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I “share” a pedal-based powermeter between my road bike and my spin bike. I performed an FTP test a few weeks ago on the spin bike but only just realized now that the crank lengths are slightly shorter (they are 170 but I calibrated the powermeter at 172.5 which is my road bike crank length).
FTP was calculated at 237 as a result of the test.
Am I correct in assuming that my FTP is actually slightly LOWER now that I know this error occurred? I’m assuming that it’s lower by a factor of 170/172.5 or 98.5% of the original.
This is a pretty insignificant difference (new FTP would be 233 based on this) but I just want to ensure I understand the difference.
Hi Brian,
Had to reach out to David Warden for guidance on this one.
Our understanding is that your head unit (garmin etc) will need to be adjusted for the crank length of the bike you are using as the power pedals do adjust power based on this information.
Now I am not sure if that will do the trick for the difference in FTP. Might be worth doing some experimentation to see if you notice a difference – with 5 watts only it might not be obvious.Hi Brian
Pedal based power meters use an equation that calculates Power through force applied to the pedal, and the rotational velocity (velocity = distance (d)/time (t)) of the crank. Consequently a critical input to the power equation is the length of the crank – that’s the distance (or d) part of the velocity equation.
Consequently if you did the FTP test with the wrong crank length, its likely that the power numbers generated are off and your FTP is probably inaccurate difficult to say how much – but could be 1-2% 04 could be 10%…. Thus its probably best if you do the test again…… 🙁
Sorry……
Gareth
Thanks for the feedback everyone – I will test again in a few weeks anyway and make sure I change the Crank Length this time!
Incidentally, this site keeps signing me out after just a few days in between posting – I have “keep me signed in” checked but it doesn’t seem to make a difference. Anyone else having this issue?
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