Patent Description:
Electric motors are ubiquitous, found in everything from household appliances and toys to high-performance automobiles. These motors come in various forms and categories. Magnetic hub motors include a central stationary stator to which a driving electrical current is applied, and an outer rotor that spins around the stator. These motors are useful for driving fans, wheels of vehicles, and the like. However, partly because the stator is typically enclosed, these motors have a likelihood to experience heat build-up. This heat build-up in the stator means that the full capacity of the motor cannot be utilized. For example, known hub motors that are capable of multiple kilowatts may have a power output limited to approximately <NUM> to <NUM> Watts. Vehicles and other devices utilizing these motors are therefore limited as well.

<CIT> discloses a moving apparatus provided with a drive wheel driven by human power and a drive wheel assist device. <CIT> discloses a stator ring for an electric motor. <CIT> discloses a self-stabilizing skateboard.

Apparatuses and methods relating to thermally enhanced hub motors disclosed herein overcome the problems described above through improved axle geometries, materials, and/or manufacturing processes. Benefits may include extending the performance envelope for a motor, reducing motor weight, reducing operating temperatures, increasing power output, and/or extending motor lifetime. Corresponding benefits are applicable to a vehicle or other device driven by these motors.

Accordingly, the present disclosure provides an electric vehicle according to the appended set of claims. In some embodiments, an electric vehicle may comprise a board including a frame supporting first and second deck portions, each deck portion configured to receive a left or right foot of a rider oriented generally perpendicular to a direction of travel of the board; a wheel assembly including a rotatable ground-contacting element disposed between and extending above the first and second deck portions; a hub motor configured to rotate the ground-contacting element to propel the electric vehicle, the hub motor including a rotor; a stator disposed within the rotor; a motor casing housing the rotor and the stator; and an axle extending from the stator through the motor casing, the axle being coupled to the frame of the board; wherein the axle comprises aluminum.

In some embodiments, an electric vehicle may comprise a board including a frame supporting first and second deck portions, each deck portion configured to receive a left or right foot of a rider oriented generally perpendicular to a direction of travel of the board; a wheel assembly including a rotatable ground-contacting element disposed between and extending above the first and second deck portions; a hub motor configured to rotate the ground-contacting element to propel the electric vehicle, the hub motor including a rotor, a stator disposed concentrically within the rotor, such that the rotor is configured to rotate about the stator, and an axle portion coupled to the frame of the board, the axle portion having a central shaft extending axially from a mandrel fixed concentrically within the stator, an outer surface of the mandrel in direct contact with the stator; wherein the central shaft and the mandrel together form a unit consisting of a single material having a thermal conductivity substantially higher than the stator.

In some embodiments, an electric vehicle may comprise a board including a frame supporting first and second deck portions, each deck portion configured to receive a left or right foot of a rider oriented generally perpendicular to a direction of travel of the board; a wheel assembly including a rotatable ground-contacting element disposed between and extending above the first and second deck portions; a hub motor configured to rotate the ground-contacting element to propel the electric vehicle, the hub motor including a stator disposed concentrically within the ground-contacting element, and an axle portion having a central shaft extending axially from a mandrel fixed concentrically within the stator, an outer surface of the mandrel in direct contact with the stator, the central shaft and the mandrel comprising aluminum; wherein the axle portion of the hub motor is in thermal communication with the frame of the board.

Features, functions, and advantages may be achieved independently in various embodiments of the present disclosure, or may be combined in yet other embodiments, further details of which can be seen with reference to the following description and drawings.

Various aspects and examples of an electric hub motor having a high-thermal conductivity central axle and a vehicle utilizing such a hub motor, as well as related methods, are described below and illustrated in the associated drawings. Unless otherwise specified, a vehicle including a thermally enhanced hub motor and/or its various components may, but are not required to, contain at least one of the structure, components, functionality, and/or variations described, illustrated, and/or incorporated herein. Furthermore, unless specifically excluded, the process steps, structures, components, functionalities, and/or variations described, illustrated, and/or incorporated herein in connection with the present teachings may be included in other similar devices and methods, including being interchangeable between disclosed embodiments. The following description of various examples is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. Additionally, the advantages provided by the examples and embodiments described below are illustrative in nature and not all examples and embodiments provide the same advantages or the same degree of advantages.

"Thermal conductivity" of a material refers to a property quantifying the ability of the material to conduct heat. Thermal conductivity may be abbreviated herein as k, and is typically measured in units of Watts per meter per Kelvin (W/(mK)). Heat transfer occurs at lower rates through materials with lower k, and at higher rates through materials with higher k. In metals, thermal conductivity may vary with temperature. Accordingly, comparisons between different materials should be understood to be in the context of substantially the same temperature or range of temperatures.

