Abstract:
A sprocket wheel and drive track assembly of a tracked vehicle includes at least one drive sprocket having a sprocket wheel. A plurality of internal teeth extending laterally therefrom is distributed in a circumferential direction on a periphery thereof at a sprocket pitch from each other. A belt in contact with the at least one drive sprocket has an inner side, an outer side, a longitudinal and a lateral direction. At least two of a plurality of internal lugs distributed on the inner side thereof are engaged by at least two of the plurality of internal teeth of the at least one drive sprocket. The plurality of internal lugs is disposed to form at least one longitudinal row. At least two consecutive internal lugs are disposed at a first pitch from each other. The first pitch is longer than an arc length corresponding to the sprocket pitch.

Description:
CROSS-REFERENCE 
       [0001]    The present application is a continuation of U.S. patent application Ser. No. 13/017,787, filed on Jan. 31, 2011, the entirety of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention generally relates to tracks and drive sprockets for tracked vehicles. 
       BACKGROUND 
       [0003]    Tracked vehicles such as snowmobiles and snow groomers drivingly engage the ground through one or more endless tracks. Endless tracks conventionally include an outer side with a pattern of projecting lugs (or ribs) that are designed to engage the snow or other ground surface, apply traction, and propel the vehicle. Conventional endless tracks also include an inner side that engages one or more drive sprockets, which, in turn, are operatively connected to a propulsion system of the vehicle. The drive sprockets and the inner side of the endless track typically include mating teeth that provide traction between the drive sprockets and the endless track. Conventional drive sprockets use either external (radially extending) or internal (axially extending) teeth or both. 
         [0004]      FIG. 1  illustrates a conventional arrangement of four drive sprockets  1050 ,  1060 . Two drive sprockets  1050  are to be disposed on outer sides of an endless track  1080 , and two drive sprockets  1060  are to be disposed on a central portion of the endless track  1080 . The drive sprockets  1050  include a plurality of axially-extending sprocket teeth  1070  (9 teeth) on one side thereof, while the drive sprockets  1060  include a plurality of axially-extending sprocket teeth  1070  (9 teeth) on each side thereof. In some cases, the sprockets  1050  or  1060  also have radially extending teeth. In other cases, the drive sprockets  1050 ,  1060  have 8 or 10 sprocket teeth  1070 . The drive sprockets  1050  are used in combination with the drive sprockets  1060  to ensure sufficient application of the engine torque to the endless track  1080  without allowing ratcheting (rotation of the sprocket without equivalent rotation of the track). Only two sprockets  1050  or  1060  could also be used, if the sprockets  1050  or  1060  provided sufficient application of the engine torque to the endless track  1080 . 
         [0005]    The endless track  1080  includes a plurality of longitudinally-spaced internal track lugs (or teeth)  1090  projecting inwardly from an inner side of the endless track  1080  for contacting the drive sprockets  1050 ,  1060 . The sprocket teeth  1070  engage the internal track lugs  1090  to provide traction between the sprocket  1050 ,  1060  and the endless track  1080 . Alignment cleats  1085  are laterally offset from the track lugs  1090  on the endless track  1080  and the sprocket teeth  1070 . The internal track lugs  1090  are disposed so as to form two outer rows  1091  and four inner rows  1092 , in a longitudinal direction  1002 . The rows  1091  and  1092  are disposed adjacent to each other in a lateral direction  1004 , and the internal track lugs  1090  are aligned in the lateral direction  1004 . A pitch  1093  between the track lugs  1090  is constant. The pitch  1093  is measured in the longitudinal direction  1002 . The track  1080  also features two longitudinal rows of apertures or windows  1030 . The track  1080  includes a plurality of external track lugs  1095 . The external track lugs  1095  are distributed in lateral rows, and the rows are disposed at a pitch  1097  in the longitudinal direction  1002 . The pitch  1097  of the external track lugs  1095  equals the pitch  1093  of the internal track lugs  1090 . 
