Patent Publication Number: US-6655431-B2

Title: Pneumatic tire with turnup portion wrapped along outer peripheral face of bead core

Description:
This is a divisional of application Ser. No. 09/756,879 filed Jan. 10, 2001; now U.S. Pat. No. 6,491,079 the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to pneumatic tires, particularly a pneumatic tire having an excellent bead portion durability. More particularly, it relates to a pneumatic tire capable of preventing pulling-out of a ply cord and occurrence of separation failure at an end position of a turnup portion of a carcass ply and also effectively controlling failure of a bead portion due to bending deformation of the bead portion around a rim flange under an action of a high loading to improve the bead portion durability. The invention is favorably applied to so-called heavy duty pneumatic radial tires used under conditions of a high load and a high internal pressure such as trucks, buses and the like. 
     2. Description of Related Art 
     In order to prevent pulling-out of a ply cord during the running of the pneumatic tire under loading, as shown in FIG. 1 a,  a carcass ply  100  is usually wound around a bead core  102  of a bead portion  101  from an inside of the tire toward an outside thereof in a widthwise direction and a turnup portion  103  thereof is embedded and fixed in rubber. 
     In the conventional tire having such a bead portion structure, however, a sidewall portion  104  is subjected to a large bending deformation during the running of the tire under loading, particularly under an action of a large loading and hence pulling force F is applied to the carcass ply  100  in a direction shown by an arrow in FIG. 1 a,  while a step difference in stiffness is caused on inside and outside of the tire in a radial direction thereof about an end position  105  of the turnup portion  103  of the carcass ply  100  and also stress concentration is repeatedly caused in the end position  105  and a neighborhood thereof. As a result, there is a problem that separation failure of the ply cord from rubber is apt to be caused and results in the occurrence of crack Cr in the bead portion as shown in FIG. 1 b.    
     As means for preventing such a separation failure to improve the bead portion durability, there are various proposals that it is useful to render a turnup portion  107  of a carcass ply  106  into a wrap part  109  winding on the bead core along its peripheral face as shown in FIG. 2 a.    
     However, when a reinforcing layer such as a wire chafer or the like is not arranged in the bead portion, a large bending deformation is created in a direction shown by an arrow A about a rim flange  110  as shown by a phantom line in FIG. 2 b.  As a result, crack is apt to be created in the bead portion of the tire contacting with the rim flange by repeating such a bending deformation and in the worst case, there is a risk of causing the bead portion failure by the growth of such a crack. 
     As means for improving the bead portion durability of the tire having the above bead portion structure, it is useful that at least one reinforcing layer  112  wound around a bead core  111  from an inside of the tire toward an outside thereof in a widthwise direction at a position outside the carcass ply  110  is arranged in the bead portion to control the bending deformation of the bead portion under the action of high loading applied to the tire as shown in FIG. 3 a.    
     As shown in FIG. 3 b,  however, stress concentrates in an end position  113  of the reinforcing layer  112  and there is caused a fear of creating troubles such as crack Cr of bead portion and the like in such a position  113 . As a result, such a means can not effectively improve the bead portion durability. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the invention to provide a pneumatic tire capable of preventing pulling-out of a ply cord and occurrence of separation failure at an end position of a turnup portion of a carcass ply and also effectively controlling failure of a bead portion due to bending deformation of the bead portion around a rim flange under an action of a high loading to improve the bead portion durability. 
     According to a first aspect of the invention, there is the provision of a pneumatic tire comprising at least one carcass ply of steel cords constituted with a main body portion toroidally extending from a tread portion through a sidewall portion to a bead portion and a turnup portion wound around a bead core embedded in the bead portion, in which a wrap part wound on the bead core along its outer peripheral face is disposed in the turnup portion of the carcass ply and at least one wire chafer wound around the bead core from the main body portion of the carcass ply toward the turnup portion thereof in a widthwise direction is embedded in the bead portion, and a shortest distance of a start end of the wire chafer located at the side of the main body portion of the carcass ply as measured outward from a normal line drawn to an outer face of the bead portion at a first rim line position in the radial direction of the tire is located to be within a range of 15-25 mm and a terminal end of the wire chafer located at the side of the turnup portion of the carcass ply is arranged within a range defined between a vertical line drawn from an outermost end position of the bead core in the radial direction to the outer face of the bead portion and the above normal line. 
     The term “first rim line position”used herein means an outermost end in the radial direction of that portion of an outer face of the tire which contacts with a rim when the tire is mounted onto an approved rim and applied under a maximum air pressure and a maximum load capacity. 
