You're using an estimation for an extrapolation.

Here's what I don't get.....all the talk about left vs dual, 2% vs 4%, etc etc....

AS long as it's consistent, and you use the same PM to measure your FTP as you do to do your FTP based workouts, isn't that what counts for training?

What happens if your PM dies and you need to get a new one?

re-test so you have a number for the new PM. You "should" be able to figure out what the numbers should be, as long as the meter is accurate. In theory anyway, I don't have a PM "yet".

It's a little like switching between short course and long course in the pool. The times will be different, but you still can use them once you know what you are capable of in both formats.

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Yes, mostly. If your L/R balance is generally consistent, then a single-sided PM is pretty good for a training and race management point of view. But, if your L/R balance varies, then it can get wonky, and accuracy can easily swing by 5%-10%. That could be an error of 20W when you are doing a ride in the mid 200s, and it could mess with your mind. Say, on a day when you are rested and your balance is 50-50, intervals at 250W feel easy. But, on a day when you are fatigued a little, and your balance is 47-53, it would double-down on you, and the PM would show 250W when you were actually riding 266W. That is just mean.

In general, a single-sided PM is better than no PM. But, these threads do tend to get binary, and single-sided gets labeled as worse than no PM. You just have to know the limitations of a single-sided PM-- that if your L/R balance fluctuates, then the consistency of the PM can be way off from ride to ride.

I think that what AC said was (paraphrasing) that for relatively steady state efforts (where a single sided PM is pretty good) a HR monitor works just as well. If they aren't steady state, then the LR balance can change quite a bit.

With the price of power meters that actually measure total power now, I don't see the point in single sided.

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This was going to be my answer as well. Just like if someone decided to go from one dual sided power meter to another (say P2M to Garmin Vectors), you'd always do a fresh ftp test to get an accurate baseline.

Going back to the pool analogy, it's like swimming at Centennial. You need a different standard for swimming in lane 5. (or is it 2? the second one from the far wall in the shallow end, anyway). The current. Oh god that current....

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Weird, I always noticed it mostly in the far lane (whatever number it is) in the deep end!
I also enjoy getting cloths-lined by others in our club with a wonderfully wide recovery stroke....

I rarely swim in that lane, so I never noticed it in the deep end.

Sorry for the hijack, folks. continue on.

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Which is exactly what a single sided PM does. Extrapolates your total power based on the assumption that your L/R power is even and doesn't change within a ride, or from ride to ride.

The difference is that I don't use the L/R power estimation from my Quarq to do anything, where people who chose single sided PMs use their estimation and extrapolations to guide their training and racing.

This is what I was thinking too. I'm still confused about what to do. :)

I've been running left crank Stages on my bikes for 4 years, and have enjoyed it tremendously. It's helped my training and also the few races I've done so far. Now I'm trying to figure out if I should put a LR Stages on my new bike, or just single side again. It's really just a matter of money. Is it worth the extra money, will it make a significant difference in my training and racing, or is it just a nice-to-have (which would certainly be a gotta-have for a pro)?

I have a Stages and can't recommend it. While its a solid product I have 2 issues with it:

I have a hypermobile SI joint in my left leg, which means after being on the bike I get bad sciatica and snapping hip effecting my left leg power. Last hour effort I did for the last 15 minutes I could only put power into my right leg, so whatever power the Stages showed was off. While that is an extreme example many cyclists have similar issues, sometimes being so minor its unnoticeable.

For TT's its fine but for drafting, it can be off. For example if you accelerate in to draft and pedal 5 times, that first one always has the most force. So the Stages could read 1000w, 500w, 500w, and normalize that or 500w/ 500w. That sounds petty, but in a crit I could do that 50 times and depending on what leg I use first, the reading are going to be different.

What's missing from the bolded statement above is "and as long as your L/R balance is consistent within a given ride, day to day, week to week, at different points in your season, fatigued vs tapered, and across all effort levels."

That is a huge unknown for most people, because they only way you can find out for yourself is to use a true L/R power meter for an extended amount of time (months or even years)...in which case you're already gotten past the point of considering a single sided PM. And most studies that have been done to test L/R balance across multiple athletes show that most athletes are not consistent in their L/R balance in any given ride, day to day, or across multiple effort levels...nor is it predictable.

Accuracy matters. Not just precision. Why?

