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An improved coast down method that incorporates the input of data measured with an onboard anemometer into the governing equation of motion is used to evaluate models representing the aerodynamic and mechanical drag characteristics of a wide variety of vehicles. Results from the application of the improved technique are presented to demonstrate the repeatability and precision of the analyzed aerodynamic drag coefficients, to address the merits of different wind tunnel blockage correction methods, to illustrate the method's usefulness in the study of tractor-trailer and light truck-travel trailer aerodynamic drag, to evaluate the influence of floor blowing in wind tunnel studies of the effects of various changes to the underbody of an automobile, and to compare track and dynamometer determinations of the variation of mechanical drag with speed on a vehicle equipped with a four-speed automatic transmission.

The problem of evaluating the fuel savings benefit of a truck aerodynamic drag-reducing device in an over-the-road test is addressed. A demonstration of the limitations of current SAE short-term test methods led to the development of a modified, wind-monitoring-based procedure which facilitates a more fundamental analysis of the test data from which a more meaningful estimate of the long-term fuel savings benefit can be derived. Sample results from the application of the new test and analysis procedure are reported to demonstrate the usefulness of the improved test technique. The procedure also provides findings that are of interest in wind-tunnel-to-road correlation exercises.

Wind tunnels which are large enough for full-scale trucks are rare, and the cost of satisfactorily-detailed models for smaller tunnels is high. The work presented shows the results from the application of a method which provides an over-the-road evaluation of the incremental changes in fuel consumption and drag coefficient produced following the addition of a variety of aerodynamic drag reducing devices to a tractor-trailer truck combination. The devices tested were an aerodynamic sunvisor, a roof-mounted air deflector, cab extenders, cab skirts, a trailer nose fairing, a set of trailer quads (quarter-rounds), and trailer skirts which were mounted on a low-forward-entry tractor and high box-van trailer. The significant differences between the wind tunnel and on-road drag reductions suggest that the effects of on-road wind turbulence can substantially reduce the wind tunnel results even though a 1.5% turbulence intensity level was used in the tunnel experiments.