Visual inspections of selected semitrailers during routine equipment checks revealed that the kingpin bent in the direction of 180 degrees from the direction that the semitrailer is towed. Confirmation from semitrailer repair facilities found that in some cases the semitrailer’s supporting structure developed unexpected cracks. These cracks were not thought to be age related but were most likely caused by high stresses from unknown high loads. In an effort to determine the forces at the kingpin and fifthwheel, TruckSim® modeling and simulation software was utilized to predict the forces in all three directions during various operating maneuvers. Computer simulations suggest the largest forces are experienced during coupling operations as opposed to severe maneuvering or braking. The development of a Finite Element Analysis (FEA) model of the tractor-semitrailer coupling determined that high coupling speeds would overload the kingpin-fifthwheel structure. The FEA model also allowed researchers to determine that a damping system would lower the forces at the kingpin-fifthwheel interface to the magnitude of forces experienced during normal operations. A literature search found no valid documented tests, and determined the SAE J133 kingpin loading requirements were incorrect.

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