Mr. G.T. Bowman
Manager, Product Integrity
Rockwell International Corporation
2135 West Maple Road
Troy, MI 48084-7186

Dear Mr. Bowman:

This responds to your request for the National Highway Traffic Safety Administration (NHTSA) to reconsider a previous interpretation of the dynamometer test procedures set forth in S5.4.2.2 of Standard No. 121. 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. In a June 29, 1995, interpretation to Advance Engineered Products, Mr. John Womack, the Acting Chief Counsel, stated that "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."

In support of your request for the agency to reconsider its interpretation, you reference the March 1, 1976 final rule which states that

The "hot stop"deletion was also supported, though most comments on the proposal stated that the whole section should be deleted and not simply the last sentence. The NHTSA purposely did not delete the whole section, so that the sequence of testing would remain as in the past, to preserve the data on recovery that was developed following "hot stop" testing. Therefore the required test level is deleted as proposed, but the testing remains in the standard to maintain the same sequence as in the past. (41 FR 8733, 8787)

Amendatory language in the final rule states that "Section S4.2.2 is amended by deletion of the last sentence of the text." As a result, that provision reads as follows:

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. The service brake line air pressure shall not exceed 108 p.s.i.

In the proposal leading to the March 1976 final rule, NHTSA had stated that "For all vehicles, the dynamometer brake power and recovery requirements would be modified by increasing the upper pressure limit. The 'hot stop' dynamometer requirement (S5.4.2.2) would be deleted, since the 14 fpsps deceleration rate is not comparable to the new stopping distance requirements. (40 FR 59222, December 22, 1975)

Based on these prior agency statements, I have decided to modify NHTSA's June 1995 interpretation. Upon further reflection, Standard No. 121 does not specify any limitation on the pressure that can be used to achieve the required deceleration rate specified in S5.4.2.2.

I hope this information is helpful. If you have any further questions about NHTSA's safety standards, please feel free to contact Marvin Shaw at this address or by telephone at (202) 366-2992.


Samuel J. Dubbin

Chief Counsel