Experimental evaluation of lateral transfer functions and structural modes of two-wheeled vehicles

The dynamic behaviour of two-wheeled vehicles is simulated by means of powerful multi-body codes in which the vehicle is described as a system ofrigid bodies with elastic suspensions and tires [l]. The structural modes of vibration of motorcycle and scooter chassis are usually neglected, but they may bave a significant influence on severa! features of vehicle's behaviour such as comfort, shockabsorption and braking, which are related to dynamics in the symmetry piane, and handling and stability, which are related to out-of-plane dynamics [2]. The purposes of this research program are: the identificati an of the structural modes of vibrati an of two-wheeled vehicles, the measurement of the latera) transfer functions (between latera! accelerations and latera! tire forces) and, eventually, the integration of experimental modal analysis with multi-body simulation. Structural moda! analysis of the vehicle's subsystems (such as chassis, forks and tires [3]) and ofthe whole vehicle was carried out. Structural moda! analysis of the whole vehicle is very interesting, since the identified moda! parameters (natura! frequencies, mode shapes) are related to vehicle stability and handling. When performing moda! testing of the whole vehicle, constraints similar to the ones caused by the contact between the rolling tire and the road bave to be introduced. Far this reason the modal analysis was carried out exciting one tire with a special equipment and with the other tire in free condition or in contact with a rough surface. The structural modification method was then used to correlate the results achieved in the two testing conditions and to simulate the constraints that are present when the tire rolls on the road. Moda! analysis was carried out with global methods and results are presented in terms of lateral transfer functions of significant points ( e.g. handle-bars), natural frequencies, damping factors and mode shapes. Finally the influence of structural behaviour on stability and handling is discusse d. The different ranges of motorcycle and scooter modes are highlighted and the effect of low frequency torsi an modes of scooters is discussed too.