Patent Publication Number: US-2021178834-A1

Title: Pneumatic tire

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of Japanese Patent Application No.: 2019-225709 filed on Dec. 13 2019, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Technical Field 
     The present invention relates to a pneumatic tire. 
     Related Art 
     A carcass ply of a pneumatic tire disclosed in JP 2017-109517 A includes a first ply that is continuous between a pair of bead portions and a discontinuous second ply that is disposed on the outer side in the tire radial direction of the first ply. The second ply includes a pair of ply pieces respectively extending from a tread portion to any one of a pair of sidewall portions. In the center of the tread portion, more specifically between inner end portions of the pair of ply pieces, a region where neither of two ply pieces exists, that is, a hollow portion is provided. 
     SUMMARY 
     Since a region in which the ply piece does not exist is formed between the inner end portions of the pair of ply pieces, there is a possibility that air entry in which air enters and remain in the hollow portion of the second ply may occur. No consideration is given to Patent Literature 1 regarding the air entry into the hollow portion. 
     An object of the present invention is to suppress air entry into a hollow portion in a pneumatic tire including a carcass ply having the hollow portion. 
     One aspect of the present invention provides a pneumatic tire including: a tread portion; a pair of sidewall portions extending from both ends of the tread portion to an inner side in a tire radial direction; a first ply including a central portion positioned on the inner side in the tire radial direction of the tread portion and a pair of first side portions extending from both ends of the central portion to the inner side in the tire radial direction and positioned on an inner side in a tire width direction of the sidewall portion; and a discontinuous second ply including a pair of ply pieces, each of the pair of ply pieces having an inner end portion that is disposed on an outer side in the tire radial direction of the central portion in the tread portion and a second side portion that extends from the inner end portion to the inner side in the tire radial direction and is positioned on an outer side in the tire width direction of one of the pair of first side portions. The first ply is provided with a plurality of ventilation holes, and a density of the ventilation holes in the central portion is higher than a density of the ventilation holes in the pair of first side portions. 
     The second ply includes the pair of ply pieces and is discontinuous. That is, between respective inner end portions of the pair of ply pieces, there is a hollow portion in which no ply piece exists. By employing the second ply having the hollow portion, weight reduction and reduction in rolling resistance due to the weight reduction can be achieved as compared with the case where the second ply is one continuous ply. Further, since the first ply and the second ply include the side portions positioned on the inner side in the tire width direction of the sidewall portion, the rigidity of the sidewall portion and the steering stability and cut resistance due to the rigidity can be ensured. 
     Most of air existing in the hollow portion is pushed out by the pressure welding of the tire components performed during the molding of the pneumatic tire (green tire). A plurality of the ventilation holes are formed in the first ply, and the density of the ventilation holes in the central portion adjacent on the inner side in the tire radial direction to the hollow portion is higher than the density of the ventilation holes in the first side portion. That is, the ventilation holes (space) through which air can escape are scattered in the central portion of the first ply adjacent on the inner side in the tire radial direction to the hollow portion. Therefore, air that cannot be completely discharged by the pressure welding is dispersed in a plurality of the ventilation holes, and is not concentrated or interposed between the first ply and the facing member. Therefore, it is possible to significantly suppress the local air entry into the hollow portion. 
     The density of the ventilation holes of the pair of first side portions includes zero. The pair of first side portions may have no ventilation holes. 
     That is, the present aspect includes a configuration in which the ventilation hole is not formed in the first side portion of the first ply. Therefore, it is possible to prevent the rigidity of the first side portion from being lowered due to the formation of the ventilation holes. As a result, it is possible to ensure the rigidity of the sidewall portion and the steering stability and cut resistance due to the rigidity. 
     The central portion includes a pair of first regions respectively being adjacent on the inner side in the tire radial direction to respective inner end portions of the pair of ply pieces and a second region adjacent on the inner side in the tire width direction to the pair of first regions, and the density of the ventilation holes in the pair of first regions is higher than the density of the ventilation holes in the second region. The density of the ventilation holes in the pair of first regions may be 100 or more and 300 or less per one square meter, and the density of the ventilation holes in the second region may be 0 or more and 200 or less per one square meter. The density of the ventilation holes in the second region may be higher than the density of the ventilation holes in the pair of first side portions. 
