Research on the unsprung mass effect of in-wheel motor drives based on a half-vehicle model

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.