an axle body having axially opposed ends provided with a respective wheel hub hinged about a respective first steering axis, said axle body being in its turn hinged on to a bearing structure of the vehicle by means of an articulated joint spaced from the axle body for an angular displacement of said axle body with respect to the bearing structure about a second steering axis,

steering means acting between said wheel hubs and the axle body for steering said hubs about said first steering axis, said steering means including means for angularly displacing said axle body about the second steering axis as a consequence of the relative displacement between the wheel hubs and the axle body,

vertical securing means extending vertically between the axle body and the bearing structure in order to hold said axle body secured vertically to the bearing structure during said lateral displacement, wherein said vertical securing means comprise an extensible member, the extension of which is correlated to the position of the axle body with respect to the second steering axis.