== torsional stiffness == 

 I removed a factor of 4 from the stiffness equation. i suspect the original author was applying a torque Mx to the MID POINT OF THE length axis X OF A BEAM ENCASTRE at both ends. Unless that is explicitly stated then it is not obvious, and is not the usual form of the shaft stiffness equation. sorry about the caps. Also this article perpetuated the myth that the torsional constant is the second moment of area. It is but only for a few shapes.