Interpretation ID: nht76-4.12

DATE: 08/04/76

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: Blue Bird Body Company

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Blue Bird Body Company's June 21, 1976, question whether S5.3.1 of Standard No. 121, Air Brake Systems, requires the installation of an antilock system, or whether the stopping distance and "no lockup" requirements may be met using modulation of the service brake control by the vehicle driver.

Section S5.3.1 requires that vehicles subject to it must be capable of stopping under specified conditions, within the stopping distance set forth in Table II without leaving a 12-foot-wide lane and without "uncontrolled" lockup of certain wheels of the vehicle. There is no requirement for the installation of an antilock system. Also, there is no test condition that specifies a full brake application and modulation of the service brake may be used to reduce or eliminate lockup.

Yours truly,

ATTACH.

BLUE BIRD BODY COMPANY

June 21, 1976

Thomas Herlihy -- Office of Chief Counsel, National Highway Traffic Safety Administration

Dear Mr. Herlihy:

SUBJECT: FMVSS 121 - AIR BRAKE SYSTEMS

It is our understanding that the subject standard does not currently specify maximum allowable stopping distances. We also understand that maximum allowable stopping distances are specified for future vehicles.

Our question is, when and if the stopping distance requirements of Paragraphs S5.3.1 and Table 2 are reinstated for buses, will it be permissible to meet those stopping requirements without an anti-lock system, provided we can meet the requirements stated in S5.3.1 ". . . . without any part of the vehicle leaving the roadway and without lock up of any wheel at speeds above 10 MPH . . . .?" Obviously, the driver would have to modulate the braking force to prevent wheel lock up on a system not equipped with an automatic anti-lock system.

We would appreciate your early reply.

Thank you.

Yours very truly, W. G. Milby -- Staff Engineer

c Ben Newberry