Patent Publication Number: US-2021170799-A1

Title: Tire for running on rough terrain

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a Divisional of U.S. application Ser. No. 15/802,810 filed Nov. 3, 2017, which claims which claims the benefit under 35 U.S.C. § 119(a) to Japanese Patent Application No. 2016-229220 filed Nov. 25, 2016, all of which are hereby expressly incorporated by reference into the present application. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a tire for running on rough terrain capable of exerting large traction on muddy roads. 
     BACKGROUND ART 
     For example, Japanese Unexamined Patent Application Publication No. 2016-60347 (Patent Literature 1) has proposed a tire for running on rough terrain provided with axially-long crown blocks and middle blocks in order to obtain large traction. 
     However, in the tire disclosed in Patent Literature 1, mud scraped by crown blocks when they contact the ground easily moves in lateral directions during running on muddy roads, therefore, there is room for further improvement of traction. 
     SUMMARY OF THE INVENTION 
     The present invention was made in view of the above, and a primary object thereof is to provide a tire for running on rough terrain capable of exerting large traction on muddy roads. 
     In one aspect of the present invention, a tire comprises a tread portion bound with an intended tire rotational direction and comprising a row of crown blocks arranged on a tire equator and a pair of rows of middle blocks arranged in a tire circumferential direction on both sides of the row of the crown blocks, wherein each of the crown blocks and the middle blocks has an axially-elongated shape in which a length thereof in a tire axial direction is longer than a length thereof in the tire circumferential direction, a ground contacting surface of each of the crown blocks has a crown front edge positioned on a heel-side in the tire rotational direction, the crown front edge comprises a most concave portion positioned at the most toe-side thereof in the tire rotational direction and a pair of inclined edges extending from the most concave portion toward both sides thereof in the tire axial direction and toward the heel-side in the tire rotational direction, an inclination angle of each of the inclined edges is in a range of from 10 to 45 degrees with respect to the tire axial direction, a ground contacting surface of each of the middle blocks has a middle front edge positioned on the heel-side in the tire rotational direction, and the middle front edge extends axially outwardly and is inclined to the heel-side. 
     In another aspect of the invention, it is preferred that the middle front edge is inclined at an angle in a range of from 10 to 45 degrees with respect to the tire axial direction. 
     In another aspect of the invention, it is preferred that a pair of the crown block and the middle block adjacent to each other in the tire circumferential direction are arranged so that an imaginary straight line obtained by connecting a centroid of the ground contacting surface of the crown block and a centroid of the ground contacting surface of the middle block is inclined at an angle in a range of from 20 to 45 degrees with respect to the tire axial direction. 
     In another aspect of the invention, it is preferred that in a pair of the crown block and the middle block adjacent to each other in the tire circumferential direction, a gap in the tire axial direction between the crown block and the middle block is in a range of from 0.05 to 0.10 times a tread development width, and a gap in the tire circumferential direction between the crown block and the middle block is in a range of from 0.50 to 1.00 times the gap in the tire axial direction. 
     In another aspect of the invention, it is preferred that a width in the tire axial direction of the ground contacting surface of each of the crown blocks is in a range of from 0.25 to 0.35 times the tread development width, and a width in the tire axial direction of the ground contacting surface of each of the middle blocks is in a range of from 0.10 to 0.15 times the tread development width. 
     In another aspect of the invention, it is preferred that the ground contacting surface of each of the middle blocks comprises a substantially rectangular block main body and a convex portion protruding toward the toe-side from the block main body. 
     In another aspect of the invention, it is preferred that the convex portion is provided axially outside a center position in the tire axial direction of the ground contacting surface of the middle block. 
     In another aspect of the invention, it is preferred that the middle front edge is concave toward the toe-side. 
     In another aspect of the invention, it is preferred that the ground contacting surface of each of the middle blocks has a middle rear edge, and the middle rear edge is concave toward the heel-side. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a lateral cross-sectional view of a tire for running on rough terrain according to one embodiment of the present invention. 
         FIG. 2  is a development view showing a tread pattern of a tread portion of  FIG. 1 . 
         FIG. 3  is an enlarged partial view of crown blocks and middle blocks. 
         FIG. 4  is a development view of the tread portion of a tire for running on rough terrain according to second embodiment of the present invention. 
