KellyT said:
No doubt at all, aluminium can be shaped to give ultimately similar properties to steel. But, since it is a material that behaves quite differently to repeated flexing, it will fail a lot quicker. It's a bit of a catch 22, that the 'filling rattling' very rigid aluminium frames are likely to be the one's that will suffer fatigue damage more slowly, since they are resisting any movement at all.
Kelly, these old cliches about al vs fe have been spouted and debated many times, but they just don't hold up. Comparing frames of equal weight, aluminum should have at least as good a fatigue life as steel. Super thinwall steel, the stuff down to 0.3mm thickness, may be close in weight to aluminum, but I've not heard anyone claim it's going to last forever.
My old Raleigh 531 Gran Sport may have a "lifetime" lugged steel frame and fork, but who wants to ride a heavy clunker like that anymore? I get it out every now and then to keep in practice with the Simplex friction shifting and toe clips
Seriously, how many frames have you seen of any material that failed due to fatigue? A randonneur type here is still riding his CAAD3 from the mid 90's, with~ 60K miles now. He takes it on the 1200 km brevets rather than his heavier steel touring bike.
Not trying to put down steel here. I rode and loved 531 steel frames for 30 years. The steel tradition has a lot going for it, but recognize that millions of great frames are now built out of the newer materials....aluminum, Ti, and CF as well.