JP5615665B2 - Pneumatic radial tire - Google Patents

Description

  The present invention relates to a single inclined belt layer formed by inclining a plurality of metal wires with respect to the tire equatorial plane, and the organic fiber cord disposed on the inclined belt layer substantially parallel to the tire equatorial plane. The present invention relates to a pneumatic radial tire (hereinafter, also simply referred to as “tire”) having a reinforcing belt comprising at least one circumferential belt layer that is arranged, and more particularly, to a pneumatic radial tire that is suitable as a radial tire for passenger cars. .

  In recent years, there has been an increasing demand for reducing the weight of tires in order to improve the fuel efficiency of automobiles. In general, a pneumatic radial tire has a so-called cross belt in which at least two inclined belt layers are laminated on the outer periphery of a crown portion of a carcass so that respective reinforcing members cross each other. From the viewpoint of reducing the weight of the tire, there is a single inclined belt layer and a circumferential belt layer that is positioned on the inclined belt layer and in which light-weight organic fiber cords are arranged substantially parallel to the tire equatorial plane. Tires that make up belts have been developed.

  Tires to which a belt composed of a combination of such an inclined belt layer and a circumferential belt layer is applied are disclosed in Patent Documents 1 to 4, for example.

JP-A-7-323704 (Claims etc.) JP-A-8-164703 (Claims etc.) JP-A-9-156313 (Claims etc.) JP-A-9-156315 (Claims)

  However, none of the above prior arts is still sufficient, and there has been a demand for realizing a tire that is further reduced in weight.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a pneumatic radial tire that solves the above problems and achieves both good steering stability and durability, and further weight reduction of the tire.

  As a result of intensive studies on the structure, material, and arrangement of a reinforcing belt using a metal wire, the present inventor has applied the metal wire having a special cross-sectional shape to the inclined belt layer as a single wire or a bundle bundled together. As a result, the present invention has been completed.

That is, the pneumatic radial tire of the present invention is a single layer formed by arranging a plurality of metal wires in an inclined arrangement with respect to the tire equatorial plane on the outer periphery of the crown portion of a carcass that forms a toroid shape between at least a pair of bead cores. The belt is reinforced by a reinforcing belt comprising: an inclined belt layer; and at least one circumferential belt layer formed on the inclined belt layer, the organic fiber cord being arranged substantially parallel to the tire equatorial plane. In a pneumatic radial tire having a tread portion,
The inclined belt layer has one or more metal wires having a flat shape that is long in the belt width direction when at least most of the metal wires in the inclined belt layer are viewed in a cross section perpendicular to the longitudinal direction. It exists as a single metal wire or a bundle of metal wires that are arranged in parallel in the belt width direction without twisting, and the single wire or bundle of flat metal wires are parallel and flat with a gap between the single wires or between the bundles in the belt width direction. Are arranged in the belt width direction and embedded in rubber,
The width W (mm) and the thickness T (mm) of the single wire or bundle of the flat metal wire are expressed by the following formula (1),
0.334 × (W / T) ^−0.3449 ≦ T ≦ 0.342 × (W / T) ^−0.2539 (1)
(W is represented by W = N × Wf, where N is 2 or 3, where the width of the flat metal wire is Wf (mm) and the number of bundles is N). Satisfying the relationship, the ratio of the width W (mm) and the thickness T (mm) of the single wire or bundle of the flat metal wire is represented by the following formula:
0.18 ≦ T / W ≦ 0.34
Satisfying the relationship represented by
The number P (lines / 50 mm) of single wires or bundles of the flat-shaped metal wires for the inclined belt layer is expressed by the following equation (2):
−9.258 × Ln (W / T) + 40.187 ≦ P ≦ −10.487 × Ln (W / T) +45.848 (2)
(Wherein W and T are the same as those described above, and Ln represents a natural logarithm).

  In the present invention, the cross-sectional shape of the flat-shaped metal wire is preferably a track shape having a pair of parallel straight portions and a pair of arc portions that protrude outwardly and face each other. , The cross-sectional shape of the flat metal wire is a pair of parallel straight portions, a pair of arc portions projecting outwardly and facing each other, and another pair of portions that transition from the straight portions to the arc portions. A track shape having an arc portion is more preferable.

Furthermore, in the present invention, the flat metal wire has a thickness Tf of 0.15 to 0.26 mm, and a ratio of the width Wf to the thickness Tf is represented by the following formula:
0.18 ≦ Tf / Wf ≦ 0.34
It is preferable to satisfy the relationship represented by these. Furthermore, the tensile strength of the flat metal wire is advantageously 3300 to 4000 MPa.

