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. Specifically, this can be done by using a spreadsheet which allows parameter identification by fitting routines applied to a “user defined function” (the latter being obtained from the solution of the equation of motion for the vehicle, including properly defined biased and randomly disturbed environmental conditions). Fitting routines, such as given by Levenberg-Marquardt and others are part of some spreadsheets. Thus, information on tire drag and aerodynamic drag contributions can be promptly evaluated at the end of each coast down run. Special concern arises from the sensitivity of aerodynamic drag determination under- even light- prevailing winds. It is shown how to account for such disturbances by introducing the concept of “effective” wind correction by conducting coast down runs in opposing directions. Additional difficulties arise due to the lack of separability of the drag parameters (speed dependency of tire drag); in which case one has to use information provided by tire manufacturers.
The efficiency of different data processing methods is tested by the recovery of drag components from a generating program, which simulates coast down subjected to biased and randomly disturbed conditions. Examples of actual coast down runs with 18 wheelers and their road load evaluations are given to demonstrate the capability of the approach.
Since heavy trucks and, especially 18-wheeler rigs come in a great variety of configuration, selection of a reference (frontal) area for extracting drag coefficients may produce misleading results; as one observes that all important criteria for truck operation depend on the product A0 times CD, it is conventional to define this product as Drag Area, as the most appropriate criterion for aerodynamic performance.