     In the present aspect, the density of the ventilation holes in the first region where air entry is likely to occur due to a step formed by the inner end portion of the ply piece and the first ply is set to be higher than the density of the ventilation holes in the second region where air entry is less likely to occur than in the first region. Therefore, it is possible to suppress the air entry into the hollow portion, and to prevent the rigidity of the tread portion from being lowered due to excessive formation of the ventilation holes. 
     The central portion includes a pair of first regions respectively being adjacent on the inner side in the tire radial direction to respective inner end portions of the pair of ply pieces, a pair of second regions respectively being adjacent on the inner side in the tire width direction to the pair of first regions, and a third region adjacent on the inner side in the tire width direction to the pair of second regions, and the density of the ventilation holes is increasing in order of the pair of second regions, the third region, and the pair of first regions. The density of the ventilation holes in the pair of first regions may be 100 or more and 300 or less per one square meter, the density of the ventilation holes in the pair of second regions may be 0 or more and 200 or less per one square meter, and the density of the ventilation holes in the third region may be 50 or more and 250 or less per one square meter. The density of the ventilation holes in the pair of second regions may be higher than the density of the ventilation holes in the pair of first side portions. 
     According to the present aspect, the density of the ventilation holes in the first region where air entry is likely to occur due to a step formed by the inner end portion of the ply piece and the first ply is set to be higher than the density of the ventilation holes in the second region and the third region where air entry is less likely to occur than in the first region. Therefore, it is possible to suppress the air entry in the vicinity of the inner end portion of the ply piece, and to prevent the rigidity of the tread portion from being lowered due to excessive formation of the ventilation holes. 
     Further, in a case where the tire components are pressurized by the pressure roller having a both ends supported structure during the molding of a green tire, the pressure at the center of the pressure roller is smaller than the pressure at both ends. In contrast, in the present aspect, the density of the ventilation holes in the third region positioned at the center of the pressure roller is made higher than the density of the ventilation holes in the second region. Therefore, it is possible to suppress the interposition of air, that is, the air entry between the third region of the first ply and the facing member. 
     The first region has a width from a first portion closer to the inner side in the tire width direction than the inner end portion to a second portion closer to the outer side in the tire width direction than the inner end portion in the central portion, and the width of the first region is larger than 0 mm and 50 mm or less. 
     In the present aspect, air entry in the vicinity of the inner end portion of the ply piece can be effectively suppressed. 
     A bleeder cord composed of a plurality of fibers is disposed on a surface of the first ply and a surface of the second ply, and the density of the bleeder cord disposed on the first ply is higher than the density of the bleeder cord disposed on the second ply. 
     In the present aspect, even if unintended air entry occurs, air can be dispersed through a gap between the fibers constituting the bleeder cord. Therefore, it is possible to effectively suppress a problem caused by local air entry. 
     In the present invention, air entry into a hollow portion in a pneumatic tire including a carcass ply having the hollow portion can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and the other features of the present invention will become apparent from the following description and drawings of an illustrative embodiment of the invention in which: 
         FIG. 1  is a meridian cross-sectional view of a pneumatic tire according to a first embodiment of the present invention; 
         FIG. 2  is a meridian cross-sectional view of a tread portion and its vicinity of the pneumatic tire according to the first embodiment of the present invention; 
         FIG. 3  is a development view of a first ply and a second ply in the first embodiment; 
         FIG. 4A  is a cross-sectional view taken along line A-A of  FIG. 3 ; 
         FIG. 4B  is a sectional view taken along line B-B in the second ply of  FIG. 3 ; 
         FIG. 5  is a development view of the first ply and the second ply in a second embodiment; 
         FIG. 6  is a development view of the first ply and the second ply in a third embodiment; and 
         FIG. 7  is a development view of the first ply and the second ply in a variation. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. 
     First Embodiment 
       FIGS. 1 and 2  show a pneumatic tire (hereinafter referred to as tire)  1  made from rubber according to a first embodiment of the present invention. The tire  1  includes a tread portion  2 , a pair of sidewall portions  3 , and a pair of ring-shaped bead portions  4 . 
