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.