TMonk Back in the day of team structured workouts and coach sponsored training camps, there was always a discussion of what position and on what course does one test for FTP. And I do not remember anyone that used a TT bike on a flat to rolling course. In reality, all those the discussed the matter felt that they were 10s of watts lower on the TT bike and these were the ones that were good TT riders.
FTP SmefTP, it is just harder to make power in a TT or pursuit position. What I end up doing is using my TT or pursuit position for the harder intervals. At least that way, I am training the position for more power as well as the physiology.
If it is pure power you want, nothing beats a good fitting road bike working against a grade riding on the hoods. I bet some of the Zwifters have oxygen generators and breathe pure oxygen as they ride.
