## 1 Introduction  
Light rail vehicles have been popularly applied in urban transportation systems due to their high capacity, safety, and reliability, which satisfy the needs of mass passenger transportation. To meet low-floor requirements and lightweight demands, independently rotating wheelset (IRW) technology is introduced in running mechanisms and widely studied. At the initial stage of IRW technology development, some advantages were thought to exist with the IRW, such as increased critical hunting speed, reduced rail and wheel flange wear, and improved curve negotiation performance [1; 2]. However, with more and more research and attempts for IRW's applications, some fatal disadvantages have been discovered, including unique zigzag-type vibration, severe flange contact, and aggravated wear problems [3; 4; 5]. In contrast to the conventional solid wheelset (SW), the two wheels of an IRW can rotate at different angular speeds. Then, the compensation of the path difference between the inner and outer wheel unnecessarily depends on the lateral displacement and the wheel tread conicity, which theoretically improves the curve negotiation performance of the IRW. However, decoupling the two wheels in the rotational direction leads to a loss of longitudinal creep moment, causing self-guidance-ability deficiency on the IRW. Although many methods were investigated and developed to improve the self-guidance ability of the IRW [6; 7; 8; 9; 10; 11; 12; 13; 14], only a few studies have focused on the intrinsic dynamic characteristics and the fundamental reasons why the self-guidance ability deficiency happens on the IRW. [PERSON] and [PERSON] [15] analyzed an unconstrained IRW model using the block diagram method and made a comparison with the SW model on stability issues. The theoretical frequency expression shows that on the IRW, the dynamic coupling via inertial effects can cause a kinematic oscillation. [PERSON] et al. [16] developed a multibody IRW model and conducted numerical simulations. The results demonstrated that in the change in wheel rolling radius, longitudinal slip can occur and contribute to a weak coupling of the lateral, yaw, and pitch motions. [PERSON] and [PERSON] [5] deduced mathematical IRW wheelset models and IRW bogie models with different settings of DOF and under consideration of gravitational restoring forces. According to simulation results on a tangent track, flange contact can be avoided under the restoration effects generated by gravitational restoring forces. To some extent, these theoretical and mathematical analyses of the IRW model are intrinsically and logically reasonable. However, due to some issues with hypothesis and research focus, they remain to be verified by specialized simulation software and experiments. Most researchers and engineers considered that the hunting motion was eliminated on the IRW in previous research, but the hunting phenomenon on the IRW has already been observed in some simulations or practical experiments conducted by different research groups. [PERSON] and [PERSON] [17] built an IRW simulation model in Simulink and an IRW experiment system with a test rig. They found that lateral oscillation would occur at high speeds without an active motor control. [PERSON] et al. [18] developed an active steering control method for the IRW using hub motor wheels with rotating speed difference feedback. Their IRW model was built in the software SIMPACK. In their simulation results, when the output rotating speed fluctuation of the motor was 0.5%, the lateral displacement of the wheelset fluctuated back and forth around the center line of the rail, which is similar to the hunting motion on the SW. [PERSON] et al. [19] designed a new type of bogie with four independently rotating wheels by taking advantage of the caster angle. They also found similar hunting motion on their experimental scaled IRW bogie.