Driveline Vibration Analysis for Successful Clutch Spring Pack Design for a 4 Cylinder Motorcycle Engine

The design of the spring pack providing torsional compliance within motorcycle clutch assemblies is often determined from the mean torque and a factor to allow for torque fluctuations. While this approach may work for many applications the selected spring rates can sometimes cause a driveline resonance within the operating speed range with major implications for NVH. For motorcycle engines it is also often impossible to select a spring rate low enough to shift the resonance below the operating speed range due to the requirement to transmit torques under steady load and transient events within tight spring package constraints.

This paper demonstrates the approach of using a linear frequency domain analysis to model the entire driveline, from the crankshaft to the bike mass, to provide a relatively quick assessment of the driveline torsional vibration and assess potential design solutions. The paper also demonstrates a novel method for the calculation of an optimum variable rate clutch spring design to minimise the resonant vibration for 4 cylinder engines where isolation is not possible.

This method was applied to the development of the 1.3 liter 4 cylinder BMW K1300 motorcycle engine in order to avoid a potential NVH issue caused by a resonance across the clutch system. Measurements of the torsional vibration were taken from the motorcycle on a rolling road and used to sufficiently correlate the baseline model and to confirm the significant improvements predicted by the analysis of a modified spring pack design.