         FIG. 5  is a development view of the tread portion of a tire for running on rough terrain according to third embodiment of the present invention. 
         FIG. 6  is a development view of the tread portion of a tire for running on rough terrain according to reference  1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the present invention will now be described in conjunction with accompanying drawings. 
       FIG. 1  is a lateral cross-sectional view of a tire for running on rough terrain (hereinafter may be simply referred to as “tire”)  1  according to an embodiment of the present invention in a standard state.  FIG. 2  is a development view of a tread portion  2  of the tire  1  showing a tread pattern thereof.  FIG. 1  is a cross-sectional view taken along A-A line of  FIG. 2 . 
     The “standard state” is a state in which the tire  1  is mounted on a standard rim (not shown), inflated to a standard pressure, and loaded with no tire load. In this specification, dimensions and the like of various parts of the tire are values measured in the standard state unless otherwise noted. 
     The “standard rim” is a wheel rim specified for the concerned tire by a standard included in a standardization system on which the tire is based, for example, the “normal wheel rim” in JATMA, “Design Rim” in TRA, and “Measuring Rim” in ETRTO. 
     The “standard pressure” is air pressure specified for the concerned tire by a standard included in a standardization system on which the tire is based, for example, the “maximum air pressure” in JATMA, maximum value listed in the “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” table in TRA, and “INFLATION PRESSURE” in ETRTO. 
     As shown in  FIG. 1 , the tire  1  in this embodiment is a tire for a motorcycle for running on rough terrain, and is used, for example, for motocross competition. Thereby, in the lateral cross-section of the tread portion  2  of the tire  1  in this embodiment, an outer surface thereof is curved in an arc shape protruding outwardly in a tire radial direction. However, the tire of the present invention is not limited to a tire for a motorcycle, but may be used for a three-wheel buggy or a 4WD-car, for example. 
     The tire  1  in this embodiment is provided with, for example, a carcass  6  and a belt layer  7 . Known configurations are appropriately applied to these. 
     As shown in  FIG. 2 , the tread portion  2  has a directional pattern bound with an intended rotational direction (R). The rotational direction (R) is indicated by a letter or a symbol on at least one of the sidewall portions  3  (shown in  FIG. 1 ), for example. 
     The tread portion  2  is divided into a crown region (Cr), a pair of middle regions (Mi), and a pair of shoulder regions (Sh), for example. 
     The crown region (Cr) is a region having a width of ⅓ of a tread development width (TWe) with a tire equator (C) in a center thereof. 
     The shoulder regions (Sh) are regions each having a width of ⅙ of the tread development width (TWe) and extending from each of tread edges (Te) toward the tire equator (C). 
     Each of the middle regions (Mi) is a region between the crown region (Cr) and each of the shoulder regions (Sh). 
     The tread development width (TWe) is a distance in a tire axial direction between tread edges (Te) when the tread portion  2  is developed into a plane. The “tread edges (Te)” means axially outermost edges of blocks arranged axially outermost among the blocks provided in the tread portion  2 . 
     The tread portion  2  is provided with a row of crown blocks  10  arranged along the tire equator (C), a pair of rows of middle blocks  11  arranged in a tire circumferential direction on both sides in the tire axial direction of the row of the crown blocks  10 , and a pair of rows of shoulder blocks  12  each arranged in the tire circumferential direction on an axially outer side of the corresponding row of the middle blocks  11 . 
     In each of the crown blocks  10 , for example, the entire block is provided in the crown region (Cr) and at least a part thereof extends across the tire equator (C). In each of the crown blocks  10 , for example, a center portion thereof in the tire axial direction is located on the tire equator (C). As a preferred embodiment, each of the crown blocks  10  in this embodiment has a ground contacting surface that is line symmetrical with respect to the tire equator (C) with a centroid ( 10   c ) of the ground contacting surface located on the tire equator (C). 
     In each of the middle blocks  11 , for example, a centroid ( 11   c ) of a ground contacting surface thereof is located within the middle regions (Mi). A part of each of the middle blocks  11  in this embodiment extends across a boundary (not shown) between one of the middle regions (Mi) and its adjacent one of the shoulder regions (Sh). The centroids ( 11   c ) of the ground contacting surfaces of the middle blocks  11  in this embodiment are provided, for example, in regions axially outside the center positions in the tire axial direction of the middle regions (Mi). More specifically, a distance L 1  in the tire axial direction (that is, a distance along the outer surface of the tread portion  2 , the same applies below) between the centroid ( 11   c ) of the ground contacting surface of each of the middle blocks  11  and the tire equator (C) is, for example, in a range of from 0.25 to 0.35 times the tread development width (TWe). 
