Zero Offset for Power Meters


(Susan) #21

My point about Zwift knowing what each PM’s offset is supposed to be was in response to someone else who I thought was suggesting Zwift would know whether the offset had been fouled with if the Zwift program was the one promlting for the calibration to be done. People are always griping over the legitimacy of rider’s W/kg, so that’s what I thought was happening in this thread.


(Susan) #22

If you think there’s exrta torque going to the pm during stationary-but-rider-aboard zeroing, then I’m going to argue it would be more realistic to zero with rider on since no power should ever be generated while rider is not on board, and it is their effort, not their stationary weight sitting there that should be reflected by any number the PM shows.


(Steve) #23

I think you’ll find that all crank mounted power meters require the rider to not have their foot on the pedal(s) because any pressure, even just the weight of the rider’s foot, is transferring torque through the crank so a correct calibration will not be possible.

A crank mounted power meter uses cadence to decide if the rider is actively applying power. This is why all crank based power meters transmit cadence information as well as power. In simple terms, if the cadence is zero then the power meter ignores the torque reported by its strain gauge and reports 0 watts. This explains, for example, why standing on the pedals (out of the saddle) but not pedalling is reported as 0 watts even though the strain gauge will be registering the rider’s weight on the pedals.

All that said. I’m not sure why this level of discussion is necessary - all I’m suggesting is that it would be convenient for users to have a calibration command available in Zwift in the same way it is on, for example, a Garmin Edge. It’s then down to each user to follow the calibration procedure advised by the manufacturer of the power meter they’re using.


(Johnathan) #24

:man_facepalming:


(Susan) #25

Agree your suggestion would be convenient, Steve, and never suggested the pedals should be touched during calibration. I was pointing out I don’t see a need to get off and step away from the bike to calibrate a PM. Sitting on the saddle and holding feet away from the pedals should be sufficient.

No face palm needed, Johnathan.


(Steve) #26

Apologies but I thought you were suggesting that the calibration should be done with a foot resting on the pedal because that would be the correct reference for ‘0 watts’.

Glad we’re all agreed that this is incorrect!

Like you, I don’t get off the bike to calibrate when it’s mounted on the trainer - I just make sure my feet are well away from the pedals (and that the crank is in the correct position, of course).


(Susan) #27

:+1:
(agreed)


(Johnathan) #28

My apologies for the face palm. If you are letting the trainer come to rest, unclipping and not touching the PM, that is sufficient in my book for the calibration. I read your last message as if you were staying clipped in. I just get off so I can multitask for the 30 seconds that it takes.