JP2010137601A - Pneumatic tire - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pneumatic tire with excellent uniformity not disordering a carcass ply after vulcanization even if fiber cords of different materials are used for the carcass ply. <P>SOLUTION: The pneumatic tire 1 has a pair of bead parts 2, a pair of side wall parts 3 and a tread part 4 continued to both side wall parts 3 and is provided with a carcass 6 comprising two sheets or more of carcass plies 5a, 5b troidally extending between the pair of bead parts 2. In the pneumatic tire 1, the carcass 6 comprises two kinds or more of carcass plies 5a, 5b of different kinds of the fiber cord, and in the fiber cord constituting the carcass plies 5a, 5b, thermal contraction ratio at 170°C under 50 g load is 7.0% or lower. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a pneumatic tire, and more particularly to a pneumatic tire excellent in uniformity.

  In order for a tire to exhibit sufficient durability and performance, it is necessary to reinforce the tire with various organic fiber cords and steel cords. In particular, the carcass ply constituting the carcass of a tire has, for example, rayon, nylon, or polyester to ensure or improve the strength for maintaining the internal pressure and the fatigue resistance for maintaining the strength as basic characteristics. Various organic fibers such as are used as the cord material.

  In addition, when the carcass of the tire is composed of a plurality of carcass plies, an organic fiber cord generally made of the same material is used for the carcass ply cord, particularly from the viewpoint of improving the durability of the tire, Polyester cords are frequently used. However, there is room for improvement in polyester cords in terms of adhesion to rubber and high-temperature stability during high-speed rolling.

  On the other hand, in a tire having a carcass composed of a plurality of carcass plies, when all of the carcass ply cords are made of rayon cords, the high-speed durability is improved as compared with the case of using polyester cords. The rigidity tends to be high, and the driving performance tends to become peaky and the steering stability tends to decrease.

  In order to solve these problems, a method of applying a cord made of different materials to the carcass ply has been proposed. For example, in Japanese Patent Application Laid-Open No. 64-32902 (Patent Document 1), in a tire having a carcass composed of a plurality of carcass plies, a nylon cord is used for the carcass ply inside the tire, and the carcass ply outside the tire is used. A technology for improving high load durability and interlayer adhesion by using a polyester cord is disclosed. However, in the combination of organic fiber cords disclosed in Japanese Patent Application Laid-Open No. 64-32902, since the heat shrinkage rates of the two materials are different, the shrinkage behavior of each organic fiber cord during vulcanization is different, and the carcass ply at the time of product is different. There is a problem in that it is easy to disturb, and sufficient uniformity cannot be secured.

JP-A-64-32902

  Accordingly, an object of the present invention is to provide a pneumatic tire that solves the above-described problems of the prior art and has excellent uniformity without causing the carcass ply to be disturbed after vulcanization even when fiber cords of different materials are used for the carcass ply. It is in.

  As a result of intensive studies to achieve the above object, the inventor of the present invention, in a carcass using a total of at least two types of carcass plies of two or more different types of fiber cords, the fiber cords constituting these carcass plies. It has been found that by controlling the thermal shrinkage rate to a specific value or less, the turbulence of the carcass ply after vulcanization can be suppressed and the uniformity of the tire can be improved, and the present invention has been completed.

  That is, the pneumatic tire of the present invention has a pair of bead portions and a pair of sidewall portions, and a tread portion connected to both sidewall portions, and extends in a toroidal shape between the pair of bead portions. In a pneumatic tire having a carcass composed of at least one carcass ply, the carcass is composed of two or more types of carcass plies having different fiber cord materials, and the fiber cord constituting the carcass ply has a load of 50 g at 170 ° C. The thermal shrinkage rate below is 7.0% or less.

  In the pneumatic tire of the present invention, the fiber cord constituting the carcass ply preferably has a thermal shrinkage rate of 5.0% or less under a 50 g load at 170 ° C.

