Interpretation ID: nht74-1.47

DATE: 11/12/74

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

TO: Oshkosh

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your October 8, 1974, question whether a front axle automatic pressure limiting valve may be removed during the burnish procedure to permit effective burnish of the front brakes.

The answer to your question appears in Notice 6 to Docket 74-10 in response to a similar inquiry from International Harvester. A copy of that notice is enclosed for your information. It amends S6.1.8.1 to require that any automatic pressure limiting valve be in use except in the case where the temperature of the hottest brake on a rear axle exceeds the temperature of the hottest brake on a front axle by more than 125 degrees F. A bypassed valve is reconnected if the temperature of the hottest brake on a front axle exceeds the temperature of the hottest brake on a rear axle by 100 degrees F.

Yours truly,

Enclosure

ATTACH.

OSHKOSH TRUCK CORPORATION

October 9, 1974

Richard Dyson -- U.S. Dept. of Transportation, National Highway Traffic Safety Administration

Subject: 49 CFR Part 571, FMVSS 121 - Air Brake Systems

Dear Mr. Dyson:

Per Federal Bureau of Motor Carrier Safety Regulations, Part 393, Section 393.48, use of automatic devices for reducing front wheel brake effort is permitted on vehicles complying to FMVSS 121.

With the automatic pressure reduction valve in the front axle, Oshkosh Truck Corporation has found that during burnishing of some vehicles per Section 6.1.8.1 of FMVSS 121, the rear brakes reach 500 degrees F, much earlier than the front axle brakes because of the automatic reduction of brake force to the front axle and, therefore, effective burnishing of the front brakes is not achieved.

Please advise if the automatic device to reduce the front wheel brake force can be removed during the burnishing procedure.

Very truly yours,

P. K. Kamath -- Sr. Safety Engineer