  According to the present invention, the configuration described above makes it possible to realize a pneumatic radial tire that achieves both good steering stability and durability and further weight reduction of the tire.

1 is a cross-sectional view in the width direction showing an example of a pneumatic radial tire of the present invention. It is width direction sectional drawing which shows the example of three bundles of the flat-shaped metal wire which concerns on this invention. It is width direction sectional drawing which shows the example of the single wire of the flat-shaped metal wire which concerns on this invention. It is width direction sectional drawing which shows the example of the two bundles of the flat-shaped metal wire which concerns on this invention. It is width direction sectional drawing which shows the example which twisted the conventional cross-section metal wire to 1x3 structure. It is sectional drawing of the width direction which shows the example which arranged the conventional metal wire of a circular cross section at equal intervals. It is width direction sectional drawing which shows the example of the three bundles of the conventional metal wire of a circular cross section. It is a graph which shows the relationship between ratio W / T of the width | variety and thickness of the single line | wire or bundle | flux of the metal wire in each Example and a comparative example, and thickness T. It is a graph which shows the relationship between ratio W / T of the width | variety and thickness of the single line | wire or bundle | flux of a metal wire in each Example and a comparative example, and the number P of implantation.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
In FIG. 1, the cross-sectional view of the width direction of an example of the pneumatic radial tire of this invention is shown. As shown in the figure, the pneumatic radial tire 10 of the present invention has at least one pair, and in the illustrated example, a single inclined belt layer 3 on the outer periphery of a crown portion of a carcass 2 that forms a toroid shape across a pair of bead cores 1. And a tread portion reinforced by a reinforcing belt including at least one layer located on the inclined belt layer 3, in the illustrated example, one circumferential belt layer 4.

  In the present invention, the inclined belt layer 3 is formed by arranging a plurality of metal wires in an inclined arrangement with respect to the tire equatorial plane. More specifically, in the inclined belt layer 3 according to the present invention, at least most of the metal wires in the inclined belt layer have a flat shape that is long in the belt width direction when viewed in a cross section orthogonal to the longitudinal direction. There is a single metal wire or a bundle of metal wires that are arranged in parallel in the belt width direction without twisting one or a plurality of metal wires present, and the single wire or bundle of flat metal wires is arranged in the belt width direction. The inclined belt layer 3 is formed by being arranged in the belt width direction in parallel and in a plane with a space between the single wires or between the bundles and embedded in rubber.

  FIG. 2 shows a cross-sectional view of an example of the flat metal wire 20 according to the present invention. Conventionally, the metal wire used for the inclined belt layer 3 has a substantially circular cross-sectional shape perpendicular to its length direction as shown in FIGS. As shown in FIGS. 2 to 4, the metal wire used for the inclined belt layer 3 is characterized in that its cross section is a flat shape. When compared with the same cross-sectional area, such flat-shaped metal wires have substantially the same tensile modulus in the length direction as compared to the circular cross-section metal wires, but have a lower flexural modulus in bending deformation in the thickness direction. Also, the distortion generated on the surface is small. Therefore, as shown in FIG. 2, the inclined belt layer 3 according to the present invention in which the long metal wires 20 are arranged side by side so as to be parallel to the belt layer has sufficient circumferential rigidity as a reinforcing belt. While being held, the belt is highly durable against repeated bending deformation, because the surface distortion is small, so that the wire breakage hardly occurs.

  Another advantage of using a flat metal wire having a flat cross-sectional shape as a reinforcing material for the inclined belt layer 3 is that the cross-sectional area can be increased and the productivity of wire drawing can be improved. And the surface irregularity of the cord is reduced, which is useful for enhancing the belt tension rigidity and improving the handling stability.

