With ride comfort in a motorcycle gaining significance, it is important to minimize vibration levels at the customer touch points. The reciprocating piston imparts rotary motion to the crankshaft which in turn induces unbalance forces and produces vibration in the vehicle, thus influencing the ride quality. Generally, the primary inertial forces are balanced by a combination of balancer body and crank web. However, being a commuter bike, a balancer body could not be accommodated due to cost and space constraints. In such scenario, the first order unbalance force cannot be completely eliminated but can only be redistributed by adding counterweight to the crankshaft. Proper distribution of these forces is required for optimum vibration levels at motorcycle touch sensitive points (TSP) such as handle bar, footrest etc.
In the current study, crankshaft of a single cylinder motorcycle engine is optimized for balancing to reduce vibration at the TSP through multi body dynamics (MBD) and finite element (FE) simulation tools. The complete crank train comprising of piston assembly, connecting rod, bearing, crankpin and crankshafts are modelled with accurate mass and inertia in a commercially available MBD software. Crankshaft balancing factor and the angle of unbalance force are varied by changing different design parameters of the crankshaft such as web radius and width, modification in shape etc. Inertia forces at engine mounting locations due to the first order unbalance of crankshaft are predicted using MBD simulation. These forces are then given as input to full vehicle FE model to predict the vibration response at TSP for the operating speed range of vehicle. Crankshaft design is finalized based on optimal vibration response at TSP.