powertap calibration

[ivanooze]

so... i have been using the powertap g3 hub for quite some time now, up until recently i started calibrating it via my garmin forerunner 920.

- when i would calibrate it, i would get a number like -145 all the way up to 12,454 (yeah my hub really effed up that day).

-i just recently calibrated it and as of right now the offset is at 515. I've never gotten a 0.

-are you expected to see a 0 when you calibrate a power meter? if so, does the 0 mean that your power meter is on point in terms of accuracy?
-also, what can i do (besides sending it in to powertap) to try and fix this whacky offset calibration number?

[OldCrank]

I'd guess this 'offset' is something to be lived with.
I wish the hub would self-calibrate but oh well.
Even if Powertap adjusted it to zero, after some time it would probably drift.
Then there's temperature effects.

I've found Powertap's customer service pretty helpful, they might have a more complete explanation.

[dem]

Before this thread spins out of control.. :)

The "Zero Offset" is not "Calibration" - when you press calibrate with no load, a power meter should read.. zero. If it does not, the device applies an offset so that it says zero. Then all power readings after that are "correct"

On top of that, some power meters don't report the value, and just say "Successful" or "0"

Most modern power meters (including the Powertap G3) will do temperature compensation and offset reset when you freewheel for some period of time.

Your first scenario is actually worse - if you get crazy values all over the place, something is wrong with your power meter.

[ivanooze]

Your first scenario is actually worse - if you get crazy values all over the place, something is wrong with your power meter.

well i noticed that it went crazy after a hard ride that involved allot of descents and braking, and plus the rim that it's laced to is on the verge of exploding. Spokes are twisted, the wheel is out of true, the brake surface is terrible etc.

[shovelhd]

well i noticed that it went crazy after a hard ride that involved allot of descents and braking, and plus the rim that it's laced to is on the verge of exploding. Spokes are twisted, the wheel is out of true, the brake surface is terrible etc.

That would be problem for a Powertap hub.

[commonguy001]

515 is a valid offset number for a hub without the latest firmware. You should pull the cap and download a copy of the PT software so you can update the firmware if you haven't already. After you update it the offset should be near zero for a hub that is within specs. If something isn't correct the new firmware should fail the zero offset process and you'll need to have it serviced.