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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.

When reconstructing a highway accident involving a heavy truck, an accurate deceleration rate is often an elusive piece of the puzzle. This paper presents a method for calculating the deceleration rate of a heavy truck equipped with an air-mechanical brake system. Traditionally a deceleration rate had been more an estimate of the percent of wheel lock-up based on the brake adjustment, and then proportioned to the loading on the axles. Equations for two configurations of brake chamber size and slack adjuster length were written by engineers at the National Highway Traffic Safety Administration's (NHTSA) Vehicle Research and Test Center (VRTC) to describe braking torque as it varies with pushrod stroke. These equations were developed by means of a regression analysis, to fit a curve to dynamometer data produced at the VRTC in East Liberty, Ohio. Once this braking torque is converted to a braking force, it can be compared to the braking force available at the tire/road interface.