The effect of unsteady aerodynamics on the motion of a heavy duty truck subjected to sudden crosswinds was analyzed in vehicle-dynamics simulations. Large eddy simulation based on high-performance computing (LES-HPC) was applied to evaluate the effect of unsteady external forces on vehicle motion as a first step toward a more reliable vehicle motion analysis. Before the vehicle-dynamics simulations, the steady and unsteady aerodynamics of a simplified model of a heavy truck developed in our first report were analyzed by HPC-LES for various aerodynamic yaw angle conditions. On the basis of these aerodynamic analyses, two vehicle-dynamics simulations were conducted for transient crosswind conditions. One simulation was coupled with unsteady aerodynamic forces and the other applied a conventional approach with quasi-steady aerodynamics. Their numerical results, such as the trajectory of the vehicle, were compared and the unsteady-aerodynamics effects on vehicle stability or drivability were investigated. For strong, sudden crosswind at an aerodynamic yaw angle up to 45°, the unsteady aerodynamics significantly affected vehicle motion, and the importance of considering the unsteady aerodynamic effect on the truck's motion was apparent. The effect was clearer for the yaw motion of the truck because of the strong unsteadiness of the yawing moment. The unsteady aerodynamics also changed the trajectory of the truck because of the changing yaw motion and unsteady aerodynamic lateral force. The amplitude of the effect depended on the crosswind intensity and also on the truck's load. The effect of unsteady aerodynamics was more significant under a no-load condition than under a full-load condition.