Originally Posted by
unterhausen
I'm sorry, this makes no sense at all. The tube is going to be supporting axial stresses, and to a lesser degree, bending stresses. A tube is a long, thin shell that has the potential for buckling if the forces get too far out of plane. The downtube is primarily in tension along its length. The bending stresses will induce compressive stresses and tensile stresses that cancel/add to the tensile stresses that the tube must support. I don't think it's a great idea to do stress analysis by thought experiment, but it seems like the bending stresses would primarily increase the stress on the bottom of the down tube and reduce the stress on the top of the downtube. I'm not sure what you are getting at anyway, but your stress analysis is all wrong.
Given the main loading of a bike is at the seat tube, we'll say the seat lug is pin/pivot connection for the seat stays and top tube. When load is applied to the seat tube it attempts to move in a downward direction. There is moment on the seat stay and top tube that makes them want to rotate upward. Assuming everything stays attached together and no part is allowed to elongate/fail, axial forces are not the only ones the downtube should encounter. Have you ever seen a bike that suffered damage from ahead on collision that didn't bend? Axial forces in this instance do cause the top tube to elongate, but it also bends noticeably downward. The reaction of a failing downtube should be to elongate and bend upward as the forces exerted on it are not just axial.
Right now I think I'm dealing with a stomach ulcer and am not quite on top of my game but it doesn't seem like there's any way the tendency of the downtube should only be to elongate. Rotational forces should cause a bend as well. The guerciotti in the picture did exactly what I claimed. The section above the break is no longer in the line it should be.
Somewhere a tube is going to bend before it fails due to elongation. Maybe the top tube bends and then the downtube fails at the point shown due to elongation, but I guarantee there's one hellacious bend at the other end of the down tube after the top tube failed.