Interpretation ID: aiam4063
Director of Technology
Paccar Technical Center
1261 Highway 237
Mount Vernon
WA 98237;
Dear Mr. Stephens: This responds to your letter of September 13, 1985, asking tw questions regarding the legality of an air brake system which proportions the amount of air brake pressure delivered to the drive axles of a tandem drive tractor. By 'proportions', you refer to the reduction of the applied air pressure at all treadle positions in rough proportion to the load carried by the tires. Your first question is whether an air brake system such as the one you described would be legal if it otherwise complies with Standard No. 121, *Air Brake Systems*, (49 CFR S571.121). You also would like to know if it would be legal for you to remove the front axle limiting valve in the design you propose.; By way of background information, this agency does not give approval of motor vehicles or equipment. The National Traffic and Motor Vehicle Safety Act places the responsibility on the manufacturer to ensure that its vehicles or equipment comply with applicable requirements. A manufacturer then certifies that its vehicles and equipment comply with all applicable standards. The following represents our opinion based on the facts provided in your letter.; In response to your first question, the NHTSA's regulations do no address the matter of proportioning. Standard No. 121 does not specify the design of brake system components, rather, it establishes a performance level. A brake system which proportions the amount of air brake pressure delivered to the drive axles of a tandem drive tractor is not prohibited if it otherwise meets Standard No. 121.; As to your second question, NHTSA regulations do not require nor d they prohibit a vehicle from having a front axle limiting valve. Therefore, vehicles need not have these valves in order to meet our standard and you are not prohibited from removing the front axle limiting valve in the design you propose. In fact, an interim technical report of the NHTSA Heavy Duty Vehicle Brake Research Program, published April, 1985, concluded that automatic front axle limiting valves in many vehicles significantly degrade straight line performance in the empty mode and on wet surfaces. However, if a vehicle does have an automatic pressure limiting valve, it must meet the requirements set out in S6.1.8.1 of Standard 121.; Sincerely, Erika Z. Jones, Chief Counsel