         [0006]    While the above endless track and sprocket assembly functions to provide torque transfer while avoiding ratcheting, a greater number of sprockets is required when the sprockets do not have radially extending teeth or there is a greater amount of torque to be transferred to the track than can be applied by the sprocket assembly without ratcheting. On the other hand, when using sprockets with radially extending teeth, these sprockets are aligned with the windows of the track, and travel over the alignment cleats  1085  over which the slide rails glide. The contact between the alignment cleats  1085  and the sprocket causes undesired noise and vibration. It also requires the slide rails to be shortened due to the space required for the sprockets. 
       SUMMARY 
       [0007]    It is an object of the present invention to ameliorate at least some inconveniences of the past. It is also an object to provide a sprocket and track combination that reduces the level of noise and vibration normally generated by the interaction of the two. 
         [0008]    In one aspect, a sprocket wheel and drive track assembly of a tracked vehicle includes at least one drive sprocket, the at least one drive sprocket having a sprocket wheel and a plurality of internal teeth extending laterally therefrom. The plurality of internal teeth are distributed in a circumferential direction on a periphery of at least one side of the at least one drive sprocket. The plurality of internal teeth is disposed at a sprocket pitch from each other. A belt is in contact with the at least one drive sprocket. The belt has an inner side and an outer side. The belt has a longitudinal direction and a lateral direction. A plurality of internal lugs is distributed on the inner side of the belt. At least two of the plurality of internal lugs are engaged by at least two of the plurality of internal teeth of the at least one drive sprocket. The plurality of internal lugs is disposed to form at least one longitudinal row. At least two consecutive internal lugs of the plurality of internal lugs are disposed at a first pitch from each other. The first pitch is longer than an arc length corresponding to the sprocket pitch. 
         [0009]    In another aspect, the first pitch is a multiple of the arc length corresponding to the sprocket pitch. 
         [0010]    In another aspect, the first pitch is double of the arc length corresponding to the sprocket pitch. 
         [0011]    In yet another aspect, the at least one drive sprocket is adapted to be rotated so as to engage the belt in rotation. When the at least one sprocket wheel is rotated to complete one revolution, only some of the plurality of internal teeth of the at least one drive sprocket have transferred torque to an internal lug. 
         [0012]    In a further aspect, the at least one sprocket wheel has 16 internal teeth on each side thereof. 
         [0013]    In an additional aspect, at least two other consecutive internal lugs of the at least one longitudinal row are disposed at a second pitch from each other. The second pitch is shorter than the first pitch. The plurality of internal lugs of the at least one longitudinal row is disposed so as to form a repeating sequence where two second pitches are followed by one first pitch consecutively. 
         [0014]    In another aspect, the second pitch is one of equal to and smaller than the arc length corresponding to the sprocket pitch. 
         [0015]    In another aspect, the second pitch is equal to the arc length corresponding to the sprocket pitch. 
         [0016]    In an additional aspect, the belt further comprises a plurality of studs extending from the outer side. At least some of the studs of the plurality of studs are positioned in the longitudinal direction between consecutive internal lugs of the plurality of internal lugs that are disposed at the first pitch from each other. 
         [0017]    In another aspect, the at least one longitudinal row includes a first row and a second row adjacent to each other in the lateral direction. The first row and the second row each have the repeating sequence. The repeating sequences of the first and second rows are longitudinally offset from each other. 
         [0018]    In yet another aspect, the second pitch is half of the first pitch, and the repeating sequence of the first row is offset from the repeating sequence of the second row such that at least one of the two consecutive lugs of the first row disposed at the first pitch from each other is longitudinally aligned with one of the two consecutive lugs of the second row disposed at the second pitch from one another. 
         [0019]    In a further aspect, the at least one drive sprocket includes a first drive sprocket and a second drive sprocket. The at least one longitudinal row includes a pair of first rows and a pair of second rows. The pairs of first and second rows are adjacent to each other in the lateral direction. The first drive sprocket is engaging internal lugs of the pair of first rows, and the second drive sprocket is engaging internal lugs of the pair of second rows. 
         [0020]    In an additional aspect, at any given time, at least one sprocket tooth is immediately surrounded by two consecutive inner track lugs, and at least one other sprocket tooth of the at least one side of the drive sprocket is immediately surrounded by only one other inner track lug. 
