Evaluation of the unsprung mass effect on ride comfort of in-wheel motor driving vehicles

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As an innovative powertrain configuration for electric vehicles, in-wheel motor drive has tremendous potentials. However, the influence of excessive unsprung mass related to the in-wheel motor drive on vehicle performance remains ambiguous or even controversial. This paper tries to address the issue based on a full-vehicle model, taking into consideration different road profiles, body positions and vehicle speeds. A full-vehicle model is established for an in-wheel motor driving vehicle, and a centralized driving vehicle with lighter unsprung mass is taken as the baseline vehicle. Simulation studies are carried out for both vehicle configurations at various speeds on random and bumpy road profiles, respectively. Various observations about the effects of the increased unsprung mass are obtained and clarified, including those mentioned in the literature and some new ones. The results indicate that, for random roads, the increased unsprung mass enlarges the wheel dynamic load and suspension travel, and thus deteriorates the road holding and suspension performances. Interestingly, the effects of the increased unsprung mass on body vertical acceleration vary with body position due to the wheelbase filtering property. Front and rear body accelerations are magnified with the increased unsprung mass at all speeds, whereas the body centroid acceleration exhibits an alternating pattern of increase and decrease throughout the speed range. Moreover, the increased unsprung mass reduces the body roll acceleration at all speeds. For bump roads on the other hand, the increased unsprung mass deteriorates all metrics at low speeds, but improves them at high speeds.