| Summary: | B-double is one kind of vehicle combinations with three vehicle units structurally and widely applied in cargo transport, so its directional stability analysis is crucial but more complex than a car. Directional stability was related with vehicle operating conditions and structure parameters. A basic linear dynamic model with four degrees of freedom for B-double was developed with a special method for simplification in a calculation process. The method was verified from the simulation results with MATLAB. The step input is tractor's steering wheel angle, so the steady-state yaw rate gains with understeer gradient for three vehicle units were solved. Steady-state relative gain was also derived and has an impact on directional stability, which could be influenced by the structure parameters, such as body mass (cargo weight), tire cornering stiffness of rear axle, location of mass center, wheelbase, and location of articulated point. The results indicate that increasing body mass and tire cornering stiffness of rear axle appropriately, moving backward mass center, and moving forward articulated points could increase the understeer gradient, which results in the improvement of directional stability. It is theoretical basis for the stability tests and structure design of B-double.
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