Interpretation ID: nht76-4.17

DATE: 12/10/76

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: Great Dane Trailers, Inc.

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Great Dane Trailer's November 4, 1976, question whether the "controlled lockup" exception in S5.3.2 of Standard No. 121, Air Brake Systems, is available to a manufacturer who equips a "tri-axle" trailer suspension with one set of wheel speed sensors on the center axle (or, alternatively, on the front axle) and one antilock valve and logic module that meters air pressure to the two rearmost axles.

Your question appears to be similar to questions about tandem-axle suspensions that prompted an interpretation of the "controlled lockup" exception to the "no lockup" requirements of S5.3.1 and S5.3.2. A copy is enclosed for your information. You will note that the closing statement in the interpretation states that ". . . the controlled lockup exception is not dependent on the number or location of sensors used in an antilock installation." It is left to the manufacturer to decide what placement of the sensors will provide the best performance.

Sincerely,

Enclosure

Great Dane Trailers, Inc.

November 4, 1976

Thomas Herlihy Office of Chief Counsel National Highway Traffic Safety Administration

Subject: FMVSS 121-Air Brake Systems

Dear Mr. Herlihy:

In Section S5.3.2 of the 121 requirements, there are indications that anti-lock controls be applied to the two rearmost, nonliftable, nonsteerable axles. In the more recent interpretation, there are indications that if it is possible to predict which of the two axles will lock first during braking, sensors may be placed on this axle only; however, they both must be controlled.

My question involves a tri-axle unit. In this case, where must the sensors be used? By the first interpretation, controls need not be on the front axle of this suspension, only on the last two. Of these last two, the center axle will always lock before the rear axle, meaning that the control sensors should be installed on this center axle. However, in a tri-axle suspension, the front axle will lock first, the center axle will lock second and the rear axle will be the last to lock.

We would appreciate your early reply.

Sincerely,

Dudley E. DeWitt Manager/R & D