The transient aerodynamic forces on automotive vehicles traveling in close proximity to each other are investigated in a wind tunnel. Scale vehicle models are longitudinally aligned in a "platoon" configuration with various separation distances between the models. One model is actuated such that it is capable of lateral motion at several accelerations with respect to the platoon. The location of the mobile model in the platoon is varied to simulate various lane changing maneuvers of full scale vehicles. The drag force, side force, and yawing moment are measured with strain gage balances to quantify the transient interactions of the vehicle flow fields. Several important general trends can be observed. The drag coefficients for the vehicles in a platoon are lower than that of a single vehicle, and the coefficients decrease with smaller vehicle spacings. In addition, the drag coefficients on the remaining vehicles increase as a vehicle leaves the platoon. The side force coefficients also increase as a vehicle leaves the platoon. The increase in the side force is on the same order as the increase in the drag. The side force also increases with decreasing vehicle separation. Although the yawing moment is small and, thus, difficult to quantify, the moment also reflects influences of the transient flow field. A significant conclusion of the results is that the transient forces differ depending upon whether a vehicle leaves or joins a platoon. This hysteresis in the flow dynamics is not observable in steady-state measurements. These results also apply to the more general case of flow over bluff bodies,
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