In general, a thermally enhanced hub motor may include a high-k central axle, relative to a k of the motor's stator, where a mandrel of the axle is interference fit into the stator. The mandrel and shaft of the axle have a thermal conductivity substantially higher than the stator, such that heat will be transferred from the stator, through the mandrel and shaft. This heat can then be transferred to a heat sink outside the motor.

The stator body of a hub motor typically includes a plurality of steel laminations. A central axle, according to the present teachings, may be constructed from an aluminum or magnesium alloy, thus having a substantially higher thermal conductivity than the lamination stack. The external heat sink may comprise an aluminum alloy or other suitable material, and may include a portion (e.g., a frame) of an apparatus to which the motor is mounted. Accordingly, heat from the stator will flow readily from the stator through the mandrel and shaft of the central axle, to the heat sink, thereby cooling the stator and facilitating improved motor performance characteristics.

A vehicle or other apparatus having a thermally enhanced hub motor includes a frame to which the motor is directly or indirectly coupled. As is typical in a hub motor, the electric motor includes a rotor and a stator disposed concentrically within the rotor, such that the rotor is configured to rotate about the stator. In some examples, a tire or other ground-contacting element is coupled to the rotor, allowing the motor to propel the vehicle across a support surface.

The hub motor includes an axle portion that is coupled to the frame of the vehicle. A central shaft of the axle portion extends axially from a larger mandrel fixed concentrically within the stator. The outer surface of the mandrel is in direct contact with the stator, to facilitate heat transfer between the two. The central shaft and the mandrel are unitary. In other words, the shaft and mandrel form a unit made out of a single material. The mandrels described below are generally cylindrical. However, the outer shape of the mandrel may have other shapes (e.g., octagonal or hexagonal prisms), have keyed (e.g., castellated) surfaces, and/or the like, or any combination of these. A convoluted or shaped mandrel surface may be helpful for alignment, or for increasing the contact surface area between the mandrel and a corresponding inner surface of the stator, thereby altering heat transfer and/or torque transfer characteristics.

The following sections describe selected aspects of exemplary thermally enhanced hub motors as well as related apparatuses, systems, and methods. The examples in these sections are intended for illustration and should not be interpreted as limiting the entire scope of the present disclosure. Each section may include one or more distinct inventions, and/or contextual or related information, function, and/or structure.

As shown in <FIG>, this section describes an illustrative vehicle <NUM> having a thermally enhanced hub motor.

Vehicle <NUM> is a one-wheeled, self-stabilizing skateboard substantially similar to the electric vehicles described in <CIT> (the '<NUM> patent). Accordingly, vehicle <NUM> includes a board <NUM> having a frame <NUM> supporting a first deck portion <NUM> and a second deck portion <NUM>. Each deck portion <NUM>, <NUM> is configured to receive a left or right foot of a rider oriented generally perpendicular to a direction of travel of the board, said direction of travel generally indicated at <NUM>.

Vehicle <NUM> also includes a wheel assembly <NUM>. Wheel assembly <NUM> includes a rotatable ground-contacting element <NUM> (e.g., a tire, wheel, or continuous track) disposed between and extending above the first and second deck portions <NUM>, <NUM>, and a hub motor <NUM> configured to rotate ground-contacting element <NUM> to propel the vehicle. As shown in <FIG>, vehicle <NUM> may include exactly one ground-contacting element.

Frame <NUM> may include any suitable structure configured to rigidly support the deck portions and to be coupled to an axle of the wheel assembly, such that the weight of a rider may be supported on tiltable board <NUM> having a fulcrum at the wheel assembly axle. Frame <NUM> may include one or more frame members <NUM>, on which deck portions <NUM> and <NUM> may be mounted, and which may further support additional elements and features of the vehicle, such as a charging port <NUM>, and end bumpers <NUM>, <NUM>, as well as lighting assemblies, battery and electrical systems, electronics, controllers, and the like (not shown).

Deck portions <NUM> and <NUM> may include any suitable structures configured to support the feet of a rider, such as non-skid surfaces, as well as vehicle-control features, such as a rider detection system, and the like. Illustrative deck portions, including suitable rider detection systems, are described in the '<NUM> patent, as well as in <CIT>, the entirety of which is hereby included herein for all purposes.

