Pretty much what everybody else has said...it's road bikes that make the big difference in weight because of climbing...that's why the high end road bikes aren't nearly as aero.

To make an aero bike you add weight to create the teardrop shape that cars and planes use...Essentially turn a regular tube from O into <O and it adds weight but reduces drag.

Weight really only matters when you accelerate and decelerate and in TT events that is pretty rare.


Lighter will help but in TT probably not much unless the course is hilly. I could see it MAYBE being a factor if the race is draft legal and you have a really strong field that is going to try and create distance by attacking and having a lighter bike helps to keep up...but not significantly and from what I understand racers will sometimes have regular road bikes because they have each other to break the wind and reduce drag.