         [0021]    In another aspect, the at least one side of the at least one drive sprocket includes a first side and a second side. At any given time, at least one sprocket tooth on the first side of the at least one drive sprocket is immediately surrounded by two consecutive inner track lugs, and at least one other sprocket tooth on the second side of the at least one drive sprocket is immediately surrounded by only one other inner track lug. The at least one other sprocket tooth is aligned with the at least one sprocket tooth. 
         [0022]    In another aspect, a plurality of external lugs is distributed on the outer side of the belt, the plurality of external lugs being disposed so as to form a plurality of lateral rows. The plurality of lateral rows is disposed at an external lug pitch from each other. The first pitch equals the external lug pitch. 
         [0023]    In an additional aspect, a sprocket wheel and drive track assembly of a tracked vehicle includes at least one drive sprocket. The at least one drive sprocket has a sprocket wheel and a plurality of internal teeth extending laterally therefrom. The plurality of internal teeth is distributed in a circumferential direction on a periphery of at least one side of the at least one drive sprocket, the plurality of internal teeth is disposed at a sprocket pitch from each other. A belt is in contact with the at least one drive sprocket. The belt has an inner side and an outer side. The belt has a longitudinal direction and a lateral direction. A plurality of internal lugs is distributed on the inner side of the belt. At least two of the plurality of internal lugs is engaged by at least two of the plurality of internal teeth of the at least one drive sprocket. The plurality of internal lugs is disposed to form at least one longitudinal row. At least two consecutive internal lugs of the plurality of internal lugs is disposed at a first pitch from each other. A plurality of external lugs is distributed on the outer side of the belt. The plurality of external lugs is adapted to be in contact with a ground. The plurality of external lugs is disposed so as to form a plurality of lateral rows. The plurality of lateral rows is disposed at an external lug pitch from each other over the entirety of the outer side of the belt. The external lug pitch is longer than an arc length corresponding to the sprocket pitch. 
         [0024]    In another aspect, at least two other consecutive internal lugs of the at least one longitudinal row are disposed at a second pitch from each other. The second pitch is shorter than the first pitch. The plurality of internal lugs of the at least one longitudinal row is disposed so as to form a repeating sequence where two second pitches are followed by one first pitch consecutively. The external lug pitch is longer than the second pitch. The external lug pitch is equal to the first pitch. 
         [0025]    In yet another aspect, the first pitch is equal to the external lug pitch, and at least one internal lug of the at least two consecutive internal lugs is longitudinally aligned with one lateral row of the plurality of lateral rows of external lugs. 
         [0026]    In a further aspect, at least three other consecutive internal lugs of the at least one longitudinal row are disposed at a second pitch from each other. The at least three other consecutive internal lugs including a first internal lug, a second internal lug and a third internal lug. The first internal lug is longitudinally aligned with a first lateral row of the plurality of lateral rows of external lugs. The third internal lug is longitudinally aligned with a second lateral row of the plurality of lateral rows of external lugs, the first lateral row and the second lateral row being disposed consecutively. The second internal lug is disposed between the first and third internal lug. 
         [0027]    In another aspect, the external lug pitch is double of the arc length corresponding to the sprocket pitch. 
         [0028]    For purposes of this application, terms related to spatial orientation such as forwardly, rearwardly, upwardly, downwardly, left, and right, are as they would normally be understood by a driver of the vehicle sitting thereon in a normal riding position. 
         [0029]    Embodiments of the present invention each have at least one of the above-mentioned objects and/or aspects, but do not necessarily have all of them. It should be understood that some aspects of the present invention that have resulted from attempting to attain the above-mentioned objects may not satisfy these objects and/or may satisfy other objects not specifically recited herein. 
