Interpretation ID: 1984-2.31
TYPE: INTERPRETATION-NHTSA
DATE: 07/20/84
FROM: AUTHOR UNAVAILABLE; Frank Berndt; NHTSA
TO: Southwest Research Institute
TITLE: FMVSS INTERPRETATION
TEXT:
Mr. Louis F. Klusmeyer, Senior Research Scientist Southwest Research Institute P.O. Drawer 28510 6220 Culebra Road San Antonio, Texas 78284
Dear Mr. Klusmeyer:
This responds to your letter dated May 15, 1984, regarding the applicable Federal motor vehicle safety standard for compliance testing of the "air over hydraulic" braking system used on certain Nissan heavy duty trucks.
After examining the information provided in your letter and the enclosed diagram of the air over hydraulic braking system, we note that air pressure is used to transmit braking pressure from the driver, not merely to assist the driver in applying muscular force to hydraulic or mechanical components.
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. 1O5a (now, Standard No. 1O5), 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 17917. Copies of these pages are enclosed.
Accordingly, the air over hydraulic diesel truck system described in your letter would have to meet the requirements of Standard No. 121 , Air Brake Systems.
You ask whether the air chamber volumes at the "air booster," as it appears in your diagram, could be used in calculating the required air service reservoir capacity or in determining reaction time. This agency agrees that, if the brake actuation to the wheels is hydraulic, then the requirements of Standard No. 121 for air service reservoir capacity and brake application and release timing can be measured at the "air booster" for compliance testing. When a heavy duty truck is equipped with an air over hydraulic brake system, the air booster can be considered the equivalent of the air brake chamber in Standard No. 121.
Sincerely,
Frank Berndt Chief Counsel
Enclosures
15 May 1984
Mr. Frank Berndt, Chief Counsel National Highway Traffic Safety Administration 400 7th Street, S.W. Washington, DC 20590
Dear Mr. Berndt:
Southwest Research Institute (SwRI) has been asked to examine heavy duty trucks produced by Nissan Diesel Motor Co., Ltd. in order to determine compliance with United States standards.
These trucks use a "air-over hydraulic" brake system and there is some question as to the appropriate Federal Motor Vehicle Safety Standard (FMVSS). We are, therefore, requesting a decision from NHTSA as to whether FMVSS standard 105 or FMVSS standard 121 is applicable to these trucks.
The following information may help in this decision:
o The driver uses a treadle valve to operate and control the service brakes.
o Compressed air is used from the treadle valve to the "air booster" and hydraulic brake fluid is used from the "air booster" to the service brakes.
o No hydraulic or mechanical means is used to transmit force from the drivers control to the "air booster" and no air is used to supply actuating force at the individual wheels.
o A complete loss of air pressure causes a complete loss of braking from the service brake system, rather than a reduced capability as would be normal with a "brake power assist unit" or a "brake power unit".
o "Air brake chambers," as such, are not used since brake actuation at the wheels is hydraulic. The air chamber volumes at the "air booster" could possibly be used in calculating required aiu service reservoir capacity or determining reaction time.
A diagram of the braking system is included with this letter as a aid in understanding the system.
If I can provide further information or answer questions for you, please call (512) 684-5111, extension 3017.
Sincerely,
Louis F. Klusmeyer Senior Research Scientist Vehicle Systems Department of Engine and Vehicle Research Engines, Emissions and Vehicle Research Division
LFK/dg Attachment
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