Methodology to optimize engine mounts design in order to minimize inertial unbalances vibration propagation

Motor vehicle riding comfort is mainly affected by road roughness and volumetric engine inertial unbalances. The vibration generated by these last cause propagates till the rider passing through engine mounts, main frame and auxiliary sub systems physically in contact with driver (handlebar, footrest, seat, fuel tank, etc.). In commercial motorcycle engines, due to higher performances, costs and downsizing requirements, often no balance countershafts are installed. Thus, an innovative approach to reduce the negative effects of inertial unbalances could consist in a optimal design of mounting system. The aim of this work is to develop a methodology able to numerically predict inertial engine unbalances due to the crank mechanism and eventually to the countershaft, and to reduce the consequent propagating vibrations transmitted to the frame via engine mounts optimizing their layout. The methodology can be applied to different layouts of single/multi-cylinder engine with/without a balancing countershaft and characterized by different angles of bench engine and angular phases of the cranks. To introduce an important step ahead about the ride comfort of a motorcycle during the design process of its frame, it becomes necessary to know both the optimal positions, the number and the viscous elasticity characteristics of the engine mounts. A numerical tool was realized to calculate engine inertial unbalances and to investigate the dynamical behavior of the engine block forced by such inertial actions. The optimal layout of engine mounts is finally defined trying to minimize the shaking effects of engine block in terms of forces transmitted to the frame. The proposed methodology can consider the frame as rigid or deformable. The methodology has been applied to an existing vehicle knowing engine parameters, dimensions and masses of crank mechanism components and both frame geometry and material. The obtained results from the application of the methodology showing the provided improvement are reported in the article.