Power increases at 30 mins?

I run Zwift from a PC, with an ANT+ dongle on an extension cable, right underneath my rig, and a Kinetic smart trainer, and Stages crank arm power meter (which I use for cadence). I spindown calibrate once a week, but that doesn’t seem to make any difference with my issue.

I’ve noticed on hill climbs that my power seems correct for about 30 mins, but then after 30 mins I simultaneously gain a lot of power and my heart rate drops. I can feel in my legs that I’m not pushing that much power. I’ve seen this 4 times now, it’s very consistent.

For example, just ran up the Insbruck KOM. Prior to 30 mins, I was pushing about 220w or so, and I was struggling to keep my heart rate around 160. After that 30 min barrier, however, I was pushing 300+ watts and my heart rate dropped. Going down the hill on the other side, I hit over 50mph and 380w or so without much effort.

The thing I noticed, at least on the downhill, is that if I freewheel, the cadence got “stuck” at whatever I stopped pedaling at. So if I was pedaling at 75rpm, if I then stopped to just freewheel, the cadence would read 75rpm until I started pedaling again.

So I can’t tell at this point, do I have a problem with the cadence meter, or is the power reading from the smart trainer just not right? Is the rpm supposed to “stick” like that? I’m just not sure what’s going on.

Thanks all!

Hi @Tim_Dunnington and welcome to the forums.
No the RPM should not stick like that. I wonder if you unpaired the stages PM if it will stop the issue. Does the trainer also have cadence? You might need to test this without having a cadence reading just to rule it out.

Or, is pairing the Stages PM as the power source and cadence instead of the trainer an option?

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Thanks @Mike_Rowe_PBR!

My Kinetic trainer does not have cadence. But I do have another cadence meter, I could try that, and I could also try monitoring power via Stages. I’ll give both a shot and see if they help.