Interpretation ID: 77-2.22

TYPE: INTERPRETATION-NHTSA

DATE: 04/28/77

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: The Bendix Corporation

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Bendix Corporation's March 29, 1977, request for confirmation that the manual adjustment of automatic adjusters is acceptable following the 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.

I have enclosed a copy of a similar interpretation made to Rockwell International.

Sincerely,

ATTACH.

Bendix

Heavy Vehicle Systems Group

John Snow -- Administrator, National Highway Traffic Safety Administration

March 29, 1977

Subject: Request for Interpretation: FMVSS '121'

Ref.: Brake Burnish Procedures S6.1.8.1 and S6.1.8.2

Dear Mr. Snow:

The Bendix Corporation, Heavy Vehicle Systems Group (HVSG) respectfully requests clarification of the portions of S6.1.8.1 and S6.1.8.2, which permit adjustment of the brakes as recommended by the vehicle manufacturer after the burnish procedure. Specifically, clarification is requested that either S-Cam or wedge brakes equipped with automatic adjusters can be adjusted as recommended by the vehicle manufacturer, as well as those equipped with manual brake adjusters.

As permitted by the subject paragraphs, vehicle manufacturers have, in the past, manually adjusted the brakes; especially S-Cam brakes, after the burnish procedure. The affect of this has been to generate a large amount of background data and vehicle compliance testing starting with given brake chamber strokes.

In an effort to promote safety by maintaining a predictable level of good braking, assuring good brake balance and eliminating the possibility of vehicles being operated with S-Cam brakes badly out of adjustment, Bendix has introduced an automatic slack adjuster. Since the adjustment maintained by an automatic slack adjuster is influenced by several factors, including brake and drum temperature, and lining and drum design and conditions, the stroke maintained by an automatic slack adjuster after the burnish procedure will likely be different than that which has been used by the vehicle manufacturers for compliance testing.

It is the opinion of the Bendix Corporation (HVSG) that the use of automatic slack adjusters does not prohibit the vehicle manufacturer from adjusting the brakes, if desired, after the burnish procedure. A vehicle manufacturer advised us that it is reluctant to introduce this device, unless assurance can be given that requalification testing will not be required with the strokes that exist after the burnish test procedure.

We would be pleased to discuss the details of this matter if any additional information is necessary.

Very truly yours,

R. W. Hildebrandt -- Group Director of Engineering