Patent Publication Number: US-11034189-B2

Title: Pneumatic tire

Description:
TECHNICAL FIELD 
     The present technology relates to a pneumatic tire, and more particularly relates to a pneumatic tire, which is reduced in weight and is capable of securing rigidity desired for each of a tread region and side regions by employing a curved carcass structure in which an angle of a carcass cord continuously changes in the tread region and the side regions. 
     BACKGROUND ART 
     Generally, as a method of reducing weight of a tire, the thickness is reduced of rubber used for, for example, a cap tread and sides, which are portions of the tire large in area. However, from the perspective of wear resistance and durability, it is difficult to achieve further reduction in weight with the method of reducing the thickness of each portion of the tire. 
     Meanwhile, for reduction in weight of the tire, there have been proposed various types of a tire which does not require an independent belt layer of a related-art radial structure. For example, in Japan Unexamined Patent Publication No. 2005-22537, the following pneumatic tire can be obtained. That is, in each of an inner side cord layer and an outer side cord layer, a large number of cords are arrayed in parallel with each other. A cord angle in each cord layer is set to a range from 80° to 90° in sidewall portions and to a range from 15° to 50° in a center of a tread portion, and the cords are tilted reversely with each other between both the cords layers. As a result, the pneumatic tire can have both an internal-pressure retaining function and a hoop function without a belt layer. However, the related art relates to a complicated manufacturing method in which one cord continuously reciprocates between right and left bead cores, and hence practical utility is not high. 
     Further, in Japan Unexamined Patent Publication No. 2002-127711, the following pneumatic tire can be obtained. That is, the pneumatic tire includes two or more carcass layers for reinforcing a space between a pair of beads. Cords of the carcass layers are arranged substantially in a tire radial direction from the bead to a position near a tire maximum width position. From this position to a ground contact edge, an angle of the cords with respect to a circumferential direction gradually changes. With respect to the circumferential direction, the cords are arranged at an angle from 20° to 60° in the vicinity of the ground contact edge, and are arranged at an angle from 20° to 50° at a tread portion. However, in the case where reinforcement is performed through use of the two or more carcass layers, as compared to a case where a single carcass layer is used, portions such as end portions of the carcass layers, which are wound about the bead cores, do not substantially function as reinforcing materials. Thus, there is a problem in that effective reinforcement cannot be performed while reducing the tire in weight. 
     SUMMARY 
     The present technology provides a pneumatic tire, which is reduced in weight and is capable of securing rigidity desired for each of a tread region and side regions by employing a curved carcass structure in which an angle of a carcass cord continuously changes in the tread region and the side regions. 
     An embodiment of the present technology is a pneumatic tire including a tread portion, a pair of sidewall portions, a pair of bead portions, and a carcass layer. The tread portion extends in a tire circumferential direction and has an annular shape. The pair of sidewall portions are arranged on both sides of the tread portion. The pair of bead portions are arranged inward in a tire radial direction of the pair of sidewall portions. The carcass layer includes a plurality of carcass cords, which are mounted between the pair of bead portions, and are wound from a tire inner side to an outer side about bead cores of the pair of bead portions. When a tread region indicates a region corresponding to a ground contact width of the tread portion, a tread central region indicates a region corresponding to 80% of a center of the ground contact width of the tread portion, and a side region indicates a region inward in the tire radial direction with respect to a tire maximum width position, both terminals of the carcass layer are overlapped with each other in the tread portion. The carcass layer has a triple layer structure including an inner layer positioned most inward in the tire radial direction in the tread region, an outer layer positioned most outward in the tire radial direction in the tread region, and an intermediate layer positioned between the inner layer and the outer layer in the tread portion. A cord angle in the carcass layer in the tread central region with respect to the tire circumferential direction differs in at least one of the inner layer, the outer layer, and the intermediate layer from a cord angle in the carcass layer in the side region with respect to the tire circumferential direction. 
     The present technology has the following structure. That is, both the terminals of the carcass layer overlap with each other in the tread portion. The carcass layer has a triple layer structure including the inner layer positioned most inward in the tire radial direction in the tread region, the outer layer positioned most outward in the tire radial direction in the tread region, and the intermediate layer positioned between the inner layer and the outer layer in the tread portion. The cord angle in the carcass layer in the tread central region with respect to the tire circumferential direction differs in at least one of the inner layer, the outer layer, and the intermediate layer from the cord angle in the carcass layer in the side region with respect to the tire circumferential direction. Thus, rigidity different for each of the tread region and the side region can be achieved, and the rigidity desired for each of the tread region and the side region can be secured. Further, as compared to the related-art pneumatic tire including the plurality of belt layers, the tire can be reduced in weight. Further, as compared to the pneumatic tire including the carcass layers, which are not single but have two or more layers, portions which do not substantially function as reinforcing materials can be reduced, and hence the space between the bead portions can be reinforced with the minimum number of members. 