In the present invention, the width W (mm) and the thickness T (mm) of the flat or bundle of flat metal wires are expressed by the following formula (1),
0.334 × (W / T) ^−0.3449 ≦ T ≦ 0.342 × (W / T) ^−0.2539 (1)
(W is represented by W = N × Wf, where N is 2 or 3, where the width of the flat metal wire is Wf (mm) and the number of bundles is N). It is necessary to satisfy the relationship. Here, when the belt ply is made of a single wire of the flat metal wire 11, the width of the single wire is Wf = W and the thickness is Tf = T. The reason why the thickness T of the bundle is not always T = Tf is that, in an actual treat material, it is considered that the flat metal wires constituting the bundle are slightly shifted. When the thickness T is less than 0.334 × (W / T) ^−0.3449, it is necessary to increase the tensile strength of the metal wire in order to ensure the strength when used as a tire reinforcing belt. It becomes difficult. On the other hand, if the thickness T exceeds 0.342 × (W / T) ^−0.2539 , the wire surface distortion becomes large at the time of large bending deformation, and the wire is likely to be broken at the time of sudden turning of the vehicle. End up. In addition, the number N of the bundles of the flat metal wires is set to two or three because if the number of the bundles is four or more, it is difficult to perform the processing work of arranging the flat materials in the process of manufacturing the treat material. It is.

Furthermore, in the present invention, the number P (lines / 50 mm) of a single wire or bundle of flat metal wires for the inclined belt layer is expressed by the following equation (2):
−9.258 × Ln (W / T) + 40.187 ≦ P ≦ −10.487 × Ln (W / T) +45.848 (2)
It is also necessary to satisfy the relationship represented by the formula (W and T are the same as described above, and Ln represents a natural logarithm). When the number P of driving is less than −9.258 × Ln (W / T) +40.187, the strength when used as a tire reinforcing belt is insufficient. On the other hand, when the number P to be driven exceeds -10.487 × Ln (W / T) +45.848, the flat metal wires are arranged in parallel and in a plane in the belt width direction at intervals between single wires or bundles. The gap between adjacent single wires or bundles becomes narrow, and the generation and propagation of the belt layer end cannot be suppressed.

In the present invention, in addition to the above, the ratio of the width W (mm) and the thickness T (mm) of a single wire or bundle of flat metal wires is represented by the following formula:
0.1 ≦ T / W ≦ 0.34
It is preferable to satisfy the relationship represented by these. If the T / W is less than 0.1, it is difficult to perform the processing work for aligning in a plane in the process of manufacturing the treat material. On the other hand, when T / W exceeds 0.34, the thickness Tf of the flat metal wire is increased in order to ensure the strength when used as a tire reinforcing belt, and as a result, the flat metal wire is a single wire or The bundle thickness T is also increased, which is disadvantageous in reducing the weight of the tire.

Specifically, as the cross-sectional shape of the flat metal wire 20, for example, as shown in FIG. 2, a pair of parallel straight portions 11 and a pair of arc portions 12 that are convex outward and face each other, The track shape is preferable. In this case, the radius of curvature R (mm) of the arc portion 12 is preferably the following formula with respect to the thickness T (mm):
0.6354 × T ≦ R ≦ 0.77 × T + 0.019
The relationship expressed by When the radius of curvature R of the arc portion 12 is less than 0.6354 × T, the radius of curvature is too small, so that the shear stress generated at the bonding interface between the metal wire and the rubber is locally increased, and separation tends to occur. On the other hand, when the radius of curvature R exceeds 0.77 × T + 0.019, the shear stress generated at the bonding interface between the metal wire and the rubber in the boundary region between the straight portion 11 and the arc portion 12 is locally increased. Separation easily occurs.

In the present invention, in particular, as shown in FIG. 3, a pair of parallel straight line portions 21, a pair of arc portions 22 that are convexly opposed to each other, and a straight portion 21 to a circular arc portion 22. By using a flat-shaped metal wire 30 having a track-shaped cross-section having another pair of arc portions 23 at a portion that transitions to, the bending of the wire at the time of a large bending deformation at the time of sudden turning of the vehicle or the like can be achieved. It can be suppressed more effectively and is suitable. In this case, the radius of curvature Ra (mm) of the arc portion 22 is preferably the following formula with respect to the thickness T (mm):
0.5 × T ≦ Ra ≦ 0.77 × T + 0.019
It is assumed that the relational expression represented by If the radius of curvature Ra is less than 0.5 × T, the radius of curvature is too small, so that the shear stress generated at the bonding interface between the metal wire and the rubber is locally increased, and separation tends to occur. On the other hand, if the curvature radius Ra exceeds 0.77 × T + 0.019, the shear stress generated at the bonding interface between the metal wire and the rubber in the boundary region with the arc portion 23 is locally increased, and separation is likely to occur. Become. Furthermore, the radius of curvature Rb of the arc portion 23 is preferably the following formula with respect to the thickness T (mm):
T ≦ Rb ≦ 2.5 × T
It is assumed that the relational expression represented by When the radius of curvature Rb of the arc is less than T, the radius of curvature is too small, so that the local distortion of the arc portion 23 becomes large at the time of large bending deformation, and the wire is likely to be broken at the time of sudden turning of the vehicle. End up. On the other hand, when the curvature radius Rb exceeds 2.5 × T, the shear stress generated at the bonding interface between the metal wire and the rubber is locally increased in the boundary area between the arc portion 22 of the wire and separation is likely to occur. Become. In addition, the strain generated at the time of large bending deformation in the boundary region with the straight portion 21 is locally increased, and the metal wire is likely to be broken at the time of sudden turning of the vehicle.

