I'm guessing you've never brought a commercial measurement device to market. The theory is simple. Making a reliable, maintainable, accurate, weather-proof, lower power and low-cost electronic measurement device is absolutely brutal. There's a reason why some of these things are crappy, several have tried and failed, and Metron/Vector took the better part of a decade (and still isn't that great).

I'm not talking about what it takes to develop the PMs, I'm talking about what it takes to have reasonable confidence in the output. THAT ain't rocket surgery...

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I just think hanging weights off the crank arm is kind of crude. :)

And yet, just upthread you seem to hold DCRainmaker up as some sort of expert on accuracy/precision, when in fact 1) he's had to reach out to people like Tom A and Robert Chung for help with doing a good job of it, and 2) he's never tested an SRM. Sorry, but that makes me think that, at least in actual practice, your standards aren't nearly as high as you pretend them to be.

It does matter.

What if the other sensors on your bike were consistent but not accurate per your example.

• Heart rate reading 1.5x too high?
• Speed is 10kph low/high?
• Distance traveled on rides is 15% low?
• Elapsed time for rides or intervals is 20% fast?

I don't think anyone would make the argument that consistent is good enough for the above issues. Why should it be OK for a powermeter?

Except since it's not accurate you don't know if 10 watts more power is actually what you think it is.

10 watts may literally mean 10 watts and if you're actually pushing 200 watts it was a 5 percent gain instead of a 10 percent.

If you have multiple bikes having them all wrong is going to give you horrible data to train off. A lot of us have road bikes and tri bikes. Without them being accurate it's impossible to know if your tri position is sapping watts compared to your road position. I ride both bikes during the week so it's very important to know they are accurate.

...and yet, effective enough for the task at hand.

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You’re assuming that the inaccuracy is linear. What if when you’re pushing 200 watts your pm reads 100, and when you’re pushing 220 watts your pm reads 105, and when you’re pushing 240 it reads 327, 250 reads 318, and so on. As long as it does this consistently you’re fine right?

Unless you win every race you are in - you need to improve. Its really that simple.

Linear is consistent. There is no consistency error in your example.

200 to 100 is 50% off (or 100% depending on the direction you are taking)

220 to 105 is not 50%.

Therefore not consistent.

If the value displayed for any given actual power output, X, is ALWAYS the same, Y, I'd say that's consistent.

By your apparent definition the displayed value could be based on a 3rd degree polynomial and would be OK. The relationship between actual power and power displayed would be 'consistent'.

That's what he said...

Well there's worrying about other people, which is a distraction. Or learning from them - in which case they're a resource.

On race day, if some dude goes flying by - absolutely forget about them and focus on what you can control.

But, later, if you figure out that guy flew by you doing the same W as you, it's an opportunity to learn and improve. I learned a lot from this forum when reading about guys who said they could break the 1-hour 40K on less than 250W, at a time when it took me 300W. At the time I thought I was all smart and aero'd-up. But I wasn't. So I learned, and got a lot better at all the little things. Now I can go 24MPH on 200W, like echappist, jeffp or other near-best-in-class at W/CdA. Don't hate the player....

thanks for the kind words, but i shouldn't be in the same sentence as JeffP, who is probably in the 0.18x range

also as an aside for people who think they should be going faster at the watts they are generating, check your drive train. For the longest time, i was woefully inefficient on my current bike (CdA around 0.235-0.24), and i never bothered to check the one place people just take for granted: the rear derailleur pulleys. I bought my RD used and never bothered checking how smooth the pulleys were. It wasn't until after i swapped chains (two years after i bought my bike) that i realized that the pulleys were completely shot. While i don't have good before and after comparisons that can give me a good estimate of how much power was wasted, i seemed to have been able to go from mid 25 to mid 26 at 300W around a local loop (without using other aero aids). Moral of the story is: check your chains, pulleys, and chainrings as

I can map my power data to the length of the group behind me pretty accurately ;-) Desert Dude will testify.

amusing.....

yeah, drive train, I had to take to installing my own BBs this year as the shops kept giving me things that would not spin one revolution with chain removed. Last item I had shop working on I now do myself. Ok I use them to remove shredded shift cables in Shimano shifters. (those things eat cables)

No 0.18x's here, not yet............

I was fortunate to stumble onto a decent position before I started refining and I strive to help others when I can

1) As a general rule in life, when there are smart people that are experts on certain aspects of a topic, I like to chat with those smart people. I find that doing so usually yields interesting things. Both Tom and Robert have reached out and discussed ideas in the past. They have ranged from producing interesting graphs for certain power meters, to adding ideas for testing in general. I don't always agree with everything they state, and that's OK. I'm not sure I "had to reach out" for anything, I just think it's better to chat with smart people than live in a small bubble of my own thoughts.

2) I own an SRM, and have done tests with it. Not sure your point there.

As an aside - where's your published side by side accuracy data on all the power meters in the market today? I may have missed it.

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My tiny little slice of the internets: dcrainmaker.com

Wow, I really wish that was the case for me: a PM reading low. Why wouldn't the converse also be true that the slower guys have PM's that are reading too high?

I'm in the sub .200 camp and have rolled a sub hour 40K at 201 watts (normally I'm around 56 flat @230ish). I have also been to the tunnel and come away with a .195 and .185 at 0º and 10º respectively. When looking at these numbers, my times, and power output they seem to correlate pretty well. I'm assuming that most other "fast" guys have a similar correlation.

But to answer your question: Yes. Those with fast times may have a PM that is reading low. However, those with slow speeds and high watts might have a PM that reads high. More likely, the fast guys have things dialed in and the slow guys don't. That being said there is only so much one can do.

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Have you ever posted said tests? Honest question here cause I don't think I have ever seen them.

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I posted one test a while back on an initial 4iiii test ride I did a few years ago when they first announced their unit. In that case it was interesting because very few people noticed/caught the drift the SRM actually had on that ride. It was sorta prime 'weather' for SRM drift - an early morning ride in a location where the temp shifts quickly at sunrise during the ride.

I'll probably start including more in upcoming reviews, simply because I've expanded my bike testing fleet a bit more.

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My tiny little slice of the internets: dcrainmaker.com

No, man, it's a trade craft. There are always little avenues for improvement, always things to test. That's half the fun. I don't think many of the uber-aero guys just up and go "OK, I'm done!" I think we're the guys wanting to try the new Supersonic with the textured sidewall because maybe that texture creates a better boundary layer around the tire/wheel interface.

please test that on an H3 and report back :) I still have too many regular SS tires to use up before I can switch(if there was a reason to switch) don't want to switch prematurely and become slower in the process :)

Yeah, that's my problem. I have 4 new-in-box and 4 more on two wheelsets. That's good for at least two years unless I get super-unlucky.

Agreed, but with one caveat: I think chasing that speed is where all the fun lies. Also, what I meant is that a guy 6'5" 220lbs is not likely to achieve the same CdA as someone who is 5'6" 140lbs. And they might not need to, as their power differential might be great enough to overcome the little guy's aero advantage. Lastly, if you count me in the uber-aero group then I think I am one that is pretty close to saying "ok, I'm done".

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I am pretty much done now too, for a while not looking for anything new for a bit