Interpretation ID: aiam3196
Supervisor
Mechanical Engineering
Motor Coach Industries
Pembina
ND 58271;
Dear Mr. Szkolnicki: This responds to your December 11, 1979, letter asking whether yo would be permiited to install a valve in your braking system that would prevent air from reaching the front axle brakes when your vehicle is in reverse. You want to make such a modification to prevent brake chatter when your vehicle is in reverse and question whether such a modification would comply with the requirements of Standard No. 121, *Air Brake Systems*.; Sections S5.3.3 and S5.3.4 of the standard establish brake actuatio and release times. In an interpretation of those sections (July 23, 1976), the agency stated that the air pressures of 60 psi and 95 psi were only benchmarks, and that the agency would use either of those values or 70 percent of the maximum pressure in the brake chamber, whichever is lower. You state that this interpretation allows you to install a valve, because the maximum air pressure reaching the front brake chamber when the vehicle is in the reverse gear would be 0 and 70 percent of 0 is 0. Therefore, you suggest that your vehicle would pass the tests in these two sections if tested at 0 psi.; The intent of the July 23 interpretation of the sections was to provid flexibility of designs that incorporate lower air pressures than originally contemplated by the air brake standard. The interpretation was not intended as a device to escape from compliance with the air brake standard by creating a situation where front brakes would be rendered inoperative. Accordingly, the agency limits its July 23, 1976, interpretation to those instances where air brakes are receiving air pressure and are performing as designed to stop the vehicle. Using this limitation on our July 23 interpretation, the NHTSSA concludes that your new brake design would violate the standard and, therefore, will not permit the use of the valve that you recommend.; The brake chatter that you refer to in your letter appears to be problem that occurs only in your vehicles. We have not been made aware of similar problems affecting other manufacturers' vehicles. Accordingly, we must assume that something in your design is creating the chatter problem. We suggest that you alter your brake design in a way that elinates the chatter problem while maintaining the vehicle's compliance with the air brake standard.; Sincerely, Frank Berndt, Chief Counsel