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There are many aspects of the unsteady flow around fastback passenger cars that remain to be understood. These include the source and nature of unsteady flow structures, the relevant time-scales, the effect of geometric parameters and the impact of the unsteadiness in terms of steady and unsteady forces on the vehicle. This paper investigates large scale unsteady structures in the wake of the Ahmed form and of a scale model of a real car shape using two wind tunnels and model scales between 12.5% and 40%. The unsteadiness demonstrated only low coherence and weak periodicity and the Strouhal number of a given structure varied from tunnel to tunnel indicating a high sensitivity to external influences. Nevertheless, a novel visualization technique, used to display the results of time-accurate pressure probe measurements, was able to reveal structures involving both symmetric and anti-symmetric oscillations in the strength of the rear-pillar vortices.

Side force has an influence on the behaviour of passenger cars in windy conditions. It increases approximately linearly with yaw angle over a significant range of yaw for almost all cars and the side force derivative, (the gradient of side force coefficient with yaw angle), is similar for vehicles of a given category and size. The shape factors and components which affect side force for different vehicle types are discussed. The dominant influence on side force, for most cars, however, is shown to be the vehicle height which is consistent with slender wing theory if the car and its mirror image are considered. This simple theory is shown to apply to 1-box and 2- box shapes, covering most MPVs, hatchbacks and SUVs, but does not adequately represent the side forces on notchback and fastback car shapes. Data from simple bodies is used to develop a modification to the basic theory, which is applied to these vehicle types.