         [0030]    Additional and/or alternative features, aspects, and advantages of embodiments of the present invention will become apparent from the following description, the accompanying drawings, and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    For a better understanding of the present invention as well as other objects and further features thereof, reference is made to the following description which is to be used in conjunction with the accompanying drawings, where: 
           [0032]      FIG. 1  is a partially exploded view of a plurality of drive sprockets and an endless belt according to the prior art; 
           [0033]      FIG. 2  is a left side elevation view of a snowmobile with a portion of the tunnel broken away to show the endless track and associated components; 
           [0034]      FIG. 3  is a perspective view taken from a rear, left side of a drive sprocket of the snowmobile of  FIG. 2 ; 
           [0035]      FIG. 4  is a perspective view taken from a rear, left side of drive sprockets, a drive axle, and a portion of an endless belt of the snowmobile of  FIG. 2  according to a first embodiment; 
           [0036]      FIG. 5  is a left side elevation view of the drive sprockets, the drive axle, and the endless belt of  FIG. 4 ; 
           [0037]      FIG. 6  is a top plan view of the drive sprockets, the drive axle, and the endless belt of  FIG. 4 ; 
           [0038]      FIG. 7  is a bottom plan view of the drive sprockets, the drive axle, and the endless belt of  FIG. 4 ; 
           [0039]      FIG. 8  is a perspective view taken from a rear, left side of drive sprockets, a drive axle, and a portion of an endless belt of the snowmobile of  FIG. 2  according to a second embodiment; 
           [0040]      FIG. 9  is a top plan view of the drive sprockets, the drive axle, and the endless belt of  FIG. 8 ; 
           [0041]      FIG. 10  is a cross-sectional view taken along line  10 - 10  in  FIG. 9  of the drive sprockets, the drive axle, and the endless belt of  FIG. 8 ; 
           [0042]      FIG. 11  is a perspective view taken from a rear, left side of drive sprockets, a drive axle, and a portion of an endless belt of the snowmobile of  FIG. 2  according to a third embodiment; 
           [0043]      FIG. 12  is a top plan view of the drive sprockets, the drive axle, and the endless belt of  FIG. 11 ; 
           [0044]      FIG. 13  is a bottom plan view of the drive sprockets, the drive axle, and the endless belt of  FIG. 11 ; 
           [0045]      FIG. 14  is a left side elevation view of the drive sprockets, the drive axle, and endless belt of  FIG. 11 ; 
           [0046]      FIG. 15  is a graph illustrating level of vibration for different sprockets and endless belt combinations; and 
           [0047]      FIG. 16  is a perspective view of a portion of a prior art endless belt. 
       
    
    
     DETAILED DESCRIPTION 
       [0048]    As illustrated in  FIG. 2 , a snowmobile  10  according to an embodiment of the present invention includes a frame  15  that supports a pair of selectively steerable skis  20  and a straddle-type seat  50 . An endless track  30  is supported by the frame  15  through a slide rail suspension system  40 . The frame  15  includes a tunnel  52  onto which the seat  50  is disposed. The slide rail suspension system  40  includes at least slide rails  42  and several idler wheels  86 . Other components of the suspension system, being well known in the art, it will not be described herein. 
         [0049]    The snowmobile  10  includes a propulsion unit  60  (shown in phantom), such as an internal combustion engine, that is operatively connected to the endless track  30  via a drive axle  70 . A continuously variable transmission (not shown) connects the propulsion unit  60  to the drive axle  70 . Two drive sprockets  80  (only a left one being shown in  FIG. 2 ) are mounted to the drive axle  70  for common rotational movement about a drive sprocket axis  85 . It is contemplated that only one or more than two drive sprockets  80  could be mounted on the drive axle  70 . Each sprocket  80  includes two sets of sprocket teeth  90 ,  100  (shown in  FIG. 3 ) that engage corresponding internal lugs (or track lugs, or track teeth)  260  of the endless track  30  to provide traction between the sprockets  80  and the endless track  30 . As illustrated in  FIG. 4 , because the sprocket teeth  90 ,  100  and inner track lugs  120 ,  130  provide substantial traction between each sprocket  80  and the endless track  30 , only two sprockets  80  are required. This reduces the weight of the moving parts of the snowmobile  10  as compared to conventional foursprocket arrangements (see e.g.  FIG. 1 ). While two sprockets  80  are used in the illustrated embodiments, greater or fewer sprockets  80  may alternatively be used. Additionally, a sprocket  80  may be used in conjunction with one or more conventional sprockets such as the previously described sprockets  1050 ,  1060 . 