     In the present technology, it is preferred that the cord angle in at least one of the inner layer, the outer layer, and the intermediate layer with respect to the tire circumferential direction fall within a range from 10° to 75° in the tread central region, and that the cord angle in the carcass layer with respect to the tire circumferential direction fall within a range from 85° to 90° in the side region. Thus, the carcass layer can sufficiently share the function as the belt layer. Further, near a boundary between the tread region and the side region, the carcass cords are allowed to be gently changed in angle, and hence the durability can be improved. More preferably, the cord angle preferably falls within a range from 15° to 70° and more preferably a range from 15° to 65° in the tread central region. 
     In the present technology, it is preferred that a cord count per unit width, which is 50 mm, for the plurality of carcass cords forming the carcass layer at least in the tread region be from twenty to seventy, and that a diameter of each of the plurality of carcass cords fall within a range from 0.2 mm to 1.5 mm. Thus, increase in mass of the carcass layer is suppressed, which contributes to reduction in weight of the tire. At the same time, degradation in rigidity and durability can be suppressed. 
     In the present technology, it is preferred that the belt layer be provided on an outer circumferential side of the carcass layer in the tread region. As a result, the rigidity of the tread portion can sufficiently be secured, and satisfactory steering stability can be exerted. 
     In the present technology, it is preferred that the belt layer be a single layer. Thus, increase in mass of the belt layer can be suppressed to a minimum extent, and the hoop effect of the tread portion can be increased. 
     In the present technology, it is preferred that the cord angle in the belt layer with respect to the tire circumferential direction fall within a range from 15° to 45°. Thus, the belt layer can be caused to have the hoop function required as the belt layer. 
     In the present technology, it is preferred that an intermediate rubber layer having a thickness from 0.2 mm to 2.0 mm be arranged between the carcass layer and the belt layer. Thus, degradation in out-of-plane flexural rigidity of the tread region can be complemented, and plane flexural rigidity of the tread region can be improved. 
     In the present technology, it is preferred that a belt reinforcing layer be provided outward in the tire radial direction of the belt layer. Thus, high-speed durability can be improved. 
     In the present technology, a ground contact region of the tread portion is specified based on a tread ground contact width in a tire axial direction, which is measured when the tire is mounted on a regular rim, is inflated to a regular internal pressure, and is placed vertically upon a flat surface with a regular load applied thereto. Ground contact edges are outer most positions of the ground contact region in the tire axial direction. “Regular rim” is a rim defined by a standard for each tire according to a system of standards that includes standards on which tires are based, and refers to a “standard rim” in the case of JATMA (Japan Automobile Tyre Manufacturers Association, Inc.), refers to a “design rim” in the case of TRA (Tire and Rim Association, Inc.), and refers to a “measuring rim” in the case of ETRTO (European Tyre and Rim Technical Organization). “Regular internal pressure” is an air pressure defined by standards for each tire according to a system of standards that includes standards on which tires are based, and refers to a “maximum air pressure” in the case of JATMA, refers to the maximum value in the table of “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” in the case of TRA, and refers to the “INFLATION PRESSURE” in the case of ETRTO. “Regular internal pressure” is 180 kPa for a tire on a passenger vehicle. “Regular load” is a load defined by standards for each tire according to a system of standards that includes standards on which tires are based, and refers to “maximum load capacity” in the case of JATMA, refers to the maximum value in the table of “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” in the case of TRA, and refers to “LOAD CAPACITY” in the case of ETRTO. When the tire is for use with a passenger vehicle, a load corresponding to 88% of the load described above is used. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a meridian cross-sectional view illustrating a pneumatic tire according to an embodiment of the present technology. 
         FIG. 2  is a plan view in which a carcass layer, a belt layer, and a belt reinforcing layer of the pneumatic tire according to the embodiment of the present technology are extracted and illustrated. 
         FIG. 3  is a plan view partially illustrating the carcass layer, the belt layer, and the belt reinforcing layer in  FIG. 2  for the purpose of defining cord angles of carcass cords and belt cords. 
         FIG. 4  is an explanatory diagram schematically illustrating an angle change of the carcass cords in each portion of the carcass layer in  FIG. 1 . 
         FIG. 5  is a plan view in which a carcass layer, a belt layer, and a belt reinforcing layer in a modified example of the pneumatic tire according to the embodiment of the present technology are extracted and illustrated. 
         FIG. 6  is an explanatory diagram schematically illustrating an angle change of the carcass cords in each portion of the carcass layer in  FIG. 5 . 
         FIG. 7A  is an explanatory diagram schematically illustrating positions of both ends of the carcass layer in  FIG. 1 , and  FIG. 7B  and  FIG. 7C  are explanatory views illustrating modified examples. 
     