     In this case, the terms “maximum air pressure” and “maximum load capacity” used herein are defined according to “Year Book of The Tire and Rim Association Inc.” in USA, “Standard Manual of The European Tire and Rim Technical Organization” in Europe or “JATMA Year Book” in Japan. 
     And also, the term “turnup portion of carcass ply” used herein includes a case that the carcass ply is wound around the bead core from an outside of the tire toward an inside thereof in the widthwise direction in addition to a case that the carcass ply is wound around the bead core from an inside of the tire toward an outside thereof in the widthwise direction. 
     In addition, the carcass ply is a ply wherein many steel cords each having both cut ends are arranged substantially in the radial direction but also includes a ply wherein a single steel cord is folded at a position corresponding to the turnup portion of the carcass ply and continuously extended while detouring in a circumferential direction of the tire. 
     When it is required to more control the occurrence of separation failure at the terminal end position of the wire chafer, it is favorable to arrange at least one organic fiber chafer covering the terminal end of the wire chafer at the side of the turnup portion of the carcass ply, and in this case the organic fiber cords in the organic fiber chafer are preferable to be arranged at an angle of 15-75° with respect to an arranging direction of the steel cord in the wire chafer. 
     When it is required to more control separation failure at the start end position of the wire chafer, it is favorable to arrange a cushion rubber layer between the main body portion of the carcass ply and the start end portion of the wire chafer, and in this case a rubber thickness of the cushion rubber layer at the start end position of the wire chafer is preferable to be 1.5-2.0 mm at a section in the widthwise direction of the tire. 
     According to a second aspect of the invention, there is the provision of a pneumatic tire comprising at least one carcass ply of steel cords constituted with a main body portion toroidally extending from a tread portion through a sidewall portion to a bead portion and a turnup portion wound around a bead core embedded in the bead portion, a part or whole of which turnup portion being a wrap part wound along a peripheral face of the bead core, in which the main body portion of the carcass ply has a convexly curved shape toward an outer face of the tire or a combined shape of the curve and straight line at a section in a widthwise direction of the tire when the tire is mounted onto an approved rim and inflated at a maximum air pressure under no load, and a straight line or a chord connecting both ends of an arc drawn by the main body portion of the carcass ply over a zone ranging from a turning point of the main body portion on the bead core to any point in a region corresponding to 15-35% of a length in the radial direction of the tire defined between a standard line passing through an axial center of the bead core and drawn in parallel to a rotating axis of the tire and an outermost side of the main body portion in the radial direction and separated from the standard line is inclined at an angle of 45-65 °with respect to an equatorial plane of the tire. 
     In a preferable embodiment of the second aspect of the invention, the main body portion of the carcass ply has an arc shape convexly extending toward the outer surface of the tire over a zone ranging from the turning point of the main body on the bead core to any point in a region corresponding to 17-25% of the length in the radial direction of the tire separated from the standard line. 
     In another preferable embodiment, a maximum length of a vertical line drawn from the arc to a chord connecting both ends of the arc is not less than 2 mm. 
     According to the second aspect of the invention, the occurrence of separation failure at the turnup end of the carcass ply can be prevented by wrapping the turnup portion of the carcass ply on the peripheral face of the bead core, while the shape of the main body portion of the carcass ply in the bead portion is regulated to increase lateral stiffness of the bead portion, whereby outward fall-down deformation of the bead portion in the widthwise direction during the running of the tire under loading or the like can effectively be controlled to sufficiently prevent the occurrence of fatigue breakage in the bead portion contacting with a rim flange or the like. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described with reference to the accompanying drawings, wherein: 
     FIG. 1 is a diagrammatically partial section view illustrating a problem point in a bead portion structure of the conventional tire; 
     FIG. 2 is a diagrammatically partial section view illustrating a problem point in the conventional tire adopting such a bead portion structure that a carcass ply is wound around a bead core; 
     FIG. 3 is a diagrammatically partial section view illustrating a problem point caused at an end position of a wire chafer arranged for reinforcing the bead portion of FIG. 2; 
     FIG. 4 is a diagrammatically partial section view illustrating a state of mounting a bead portion of a first embodiment of the pneumatic tire according to the invention onto a rim; 
     FIG. 5 is a diagrammatically partial section view illustrating a state of mounting a bead portion of a second embodiment of the pneumatic tire according to the invention onto a rim; 
