The suspended in-wheel motor drive unit (SIWMDU) is a new electromechanical integration method with a drive system that is elastically mounted within the wheel through spring-damper elements. This configuration delivers important benefits, such as extended motor service life and avoidance of negative effects caused by high unsprung mass. However, the implementation of internal suspension not only raises the number of components but also considerably decreases available space within the wheel envelope. These factors greatly elevate the complexity of mechanical architecture designs. Most current studies in the literature emphasize performance analysis and parameter optimization of specific configurations while ignoring the underlying logic of configuration selection and the possibility of discovering novel and more optimal design solutions. To fill this gap, this study proposes a systematic method to analyze, enumerate, and evaluate various SIWMDU configurations under a unified methodological framework.

This research introduces a configuration synthesis and evaluation methodology that incorporates morphological analysis, topological modeling, and quantitative assessment using a preference matrix. Initially, a functional requirement table is established based on a detailed analysis of existing SIWMDU architectures, which identifies four essential system-level functions: driving, braking, propulsion, and vibration isolation. Then, the main mechanical components are categorized into two morphological classifications according to the rotational behavior of the motor (inner-rotor vs. outer-rotor), and a morphological matrix is constructed to map functional elements to feasible component forms. Thereafter, a set of topological connectivity diagrams is built to correspond to mechanical interfaces and constraints among components, which enables the systematic and exhaustive generation of valid configuration candidates. Ultimately, three evaluation metrics, namely, suspension space, thermal performance, and upright complexity, are chosen to reflect the core difficulties in an SIWMDU design, and these criteria are utilized in a preference matrix to quantitatively assess and compare the performance of each configuration candidate.

Using the proposed methodology, the study specifies a total of 18 feasible configurations (6 inner-rotor-based and 12 outer-rotor-based). Among these configurations, well-documented benchmark designs and previously unidentified new configurations are obtained. A novel configuration is emphasized and analyzed comprehensively. It employs an outer disc brake and thin-section bearings, which eliminates the need for a wheel hub or spokes; this greatly expands the usable internal space. The motor in this configuration is positioned outside the wheel and is connected to the rim through a newly designed suspended coupling, which streamlines the upright structure and offers greater flexibility for suspension layout. Comparative evaluation reveals that this configuration realizes notable enhancements in suspension space utilization, heat dissipation, and upright simplification; thus, it provides a promising solution to the space and packaging limitations of current SIWMDU designs.

The results verify that the proposed approach allows structured and exhaustive configuration exploration while enabling rigorous multi-criteria evaluation to recognize optimal mechanical solutions. This study exhibits the value of integrating systematic design methodologies, such as morphological matrices and topological modeling, in the conceptual design and innovation of complex electromechanical systems such as the SIWMDU. The method presented herein not only offers immediate practical value for SIWMDU designers but also launches a methodological foundation that can be extended to other complex mechatronic systems encountering similar integration complexities. By formalizing the configuration synthesis process and proposing quantitative evaluation criteria, this work contributes to filling the gap between abstract design theory and practical engineering implementation in the rapidly evolving field of electric vehicle propulsion systems.

In-wheel motor drive systems offer significant advantages for electric vehicles, including large chassis space, high transmission efficiency, and great control flexibility. However, in current mainstream in-wheel motor driving vehicles, the unsprung mass is significantly increased because the motor or the driving unit is rigidly connected to the wheel hub. The increased unsprung mass not only deteriorates vehicle ride comfort and road holding performance, but also results in heavy motor vibration. To mitigate these negative effects, configurations with suspended motor or driving unit have been proposed. It is thus desirable to explore the potential of these new configurations in this regard.

This paper aims to mitigate the negative effects of unsprung mass by optimizing vehicle and motor suspension parameters simultaneously. To this end, it examines two typical in-wheel motor drive configurations with motor suspension: the dynamic vibration absorber configuration and the two-stage suspension configuration. Half-vehicle models are established respectively for both configurations, and key indices for vehicle dynamic performance are selected or defined. Drawing on earlier studies on how the increased unsprung mass impacts vehicle performance at various speeds, and considering the trade-off among ride comfort, road holding, and motor vibration, a multiobjective optimization strategy is proposed for parameter optimization of vehicle suspension and motor suspension. In the strategy, the goal is to minimize body vertical acceleration, wheel dynamic load, and motor acceleration at medium speeds while reducing body pitch acceleration, wheel dynamic load, and motor acceleration at high speeds. Constraints include the natural frequency and dynamic deflection of the vehicle suspension. Using the NSGA-Ⅱ algorithm, Pareto optimal solution sets are derived respectively for the two configurations. The entropy weight method is then applied to determine the optimal parameters for vehicle and motor suspensions. With the optimal suspension parameters, dynamic simulations are conducted on a random road, and the dynamic performance is evaluated based on the predefined indices.

The results indicate that, compared to the fixed hub motor configuration, both motor suspension configurations achieve a substantial performance enhancement in vehicle ride comfort, road holding, and motor vibration. Specifically, the dynamic vibration absorber configuration delivers greater enhancements in vehicle body vertical and pitch vibrations, as well as wheel dynamic load. Specifically, it reduces body vertical and pitch accelerations by 36.9% and 33.09%, respectively, at medium and high speeds. The wheel dynamic load is decreased by 18.42% and 18.55% at medium and high speeds, respectively. By contrast, the two-stage suspension configuration excels in reducing motor vertical vibration. It reduces motor vertical acceleration by 67.48% and 65.43% at medium and high speeds, respectively.

