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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.

This paper discusses dry stopping performance comparisons of various foundation brake systems on Class-8 truck tractors (having a GVWR greater than 33,000 lbs.). For these studies, four configurations of foundation brakes were fitted to two modern 6x4 conventional truck tractors without modification to the control, application, or ABS systems. Foundation brakes compared include standard S-cam drum brakes on all six positions, high-output S-cam drum and then air disc brakes on the steer axles, and air disc brakes on all six brake positions. Discussions include analyses of stopping distance from 60 mph (96.6 kph) for all test conditions. The truck tractors were tested in two weight configurations, LLVW (i.e., bobtail) and GVWR (50,000 lbs. total axle weight, using an unbraked control semitrailer).