     FIG. 6 is a diagrammatically partial section view illustrating a state of mounting a bead portion of a third embodiment of the pneumatic tire according to the invention onto a rim; 
     FIG. 7 is a diagrammatically partial section view illustrating a state of mounting a bead portion of a fourth embodiment of the pneumatic tire according to the invention onto a rim; 
     FIG. 8 is a diagrammatically partial section view illustrating a state of mounting a bead portion of a fifth embodiment of the pneumatic tire according to the invention onto a rim; and 
     FIG. 9 is a schematic view illustrating a shape of a main body portion of a carcass ply according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 4 is shown a part of a bead portion in a first embodiment of the pneumatic tire according to the invention, wherein numeral  1  is a bead portion, numeral  2  a carcass ply, numeral  3  a sidewall portion, numeral  4  a bead core, and numeral  5  a wire chafer. 
     The tire having the bead portion  1  shown in FIG. 4 comprises at least one rubberized carcass ply  2  of steel cords arranged substantially in a radial direction of the tire (concretely, arranged at a cord angle of 70-90° with respect to an equatorial plane of the tire), which is constituted with a main body portion  2   a  toroidally extending from a tread portion (not shown) through the sidewall portion  3  to the bead portion  1  and a turnup portion  2   b  wound around the bead core  4  embedded in the bead portion  1 . And also, a belt (not shown) of one or more rubberized cord layers is arranged on a crown portion (not shown) of the carcass ply  2  for reinforcing the tread portion. 
     A main feature of the invention lies in that the bead structure of winding the carcass ply  2  around the outer surface of the bead core  4  is adopted and further the wire chafer  5  is arranged for adequately controlling the bending deformation of the bead portion in the application of a high loading to the tire. More concretely, the wrap part  6  wound around the bead core  4  along its outer peripheral face is arranged in the turnup portion  2   b  of the carcass ply  2 , whereby the bead portion durability can considerably be improved. 
     That is, the pulling-out of the carcass ply  2  and the occurrence of separation failure at the turnup end position  8  of the carcass ply  2  can be prevented by winding the carcass ply  2  around the outer peripheral face of the bead core  4 . In this case, the bead portion is considerably reinforced as compared with the conventional tire having the usual turnup structure (FIG. 1 a ). 
     However, when the high loading is applied to the tire, a large bending deformation about the rim flange as a fulcrum can not effectively be controlled only in the above structure, so that crack is apt to be caused in the bead portion  1  of the tire contacting with the rim flange  14  by repetition of such a bending deformation. 
     For this end, according to the invention, at least one wire chafer  5  wound around the bead core  4  from an inside of the tire toward an outside thereof in the widthwise direction  7  is arranged in the bead portion  1  from the main body portion  2   a  of the carcass ply  2  toward the turnup portion  2   b  thereof in addition to the above structure, and a shortest distance L of a start end  8  of the wire chafer  5  located at the side of the main body portion  2   a  of the carcass ply  2  as measured outward from a normal line n drawn to an outer face  13  of the bead portion  1  at a first rim line position  9  in the radial direction  10  of the tire is located to be within a range of 15-25 mm and a terminal end  11  of the wire chafer  5  located at the side of the turnup portion  2   b  of the carcass ply  2  is arranged within a range defined between a vertical line m drawn from an outermost end position  12  of the bead core  4  in the radial direction to the outer face  13  of the bead portion  1  and the above normal line n. According to such a structure, the above bending deformation can effectively be controlled and hence the bead portion durability can considerably improved. 
     The reason why the shortest distance L of the start end  8  of the wire chafer  5  is located within a range of 15-25 mm is due to the fact that when the shortest distance L is less than 15 mm, the deformation of the carcass ply in the circumferential direction can not sufficiently be controlled and hence the bending stiffness of the bead portion becomes large to facilitate the occurrence of troubles in the bead portion, while when it exceeds 25 mm, the start end  8  approaches to a flex zone, particularly in case of a low-section profile tire and stress concentrates in the start end  8  of the wire chafer  5  to facilitate the occurrence of separation failure. 
     And also, the reason why the terminal end  11  of the wire chafer  5  is arranged within the range defined between the vertical line m drawn from the outermost end position  12  of the bead core  4  in the radial direction to the outer face  13  of the bead portion  1  and the normal line n is due to the fact that when the terminal end  11  locates downward from the vertical line m in the radial direction  10  of the tire, the turnup portion  2   b  of the carcass ply  2  can not sufficiently be protected and separation failure is apt to be caused at an interface between the terminal end  11  and the turnup portion  2   b,  while when it locates upward from the normal line n in the radial direction  10  of the tire, a large shearing strain is applied to the terminal end  11  to facilitate the occurrence of troubles in the bead portion. 
