Something that gets lost in the hysteria surrounding frame failures is that it ALL fails by cracking. Any failure from fatigue always manifests as a crack, whether the material is aluminum, steel or titanium.
Originally Posted by
Six jours
In my experience most of the frames that come into bike shops with cracks are aluminum. I suspect that aluminum is somewhat more prone to cracking than steel. But I also suspect the increase in failure rate is so slight as to be worth ignoring.
A somewhat more pressing problem, IMO, is that aluminum tends to fail suddenly whereas steel tends to bend first. That makes me nervous, but again, is probably too small a concern to actually worry about. (Carbon, of course, fails suddenly and catastrophically too, so choosing carbon instead of aluminum because you're worried about cracks is stupid.)
"Catastrophic failure" is a classic shibboleth of aluminum. The reality is that failure from fatigue (which is the source of the vast majority of frame failures, not impacts) looks exactly the same in both aluminum and steel and is not immediate in either one. Once a stress riser develops in an aluminum frame, it WILL propagate faster than it will in steel, because the material is not as tough, but you don't typically go from zero to broken frame immediately even with aluminum. In any case, most frame failures happen either on a chainstay near the bottom bracket, or the bottom of the seattube, again very near the bottom bracket, and these usually go unnoticed until they reach the point where they fail completely. Whether you're riding steel or aluminum, this might as well be a catastrophic failure, if you don't notice until the tube actually breaks. But that doesn't particularly matter, because this kind of failure isn't particularly dangerous. The tube cracks and the frame suddenly goes all noodly, but it will hold its shape and not fall apart. A dangerous failure would be at the headtube or downtube, but these are comparatively very rare and usually caused by crashes, not fatigue. The fatigue failure to worry about would be the fork. But fortunately, these are rare as well.
If you're worried about catastrophic failure, I wouldn't be concerned about a frame, I would be concerned about handlebars and such (though, again, the failure probably isn't catastrophic so much as it a gradual failure from fatigue or corrosion that goes unnoticed until the part suddenly breaks). Most of us are riding aluminum bars, stems and seatposts, though, whether we are on steel, aluminum, Ti or carbon frames, and few people make much of this. I just don't think that there's much to be made out of how a given material is believed to break. Even if aluminum frames break more often, I don't think that necessarily makes them more dangerous.