Crank length settings on a power meter

Equipment
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I have a Garmin Rally power-meter that I use on 3 different bikes, each with a different crank length. I also use 3 devices (Garmin 1030+, TrainerRoad on two different Android phones) to record rides
Question: When I change the crank length using my Garmin, will TrainerRoad use the new crank length?
In other words, does the power-meter itself store the crank length, or does the device or app store the crank length?

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F/U question to the original posters question. If I take my Rally pedals off my TT bike (155 mm) and put it straight onto my trainer dedicate bike (179 mm), then fire up a TR workout, will the calibration be off?

I know every time I go to calibrate the pedals, my Garmin head unit always asks what length the cranks are that I am using. If I don’t change that through my Garmin head unit prior to starting up the TR workout, would the calibration be off?

Asking the same question another way, after switching cranks to my trainer bike, prior to starting a TR workout, do I need to make sure that the Rally pedals are calibrated through my head unit with the proper crank length input?

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  • The zero offset (what is actually happening, not a “calibration”) may be “correct” since it is essentially just setting the default ‘no-load’ case. But that is not the only part that matters here, so don’t assume a calibration fixes the issue below.

  • But any power data measurement will be entirely flawed if you use a crank arm setting in the device/power meter that does not match your physical crank arm exactly.

  • That distance is an essential part of the torque calculation used to give the power data, so it must match the physical arm in use for any data to be useful.

  • Yes, you have to match the length to the arms anytime you switch bikes that don’t have identical length crank arms.
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