A shaft <NUM> of an axle portion <NUM> of hub motor <NUM> is coupled to frame <NUM>, as shown in <FIG>. Shaft <NUM> is coupled to frame <NUM> such that the axle portion of the hub motor is in thermal communication with the frame of the board. For example, the shaft may be directly attached to frame <NUM> (see description of <FIG> below), or may be coupled to the frame through a connection or mounting block <NUM> (also referred to as an axle mount). Shaft <NUM> may be bolted or otherwise affixed to mounting block <NUM>, which in turn may be bolted or affixed to frame <NUM> (e.g., by bolts <NUM>, <NUM>). A through hole <NUM> may be provided in frame <NUM> for access to the connector of shaft <NUM> to block <NUM>. Coupling of shaft <NUM> to frame <NUM> facilitates use of frame <NUM> as a heat sink.

Turning to <FIG>, hub motor <NUM> is shown isolated from the remainder of vehicle <NUM>. Hub motor <NUM> is suitable for use in any number of vehicles or other apparatuses, and is not limited to the present contextual example. Hub motor <NUM> is a brushless, direct-drive electric motor, and may be operated using any suitable electrical controls and wiring. For example, when used in vehicle <NUM>, phase wires (not shown) may electrically connect one or more electric coils of the motor with one or more other electrical components of vehicle <NUM>, such as a power stage, motor controller, battery, and the like (not shown). The one or more electrical components may drive hub motor <NUM> based on rider inputs to propel and actively balance vehicle <NUM>. For example, the one or more electrical components may be configured to sense tilting of board <NUM> about a pitch axis, and to drive hub motor <NUM> to rotate element <NUM> in a similar direction about the pitch axis.

Hub motor <NUM> has an outer motor casing <NUM> including a pair of end bells <NUM> and <NUM> bolted to a central cylinder portion <NUM>. Shaft <NUM> extends from a stator <NUM> of motor <NUM>, through casing <NUM>, and the casing is supported on the shaft by one or more bearings <NUM>. In addition to forming an outer shell for motor <NUM> and a mounting surface for ground-contacting element <NUM>, cylinder portion <NUM> of casing <NUM> is an outer wall of a rotor <NUM> of hub motor <NUM>. Accordingly, cylinder portion <NUM> and casing <NUM> as a whole are configured to rotate about stator <NUM> when the motor is operated.

Rotor <NUM> is a generally cylindrical structure having a plurality of magnets <NUM> fixedly arranged around an inner wall <NUM>. Magnets <NUM> may be permanent magnets, and may be generally rectangular, with long axes of the magnets extending along a length of the rotor. Rotor <NUM> may be configured to rotate around stator <NUM>, leaving an air gap <NUM> between the two structures.

Stator <NUM> is a generally cylindrical structure having a plurality of projections <NUM> (also referred to as teeth) forming slots therebetween. Projections <NUM> are configured to hold electrical windings or coils (not shown). Energizing the coils sets up a rotating magnetic field, which exerts a force on magnets <NUM>, thereby spinning the rotor. Stator <NUM> includes metal (usually steel) sheets or laminations <NUM> layered together, an outermost of which can be seen in <FIG>. These laminations typically comprise so-called electrical steel or silicon steel, and are collectively referred to as a lamination stack. Electrical steel and the lamination topology function to produce a magnetically permeable structure, while reducing core losses by interrupting induced eddy currents. However, the steel structure has a low thermal conductivity, and tends to retain heat, especially when enclosed in motor casing <NUM>.

Axle portion <NUM> is disposed concentrically within stator <NUM> in an interference fit. Axle portion <NUM> includes a mandrel portion <NUM> and shaft <NUM>, an outer surface <NUM> of mandrel portion <NUM> being in direct contact with the stator. With continuing reference to <FIG>, <FIG> is an isometric view of the mandrel and shaft of axle portion <NUM> isolated from remaining components of hub motor <NUM>. Mandrel portion <NUM> may have a larger diameter than the shaft of the axle portion, as shown in <FIG>. Shaft <NUM> is a central shaft extending axially from mandrel portion <NUM>, which is fixed concentrically within the stator. The central shaft and the mandrel together form a unit, which consists of a single material having a thermal conductivity substantially higher than the stator. In some examples, this unit is made of aluminum, or of a single alloy of aluminum. In some examples, this unit is made of magnesium, or of a single alloy of magnesium.

