The suspension system of the present invention broadly comprises a method of retrofitting an existing torsion axle suspension carried by a frame with an air spring comprising the steps of removing an original torsion axle that was fixed to a frame from the frame and mounting the torsion axle to at least one rotatable arm such that rotation of the arm causes vertical translation of the original torsion axle. The suspension system also comprises a retrofit kit for use with a torsion axle suspension system having a torsion axle and a spindle comprising a hanger bracket adapted to be attachable to the frame, a pivot arm for mounting to the hanger bracket and adapted for rotatably supporting the existing torsion axle, an air spring, a support assembly secured to the pivot arm for supporting the air spring, and an attachment for securing the pivot arm to the support assembly.