Torsional stiffness per unit twist
= Torsional stiffness per unit twist
{title2=$K_\tau=dN/d\tau$}
For a uniform rod of length $L$ and twist per unit length $\tau$, the end-rotation <angle> is $\varphi=L\tau$. The <chain rule> gives $K_\tau=LK_\varphi$, where $K_\varphi$ is the <torsional stiffness>. If a source defines a normalized <torque> $M=N/(2\pi)$, its derivative is $dM/d\tau=K_\tau/(2\pi)$.