Some crude forensics. Let's say the down tube separated from the head tube. (The initial "pop" you heard.) Some steerage is lost. The top tube is levered up by fork which is now rotating forward. Top tube starts to peel off the seat tube. (You can see it lifting the entire weld off at the bottom.) Front wheel hits the curb. Fork is immediately yanked back, hard. Top tube bends, back portion comes up more and snaps off at the remaining metal along the top, just forward of the weld. Front wheel, now completely detached and stopped by the curb, comes back along the down tube, now just an unsupported strut sticking forward and forces it up and to the left, breaking it off at the bottom bracket.

I think this was one break and a lot of collateral damage. I think a large part of the collateral damage is the use to joints that have ultimate failure loads very close to the yield loads. In other words, joints that break instead of bend when they are deflected. This also means the energy that is not absorbed by bending is passed on to the next joint which in turn does the same to the next.

As backwards and heavy as the technology is, there is something to be said for the time honored lugged steel construction. Odds are good that this bike, built that way, could have been ridden to a standstill with the same complete failure of the that joint.

I wonder how my two welded ti bikes would fair. Now, neither one of those bikes was built with especially thin tubing. They are not light. I do consider everything forward of mid top and down tube to be "critical". In other words, failure there being very destructive and quite likely causing major injury or worse. On a welded bike, I want to know who did those welds.

Ben