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Which powermeter is accurate?

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Which powermeter is accurate?

Old 07-19-21, 09:06 AM
  #1  
oik01
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Which powermeter is accurate?

I bought a used garmin vector S a few months ago. I enjoyed having power and the numbers seemed believable out on the road but when compared to my Saris CycleOps Trainer always seemed to consistently read 20 W below the saris powermeter no matter how often I calibrated either device ( remarkable always displaced by that amount when using the dc rainmaker tool as if the graph is shifted down). Ultimately I was curious enough and bought a pair of Vector ( dual side power meter). My original thought was that my left right balance must be off. With the new powermeter, I have noticed that it actually matches the cycleops trainer pretty well ( occasionally lower, occasionally higher by a couple points but over a long ride get an average 2 watts higher as is expected from drivetrain losses and general inaccuracies). The left-right power are very much balanced making me think it could just be calibration? All of these things are pointing towards this new powermeter being accurate ( Each pedal side matches the other and they match the trainer). However when I compare strava segment times to others, my power more closely matches that of others when I use the lower reads from the vector S (the one that doesn't match my trainer or the new unit). Keep in mind Im probably of average weight for cyclists ( 165 pounds 182 cm tall). On my last ride with the new vector I got an average of 186 watts for 16.6 miles per hour average speed. On the vector S and a similar route I averaged 173W and 18.2 miles per hour. I want to decide on which powermeter to keep. Id rather know which one is accurate and keep that one. I feel like the consistency between indoor trainer and outdoor from the new unit is good to allow me to better pace myself but the consistency compared to other riders with the vector S is also nice because I can predict what I need to keep up on group rides or improve my ranking on segments etc. Any thoughts?

New dual sided powermeter ride:
https://www.strava.com/activities/5649460499

Single sided ride:
https://www.strava.com/activities/5303631941
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Old 07-19-21, 09:43 AM
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I borrowed a pair of Vector 3's when my coach questioned my indoor vs outdoor performance.
I use different PM's indoor/outdoor, so a discrepancy was possible.

I installed the V3's on my Wahoo Kickr bike (has it's own PM) and compared.
I installed the V3's on my road bike (Has Stages Gen3 dual) and compared

Then I compared the results of the two tests.
The V3's constantly read 3% different from the Stages & Wahoo PM's at a variety of power levels.

With all 3 PM's within 3% and the two I actually own even closer, I figured that was a Win !
Assuming you Calibrated your new dual PM's after 15min's of riding to allow the temp to stabilize, I'd trust the new dual PM's.

All the best

Barry
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Old 07-19-21, 10:11 AM
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PeteHski
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I wouldn’t put much weight on comparing Strava power with other riders. There are way too many variables. The most important thing is consistency and if your pedals and trainer are consistently reporting the same power then that is a pretty good result. It doesn’t really matter if they both report slightly high or low watts in absolute terms. As long as they remain consistent you will be able to track your progress. Having 2 or more power sources is good for monitoring consistency as it becomes more obvious if something is going wrong.

For example I regularly compare my crank power meter to the optical power meter on my Elite trainer and they are usually within 10W on a 200W average ride and the crank power reads higher as you would expect with drivetrain losses (approx 4%).
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Old 07-19-21, 11:08 AM
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All of the modern PM's from major manufacturers are accurate to about 2-4%. The best way to check your PM for "accuracy" is to put your pedals on a smart trainer and have the trainer broadcast in BLE and your pedals broadcast in Ant+ then ride an entire workout with long steady efforts, hard punchy efforts, and up and down sprint efforts and then you'll have 2 power files. Put those files into this website:

https://analyze.dcrainmaker.com/

And you can compare the two and see what's what.

Ignore power estimated from Strava...it's nothing more than a random number generator.
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Old 07-19-21, 04:49 PM
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Accuracy is good, but what's really important from a training perspective is repeatability.

At the end of the way it doesn't matter if it reads high or low, as long as it's consistent you can use that number for training, pacing and gauging improvement.

That said, I'd keep the dual sided powermeter.
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Old 07-21-21, 06:39 AM
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RChung
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Virtual elevation was originally developed as a way to check power meter accuracy; for this purpose, consistency is insufficient, you need accuracy too. If you race in Zwift, consistency is also insufficient, as well as if you're trying to use power to estimate calories expended for nutritional purposes. Finally, although people will often claim that for training accuracy doesn't matter, only consistency does, I've rarely seen a power meter that's widely inaccurate but narrowly consistent. Almost always, if it's inaccurate its consistency is at question.
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Old 07-21-21, 07:04 AM
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I have three Powertap G3 hub meters. In terms of accuracy, precision, and repeatability, they seem good. Allegedly, SRM is top dog but they are kind of expensive

If you hang a known weight from the crank set horizontally, you can check the accuracy. If you use several different weights, you can check linearity. When I repeat the checks, the results are the same. Very crude methods. The meters vary a little from one to the next. One is 2% high and another is 1% low and the third is dead on. The "calibration" feature is just a nulling of the zero.

I was getting dropouts on Zwift team time trials. Drove me crazy. I disassembled and put new bearings in. New batteries a million times. I could not swap powercaps because the others were older FW. I bit the bullet and bought a new Powercap and after many months of trying to fix it, problem solved. I guess this would have been a consistency issue. It was a pain to solve.
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Old 07-21-21, 07:05 AM
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Originally Posted by RChung
Virtual elevation was originally developed as a way to check power meter accuracy; for this purpose, consistency is insufficient, you need accuracy too. If you race in Zwift, consistency is also insufficient, as well as if you're trying to use power to estimate calories expended for nutritional purposes. Finally, although people will often claim that for training accuracy doesn't matter, only consistency does, I've rarely seen a power meter that's widely inaccurate but narrowly consistent. Almost always, if it's inaccurate its consistency is at question.
Good points. Having several different power meters gives me some confidence in both their accuracy and repeatability, which actually appears to be very good - better than I expected. Both my current power meters are rated at +/- 1.5% and appear to be well within that range - at least as an average power over a ride. Putting my power numbers into the likes of BBS gives accurate time estimates too without much fudging of aero etc.

Zwift racing is fine if you don't consider you or your competitors to be "real". In other words if I'm competing against a rider with a very optimistic power meter it's no different to simply competing against a more powerful real rider. They could all be bots for all I care. But obviously that would be very different if it was a serious Zwift competition with prize money etc. Regardless I still find Zwift racing provides the necessary motivation associated with competitive riding. I will push that little bit harder when chasing someone down or sprinting for the line.
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Old 07-21-21, 07:07 AM
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The power meter that reads lower is always the one that is right.
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Old 07-21-21, 07:18 AM
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The powermeter on my fluid trainer was accurate at 150 watts but was almost 100 watts high at 350. I use that one when disgusted with my fitness.
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