    
    
     DETAILED DESCRIPTION 
     Now, with reference to the accompanying drawings, detailed description is made of the configuration of the present technology.  FIG. 1  and  FIG. 2  are illustrations of a pneumatic tire according to an embodiment of the present technology. Note that, the line CL indicates a tire center line in  FIG. 1  and  FIG. 2 . 
     As illustrated in  FIG. 1 , the pneumatic tire of the present embodiment includes a tread portion  1  extending in a tire circumferential direction to have an annular shape, a pair of sidewall portions  2  and  2  arranged on both sides of the tread portion  1 , and a pair of bead portions  3  and  3  arranged inward of the sidewall portions  2  in a tire radial direction. 
     A single carcass layer  4  is mounted between the pair of bead portions  3  and  3 . The carcass layer  4  includes a plurality of carcass cords  41  extending in the tire radial direction, and is folded back around bead cores  5  arranged in each of the bead portions  3  from a tire inner side to a tire outer side. A bead filler  6 , which has a triangular cross-sectional shape and is formed from rubber composition, is arranged on the outer circumference of the bead core  5 . 
     In this case, a tread region Rt indicates a region corresponding to a ground contact width W of the tread portion  1 . A side region Rs indicates at least a region inward in the tire radial direction from a tire maximum width position P max . A tread central region Rc indicates a region corresponding to 80% of a center of the ground contact width W of the tread portion  1 . The carcass layer  4  includes an inner layer  4 A arranged inward in the tire radial direction in the tread region Rt, an outer layer  4 B arranged outward in the tire radial direction in the tread region Rt, and an intermediate layer  4 C arranged between the inner layer  4 A and the outer layer  4 B in the tread region Rt, and has a triple layer structure in the tread region Rt. 
     A belt layer  7  having a single layer is embedded on an outer circumferential side of the carcass layer  4  in the tread portion  1 . When the belt layer  7  has a single layer as described above, an increase in mass of the belt layer  7  can be suppressed to a minimum extent, and a hoop effect of the tread portion  1  can be increased. The belt layer  7  includes a plurality of belt cords  71  that are inclined with respect to the tire circumferential direction. As the belt cords  71  of the belt layer  7 , steel cords or organic fiber cords of nylon, aramid, or the like are preferably used. 
     For the purpose of improving high-speed durability, a belt reinforcing layer  8  is arranged on an outer circumferential side of the belt layer  7 . The belt reinforcing layer  8  has a plurality of (two in  FIG. 1 ) layers in which fiber cords are oriented in the tire circumferential direction. The belt reinforcing layer  8  may have, for example, a jointless structure in which a strip material formed by aligning at least one rubber-coated fiber cord is spirally wound in the tire circumferential direction. A cord angle of the belt reinforcing layer  8  with respect to the tire circumferential direction is 5° or less and more preferably 3° or less. As the fiber cord of the belt reinforcing layer  8 , there are suitably used an organic fiber cord of, for example, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), nylon, rayon, or aramid, an aramid fiber cord with high elasticity, or a composite cord in which an aramid fiber with high elasticity and a nylon fiber with low elasticity are intertwined. Note that, in the mode in  FIG. 1 , a portion of the belt reinforcing layer  8 , which is positioned inward in the tire radial direction, forms a full cover for covering the entire width of the belt layer  7 , and portions of the belt reinforcing layer  8 , which are positioned outward in the tire radial direction, form edge cover layers for covering only edges of the belt layer  7 . 
     Further, an intermediate rubber layer  9  is arranged between the carcass layer  4  and the belt layer  7 . It is preferred that the intermediate rubber layer  9  have a modulus of 10 MPa or greater and a dissipation factor (tan 6) of 0.2 or less at 60° C. Further, short fibers may be blended in a rubber sheet forming the intermediate rubber layer  9 , but long fibers are excluded. A thickness of the intermediate rubber layer  9  may be formed to fall within a range from 0.2 mm to 2.0 mm. When the thickness of the intermediate rubber layer  9  is appropriately set as described above, degradation in out-of-plane flexural rigidity of the tread region Rt can be complemented, and plane flexural rigidity of the tread region Rt can be improved. 