     In a second embodiment of the invention, as shown in FIG. 5, at least one organic fiber chafer covering the terminal end  11  of the wire chafer  5  (two chafers  15   a,    15   b  in the illustrated embodiment) may be arranged at the side of the turnup portion  2   b  of the carcass ply  2 , whereby the shearing strain can be more controlled at the terminal end  11  of the wire chafer  5  to further improve the bead portion durability. 
     Furthermore, when organic fiber cords  16  in the organic fiber chafers  15   a,    15   b  are arranged at a cord angle of 15-75° with respect to the extending direction of steel cords  17  in the wire chafer  5 , forces applied from various directions to a terminal end portion  5   b  of the wire chafer  5  can be born by the organic fiber cords in the organic fiber chafers, whereby the shearing strain at the position of the terminal end  11  of the wire chafer  5  can be more controlled. 
     In addition, when it is required to more suppress separation failure at the start end  8  of the wire chafer  5 , as shown in FIG. 6, it is favorable to interpose a cushion rubber layer  18  between the main body portion  2   a  of the carcass ply  2  and the start end portion  5   a  of the wire chafer  5 . 
     In this case, it is preferable that a rubber thickness t of the cushion rubber layer  18  at the start end  8  of the wire chafer  5  is 1.5-2.0 mm viewing at a section in the widthwise direction of the tire. When the rubber thickness t is less than 1.5 mm, the effect of suppressing separation failure by the arrangement of the cushion rubber layer  18  is not sufficiently obtained, while when it exceeds 2.0 mm, there is caused a fear that the effect of controlling the deformation in the circumferential direction through the wire chafer  5  is not sufficiently obtained. 
     In FIG. 7 is shown a fourth embodiment of the bead portion according to the invention, wherein numeral  21  is a main body portion of a carcass ply toroidally extending from a tread portion (not shown) through a sidewall portion (not shown) to a bead portion  22 . The main body portion  21  of the carcass ply is comprised of steel cords arranged at a cord angle of, preferably, about 70-90° with respect to an equatorial plane of the tire. 
     Further, a side portion of the carcass ply is turned around a bead core  23  embedded in the bead portion  22  from an inside of the tire toward an outside thereof in a radial direction of the tire and then wound along an outer peripheral face of the bead core  23  to form a wrap part  24 . 
     In the winding of the turnup portion, it is favorable that at least one site of a portion of the wrap part  24  corresponding to corner parts of the bead core  23  is subjected to a plastic deformation so as to more accurately extend along the peripheral face of the bead core. In this way, the wrap part  24  is effectively restrained by the bead core  3 , whereby a fear of causing the pulling-out of the carcass ply and the separation failure of the wrap part can advantageously be removed. 
     Moreover, the plastic deformed site is favorable to be previously formed in a portion corresponding to the wrap part  24  prior to the winding of the carcass ply around the peripheral face of the bead core  23 . In this case, the plastic deformation can be always and accurately conducted as it is expected, whereby the above effect can be more enhanced. 
     In the invention, the wrap part  24  can be more closed to the bead core  23  through a bead filler  25  located side the outer periphery of the bead core by sandwiching the wrap part  24  between the bead core  23  and the bead filler  25 , so that the pulling-out of the carcass ply and the separation failure of the wrap part can be more advantageously prevented. 
     It is more favorable that the wrap part  24  is extended along the peripheral face of the bead core over a half periphery of the sectional profile of the bead core. 
     In the invention, it is important that the main body portion  21  of the carcass ply has a convexly curved shape toward an outer face of the tire or a combined shape of the curve and straight line at a section in a widthwise direction of the tire when the tire is mounted onto an approved rim and inflated at a maximum air pressure under no load, and a straight line or a chord connecting both ends of an arc drawn by a part a of the main body portion  21  over a zone ranging from a turning point P of the main body portion on the bead core  23  to any point (H 2 ) in a region corresponding to 15-35% of a length H 1  in the radial direction of the tire defined between a standard line S passing through an axial center of the bead core and drawn in parallel to a rotating axis of the tire and an outermost side of the main body portion in the radial direction and separated from the standard line is inclined at an angle θ of 45-65° with respect to an equatorial plane O of the tire. 
     The turning point P of the carcass ply is a turning start point of the carcass ply against the bead core  23 . More concretely, it means a point that the carcass ply extending straight toward the bead core  3  or convexly extending toward the outer surface of the tire is turned about the bead core  3  and convexly inflected toward the inner surface of the tire. 
