I haven’t done the structured test since my report above but did have an FTP improvement following one of the KISS 10 mile time trial races which I think is an ideal way of testing yourself as I found my focus was more on the “race” and not just staring at a number. With no drafting on TT bikes it is a “fair” effort too.
I would not do an FTP test in ERG mode as it kind of predicts the result before you do it and anyway I found some “strange” things happening in ERG mode on my trainer (Tacx Genius) when I tried an interval session. Wattbikes don’t do ERG mode anyway.
Ultimately you are riding hard for 20 mins to predict something that is actually meant to be a metric for approximately an hour. I then look at the computed FTP and think to myself could I really keep that going for an hour. Might need to enter some Zwift races around the 40K mark to check this out!
Another factor is repeatibility. When I did those first FTP tests, they were actually on a Wattbike at the gym. I am very tall and even bought an extended seat pin so can get to the right height but then I find myself so far back that I just don’t feel am in an A1 optimum position. It is handy to use them at the gym however as are right next to the treadmills when want to try Zwift running and I don’t have my own treadmill. When I use my own bike on the trainer at home it just feels “better” and indeed the watts I can put out on the trainer on Zwift are more similar to what I see on my TT bike with a PM on it. I think I might “lose” 20-30W on the Wattbike for what seems a comparable effort. Those tests on the Wattbike were trying to do the “warm up” intervals at a FTP level from my own bike and perhaps not surprising I struggled.
Given the fact that Zwift only seems to “care” about the 20 min effort then I think that is the focus but ultimately you can become good about doing 20 min efforts. It is a bit like saying I want to know how well I can do in a 40K/25M TT but I only want to ever do 10M TTs.
Of course for any hard effort you need to do a decent warm up. I would not want to do 3 mins way over FTP as part of that however. I did discuss this in the context of the FTP test with a few people however and the consensus was that the hard intervals are meant to introduce some level of fatigue to make the 20 min simulation that much more realistic.
I saw on GCN they did an FTP test on Trainer Road the other week. I only recall the 20 min bit however. Trainer Road says “During an assessment, riders first follow a consistent, structured warm-up before riding at a challenging effort level for an extended period of time, e.g. 20 minutes or 8 minutes.” Anybody got an idea whether their “structured warm-up” includes a 3 min “very hard” effort?