| Although modern types of the tractor are various, but most of the main are front wheel steering (FWS) types. Four wheel steering (4WS) system more general has been applied in the sphere of production less in tractor. Its types are smaller and the studies are mainly to the low-speed maneuverability. 4WS vehicle can improve the mobility and reduce the turning radius at low speed by the reason of reverse phase shift between front and rear wheel; since rear for the same phase shift with front wheel, a moving vehicle's direction has been changed and the yaw rate and lateral acceleration achieve steady-state response soon, which improve vehicle handling and stability at high speed. So, if the technology of 4WS is applied in the tractor-trailer reasonably, the increase in its handling and stability have been achieved a large extent.1. Dynamic model analysisIn this paper, simplify analysis of this system of the tractor-trailer and the four-wheel steering technology (4WS) have been adopted for tractor, the handling and stability, especial lateral stability characteristics of 4WS train have been simulated and analyzed.Firstly, the author analyzes the degree of freedom of the tractor-trailer's movement and dynamics, then, the sideslip characteristics of the tire various theories and empirical model of 4WS train are deduced. Finally, through simplifying, the bicycle model of three degree of freedom (3-DOF) 4WS tractor-trailer system is deduced and the author specifies the model equations and transforms the movement into a state equation.2. Simulation descriptionThe yaw moments of inertia for the tractor and the trailer can be derived from empirical formula and then the author calculates the matrix parameters of the equations of state, again with the software of MATLAB, the given four-wheel steering wheel angle ratio coefficient of tractor-trailer's lateral sway response characteristic is simulated in the domains of time. The linear lateral stability of the 4WS system tractor-trailer, the tractor sideslip velocity, yaw rate and parameters of the trailer are analyzed.3. Similar model testIn this paper the model vehicle is used to calibrate the mathematic model. The simulation experiment is based on the principle of similitude. After model build and the experiment results of simulation experiment analyze, the model vehicle experiment have be carried out other than full-scale trailer test which can not be tested because the test is very dangerous at the critical status.The VB program has been applied to realize the control to transducer's serial communication that ensure the need of speed changing and speed stability. This paper retrieves and improves the simulating test bench that can provide the good assurance and the experimental basis for the following simulation experiment. In experiments, some parameters of the model trailer can be changed to find the effect of stability caused by velocity, the position of towing point, the position of suspension mass center and so on.4. Analyze stability by the changing of parameters of the tractor-trailerAt last, the author analyzes the stability capability infection by the changing of parameters of the tractor-trailer in this paper. Through changing the construction parameters such as the center of gravity (CG), location of bicycle saddle, etc and kinematics parameters such as velocity, etc, respectively, which how to effect the lateral sway has bee deduced.According to simulation experiments and analysis, some characteristics of lateral sway phenomenon have been found.(1) The sway appears in the straight roadway(2) The sway appears easily in critical speedStrictly speaking, the sway phenomenon occurs at a certain critical speed. The sway only appears when practical speed is higher than critical speed. After the sway appeared, the speed must be lower than critical speed in order to eliminate or fade the sway.(3) The sway is caused by the vehicle's self-excited vibrationThe simulation experiment has good experimental condition in the laboratory. The sway phenomenon appears in good condition that indicates that self-excited vibration is the fundamental reason of vehicle's sway.(4) The sway possible persists in a long distanceWhen the sway appears, if the condition of road and swing speed still exists, the sway can persist in a long distance.(5) Some effects of longitudinal force on the sway.In simulation experiments, when the tractor vehicle was during towing acceleration, the sway was not easy to appear. But when the speed's desired value once has inert condition, the sway happens. By preliminary analyzing, when the force of towing point in the direction of vehicle's traveling is larger than zero or equal to zero, the sway phenomenon will appear. The driver has such impression that when the vehicle brakes slightly at high running speed, the vehicle will sway seriously.The result shows that the tractor-trailer can achieve the handling and stability more at high-speed and little hitch angle turning with 4WS system which is same as at high-speed straight driving. Obviously, the tractor-trailer's lateral sway phenomena in itself, which is not effect by the maneuverability of rear steering and can be to stability soon all the same. The favorable stability capability can be deduced through analysis of the simulation.
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