Joe, this is a very interesting topic and one you've covered before in some detail with plenty of comments but no definative conclusions.. https://www.joefrielsblog.com/2011/01/indoor-vs-outdoor-bike-performance.html My own experience using a basic resistance roller trainer with power pedal, my RPE is much higher for any given power when indoors on the trainer vs outdoors? Basically I find it much easier to put down the same power outdoors vs on the trainer. Id compare the feeling to going uphill vs downhill, it's "easier" to stay in the higher power zones uphill. I'm using the same power pedal indoors and outdoors which rules out a few variables. What's also puzzling is a recent FTP test on the basic trainer, result: 256W, but a laboratory blood lactate test gave a result of 270W? The lab test was done using this ergometer.. https://www.lode.nl/en/product/excalibur-sport/3 The lab test was done in December 2018, trainer test in February 2019, so was expecting some improvement after completing a 2months of base training! I'm wondering if this result is in line with your comments on flywheel vs roller? I enjoy reading your blog and books, thanks for all you expert guidance, much appreciated.