Interpretation ID: nht76-4.26

DATE: 09/09/76

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: B. F. Goodrich Engineered Systems Company

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your July 26, 1976, question whether the "no lockup" requirement of S5.3.1 and S5.3.2 of Standard No. 121, Air Brake Systems, requires wheel sensors on both axles of a tandem axle system in those cases where the "no lockup" performance is provided by means of an antilock system. I have enclosed a detailed discussion of this issue that responded to a similar question from another manufacturer. The response should answer your question.

Yours truly,

Enclosure

ATTACH.

B.F. Goodrich Engineered Systems Company

July 26, 1976

JOHN W. SNOW -- ADMINISTRATOR -- National Highway Traffic Safety Administration

Subject: REQUEST FOR CLARIFICATION OF MVSS 121

Gentlemen:

Section S.5.3.1.A (for trucks and buses) and Section S.5.3.2.A (for trailers) of MVSS 121 states that wheel lock-up shall not occur on any wheel above 10 M.P.H., except for controlled lock-up of wheels allowed by an anti-lock system.

It is understood that a manufacturer is not required to utilize an anti-lock system to meet the "no lock-up" requirement, however, if a manufacturer does choose to utilize the "controlled lock-up" exception of S.5.3.1.A and S.5.3.2.A, by installing an anti-lock system, it must control the lock-up of the wheels which are pneumatically controlled by that system (reference attached copy of letter dated March 7, 1975 to Harold D. Shall from James C. Shultz). Therefore, it is our interpretation that the vehicles' anti-lock system, if included, must include a wheel speed sensor on each wheel, that is pneumatically controlled by the anti-lock logic module valve. Is this correct?

We will appreciate your official ruling on this pressing matter at your earliest convenience.

Very truly yours,

Richard J. Brandewie -- Program Manager, Highway Products

cc: D. L. Haines; C. D. McCarty