     The tread portion  2  includes tread rubber  5  extending in the tire width direction (indicated by the reference numeral TW in  FIG. 1 ). A groove  5   a  is provided on the surface, that is, the tread surface of the tread rubber  5 . 
     Each of the pair of sidewall portions  3  includes side rubber  6  extending from each of both ends of the tread portion  2  to the inner side in the tire radial direction (reference numeral TR in  FIG. 1 ). 
     The pair of bead portions  4  are respectively disposed in end portions on the inner side in the tire radial direction of the pair of sidewall portions  3 . Each of the bead portions  4  includes a bead core  7  and a bead filler  8 . The bead core  7  includes a large number of steel wires bundled in a ring shape. The bead filler  8  has a ring shape and is made from rubber that is harder than rubber that constitutes the tread portion  2  and the sidewall portion  3 . The bead filler  8  includes a base end  8   a  disposed adjacent to the outer side in the tire radial direction of the bead core  7  and a tip end  8   b  on the side opposite to the base end  8   a,  and extends in a tapered shape from the base end  8   a  to the outer side in the tire radial direction toward the tip end  8   b.  Each of the bead portions  4  includes strip rubber  9  provided to wrap the bead core  7  and the bead filler  8 . 
     The tire  1  includes a carcass  12  laid over between the bead portions  4  in a toroidal shape. In the present embodiment, the carcass  12  includes a first carcass ply (hereinafter, referred to as “first ply”)  13  and a second carcass ply (hereinafter, referred to as “second ply”)  14 . The second ply  14  is a ply having a hollow portion  14   a,  while the first ply  13  is a normal ply having no hollow portion. The first ply  11  and the second ply  12  will be described in detail later. An inner liner  10  is provided on the inner side of the carcass  12 , that is, on the innermost peripheral surface of the tire  1 . 
     With reference to  FIG. 2 , an endless belt layer  18  is provided on the tread portion  2 , more specifically, between the carcass  12  and the tread rubber  5 . In the present embodiment, the belt layer  18  includes two belts  19  and  20 . The belt  19  is disposed adjacent to the outer side in the tire radial direction of the carcass  12 , and the belt  20  is disposed adjacent to the outer side in the tire radial direction of the belt  19 . Further, the dimension in the tire width direction of the belt  19  in the lower layer is larger than the dimension in the tire width direction of the belt  20  in the upper layer, and an end portion  19   a  of the belt  19  is positioned closer to the outer side in the tire width direction than an end portion  20   a  of the belt  20 . The belts  19  and  20  are formed by covering a belt cord made from steel or organic fiber with rubber. The belt layer  18  may be composed of one belt, or may include three or more belts. 
     An endless cap layer  22  is provided adjacent to the outer side in the tire radial direction of the belt layer  18 . The cap layer  22  includes a pair of narrow edge plies  23  that directly cover either of the end portions  19   a  and  20   a  of the belts  19  and  20 . The cap layer  22  of the present embodiment includes a wide cap ply  24  that is disposed adjacent to the outer side in the tire radial direction of the edge ply  23  and  24  that covers the entire belts  19  and  20  including the end portions  19   a  and  20   a  as a single sheet. The cap layer  22  may include one or three or more plies. Further, the cap layer  22  may be eliminated. 
     A pair of endless pads  26  made from rubber are respectively disposed between both ends on the outer side in the tire width direction of the belt layer  18  and the carcass  12 . The cross-sectional shape of the pad  26  is a flat triangular shape. The positions in the tire width direction of the end portions  19   a  and  20   a  of the belts  19  and  20 , the end portion  23   a  on the outer side in the tire width direction of the edge ply  23 , and an end portion  24   a  of the cap ply  24  are set in a region between an end portion  26   a  on the outer side in the tire width direction of the pad  26  and an end portion  26   b  on the inner side, that is, a region where the pad  26  exists. The pad  26  may be eliminated. 
     Hereinafter, the first ply  13  and the second ply  14  constituting the carcass  12  will be described. 
     As shown in  FIGS. 1 and 2 , the first ply  13  is a single ply, while the second ply  14  is a discontinuous ply having the hollow portion  14   a  as described above. The second ply  14  is composed of a pair of ply pieces  15 . Referring to  FIG. 3 , all of the first ply  13  and the ply pieces  15  of the second ply  14  are a strip-shaped sheet in which a plurality of cords  30  and  31  arranged in parallel at intervals are covered with rubber. 
