This document describes a rigorous engineering test procedure that utilizes industry-accepted data collection and statistical analysis methods to determine the road load and to estimate the aerodynamic drag area of trucks and buses weighing more than 10000 pounds. The test procedure may be conducted on a test track or on a public road under controlled conditions and supported by extensive data collection and data analysis constraints. The estimated aerodynamic-drag-area result represents a single-speed and single-yaw-angle condition. Test results that do not rigorously follow the method described herein shall not be represented as an SAE J2978 result.
Coastdown testing of trucks and buses has become common practice over the past decade as a means to quantify the magnitude of the road-load forces exerted on a production vehicle or to quantify the difference in road load forces between vehicles or vehicle configurations. Environmental regulations and energy-savings verification programs have adopted the coastdown method for evaluating the aerodynamic-drag performance of vehicles and the drag reduction associated with aerodynamic technologies. Using industry-accepted best practices, this SAE Recommended Practice establishes a procedure for the coastdown testing of trucks and buses weighing more than 10000 pounds.