Abstract:
Provided is a motorcycle tire ( 1 ) for traveling on rough terrain, the tire having a block ( 10 ) provided to a tread section ( 2 ). The block ( 10 ) has the following: a block lateral surface body part ( 15 ) that extends straight from the edge of a tread surface ( 12 ) inward in the tire radial direction; and a skirt section ( 20 ) that connects the block lateral surface body part ( 15 ) and a groove bottom surface ( 9 ). The skirt section ( 20 ) has the following: a flat section ( 21 ) that extends in a straight line on the groove bottom surface ( 9 ) side; and a curved section ( 22 ) that connects, in a beveled arc, the area between the flat section ( 21 ) and the block lateral surface body part.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a motorcycle tire for running on rough terrain capable of improving durability of blocks. 
       BACKGROUND ART 
       [0002]    For instance, motorcycle tires for running on rough terrain for using motocross or the like include a tread portion provided with a plurality of blocks. When these tires travel on off-road such as sandy, soil or muddy road, the blocks can bite the road to maintain grip performance. 
         [0003]    In general, the blocks are molded using a recess of a mold through a vulcanization process. In the vulcanization process, tread rubber around blocks flows significantly. Such a significant rubber flow may cause a problem that a tread rubber thickness tends to be thin locally around the blocks and deteriorates durability of the blocks. 
         [0004]    Furthermore, when traveling, a large stress and bending moment acts on the base portion of the respective blocks. Thus, there was a problem that damage tends to concentrate the base portion of the blocks. 
         [0005]    In order to improve durability of tread blocks, the following patent document 1, for instance, has proposed a motorcycle tire for running on rough terrain having an improved shape of a base portion of a block. 
       CITATION LIST 
     Patent Literature 
       [0006]    Patent document 1: Japanese Unexamined Patent Application Publication No. 2004-351956 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    Unfortunately, there has been room for further improvement of durability of blocks even on the motorcycle for running on rough terrain tire in accordance with the above mentioned patent document 1. 
         [0008]    The present invention has been made in view of circumstances described above, and has a main object to provide a motorcycle tire for running on rough terrain capable of improving durability of blocks. 
       Solution to Problem 
       [0009]    The present invention provide a motorcycle tire for running on rough terrain including a tread portion provided with a plurality of blocks protruding from a virtual block bottom which extends along a groove bottom, in a block cross section passing a center of gravity of a ground contact surface of the blocks and being perpendicular to the virtual block bottom, the blocks including a block-sidewall main portion extending radially inwardly in a straight shape from an edge of the ground contact surface and a skirt portion that connects the block-sidewall main portion to the groove bottom, and the skirt portion including a planar portion extending in a straight shape on the side of the groove bottom and a curved portion so as to connect the block-sidewall main portion to the planar portion in a chamfered manner. 
         [0010]    In the motorcycle tire for running on rough terrain according to the present invention, a skirt length (L 1 ) measured along the virtual groove bottom from an intersection (P 1 ) between the virtual block bottom and a virtual extension of the block-sidewall main portion to an intersection (P 2 ) between the groove bottom and the skirt portion is preferably in a range of from 0.19 to 1.00 times a block height (Hb) from the virtual block bottom to the ground contact surface. 
         [0011]    In the motorcycle tire for running on rough terrain according to the present invention, a skirt height (Hs) from the virtual block bottom to an intersection (P 3 ) between a virtual extension of the planar portion and a virtual extension of the block-sidewall main portion is preferably in a range of from 0.02 to 0.13 times a block height (Hb) from the virtual block bottom to the ground contact surface. 
