Published NHTSA rulemaking plans propose significant reduction in the maximum stopping distance for loaded Class-VIII commercial vehicles. To attain that goal, higher torque brakes, such as air disc brakes, will appear on prime movers long before the trailer market sees significant penetration. Electronic control of the brakes on prime movers should also be expected due to their ability to significantly shorten stopping distances. The influence upon jackknife stability of having higher performance brakes on the prime mover, while keeping traditional pneumatically controlled s-cam drum brakes on the trailer, is discussed in this paper. A hybrid vehicle dynamics model was applied to investigate the jackknife stability of tractor-semitrailer rigs under several combinations of load, speed, surface coefficient, and ABS functionality.
These simulations were run to simulate brake-in-turn (B.I.T.) scenarios for a tractor-semitrailer articulated vehicle which is near the maximum drive-through speed limit for various vehicle weight / surface coefficient conditions. ECBS-disc brake equipped tractors were directly compared to those having s-cam drum brakes.
This study shows that the simulated presence of ECBS-disc brakes on the tractor results in no degradation of the performance of the rig, in terms of jackknife stability, while braking in a turn. Furthermore, the elaborate vehicle simulations showed significant reduction in the tractor maximum yaw rate and hitch articulation angle seen during the simulation, for those simulated vehicles equipped with disc brakes and electronically controlled braking systems (ECBS).