Large discrepancy in power between Zwift Hub and Stages Powermeter

Bugs and Support
1

I have a very large discrepancy in power readings from these two devices.
When riding to 250w from the Zwift data, my Stages is reading 270w (or vice versa where I’m riding at 270w per Stages and the Hub is showing 250w). I have performed the calibration procedure for the Stages meter.

I’m trying to figure out which is correct. Since the last firmware update the Hub readings seem quite buggy/erratic. I am not using ERG, I have the trainer set to max resistance.
The calibration option is now extremely buggy where the little wrench icon to perform a calibration will disappear, or not appear at all.
I see the new firmware has implemented auto calibration, but this occurs at 10mph instead of 20mph…isn’t that way too low of a power reading for this to perform an accurate calibration? At these very low watts around 10mph, the Hub and Stages meter seem to line up, with widening discrepancy the higher the power goes. There also seems to be an odd reading where I can be pedaling at a set cadence, I will move up the cassette to put out more watts and the reading displayed by Zwift only moves 10w or so (in reality it should be 30-50w+).

Anyone else having issues with the Hub since the last firmware update?

2

Whilst an 8% difference is high, there will always be a difference. The Stages is measuring at the cranks, the hub is measuring at the hub, and there is a mechanical power loss between the two points.

The usual expectation is a 2 or 3% difference due to drive train losses on even a well setup bike. But that gets compounded when you measure power in both locations, due to the +/- X% accuracy of each power meter. Zwift claim +/-2.5% and Stages +/-1.5%. So you can see how even when working perfectly, the power meters can add another 4% difference onto the drive train loss.

The simplest rule for sanity, is measure in one place and don’t look at the other, becuase they will never agree (and it would be very odd if they did). Purists would argue that the hub measurement is nearer to what you’re actually putting out to the road. But an e-racer might appreciate telling Zwift the better figure for what you put into the system at the cranks.

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