1992-11-01

Air Brake Inspections on Five-Axle Combinations 922443

Numerous government studies have documented the need for improvement in the condition of heavy vehicle air brake systems. There is an apparent lack of good data indicating the condition of the air brake system on heavy trucks in the United States. Until the National Transportation Safety Board (NTSB) conducted its heavy truck inspection project, there was no database which could accurately portray the type and size of braking components used, the adjustment level, and the condition of these braking components.
A careful analysis of the 1520 five-axle heavy trucks which the NTSB inspected provided needed insight into a severe problem. These inspections also demonstrated that current state truck inspections are not always able to identify these brake problems. There is a major discrepancy between the figures the states submit on the condition of air brakes compared to what the NTSB found. The Motor Carrier Safety Assistance Program (MCSAP) reported 18 percent of the vehicles the states inspected in 1990 were placed out of service for brake defects. The NTSB's random inspections placed 53 percent of the vehicles out of service for brake defects from the interstate portion of the sample.
By identifying the magnitude and scope of the air brake problem on heavy trucks, the solution should be more readily discovered. The inspection sample was broken down into an interstate and an off-interstate portion. Over 128 items of data were gathered for each vehicle. A complete description of the brake system was recorded, including the type of slack adjuster, chamber size, and each measured pushrod stroke.
This paper examines this brake problem primarily as it relates to the adjustment of the brakes. It compares the adjustment to location on the vehicle, the type of cab, type of trailer, the tractor year, the type of slack adjuster, the carrier size, the trip distance, and who is responsible for the adjustment. Finally possible reasons for the differing results between the NTSB and MCSAP data are discussed, as well as some suggested improvements.

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