Innovative Vehicle-Trailer Connection for Yaw Stability Improvement

As well known, dangerous conditions may occur for the lateral stability of the road vehicles on increasing the running velocity. While it is easy to make up for this drawback in the case of a vehicle alone, by a proper weight distribution, the unstable trend may worsen unfavourably for the vehicle-trailer configuration, imposing a very hard correction task to the driver, who has to cope with the development of two types of instability, exponentially divergent or swaying with increasing amplitude. The critical velocity depends on the geometrical and inertial characteristics of the car-trailer system, on the cornering stiffness of the tires and on the weight distribution among the axles. The theoretical models are generally simplified to a linear formulation and the stability thresholds are searched by conventional inspection techniques on the sign of the characteristic roots. These techniques permit also detecting the single influence of several parameters on the general system stability. The present approach considers a new kind of connection system in place of the conventional hitch, consisting in linking the vehicle and the trailer by a Robert's four-bar scheme like in Fig. 1. The effect of various sources of resistance is also taken into account, like for example the aerodynamic forces, and often proves to be favourable in terms of stable running. The two types of instability thresholds, divergent or oscillating, may be analyzed by the Routh-Hurwitz criterion or by the direct analysis of the characteristic equation: the divergent instability threshold implies the vanishing of the constant term of this equation (vanishing of a real root), while the oscillating instability threshold splits it into two lower-degree algebraic "sub-equations" (pure imaginary pair of roots). A large field of geometrical configurations of the four-bar linkage may be tested by numerical search procedures, including the configuration where the linkage side bars are forward directed, i. e. towards the inside of the drawing car. Plot of the critical velocity as a function of the distance d between the mass centre GT and the axle of the trailer may be traced. They clearly show the favourable influence of the forward four-bar connection for small values of this distance and of the backward connection for larger distances, passing through a sort of inversion point, or strictly speaking through a very narrow inversion region, where no benefit can be achieved in comparison with the conventional hitch. Therefore, it is conceivable to increase the critical speed by optimizing the four-bar design depending on the weight distribution on the axles. Moreover, the under-steer or over-steer behaviour may also be corrected by this type of connection assembly.