In the pneumatic tire of the present invention, the fiber cord constituting the carcass ply has the following formula (I):
N1 = n1 × (0.125 × D1 / ρ) ^1/2 × 10 ⁻³ (I)
[Where N1 is the lower twisting factor, n1 is the number of lower twists (times / 10 cm), D1 is the indicated decitex number (dtex) of the lower twisted yarn, and ρ is the specific gravity of the fiber (g / cm ³ )] The lower twist coefficient N1 defined by the following formula (II):
N2 = n2 × (0.125 × D2 / ρ) ^1/2 × 10 ⁻³ (II)
[Where N2 is the upper twisting factor, n2 is the number of upper twists (times / 10 cm), D2 is the total number of decitex (dtex) of the fiber cord, and ρ is the specific gravity of the fiber (g / cm ³ ) The upper twist coefficient N2 defined by the following formula (III):
0.81 <N2 / N1 ≤ (D2 / D1) ^1/2 ... (III)
[Wherein, N1, N2, D1 and D2 have the same meanings as the above formulas (I) and (II)], and the upper twist coefficient N2 is represented by the following formula (IV):
0.1 ≦ N2 ≦ 1 (IV)
It is preferable that the relationship of [wherein N2 is synonymous with the formula (II)] is satisfied.

  In a preferred example of the pneumatic tire of the present invention, at least one of the carcass plies is made of a carcass ply in which a plurality of cellulosic fiber cords are coated with a coating rubber. In this case, the high temperature durability of the tire can be improved.

  In the pneumatic tire of the present invention, the cellulosic fiber cord preferably has a heat shrinkage rate of 1.0% or less under a 50 g load at 170 ° C. In the pneumatic tire of the present invention, it is preferable that at least one of the carcass plies other than the carcass ply using the cellulose fiber cord is made of a carcass ply in which a plurality of polyester cords are coated with a coating rubber. In this case, by combining a polyester cord with a cellulosic fiber cord, an appropriate rigidity as a carcass ply cord is ensured, and steering stability can be improved.

  In the pneumatic tire of the present invention, the fiber cord constituting the carcass ply is preferably such that the difference between the maximum value and the minimum value of the heat shrinkage rate under a 50 g load at 170 ° C. is 3.0% or less, and 1.0% or less. More preferably. In this case, the disturbance of the carcass ply can be reliably suppressed, and the tire uniformity can be greatly improved.

  In the pneumatic tire of the present invention, the polyester cord preferably has a thermal shrinkage rate of 2.0% or less under a 50 g load at 170 ° C.

  In the pneumatic tire of the present invention, the polyester cord is preferably a cord that has been dipped by a low tension treatment.

  In another preferred embodiment of the pneumatic tire according to the present invention, at least one of the carcass plies is wound around the bead core and a main body portion extending in a toroid shape between bead cores embedded in the bead portion. A first carcass ply having a winding portion, and at least one of the first carcass plies is a carcass ply in which a plurality of cellulosic fiber cords are coated with a coating rubber. In this case, the adhesiveness of the end portion of the turn-up portion of the carcass ply is improved, and the durability of the tire can be improved.

  In another preferred embodiment of the pneumatic tire of the present invention, the cellulosic fiber cord is a rayon cord or a lyocell cord.

  In another preferred embodiment of the pneumatic tire of the present invention, the polyester cord is made of polyethylene terephthalate or polyethylene naphthalate.

  In another preferable example of the pneumatic tire according to the present invention, the pneumatic tire further includes a pair of side reinforcing rubber layers disposed on the inner side of the carcass located in the sidewall portion. In this case, the rigidity of the sidewall portion can be improved without increasing the rigidity of the fiber cord.

  According to the present invention, in a carcass using a total of at least two types of carcass plies having different fiber cord materials, the thermal contraction rate of the fiber cords constituting these carcass plies is controlled to a specific value or less. By doing so, the carcass ply is not disturbed after vulcanization, and a pneumatic tire excellent in uniformity can be provided.

