Using left crank power meter and now adding Garmin Rally pedal PM
 

So I currently have a stages left crank power meter and that is paired with my edge 830 for power and cadence. I want to move to a dual sides and trying the pedals for the ease of switching etc. they seem to get good reviews.

my question is this. When I pair my pedals, should I unpair my stages first? Or is there a way to leave both paired them select which to pull power and cadence data from? Would also be interesting to compare the two PMs, but I assume this isn’t possible?

You can pair them both simultaneously, but you can only select one power source at a time on your head unit. If you want to swap them over, you just have to disable one and enable the other in your sensor settings. So when you come to pair your new pedals, just disable your Stages PM in the sensor settings first. Once paired you can then toggle between them.

You might be able to use your phone to record one or both of them.

If you have only the Garmin 830, you'll have to use a phone to record the other with BT while using the 830 to record ANT+. Syncing up the data streams by time stamp alone is a pain; the easiest way is if you have a speed sensor that dual transmits on both ANT+ and BT. The way I compare dual data streams is via a dual VE comparison: the total mass is the same, the air density is the same, the wind is the same, your use of the brakes is the same, the speed is the same (if you use the same speed sensor) so differences in VE profile will be due to differences in the power transmission and recording.

Now you have two watches. Which has the correct time?

People say this all the time but, unless you're doing something that's a lot more important than riding a bike, you really don't need much more precision and accuracy than you can easily to achieve with most modern watches to know the "correct time." The work it takes to raise a mass m to a height h is m*h*g so if you can run a test with your two power meters that will tell you how much "work" you performed to raise your mass m to a height h, you can generally tell which power meter's measurement of work is "correct." The only trick is in formulating the proper test.

There is a connectIQ data field for Garmin devices that allows you to dual record multiple power meters to the same fit file. (ANT+ Power Meter by takura87 - https://apps.garmin.com/en-US/apps/7...a-23f029a75e3b)

I use this to record both a P2M NGeco and Assioma pedals when I ride outdoors. The Assiomas are connected to the Garmin the normal way, and I put the ANT+ ID of the NGeco in the settings of the ANT+ Power Meter ConnectIQ app via Garmin Express. Then you have to add the data field to a data screen in your Outdoor Cycling activity profile so the Garmin knows to look for the extra power meter.

The data for the second PM is recorded in a developer field in the fit file, and I use either DCRainmaker's Analysis tool or Golden Cheetah to see the info if I want to compare the two PM's.

Pretty much my point by making the comment, but in a round about and obscure way to get there.

Even if they don't agree, consistency is more important. Even with inaccurate wind up watches of my child hood days, we soon learned that if we were late for a meal or too early, we just showed up that much later or sooner by the watch. So it really didn't matter if what we set our watches by was correct or not.

Not many of us had a short-wave radio to get a time signal. And banks and other businesses that provided time were frequently wrong by a few minutes.

Power meter data is much the same. Knowing the exact number isn't as important as just knowing that it is giving a consistent reading and that your numbers are improving or getting worse. If the exact number was all important, there'd be no need to have races. We'd just see who had the higher FTP or maybe best power for so many hours and maybe do a weight to power ratio or something.

People say this all the time, too, and you can pretty much tell who doesn't use power data to their fullest extent when they say this. You know what's consistent? A wristwatch. If all you needed is consistency, a wristwatch works well and is a lot cheaper.

There are lots of things one can do with power data; some of them don't require much accuracy, some of them do. Training FTP, for example, is one of the least demanding things you can do with a power meter so if that's all you're doing you can pretty much get away with an ordinal measurement. That's because FTP is a single point estimate of a single quantity. However, there are things you can do with power data that depend on the shape and changing relationship between quantities: for example, the changing relationship between power and speed at different levels of power or speed. If you do any of those things, you'll need accuracy across the entire range of power, not just consistency at a single point.

Power meters are expensive so if all you're doing is treating your power data like you would a wristwatch, or a HRM, you're missing out on their potential benefits.

If my power meter were to consistently report everything 5% less than my actual power or heck lets say 20% less than my actual power, then how would that affect the shape and changing relationship between quantities? It's all moving relative to each other so from my viewpoint my numbers still show if I'm getting better or worse.

The only downside to not being accurate is when one compares their numbers to another persons numbers. Even then numbers mean nothing until we know who crossed the finish line first. Not who could cross the finish line first.

No.

Why?

Because

1. What happens if you change power meters for any reason? To use a different wheel, kind of pedal, because you sold your bike or it got stolen, they have them at the gym, or you use a smart trainer? How can they be consistent with each other except by measuring accurately?
2. Work done on a road bike gets you calories burned with a maximum error of +/- 2.5%. If you want to use that to make weight for an event being consistent is much less useful than being accurate.
3. If you want to use your power meter to refine your position and clothing you need good data to do the math with.
4. Races can be as much about tactics and other choices (smarts) as fitness.

1) That would be a good reason for having accuracy between your power meters. But still I don't see how that would affect training by a power meter. You still look for the amount of change from one ride or group of rides to the next don't you?

2) I've never understood why people think the accuracy of the number burned for Calories needs to be double, triple or maybe even 10 times the precision of the numbers they use for calculating their Calorie intake.

3) I suppose. But why wouldn't a power meter that consistently reports by the same percentage off of the real number not tell you which clothing you were faster in?

4) Yes they are. It's all about who crosses the line first. And I don't need accurate numbers to do so that I'm aware of. I just need my numbers to be reported consistently across their range so I can know by that PM how much power (according to it) I can use and still have some left for the finish line.

