A Comparison of the Coastdown and Steady State Torque Methods of Estimating Vehicle Drag Forces

Accurate measurement of a vehicle's resistance to straight line motion on a road (road load), and the separation of this resistive force into its contributory components is of fundamental importance for the calibration of a modern chassis dynamometer and to provide the data required for vehicle performance assessment.

The coastdown and steady-state torque tests are the established means of determining the road load on a test track. Differences in vehicle operating conditions and the instrumentation used during the tests lead to variations in the values obtained for the coefficients in the road load equation. This paper describes an investigation into these test methods to determine their relative accuracies, and to compare the results obtained in the different modes. On vehicle anemometry is used to improve the overall accuracy achieved in both types of test.

The results show that eliminating the driveline losses from the analysis yields comparable coefficients in the road load equation for the two test modes. The driveline losses are also measured in each mode and compared.