1997-02-24

Coast Down Method in Time-Distance Variables 970408

The differential equation of motion of a freely coasting vehicle is analysed and its integral solution in the form of an explicit speed-time function and then a distance-time one is derived. The latter form eliminates the traditional speed term from the equation. By making speed or deceleration measurements unnecessary, a whole set of potential error sources is eliminated, thus improving the sensivity and accuracy of the coast down method. By replacing the distance measurements with precision test track markings, the coasting data requirements are reduced to accurate measurements of time only. This simplifies both the method and the test facility, and improves the reliability of the test data. Because of the efficiency of this a significant volume of test data has been gathered, and included in this report, which characterizes vehicle aerodynamic drag and rolling resistance. In addition to smaller vehicles, these data also include trucks, buses, and articulated vehicles in production and under development which are too large and too heavy to be tested in wind tunnels or on tire dynamometers. For the smaller vehicles, road-to-tunnel and road-to-dynamometer correlations are also presented.

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