It has long been considered that there is a limit to the eigenvalue analysis used to discuss the straight running stability of motorcycles. The energy flow method was proposed to solve this problem. However, it is found that the calculation method used in the energy flow method and the eigenvector equation are exactly the same. Based on this fact, a new formulation method is proposed that allows detailed discussion of straight running stability using only the eigenvector equations. This means that the traditionally used eigenvalue analysis contained useful and detailed information, but for a long time it was only unknown how to use that information.