Interpretation ID: 7745
Oshkosh Chassis Division
Oshkosh Truck Corporation
P.O. Box 2508
Oshkosh, WI 54903
Dear Mr. Westphal:
This letter responds to your inquiry about which Federal Motor Vehicle Safety Standards would be applicable to certain incomplete vehicles (chassis less cab) that you manufacture for motor homes. You anticipate that the motor homes will have a gross vehicle weight rating (GVWR) over 10,000 pounds but less than 26,000 pounds.
Your letter indicated that Oshkosh plans to install brake systems in the two models which use compressed air to provide braking power, and hydraulic fluid to transmit the energy to the hydraulically activated disc brakes at each wheel. You stated that this system is commonly known as "air-over-hydraulic."
The following is in response to your four specific questions:
1. Must the brake system comply with the requirements of Standard No. 121 applicable to trucks?
The answer to question number one is yes. The agency classifies air-over-hydraulic brake systems as air brake systems. Accordingly, vehicles equipped with air-over- hydraulic brake systems are required to comply with the requirements of Standard No. 121. I am enclosing a July 20, 1984 interpretation letter to Ms. Margaret Moore Oba which discusses this issue at length.
2. Must the brake system comply with the requirements of Standard No. 105 applicable to multipurpose passenger vehicles?
The answer to question number two is no. Standard No. 105 only applies to vehicles with hydraulic brake systems. Since your system is air-over-hydraulic, it is considered to be an air brake system and not a hydraulic brake system.
3. If Standard No. 121 compliance is required must the hydraulically powered disc brakes comply with Section S5.4 Service brake system--dynamometer tests?
The answer to question number three is yes. The requirements of S5.4 are among the requirements specified in Standard No. 121 for each vehicle equipped with air brakes.
4. If compliance to parts of both Standards 121 and 105 is required, must the system meet the requirements of the following sections in Standard No. 105: S5.1.2 Partial Failure, S5.1.3 Inoperative brake power assist or brake power unit, and/or S5.3 Brake system indicator lamp.
As indicated above, air-over hydraulic brake systems are not required to meet the requirements of Standard No. 105.
I hope this information is helpful. If you have any further questions about NHTSA's safety standards, please feel free to contact Marvin Shaw of my staff at this address or by telephone at (202) 366-2992.
Sincerely,
Paul Jackson Rice Chief Counsel Enclosure
Ref: 121 d:11/3/92