Au contraire, mon freire. You get a stress riser (i.e. a crack), whether by trauma or fatigue, and that crack grows until it is a break. Breaking by bending or snapping is VERY rare.


No, it's not unreasonable. The OP is asking about aluminum frames cracking, and the vast majority of cracks come from fatigue failure. Impact (for example, if you're riding a frame that gets a lot of abuse, like a mountain bike) can definitely accelerate the process, or finish off a weakened spot, but you basically never get an abrupt failure that isn't from fatigue except in the case of a major crash.

Baloney. You have no way of knowing that this is true. Crack a few aluminum frames before you start talking about how much worse you would have been hurt by it. Check out CDR's explanation upthread. Break a chainstay or seattube and you're probably going to be okay, no matter what you're riding. Break a top tube, you've got a scary situation. Break a downtube or headtube, and you're in for a world of hurt, no matter what you're riding. Frames broken in a particular place all behave in the same way, no matter what they're made from.

Actually, I do - experience, a rudimentary understanding of how the universe functions, and sheer common sense. Impact accelerates the process, but it's extremely difficult to take a brand-new frame and break it by smacking it into something in the course of normal riding. With accumulated wear and tear, all those sweet jumps will eventually do the job, but again, that's because of the continual process of pushing the material too far, weakening it and eventually starting micro-cracks that propagate into full-blown cracks.

EDIT: Actually, let me back off on this - I still think this is basically true, but I want to clarify that, yes, the distinction between impact damage and fatigue in the kind of case I'm describing can seem a bit academic. When I say that the culprit for most frame failures is fatigue, what I'm saying is that a single impact that snaps something on a frame is rare. It's a lot more common that all those rough landings start to add up and start a crack that ultimately leads to a crack. And Sixty Fiver is right on, of course, to say that most failures stem from manufacturing defects (and impacts, but I'm not sure we mean the same thing by that, and I beg to differ on single impacts doing in a frame without defects). Most frames, no matter what they're made of, will never break. I stand by my statement that any claim that you'll be hurt worse by a breaking aluminum or carbon frame than a breaking steel frame is complete and utter baloney.