       FIG. 2  is a view in which the carcass layer  4 , the belt layer  7 , and the belt reinforcing layer  8  of the pneumatic tire according to the embodiment of the present technology are extracted and illustrated. The tire radial direction is denoted by Tr, and the tire circumferential direction is denoted by Tc. As illustrated in  FIG. 2 , the carcass cords  41  forming the carcass layer  4  are inclined with respect to the tire radial direction, that is, a tire width direction in the tread region Rt, and at the same time, are arranged to extend along the tire radial direction in the side region Rs. In the mode in  FIG. 2 , the carcass cords  41  in each of the inner layer  4 A, the outer layer  4 B, and the intermediate layer  4 C are inclined with respect to the tire width direction in the tread region Rt. However, in an embodiment of the present technology, the carcass cords  41  in at least one layer of the inner layer  4 A, the outer layer  4 B, and the intermediate layer  4 C may be arranged to be inclined with respect to the tire width direction in the tread region Rt. 
     Meanwhile, the belt cords  71  forming the belt layer  7  are inclined in the same direction with respect to the tire radial direction in the tread region Rt. In the mode in  FIG. 2 , the carcass cords  41  in the inner layer  4 A and the outer layer  4 B, and the belt cords  71  cross with each other. An embodiment of the present technology has the structure in which, in the tread region Rt, the carcass cords  41  in at least one layer of the inner layer  4 A, the outer layer  4 B, and the intermediate layer  4 C, and the belt cords  71  are arranged to cross with each other. 
     As illustrated in  FIG. 3 , an angle of the carcass cords  41  in the carcass layer  4  with respect to the tire circumferential direction is indicated as a cord angle θ 1 . The cord angle θ 1  indicates an average angle in each of the tread central region Rc and the side region Rs. In this case, as for the cord angle θ 1  in the carcass layer  4  in the tread central region Rc, the cord angle θ 1  in at least one layer of the inner layer  4 A, the outer layer  4 B, and the intermediate layer  4 C is set different from the cord angle θ 1  in the side region Rs. Further, in the mode in  FIG. 2 , there is exemplified an example in which the cord angle θ 1  in the inner layer  4 A or the outer layer  4 B and the cord angle θ 1  in the intermediate layer  4 C are different from each other in the tread central region Rc, and in which the carcass cords  41  in the inner layer  4 A or the outer layer  4 B and the carcass cords  41  in the intermediate layer  4 C are arranged to be orthogonal with each other in the tread central region Rc. However, when the belt layer  7  is provided, the carcass cords  41  may be arranged to have the same cord angle θ 1  in the inner layer  4 A, the outer layer  4 B, and the intermediate layer  4 C in the tread central region Rc. 
       FIG. 4  is a view schematically illustrating an angle change of the carcass cords  41  in each portion of the carcass layer  4  in  FIG. 1 . That is,  FIG. 4  is a view continuously illustrating the change of the cord angle θ 1  of the carcass cords  41  between the one terminal  40 A and the other terminal  40 B when the entire single carcass layer  4  is viewed as one member in plan view. When the carcass cords  41  are parallel with the tire radial direction, the cord angle θ 1  is 90°, and when the carcass cords  41  are inclined, the cord angle θ 1  is other than 90° (45° or −45° in  FIG. 4 ). The carcass layer  4  mainly includes three portions, which are a turn-up portion Pu, a side portion Ps, and a tread portion Pt. That is, as for the carcass cords  41  illustrated in  FIG. 4 , the cord angle θ 1  is 45° or −45° in the tread portion Pt, and the cord angle θ 1  is 90° in the turn-up portion Pu and the side portion Ps. Accordingly, the carcass layer  4  is folded back on the tire, and as illustrated in  FIG. 2 , the carcass cords  41  in the inner layer  4 A or the outer layer  4 B and the carcass cords  41  in the intermediate layer  4 C are arranged to be orthogonal with each other. 
       FIG. 5  is a view of a modified example of the pneumatic tire according to the embodiment of the present technology. In the mode in  FIG. 5 , the carcass cords  41  in each layer of the outer layer  4 B and the intermediate layer  4 C are inclined with respect to the tire width direction in the tread region Rt. The carcass cords  41  in the inner layer  4 A extend in the tire radial direction, and the carcass cords  41  in the outer layer  4 B and the carcass cords  41  in the intermediate layer  4 C are arranged to be orthogonal with each other.  FIG. 6  is a view schematically illustrating an angle change of the carcass cords  41  in each portion of the carcass layer  4  in  FIG. 5 . Only in the tread portions Pt of the tread portions Pt illustrated in  FIG. 6 , which are positioned on both the end portions when the entire single carcass layer  4  is viewed as one member in plan view, the cord angle θ 1  is 45° or −45°. The cord angle θ 1  is 90° in all the other portions. In this case, the carcass layer  4  is folded back on the tire, and as illustrated in  FIG. 5 , the carcass cords  41  in the outer layer  4 B and the carcass cords  41  in the intermediate layer  4 C are arranged to be orthogonal with each other. In an embodiment of the present technology, as described above, the cord angle θ 1  is appropriately set only in the tread portions Pt positioned on both the end portions when the entire single carcass layer  4  is viewed as one member in plan view. Thus, only both the end portions of the carcass layer  4  can be formed to have a curved carcass structure, which is suitable for preparing members at the time of tire formation. 