Aluminum, for example, has a significantly higher thermal conductivity than does the steel of stator <NUM>, with aluminum being on the order of <NUM> W/(mK), while steel is typically less than <NUM>. Similarly, magnesium has a thermal conductivity on the order of <NUM> W/(mK). In some examples, the thermal conductivity of the stator is less than approximately <NUM> W/(mK), and the thermal conductivity of the unit formed by the central shaft and the mandrel is greater than approximately <NUM> W/(mK) (e.g., greater than approximately <NUM> W/(mK)).

Mandrel portion <NUM> has two opposing end portions <NUM>, <NUM>, and central shaft <NUM> extends axially from both of the end portions of the mandrel. In some examples, shaft <NUM> may extend from only one of the end portions, such as when a hub motor is side-mounted to a supporting structure, leaving one side free.

Mandrel portion <NUM> is connected to shaft <NUM> by a plurality of radial connectors <NUM>, (also referred to as connection members) which run the length of mandrel portion <NUM> and form a plurality of lengthwise cavities <NUM> therebetween. Radial connectors <NUM> are unitary with the mandrel portion and the central shaft, and are made of the same material (e.g., aluminum alloy). Although axle portion <NUM> is shown in the drawings as having five such radial connectors <NUM>, more or fewer may be used. In some examples, mandrel portion <NUM> may be radially continuous with shaft <NUM>, forming a solid structure without any cavities <NUM>.

Shaft <NUM> may include one or more end apertures <NUM>, which may be threaded, end chamfers <NUM>, one or more stepped abutments <NUM>, and/or a lateral recess <NUM>. End apertures <NUM> and/or chamfers <NUM> may be utilized for facilitating the attachment of shaft <NUM> to a mounting structure, such as mounting block <NUM> or frame <NUM> (see additional discussion below regarding <FIG>). Abutments <NUM> may include a slight step change in the diameter of shaft <NUM>, and may be utilized for proper positioning and interfacing with one or more bearings, such as bearings <NUM>. Finally, lateral recess <NUM> may function as a mounting surface for one or more electronic components and/or a through-way for electrical wiring (when installed in hub motor <NUM>).

As shown in <FIG>, this section describes an alternative coupling between hub motor <NUM> and a frame or other mounting surface. An axle portion of a hub motor is shown in <FIG>, which is substantially identical to axle portion <NUM>, described above. Accordingly, the axle portion of <FIG> will be referred to as axle portion <NUM>', with primed reference numbers for its component elements corresponding to those of axle portion <NUM>. Features and component elements of axle portion <NUM>' not described directly below are understood to be substantially as described above.

Axle portion <NUM>' may include end bolts <NUM>, <NUM>, for coupling shaft <NUM>' of the hub motor directly to a frame or other mounting support structure. In the example shown in <FIG>, axle portion <NUM>' is attached to a pair of frame members <NUM> and <NUM>. Frame members <NUM> and <NUM> may include any suitable support structures forming a part of (or attached to) a larger apparatus, thereby providing a heat sink for heat being conducted through axle portion <NUM>'. Frame members <NUM> and <NUM> may comprise a same material as axle portion <NUM>', such as an aluminum or magnesium alloy, to further enhance heat transfer.

As depicted in <FIG>, frame members <NUM> and <NUM> each have an opening <NUM>, <NUM>, having angled walls <NUM> configured to receive a respective chamfered end of shaft <NUM>'. Bolts <NUM> and <NUM> are used to draw their respective ends into the openings. In this example, the depths of openings <NUM> and <NUM> are such that chamfers <NUM>' will abut against angled walls <NUM> before the end of the shaft bottoms out against the frame member. In other words, a gap <NUM> exists between the end of the shaft and the inner wall of the frame member. Accordingly, tightening of the end bolts causes the shafts to more firmly wedge into the openings. Chamfers <NUM>' and angled walls <NUM> are configured such that the axle portion and stator of the hub motor are properly oriented and maintained in a proper orientation with respect to the remainder of the apparatus when shaft <NUM>' is fixed to frame members <NUM> and <NUM>.

As shown in <FIG>, this section describes an intermediate article of manufacture suitable for use in the manufacturing of a mandrel and axle shaft for a thermally enhanced hub motor.

<FIG> is an isometric view of an elongate cylinder <NUM> having an outer surface <NUM>, a long axis <NUM>, and an outer diameter <NUM>. <FIG> is an end elevation view of cylinder <NUM>, showing various dimensions thereof. As described below, cylinder <NUM> may be extruded as a single piece in the form shown in <FIG>, and may comprise aluminum or magnesium. In some examples, cylinder <NUM> is made of an aluminum alloy or a magnesium alloy.