     As shown in  FIGS. 1 and 2 , the first ply  13  includes a central portion  13   a  positioned on the inner side in the tire radial direction of the tread portion  2 , and a pair of side portions (first side portions)  13   b  extending from both ends in the tire width direction of the central portion  13   a  to the inner side in the tire radial direction. Further, the first ply  13  includes a wind-up portion  13   c  that is provided continuously with the side portion  13   b  and wound up from the inner side to the outer side in the tire width direction with respect to the bead portion  4 . 
     The central portion  13   a  is disposed adjacent to the outer side in the tire radial direction of the inner liner  10  in the tread portion  2 . 
     The side portion  13   b  is provided continuously with the central portion  13   a,  and is disposed adjacent to the outer side in the tire width direction of the inner liner  10  in the sidewall portion  3 . 
     The wind-up portion  13   c  is terminated in the sidewall portion  3 . More specifically, the wind-up portion  13   c  includes an inner portion  13   d,  a winding portion  13   e,  and an outer portion  13   f.  The inner portion  13   d  is disposed closer to the inner side in the tire width direction than the bead portion  4 , that is, the bead core  7  and the bead filler  8 , with the ply piece  15  interposed between them. The winding portion  13   e  is provided continuously with the inner portion  13   d,  and is wound around the bead core  7  with the ply piece  15  interposed between them. The outer portion  13   f  is provided continuously with the winding portion  13   e,  and is disposed closer to the outer side in the tire width direction than the bead portion  4  with the ply piece  15  interposed between them. An end portion of the outer portion  13   f  constitutes an end portion  13   g  of the first ply  13 . The end portion  13   g  is positioned closer to the outer side in the tire radial direction than the tip end  8   b  of the bead filler  8  and is positioned closer to the inner side in the tire radial direction than the tread portion  2 . 
     The second ply  14  is disposed adjacent on a tire outer surface side with respect to the first ply  13 , and is a discontinuous ply composed of the pair of ply pieces  15 . The ply piece  15  has an inner end portion  15   a  disposed between the belt layer  18  and the central portion  13   a  of the first ply  13 . The pad  26  is interposed between the inner end portion  15   a  and the belt layer  18 . 
     The position in the tire width direction of the inner end portion  15   a  of the ply piece  15  is set in the region on the outer side in the tire width direction of the tread portion  2 , more specifically, in the region closer to the inner side in the tire width direction to both the end portions  19   a  and  20   a  of the belts  19  and  20  constituting the belt layer  18 . The hollow portion  14   a  is provided in the region at the center in the tire width direction of the tread portion  2 , more specifically, in the region between the inner end portions  15   a  of the pair of ply pieces  15 . In the hollow portion  14   a,  the second ply  14  does not exist, and only the central portion  13   a  of the first ply  13  exists. 
     The ply piece  15  includes a side portion (second side portion)  15   b  extending to the inner side in the tire radial direction from the inner end portion  15   a,  and a wind-up portion  15   c  wound up from the inner side to the outer side in the tire width direction with respect to the bead core  7 . 
     The side portion  15   b  is disposed adjacent to the outer side in the tire width direction of the side portion  13   b  of the first ply  13  in the sidewall portion  3 . 
     The wind-up portion  15   c  is provided continuously with the side portion  15   b  and is terminated in the sidewall portion  3 . The wind-up portion  15   c  includes an inner portion  15   d,  a winding portion  15   e,  and an outer portion  15   f.  The inner portion  15   d  is disposed on the inner side in the tire width direction of the bead portion  4 , more specifically, between the bead portion  4  and the inner portion  13   d  of the first ply  13 . The winding portion  15   e  is provided continuously with the inner portion  15   d  and is wound around the bead core  7 . More specifically, the winding portion  15   e  is disposed between the bead core  7  and the winding portion  13   e  of the first ply  13 . The outer portion  15   f  is provided continuously with the winding portion  15   e,  and is disposed closer to the outer side in the tire width direction than the bead portion  4 . Of the outer portion  15   f,  a part positioned on the inner side in the tire radial direction is disposed so as to overlap the outer side in the tire width direction of the bead portion  4 , and the rest is disposed so as to overlap the outer side in the tire width direction of the side portion  15   b.  An end portion of the outer portion  15   f  constitutes an outer end portion  15   g  of the ply piece  15 . The outer end portion  15   g  is positioned closer to the outer side in the tire radial direction than the tip end  8   b  of the bead filler  8  and is positioned closer to the inner side in the tire radial direction than the tread portion  2 , more specifically, the end portion  13   g  of the first ply  13 . 
