Originally Posted by
johnwilson09
What are the reasons for differently-shaped top and down tubes on road bicycle frames? They can be straight, curved or tapered, with either a circular, triangular or square cross-section. These can be in different combinations on different bikes (as shown in the following images). Is this purely stylistic, or does it have some functional importance?
The differences can be just styling and marketing flair, but shaped tubing can and does effect ride characteristics.
By shaping the tubes manufacturers can tune the stiffness of the bottom bracket, tweak aerodynamics, tune out unwanted lateral flex, give more road compliance, allow for more standover clearance while maintaining the same geometry as a bike with a longer or shorter seat tube...or just give the bike a unique look.