Cylinder <NUM> includes an outer tube <NUM>, which has an inner diameter <NUM>, surrounding and connected to an inner shaft <NUM>. At least one lengthwise cavity or gap <NUM> is formed between the outer tube and the inner shaft. Outer tube <NUM> is connected to inner shaft <NUM> by a plurality of radial members <NUM> (also referred to as radial connectors or spokes). Inner shaft <NUM> has an outer diameter <NUM>, which is smaller than outer diameter <NUM> and inner diameter <NUM>. Inner shaft <NUM> is hollow, having a lengthwise axial aperture <NUM>.

Referring back to axle portion <NUM> of <FIG>, it may be understood that cylinder <NUM> has the same structural form as mandrel portion <NUM>. Similarly, it may be understood that the extruded form of cylinder <NUM> may be machined to remove a portion of outer tube <NUM> and radial members <NUM> to produce shaft <NUM>. Accordingly, extruded cylinder <NUM> may be an intermediate article in the manufacturing of axle portion <NUM>, or the like.

As shown in <FIG>, this section describes the results of a simulation of thermal effects in a standard-construction hub motor having a steel axle, as compared with a thermally enhanced hub motor in accordance with the present teachings. Additional features and benefits are also described.

This simulation was performed assuming <NUM> Watts (W) of total output in each of a prior art motor <NUM> (see <FIG>) and a thermally enhanced hub motor <NUM> (see <FIG>). Motor <NUM> includes a rotor <NUM> having magnets <NUM>, and a steel stator <NUM> having a central steel axle <NUM>. Motor <NUM> includes a rotor <NUM> having magnets <NUM>, and a steel stator <NUM> having an aluminum alloy axle portion <NUM> including a mandrel and shaft, substantially as described above.

The <NUM> W was divided equally among the <NUM> coils/windings in each motor, such that each projection of the respective stator had an applied <NUM> W. Ambient temperature was assumed to be <NUM> C, and steady-state equilibrium conditions were simulated.

As shown in <FIG>, and in Table <NUM> below, significant differences were found between the thermal performances of the two motors. Average stator temperature in standard hub motor <NUM> was about <NUM> C, while average stator temperature in thermally enhanced motor <NUM> was about <NUM> C. Moreover, the range of temperatures in stator <NUM> was much wider than in stator <NUM>, due to the thinner walls and aluminum construction. As shown in <FIG>, the range of temperatures in the standard motor was approximately <NUM> C, while enhanced motor <NUM> varied by less than half of a degree C. Thermally enhanced motor <NUM> also experienced a reduced average temperature of magnets <NUM> as compared to magnets <NUM>.

Additional benefits are present in the thermally enhanced hub motor. Hub motor <NUM> is suitable for use in a vehicle such as vehicle <NUM>, and has a substantially reduced stator, axle, and combined mass, as compared with the standard hub motor that would be used for the same application. See Table <NUM>.

This section describes steps of an illustrative method for manufacturing a hub motor; see <FIG>. Aspects of vehicles, hub motors, and/or intermediate articles of manufacture described above may be utilized in the method steps described below. Where appropriate, reference may be made to previously described components and systems that may be used in carrying out each step. These references are for illustration, and are not intended to limit the possible ways of carrying out any particular step of the method.

<FIG> is a flowchart illustrating steps performed in an illustrative method, and may not recite the complete process or all steps of the method. <FIG> depicts multiple steps of a method, generally indicated at <NUM>, which may be performed in conjunction with thermally enhanced hub motors according to aspects of the present disclosure. Although various steps of method <NUM> are described below and depicted in <FIG>, the steps need not necessarily all be performed, and in some cases may be performed in a different order than the order shown.

Step <NUM> of method <NUM> includes extruding an elongate member out of relatively high-k material, such as an aluminum alloy or a magnesium alloy. The elongate member may be cylindrical, and may include an outer surface, a long axis, and a first outer diameter. Cylinder <NUM> of <FIG> is an example of an elongate member that may result from step <NUM>. Accordingly, the extruded member may comprise an outer tube surrounding and connected to an inner shaft, such that at least one lengthwise cavity or gap is formed between the outer tube and the inner shaft. The outer tube may be connected to the inner shaft by a plurality of radial members, which may also be referred to as connectors, connector members, fins, or spokes.

Extrusion of the elongate member, as compared, for example, to casting methods, allows otherwise infeasible alloys of aluminum or magnesium to be utilized. These alloys may have higher thermal conductivities. The extrusion process also results in a more uniform grain structure in the metal, which further enhances the heat transfer capacity of the material.

