gtluke
The Moped
The biggest reason why aluminum hasn't been used for chassis is it's so hard to work with. Welding is harder to do compared to the factory spot-welds done in the past. And it's nearly impossible to do a cheap/fast steel to aluminum fastening method. Like if you want to use aluminum frame rails but steel floors and bulkheads. Or ABC pillars made of steel but aluminum bulkheads, etc. Not to even mention galvanic corrosion. Plus aluminum has historically been expensive.
Much easier to make bolt-on parts that are heavy in steel out of aluminum to minimize weight. Crossmembers, suspension arms, door impact supports, etc. Body panels are actually hard as Aluminum is hard to stamp. Castings are much easier.
For this steerer tube, I'd want to see more close up pictures. It could have been a scratch on the tube 10 years ago from a loose & then spun stem creating a stress riser that took a long time to materialize.
-Steve
more difficult than fiberglass though?
now I can't remember but there was some car recently that had aluminum fenders that was recalled or problematic because after a few years the fenders actually develop big cracks in them. I want to say subaru, they have been using aluminum for the hoods for a while.
someone recently on jay leno's garage was explaining that the reason that all the 60's race cars were iconically round is because they had aluminum bodies. That the nature of aluminum is to be round, and that it's nearly impossible to get straight lines and flat panels with sheet aluminum. What came to mind was this is probably why the deloreon was stainless and not aluminum. This dude today couldn't get the body made out of aluminum because it had too many straight lines, nobody could do it but he said there were some craftsman in italy that may be able to do it.
It was this car:
http://www.kenokuyamadesign.com/oneoff/
i'm sure with production presses you could do it though. But man if you try to shape a flat seam in aluminum and take more than one try it's going to work harden and tear right off.