1. With the bike held stable in an upright position, and the crank arm
horizontal, use the “Calibrate” feature on the Garmin and note down the
number displayed (-806 in my case)
2. With the rear brake applied, attach a known weight to the pedal spindle
and repeat (gives a number of –728 in my case)
3. Calculate the actual torque being applied as
W * 9.80665 * C / 1000
where W = weight in kg, and C = crank length in mm
(actual torque = 13.337044 in my case with an 8kg weight and 170mm cranks)
4. The Power2Max appears to use some sort of arbitrary units of “ppm” for
the number displayed on the Garmin. It doesn’t really matter what they
represent, as all we need to do is calculate the slope as
T / (Wppm – Uppm)
where T is our calculated actual torque, Wppm is the number displayed by the
Garmin with the weight attached, and Uppm is the unweighted number.
In my case, this works out as 13.337044 / (-728 –(-806)) = 0.1709877436
5. The Power2Max includes a calibration certificate in the box that you can
compare this number against. In my case, it specifies 0.170035 Nm/ppm, so
the error in my slope measurement is 0.6% with the large chainring. With the
small chainring the error worked out at 3.1%.
However, a problem with this is sensitivity to changes in the numbers. If my
2nd number had changed by 1 digit, to –729, that would change the calculated
error with the large chainring to 1.9%.
