I'm not an metallurgical expert by any means, but my under standing is that
because aluminum is lighter than steel, one can make aluminum tubing that
weighs the same as steel, but is a good deal larger in diameter and in wall
thickness. This results in a stiffer/stronger tube while weighing the same
(or less, depending what trade offs get made).

When aluminum frame bicycles first appeared using tubing of the same size as
steel tubing (and bonded lugs IIRC), they were horribly flexible and while
comfortable, were pretty scary in a pack sprint or while going down hill
quickly. When Klien came out with the first big tube aluminum bike in the
early eighties (?) it was a revelation. The bikes were light and really
stiff - too stiff for many. anyhow, the market (mostly Cannondale for a
while) took off and now the world is lousy with aluminum frames.

Peter

"Eric R Snow" wrote in message
...
From what I've read the stiffness, by weight, of steel and aluminum is
almost equal. So when building a tubular space frame there's really no
advantage, from a weight standpoint, to using aluminum over steel. But
modern aircraft are built using aluminum almost exclusively for the
frame and skin. Is the reason this makes sense due to the skin? By how
much does making the skin a structural component favor aluminum? And
what are the other factors that favor aluminum over steel for the
frame? Is this because of the frame shape?
Thanks,
Eric