Torsional stiffness calculation of Coupling

http://www.google.co.uk/search?sourceid=navclient-menuext&ie=UTF-8&q=Torsional+stiffness+calculation

What kind of coupling?

Such a generic question can only lead to a frustrating answer as for instance : divide torque by torsional angle obtained when the torque is applied.

Is it a special design ? As far as I know manufacturers write in the catalogues how big the torsional stiffness is since it is needed for resonance computing.