     That is, when the convexly curved shape toward the outer face of the tire or the combined shape of the curve and straight line is given to the main body portion  21  of the carcass ply at the section in the widthwise direction of the tire, a carcass line under an inflation of an internal pressure can be closed to a natural equilibrium shape capable of controlling the inward falling deformation in the section as compared with the conventional one. Moreover, the term “carcass line” used herein means a center line in a thickness direction of the carcass ply constituting the main body portion at a section in the widthwise direction of the tire. Concretely, the carcass lien means a line at a center of the arranging position of the cord embedded in the carcass ply when using the one carcass ply, or a line of a center in total ply thickness when using two or more carcass plies. 
     Furthermore, the inclination angle θ of the zone  21   a  of the main body portion  21  ranging from the position of any points (H 2 ) measured from the standard line S in the radial direction of the tire up to the turning point P at the section in the widthwise direction of the tire is made to 45-65° with respect to an equatorial plane O of the tire, whereby the main body portion  21  in the bead portion is arranged so as to more shift toward the outer surface of the tire as compared with the conventional carcass line shown in FIG. 2 a.  As a result, the main body  21  is disposed in the vicinity of thickness center of the bead portion, so that the rubber thickness of from the main body portion  21  to the outer surface of the tire can be thinned as compared with that of the conventional tire to control the deformation of an outer surface part of the bead portion in the circumferential direction of the tire. 
     When the inclination angle θis less than 45°, the carcass lien approaches to the inner surface of the tire and becomes large in the distance from the natural equilibrium shape, while when it exceeds 65°, the distance from the natural equilibrium shape becomes also large and the effect of suppressing the falling-down is decreased. 
     Furthermore, the reason why the position of any points (H 2 ) from the standard line S in the radial direction of the tire is restricted to a range of 15-35% of the length in the radial direction of the tire (H 1 ) is due to the fact that when the position is within the above range, it is advantageous to make the carcass line in the inflation of the internal pressure close to the natural equilibrium shape to suppress the falling-down. 
     Moreover, when the zone  21   a  of the main body portion  21  extends straight, the inclination angle θ is an angle of a straight line with respect to the equatorial plane O, while when the zone  21   a is curved as shown in FIG. 9, the inclination angle θ is an angle of a chord  21   b  connecting both ends of an arc corresponding to the curved line of the zone  21   a  to each other with respect to the equatorial plane O. 
     And also, when the zone  21   a  of the main body portion  21  is curved as shown in FIG. 9, it is favorable that a maximum length of a vertical line h drawn from the arc to the chord connecting both ends of the arc to each other is not less than 2 mm, because when the maximum length of the vertical line h is not less than 2 mm, the carcass line becomes more close to the natural equilibrium shape and the rubber thickness between the main body portion and the outer surface of the tire can be more thinned. 
     From the same reason as mentioned above, it is more favorable that when the zone  21   a  of the main body portion  21  is curved, the position of any points (H 2 ) in the radial direction of the tire corresponds to 17-25% of the length in the radial direction of the tire (H 1 ) from the standard line S defining the zone  21   a.    
     The following examples are given in illustration of the invention and are not intended as limitations thereof. 
     There are provided tires of Examples 1-7 each having a tire size of TBR 315/60R22.5, wherein start end position  8  and terminal end position  11  of a wire chafer  5 , presence or absence of an arrangement of a cushion rubber layer  18  and a rubber gauge t thereof, and presence or absence of an arrangement of an organic fiber chafer and a cord angle thereof are shown in Table 1. 
     For the comparison, there are provided tires of Comparative Examples 1-5 having a wrap structure in a bead portion and using a wire chafer provided that either start end position  8  or terminal end position  11  of the wire chafer is outside the range defined in the invention, and a tire of Conventional Example having the usual turnup structure of a carcass ply. 
     Each of the above tires is mounted onto a rim of 9.00×22.5 and subjected to the following test for the evaluation of bead portion durability. 
     The bead portion durability is evaluated from results by a test for measuring deformation in circumferential direction and drum test for bead portion durability. 
     In the test for measuring the deformation in circumferential direction, a flat plate is pushed downward onto the tire under a force of 66.15 kN at a free state to measure a deformation quantity in circumferential direction. 
     In the drum test for bead portion durability, the tire inflated under an internal pressure of 900 kPa is run on a rotating drum at a speed of 60 km/h under a load of 66.15 kN to measure a running distance until the occurrence of troubles in the tire. 
     The evaluation results are shown in Table 1. Moreover, the numerical values in Table 1 are represented by an index on the basis that the conventional example is 100. In this case, the deformation in circumferential direction is smaller as the index value becomes smaller, while the bead portion durability by the drum test is better as the index value becomes larger. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                   Evaluation result of 
               