         [0012]    In the motorcycle tire for running on rough terrain according to the present invention, a length of the planar portion along the virtual block bottom is preferably greater than a length of the curved portion along the virtual block bottom. 
         [0013]    In the motorcycle tire for running on rough terrain according to the present invention, the planar portion is preferably inclined at an angle of from 5 to 15 degrees with respect to the virtual block bottom. 
         [0014]    In the motorcycle tire for running on rough terrain according to the present invention, the planar portion preferably has an angle of less than 5 degrees with respect to the virtual block bottom. 
       Advantageous Effects of Invention 
       [0015]    The motorcycle tire for running on rough terrain in accordance with the present invention includes the tread portion provided with a plurality of blocks protruding from the virtual block bottom which extends along the groove bottom. In a block cross section passing the center of gravity of the ground contact surface of the blocks and being perpendicular to the virtual block bottom, the blocks include the block-sidewall main portion extending radially inwardly in a straight shape from the edge of the ground contact surface and the skirt portion that connects the block-sidewall main portion to the groove bottom. Furthermore, the skirt portion includes the planar portion extending in a straight shape on the side of the groove bottom and the curved portion so as to connect the block-sidewall main portion to the planar portion in a chamfered manner. 
         [0016]    The blocks having the skirt portion may effectively suppress crack locally generated on the base portion of the blocks by bending the curved portion and the planar portion when traveling on rough terrain. 
         [0017]    Since the blocks in accordance with the present invention include the skirt portion, the virtual block bottom is greater than the conventional one. Accordingly, a recess of a vulcanization mold for molding each block has an opening greater than the conventional one. Thus, when each block is molded through the vulcanization process, rubber gathered from a wide area flows into the recess. Therefore, the blocks in accordance with the present invention may suppress a significant local rubber flow, and then may prevent the problem that a tread rubber thickness around the blocks tends to be thin locally. 
         [0018]    As described above, the motorcycle tire for running on rough terrain in accordance with the present invention may improve durability of blocks. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is a development view of a tread portion of a pneumatic tire in accordance with an embodiment of the invention. 
           [0020]      FIG. 2  is a cross-sectional view taken along lines A-A of  FIG. 1 . 
           [0021]      FIG. 3  is an enlarged perspective view of a block illustrated in  FIG. 1 . 
           [0022]      FIG. 4  is a cross-sectional view taken along lines B-B of the block illustrated in  FIG. 3 . 
           [0023]      FIG. 5A  is a cross-sectional view of the block during vulcanization process. 
           [0024]      FIG. 5B  is a cross-sectional view of the block after vulcanization process. 
           [0025]      FIG. 6  is a cross-sectional view of the block in accordance with another embodiment. 
           [0026]      FIG. 7  is a cross-sectional view taken along lines C-C of  FIG. 6 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0027]    An embodiment of the present invention will be explained below with reference to the accompanying drawings.  FIG. 1  illustrates a tread portion  2  of a motorcycle tire for running on rough terrain (hereinafter, simply referred to as “tire”)  1  in accordance with an embodiment of the present invention. The tire  1  according to the embodiment, for instance, is illustrated as a tire for motocross race. 
         [0028]      FIG. 2  illustrates a cross-sectional view taken along lines A-A of the tread portion of  FIG. 1 .  FIG. 2  is a cross-sectional view of the tire  1  under a standard state. The standard state is such that the tire is mounted on a standard wheel rim (not shown) with a standard pressure, but is loaded with no tire load. In this description, unless otherwise noted, dimensions of respective portions of the tire are values specified in the standard state. 
