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Interpretation ID: Sens_a_brake003065

    Mr. Gregory Gibb
    Edge International Limited
    PO Box 5682
    Frankton, Hamilton
    New Zealand


    Dear Mr. Gibb:

    This responds to your inquiry regarding the regulations applicable to a trailer brake system that relies on a combination of air and hydraulics. You asked if a system manufactured by your company, the "Sens-a-Brake" system, would be classified as an air brake system under Federal Motor Vehicle Safety Standard (FMVSS) No. 121, Air brake systems. As explained below, the "Sens-a-Brake" system would be defined as an air-over-hydraulic brake subsystem, and therefore would be an airbrake system under FMVSS No. 121.

    I am pleased to have this opportunity to explain our regulations to you. FMVSS No. 121 specifies performance requirements for trucks, buses and trailers equipped with air brake systems. S4 of the standard defines "air brake system" as follows:

    Air brake system means a system that uses air as a medium for transmitting pressure or force from the driver control to the service brake, including an air-over-hydraulic brake subsystem, but does not include a system that uses compressed air or vacuum only to assist the driver in applying muscular force to hydraulic or mechanical components. (Emphasis added.)

    S4 of the standard defines "air-over-hydraulic brake subsystem" as:

    [A] subsystem of the air brake system that uses compressed air to transmit a force from the driver control to a hydraulic brake system to actuate the service brakes.

    In your letter you explained that the "Sens-a-Brake" system consists of an electronic brake controller and an air compressor mounted on a trailer, which supplies air for the application of the trailers braking mechanism. Your companys web site states that the electric brake controller incorporates a "pressure sensitive film that enables accurate measurement of the force being applied to the towing vehicles brake pedal."The website further explains that this pad attaches directly to the brake pedal of the tow vehicle. The pedal controller then modulates the air pressure supplied by the air compressor based on the pressure applied to the brake by the vehicle driver. The air pressure then transmits a force to the mechanism used to apply or release the hydraulic trailer brakes.

    Based on the description you provided in your letter and the information on your companys website, the "Sens-a-Brake" system comes within definition of "air-over-hydraulic brake subsystem." The "Sens-a-Brake" system transmits a force applied by a vehicle driver to the brake pedal, first through an electrical signal and then through compressed air, to a hydraulic brake system. While the system does not use air as the initial means for transmitting force from the driver control to the hydraulic system, compressed air is used in the system to transmit force from the driver control. Therefore, the "Sens-a-Brake" system would be an "air-over-hydraulic system," which is regulated as an air brake system under FMVSS No. 121.

    In your letter, you referenced a previous interpretation letter the agency issued to Mr. Tom Brunson, in which the agency concluded that a system with some similarities to the "Sens-a-Brake" system was not an air brake system (April 4, 2000). However, the system addressed by the letter to Mr. Brunson had one significant difference; the air pressure in that system was controlled primarily through an inertial controller mounted on the back of the tow vehicle. The system in the Brunson letter permitted a driver to control braking through a dash-mounted switch, but the switch was not intended to modulate the pressure applied to the brake mechanism. The primary control for actuating the air compressor in the Brunson letter system was the inertial controller mounted on the rear of the tow vehicle and not the vehicle driver. In contrast, the "Sens-a-Brake" system relies on force generated at the driver control (the force applied to the brake pedal pad) to modulate the application of compressed air to a hydraulic brake.

    Similarly, in an October 22, 2001 letter to Mr. Gary Rudnik the agency concluded that a trailer brake system was not an air brake system under FMVSS No. 121 if a trailers brakes activated as a result of the tow vehicles brake lamps illuminating (copy enclosed). Under the system described in the Rudnick letter, the brake pedal would function solely as an on-off switch for the trailer braking system, with the trailers brakes being activated upon illumination of the tow vehicles brake lamps when the driver pressed the tow vehicles brake pedal. However, the system would not transmit force applied by the driver via air to modulate the brakes; the brake pedal merely served as an on-off switch for the trailer braking system. Again in contrast, the "Sens-a-Brake" system relies on force applied to the brake pedal to modulate the application of compressed air to a hydraulic brake.

    Thus, if a system were to modulate the amount of air pressure applied to a hydraulic trailer brake mechanism in relation to force applied to a driver control, then that system would be an air brake system. If, however, a driver control were merely to function as an on-off switch, then that system would not be an air brake system under FMVSS No. 121.

    We note that when the agency incorporated the definition of "air-over-hydraulic brake subsystem" into FMVSS No. 121, we did not anticipate its application to light duty trailers. At that time, the agency indicated that air-over-hydraulic brake subsystems were installed exclusively on single-unit vehicles with a gross vehicle weight rating greater than 19,500 pounds (See 60 Federal Register 36741; July 18, 1995; copy enclosed). Unless the standard were amended through the rulemaking process, the Sens-a-Brake system is an air brake system under FMVSS No. 121. If you have any further questions, please contact Mr. Chris Calamita, of my staff at (202) 366-2992

    Sincerely,

    Jacqueline Glassman
    Chief Counsel

    Enclosures
    ref:121
    d.6/25/04