Solid 17mm bar will not be as stiff or rigid as 26mm hollow bars (of similar mass or amount of material). Skin/surface of tubes, solid or hollow, takes vast majority of load. Rigidity/stiffness of tubes goes up by 4th-power of diameter. So 26mm hollow tube will be +400% stiffer than solid 17mm round bar.
Test it yourself. Clamp 40cm hollow 26mm steel tube (bicycle tubing?) in vise vertically. Slide long floor-jack handle over it and bend. Now do same with 17mm solid steel bar of same length and you’ll find it bends much easier with same force.
This Area Moment of Inertia property is why steel tubular moto frames steadily increased in diameter over the decades (while decreasing wall-thickness). Makes for stiffer
and lighter frames.
Also why wheel-axles evolved from heavy thin solid bars to much stiffer and lighter hollow tubes.
Typical cross sections and their Area Moment of Inertia.
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