[ItBites]

Silverback (and others):

With frames, we usually care about torsional stiffness, not bending stiffness. Ix or Iy is normally the cross-sectional moment of inertia with respect to the neutral bending axis (what you calculated).

Ip is typically the abbreviation for torsional cross-section moment of inertia. I think if you look at torsional stiffness, you will find the 4" round tube is superior to any other shape with dimensions less than 4 x 4 (given similar wall thicknesses). The .120 wall thick 4" round tube is still likely better than the SPF .160 wall thickness, due to the 2" dim on the SPF frame (and with less mass).

Have a nice day