An air spring for a cab of a heavy truck with automatic height adjustment, in which the air spring is provided between the cab and a frame of a truck and supports the cab with a pressure of air filling in the air spring, includes a canister in which an internal space is formed, and a piston movably mounted in the internal space of the canister and configured to ascend and descend relative to the canister. A port opening unit is provided adjacent to an intake port and an exhaust port of the piston, respectively. When the cab ascends or descends, each of the port opening units comes into contact with guides, each having an inclined surface, formed in the canister to open either the intake port or the exhaust port.

Aspects of the disclosure relate to a mechanical joint with five degrees of freedom. In certain embodiments, the mechanical joint includes first and second triangular linkages. The first triangular linkage includes a base end configured to hingedly couple to a first body to pivot relative to the first body and a vertex end that includes a first rotational member. The second triangular linkage includes a base end configured to hingedly couple to a second body to pivot relative to the second body and a vertex end that includes a second rotational member. The first rotational member and the second rotational member are rotationally coupled to form a all joint. With this joint, the second body is moveable in two translational degrees of freedom and restricted in one translational degree of freedom relative to the first body. Such a configuration allows vertical movement and/or reduces stress on the joint.

Aspects of the disclosure relate to a mechanical joint with five degrees of freedom. In certain embodiments, the mechanical joint includes first and second triangular linkages. The first triangular linkage includes a base end configured to hingedly couple to a first body to pivot relative to the first body and a vertex end that includes a first rotational member. The second triangular linkage includes a base end configured to hingedly couple to a second body to pivot relative to the second body and a vertex end that includes a second rotational member. The first rotational member and the second rotational member are rotationally coupled to form a all joint. With this joint, the second body is moveable in two translational degrees of freedom and restricted in one translational degree of freedom relative to the first body. Such a configuration allows vertical movement and/or reduces stress on the joint.

A trailer convertible between step deck trailer and flatbed trailer configurations includes a frame, wheel and axle assemblies supported on the frame, an upper deck at the front of the frame, a lower deck configured for vertical movement on a vertical slide rail from a first step deck position to a second raised flatbed position wherein the lower deck and the upper deck are coplanar; a controller actuatable for upward and downward movement of the lower deck; at least one locking support beam; and landing gear deployable to support the convertible trailer while it is parked for loading. An air lift jack assembly is configured to raise the lower deck, and an air pressure release valve is configured to release air pressure to lower the lower deck, in response to actuation of the controller. The wheel and axle assemblies may be configured for forward-backward movement and/or upward-downward movement.

A trailer convertible between step deck trailer and flatbed trailer configurations includes a frame, wheel and axle assemblies supported on the frame, an upper deck at the front of the frame, a lower deck configured for vertical movement on a vertical slide rail from a first step deck position to a second raised flatbed position wherein the lower deck and the upper deck are coplanar; a controller actuatable for upward and downward movement of the lower deck; at least one locking support beam; and landing gear deployable to support the convertible trailer while it is parked for loading. An air lift jack assembly is configured to raise the lower deck, and an air pressure release valve is configured to release air pressure to lower the lower deck, in response to actuation of the controller. The wheel and axle assemblies may be configured for forward-backward movement and/or upward-downward movement.