Patent Publication Number: US-9840233-B2

Title: Wheel and tire assembly with adjustable spacer system

Description:
RELATED APPLICATION 
     The present application is a divisional application of co-pending U.S. Patent Application titled WHEEL AND TIRE ASSEMBLY WITH ADJUSTABLE SPACER SYSTEM, Ser. No. 13/455,699 filed Apr. 25, 2012. The co-pending patent application is hereby incorporated by reference in its entirety into the present application. 
    
    
     BACKGROUND 
     1. Field 
     Embodiments of the present invention relate to wheel and tire assemblies for use with airless tires. More particularly, embodiments of the present invention relate to airless wheel assemblies with certain performance characteristics of pneumatic tires. 
     2. Related Art 
     Irrigation systems include elevated water conduits supported by mobile towers. Such mobile towers are mounted on wheels that propel the towers along the ground to be irrigated. The wheels typically include pneumatic tires that require periodic maintenance, including adjusting air pressure, repairing tires that develop holes or other damage, and replacing old or damaged tires that are beyond repair. 
     Because irrigation systems and similar agricultural equipment are typically used in fields or other remote locations, monitoring the tires for problems and reaching the tires to perform maintenance and repairs can be inconvenient or difficult. If a tire loses air pressure and is not repaired in a timely manner, damage to the tire, to the equipment mounted on the tire, or both may result. 
     One solution to the challenges presented by the use of pneumatic tires involves using wheels without tires. While this approach addresses most of the problems of tire maintenance, repair and replacement, it presents other challenges. Tireless wheels, for example, are rigid and experience greater ground penetration than a flexible tire, thereby creating ruts or otherwise disturbing the land more than a pneumatic tire. 
     Similar problems exist for tractors, automobiles, and other vehicles that typically use pneumatic tires. 
     Accordingly, there is a need for a solution which overcomes the limitations described above. 
     SUMMARY 
     A wheel assembly in accordance with an embodiment of the invention comprises a rigid wheel and an airless tire mounted on the wheel, a portion of the tire being separated radially from the wheel by a space and configured to flex inwardly toward the wheel when subject to ground engaging pressure. A spacer is interposed between the wheel and the portion of the tire separated radially from the wheel by a space, the spacer configured to limit the inward flexing of the tire. 
     A wheel assembly in accordance with another embodiment of the invention comprises a rigid wheel including a radially outer cylindrical rim wall, the wheel including a plurality of mounting elements extending radially outwardly from the rim wall, and an airless cylindrical flexible tire mounted on the rim wall and engaging the mounting elements such that portions of the tire not engaging the mounting elements are separated from the rim wall by a space. The portions of the tire not engaging the mounting elements are configured to flex inwardly toward the rim wall when subject to ground engaging pressure. 
     A method of exchanging spacers in a wheel assembly in accordance with yet another embodiment of the invention comprises removing a first spacer from a first position on the wheel assembly, the wheel assembly including a rigid wheel and a flexible tire mounted on the wheel, a first portion of the tire being separated radially from the wheel by a space and configured to flex inwardly toward the wheel when subject to ground engaging pressure, the first position being between the wheel and the first portion of the tire configured to flex inwardly toward the wheel. The method further comprises securing a second spacer in the first position on the wheel assembly, the second spacer configured to allow the first portion of the tire to flex inwardly a greater distance than allowed by the first spacer. 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Other aspects and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments and the accompanying drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an exemplary irrigation system including wheel assemblies constructed in accordance with embodiments of the invention; 
         FIG. 2  is a first side perspective view of one of the wheel assemblies of  FIG. 1 , the wheel assembly including a rigid wheel and a flexible airless tire mounted on the wheel; 
         FIG. 3  is a second side perspective view of the wheel assembly of  FIG. 2 ; 
         FIG. 4  is a perspective fragmentary view of the wheel assembly of  FIG. 2 ; 
         FIG. 5  is an exploded view of the wheel assembly of  FIG. 2 ; 
         FIG. 6  is a side elevation view of the wheel assembly of  FIG. 2 , illustrating the wheel assembly mounted on the irrigation system of  FIG. 1  and the tire flexing in response to ground engaging pressure; 
         FIG. 7  is a side perspective view of a wheel assembly constructed according to another embodiment of the invention, the wheel assembly including a rigid wheel with a polygonal outer rim wall and a flexible airless tire mounted on the wheel; 
         FIG. 8  is an exploded view of the wheel assembly of  FIG. 7 ; 
         FIG. 9 a    is a side elevation view of the wheel assembly of  FIG. 7 , illustrating the wheel assembly mounted on the irrigation system of  FIG. 1  and the tire flexing in response to ground engaging pressure; 
         FIG. 9 b    is a side elevation view of the wheel assembly of  FIG. 9 a   , the wheel assembly including spacers between the wheel and the tire, the spacers limiting the amount the tire flexes in response to the ground engaging pressure; 
         FIG. 10  is a side perspective view of a wheel assembly constructed according to another embodiment of the invention, the wheel assembly including a rigid wheel presenting a hub and spoke configuration and a flexible airless tire mounted on the wheel; 
         FIG. 11  is an exploded view of the wheel assembly of  FIG. 10 ; 
         FIG. 12 a    is a side elevation view of the wheel assembly of  FIG. 10 , illustrating the wheel assembly mounted on the irrigation system of  FIG. 1  and the tire flexing in response to ground engaging pressure; 
         FIG. 12 b    is a side elevation view of the wheel assembly of  FIG. 12 a   , the wheel assembly including spacers between wheel spokes and the tire, the spacers limiting the amount the tire flexes in response to the ground engaging pressure; 
         FIG. 13  is a side perspective view of a wheel assembly constructed according to another embodiment of the invention, the wheel assembly including a modular airless tire; 
         FIG. 14  is a fragmented, partially exploded view of the wheel assembly of  FIG. 13 ; 
         FIG. 15  is a cross-sectional view of a wheel assembly constructed according to another embodiment of the invention, the wheel assembly including a tire presenting a transversely concave outer side; 
         FIG. 16 a    is a fragmented side elevation view of a wheel assembly constructed according to another embodiment of the invention, the wheel assembly including a rigid wheel and a flexible airless tire and further including a plurality of first removable spacers interposed between the wheel and the tire; 
         FIG. 16 b    is a fragmented side view of the wheel assembly of  FIG. 16 a   , the wheel assembly including a plurality of second removable spacers, the second removable spacers being smaller than the first removable spacers; 
         FIG. 17 a    is a fragmented side elevation view of a wheel assembly constructed according to another embodiment of the invention, the wheel assembly including a rigid wheel and a flexible airless tire and further including a plurality of inflatable spacers interposed between the wheel and the tire, the spacers being inflated to a first, large size; 
         FIG. 17 b    is a fragmented side view of the wheel assembly of  FIG. 17 a   , the spacers being inflated to a second, small size; 
         FIG. 18 a    is a fragmented side elevation view of a wheel assembly constructed according to another embodiment of the invention, the wheel assembly including a rigid wheel and a flexible airless tire and further including a spacer component with a plurality of inflatable sections interposed between the wheel and the tire, the inflatable sections being inflated to a first, large size; 
         FIG. 18 b    is a fragmented side view of the wheel assembly of  FIG. 18 a   , the inflatable sections being inflated to a second, small size; 
         FIG. 19  is a side perspective view of a wheel assembly constructed according to another embodiment of the invention, the wheel assembly presenting an outer side with a truncated conical shape; 
         FIG. 20  is an end elevation view of the wheel assembly of  FIG. 18 ; 
         FIG. 21  is a fragmented, side perspective view of the wheel assembly of  FIG. 18 ; and 
         FIG. 22  is a side elevation view of a wheel assembly constructed according to another embodiment of the invention, the wheel assembly including a rigid wheel and a flexible airless tire mounted on the wheel, the tire including a plurality of traction lugs of varying size. 
     
    
    
     The drawing figures do not limit the present invention to the specific embodiments disclosed and described herein. The drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention. 
     DETAILED DESCRIPTION 
     The following detailed description references the accompanying drawings that illustrate specific embodiments in which the invention may be practiced. The embodiments are intended to describe aspects of the invention in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments can be utilized and changes can be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense. The scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled. 
     In this description, references to “one embodiment”, “an embodiment”, or “embodiments” mean that the feature or features being referred to are included in at least one embodiment of the technology. Separate references to “one embodiment”, “an embodiment”, or “embodiments” in this description do not necessarily refer to the same embodiment and are also not mutually exclusive unless so stated and/or except as will be readily apparent to those skilled in the art from the description. For example, a feature, structure, act, etc. described in one embodiment may also be included in other embodiments, but is not necessarily included. Thus, the present technology can include a variety of combinations and/or integrations of the embodiments described herein. 
     Turning now to the drawing figures, and initially  FIG. 1 , an exemplary irrigation system  10  is illustrated including a plurality of wheel assemblies constructed in accordance with embodiments of the invention. The illustrated irrigation system  10  is a central pivot irrigation system that broadly comprises a fixed central pivot  12  and a main section  14  pivotally connected to the central pivot  12 . The irrigation system  10  may also comprise an extension arm (also commonly referred to as a “swing arm” or “corner arm”) pivotally connected to the free end of the main section. 
     The fixed central pivot  12  may be a tower or any other support structure about which the main section  14  may pivot. The central pivot  12  has access to a well, water tank, or other source of water and may also be coupled with a tank or other source of agricultural products to inject fertilizers, pesticides and/or other chemicals into the water for application during irrigation. 
     The main section  14  may comprise a number of mobile support towers  16 A-D, the outermost  16 D of which is referred to herein as an “end tower”. The support towers are connected to the fixed central pivot  12  and to one another by truss sections  18 A-D or other supports to form a number of interconnected spans. The irrigation system  10  illustrated in  FIG. 1  includes four mobile support towers  16 A-D; however, it may comprise any number of mobile support towers without departing from the scope of the present invention. 
     Each mobile tower may include a drive tube  20 A-D on which a pair of wheel assemblies  22 A-D is mounted. Embodiments of the wheel assemblies  22 A-D are described in more detail below. A drive motor  24 A-D is mounted to each drive tube  20 A-D for driving the wheel assemblies  22 A-D. The motors  24 A-D may include integral or external relays so they may be turned on, off, and reversed. The motors may also have several speeds or be equipped with variable speed drives. 
     Each of the truss sections  18 A-D carries or otherwise supports a conduit section  26 A-D or other fluid distribution mechanism that is connected in fluid communication with all other conduit sections. A plurality of sprinkler heads, spray guns, drop nozzles, or other fluid-emitting devices are spaced along the conduit sections  26 A-D to apply water and/or other fluids to land underneath the irrigation system. 
     The irrigation system  10  may also include an optional extension arm (not shown) pivotally connected to the end tower  16 D and supported by a swing tower with steerable wheels driven by a motor. The extension arm may be joined to the end tower by an articulating pivot joint. The extension arm is folded inward relative to the end tower when it is not irrigating a corner of a field and may be pivoted outwardly away from the end tower while irrigating the corners of a field. 
     The irrigation system  10  may also include one or more high pressure sprayers or end guns  28  mounted to the end tower  16 D or to the end of the extension arm. The end guns  28  may be activated at the corners of a field or other designated areas to increase the amount of land that can be irrigated. 
     It will be appreciated that the irrigation system  10  is illustrated and described herein as one exemplary implementation of the wheel assemblies  22  described in detail below. Other, equally preferred implementations of the wheel assemblies  22  not shown or discussed in detail herein may include, without limitation, other types of irrigation systems, such as lateral irrigation systems, other types of agricultural equipment, such as wagons, carts, implements, and so forth, or other types of vehicles such as buses, trucks, and automobiles. However, embodiments of the invention are especially suited for irrigation systems and other vehicles or systems that travel over un-paved or un-finished ground. 
     Referring now to  FIGS. 2-6 , a wheel assembly  22  constructed in accordance with embodiments of the invention is illustrated. The wheel assembly  22  broadly includes a rigid wheel  30  and a flexible airless tire  32  mounted on the wheel  30  in a generally circumscribing relationship to the wheel  30 . The wheel  30  and the tire  32  are configured such that the tire  32 , while being airless, has some characteristics of a pneumatic tire that improve the performance of the tire  32 . For example, the wheel  30  and tire  32  are configured such that portions of the tire  32  flex radially inwardly toward the wheel  30  in response to ground engaging pressure, as illustrated in  FIG. 6 . It will be appreciated that this flexing action increases the total ground engaging footprint of the wheel assembly  22 , thus distributing the weight of the wheel assembly  22  and the weight of any machinery supported by the wheel assembly  22  over a larger area and limiting the total amount of ground penetration of the wheel assembly  22 . Additionally, penetration of traction lugs or other ground-gripping elements of the tire is limited, as explained below in greater detail. 
     As used herein, “ground engaging pressure” refers to pressure exerted on the wheel assembly  22  by the ground when the wheel assembly  22  is at rest on the ground or rolling on the ground. Ground engaging pressure may include pressure exerted on the wheel assembly  22  from different directions or from multiple directions simultaneously, such as where the wheel assembly  22  is on inclined terrain or rolling over an obstacle. Ground engaging pressure is related to the weight of the wheel assembly  22  and to the weight of any machinery supported by the wheel assembly  22 , and thus will vary from one embodiment of the invention to another and from one implementation to another. 
     Generally, the wheel  30  is configured to engage the tire  32  at circumferentially spaced locations such that portions of the tire  32  between the points of engagement flex inwardly in response to ground engaging pressure. In the illustrated embodiment, the wheel  30  includes an innermost hub  34  with a plurality of apertures  36  for attaching to, for example, lug nuts or similar attachment components. A circular or disc-shaped radial wall  38  or similar structural element connects the innermost hub  34  with a radially outer cylindrical rim wall  40 . The rim wall  40  includes a first shoulder  42  corresponding to a first axial margin of the rim wall  40 , a second shoulder  44  corresponding to a second axial margin of the rim wall  40 , and a substantially flat transverse portion  46  interconnecting the shoulders  42 , 44 . The rim wall  40  is generally cylindrical in shape about an axis that corresponds to an axis of rotation of the wheel assembly  22 . Thus, the rim wall  40  is generally perpendicular or nearly perpendicular to the radial wall  38 . 
     A plurality of mounting elements  48  are approximately equally spaced around the rim wall  40  and extend radially outwardly from the rim wall  40 . When the tire  32  is mounted on the wheel  30  the mounting elements  48  engage portions of the tire  32  such that the tire  32  is separated radially from the rim wall  40  by a space, allowing portions of the tire  32  to flex inwardly toward the rim wall  40  in response to ground engaging pressure as explained below in greater detail. 
     In the illustrated embodiment, the wheel assembly  22  includes twelve mounting elements  48  approximately equally spaced around an outer periphery of the rim wall  40 . Each mounting element  48  generally presents an elongated shape and is oriented transversely on the rim wall  40 , that is, oriented parallel with the axis of rotation of the wheel assembly  22 . Each of the illustrated mounting elements  48  includes a first side wall  50 , a second side wall  52 , and an outer wall  54 . While each of the illustrated mounting elements  48  includes substantially flat, rectangular side walls  50 , 52  and outer wall  54 , it will be appreciated that the particular size and shape of the mounting elements  48  is not important to the invention and may vary from one embodiment to another without departing from the spirit or scope of the invention. By way of example, each of the mounting elements  48  may be defined by a single, continuous rounded wall or a plurality of separate elements positioned to cooperatively perform the functions of the spacer elements  48  as described herein. 
     As explained above, the illustrated embodiment of the wheel assembly  22  includes twelve mounting elements  48  approximately equally spaced along an outer circumference of the wheel  30 , or separated by an angle of approximately thirty degrees. Thus, if the rim wall  40  is about forty-eight inches in diameter, the mounting elements  48  are spaced approximately twelve and one-half inches apart. If the rim wall  40  is about fifty inches in diameter, the mounting elements  48  are spaced approximately thirteen inches apart. If the rim wall  40  is about twenty-four inches in diameter, the mounting elements  48  are spaced approximately six inches apart. These are but a few examples. 
     Each mounting element  48  is configured to engage the flat transverse portion  46  of the rim wall  40  and each of the first  42  and second  44  shoulders of the rim wall  40 . End caps  56  may be placed on opposing ends of each mounting element  48  and secured in place with a fastener, such as a nut and bolt combination or similar fastener. Each end cap  56  includes a pair of flanges  58  for engaging drive lugs of the tire  32  to prevent lateral movement of the tire  32  relative to the wheel  30 . Each pair of flanges  58  extends laterally relative to the respective mounting element  48 , longitudinally relative to the rim wall  40 . The end caps  56  may also perform other functions, such as preventing soil and debris from entering the mounting elements  48 . The illustrated mounting elements  48  are constructed separately from the wheel  30  and may be welded or otherwise bonded or attached to the wheel  30 . Alternatively, the mounting elements  48  may be integrally formed as part of the wheel  30 . 
     The tire  32  is configured to be mounted on the wheel  30  such that at least a portion of the tire  32  engages the mounting elements  48  and the tire  32  presents a generally circular or nearly circular outer profile. Advantageously, the wheel  30  is configured for use with an airless tire. As used herein, a “tire” is a flexible component positioned and configured to engage the ground during use of the wheel assembly  22 . An “airless tire” is a tire that does not require trapped or compressed air for normal and proper use. An airless tire may be constructed of a single, unitary piece of material or multiple pieces of material. For purposes of this document, a tire used with inflatable spacers (explained below) is considered airless if the tire does not otherwise use or require trapped or compressed air for normal and proper use, even if the inflatable spacers are attached to or integral with the tire. 
     The illustrated tire  32  is generally cylindrical in shape with a plurality of traction lugs  60  extending radially outwardly from an outer side  62  of the tire  32  and a plurality of drive lugs  64 , spacers  66 , or both extending radially inwardly from an inner side  68  of the tire  32 . The outer side  62  of the tire  32  is generally transversely flat, that is, the outer side  62  of the tire  32  presents little or no curvature from a first edge  70  of the tire  32  to a second edge  72  of the tire  32 . Similarly, the inner side  68  of the tire  32  is also generally transversely flat. 
     The traction lugs  60  engage the ground and help prevent the wheel assembly  22  from slipping relative to the ground. The illustrated traction lugs  60  are generally rectangular in shape and transversely oriented on the tire  32 , are integrally formed with the tire  32  and may be approximately equally spaced circumferentially around the outer side  62  tire  32 . As illustrated in  FIG. 6 , the tire  32  may be configured such that each traction lug  60  is positioned intermediate two consecutive mounting elements  48 . This configuration allows the portion of the tire  32  bearing the traction lug  60  to flex inwardly in response to ground engaging pressure. This performance characteristic allows each traction lug  60  to engage the ground, yet limits the amount of ground penetration according to the amount of inward flex permitted by the spacer  66 . It may be desirable in some implementations to position the traction lugs  60  proximate or in direct radial alignment with the mounting elements  48 . Such alternative configurations of the tire  32  are within the ambit of the present invention. 
     The drive lugs  64  engage the wheel  30  and prevent the tire  32  from slipping on the wheel  30 . In the illustrated embodiment, each of the drive lugs  64  is integrally formed in the tire  32  and presents a generally elongated body with a triangular cross section, transversely oriented on the tire  32 . A pair of drive lugs  64  engages either side of each mounting element  48 , wherein sides of the drive lugs  64  are configured and angled for optimal contact with the angled side walls  50 , 52  of the mounting elements  48 . Because a drive lug  64  is positioned on each side of each mounting element  48 , the drive lugs  64  engage the mounting element  48  and prevent the tire  32  from rotating relative to the wheel  30  regardless of whether the wheel  30  is moving in a forward or reverse direction. Similarly, the flanges  58  of the mounting element end caps  56  engage axial ends of the drive lugs  64  and prevent the tire  32  from shifting axially relative to the wheel  30 . 
     The plurality of spacers  66  extend inwardly from the inner side  68  of the tire  32  and regulate the amount of inward flex of the tire  32 . In the illustrated embodiment, the spacers  66  are integrally formed as part of the tire  32  and are similar in size and shape to the drive lugs  64 , and each spacer  66  is positioned to be in radial alignment with one of the traction lugs  60 . As best illustrated in  FIG. 6 , as the tire  32  flexes inward in response to ground engaging pressure, a spacer  66  limits the inward flex as the spacer  66  contacts the rim wall  40 . While the spacers  66  of the illustrated embodiment are similarly configured to the drive lugs  64 , it will be appreciated that the spacers  66  may be of various sizes and shapes without departing from the spirit or scope of the present invention. Particularly, the spacers  66  may be configured according to a particular size to allow the tire to flex inwardly a desired amount. Similarly, the spacers  66  need not be aligned axially with the traction lugs  60 , but may be offset from the position in the illustrated embodiment. Such a configuration may be desirable, for example, to further regulate the performance of the traction lugs  60  when the tire  32  flexes in response to ground engaging pressure. 
     The tire  32  may be configured such that as the tire  32  flexes inwardly toward the wheel  30 , the flexed portion of the tire  32  remains transversely flat or substantially transversely flat. This may be desirable, for example, to preserve a wide footprint or otherwise limit the amount of ground penetration. 
     The tire  32  may be mounted on the wheel  30  according to any of various methods. For example, the tire  32  may be press fitted to the wheel  30 , wherein the tire  32  must be expanded or stretched to be placed over the wheel  30  and remains partially stretched while mounted on the wheel  30 . If the tire  32  is press fitted to the wheel  30 , tension remaining in the tire  32  after it is mounted on the wheel  30  helps retain the tire  32  on the wheel  30 , in addition to the mounting element end caps  58 , as explained above. Alternatively, the tire  32  may be loose fitted on the wheel  30 , wherein the tire  32  is of an appropriate size relative to the wheel  30  that the tire  32  need not be stretched for placement on the wheel  30  but may be slid onto the wheel  30 . If the tire  32  is loose fitted to the wheel  30 , little or no tension remains in the tire  32  once it is mounted on the wheel  30  such that the mounting element end caps  58  or similar elements may be required to hold the tire  32  in place on the wheel  30 . In some embodiments, the tire  32  may be bonded to the wheel  30 , although bonding presents some disadvantages, namely, the tire  32  cannot be easily removed for repair or replacement. 
     The tire  32  is constructed of a flexible material, such as rubber, PVC or plastic. The tire  32  may further include reinforcing elements that limit stretch and strengthen the tire  32 . Such reinforcing elements may include, for example, fabric webbing or steel belts. The tire  32  may be sufficiently resilient that it retains a circular shape along portions of the tire  32  not supported by the mounting elements  48 . 
     The wheel  30  is constructed of a rigid material such as, for example, metal, plastic or a composite material. The size of the wheel assembly  22  may vary substantially from one embodiment of the invention to another without departing from the scope of the invention. Dimensions and ranges of various preferred embodiments will now be discussed with the understanding that the dimensions and ranges are exemplary, and not limiting, in nature. The diameter of the wheel  30 , including the mounting elements  48 , is preferably within the range of from about twenty-four inches to about sixty inches and more preferably within the range of from about thirty-six inches to about forty-eight inches. The width of the wheel  30  is preferably within the range of from about six inches to about eighteen inches and more preferably within the range of from about eight inches to about sixteen inches. The height of the mounting elements  48  is preferably within the range of from about one inch to about four inches, more preferably within the range of from about two inches to about three inches. 
     The thickness of the tire  32 , excluding the traction lugs  60 , is preferably within the range of from about one-half inch to about three inches, more preferably within the range of from about one inch to about two inches. The height of the traction lugs  60  is preferably within the range of from about one-quarter inch to about four inches, more preferably within the range of from about one-half inch to about three inches. In one exemplary embodiment, the wheel assembly  22  is about eleven inches wide and about fifty-two inches in diameter. 
     Turning now to  FIGS. 7-9 , a wheel assembly  100  constructed according to an alternative embodiment of the invention is illustrated. The wheel assembly  100  is similar in size, shape and function to the wheel assembly  22  described above, except that the wheel assembly  100  includes a wheel  102  with a radially outer rim wall  104  presenting a polygonal shape. 
     The wheel  102  includes an innermost hub  106  with a plurality of apertures  108  for attaching to, for example, lug nuts or similar attachment components. A disc-shaped radial wall  110  or similar structural element extends radially outwardly from the hub  106  and includes peripheral apertures  112  for attaching to elements of the rim wall  104 . 
     The rim wall  104  is defined by a plurality of outermost vertices  114  connecting a plurality of planar faces  116 . In the illustrated embodiment, the rim wall  104  is defined by twelve vertices  114  connecting twelve faces  116 . The rim wall  104  comprises eight sections  118 , including four sections  118   a  corresponding to a first side of the wheel  102  and four sections  118   b  corresponding to a second side of the wheel  102 . Each section  118  defines a portion of each of three consecutive faces and a portion of a lug aperture  120 . Each section  118   a  cooperates with a corresponding section  118   b  from the opposite side to fully define the three consecutive faces and the lug aperture. Each section  118  also includes one or more inwardly-extending flanges corresponding to an axially inner edge of the section  118  and including attachment apertures  124  corresponding to the peripheral apertures  112  of the radial wall  110  for attaching the sections to the radial wall  110 . 
     When the wheel  102  is assembled the vertices  114  are the outermost structural elements of the wheel  102 . The wheel  102  is configured such that the tire  126  engages and is supported by the vertices  114  of the rim wall  104  and portions of the tire  126  between the vertices  114  are separated from the faces  116  by a space. Portions of the tire  126  between the vertices  114  are configured to flex inwardly toward the faces  116  in response to ground engaging pressure, as illustrated in  FIG. 9 a   . The wheel  102  need not include mounting elements, as the tire  126  is mounted on the vertices  114  of the rim wall  104 . The lug apertures  120  are used to secure the tire  126  to the wheel  102  such that the tire  126  does not rotate or shift laterally relative to the wheel  102 . 
     A plurality of spacers  128  may be interposed between the wheel  102  and the tire  126  to regulate the degree to which the tire  126  flexes in response to ground engaging pressure. In particular, the spacers  128  may be positioned on the faces  116  at approximately a midpoint between consecutive vertices  114  where separation of the tire  126  from the wheel  102  is the greatest. As illustrated in  FIG. 9 b   , the spacers  128  are attached to the wheel  102  and, as explained below, may be removably attached to the wheel  102 . 
     The tire  126  is similar to the tire  32  described above, except that the tire  126  includes four drive lugs  130  for engaging the drive lug apertures  120  of the rim wall and may not include spacer elements attached thereto or integrally formed in the tire  126 . The four drive lugs  130  may be integrally formed with the tire  126  and are approximately equally spaced around an inner side  132  of the tire  126 . The drive lugs  130  are configured and positioned to engage the lug apertures  120  of the wheel  102 . The tire  126  may be positioned on the wheel  102  such that each traction lug  130  is positioned approximately over a center of a face  116  and in radial alignment with a spacer  128 , if spacers are present. 
     Turning now to  FIGS. 10-12 , a wheel assembly  200  constructed according to another embodiment of the invention is illustrated. The wheel assembly  200  is similar to the wheel assembly  100  described above, except that the wheel assembly  200  includes a wheel  202  defined by a hub and spoke configuration. In particular, the wheel  202  comprises a hub  204  and a plurality of spoke elements  206  extending radially outwardly from the hub  204 . The hub  204  includes an inner hub ring  208  with a plurality of apertures  210  for attaching to, for example, lug nuts or similar attachment components. A disc-shaped radial wall  212  or similar structural element extends radially outwardly from the inner hub ring  208 . The radial wall  212  includes a plurality of outer peripheral apertures  214  for attaching the plurality of spoke elements  206  to the radial wall  212 . The spoke elements  206  extend radially outwardly from the radial wall  212  and support the tire  216 . 
     The illustrated embodiment includes six spoke elements  206  approximately equally spaced around the radial wall  212 . Each spoke element  206  includes a planar radial portion  218  and a planar transverse portion  220  that together form a T-shaped cross section. The planar radial portion  218  attaches to the radial wall  212  and extends radially outwardly toward the transverse portion  220 . Each transverse portion  220  represents a radially outer, rectangular planar face that lies in a plane that is approximately parallel with an axis of rotation of the wheel assembly  200 . 
     The tire  216  may be similar to the tire  126  described above. The tire  216  is mounted on the spoke elements  206  and engages opposing forward  222  and trailing  224  edges of the transverse portion  220  of each spoke element  206 . The edges  222 , 224  support the tire  216  and portions of the tire  216  between consecutive spoke elements  206  and between the edges  222 , 224  of each spoke element  206  flex inwardly toward a center of the wheel  202  in response to ground engaging pressure, as illustrated in  FIG. 12 a   . Spacers  226  may be placed between the spoke elements  206  and the tire  216 , as illustrated in FIG.  12   b , thus limiting the amount of inward flex of the tire toward the face of each spoke element. 
     Portions of the tire  216  between the spoke elements  206  may flex inwardly more than portions of the tire  216  corresponding to the spoke elements  206 , particularly if spacers  226  are present on the spoke elements  206 . This may result in traction lugs  228  positioned in alignment with the spoke elements  206  experience greater ground penetration than traction lugs  228  positioned between the spoke elements  206 . 
     A first, outer diameter of the wheel  202  including the spoke elements  206  is greater than a second, inner diameter of the wheel  202  not including the spoke elements  206 . By way of example, the inner diameter may be about 25% of the outer diameter, may be about 50% of the outer diameter, or may be about 75% of the outer diameter. 
     Turning now to  FIGS. 13-14 , a wheel assembly  300  constructed according to another embodiment of the invention is illustrated. The wheel assembly  300  is similar in size, shape and function to the wheel assembly  100  described above, except that the wheel assembly  300  includes a wheel  302  with a plurality of attachment points  304  for attaching modular portions of a tire  306  to the wheel  302 . More particularly, the wheel  302  includes four attachment points  304  approximately equally spaced around a radially outer rim wall  308  of the wheel  302 . Each attachment point  304  corresponds to a vertex of the rim wall  308  and includes an aperture  310  and a pair of eyelets  312 . The eyelets  312  are positioned in axial alignment on opposing axial margins of the rim wall  308  to receive a pin  314 . 
     The tire  306  is similar in size and shape to the tire  126  describe above, except that the tire  306  comprises a plurality of separate tire sections  316 , 318 , 320 , 322 . Each of the tire sections  316 - 322  corresponds to a circumferential portion of the tire  306 , in the illustrated embodiment each section  316 - 322  corresponds to about one-fourth of the circle defined by the tire, or about ninety degrees of curvature of the tire  306 . The tire sections  316 - 322  may be removably attached to the wheel  302  and, when attached to the wheel  302 , generally form a circular shape in circumscribing relationship to the wheel  302 . 
     Each of the tire sections  316 - 322  may be identically configured, therefore only section  316  will be described in further detail with the understanding that the other sections  318 , 320 , 322  may be similarly or identically configured. A first end  324  of the section  316  and a second end  326  of the section  316  each include a plurality of loops  328  in axial alignment and configured to receive a pin  314 . When the tire section  316  is attached to the wheel  302 , the loops  328  of the first end  324  are interdigitated with end loops  328  of section  318  and positioned in axial alignment with the eyelets  312  of the attachment point  314  and the end loops  328  of section  318  such that a pin  314  may be inserted through the eyelets  312  and the loops  328  of both sections  316 , 318 , thereby securing the loops  328  in place. The second end  326  of the section  316  is similarly secured to an attachment point  304  with an end of section  322 . 
     Because each tire section  316 - 322  is secured to the wheel  302  via a pin  314 , drive lugs are not necessary. Thus, an inner side of each of the tire sections  316 - 322  may be substantially smooth while an outer side may include traction lugs  330  similar to the traction lugs  60  described above. While the illustrated modular tire  306  includes four sections  316 - 322 , it will be appreciated that the tire  306  may comprise more than four sections or fewer than four sections. By way of example and not limitation, the modular tire  306  may include only two sections, three sections, or five sections. Furthermore, the wheel assembly  300  may include spacers (not shown) positioned between the tire  306  and the wheel  302  to regulate the flexing action of the tire  306 . Such spacers may be attached to the wheel  302  or the tire  306 . 
       FIG. 15  illustrates a wheel assembly  400  constructed according to another embodiment of the invention. The wheel assembly  400  is similar in size, shape and function to the wheel assembly  22  described above, except that the wheel assembly  400  includes a tire  402  with an outer side  404  that is transversely concave. In other words, the outer side  404  of the tire  402  presents a concave profile between a first edge  406  and a second edge  408 . The tire  402  also differs from the tire  32  in that the tire  402  does not include traction lugs. In the illustrated embodiment, the concave profile results from the tire  402  being thicker on the edges  406 , 408  than in a middle portion between the edges  406 , 408 . Such a configuration may be desirable, for example, to prevent the lateral displacement of soil beneath the tire  402 . 
     Turning now to  FIGS. 16-18 , a wheel assembly  500  constructed according to another alternative embodiment of the invention is illustrated. The wheel assembly  500  includes a wheel  502  and a tire  504  and is similar to the wheel assembly  22  described above, except that the wheel assembly  500  includes spacers that are interchangeable, adjustable, or both. Removing, exchanging or adjusting the spacers may be desirable to adjust the amount the tire flexes. If the wheel assembly  500  will be used in an environment with softer ground, it may be desirable to use smaller spacers that allow greater inward flex to increase the ground engaging footprint of the wheel assembly  500 , decrease penetration of the traction lugs, or both. Similarly, if the wheel assembly  500  will be used in an environment with harder ground, it may be desirable to use larger spacers that allow less inward flex to decrease the ground engaging footprint of the wheel assembly  500 , increase penetration of the traction lugs, or both. 
     In a first configuration illustrated in  FIGS. 16 a  and 16 b   , the wheel assembly  500  includes interchangeable spacers  506 , 508 . A first set of spacers  506  are larger and may be removed and replaced with a second, smaller set of spacers  508 . Each of the spacers  506 , 508  is elongated with a generally rectangular cross section and is oriented generally transversely on the wheel  502 . The spacers  506 , 508  may be removably attached to the wheel  502  using any of various removable fasteners such as, for example, a nut and bolt combination. 
     In a second configuration illustrated in  FIGS. 17 a  and 17 b   , the wheel assembly  500  includes adjustable spacers  510 . The spacers  510  are secured to the wheel  502  in radial alignment with the traction lugs and are adjustable between a first, larger size ( FIG. 17 a   ) and a second, smaller size ( FIG. 17 b   ). Adjusting the spacers between the first size and the second size may be performed without removing the spacers  510  from the wheel assembly  500 . By way of example, the adjustable spacers  510  may be inflatable such that inflating the spacers  510  causes them to enlarge and deflating the spacers causes them to decrease in size. Alternatively or additionally, adjusting the air pressure in the spacers  510  may alter the hardness or flexibility of the spacers  510 , such that increasing air pressure hardens the spacers  510  and decreases the amount of inward flex of the tire  504  and decreasing air pressure softens the spacers  510  and increases the amount of inward flex of the tire  504 . 
     Each of the spacers  510  is adjustable separately from the other spacers, wherein each of the spacers  510  may include a valve and valve stem (not shown) similar or identical to a conventional valve and a valve stem used on a pneumatic tire. Alternatively, a single spacer component  512  with a plurality of inflatable sections  514  may be used. The spacer component  512  functions similarly to spacers  510 , except that the inflatable sections  514  are in fluid communication such that all of the sections  514  are inflated simultaneously as a single unit. A recess or break in each of the mounting elements  516  my accommodate a portion of the spacer component  512  enabling fluid communication between sections  514 . For example, the spacer component  512  may take the form of an inflatable tube with a single inflatable chamber comprising all of the sections  514 . Alternatively, the inflatable sections  514  may be interconnected via rigid channels in the spacer component  512  that permit fluid flow therethrough. 
     It will be observed that the combination of the tire  504  and the single spacer component  512  described above is similar in some respects to a traditional pneumatic tire or tube/tire assembly in form and function. Specifically, the spacer component  512  is configured to be inflated and deflated as a single unit, is interposed between the tire and the wheel, and regulates the inward flex of portions of the tire during operation. One notable difference between the wheel assembly  500  with the spacer component  512  and a conventional pneumatic tire is that the tire  504  is supported by the mounting elements  516  and the spacer component  512  only regulates the inward flex of certain portions of the tire  504 . Another notable difference between the wheel assembly  500  with the spacer component  512  and a conventional pneumatic tire is that the wheel assembly  500  is configured for normal operation even if the spacer component  512  is completely deflated. Thus, if the spacer component  512  should become damaged or otherwise deflate unexpectedly during use, the wheel assembly  500  would continue to operate without sustaining damage and with minimal impact on the performance of the wheel assembly  500 . 
     Turning now to  FIGS. 19-21 , a wheel assembly  600  constructed according to another alternative embodiment of the invention is illustrated. The wheel assembly  600  includes wheel  602  that is similar to the wheel  30  except that the wheel  602  does not have mounting elements. The wheel assembly  600  also includes a tire  604  that is similar to the tire  32  except that the tire  604  does not include spacer elements or drive lugs, and the tire  604  presents a truncated conical outer shape wherein a first edge  606  of the tire  604  presents a larger outer diameter than a second edge  608  of the tire  604 . An outer side  610  of the tire  604  slopes radially inwardly from the first edge  606  to the second edge  608  along a straight or arcuate path. 
     The truncated conical configuration may be achieved through design configurations associated with the wheel  602 , the tire  604  or both. In the illustrated embodiment, the wheel  602  includes a radially outer rim wall  612  similar or identical to the rim wall  40  described above, wherein the rim wall  612  defines a cylinder about an axis that corresponds to an axis of rotation of the wheel assembly  600 . The rim wall  612  includes first  614  and second  616  shoulders and a transverse portion  618  intermediate the shoulders  614 , 616 . The tire  604  is configured such that a first axial margin of the tire  604  corresponding to the first edge  606  rests on the first shoulder  614  and is separated from the transverse portion  618  by a space. A second axial margin of the tire  604  corresponding to the second edge  608  rests in part on the transverse portion  618  of the rim wall  612  proximate the second shoulder  616 . This is but one, exemplary configuration of the wheel assembly  600 . 
     The slope of the outer side  610  of the tire  604  relative to the axis of rotation of the wheel assembly  600  is preferably within the range of from about 0.01 to about 1.0, and more preferably within the range of from about 0.05 to about 0.5 and may particularly be about 0.1, about 0.2 or about 0.3. By way of example, the width of the tire  604  may be about eleven inches, the first edge  608  may be about fifty-one inches in diameter and the second edge  606  may be about fifty three inches in diameter. 
     The truncated conical configuration of the embodiment illustrated in  FIGS. 19-21  may be desirable, for example, where the wheel is used on a sloped surface or otherwise is not perpendicular with the surface of the ground. 
       FIG. 21  illustrates a wheel assembly  700  constructed according to another alternative embodiment of the invention. The wheel assembly  700  may be similar or identical to the wheel assembly  22  described above, except that the wheel assembly  700  includes some traction lugs  702  of a first configuration and some traction lugs  704  of a second configuration. More particularly, the wheel assembly  700  includes a total of twelve traction lugs, wherein three of the traction lugs  704  are larger than the remaining nine traction lugs  702 . The larger traction lugs  704  have a greater radial reach than the smaller traction lugs  702 , such that the larger traction lugs  704  may experience greater ground penetration than the smaller traction lugs  702 . This may be desirable, for example, on soft or muddy ground where the wheel assembly  700  is susceptible to slipping on the ground. 
     Although the invention has been described with reference to the exemplary embodiments illustrated in the attached drawings, it is noted that equivalents may be employed and substitutions made herein without departing from the scope of the invention as recited in the claims. For example, the interchangeable and adjustable spacers  506 , 508 , 510  may be used with any of the embodiments of the wheel assembly described herein. Furthermore, spacers may be entirely omitted from any of the embodiments of the wheel assembly described herein. Additionally, any number of traction lugs may be used with the various embodiments of the wheel assembly, the traction lugs may be of virtually any size and shape, and a conventional tire tread may be used instead of traction lugs.