Identification of the critical stiffnesses of motorcycles in static and dynamic conditions

Stiffness of structural elements has a significant effect on the dynamics of single-track vehicles, because it influences the stability of the typical modes of this class of vehicles (weave and wobble). Up to date no specific method for measuring the critical stiffnesses of front fork, chassis and swingarm is universally recognized. This measurement is difficult chiefly for two reasons. When a structural element of a single-track vehicle is loaded at one end it undergoes both bending and torsion deformation and stiffness has to be decomposed into the bending and torsion components. The stiffness characteristics measured in static conditions may be rather different from the ones measured in the presence of dynamic loads, owing to the excitation of vibration modes. The concept of Mozzi or twist axis is used in this paper for giving a lumped element representation of stiffness of structural elements of single-track vehicles. Then the differences between stiffness characteristics measured in static and dynamic conditions are highlighted and analyzed. Finally, a novel method is proposed for the decomposition of stiffness, it makes use of the axes of the bending and torsion modes. These axes are identified by means of impulsive tests measuring the frequency response functions of three points of a rigid plate that moves with the loaded end of the structural element. Experimental results dealing with swingarm, chassis and front fork are presented.