The limit yaw stability of heavy vehicles while cornering is examined by means of computer simulation and full-scale tests. Research results are presented from as-designed vehicles as well as vehicles which have been modified with increased frame and front roll stillness. A discussion of vehicle dynamics is used to explain the limit yaw stability behavior of trucks and tractor-semitrailers. Particular attention is given to the effects of overturning moment reaction by the front and rear suspension systems and how this distribution influences vehicle yaw stability.

Research results show that while as-designed vehicle may exhibit yaw instability well below the vehicle rollover threshold for certain vehicles, increased frame stiffness and modilied roll stiffness distribution can greatly reduce such premature yaw instability. Computer simulation studies of the influence of vehicle design and operating variables on tractor yaw stability demonstrated the relative importance of different roll stiffness options, as well as the effects of tire mix, fifth wheel placement, and trailer loading practices on vehicle yaw stability. It was discovered that low levels of tractor yaw stability are possible with certain combinations of vehicle design and in-use variables. Vehicle design and in-use criteria is presented that will yield increased vehicle yaw stability, especially during steady state cornering.