I'm pretty sure that there literally isn't another topic that has been as extensively litigated on this forum as this one, and as a result an appeal to the search bar will be far more useful than anything I could possibly add to the debate at this late point in time.

---

Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
Coaching and bike fit - http://source-e.net/ Cyclocross blog - https://crosssports.net/ BJJ instruction - https://ballardbjj.com/

Per fredly, lots of discussion, but, I think the consensus is that a left-side PM is just fine for lots of purposes, like general measuring of training stress.

If you want really accurate/precise measurements, and want to capture like a 3-4W increase in your FTP with good certainty, then no. Some documented risk of uncertain likelihood that a ~50/50 balance can go out of whack when you're really fatigued.

I have 3 dual-sided and one single-sided Stages, and never felt like I was really missing anything with the Stages. But I was just using to capture training stress and occassional race forensics.

Or if you think you will do aero testing so I am have read

Is that true do you know ?

Yeah, for aero testing, like A/B testing probably go two-sided if you can.

This is absolutely the case.

---

Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
Coaching and bike fit - http://source-e.net/ Cyclocross blog - https://crosssports.net/ BJJ instruction - https://ballardbjj.com/

If we assume an FTP of around 300w, 4W is inside the standard deviation of essentially all perfectly functioning power meters, even those that don't simply double the measured output of one leg. You'd have to be incredibly lucky to be consistently in that range with a single sided meter. In practice, the number is significantly larger. Very much significantly larger for most.

---

Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
Coaching and bike fit - http://source-e.net/ Cyclocross blog - https://crosssports.net/ BJJ instruction - https://ballardbjj.com/

Maybe. I spent a month comparing my Stages to my smart trainer, and the Stages tracked like a bloodhound. Maybe Im just super lucky, though. I never saw the wild variations warned about

Hey, didn't you say we weren't re-litigating this? :)

Just fwiw, ime (and again fwiw, I'm lucky enough to have many thousands of training client files to draw samples from) low inertia pedaling tends to be one of the "bins" that variances in power balance sorts into. This is very confounding to attempts to analyze these issues effectively on a trainer.

---

Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
Coaching and bike fit - http://source-e.net/ Cyclocross blog - https://crosssports.net/ BJJ instruction - https://ballardbjj.com/

As fredly alluded to, I have files from sessions where if I had used a left only PM my first interval would have shown as more than 35w higher than my final interval, when actually my final interval was a couple of watts higher than my first.

Xcadey power meter crank from Aliexpress works better than my Stages, is chargeable, and only costs $165.

I bought a Magene PEC P505 powermeter in the fall which is compatible with Shimano 24mm cranks. It was $299 and has been super reliable.