When is it too Late to Brake?

This paper provides a simulation analysis of the braking action that would prevent untripped rollovers of long combination vehicles (LCV) in turns when the entry speed into a turn exceeds the vehicle’s threshold. A co-simulation model is used to integrate the details of truck pneumatic brakes into a TruckSim® model. The brake system model is developed in Simulink. Both the TruckSim® and Simulink models are validated using data from field tests. Using the validated models, various braking initiation times (relative to the start of steering) are performed for a 150-ft J-turn. The J-turn simulates an exit ramp or curved roadway. The simulation results reveal that at higher speeds, there is very little time for the driver to initiate braking before it is too late to avoid a rollover, referred to as the Critical Brake Initiation Time (CBIT). For instance, at an entry speed of 40 mph (64 km/hr), the driver of a fully-loaded truck has approximately 1.0s before recognizing that the speed is too excessive for the turn. Applying the brakes beyond this time would not prevent a rollover. The challenge often stems from the fact that for long combination vehicles, the driver can not accurately sense the trailer’s roll dynamics, which can greatly hinder the driver’s timely response to reducing speed to avoid rollovers. A key question is “when it is too late to brake?” To provide answers, various loading conditions and entry speeds are simulated. The results indicate that heavier loads that result in higher trailer CG require both lower entry speeds and sooner braking to avoid rollovers, somewhat as expected. The CBIT is highly influenced by the entry speed into a turn. For instance, for a fully-loaded LCV, increasing the speed by 20% from 40 mph (64 km/hr) to 50 mph (80 km/hr), reduces CBIT by 90%, from 1.0s to 0.1s. The effect of load on CBIT is less dramatic than speed. At 40 mph (64 km/hr), increasing the cargo load by 47%, from 15000 lb. (6800 kg) to 22000 lb. (10000 kg), decreases the CBIT by 20%, from 1.2s to 1.0s.