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Ensuring Power Meter Accuracy

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Ensuring Power Meter Accuracy

Old 03-31-21, 10:16 PM
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Ensuring Power Meter Accuracy

I recently had installed a Stages 105, L crank-arm paired with Wahoo Elemnt Bolt and the readings I am receiving during climbs are consistently 50 watts higher than my riding partners, who are running Dura Ace dual-sided power meters. As they average 280 watts, I am putting out 330+. Including bike weight, I am 12-15 lbs heavier than them, they're identical in weight. We calibrated our power meters together through our head units and all received similar torque-offset numbers and are using 5sec avg Power to read our wattage on the head unit. Am I mistaken in thinking that the difference in weight is too slight to account for the discrepant power readings?
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Old 03-31-21, 10:34 PM
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Either your Stages is reading wrong or you have a significantly stronger left leg.
If one of their bikes is close to yours in set up you could swap for a short time to compare readings.
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Old 03-31-21, 11:12 PM
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Your system takes three reading from your left leg and doubles it. If both legs always put out exactly the same power, this will give you accurate data. If your left leg puts out 10% more power than your right, your PM will be 20% too high. That's my guess as to what's going on. Does your Stages have a calibration procedure? It would be different from zeroing it before a ride, with pedals you can hang a known weight and check the torque reading.

(A 10% difference between legs is a lot, I picked that for easy math.)
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Old 03-31-21, 11:21 PM
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Also, you can't really compare 5s averages in the moment to know if your PM works. Look at the second by second power chart for your last ride, it's real choppy, almost random. That's why we use rolling averages. 30s would give you a better idea. People also get different speed out of the same power. You mentioned weight, tires and aerodynamics can make a lot of difference in how many watts you need to go X mph.
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Old 04-01-21, 05:45 AM
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You’re 15 pounds heavier, climbing at the same speed and putting out 50w more? Doesn’t seem that far off, especially since some folks have reported that dual sided hollowtech II crank based PMs show too low of a power number at times (I think it was gplama)
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Old 04-01-21, 07:02 AM
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For what its worth I have the same Stages power meter and it has aligned reasonably well with my Wahoo Kickr power numbers.

Along with the left side issue and the weight issue there is also the efficiency issue (tires, aero, etc). I agree swapping bikes on a climb would be the best way to diagnose it.
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Old 04-01-21, 07:16 AM
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I don't believe you should be expecting a similar reading to your friends' power meters given that you have a single sided PM and they have dual sided.

Also, that fact that ou are 15lbs heavier probably makes a difference when you get out of the saddle and start climbing. I would definitively swap out bikes and see what you get in terms of number.
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Old 04-01-21, 07:36 AM
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There are a lot of variables to consider before coming to the conclusion you have. Some have already been mentioned. I would add position in the bike to the list. 15lb. Difference is considerable on a steep climb. You will be producing a lot more wattage to go the same speed as your riding partners.
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Old 04-01-21, 07:39 AM
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Yeah, that’s not a reliable comparison methodology, and comparing different riders on different bikes will never be a way to validate— or challenge— power meter accuracy. Saying those folks put out about 300w during that 5sec is all you can say about that.

Further complicating this are the known issues with 3rd party, dual sided powermeters on DuraAce R9100 cranksets. Are the friends running those?
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Old 04-01-21, 07:50 AM
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Ride your friends bike up the same climb and compare? I would look at the average numbers over the entire climb, not just 5 second snips.
Also, 15lb weight difference should be considered in terms of percentage. The difference between 135lbs and 150lbs will be bigger than the difference between 215lbs and 230lbs, for example.
Watts per kilo is a better way to compare between riders than just comparing power output.
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Old 04-01-21, 08:30 AM
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GCN+ did a very interesting lab test about power needed vs weight added on a climb. They added 10 kilos to the body, then 2 kilos to the bike. Measured from 1 to 8 percent on a climb. It was on the end of the "Building the lightest bike" documentary.
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Old 04-01-21, 09:02 AM
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This post makes me sad.