  Moreover, it is preferable that thickness Tf of the cross section orthogonal to a longitudinal direction of a flat-shaped metal wire is 0.15-0.26 mm. When the thickness Tf is less than 0.15 mm, the rigidity of the belt layer is insufficient, and the steering stability of the tire is impaired. On the other hand, when the thickness Tf exceeds 0.26 mm, the surface distortion of the flat metal wire at the time of large bending deformation increases, and the metal wire is likely to be broken at the time of sudden turning of the vehicle.

Furthermore, the ratio between the width Wf and the thickness Tf of the flat metal wire is as follows:
0.12 ≦ Tf / Wf ≦ 0.34
It is preferable to satisfy the relationship represented by these. In the rolling process of a high flattened metal wire having Tf / Wf of less than 0.12, cracking of the metal wire is likely to occur. On the other hand, if Tf / Wf exceeds 0.34, the thickness Tf of the metal wire is increased in order to ensure the strength when used as a tire reinforcing belt, and as a result, the thickness of the single wire or bundle of metal wires. T also becomes thick, which is disadvantageous for weight reduction of the tire.

  Furthermore, the tensile strength of the flat metal wire used in the present invention is preferably 3300 to 4000 MPa. When the tensile strength is less than 3300 MPa, the amount of metal wire used is increased in order to ensure the belt layer strength, which is disadvantageous for weight reduction of the tire. Alternatively, it is necessary to increase the number of driven wires, the distance between adjacent metal wires or bundles becomes narrow, and it becomes impossible to inhibit the occurrence and propagation of the belt layer end. On the other hand, a metal wire having a tensile strength exceeding 4000 MPa is difficult to manufacture and is not suitable for mass production.

  The flat-shaped metal wire according to the present invention is rolled between rollers in the latter half of the wire drawing process using the conventional equipment and process for producing a metal wire having a normal circular cross section as it is, or By flattening, for example, by passing a flat hole die, it is possible to manufacture economically and easily.

  In the present invention, the metal wire used for the inclined belt layer 3 only needs to satisfy the above conditions, and the details of the other tire structures and the material of each member are not particularly limited. It can be configured by appropriately selecting from among them. For example, the illustrated tire includes a tread portion 5 and a pair of sidewall portions 6 and bead portions 7 connected to both sides thereof. The carcass 2 includes a pair of bead cores 1 each embedded in the bead portion 7. It is reinforced throughout.

  In the present invention, the cord angle of the belt ply constituting the inclined belt layer 3 is preferably 40 ° to 60 ° with respect to the tire circumferential direction. Furthermore, in the present invention, as the reinforcing cord of the carcass 2, cords of various organic fibers such as nylon, polyester, rayon or aromatic polyamide can be used. The carcass 2 is composed of at least one layer of carcass ply, but the carcass ply may be arranged in two or more layers, and is usually folded around the bead core 1 from the inside of the tire to the outside and locked. .

  Furthermore, in the present invention, the circumferential belt layer 4 disposed on the inclined belt layer 3 is formed by arranging organic fiber cords substantially parallel to the tire equatorial plane, and is preferably at least one layer from the viewpoint of weight reduction. Are arranged in one layer. Examples of the organic fiber cord used for the circumferential belt layer 7 include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), nylon, vinylon, and aramid. Since the circumferential belt layer 4 is usually formed by spirally winding a belt-like belt member in the tire circumferential direction, it actually has a slight inclination angle with respect to the tire equator line. In the present invention, the number of cords to be driven in the circumferential belt layer 4 is preferably 40 to 70/50 mm from the viewpoint of reinforcement.

