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Over the past decades substantial advances have been made in reducing the drag of automobiles. As might have been expected, the early performance gains were relatively easy but future gains are becoming increasingly difficult to achieve. A diagnostic method which is both quantitative and independent of the model is sorely needed. This paper discusses experimental measurements of wakes behind. 154-scale model production cars and comparisons to balance measurements. Models, test equipment, and data analysis are discussed. Explanations of the wake flow results in relation to the car modifications are included. Correlation of the wake drag integral to the balance drag measurement is good; however, some questions remain relative to the influence of fixed-ground skin friction.

Wind tunnel force-balance and wake-traverse tests were made on .154-scale car models with various degrees of streamlining to determine the significance of ground treatment for increasing levels of aerodynamic cleanness. The wake-traverse analysis included investigations of spanwise distributions of vortex and viscous drag, which gave insight into the flow mechanisms involved. It was found from these tests that thick, uncontrolled tunnel-floor boundary layers yielded wakes with viscous “side-lobes” at floor level, which were absent with a moving ground representation. For “bluff car designs, this was the only effect. For “slippery” designs, more typical of modern design practice, other more significant changes were noted. Measured changes in trailing-vortex strength and the associated vortex drag suggested that very low-drag designs experience an increase in effective angle-of-attack when the ground is fixed.