New power meter...pleas help me understand
#51
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You mean with an inaccurate, but precise, PM? I am not an engineer, but I would say as long as the error was just a constant offset over the operating range, yes. However, in my experience instrument errors, (when they're even linear) are usually multipliers rather than simple offsets.
Ah, I see what you mean now. Yes, some sort of personal benchmarking is how the whole thing works.
Ah, I see what you mean now. Yes, some sort of personal benchmarking is how the whole thing works.
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One other thought: if you're interested in tracking your power output progress, the actual power level you're putting out is less important than the relative change in power output over time. You want a power meter that is consistent in measurement so that you can see how the metrics change over time. This is why many experts say the accuracy of a given power meter is less important than the consistency of a given power meter.
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very interesting experiment. unfortunately i have a crank based power meter so can't do the same thing. let us know if you do more testing! I also don't know how the normalized power calculation works exactly, but given there is coasting and downhills in the real world, that definitely reduces output relative to the Peloton where you never stop pedaling. Just taking a quick observation, on flats I am pushing at least 240-260 watts on the road bike and going up hills in the 300's at least. hard to keep those kind of watts downhill which is clearly going to impact average.
Anyway, a lot of people think NP is basically avg pwr without the zeros, that idea came up in this thread. The zeros affect your NP in two ways: they bring the number down, but they also allow you to make more power afterwards because they're rest, if you're doing zeros because you stopped you have to start again which means bigger than normal numbers, and those are weighted heavily in NP. The details don't really matter that much, the moral of the story is NP is a prediction, and it's not just ignoring the zeros.
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Normalized power is a rolling 30 second average raised to the 4th power. Typically not that useful for efforts under 20 minutes.
Can be gamed significantly depending on the rider. For example, 6-7 max one minute efforts with easy pedaling in between for an hour can result in an NP significantly higher than FTP for some.
Conversely, long, very steady, high power efforts (30 minutes at threshold, for example) can result in an NP a few watts lower than AP.
TSS/ATL/CTL are all based on NP.
Can be gamed significantly depending on the rider. For example, 6-7 max one minute efforts with easy pedaling in between for an hour can result in an NP significantly higher than FTP for some.
Conversely, long, very steady, high power efforts (30 minutes at threshold, for example) can result in an NP a few watts lower than AP.
TSS/ATL/CTL are all based on NP.
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I do TT. Having the meter say "250w" and it is actually 250w matters. I use the meter with a speed sensor and pitot to do aero testing. If the power reading is garbage, the aero testing is garbage.
So, consistently seeing the wrong number doesn't help me one bit.
I went to a Quarq on the TT bike and now will ask for one for Christmas for my road bike also to replace the abhorrent Stages left-only.
So, consistently seeing the wrong number doesn't help me one bit.
I went to a Quarq on the TT bike and now will ask for one for Christmas for my road bike also to replace the abhorrent Stages left-only.
#56
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NP is not a prediction of anything. It is the fourth norm of 30 second average power. That means you average power readings from the previous 30 seconds, raise that to the fourth power, calculate the average of those values, then take the fourth root of that average. People can choose to use NP however they want. Andy Coggan developed the concept as a measure of the metabolic cost of a workout. In other words, he considered the metabolic cost of all workouts with the same duration and NP equal independent of how that NP was achieved.
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The discussion above about the power to hold 20 mph, assuming flat and no wind, is pretty interesting. Again, I realize power meters are not always accurate, but I’m surprised by the result above showing 200 NP watts to hold 20 mph. I would think that 200 watts would result in more than that. Many variables I’m sure.
On my next ride, I’m going to see how many watts it takes for me to hold 20 mph, on the hoods and in the drops.
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NP is not a prediction of anything. It is the fourth norm of 30 second average power. That means you average power readings from the previous 30 seconds, raise that to the fourth power, calculate the average of those values, then take the fourth root of that average. People can choose to use NP however they want. Andy Coggan developed the concept as a measure of the metabolic cost of a workout. In other words, he considered the metabolic cost of all workouts with the same duration and NP equal independent of how that NP was achieved.
But as to your first sentence, NP is really a prediction of the metabolic cost. It isn't the actual cost, that's unknown. The take home point for the OP who is learning to use power data is that NP can be a useful metric, but it's not the end all be all and it's not better than avg for a lot of things.
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Two possibilities:
1) The Peloton, like many exercise devices, over-reports power.
2) When you ride the Peloton, you are pedaling at all times. You are riding to your power numbers. There is no coasting, no downhills, no tail winds, etc. When you are riding outside, you are riding to speed, and all those things allow you to maintain speed at lower power during parts of your ride.
1) The Peloton, like many exercise devices, over-reports power.
2) When you ride the Peloton, you are pedaling at all times. You are riding to your power numbers. There is no coasting, no downhills, no tail winds, etc. When you are riding outside, you are riding to speed, and all those things allow you to maintain speed at lower power during parts of your ride.
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Adding fuel to the fire...why do different devices show power differently. My wahoo, strava and rwgps showed average power as 206, 213, and 237 respectively for the same ride...
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Depends. You might be looking at Average Power or Normalized Power or Weighted Average Power (Strava). There's also the possibility that the highest (rwgps) isn't factoring in the 0s.
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Wahoo and Strava both agreed on Average Power, as is typical: 238w.
Wahoo's NP was 271w while Strava's Weighted Average was 257w. Strava's WA being a little lower than NP is also typical.
