Abstract:
A pneumatic tire, comprising a bead part ( 4 ) having a bead reinforcement layer ( 10 ) continuously extending from the inside part ( 10   a ) of a bead core ( 5 ) positioned on the axial inside of the tire to the outside part ( 10   c ) thereof on the axial outside of the tire through a bottom part ( 10   b ) passing the radial inside of the tire, the bead reinforcement layer ( 10 ) further comprising reinforcement cords ( 11 ) arranged at an angle of 20 to 60° relative to the circumferential direction of the tire, the outside part ( 10 c) further comprising a main part (C 1 ) extending to an area (Y) adjacent to a bead apex rubber ( 8 ) and a folded part (C 2 ) folded up at the radial outer end (Ce) thereof, wherein the folded part (C 2 ) terminates on the radial inside of the radial outer surface ( 5   s ) of the bead core ( 5 ).

Description:
TECHNICAL FIELD 
   The present invention relates to a pneumatic tire which enhances steering stability by providing a bead reinforcing layer having a reinforcing cord between a bead core and a carcass ply. 
   BACKGROUND TECHNIQUE 
   In recent years, as output and performance of automobiles are enhanced, tires are also strongly required to enhance the steering stability together with riding comfort. 
   In a pneumatic tire, it is known that the steering stability can be enhanced by increasing the tire lateral rigidity. Therefore, a reinforcing layer using steel cord or organic fiber cord is conventionally provided over a wide range from a bead portion to a sidewall portion to enhance the flexural rigidity of the sidewall. 
   However, if such a reinforcing layer is used, the tire vertical rigidity is increased and thus, the riding comfort is deteriorated. Further, stress is prone to be concentrated on an outer end of the reinforcing layer. Especially when the reinforcing layer is used for a high performance tire in which a tire aspect ratio is reduced to 55% or lower and a ground-contact width and a ground-contact area are increased, a flexible region of the sidewall portion becomes narrow and stress is concentrated more. Thus, there is a tendency that the endurance is deteriorated. 
   Thereupon, the present inventor focused attention on a fact that not only the tire lateral rigidity but also twisting rigidity of the bead portion largely affected the steering stability, and researched. As a result, the inventor found that the tire vertical rigidity could be maintained at low level and the steering stability could be enhanced without deteriorating the riding comfort if a reinforcing layer in which reinforcing cords were arranged at a predetermined angle and which had a folded-back portion was formed around the bead core without projecting the reinforcing layer from a bead apex rubber, and the bead core and the bead apex rubber were formed integral with each other, and the twisting rigidity of the bead portion was enhanced. 
   The steering stability can be enhanced also by increasing the width of the bead core to increase the twisting rigidity and to enhance the fitting performance between the bead core and a rim. In such a case, however, since the steel amount of the bead core and a rubber amount of the bead apex rubber are increased, there is a problem that the tire weight is increased and the fuel performance is deteriorated. 
   Based on an idea that a predetermined reinforcing layer is provided around a bead core, it is an object of the present invention to provide a pneumatic tire capable of enhancing the steering stability without deteriorating the riding comfort and without excessively increasing the tire weight. 
   SUMMARY OF THE INVENTION 
   To achieve the object, the present invention provides a pneumatic tire, comprising a carcass including a carcass ply having a ply body portion which extends from a tread portion to a bead core of a bead portion through a sidewall portion and which is continuously provided with a ply folded-up portion folded up around the bead core from its inner side to its outer side in the tire axial direction, and a bead apex rubber passing between the ply body portion and the ply folded-up portion and extending radially outward of the tire from a radially outer surface of the bead core, wherein 
   the bead portion is provided with a bead reinforcing layer which is arranged from an inner portion of the bead core located inward in the tire axial direction through a bottom portion passing radially inward and which is connected to an outer portion of the bead core outward in the tire axial direction and which is arranged at an angle of 20 to 60° with respect to a tire circumferential direction, 
   the outer portion includes a main portion which extends radially outward of the tire beyond the radially outer surface of the bead core to a region adjacent to the bead apex rubber, and a folded-back portion which is folded back at a radially outer end and terminated radially inward of the radially outer surface of the bead core, 
   the inner portion passes between the bead core and the ply body portion, and the outer portion passes between the bead core and the ply folded-up portion. 