  Furthermore, in the present invention, a tread pattern is appropriately formed on the surface of the tread portion 5, a bead filler (not shown) is disposed on the outer side of the bead core 1 in the tire radial direction, and an inner liner is disposed on the innermost layer of the tire. Established. Furthermore, in the tire of the present invention, as the gas filled in the tire, it is possible to use normal or air having a changed oxygen partial pressure, or an inert gas such as nitrogen.

Hereinafter, the present invention will be described in more detail with reference to examples.
A pneumatic radial tire (tire size 195 / 65R15) having the structure shown in FIG. 1 is substantially parallel to the tire equatorial plane, with one inclined belt layer 3 inclined at an angle of 50 ° to the right with respect to the tire equatorial plane. It was produced by applying one circumferential belt layer 4. For each inclined belt layer, cords according to the specifications shown in the cord specification column in the table below were used.

  As shown in FIG. 5, the belt cord used in Conventional Example 1 is formed by twisting a conventional metal wire having a circular cross section into a 1 × 3 structure. Moreover, the belt used in Comparative Examples 1 and 2 is a conventional metal wire having a circular cross-section, in which Comparative Example 1 is uniformly driven as shown in FIG. 6, and Comparative Example 2 is a bundle of six metal wires. It is the one that was driven in (not shown). Further, the flat metal wire is uniformly used for the belts used in Examples 1 to 4, Reference Example 1 and Comparative Examples 3 and 5, and 2 for the belts used in Reference Example 2 and Comparative Examples 6, 8, and 10. In this bundle arrangement, the belts used in Reference Examples 3, 4 and Comparative Examples 4, 7, and 11 are arranged in three bundles, the belt used in Comparative Example 9 is arranged in four bundles, and used in Comparative Example 12. The belts we had were driven in a 6-bundle arrangement. Furthermore, flat metal wires having high tensile strengths for Comparative Examples 3 and 6 and low tensile strengths for Comparative Examples 4 and 11 were used.

  Each performance test was carried out according to the following for each obtained tire. The results are also shown in the table below. 8 and 9 are graphs showing the relationship between the thickness T and the ratio W / T of the width and thickness of a single wire or bundle of metal wires in each of Examples and Comparative Examples (excluding Comparative Examples 1 and 2). , And a graph showing the relationship between the ratio W / T of the width and thickness of a single wire or bundle of metal wires and the number P of driven wires, respectively. The two curves in each graph correspond to the upper limit value and the lower limit value of the expressions (1) and (2), respectively.

<Belt end peel resistance test>
Each test tire is assembled to a regular rim, filled with an internal pressure of 1.5 kgf / cm ² and mounted on a test passenger car. After running 60,000 km on a general road, the tire is dissected and the end of the belt The length of cracks occurring in the film was measured. The result was expressed as an index with the reciprocal of the crack length of each test tire calculated and the reciprocal value of the tire of Conventional Example 1 being 100. The larger the index, the better the belt end separation resistance. If this index is 90 or more, there is no practical problem.

<Steering stability test>
Each test tire adjusted in accordance with JIS standard D4202 is installed in a drum testing machine having an outer diameter of 3 m, loaded with a load determined by a predetermined size and internal pressure, and preliminarily run for 30 minutes at a speed of 30 km / h. After that, the internal pressure was readjusted to the standard value excluding the load in order to eliminate the influence of the increase in internal pressure due to the temperature rise. After that, the slip angle is continuously added every 1 ° from ± 1 ° to ± 4 ° under the same speed and the same load, and the cornering force (CF) per unit angle at each positive and negative angle is measured. The cornering power (CP) was obtained by calculating the average value of these. The results were expressed by dividing the CP of each test tire by the CP of the tire of Conventional Example 1 and indexing it. The larger this value, the better the steering stability.

<Belt break test>
Each test tire is mounted on an actual vehicle, and after a foldable road running at 20,000 km at a speed of 60 km per hour, the tire is dissected and the belt cord in the inclined belt layer is collected and folded The number of belt cords in the above was investigated, and the reciprocal number was indexed with the tire of Conventional Example 1 being 100. The larger this value, the better the belt folding resistance. If this value is 95 or more, there is no practical problem.

<Rolling workability test>
Measure the time required for belt cord preparation work and rolling (calendaring) work before combining belt cord and rubber, and increase the time within 20% compared to the working time of the belt cord of Conventional Example 1 Is indicated by “◯”, and further time increase is indicated by “×”.

<Belt weight>
The weight per unit area of the inclined belt layer used for each test tire was measured, and the index was displayed with the conventional example 1 as 100. It can be said that the smaller this value is, the lighter and lighter the inclined belt layer is.