     In a case where the second ply  14  includes the hollow portion  14   a,  a region where the ply piece  15  does not exist is formed between the pair of inner end portions  15   a,  so that air entry in which air enters the region (hollow portion  14   a ) may occur. 
     In the present embodiment, as shown in  FIG. 3 , the first ply  13  is provided with a plurality of ventilation holes  33  in order to suppress air entry into the hollow portion  14   a.  Further, as shown in  FIGS. 4A and 4B , bleeder cords  34  are respectively disposed on a surface of the first ply  13  and a surface of the second ply  14 . The ply piece  15  of the second ply  14  is not provided with the ventilation hole.  FIG. 3  shows a developed state of the carcass  12  of the first embodiment, and in  FIG. 3  the ply piece  15  on left side is omitted and only the inner end portion  15   a  thereof is shown by a dotted line.  FIG. 4A  shows a cross-sectional view taken along line A-A of  FIG. 3 , and  FIG. 4B  shows a sectional view taken along line B-B of only the second ply  14  in  FIG. 3 . These diagrams are exaggerated to clearly show the ventilation holes  33  and the bleeder cord  34 . 
     As shown by a broken line in  FIGS. 3 and 4A , a plurality of the cords  30  are embedded in the first ply  13  at intervals in the tire circumferential direction (vertical direction in  FIG. 3 ). The individual cords  30  extend in the tire width direction TW in the tread portion  2  and extend in the tire radial direction in the sidewall portion  3 . With reference to  FIGS. 3 and 4B , a plurality of the cords  31  are arranged side by side on the ply piece  15  of the second ply  14 . The individual cords  31  extend parallel to the cords  30  and are disposed at intervals in the tire circumferential direction. All of the cords  30  and  31  are disposed at intervals of 0.1 mm or more and 3 mm or less. The diameter of the individual cords  30  and  31  is approximately 0.6 mm. 
     The ventilation hole  33  is a through-hole penetrating from one surface to the other surface of the first ply  13 , and is provided so as to avoid the cord  30 . The diameter of the ventilation hole  33  is less than the interval between adjacent ones of the cords  30 . For example, the ventilation holes  33  are formed with a diameter of about 1 mm by a large number of needles disposed on a roller that sends out the first ply  13  during forming of a green tire before vulcanization. By changing the number of needles to be disposed depending on a portion of a delivery roller, the density of the ventilation holes  33  can be changed depending on a portion of the first ply  13  in the width direction. 
     In the present embodiment, the density of the ventilation holes  33  formed in the central portion  13   a  and the density of the ventilation holes  33  formed in the side portions  13   b  are different. Further, the wind-up portion  13   c  connected to the side portion  13   b  is provided with the ventilation hole  33  as in the side portion  13   b.  Note that it is not necessary to provide the ventilation hole  33  in the wind-up portion  13   c.    
     As shown in  FIG. 2 , the central portion  13   a  of the present embodiment is between a pair of shoulder portions  2   a  positioned on the outer side in the tire width direction of the tread portion  2 , more specifically, a range R 1  in the tire width direction between the pair of pads  26 . In the tire width direction, an outer end  13   h  of the central portion  13   a  is positioned closer to the outer side than the inner end portion  15   a  of the ply piece  15 . A range R 2  from the outer end  13   h  of the central portion  13   a  to the end portion  13   g  constitutes the side portion  13   b  including the wind-up portion  13   c.  Among them, the side portion  13   b  is a range from the outer end  13   h  of the central portion  13   b  to the tip end  8   b  of the bead filler  8 . 
