Originally Posted by
20grit
Alright, now I get to throw out some more thoughts:
A bicycle downtube should be in tension (reference wire bikes). If the tube sheared/unraveled as we're lead to believe while riding, it would have been under its typical peak load (weight of rider (live load), weight of bike (dead load), and other small live loads. Assuming all other tubes were in their proper places and the addition of other tubes never took place, the break as pictured makes little sense to me. Assuming forces pulling to the upper right and lower left of the picture as we see it, the break should occur as pictured. However, this tube is supposedly part of a frame. This then should change the direction of tensile forces, pulling them more in the vertical direction (as in, if the downtube failed, the bike could potentially fold in half.). The break then, should bow outward to the bottom right of the picture. Placing a straight edge on the picture shows it does not bow in that direction. In fact, it bows in the opposite direction, which I'll attribute to the lens of the camera. The break as pictured appears to have been generated by forces exactly parallel to the tube. Forces that I don't believe exist on that tube in a bicycle.
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.
The guercotti shown in this thread failed in the heat affected zone. This Kogswell didn't, although I wonder if it failed at the very end of the butting.
Originally Posted by
cudak888
Jim's interpretation of the photo makes quite a bit of sense - it looks as if the tube was under tension when installed.
-Kurt
when the down tube fails, what is going to take up the forces that it was holding? It's clear that top tubes on both of these bicycles bent a little. Not so much that it would be an issue once repaired, but they underwent some plastic deformation