Just thought I should round out this discussion with some real engineering data. So I opened up TubeComp on my work computer and found two profiles that have the same cross sectional area and compared them for (A) canter levered and (B) load bearing deflection. Canter lever is attached on one side with the load on the other, load bearing, is attached on both ends with the weight in the middle.
The profiles were 35 x 35 x 2.0mm box and 42.4 OD x 2.0 mm pipe. A bit thicker than what you would typically use in a frame, but both have a cross sectional area of 245mm squared.
Box
Pipe
Yield
450Mpa
350Mpa
Tensile
500Mpa
430Mpa
Mx & My
1.25KN
1.03KN
Mt (torsion)
.945KN
.926KN
Deflection A
37mm
32.1mm
Deflection B
2.31mm
2.01
So what does this mean? The box is stronger, and can withstand about 20%
higher loads before permanently bending. The box can also withstand the
same torsional loads as the pipe, but this is because it has the
elongated lattice and has the higher yield strength and ultimate tensile
strength.
However, and this is an interesting point of difference, despite being weaker, the pipe is stiffer, and experienced less deflection when the force is applied. Now the pipe here because it has a thinner wall, has been drawn twice and so is stronger than regular thicker walled pipes which have 250Mpa yield strength. From this I image that seamless tube, which as good or better than the box in strength for the same cross sectional area so would be much stiffer.
Never the less, I agree entirely with 25mhz, if you are a home builder stick with box. If you are building a recumbent to get active and loose a few pounds, you have more chance of shedding weight from your body than from a frame by using exotic materials. Even Aluminum can be a trap. I notice that the Hipparion http://personal.eunet.fi/pp/davinci/fill.html which has a frame made of Aluminum actually weighs only slightly less than the steel frame I am currently building.
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