  Hereinafter, the pneumatic tire of the present invention will be described in detail with reference to the drawings. 1 to 4 are sectional views of embodiments of the pneumatic tire of the present invention. 1-4, the same code | symbol shows that it is the same member.

  The tire shown in FIG. 1 is a radial tire 1, which has a pair of bead portions 2 and a pair of sidewall portions 3, and a tread portion 4 connected to both sidewall portions 3. And a radial carcass 6 composed of two or more carcass plies 5 extended in a toroid shape. Here, in the pneumatic tire of the present invention, the carcass 6 is composed of two or more types of carcass plies 5 having different fiber cord materials. The fiber cords constituting the carcass ply 5 are under a 50 g load at 170 ° C. The heat shrinkage ratio of each is 7.0% or less. In this way, even when the material of the fiber cords constituting each carcass ply is different, by suppressing the heat shrinkage rate of all types of fiber cords constituting the carcass ply to 7.0% or less, the fibers at the time of vulcanization It is possible to prevent the cord from biting into the inside of the tire. For this reason, the pneumatic tire of the present invention is excellent in uniformity because the carcass ply is not disturbed after vulcanization.

  In the illustrated tire, the carcass 6 includes two carcass plies 5, and the carcass ply 5 includes a main body portion that extends in a toroidal manner between bead cores 7 embedded in the bead portion 2, and the carcass ply 5. A first carcass ply 5a having a rewinding portion that is wound from the inner side to the outer side in the tire width direction around the bead core 7; The second carcass ply 5b. In the pneumatic tire of the present invention, the number of carcass plies may be at least two, and the number and structure of the carcass plies are not limited to the carcass ply shown in FIG. For example, the tire shown in FIGS. 2 and 4 includes a carcass 6 composed of two first carcass plies 5a.

  The carcass ply 5 is formed by coating a plurality of fiber cords with a coating rubber. However, in the pneumatic tire of the present invention, it is necessary to use two or more types of carcass plies having different fiber cord materials. In this way, by combining carcass plies using different types of fiber cords, it is possible to eliminate the drawbacks seen when using a single fiber cord or to impart various characteristics to the tire. Although not shown, in a tire having a carcass composed of three carcass plies, the present invention is not limited to the case where different types of fiber cords are used for all carcass plies, and fibers of the same material for two carcass plies. A cord may be used, and a fiber cord of a different material may be used for the remaining sheet. The coating rubber constituting the carcass ply 5 is not particularly limited, and a normal rubber composition for coating rubber can be used.

  For example, in order to improve the high-speed durability of the tire, a carcass ply in which a plurality of cellulosic fiber cords are coated with a coating rubber can be applied to at least one of the carcass plies 5. In addition, as described above, the carcass ply using the cellulosic fiber cord has a possibility that the cord rigidity is too high or the steering stability is lowered, but according to the pneumatic tire of the present invention, other than the cellulosic fiber cord Such a problem can be solved by combining a carcass ply using a fiber cord. Specifically, a carcass ply in which a plurality of polyester cords are coated with a coating rubber can be applied to at least one of the carcass plies other than the carcass ply using the cellulose fiber cord. Thus, by combining the carcass ply using the polyester cord with the carcass ply using the cellulosic fiber cord, moderate rigidity is imparted as the carcass ply cord, and the steering stability can be improved.

