Maybe this extra stiffness of the box section is why the likes of axyz and chinese makers use this design for the y axis?
Do they use aluminium or mild steel box section. I bet its steel, stronger and easy to weld plates onto it as well as better stiffness?
what is the main reason diy builders use aluminium, softer so easier to machine, lightness or does not rust?
I am not American!
You might be reading off the wrong rows.
I've compared the 2 shape:
'I beam' width 100mm, depth 200mm, flange 10mm, web 7mm
RHS width 120mm, depth 80mm, wall 3mm
Assuming length 2000mm, and your weight 60kg, and vertical Z deflections are:
I beam 23 microns
RHS 393 microns
Which tallies with your jumping experiment, showing the vertical stiffness difference, where the I beam is much stiffer.
Unless you had a lever arm to jump on (which was very stiff), then you would not be comparing the torsional performance which I would expect to be worse for the I beam.
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