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Power Meter Accuracy Study

 
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Jorgan




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PostPosted: Thu May 18, 2017 7:56 am    Post subject: Power Meter Accuracy Study Reply with quote

Anyone see the recent CW article?
http://www.cyclingweekly.com/news/latest-news/huge-scientific-study-shows-inaccuracies-popular-power-meters-330322

I did an event yesterday, and looking at the Strava files of some other guys, there is a huge delta between power numbers for similar performances. Guy racked next to me was using a PT, similar size, all the aero kit, and he was pushing out 50w+ more than me for segments that were split by a few seconds. On one segment he was pushing 340w to my 234w, and he was about 5s quicker over 2 (flat) miles!

I know the P2M has a reputation of reading a tad low, but his PT must be smoking crack.
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explorerJC




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PostPosted: Thu May 18, 2017 8:09 am    Post subject: Re: Power Meter Accuracy Study Reply with quote

Jorgan wrote:
Anyone see the recent CW article?
http://www.cyclingweekly.com/news/latest-news/huge-scientific-study-shows-inaccuracies-popular-power-meters-330322

I did an event yesterday, and looking at the Strava files of some other guys, there is a huge delta between power numbers for similar performances. Guy racked next to me was using a PT, similar size, all the aero kit, and he was pushing out 50w+ more than me for segments that were split by a few seconds. On one segment he was pushing 340w to my 234w, and he was about 5s quicker over 2 (flat) miles!

I know the P2M has a reputation of reading a tad low, but his PT must be smoking crack.


which is one of the reasons why it is very hard to make judgements or give guidance on power alone...

that said, he could have been of much greater BM and/or really poor set up/positioning...
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Jorgan




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PostPosted: Thu May 18, 2017 8:17 am    Post subject: Re: Power Meter Accuracy Study Reply with quote

explorerJC wrote:


which is one of the reasons why it is very hard to make judgements or give guidance on power alone...

that said, he could have been of much greater BM and/or really poor set up/positioning...


Yes, that would be my first thought, but he would have to be using the base bars on his Trek SC9 series! I would estimate his body mass to be within 5kg of mine. Looking at most of the other guys with PMs and similar performances, we were a lot closer (within 10w).
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explorerJC




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PostPosted: Thu May 18, 2017 8:56 am    Post subject: Re: Power Meter Accuracy Study Reply with quote

Jorgan wrote:
explorerJC wrote:


which is one of the reasons why it is very hard to make judgements or give guidance on power alone...

that said, he could have been of much greater BM and/or really poor set up/positioning...


Yes, that would be my first thought, but he would have to be using the base bars on his Trek SC9 series! I would estimate his body mass to be within 5kg of mine. Looking at most of the other guys with PMs and similar performances, we were a lot closer (within 10w).


he probably needs to recalibrate then... Smile
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Tin Pot




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PostPosted: Thu May 18, 2017 11:12 am    Post subject: Reply with quote

I thought it was already well known that power measurement accuracy varies fairly significantly.


Somethings are unclear on the test;

Are they talking about absolute accuracy or consistency?

In the intro they say the dynamic calibration rig can't test power meters that measure power before it hits the cranks, what was done to test pedals like my P1S?
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chickenboy




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PostPosted: Thu May 18, 2017 11:23 am    Post subject: Reply with quote

I've a stages PM on my TT and road bike. I've been measuring and comparing them recently to wahoo kickr running them in tandem with a traineroad workout.

They've all been calibrated. The TT stages meter consistently reads 5% lower than kickr, and the road bike stages 5% above. I've done this over a number of turbo rides and the numbers are pretty consistent. That's a huge discrepancy between stages meters. My wahoo kickr FTP is 320 but I suspect I'm going to have to retest each stages separately for their own specific FTP reading.
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explorerJC




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PostPosted: Thu May 18, 2017 11:24 am    Post subject: Reply with quote

Tin Pot wrote:
I thought it was already well known that power measurement accuracy varies fairly significantly.




Are they talking about absolute accuracy or consistency?

?


that...plus it is very disingenuous to suggest that an athlete cant be sure of improvement...
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Chrace




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PostPosted: Thu May 18, 2017 12:42 pm    Post subject: Re: Power Meter Accuracy Study Reply with quote

explorerJC wrote:
Jorgan wrote:
explorerJC wrote:


which is one of the reasons why it is very hard to make judgements or give guidance on power alone...

that said, he could have been of much greater BM and/or really poor set up/positioning...


Yes, that would be my first thought, but he would have to be using the base bars on his Trek SC9 series! I would estimate his body mass to be within 5kg of mine. Looking at most of the other guys with PMs and similar performances, we were a lot closer (within 10w).


he probably needs to recalibrate then... Smile

No way! He has the perfect setup to brag about his FTP at the club.
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Cat5 in the Hat




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PostPosted: Thu May 18, 2017 1:11 pm    Post subject: Reply with quote

chickenboy wrote:
I've a stages PM on my TT and road bike. I've been measuring and comparing them recently to wahoo kickr running them in tandem with a traineroad workout.