Well you're intending to do your sweet spots at 95% of your FTP, so ...

​​​​​​It's ok for you not to understand other people's goals and methods, as long as you don't need to use them. 🙂

​​​​​​Was it because the winds were different that day, you had different luck with the traffic lights, or because you put out more effort? If you can accurately measure your effort, you'll know the important part of the answer, if you can't you won't.

Sigh. People say this all the time too, but those people have never actually tried to use their power meters to do anything where the relationship between power and speed varies. Are you familiar with the DCRAnalyzer? That's a simple tool to compare two different power meter data streams--and it doesn't measure accuracy it *only* measures consistency, which is what you're saying is the only thing that matters. If you've ever looked at a comparison on Youtube (for example, Shane Miller's comparisons) you'd see that *even in those cases* where the power meters are close to consistent, they're *never* a constant multiple of each other. Even in this easiest of all standards, the kind of deviation you're talking about (one being a constant percentage multiple of the other) doesn't exist. So that's a straw man argument: you've set up an easy-to-knock-down hypothesis and then succeeded in knocking it down. But it wasn't real, it was always a straw man.

In the real world, when you look at real data from real power meters, they will differ by differing amounts at different times under differing conditions. So what's important for these other uses (not training, which is arguably the least demanding use for a power meter) is knowing when they're off, by how much, and how much that affects the results.


https://cimg9.ibsrv.net/gimg/bikefor...55cdb96b55.png

To clarify the graphic: I did this a few years ago but one of the power meters was single-sided while the other measured total both-side power. I don't remember which was which. The average difference between PM1 and PM2 was smallish, like 3 or 4%, so the dots in the left panel have a slope of 1.03 or 1.04. The red line in both panels has a slope of 1. People think that a 3% difference is small because they think the comparison looks like the left panel.

The real question is, can a more accurate PM on the market make a weekend athlete any better than a less accurate PM. I'm referring to weekend athletes, the people in this discussion. I doubt it. To this end, the discussion and dissection of PM... probably doesn't mean much for weekenders than say a pro pursuit cyclists. They should really stop pushing weekenders into buying PMs and into thinking that PMs will somehow enable them to do something that requires genetic blessing. No PM is gonna overcome a genetic deficit.

That's a good and important question.

There are two ways to get faster: more power or less drag. I'm now at a stage of my life where my power is way down from my peak -- but I'm only a tiny bit down from my peak speeds. All this crazy stuff I do with power data? I have to do it or else I'd be even slower than I am.

Your plot of what your data actually looks like is exactly what I expected.

And still none have shown me why data with a PM that consistently reports 5% lower values or higher values wouldn't be just as useful as one that is considered accurate every time. Aren't you still looking at the data relative to all the data previously collected with the same PM?

Woah. So, that's exactly what you expected but you still think that the relationships between power and speed aren't going to be affected? Hmmm. You don't do a lot of statistical estimation, it seems. (Which isn't a value judgement -- not everyone does).

Maybe I'm confused then as to your use of the PM. Is it for training. Or for predicting how fast you'll be at the finish line or points in between?

If the latter, then I'd agree you need to be as accurate as possible.

But for training, as long as you aren't comparing to others, then a consistently off by a certain percent PM will do just as well as another.

295w target power, 1.16 kg/m3, .0035 crr, .218 CdA, 10mi TT, pan flat, .98 drivetrain loss:
100%, minus 5%, minus 20%: 45.45 kph vs 44.60 kph vs 41.90 kph

200w target power, 1.16 kg/m3, .0035 crr, .218 CdA, 10mi TT, pan flat, .98 drivetrain loss:
100%, minus 5%, minus 20%: 39.41 kph vs 37.89 kph vs 36.25 kph

The 295w power target has a delta of .85kph and 3.55kph.
The 200w power target has a delta of .52kph and 3.16kph.

The relationship isn't a nice linear 5% due to the non linear nature of aero drag at different speeds. For a 40k, the 5% you'd probably be off on accuracy by a whole kilometer over the course of the event (relating the error to a visual cue like distance).

Additionally, in training, if you're doing intervals right on the limit.......being off a hair can mean you don't meet your workout targets. Either underperforming and not adapting or trying to overperform and not adapting because you tried to hit targets you thought you should. The wisdom there is adjusting the target mid workout by knowing your body and how the workout should be going. But most folks do ERG workouts refusing to change the target. Probably the same folks who have the fever dream of thinking their ftp is actually 95% of a 20min test or a ramp that they didn't even perform correctly.

Also, with the linear assumption of 5%, 5% of a larger training target is larger than a small one. So 5% error on a 200w warmup is less consequential than 5% error on a 300w 8min effort where the larger error is more watts. And therefore more consequential.

So yeah, IMO a routinely wrong 5% can toss you in aero or training. Especially knowing most folks don't have the wisdom to hit the little minus button on their training program or control unit in ERG mode when the workout isn't going how it should.

There are lots of things you can do with power meter data. Training is certainly one of them, but there are others. It turns out that training (especially training FTP) is one of the least demanding things you can do with power data (that's why riders have been able to train effectively with just a wristwatch and a regular training route since there have been wristwatches). There are things you can do with power data that you can't do with a Timex wristwatch. That you don't do any of these things and instead only do the simplest possible thing with power data is probably why you think that consistency is all one needs.

And, I don't look at that right hand panel and think "oh, that shows that one is consistently off by a certain percent." I think that's what the left hand panel shows.