  Further, the carcass 6 of a pneumatic radial tire often has one or more first carcass plies 5a. The first carcass plies 5a are beaded along with the deformation of the bead portion 2 that occurs during traveling. As a result of large distortion occurring in the end portion of the rewinding portion located in the portion 2 and in the vicinity thereof, the coating rubber is peeled off from the cord in the vicinity of the end portion, separation occurs, and the durability of the end portion of the carcass ply is reduced. There is a fear. Therefore, in the pneumatic tire of the present invention, when at least one of the carcass plies 5 is composed of the first carcass ply 5a, a plurality of cellulosic fiber cords are coated with a coating rubber on at least one of the first carcass plies 5a. A coated carcass ply can be applied. Cellulosic fiber cords are excellent in adhesion to rubber and heat resistance, and by applying this to the first carcass ply 5a, it is possible to suppress the peeling of the coating rubber at the end of the rolled-up portion and to provide elasticity at high temperatures. Reduction in rate can be suppressed. For this reason, the adhesiveness and heat resistance of the carcass ply 5 are improved, and the high-speed durability of the tire can be improved.

  The cellulosic fiber cord is a cord produced using cellulosic fibers as a raw material, and the cellulosic fiber cord is chemically esterified with cellulose, which is a natural polymer, in addition to rayon cord or lyocell cord. Also included is a cord produced using a cellulose derivative obtained by crystallization or etherification as a raw material. On the other hand, examples of the polyester cord include a cord made of polyethylene terephthalate (PET) or polyethylene naphthalate (PEN). The fiber cords constituting the carcass ply 5 are not limited to these, and for example, various nylon cords, aromatic polyamide (aramid) cords, polyketone fiber cords, carbon fiber cords, glass fiber cords, steel cords and the like are used. Also good.

  In the pneumatic tire of the present invention, the fiber cord constituting the carcass ply needs to have a heat shrinkage rate of 7.0% or less under a load of 50 g at 170 ° C., but from the viewpoint of improving the uniformity, 5.0% or less It is preferable that From the same viewpoint, when the fiber cord constituting the carcass ply is a cellulosic fiber cord, the heat shrinkage rate under a load of 50 g at 170 ° C. is preferably 1.0% or less, and the fiber cord is polyester. In the case of a cord, the thermal shrinkage rate under a 50 g load at 170 ° C. is preferably 2.0% or less. Also, if the thermal contraction rate of the fiber cords that make up the carcass ply located at the outermost side of the tire under a load of 50 g at 170 ° C is controlled to 2.0% or less, the gauge width of the coating rubber and the drum width during expansion can be adjusted. In combination with the adjustment or the like, the carcass ply disturbance in the product can be surely suppressed, and the uniformity of the tire can be greatly improved.

  Further, as a result of investigation by the present inventor, it is possible to control the difference between the maximum value and the minimum value of the thermal contraction rate of the fiber cord after controlling the thermal contraction rate of the fiber cord constituting the carcass ply to a specific value or less. It has been found that the tire uniformity can be greatly improved. Accordingly, in the pneumatic tire of the present invention, the fiber cord constituting the carcass ply preferably has a difference between the maximum value and the minimum value of the heat shrinkage rate under a load of 50 g at 170 ° C. of 3.0% or less, 1.0% More preferably, it is as follows.

Furthermore, in the pneumatic tire of the present invention, the fiber cord constituting the carcass ply has a lower twist coefficient N1 defined by the above formula (I) and an upper twist coefficient N2 defined by the above formula (II). It is preferable that the relationship of (III) is satisfied and the upper twist coefficient N2 satisfies the relationship of the above formula (IV). When the twisting coefficient of the fiber cord for carcass ply satisfies the relationship of the above formulas (III) and (IV), the performance required for the tire cord such as fatigue resistance, adhesion, and bending resistance can be improved. When the upper twist coefficient N2 is less than 0.1, the durability of the tire may be lowered. On the other hand, when the upper twist coefficient N2 is more than 1, the twist of the twisted cord is greatly unwound and workability is lowered. Further, when N2 / N1 is 0.81 or less, the difference in torque between the lower twist and the upper twist is too large, and, for example, workability in the dipping process is lowered. On the other hand, when N2 / N1 exceeds (D2 / D1) ^1/2 , workability is similarly reduced.

