Stiffness of helical torsion spring is usually measured using the Newton Millimeter per Radian[Nmm/rad] for Roll Stiffness. Newton Meter per Radian[Nmm/rad], Newton Millimeter per Degree[Nmm/rad], Newton Meter per Degree[Nmm/rad] are the few other units in which Stiffness of helical torsion spring can be measured.