Originally Posted by
DrWJODonnell
A TT bike will have a steeper seat tube thus shortening it relative to standover height. In addtion, the Top tube shortens. So, no, they should not keep things the same for the reason outlined above. In addition, because of the position one tends to achieve being the same as though you were in a road bike "drop" position, the top tube is lower on the triangle to accomodate a lower stem, and thus both the top tube and seat tube are shorter. We are not talking about randomly assigned "small, medium, and large" but actual numbers whose values have real meanings.
I get your logic, I hadn't thought about it that way. I busted out the ol' trig skills, and the difference from 73 to 76 degrees would make the seat tube a little under a centimeter shorter. I still kinda agree with the OP though. It would seem like a company would get more repeat customers if someone who rode a 58 cm cervelo road bike could go into the bike shop and say, "I want a 58 centimeter cervelo time trial bike" or vice versa.