  Moreover, the fiber cord constituting the carcass ply has a tensile strength of 3.0 to 40 cN / dtex of the original yarn in order to enable use in a field where strength is required or to reduce the amount of fiber cord used. Is preferably 4.0 to 10 cN / dtex. In addition, the tensile strength (cN / dtex) of the original yarn of the fiber cord is a value measured based on the JIS standard. Furthermore, the higher the elastic modulus of the fiber cord, the smaller the dimensional change under the same load and the better the form stability. Therefore, the fiber cord constituting the carcass ply has an elastic modulus of the raw yarn of 50 to 2500 cN / It is preferably dtex, more preferably 100 to 400 cN / dtex. The elastic modulus (cN / dtex) of the yarn of the fiber cord is an elastic modulus at a load of 49 N at 25 ° C., and is a value calculated from a tangent at 49 N of the SS curve by a JIS cord tension test.

  The fiber cord constituting the carcass ply may have a single twist structure in which one filament bundle of fibers is twisted, and two or more filament bundles of fibers that have undergone a lower twist are combined to be twisted together. You may have the twisted structure which gave. Here, the number of twists and the twisted structure imparted to the cord during the lower twist and the upper twist are determined in consideration of the strength, convergence, and other physical properties of the cord.

  As described above, a normal rubber composition for a coating rubber can be used as the coating rubber constituting the carcass ply 5. For example, the rubber component used in the rubber composition for the coating rubber is not particularly limited. In addition to natural rubber (NR), polyisoprene rubber (IR), styrene-butadiene copolymer rubber (SBR), polybutadiene rubber ( BR), ethylene-propylene-diene rubber (EPDM), chloroprene rubber (CR), halogenated butyl rubber, acrylonitrile-butadiene rubber (NBR), etc. can be used. These rubber components are used alone. You may use, and you may blend and use 2 or more types. The rubber composition for coating rubber includes, in addition to the rubber component, for example, filler, softener, anti-aging agent, vulcanizing agent, vulcanization accelerator, scorch preventing agent, zinc white, stearic acid, silane coupling. A compounding agent usually used in the rubber industry such as an agent can be appropriately selected and compounded within a range not impairing the object of the present invention. As these compounding agents, commercially available products can be suitably used. The rubber composition for a coating rubber can be produced by blending a rubber component with various compounding agents appropriately selected as necessary, kneading, heating, extruding, and the like.

  The tire shown in FIG. 1 includes a belt 8 positioned on the outer side in the tire radial direction of the crown portion of the carcass 6. The belt 8 is composed of two belt layers, but the number of belt layers constituting the belt in the tire of the present invention is not limited to this. The belt layer is formed by coating a plurality of cords with a coating rubber. For the cord constituting the belt layer, for example, an organic fiber cord or a steel cord is preferably used, and the coating rubber constituting the belt layer is used. Is not particularly limited, and a normal rubber composition for coating rubber can be used. Further, as shown in FIG. 1, the pneumatic tire of the present invention has a belt reinforcing layer 9 made of a rubberized layer of cords arranged substantially parallel to the tire circumferential direction on the outer side in the tire radial direction of the belt 8. It may be provided. The number and structure of the belt reinforcing layers are not particularly limited. For example, the belt reinforcing layer 9 shown in FIG. 1 has only one first belt reinforcing layer 9 a covering the entire belt 8 and only the end of the belt 8. It consists of a pair of covering and one second belt reinforcing layer 9b.

  In addition, the pneumatic tire of this invention can further provide a well-known tire member as needed. For example, as shown in FIGS. 3 and 4, the pneumatic tire of the present invention can include a pair of side reinforcing rubber layers 10 disposed inside the carcass 6 located in the sidewall portion 3. In this case, the rigidity of the sidewall portion can be improved without increasing the rigidity of the fiber cord.

