Power is proportional to Torque * Cadence.

Cadence doesn't change with the crank length setting; it is measured in real time.

Torque is force (i.e. what the PM measures) * crankarm length.

So, if the PM thinks the crankarm is 172.5/165 = 104.5% of its actual length then it will overestimate the power being produced by 4.5%. 300W measured with the mis-calibration would be 287W.


You can test this empirically by putting your bike on the trainer in erg mode at 300W, and see what the pedals read at the different crankarm length settings. The readings should be higher than 300W when correctly calibrated, since the pedals measure power before the drivetrain losses.

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

Torque = distance (of crank arm) * force

So if your PM thinks it has a 172.5mm lever arm, it's going to estimate a larger amount of torque (and therefore more power) than you really are. By about 4-5% off the top of my head.

From basic engineering I'd expect it's just pro-rata that it's over reading by 172.5 / 165, ie reading 1.045 times higher than real.


So 300w reading becomes 300 / 1.045 = 287 watts.

(I'm making the assumption here the pedals really measure the force + rpm, and use the arm length input to give the distance travelled by the pedal per second [ ( 3.14 x crank length x rpm/ 60) x force] = Power.
And not something funky or fancy)

As Titanflexr said, The PM will overstate your power if you were on 165's and you had the PM set to 172.5's. I made the same mistake in a race a couple years ago. I moved my Garmin Vector pedals from my road bike to my tri bike just before the race but forgot to recalibrate/reset crank length. Come race day I felt like the bike leg was just so easy... it must have been because I was tapered and on fresh legs, right?! Sure, except my bike split sucked. On a positive note, I felt great on the run. Didn't realize what happened until a few days later, at which time I was kicking myself.

If my brain is working correctly tonight - 165/172.5 X 300 = ~287 watts for an indicated 300 watts.

YMMV

Hugh

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Genetics load the gun, lifestyle pulls the trigger.

Seems to be the consensus. I would swear power meters never read high though lol

Thanks all for the sanity check!