Improving the NVH performance of a motorbike powertrain

A CAE investigation of a motorbike showing an unsatisfactory NVH behavior during gear shifts: identification of the root causes and definition of the most effective design modifications.

The study’s motivation was the assessment of the NVH behavior of a motorbike engine driveline, during its working condition, in first gear. More precisely, its behavior was considered in transient conditions during relative rotations between the first and fifth gear of the driven shaft. The study was carried out to investigate the possible causes of an undesired noise observed during road and lab tests and to identify the most effective design changes to remove it.

A CAE campaign was carried out to assess the effectiveness of design variants devised to reduce the mechanical excitation due to the impacts between the gears and to analyse the behaviour of the engine casing, whose vibrations are the acoustic source.

The CAE results showed that following design variants are the most effective in terms of mechanical excitation for the powertrain system:
• reduction of the circumferential gap between the gears’ teeth and the slots;
• adoption of different driving shaft’s cross section and of a cam coupler.

The behaviour of the engine casing as an acoustic source was quantified by a loudness index related to the casing’s eigenmodes. A comparison between different engines based on that index matched test data.