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 to each other and to the forecarriage frame by means of a first kinematic mechanism which enables the front wheels (10',10") to roll in a synchronous and specular manner, each wheel (10',10") being connected to said first kinematic mechanism by means of a respective axle journal, said axle journal being mechanically connected to a rotation pin of the wheel in order to support it rotatably around an axis of rotation (R'-R', R"-R"), - a roll block system, - suspension means which guarantee each axle journal at least one spring suspension movement with respect to said first kinematic mechanism, - a steering device (36, 86) suitable to command the rotation of the axle journals around respective steering axes (S'-S',S"-S") of each front wheel (10', 10"), characterised in that said roll block system comprises a second kinematic mechanism which directly connects the two front wheels (10',10") to one another at the respective axle journals, by means of hinging means (71, 72, 73), between two connection points the distance of which varies in the case of rolling of the two front wheels (10', 10") but does not vary on steering, said second kinematic mechanism being susceptible to adopt at least two configurations: - a free configuration, wherein said second kinematic mechanism is configured to passively follow the movements of said two wheels (10',10") with respect to each other and with respect to the forecarriage frame without interfering with them, and - a blocked configuration, wherein said second kinematic mechanism is configured to set the distance between said two points thus preventing rolling movements between the two wheels and at the same time allowing the pitching and steering movements; and said roll block system comprising a control device of the configuration of the second kinematic mechanism which is suitable to act on the second kinematic mechanism to take it from said free configuration to said blocked configuration and vice versa. - a forecarriage frame, - at least one pair of front wheels (10',10") kinematically connected to each other and to the forecarriage frame by means of a first kinematic mechanism which enables the front wheels (10',10") to roll in a synchronous and specular manner, each wheel (10',10") being connected to said first kinematic mechanism by means of a respective axle journal, said axle journal being mechanically connected to a rotation pin of the wheel in order to support it rotatably around an axis of rotation (R'-R', R"-R"), - a roll block system, - suspension means which guarantee each axle journal at least one spring suspension movement with respect to said first kinematic mechanism, - a steering device (36, 86) suitable to command the rotation of the axle journals around respective steering axes (S'-S',S"-S") of each front wheel (10', 10"), - a free configuration, wherein said second kinematic mechanism is configured to passively follow the movements of said two wheels (10',10") with respect to each other and with respect to the forecarriage frame without interfering with them, and - a blocked configuration, wherein said second kinematic mechanism is configured to set the distance between said two points thus preventing rolling movements between the two wheels and at the same time allowing the pitching and steering movements; and