Forecarriage of a rolling motor vehicle with three or four wheels, comprising: - a forecarriage frame, - at least one pair of front wheels (10',10") kinematically connected one to each other and to the forecarriage frame by means of a kinematic rolling mechanism which enables the front wheels (10', 10") to roll in a synchronous and specular manner, each wheel (10',10") being connected to said kinematic rolling mechanism by means of a respective axle journal being mechanically connected to a rotation pin of a respective wheel (10',10") in order to rotatably support said respective wheel (10',10") around a rotation axis (R'-R', R"-R"), - a roll block system, - suspension means which provide each axle journal at least one spring suspension movement with respect to said kinematic rolling mechanism, - a steering device suitable to command the rotation of the axle journals about respective steering axes (S'-S', S"-S") of each front wheel (10', 10"), said roll block system comprises a length extensible rod, characterised in that said length extensible rod directly connects one of said two front wheels (10', 10") in correspondence of the respective axle journal to said forecarriage frame by means of hinging means (71, 72, 73) at both ends of said length extensible rod, said forecarriage frame being suitable to follow said respective axle journal in rolling movements which are allowed by said kinematic rolling mechanism, said length extensible rod being suitable to passively follow the movements of said respective axle journal and of said forecarriage frame by varying its orientation in space with respect to them, and in that said roll block system comprises reversible blocking means (120, 130) which are suitable to reversibly block the lying position of said extensible rod on a rolling plane, with respect to said respective axle journal and/or with respect to said forecarriage frame, so as to be suitable to block the rolling movements allowed by said kinematic rolling mechanism, said lying position being defined by the angle (α) formed by the projection of the rod on a vertical projection plane with a reference plane orthogonal to the rotation axis (R'-R', R"-R") of the wheel associated to said respective axle journal, wherein said vertical projection plane is transverse to a centreline plane (M-M). - a forecarriage frame, - at least one pair of front wheels (10',10") kinematically connected one to each other and to the forecarriage frame by means of a kinematic rolling mechanism which enables the front wheels (10', 10") to roll in a synchronous and specular manner, each wheel (10',10") being connected to said kinematic rolling mechanism by means of a respective axle journal being mechanically connected to a rotation pin of a respective wheel (10',10") in order to rotatably support said respective wheel (10',10") around a rotation axis (R'-R', R"-R"), - a roll block system, - suspension means which provide each axle journal at least one spring suspension movement with respect to said kinematic rolling mechanism, - a steering device suitable to command the rotation of the axle journals about respective steering axes (S'-S', S"-S") of each front wheel (10', 10"), said roll block system comprises a length extensible rod, characterised in that said length extensible rod directly connects one of said two front wheels (10', 10") in correspondence of the respective axle journal to said forecarriage frame by means of hinging means (71, 72, 73) at both ends of said length extensible rod, said forecarriage frame being suitable to follow said respective axle journal in rolling movements which are allowed by said kinematic rolling mechanism, said length extensible rod being suitable to passively follow the movements of said respective axle journal and of said forecarriage frame by varying its orientation in space with respect to them,