               
                   
                 bead portion durability 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Cushion rubber 
                 Organic fiber 
                 index of 
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Wire chafer 5 
                   
                 layer 18 
                 chafer 
                 deformation in 
                   index of bead portion 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 start end 
                 terminal end 
                   
                 rubber 
                   
                 cord 
                 circumferential 
                 durability by 
               
               
                   
                 position 8 
                 position 11 
                 arrangement 
                 gauge 
                 arrangement 
                 angle 
                 direction 
                 drum test 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Conventional 
                 L = 0 mm 
                 +10 mm of 
                 absence 
                 — 
                 absence 
                 — 
                 100 
                 100 
               
               
                 Example 
                   
                 normal line n 
               
               
                 Comparative 
                 L = +30 mm 
                 −5 mm of 
                 presence 
                 1.5 
                 absence 
                 — 
                 61 
                 105 (peculiar 
               
               
                 Example 1 
                   
                 normal line n 
                   
                   
                   
                   
                   
                 trouble) 
               
               
                 Comparative 
                 L = +20 mm 
                 −5 mm of 
                 absence 
                 — 
                 absence 
                 — 
                 80 
                 105 (peculiar 
               
               
                 Example 2 
                   
                 vertical line m 
                   
                   
                   
                   
                   
                 trouble) 
               
               
                 Comparative 
                 L = +20 mm 
                 +5 mm of 
                 presence 
                 1.0 
                 absence 
                 — 
                 81 
                 110 (peculiar 
               
               
                 Example 3 
                   
                 normal line n 
                   
                   
                   
                   
                   
                 trouble) 
               
               
                 Comparative 
                 L = +20 mm 
                 +5 mm of 
                 presence 
                 2.5 
                 absence 
                 — 
                 90 
                 112 (peculiar 
               
               
                 Example 4 
                   
                 normal line n 
                   
                   
                   
                   
                   
                 trouble) 
               
               
                 Comparative 
                 L = +20 mm 
                 −5 mm of 
                 absence 
                 — 
                 absence 
                 — 
                 55 
                 88 (peculiar 
               
