Measuring vehicle drag forces using an on-board microcomputer

The force resisting the straight-line motion of a road vehicle on a flat track consists of mechanical resistance and aerodynamic drag. It is necessary to measure these component forces over the speed range of the vehicle to obtain data for performance calculations and for the accurate calibration of modern chassis dynamometers used in fuel economy and exhaust emission tests. Continuous measurement of wheel torque, vehicle speed, airspeed and direction enables estimates to be made of the various drag forces that bring the vehicle to rest during a coastdown test. The data are analysed using a sophisticated parameter optimization routine to determine the numerical values of the coefficients in a mathematical model of the total road load. Repeatability of the results are assessed by calculating the uncertainties in the values obtained from a number of track tests. A brief description of the on-vehicle data acquisition and processing system is given, and the advantages of an advanced test and analytical process are demonstrated.