       FIG. 7A  is a view schematically illustrating positions of both terminals  40 A and  40 B of the carcass layer  4  in  FIG. 1 . The terminals  40 A and  40 B illustrated in  FIG. 7A  extend to the parts near the shoulder portions of the tread portion  1 , which are positioned on opposite sides with respect to the tire center line CL as the reference.  FIG. 7B  and  FIG. 7C  are modified examples of  FIG. 7A . The terminals  40 A and  40 B illustrated in  FIG. 7B  extend beyond the shoulder portions of the tread portion  1 , which are positioned on the opposite sides with respect to the tire center line CL as the reference, and extend to the parts near the bead cores  5 . Further, the one terminal  40 A illustrated in  FIG. 7C  extends to the ground contact edge of the tread portion  1  positioned on the opposite side with the tire center line CL as the reference, and the other terminal  40 B is overlapped with the terminal  40 A to be terminated without reaching the tire center line CL. Particularly, the positions of both the terminals  40 A and  40   b  are suitably set as illustrated in  FIG. 7A  and  FIG. 7B . In an embodiment of the present technology, both the terminals  40 A and  40 B may be arranged so that the carcass layer  4  has a triple structure in the entire width in the tread region Rt. Such arrangement of both the terminals  40 A and  40 B contributes to improvement in steering stability. 
     In the above-described pneumatic tire, the following structure is provided. That is, both the terminals  40 A and  40 B of the carcass layer  4  are overlapped with each other in the tread region Rt. The carcass layer  4  has a triple layer structure including the inner layer  4 A, the outer layer  4 B, and the intermediate layer  4 C. The inner layer  4 A is positioned most inward in the tire radial direction in the tread region Rt. The outer layer  4 B is positioned most outward in the tire radial direction in the tread region Rt. The intermediate layer  4 C is positioned between the inner layer  4 A and the outer layer  4 B in the tread region Rt. The cord angle θ 1  in the carcass layer  4  in the tread central region Rc with respect to the tire circumferential direction differs in at least one layer of the inner layer  4 A, the outer layer  4 B, and the intermediate layer  4 C from the cord angle θ 1  in the carcass layer  4  in the side region Rs with respect to the tire circumferential direction. With this structure, rigidity different for each of the tread region Rt and the side region Rs can be achieved, and the rigidity desired for each of the tread region Rt and the side region Rs can be secured. Further, as compared to the related-art pneumatic tire including the plurality of belt layers, the tire can be reduced in weight. Further, as compared to the pneumatic tire including the carcass layers, which are not single but have two or more layers, portions which do not substantially function as reinforcing materials can be reduced, and hence the space between the bead portions  3  can be reinforced with the minimum number of members. 
     Particularly, it is preferred that the cord angle θ 1  in the carcass layer  4  in the tread central region Rc with respect to the tire circumferential direction be smaller than the cord angle θ 1  in at least one layer of the inner layer  4 A, the outer layer  4 B, and the intermediate layer  4 C of the carcass layer  4  in the side region Rs with respect to the tire circumferential direction. As described above, when the cord angle θ 1  in each of the tread central region Rc and the side region Rs is appropriately set, the carcass layer  4  can share the function as the belt layer. 
     Further, the cord angle θ 1  is preferably set in the following manner. That is, the cord angle θ 1  falls within a range from 10° to 75° in the tread central region Rc, is gradually increased as approaching outward in the tire width direction with respect to the edge of the belt layer  7 , and falls within a range from 85° to 90° in the side region Rs. Particularly, in the tread central region Rc, the cord angle θ 1  preferably falls within a range from 15° to 70° and more preferably a range from 15° to 65°. When the cord angle θ 1  is appropriately set as described above, the carcass layer  4  can sufficiently share the function as the belt layer. Further, the carcass cords  41  are allowed to be gently changed in angle between the tread region Rt and the side region Rs, and hence the durability can be improved. 