   It is preferable that a width H 1  of the folded-back portion of the bead reinforcing layer in the radial direction is 5 to 40 mm. It is preferable that a radial height H 2  of the outer portion of the bead reinforcing layer from a bead base line is 1.5 to 4.5 times a radial height H 3  of the bead apex rubber, and is smaller than a radial height H 4  of the bead apex rubber. It is preferable that a radial height H 5  of the ply folded-up portion of the carcass ply is greater than the height H 4  of the bead apex rubber. 
   In this specification, the term “bead base line” means a line in the tire axial direction passing through a bead diameter position defined in a standard on which tires are based. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view of a tire according to an embodiment of the present invention; 
       FIG. 2  is an enlarged sectional view of a bead portion together with a reinforcing layer; and 
       FIG. 3  is a perspective view showing the reinforcing layer together with a bead core and a bead apex rubber. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An example of the present invention will be explained with reference to the drawings. In this example, the pneumatic tire of the invention is a high performance passenger automobile tire having a tire aspect ratio of 55% or lower. 
   As shown in  FIG. 1 , the pneumatic tire  1  includes a carcass  6  extending from a tread portion  2  to a bead core  5  of a bead portion  4  through a sidewall portion  3 , and a belt layer  7  disposed on the inner side of the tread portion  2  and on the outer side of the carcass  6 . The bead portion  4  is provided with a bead apex rubber  8  extending from a radially-outer surface of the tire of the bead core  5  to radially outward of the tire. 
   The belt layer  7  comprises two or more, in this example, two belt plies  7 A and  7 B in which high elastic belt cords are arranged in a tire circumferential direction at an angle of 10 to 35° for example. Inclination directions of the belt plies  7 A and  7 B are changed such that the belt cords intersect with each other between the plies. With this configuration, the belt rigidity is enhanced, and substantially the entire width of the tread portion  2  is strongly reinforced with a hoop effect. A steel cord or an organic fiber cord having high modulus such as aromatic polyamide fiber is preferably used as the belt cord. 
   In this example, in order to enhance a binding force with respect to the belt layer  7  and to enhance the high speed endurance, a band layer  9  is disposed on an outer side of the belt layer  7 . The band layer  9  includes a band cord which is helically wound in the tire circumferential direction at an angle of 5° or less for example. The band layer  9  extends such as to wrap at least an outer end of the belt layer  7  in the tire axial direction. 
   The carcass  6  comprises one or more, in this example, one carcass ply  6 A in which a carcass cord is arranged at an angle of 75 to 90° with respect to the tire circumferential direction. The carcass ply  6 A is integrally provided with a ply body portion  6   a  extending between the bead cores  5  and  5 , and with a ply folded-up portion  6   b  which is folded up around the bead core  5  from the inner side to the outer side in the tire axial direction on both ends of the ply body portion  6   a . An organic fiber cord such as nylon, rayon, polyester, and aromatic polyamide can preferably be used as the carcass cord. 
   Next, the bead apex rubber  8  passes between the ply body portion  6   a  and the ply folded-up portion  6   b  of the carcass  6 , and has a triangular cross section which is tapered radially outward of the tire. 
   In this example, in order to secure the tire rigidity required for the high performance tire, a radial height H 4  of the bead apex rubber  8  from the bead base line BL is set in a range of 0.25 to 0.5 times a height HT of a tire cross section. A radial height H 5  of the ply folded-up portion  6   b  from the bead base line BL is set greater than the height H 4 , thereby forming an extending portion  6   b   1  extending beyond the bead apex rubber  8 . The extending portion  6   b   1  and the ply body portion  6   a  are adjacent to each other. With this configuration, the bead apex rubber  8  is completely covered and the tire rigidity is further enhanced. 
   In this invention, in order to further enhance the steering stability, the bead portion  4  is provided with a bead reinforcing layer  10 . 
   As shown in  FIGS. 2 and 3 , the bead reinforcing layer  10  comprises one cord ply in which reinforcing cords  11  are arranged at an angle θ (20 to 60°) with respect to the tire circumferential direction. Organic fiber cord such as nylon, polyester, rayon, and aromatic polyamide can be used as the reinforcing cord  11 . Nylon cord is especially preferable because the thickness of the cord can be increased by folding the same. 