     As most clearly shown in  FIG. 3 , the density of the ventilation holes  33  formed in the central portion  13   a  is higher than the density of the ventilation holes  33  formed in the side portion  13   b  including the wind-up portion  13   c.  That is, the number of the ventilation holes  33  per unit area (one square meter) formed in the first range R 1  of the first ply  13  is larger than the number of the ventilation holes  33  per unit area formed in the second range R 2  of the first ply  13 . 
     For example, the first range R 1  of the first ply  13  is provided with 50 or more and 300 or less of the ventilation holes  33  per unit area (one square meter). On the other hand, the second range R 2  of the first ply  13  is provided with 0 or more and 250 or less of the ventilation holes  33  per unit area. That is, the side portion  13   b  including the wind-up portion  13   c  includes a configuration in which the ventilation holes  33  are not formed, that is, a configuration in which the density of the ventilation holes  33  is zero (see  FIG. 7 ). In the present embodiment, the first range R 1  of the first ply  13  is provided with the ventilation holes  33  in a matrix shape at intervals of 5 mm or more and 50 mm or less. The second range R 2  of the first ply  13  is provided with the ventilation holes  33  in a matrix shape at intervals of 20 mm or more and 100 mm or less. 
     Subsequently referring to  FIGS. 3 and 4A , the bleeder cord  34  is respectively provided on an inner surface positioned on the inner side in the tire radial direction and an outer surface positioned on the outer side in the tire radial direction in the first ply  13 . The bleeder cord  34  of the first ply  13  extends in the tire width direction in the tread portion  2  and extends in the tire radial direction in the sidewall portion  3 . Referring to  FIGS. 3 and 4B , in the second ply  14 , the bleeder cord  34  is respectively provided on an inner surface positioned on the inner side in the tire radial direction and an outer surface positioned on the outer side in the tire radial direction like in the first ply  13 , and extends in the tire radial direction in the sidewall portion  3 . The bleeder cord  34  is composed of a plurality of fibers such as cotton yarn and polyester yarn, and has a function of reducing local air entry by allowing air to pass through. The diameter of each of the bleeder cords  34  is approximately 0.1 mm. 
     As most clearly shown in  FIG. 3 , the density of the bleeder cord  34  disposed on the first ply  13  is higher than the density of the bleeder cord  34  disposed on the second ply  14 . That is, an interval I 1  of the bleeder code  34  disposed in the first ply  13  and an interval I 2  of the bleeder code  34  disposed in the second ply  14  are different, and the interval I 1  is smaller than the interval I 2 . In the present embodiment, each of the bleeder cords  34  of the first ply  13  is disposed between the ventilation holes  33  adjacent to each other in the tire circumferential direction so as to extend along the cord  30 . The bleeder cords  34  of the second ply  14  are disposed every other one of spaces between the cords  31  adjacent to each other in the tire circumferential direction so as to extend along the cord  31 . 
     Here, a green tire before vulcanization constituting the pneumatic tire  1  is formed into a cylindrical shape by laminating a plurality of the tire components described above on a forming drum in a predetermined order. By pressurizing the laminated tire components with pressure contact members, most of air existing between the members can be pushed out. For example, a pressure roller having a both ends supported structure is used to pressurize a member having a wide width in the tire width direction such as the first ply  13 , the second ply  14  composed of the pair of ply pieces  15 , and the belt  19 . 
     The pressure welding of the ply piece  15  by the pressure roller pushes out most of air existing between the first ply  13  and the second ply  14 . The pressure welding of the belt  19  by the pressure roller pushes out most of air existing in the hollow portion  14   a.  At this time, the ventilation holes  33  formed in the first ply  13  function as space for air to escape. Therefore, air that cannot be completely discharged by the pressure welding is dispersed in a plurality of the ventilation holes  33 , and is not concentrated or interposed between the first ply  13  and the facing members  15  and  19 . 
     The pneumatic tire  1  configured as described above has a feature described below. 
     The second ply  14  includes the pair of ply pieces  15  and is discontinuous. That is, between the inner end portions  15   a  of the pair of ply pieces  15 , there is the hollow portion  14   a  in which no ply exists. By employing the second ply  14  having the hollow portion  14   a,  weight reduction and reduction in rolling resistance due to the weight reduction can be achieved as compared with the case where the second ply  14  is one continuous ply. Further, since the first ply  13  and the second ply  14  include the side portions  13   b  and  15   b  positioned on the inner side in the tire width direction of the sidewall portion  3 , the rigidity of the sidewall portion  3  and the steering stability and cut resistance due to the rigidity can be ensured. 
