Interpretation ID: aiam2463
Manager/R & D
Great Dane Trailers
Inc.
Lathrop Avenue
P.O. Box 67
Savannah
GA 31402;
Dear Mr. DeWitt: This responds to Great Dane Trailer's November 4, 1976, questio 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-axl 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, Frank A. Berndt