares the output on power pedals (Garmin I think) vs a Quarq power meter. The quarq only goes down to something like 75 / 25 balance even when power is only applied to one pedal with the other one unclipped, whereas the power pedals clearly go to 100 / 0.
Here is the video.
I did regret getting a single sided power meter. I was having a lot of trouble to hit the required power on some intervals because my left/right offset is not constant. When I need to push high wattages at high cadences my left leg starts to slack compared to my right leg, which means that when TR wants me to hit 300w, I really need to hit 315w so the left sides power meter can reach 300w. This was really affecting my training so I recently upgraded to a dual sided one and not only that problem went away, but also is now much easier to hit and maintain the required power at all intervals.
In the end, I would say that one sided power meter is much better than virtual, but there is a significant jump from one sided to two sided.
My Assioma duo upgrade pedal arrived, and I am able to say conclusively that in my case, only having Left sided power grossly overstates my power due to my stronger left leg.
Here is a comparison of a ride between my Quarq dzero and Assioma duos, very closely aligned.
Here is the left right balance, with a large discrepancy between my left and right on the Assiomas. Interestingly the Quarq has me very even, which shows that its left right measurements are just guesses. The darker purple lines at the top and bottom are the assiomas.
And finally, here was a ride from a few days ago comparing Assioma Uno with Quarq Dzero which was perplexing me and causing me to question the accuracy of both my power meters as the Assioma is always at least 10% higher:
So my take, in my situation with more than one bike / power meter and training indoors as well, I need consistency and my left right balance means that having one of them being single sided screws up my data to a material level. That said I did have a stages left only for a couple of years when I used only one bike and it was perfectly fine for training. Left only for me would put my FTP at 330 rather than 300 ish, which although that would be nice, is clearly not accurate.
I have been so impressed with the Assioma in the few days I have been using them, and with the ability to switch between bikes so quickly I am questioning if I even need the Dzero now, even though it is a great power meter.
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Looking to pick the collective brain. I’m seriously thinking about adding power meters to my 'bent trike. I’m not looking for anything super accurate. Just good enough to give an indication of how I’m doing. I use TrainerRoad for indoor rides. For those rides, power is calculated by speed and the power curve for my double overdrive trike rollers. What I’m wondering is, is whether or not a single sided crank based meter is going to do the trick. I don’t cringe at the thought of $3-400. $1000 makes me flinch.
Thoughts?
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Yes. Single sided is fine.
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Yes.
You might have a discrepancy between your legs, meaning that the L-side Watts x2 is not strictly ‘accurate’ but it will be completely consistent as you continue to train.
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Nope. Dual power or death.
And nothing under $1K.
You want to fast don’t you?
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Until you can push 53x11 at 95 RPM or your FTP is over 400w…single is good enough. 
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Unless, of course, his L/R balance varies with power, cadence, fatigue, etc… which it usually does
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On that basis single sided is absolutely fine. Full disclosure, it’s what I use and I love the data it generates.
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