"Tim Williams" wrote in message
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"Ted Edwards" wrote in message
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Through your reading, keep in mind that aluminum is 1/3 the weight of
steel but also has 1/3 the elastic modulus.

The nice thing about it though, is if you can give up stiffness in one
dimension, you gain 3 times the stiffness in the other (think edgewise).

It doesn't apply with space frames, Tim. A "pure" space frame loads its
members only in tension and in compression. Thus, aluminum versus steel is a
wash. Aluminum offers no weight advantage, neither for stiffness nor for
strength.

In practice, the reality is not far from that, if the space frame is
properly triangulated throughout. Most space frame cars are good at the
front and rear bays, but they depart from a true space frame in the cockpit
bay.

There are several issues here, which Ted has alluded to. First, there is
some torsional loading in actual space-frame members. In the cockpit bay,
there often is a *lot* of torsional loading. With round or square tubes,
once again, aluminum offers no advantage here. But it can offer an advantage
with other structure shapes, such as the L-channels (angle aluminum) that
were being discussed. There, the PLATE stiffness and strength, which is what
you're talking about, comes into play. The stiffness and strength in a plate
vary with the cube of the thickness. An L-channel subject to bending or
torsion presents a complex resistance, and plate stiffness is one part of
it.

Again, that isn't true with tubes. If the limiting factor in a space-frame
design is resistance to columnar buckling, aluminum's advantage in plate
stiffness is a help, because thicker-wall or larger-diameter tubes have more
resistance to buckling. This is something that designers try to avoid today.
But the Lotus 7, like other Chapman designs before the early '60s, tend to
have long, thin tubes loaded in compression. Aluminum could help,
theoretically.

But a better answer is to produce a better design, in which the frame is
less likely to fail by buckling. The Caterham folks, who build the Lotus 7
under license, have done just that. They re-designed the frame with the aid
of finite-element analysis.

Ed Huntress