RChung

It's from this study on the accuracy of power meters. Each dot is a separate device, so there are a total of 54 different devices from 9 manufacturers (and owned or used by 32 different individuals). The analysis wasn't just a comparison like Ray or GPLama do, the test was actually on a known and calibrated "bike treadmill," so this wasn't just a "relative to each other" test, it was an absolute accuracy test.

BTW, on the "consistency with itself is all that matters:" several years ago I discovered that the cheap plastic measuring cups I had been using were off. Two of the half cup measures didn't equal the one cup measure, etc. Obviously, each of the cups was consistent with itself, what was off was the relationship between each cup. You may know that baking is more demanding of accurate measurements than regular cooking. Even though I didn't know the measuring cups were off, over the years I had noticed that my baking was inconsistent *even though the cups were consistent unto themselves*. When I replaced them with new, accurate, measuring cups my baking improved. I'm not a professional baker, I just like eating. OTOH, if I never baked I may never have known. The need for accuracy and precision depends on what you're doing with the data.

Barry2 

This subject keeps coming up.
This is the difference between "Accuracy" and "Precision"... Terms that we incorrectly tend to use interchangeably.

You place a 1 pound weight on the same scale 3 times and get the following readings...
2.001 pounds
2.002 pounds
1.999 pounds

That scale IS a precision instrument, but it's sure NOT accurate.
With most of us comparing PM readings to our own prior performance, precision is required, accuracy would be nice.

My coach accused my Wahoo Kickr Bike PM of being wildly inaccurate when compared with my Road bike dual Stages PM.
I installed Garmin Vector pedals on the Road bike and compared. The Vectors read 3% high across a range of powers.
I then installed the same Garmin pedals on the Wahoo Kickr bike and compared. Again the Vectors read 3% high across the range of powers.


I was glad both my PM's read the same. But even with a 3% difference, I'd consider all 3 PMs accurate enough for my usage.

Barry

RChung

Yup. The need for accuracy and precision depends on usage. Sometimes needs change over time, so usage changes; sometimes they don't.

That said, post #14 above shows what a 4% average difference between two power meters looked like.

[Edited to add:] There are 28 posts in this thread and a quarter of them are by me. Damn you asgelle.

asgelle

And here I thought I was saving you from all that by referring people to what had already been posted.

PeteHski

Time for a new coach?

Barry2 

Nope, coach is a keeper.
I told him I don’t use an indoor trainer, “I have an indoor bike”, and I think he assumed the worse.
Even the Peloton does not have an actual PM. Those estimate power from resistance and cadence.
I was surprised to find out that even with Pelotons calibration kit, expected PM accuracy is about 15%.

Barry

PeteHski

Why didn't you just tell him you had a Kickr Bike? Well known to be accurate and consistent. According to DC Rainmaker's detailed analysis, the Peloton Bike+ also has very accurate power measurement, but not the standard Peloton.

OshkoshBiker

As someone who studies the data from power meters vs. just using the sensors in your smartphone to calculate power, I agree that most users are putting way to much importance on a power meters claim they are super accurate. There are so many variables in the real world that mess with your power meter data. Anyone in a controlled environment can claim +- 1%, but my research tells me that in real riding conditions +- 10% is pretty good. There is a lot of noise in the data based on road condition, wind variances and so on.

Kind regards,
Chuck
Cykelstrom.com

asgelle

How do road conditions or changes in wind affect power meters? The ones I use comprise strain gauges and a reed switch. I don't see how road conditions or wind would compromise their accuracy.

OshkoshBiker

There is a certain amount of noise due to impulses that the strain gauges pick up from just going over a rough road or bump which are not a result of power dissipation. Depending on how the signal is filtered or averaged to eliminate the noise affects the power measurement. Also, if you pedal hard and fast vs. say a smooth pedaling motion, the rider can induce a different kind of noise in the signal. Wind can affect power measurement in high gusts and cross winds which catch the cyclist and bicycle creating an impulse which can also be picked up by the strain gauges through the movement of the cyclists mass on the crank. There are a myriad of things that happen while riding and my experience is that the sensors are super sensitive, in some cases too sensitive and the accuracy can depend on the signal processing to eliminate those noise sources which impact accuracy.

Kind regards,
Chuck
Cykelstrom.com

RChung

That's interesting. Have you taken the data from different power meters and back-calculated what the gusts, cross winds, pedaling type, and road roughness must have been?

OshkoshBiker

I have not done such a study as my resources are limited. My observations are based on looking at opposing force power meter estimates, smartphone sensors, and a crank based power meter as the standard.

Kind regards,
Chuck
Cykelstrom.com

Jrasero

I just got some Assioma Duo-Shi power meter pedals that work with Shimano SPD-L pedals and I personally would highly recommend.

I am 5'5" and pretty darn narrow so I was pretty nervous once I ordered these since I kept hearing about the massive Q-Factor, plus the bike these went on has a fairly large Q-factor from the big bottom bracket. I am decent at wrenching but a lot of stuff I take to the LBS but assembling these on my Shimano 105 SPD-L pedals was quick and painless. First ride I did notice the increase Q-Factor but to my surprise I was totally comfortable but maybe this shouldn't have come as a surprise since I grew up riding hardtails. The only issue is reclipping in was sometimes cumbersome due to the extra Q-Factor, but again no discomfort at all.

I am new to power but the accuracy seems right on par with my single sided SRAM Force D2 Quark Left Crank power meter on my other bike.

At $500 I think you are getting a pretty great value even if you are not including the pedal body cost since I would assume most people already have SPD-L pedals to begin with, this is probably one of the cheapest and easiest way to get into power with also having the most versatility. I can put these on any bike in a matter of minutes.

Jrasero

For anyone wondering, about dual vs single sided power meter, the difference in terms of accuracy is minor. Any power meter is better than no power meter since you have some sort of baseline to measure power regardless of accuracy. Now after you know your average output, then yeah that's when accuracy starts to matter but like anything in cycling it's marginal gains especially when your factor in money.

IMO Favero Assioma DUO-SHI are one of the best value dual sided power meters out there ($520) if you have Shimano Ultegra SPD-SL pedals or lower and you are okay with the added Q-Factor