The code provides test procedures and methods of calculating a brake rating from the data obtained for brakes used in highway commercial vehicles over 4.5 T (10 000 lbs) GVWR air and hydraulic. Some general correlation may be expected between brake ratings established by this means and those obtained from vehicle tests such as outlined in SAE J880. The brake rating power, kW (hp) calculated by conduct of this code is an arbitrary index of performance of the brake and drum when tested by this procedure and may be appreciably different from the values obtained by other techniques.
J971 is a dynamometer test to determine the brake horsepower rating. This rating is best determined via vehicle test as it is highly dependent upon the brake cooling characteristics that are not simulated on the dynamometer. Certain conditions specified currently in J971, such as the percentage of lining to drum contact, may not allow the test to be run as specified. Testing per J971 is not being done, having been replaced by testing to the procedures specified by FMVSS-121 and SAE J2115.