This question might have been asked before but anyway I have 2 bikes (both are road bikes) with different power meter attached to them. A Shimano Dura Ace R9100-P and a Quarq DFour in case anyone was wondering.
I believe that a separate FTP is needed for each device ( covered on the podcast a few times) to take into account the difference in accuracy. My question is should I do the tests on the same day ? How far apart should I schedule the tests in order to get the most accurate results.
Thanks in advance, cheers
I wouldn’t do them the same day, because fatigue from the first test will impact the second. The day after should be ok, if you do the ramp test. If you do a 20 min test, depends on your fitness if you can two in a row.
To be honest I’m not convinced just one test for each will be enough, because the variance can be as high as the accuracy. At 250W ftp, a 2% error is 5 W. Lets say your tests come out with a difference of 3 W, what does that say? Does one read higher, were you just a tiny bit more tired, or is it just the normal margin of error?
I’d probably do the tests to see if there’s any dbig differences, but otherwise keep an eye out for perceived differences - if VO2max feels constantely harder on one bike, its likely the power reading is just different.
Do you have a smart trainer that also measures power, or access to one?
If you do, you can do a session with both bikes where you use the smart trainer to control power (without power match), then see how the 2 on-bike power meters compare. Do a few intervals in your different power zones.
(Or possibly, do a session with erg mode turned off where you hold a target power on your on-bike power meter and then record what power the smart turbo says it is.)
You should then be able to see the difference between the 2 power meters - and crucially, see how they differ at different intensities (the issue that @splash points out).
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I have a powertap hub and a power2max PM and wonder that it fells so different. I made some tests where I use both for the same intervals. The difference was “only” 6-8 watts on 300 Watt intervals. So the difference was not so bin an can inside the tolerances for accuracy.
However if one perform V02max intervals is not possible for me, just ride with 2-3% more power. I don’t think one need a own FTP, but is very useful to “calibrate” the PM against each other, and take the difference into account.
Beside the mentioned smart trainer maybe you can borrow a powertap wheel
I do my workouts on a Elite Direto which has a built in power meter, will definitely try the methods suggested as it sounds like and ideal place to start in order to get the measurements right.
Knowing the difference between the two devices is better than doing 2 separates tests.
Thank you
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I would think a ramp test on back to back days would be fine.
Is there any possibility you can test them against a separate meter? I think that would give you much better results. I.e., if you had a smart trainer, or a power hub and ride a steady 20 minutes or something, and check the difference each of them have to the third meter. So if the hub reads 200w for 20 minutes, one of the crank meters reads 200w and the other reads 190, you know you have a 5% difference between the two. I know the percentage difference can vary depending on the power range as well, but i think that would give you a more objective measurement than introducing all the variability of two separate ramp tests.
