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Strava with two power meters

Old 05-27-22, 11:40 PM
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Strava with two power meters

this is a variant on “a man with one watch knows the time….”

i really like strava’s power curve feature, primarily for comparing different types of efforts over long periods of time. for the first year of data, i used only one bike. it’s PM was probably a bit high, but very consistent. now i have a second bike which i ride 90% of the time for the last 3 months, and the PM reads around 10% lower. to add to that, for a while it was functioning one sided, reading another 10% lower since my left leg is weaker. there are a also a couple rides before I zeroed it where it was almost 15% HIGHER.

so now my power curve is showing historical points/peaks that are quite high in some places, and I have quite a few rides which are low. I truly don’t care which is right, I assume the brand new dual sided 4iiii is right once zeroed, but it really doesn’t matter. is there a way to adjust future rides with the +10% bike to keep everything more comparable? is there an easy way to do the same to some of the +15% outliers, or should I just download the GPX, strip the power channel, and reupload?

the PM on the original bike can’t be adjusted, it’s a fundamentally different type than the crank or pedal based ones.
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Old 05-28-22, 12:44 AM
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I think Garmin units have a scale factor that will do exactly what to want. If you're using a Garmin go into the menu, find your PM under sensors and accessories, and see if you have that option?

Far from ideal, but you can use software to change the power data in your files too.
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Old 05-28-22, 06:13 AM
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Originally Posted by mschwett View Post
the PM on the original bike can’t be adjusted, it’s a fundamentally different type than the crank or pedal based ones.
What type of power meter is the other type?

A difference of 10% between two different type power meters is very good. I have a dual sided 4iiii Precision Pro 105 and I use it as a standard to test my power meter app. I wouldn’t worry about normalizing the data in Strava. Just keep adding data and let Strava do its thing.

Chuck
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Old 05-28-22, 07:29 AM
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I have five different power meters and would estimate the range to be 4%. 10% error is rather large.

I am pretty sure you cannot scale the power values with my Garmin 1030.

I can scale them in GoldenCheetah, it makes my coach feel good. I cannot scale the power in Strava in my account. I think you have to go to Zwift, there is a lot of expertise on tweaking power realtime over there.
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Old 05-28-22, 08:56 AM
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Originally Posted by Seattle Forrest View Post
I think Garmin units have a scale factor that will do exactly what to want. If you're using a Garmin go into the menu, find your PM under sensors and accessories, and see if you have that option?

Far from ideal, but you can use software to change the power data in your files too.
any software suggestions? i might just download, scale, re-upload the couple crazy high ones and then going forward scale all the ones from the +10 bike before upload. pain in the butt since normally they’re just auto uploaded, but i don’t ride it all that often.
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Old 05-28-22, 09:02 AM
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Originally Posted by OshkoshBiker View Post
What type of power meter is the other type?

A difference of 10% between two different type power meters is very good. I have a dual sided 4iiii Precision Pro 105 and I use it as a standard to test my power meter app…
Originally Posted by GhostRider62 View Post
I have five different power meters and would estimate the range to be 4%. 10% error is rather large.
….
one is a factory install precision pro 4iiii on 12 speed DA cranks. hopefully “accurate.” the other is the internal power meter of a specialized turbo creo; it’s part of the system that measures torque and cadence at the crank in order to determine what the motor should do. i ride the bike with the motor off for workouts (i use the motor for commutes and other kinds of rides) but all the electronics are still active and the data output is actually very good. specialized/brose won’t say how it works, other than that it’s “different” than a standard strain based PM.
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Old 05-28-22, 09:24 AM
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Whether a 10% lower indicated power level is significant depends on how you train or how you use your PM.

Being off by 10% is basically being off by one zone. If I think I am riding in the endurance sone (zone 2), I could actually be doing tempo in zone 3. Worse, a sweet spot session becomes a threshold affair. I do not use a PM this way, so, it would not affect me.
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Old 05-28-22, 09:33 AM
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Originally Posted by GhostRider62 View Post
Whether a 10% lower indicated power level is significant depends on how you train or how you use your PM.

