Hello, been training with TR for a little over a year now. I recently ramp tested about 10% lower on my TT bike compared to my road bike. My question is, should I train with my road bike FTP set or the lower power of the TT bike FTP. Correct me if I’m wrong but my theory is that since my body can probably physiologically handle the stress from the higher FTP, will it make sense to train with the road bike FTP in my TT position, granted i can probably hold the higher power numbers for shorter periods like intervals.
I do not know if there is a correct answer to this. Some pros train road bike only when on the trainer. Some pros are road or TT bike on trainer depending on what type of workout and/or how close to race. Also if you cannot generate the same power in your TT position/fit as you can on your road bike, then why would you use your road FTP to set your TR workouts on your TT bike? 
You definitely need time in the aero position especially if you are doing full distance triathlon. How much time you need depends on you. Much like some people do not need to run bricks, some do not have a lot of issues going from road to TT bike in terms of comfort and power.
I think it depends on your racing goals, that is to say how much of your racing will be on your TT bike.
I’m competing only in TTs so I do as much of my TR workouts as possible on my TT bike. The exceptions are VO2Max type workouts that for me have no relevance to a TT position, so I do them on the road bike.
My TT FTP is around 5-10W lower than my road bike FTP so as a compromise I took my road bike FTP and reduced it by 5W. I guess you should aim to have an FTP setting on TR that allows you to “only just” complete workouts such as over/unders on your TT bike. That’s what I do, anyway.
As I use TR more and more I realise that the actual FTP number is less important than I first thought and it’s far more critical to get the appropriate intensity for each workout based on your own abilities/strengths/weaknesses.
For example with my FTP now set at “TT bike level” I can bash out super hard threshold and over/under workouts (only just!) on my TT bike but at this same FTP setting I still struggle on VO2Max workouts even when on my road bike, and have to turn down the intensity from 100%.
There are 3 crucial questions to ask yourself before you try to answer this.
- What are your goals for the year (ie road races, TTs, both)?
- When are your goal events?
- What are your limiters?
If you primarily race TTs (or triathlons):
- If you have been training for a while so your FTP gains are small, I would train (and test) on the TT bike, in position, and try to drag the TT FTP up towards the Road FTP. Especially if your target TTs are soon.
- If you’ve not been training long and so you are still seeing good FTP gains, I’d stick to the road bike on the turbo and try to drag the road FTP upwards (this will drag the TT FTP up as well). Especially if your target TTs are far away.
If you primarily race road/crit, or if you mix and match, I’d leave the road bike on the turbo and use your road FTP - if you have events that need TTing then I’d just mix in TT specific stuff in the fortnight beforehand.