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Recently, it has been widely practiced in motorcycle developments that the same type of engine is commonly applied to various vehicle categories. Accordingly, it is drawing more attention to develop the methodology for creating the best suitable sound for each individual vehicle category regardless of restriction from the engine configurations. In our study, we aimed to establish a procedure to control exhaust sounds beyond the borders across the inherent sound qualities originated from their engine configurations. Firstly, we conducted subjective tests in order to extract essential factors, depicted by adjectives that appear in verbal expressions commonly used to illustrate sound qualities in general. The results enabled us to conduct quantitative evaluations of the exhaust sound qualities of various motorcycles. Next, we clarified the relationships among the individual factors of sound qualities under our study and physical parameters in waveforms of the sounds.

Recently, rider comfort at highway driving has become an important issue in the performance improvement of motorcycles. Comfort includes windbreak, pressure on the rider, wind noise, visibility, and steering characteristics. However, most of these factors cannot be analyzed conventionally. Therefore we used CFD (Computational Fluid Dynamics) to examine the rider’s airflow environment. The environment of a rider on a motorcycle is an open space, very different from the passenger environment in an automobile. There are many airflow paths in a motorcycle with a cowl and a windscreen; thus, airflow behavior is constituted with a delicate balance. Though wind tunnel tests can give us an outline of airflow, CFD is a useful way to visualize the airflow of the rider’s environment and clarify the details. As the results, a lot of helpful knowledge was obtained for the development of new motorcycles.

Examining the noise reduction of a motorcycle, the requirement of an effective method of reducing a drive chain noise has been a pending issue similarly to noise originating from an engine or exhaust system, etc. Through this study, it became clear that the mechanism of chain noise could be classified into two; low frequency noise originated from cordal action according to the degree of chain engagement and high frequency noise generated by impact when a chain roller hits sprocket bottom. An improvement of urethane resin damper shape, mounted on a drive side sprocket, was effective for noise reduction of the former while our development of a chain drive that combined an additional urethane resin roller with an iron roller worked well for the latter. The new chain system that combined this new idea has been proven to be capable of reducing the chain noise to half compared with a conventional system.