Interpretation ID: nht92-1.4

DATE: 12/29/92

FROM: PAUL JACKSON RICE -- CHIEF COUNSEL, NHTSA

TO: LEO WENSTRUP -- MANAGER - TECHNICAL SERVICES, EATON CORPORATION, AXLE AND BRAKE DIVISION

ATTACHMT: ATTACHED TO LETTER DATED 11-3-92 FROM LEO WENSTRUP TO NHTSA OFFICE OF CHIEF COUNSEL (OCC 7963)

TEXT: This responds to your letter asking about the dynamometer requirements of Federal Motor Vehicle Safety Standard No. 121, Air Brake Systems (49 CFR 571.121). You explained that you have experienced problems in conducting certification testing of brake lining for vehicles in a city refuse fleet. Specifically, you stated that while the fade portion of the dynamometer test specifies decelerations from 50 miles per hour (mph) to 15 mph, the trucks in question have a maximum vehicle speed governed to 45 mph. You asked at what speed should the fade portion of the test be conducted given that these trucks are governed to a lower speed. I am pleased to have this opportunity to explain our regulations to you.

Some background information on Federal motor vehicle safety laws and regulations may be helpful. The National Traffic and Motor Vehicle Safety Act (15 U.S.C. @ 1381 et seq., Safety Act) authorizes this agency to issue safety standards applicable to new motor vehicles and items of motor vehicle equipment. NHTSA, however, does not approve motor vehicles or motor vehicle equipment, nor do we endorse any commercial products. Instead, the Safety Act establishes a "self-certification" process under which each manufacturer is responsible for certifying that its products meet all applicable safety standards.

Manufacturers must have some basis for their certification that a product complies with all applicable safety standards. This does not necessarily mean that a manufacturer must conduct the specific tests set forth in an applicable standard. Certifications may be based on, among other things, engineering analyses, actual testing, and computer simulations. Whatever the basis for certification, however, the manufacturer must certify that the product complies with a standard as it is written, i.e., that the vehicle will pass all applicable requirements if it is tested exactly according to the standard's test conditions and other specifications.

Standard No. 121's dynamometer test requirements are set forth in section S5.4. That section specifies that brake assemblies must meet the requirements of S5.4.1 (brake retardation force--relevant only to towed vehicles), S5.4.2 (brake power), and S5.4.3 (brake recovery), under the conditions of S6.2. The purpose of the dynamometer test requirements is to help ensure that brakes retain adequate stopping capacity during and after exposure to conditions caused by prolonged or severe use, such as long, downhill driving. In particular, the specified decelerations are designed to heat the brakes to simulate such severe driving conditions.

In conducting a compliance test, NHTSA would follow the procedures set forth in Standard No. 121. The standard expressly provides that decelerations are made from 50 mph to 15 mph in the dynamometer tests. See, for example, S5.4.2 and S5.4.2.1. Nothing in Standard No. 121 specifies an exception for vehicles with a maximum speed below 50 mph. Moreover, since the tests in question are dynamometer tests and not road tests, there would be no difficulty in conducting the tests at a speed higher than the vehicle's maximum speed.

I note that this opinion assumes that Standard No. 121 applies to the vehicles in question. Certain vehicles with slow maximum speeds are excluded from the standard's coverage. See S3. However, these exclusions do not appear to apply to the refuse trucks in question.

Your letter appears to imply that Standard No. 121's dynamometer tests are inappropriate for vehicles which have a maximum speed governed to less than 50 mph. We note, however, that a governor set at 45 mph does not prohibit a vehicle from achieving speeds greater than 50 mph in certain situations such as driving downhill. In addition, as indicated above, the specified decelerations are designed to heat the brakes to simulate the severe driving conditions caused by prolonged and severe use and long downhill driving. The same degree of heating can occur at speeds below 50 mph. Therefore, we believe that the specified tests are appropriate for vehicles with a maximum speed governed to below 50 mph.

If you have any further questions, please feel free to call Mr. Marvin Shaw of my staff at (202) 366-2992.