Interpretation ID: 7742-2
Senior Engineer - Friction Materials
Lucas Heavy Duty Braking Systems
Grange Road
Cwmbran, Gwent
South Wales NP44 3XU
Great Britain
Dear Mr. Gould:
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 requested clarification of the term "average deceleration rate" and its tolerance, particularly with respect to the brake power test (S5.4.2). You stated that you view the specified deceleration rate as "only a target" in order to fade the linings, and believe that it is acceptable to conduct tests at five percent below the specified rate. 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 independent 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.
With respect to your question about the meaning of "average deceleration rate," that term is used in both S5.4.2 and S5.4.3. Section S5.4.2 specifies, for example, that each brake shall be capable of making 10 consecutive decelerations at an average rate of 9 f.p.s.p.s. from 50 mph to 15 mph, and shall be capable of decelerating to a stop from 20 mph at an average deceleration rate of 14 f.p.s.p.s. after the 10th deceleration. In S5.4, the meaning of average deceleration rate is explained as follows:
For purposes of the requirements of S5.4.2 and S5.4.3, an average deceleration rate is the change in velocity divided by the decleration time measured from the onset of deceleration.
We do not agree with your suggestion that the deceleration rates specified in Standard No. 121 are "only a target" in order to fade the linings. As indicated above, manufacturers must certify that each vehicle 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. Thus, if a vehicle was unable to pass Standard No. 121's test requirements at the specified deceleration rates, it would not comply with the standard, notwithstanding the fact that it might be able to pass the standard's requirements at slightly lower deceleration rates.
We recognize, however, that it may be difficult to achieve any exact deceleration rate in conducting a brake test. For this reason, the agency's Office of Vehicle Safety Compliance (OVSC) specifies tolerances in its Laboratory Test Procedures developed for use by contractors in conducting compliance tests for the agency. For the brake power and brake recovery tests (S5.4.2 and S5.4.3), the agency's current Laboratory Test Procedure specifies the following tolerances on deceleration rates: +0 to -1 ft/s/s, except for 12 ft/s/s: +0.5 ft/s/s.
Enclosed for your information is a copy of the agency's Laboratory Test Procedure for Standard No. 121's dynamometer tests. On the issue of tolerances, I call your attention to the following statement at the beginning of the Laboratory Test Procedure:
The OVSC Laboratory Test Procedures, prepared for use by independent laboratories under contract to conduct compliance tests for the OVSC, are not intended to limit the requirements of the applicable FMVSS(s). In some cases, the OVSC Laboratory Test Procedures do not include all of the various FMVSS minimum performance requirements. Sometimes, recognizing applicable test tolerances, the Test Procedures specify test conditions which are less severe than the minimum requirements of the standards themselves. Therefore, compliance of a vehicle or item of motor vehicle equipment is not necessarily guaranteed if the manufacturer limits certification tests to those described in the OVSC Laboratory Test Procedures.
If you have any further questions, please feel free to call Mr. Marvin Shaw of my staff at (202) 366-2992.
Sincerely,
Paul Jackson Rice Chief Counsel
Enclosure
ref:121 d:11/19/92