Interpretation ID: nht92-4.20

DATE: 09/02/92

FROM: JAMES A. WESTPHAL --OSHKOSH CHASSIS DIVISION, OSHKOSH TRUCK CORPORATION

TO: ADMINISTRATOR -- NHTSA

ATTACHMT: ATTACHED TO LETTER DATED 11-03-92 FROM PAUL J. RICE TO JAMES A. WESTPHAL (A40; STD. 121)

TEXT: The purpose of this letter is to seek clarification as to which Federal Motor Vehicle Safety Standards for brake systems must the vehicles described herein comply.

Oshkosh Chassis Division of Oshkosh Truck Corporation is a manufacturer of incomplete vehicles (chassis less cab). The chassis being addressed in this letter are sold to manufacturers who complete and sell the vehicles as "motor homes". The chassis for which we are requesting clarification are the "X" and "V" models both of which are over 10,000 pounds GVWR but no larger than 26,000 pounds GVWR.

We plan to install brake systems in these two models which use compressed air to provide braking power, and hydraulic fluid to transmit the energy to the hydraulically activated disk brakes at each wheel. This system is commonly known as "air-over-hydraulic" brakes. Please refer to the attached diagram showing the service brake system logic. The air components of the system include an air compressor and pressure governor, air dryer, air reservoirs, service brake control (treadle) valve, parking brake control valve and spring applied/air release parking brake chamber, and piping/valves/gauges/switches as required. Energy from the compressed air is transmitted to the hydraulic fluid through two air/hydraulic converters (brake boosters). The converter/booster increases the output hydraulic pressure approximately 16 times greater than the input air pressure. The hydraulic components of the system include disk brakes at each wheel, fluid reservoirs, and piping between the air hydraulic converters and disk brakes. Split service brake systems will be used.

Please answer the following compliance questions concerning the vehicles and brake system described above:

1. Must the brake system comply with the requirements of FMVSS 121 applicable to trucks?

2. Must the brake system comply with requirements of FMVSS 105 applicable to multipurpose passenger vehicles?

3. If SS121 compliance is required must the hydraulically powered disk brakes comply with Section S5.4 Service brake system-dynamometer tests?

4. If compliance to parts of both SS121 and SS105 is required must the system meet the requirements of SS105 Sections S5.1.2 Partial failure, S5.1.3 Inoperative brake power assist or brake power unit, and/or S5.3 Brake system indicator lamp?

Thank you for your consideration and response to these questions.ATTACHMENTS (TEXT AND GRAPHICS OMITTED.)