If you are only measuring the left side and doubling the value, then the error is twice the deviation from 50/50.
For example, suppose one day you do a workout during which your left leg generates 100 W and your right leg generates 110 W (so 48/52). On another day, you do the exact same workout, but this time your left leg generates 100 W and your right leg only 90 W (so 53/47). Your left-leg-only power meter would tell you that the workouts were identical, but in fact the first was 22% (!) harder for your right leg, and 10% harder for your body as a whole.
Note that the above is a common observation, not some extreme example just to prove a point.
I went through phases of regret, now I’m happy I have my single sided. I have a 4iiii single sided that I used to use on a fluid trainer, then got a flux s because I wanted to use ERG mode and noticed my numbers dropped a bit. I estimate now that I am 53/47 just looking at both power curves from a few rides, which checks out because I’ve always been left leg dominant. Anyway, I scaled the 4iiii to kind of match the flux and “dyno’d” it. It tracks well enough - as I get tired the balance changes slightly, sprints, and things out of the saddle heavily throw it off. It works great for keeping me on track in outdoor workouts now that I don’t want to be on the trainer. Basically I had to get over my obsession with the numbers and just use it as a tool. I added an assioma to my mtb which tracks slightly better with the flux, but mainly use that for looking at TSS and a birdseye view of my power profile on the trail.
A 20w swing on one leg @100w target to me is an extreme example. Most people (I’m willing to bet nearly all) will not experience such variation between workouts.
Single side isn’t a power meter so much as a power approximator. If you have to apply a correction factor of 200% to get a number that number is going to be very prone to inaccuracy.
I have a single-side 4iii for about 3 months now. I am quite impressed. I don’t know whether I have an imbalance (I think I have a slight one, as I have had slight left knee injuries, and my right leg is dominant which I know from other sports such as martial arts).
However, compared to my Tacx Neo 2, it is only 10-15 watts higher outside, which makes sense since outside riding is easier.
So overall, for me, it is “good enough” to track my performance compared to inside and to track progress. Also, when I compare power curves after rides, it matches HR levels. So I believe it is at least accurate enough to estimate the zones I’m riding in.
But it isn’t. Just read enough forums like this one, or some of the scientific literature, and you will come across plenty of examples such as the one I described.
Here, I found a good review for you:
“During cycling, bilateral differences are frequently found and vary with the competitive situation, pedaling cadence, exercise intensity and exercise duration.”
Single sided pm are still quite expensive. Especially when you consider that you don’t know if they work for you or not. Who knows about a potenial imbalance beforehand?
I started with a Stages too. And really regret it. Only figured out later that my legs fatigue differently causing crappy data later in a ride. Or in the last set of a hard interval session. Furthermore, imbalance was most prominent in the SST range. This is where I race mainly.
Figuring this out took quite a while but I was really glad that I had my dual pedal pm. Learned quite a lot about myself, my cramping and so. I would rather invest the money in a new wheelset than a single sided pm.
Coming to this thread late so probably repeating a point that’s been made before. But I tend to go right-leg heavy at high intensities, especially fatigued.
So on the final minute of my ramp test, my Favero Assioma duos recorded 368 watts, giving me an FTP of 276.
But the L/R balance was 47.8/52.2. Which means on a left-sided stages, it would have been recording 176 watts and doubling it, giving me a power of 352, and an FTP of 264.
So if I’d been on Stages, I might now be doing sweetspot intervals (where my legs actually balance much more evenly) 12 watts lower than they should be.
I just sent stages a ticket in regard to low reading. My Kickr direct drive and Stages L are about 20-25 watts different. stages being lower. I’ve consistently trained in ERG for years now on the Kickr and only use the Stages for Cadence on the trainer. Whether or not one reads high or low is irrelevant to me as I’m looking for consistency. My Kickr has been consistent and I’ve seen gains appropriate to my training over the past several years. However, I’ve started to notice that my gains arent realized in power outside with readings from my Stages. Not looking for the same numbers inside and outside but there should be an increase regardless. Essentially my power doesnt necessarily represent the effort i put in. As hard as I try to be consistent outside with even pedal strokes and holding power the Stages always seems to drop 30-40 watts. Did a side by side with Kickr to computer and Stages to head unit earlier this week doing TAKU and really saw the difference.
The number and head games are brutal when this is the case. I’d consider myself fairly fast as I ride multiple group rides with very fast guys and racers so my 4 w/kg TR determination seems incredibly valid. Losing 30 watts is heart breaking especially considering the work I’ve put in to reach my current state. Big picture it doesn’t really matter. . .but it does.
If I had the money at the time I would have bought a dual sided power meter. But I didn’t and got a Stages left crank arm. I have since bought a P2M for my mountain bike. I notice that the power readings are slightly less spiky with the P2M but overall I don’t regret getting the single sided PM. Sure it may not be 100% as accurate as a dual sided but it is much better than no PM and until I feel like giving my bike a complete overhaul I don’t see myself exchanging the stages for a dual PM.
We are in exactly the same boat but I’m like 60-80 watts off. It’s a blow to the ego and doubly frustrating that I have no clue what my actual numbers are now.
I don’t know know what has happened but suddenly my stages L and Kickr are within 5% again. I have not done anything different but for the workouts in the last 2 and a half weeks, they’re mostly within 5% of each other. Same firmware, same spindowns and offsets so it’s a mystery to see. Ex, Gendarme+3 below.
Which is, unfortunately, difficult to determine without the dual meter in the first place. I got one coming off a serious knee injury and wanted to address any issues in PT.
If you get a power meter you have to accept that there’s a margin of error in any case. And if you compare different systems, the difference can easily get quite large.
For instance, I have a dual Powertap P1 and it reads much higher than my Tacx Neo trainer (15-20W higher).
I find the point about L/R is greatly exaggerated for most people. It’s really hard to train the L/R balance and the truth is that for most people the difference isn’t much bigger than the margin of error itself.
Yes, there is a small number of people who can actually learn a lot from a dual system and you won’t know if you are one unless you buy the dual option.
Going back I would probably buy a Left only PM instead.
I’ve two left sided 4iii’s on two different bikes, and I’m happy with them. However, if I was buying now I’d go Assioma, even an uno with a view to upgrading.
fwiw I’ve only found within stated accuracy variances with my original Hammer. If money was no object I’d go total power rather than dual sided most likely!
I changed from single sided PM (Powertap P1S and Stages) to a single set of Vector 3 dual sided pedals. I have noticed absolutely zero difference in my performance.
(Interesting to note though - I had the Vector pedals replaced under warranty. The original set had me at a 50/50 L/R balance. The new set are 46/54 L/R. Which is correct? Again - zero difference in my performance.)