Step <NUM> of method <NUM> includes machining a first end portion of the extruded member to produce a shaft extending coaxially from a remaining unmachined portion of the elongate member. The shaft may have a second outer diameter smaller than the first outer diameter. Step <NUM> may include additional machining, milling, lathing, and/or the like, and may produce an article such as the unitary mandrel and shaft shown in <FIG>.

Step <NUM> of method <NUM> includes inserting the elongate member into a corresponding central opening in a stator portion of an electric motor. The elongate member is inserted such that the elongate member is coupled to the stator portion in a concentric interference fit. When inserted, the shaft extends beyond the stator portion, as shown in the example of <FIG>.

In some examples, step <NUM> may include inserting the elongate member (e.g., a mandrel) into the stator by pressing the elongate member into an opening of the stator. In some examples, step <NUM> may instead include compression shrink fitting, also referred to as cryo-fitting. In the compression shrink fitting examples, the elongate member may be cooled using a cryogen (e.g., liquid nitrogen) before insertion into the opening of the stator. Cooling the elongate member shrinks it slightly, resulting in a reduced outer diameter. Following insertion, a subsequent return of the elongate member to ambient temperature causes the elongate member to expand to its original size. The inner diameter of the stator opening is configured such that outward/lateral expansion of the elongate member results in a tight interference fit between the two components. This method may be preferred over press-fitting, as it results in less galling, scoring, and damage to the mating surfaces of the components, thereby resulting in better heat transfer.

Step <NUM> of method <NUM>, which is optional, may include installing the joined stator portion and elongate member into an electric motor. Step <NUM> may include inserting the stator assembly concentrically within an outer rotor of the motor. Step <NUM> may include encasing the rotor and stator in a motor casing, with the shaft(s) of the elongate member protruding through the motor casing.

This section describes steps of an illustrative method for reducing heat in a hub motor; see <FIG>. Aspects of vehicles, hub motors, and/or intermediate articles of manufacture described above may be utilized in the method steps described below. Where appropriate, reference may be made to previously described components and systems that may be used in carrying out each step. These references are for illustration, and are not intended to limit the possible ways of carrying out any particular step of the method.

Step <NUM> of method <NUM> includes operating a hub motor, such that an outer rotor rotates around an inner stator of the hub motor, thereby generating heat in the stator. The hub motor is coupled to a device, which may include a vehicle such as vehicle <NUM>, described above. In some examples, the hub motor may be configured to rotate a wheel of the vehicle.

Step <NUM> of method <NUM> includes transferring at least a portion of the heat from the stator to a heat sink outside the motor. This is accomplished by conducting heat from the stator to the heat sink through an axle of the motor. The axle has a substantially higher thermal conductivity than the stator, and the heat sink comprises a portion of the device to which the hub motor is mounted. For example, the axle may comprise aluminum or an aluminum alloy, while the stator comprises steel, such as electrical steel, having a much lower thermal conductivity (k).

Claim 1:
An electric vehicle (<NUM>) comprising:
a frame (<NUM>);
at least one wheel assembly (<NUM>) coupled to the frame (<NUM>) and including a rotatable ground-contacting element (<NUM>); and
a hub motor (<NUM>) corresponding to each wheel assembly (<NUM>) and configured to rotate the corresponding ground-contacting element (<NUM>) to propel the electric vehicle (<NUM>), each hub motor (<NUM>) including: a stator (<NUM>) disposed concentrically within the ground-contacting element (<NUM>),
wherein an axle portion (<NUM>) of the hub motor (<NUM>) has a central shaft (<NUM>) extending axially from a mandrel (<NUM>) fixed concentrically within the stator (<NUM>), the mandrel (<NUM>) having an outer diameter larger than the central shaft (<NUM>), an outer surface of the mandrel (<NUM>) in direct contact with the stator (<NUM>), the central shaft (<NUM>) and the mandrel (<NUM>) being made out of a single material having a thermal conductivity higher than a thermal conductivity of the stator (<NUM>), wherein the central shaft (<NUM>) and the mandrel (<NUM>) are formed as a single piece; and
wherein the central shaft (<NUM>) is coupled to the frame (<NUM>), such that the axle portion (<NUM>) of each hub motor (<NUM>) is in thermal communication with the frame (<NUM>), and the mandrel (<NUM>) is connected to the central shaft (<NUM>) by a plurality of radial connectors (<NUM>) unitary with the mandrel (<NUM>) and the central shaft (<NUM>).