The cross-section of a beam determines the moment of inertia, which is used in the bending calculation. It also defines the volume of the beam, which, dependant on material, determines how much the tube weighs and costs.
Pick a cross-section type from the pop-down list, then adjust its size using the sliders. When you've done that, visit Bending, Cost, or Weight to see how the tube you have designed affects the properties of the frame.
However modern cycle tubes are of much more complex design than this. What ways could be used to make the tube lighter? Answer.
Links to Cycle tube manufacturers.