     In each of the shoulder blocks  12 , for example, a centroid ( 12   c ) of a ground contacting surface thereof is located within the shoulder regions (Sh). The centroids ( 12   c ) of the ground contacting surfaces of the shoulder blocks  12  in this embodiment are provided, for example, in regions axially outside the center positions in the tire axial direction of the shoulder regions (Sh). More specifically, a distance L 2  in the tire axial direction between the centroid ( 12   c ) of the ground contacting surface of each of the shoulder blocks  12  and the tire equator (C) is, for example, in a range of from 0.40 to 0.48 times the tread development width (TWe). 
     Each of the crown blocks  10  and the middle blocks  11  has an axially-elongated shape in which a length thereof in the tire axial direction is larger than a length thereof in the tire circumferential direction. The crown blocks  10  and the middle blocks  11  configured as such can exert large traction upon shearing mud on a road surface during running on a muddy road. 
     It is preferred that a ratio W 2 /W 1  of a length W 2  in the tire circumferential direction and a width W 1  in the tire axial direction of the ground contacting surface of each of the crown blocks  10  is in a range of from 0.25 to 0.45, for example. 
     Further, it is preferred that the width W 1  in the tire axial direction of the ground contacting surface of each of the crown blocks  10  is in a range of from 0.25 to 0.35 times the tread development width (TWe), for example. 
     The crown blocks  10  configured as such have sufficient rigidity in the tire circumferential direction, therefore, it is possible that large traction is exerted under various road surface conditions. 
     It is preferred that a ratio W 4 /W 3  of a length W 4  in the tire circumferential direction and a width W 3  in the tire axial direction of the ground contacting surface of each of the middle blocks  11  is in a range of from 0.50 to 0.90, for example. 
     Further, it is preferred that the width W 3  in the tire axial direction of the ground contacting surface of each of the middle blocks  11  is smaller than ½ of the width W 1  of the crown block  10 , for example. More specifically, it is preferred that the W 3  is in a range of from 0.10 to 0.15 times the tread development width (TWe). 
       FIG. 3  shows an enlarged partial view of the crown blocks  10  and the middle blocks  11 . As shown in  FIG. 3 , a ground contacting surface ( 10   s ) of each of the crown blocks  10  comprises, for example, a first surface  13  disposed on one side of a center position of the ground contacting surface ( 10   s ) in the tire axial direction and a second surface  14  disposed on the other side of the center position and connected with the first surface  13 . Each of the first surface  13  and the second surface  14  is formed in an axially-elongated shape in which a width thereof in the tire circumferential direction is constant in a longitudinal direction of the ground contacting surface in this embodiment. Further, each of the first surface  13  and the second surface  14  is, for example, formed in a rectangular shape, and formed in a parallelogram shape in this embodiment. As a more preferred embodiment, the first surface  13  and the second surface  14  are configured in line symmetry. However, the ground contacting surfaces of the crown blocks  10  are not limited to such a configuration. 
     The ground contacting surface of each of the crown blocks  10  includes a crown front edge  15  positioned on a heel-side in the rotational direction (R) (hereinafter may be simply referred to as “heel-side”) and a crown rear edge  16  positioned on a toe-side in the rotational direction (R) (hereinafter, may be simply referred to as “toe-side”), and a pair of crown side edges  17  extending therebetween. 
     The crown front edge  15  is concave toward the toe-side and includes a most concave portion  18  positioned at the most toe-side. The most concave portion  18  in this embodiment is provided, for example, at the center position in the tire axial direction of the block. However, the crown front edge is not limited to such a configuration, and the most concave portion  18  may be provided, for example, at a position different from the center position. 
     The crown front edge  15  includes a pair of inclined edges  20  each extending from the most concave portion  18  positioned at the most toe-side toward both sides of the most concave portion  18  in the tire axial direction and toward the heel-side. The inclined edges  20  include an inclined edge ( 20   a ) on one side and an inclined edge ( 20   b ) on the other side in the tire axial direction, and they are inclined in opposite directions to each other. 
     Each of the inclined edges ( 20   a ) and ( 20   b ) in this embodiment extends straight from the most concave portion  18  to respective ends in the tire axial direction of the crown front edge  15 , for example. That is, the entire crown front edge  15  is composed of a pair of the inclined edges  20 . Thereby, the pair of inclined edges  20  is configured, for example, in a V-shape that is concave toward the toe-side. 
     The inclined edge ( 20   a ) on one side and the inclined edge ( 20   b ) on the other side in this embodiment are formed line-symmetrically. However, they are not limited to such a configuration and the inclined edge ( 20   a ) and the inclined edge ( 20   b ) may have shapes and lengths different from each other. 
