There have been a number of attempts to clarify the relationship between motorcycle specifications and shimmy phenomenon. Some of such efforts are based on equations of motion. The methods used in those efforts are suitable for analyzing motions in a fundamental structure. However, when the degree of freedom is large, it is extremely difficult to deliver an equation of motion. Therefore, a practical method cannot be found generally when applying the methods employing equations of motion.We also conducted the analysis of shimmy using multi-body dynamics simulation. The yielded results were useful only for clarifying the differences in shimmy levels among motorcycles. However, they were not helpful to understand the relationships between specifications and shimmy phenomenon.In this study, we focused clarifying these relationships and we took four study steps shown below: 1Narrowing down the motorcycle specifications affecting shimmy2Determining physical parameters influential to shimmy3Investigating how a change of physical parameters affects shimmy using simplified model4Analyzing how the changes of motorcycle specifications affect the shimmyFollowing these steps, we clarified the relationships between motorcycle's specifications and shimmy by using only three physical parameters.