For thin walled tubing, you get way more strength increasing the diameter of the tube rather than the wall thickness.

Area moment of inertia (the resistance to bending of a hollow tube) is approximately pi x R^3 x T. R is OD and T is thickness. So stiffness is porportional to T, but porportional to the cube of radius.

Torsional resistance or polar moment of inertia is similar

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that's true

but i'm not talking about the frame strength/stiffness here, i'm talking about the tube resisting kinking. The larger the diameter for a given thickness, the more liable it is to kink. As the diameter increases, the curvature decreases, so you get less support from the arc and it behaves more and more like a flat piece. This is only for localized stress though.

so i redid the sketch of the frame to include a beam for the forks and a few for the swingarm so that i could place the appropriate loads and constraints

3 tests on the frame with no engine support.

enough force on the lateral test to bottom out the planned spring rate.

and substantially more on the other 2

1400 ftlbs of torque twisting the neck with the SA pivot pinned

and 900lbf pushing the neck across with the pivot pinned.

if the frame ever sees these magnitudes, the frame holding is the least of my worries.

i am content with the results

Edit: the images do not show the actual displacement of the tubes, they are adjusted to for easier viewing.