A ball bearing reaction wheel (RW) is a key attitude control actuator of spacecrafts, but it is also a major source of inner disturbances. Future space mission requires high attitude stability, and disturbance property of the RW must be improved. There are some disturbance sources inside the RW, and abnormal motion of a retainer is one of the most significant ones. The retainer is one of mechanical parts of a ball bearing supporting a rotor spin axis. It is used to keep the ball intervals. Therefore it is nonholonomically constrained with balls, an inner race, and an outer race, and its complex motion causes disturbances which are difficult to be effectively removed. In this paper, dynamics of the retainer is investigated through experimental tests and numerical simulations. Disturbances of normal and abnormal RWs are compared, and relation between retainer mass imbalances and their dynamics are investigated. As results, a trade-off relation between instability reduction and disturbance reduction is verified and one of the criteria to decide the appropriate mass imbalance is proposed.