  The pneumatic tire of the present invention can be manufactured by a conventional method by applying a rubber-coated cord formed by rubber-drawing a fiber cord satisfying the above conditions to a carcass ply. Prior to rubberizing the fiber cord, the fiber cord may be dipped (adhesive treatment) to improve the adhesion with the coating rubber. Further, when using a polyester cord as the fiber cord, for example, by performing a dip treatment by a low tension treatment with a tension of 1.5 kg / piece or less, preferably 1.0 kg / piece or less, the low heat shrinkage as described above. Rate can be achieved. In addition, as gas with which the pneumatic tire of this invention is filled, inert gas, such as nitrogen, argon, helium other than the air which adjusted normal or oxygen partial pressure, can be used.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples.

  Next, a plurality of fiber cords shown in Tables 1 to 3 are aligned as warp yarns, and a low-strength treatment (tensile: 1.0 kg / piece) is applied as necessary to the fiber cords in a slender shape in which thin, weak wefts are roughly driven. A rubber-coated cord was prepared by applying the dip treatment according to the above and coating with a coating rubber. The obtained rubber-coated cords were used in carcass plies in the combinations shown in Tables 1 to 3, and the tires having the structures shown in Tables 1 to 3 and having a tire size of 195 / 65R15 were made as trial products. The tire was evaluated for uniformity, high-speed durability, and steering stability by the following methods. The results are shown in Tables 1-3.

(1) Uniformity According to the uniformity measurement method defined in JASO C 607 (2000), radial force variation (RFV) and lateral force variation (LFV) are measured, and the index of RFV and LEV of the tire of Comparative Example 1 is 100. displayed. A smaller index value indicates higher tire uniformity (uniformity).

(2) High-speed durability The test tire is assembled on a rotating drum and run at a speed of 150 km / h for 30 minutes. If there is no failure, the speed is increased by 6 km / h, and the durability time until the failure occurs is measured. The index was displayed with the endurance time of Comparative Example 1 as 100. A larger index value indicates a longer durability time and better high-speed durability.

(3) Steering stability The test tires were mounted and the steering stability was evaluated based on an actual vehicle sensory test by a specialized driver. The larger the index value, the better the steering stability.

* 1 66Ny is an abbreviation for 66 nylon, 6Ny is an abbreviation for 6 nylon, PET is an abbreviation for polyethylene terephthalate, and PEN is an abbreviation for polyethylene naphthalate.
* 2 Thermal contraction rate when left at 170 ° C and 50g load for 5 minutes.
* 3 The elastic modulus at 25 ° C calculated from the tangent at 49N of the SS curve in the JIS cord tension test.
* 4 Tensile strength measured based on JIS cord tensile test.
* 5 4a is a first carcass ply having a main body portion and a winding portion, and 4b is a second carcass ply covering the winding portion of the first carcass ply.

  From the comparison between Comparative Example 1 and Example, in a tire having a carcass made of two or more types of carcass plies having different fiber cord materials, the uniformity of the fiber cords constituting the carcass ply is reduced by reducing the heat shrinkage rate. It can be seen that it can be improved. Furthermore, it can be seen that the effect of improving the uniformity is increased by narrowing the difference in the thermal contraction rate of the fiber cords constituting each carcass ply.

  Moreover, from the results of Comparative Example 2 and Examples 6 to 14, a tire in which a carcass ply using a polyester cord is combined with a carcass ply using a cellulose fiber cord is composed of only a carcass ply using a cellulose fiber cord. It can be seen that the steering stability can be improved together with the uniformity as compared with the tire.