         [0050]    Referring specifically to  FIG. 3 , the two drive sprockets  80  (left and right) being identical, only a left drive sprocket  80  will be described. The drive sprocket  80  comprises a generally disc-shaped sprocket wheel  150  that has an outer perimetrical surface  160  and two opposing axial surfaces  170 ,  180 . The sprocket wheel  150  has a central bore  190  through which the drive axle  70  extends. The bore  190  and the drive axle  70  have mating cross-sections that rotationally secure the sprocket  80  to the drive axle  70 . Alternative methods of rotationally securing the sprocket  80  to the drive axle  70  may also be used (e.g., a key and keyway, square cross-sections, radial pins, etc.). The sprocket wheel  150  has a diameter of 7 inches (17.78 cm). It is contemplated that the diameter of the sprocket wheel  150  could be other than 7 inches. For example, the diameter of sprocket wheel  150  could be 6 inches (15.24 cm). 
         [0051]    The sprocket teeth  90  comprise circumferentially-spaced teeth that project axially outwardly from the axial surface  170 , and the sprocket teeth  100  comprise circumferentially-spaced teeth that project axially outwardly from the axial surface  180 . The sprocket teeth  100  are mirror images of the sprocket teeth  90 . It is contemplated that the sprocket teeth  90 ,  100  could be different from each other. The drive sprocket  80  has 16 sprocket teeth  90  and 16 sprocket teeth  100 , which is a higher number than previously seen in the prior art (e.g. see  FIG. 1 ). It is contemplated that the drive sprocket  80  could have more or less than 16 sprocket teeth  90  and 16 sprocket teeth  100 . 
         [0052]    Consecutive sprocket teeth  90  are disposed at a sprocket pitch  8  from each other. Consecutive sprocket teeth  100  are also disposed at a sprocket pitch  8  from each other. The sprocket pitch  8  is 22.5 degrees. It is contemplated that the sprocket pitch  8  could be more or less than 22.5 degrees depending on the number of sprocket teeth  90 ,  100 . For example the sprocket pitch  8  could be 40 degrees if the sprocket  80  had nine teeth. It is also contemplated that the sprocket teeth  90  could have a sprocket pitch different from a sprocket pitch of the sprocket teeth  100 . The sprocket pitch  8  is determined in conjunction with a pitch between the plurality of internal lugs  260  in order to ensure traction. The sprocket teeth  90 ,  100  are preferably axially aligned to each other such that each sprocket tooth  90  is disposed at the same circumferential position as a corresponding one of the sprocket teeth  100 . It is contemplated that the sets of sprocket teeth  90 ,  100  could be offset in the circumferential direction . Sprocket valleys  200  are formed between consecutive sprocket teeth  90 , and between consecutive sprocket teeth  100 . 
         [0053]    The sprocket wheel  150  and the sprocket teeth  90 ,  100  are integrally formed. To reduce the weight of the snowmobile  10 , the sprocket  80  is made of a strong, light material (such as plastic). It is contemplated that the sprocket  80  could be made of any other material including steel or a composite material including aluminum. It is also contemplated that the sprocket  80  could be made from a composite including carbon fibers. A composition of the sprocket  80  may be selected from a wide variety of substances. It is contemplated that the sprocket teeth  90 ,  100  could be formed separately from the sprocket wheel  150  and subsequently rigidly fastened (via rivets, welds, bolts, etc.) to the sprocket wheel  150 . 
         [0054]    Referring more specifically to  FIGS. 4-7 , a first embodiment of the endless track  30  will be described. The endless track  30  has an endless flexible belt  220  which has an inner side  220   a  (best shown in  FIG. 6 ) and an outer side  220   b  (best shown in  FIG. 7 ). The endless track  30  includes a plurality of external lugs (or ribs)  230  that project from the outer side  220   b  to give the endless track  30  traction against the snow as the endless track  30  propels the snowmobile  10 , and a plurality of internal lugs  260  that project from the inner side  220   a.  The endless track  30  defines a longitudinal direction  2  (shown in  FIG. 4 ) and a lateral direction  4  (shown in  FIG. 4 ). The endless track  30  comprises a strong, flexible material such as rubber reinforced with fabric and metal. The endless belt  220 , the external lugs  230  and the internal lugs  260  are integrally formed with each other. The endless belt  220  has a width (measured in the lateral direction  4 ) of 16 inches (40.64 cm). It is contemplated that the width of the endless belt  220  could be more or less than 16 inches. For example, the width of the endless belt  220  could be 15 inches (38.1 cm), 20 inches (50.8 cm) or 24 inches (60.96 cm). A length of the endless belt (measured in the longitudinal direction  2 ) is 137 inches (347.98 cm). It is contemplated that the length of the endless belt  220  could be more or less than 137 inches. For example, the length of the endless belt  220  could be 121 inches (307.34 cm), 154 inches (391.16 cm), 156 inches (396.24 cm) or 163 inches (414.02 cm). 