     With the inclined edges  20  configured as such, each of the crown blocks  10  has block walls extending radially inwardly from each of the inclined edges  20  on the heel-side. The block walls disposed on the heel-side include two surfaces inclined in opposite directions to each other on both sides of the most concave portion  18 , and are concave toward the toe-side. 
     An angle θ 1  of each of the inclined edges ( 20   a ) and ( 20   b ) with respect to the tire axial direction is in a range of from 10 to 45 degrees. The crown blocks  10  configured as such can scrape mud toward the center of the block and then can shear it by the inclined edges  20  during running on rough terrain. Thereby, mud does not escape in the left and right sides of the inclined edges, therefore, larger traction can be expected. 
     The angle θ 1  is preferably not larger than 30 degrees, more preferably not larger than 20 degrees. Thereby, it is possible that large traction is obtained even on a relatively hard road surface. 
     The crown rear edge  16  extends along the crown front edge  15 , for example. Thereby, the crown rear edge  16  is configured in a V-shape convex toward the toe-side. As a more preferred embodiment, the crown rear edge  16  and the crown front edge  15  extend in parallel with each other. Thereby, each of the crown blocks  10  extends in the tire axial direction with a constant width, therefore, partial damage of the blocks during running is suppressed. 
     The crown side edges  17  are disposed on both sides in the tire axial direction of the crown block  10  and extend in the tire circumferential direction between the crown front edge  15  and the crown rear edge  16 . The crown side edges  17  in this embodiment extend straight in the tire circumferential direction. The crown side edges  17  configured as such is helpful for shearing mud in the tire axial direction, therefore, it is possible that cornering performance on rough terrain is improved. 
     For example, a plurality of the middle blocks  11  is provided in the tire circumferential direction at same pitches as the crown blocks  10 . 
     The ground contacting surface of each of the middle blocks  11  is, for example, formed in a rectangular shape, and in this embodiment, a width thereof in the tire circumferential direction is constant in a lateral direction of the block. As a preferred embodiment, the ground contacting surface of each of the middle blocks  11  in this embodiment is configured in a parallelogram shape. 
     The ground contacting surface of each of the middle blocks  11  includes a middle front edge  21  positioned on the heel-side, a middle rear edge  22  positioned on the toe-side, and a pair of middle side edges  23  extending between them. 
     The middle front edge  21  extends axially outwardly and is inclined to the heel-side. Thereby, each of the middle blocks  11  has a block wall extending radially inwardly from the middle front edge  21  on the heel-side. The block wall on the heel-side is inclined in the same direction as the middle front edge  21 . 
     The middle blocks  11  configured as such guide mud and the like to sides of the crown blocks  10  by the middle front edges  21  during running on muddy roads. 
     Thereby, it is possible that the crown blocks  10  shear more mud, therefore, larger traction is obtained. 
     The middle front edge  21  in this embodiment extends straight in its entirety, for example. Thereby, it is possible that more mud is guided toward the crown blocks  10 . 
     It is preferred that an angle θ 2  of the middle front edge  21  with respect to the tire axial direction is in a range of from 10 to 45 degrees, for example. In order to exert large traction under various road surface conditions, the angle θ 2  is preferably not greater than 30 degrees, more preferably not greater than 20 degrees. 
     The middle rear edge  22  is inclined to the same direction as the middle front edge  21 , for example. It is preferred that the middle rear edge  22  extends along the middle front edge  21 . As a more preferred embodiment, the middle rear edge  22  in its entirety in this embodiment extends in parallel with the middle front edge  21 . 
     The middle side edges  23  are arranged on both sides in the tire axial direction of each of the middle blocks  11 . 
     Each of the middle side edges  23  extends in the tire circumferential direction between the middle front edge  21  and the middle rear edge  22 , and extends straight along the tire circumferential direction in this embodiment. 
     The middle side edges  23  configured as such are helpful for shearing mud in the tire axial direction, therefore, it is possible that the cornering performance on rough terrain is improved. 
     In a pair of the crown block  10  and the middle block  11  adjacent in the tire circumferential direction, an imaginary straight line  25  obtained by connecting the centroid ( 10   c ) of the ground contacting surface of the crown block  10  and the centroid ( 11   c ) of the ground contacting surface of the middle block  11  is inclined in the same direction as the middle front edge  21 . It is preferred that the pair of the crown block  10  and the middle block  11  is arranged so that the imaginary straight line  25  is inclined at an angle θ 3  in a range of from 20 to 45 degrees with respect to the tire axial direction. By such arrangement of the blocks, it is possible that a lot of mud is guided toward the crown blocks  10 , therefore, excellent traction performance is obtained. 
