Interpretation ID: nht80-1.13

DATE: 02/08/80

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: Motor Coach Industries

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your December 11, 1979, letter asking whether you would be permitted 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 actuation 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 provide 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 NHTSA 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 a problem that occurs only in your vehicles. We have not been made aware of similar problems affecting other manufacturer's 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 eliminates the chatter problem while maintaining the vehicle's compliance with the air brake standard.

SINCERELY,

December 11, 1979

National Highway Traffic Safety Agency Chief Consul

SUBJECT: REQUEST FOR INTERPRETATION -- STANDARD #121

Gentlemen: We are manufacturers of the MC-9 Crusader Intercity bus. Some of our operators, have experienced a brake chatter, from the front axle, when the coach is in reverse. This does not affect brake performance, but can be annoying to the passengers. As this situation only occurs when the coach is backing up, we question if a value can be inserted into the braking system, to stop the air from the front axle, only when the transmission is in reverse. This control would be automatic, and could not be controlled by the driver (except by shifting into "reverse").

We question if this modification will affect the compliance to Standard #121. Specifically, we are looking at Section S5.3.3 and S5.3.4 which covers apply and Release times. Also, we refer to Mr. Frank Berndt's letter to White Motor Corporation (copy enclosed), which refers to 70% of the maximum air pressure at the brake chamber. In our proposal, this could mean 70% of zero.

We would appreciate your comments on this proposal.

MOTOR COACH INDUSTRIES

Ted J. Szkolinicki, Supervisor Mechanical Engineering