         [0029]    The standard wheel rim is a wheel rim officially approved or recommended for the tire by standards organizations, wherein the standard wheel rim is the “standard rim” specified in JATMA, the “Measuring Rim” in ETRTO, and the “Design Rim” in TRA or the like, for example. 
         [0030]    The standard pressure is a standard pressure officially approved or recommended for the tire by standards organizations, wherein the standard pressure is the “maximum air pressure” in JATMA, the “Inflation Pressure” in ETRTO, and the maximum pressure given in the “Tire Load Limits at Various Cold Inflation Pressures” table in TRA or the like, for example. 
         [0031]    As illustrated in  FIG. 2 , the tread portion  2  includes an outer surface that is curved so as to protrude radially outwardly. 
         [0032]    The tread portion  2  is provided with a plurality of blocks  10 . Each of the blocks  10  protrudes from a virtual block bottom  11  which extends along a groove bottom  9  forming a part of the outer surface of the tread portion  2 . For traveling on rough terrain, the block height Hb from the virtual block bottom  11  to a ground contact surface  12  of the block  10 , for example, is in a range of from 6 to 22 mm. The axial width Wb of the ground contact surface  12  of the block  10 , for example, is in a range of from 5% to 15% of a tread development width TWe. Note that the block height Hb and the width Wb of the block  10  are not limited to these ranges. 
         [0033]    The tread development width TWe is a length in a width direction of the tire portion between tread edges Te and Te measured along the outer surface of the tread portion  2 . Each tread edge Te means an axially outer edge  10   e  of the block  10  arranged axially outermost of the tread portion  2 . 
         [0034]    As illustrated in  FIG. 1 , the ground contact surface  12  of the block  10  has a substantially square shape. Alternatively, any shapes may be employed for the shape of the ground contact surface  12  of the block  10 . For instance, a polygonal shape, a circular shape, an oval shape and the like may be used as the ground contact surface  12  of the block  10 . 
         [0035]      FIG. 3  illustrates an enlarged perspective view of the block  10 .  FIG. 3  is an enlarged perspective view of a region  13  surrounded by two-dot chain line shown in  FIG. 1 . As illustrated in  FIG. 3 , the block  10  includes a block-sidewall main portion  15  and a skirt portion  20 . In order to help understanding, a boundary between the block-sidewall main portion  15  and the skirt portion  20  is indicated using two-dot chain line in  FIG. 3 . 
         [0036]      FIG. 4  illustrates a cross-sectional view taken along lines B-B of the block  10  illustrated in  FIG. 3 . This B-B cross section is a section that passes the center  12   g  (shown in  FIG. 3 ) of gravity of the ground contact surface  12  of the block  10 , and is perpendicular to the virtual block bottom  11  and an edge  12   e  of the ground contact surface  12 . Although the following description relates to the B-B cross section, the same applies with respect to the section that is perpendicular to the B-B cross section shown in  FIG. 3 . 
         [0037]    As illustrated in  FIG. 4 , the block-sidewall main portion  15  extends axially inwardly in a straight shape from the edge  12   e  of the ground contact surface  12 . The skirt portion  20  connects between the block-sidewall main portion  15  and the groove bottom  9 . 
         [0038]    The skirt portion  20  includes a planar portion  21  and a curved portion  22 . In  FIG. 3 , to help understanding, a boundary between the planar portion  21  and the curved portion  22  is indicated using two-dot chain line. As illustrated in  FIG. 4 , the planar portion  21  extends in a straight manner on the side of the groove bottom  9 . The curved portion  22  is formed as an arc shape (like a fillet) so as to chamfer the corner between the planar portion  21  and the block-sidewall main portion  15 . 
         [0039]    Such a block  10  having the skirt portion  20  may effectively suppress crack locally generated on the base portion of the block  10  by bending the curved portion  22  and the planar portion  21  as a whole, when traveling on rough terrain. Accordingly, damage of the block  10  may be effectively prevented to improve durability of the block  10 . 
