Is my power meter bad? Or am I?

New Zwifter here. I find I’m having a hard time maintaining a steady power output with a (relatively) constant cadence. Not sure if it’s me, the bike, or the power meter.

I should note I’m using a dumb trainer—it’s a Stages stationary bike with power meter (SC3).

Looks like my power fluctuates as much as 30% (+/- 20W in this example) while I held my cadence around 95 rpm. Is this just what happens with a dumb trainer?

I find holding a constant power with a dumb trainer and power meter pretty easy ( I travel a lot and take my dumb trainer with me). Ironically I can hold smoother power than when using my smart trainer in ERG mode with power matching i.e. the power meter controlling the resistance to match the target power.

That said you have to maintain a constant cadence and pretty much watch your power the entire time. With ERG mode on a smart trainer you don’t have this to worry about.

I’m not sure. If this is the first time, try a new battery, make sure your connection/dongle is close enough to work, and try, in settings, 3 second averaging instead of instantaneous reading.

Your heart rate was going up and down following the power going up and down so I would say the power fluctuations are real. but i have no idea why the resistence would jump around on a dumb trainer.

I have noticed that when using ERG mode on Zwift with Smart
Trainer and Zwift with 3 sec smoothing the power is never as
smooth as the readings my Garmin are displaying (also with
3 second smoothing)
Garmin will read literally 1 or 2 watts either side but Zwift can
be 10 watts either side.

Not sure what that is all about.

Looks to me like your heart rate is fluctuating fairly close to in step with the power. Given a constant cadence, I would assume you shifted gears? If so, your power is going to change.

That is what non smoothed power readings look like. Completely normal.

Thanks Chris! I suppose I’ll just have to deal with a messy effort graph. I had anticipated more smoothing with 3 sec averaging.

Contributing to the problem is that my FTP is so low that a +/- 15W variation in power bounces me through several zones!

The step increases are from changing the resistance on the dumb trainer. The issue is when trying to target a specific power output, ( say 150W in this case) the displayed power fluctuates +/- 20W even with a constant cadence.

I’ve had similar issues with constant cadence the w/kg varying by up to +/- 0.7 W/kg. Having changed cadence sensor, changed WiFi channel, turned off Bluetooth in same room (ant+ is in same frequency band), Zwift support looked at logs and blamed broadband dropouts or high latency.

I’m still asking BT to investigate connection, they did replace router but not improved. The problem seems to be worse at lower powers, if > 250 W seems slightly better.

Whilst I can see connection issues in logs I would have expected Zwift to be less susceptible to latency.

Look at your log files.