Ok, so hypothetically if I have been using Garmin Vector 3 power pedals for the past year and had the wrong crank length entered, how would this affect the power reading?
Scenario: Garmin calibrated to 172.5mm crank length, when in reality I have 165mm crank arms
For example, if I measured 300w for 20min, would it really be 280 or 320 or have little to no impact?
Fortunately, I do 95% of my riding on the KICKR so all that data is accurate. But this means every race I’ve done in the last year was on an improperly calibrated PM, ugh
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.
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 = 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.
Ok, so hypothetically if I have been using Garmin Vector 3 power pedals for the past year and had the wrong crank length entered, how would this affect the power reading?
Scenario: Garmin calibrated to 172.5mm crank length, when in reality I have 165mm crank arms
For example, if I measured 300w for 20min, would it really be 280 or 320 or have little to no impact?
Fortunately, I do 95% of my riding on the KICKR so all that data is accurate. But this means every race I’ve done in the last year was on an improperly calibrated PM, ugh
If my brain is working correctly tonight - 165/172.5 X 300 = ~287 watts for an indicated 300 watts.