Nhtsa Research Efforts to Significantly Improve Braking Performance of Medium and Heavy Trucks

In 1999, National Highway Traffic Safety Administration (NHTSA) researchers theorized that substantial improvements could be made in the braking performance of medium and heavy trucks. Therefore, NHTSA initiated a multi-year research program to learn what improvements in stopping performance could be achieved using advanced, but currently available, brake technology for medium and heavy trucks. Truck tractors were the first type of heavy truck studied. Tractor testing results, including dry stopping distance, wet brake-in-curve stability evaluations, and wet split coefficient of friction stopping distances are presented. Testing results showed that a 30 percent reduction in maximum permissible dry stopping distances is possible for U.S. truck tractors, with no degradation in other performance areas. Objective measurements of brake torque, measured on NHTSA’s inertial brake dynamometer at speeds up to 112.7 kph, are presented. Vehicle dynamics simulation results were used to understand effects that higher-torque brakes might have on jackknife stability during braking of tractor-semitrailer rigs. Changing tractors to have all air disc brakes make braking performance improvements attainable with incremental costs that are outweighed by the expected benefits. Unforeseen improvements include a nominal 5 to 8 percent improvement in stopping distance during ABS-controlled stops on wet pavement, a result of significantly lower brake hysteresis with air disc brakes. Hybrid brake configurations, utilizing larger, more powerful S-cam drum brakes or air disc brakes on the steer axle only, are also shown to provide significant performance improvements over current foundation brakes. Based on this research, NHTSA has proposed revising FMVSS 121; shortening the maximum permitted stopping distance for truck tractors by 20 to 30 percent. The paper concludes by briefly discussing NHTSA’s research to improve the stopping performance of medium and heavy straight trucks. INTRODUCTION In the United States, Federal Motor Vehicle Safety Standards (FMVSS) 105 and 121 currently require medium and heavy trucks (vehicles with Gross Vehicle Weight Ratings (GVWR) of 4,537 kg to 11,794 kg are medium trucks, ones with a GVWR of more than 11,794 kg are heavy trucks) to stop from 96.6 kph, on a high coefficient of friction pavement and with properly working brakes, in the distances shown in Table 1. In comparison, FMVSS 135 requires light vehicles (vehicles with a GVWR of 4,536 kg or less except for motorcycles) to stop, in similar conditions, in 70.0 meters from 100.0 kph. These standards also set required failed system/emergency brake stopping distances (not shown). Required failed system/ emergency brake stopping distances are substantially longer for medium and heavy trucks than for light vehicles. Table 1: Current Stopping Distance Requirements for Medium and Heavy Trucks Type of Vehicle Empty Stopping Distance Loaded Stopping Distance