Being off by 10% is basically being off by one zone. If I think I am riding in the endurance sone (zone 2), I could actually be doing tempo in zone 3. Worse, a sweet spot session becomes a threshold affair. I do not use a PM this way, so, it would not affect me.
i like to understand my overall effort for a ride, which is pretty much impossible to gauge by any other method. wind varies so much that even on the same route speed isn’t a great indicator. plus different routes, traffic, and the fact that my HR is +/- 10% depending on time of day for the same effort.

when i look at the power curve, i’m really just trying to understand if, for example, what felt like a very hard 5 minute push during a generally easy three hour ride was in fact max-output for 5 min, or did it just feel that way because the 2 hours before were at my sustained max, etc. or, was a 30 minute ride after work that felt super strong actually strong or just because of a big tailwind 😂😂

having the historical peak of the curve based on rides that might be 10% high (or vice versa) definitely messes it up, since my range for anything last a few minutes is really narrow.

i *think* i’m getting faster, since i get PRs on segments i’ve done dozens of times pretty often, but that could also be lucky wind…
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Old 05-28-22, 09:46 AM
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Originally Posted by mschwett View Post
i like to understand my overall effort for a ride, which is pretty much impossible to gauge by any other method. wind varies so much that even on the same route speed isn’t a great indicator. plus different routes, traffic, and the fact that my HR is +/- 10% depending on time of day for the same effort.

when i look at the power curve, i’m really just trying to understand if, for example, what felt like a very hard 5 minute push during a generally easy three hour ride was in fact max-output for 5 min, or did it just feel that way because the 2 hours before were at my sustained max, etc. or, was a 30 minute ride after work that felt super strong actually strong or just because of a big tailwind 😂😂

having the historical peak of the curve based on rides that might be 10% high (or vice versa) definitely messes it up, since my range for anything last a few minutes is really narrow.

i *think* i’m getting faster, since i get PRs on segments i’ve done dozens of times pretty often, but that could also be lucky wind…
Everyone measures improvement differently. I have two basic metrics.

1. Power average and time on a standard 4-5 minute hill. This is sort of a proxy for VO2 max power. I go as hard as I can but only once every 10-14 days.

2. Power at 120 beats per minute, this is approximately my first ventilatory VT1 point. For me, this is the most important metric.

10% off in power has a big effect on TSS and TSB because it is based on IF squared. 0.8 vs 0.7IF is about 15 TSS points per hour. That is huge.
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Old 05-28-22, 10:06 AM
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Also, if the bikes give you two different riding positions, then you might just not be getting the best transfer of power out of one bike.
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Old 05-28-22, 10:23 AM
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Originally Posted by GhostRider62 View Post
Everyone measures improvement differently. I have two basic metrics.

1. Power average and time on a standard 4-5 minute hill. This is sort of a proxy for VO2 max power. I go as hard as I can but only once every 10-14 days.

2. Power at 120 beats per minute, this is approximately my first ventilatory VT1 point. For me, this is the most important metric.

10% off in power has a big effect on TSS and TSB because it is based on IF squared. 0.8 vs 0.7IF is about 15 TSS points per hour. That is huge.
yes, huge. for me 10% is the difference between a fairly casual workout ride and a nearly-best-ever effort. 200w average vs 220w average over two hours is a huge difference.

maybe i should get power pedals for the creo, they’d probably be a closer match. but a pain.
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Old 05-28-22, 10:49 AM
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Originally Posted by mschwett View Post
any software suggestions? i might just download, scale, re-upload the couple crazy high ones and then going forward scale all the ones from the +10 bike before upload. pain in the butt since normally they’re just auto uploaded, but i don’t ride it all that often.
What file formats can you work with?
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Old 05-28-22, 05:44 PM
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Originally Posted by Seattle Forrest View Post
What file formats can you work with?
one app exports TCX; the other GPX and FIT. In the few cases where I’ve had data problems before (split rides, mostly) I’ve used the tools on gotoes dot org to fix them. Looked around there a bit but don’t see anything like a “power scaler,” although i could export to CSV, edit the data in excel/sheets, and then go back. But that would be a serious PITA.
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Old 05-28-22, 05:46 PM
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Originally Posted by Iride01 View Post
Also, if the bikes give you two different riding positions, then you might just not be getting the best transfer of power out of one bike.
possible. the geometry is pretty close between the two bikes, and generally i get much faster segments on the newer, lighter bike with the lower power readings. I’m pretty sure the power meter in the creo is high or the 4iii unit is low (seems less likely.)
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Old 05-28-22, 09:49 PM
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here's an illustration of the issue, although with a little closer inspection a slightly different picture emerges. the chart represents 22 segment attempts of the first part of a nearby hill. although there can be occasional gusts, it's quite sheltered from the wind by a high ridge on the windward side. the grade is relatively steady and averages 10.6%, per google earth. strava thinks it's a slightly different length and quite a bit steeper, but i believe the google earth data to be more reliable. the discrepancy observed with the power meters would actually be far more if strava was right. my speeds are low enough (6mph ish!) here that rolling resistance and aerodynamics should not be a large factor at all. i always climb this in my lowest gear, without stopping, rarely out of the saddle. there's no traffic and the surface is pristine, should be a pretty good test case assuming the map/elevation data is in the ballpark.