   The bead reinforcing layer  10  comprises an inner portion  10   a  located on the inner side of the bead core  5  in the tire axial direction, a bottom portion  10   b  passing radially inward of the bead core  5 , and an outer portion  10   c  located on the outer side of the bead core  5  in the tire axial direction. These portions  10   a ,  10   b  and  10   c  are continuously connected to one another to form the U-shape. 
   The inner portion  10   a  passes between the bead core  5  and the ply body portion  6 , and the outer portion  10   c  passes between the bead core  5  and the ply folded-up portion  6   b . At least the outer portion  10   c , in this example, both the outer portion  10   c  and the inner portion  10   a  extend beyond the radially outer surface  5   s  of the bead core  5 , radially outward, and extends to a region Y which is adjacent to the bead apex rubber  8 . 
   Thus, the bead reinforcing layer  10  brings the bead core  5  and the bead apex rubber  8  into tight contact with each other to wrap them in the U-shape and integrally and strongly couples the bead core  5  and the bead apex rubber  8  to each other. 
   The outer portion  10   c  includes a main portion C 1  which is connected to the bottom portion  10   b  and extends to the region Y which is adjacent to the bead apex rubber  8 , and a folded-back portion C 2  which is folded back at a radial outer end Ce of the main portion C 1  and superposed on the main portion C 1  and extends radially inward. The folded-back portion C 2  is terminated radially inward of the radially outer surface  5   s  of the bead core  5 . 
   A cord angle θ of the bead reinforcing layer  10  is in a range of 20 to 60° . Thus, the reinforcing cords  11  intersect with each other at an angle of 2×θ to form a strong intersecting structure between the main portion C 1  and the folded-back portion C 2 . With this structure, and with effect of the integral structure between the bead core  5  and the bead apex rubber  8 , the twisting rigidity of the bead portion  4  can be effectively enhanced, and the steering stability can be enhanced. 
   In the bead reinforcing layer  10 , since radial heights H 6  and H 2  of the inner portion  10   a  and the outer portion  10   c  from the bead base line BL are smaller than the radial height H 4  of the bead apex rubber  8 , the tire&#39;s vertical rigidity is restrained from being increased, and the deterioration of the riding comfort is suppressed. 
   In order to enhance the steering stability, it is preferable that a width H 1  of the folded-back portion C 2  in the radial direction is in a range of 5 to 40 mm. If the width H 1  is less than 5 mm, the twisting rigidity is insufficient, and if the width H 1  exceeds 40 mm, the riding comfort is deteriorated. 
   For the same reason, it is preferable that the radial height H 2  of the outer portion  10   c  is 1.5 to 4.5 times the radial height H 3  of the bead core  5 . If the height H 2  is less than 1.5 times, the twisting rigidity is insufficient, and if the height H 2  exceeds 4.5 times, the riding comfort is deteriorated. 
   Since the outer portion  10   c  of the bead reinforcing layer  10  is lower than the bead apex rubber  8 , the stress concentration on the outer end Ce can be reduced. Since the ply folded-up portion  6   b  covers the outer end Ce, bead damage such as cord loose can be suppressed. 
   Since the bead reinforcing layer  10  is formed with the folded-back portion C 2 , the bead core  5  is displaced axially inward by a distance corresponding to the thickness of the folded-back portion C 2 . As a result, the fitting performance with respect to the rim is enhanced, and this configuration also enhances the steering stability. 
   Although the preferred example of the present invention has been described in detail, the invention is not limited to the illustrated example, and the various changes and modifications may be made in the invention. 
   EMBODIMENT 
   Tires of 215/45ZR17 size were prototyped based on the specifications shown in Table 1. The steering stability, riding comfort and weight of the prototyped tire were measured and compared with each other. Specifications not shown in Table 1 are substantially the same. The test method is as follows: 
   (1) Steering Stability 
   The tires were mounted on all rims (17×7JJ) of a passenger vehicle (Japanese FR vehicle, displacement of 2500 cc) under internal pressure of 200 kPa, and the vehicle was run on a dry asphalt road at a speed of 120 km/H. The straight running stability and lane change stability at that time were evaluated by a driver&#39;s sensory evaluation on a scale of 10 while a comparative example was defined as 5. A greater value indicates more excellent result. 
   (2) Riding Comfort 
   The same test vehicle was run on an asphalt road (good road), and the riding comfort was evaluated by a driver&#39;s sensory evaluation on a scale of 10 while a comparative example was defined as 5. A greater value indicates more excellent result. 
   (3) Tire Weight 
   The weight of one tire was measured. 
   