     In order to exert large traction under various road surface conditions, the angle θ 3  is preferably not more than 40 degrees, more preferably not more than 30 degrees. 
     As shown in  FIG. 2 , the crown block  10  and the middle block  11  are arranged so as to have a gap L 3  in tire axial direction between the ground contacting surfaces thereof, for example. In other words, an area obtained by projecting the ground contacting surface of the crown block  10  in the tire circumferential direction does not overlap the ground contacting surface of the middle block  11 . It is preferred that the gap L 3  is in a range of from 0.05 to 0.10 times the tread development width (TWe), for example. By such arrangement of the crown blocks  10  and the middle blocks  11 , it is possible that clogging of mud between the blocks is suppressed. 
     The centroids of the ground contacting surfaces of the crown block  10  and the middle block  11  in this embodiment are misaligned in the tire circumferential direction, for example. 
     As a preferred embodiment, the crown block  10  and the middle block  11  in this embodiment are misaligned in their entirety in the tire circumferential direction. 
     That is, the crown block  10  and the middle block  11  are arranged so as to have a gap L 4  in the tire circumferential direction between the ground contacting surfaces thereof, and an area obtained by projecting the ground contacting surface of the crown block  10  along the tire axial direction does not overlap the ground contacting surface of the middle block  11 . 
     The gap L 4  is preferably not smaller than 0.50 times the above described gap L 3  in the tire axial direction, more preferably not smaller than 0.65 times the gap L 3 , and preferably not more than 1.00 times the gap L 3 , more preferably not more than 0.85 times the gap L 3 . By such arrangement of the crown blocks  10  and the middle blocks  11 , it is possible that the middle blocks  11  guide a lot of mud toward the crown blocks  10 , therefore, it is possible that clogging of mud is suppressed. 
     For example, a plurality of the shoulder blocks  12  is provided in the tire circumferential direction at the same pitches as the middle blocks  11 . 
     The ground contacting surface of each of the shoulder blocks  12  is formed in a rectangular shape, for example, and in this embodiment, a length thereof in the tire circumferential direction is constant in the lateral direction of the block. As a preferred embodiment, the ground contacting surface of each of the shoulder blocks  12  in this embodiment is configured in a parallelogram shape. 
     It is preferred that a ratio W 6 /W 5  of a length W 6  in the tire circumferential direction and a length W 5  in the tire axial direction of the ground contacting surface of each of the shoulder blocks  12  is in a range of from 0.90 to 1.30, for example. Further, it is preferred that the length W 5  in the tire axial direction of the ground contacting surface of the shoulder block  12  is smaller than the width W 3  in the tire axial direction of the ground contacting surface of the middle block  11 , for example, and specifically, it is preferred that the length W 5  is in a range of from 0.05 to 0.10 times the tread development width (TWe). 
     The ground contacting surface of each of the shoulder blocks  12  has a shoulder front edge  27  located on the heel-side thereof. The shoulder front edge  27  is inclined in the same direction as the middle front edge  21 , for example. It is preferred that the shoulder front edge  27  is inclined at the same angle as the middle front edge  21  with respect to the tire axial direction. As a further preferred embodiment, the shoulder front edge  27  in this embodiment is provided at a position where the middle front edge  21  is extended in its longitudinal direction. Thereby, it is possible that the shoulder blocks  12  and the middle blocks  11  shear mud together as one body, therefore, excellent traction performance is obtained. 
     The middle block  11  and the shoulder block  12  are arranged so as to have a gap L 5  in tire axial direction between the ground contacting surfaces thereof, for example. In other words, an area obtained by projecting the ground contacting surface of the middle block  11  in the tire circumferential direction does not overlap the ground contacting surface of the shoulder block  12 . It is preferred that the gap L 5  is in a range of from 0.05 to 0.10 times the tread development width (TWe), for example. By such arrangement of the middle blocks  11  and the shoulder blocks  12 , it is possible that clogging of mud is suppressed while exerting the above described effects. 
     A projection area in the tire axial direction of the ground contacting surface of the middle block  11  in this embodiment overlaps the ground contacting surface of the shoulder block  12 . On the other hand, the centroid ( 11   c ) of the ground contacting surface of the middle block  11  and the centroid ( 12   c ) of the ground contacting surface of the shoulder block  12  are misaligned in the tire circumferential direction. Specifically, the centroid ( 12   c ) of the shoulder block  12  is positioned on the heel-side of the centroid ( 11   c ) of the middle block  11 . Thereby, the shoulder blocks  12  guide mud toward the middle blocks  11 , therefore, it is possible that the traction performance during cornering is improved. 
