What Is a Wheel Motor?
A wheel motor is an electric motor that’s installed in each wheel of a vehicle. It eliminates energy-robbing drive shafts and provides more control.
It also reduces unsprung weight, which can improve handling and ride quality. However, the motors can get pelted with road shocks, water, dust, and other debris on a regular basis.
Size
The size of a wheel motor is important because the power delivered to the wheels is proportional to the volume of the rotor. A big rotor can deliver lots of torque, but it will also be heavy and expensive. Consequently, manufacturers have a dilemma: they need to make their motors larger or they need to use a smaller rotor in order to deliver enough power.
The solution lies with the concept of in-wheel drive systems, which are motors integrated directly into a wheel hub. This allows the motor to be smaller, more compact, and provides much more direct mechanical torque transfer compared to traditional systems. In-wheel drives can also be used to provide rear or all-wheel drive vehicles without the need for a conventional transmission or differential.
Examples of in-wheel motors include Protean Electric’s Pd18, which fits into an 18-inch (46 cm) wheel rim and can deliver up to 75 kW per powered wheel. This can significantly boost the efficiency of a vehicle and improve its performance. However, there are certain risks associated with this type of system, including unsprung mass, torque offset and bearing failures.
Design
The motor’s rotor connects directly to the wheel hub, delivering torque without the need for transmission or gearing. This helps maximize efficiency and compactness.
The rotor’s geometry has to be precisely engineered to meet a number of different requirements. Static eccentricity, for instance, can reduce a motor’s efficiency by altering its minimum radial air gap; dynamic eccentricity, on the other hand, can generate vibrations and unbalanced electromagnetic forces. To minimize eccentricity, engineers use a process called topological optimization, which starts with a vague CAD approximation and iteratively refines the geometry to reach its ideal form.
Other factors that influence a wheel motor’s design include external loads, which can vary significantly depending on the kind of vehicle it will be installed in. In-wheel motors, for example, are constantly in contact with the road and therefore must withstand harsher interactions between tyre and surface. To test these loads, Elaphe uses software scenarios and dedicated test tracks to wheel motor expose the motor to a wide range of impacts and stresses. These tests help ensure the motor will continue to operate smoothly and without damage.
Weight
Motor wheels can add a significant amount of weight to a robot, and this is one reason why it is important to first determine the torque and speed requirements for your application. Once you have this information, you can choose a wheel motor that can match your needs.
Most vehicle manufacturers try to keep unsprung mass (the portion of the total vehicle weight that is not supported by the suspension) to a minimum, as it can affect ride quality. Adding in-wheel motors can significantly increase unsprung mass, as they add weight to the axles, bearings, and tires of the vehicle.
Standard motors have too high of a speed for direct drive to the wheels, so they must be coupled through a gear box speed reducer or a reduction roller chain drive. Additionally, only one wheel can be powered by a single motor, as they do not generate enough power to provide traction for multiple wheels. Traction for any wheel is dependent upon the total vehicle and load weight and the coefficient of road traction.
Offset
When it comes to RC cars, wheel offset is a crucial factor that affects more than just how your car looks. It also influences the handling and performance of your vehicle, as it dictates how much space exists between the outside “lip” of the wheel and the suspension components.
The amount of offset is calculated by measuring the distance between the wheel hub mounting surface and the centerline of the wheel. This measurement can be positive, negative, or zero.
A zero offset means that the hub mounting surface is directly even with the wheel centerline. This type of offset is commonly found on stock wheels. A positive offset moves the mounting surface closer to the front of the wheel, tucking the wheel and tire assembly in toward the vehicle body and suspension. A negative offset pushes the wheel and tire assembly further back, resulting in the popular “deep dish” look.
You can find the offset of a wheel by measuring it yourself with a tape measure and straight-edge ruler. Ideally, the wheel motor manufacturer measurements should be taken from the inside of the wheel, without the tire.
Centerbore
The center bore is the size of the machined opening on the back of a wheel that mates to the hub of your vehicle. It’s machined to exactly match the hub size, which helps keep the wheel properly centered as the lug nuts are tightened, minimizing the chance of vibration. Some wheels are vehicle model specific and come with a center bore machined to the hub; others use centering rings or spigot rings to reduce the bore to match different vehicle hubs.
In-wheel motors, also known as in-hub or hub motors, are a type of electric vehicle drive system that installs an in-wheel motor right on the hub of each wheel. This design eliminates the need for a transmission, differential or constant-velocity joints (CVJs), which can reduce efficiency and increase overall vehicle weight. In-wheel motors also offer regenerative breaking, which captures kinetic energy during braking and uses it to charge the battery.