Originally Posted by RChung

TL;DR summary: for most people, single-sided is *probably* fine.

I usually try to stay out of these arguments, and my opinion and attitude about the single-sided vs. both-sided debate have changed over time. I've been using a power meter since the last millenium (this year marks a quarter century that I've been using power data). I did what I think was the first close analysis of a single-sided power meter (the late and not lamented at all Ergomo BB), and I also did what I think was the very first analysis of the Stages single-sided crank, for Ray Maker (DC Rainmaker). Importantly, one of the remits that Ray had set when he asked me to help him was that the single- vs. both-sided issue was off the table. Because of Ray's influence (and my analysis), although Stages had already announced a release date, they held their product back for another six weeks and revised firmware to address (not quite fix) some of the problems I'd identified. I consider that a good outcome: the public got a better device when it was finally released than they otherwise would have had.

But the reality is that Stages changed the market not by producing a superior product but by undercutting the price. They sold power meters that were roughly 60% the price of existing products, and sold people on the idea that accuracy didn't matter, only consistency does.

I thought then, and think now, that this is myopic. But over the years, it's become clear to me that I'm in a minority (as I am on so many things), and they won that war. Although there are many things you can do with power data, people have been convinced that these still-expensive devices are the best tool for training, and so they use it for training and nothing else. In fact, training (especially training FTP) is arguably the least demanding use of power data: that's why people trained successfully for about a century before the first power meters hit the market, and then trained with stop watches, speedometers, and heart rate monitors when each became affordable and available. There are things you can do with a power meter that you can't do with a stop watch, speedometer, or heart rate monitor, but the hard fact is that people don't do them. Recently, people have been eschewing on-bike power meters in favor of smart home trainers, which is pretty clear evidence of their uses and preferences.

So, I've had to change to mind about single-sided PMs. I think that as long as you only do training for FTP and you're not obviously physically impaired, single-sided is probably fine. You see people buying an additional power meter either for another bike, or as a replacement, or inside a smart home trainer, that gives different data -- but they think it's perfectly acceptable that they've been training for years with inaccurate data. That's because training is the least demanding use for power data. To me, that should trigger the thought that maybe they didn't really need a power meter at all, but that's just me.

Bottom line: if all you ever expect to do is train FTP, single-sided is probably fine; a heart rate monitor or a speedometer and a stopwatch is also probably fine. You may very well have challenges down the road when you buy a second (or third) power meter, but that's in the future and hardly anyone thinks that far. There are a handful of people who have obvious physical impairments (I have a friend whose left leg had to be amputated -- single-sided wouldn't work for him at all), and another handful who do things that require higher data fidelity, and a few who think ahead to a time when they need to replace or buy an additional power meter, but if you're not in those groups, single-sided is probably fine.