A Novel Method for Evaluating on-Road Drag Changes Using Constant Power Measurements

This paper introduces a new approach for measuring changes in drag force across different vehicle configurations using an on-road testing technique. The method involves fixing the vehicle’s power across configurations and then measuring the resulting speed differences. A detailed formulation is provided on how these speed variations can be used to calculate the change in drag force for each configuration. The OBD II port is used to access and record additional data necessary for the calculations. The method is applied to both a passenger car and a commercial van to evaluate drag changes for different vehicle add-ons. A roof sign was installed at various positions along the roof of the vehicles to assess drag increases, while novel rear appendages were fitted to both vehicles to evaluate the resulting drag reductions. Detailed CFD simulations were performed on the road-tested configurations to compare the simulated drag changes with those measured on the road. Excellent agreement was found with the CFD results for both vehicles, with particularly strong correlation observed for the passenger car’s configurations. Finally, the paper describes how the method can be adapted to measure the baseline drag coefficient of a given vehicle. This involves attaching an elevated body to the vehicle’s towbar, which adds a known drag force to the setup. By measuring the speed change for the configured vehicle on the road, the baseline drag coefficient can be estimated.