It is sectional drawing of an example of the pneumatic tire of this invention. It is sectional drawing of the other example of the pneumatic tire of this invention. It is sectional drawing of the other example of the pneumatic tire of this invention. It is sectional drawing of the other example of the pneumatic tire of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Radial tire 2 Bead part 3 Side wall part 4 Tread part 5 Carcass ply 5a First carcass ply 5b Second carcass ply 6 Carcass 7 Bead core 8 Belt 9 Belt reinforcement layer 9a First belt reinforcement layer 9b Second belt reinforcement layer 10 Side Reinforced rubber layer

Claims (14)

A carcass comprising two or more carcass plies having a pair of bead portions and a pair of sidewall portions, and a tread portion connected to both sidewall portions and extending in a toroidal shape between the pair of bead portions. In a pneumatic tire
The carcass is composed of two or more types of carcass plies with different fiber cord materials,
A pneumatic tire characterized in that the fiber cord constituting the carcass ply has a heat shrinkage rate of 7.0% or less under a load of 50 g at 170 ° C.   The pneumatic tire according to claim 1, wherein the fiber cord constituting the carcass ply has a thermal shrinkage rate of 5.0% or less under a load of 50 g at 170 ° C. The fiber cord constituting the carcass ply has the following formula (I):
N1 = n1 × (0.125 × D1 / ρ) ^1/2 × 10 ⁻³ (I)
[Where N1 is the lower twisting factor, n1 is the number of lower twists (times / 10 cm), D1 is the indicated decitex number (dtex) of the lower twisted yarn, and ρ is the specific gravity of the fiber (g / cm ³ )] The lower twist coefficient N1 defined by the following formula (II):
N2 = n2 × (0.125 × D2 / ρ) ^1/2 × 10 ⁻³ (II)
[Where N2 is the upper twisting factor, n2 is the number of upper twists (times / 10 cm), D2 is the total number of decitex (dtex) of the fiber cord, and ρ is the specific gravity of the fiber (g / cm ³ ) The upper twist coefficient N2 defined by the following formula (III):
0.81 <N2 / N1 ≤ (D2 / D1) ^1/2 ... (III)
[Wherein, N1, N2, D1 and D2 have the same meanings as the above formulas (I) and (II)], and the upper twist coefficient N2 is represented by the following formula (IV):
0.1 ≦ N2 ≦ 1 (IV)
2. The pneumatic tire according to claim 1, wherein a relationship of [wherein N <b> 2 is synonymous with the formula (II)] is satisfied.   The pneumatic tire according to claim 1, wherein at least one of the carcass plies is formed of a carcass ply in which a plurality of cellulosic fiber cords are coated with a coating rubber.   The pneumatic tire according to claim 4, wherein the cellulose fiber cord has a heat shrinkage rate of 1.0% or less under a load of 50 g at 170 ° C.   5. The pneumatic tire according to claim 4, wherein at least one of the carcass plies other than the carcass ply using the cellulose fiber cord is made of a carcass ply in which a plurality of polyester cords are coated with a coating rubber.   2. The pneumatic tire according to claim 1, wherein the fiber cord constituting the carcass ply has a difference between a maximum value and a minimum value of heat shrinkage under a load of 50 g at 170 ° C. of 3.0% or less.   The pneumatic tire according to claim 6, wherein the polyester cord has a thermal shrinkage rate of 2.0% or less under a load of 50 g at 170 ° C.   The pneumatic tire according to claim 6, wherein the polyester cord is a cord that has been dipped by a low tension treatment. At least one of the carcass plies is composed of a first carcass ply having a main body portion extending in a toroid shape between bead cores embedded in the bead portions and a rewinding portion that is wound around the bead cores,
The pneumatic tire according to claim 1, wherein at least one of the first carcass plies is formed of a carcass ply in which a plurality of cellulosic fiber cords are coated with a coating rubber.   The pneumatic tire according to claim 4 or 10, wherein the cellulosic fiber cord is a rayon cord or a lyocell cord.   The pneumatic tire according to claim 6, wherein the polyester cord is made of polyethylene terephthalate or polyethylene naphthalate.   The pneumatic tire according to claim 7, wherein the fiber cord constituting the carcass ply has a difference between a maximum value and a minimum value of a heat shrinkage rate under a load of 50 g at 170 ° C of 1.0% or less.   The pneumatic tire according to claim 1, further comprising a pair of side reinforcing rubber layers disposed inside the carcass positioned in the sidewall portion.

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