RwGPS was all messed up - Average Power was 299w. They must not be counting 0s (coasting).
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I've never had Strava report a different average power than my Garmin.
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After seeing the passion for life just simply vanish from rider's eyes while they're in the midst of 6 months of extremely specific power driven training.... I tend not to hold it up for any use other than for specific targeted competition. After having watched a ton of riders improve greatly without the use of power it just isn't needed at the entry or lower levels of the sport.
The only thing i disagree with you on, Rob, is your statement that powermeters are not that accurate. Looking at DCRainmaker's tests, various meters all tend to be fairly close to each minor (minor spikes excepted). So either they are all inaccurate by the same degree, or in fact they are close enough - within a couple of percents, as claimed.
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Despite being a devoted power meter user, I agree that power-related jackassery bores me to tears (and I am a guy who likes geeking out on numbers and such). People that talk of rides in terms of TSS, and their weekly CTLs... dudebros, no one cares. Show it on raceday.
The only thing i disagree with you on, Rob, is your statement that powermeters are not that accurate. Looking at DCRainmaker's tests, various meters all tend to be fairly close to each minor (minor spikes excepted). So either they are all inaccurate by the same degree, or in fact they are close enough - within a couple of percents, as claimed.
The only thing i disagree with you on, Rob, is your statement that powermeters are not that accurate. Looking at DCRainmaker's tests, various meters all tend to be fairly close to each minor (minor spikes excepted). So either they are all inaccurate by the same degree, or in fact they are close enough - within a couple of percents, as claimed.
As for switching between devices - very common to hear athletes say, "Oh I switched over to my cross bike and the power meter on that one seems to read about 5-10W higher than the one on my road bike so I just adjust." Many times when riding at a constant effort even the 3s averaged power readings can vary to a degree that is larger than the PM's tolerance range. I honestly think a lot of riders just can't tell the small differences between devices.
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Most are generally accepted to be within +-5% or less. Computrainers are +-2-3%. Infocrank was something like +/-1%. That means they will all "seem" close to each other at lower power numbers. The reality is that they are all "close enough". From that point on as long as they are consistent then that's all that maters.
As for switching between devices - very common to hear athletes say, "Oh I switched over to my cross bike and the power meter on that one seems to read about 5-10W higher than the one on my road bike so I just adjust." Many times when riding at a constant effort even the 3s averaged power readings can vary to a degree that is larger than the PM's tolerance range. I honestly think a lot of riders just can't tell the small differences between devices.
As for switching between devices - very common to hear athletes say, "Oh I switched over to my cross bike and the power meter on that one seems to read about 5-10W higher than the one on my road bike so I just adjust." Many times when riding at a constant effort even the 3s averaged power readings can vary to a degree that is larger than the PM's tolerance range. I honestly think a lot of riders just can't tell the small differences between devices.
#67
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Which is fine until they get home and want to use their power/duration curve for post hoc analysis.
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If you look at those analytics, you will notice that while they do give similar numbers under steady out put, they can vary greatly under acceleration and short high intensity efforts.
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I assume vary greatly means different things to different people. This is the first graph I found on his site and I am sure you can find a graph with flyer readings. The point is most meters track very closely considering all the variabilities involved.
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Without zooming in it looks like that last peak in there the power meters were giving difference of up to 30W. That's varying greatly.
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Here is a power meter study (and references to previous studies). After testing 54 meters they claim an accuracy of -0.9% to true value (slight under reporting of true effort) and coefficient of variance (relative standard deviation) of 1.2% for all meters. This was surprising as I thought the accuracy would be worse, also this is a snap shot of meters built around 2015 so they could be better or worse now.
If you believe this study you can repeat the same effort at exactly 100w one hundred times on one hundred different meters and 95% of your values should be between 97W and 103W. And for a bigger effort of a 1100W sprint values between 1075W and 1125W are essentially “the same”, making the meter appear to vary more, but the relative variation can still be the same.
Also, means if your FTP is 250W and you do a new test and it spits out 253W it’s probably not significant.
Accuracy of Cycling Power Meters against a Mathematical Model of Treadmill Cycling
If you believe this study you can repeat the same effort at exactly 100w one hundred times on one hundred different meters and 95% of your values should be between 97W and 103W. And for a bigger effort of a 1100W sprint values between 1075W and 1125W are essentially “the same”, making the meter appear to vary more, but the relative variation can still be the same.
Also, means if your FTP is 250W and you do a new test and it spits out 253W it’s probably not significant.
Accuracy of Cycling Power Meters against a Mathematical Model of Treadmill Cycling
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I see you have never visited a Zwift forum....
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I've watched triathletes climb off of bikes and quit their workout because one meter was giving them a 5W different reading.
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One other thought: if you're interested in tracking your power output progress, the actual power level you're putting out is less important than the relative change in power output over time. You want a power meter that is consistent in measurement so that you can see how the metrics change over time. This is why many experts say the accuracy of a given power meter is less important than the consistency of a given power meter.
In general, equipment that "estimates" your power (and even calories burned) is almost always higher than the actual numbers. It's like they err on the high side (like car dealerships estimate MPG).
To see what your average power is during pedaling, you can look at the power graph and only select the time periods where you are pedaling (the lap feature on your bike computer is useful here too). You can use the lap feature to mark specific portions of your route and then compare the portions.
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For those people who are using a power meter like a hrm, then having numbers close enough is probably fine. If a rider is trying to improve on their flying 200, sprint or standing start, They will probably want something more accurate.