my weight per my digital scale has been within a few pounds this whole time - 185 - and the 190 is including shoes, shorts, shirt, keys, helmet. the bike weights are within a pound or so. the maximum variation here might be a couple pounds either way, perhaps 1.5 to 2 percent.

the yellow rows are with the turbo creo power meter, which is generally believed to be high and not totally consistent with more traditional strain based meters.

the reddish row is the first ride on a new bike, half the weight, with a 4iiii precision pro dual meter that i had not yet zero'd. when riding, it was immediately obvious that the numbers were quite a bit higher than the creo, which is not what i would have expected.

the blue rows are the same bike and meter, but zero'd. unfortunately i also mistakenly split the left and right halves (believing "pair" meant pairing the meter to my phone haha) so these are left side only. my left/right balance averages 47/53, so you'd expect these numbers to be lower. but not that much lower. note the power drops from 282 to 245 for a ride with a higher average heart rate that was 5 seconds faster.

the green rows are since i got my head unstuck, repaired the left and right, and zero'd the meter before most every ride.

so what's the punchline? if you plug all the same data into bikecalc (or any of the other online physics based calculators) the only result which matches the observed time to complete the segment is the uncalibrated 4iiii, which is pretty much dead on! the current "correct" configuration deviates from the physics by almost 15% on average, as does the creo. the left only 4iiii is closer to 20%, but that's not surprising if it's off by 15% when considering both legs.




for reference, the segment:



setting aside all the power and junk, for the same heart rate at the same time of day, the lighter bike is around 10 seconds faster (7%) up the hill for the same HR, which happens to be exactly the proportional difference in total weight!
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Old 05-28-22, 10:21 PM
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What operating system do you use? If there's a way to batch update a bunch of tcx files?
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Old 05-28-22, 11:09 PM
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Originally Posted by Seattle Forrest View Post
What operating system do you use? If there's a way to batch update a bunch of tcx files?
I use windows but have access to others. there aren’t so many that it would need to be automated, if there was an app that easily scaled the power values.
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Old 05-29-22, 04:11 PM
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Originally Posted by mschwett View Post
I use windows but have access to others. there aren’t so many that it would need to be automated, if there was an app that easily scaled the power values.
I know there are some cumbersome ways to go about it, like using Golden Cheetah or Excel. I spent five minutes on Google and didn't find anything definite. It's a very easy technical problem, so I spent another 15 minutes writing a TCX file power scaler. I can either post the source code for you if you're able to build it (C#) or I can email you an exe file.

It will scale the power field for every TCX file in a folder by some % that you choose.
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Old 05-29-22, 07:56 PM
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Originally Posted by Seattle Forrest View Post
I know there are some cumbersome ways to go about it, like using Golden Cheetah or Excel. I spent five minutes on Google and didn't find anything definite. It's a very easy technical problem, so I spent another 15 minutes writing a TCX file power scaler. I can either post the source code for you if you're able to build it (C#) or I can email you an exe file.

It will scale the power field for every TCX file in a folder by some % that you choose.
dude! that's awesome. and super helpful. i haven't used compiled languages in probably 20 years, so i made a quick script that does it after seeing how simple the format is. but your .exe would be much appreciated. PM'd you my email.
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Old 05-31-22, 08:49 AM
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Dual sided shimano power meters are known to be inaccurate. The shape of the drive side arm is too complex for power meters, even the one that shimano sells. Every engineer thinks they can design a power meter, and so I think the entire industry is failing to try hard enough. But this is a known issue
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Old 05-31-22, 09:24 AM
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Originally Posted by unterhausen View Post
Dual sided shimano power meters are known to be inaccurate. The shape of the drive side arm is too complex for power meters, even the one that shimano sells. Every engineer thinks they can design a power meter, and so I think the entire industry is failing to try hard enough. But this is a known issue
https://youtu.be/M0tWTUwpt1k
my understanding is that shimano “fixed” this by removing the asymmetry from the design in the latest generation.

it doesn’t appear any more complex in shape than any other crank arm now?

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Old 05-31-22, 03:31 PM
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In case anybody else has this problem.

https://github.com/CascadePass/TCXPo...eases/tag/v1.0
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