Forecarriage of a rolling motor vehicle with three or four wheels, comprising: - a forecarriage frame, - at least one pair of front wheels (10") kinematically connected one to each other and to the forecarriage frame by means of kinematic rolling mechanism which enables the front wheels (10")to roll in a synchronous and specular manner, each wheel (10") being connected to said kinematic rolling 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"), - suspension means which provide each axle journal at least one spring suspension movement with respect to said kinematic rolling mechanism, - a steering device (36, 86) that kinematically connects the axle journals one to each other so as to command the rotation of the axle journals about respective steering axes (S'-S', S"-S") of each front wheel (10"), - a roll block system, wherein said steering device comprises a horizontal steering bar, said steering bar being in turn hinged to a steering column at an intermediate point between its two ends so as to oscillate with respect to said steering column about a first rotation axis (Z-Z) substantially orthogonal to a rolling plane of the two front wheels (10") defined by said kinematic rolling mechanism, and in that the roll block system comprises releasable blocking means adapted to reversibly block the lying position of said steering bar with respect to a vertical projection plane, which is transverse to a centreline plane (M-M) of the motor vehicle, wherein the lying position of the steering bar with respect to a vertical projection plane, which is transverse to a centreline plane M-M of the motor vehicle, means the angle formed by the projection of the steering bar on said vertical plane, when said releasable blocking means are deactivated said steering bar being free to change its lying position with respect to said projection plane passively following the movements of said two wheels (10") with respect to each other and to the frame without interfering with them except to control the steering, when said releasable blocking means are activated said steering bar being prevented from changing its lying position with respect to said projection plane in this way constraining the relative position between the two axle journals, characterised in that the steering bar directly connects the two axle journals one to each other at its two ends by means of hinging means (72, 73). - a forecarriage frame, - at least one pair of front wheels (10") kinematically connected one to each other and to the forecarriage frame by means of kinematic rolling mechanism which enables the front wheels (10")to roll in a synchronous and specular manner, each wheel (10") being connected to said kinematic rolling 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"), - suspension means which provide each axle journal at least one spring suspension movement with respect to said kinematic rolling mechanism, - a steering device (36, 86) that kinematically connects the axle journals one to each other so as to command the rotation of the axle journals about respective steering axes (S'-S', S"-S") of each front wheel (10"), - a roll block system, wherein said steering device comprises a horizontal steering bar,