On Saturday, May 23, 2015 at 7:18:03 PM UTC-7, jack wrote:
Is there a general rule(s) that describe the stiffness of a round tubing
to the stiffness of a solid rod? For instance: is 1" dia. round rod
stiffer than 1 " dia. x .125" wall tubing? How much larger dia. tubing
equals a solid rod of a certain dia.?

For a full treatment, see Roark, _Formulas for Stress and Strain_.
Plan on spending a few days...
For a given material, and quantity of material (pounds per foot),
the tube will be stiffer as long as you keep the deflection low (don't
buckle the material of the tube).