     Further, as illustrated in  FIG. 3 , an angle of the belt cords  71  forming the belt layer  7  with respect to the tire circumferential direction is indicated as a cord angle θ 2 . The cord angle θ 2  preferably falls within a range from 15° to 45°. When the cord angle θ 2  is appropriately set as described above, the belt layer  7  can be caused to have the hoop function required as the belt layer. 
     In the present technology, a cord count per unit width, which is 50 mm, for the carcass cords  41  at least in the tread region Rt is preferably from twenty to seventy, and a diameter of the carcass cord  41  preferably falls within a range from 0.2 mm to 1.5 mm. When a dimension of the carcass cord  41  is appropriately set as described above, increase in mass of the carcass layer  4  is suppressed, which contributes to reduction in weight of the tire. At the same time, degradation in rigidity and durability can be suppressed. 
     EXAMPLES 
     A tire for each of Examples 1 to 8 was manufactured in the following manner. The pneumatic tire had a tire size of 235/40R18, and included a tread portion, a pair of sidewall portions, a pair of bead portions, and a carcass layer. The tread portion extended in the tire circumferential direction, and had an annular shape. The pair of sidewall portions were arranged on both sides of the tread portion. The pair of bead portions were arranged inward in the tire radial direction of the sidewall portions. The carcass layer included a plurality of carcass cords mounted between the pair of bead portions. The pneumatic tire had a structure in which the carcass layer was wound from the tire inner side to the outer side about the bead cores of the bead portions. The pneumatic tire also had the following structure. That is, both terminals of the carcass layer were overlapped with each other in the tread region. The carcass layer had a triple layer structure including an inner layer positioned most inward in the tire radial direction in a tread region, an outer layer positioned most outward in the tire radial direction in the tread region, and an intermediate layer positioned between the inner layer and the outer layer in the tread region. A cord angle in the carcass layer in a tread central region with respect to the tire circumferential direction was different from a cord angle of the carcass layer in a side region with respect to the tire circumferential direction. 
     In each of Examples 1 to 8, as shown in Table 1, the cord angle in the inner layer in the tread central region, the cord angle in the inner layer in the side region, the cord angle in the intermediate layer in the tread central region, the cord angle in the intermediate layer in the side region, the cord angle in the outer layer in the tread central region, the cord angle in the outer layer in the side region, the cord angle in a belt layer, positions of both the terminals of the carcass layer, and presence or absence of an intermediate rubber layer were set. The respective cord angles were inclination angles with respect to the tire circumferential direction. 
     Note that, as for the presence or absence of the intermediate rubber layer, a rubber sheet having a thickness of 1 mm was used for the tire including the intermediate rubber layer in Example 8. 
     For the purpose of comparison, a tire of a conventional example, which included two carcass layers in which carcass cords were oriented in the tire radial direction and two belt layers, was prepared. In the conventional example, the inner belt layer and the outer belt layer had the cord angle of 24° and the cord angle of −24°, respectively. 
     The test tires were evaluated for tire weight, rolling resistance, and steering stability according to the following evaluation methods, and results are shown in Table 1. 
     Tire Weight 
     The weight of each test tire was measured. Evaluation results are expressed as index values with the value of the conventional example being defined as 100. Smaller index values indicate lower tire weight. 
     Rolling Resistance 
     Each test tire was assembled on a wheel having a rim size of 18×8.5 J, and was inflated to an air pressure of 230 kPa. Then, according to the ISO standard, the rolling resistance was measured through use of a drum test device having a drum diameter of 2000 mm. Evaluation results are expressed as index values with the value of the conventional example being defined as 100. Smaller index values indicate less rolling resistance. 
     Steering Stability 
     Each test tire was assembled on a wheel having a rim size of 18×8.5 J, and mounted to a vehicle having a displacement of 2400 cc. Then, under a condition of an air pressure of 230 kPa, sensory evaluations were performed by a test driver on steering stability. Evaluation results are expressed by a grading scale out of 10 with 5 as a reference point. Larger evaluation scores indicate superior steering stability. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Conventional  
                 Example 
                 Example 
                 Example 
                 Example 
                 Example 
                 Example 
                 Example 
                 Example 
               