         [0040]      FIG. 5  illustrate cross-sectional views of the block  10  in accordance with the present embodiment formed through a vulcanization process.  FIG. 5A  is the cross-sectional view when a tread rubber Tg of a green tire is flowing into a recess H of a vulcanization mold M. The recess H is decompressed by discharging the air through a vent hole Bh of the vulcanization mold M.  FIG. 5B  is a cross-sectional view of the block  10  removed from the mold after the vulcanization process. Note that a carcass ply and a tread reinforcing layer, which are disposed in the tread rubber Tg of the green tire, are not illustrated in  FIGS. 5A and 5B . 
         [0041]    As illustrated in  FIG. 5A , the recess H has a revere shape of the block  10 . Thus, the width A of the opening for forming the virtual block bottom  11  is greater than the conventional opening width A. When the block  10  is molded during the vulcanization process, rubber G gathered from a wide area flows into the recess H. Furthermore, the rubber G may flows into the recess H smoothly along an outer surface of the recess H. 
         [0042]    Therefore, a significant local rubber flow may be prevented, and then the block  10  in accordance with the invention may effectively prevent that the thickness t 1  of the tread rubber Tg at around the base portion of the block  10  tends to be thin locally, as shown in  FIG. 5B . Accordingly, the thickness of the tread rubber Tg tends to be uniform, thereby improving durability of the block  10 . 
         [0043]    In order to further improve the effects, as shown in  FIG. 4 , the skirt portion  20  preferably has a skirt length L 1  in a range of not less than 0.19 times, more preferably not less than 0.38 times, still further preferably not less than 0.50 times, but preferably not more than 1.00 times, more preferably not more than 0.75 times, still further preferably not more than 0.63 times the block height Hb. Such a skirt portion  20  may offer a smooth flow of the rubber G into the recess H to have an uniform thickness of the tread rubber Tg while ensuring a sufficient biting amount of the block  10  into a muddy road. The skirt length L 1  is a length measured along the virtual groove bottom  11  from an intersection P 1  between the virtual block bottom  11  and a virtual extension  15   i  of the block-sidewall main portion  15  to an intersection P 2  between the groove bottom  9  and the skirt portion  20 . 
         [0044]    In the same point of view, the skirt height Hs of the skirt portion  20  is preferably in a range of not less than 0.02 times, more preferably not less than 0.05 times, but preferably not more than 0.13 times, but preferably not more than 0.08 times the block height Hb. The skirt height Hs is a height from the virtual block bottom  11  to an intersection P 3  between a virtual extension  21   i  of the planar portion  21  and the virtual extension  15   i  of the block-sidewall main portion  15 . 
         [0045]    The angle θ 1  of the planar portion  21  with respect to the virtual block bottom  11  is preferably in a range of not less than 5 degrees, more preferably not less than 8 degrees, but preferably not more than 15 degrees, more preferably not more than 12 degrees. Such a planar portion  21  may improve durability of the block  10  while ensuring a sufficient biting amount of the block  10  into a muddy road. 
         [0046]    Preferably, the length L 2  of the planar portion  21  along the virtual block bottom  11  is greater than the length L 3  of the curved portion  22  along the virtual block bottom  11 . Such a planar portion  21  may offer a smooth flow of rubber G during vulcanization process. The ratio L 3 /L 2  of the length L 3  of the curved portion  22  to the length L 2  of the planar portion  21  is preferably in a range of not less than 0.25, more preferably not less than 0.30, but preferably not more than 0.45, more preferably not more than 0.40. If the ratio L 3 /L 2  is less than 0.25, flow resistance of rubber G into the recess during vulcanization tends to increase. On the other hand, if the ratio L 3 /L 2  is more than 0.45, grip performance tends to deteriorate due to lowering of biting amount of the block  10  into muddy road. 
         [0047]    The radius of curvature r 1  of the curved portion  22  is preferably in a range of not less than 0.25 times, more preferably not less than 0.30 times, but preferably not more than 0.45 times, but preferably not more than 0.40 times the block height Hb. Such a curved portion  22  may reduce flow resistance of rubber G into the recess while ensuring a sufficient biting amount of the block into muddy road. 
