07 - Square Tubing vs Round Tubing
I wasn’t sure when I was going to talk about this but I might as well do it now. You love math right?
The thing is, he’s not wrong but there’s a lot more to this story. I think it really helps to understand what’s going on so we can all make the best decisions when designing and building our own frames. I heard this debate all the time as a welder. There are times to use square tubing and times to use round tubing. I made the following graphic to simplify this concept:
So, Mike is right if you compare 16mm square tubing to 16mm OD round tubing, both made from the same material and both with 1.0mm wall thickness. The 16mm round tube is only 59% as strong as the square tubing. BUT, the round tube is 22% LIGHTER!
The next thing to notice is the 20mm round tube on the right. It weighs the same as the square tube but its 20% STRONGER!
Now this doesn’t mean you should always use round tubing. There will be times when the strength of the square tube is more important than the weight savings of the round tube. Going to a larger tube diameter may not always be possible. A larger tube will be more expensive and may require heavier motor mounts.
I hope this helps everyone. If you like my graphic, please feel free to share it anywhere!
Math
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Cross section area, round tube: PI/4 (OD2 - ID2)
for 16mm tubing its 3.1415/4(162-142) = 47.12mm2 -
Cross section area, square tube: OD2 - ID2
that’s just the outer square minus the inner square, 256-196 = 60mm2 -
Moment of inertia, round tube: PI/64 (OD4 - ID4)
for 16mm round tubing its 3.1415/64(164-144) = 1331mm4 -
Moment of inertia, square tube: 1/12 (OD4 - ID4)
for the 16mm square tube its 0.08333(164-144) = 2260mm4
Sources
The first two links are online calculators:
http://www.amesweb.info/SectionalPropertiesTabs/SectionalPropertiesHollowCircle.aspx
http://www.amesweb.info/SectionalPropertiesTabs/SectionalPropertiesHollowRectangle.aspx
http://mnhpva.org/tech/frame_tubes.html
https://www.engineeringtoolbox.com/pipe-formulas-d_1335.html
https://en.wikipedia.org/wiki/Moment_of_inertia
https://en.wikipedia.org/wiki/Polar_moment_of_inertia