       FIG. 4  shows a development view of the tread portion  2  according to second embodiment of the present invention and  FIG. 5  shows a development view of the tread portion  2  according to third embodiment of the present invention. The embodiments shown in  FIGS. 4 and 5  have the same configuration as the above-described first embodiment except for the differences described below. In  FIGS. 4 and 5 , the same reference numbers are given to the configurations common to the above-described first embodiment. 
     As shown in  FIG. 4 , the crown front edge  15  in this embodiment is formed in an arc shape concave toward the toe-side. Similarly, the crown rear edge  16  is formed in an arc shape convex toward the toe side. As a preferred embodiment, the crown front edge  15  and the crown rear edge  16  extend in parallel with each other. Each of the crown blocks  10  configured as such is deformed in its entirety when shearing mud, therefore, it is possible that partial damage of the block is suppressed. 
     The ground contacting surface of each of the middle blocks  11  in this embodiment includes a substantially rectangular block main body  28  and a convex portion  29  protruding toward the toe-side from the block main body  28 . In the middle block  11  configured as such, deformation in the tire circumferential direction of the block is suppressed by the convex portions  29 , therefore, it is possible that large reaction force is obtained when the block shears mud. 
     It is preferred that the convex portion  29  is provided axially outside the center position in the tire axial direction of the ground contacting surface of the middle block  11 . More preferably, the convex portion  29  is provided at an outer end in the tire axial direction of the middle block  11 , and a part of the convex portion  29  constitutes a part of the middle side edges  23 . Thereby, axially inner part of each of the middle blocks  11  is relatively easy to deform, therefore, it is possible that the middle blocks  11  guide mud toward the crown blocks  10  more effectively. 
     As shown in  FIG. 5 , the ground contacting surface of each of the crown blocks  10  in this embodiment includes an axially-elongated block main body  31  and a convex portion  32  protruding toward the toe-side from the block main body  31 . The convex portion  32  is provided, for example, at the center position in the tire axial direction of the crown block  10 . The convex portions  32  configured as such are helpful for suppressing deformation in the tire circumferential direction of the crown blocks  10 . 
     It is preferred that the middle front edge  21  in this embodiment is concave toward the toe-side. Thereby, it is possible that the middle blocks  11  provide larger reaction force in the tire circumferential direction when shearing mud. 
     Further, it is preferred that the middle rear edge  22  in this embodiment is concave toward the heel-side. Thereby, mud positioned on the toe-side of the middle block  11  is easy to move in the tire axial direction, therefore, it is possible that the motorcycle is easy to lean when running on muddy roads. 
     In the embodiments of  FIGS. 4 and 5 , it is preferred that a length W 7  in the tire circumferential direction of an outer end in the tire axial direction of each of the middle blocks  11  is larger than a length W 8  in the tire circumferential direction of an inner end in the tire axial direction of each of the middle blocks  11 . 
     Thereby, axially inner part of each of the middle blocks  11  is easy to deform, therefore, it is possible that mud is surely guided to the crown blocks  10 . 
     While detailed description has been made of the tire for running on rough terrain as embodiments of the present invention, the present invention can be embodied in various forms without being limited to the illustrated embodiments. 
     Working Example (Example) 
     Tires for running on rough terrain, to be mounted on a rear wheel of a test vehicle, having the basic structure shown in  FIG. 2, 4 , or  5  were made by way of test according to the specifications listed in Table 1. As reference  1 , a tire having the crown front edges extending straight in the tire axial direction as shown in  FIG. 6  is made by way of test. Then each of the test tires was tested for the traction performance. Common specifications and test methods of the test tires are as follows. 
     Test vehicle: motorcycle for motocross competition with displacement of 450 cc 
     Tire size: 110/90-19 
     Rim size: 2.15×19 
     Tire pressure: 80 kPa 
     Test methods are as follows. 
     &lt;Traction Performance&gt; 
     A test rider drove the above-mentioned test vehicle on a soft road surface fully covered with mud containing a lot of water, a medium road surface including mixture of a road surface covered with the above mud and dry rough terrain, and a sandy road surface, and the test driver evaluated the traction performance of each of the test tires by the driver&#39;s feeling during the test drive. The results are indicated by an evaluation point based on Reference  1  being 100, wherein the larger the numerical value, the better the traction performance. 
     Test results are shown in Table 1. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                   
                 Ref. 1 
                 Ref. 2 
                 Ref. 3 
                 Ex. 1 
                 Ex. 2 
                 Ex. 3 
                 Ex. 4 
                 Ex. 5 
               
