Interpretation ID: 1984-2.34

TYPE: INTERPRETATION-NHTSA

DATE: 07/20/84

FROM: AUTHOR UNAVAILABLE; Frank Berndt; NHTSA

TO: Hino Motors (U.S.A.) Inc.

TITLE: FMVSS INTERPRETATION

TEXT:

Ms. Margaret Moore Oba Hino Motors (U.S.A.) Inc. 200 Park Avenue Suite 4114-12 New York, N.Y. 10166

Dear Ms. Oba:

This responds to your letter regarding the Federal motor vehicle safety standard on brake systems applicable to a diesel truck equipped with an "air over hydraulic" brake system.

After examining your letter and the enclosed diagram of the braking system, we note that air pressure is used to transmit braking force from the driver, not merely to assist the driver in applying muscular force to hydaulic or mechanical components. A failure in air pressure would result in loss of braking force. Therefore, this brake system falls within the definition of hair brake system" in paragraph S4 of Standard No. 121, Air Brake Systems.

When the original final rule was issued on Standard No. 121 , Air Brake Systems, the preamble stated:

It should be noted that the term "air brake system" as defined in the standard applies to the brake configuration commonly referred to as "air over hydraulic," in which failure of either medium can result in complete loss of braking ability.

See Federal Register, February 27, 1971, at page 3817.

In 1972, the agency reiterated this interpretation in the preamble to the original final rule on Standard No. 105a (now, Standard No. 105), Hydraulic Brake Systems:

Standard No. 105a does not apply to vehicles equipped with "air over hydraulic" systems, which remain within the purview of Standard No. 121 , Air Brake Systems.

See Federal Register, September 2, 1971, at page 17971. Copies of these pages are enclosed.

Accordingly, the air over hydraulic diesel truck braking system described in your letter would have to meet the requirements of Standard No. 121, Air Brake Systems.

Sincerely, Frank Berndt Chief Counsel Enclosures

January 26, 1984 Mr. Frank Berndt Chief Counsel National Highway Traffic Safety Administration 400 Seventh Street, S.W. Washington, D.C. 20590

Dear Mr. Berndt:

I would like to request clarification regarding F.M.V.S.S. classification of brake systems. In a new model of diesel truck which we intend to import to the United States, we use a system called the "air over hydraulic" brake system. It is rarely used in America, and we are unsure whether it should fit within the definition of a hydraulic brake or an air brake, as it contains elements of both designs.

I am enclosing a simple diagram for your reference. In it, the blue line represents air, the red, fluid. As you can see, pressure exerted by the driver is carried by air to the hydraulic air serve. From there,force is then transmitted by fluid to the wheel brakes. The descriptions of brake systems in F.M.V.S.S. 105 (hydraulic) and 121 (air) define each category according to the medium transmitting force from the service brake control to the brake itself. If the unit which uses air is considered only as a form of power assistance, the air-over system should be included in the hydraulic brake definition.

However, if the air system from the pedal to the hydraulic servo is considered as an integral part of the brake and of equal value, then it would not be merely an assist to the hydraulic.

We would like to know into which category our brake system should fall. If my explanations or diagram are incomplete or unsatisfactory in any way, please do not hesitate to contact us for further clarification.

Sincerely yours, Margaret Moore Oba Encl: