Patent Publication Number: US-2020277012-A1

Title: Track system for traction of a vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. Provisional Patent Application 62/268,309 filed on Dec. 16, 2015 and hereby incorporated by reference herein. 
    
    
     FIELD 
     The invention relates generally to track systems for tractions of vehicles, such as all-terrain vehicles (ATVs) and other off-road vehicles. 
     BACKGROUND 
     Certain vehicles, such as all-terrain vehicles (ATVs), agricultural vehicles (e.g., tractors, harvesters, etc.), may be equipped with track systems which enhance their traction and floatation on soft, slippery and/or irregular grounds (e.g., soil, mud, sand, ice, snow, etc.) on which they operate. A track system comprises a track disposed around a track-engaging assembly including a frame and track-contacting wheels that drive and guide the track around the track-engaging assembly. 
     As they travel on the ground, tracked vehicles can encounter various obstacles (e.g., rocks, portions of trees, debris, ice blocks, bumps or other abrupt changes in ground level, etc.) depending on their environment. Such obstacles can create shocks in their track systems. Depending on their intensity, these shocks may affect ride quality and/or structural integrity of the track systems. Also, some obstacles may sometimes prove more difficult to overcome. This can negatively impact a tracked vehicle&#39;s performance by slowing it down or in some cases bringing it to a standstill. 
     In some cases, shock absorption of track systems may be enhanced by providing some resilience within the track systems (e.g., wheels or other components that are less stiff). However, this may cause issues in view of track tensioning. 
     For these and other reasons, there is a need to improve track systems for traction of vehicles. 
     SUMMARY 
     According to various aspects of the invention, there is provided a track system for traction of a vehicle. The track system may be designed (e.g., may comprise non-pneumatic tires) to enhance its use or performance and/or that of the vehicle such as, for example, by being lightweight and/or by better handling loads, including, for instance, those resulting from track tension within the track system and/or from unevenness or other aspects of the ground, including encounters (e.g., impacts) with obstacles on the ground (e.g., rocks, portions of trees, debris, bumps, abrupt changes in ground level, etc.). For instance, in some embodiments, the track system may comprise tension-based non-pneumatic tires. 
     For example, according to an aspect of the invention, there is provided a track system for traction of a vehicle. The track system comprises: a track for engaging the ground; and a track-engaging assembly for driving and guiding the track around the track-engaging assembly. The track-engaging assembly comprises: a drive wheel for driving the track; and an idler wheel contacting a bottom run of the track. The idler wheel comprises a non-pneumatic tire. The non-pneumatic tire comprises: an annular beam configured to deflect at an area of contact of the non-pneumatic tire with the track; and an annular support disposed radially inwardly of the annular beam and configured to resiliently deform under loading on the idler wheel for supporting the loading on the idler wheel by tension. 
     According to an aspect of the invention, there is provided a track system for traction of a vehicle. The track system comprises: a track for engaging the ground; and a track-engaging assembly for driving and guiding the track around the track-engaging assembly. The track-engaging assembly comprises: a drive wheel for driving the track; and an idler wheel contacting a bottom run of the track. The idler wheel comprises a non-pneumatic tire. The non-pneumatic tire comprises an annular beam configured to deflect at an area of contact of the non-pneumatic tire with the track. The annular beam is configured to deflect more by shearing than by bending at the area of contact of the non-pneumatic tire with the track when the idler wheel encounters an obstacle on the ground. The non-pneumatic tire comprises an annular support disposed radially inwardly of the annular beam and configured to resiliently deform under loading on the idler wheel. 
     According to an aspect of the invention, there is provided a track system for traction of a vehicle. The track system comprises: a track for engaging the ground; and a track-engaging assembly for driving and guiding the track around the track-engaging assembly. The track-engaging assembly comprises: a drive wheel for driving the track; and an idler wheel contacting a bottom run of the track. The idler wheel comprises a non-pneumatic tire. The non-pneumatic tire comprises an annular beam configured to deflect at an area of contact of the non-pneumatic tire with the track. The annular beam is configured to deflect more by shearing than by bending at the area of contact of the non-pneumatic tire with the track when the idler wheel is loaded against a flat surface to a contact length of at least 40% of a diameter of the idler wheel. The non-pneumatic tire comprises an annular support disposed radially inwardly of the annular beam and configured to resiliently deform under loading on the idler wheel. 
     According to an aspect of the invention, there is provided a track system for traction of a vehicle. The track system comprises: a track for engaging the ground; and a track-engaging assembly for driving and guiding the track around the track-engaging assembly. The track-engaging assembly comprises: a drive wheel for driving the track; and an idler wheel contacting a bottom run of the track. The idler wheel comprises a non-pneumatic tire. The non-pneumatic tire comprises an annular beam configured to deflect at an area of contact of the non-pneumatic tire with the track. The annular beam comprises a plurality of openings distributed in a circumferential direction of the non-pneumatic tire. Each of the openings extends from a first lateral side of the non-pneumatic tire to a second lateral side of the non-pneumatic tire. The non-pneumatic tire comprises an annular support disposed radially inwardly of the annular beam and configured to resiliently deform under loading on the idler wheel. 
     According to an aspect of the invention, there is provided a track system for traction of a vehicle. The track system comprises: a track for engaging the ground; and a track-engaging assembly for driving and guiding the track around the track-engaging assembly. The track-engaging assembly comprises: a drive wheel for driving the track; and an idler wheel contacting a bottom run of the track. A radial stiffness of the idler wheel is different under different types of loading on the idler wheel. 
     According to an aspect of the invention, there is provided a track system for traction of a vehicle. The track system comprises: a track for engaging the ground; and a track-engaging assembly for driving and guiding the track around the track-engaging assembly. The track-engaging assembly comprises: a drive wheel for driving the track; and a leading idler wheel and a trailing idler wheel that contact a bottom run of the track. An axis of rotation of the drive wheel is located between an axis of rotation of the leading idler wheel and an axis of rotation of the trailing idler wheel in a longitudinal direction of the track system. The leading idler wheel is structurally different from the trailing idler wheel. 
     These and other aspects of the invention will now become apparent to those of ordinary skill in the art upon review of the following description of embodiments in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A detailed description of embodiments is provided below, by way of example only, with reference to the accompanying drawings, in which: 
         FIGS. 1A and 1B  show an example of an all-terrain vehicle (ATV) comprising track systems in accordance with an embodiment of the invention; 
         FIGS. 2A and 2B  show another example in which the ATV is a utility terrain or utility task vehicle (UTV) in accordance with another embodiment of the invention; 
         FIGS. 3A and 3B  show the ATV equipped with ground-engaging wheels instead of the track systems; 
         FIGS. 4 and 5  respectively show perspective and side views of a rear one of the track systems; 
         FIG. 6  shows a bottom view of the track system; 
         FIGS. 7 and 8  respectively show perspective and side views of the track system without its track; 
         FIGS. 9 and 10  show perspective views of a segment of the track of the track system, which depict features of an inner side and a ground-engaging outer side of the track; 
         FIG. 11  shows a side view of the track of the track system; 
         FIG. 12  shows a partial cross-sectional view of an embodiment of the track of the track system; 
         FIG. 13  shows a partial cross-sectional view of another embodiment of the track of the track system; 
         FIGS. 14A and 14B  respectively show a side view and a perspective view of an idler wheel of the track system; 
         FIG. 15  shows a close-up view of part of a non-pneumatic tire of the idler wheel; 
         FIG. 16  shows a cross-sectional view of the idler wheel; 
         FIGS. 17 to 19  show representations of the idler wheel in different conditions; 
         FIG. 20  shows a system for measuring a radial stiffness of the idler wheel under a track tension load; 
         FIG. 21  shows a representation of an embodiment of the idler wheel and the track; 
         FIG. 22  shows the idler wheel under the track tension load; 
         FIG. 23  shows the idler wheel under an obstacle load; 
         FIG. 24  shows a chart that relates load and displacement for the idler wheel of  FIGS. 22 and 23 ; 
         FIGS. 25 to 27  respectively show structural modeling results for another embodiment of the idler wheel under no load, under the track tension load, and under the obstacle load; 
         FIG. 28  shows a side view of a rear one of the track systems where a leading idler wheel and a trailing idler wheel are made of different materials; 
         FIG. 29  shows a side view of the rear one of the track systems where a number of openings of an annular beam of a non-pneumatic tire of the leading idler wheel is greater than that of a non-pneumatic tire of the trailing idler wheel. 
         FIG. 30  shows a side view of a rear one of the track systems where a size of one or more of the openings of the annular beam of the non-pneumatic tire of the leading idler wheel is greater than that of the non-pneumatic tire of the trailing idler wheel; 
         FIG. 31  shows a side view of the rear one of the track systems where a number of spokes of the non-pneumatic tire of the leading idler wheel is less than that of the non-pneumatic tire of the trailing idler wheel. 
         FIG. 32  shows a side view of the rear one of the track systems where a size of one or more of the spokes of the non-pneumatic tire of the leading idler wheel is less than that of the non-pneumatic tire of the trailing idler wheel. 
         FIG. 33  shows an example of an embodiment in which the annular beam of the non-pneumatic tire comprises a reinforcing layer; 
         FIG. 34  shows an example of an embodiment of the reinforcing layer; 
         FIG. 35  shows an example of another embodiment of the reinforcing layer; 
         FIG. 36  shows an example of an embodiment in which a thickness of the annular beam of the non-pneumatic tire is increased; 
         FIG. 37  shows an example of an agricultural vehicle comprising a track system in accordance with an embodiment of the invention; and 
         FIG. 38  shows a perspective view of the track system of  FIG. 37 . 
     
    
    
     It is to be expressly understood that the description and drawings are only for the purpose of illustrating certain embodiments and are an aid for understanding. They are not intended to be a definition of the limits of the invention. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIGS. 1A and 1B  show an example of a vehicle  10  comprising track systems  20   1 - 20   4  in accordance with an embodiment of the invention. In this embodiment, the tracked vehicle  10  is an all-terrain vehicle (ATV), which is a small open vehicle designed to travel off-road on a variety of terrains, including roadless rugged terrain, for recreational, utility and/or other purposes. The ATV  10  comprises a frame  12 , a powertrain  14 , a steering system  16 , a suspension  18 , the track systems  20   1 - 20   4 , a seat  22 , and a user interface  24  which enable a user of the ATV  10  to ride the ATV  10  on the ground. The ATV  10  has a longitudinal direction, a widthwise direction and a height direction. 
     As further discussed later, in various embodiments, the track systems  20   1 - 20   4  may be designed (e.g., may comprise non-pneumatic tires, such as tension-based non-pneumatic tires) to enhance use or performance of the track systems  20   1 - 20   4  and/or the ATV  10 , such as, for example, by being lightweight and/or by better handling loads, including, for instance, those resulting from track tension within the track systems  20   1 - 20   4  (e.g., at low speed and high torque) and/or from unevenness or other aspects of the ground, including encounters (e.g., impacts) with obstacles (e.g., rocks, portions of trees, debris, bumps, abrupt changes in ground level, etc.) on the ground (e.g., at high speed). 
     The powertrain  14  is configured for generating motive power and transmitting motive power to respective ones of the track systems  20   1 - 20   4  to propel the ATV  10  on the ground. To that end, the powertrain  14  comprises a prime mover  15 , which is a source of motive power that comprises one or more motors. For example, in this embodiment, the prime mover  15  comprises an internal combustion engine. In other embodiments, the prime mover  15  may comprise another type of motor (e.g., an electric motor) or a combination of different types of motor (e.g., an internal combustion engine and an electric motor) for generating motive power to move the ATV  10 . The prime mover  15  is in a driving relationship with one or more of the track systems  20   1 - 20   4 . That is, the powertrain  14  transmits motive power generated by the prime mover  15  to one or more of the track systems  20   1 - 20   4  (e.g. via a transmission and/or a differential) in order to drive (i.e. impart motion to) these one or more of the track systems  20   1 - 20   4 . 
     The steering system  16  is configured to enable the user to steer the ATV  10  on the ground. To that end, the steering system  16  comprises a steering device  26  that is operable by the user to direct the ATV  10  along a desired course on the ground. In this embodiment, the steering device  26  comprises handlebars. The steering device  26  may comprise a steering wheel or any other steering component that can be operated by the user to steer the ATV  10  in other embodiments. In this embodiment, the steering system  16  responds to the user interacting with the steering device  26  by turning respective ones of the track systems  20   1 - 20   4  to change their orientation relative to the frame  12  of the ATV  10  in order to cause the ATV  10  to move in a desired direction. In this example, the track systems  20   1 ,  20   2  (i.e., front ones of the track systems  20   1 - 20   4 ) are turnable in response to input of the user at the steering device  26  to change their orientation relative to the frame  12  of the ATV  10  in order to steer the ATV  10  on the ground. More particularly, in this example, each of the track systems  20   1 ,  20   2  (i.e., each of the front ones of the track systems  20   1 - 20   4 ) is pivotable about a steering axis  28  of the ATV  10  in response to input of the user at the steering device  26  in order to steer the ATV  10  on the ground. The track systems  20   3 ,  20   4  (i.e. rear ones of the track systems  20   1 - 20   4 ) are not turned relative to the frame  12  of the ATV  10  by the steering system  16 . 
     The suspension  18  is connected between the frame  12  and the track systems  20   1 - 20   4  to allow relative motion between the frame  12  and the track systems  20   1 - 20   4  as the ATV  10  travels on the ground. For example, the suspension  18  enhances handling of the ATV  10  on the ground by absorbing shocks and helping to maintain traction between the track systems  20   1 - 20   4  and the ground. The suspension  18  may comprise an arrangement of springs and dampers. A spring may be a coil spring, a leaf spring, a gas spring (e.g., an air spring), or any other elastic object used to store mechanical energy. A damper (also sometimes referred to as a “shock absorber”) may be a fluidic damper (e.g., a pneumatic damper, a hydraulic damper, etc.), a magnetic damper, or any other object which absorbs or dissipates kinetic energy to decrease oscillations. In some cases, a single device may itself constitute both a spring and a damper (e.g., a hydropneumatic, hydrolastic, or hydragas suspension device). 
     In this embodiment, the seat  22  is a straddle seat and the ATV  10  is usable by a single person such that the seat  22  accommodates only that person driving the ATV  10 . In other embodiments, the seat  22  may be another type of seat, and/or the ATV  10  may be usable by two individuals, namely one person driving the ATV  10  and a passenger, such that the seat  22  may accommodate both of these individuals (e.g., behind one another or side-by-side) or the ATV  10  may comprise an additional seat for the passenger. For example, in other embodiments, as shown in  FIGS. 2A and 2B , the ATV  10  may be a side-by-side ATV, sometimes referred to as a “utility terrain vehicle” or “utility task vehicle” (UTV), an example of which is shown in  FIGS. 2A and 2B . 
     The user interface  24  allows the user to interact with the ATV  10 . More particularly, the user interface  24  comprises an accelerator, a brake control, and the steering device  26  that are operated by the user to control motion of the ATV  10  on the ground. The user interface  24  also comprises an instrument panel (e.g., a dashboard) which provides indicators (e.g., a speedometer indicator, a tachometer indicator, etc.) to convey information to the user. 
     The track systems  20   1 - 20   4  engage the ground to provide traction to the ATV  10 . More particularly, in this example, the track systems  20   1 ,  20   2  (i.e., the front ones of the track systems  20   1 - 20   4 ) provide front traction to the ATV  10  while the track systems  20   3 ,  20   4  (i.e. the rear ones of the track systems  20   1 - 20   4 ) provide rear traction to the ATV  10 . 
     In this embodiment, as shown in  FIGS. 3A and 3B , the track systems  20   1 - 20   4  are respectively mounted in place of ground-engaging wheels  21   1 - 21   4  with tires that may otherwise be mounted at positions of the track systems  20   1 - 20   4  to propel the ATV  10  on the ground. Basically, in this embodiment, the track systems  20   1 - 20   4  may be used to convert the ATV  10  from a wheeled vehicle into a tracked vehicle, thereby enhancing its traction and flotation on the ground. 
     With additional reference to  FIGS. 4 to 8 , in this embodiment, each track system  20   i  comprises a track-engaging assembly  17  and a track  41  disposed around the track-engaging assembly  17 . In this example, the track-engaging assembly  17  comprises a frame  44  and a plurality of track-contacting wheels which includes a drive wheel  42  and a plurality of idler wheels  50   1 - 50   4 ,  55   1 - 55   8 , which includes leading (i.e., front) idler wheels  50   1 ,  50   2 , trailing (i.e., rear) idler wheels  50   3 ,  50   4 , and support wheels  55   1 - 55   8  between the leading idler wheels  50   1 ,  50   2  and the trailing idler wheels  50   3 ,  50   4 . The track system  20   i  has a front longitudinal end  57  and a rear longitudinal end  59  that define a length of the track system  20   i . A width of the track system  20   i  is defined by a width of the track  41 . The track system  20   i  has a longitudinal direction, a widthwise direction, and a height direction. 
     The track  41  engages the ground to provide traction to the ATV  10 . A length of the track  41  allows the track  41  to be mounted around the track-engaging assembly  17 . In view of its closed configuration without ends that allows it to be disposed and moved around the track-engaging assembly  17 , the track  41  can be referred to as an “endless” track. Referring additionally to  FIGS. 9 to 11 , the track  41  comprises an inner side  45  facing the wheels  42 ,  50   1 - 50   4 ,  55   1 - 55   8  and defining an inner area of the track  41  in which these wheels are located. The track  41  also comprises a ground-engaging outer side  47  opposite the inner side  45  for engaging the ground on which the ATV  10  travels. Lateral edges  63   1 ,  63   2  of the track  41  define the track&#39;s width. The track  41  has a top run  65  which extends between the longitudinal ends  57 ,  59  of the track system  20   i  and over the track-engaging assembly  17 , and a bottom run  66  which extends between the longitudinal ends  57 ,  59  of the track system  20   i  and under the track-engaging assembly  17 . The track  41  has a longitudinal direction, a widthwise direction, and a thickness direction. 
     The track  41  is elastomeric, i.e., comprises elastomeric material, allowing it to flex around the wheels  42 ,  50   1 - 50   4 ,  55   1 - 55   8 . The elastomeric material of the track  41  can include any polymeric material with suitable elasticity. In this embodiment, the elastomeric material includes rubber. Various rubber compounds may be used and, in some cases, different rubber compounds may be present in different areas of the track  41 . In other embodiments, the elastomeric material of the track  41  may include another elastomer in addition to or instead of rubber (e.g., polyurethane elastomer). The track  41  can be molded into shape in a mold by a molding process during which its elastomeric material is cured. 
     More particularly, the track  41  comprises an elastomeric belt-shaped body  36  underlying its inner side  45  and its ground-engaging outer side  47 . In view of its underlying nature, the body  36  can be referred to as a “carcass”. The carcass  36  comprises elastomeric material  37  which allows the track  41  to flex around the wheels  42 ,  50   1 - 50   4 ,  55   1 - 55   8 . 
     As shown in  FIG. 12 , in this embodiment, the carcass  36  comprises a plurality of reinforcements embedded in its elastomeric material  37 . One example of a reinforcement is a layer of reinforcing cables  38   1 - 38   C  that are adjacent to one another and that extend in the longitudinal direction of the track  41  to enhance strength in tension of the track  41  along its longitudinal direction. In some cases, a reinforcing cable may be a cord or wire rope including a plurality of strands or wires. In other cases, a reinforcing cable may be another type of cable and may be made of any material suitably flexible longitudinally (e.g., fibers or wires of metal, plastic or composite material). Another example of a reinforcement is a layer of reinforcing fabric  40 . Reinforcing fabric comprises pliable material made usually by weaving, felting, or knitting natural or synthetic fibers. For instance, a layer of reinforcing fabric may comprise a ply of reinforcing woven fibers (e.g., nylon fibers or other synthetic fibers). Various other types of reinforcements may be provided in the carcass  36  in other embodiments. 
     The carcass  36  may be molded into shape in the track&#39;s molding process during which its elastomeric material  37  is cured. For example, in this embodiment, layers of elastomeric material providing the elastomeric material  37  of the carcass  36 , the reinforcing cables  38   1 - 38   C  and the layer of reinforcing fabric  40  may be placed into the mold and consolidated during molding. 
     In this embodiment, the inner side  45  of the track  41  comprises an inner surface  32  of the carcass  36  and a plurality of wheel-contacting projections  48   1 - 48   N  that project from the inner surface  32  to contact at least some of the wheels  42 ,  50   1 - 50   4 ,  55   1 - 55   8  and that are used to do at least one of driving (i.e., imparting motion to) the track  41  and guiding the track  41 . In that sense, the wheel-contacting projections  48   1 - 48   N  can be referred to as “drive/guide projections”, meaning that each drive/guide projection is used to do at least one of driving the track  41  and guiding the track  41 . Also, such drive/guide projections are sometimes referred to as “drive/guide lugs” and will thus be referred to as such herein. More particularly, in this embodiment, the drive/guide lugs  48   1 - 48   N  interact with the drive wheel  42  in order to cause the track  41  to be driven, and also interact with the idler wheels  50   1 - 50   4 ,  55   1 - 55   8  in order to guide the track  41  as it is driven by the drive wheel  42 . The drive/guide lugs  48   1 - 48   N  are thus used to both drive the track  41  and guide the track  41  in this embodiment. 
     The drive/guide lugs  48   1 - 48   N  are spaced apart along the longitudinal direction of the track  41 . In this case, the drive/guide lugs  48   1 - 48   N  are arranged in a plurality of rows that are spaced apart along the widthwise direction of the track  41 . The drive/guide lugs  48   1 - 48   N  may be arranged in other manners in other embodiments (e.g., a single row or more than two rows). Each of the drive/guide lugs  48   1 - 48   N  is an elastomeric drive/guide lug in that it comprises elastomeric material  68 . The drive/guide lugs  48   1 - 48   N  can be provided and connected to the carcass  36  in the mold during the track&#39;s molding process. 
     The ground-engaging outer side  47  of the track  41  comprises a ground-engaging outer surface  31  of the carcass  36  and a plurality of traction projections  61   1 - 61   M  that project from the outer surface  31  and engage and may penetrate into the ground to enhance traction. The traction projections  61   1 - 61   M , which can sometimes be referred to as “traction lugs” or “traction profiles”, are spaced apart in the longitudinal direction of the track system  20   i . The ground-engaging outer side  47  comprises a plurality of traction-projection-free areas  71   1 - 71   F  (i.e., areas free of traction projections) between successive ones of the traction projections  61   1 - 61   M . In this example, each of the traction projections  61   1 - 61   M  is an elastomeric traction projection in that it comprises elastomeric material  69 . The traction projections  61   1 - 61   M  can be provided and connected to the carcass  36  in the mold during the track&#39;s molding process. 
     Each traction projection  61   i  extends transversally to the longitudinal direction of the track  41 . That is, the traction projection  61   i  has a longitudinal axis  54  extending transversally to the longitudinal direction of the track  41 . In this example, the longitudinal axis  54  of the traction projection  61   i  is substantially parallel to the widthwise direction of the track  41 . In other examples, the longitudinal axis  54  of the traction projection  61   i  may be transversal to the longitudinal direction of the track  41  without being parallel to the widthwise direction of the track  41 . 
     In this example, the carcass  36  has a thickness T c  which is relatively small. The thickness T c  of the carcass  36  is measured from the inner surface  32  to the ground-engaging outer surface  31  of the carcass  35  between longitudinally-adjacent ones of the traction projections  61   1 - 61   M . For example, in some embodiments, the thickness T c  of the carcass  36  may be no more than 0.375 inches, in some cases no more than 0.325 inches, in some cases no more than 0.275 inches, in some cases no more than 0.225 inches, in some cases no more than 0.200 inches, and in some cases even less (e.g., 0.180 or 0.170 inches). The thickness T c  of the carcass  36  may have any other suitable value in other embodiments. 
     In this embodiment, as shown in  FIG. 12 , the track  41  is free of transversal stiffening rods embedded in its elastomeric material. That is, the track  41  does not comprise transversal stiffening rods embedded in its elastomeric material and extending transversally to its longitudinal direction.  FIG. 13  shows a variant in which the track  41  may comprise transversal stiffening rods  53   1 - 53   M  embedded in its elastomeric material and extending transversally to its longitudinal direction in other embodiments. This absence of transversal stiffening rods makes the track  41  more flexible in its widthwise direction than if the track  41  had the transversal stiffening rods  53   1 - 53   M  but was otherwise identical. 
     The track  41  may be constructed in various other ways in other embodiments. For example, in some embodiments, the track  41  may comprise a plurality of parts (e.g., rubber sections) interconnected to one another in a closed configuration, the track  41  may have recesses or holes that interact with the drive wheel  42  in order to cause the track  41  to be driven (e.g., in which case the drive/guide lugs  48   1 - 48   N  may be used only to guide the track  41  without being used to drive the track  41 ), and/or the ground-engaging outer side  47  of the track  41  may comprise various patterns of traction projections. 
     The drive wheel  42  is rotatable about an axis of rotation  49  for driving the track  41  in response to rotation of an axle of the ATV  10 . In this example, the axis of rotation  49  corresponds to the axle of the ATV  10 . More particularly, in this example, the drive wheel  42  has a hub which is mounted to the axle of the ATV  10  such that power generated by the prime mover  15  and delivered over the powertrain  14  of the ATV  10  rotates the axle, which rotates the drive wheel  42 , which imparts motion of the track  41 . In this embodiment in which the track system  20   i  is mounted where a ground-engaging wheel  21   i  could otherwise be mounted, the axle of the ATV  10  is capable of rotating the drive wheel  42  of the track system  20   i  or the ground-engaging wheel  21   i . 
     In this embodiment, the drive wheel  42  comprises a drive sprocket  43  engaging the drive/guide lugs  48   1 - 48   N  of the inner side  45  of the track  41  in order to drive the track  41 . In this case, the drive sprocket  43  comprises a plurality of teeth  46   1 - 46   T  distributed circumferentially along its rim to define a plurality of lug-receiving spaces therebetween that receive the drive/guide lugs  48   1 - 48   N  of the track  41 . The drive wheel  42  may be configured in various other ways in other embodiments. For example, in embodiments where the track  41  comprises recesses or holes, the drive wheel  42  may have teeth that enter these recesses or holes in order to drive the track  41 . As yet another example, in some embodiments, the drive wheel  42  may frictionally engage the inner side  45  of the track  41  in order to frictionally drive the track  41 . 
     The idler wheels  50   1 - 50   4 ,  55   1 - 55   8  are not driven by power supplied by the prime mover  15 , but are rather used to do at least one of supporting part of the weight of the ATV  10  on the ground via the track  41 , guiding the track  41  as it is driven by the drive wheel  42 , and tensioning the track  41 . 
     Each of the idler wheels  50   1 - 50   4 ,  55   1 - 55   8  has an axial direction defined by an axis of rotation  80  of that idler wheel (also referred to as a “Y” direction), a radial direction (also referred to as a “Z” direction), and a circumferential direction (also referred to as a “X” direction). Each of the idler wheels  50   1 - 50   4 ,  55   1 - 55   8  has an outer diameter D W  and a width W W  and comprises an inboard lateral side  47  for facing a center of the ATV  10  in the widthwise direction of the ATV  10  and an outboard lateral side  49  opposite the inboard lateral side  47 . Each of the idler wheels  50   1 - 50   4 ,  55   1 - 55   8  has an area of contact  25  with the inner side  45  of the track  41  that has a dimension L C , referred to as a “length”, in the circumferential direction of that idler wheel and a dimension W C , referred to as a “width”, in the axial direction of that idler wheel. 
     More particularly, in this embodiment, the leading idler wheels  50   1 ,  50   2  and the trailing idler wheels  50   3 ,  50   4  maintain the track  41  in tension and can help to support part of the weight of the ATV  10  on the ground via the track  41 . The leading idler wheels  50   1 ,  50   2  are spaced apart along the widthwise direction of the track system  20   i , and so are the trailing idler wheels  50   3 ,  50   4 . 
     Each idler wheel  50   x  of the leading and trailing idler wheels  50   1 - 50   4  contacts the inner side  45  of the track  41  such that a longitudinal end segment  52  of the track  41  turns about the idler wheel  50   x . That is, the idler wheel  50   x  contacts both the top run  65  of the track  41  and the bottom run  66  of the track  41  such that the longitudinal end segment  52  of the track  41  includes a longitudinal end part  67  of the top run  65  of the track  41  and a longitudinal end part  73  of the bottom run  66  of the track  41 . In this embodiment, the longitudinal end segment  52  of the track  41  is located between the axis of rotation  80  of the idler wheel  50   x  and a given one of the longitudinal ends  57 ,  59  of the track system  20   i  that is adjacent to the idler wheel  50   x . Thus, in this example, the longitudinal end segment  52  of the track  41  is that segment of the track  41  between points A 1  and B 1  and defines an an angle of wrap a of the track  41  about the idler wheel  50   x . 
     The idler wheels  55   1 - 55   8  are roller wheels that roll on the inner side  45  of the track  41  along the bottom run  66  of the track  41  to apply the bottom run  66  on the ground. The idler wheels  55   1 - 55   8  move on respective ones of a plurality of idler wheel paths  51   1 ,  51   2  of the inner surface  32  of the carcass  35  of the track  41 . Each of the idler wheel paths  51   1 ,  51   2  extends adjacent to respective ones of the drive/guide lugs  48   1 - 48   N  to allow these lugs to guide motion of the track  41 . As the roller wheels  55   1 - 55   8  roll on respective ones of the idler wheel paths  51   1 ,  51   2 , these paths can be referred to as “rolling paths”. 
     In some embodiments, one or more of the idler wheels  50   1 - 50   4 ,  55   1 - 55   8  may be resiliently deformable as the track  41  moves around them, including in response to encountering obstacles of the ground (e.g., rocks, portions of trees, debris, bumps, abrupt changes in ground level, etc.). This may help to absorb shocks when the track system  20   i  encounters obstacles and/or may make it easier for the track system  20   i  to surmount obstacles. 
     That is, in some embodiments, an idler wheel  50   i  or  55   i  may deform under load and regain its original shape upon removal of the load. The idler wheel  50   i  or  55   i  comprises a resiliently-deformable wheel portion  77 . For example, in some embodiments, the resiliently-deformable wheel portion  77  may comprise a tire. For instance, in some embodiments, the tire may be a non-pneumatic tire. In other embodiments, the tire may be a pneumatic tire. 
     For example, in some embodiments, each idler wheel  50   x  of the leading and trailing idler wheels  50   1 - 50   4  may be resiliently deformable in response to encountering obstacles. By resiliently deforming, the idler wheel  50   x  allows a change in curvature of the longitudinal end segment  52  of the track  41  when the longitudinal end segment  52  of the endless track  41  contacts an obstacle on the ground. For instance, when the longitudinal end segment  52  of the track  41  adjacent to the idler wheel  50   x  contacts an obstacle as the ATV  10  moves, the idler wheel  50   x  can elastically deform, by being elastically compressed under load, to allow a change in curvature of the longitudinal end segment  52  of the track  41  in order to generally conform to a contacted part of the obstacle. This elastic deformation of the idler wheel  50   x  absorbs at least part of a shock resulting from contact with the obstacle. Also, the change in curvature of the longitudinal end segment  52  of the track  41  may enhance its traction on the obstacle and can thus facilitate climbing of the track system  16   i  over the obstacle. As the obstacle is surmounted and stress on the idler wheel  50   x  that had been compressed is reduced, the idler wheel  50   x  can regain its original shape. 
     The idler wheels  50   1 - 50   4 ,  55   1 - 55   8  may be arranged in other configurations and/or the track system  20   i  may comprise more or less idler wheels in other embodiments. 
     The frame  44  supports components of the track system  20   i , including the idler wheels  50   1 - 50   4 ,  55   1 - 55   8 . More particularly, in this embodiment, the front idler wheels  50   1 ,  50   2  are mounted to the frame  44  in a front longitudinal end region of the frame  44  proximate the front longitudinal end  57  of the track system  20   i , while the rear idler wheels  50   3 ,  50   4  are mounted to the frame  44  in a rear longitudinal end region of the frame  44  proximate the rear longitudinal end  59  of the track system  20   i . The roller wheels  55   1 - 55   8  are mounted to the frame  44  in a central region of the frame  44  between the front idler wheels  50   1 ,  50   2  and the rear idler wheels  50   3 ,  50   4 . Each of the roller wheels  55   1 - 55   8  may be rotatably mounted directly to the frame  44  or may be rotatably mounted to a link which is pivotally mounted to the frame  44  to which is rotatably mounted an adjacent one of the roller wheels  55   1 - 55   8 , thus forming a “tandem”. 
     The frame  44  is supported at a support area  39 . More specifically, in this case, the frame  44  is supported by the axle of the ATV  10  to which is coupled the drive wheel  42 , such that the support area  39  is intersected by the axis of rotation  49  of the drive wheel  42 . 
     In this embodiment, the track system  20   i  is movable relative to the frame  12  of the ATV  10 , such as when the ATV  10  travels on uneven terrain. More particularly, the frame  44  of the track system  20   i  is movable relative to the frame  12  of the ATV  10  to facilitate motion of the track system  20   i  on uneven terrain and enhance its traction on the ground. The frame  44  of the track system  20   i  is pivotable relative to the frame  12  of the ATV  10  about a pivot axis  51 . More specifically, in this embodiment, the pivot axis  51  corresponds to the axis of rotation  49  of the drive wheel  42  and the frame  44  can pivot about the axle of the ATV  10  to which the drive wheel  42  is coupled. In other embodiments, the pivot axis  51  of the frame  44  may be located elsewhere (e.g., lower) than the axis of rotation  49  of the drive wheel  42 . 
     The idler wheels  50   1 - 50   4 ,  55   1 - 55   8  may be designed to enhance use or performance of the track system  20   i  and/or the ATV  10 , such as, for example, by being lightweight and/or by better handling loads, including, for instance, those resulting from tension of the track  41  (e.g., at low speed and high torque) and/or from unevenness or other aspects of the ground, including impacts with obstacles on the ground (e.g., at high speed). 
     Examples of embodiments in which this may be achieved in respect of the leading and trailing idler wheels  50   1 - 50   4  are discussed below. 
     1. Idler Wheel Comprising a Non-Pneumatic Tire (NPT) 
     In some embodiments, as shown in  FIGS. 14 to 18 , each idler wheel  50   x  of the leading and trailing idler wheels  50   1 - 50   4  comprises a non-pneumatic tire  58  and a hub  75  for connecting the idler wheel  50   x  to the frame  44  of the track system  20   i . The non-pneumatic tire  58  is a compliant wheel structure that is not supported by gas (e.g., air) pressure and that is resiliently deformable (i.e., resiliently changeable in configuration) as the track system  20   i  contacts the ground. 
     The non-pneumatic tire  58  comprises an annular beam  136  and an annular support  141  that is disposed between the annular beam  136  and the hub  75  and configured to support loading on the idler wheel  50   x  as the track system  20   i  engages the ground. In this embodiment, the non-pneumatic tire  58  is tension-based such that the annular support  141  is configured to support the loading on the idler wheel  50   x  by tension. That is, under the loading on the idler wheel  50   x  (i.e., due to loading on the track system  20   i , the tension of the track  41 , etc.), the annular support  141  is resiliently deformable such that a portion  127  of the annular support  141  between the axis of rotation  80  of the idler wheel  50   x  and the area of contact  25  of the idler wheel  50   x  with the track  41  is compressed and a portion  129  of the annular support  141  between the axis of rotation  80  of the idler wheel  50   x  and a peripheral part  27  of the idler wheel  50   x  not in contact with the track  41  is in tension to support the loading. 
     The annular beam  136  of the non-pneumatic tire  58  is configured to deflect under the loading on the idler wheel  50   x  at the area of contact  25  of the idler wheel  50   x  with the track  41 . in some situations, depending on the loading on the idler wheel  50   x  (e.g., such as when the idler wheel  50   x  encounters an obstacle on the ground), as discussed later, the annular beam  136  can function like a beam in transverse deflection. An outer peripheral extent  146  of the annular beam  136  and an inner peripheral extent  148  of the annular beam  136  deflect at the area of contact  25  of the idler wheel  50   x  with the inner side  45  of the track  41  under the loading on the idler wheel  50   x . 
     More particularly, in this embodiment, the annular beam  136  comprises a shear band  139 . In some situations, such as when the idler wheel  50   x  encounters an obstacle on the ground, the shear band  139  is configured to deflect predominantly by shearing at the area of contact  25  of the idler wheel  50   x  with the track  41  under the loading on the idler wheel  50   x . That is, under the loading on the idler wheel  50   x  when encountering an obstacle on the ground, the shear band  139  deflects significantly more by shearing than by bending at the area of contact  25  of the idler wheel  50   x  with the track  41 . The shear band  139  is thus configured such that, at a center of the area of contact  25  of the idler wheel  50   x  with the track  41  in the circumferential direction of the idler wheel  50   x , a shear deflection of the shear band  139  is significantly greater than a bending deflection of the shear band  139 . For example, in some embodiments, at the center of the area of contact  25  of the idler wheel  50   x  with the track  41  in the circumferential direction of the idler wheel  50   x , a ratio of the shear deflection of the shear band  139  over the bending deflection of the shear band  139  may be at least 1.2, in some cases at least 1.5, in some cases at least 2, in some cases at least 3, and in some cases even more (e.g., 4 or more). For instance, in some embodiments, the annular beam  136  may be designed based on principles discussed in U.S. Patent Application Publication 2014/0367007, which is hereby incorporated by reference herein. 
     The annular beam  136  therefore has a high bending stiffness and a comparatively low shear stiffness. As such, the annular beam  136 , including the shear band  139 , will deflect due to a shear force more easily than it will deflect due to a bending moment. The loading to which the annular beam  136  of the idler wheel  50   x  is subjected to can be at least predominantly bending-based or at least predominately shear-based in various situations, explained as follows:
         At least predominantly bending-based (i.e., predominantly or entirely bending-based): with reference to  FIGS. 21 and 22 , a portion  117  of the annular beam  136  corresponding to the peripheral part  27  of the idler wheel  50   x  not in contact with the track  41  functions like an arch. This arch supports tension loads from the portion  129  of the annular support  141  between the axis of rotation  80  of the idler wheel  50   x  and the peripheral part  27  of the idler wheel  50   x  not in contact with the track  41  and transmits a large bending moment. This can occur when the loading on the idler wheel  50   x  is at least predominantly due to the tension of the track  41  in contact with it in normal operation without any obstacle encountering the idler wheel  50   x .   At least predominantly shear-based (i.e., predominantly or entirely shear-based): in  FIGS. 18, 19, and 23 , a portion  119  of the annular beam  136  is subjected to a transverse deflection which is accompanied by large shear forces acting on the annular beam  136 . This may occur, for instance, when the idler wheel  50   x  encounters an obstacle on the ground. In the case of  FIGS. 18 and 19 , the annular beam  136  is loaded against a flat surface, which can represent what happens when the obstacle is mild. In the case of  FIG. 23 , the annular beam  136  is loaded against a sharp object, which can represent what happens when the obstacle is more severe.       

     Since it is stiff in bending and compliant in shear, the annular beam  136  will be stiff, with little deflection, in the load case of  FIG. 22 , and, conversely, will deflect comparatively easily for the load cases shown in  FIGS. 18, 19, and 23  in which the transverse beam deflection due to shear will be higher than the deflection due to bending. 
     The idler wheel  50   x  does, however, deflect when subjected to the load of  FIG. 22 . This is due to bending of elements (e.g., spokes) of the portion  127  of the annular support  141  between the axis of rotation  80  of the idler wheel  50   x  and the area of contact  25  of the idler wheel  50   x  with the track  41  that are compressed. 
     The annular beam  136  remains basically an annular form which is translated (e.g., up in this figure) due to tensile forces of the track  41  shown in  FIG. 21 . This movement can sometimes be referred to a counter deflection. Yet, as the load is also supported in tension by elements (e.g., spokes) of the portion  129  of the annular support  141  between the axis of rotation  80  of the idler wheel  50   x  and the peripheral part  27  of the idler wheel  50   x  not in contact with the track  41 , the idler wheel  50   x  develops only a small deflection, even when subjected to large track loads under normal operation. 
     Through extensive numerical and physical testing, the inventors have developed useful relationships between deflection due to bending and deflection due to shear, for cases where the annular beam  136  is subjected to a transverse deflection. This can occur during an encounter with an obstacle on the ground, such as an impact with the obstacle. While the physics of an obstacle impact, such as shown in  FIG. 23 , may be complex, this can be related to the simple case shown in  FIG. 19  where an impact occurs when loaded against a flat surface. For example, in some embodiments, a contact length L IMPACT  equal to 40% of the outer diameter D W  of the idler wheel  50   x  correlates to a moderate to severe impact deflection. 
     When the deflection due to shear is higher than the deflection due to bending, the annular beam  136  becomes fairly complaint in transverse beam deflection. This may be very favorable to operation of the track system  20   i , as it may decouple a stiffness of the idler wheel  50   x  in normal operation to the stiffness of the idler wheel  50   x  in impact loading, which will be further discussed in a later section. 
     For instance, in some embodiments, the inventors have found that a ratio of a deflection due to bending to a deflection due to shear of 2:1 when the annular beam  136  is loaded against a flat surface to a contact length of 40% of the outer diameter of the idler wheel  50   x  may provide good wheel performance. In some embodiments, this ratio can be 3:1, in others 4:1, and in some cases even higher. 
     In this example of implementation, the shear band  139  comprises an outer rim  131 , an inner rim  133 , and a plurality of openings  156   1 - 156   N  between the outer rim  131  and the inner rim  133 . The shear band  139  comprises a plurality of interconnecting members  137   1 - 137   P  that extend between the outer rim  131  and the inner rim  133  and are disposed between respective ones of the openings  156   1 - 156   N . The interconnecting members  137   1 - 137   P  may be referred to as “webs” such that the shear band  139  may be viewed as being “web-like” or “webbing”. The shear band  139 , including the openings  156   1 - 156   N  and the interconnecting members  137   1 - 137   P , may be arranged in any other suitable way in other embodiments. 
     The openings  156   1 - 156   N  of the shear band  139  help the shear band  139  to deflect predominantly by shearing at the area of contact  25  of the idler wheel  50   x  with the track  41  under the loading on the idler wheel  50   x  in some situations as discussed above. In this embodiment, the openings  156   1 - 156   N  extend from the inboard lateral side  147  to the outboard lateral side  149  of the non-pneumatic tire  58 . That is, the openings  156   1 - 156   N  extend laterally though the shear band  139  in the axial direction of the idler wheel  50   x . The openings  156   1 - 156   N  may extend laterally without reaching the inboard lateral side  147  and/or the outboard lateral side  149  of the non-pneumatic tire  58  in other embodiments. The openings  156   1 - 156   N  may have any suitable shape. In this example, a cross-section of each of the openings  156   1 - 156   N  is circular. The cross-section of each of the openings  156   1 - 156   N  may be shaped differently in other examples (e.g., polygonal, partly curved and partly straight, etc.). In some cases, different ones of the openings  156   1 - 156   N  may have different shapes. In some cases, the cross-section of each of the openings  156   1 - 156   N  may vary in the axial direction of the idler wheel  50   x . For instance, in some embodiments, the openings  156   1 - 156   N  may be tapered in the axial direction of the idler wheel  50   x  such that their cross-section decreases inwardly axially (e.g., to help minimize debris accumulation within the openings  156   1 - 156   N ). 
     In this embodiment, the non-pneumatic tire  58  comprises a tread  150  for engaging the track  41 . The tread  150  may enhance frictional engagement (e.g., “gripping”) of the idler wheel  50   x  on the inner surface  32  of the track  41 . The tread  150  is disposed about the outer peripheral extent  146  of the annular beam  136 , in this case about the outer rim  131  of the shear band  139 . More particularly, in this example the tread  150  comprises a tread base  143  that is at the outer peripheral extent of the tread  150  and a plurality of tread recesses  152   1 - 152   T  that project from the tread base  143  towards the axis of rotation  80  of the idler wheel  50   x  in the radial direction of the non-pneumatic tire  58 . The tread recesses  152   1 - 152   T  are spaced apart in the circumferential direction of the non-pneumatic tire  58  and extend across at least a majority of a width W T  of the non-pneumatic tire. More particularly, in this example, the tread recesses  152   1 - 152   T  extends across substantially an entirety of the width W T  of the non-pneumatic tire  58 . The tread  150  may be implemented in any other suitable way in other embodiments (e.g., may comprise a plurality of tread projections, etc.). 
     The annular support  141  is configured to support the loading on the idler wheel  50   x  as the track system  20   i  engages the ground. As mentioned above, in this embodiment, the annular support  141  is configured to support the loading on the idler wheel  50   x  by tension. More particularly, in this embodiment, the annular support  141  comprises a plurality of support members  142   1 - 142   T  that are distributed around the non-pneumatic tire  58  and resiliently deformable such that, under the loading on the idler wheel  50   x , respective ones of the support members  142   1 - 142   T  in the portion  127  of the annular support  141  between the axis of rotation  80  of the idler wheel  50   x  and the area of contact  25  of the idler wheel  50   x  with the track  41  are compressed and bend while other ones of the support members  142   1 - 142   T  in the portion  129  of the annular support  141  between the axis of rotation  80  of the idler wheel  50   x  and the peripheral part  27  of the idler wheel  50   x  not in contact with track  41  are tensioned to support the loading. As they support load by tension when in the portion  129  of the annular support  141 , the support members  142   1 - 142   T  may be referred to as “tensile” members. 
     In this embodiment, the support members  142   1 - 142   T  are elongated and extend from the annular beam  136  towards the hub  75  generally in the radial direction of the idler wheel  50   x . In that sense, the support members  142   1 - 142   T  may be referred to as “spokes” and the annular support  141  may be referred to as a “spoked” support. 
     More particularly, in this embodiment, the inner peripheral extent  148  of the annular beam  136  is an inner peripheral surface of the annular beam  136  and each spoke  142   i  extends from the inner peripheral surface  148  of the annular beam  136  towards the hub  75  generally in the radial direction of the idler wheel  50   x  and from a first lateral end  155  to a second lateral end  158  in the axial direction of the idler wheel  50   x . In this case, the spoke  142   i  extends in the axial direction of the idler wheel  50   x  for at least a majority of a width W T  of the non-pneumatic tire  58 , which in this case corresponds to the width W W  of the idler wheel  50   x . For instance, in some embodiments, the spoke  142   i  may extend in the axial direction of the idler wheel  50   x  for more than half, in some cases at least 60%, in some cases at least 80%, and in some cases an entirety of the width W T  of the non-pneumatic tire  58 . Moreover, the spoke  142   i  has a thickness T S  measured between a first surface face  159  and a second surface face  161  of the spoke  142   i  that is significantly less than a length and width of the spoke  142   i . 
     When the track system  20   i  moves on the ground, respective ones of the spokes  142   1 - 142   T  that are disposed in the portion  129  of the spoked support  141  between the axis of rotation  80  of the idler wheel  50   x  and the peripheral part  27  of the idler wheel  50   x  not in contact with track  41  are placed in tension while respective ones of the spokes  142   1 - 142   T  that are disposed in the portion  127  of the spoked support  141  between the area of contact  25  of the idler wheel  50   x  with the track  41  are placed in compression. The spokes  142   1 - 142   T  in the portion  127  of the spoked support  141  which are in compression bend in response to the load. Conversely, the spokes  142   1 - 142   T  in the portion  129  of the spoked support  141  which are placed in tension support the load by tension. 
     The non-pneumatic tire  58  has an inner diameter D TI  and an outer diameter D TO , which in this case corresponds to the outer diameter D W  of the leading idler wheels  50   1 ,  50   2 . A sectional height H T  of the non-pneumatic tire  58  is half of a difference between the outer diameter D TO  and the inner diameter D TI  of the non-pneumatic tire  58 . The sectional height H T  of the tire may be significant in relation to the width W T  of the non-pneumatic tire  58 . In other words, an aspect ratio AR of the non-pneumatic tire  58  corresponding to the sectional height H T  over the width W T  of the non-pneumatic tire  58  may be relatively high. For instance, in some embodiments, the aspect ratio AR of the non-pneumatic tire  58  may be at least 70%, in some cases at least 90%, in some cases at least 110%, and in some cases even more. Also, the inner diameter D TI  of the non-pneumatic tire  58  may be significantly less than the outer diameter D TO  of the non-pneumatic tire  58  as this may help for compliance of the idler wheel  50   x . For example, in some embodiments, the inner diameter D TI  of the non-pneumatic tire  58  may be no more than half of the outer diameter D TO  of the non-pneumatic tire  58 , in some cases less than half of the outer diameter D TO  of the non-pneumatic tire  58 , in some cases no more than 40% of the outer diameter D TO  of the non-pneumatic tire  58 , in some cases no more than 30% of the outer diameter D TO  of the non-pneumatic tire  58 , and in some cases even a smaller fraction of the outer diameter D TO  of the non-pneumatic tire  58 . 
     In this embodiment, the width W T  of the non-pneumatic tire  58 , which corresponds to the width W w  of the idler wheel  50   x  in this case, may be significant in relation to the width W TR  of the track  41 . For instance, in some embodiments, a ratio of the width W T  of the non-pneumatic tire  58  over the width W TR  of the track  41  may be at least 0.2, in some cases at least 0.3, in some cases at least 0.4, in some cases at least 0.5, and in some cases even more. For example, in some embodiments, a total width W wt  of the leading idler wheels  50   1 ,  50   2  or of the trailing idler wheels  50   3 ,  50   4  (i.e., a sum of the widths W w  of the leading idler wheels  50   1 ,  50   2  or of the widths W w  of the trailing idler wheels  50   3 ,  50   4 ) may correspond to a majority of the width of the track  41 . For instance, in some embodiments, the total width W wt  of the leading idler wheels  50   1 ,  50   2  or of the trailing idler wheels  50   3 ,  50   4  may correspond to at least 60%, in some cases at least 70%, in some cases at least 80%, and in some cases an even greater part (e.g., up to an entirety) of the width of the track  41 . 
     The hub  75  is disposed centrally of the tire  34  and connects the idler wheel  50   x  to an axle  56  about which the idler wheel  50   x  rotates relative to the frame  44  of the track system  20   i . In this embodiment, the hub  75  comprises an inner member  162 , an outer member  164  radially outward of the inner member  162 , and a plurality of interconnecting members  166   1 - 166   A  that interconnect the inner member  162  and the outer member  164  and define openings  168   1 - 168   A  there between. The hub  75  comprises an opening  175  for receiving the axle  56 . The opening  175  may also receive another element (e.g., a bushing) in addition to the axle  56  in some embodiments. The hub  75  may be implemented in any other suitable manner in other embodiments (e.g., it may have any other suitable shape or design). 
     The idler wheel  50   x  may be made up of one or more materials. The non-pneumatic tire  58  comprises a tire material  145  that makes up at least a substantial part (i.e., a substantial part or an entirety) of the non-pneumatic tire  58 . The hub  75  comprises a hub material  172  that makes up at least a substantial part of the hub  75 . In some embodiments, the tire material  145  and the hub material  172  may be different materials. In other embodiments, the tire material  145  and the hub material  172  may be a common material (i.e., the same material). 
     In this embodiment, the tire material  145  constitutes at least part of the annular beam  136  and at least part of the spokes  142   1 - 142   T . Also, in this embodiment, the tire material  145  constitutes at least part of the tread  150 . More particularly, in this embodiment, the tire material  145  constitutes at least a majority (e.g., a majority or an entirety) of the annular beam  136 , the tread  150 , and the spokes  142   1 - 142   T . In this example of implementation, the tire material  145  makes up an entirety of the tire  134 , including the annular beam  136 , the spokes  142   1 - 142   T , and the tread  150 . The non-pneumatic tire  58  is thus monolithically made of the tire material  145 . In this example, therefore, the annular beam  136  is free of (i.e., without) a substantially inextensible reinforcing layer running in the circumferential direction of the idler wheel  50   x  (e.g., a layer of metal, composite (e.g., carbon fibers, other fibers), and/or another material that is substantially inextensible running in the circumferential direction of the idler wheel  50   x ). In that sense, the annular beam  136  may be said to be “unreinforced”. 
     The tire material  145  is elastomeric. For example, in this embodiment, the tire material  145  comprises a polyurethane (PU) elastomer. For instance, in some cases, the PU elastomer may be composed of a TDI pre-polymer, such as PET-95A, cured with MCDEA, commercially available from COIM. Other materials that may be suitable include using PET95-A or PET60D, cured with MOCA. Other materials available from Chemtura may also be suitable. These may include Adiprene E500X and E615X prepolymers, cured with C3LF or HQEE curative. Blends of the above prepolymers are also possible. Prepolymer C930 and C600, cured with C3LF or HQEE may also be suitable, as are blends of these prepolymers. 
     Polyurethanes that are terminated using MDI or TDI are possible, with ether and/or ester and/or polycaprolactone formulations, in addition to other curatives known in the cast polyurethane industry. Other suitable resilient, elastomeric materials would include thermoplastic materials, such as HYTREL co-polymer, from DuPont, or thermoplastic polyurethanes such as Elastollan, from BASF. Materials in the 95A to 60D hardness level may be particularly useful, such as Hytrel 5556, Hytrel 5526, Laripur 6260, Texin 970 and Elastollan C98A. Some resilient thermoplastics, such as plasticized nylon blends, may also be used. The Zytel line of nylons from DuPont may be particularly useful. The tire material  145  may be any other suitable material in other embodiments. 
     In this embodiment, the tire material  145  may exhibit a non-linear stress vs. strain behavior. For instance, the tire material  145  may have a secant modulus that decreases with increasing strain of the tire material  145 . The tire material  145  may have a high Young&#39;s modulus that is significantly greater than the secant modulus at 100% strain (a.k.a. “the 100% modulus”). Such a non-linear behavior of the tire material  145  may provide efficient load carrying during normal operation and enable impact loading and large local deflections without generating high stresses. For instance, the tire material  145  may allow the non-pneumatic tire  58  to operate at a low strain rate (e.g., 2% to 5%) during normal operation yet simultaneously allow large strains (e.g., when the ATV  10  engages obstacles) without generating high stresses. This in turn may be helpful to minimize vehicle shock loading and enhance durability of the non-pneumatic tire  58 . 
     The non-pneumatic tire  58  may comprise one or more additional materials in addition to the tire material  145  in other embodiments (e.g., different parts of the annular beam  136 , different parts of the tread  150 , and/or different parts of the spokes  142   1 - 142   T  may be made of different materials). For example, in some embodiments, different parts of the annular beam  136 , different parts of the tread  150 , and/or different parts of the spokes  142   1 - 142   T  may be made of different elastomers. As another example, in some embodiments, the annular beam  136  may comprise one or more substantially inextensible reinforcing layers running in the circumferential direction of the idler wheel  50   x  (e.g., one or more layers of metal, composite (e.g., carbon fibers, other fibers), and/or another material that is substantially inextensible running in the circumferential direction of the idler wheel  50   x ). 
     In this embodiment, the hub material  172  constitutes at least part of the inner member  162 , the outer member  164 , and the interconnecting members  166   1 - 166   A  of the hub  75 . More particularly, in this embodiment, the hub material  172  constitutes at least a majority (e.g., a majority or an entirety) of the inner member  162 , the outer member  164 , and the interconnecting members  166   1 - 166   A . In this example of implementation, the hub material  172  makes up an entirety of the hub  75 . 
     In this example of implementation, the hub material  172  is polymeric. More particularly, in this example of implementation, the hub material  172  is elastomeric. For example, in this embodiment, the hub material  172  comprises a polyurethane (PU) elastomer. For instance, in some cases, the PU elastomer may be PET-95A commercially available from COIM, cured with MCDEA. 
     The hub material  172  may be any other suitable material in other embodiments. For example, in other embodiments, the hub material  172  may comprise a stiffer polyurethane material, such as COIM&#39;s PET75D cured with MOCA. In some embodiments, the hub material  172  may not be polymeric. For instance, in some embodiments, the hub material  172  may be metallic (e.g., steel, aluminum, etc.). 
     The hub  75  may comprise one or more additional materials in addition to the hub material  172  in other embodiments (e.g., different parts of the inner member  162 , different parts of the outer member  164 , and/or different parts of the interconnecting members  166   1 - 166   A  may be made of different materials). 
     The idler wheel  50   x  may be manufactured in any suitable way. For example, in some embodiments, the non-pneumatic tire  58  and/or the hub  75  may be manufactured via centrifugal casting, a.k.a. spin casting, which involves pouring one or more materials of the idler wheel  50   x  into a mold that rotates about an axis. The material(s) is(are) distributed within the mold via a centrifugal force generated by the mold&#39;s rotation. In some cases, vertical spin casting, in which the mold&#39;s axis of rotation is generally vertical, may be used. In other cases, horizontal spin casting, in which the mold&#39;s axis of rotation is generally horizontal, may be used. The idler wheel  50   x  may be manufactured using any other suitable manufacturing processes in other embodiments. 
     The idler wheel  50   x  may be lightweight. That is, a mass M W  of the idler wheel  50   x  may be relatively small. For example, in some embodiments, a ratio M normalized  of the mass M W  of the idler wheel  50   x  in kilograms over the outer diameter D W  of the idler wheel  50   x  normalized by the width W W  of the idler wheel  50   x , 
     
       
         
           
             
               M 
               normalized 
             
             = 
             
               
                 ( 
                 
                   
                     M 
                     w 
                   
                   
                     D 
                     w 
                   
                 
                 ) 
               
               
                 W 
                 w 
               
             
           
         
       
     
     may be no more than 0.0005 kg/mm 2 , in some cases no more than 0.0004 kg/mm 2 , in some cases no more than 0.0003 kg/mm 2 , in some cases no more than 0.0002 kg/mm 2 , in some cases no more than 0.00015 kg/mm 2 , in some cases no more than 0.00013 kg/mm 2 , in some cases no more than 0.00011 kg/mm 2 , and in some cases even less (e.g., no more than 0.0001). 
     For instance, in some embodiments, the outer diameter of the idler wheel  50   x  may be 120 mm (4.7″), the width of the idler wheel  50   x  may be 60 mm (2.4″), and the mass M W  of the idler wheel  50   x  may be less than 3 kg, in some cases no more than 2 kg, in some cases no more than 1 kg, and in some cases even less. 
     While actual track operation is more complex, as previously discussed, good correlation to loading of the idler wheel  50   x  against a flat surface has been established by the inventors. Thus, in some embodiments, a radial compliance C z  of the idler wheel  50   x  may be significant, when loaded against a flat surface. That is, a radial stiffness K z  of the idler wheel  50   x  may be relatively low for shock absorption (e.g., ride quality). The radial stiffness K z  of the idler wheel  50   x  is a rigidity of the idler wheel  50   x  in the radial direction of the idler wheel  50   x , i.e., a resistance of the idler wheel  50   x  to deformation in the radial direction of the idler wheel  50   x  when loaded. The radial compliance C z  of the idler wheel  50   x  is the inverse of the radial stiffness K z  of the idler wheel  50   x  (i.e., C z =1/K z ). 
     For example, in some embodiments, a ratio K z normalized  of the radial stiffness K z  of the idler wheel  50   x  over the outer diameter D W  of the idler wheel  50   x  normalized by the width W W  of the idler wheel  50   x   
     
       
         
           
             
               K 
               
                 Z 
                  
                 
                     
                 
                  
                 normalized 
               
             
             = 
             
               
                 
                   K 
                   z 
                 
                 
                   D 
                   W 
                 
               
               
                 W 
                 W 
               
             
           
         
       
     
     may be between 0.0001 kgf/mm 3  and 0.0002 kgf/mm 3 , where the radial stiffness K z  of the idler wheel  50   x  is taken at a design load F DESIGN  of the idler wheel  50   x , i.e., a normal load expected to be encountered by the idler wheel  50   x  in use such that only the tire  34  deflects by a normal deflection. Herein, a force or load may be expressed in units of kilogram-force (kgf), but this can be converted into other units of force (e.g., Newtons). 
     The radial stiffness K z  of the idler wheel  50   x  may be evaluated in any suitable way in various embodiments. 
     For example, in some embodiments, the radial stiffness K z  of the idler wheel  50   x  may be gauged using a standard SAE J2704. 
     As another example, in some embodiments, the radial stiffness K z  of the idler wheel  50   x  may be gauged by standing the idler wheel  50   x  upright on a flat hard surface and applying a downward vertical load F z  on the idler wheel  50   x  at the axis of rotation  35  of the idler wheel  50   x  (e.g., via the hub  32 ). The downward vertical load F z  causes the idler wheel  50   x  to elastically deform from its original configuration to a biased configuration by a deflection D z . The deflection D z  is equal to a difference between a height of the idler wheel  50   x  in its original configuration and the height of the idler wheel  50   x  in its biased configuration. The radial stiffness K z  of idler wheel  50   x  is calculated as the downward vertical load F Z  over the measured deflection D Z . 
     For instance, in some embodiments, the radial stiffness K z  of the idler wheel  50   x  may be no more than 15 kgf/mm, in some cases no more than 11 kgf/mm, in some cases no more than 8 kgf/mm, and in some cases even less. 
     The radial compliance C z  of the idler wheel  50   x  is provided at least by a radial compliance C zt  of the non-pneumatic tire  58 . For instance, in this embodiment, the spokes  142   1 - 142   T  can deflect significantly in the radial direction of the idler wheel  50   x  under the loading on the idler wheel  50   x . 
     For example, in some embodiments, a volume fraction V fs  of the spoked support  141  comprising the spokes  142   1 - 142   T  may be minimized. The volume fraction V fs  of the spoked support  141  refers to a ratio of a volume occupied by material of the spoked support  141  (i.e., a collective volume of the spokes  142   1 - 142   T ) over a volume bounded by the annular beam  136  and the hub  75 . A high value of the volume fraction V fs  increases the amount of material between the outer diameter D TO  and the inner diameter D TI  of the non-pneumatic tire  58 , whereas a low value of the volume fraction V fs  decreases the amount of material between the outer diameter D TO  and the inner diameter D TI  of the non-pneumatic tire  58 . At very high deflections, as shown in  FIG. 19 , the spokes  142   1 - 142   T  begin to self-contact. This, then, enables load transfer from the ground to the hub  75  via compression. While this may be counterintuitive, minimizing material in the spoked support  141  may be beneficial to robustness of the idler wheel  50   x  in off-road use. 
     For instance, in some embodiments, the volume fraction V fs  of the spoked support  141  may be no more than 15%, in some cases no more than 12%, in some cases no more than 10%, in some cases no more than 8%, in some cases no more than 6%, and in some cases even less. For example, in some embodiments, the volume fraction V fs  of the spoked support  141  may be between 6% and 9%. 
     The non-pneumatic tire  58  of the idler wheel  50   x  may be implemented in any other suitable way in other embodiments. 
     For example, in some embodiments, the annular beam  136  of the non-pneumatic tire  58  of the idler wheel  50   x  may comprise one or more reinforcing layers running in the circumferential direction of the idler wheel  50   x  to reinforce the annular beam  136 , such as one or more substantially inextensible reinforcing layers running in the circumferential direction of the idler wheel  50   x  (e.g., one or more layers of metal, composite (e.g., carbon fibers, other fibers), and/or another material that is substantially inextensible running in the circumferential direction of the idler wheel  50   x ). For instance, this may reinforce the annular beam  136  by protecting it against cracking and/or by better managing heat generated within it as it deforms in use. 
     For example, in some embodiments, as shown in  FIG. 33 , the annular beam  136  may comprise a reinforcing layer  247  running in the circumferential direction of the idler wheel  50   x . 
     The reinforcing layer  247  is unnecessary for the annular beam  136  to deflect predominantly by shearing in some situations (e.g., when encountering an obstacle on the ground), i.e., unnecessary for the shear band  139  to deflect significantly more by shearing than by bending at the area of contact  25  of the idler wheel  50   x  with the track  41 . That is, the annular beam  136  would deflect predominantly by shearing even without the reinforcing layer  247 . In other words, the annular beam  136  would deflect predominantly by shearing if it lacked the reinforcing layer  247  but was otherwise identical. Notably, in this embodiment, this is due to the openings  156   1 - 156   N  and the interconnecting members  137   1 - 137   P  of the shear band  139  that facilitate deflection predominantly by shearing. 
     The annular beam  136  has the reinforcing layer  247  but is free of any equivalent reinforcing layer running in the circumferential direction of the idler wheel  50   x  and spaced from the reinforcing layer  247  in the radial direction of the idler wheel  50   x . That is, the annular beam  136  has no reinforcing layer that is equivalent, i.e., identical or similar in function and purpose, to the reinforcing layer  247  and spaced from the reinforcing layer  247  in the radial direction of the idler wheel  50   x . The annular beam  136  therefore lacks any reinforcing layer that is comparably stiff to (e.g., within 10% of a stiffness of) the reinforcing layer  247  in the circumferential direction of the idler wheel  50   x  and spaced from the reinforcing layer  247  in the radial direction of the idler wheel  50   x . 
     In this embodiment, the annular beam  136  has the reinforcing layer  247  but is free of any substantially inextensible reinforcing layer running in the circumferential direction of the idler wheel  50   x  and spaced from the reinforcing layer  247  in the radial direction of the idler wheel  50   x . Thus, the reinforcing layer  247  is a sole reinforcing layer of the annular beam  136 . 
     More particularly, in this embodiment, the annular beam  136  has the reinforcing layer  247  located on a given side of a neutral axis  257  of the annular beam  136  and is free of any substantially inextensible reinforcing layer running in the circumferential direction of the idler wheel  50   x  on an opposite side of the neutral axis  257  of the annular beam  136 . That is, the reinforcing layer  247  is located between the neutral axis  257  of the annular beam  136  and a given one of the inner peripheral extent  148  and the outer peripheral extent  146  of the annular beam  136  in the radial direction of the idler wheel  50   x , while the annular beam  136  is free of any substantially inextensible reinforcing layer running in the circumferential direction of the idler wheel  50   x  between the neutral axis  257  of the annular beam  136  and the other one of the inner peripheral extent  148  and the outer peripheral extent  146  of the annular beam  136  in the radial direction of the idler wheel  50   x . 
     The neutral axis  257  of the annular beam  136  is an axis of a cross-section of the annular beam  136  where there is substantially no tensile or compressive stress in the circumferential direction of the idler wheel  50   x  when the annular beam  136  deflects at the contact patch  25  of the idler wheel  50   x . In this example, the neutral axis  257  is offset from a midpoint of the annular beam  136  between the inner peripheral extent  148  and the outer peripheral extent  146  of the annular beam  136  in the radial direction of the idler wheel  50   x . More particularly, in this example, the neutral axis  257  is closer to a given one of the inner peripheral extent  148  and the outer peripheral extent  146  of the annular beam  136  than to an opposite one of the inner peripheral extent  148  and the outer peripheral extent  146  of the annular beam  136  in the radial direction of the idler wheel  50   x . 
     In this embodiment, the reinforcing layer  247  is disposed radially inwardly of the neutral axis  257  of the annular beam  136 , and the annular beam  136  is free of any substantially inextensible reinforcing layer running in the circumferential direction of the idler wheel  50   x  radially outwardly of the neutral axis  257  of the annular beam  136 . 
     In this example, the reinforcing layer  247  is disposed between the inner peripheral extent  148  of the annular beam  136  and the openings  156   1 - 156   N  in the radial direction of the idler wheel  50   x , while the annular beam  136  is free of any substantially inextensible reinforcing layer running in the circumferential direction of the idler wheel  50   x  between the outer peripheral extent  146  of the annular beam  136  and the openings  156   1 - 156   N  in the radial direction of the idler wheel  50   x . 
     The reinforcing layer  247  may be implemented in any suitable way in various embodiments. 
     For example, in some embodiments, as shown in  FIG. 34 , the reinforcing layer  247  may include a layer of elongate reinforcing elements  262   1 - 262   E  that reinforce the annular beam  136  in one or more directions in which they are elongated, such as the circumferential direction of the idler wheel  50   x  and/or one or more directions transversal thereto. 
     For instance, in some embodiments, the elongate reinforcing elements  262   1 - 262   E  of the reinforcing layer  247  may include reinforcing cables  263   1 - 263   C  that are adjacent and generally parallel to one another. For instance, the reinforcing cables  263   1 - 263   C  may extend in the circumferential direction of the idler wheel  50   x  to enhance strength in tension of the annular beam  136  along the circumferential direction of the idler wheel  50   x . In some cases, a reinforcing cable may be a cord or wire rope including a plurality of strands or wires. In other cases, a reinforcing cable may be another type of cable and may be made of any material suitably flexible longitudinally (e.g., fibers or wires of metal, plastic or composite material). 
     In some embodiments, the elongate reinforcing elements  262   1 - 262   E  of the reinforcing layer  247  may include constitute a layer of reinforcing fabric  265 . Reinforcing fabric comprises pliable material made usually by weaving, felting, knitting, interlacing, or otherwise crossing natural or synthetic elongated fabric elements, such as fibers, filaments, strands and/or others. For instance, as one example, in some embodiments such as that of  FIG. 34 , the elongate reinforcing elements  262   1 - 262   E  of the reinforcing layer  247  that include the reinforcing cables  263   1 - 263   C  may also include transversal fabric elements  273   1 - 273   T  that extend transversally (e.g., perpendicularly) to and interconnect the reinforcing cables  263   1 - 263   C . Thus, in this example, the reinforcing layer  247 , including its reinforcing cables  263   1 - 263   C  and its transversal fabric elements  273   1 - 273   T , can be viewed as a reinforcing fabric or mesh (e.g., a “tire cord” fabric or mesh). As another example, in some embodiments, as shown in  FIG. 35 , the reinforcing fabric  247  may include textile  275  (e.g., woven or nonwoven textile). 
     In other examples of implementation, the reinforcing layer  247  may include a reinforcing sheet (e.g., a thin, continuous layer of metallic material, such as steel or aluminum that extends circumferentially). 
     The reinforcing layer  247  may be made of one or more suitable materials. A material  277  of the reinforcing layer  247  may be stiffer and stronger than the elastomeric material  45  of the annular beam  136  in which it is disposed. For instance, in some embodiments, the material  277  of the reinforcing layer  247  may be a metallic material (e.g., steel, aluminum, etc.). In other embodiments, the material  277  of the reinforcing layer  247  may be a stiff polymeric material, a composite material (e.g., a fiber-reinforced composite material), etc. 
     In this example of implementation, the reinforcing layer  247  comprises the reinforcing mesh or fabric that includes the reinforcing cables  263   1 - 263   C  and the transversal fabric elements  273   1 - 273   T  which are respectively 3 strands of steel wire of 0.28 mm diameter, wrapped together to form a cable, and high tenacity nylon cord of 1400×2. 
     In some embodiments, the reinforcing layer  247  may allow the elastomeric material  45  (e.g. PU) of the annular beam  136  to be less stiff, and this may facilitate processability in manufacturing the tire  34 . For example, in some embodiments, the modulus of elasticity (e.g., Young&#39;s modulus) of the elastomeric material  45  of the annular beam  136  may be no more than 200 MPa, in some cases no more than 150 MPa, in some cases no more than 100 MPa, in some cases no more than 50 MPa, and in some cases even less. 
     The reinforcing layer  247  may be provided in the annular beam  136  in any suitable way. In this embodiment, the reinforcing layer  247  may be formed as a hoop and placed in the mold before the elastomeric material  45  of the tire  34  is introduced in the mold. As the elastomeric material  45  is distributed within the mold via the centrifugal force generated by the mold&#39;s rotation, the reinforcing layer  247  is embedded in that portion of the elastomeric material  45  that forms the annular beam  136 . 
     The reinforcing layer  247  may provide various benefits to the idler wheel  50   x  in various embodiments. 
     For example, in this embodiment, the reinforcing layer  247  may help to protect the annular beam  136  against cracking. More particularly, in this embodiment, as it reinforces the annular beam  136  proximate to the inner peripheral extent  148  of the annular beam  136  that experiences tensile stresses when the annular beam  136  deflects in use, the reinforcing layer  247  may help the annular beam  136  to better withstand these tensile stresses that could otherwise increase potential for cracking to occur in the elastomeric material  45  of the annular beam  136 . 
     As another example, in this embodiment, the reinforcing layer  247  may help to better manage heat generated within the annular beam  136  as it deforms in use. A thermal conductivity of the material  277  of the reinforcing layer  247  may be greater than a thermal conductivity of the elastomeric material  45  of the annular beam  136 , such that the reinforcing layer  247  can better conduct and distribute heat generated within the tire  34  as it deforms in use. This may allow a highest temperature of the elastomeric material  45  to remain lower and therefore allow the idler wheel  50   x  to remain cooler and/or run faster at a given load than if the reinforcing layer  247  was omitted. 
     More particularly, in this embodiment, a ratio of the thermal conductivity of the material  277  of the reinforcing layer  247  over the thermal conductivity of the elastomeric material  45  of the annular beam  136  may be at least 50, in some cases at least 75, in some cases at least 100, and in some cases even more. For instance, in some embodiments, the thermal conductivity of the material  277  of the reinforcing layer  247  may be at least 10 W/m/° C., in some cases at least 20 W/m/° C., in some cases at least 30 W/m/° C., in some cases at least 40 W/m/° C., and in some cases even more. 
     A thermal conductivity of a unidirectional composite layer can be calculated by the following equation: 
         K     =V     K   +(1− V   ) K         (10)
         Where: Ki=thermal conductivity of the ply in direction i
           V C =cable volume fraction in direction i   K C =cable thermal conductivity   K M =matrix thermal conductivity   
               

     From Equation (10) the thermal conductivity of a composite is orthotopic; i.e., it is different in different directions. The tire designer can thus tune the composite layer to have the desired conductivity in the circumferential direction (say, the “1” direction) independently of the lateral direction (say, the “2”) direction. 
     Most elastomers, such as rubber and polyurethane, are good thermal insulators. The inventors have found that even a fairly low cable volume fraction is sufficient to raise the thermal conductivity to a level that adequately evacuates heat. With a steel cable, Equation (10) shows that a cable volume fraction of 0.10 gives a composite layer thermal conductivity of 5.2 W/m/° C. This value, or even a value as low as 2.0 W/m/° C. may be sufficient to improve thermal performance. 
     In some embodiments, steel may be used as the reinforcing material in both the circumferential and lateral directions. For example, to better dissipate heat and homogenize temperature, a steel cable of 3 strands of 0.28 mm diameter at a pace of 1.8 mm could be used in both the vertical and lateral directions. Such a composite layer has an average thickness of about 1.0 mm, and a steel volume fraction of about 0.10 in both vertical and lateral directions. As previously stated, this yields a thermal conductivity of about 5.2 W/m/° C. for the composite layer. 
     In some embodiments, in addition to or instead of including the reinforcing layer  247 , as shown in  FIG. 36 , a thickness T b  of the annular beam  136  in the radial direction of the idler wheel  50   x  may be increased in order to reinforce the annular beam  136 . More particularly, in this embodiment, the inner rim  133  may be increased in thickness. For instance, the inner rim  133  of the annular beam  136  may be thicker than the outer rim  131  of the annular beam  136  in the radial direction of the idler wheel  50   x . This may help the annular beam  136  to better withstand tensile stresses proximate to the inner peripheral extent  148  of the annular beam  136  when the annular beam  136  deflects in use. 
     For example, in this embodiment, a ratio of a thickness T b  of the annular beam  136  in the radial direction of the idler wheel  50   x  over the outer diameter D W  of the idler wheel  50   x  may be at least 0.05, in some cases at least 0.07, in some cases as least 0.09, and in some cases even more. 
     As another example, in this embodiment, a ratio of a thickness T ib  of the inner rim  133  of the annular beam  136  in the radial direction of the idler wheel  50   x  over a thickness T ob  of the outer rim  131  of the annular beam  136  in the radial direction of the v may be at least 1.2, in some cases at least 1.4, in some cases as least 1.6, and in some cases even more. 
     In other embodiments, the non-pneumatic tire  58  may comprise foam. Various types of foam may be used. For instance, in some embodiments, the foam may be polyurethane foam (e.g., closed-cell polyurethane foam). 
     2. Leading and Trailing Idler Wheels Having Different Behaviors 
     In some embodiments, the leading idler wheels  50   1 ,  50   2  may behave differently from the trailing idler wheels  50   3 ,  50   4  in order to accommodate different loading conditions to which these idler wheels may be subjected. 
     For example, in some embodiments, a given one of the trailing idler wheels  50   3 ,  50   4 , say the trailing idler wheel  50   3 , may be subject to a relatively high load (e.g., as much as 450 kgf) due to the tension of the track  41  when the track system  20   i  is operated at low speed and high torque. It may thus be desirable for the trailing idler wheel  50   3  to be relatively rigid in that case. Otherwise, excessive deformation may result in “ratcheting” where the track  41  would slip over the trailing idler wheel  50   3  if the trailing idler wheel  50   3  would deflect too much. 
     Meanwhile, in some embodiments, a given one of the leading idler wheels  50   1 ,  50   2 , say the leading idler wheel  50   1 , may be subject to a relatively high load when the track system  20   i  encounters an obstacle on the ground at high speed. It may thus be desirable for the leading idler wheel  50   1  to be relatively flexible in that case. Otherwise, if the leading idler wheel  50   1  would be too rigid, a significant shock may be transmitted to the ATV  10  and the vehicle&#39;s ability to envelop and pass over the obstacle may be reduced. 
     Accordingly, in some embodiments, the leading idler wheel  50   1  and the trailing idler wheel  50   3  may behave differently, such as, for example, by designing them such that the radial stiffness K z  of each of the leading idler wheel  50   1  and the trailing idler wheel  50   3  varies depending on a type of loading on that idler wheel and/or the radial stiffness K z  of the leading idler wheel  50   1  is different from the radial stiffness K z  of the trailing idler wheel  50   3  for a given load. 
     2.1 Idler Wheel Having Different Behaviors in Response to Different Types of Loading 
     In some embodiments, each idler wheel  50   x  of the leading idler wheel  50   1  and the trailing idler wheel  50   3  may have different behaviors in response to different types of loading, such as a first type of loading when the tension of the track  41  is high (i.e., “high track tension loading”), like when the track system  20   i  is operated at low speed and high torque, and a second type of loading when the track system  20   i  encounters an obstacle on the ground (i.e., “obstacle loading”), like an impact with the obstacle at high speed. This may allow the leading idler wheel  50   1  and the trailing idler wheel  50   3  to be structurally identical but yet behave differently as needed. 
     For example, in some embodiments, the radial stiffness K z  of the idler wheel  50   x  under the high track tension loading, denoted K z1 , may be different from the radial stiffness K z  of the idler wheel  50   x  under the obstacle loading, denoted K z2 . For instance, the radial stiffness K z1  of the idler wheel  50   x  under the high track tension loading may be greater than the radial stiffness K z2  of the idler wheel  50   x  under the obstacle loading such that the idler wheel  50   x  is relatively stiffer under the high track tension loading to resist excessive deformation and track ratcheting issues and relatively more flexible (i.e., less stiff) under the obstacle loading to better handle the obstacle. 
     Thus, the idler wheel  50   x  may be viewed as having “decoupled” radial stiffness characteristics whereby the radial stiffness K z  of the idler wheel  50   x  has a given value K z1  under the high track tension loading and another value K z1  under the obstacle loading. This “tuning” of the radial stiffness K z  of the idler wheel  50   x  may be implemented in any suitable ways in various embodiments. 
     In some embodiments, a ratio K z1 /K z2  of the radial stiffness K z1  of the idler wheel  50   x  under the high track tension loading over the radial stiffness K z2  of the idler wheel  50   x  under the obstacle loading may be at least 2, in some cases at least 3, in some cases at least 4, in some cases at least 5, and in some cases even more (e.g., 6 or more). 
     A test can be conducted to measure the ratio K z1 /K z2  of the radial stiffness K z1  of the idler wheel  50   x  under the high track tension loading over the radial stiffness K z2  of the idler wheel  50   x  under the obstacle loading. 
     For example, in some embodiments, the radial stiffness K z1  of the idler wheel  50   x  under the high track tension loading may be measured by generating load v. deflection data when loading the idler wheel  50   x  on a system as depicted in  FIG. 20 . With reference to  FIG. 20 , the idler wheel  50   x  may be loaded in the Z direction against a section of the track  41  suspended between two fixtures  99 , the two fixtures  99  being movable in the X direction and the distance D x  between the two fixtures  99  in the X direction being set to approximate the geometry of the track system  20   i . The radial stiffness K z2  of the idler wheel  50   x  under the obstacle loading may be measured by generating load v. deflection data when loading the idler wheel  50   x  against an obstacle. An obstacle as shown in  FIG. 23  or  FIG. 27  may be affixed onto a loading plate with an adaptor for a load v. deflection testing system such as an Instron testing system. 
     In some embodiments, the decoupled radial stiffness characteristics of the idler wheel  50   x  may be achieved by constructing the idler wheel  50   x  according to principles described above in section 1 such that the idler wheel  50   x  comprises the non-pneumatic tire  58  comprising the annular beam  136  and the spoked support  141 . 
       FIGS. 21 to 23  show examples of a finite element model of the idler wheel  50   x  in some embodiments. In one embodiment, the non-pneumatic tire  58  of the idler wheel  50   x  has a width W T =70 mm and an outer diameter D TO =240 mm. These dimensions are suitable for use of the non-pneumatic tire  58  in the track system  20   i  of the tracked vehicle  10  (ATV or UTV). In this embodiment, the tire material  145  of the non-pneumatic tire  58  is Hytrel 5556 (Dupont) and has a Young&#39;s modulus of 207 MPa at ambient temperature. 
       FIG. 21  shows the idler wheel  50   x  and the track  41  under no load. In this embodiment, the track  41  is inextensible relative to the non-pneumatic tire  58 . The track  41  wraps around approximately 165 degrees of the tread  150  of the non-pneumatic tire  58 .  FIG. 22  shows the idler wheel  50   x  being loaded against the track  41  (i.e. under the track tension loading), which results in the development of tensile forces in the track  41  which resiliently deform the non-pneumatic tire  58 . That is, in a track tension loading state of the idler wheel  50   x , a portion  127  of the annular support  141  between the axis of rotation  80  of the idler wheel  50   x  and the area of contact of the idler wheel  50   x  with the track  41  is compressed. 
       FIG. 23  shows one of the leading idler wheels  50   1 ,  50   2  when the track system  20   i  responds to an impact with an obstacle at high speed (i.e. under the impact loading). The obstacle is a rectangular object with a width=30 mm=12.5% of the outer diameter D TO  of the non-pneumatic tire  58  and a height=80 mm. Due to the design of the non-pneumatic tire  58 , the non-pneumatic tire  58  can resiliently deform as shown in  FIG. 22  and envelop the obstacle. This may be very beneficial to off-road vehicle performance. The non-pneumatic tire  58  represents un-sprung mass; as such, the speed with which it can deform is much faster than the speed with which the suspension  18  can displace one of the leading idler wheels  50   1 ,  50   2 , or the speed with which a center of gravity of the ATV  10  can change. Thus, the ability of the non-pneumatic tire  58  to resiliently deform as shown in  FIG. 22  is a critical improvement in off-road vehicle behavior. 
       FIG. 24  shows an example of a load vs. deflection plot for the FEA model shown in  FIGS. 22 and 23 . The loading of the idler wheel  50   x  against the track  41  is predicted to result in a radial stiffness K z1  of about 30 kgf/mm while the impact with the obstacle is predicted to result in a radial stiffness K z2  of about 5 kgf/mm. Accordingly, the ratio K z1 /K z2  of the radial stiffness K z1  of the idler wheel  50   x  under the track tension loading over the radial stiffness K z2  of the idler wheel  50   x  under the obstacle loading is 6 where the obstacle has a width=30 mm=12.5% of the outer diameter D TO  of the non-pneumatic tire  58 . 
     Because of the different loading behavior of the idler wheel  50   x  under the track tension loading and under the obstacle loading, the track system  20   i  may use identical leading idler wheels  50   1 ,  50   2  and trailing wheels  50   3 ,  50   4  with no performance compromise. Under normal use, the radial stiffness K Z2  of the idler wheels  50   x  will ensure track tensioning while the radial stiffness K Z1  of the idler wheels  50   x  will enable the idler wheels  50   x  to resiliently deform on impact with an obstacle. 
     Structural modeling results are shown in  FIGS. 25 to 27  for an example of another embodiment of the idler wheel  50   X . The maximum strains are shown for an idler wheel  50   X  of 120 mm diameter and 60 mm in width at 20° C. The non-pneumatic tire  48  of the idler wheel  50   X , that is the annular beam  136 , the spoked support  141  and the hub  75  are all made from Hytrel 5526 or Ellastollan S98A. The track  41  is modeled as a 1 mm thick steel belt in frictionless contact with the idler wheel  50   X . 
       FIG. 25  shows the idler wheel  50   X  and the track  41  under no load (that is, under no initial tension on the track  41 ) at a rotation of the track  41  of 200 rad/s. The maximum principle strain of the idler wheel  50   X  is predicted to be about 0.9% at the outer peripheral extent  146  of the annular beam  136  and the radial displacement of the idler wheel  50   X  is predicted to be about 1 mm.  FIG. 26  shows the idler wheel  50   X  and the track  41  under the track tension loading with a load of 450 kgf at a rotation of the track  41  of 50 rad/s. The maximum principle strain in the spoked support  141  is predicted to be about 3% in the portion  129  of the spoked support  141  between the axis of rotation  80  of the idler wheel  50   x  and the peripheral part  27  of the idler wheel  50   x  not in contact with the track  41 .  FIG. 27  shows the idler wheel  50   X  under the impact loading when responding to an impact with an obstacle modeled as a semi-sphere having a diameter of 30 mm. The maximum principle strain of the idler wheel  50   X  is predicted to be about 15% at the inner rim  133  of the shear band  139  of the annular beam  136  in the region of contact with the obstacle. 
     2.2 Leading and Trailing Idler Wheels that are Structurally Different 
     In some embodiments, as shown in  FIGS. 28 to 32 , the leading idler wheel  50   1  and the trailing idler wheel  50   3  may be structurally different to accommodate different types of loading, such the high track tension loading and the obstacle loading which are discussed above in section 2.1. 
     For example, in some embodiments, the radial stiffness K z  of the leading idler wheel  50   1  may be different from the radial stiffness K z  of the trailing idler wheel  50   3  for a given load. For instance, in some embodiments, the radial stiffness K z  of the leading idler wheel  50   1  may be less than the radial stiffness K z  of the trailing idler wheel  50   3  for a given load, such that the trailing idler wheel  50   3  is relatively stiffer under the high track tension loading to resist excessive deformation and track ratcheting issues and the leading idler wheel  50   1  is relatively more flexible (i.e., less stiff) under the obstacle loading to better handle the obstacle. 
     In some embodiments, a ratio of the radial stiffness K z  of the leading idler wheel  50   1  over the radial stiffness K z  of the trailing idler wheel  50   3  for a given load may no more than 0.9, in some cases no more than 0.7, in some cases no more than 0.5, and in some cases even lower. 
     The leading idler wheel  50   1  and the trailing idler wheel  50   3  may structurally differ in any suitable way. 
     For example, in some embodiments, as shown in  FIG. 28 , a material M 1  of the leading idler wheel  50   1  may be different from a material M 2  of the trailing idler wheel  50   3 . For instance, in embodiments where each of the leading idler wheel  50   1  and the trailing idler wheel  50   3  is constructing according to principles described above in section 1 such that it comprises the non-pneumatic tire  58  comprising the annular beam  136  and the spoked support  141 , the material M 1  of the leading idler wheel  50   1  may be the tire material  145  of the non-pneumatic tire  58  of the leading idler wheel  50   1  and the material M 2  of the trailing idler wheel  50   3  may be the tire material  145  of the non-pneumatic tire  58  of the trailing idler wheel  50   3 . 
     For instance, in some embodiments, a modulus of elasticity (i.e., Young&#39;s modulus) of the material M 1  of the leading idler wheel  50   1  may be different from a modulus of elasticity of the material M 2  of the trailing idler wheel  50   3 . In this example, the modulus of elasticity of the material M 1  of the leading idler wheel  50   1  may be less than the modulus of elasticity of the material M 2  of the trailing idler wheel  50   3 . In some embodiments, a ratio of the modulus of elasticity of the material M 1  of the leading idler wheel  50   1  over the modulus of elasticity of the material M 2  of the trailing idler wheel  50   3  may be no more than 0.9, in some cases no more than 0.7, in some cases no more than 0.5, and in some cases even less. 
     Alternatively or additionally, in some embodiments, a shape of the leading idler wheel  50   1  may be different from a shape of the trailing idler wheel  50   3 . For instance, in embodiments where each of the leading idler wheel  50   1  and the trailing idler wheel  50   3  is constructing according to principles described above in section 1 such that it comprises the non-pneumatic tire  58  comprising the annular beam  136  and the spoked support  141 , a shape of the non-pneumatic tire  58  of the leading idler wheel  50   1  may be different from a shape of the non-pneumatic tire  58  of the trailing idler wheel  50   3 . 
     For instance, a shape of the annular beam  136  (e.g., a number and/or a configuration of the openings  156   1 - 156   N ) and/or a shape of the spoked portion  141  (e.g., a number and/or a configuration of the spokes  142   1 - 142   T ) of the non-pneumatic tire  58  of the leading idler wheel  50   1  may be different from a shape of the annular beam  136  (e.g., a number and/or a configuration of the openings  156   1 - 156   N ) and/or a shape of the spoked portion  141  (e.g., a number and/or a configuration of the spokes  142   1 - 142   T ) of the non-pneumatic tire  58  of the trailing idler wheel  50   3 . 
     Examples of this are shown in  FIGS. 29 to 32 . For instance, in some embodiments: as shown in  FIG. 29 , the number of openings  156   1 - 156   N  of the annular beam  136  of the non-pneumatic tire  58  of the leading idler wheel  50   1  may be greater than that of the non-pneumatic tire  58  of the trailing idler wheel  50   3 ; as shown in  FIG. 30 , a size of one or more of the openings  156   1 - 156   N  of the annular beam  136  of the non-pneumatic tire  58  of the leading idler wheel  50   1  may be greater than that of the non-pneumatic tire  58  of the trailing idler wheel  50   3 ; as shown in  FIG. 31 , the number of spokes  142   1 - 142   T  of the non-pneumatic tire  58  of the leading idler wheel  50   1  may be less than that of the non-pneumatic tire  58  of the trailing idler wheel  50   3 ; and/or, as shown in  FIG. 32 , a size of one or more of the spokes  142   1 - 142   T  of the non-pneumatic tire  58  of the leading idler wheel  50   1  may be less than that of the non-pneumatic tire  58  of the trailing idler wheel  50   3 . This may allow the trailing idler wheel  50   3  to be relatively stiffer than the leading idler wheel  50   1  to accommodate the high track tension loading to resist excessive deformation and track ratcheting issues, and the leading idler wheel  50   1  to be relatively more flexible (i.e., less stiff) than the trailing idler wheel  50   3  to accommodate the obstacle loading to better handle the obstacle. 
     The track system  20   i  may be configured in various other ways in other embodiments. 
     For example, in some embodiments, the track system  20   i  may comprise more or less than two leading idler wheels such as the leading idler wheels  50   1 ,  50   2  and/or more or less than two trailing idler wheels such as the trailing idler wheel  50   3 ,  50   4  adjacent to each of its longitudinal ends. For instance, in some embodiments, the track system  20   i  may have a single leading idler wheel adjacent to its front longitudinal end and/or a single leading idler wheel adjacent to its rear longitudinal end. 
     As another example, in some embodiments, one or more of the support wheels  55   1 - 55   8  may be resilient wheels that are resilient deformable as discussed above in connection with the idler wheels  50   1 , 50   4 . For instance, in some cases, one or more of the support wheels  55   1 - 55   8  may be resilient wheels similar in construction to, but smaller than, the idler wheels  50   1 - 50   4 . 
     While in this embodiment the track systems  20   1 - 20   4  are part of the ATV  10 , in other embodiments, track systems constructed according to principles discussed herein in respect of the track systems  20   1 - 20   4  may be part of other types of vehicles. 
     For example, with additional reference to  FIGS. 37 and 38 , in some embodiments, an agricultural vehicle  510  may comprise track systems  520   1 - 520   4  constructed according to principles discussed herein in respect of the track systems  20   1 - 20   4 . The agricultural vehicle  510  is a heavy-duty vehicle designed to travel in agricultural fields to perform agricultural work using a work implement  598 . In this embodiment, the agricultural vehicle  510  is a tractor. In other embodiments, the agricultural vehicle  510  may be a combine harvester, another type of harvester, or any other type of agricultural vehicle. 
     The agricultural vehicle  510  comprises a frame  512 , a powertrain  514 , the track systems  520   1 - 520   4 , the work implement  598 , and an operator cabin  584 , which enable an operator to move the agricultural vehicle  510  on the ground and perform agricultural work using the work implement  598 . The operator cabin  584  is where the operator sits and controls the agricultural vehicle  510 . More particularly, the operator cabin  584  comprises a user interface that allows the operator to steer the agricultural vehicle  510  on the ground and perform agricultural work using the working implement  598 . 
     The working implement  598  is used to perform agricultural work. For example, in some embodiments, the work implement  598  may be a combine head, a cutter, a scraper pan, a tool bar, a planter, or any other type of agricultural work implement. 
     Each track system  520   i  may be constructed according to principles described herein in respect of the track systems  20   1 - 20   4 , by comprising a track-engaging assembly  517  and a track  541  disposed around the track-engaging assembly  517 , wherein the track-engaging assembly  517  comprises a frame  544  and a plurality of track-contacting wheels which includes a drive wheel  542  and a plurality of idler wheels  550   1 - 550   4 ,  555   1 - 555   4 , which includes leading idler wheels  550   1 ,  550   2 , trailing idler wheels  550   3 ,  550   4 , and support wheels (i.e., mid-rollers)  555   1 - 555   8 , and wherein each of the idler wheels  550   1 - 550   4 ,  555   1 - 555   4  may comprise a non-pneumatic tire  558  and a hub  575  constructed according to principles described herein in respect of the non-pneumatic tire  58  and the hub  75 . The non-pneumatic tire  558  comprises an annular beam  536  and an annular support  541  that may be constructed according principles described herein in respect of the annular beam  136  and the annular support  141 . For instance, the annular beam  536  comprises a shear band  539  comprising openings  556   1 - 556   B  and the annular support  541  comprises spokes  542   1 - 542   J  that may be constructed according to principles described herein in respect of the shear band  139  and the spokes  142   1 - 142   T . In this embodiment, the shear band  539  comprises intermediate rims  551 ,  553  between an outer rim  531  and an inner rim  533  such that the openings  556   1 - 556   N  and interconnecting members  537   1 - 537   P  are arranged into three circumferential rows between adjacent ones of the rims  531 ,  551 ,  553 ,  533 . 
     In this embodiment, the non-pneumatic tire  558  of each of idler wheels  550   1 - 550   4 ,  555   1 - 555   4 , including the mid-rollers  555   1 - 555   4 , may help the idler wheels  550   1 - 550   4 ,  555   1 - 555   4  and the track  541  better accommodate the ground on which the agricultural vehicle  10  travels. For example, in some embodiments, the non-pneumatic tire  558  of each of the idler wheels  550   1 - 550   4 ,  555   1 - 555   4 , including the mid-rollers  555   1 - 555   4 , may deflect when the ground is uneven in a widthwise direction of the track system  520   i . For instance, this may occur when the agricultural vehicle  10  is travelling on uneven soil of an agricultural field and/or travelling (i.e., “roading”) on a road (i.e., a paved road having a hard surface of asphalt, concrete, gravel, or other pavement), such as between agricultural fields, where the road has a cross slope (e.g., a “crown”) for leading water away from the road (i.e., to avoid water accumulation on the road). 
     As another example, in some embodiments, track systems constructed according to principles discussed herein in respect of the track systems  20   1 - 20   4  may be used as part of a construction vehicle (e.g., a loader, a bulldozer, an excavator, a dump truck, etc.), a forestry vehicle, or a military vehicle. 
     Certain additional elements that may be needed for operation of some embodiments have not been described or illustrated as they are assumed to be within the purview of those of ordinary skill in the art. Moreover, certain embodiments may be free of, may lack and/or may function without any element that is not specifically disclosed herein. 
     Any feature of any embodiment discussed herein may be combined with any feature of any other embodiment discussed herein in some examples of implementation. 
     In case of any discrepancy, inconsistency, or other difference between terms used herein and terms used in any document incorporated by reference herein, meanings of the terms used herein are to prevail and be used. 
     Although various embodiments and examples have been presented, this was for the purpose of describing, but not limiting, the invention. Various modifications and enhancements will become apparent to those of ordinary skill in the art and are within the scope of the invention, which is defined by the appended claims.