         [0048]    As for a motocross tire, the land ratio Lr is preferably in a range of not less than 5%, more preferably not less than 15%, but preferably not more than 35%, more preferably not more than 25%. When the land ratio Lr is less than 5%, bending moment as well as shearing force acting on one block  10  tends to be high, and there is a possibility to be generated a crack on the base portion of the block  10 . On the other hand, when the land ratio is more than 35%, grip performance tends to deteriorate due to lowering of biting amount of the block  10  into muddy road. As used herein, the land ratio Lr is a ratio Sb/St of a total area Sb of the ground-contact surfaces  12  of the blocks  10  to the total area St of the outer surface of the tread portion when the all grooves are perfectly filled up virtually. 
         [0049]      FIG. 6  is a cross-sectional view of the block  10  in accordance with another embodiment. The block  10  illustrated in  FIG. 6  includes the planar portion  21  extending at an angle of less than 5 degrees with respect to the virtual block bottom  11 , and a step portion  25  connecting between the planar portion  21  and the groove bottom  9 . Such a block  10  may offer a sufficient rubber volume at the skirt portion  20  due to the planar portion  21 . Thus, crack locally generated on the base portion of the block  10  may effectively be prevented. 
         [0050]      FIG. 7  illustrates a cross-sectional view of the block  10  taken along lines C-C of  FIG. 6 . As illustrates in  FIG. 7 , the thickness H 2  of the planar portion  21  according to the embodiment is preferably in a range of from not less than 0.05 times, more preferably not less than 0.08 times, but preferably in a range of not more than 0.15 times, more preferably not more than 0.12 times the block height Hb. The planar portion  21  may effectively improve durability of the block  10  by offering an uniform thickness of the groove bottom. 
         [0051]    The step portion  25  is formed as an arc shape to connect between the groove bottom  9  to the planar portion  21  in a chamfered manner. The step portion  25  can effectively suppress the damage at the outer edge  21   e  of the planar portion  21 . 
         [0052]    While the embodiments in accordance with the present invention have been described in detail, the present invention is not limited to the illustrated embodiments, but can be modified and carried out in various aspects. 
       Example 
       [0053]    Tires having a basic tread pattern illustrated in  FIG. 1  and a block shape illustrated in  FIG. 3  or  FIG. 6  were manufactured based on details shown in Table 1. For the comparative example, a tire having the basic tread pattern illustrated in  FIG. 1  and a block without having the skirt portion was manufactured. These tires were mounted on the following rim with the inner pressure, and then were tested with respect to block durability and thickness uniformity of the tread rubber. The common specifications of tires and test procedures are as follows. 
         [0054]    Tire size: 120/80-19 
         [0055]    Rim size: 2.15×19 
         [0056]    Internal pressure: 80 kPa 
         [0057]    Block Durability: 
         [0058]    Each test tire was made to run on a drum having an outer diameter of 1.7 m with a vertical load of 1.95 kN at a speed of 50 km/h. Then, the traveling distance was measured until a block chipping happens. The results are indicated using an index based on Ref. 1 being 100. The larger the value, the better the block durability maintain. 
         [0059]    Thickness Uniformity of Tread Rubber: 
         [0060]    The difference between a rubber thickness of the groove bottom where no block is provided and a rubber thickness at an outer edge of the skirt portion was measured. The results are indicated using an index based on Ref. 1 being 100. The smaller the value, the better the sufficient rubber volume around the skirt portion is. 
         [0061]    The test results are shown in table 1. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                   
                 Ref. 1 
                 Ex. 1 
                 Ex. 2 
                 Ex. 3 
                 Ex. 4 
                 Ex. 5 
                 Ex. 6 
                 Ex. 7 
                 Ex. 8 
                 Ex. 9 
               