               
                   
                 Example 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
               
               
                   
               
             
            
               
                 Cord angle 
                 90° 
                 −60° 
                   90° 
                 −45° 
                 −45° 
                 −45° 
                   90° 
                 −60° 
                 −45° 
               
               
                 in inner 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 layer in 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 tread 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 central 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 region 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Cord angle 
                 90° 
                   90° 
                   90° 
                   90° 
                   90° 
                   90° 
                   90° 
                   90° 
                   90° 
               
               
                 in inner 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 layer in side 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 region 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Cord angle 
                 90° 
                   60° 
                 −60° 
                   45° 
                   45° 
                   45° 
                 −45° 
                   60° 
                   45° 
               
               
                 in 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 intermediate 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 layer in 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 tread 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 central 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 region 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Cord angle 
                 90° 
                   90° 
                   90° 
                   90° 
                   90° 
                   85° 
                   90° 
                   90° 
                   90° 
               
               
                 in 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 intermediate 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 layer in side 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 region 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Cord angle 
                 90° 
                 −60° 
                 −60° 
                 −45° 
                 −45° 
                 −45° 
                 −45° 
                 −60° 
                 −45° 
               
               
                 in outer 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 layer in 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 tread 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 central 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 region 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Cord angle 
                 90° 
                   90° 
                   90° 
                   90° 
                   90° 
                   90° 
                   90° 
                   90° 
                   90° 
               
               
                 in outer 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 layer in side 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 region 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Cord angle 
                 24°/−24° 
                   24° 
                   24° 
                   24° 
                   24° 
                   24° 
                   24° 
                   24° 
                   24° 
               
               
                 in belt layer 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Positions of 
                 — 
                 FIG. 7A 
                 FIG. 7A 
                 FIG. 7A 
                 FIG. 7B 
                 FIG. 7B 
                 FIG. 7B 
                 FIG. 7C 
                 FIG. 7A 
               
               
                 terminals of 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 carcass 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 layer 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Present or 
                 — 
                 Absent 
                 Absent 
                 Absent 
                 Absent 
                 Absent 
                 Absent 
                 Absent 
                 Present 
               
               
                 absence of 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 intermediate 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 rubber layer 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Tire Weight 
                 100 
                 93 
                 93 
                 93 
                 94 
                 94 
                 94 
                 92 
                 94 
               
               
                 Rolling 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Resistance 
                 100 
                 95 
                 95 
                 95 
                 96 
                 99 
                 96 
                 95 
                 96 
               
               
                 Steering 
                 5 
                 5 
                 5 
                 5 
                 6 
                 7 
                 5 
                 5 
                 6 
               
               
                 stability 
               
               
                   
               
            
           
         
       
     
     Both the terminals of the carcass layer are overlapped with each other in the tread region. The carcass layer had a triple layer structure including the inner layer positioned most inward in the tire radial direction in the tread region, the outer layer positioned most outward in the tire radial direction in the tread region, and the intermediate layer positioned between the inner layer and the outer layer in the tread region. The cord angle in the carcass layer in the tread central region with respect to the tire circumferential direction was different from the cord angle in the carcass layer in the side region with respect to the tire circumferential direction. With this structure, as apparent from Table 1, the tires in Examples 1 to 8 maintain the steering stability equivalent to that in the conventional example, and at the same time, the tire weight is reduced. Further, the rolling resistance is improved.