The additional factor related to the flywheel is trainer mode. From all the discussion here, I expect that ERG vs Resistance vs Standard vs Level vs Simulation plays a part too.
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It may also impact riders differently between steady state efforts and power jumps but I bet it is another piece to the puzzle.
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For those aiming to get closer to their outside power, I’d suggest a basic test using Zwift, Rouvy or other apps in their Simulation mode and aiming to follow a workout as if they were outside. I don’t have any info on whether the Simulation modes differ between apps, but that could be another level of delta here.