Exactly, when you go to the calibration screen, let the Garmin do the zero offset calibration and note the number. Do it several times in a row and see if that number changes.
I understand your reasoning that speed to power should always be constant on a trainer. The thing is that trainers are uncalibrated resistance units and subject to many influences that can slightly change resistance over the course of the ride and that makes it pretty much impossible to draw conclusions from trainer rides. If you feel uncomfortable we will test your power meter on a calibrated Cyclus2 scientific erg for precision.
Nicolas, let me reply....
- I messaged customer support first thing this morning, nobody got back to me. Their is no phone number on your site. Can you have someone call me asap
-What I'm not explaining well, is how I can calibrate at any point in a ride and it will shift my readings. For example, I can be biking for 5 minutes steady on a trainer, same gear, same cadence and the power will read on my garmin 200 watts over the 5 minute period. I'm not talking about the average over 5 minutes, I'm saying my 3s average that I read on my Garmin 500 will be at 200 (or within 1 watt) the whole time, it stays very steady. If at any point I stop pedaling for 3 seconds, which it obviously calibrates, and then I resume at exactly what I was doing before (same cadence, same gear etc) the power number will change and can easily be +10 or -10 what I was just seconds before, again it will be very steady, just at a new calibrated steady number. It will then stay consistent at the +10/-10 until I calibrate again and it will jump again. It's almost like the 0 balance gets shifted. I can recalibrate every minute and it can jump around each time I calibrate.
- Something is not right, if you used it for a ride, common sense would make it clear something is off.