  As shown in the above table, in the tires of Examples 1 to 4 that satisfy the conditions of the present invention, the belt end peel resistance and belt folding resistance are suppressed to a level that does not cause a problem in practice, and Weight reduction has been achieved in comparison with the conventional example 1. In contrast, the tire of Comparative Example 3 is difficult to manufacture because the tensile strength of the metal wire exceeds 4000 MPa, and is not suitable for mass production. In the tire of Comparative Example 4, the tensile strength of the metal wire is less than 3300 MPa, and the use amount of the metal wire is increased in order to ensure the strength of the belt layer. Furthermore, for the tires of Comparative Examples 5 to 12, since the conditions of the present invention are not satisfied, any of belt end peel resistance, belt folding resistance, and rolling workability is poor.

  As described above, according to the pneumatic radial tire of the present invention, various problems caused by weight reduction, that is, light weight with improved performance such as peeling resistance, handling stability, and belt folding resistance at the belt end. It was confirmed that a pneumatic radial tire was obtained.

DESCRIPTION OF SYMBOLS 1 Bead core 2 Carcass 3 Inclined belt layer 4 Circumferential belt layer 5 Tread part 6 Side wall part 7 Bead part 10 Pneumatic tires 11, 21 Linear part 12, 22, 23 Arc part 20, 30 Flat shape metal wire W of metal wire Single wire or bundle width T Metal wire single wire or bundle thickness Wf Metal wire width Tf Metal wire thickness Ra Curvature radius of metal wire arc portion 22 Rb Curvature radius of metal wire arc portion 23

Claims (5)

A single inclined belt layer in which a plurality of metal wires are inclinedly arranged with respect to the tire equatorial plane on the outer periphery of a crown portion of a carcass that forms a toroidal shape straddling between at least a pair of bead cores, and on the inclined belt layer In a pneumatic radial tire comprising a tread portion reinforced by a reinforcing belt, and at least one circumferential belt layer formed by arranging organic fiber cords substantially parallel to the tire equatorial plane,
The inclined belt layer has one or more metal wires having a flat shape that is long in the belt width direction when at least most of the metal wires in the inclined belt layer are viewed in a cross section perpendicular to the longitudinal direction. It exists as a single metal wire or a bundle of metal wires that are arranged in parallel in the belt width direction without twisting, and the single wire or bundle of flat metal wires are parallel and flat with a gap between the single wires or between the bundles in the belt width direction. Are arranged in the belt width direction and embedded in rubber,
The width W (mm) and the thickness T (mm) of the single wire or bundle of the flat metal wire are expressed by the following formula (1),
0.334 × (W / T) ^−0.3449 ≦ T ≦ 0.342 × (W / T) ^−0.2539 (1)
(W is represented by W = N × Wf, where N is 2 or 3, where the width of the flat metal wire is Wf (mm) and the number of bundles is N). Satisfying the relationship, the ratio of the width W (mm) and the thickness T (mm) of the single wire or bundle of the flat metal wire is represented by the following formula:
0.18 ≦ T / W ≦ 0.34
Satisfying the relationship represented by
The number P (lines / 50 mm) of single wires or bundles of the flat-shaped metal wires for the inclined belt layer is expressed by the following equation (2):
−9.258 × Ln (W / T) + 40.187 ≦ P ≦ −10.487 × Ln (W / T) +45.848 (2)
A pneumatic radial tire characterized by satisfying a relationship represented by the formula (W and T are the same as described above, and Ln represents a natural logarithm). The cross-sectional shape of the flat-shaped metal wire, pneumatic radial tire according to claim 1, wherein the track-shaped having a pair of parallel linear portions, and a pair of arcuate portions which face each other is convex on the outside, a. The cross-sectional shape of the flat metal wire is a pair of parallel straight lines, a pair of circular arcs that protrude outward from each other, and another pair of circular arcs at a portion that transitions from the straight line to the circular arc. The pneumatic radial tire according to claim 2 , wherein the pneumatic radial tire has a track shape. The flat metal wire has a thickness Tf of 0.15 to 0.26 mm, and the ratio of the width Wf to the thickness Tf is:
0.18 ≦ Tf / Wf ≦ 0.34
The pneumatic radial tire according to any one of claims 1 to 3 , satisfying a relationship represented by: The pneumatic radial tire according to any one of claims 1 to 4 , wherein a tensile strength of the flat metal wire is 3300 to 4000 MPa.

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