1. If you ever ride a smart trainer or upgrade/swap power meters. You don't need to adjust your FTP, zones, power numbers, etc. This could have a big effect on TSS, CTL, etc.
2. Estimating or getting an idea of how aero you are (whether that's just comparing wattage and race times with other folks, the chung method, or best bike split)

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Good points. So...accuracy...on that topic, I wonder what the current factual knowledge is about how accurate Stages PM's are? I've read some reports in the past and heard about inaccurate power readings (not just data dropouts, but current, avg and peak power measurements), but haven't really seen anything that definitively supports the complaint. Was this a past issue that's now been resolved with the newest tech they offer? I'm used to using Stages, and have it on my other bike, and don't want to switch unless there's a solid reason to do so. No need to switch unless there is some advantage to making a change. If we're talking about accuracy, and the left-only VS L-and-R is in discussion, we should also take into account the overall accuracy of the PM itself.

The instant your L/R balance is not 50/50, Stages total power reading are inaccurate, no matter how accurate the strain gauges in the left crank arm are.

From what I seen though, Stages strain gauges operate at the same type of accuracy (1-2%) in measuring power on the left arm as most other power meters. The problem lies that they take that left arm reading and change that into a total power reading, which may or may not be accurate.

So let's say you're within 5% avg left-to-right in terms of power balance across your training, and there are exceptions during fatigued instances when the imbalance becomes more significant...If that's your natural physiology, and if you're not riding professionally where it's critical, where does the practical benefit of a L-R setup come into reality? I'm a little geeky and have learned a lot from using my Stages PM and power graph during training, but I guess I'm having a bit of a hard time justifying whether a L-R setup is a nice-to-have or a gotta-have. I know that can be subjective of course, but unless there's something compelling that puts it into the gotta-have column for club racers who use PM's, it would seem the money could be allocated in other areas of need where there could be more realizable gains. (If money was no object, we'd all just get everything, wouldn't we, and get plenty of sideways glances from our spouses, haha.)

A L-R setup isn't what is important, but rather one that gives you total power. You don't need to know what your balance is, just what your total power is.

My PM gives me a L/R balance and I have examples where (on the trainer) I shifted gears in the middle of an interval and simply changing my cadence flipped my balance from left dominant to right dominant. Even though I was putting out the exact same total power, a left only PM would have reported a wattage difference of 20w.

5% is actually a pretty huge swing, but lets go with that for the sake of this example.

When you do your FTP test, lets say you are putting out 50/50 power. 125 watts in each leg, so your test result is 250 watts regardless of whether you were using a one sided PM, or a PM that measures total power. For simplicity's sake, lets just say your FTP is 250.

Based on that test, you set up intervals to do at 110%, so your target is now 275 watts. But while doing intervals, you have a 5% swing in balance as you suggested. So lets say 45% on the left leg and 55% on the right leg. In order for you to hit 275 watts using a left only PM, you would have to put out 137.5 watts on the left leg. But if 137.5 watts only represents 45% of your total power, you're really putting out 305.5 watts (137.5 left, and 168 right). Your wattage output would be telling you 275 watts, which is your target, but in reality you're doing 305 watts, or over 120%. Lets just say you're likely going to blow up before your intervals are done, and left with more questions than answers as to why you failed your workout so miserably. You're going to start blaming your running or swimming schedule, how much sleep you got, your diet, hydration, etc. Then next week your L/R balance is going to be closer to 50/50 and you're going to wonder if all those changes you made to your running, swimming, sleep, diet, and hydration were the reason why that same workout is now much more doable. When in reality those things had nothing to do with it. You just had a PM that was giving you inaccurate power numbers due to unknown and unpredictable changes to your L/R balance.

If that's a tool you feel worth spending money on...go for it I guess.

If you are new to power, I think a left sided powermeter is a fine way to start. Not everyone needs a $1000 option right away. Obviously if you get more serious, I think moving to a L/R power meter makes sense.

The good news is all these new systems Vector 3, 4iii, Stages are all offering upgrade paths where you can start off with a left-sided powermeter and then move to a dual-sided one down the road. Just make sure you get the 2018 models of 4iii or Stages.

That's an enlightening way to paint the picture, thank you!

I would agree with everything that JasonN has said, and for reference my L-R balance can vary by up to 8% (though it tends to vary by closer to 4%) both within rides and day to day.

I think it really depends on your intended purpose. If you just want to track your changes in fitness so you know how well your training programme is working, that's one thing. If you are going to be using it to modulate power output for pacing purposes in a race, that's another thing. You just have to look at how flat the TTE curve is to see how small wattage differences have a huge effect on how long you can hold that pace for.

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I would disagree (respectfully!!). As JasonN's example, if I aim to do an FTP session at 100% FTP and my balance is 4% different (which it is every ride), I could be trying to hold 30 watts too many at FTP. I'll fail, and wonder why I'm going so badly. If your balance always stays constant, then I agree with you, but if your balance changes then I think a single sided power meter is worse than no power meter. Unfortunately you won't know unless you have a dual sided power meter!!!