         [0055]    The plurality of internal lugs  260  includes inner track lugs  120 ,  130  and outer track lugs  140  which project inwardly from the inner side  220   a  of the endless belt  220 . The inner track lugs  120 ,  130  come in contact with the two sprockets  80  for providing traction to the snowmobile  10 , while the outer track lugs  140  are in contact with the slide rails  42  for ensuring that the endless belt  220  stays in alignment. The slide rails  42  are sliding in a space defined between the outer track lugs  140  and inner track lugs  120  on the left side, and between the outer track lugs  140  and inner track lugs  130  on the right side. It is contemplated that that the outer track lugs  140  could be in contact with sprockets other than the sprockets  80  mounted onto the drive axle  70 . A height  15  (measured in a direction perpendicular to the longitudinal direction  2  and the lateral direction  4 ) of the internal lugs  230  is about 0.5 inches (1.27 cm). It is contemplated that the height  15  could be more or less than 0.5 inches. 
         [0056]    As best seen in  FIG. 6 , the inner track lugs  120 ,  130  each form longitudinal rows  122 ,  132 , and the outer track lugs  140  each form longitudinal rows  142 . The inner track lugs  120  are aligned with the inner track lugs  130  in the longitudinal direction  2 . 
         [0057]    The outer internal lugs  140  are spaced at an outer lug pitch  6  of 2.86 inches (7.26 cm). The outer lug pitch  6  is measured in the longitudinal direction  2 . It is contemplated that the outer lug pitch  6  could be more of less than 2.86 inches. For example, the outer lug pitch  6  could be 2.52 inches (6.4 cm). The inner track lugs  120  and the inner track lugs  130  are spaced at an inner lug pitch  12  of 1.43 inches (3.63 cm). The inner lug pitch  12  is measured in the longitudinal direction  2 . It is contemplated that the inner lug pitch  12  could be more of less than 1.43 inches. For example, the inner lug pitch  12  could be 1.26 inches (3.23 cm). It is also contemplated that the inner lug pitch of the inner track lugs  120  could be different from the inner lug pitch of the inner track lugs  130 . As will be described below for a second and third embodiment of the endless drive track  30 , it is also contemplated that more than one inner lug pitch could define distances between two consecutive inner track lugs  120  and/or two consecutive inner track lugs  130 . 
         [0058]    A plurality of longitudinally spaced apertures (or windows)  240  are defined in the endless belt  220 . The plurality of spaced apertures  240  is disposed into two longitudinal rows  242 . It is contemplated that the plurality of longitudinally spaced apertures  240  could be omitted. It is contemplated that the plurality of longitudinally spaced apertures  240  could be disposed in a fashion other than in a longitudinal row. It is also contemplated that the plurality of longitudinally spaced apertures  240  could form only one row  242  or more than two rows  242 . 
         [0059]    The endless track  30  also comprises a plurality of alignment cleats  250  that are mounted onto the outer track lugs  140  and extend in between the apertures  240 . Each cleat  250  includes a base portion  250   a  and a cleat portion  250   b.  The base portion  250   a  has a generally C-shaped cross-section that wraps around the inner track lugs  120 ,  130 . The cleat portion  250   b  projects inwardly away from the inner side  220   a  of the belt  220 . The alignment cleats  250  preferably comprise a strong, light, stamped sheet of metal such as steel. As would be appreciated by those skilled in the art, however, any other suitable material (e.g., aluminum, etc.) may be used. Moreover, the cleats  250  need not be stamped from a metal sheet but may be cast or molded into the appropriate configuration. 
