Comparing Grade Severity Rating System Models for Trucks Fitted with Drum Brakes versus Disc Brakes

Excessive brake heating of trucks on downgrades is a cause of continuing concern for the Wyoming Department of Transportation (WYDOT). Brake failure on downgrades characteristically takes a catastrophic toll on lives and property. The Grade Severity Rating System (GSRS) developed by the Federal Highway Administration (FHWA) recommends a maximum safe speed limit that has been identified as a feasible remedy for reducing the incidence of downgrade truck crashes. However, truck characteristics and roadway geometrics have changed over the years following the development of the GSRS. To deal with this development, a research project was initiated by the WYDOT in 2016 to update the GSRS model. The test truck used for the field tests in the prior research project was fitted with disc brakes on the front axle and drum brakes on the rear axle. However, disc brakes represent only about 20% of the brake market. This article discusses the validation of the GSRS model for trucks fitted with only drum brakes. The procedure was achieved by conducting field tests, specifically the Hill-Descent (HD) and Validation Tests with a fully loaded truck fitted with only drum brakes. The main objective of the field tests was to derive an equation for the heat coefficient, K ₂, and then compare it to the K ₂ obtained from the field tests conducted in 2016. The K ₂ value derived from the tests was minimally different from that computed for the scenario of the test truck equipped with both disc and drum brakes. This was established by examining the maximum safe descent speeds generated by the previous updated model and the model developed in this study. They were essentially the same.