               
                 Example 5 
                   
                 vertical line m 
                   
                   
                   
                   
                   
                 trouble) 
               
               
                 Example 1 
                 L = +15 mm 
                 −5 mm of 
                 presence 
                 1.5 
                 absence 
                 — 
                 89 
                 158 
               
               
                   
                   
                 normal line n 
               
               
                 Example 2 
                 L = +20 mm 
                 −5 mm of 
                 presence 
                 2.0 
                 absence 
                 — 
                 83 
                 193 
               
               
                   
                   
                 normal line n 
               
               
                 Example 3 
                 L = +25 mm 
                 −5 mm of 
                 presence 
                 1.5 
                 absence 
                 — 
                 77 
                 173 
               
               
                   
                   
                 normal line n 
               
               
                 Example 4 
                 L = +20 mm 
                 on normal line n 
                 absence 
                 — 
                 absence 
                 — 
                 75 
                 121 
               
               
                 Example 5 
                 L = +20 mm 
                 −5 mm of 
                 absence 
                 — 
                 absence 
                 — 
                 69 
                 140 
               
               
                   
                   
                 normal line n 
               
               
                 Example 6 
                 L = +20 mm 
                 on vertical line m 
                 absence 
                 — 
                 absence 
                 — 
                 64 
                 152 
               
               
                 Example 7 
                 L = +20 mm 
                 on normal line m 
                 absence 
                 — 
                 presence *1 
                 ±45° 
                 52 
                 170 
               
               
                   
               
               
                 *1: two nylon chafers  
               
            
           
         
       
     
     As seen from the results of Table 1, Examples 1-7 are superior in the bead portion durability to the conventional example. On the other hand, Comparative Examples 1-5 indicate the occurrence of peculiar trouble in the drum test for bead portion durability. 
     Then, there is provided a radial tire for truck and bus according to the invention having a structure shown in FIG. 8, a carcass line of a main body portion of a carcass ply defined in Table 2 and tire size of 435/45R22.5. For the comparison, there is provided a comparative tire having a structure of a main body portion of a carcass ply shown in FIG. 2 a  and the same tire size as mentioned above. Then, these tires are subjected to a drum test for the evaluation of durability. 
     That is, the tire is mounted onto a rim of 14.00×22.5, inflated under an internal pressure of 900 kpa and then run on a drum at a speed of 60 km/h under a load corresponding to 1.95 times a maximum load capacity to measure a running distance until the tire can not be run due to the occurrence of troubles in the bead portion. The measured values are shown in Table 2. Moreover, the bead portion durability is represented by an index of the measured value on the basis that the comparative tire is 100, in which as the larger the index value, the durability becomes better. 
     With respect to these tires, a falling-down quantity of a main body portion of a carcass ply is measured at a state that the tire is inflated under the above internal pressure and loaded under the above load based on a state that the internal pressure and load are not applied to the tire. The falling-down quantity is represented by an index on the basis that the comparative tire is 100. 
     Similarly, a deformation quantity in circumferential direction of an outer surface of a bead portion is measured at a state that the tire is inflated under the above internal pressure and loaded under the above load based on a state that the internal pressure and load are not applied to the tire. The deformation quantity in circumferential direction is represented by an index on the basis that the comparative tire is 100. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Comparative tire 
                 Invention tire 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Carcass line 
                 FIG. 2a 
                 FIG. 8 
               
               
                   
                 H 2 /H 1   
                 — 
                   22% 
               
               
                   
                 θ (°) 
                  33 
                 55 
               
               
                   
                 Falling-down quantity 
                 100 
                 75 
               
               
                   
                 Deformation quantity in 
                 100 
                 84 
               
               
                   
                 circumferential direction 
               
               
                   
                 Bead portion durability 
                 100 
                 160  
               
               
                   
                   
               
            
           
         
       
     
     As seen from the above, according to the invention, the wrap part wound around the bead core along the peripheral face thereof is formed in the carcass ply and the wire chafer is arranged or the shape of the main body portion of the carcass ply is made adequate, whereby the bead portion durability can largely be improved. Especially, the invention is useful in so-called heavy duty pneumatic radial tires used under higher internal pressure and load.