The measurement and analysis of road vehicle drag forces
Accurate measurement of a vehicle's resistance to motion on a road (the 'road load'), and the separation of this resistive force into its contributory components is of fundamental importance to generate the data required for vehicle performance assessment, the calibration of a modem chassis dynamometer and for comparing the drag of different vehicles or vehicle configurations. Established methods of determining road load on a test track are the coastdown and steady state torque tests, but environmental variability (largely due to ambient wind) and differences in the vehicle operating conditions cause wide variation in the results. This thesis describes a comprehensive study into methods of acquiring and analysing road load data at a test track. A mathematical model of the vehicle travelling in a straight line, in the presence of ambient wind, is developed and may be applied to measured data obtained in both the coastdown and the steady state test modes. The model includes the aerodynamic drag, tyre losses, transmission and un-driven wheel losses and the variation of aerodynamic drag with yaw angle. Experimental data obtained at a test track, using advanced instrumentation (including on-board anemometry and wheel torque meters) are analysed to obtain estimates of the coefficients in the road load equation. The results from an initial study demonstrate the importance of measuring the local wind at the test vehicle and the transmission losses if the total drag is to be accurately measured and separated into its contributory components. The coastdown method is shown to be more accurate and repeatable than the steady state method, and is therefore used as the basis of an advanced test procedure. Up to four coefficients can be determined from the coastdown data using a parameter optimisation routine. This routine fits the mathematical model to the measured coastdown profile to obtain estimates of the road load coefficients including the variation of aerodynamic drag coefficient with yaw angle. Results using this analytical method show that all four coefficients can be determined from coastdown data if there is sufficient ambient wind, and hence lay the basis for an advanced test method using only data from track tests. Constrained methods, where one or more of the parameters is fixed, can be used to investigate a single source of drag. The reduction in the total number of tests required to achieve an acceptable level of accuracy in the variable coefficients when using the constrained method is demonstrated.