     
       
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Comparative 
               Comparative 
               Embodiment 
               Embodiment 
               Embodiment 
               Embodiment 
               Embodiment 
             
             
                 
               example 1 
               example 2 
               1 
               2 
               3 
               4 
               5 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               Bead reinforcing 
               Absence 
               Presence 
               Presence 
               Presence 
               Presence 
               Presence 
               Presence 
             
             
               layer 
             
             
               Reinforcing cord 
               — 
               Nylon 
               Nylon 
               Nylon 
               Nylon 
               Nylon 
               Nylon 
             
             
               Cord angle θ (°) 
               — 
               50 
               50 
               50 
               50 
               20 
               60 
             
             
               Presence or 
               — 
               Absence 
               Presence 
               Presence 
               Presence 
               Presence 
               Presence 
             
             
               Absence of folded- 
             
             
               back portion 
             
             
               Heights (mm) 
             
             
               H1 
               — 
               — 
               10 
               20 
               30 
               20 
               20 
             
             
               H2 
               — 
               35 
               35 
               35 
               35 
               35 
               35 
             
             
               H3 
               6 
               6 
               6 
               6 
               6 
               6 
               6 
             
             
               H4 
               45 
               45 
               45 
               45 
               45 
               45 
               45 
             
             
               H5 
               60 
               60 
               60 
               60 
               60 
               60 
               60 
             
             
               H6 
               — 
               15 
               15 
               15 
               15 
               15 
               15 
             
             
               Bead core (*1) 
               4S × 4T 
               5S × 4T 
               4S × 4T 
               4S × 4T 
               4S × 4T 
               4S × 4T 
               4S × 4T 
             
             
               Steering stability 
               5.0 
               6.0 
               6.0 
               6.5 
               6.5 
               6.5 
               6.5 
             
             
               Riding comfort 
               7.0 
               6.5 
               6.5 
               6.5 
               6.0 
               6.0 
               5.5 
             
             
               Tire weight (kg) 
               9.8 
               10.3 
               10.1 
               10.1 
               10.1 
               10.1 
               10.1 
             
             
                 
             
             
               (*1) This is a tape bead structure, and “nS × mT” means that n-number of bead wires are arranged in the widthwise direction and m-number of bead wires are arranged in the height direction. 
             
           
        
       
     
   
   INDUSTRIAL APPLICABILITY 
   Since the pneumatic tire of the invention is provided with a bead reinforcing layer having a predetermined structure around the bead core. Therefore, it is possible to enhance the steering stability without deteriorating the riding comfort and without excessively increasing the tire weight.