Zero Offset for Power Meters


(Steve) #1

Zwift should have a zero-offset command for power meters.

This was previously requested in Power meter zero offset but has been archived. Not sure why it was archived as it seems to be a fairly basic feature to enable accurate power data.


(Tomas) #2

I agree,if possible that would be a very helpful feature!


(Daren) #3

It’s from the previous incarnation of the support and feature request forums, is all. Almost everything present there was archived. A handful made it across as popular requests:


(Gerrie Delport TeamODZ) #4

good morning @Steve_Short : I have not think about this, but why would you need a zero offset for power meters. I have a power tap Wheel and it has auto calibration, am I missing something?


(Steve) #5

Most power meters do not auto-calibrate and require a manual zero-offset to be before using them.


(Jim) #6

Not everyone has a Power Tap, Gerrie.

I have a Stages single crank arm power meter, for example. Regarding the zero-offset of this power meter, Stages Cycling says,

To ensure maximum accuracy, we recommend performing a zero reset (or “calibrate”) before each ride.


(Gerrie Delport TeamODZ) #7

Thanks guys, I had to read up on this. You got me worried that I missed something. LOL

This will be hard for Zwift to implement. I mean this will make cheating so easy.


(Johnathan) #8

I don’t think it’d be hard at all. With that said, my current work around for this is warm up, turn my garmin on, get off the bike with the drive side crank at 6 o’clock, “calibrate” on the Garmin, get ready to rumble.

Folks would need to remove torque from their PM in order to do this. A warning to dismount the bike with the crank in the MFG’s suggested position and let your trainer come to rest should be sufficient. Once you pass that screen it could show Previous calibration “x”, current calibration “y”. If it passes, you are good to go. If you fail to “calibrate” within the acceptable range, a note should say that you need to redo the “calibration” (and you should fail to correctly calibrate, the rider should have an asterisk next to their w/kg for the events that they do, until their next successful calibration).


(Johnathan) #9


(Gerrie Delport TeamODZ) #10

Good suggestion: But how will Zwit know you did not lock your crank and are pulling on it? IRL it is very important to have the most accurate reading possible, but in Zwift having a power meter that read high is an advantage.


(Susan) #11

Incidentally, I don’t get off the bike when it’s on the trainer, to do the calibration. I simply clip out, move crank arm as appropriate, and hold my feet away.

But for Zwift to do this, I don’t know how they’d know what the normal calibrated range should be. It could be different for each manufacturer and model out there. My 2 quarks, for instance, one calibrates around 2-7 and the other around a 3-digit number.


(Steve) #12

I don’t think Zwift has to know anything about the power meter - it’s a standard ANT+ command.

You could say the same thing about a Garmin Edge - how does it know about all the different power meters out there ? It doesn’t - it simply allows you to send the ‘calibrate now’ command.

I suspect that it’s the power meter itself that understands what the offset value displayed means and it uses this to offset the power values it sends after the calibration command has been done.


(Steve) #13

Zwift will not know.

And neither does your Garmin Edge (or whatever cycle computer you use IRL).

This request is to implement a power meter calibrate in Zwift, not to prevent users cheating which is a much bigger subject.


(Gerrie Delport TeamODZ) #14

But IRL it is in your best interest to have it as accurate as possible, you wont do better if you cheat your Garmin.

And in a perfect world I would say yes Zwift should have a power calibrate. But this will just open a new door to cheating.

But don’t get me wrong, If Zwift can do this it will be great. I am just point out the possible negative effects.


(Steve) #15

Not having a calibrate command is a good way to ensure that your power data is never accurate.

At the moment, anyone who cares about accurate power data must use a separate device such as a Garmin Edge to perform the calibration before they start their ride in Zwift.

If you think it’s possible to cheat in Zwift by deliberately incorrectly calibrating your power meter then people are probably already doing this using the calibrate command on a separate device.

Implementing the calibrate command in Zwift does not open a new door to cheating it simply makes it convenient to calibrate the power meter without having to use another device.


(Gerrie Delport TeamODZ) #16

This is an interesting comment that makes me rethink my whole argument. :blush:

So the way you describe it is you use your garmin edge to calibrate your power meter for Zwift, so that tell me that the calibration value gets send to the power meter.

My assumption was that the calibration only effected the device that you did the calibration on. So zwift and your power meter show the same value?


(Steve) #17

Gerrie, I believe the way the ANT+ Calibrate command works is that it asks the device to log its current offset value and then use this to modify the power values it sends. The power meter confirms that it has done the calibrate command by sending the new offset value but this is not used by the device that sent the calibrate command although the device may display the value (as the Garmin Edge does).

So, it doesn’t matter whether you do the calibrate in Zwift (if it had a calibrate command!) or a Garmin Edge - the power meter will send accurate power data.

It would just be more convenient if Zwift allowed the calibrate command to be sent rather than having to use another device.


(Johnathan) #18

A lot more data is sent from the PM’s that watts, torque and cadence. They can pull all sorts of data such as the make, model, battery voltage, slope, etc. You can see these buy using the Quarq Legacy App. The first time you pair a new PM, assuming that it’s acting as it should big assumption, start creating (locally saved?) baseline. If a rider is manipulating their PM, they will have to be extremely precise to be consistent enough to get the same value, or values close enough to have a successful calibration.

I jump off the bike so the trainer (CycleOps Hammer) can come to rest after my warm-up. The few seconds are good for a quick stretch or something like that so I can make sure that there is no added torque during the tare/“calibration”.


(Johnathan) #19

See my photo above. I do just this. you can use any compatible device to “calibrate” the PM. To my understanding the PM then updates what that value is and sends the torque/power data out with the newly calibrated value in mind.


(Gerrie Delport TeamODZ) #20

If this is the case then you have my vote. 100%. I may have misunderstood the calibration.