Interpretation ID: nht95-3.31

TYPE: INTERPRETATION-NHTSA

DATE: June 29, 1995

FROM: John Womack -- Acting Chief Counsel, NHTSA

TO: Doug Russel -- Design Engineer, Advance Engineered Products

TITLE: NONE

ATTACHMT: ATTACHED TO 05/03/95 LETTER FROM DOUG RUSSELL TO JOHN WOMACK (OCC 10888)

TEXT: Dear Mr. Russel:

This responds to your letter asking about the brake power requirements in section S5.4.2 of Federal motor vehicle safety standard No. 121, Air Brake Systems (49 CFR 571.121). You asked whether there are any limitations on the pressure which can be used to achieve the required deceleration rate specified in S5.4.2.2. As explained below, the same limitation on the pressure which can be used to achieve the deceleration rate specified in S5.4.2.1 applies to the deceleration in S5.4.2.2. You also asked wh ether brakes must be capable of meeting the specific deceleration rate specified in S5.4.2.2. The answer to that question is yes.

Some background information on Federal motor vehicle safety laws and regulations may be helpful. Congress has authorized this agency the National Highway Traffic Safety Administration (NHTSA), to issue Federal motor vehicle safety standards applicable t o new motor vehicles and new items of motor vehicle equipment. NHTSA has used this authority to establish Standard No. 121, which applies to braking systems on vehicles equipped with air brakes. The agency, however, does not approve motor vehicle or mo tor vehicle equipment, nor does it endorse any commercial products. Instead, Congress has established a "self-certification" process under which each manufacturer is responsible for certifying that its products meet all applicable safety standards.

Standard No. 121's brake power requirements are set forth in section S5.4.2. The purpose of these requirements is to help ensure that brakes retain adequate stopping capability during and after exposure to prolonged or severe use, such as long, downhill driving. The brake power procedure specified by that section is conducted on a dynamometer and is intended to replicate real world brake performance. The specified decelerations are designed to heat the brakes to simulate severe driving conditions.

Section S5.4.2 requires that each brake

shall be capable of making 10 consecutive decelerations at an average rate of 9 f.p.s.p.s from 50 m.p.h. to 15 m.p.h., at equal intervals of 72 seconds, and shall be capable of decelerating to a stop from 20 m.p.h. at an average deceleration rate of 14 f .p.s.p.s. 1 minute after the 10th deceleration.

For the first ten decelerations, section S5.4.2.1 specifies the conditions under which the decelerations must be performed, including that "The service line air pressure shall not exceed 100 p.s.i. during any deceleration." For the eleventh deceleration, S5.4.2.2 specifies that "one minute after the end of the last deceleration required by S5.4.2.1 and with the drum or disc rotating at a speed of 20 m.p.h., decelerate to a stop at an average deceleration rate of 14 f.p.s.p.s."

In asking whether there are any limitations on the pressure which can be used to achieve this eleventh deceleration, you noted that S5.4.2.2, unlike S5.4.2.1, does not explicitly specify a pressure limitation. However, reading section S5.4.2 (including S5.4.2.1 and S5.4.2.2) as a whole, the brake line pressure limitation set forth in S5.4.2.1 applies to the eleventh deceleration as well. Section S5.4.2 specifies a test procedure consisting of a series of events, i.e., preparing the dynamometer for the test including a 100 p.s.i. pressure limit, then making 10 decelerations on the dynamometer under the conditions set forth in S5.4.2.1, then making the eleventh deceleration. Section S5.4.2.2 does not specify any change in the dynamometer pressure limit for the eleventh deceleration. The dynamometer pressure limit specified for the brake power test simply reflects the fact that tractor trailer brakes are typically configured to operate at a maximum nominal brake pressure of approximately 100 psi. Use of a higher brake pressure that differed from a vehicle's actual maximum brake pressure would result in the brake power test not being representative of real world brake performance.

In response to your second question, section S5.4.2 specifies that a brake must be capable of decelerating to a stop from 20 m.p.h. at an average deceleration rate of 14 f.p.s.p.s. 1 minute after the 10th deceleration. See also S5.4.2.2. A brake which c ould not achieve this specified deceleration rate for the eleventh deceleration would not comply with the Standard's requirements.

I hope this information has been helpful. If you have any other questions, please contact Marvin Shaw of my staff at this address or by phone at (202) 366-2992.