  We solve the St.Venant torsion problem for an infinite cylindrical rod whose
behaviour is described by a family of isotropic generalized continua, including
the relaxed micromorphic and classical micromorphic model. The results can be
used to determine the material parameters of these models. Special attention is
given to the possible nonphysical stiffness singularity for a vanishing rod
diameter, since slender specimens are in general described as stiffer.
