Motor-vehicle rear suspension system, comprising: - a pair of longitudinal arms, having front ends carrying swivel joints for the articulated connection to a vehicle supporting structure and rear ends carrying respective wheel supports, - a cross-member in the form of a main tubular element, having opposite ends configured to be rigidly connected to the two longitudinal arms, wherein said main tubular element and said longitudinal arms are configured in such a way that said main tubular element can be rigidly connected to the longitudinal arms in any of two selectable mounting positions: - a first mounting position, in which said main tubular element is rigidly connected to the longitudinal arms at a position substantially coaxial with the wheel supports so as to define a rigid axle suspension, - a second mounting position, in which said main tubular element is rigidly connected to the longitudinal arms at positions intermediate between the front ends and the rear ends of the longitudinal arms, so as to define a twist axle suspension, wherein each wheel support is rigidly connected to the respective longitudinal arm through a respective auxiliary tubular element, having an outer end projecting from an outer lateral surface of the longitudinal arm and rigidly connected to the wheel support and an inner end projecting from an inner lateral surface of the longitudinal arm, wherein said first mounting position said main tubular element has its opposite ends rigidly connected to the inner ends of the two auxiliary tubular elements, wherein each longitudinal arm has a main body of sheet-metal and an auxiliary body also made of sheet-metal and configured to be welded to the main body only in the twist axle suspension configuration, so that said main body and said auxiliary body respectively define an outer lateral wall of the longitudinal arm and an inner lateral wall of the longitudinal arm, which are spaced apart from each other, and wherein in said second mounting position of the main tubular element corresponding to the twist axle suspension configuration, said main tubular element has end portions each welded to the inner lateral wall of the respective longitudinal arm. - a pair of longitudinal arms, having front ends carrying swivel joints for the articulated connection to a vehicle supporting structure and rear ends carrying respective wheel supports, - a cross-member in the form of a main tubular element, having opposite ends configured to be rigidly connected to the two longitudinal arms, wherein said main tubular element and said longitudinal arms are configured in such a way that said main tubular element can be rigidly connected to the longitudinal arms in any of two selectable mounting positions: - a first mounting position, in which said main tubular element is rigidly connected to the longitudinal arms at a position substantially coaxial with the wheel supports so as to define a rigid axle suspension, - a second mounting position, in which said main tubular element is rigidly connected to the longitudinal arms at positions intermediate between the front ends and the rear ends of the longitudinal arms, so as to define a twist axle suspension, - a first mounting position, in which said main tubular element is rigidly connected to the longitudinal arms at a position substantially coaxial with the wheel supports so as to define a rigid axle suspension, - a second mounting position, in which said main tubular element is rigidly connected to the longitudinal arms at positions intermediate between the front ends and the rear ends of the longitudinal arms, so as to define a twist axle suspension, wherein each wheel support is rigidly connected to the respective longitudinal arm through a respective auxiliary tubular element, having an outer end projecting from an outer lateral surface of the longitudinal arm and rigidly connected to the wheel support and an inner end projecting from an inner lateral surface of the longitudinal arm, wherein said first mounting position said main tubular element has its opposite ends rigidly connected to the inner ends of the two auxiliary tubular elements, wherein each longitudinal arm has a main body of sheet-metal and an auxiliary body also made of sheet-metal and configured to be welded to the main body only in the twist axle suspension configuration, so that said main body and said auxiliary body respectively define an outer lateral wall of the longitudinal arm and an inner lateral wall of the longitudinal arm, which are spaced apart from each other, and wherein in said second mounting position of the main tubular element corresponding to the twist axle suspension configuration, said main tubular element has end portions each welded to the inner lateral wall of the respective longitudinal arm.