An increase in unsprung weight is a significant hurdle to the development of in-wheel motors. This article will discuss the issue and proposes possible solutions. To understand this article better, let’s start with two fundamental concepts.
What Is an In-wheel Motor?
An in-wheel motor, as suggested by the name, is a motor located inside the wheels. Its most critical feature is that it combines the car’s power system, transmission system, and braking system, with the motor driving the wheels directly, bypassing any mechanical structures.
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What Is Unsprung Weight?
Unsprung weight is the combined weight of the wheels and the components that connect them, which includes the hubs, brakes, suspension, and transmission system. The key feature of these parts is that they vibrate upwards and downwards as the vehicle moves. The in-wheel motors incorporate several control systems, so they can’t be very lightweight, which would have a more substantial effect on the car’s handling and comfort. Studies show that under optimal conditions, losing 1lb of unsprung mass equals up to a 20 lb reduction in overall sprung weight.
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Impacts of Unsprung Weight
Vehicle handling | It directly affects a vehicle’s handling – the less the unsprung weight, the more nimble the handling. Lower unsprung weight means lower inertia, thus the motion state can change more easily. The suspension can respond quickly to uneven road surfaces, resulting in improved handling.
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Comfort | Usually, the reasonable ratio of sprung to unsprung weight is more than 2. The larger the ratio, the more comfortable the ride. When the unsprung weight is low, the suspension is flexible, allowing easy changes to the motion state. Shock springs and damping cylinders can quickly dissipate vibrations from the suspension motion; conversely, when the unsprung mass is large, the vibrations can’t dissipate quickly enough and get transferred to the cabin, leading to less comfort.
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Acceleration | Less unsprung weight equals less inertia, which means wheels can be driven more easily for the same amount of power, leading to quicker acceleration. Lower unsprung mass also results in improved braking.
New Materials | Using carbon fiber, advanced alloys, and composite materials can drastically reduce the weight of wheels, motor parts, and other unsprung weights. Thanks to advancements in material science, The weight of hub motors has been gradually decreasing, from 45kg to 36kg, and from 36kg to 23kg. That means the total weight loss is 4X45-4×23=88kg. Experts anticipate a reduction of approximately 35% in hub motors’ weight for passenger vehicles in the next five years. As the problem of unsprung weight is gradually being solved, the usage of hub motors will likely increase.
|Std E46 M3 Setup
|Std E46 M3 & Bilstein
|Effective Unsprung Weight
|Effective Unsprung Weight
|Bearing hub, Nut
|Bolts, Pads, Pins
|Strut (slider )
|Wheel (18″ OE)
|Tyre (225) Est
|Total Unsprung Weight
Optimize design, materials, and processes | Some EVs use E-shaped multi-link designs or aluminum alloy lower swingarms, while some luxury cars use double-layered pressed steel plates for increasing strength and decreasing lightness. There can also be three symmetrical spring-damping devices mounted inside the hub to form the hub-motor suspension, reducing the vibration issue caused by increased unsprung mass.
TheMotion 2.0 ULF In-wheel Hub
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The ratio of unsprung to sprung weight | While many road surfaces are uneven, passengers inside the vehicle don’t feel too many bumps thanks to shock absorbers and the sprung weight. Therefore, increasing the sprung weight can be effective, and the higher the sprung mass, the smoother the vehicle runs. While the effect of unsprung weight on an electric vehicle’s performance is noticeable, it is less prominent as the sprung mass of an electric vehicle also increases due to factors like battery weight.
The unsprung weight issue in electric vehicles (EVs) with in-wheel motors poses a substantial challenge to the development of in-wheel motors. This issue can be resolved step by step through innovative materials, optimization of motor design, and alternative drive configurations.
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