Power differences between small and large chain rings

Hi there, I have a rather complicated question… at least I think it is. Typically I ride on routes with not all too much gradient changes. I ride based on my HR zones and have found the corresponding wattages that I need to ride.

However yesterday I rode on Alpe du Zwift (large gradients) and had to shift to the small front chain ring. I still rode based on HR but the corresponding wattage was significantly higher than expected (50-60 watts more). Do you have an explanation for this?

On large chain ring HR 125 at a cadence of 95-100 is round 190-200W, yesterday on small chain ring climbing Alpe du Zwift HR 123 at a cadence of 85-90 wattage shown was 240-260W. My FTP is set at 262W (corresponding HR should be around 152).

Trainer is a Tacx Vortex smart.

Any help would be appreciated

You get your power readings from Tacx Vortex, right? My guess would be, you have an error in power measurement (or calculation, if your power is calculated from your speed using ZwiftPower) which depends on your speed. You might also want to check pressure in your rear tire and condition of the tire.

There is no way how you would ride at a power close to your FTP for an hour, and your heart rate would stay in the endurance range. This just does not happen.

Your solution would be to move the “trainer difficulty” slider to zero. This would enable you to ride pretty much any terrain in the same gear. Zwift will just read the power which you generate, and your avatar would move at a speed proportional to your power.

Not a great answer but these trainers have different calibration curves for low wheel speed, mid and high wheel speed possibly needs calibrating at a lower speed