Power meters are expensive, and we buy them to help answer questions, not raise a new one: "Is my power meter off?"

The good news is that those Dura Ace power meters can be off, too, and they paid more.
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Old 04-01-21, 09:22 AM
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Originally Posted by RChung
This post makes me sad.

Power meters are expensive, and we buy them to help answer questions, not raise a new one: "Is my power meter off?"

The good news is that those Dura Ace power meters can be off, too, and they paid more.
Weren't you the one who was arguing that the average person doesn’t need accurate power meters in the other thread?
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Old 04-01-21, 09:24 AM
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Originally Posted by Doomrider74
Weren't you the one who was arguing that the average person doesn’t need accurate power meters in the other thread?
Well, I guess you missed the point.
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Old 04-01-21, 09:37 AM
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FWIW, my Stages L pm is about 10% higher than what my Direto trainer tells me. Which one is correct, I dunno for sure. I suspect the trainer's PM should be more accurate.

Is it just climbs where the difference is read, but on flats you're all about the same? If you all upload to eg. Strava after a ride, it might be more informative to select the same portion of a ride (eg. a 1/2 mile segment) within Strava's "Analysis" section and then read the left-side returned results for the average and Max power for the selected portion.
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Old 04-01-21, 09:52 AM
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Originally Posted by colnago62
Well, I guess you missed the point.
No, he seemed pretty clear. Oh, and why don't you let him respond: he doesn't need you to stick up for him.
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Old 04-01-21, 10:04 AM
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Re left/right power balance,

absent injury, short or missing limb, etc., has anyone seen any significant, consistent difference in power between left & right legs?
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Old 04-01-21, 10:07 AM
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Originally Posted by woodcraft
Re left/right power balance,

absent injury, short or missing limb, etc., has anyone seen any significant, consistent difference in power between left & right legs?
That's my question now too, thanks for asking
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Old 04-01-21, 10:10 AM
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Almost the same discussion we used to have with Calories burned on a ride over 10 years ago with our simple HR/GPS cyclometers. Then they said power meters will end this type of thing.
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Old 04-01-21, 10:20 AM
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Originally Posted by Doomrider74
No, he seemed pretty clear. Oh, and why don't you let him respond: he doesn't need you to stick up for him.
There are lots of wingmen around here dying to be quick in the pig pile, hoping to catch the eye of their idol and get mentioned or quoted.
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Old 04-01-21, 10:35 AM
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Originally Posted by Doomrider74
No, he seemed pretty clear. Oh, and why don't you let him respond: he doesn't need you to stick up for him.
You seem to be looking for a fight. You're looking in the wrong place. @RChung knows a lot more about this than you do.
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Old 04-01-21, 10:37 AM
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Originally Posted by woodcraft
Re left/right power balance,

absent injury, short or missing limb, etc., has anyone seen any significant, consistent difference in power between left & right legs?
I do. Guess it depends how you define consistent, it goes away when I'm sprinting or going up a steep hill, but at the end of every ride it's always L dominant and always within a particular range of imbalance.
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Old 04-01-21, 10:40 AM
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Originally Posted by woodcraft
Re left/right power balance,

absent injury, short or missing limb, etc., has anyone seen any significant, consistent difference in power between left & right legs?
How much is significant? 51/49? 60/40? 75/25?
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Old 04-01-21, 11:01 AM
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Originally Posted by tomato coupe
How much is significant? 51/49? 60/40? 75/25?

Beyond the margin of error for the PM for starters. The couple of times that I looked, My PM registered 51/49, or possibly 52/48. This was not significant enough for me to check again.

I checked youtube before spouting off here, & the guys on Trainerroad seemed to agree that even if recovering from a broken leg, focusing on L/R balance is likely counterproductive,

but maybe somebody consistently puts out >5% more with one leg than the other- that would be interesting.
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Old 04-01-21, 11:05 AM
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Originally Posted by Seattle Forrest
You seem to be looking for a fight. You're looking in the wrong place. @RChung knows a lot more about this than you do.
In your head, maybe. Not in mine. Again: why don't you let him answer for himself.
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