Modern heavy-duty trucks are typically equipped with beam axles and leaf spring suspensions. Leaf springs provide a simple and economical assembly for locating and controlling articulation of the axle. Within available design space, it is often not possible to design a spring that has both the necessary load capacity and a spring rate low enough for optimal ride comfort. Also, because the roll stiffness produced by a pair of leaf springs derives mostly from their vertical stiffness, leaf springs having low spring rates may not provide adequate roll stiffness. This requires the use of supplementary roll stiffening mechanisms, such as anti-roll bars.
Air springs are commonly used on the rear axle suspensions of heavy trucks, in part because it is easier to achieve low spring rates with air springs than other types of springs. However, air springs provide no axle locating or structural functionality. As such, other assemblies for locating and controlling axle motion must be provided.
On rear axle air suspensions, various types of linkages are used to locate and control the axle motion. In most cases, lateral positioning of the axle includes the use of a transverse rod that has one end pivotally connected to the axle and the other end pivotally connected to the truck frame. Alternatively a V-link structure may be used to react lateral loads. Vehicles with front mounted engines precludes the use of these types of linkages for front axle air suspensions. Hence, lateral loads must be reacted through other types of assemblies.
U.S. Pat. No. 4,856,812, issued to Stephens et al., the disclosure of which is hereby expressly incorporated by reference, describes one approach to incorporating air springs into a front suspension to achieve a low vertical spring rate along with a linkage that locates the axle and resists lateral loads. A pair of stiff beams pivotally connects the axle to forward pivots. Two air springs are interposed between each beam and the vehicle frame and support the sprung mass. A “double shackle” linkage is used at the rear of the beam to allow the rear of the beam to move vertically without restraint. Lateral loads applied to the axle are reacted through the forward ends of each beam at the forward pivots and through the linkages at the rear of each beam at the rear frame brackets.
A portion of the suspension roll stiffness is provided by the roll couple produced by the pairs of air springs on each side of the vehicle as the chassis rolls relative to the axle. Supplemental roll stiffness is generated from the flexural stiffness of the forward ends of the beams and the torsional stiffness of the axle which together act as an anti-roll bar under roll deflection. Some additional roll stiffness is produced from direct twisting of the beams themselves.
A disadvantage of this design is the introduction of additional linkages and pivots to the rear of each beam. The pivots located at the ends of the beams are particularly troublesome since they must support a bending moment when the axle is subjected to lateral loads, introducing the potential for wear and lash in the linkage.