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Coast down testing with full-scale vehicles on level and inclined roads offers an inexpensive approach to road load determination and, in particular, aerodynamic force evaluation, provided that drag component extractions can be accurately achieved under random instrumental disturbances and biased environmental conditions. Wind tunnel testing of large vehicles, especially truck/trailers, to establish their aerodynamic drag is costly and also may produce questionable results when the effects of the moving road, blockage, wake/diffuser interaction, and rotating tires are not properly simulated. On the road, testing is now conveniently and speedily carried out using GPS-based data acquisition and file storage on laptops, allowing instantaneous on-board data processing.

Hydroplaning behavior of a single tire running in stationary, undisturbed water of constant depth is a well-studied phenomenon, and has been examined both theoretically and experimentally. Most experimental tire studies have been conducted on drum or flat-track test machines or with towed tires, and correlative expressions for hydroplaning of a single tire have been developed from such tests. Vehicle testing, on the other hand, has typically involved full-scale, proving ground experiments in which gross vehicle motion and behavior were of interest without regard to individual tire contributions. In the present work, we examine the behavior of a vehicle with rear tires running in a path partially cleared by the front tires. Under such conditions, it can no longer be assumed that the rear tires are experiencing the same hydrodynamic conditions as the front tires, nor does their behavior correlate well with conditions obtained from individual tire testing.