               
                   
               
               
                 Block shape 
                 — 
                 FIG. 3 
                 FIG. 3 
                 FIG. 3 
                 FIG. 3 
                 FIG. 3 
                 FIG. 3 
                 FIG. 3 
                 FIG. 3 
                 FIG. 3 
               
               
                 Block height Hb (mm) 
                 16.0 
                 16.0 
                 16.0 
                 16.0 
                 16.0 
                 16.0 
                 16.0 
                 16.0 
                 16.0 
                 16.0 
               
               
                 Skirt length Ll/Block height Hb 
                 — 
                 0.63 
                 0.63 
                 0.63 
                 0.63 
                 0.19 
                 0.25 
                 0.38 
                 0.50 
                 0.75 
               
               
                 Skirt height Hs/Block height Hb 
                 — 
                 0.06 
                 0.03 
                 0.13 
                 0.19 
                 0.06 
                 0.06 
                 0.06 
                 0.06 
                 0.06 
               
               
                 Curved portion length L3/Planar portion 
                 — 
                 0.36 
                 0.36 
                 0.36 
                 0.36 
                 0.36 
                 0.36 
                 0.36 
                 0.36 
                 0.36 
               
               
                 length L2 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Planar portion angle θ1 
                 — 
                 10.0 
                 10.0 
                 10.0 
                 10.0 
                 10.0 
                 10.0 
                 10.0 
                 10.0 
                 10.0 
               
               
                 Curved portion radius/Block height Hb 
                 — 
                 0.35 
                 0.35 
                 0.35 
                 0.35 
                 0.35 
                 0.35 
                 0.35 
                 0.35 
                 0.35 
               
               
                 Block durability (Index) 
                 100 
                 110 
                 105 
                 107 
                 104 
                 104 
                 105 
                 105 
                 107 
                 107 
               
               
                 Thickness uniformity of tread rubber 
                 100 
                 70 
                 85 
                 79 
                 95 
                 92 
                 73 
                 70 
                 70 
                 76 
               
               
                 (Index) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
               
               
                   
                 Ex. 10 
                 Ex. 11 
                 Ex. 12 
                 Ex. 13 
                 Ex. 14 
                 Ex. 15 
                 Ex. 16 
                 Ex. 17 
                 Ex. 18 
                 Ex. 19 
               
               
                   
               
               
                 Block shape 
                 FIG. 3 
                 FIG. 3 
                 FIG. 3 
                 FIG. 3 
                 FIG. 3 
                 FIG. 3 
                 FIG. 3 
                 FIG. 3 
                 FIG. 3 
                 FIG. 6 
               
               
                 Block height Hb (mm) 
                 16.0 
                 16.0 
                 16.0 
                 16.0 
                 16.0 
                 16.0 
                 16.0 
                 16.0 
                 16.0 
                 16.0 
               
               
                 Skirt length L1/Block height Hb 
                 100 
                 0.63 
                 0.63 
                 0.63 
                 0.63 
                 0.63 
                 0.63 
                 0.63 
                 0.63 
                 0.63 
               
               
                 Skirt height Hs/Block height Hb 
                 0.06 
                 0.06 
                 0.06 
                 0.06 
                 0.06 
                 0.06 
                 0.06 
                 0.06 
                 0.06 
                 0.06 
               
               
                 Curved portion length L3/Planar portion 
                 0.36 
                 0.25 
                 0.3 
                 0.4 
                 0.45 
                 0.36 
                 0.36 
                 0.36 
                 0.36 
                 0.36 
               
               
                 length L2 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Planar portion angle θ1 
                 10 
                 10 
                 10 
                 10 
                 10 
                 5 
                 15 
                 10 
                 10 
                 10 
               
               
                 Curved portion radius/Block height Hb 
                 0.35 
                 0.35 
                 0.35 
                 0.35 
                 0.35 
                 0.35 
                 0.35 
                 0.25 
                 0.45 
                 0.35 
               
               
                 Block durability (Index) 
                 104 
                 106 
                 108 
                 108 
                 105 
                 105 
                 107 
                 104 
                 108 
                 112 
               
               
                 Thickness uniformity of tread rubber 
                 85 
                 83 
                 76 
                 73 
                 84 
                 75 
                 87 
                 80 
                 77 
                 73 
               
               
                 (Index) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
               
             
          
         
       
     
         [0062]    From the test results, it was confirmed that the example tires has improved durability of blocks. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           2  Tread portion 
           9  Groove bottom 
           10  Block 
           11  Virtual block bottom 
           12  Ground contact surface 
           15  Block-sidewall main portion 
           20  Skirt portion 
           21  Planar portion 
           22  Curved portion