Interpretation ID: nht76-4.15

DATE: 09/30/76

FROM: AUTHOR UNAVAILABLE; R. L. Carter; NHTSA

TO: Rockwell International

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Rockwell International's February 23, 1976, request for confirmation that the deactivation of automatic adjusters is acceptable during burnish procedures of Standard No. 121, Air Brake Systems, at the option of the manufacturer.

The procedure that you recommend is not permitted by any provision of Standard No. 121. The NHTSA would consider some provision to deal with the overadjustment of automatic adjusters upon receipt of technical data showing justification for such action. Based on consideration of the data received and a petition for amendment, the agency could commence a rulemaking proceeding in accordance with established procedures.

It should be noted that the option in FMVSS 105-75 that you describe requires that the automatic adjusters be deactivated for the entire test sequence, not just the burnish procedure.

Sincerely,

ATTACH.

February 23, 1976

National Highway Traffic Safety Administration Office of Crash Avoidance

Gentlemen:

Subject: Air Brakes - Automatic Adjusters

Rockwell International requests an interpretation of FMVSS #121 testing procedures as it relates to Rockwell cam and wedge brakes with automatic adjustment features.

Automatic adjusting air brakes are premium brakes having significant safety advantages in day to day operation. The automatic adjustment feature is offered on Rockwell cam and wedge brakes, single and dual actuated air and single and dual air/hydraulic units. The automatic adjusting feature enables vehicles to maintain their stopping capability when an otherwise manually adjusting brake would exceed available adjustment or "stroke." At the same time the automatic adjustment feature keeps brakes within a favorable operating range of the air chamber. This becomes more important with the improved brake performance required by FMVSS #121. It is noted that NHTSA has made proper allowance for automatic adjusting hydraulic brakes in the test procedures of FMVSS #105-75.

The accelerated brake burnishing required by FMVSS #121 results in rapid expansion of the brake drum and to a lesser extent brake lining, thereby preventing proper operation of the adjusters. It is, therefore, necessary to manually adjust the brakes for the test procedure.

Enclosed are copies of our recommended procedure for Rockwell International brakes with automatic adjusters when tested for FMVSS #121. It is requested that NHTSA review this procedure and advise that it is acceptable for testing under FMVSS #121. Secondly, it is requested that NHTSA allow for deactivation of automatic adjusters in the FMVSS #121 similar to the provisions of FMVSS #105-75 at the next opportunity.

Very truly yours,

ROCKWELL INTERNATIONAL, CORPORATION AUTOMOTIVE OPERATIONS;

William R. Rodger -- Vice President - Advanced Programs

Encls.