Non-Circular Torsion

Only round shafts follow τ = Tr/J. A rectangle uses tabulated coefficients, a closed thin-wall tube follows Bredt's formula, and an open section (a slit tube, a channel) is dramatically weaker in torsion than the same section closed.

Closed sections are orders of magnitude stiffer in torsion than open ones — never rely on an open channel to carry torque.