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A significant reduction of the aerodynamic drag can be achieved by the refinement of the vehicle underside. But the investigation of the aerodynamic effectiveness of underfloor panels in wind tunnels turns out to be a challenging task. For this reason on-road methods like coastdown analysis are frequently employed. Most of these advanced coastdown methods determine the coefficients of the total drag by fitting a mathematical model to the measured velocity data. This requires separate time consuming tests in the laboratory to measure the wheel and transmission losses. The objective of the presented work was the development of an analysis method, which enables a simpler and more practicable on-road determination of aerodynamic improvements. In this method the rolling resistance can be factored out by employing the conservation of energy as basic approach for the investigation of air drag differences.

The objective of this work is an investigation of the aerodynamic improvements by e.g. the application of underfloor panels that might be masked by traditional, fixed-ground wind tunnel testing. Of prime importance is the simplicity and practicability of the developed analysis method, with particular emphasis on industrial application. A coastdown method with minimal instrumentation effort is chosen to determine drag coefficients on the road. Usually such experiments demand the measurement of vehicle velocity, airspeed, yaw angle and some additional environment characteristics like temperature and air pressure. However, in principle it is only required to measure the vehicle's speed-time history during the test, from which the road load results can be derived by mathematics of inversion. In this work several underfloor configurations of the test vehicle are investigated by only considering the motorcar's speed data retrieved from the control area network data bus during the coastdown.