wrote:

...

Hollow shafts are stiffer than solid shafts. The stresses are
all concentrated in the outside wall, with no central material to act
as a fulcrum to stretch the outside on bends. Try bending a piece of
5/8" bar and one of 5/8" tube sometime (same material, of course) and
see the difference. The bar will bend, but the tube will resist bending
until it suddenly kinks. Cranks need to be stiff, especially where they
are loaded with gyroscopic forces, and need to be light, so they're
mostly hollow.


More importantly a hollow shaft that is the same weight as a solid
one will have a larger diameter which gives you much better
stiffness, especially in torsion, for the same weight.

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FF