This paper presents a passive control approach to address the negative effects of unsprung mass by utilizing motor suspension configurations. The in-wheel motor drive configurations with motor suspension demonstrate significant potential for improving vehicle dynamic performance. This research serves as a valuable resource for the design of in-wheel motor driving vehicles.

In-wheel motor drive has emerged as a promising innovation in electric-vehicle-powertrain configurations. In mainstream configurations of in-wheel motor drive units, power and transmission devices are rigidly connected to wheel hubs. However, this design increases the unsprung mass of the vehicle. The implications of this added mass on vehicle performance, especially in terms of ride comfort, have been a subject of debate. Existing research, encompassing experimental and simulation studies, presents different and sometimes contradictory conclusions. Notably, many simulations fail to consider the effects of motor positioning and resultant vibrations across different areas of the vehicle.

This study tries to address these concerns using a half-vehicle model approach. These models are established for a standard passenger vehicle and an in-wheel motor-driven variant, the latter modified by adding mass to the wheels of the former. To assess ride comfort, we focused on several dynamic performance indicators: body vertical acceleration, pitch angular acceleration, relative wheel dynamic load, and suspension travel. Using the frequency domain method allowed us to convert double-wheel excitations into single-wheel excitations, from which we derived the equivalent amplitude-frequency characteristics for our chosen indicators. This step was followed by determining the power spectral density of a random road profile, which in turn facilitated the calculation of the power spectral density and root-mean-square values of the performance indicators. Simulations were then performed to compare the performance of the in-wheel motor-driven vehicle with that of the traditional vehicle on a C-class random road over a speed range varying from 1 to 50 m/s (3.6-180 km/h). The analysis considered various factors, including body position, hub motor mass, and hub motor drive mode. By integrating the amplitude-frequency characteristics of our indicators, we were able to shed light on how increased unsprung mass influences vehicle dynamics.

The results of our study can be summarized as follows: 1) In the vehicle speed range of up to 50 m/s, an increase in unsprung mass results in a larger wheel dynamic load and greater suspension travel. This, in turn, negatively affects road holding and suspension performance. 2) The impact of increased unsprung mass on body vertical acceleration varies with body position owing to the wheelbase-filtering property. Specifically, the front and rear body accelerations are exacerbated by the increased unsprung mass across all speeds. Furthermore, the vertical and pitch accelerations of the body centroid exhibit alternating patterns of increase and decrease throughout the speed range. In other words, these two indicators deteriorate at certain speeds but improve at others. 3) As the hub motor mass increases, the vertical and pitch accelerations of the body centroid intensify within the speed ranges where deterioration occurs. Conversely, within the speed ranges where improvements are noted, these accelerations diminish. 4) At typical speeds, vehicles with front-drive and four-drive hub motors experience significant increases in vertical and pitch accelerations of the body centroid owing to the added unsprung mass. The adverse effect is considerably less pronounced in vehicles equipped with rear-drive hub motors.

In summary, this study systematically reveals the influence of increased unsprung mass on vehicle ride comfort. By doing so, it aims to resolve the discrepancies and controversies found in previous research. The insights gained from this research serve as a valuable resource for informing the design of hub motor-driven vehicles.

Compared with the commercially available centralized motor drive systems, in-wheel drive motors are installed inside the wheels and provide high transmission efficiency, enhanced control flexibility, and ease of modular vehicle design. In mainstream configurations of in-wheel drive motors, the power and transmission devices are rigidly connected to the chassis, thereby increasing the unsprung mass of the vehicle and the rotational inertia of the wheels. This increase in unsprung mass and rotational inertia results in higher vertical acceleration of the vehicle body and increased dynamic loads on the wheels. Moreover, due to the lack of buffering effect from the centralized driving vehicle's half shaft, the rotational vibration of the wheels and the longitudinal vibration of the vehicle are more prominent. Electric wheels with suspended drive motors can not only extend motor life but also alleviate comfort problems as a result of excessive unsprung mass. Designing flexible coupling within limited wheel space in an electric wheel is a critical challenge.

In this paper, a flexible coupling was proposed for an electric wheel with a suspended drive motor. The coupling was a planar parallel mechanism composed of several connecting arms, each of which consisted of two parallelogram mechanisms linked in series. As only the rotational joints existed and the stiffness of the input and output disks were high, the analysis assumed that the connecting rod only experienced axial deformation and the input and output disks were treated as rigid bodies. Under these assumptions, kinematic and dynamic models were established for the coupling utilizing the geometric relationships between the rods with the principles of mechanical equilibrium. Coupling eccentricity and the static and dynamic loads of the components were then derived. To demonstrate, a set of parameters were selected for the coupling, and numerical calculations were performed for the eccentricity, static load, and dynamic load of the coupling.

The results show that the coupling can transmit power with constant velocity and allows the transmission system to jump in vertical directions during power transmission. The total eccentric stroke of this coupling reaches 58.2 mm, which is sufficient for an in-wheel suspended drive motor. Application of 400 N·m torque and 1 000 r/min to the input shaft leads to a maximum static load on the connecting rod of approximately 1 300 N and a maximum dynamic load on the output disk of approximately 35 N. Under the same conditions, the dynamic load on the proposed coupling is only 1/10 of that on the Oldham coupling.

The coupling proposed in this paper can have a large eccentric stroke while occupying little axial space and imposing negligible dynamic load. This coupling can be helpful in developing electric wheels with suspended drive motors. The flexibility of the coupling allows the motor and transmission components to move independently from the wheel, which reduces the unsprung mass and solves the problem of vibration and noise. This coupling has significant advantages over previous solutions in terms of size, dynamic load, and mechanical efficiency. This work provides a reference for designing in-wheel suspended drive motors.