This paper deals with the stability analysis of a novel four wheeled tilting motorcycle. This new kind of vehicle is equipped with two front steering wheels and two rear wheels; the front and rear suspension mechanisms allow the scooter to roll freely as a standard motorcycle. An analytical model of the vehicle with locked suspensions, rigid and lenticular tires was developed in order to study the stability of the system in straight running and to carry out a modal analysis focusing on the main normal modes. The frame and all the vehicle components were considered as rigid bodies, the driver was considered fixed to the frame and the tires behavior was described by a linear function of the camber and side-slip angles. The action of the driver on the handlebar was not taken into account, considering the vehicle running in straight without any control system. The stability analysis shows that the eigenmodes of the four-wheeler have several common features with the capsize, weave, wobble and rear wobble modes which characterize the dynamics of two and three wheeled motorcycles. A sensitivity analysis showing the influence of the front trail on stability and the vehicle eigenmodes is presented. A graphical and intuitive representation of the eigenvectors is also given.
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