         [0060]    As best seen in  FIG. 7 , the plurality of external lugs  230  is distributed into a plurality of external lug lateral rows  232 . Each external lug lateral row  232  contains either two external lugs  230  or three external lugs  230  disposed in a repeating sequence. It is contemplated that the repeating sequence could be different from the one shown in  FIG. 7 . The external lugs  230  include three different designs of external lugs. It is contemplated that the external lugs  230  could include more or less than three different designs of external lugs. The external lug lateral rows  232  are disposed at an external lug pitch  14  of 2.86 inches (7.26 cm) (shown in  FIG. 5 ). The external lug pitch  14  is measured in the longitudinal direction  2 . As best seen in  FIG. 5 , the external lug pitch  14  equals the outer lug pitch  6 . It is contemplated that the external lug pitch  14  could be smaller or greater than the outer lug pitch  6 . It is contemplated that the external lug pitch  14  could be more or less than 2.86 inches. For example, the external lug pitch  14  could be 2.52 inches (6.4 cm). It is also contemplated that more than one external lug pitch  14  could exist between the external lug longitudinal rows  232 . A height  13  (measured in a direction perpendicular to the longitudinal direction  2  and the lateral direction  4 ) of the external lugs  260  is 1.25 inches (3.17 cm). It is contemplated that the height  13  could be more or less than 1.25 inches. For example, the height of the external lugs  260  could be 0.75 (1.905 cm), 1 inch (2.54 cm), 1.5 inches (3.81 cm), 1.75 inches (4.44 cm), 2 inches (5.08 cm), 2.25 inches (5.71 cm) or 2.5 inches (5.715 cm). As will be described below, studs could be extending from the outer surface  220   b  for additional gripping to the ground. 
         [0061]    Turning now to  FIGS. 8 to 10 , a second embodiment of an endless track  30 ′ will be described. 
         [0062]    The endless track  30 ′ has features similar to the ones of the endless track  30 . These features will be referred to using the same reference numerals as the ones of the endless track  30 , and will not be described in greater details herein again. 
         [0063]    The endless track  30 ′ includes a plurality of internal lugs  260 ′ on an inner face  220   a  of the endless belt  220 . The plurality of internal lugs  260 ′ includes the outer track lugs  140  and inner track lugs  120 ′,  130 ′. The inner track lugs  120 ′ form two inner track lug longitudinal rows  122 ′, and the inner track lugs  130 ′ form two inner track lug longitudinal rows  132 ′ disposed similarly as the inner track lug longitudinal rows  120 ,  130 . 
         [0064]    The inner track lugs  120 ′ are distributed along the inner track lug longitudinal rows  122 ′ in a repeating sequence that uses a pitch  18  and at a pitch  22 , both measured in the longitudinal direction  2  between two consecutive inner track lugs  120 ′. The pitch  18  is half of the pitch  22 , and equals the pitch  6  of the inner track lugs  120 ,  130 . As best seen in  FIG. 9 , the repeating sequence of pitches is two pitches  18  followed by a pitch  22 . It is contemplated that the pitch  18  could be more or less than half of the pitch  22 . The pitch  22  equals the pitch  6  of the outer track lugs  140 . It is contemplated that the pitch  22  could be different from the pitch  6 . It is also contemplated that the repeating sequence could be different. For example, the repeating sequence could feature three different pitches. The inner track lugs  130 ′ are arranged in the same repeating sequence as the inner track lugs  120 ′. 
         [0065]    The repeating sequences of the inner track lug longitudinal rows  122 ′,  132 ′ are offset from each other. Thus at any given time, at least one side of each drive sprocket  80  has two consecutive teeth  90  or  100  in contact with the inner track lugs  120 ′ or  130 ′. It is contemplated that the inner track lug longitudinal rows  122 ′,  132 ′ could be disposed such that the repeating sequences of the inner track lugs  120 ′,  130 ′ are aligned in the longitudinal direction  2 . As best shown in  FIG. 10 , because the repeating sequence features inner track lugs  120 ′, 130 ′ that are distanced apart by the pitch  22  being twice of the corresponding sprocket pitch  8 , there are times when the drive sprocket  80  rotates where some of the sprocket teeth  90 ,  100  are not immediately surrounded by two inner track lugs  120 ′, 130 ′. As best seen in  FIG. 10  for a sprocket  90   a,  when the sprocket tooth  90   a  is not immediately surrounded (front/back) by two inner track lugs  120 ′, the sprocket tooth  100  (shown in phantom) on the opposite side of the sprocket  80 , aligned with the sprocket tooth  90   a,  is surrounded by two inner track lugs  130 ′. Similarly, when a sprocket tooth  100  is not immediately surrounded (front/back) by two inner track lugs  130 ′, the sprocket tooth  90  on the opposite side of the sprocket  80 , aligned with the sprocket tooth  100 , is surrounded by two inner track lugs  120 ′. 
