The torsion bar diameter d cannot be selected freely in order to achieve a suitable torsional rigidity ctor. A minimum diameter which is a function of the expected maximum twisting moment and the admissible shear stress, has to be specified.
In practice, to achieve a suitable spring stiffness, relatively large torsion bar lengths are necessary.
Torsion bar springs are combined both with transverse and longitudinal struts and in some cases even with semi-trailing links. They are arranged predominantly parallel to the vehicle floor.
For front wheel suspensions torsion-bars are used predominantly in combination with wishbones and arranged in the vehicle's longitudinal direction, as shown in the figure. With the axle shown in the picture a vertical adjustment is intended at the end of the bars.