               
                   
               
               
                 Figure showing Tread pattern 
                 FIG. 6 
                 FIG. 2 
                 FIG. 2 
                 FIG. 2 
                 FIG. 2 
                 FIG. 2 
                 FIG. 2 
                 FIG. 2 
               
               
                 Angle θ1 of Inclined edge of Crown 
                 0 
                 5 
                 50 
                 10 
                 15 
                 20 
                 30 
                 45 
               
               
                 block [degrees] 
               
               
                 Angle θ2 of Middle front edge 
                 15 
                 15 
                 15 
                 15 
                 15 
                 15 
                 15 
                 15 
               
               
                 [degrees] 
               
               
                 Angle θ3 of Imaginary straight line 
                 25 
                 25 
                 25 
                 25 
                 25 
                 25 
                 25 
                 25 
               
               
                 connecting between Crown block and 
               
               
                 Middle block 
               
               
                 Axial Gap L3 between Crown block and 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
               
               
                 Middle block/Tread development width 
               
               
                 (Twe) 
               
               
                 Circumferential Gap L4 between Crown 
                 0.70 
                 0.70 
                 0.70 
                 0.70 
                 0.70 
                 0.70 
                 0.70 
                 0.70 
               
               
                 block and Middle block/Axial Gap L3 
               
               
                 Traction performance on soft road 
                 100 
                 102 
                 101 
                 108 
                 109 
                 108 
                 106 
                 105 
               
               
                 surface [evaluation point] 
               
               
                 Traction performance on medium road 
                 100 
                 103 
                 103 
                 106 
                 108 
                 108 
                 107 
                 106 
               
               
                 surface [evaluation point] 
               
               
                 Traction performance on sandy road 
                 100 
                 103 
                 102 
                 106 
                 108 
                 107 
                 106 
                 104 
               
               
                 surface [evaluation point] 
               
               
                   
               
               
                   
                 Ex. 6 
                 Ex. 7 
                 Ex. 8 
                 Ex. 9 
                 Ex. 10 
                 Ex. 11 
                 Ex. 12 
                 Ex. 13 
               
