and especially in the precision of results when using the modified TMC/SAE J1321 Type II. While this recommended practice may appear to be the default procedure for a future regulation, the various compliance tests and procedures need to be evaluated to determine which one(s) are best suited to the particulars of the new regulation.

There are numerous SAE-recommended practice protocols available to evaluate the aerodynamic performance of commercial vehicles and aerodynamic devices. Three in particular address full-scale commercial vehicles in open air conditions either on the highway or at the test track.

SAE J1321 was first published in 1986. It consists of test and analysis methods to determine the change in fuel consumption for trucks and buses with GVW rating (GVWR) of more than 10,000 lb. To improve the precision of the results for use in SmartWay, EPA developed 22 additional provisions for SAE J1321. Examples of the provisions include requirements that the test must be conducted on a test track, not a roadway, with a grade change on the test track not greater than 2 degrees and the test facility not more than 4,000 ft above sea level. The latest revision occurred in 2012 as TMC/SAE J1321 Type II. This recommended practice may be used to compare entire vehicles and for easy-to-change components. The revised recommended practice specifically states that “test results that do not rigorously follow the method described herein are not intended for public use and dissemination and shall not be represented as a J1321Type II test result.”14 It is used by the SmartWay program to assess vehicle and component performance.

SAE J1526 Type III recommended practice remains a work in progress. It is “a standard test procedure for comparing the fuel economy of components or systems” that can be switched from one vehicle to another in a short period of time” and “is ideally suited for comparing the fuel consumption of one vehicle to another and one component of a combination vehicle to the same component in another” (SAE, undated). SAE J1526 Type III is an open-road moderate-distance test with a minimum test distance of 28 mi. It involves two vehicles running the same test without interplay as in SAE J1264 Type I, described below. The object or device being tested is switched from one vehicle to the other on separate runs. Because it is an open-road test, variable control will be more challenging; however, it provides an alternative means of evaluating fuel consumption away from a test track. To improve the accuracy of the results when evaluating devices for the Innovative Technology credits in the Phase I Rule, the minimum route distance for SAE J1526 Type III must be increased from 28 mi to 100 mi and the route “must be representative in terms of grade.”15 Many of the 22 additional provisions from TMC/SAE J1321 Type II could also be applied to SAE J1526 Type III to further improve accuracy. The relative fuel economy of the component or vehicle under test is expressed “as a percentage of fuel saved. This factor is calculated using relative fuel consumption while operating with and without the test component or vehicle under evaluation” (SAE, undated).

SAE J1264 Type I was revised in 2011 and is a recommended practice “providing minimum requirements for testing components or systems of the type which can be switched from one truck to another with relative ease—that is, aerodynamic devices, clutch fans, tires, and the like. The test utilizes in-service fleet vehicles, operated over representative routes.”16 The tests are conducted with two vehicles in simultaneous operation within close proximity of each other: 200-250 yd (180-230 m). Halfway along the test leg of at least 50 mi, the trailing vehicle passes the lead vehicle and remains in the lead position for the remainder of the test leg. The relative fuel effectiveness of the component or system under test is determined as a percentage improvement factor.

The Phase I Rule provides for validation of the drag coefficient (Cd) through coast-down testing. Coast-down tests are usually conducted in accordance with SAE J1263, “Road Load Measurement and Dynamometer Simulation Using Coastdown Techniques.” This method was developed for light vehicle testing in relation to dynamometer simulation and to also serve heavy vehicle requirements. An additional recommended practice for coast-down tests is SAE J2263, “Road Load Measurement Using Onboard Anemometry and Coastdown Techniques.” The final result of SAE J2263 is a model of road load force (as a function of speed) during operation on a dry, level road under reference conditions.

The precision of the results from these recommended practices is highly dependent on test controls, protocols, and environmental factors. A significant effort is required to ensure that procedures are rigorously followed and that the external factors that can influence fuel consumption are tightly controlled. The fidelity of test results from this J1263 coast-down procedure to results from the powered J1321 (or other) track test could not be established. This is believed to be the critical issue in test procedure selection for trailer regulation.

Wind tunnel testing at reduced scale and full scale is another well-developed method of aerodynamic performance evaluation. SAE J1252 is “Recommended Practice Wind Tunnel Test Procedure for Trucks and Buses.” The scope encompasses the full range of full-scale MHDVs represented as either full-scale or reduced-scale wind tunnel models. The document provides guidance for wind tunnel testing to support current vehicle characterization, vehicle development, vehicle concept development, and vehicle component development.


14 SAE. “Fuel Consumption Test Procedure—Type II.” Available at Accessed December 23, 2013.

15 40 CFR part 1037.601.

16 Society of Automotive Engineers, undated. “Joint RCCC/SAE Fuel Consumption Test Procedure (Short Term In-Service Vehicle) Type 1 (STABILIZED May 2011).” Warrendale, Pa.: SAE.

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