How do I know my new power meter is accurate?

Equipment
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I have finally got a powermeter on my big bike (SB150). My LBS found me a near new XX Quarq off a pro-riders bike and arranged a swap for me with the guy who now owns the bike. They then got it working like a dream with my Shimano Cassette. I am not sure how this magic works, but it does, and it’s running seamlessly. I don’t think I have ever had such smooth shifting!

Anyway, I did a ride today and, for the first time in 4 weeks, actually felt really great physically. I’ve been fighting a virus so it was nice to just go out and feel fab and want to ride and ride

When I got back I had a look at the data and, for the HR, the power didn’t look that great. My Garmin condition score was shocking. Now I appreciate HR doesn’t really mean much, and the Garmin condition stuff is rubbish at the best of times (it was pretty humid today, etc) but, it got me thinking…

How do I know that each of my power meters are in the right ballpark? With my XC bike and gravel bike, the power outputs do seem to be similar for the same RPE/HR.

Obviously today may just be a blip on the big bike. I felt amazing, but I have been tapering/recovering after an aborted A race so fitness probably not what it was 6 weeks ago, and that’s fine. But equally, my easy rides on the gravel bike have been good and power has been where I would expect it.

What is the best way to tell if a power meter is reading accurately? Is there a particularly good process for it? I was thinking maybe just doing a 20 minute flat out effort on it and see if it puts me somewhere near to where I would expect to be for that effort but, obviously, that’s dependent on a lot of factors. Maybe 3 of those over a number of weeks…?

Or is it better just to try and hold a steady effort for, say an hour?

Or just not worry about it - it’s not like I will be riding the big bike that often, maybe once a week, so even if it is reading low, then it won’t have much effect on AiFTP?

I am so confused!

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There is no good easy way. You can put your bike on a trainer and then compare the two outputs. You need to dual record to come up with two ride files and then you need software to compare them. I went down this rabbit hole a few years ago and use Golden Cheetah. DC Rainmaker sells or used to sell software for this purpose.

I found this free site a number of years ago but never tried it:

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Here is a topic I started a number of years ago.

It’s a PITA.

My ultimate solution was to use my Stages for everything. I use power from the Stages indoors and out. I’ve configured everything to ignore the power reported by the trainer. I even traded in that old Tacx on a used Kickr and found that it reads low by 20 watts across the board.

My conclusion was that you really can’t trust any of them to be “accurate” so just pick one and train to that power meter. Creating zones and training to one power meter works perfectly even if it’s off by 5, 10, or 20 watts because it’s all relative.

If you need to use two power meters then you have to do the comparison dance and maybe calibrate them to each other if even possible. And if they don’t agree you may need to buy a different power meter. I believe there is a pedal power meter that can be adjusted to match another power meter.

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I beg to differ.

And Chung virtual revelation is another way to evaluate whether your equipment is reading reasonably.

http://anonymous.coward.free.fr/wattage/cda/indirect-cda.pdf

A static weight test is another way to verify that your power meter is working.

I would not recommend comparing two power meters unless you have performed any of the above and set a baseline with your equipment.

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Nothing about that sounds easy. Not every power meter supports a static weight test nor a user calibration based on that.

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Do you want to answer a question (Is my power meter working correctly), or do you only want easy? I get what you’re saying, but in my humble opinion, you shouldn’t avoid doing something in a more rigorous way because it’s not easy.

All I’m saying is that if you want to know your measuring device is measuring reasonably , you need to check the measurement against something known.

I went through this in 2018 when I was getting into cycling. I hill tested my P2max, then I was comparing crank pm, spider pm, hub pm, pedal pm, and Computrainer all at once. Now I don’t even bother wondering about my power numbers much.

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Thanks, I didn’t know about the hill test. I probably should have wrote there is no easy way. :slight_smile:

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Ok, thanks all. Nothing simple about it then. :grimacing:

I can’t run the same power meter on all my bikes, that is just impossible! For a start, Stages have gone bust, and even if they hadn’t, I can’t put one on my SB150 cranks. Or the GRX gravel cranks, or, for that matter, the old race face cranks I have on the roadie on the kicker. In fact the only bike that has a stages powermeter on is the XC bike. If that would have worked, it would have saved me a vast quantity of money.

The hill test option is interesting, I will look into that a little more. Thank you!

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One way to get a sense of how they relate to each other is to put each bike on a trainer like I did and see how they compare. You can even use simple lap intervals, pedal for 5 or 10 minutes at a constant wattage and then compare the average power for the intervals.