               
                   
               
               
                 Figure showing Tread pattern 
                 FIG. 2 
                 FIG. 2 
                 FIG. 2 
                 FIG. 2 
                 FIG. 2 
                 FIG. 2 
                 FIG. 2 
                 FIG. 2 
               
               
                 Angle θ1 of Inclined edge of Crown 
                 15 
                 15 
                 15 
                 15 
                 15 
                 15 
                 15 
                 15 
               
               
                 block [degrees] 
               
               
                 Angle θ2 of Middle front edge 
                 10 
                 20 
                 30 
                 45 
                 15 
                 15 
                 15 
                 15 
               
               
                 [degrees] 
               
               
                 Angle θ3 of Imaginary straight line 
                 25 
                 25 
                 25 
                 25 
                 20 
                 30 
                 40 
                 45 
               
               
                 connecting between Crown block and 
               
               
                 Middle block 
               
               
                 Axial Gap L3 between Crown block and 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
               
               
                 Middle block/Tread development width 
               
               
                 (Twe) 
               
               
                 Circumferential Gap L4 between Crown 
                 0.70 
                 0.70 
                 0.70 
                 0.70 
                 0.70 
                 0.70 
                 0.70 
                 0.70 
               
               
                 block and Middle block/Axial Gap L3 
               
               
                 Traction performance on soft road 
                 108 
                 109 
                 107 
                 105 
                 108 
                 108 
                 107 
                 106 
               
               
                 surface [evaluation point] 
               
               
                 Traction performance on medium road 
                 109 
                 107 
                 106 
                 104 
                 107 
                 108 
                 106 
                 105 
               
               
                 surface [evaluation point] 
               
               
                 Traction performance on sandy road 
                 107 
                 108 
                 107 
                 106 
                 106 
                 107 
                 105 
                 104 
               
               
                 surface [evaluation point] 
               
               
                   
               
               
                   
                 Ex. 14 
                 Ex. 15 
                 Ex. 16 
                 Ex. 17 
                 Ex. 18 
                 Ex. 19 
                 Ex. 20 
                 Ex. 21 
               
               
                   
               
               
                 Figure showing Tread pattern 
                 FIG. 2 
                 FIG. 2 
                 FIG. 2 
                 FIG. 2 
                 FIG. 4 
                 FIG. 4 
                 FIG. 5 
                 FIG. 5 
               
               
                 Angle θ1 of Inclined edge of Crown 
                 15 
                 15 
                 15 
                 15 
                 10 
                 45 
                 10 
                 45 
               
               
                 block [degrees] 
               
               
                 Angle θ2 of Middle front edge 
                 15 
                 15 
                 15 
                 15 
                 15 
                 15 
                 15 
                 15 
               
               
                 [degrees] 
               
               
                 Angle θ3 of Imaginary straight line 
                 25 
                 25 
                 25 
                 25 
                 25 
                 25 
                 25 
                 25 
               
               
                 connecting between Crown block and 
               
               
                 Middle block 
               
               
                 Axial Gap L3 between Crown block and 
                 0.05 
                 0.10 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
                 0.07 
               
               
                 Middle block/Tread development width 
               
               
                 (Twe) 
               
               
                 Circumferential Gap L4 between Crown 
                 0.70 
                 0.70 
                 0.50 
                 1.00 
                 0.70 
                 0.70 
                 0.70 
                 0.70 
               
               
                 block and Middle block/Axial Gap L3 
               
               
                 Traction performance on soft road 
                 109 
                 108 
                 107 
                 108 
                 108 
                 105 
                 109 
                 106 
               
               
                 surface [evaluation point] 
               
               
                 Traction performance on medium road 
                 106 
                 108 
                 108 
                 107 
                 107 
                 107 
                 106 
                 105 
               
               
                 surface [evaluation point] 
               
               
                 Traction performance on sandy road 
                 109 
                 107 
                 107 
                 106 
                 105 
                 104 
                 106 
                 104 
               
               
                 surface [evaluation point] 
               
               
                   
               
            
           
         
       
     
     As is clear from Table 1, it was confirmed that the tires as the Examples exerted excellent traction performance on each of the road surfaces.