The twist axis of frames with particular application to motorcycles

The static and dynamic properties of the frame and the front fork of a single track vehicle play a critical role from the point of view of vehicle stability. A turning point in the study of motorcycle stability was established by the introduction of lumped stiffness elements to characterize the critical compliances of the motorcycle elements, this approach being still in use with advanced multibody codes. Nonetheless, up to now very few scientific studies have been carried out to identify the parameters that account for the stiffness and damping properties of motorcycle front forks and frames. This work addresses the problem of identifying the parameters needed for developing lumped element models of motorcycles from experimental results. Two motorcycle frames are studied performing static, dynamic, and modal tests by means of a specific testing equipment. The frames have been tested in two different conditions: fixing them at the steering head or at the swing-arm pivot. In the first section of the paper a general definition of the twist axis, based on the concept of ‘‘Mozzi’’ or instantaneous screw axis, is presented. The twist axis is used for characterizing the deformation patterns of the tested frames. The static twist axis is identified loading the frames at low rate by means of a servo-hydraulic actuator and measuring the deformation of a reference plate by means of three laser sensors; the dynamic twist axis is identified exerting an impulsive excitation and measuring the vibration of a reference plate by means of three accelerometers. In the last section of the paper, experimental results obtained on motorcycle frames are shown. A method to identify the stiffness properties of the frames from the measured twist axes is presented. Results obtained with the proposed method are in good agreement with the ones presented in literature..