What is a hub motor ? As the name suggests, it's a motor integrated into the wheel hub, directly driving the wheel. Hub motor technology, also known as in-wheel motor technology, is characterized by integrating the power, transmission, and braking systems into the wheel hub, thus greatly simplifying the mechanical components of electric vehicles. Hub motor technology is not new; electric vehicles equipped with hub motors on the front wheels were manufactured as early as 1900. In the 1970s, this technology was applied in fields such as mining vehicles. For hub motors used in passenger cars, Japanese manufacturers started researching and developing this technology earlier and currently hold a leading position. International automotive giants, including GM and Toyota, have also ventured into this technology. What are the advantages, disadvantages, and limitations of hub motors? Let's take a look at the pros and cons of hub motors.
Advantages of hub motors
1. By eliminating a large number of transmission components, the vehicle structure is simplified.
Traditional transmissions like the one shown in the image above are no longer seen in vehicles driven by in-wheel motors.
The hump in the rear floor of a traditional rear-wheel-drive car will disappear in an electric vehicle, freeing up more space for passengers.
For traditional vehicles, clutches, transmissions, drive shafts, differentials, and even transfer cases are all essential components. These parts are not only heavy and complex, making the vehicle's structure more intricate, but also require regular maintenance and have a high failure rate. However, in-wheel motors effectively solve this problem. In addition to their simpler structure, vehicles driven by in-wheel motors can achieve better space utilization and have significantly higher transmission efficiency.
2: It can implement a variety of complex driving methods.
Eight-wheel electric drive, like the AHED "Advanced Hybrid Electric Drive" prototype, can be easily achieved.
Because hub motors allow for independent drive of each wheel, they can easily achieve front-wheel drive, rear-wheel drive, and four-wheel drive configurations. Full-time four-wheel drive is particularly easy to implement in hub motor-driven vehicles. Furthermore, hub motors can achieve differential steering similar to tracked vehicles by varying the speeds of the left and right wheels, or even reversing their rotation, significantly reducing the vehicle's turning radius. In special circumstances, they can almost achieve turning on the spot (although this causes greater wear on the steering mechanism and tires), making them highly valuable for special-purpose vehicles.
3: Facilitates the adoption of various new energy vehicle technologies
In-wheel motors can be used to power a variety of new energy vehicles, including pure electric, hybrid, and fuel cell electric vehicles.
In-wheel motors can be used in parallel with traditional power sources, which is very significant for hybrid vehicles.
Many new energy vehicles use electric drive, making in-wheel motors extremely useful. Whether it's a pure electric vehicle, a fuel cell electric vehicle, or a range-extended electric vehicle, in-wheel motors can serve as the primary driving force. Even for hybrid vehicles, in-wheel motors can be used to assist with starting or rapid acceleration, making them versatile. Furthermore, many technologies in new energy vehicles, such as regenerative braking, can be easily implemented in in-wheel motor-driven vehicles.
Disadvantages of hub motors
1. Increasing the unsprung mass and the wheel hub's moment of inertia can affect vehicle handling.
The aluminum lower control arm was primarily used for weight reduction; adding a hub motor would render these efforts futile.
For a vehicle, we can divide it into two parts: unsprung mass and sprung mass. Unsprung mass is defined as the mass not supported by the elastic elements of the suspension system, generally including the wheels, springs, shock absorbers, and other related components. Sprung mass, on the other hand, is the mass of the remaining parts of the vehicle, generally including the frame, powertrain, transmission, and occupants. Increasing unsprung mass is extremely detrimental to both comfort and handling.
For ordinary civilian vehicles, relatively lightweight materials such as aluminum alloys are often used to make suspension components to reduce unsprung mass and improve suspension response. However, in-wheel motors significantly increase unsprung mass and wheel hub rotational inertia, which is detrimental to vehicle handling. However, considering that most electric vehicles are limited to commuting rather than pursuing power performance, this is not the biggest drawback.
2. Electric braking performance is limited, and maintaining the braking system requires a significant amount of electrical energy.
The built-in retarders in commercial vehicle axles utilize the principle of eddy current braking, and the braking of hub motors can also utilize this principle.
The second point concerns the impact on braking. Traditional cars are equipped with brake discs and brake calipers, but wheel hub motors require electric braking. Simple energy recovery is definitely not enough; stronger braking is also needed, which will also result in energy loss.
Many conventional commercial vehicles are already equipped with auxiliary deceleration devices that utilize the principle of eddy current braking (i.e., resistance braking), such as the electric retarders used in many trucks. Due to energy constraints, electric braking is also the preferred choice for electric vehicles. However, for vehicles driven by hub motors, the electric braking capacity of the hub motor system is relatively small and cannot meet the braking performance requirements of the entire vehicle, necessitating the addition of a mechanical braking system. For ordinary electric passenger vehicles, without the vacuum pump driven by a traditional internal combustion engine, an electric vacuum pump is required to provide braking assistance, which means greater energy consumption. Even though regenerative braking can recover some energy, the energy consumed by the braking system is still a significant factor affecting the driving range of electric vehicles in order to ensure the effectiveness of the braking system.
In addition, the in-wheel motor operates in a harsh environment, facing various influences such as water and dust, and has high requirements for sealing. Furthermore, how to implement the electric braking scheme and how to dissipate heat are also some challenges.
Summarize
Compared to centralized power drive by electric motors, hub motor technology has significant advantages. It offers greater flexibility in layout and eliminates the need for complex mechanical transmission systems. However, it also has its own notable shortcomings, such as the challenges of sealing, balancing starting current/torque, and differential speed issues with the drive wheels during steering. If these engineering challenges can be resolved, hub motor drive technology will have a promising future in new energy vehicles.
