How big of a difference is there between using a full suspension MTB vs a road bike on a direct drive trainer? Previously, I have been using my 100mm MTB (suspension fully locked out) for indoor riding, but now I have switched over to my road bike. Do I need to redo my half / full monty to reestablish my 4dp? I recently redid revolver and noticed that my average heart rate was 10 BPM lower vs my previous revolver workout from 2 months ago on the mtb.
Any change in equipment should warrant doing a full test. However, I would do HM first to see if there are major differences with your FTP/MAP/cTHR. There might be and if they are significant schedule the FF Prep program to ensure you are good and ready to test.
Indeed. Changing you position significantly will change what power you can generate and for how long. Often more aero positions compromise your ability to draw in oxygen and produce power, but compensate by reducing the power > velocity/resistance side of the equation, so you could even find your FTP is lower on a road bike if it compromises your breathing (or higher if it helps you engage your muscles better).
Basically, it’s not just the change of equipment and how that interacts, but also your change of position.
If you decided to install aero-bars and always train on those suddenly, but on the same bike, you’d probably be best off re-testing also.