They've all been calibrated. The TT stages meter consistently reads 5% lower than kickr, and the road bike stages 5% above. I've done this over a number of turbo rides and the numbers are pretty consistent. That's a huge discrepancy between stages meters. My wahoo kickr FTP is 320 but I suspect I'm going to have to retest each stages separately for their own specific FTP reading.


Calibrated or zeroed?
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stenard




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PostPosted: Thu May 18, 2017 2:03 pm    Post subject: Reply with quote

Worth noting the DCR comments on this study:
DC Rainmaker wrote:
Lots of chatter about this 54 units study. It's interesting stuff, and I've chatted with one of the folks doing it a bit as well this week. Overall I don't see any core issues with what they did. As they noted in an e-mail to me though, it only focuses on indoor - which limits things a bit. One of the challenges with power meters these days is that the overwhelming majority of accuracy issues actually happen outdoors (temperature changes, road conditions, humidity shifts, etc...). Note on models that most of them tended to be older units, rather than newer units. Pros and cons to that both ways actually. Still, this is an impressive pile of data to put together, good stuff!

I agree with eJC, whilst you ideally want PMs to be consistent with each other, in the same way HR can be validated as precisely accurate, (the reason being so if you swap PM's at some point, all your old data doesn't become useless), within the cycle of a single PM their results surely cannot imply that trends by a single rider using a single device cannot be measured. They even say precision was considered "high", meaning ride to ride data from the same rider on the same PM would be directly comparable.

It does beg the question as to whether my P2M Classic on the road bike and the Type S on the TT are directly comparable. The zero offset of each is different by about 50-75, but directly that doesn't say anything.

With regards to Jorgan's point about P2M generally reading low, I have heard that a lot, but my only real world reference point is DCR, and his in-depth review of the Type S had no low offset like the one's I've read mentioned:


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explorerJC




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PostPosted: Thu May 18, 2017 2:17 pm    Post subject: Re: Power Meter Accuracy Study Reply with quote

Chrace wrote:
explorerJC wrote:
Jorgan wrote:
explorerJC wrote:


which is one of the reasons why it is very hard to make judgements or give guidance on power alone...

that said, he could have been of much greater BM and/or really poor set up/positioning...


Yes, that would be my first thought, but he would have to be using the base bars on his Trek SC9 series! I would estimate his body mass to be within 5kg of mine. Looking at most of the other guys with PMs and similar performances, we were a lot closer (within 10w).


he probably needs to recalibrate then... Smile

No way! He has the perfect setup to brag about his FTP at the club.


absolutely

and bragging rights are far greater than race success Smile
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chickenboy




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PostPosted: Thu May 18, 2017 2:20 pm    Post subject: Reply with quote

Cat5 in the Hat wrote:
chickenboy wrote:
I've a stages PM on my TT and road bike. I've been measuring and comparing them recently to wahoo kickr running them in tandem with a traineroad workout.

They've all been calibrated. The TT stages meter consistently reads 5% lower than kickr, and the road bike stages 5% above. I've done this over a number of turbo rides and the numbers are pretty consistent. That's a huge discrepancy between stages meters. My wahoo kickr FTP is 320 but I suspect I'm going to have to retest each stages separately for their own specific FTP reading.


Calibrated or zeroed?


now you've brought that up i'm questioning myself. i think zeroed. is that the right or wrong thing to do?
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stenard




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PostPosted: Thu May 18, 2017 2:31 pm    Post subject: Reply with quote

chickenboy wrote:
Cat5 in the Hat wrote:
chickenboy wrote:
I've a stages PM on my TT and road bike. I've been measuring and comparing them recently to wahoo kickr running them in tandem with a traineroad workout.

They've all been calibrated. The TT stages meter consistently reads 5% lower than kickr, and the road bike stages 5% above. I've done this over a number of turbo rides and the numbers are pretty consistent. That's a huge discrepancy between stages meters. My wahoo kickr FTP is 320 but I suspect I'm going to have to retest each stages separately for their own specific FTP reading.


Calibrated or zeroed?


now you've brought that up i'm questioning myself. i think zeroed. is that the right or wrong thing to do?

Zeroing is what you do on a head unit/watch. Even though Garmin call it "calibrating". You're setting the "c" in the y = mx + c straight line equation. Certain power meters, like the P2M, cannot be calibrated (i.e. changing the slope "m") other than by sending back to the manufacturer.

That's my understanding anyway.
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Jorgan




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PostPosted: Thu May 18, 2017 2:46 pm    Post subject: Reply with quote

You don't need a scientific study though, just common sense. If you see people (of a similar morphology) tanking out way more watts than you in training, and yet they cannot best you on a bike split head-to-head, then chances are their PM is smoking something, and that my PM is maybe a little conservative. All adds up to the same thing though.
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