Lets leave kit, helmet and wheels out of this issue as I will not be changing them.
Lets say my TT bike is a Cervelo P5, fits well but heavy as a donkey (my build is 10.5kg with tool bottle).
Lets say my road bike is a shiny new Trek Madone SLR 6 ( not a featherweight but 8.3kg with saddle bag).
I have put the numbers into best bike split for Ironman 70.3 Buffalo City (South Africa) course (pretty much uphill in one direction and downhill in the other) and it seems to be much of a muchness when increasing the drag by 5% and decreasing weight by 2kg to try to get the difference between riding the two bikes. (linked)
I previously did 70.3 Turkey which is a flat as a pancake course with a cervelo s5 (8.7kg) and spent the entire race in the drops, used the best bike split calculations for the cervelo p5, and the time I ended on was not different from the prediction/calculation (calculated for cervelo p5 170 watts for 2:40, actual NP was 173 watts on cervelo s5 for a 2:38).
Is there even a significant difference between a high end aero road bike and a high end tt bike?
Would I have went faster in Turkey with the TT bike than what was calculated by bestbikesplit?
Should I take the road bike to Ironman 70.3 Buffalo City? It is a lot easier to climb with, and I am not certain of whether the TT bike will really give me an advantage or not.
For a full Ironman I will not hesitate to use the TT bike as the course is long enough for the gains to be really significant in terms of time, but more and more, for the shorter distances, I am finding the comfort and aerodynamic/weight trade off of the hyper aero road bikes to be insignificant (in terms of performance) different from the really impractical and super complicated TT bike. The road bikes are simply easier to travel with and just feel better to ride. I am running similarly off both bikes.
Please discuss to your hearts content…