fujiacerider

Doing some thinking a few days ago, I wondered about the tubing on my bike. On all the bikes I've ever seen, the tubing is round or sometimes diamond-shaped. Would star-shaped tubing increase strength? I figure probably not, since no bikes are made like that. Or does it increase strength, but have a huge downside, like more brittle, way too harsh a ride, or since it uses more metal for the same "diameter", is it heavier?

Cole

MichaelW

Creases in the metal act as stress raisers, concentrating stress , leading to failure. Hence the need for curved, basically round cross sections.
There are square section tubes, such as Pace bikes. These are externally butted by CNCing away the tube wall. They work very well, but it's a tricky process.
Reynolds 708 tubing used an octagonal rifling on the inside to strengthen the tube.

collegeskier

Circles are generally the most efficient shape for everything. They have the least amount of perimeter per internal area. Since you want the least metal (light) and largest cross-section (stiff) circles make lots of sense. Not to mention a cirlce has no weakness. They are good in twisting and bending in all directions, and in compression due to stability concerns. You would see a ton of circles in construction is joints were not such a huge problem. AH six years of college allowing me to answer what shape to use, my parents would be so proud, but don't tell them they will send you a bill.