     A plurality of the ventilation holes  33  are formed in the first ply  13 , and the density of the ventilation holes  33  in the central portion  13   a  is higher than the density of the ventilation holes  33  in the side portion  13   b.  Therefore, it is possible to significantly suppress air entry into the hollow portion  14   a  while ensuring the rigidity of the sidewall portion  3  and the steering stability and cut resistance due to the rigidity. 
     The density of the ventilation holes  33  of the pair of side portions  13   b  includes zero. Therefore, since it is possible to prevent the rigidity of the side portion  13   b  from being lowered due to the formation of the ventilation holes  33 , it is possible to ensure the rigidity of the sidewall portion  3  and the steering stability and cut resistance due to the rigidity. 
     The bleeder cord  34  composed of a plurality of fibers is disposed on the surface of the first ply  13  and the surface of the second ply  14 , and the density of the bleeder cord  34  of the first ply  13  is higher than the density of the bleeder cord  34  of the second ply  14 . Therefore, even if unintended local air entry occurs, air can be dispersed through a gap between the fibers constituting the bleeder cord  34 . As a result, it is possible to effectively suppress a problem caused by local air entry. 
     Second Embodiment 
       FIG. 5  shows a state in which the carcass  12  of the pneumatic tire  1  according to a second embodiment is developed. The second embodiment is different from the first embodiment in that the central portion  13   a  of the first ply  13  is divided into two types and three locations of regions A 1  and A 2 , and the densities of the ventilation holes  33  formed in these regions are made different from each other. 
     Specifically, the central portion  13   a  includes a pair of first regions A 1  and one second region A 2 . The ventilation holes  33  are formed in each of the regions A 1  and A 2 , and the density of the ventilation holes  33  formed in the first region A 1  is higher than the density of the ventilation holes  33  formed in the second region A 2 . The density of the ventilation holes  33  formed in the second region A 2  is higher than the density of the ventilation holes  33  formed in the side portion  13   b.    
     Each of the first regions A 1  is adjacent to the inner side in the tire radial direction of the inner end portion  15   a  of each of the ply pieces  15 . More specifically, the first region A 1  has in the central portion  13   a  a width from a first portion  13   i  positioned closer to the inner side in the tire width direction than the inner end portion  15   a  to the outer end (second portion)  13   h  closer to the outer side in the tire width direction than the inner end portion  15   a.  In the present embodiment, the first portion  13   i  and the second portion  13   h  of the first region A 1  are symmetrically positioned about the inner end portion  15   a  of the ply piece  15 . 
     The width of the first region A 1  from the first portion  13   i  to the second portion  13   h  is preferably set in the range of more than 0 mm (that is, not including 0 mm) and 50 mm or less, and more preferably set in the range of 5 mm or more and 30 mm or less. In a case where the width of the first region A 1  is excessively widened, the number of the ventilation holes  33  formed in the central portion  13   a  becomes excessive, and it becomes difficult to ensure the rigidity of the tread portion  2 . In order to prevent such inconvenience, the width of the first region A 1  is preferably set within the above range. 
     The second region A 2  is adjacent to the inner side in the tire width direction of each of the pair of first regions A 1 . The second region A 2  is a range from one of the first portions  13   i  to the other one of the first portions  13   i  of the pair of first regions A 1 . 
     In each of the pair of first regions A 1 , 100 or more and 300 or less of the ventilation holes  33  are formed per unit area (one square meter). In the second region A 2 , 0 or more and 200 or less of the ventilation holes  33  are formed per unit area. 
     In the pneumatic tire  1  of the second embodiment, the density of the ventilation holes  33  in the first region A 1  where air entry is likely to occur due to the step is set to be higher than the density of the ventilation holes  33  in the second region A 2  where air entry is less likely to occur than in the first region A 1 . Therefore, it is possible to effectively suppress the air entry in the vicinity of the inner end portion  15   a  of the ply piece  15 , and to prevent the rigidity of the tread portion  2  from being lowered due to excessive formation of the ventilation holes  33 . 
