A mechanical system for automatic levelling for a vehicle with arms, The vehicle with arms comprising a chassis substantially symmetrical relative to a median plane, at least one foot, at least one wheel and at least one arm configured to allow a user to lift said at least one foot relative to the ground in order to be able to move the vehicle by making it to roll on said at least one wheel, the chassis comprising a first part, termed front part, configured to support said at least one wheel and a second part, termed rear part, from which said at least one foot and said at least one arm extend, the system comprising three bars (11, 12, 13) arranged under the chassis of the vehicle with arms and each extending from a mechanical trunk node including a first bar extending from the node to a distal end forming a first foot of the vehicle, a second bar extending from the node to a distal end forming a second foot of the vehicle and a third bar extending from the node to a distal end configured to be secured by a mechanical connection to the first part of the chassis of the vehicle, the system further comprising at least an automatic locking and unlocking device between the first and second bars (11, 12) and the chassis, the device being secured to the second part of the chassis of the vehicle and being configured to: - keep in a suspended position said first and second bars (11, 12) when the first and second feet (221, 222) are raised relative to the ground, - allow a rotational movement and vertical translational motion of said first and second bars (11, 12) relative to the chassis when the first and second feet (221, 222) are brought to the ground, where necessary successively, and - keep the chassis in a position where it rotation is locked relative to the first and second bars (11, 12) when the first and second feet (221, 222) are resting on the ground, the three bars (11, 12, 13) extend inseparably from the trunk node, the mechanical connection between the distal end of the third bar and the first part of the chassis is a spherical joint, the system characterised in that the locking and unlocking device is configured to further allow a limited lateral movement of the trunk node when the first and second feet (221, 222) are brought to the ground, where necessary successively. the system comprising three bars (11, 12, 13) arranged under the chassis of the vehicle with arms and each extending from a mechanical trunk node including a first bar extending from the node to a distal end forming a first foot of the vehicle, a second bar extending from the node to a distal end forming a second foot of the vehicle and a third bar extending from the node to a distal end configured to be secured by a mechanical connection to the first part of the chassis of the vehicle, the system further comprising at least an automatic locking and unlocking device between the first and second bars (11, 12) and the chassis, the device being secured to the second part of the chassis of the vehicle and being configured to: - keep in a suspended position said first and second bars (11, 12) when the first and second feet (221, 222) are raised relative to the ground, - allow a rotational movement and vertical translational motion of said first and second bars (11, 12) relative to the chassis when the first and second feet (221, 222) are brought to the ground, where necessary successively, and - keep the chassis in a position where it rotation is locked relative to the first and second bars (11, 12) when the first and second feet (221, 222) are resting on the ground, - keep in a suspended position said first and second bars (11, 12) when the first and second feet (221, 222) are raised relative to the ground, - allow a rotational movement and vertical translational motion of said first and second bars (11, 12) relative to the chassis when the first and second feet (221, 222) are brought to the ground, where necessary successively, and - keep the chassis in a position where it rotation is locked relative to the first and second bars (11, 12) when the first and second feet (221, 222) are resting on the ground, the three bars (11, 12, 13) extend inseparably from the trunk node, the mechanical connection between the distal end of the third bar and the first part of the chassis is a spherical joint, the system characterised in that the locking and unlocking device is configured to further allow a limited lateral movement of the trunk node when the first and second feet (221, 222) are brought to the ground, where necessary successively.