Brake Timing Measurements for a Tractor-Semitrailer Under Emergency Braking

Abstract

The timing and associated levels of braking between initial brake pedal application and actual maximum braking at the wheels for a tractor-semitrailer are important parameters in understanding vehicle performance and response. This paper presents detailed brake timing information obtained from full scale instrumented testing of a tractor-semitrailer under various conditions of load and speed. Brake timing at steer, drive and semitrailer brake positions is analyzed for each of the tested conditions. The study further seeks to compare the full scale test data to predicted response from detailed heavy truck computer vehicle dynamics simulation models available in commercial software packages in order to validate the model’s brake timing parameters. The brake timing data was collected during several days of full scale instrumented testing of a tractor-semitrailer performed at the Transportation Research Center, in East Liberty, Ohio. Instrumented braking tests were performed at two speeds of 13.4 m/s (30 mph) and 27 m/s (60 mph) for 4 configurations including a bobtail condition, an unloaded semitrailer configuration, a half loaded semitrailer condition and a full gross vehicle weight condition. These straight-line braking tests were performed on dry concrete surfaces. In addition, brakein-turn tests and stopping tests were performed on a wet jennite surface to evaluate the vehicle response and handling for ABS and non-ABS configurations The effects of test conditions on brake timing are analyzed and are presented in this paper. The various braking configurations were simulated using detailed test parameters including brake system parameters at each wheel. Simulated vehicle kinematics were then validated against the full-scale test results and the simulation process and choices are discussed. Brake lag (delay) times and first-order model time constants are offered and discussed. The findings of this study are also compared to other testing and simulation results published in literature on this topic. INTRODUCTION Brake timing for tractor-semitrailer vehicles has been evaluated in a limited fashion in past literature aimed at either accident reconstruction or modeling using computer vehicle dynamics simulation models. In the reconstruction field, brake timing is usually identified as brake lag, or the time required for the air brakes to become fully applied. Although recognizing that wheel location has an impact on the buildup of pressure at a given axle, reconstruction analysis is usually simplified by assigning a single number meant to account for the necessary delay in obtaining full braking at all wheels. Computer simulation models on the other hand provide the ability to model braking parameters more extensively but present the opposite problem of requiring many parameters. The authors of this research wanted to offer additional and contemporary data of a real world vehicle in addition to the data already available in the public domain. The data are analyzed to provide the basis of both a simplified calculation and a more complex computer simulated analysis of tractor-semitrailer stopping performance.