This study investigated the effect of unsteady aerodynamic forces on the running stability of a road vehicle during pitching motion and lane-change maneuvering. A Large-Eddy Simulation with the moving boundary method was used to predict the time-dependent flow around the vehicle in transient motion and its surface pressure distribution. The focus was on the effect of aerodynamic parts. This effect was previously evaluated in an on-road test, and the corresponding differences in the vehicle’s motion during lane-change maneuvering have already been confirmed. In this study, a pitching or lane-change motion was imposed on a simplified vehicle model with or without aerodynamic parts, and the corresponding transient aerodynamic forces acting on the vehicle were investigated. The results indicated that there was a significant difference in aerodynamic moments between the models with and without the aerodynamic parts. It was found that the responding pitching moment works to damp the pitching motion, and the rolling moment works to prevent the rolling motion during the lane change. Moreover, in both these cases, the aerodynamic parts helped to stabilize the motion. The flow mechanisms of the stabilizing pitching or rolling motions were also analyzed.Copyright © 2013 by ASME