         [0066]    Turning now to  FIGS. 11 to 14 , a third embodiment of an endless track  30 ″ will be described. 
         [0067]    The endless track  30 ″ has features similar to the ones of the endless track  30 ′. These features will be referred to using the same reference numerals as the ones of the endless track  30 ′, and will not be described in greater details herein again. 
         [0068]    The endless belt  220  has a plurality of studs  280  extending therethrough. The studs  280  are used for enhanced contact with the ground. As best seen in  FIG. 14 , the studs  280  have a pointy end extending from the outer side  220   b  of the endless belt  220 . The studs  280  are disposed along the inner track lug longitudinal rows  122 ′ and  132 ′, in between two consecutive inner track lugs  120 ′ and  130 ′ that are spaced by the pitch  22 . The studs  280  are disposed half way in the longitudinal direction  2  between the consecutive inner track lugs  120 ′ (or  130 ′ as the case may be). A distance between a center of a stud  280  and a consecutive inner track lugs  120 ′ (or  130 ′) or a consecutive outer track lug  230  in the longitudinal direction  2  equals the pitch  18 . It is also contemplated that the studs  280  could be disposed only in between some of those consecutive inner track lugs  120 ′ (and  130 ′) that are distanced apart by the pitch  22 . As the drive sprocket  80  rotates, there are times where the sprocket teeth  90 ,  100  come into contact with a flat top of the studs  280 . 
         [0069]    As best seen in  FIG. 15  for the endless track  30 , one of the advantages of the endless tracks  30 ,  30 ′,  30 ″ and sprocket assemblies is a reduction of vibration transferred to the tunnel  52  compared with the prior art assemblies. Vibration in the tunnel  52  is measured using an acceleration sensor (not shown) attached to a vertical side portion (not shown) of the snowmobile tunnel  52  in the vicinity of the drive axle  70 . The acceleration sensor measures an amplitude of vibration in g (m.s-2). The experiment set up involves two drive sprocket mounted on a drive axle. 
         [0070]    In  FIG. 15 , a level of vibration is measured for different sprockets in contact with an endless belt  1080 ′ of the prior art (shown in  FIG. 16 ) and compared with the sprocket  80  in contact with the endless belt  220 . Sprocket #1 is a sprocket having 8 internal teeth (i.e. teeth extending axially on both sides of the sprocket) in line with windows  1030 ′ in the endless belt  1080 ′. Sprocket #2 is a sprocket having 8 internal teeth (i.e. teeth extending axially on both sides of the sprocket) in engagement with mating lugs  1090 ′ adjacent to windows in the endless belt  1080 ′. Sprocket #3 is a sprocket having no teeth in contact with the endless belt  1080 ′. Sprocket #4 is the drive sprocket  80  in engagement with the endless belt  220 . 
         [0071]    As shown in the graph displayed in  FIG. 15 , when the snowmobile  10  is driving at about 25 miles per hour (40 km per hour) the sprocket  80  in engagement with the endless track  30  has a vibration level reduced by 92% compared to the sprocket #1, 86% compared to the sprocket #2, and 88% compared to the sprocket #3. By increasing the number of sprocket teeth (doubling with respect to the sprockets #1 and #2 of the prior art) the torque transmitted from the drive sprocket to the belt is distributed among more contact points than in the past, the impacts thus get reduced and the vibration level decreased. Better traction is ensured which in turn reduces the number of drive sprockets needed on the drive axle  70 . 
         [0072]    Modifications and improvements to the above-described embodiments of the present invention may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. The scope of the present invention is therefore intended to be limited solely by the scope of the appended claims.