Adjusting for variability in power meters

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roadman
Posts: 227
Joined: Tue Nov 03, 2009 7:56 pm
Location: London

by roadman

I use a number of power meters, and most of them track quite closely (Powertaps, Neo and Computrainer) and generally seem similar in terms of perceived effort.

I recently purchased a Quarq Dfour and this consistently reads about 10-20w higher than the others for a similar effort.

Is this because of the crank vs hub based measurement? Do other brands track closer to the Powertap, such as the Power2max?

Thinking of selling the Quarq because of this.

glepore
Posts: 988
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Location: Pa USA

by glepore

Probably its drivetrain loss that you're seeing. You really should quantify it as a percentage though. All your other systems measure post drivetrain. If you're talking in the area of <5% that's probably it.
Cysco Ti custom Campy SR mechanical (6.9);Cannondale SS Evo Di2 7970 (5.79); Willier Cento Uno Air Di2 9070 (7.0); C40 Mk2 DA 7800 ; Anvil Custom steel Etap;1996 Colnago Technos Record

by Weenie


Rubik
Posts: 104
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by Rubik

My powertap hub, c1 chainrings, and SRM are all within 1-2% of each other at threshold so far as I can tell. My P2Max seemed to be as well for the 1-2 months I used it.

10-20 watts seems like too much at a threshold, in my opinion, but I guess that'd still be within 3%.
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glepore
Posts: 988
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Location: Pa USA

by glepore

Another thing, how many rides on the Quarq/ They don't stabilize for until few rides with new rings .
Cysco Ti custom Campy SR mechanical (6.9);Cannondale SS Evo Di2 7970 (5.79); Willier Cento Uno Air Di2 9070 (7.0); C40 Mk2 DA 7800 ; Anvil Custom steel Etap;1996 Colnago Technos Record

joec
Posts: 100
Joined: Wed Mar 05, 2014 9:20 pm

by joec

well you can fudge the quarq to read the same as the others using qalvin, repeatability is the main thing to be concerned about.

goodboyr
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Location: Canada

by goodboyr

Who knows which is right, without doing a slope test with a known weight. Or you could end up correcting the quarq to the wrong value.

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Dagger9903
Posts: 54
Joined: Mon May 21, 2012 7:40 pm

by Dagger9903

I have a similar issue and trying how to incorporate it into my training. My Stages trainer has a stages crank based power meter. I put on my new Assioma Duos and the pedals read ~10-15 watts lower, almost all the time. I THINK i might have something to do with flywheel, but not sure.

I'm just keeping a mental tab on the fact, but keeping track of TSS is a little annoying. Still, I don't think 10 watts makes a huge impact on TSS over 45-90 minutes.

11.4
Posts: 1100
Joined: Tue May 23, 2006 4:33 am

by 11.4

Read up on the SRM website or on YouTube about how to use calibrated-weight iron plates to get an accurate absolute measurement. You'll be able to plot a simple straight line with a y-intercept and calculate your precise slope, which lets your powermeter output the right power for a given load. If you're using multiple devices and worried about differences between them, you should be doing this on a regular basis anyway.

glepore
Posts: 988
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Location: Pa USA

by glepore

You can adjust the slope on the quarq which will make it accurate in the absolute sense, but it doesn't seem that the issue posed has to do with that.

He wants his powermeters to be consistent, and doesn't seem to care about accuracy. We could start a whole thread on why that's not the question to ask. I agree that knowing that the quarq is ok is a first step, but their slope out of the box from the factory is supposed to be pretty good. Knowing that the quarq is good though won't tell you a thing about whether another crank based meter will be equally good or "closer" to his other devices.
He states that his pt, neo and computrainer are consistent while his quarq is a bit high. If he's in the range of 5% or so, its likely not much more than normal variability and drivetrain loss. If he's training, obviously he can adjust his target power - if he did his test using the pt he simply needs to set his target x% higher on the quarq when he uses that.

The elephant in the room is that its pretty well accepted that while metabolic ftp doesn't change, indoor trainers exert a greater training load at a given target power. Who knows why, really, but most folks that have been using them for a while accept this. So there's a lot of target adjustment going on.

Yes, if you track TSS or use a training load tracker adjusting all this can be a giant pain, which is why coaches prefer that you use one power meter and stick to it. I've been using pm's since square taper wired srms pretty much it, and its only this month that I've had multiple outdoor systems at the same time. Yeah, its a bit of a pain.
Cysco Ti custom Campy SR mechanical (6.9);Cannondale SS Evo Di2 7970 (5.79); Willier Cento Uno Air Di2 9070 (7.0); C40 Mk2 DA 7800 ; Anvil Custom steel Etap;1996 Colnago Technos Record

by Weenie


roadman
Posts: 227
Joined: Tue Nov 03, 2009 7:56 pm
Location: London

by roadman

Thanks for the replies.

Yes, I assume it is partly drivetrain loss and might need to run in the Quarq a bit more with the new chainrings.

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