A semi-independent suspension system adapted for a rear or front drive and optionally steerable axle system of a low-floor heavy vehicle, comprising a twist beam axle assembly (1), a Panhard rod (8), and a pair of spring and shock absorber systems (9). The twist beam axle assembly (1) comprises a pair of rigid longitudinal trailing arms (3), a pair of wheel mounting members to lower the cross-beam relative to the wheel axis (2) and a straight cross-beam (7). The cross-beam is set in the right and left trailing arms and the wheel mounting members by an interference fit technique. The cross-beam (7) has an open-section and is rigid in bending but flexible in torsion. The shape of the cross-section is specifically tailored to increase and control each of the vertical and horizontal bending stiffness (7). The cross-section is characterized by having horizontal and vertical lines of symmetry, but with asymmetric horizontal and vertical geometries. The combination of features of the twist beam axle assembly (1) provides for a full flat floor without slope or extra height over the axle.