Abstract This study analyzed the longitudinal vibration of a vehicle body and unsprung mass. Calculations and tests verified that longitudinal vibration can be reduced using in-wheel motors, which generate torque very quickly. Despite increasing demand for measures to enhance ride comfort considering longitudinal vibration, this type of vibration cannot be absorbed or controlled using a conventional suspension. This paper describes the reduction of vehicle longitudinal vibration that cannot be controlled by conventional actuators. 1. Introduction Measures to help preserve the environment have become an essential part of vehicle development. Vehicles powered by electric motors such as hybrid and fuel-cell vehicles are an effective way of helping to reduce greenhouse gas emissions. Furthermore, in addition to environmental friendliness, motor drive allows torque to be controlled freely with high response and precision, adopting both forward and reverse rotation. As a result, motors can be used to achieve enable a high degree of controllability even for functions related to ride comfort and handling performance. In addition, in-wheel motors (IWMs), i.e., motor units that are installed inside the wheels of the vehicle, greatly increase the freedom of part layout. Consequently, IWMs have the potential to revolutionize vehicle development in ways not feasible with conventional vehicle configurations, including the way that vehicles are manufactured. Furthermore, installing IWMs in all four wheels enables control of the vehicle posture [ 1], and independent drive control of each wheel provides various advantages in terms of dynamic performance [ 2]. Structurally, IWMs are also a simple way of increasing stiffness from motor to output and are not affected by conventional drive resonance issues caused by driveshaft stiffness. For these reasons, IWMs are also capable of controlling high-frequency vibration [ 3], which is difficult to achieve with a normal actuator layout. Using these IWMs characteristics, this paper describes research that analyzed longitudinal vibration caused by road surface disturbances in a vehicle installed with IWMs, as well as the development of a control method to reduce longitudinal vehicle vibrations using simulations and tests. 2. Longitudinal Vibration and Ride Comfort Longitudinal vehicle vibration affects ride comfort by applying vibratory force to the unsprung mass in a resonance frequency range of 8 to 16 Hz. The effect of the suspension layout in helping to reduce vibration has already been verified [ 4]. The unsprung mass contacts the road surface through the tires and, in the case of a conventional suspension, it is fixed to the body through elastic elements such as bushings, the suspension arms, springs, and shock absorbers. Therefore, unsprung vibration consists of the following three elements. 1. The force acting on the tires(Ft) 2. The reaction force of the springs and elastic elements(FSP) 3. The reaction force of the shock absorbers(FSA) However, this configuration requires compromises between various performance aspects and the securing of sufficient occupant or storage space. This restricts the freedom of the suspension layout and complicates the adoption of optimum measures to reduce vibration. In addition to these three forces, this research proposed a fourth element: driving and braking force (F u) using IWMs as a means of reducing unsprung vibration ( Fig. 1). In the case of a conventional independent suspension driven by a driveshaft, driving force acts around the wheel center. However, when driven by IWMs, the reaction force to the drive torque is confined to the suspension and the drive force acts at the same ground contact point as the braking force (Fig. 2). This research assumed that the wavelength of the road surface displacement was longer than the tire ground contact length.Reduction of Longitudinal Vehicle Vibration Using In-Wheel Motors2016-