     Third Embodiment 
       FIG. 6  shows a state in which the carcass  12  of the pneumatic tire  1  according to a third embodiment is developed. The third embodiment is different from the first embodiment in that the central portion  13   a  of the first ply  13  is divided into three types and five locations of regions A 1 , A 2  and A 3 , and the densities of the ventilation holes  33  formed in these regions are made different from each other. 
     Specifically, the central portion  13   a  includes a pair of first regions A 1 , a pair of second regions A 2 , and one third region A 3 . The ventilation holes  33  are formed in each of the regions A 1 , A 2  and A 3 , and the density of the ventilation holes  33  is increasing in the order of the second region A 2 , the third region A 3 , and the first region A 1 . That is, the density of the ventilation holes  33  in the first region A 1  is the highest, and the density of the ventilation holes  33  in the second region A 2  is the lowest. The density of the ventilation holes  33  formed in the second region A 2  is higher than the density of the ventilation holes  33  formed in the side portion  13   b.    
     Each of the first regions A 1  is adjacent to the inner side in the tire radial direction of the inner end portion  15   a  of each of the ply pieces  15 . The range of the first region A 1  is similar to that of the second embodiment. Each of the second region A 2  is adjacent to the inner side in the tire width direction of each of the pair of first regions A 1 . The third region A 3  is adjacent to the inner side in the tire width direction of each of the pair of second regions A 2 . 
     The width of the third region A 3  is preferably set in the range of 50 mm or more and 200 mm or less, and more preferably set in the range of 100 mm or more and 150 mm or less. The center of this range is the center of the central portion  13   a  in the tire width direction. In the case of a pressure roller having a both ends supported structure, the pressing force at the center is smaller than the pressing force at both ends. Accordingly, if the width of the third region A 3  corresponding to the central part is made too small, it is possible that the air is not pushed out sufficiently. If the width of the third region A 3  is excessively large, the number of the ventilation holes  33  becomes excessive, and it becomes difficult to ensure the rigidity of the tread portion  2 . In order to prevent these inconveniences, the width of the third region A 3  is preferably set within the above range. 
     In each of the pair of first regions A 1 , 100 or more and 300 or less of the ventilation holes  33  are formed per unit area (one square meter). In each of the pair of second regions A 2 , 0 or more and 200 or less of the ventilation holes  33  are formed per unit area. In the third region A 3 , 50 or more and 250 or less of the ventilation holes  33  are formed per unit area. 
     In the pneumatic tire  1  of the third embodiment, the density of the ventilation holes  33  in the first region A 1  where air entry is likely to occur due to the step formed by the inner end portion  15   a  of the ply piece  15  and the first ply  13  is set to be higher than the density of the ventilation holes  33  in the second region A 2  and the third region A 3  where air entry is less likely to occur than in the first region A 1 . Therefore, it is possible to suppress the air entry in the vicinity of the inner end portion  15   a  of the ply piece  15 , and to prevent the rigidity of the tread portion from being lowered due to excessive formation of the ventilation holes  33 . 
     Further, the density of the ventilation holes  33  in the third region A 3  positioned at the center of the pressure roller having a both ends supported structure is made higher than the density of the ventilation holes  33  in the second regions A 2  positioned on both sides of the third region A 3 . Therefore, it is possible to suppress the interposition of air, that is, the air entry between the third region A 3  of the first ply  13  and the belt (facing member)  19 . 
     Note that the pneumatic tire  1  of the present invention is not limited to the configuration of the above embodiment, and various changes can be made. 
     For example, the number and arrangement of the ventilation holes  33  can be changed as needed. Further, the density of the ventilation holes  33  formed in the central portion  13   a  may be gradually increased from the center to the outer side in the tire width direction (in multiple stages). 
     As shown in  FIG. 7 , as the first ply  13  and the second ply  14 , plies on which the bleeder cord  34  is not disposed on the surface may be used. Further, the side portion  13   b  including the wind-up portion  13   c  does not have to be provided with the ventilation hole  33 . 
     In addition to the first ply  13  and the second ply  14 , a ply having the hollow portion  14   a  similar to that of the second ply  14  and/or a normal ply similar to the first ply  13  may be further included.