Originally Posted by PiratePete
I'm a little confused on your terminology. Don't you mean bending and not tension? "Loaded in tension parallel to the long sectional axis" would be like pulling on the skinny end of the oval.
What I tried to do was limit the discussion to the basic forces involved. Bending is a tensile force (and obviously at the other side of the free surface a compressive one). I simply wrote to demonstrate the difference in behaviour of, initially, an elliptic section and a circular one, and they always come down to straight tension vs. torsion. Yes I meant bending, and I was reducing the forces involved to those in the tensile half of the sample.
Originally Posted by PiratePete
What do you mean by torsion? Something like a tube twisted around its longitudinal axis at both ends. ie when you press down on the pedal the seat tube would be in bending, deflecting sideways and the down tube in torsion. It seems like the torsion force would be much less than bending forces on the down tube.
Torsion meant rotational tension/compression. Yep, your example is exactly what I meant. Yep, you're right with regards to the degree of one regime or another being exerted on the downtube.