When I did my tests I had two head units so I was able to record a Stages and a Powertap at the same time. I recorded the trainer on the Tacx app at the time.

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I know both my power meters (a 4iiii and a Favero Be Pro S) read slightly above my smart trainers (so about the same); I’ve never done anymore than that though.

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Thanks, I’ll do that next weekend with all 3 of them. I can record the kickr on the kickr app on my phone and the others on my Garmin

So if I run the intro workout (4 mins) do you think thats enough time to get a comparison?

Screenshot_20250607_223049_TrainerRoad
Screenshot_20250607_223049_TrainerRoad1080×365 27 KB

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Probably fine to just get an idea. When I tested against my Tacx trainer, the biggest difference was with sprints or high wattage intervals. The higher the power, the more the Tacx trainer would be off so ideally test a range of wattages.

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Ok cool. Maybe I’ll create something in workout creator

Ive just thought of another potential issue with this though. The kickr sets the power on the back axle, which means that the efficency of the drivechain will impact the test. I have a new chain on the big bike, but the chains (and cassettes) on the gravel bike and XC bike are well worn (knackered, tbh) and won’t sit well on the kickr cassette.. I’ll need to use the same chain on all of them. I think it should be ok if I use the new chain from the SB150, as long as it sits on the chain rings of the other two bikes :thinking:

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Generally speaking, for power meter tests, unless I’ve established ‘trust’ with that power meter, I’d throw away the first 15 minutes of data. Either way, during that trust establishment phase, after 15 minutes, do a zero offset (aka calibration) on the units (power meter, and do a spindown on the trainer if you’re not sure).

Here’s how I test power meters: How to: Troubleshooting Power Meter and Trainer Accuracy Issues | DC Rainmaker

@gpl has something relatively similar here: https://www.youtube.com/watch?v=JhheDLXRM7I

In general, the main things we do, when you boil it all down are:

A) Steady-state ERG tests are easiest to identify obviously accuracy offset issues. So aim for something like a 45-minute session ideally, but you don’t need it to be crazy. Just have a few intervals there that make it really clear (visually) the power is changing. This is more about time and temp validation than anything. So you could literally do 125w, for almost all of it then do a few 250w intervals (controlled by ERG), and that’ll give you a good picture. Do note that the lower the power, the harder it is to sniff out accuracy issues. Meaning trying to do comparisons at 75w can be really difficult.

B) We both do SIM-mode (e.g. regular Zwift) testing, which would include some hard sprints.

If you’ve only got 20 minutes, then just easy pedal (intensity doesn’t matter, you’re literally just trying to ensure room/materials temp has stabilized on all the players), for 15 minutes, then do your 4 minute test above. That’ll give you a good gut check.

Essentially though, in a theoretically perfect scenario, power meter pedals should read the highest, followed by crank arm, then crank spider, then a bit of a gap to trainer/bike (e.g. 2-5w is normal, depending on how clean your drivetrain is). So if a crankset is reading 200w, then a trainer might read 195-198w, and that’s totally normal (again, depending on exact drivetrain losses)

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Thanks for this super detailed response @dcrainmaker !! :slight_smile:

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I’m comparing relative to my Kickr:
Setting a wattage in Kickr and then reading the watts from target powermeter.

So you get an idea for the difference.

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I try not to worry about it. I have a Tacx Neo 2 trainer, and two pairs of Favero Assioma pedals. I’ve not noticed that anything is glaringly obviously wrong…

If they DIDN’T match there’s nothing I could do about it other than spend more money, so I choose not to test them against each other, lest I learn something I don’t want to know :rofl:

Of course I have some peace of mind in that both Neos and Assiomas are generally believed to be pretty accurate and reliable. :crossed_fingers:

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@dcrainmaker thank you sooooo very much for this reply. Its super useful.

I’m on holiday at the moment but will be making full use of this advice when I get home and will report back the findings for anyone interested.

@downhiller I absolutely get that point of view and had convinced myself of the same. The other powermeters are seemingly pretty close and I won’t ride the big bike that much so why bother…but then I remembered I’m an absolute geek and need to know :rofl:

At least if I know that the big bike is quite a bit off from the others I can allow for that when I am mentally assessing a ride, and I’ll be able to exclude it from the AiFTP detection.

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It’s been a while since I used them, but doesn’t the Assioma app have a manual scaling you can do to make them “equal” to another source? I feel like they did.

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Ah yes, so you can. Still too lazy to fit that bike (or the pedals) onto the trainer though. But good to know I can if I ever feel there’s an issue

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