Forensic Engineering Analysis of Commercial Vehicle Air Brake Systems Performance

Braking systems for heavy commercial vehicles differ greatly from the design for light-duty motor vehicles. For example, 49 CFR 571.121 and 49 CFR 393.52 require loaded buses, single unit commercial vehicles, and vehicle-trailer combinations equipped with air brake systems to generate sufficient braking force to meet specific stopping distance, stopping acceleration rate, and brake force-to-weight percentage performance criteria. The combination of unique design, mechanical complexity, and maintenance issues characteristic to air brake systems also pose difficulty in the analysis of air brake system performance. Air brake system performance presents a difficult problem for the forensic engineer with limited familiarity regarding air brake system functions and the elements affecting brake performance. This paper provides insight into the evolution of air brake system standards and the applicable performance criteria for heavy commercial vehicles. The methods presented allow the forensic engineer to mathematically analyze and determine the effects of brake size, mismatched components, brake adjustment, and system air pressure on the overall braking force and stopping capabilities of air brake equipped commercial vehicles.