Carl Punton wrote:
I think its probably safe to say that round section is the best compromise of wight versus strength , its pretty much standard for bicycle frames & various other vehicles that I'm not allowed to mention ! There is a reason for this , its not just coincidence. The fact is , our homebuilts are easier to build with square , yes , round would be lighter for equal strength , but much harder to align & join , hence the slight weight gain is usually accepted.
While I agree with your statement, I think the point I was making was that because of how they are manufactured, regular pipe is weaker (yields more easily) than box section which is weaker than drawn tube. So use box sections in your designs rather than regular pipe, but if you can source drawn tube, properly fishmouth butted joints, and afford it, then use drawn tube.
In the cause of Aluminum, which is most commonly extruded at elevated temperatures and quenched, all these assumptions change. With Aluminum, its the metallurgy and post welding treatment heat treatment as well as how it it manufactured that effect its yield and tensile strength, but that is a whole other story :-)
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