Interpretation ID: aiam0665
Manager
Technical Services
The Budd Company
Automotive Division
Detroit
MI 48215;
Dear Mr. Kennel: This is in reply to your letter of March 10, 1972, in which yo presented a series of questions concerning the meaning of several requirements of Federal Motor Vehicle Safety Standard No. 121, 'Air Brake Systems.' Our reply deals with the questions in the order you asked them.; >>>1. Your first question concerns the meaning of the statement i section S5.4 that 'a brake assembly that has undergone a road test pursuant to S5.3 need not conform to the requirements of this section.' To parahprase (sic) your question, the quoted language means that if a given brake assembly is subjected to the road test, the same brake assembly with the used lining need not conform to the dynamometer requirements. Conformity to the dynamometer requirements will be determined by testing an identical brake assembly with new linings. The petitions for deletion of dynamometer testing would have made the road test the only test. The standard requires both tests, even though two sets of identical brakes will be used, and our statement that the petitions were denied is therefore correct.; 2. You point out that the measurement interval used in S5.4.1.1 fo determining average torque, which begins when a specified pressure is reached, differs from the interval specified in S5.4 for measuring deceleration, which begins with the onset of deceleration. Although we agree that you may need different instrumentation for measuring average torque and average deceleration, we do not agree that their is any conflict since average torque and average deceleration are not required to be measured at the same time. We consider the present method of measuring torque and deceleration to be the correct methods.; 3. The typographical error in section S5.4.1.1, which you hav correctly edited to read 'Repeat the procedure six times, increasing the brake chamber air pressure by 10 psi each time,' has been corrected by a revision in the March 29, 1972, *Federal Register*.; 4, 5, 6. The requirements of S5.4.2, S5.4.2.1 and S5.4.3 concernin average deceleration rates should not be understood to mean that a manufacturer, in his own testing, must test at exactly that rate. It is advisable for him to test in a manner that offers assurance that the brakes will pass when tested in the manner specified in the standard. Typically, where a test value such as 9 fpsps is specified, manufacturers tend to use more adverse values in their own testing. Under the former wording of these sections, the compliance agency could have tested brakes at decelerations higher than the specified minimum, and it would have been much more difficult for a manufacturer to ascertain his 'worst case' situation.<<<; The notice ofoposing (sic) to amend the weight conditions fo truck-tractors should be issued within the next two months.; Sincerely, E. T. Driver, Director, Office of Operating Systems, Moto Vehicle Programs;