Torsional rate, twist angle and bending stress per GB/T 1239 / GB/T 23935
Formulas per GB/T 1239: kt = E·d⁴/(64·D·n) [N·mm/rad], θ = T/kt (rad, ×180/π for deg), σ = Kb·32T/(π·d³) with Kb = (4C−1)/(4C−4)+0.615/C (Wahl bending form). Torsion springs deflect by bending (E, not G); wire length L ≈ π·D·n, I = πd⁴/64. Self-consistency check: kt·θ(rad) = T.