Interpretation ID: nht74-5.23

DATE: 04/03/74

FROM: AUTHOR UNAVAILABLE; R. B. Dyson; NHTSA

TO: Great Dane Trailers, Inc.

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your November 1, 1973, letter and subsequent communication with Mr. Sidney Williams of the NHTSA Handling and Stability Division concerning the conformity of your trailers with Standard 121. You asked particularly about the diversion of service reservoir air for use in the air suspension unit, the location of the service reservoir check valve, and test conditions for extendable platform trailers. This will also acknowledge receipt of your March 14, 1974, petition for longer application and release time for special length trailers.

The NHTSA has not issued any prohibition on the use of service brake system air in auxiliary systems such as windshield wipers and air suspensions. You may tap air from the system as long as the system still meets all the requirements listed - in particular the ability of the reservoir, when pressurized to 90 psi, to release the vehicle's parking brakes at least once.

The check valve may be placed at the isolated tank to protect the trailer service reservoir as specified in S5.2.1.5, as you have detailed it in your schematic drawing.

With regard to extendable platform trailers, they should be certified in the most adverse configuration. No special configuration has been specified, and the NHTSA is free to test the vehicle at any length at which it is designed to operate.

The Fruehauf Corporation has also petitioned for relief from the application and release time requirement as it applies to extendable trailers. These petitions are under consideration and will be answered when that consideration is complete.