JPH0524413A - Pneumatic radial tire for heavy load - Google Patents

Abstract

PURPOSE:To prevent respective belt layer edge parts from being separated by extending the outer side edge of an outer side belt layer outward by a prescribed amount, layering a prescribed amount of outside belt layers width on a main belt layer by a prescribed amount, and setting the inclination angle and performance of the steel cords of the respective belt layers as a prescribed relation. CONSTITUTION:A main belt layer 5 is axially-divided into outside and central belt layers 5A, 5B, which are put between continuous belt layer 5C to form it in a three-layer structure, and the outside edge of the outer side belt layer 5A is extended toward the upper axial direction by more than 75% of a tread ground contact width TW. Besides, the continuous belt layer 5C is overlapped on the outside belt layer width L by more than 80%, and inclination angles alphaA, alphaB, alphaC of a steel cord to the tire equator O of respective belt layers 5A-5C are set as alphaB>=alphaC>alphaA, and total breaking strength/breaking extension ZA, ZB, ZC in a steel cord axial direction per unit width is set 1.2<=ZA/ZC<=2.2, 0.7<=ZB/ZC<=1.1. Therefore, it is possible to restrain separation at the belt edge parts and improve economy and durability.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heavy-duty pneumatic radial tire having a flatness ratio of 75% or less, which is mounted on a construction vehicle, a truck or the like and used under a heavy load, and particularly to a belt layer for reinforcing a tire carcass. It is intended to improve the durability of.

[0002]

2. Description of the Related Art In a tire of this type, a plurality of cord layers made of steel cords arranged in parallel with each other are provided on the outer side in the tire radial direction of a carcass made of at least one ply in which steel cords are radially arranged. It is customary to stack the belt layers arranged in such a manner as to secure the rigidity, but at both ends of the belt layer, in order to prevent the rigidity from becoming extremely high, that is, to avoid excessive stress concentration at the ends. For these reasons, since the belt layers are arranged in a stepwise manner, the number of cord layers tends to decrease at both ends of the belt layers. Then, during inflation or during running, the diameter growth of the tread shoulder portion becomes larger than that of the other tread portions, the interlayer shear strain at the belt layer end portion increases, and separation occurs at the belt layer end portion.

Therefore, Japanese Patent Application Laid-Open No. 1-175504 proposes a belt layer structure in which the belt layer is divided in the axial direction of the tire to reduce the interlaminar shear strain at the ends of the belt layer. It has been described that the interlaminar shear strain at the end of the belt layer is reduced mainly for a tire having an aspect ratio of about 90%.

By the way, in recent years, the wave of tire flattening has spread to heavy-duty tires, and tires having a flatness ratio of 75% or less have been developed. In these flat tires, too, separation occurs at the end of the belt layer. It's a problem.
However, the belt layer structure of the above publication is particularly suitable for a tire having a flatness ratio of about 90%, and a flat tire having a flatness ratio of 75% or less is not mentioned. Actually, separation of the belt layer end portion in the flat tire occurs. Although there is some effect in preventing this, it is not sufficient and the separation problem is still unsolved.

On the other hand, for flat tires having a flatness ratio of 75% or less, reinforcing belts having two cord layers laminated so that the cords cross each other are provided in Japanese Examined Patent Publication No. 2-14202, on both sides of the equator of the tire. There is a disclosure of a belt layer structure. However, since the reinforcing belt hardly contributes to the reinforcement of the end portion of the belt layer, the effect of preventing separation at the end portion of the belt layer is not sufficient. Further, the cord angle of the reinforcing belt with respect to the equator of the tire is small and the cords are laminated, so that the tension load of the reinforcing belt is large and the difference in rigidity between the reinforcing belt and the main belt layer is widened, and between them or between the reinforcing belts. I am glad to see progress in the separation.
Further, since it is an additional member, it is inevitable that the gauge of the entire tread portion becomes thicker, which causes a lot of heat generation in practical use and has a disadvantage of promoting the belt layer separation due to a high temperature.
Further, since there are many belt materials, there is a disadvantage that productivity is low and cost is high.

[0006]

SUMMARY OF THE INVENTION Therefore, according to the present invention, it is possible to advantageously avoid the occurrence of separation at the end portion of the belt layer without increasing the number of constituent materials of the belt layer, especially when the aspect ratio is reduced.
The purpose is to propose a pneumatic radial tire for heavy loads of 75% or less.

[0007]

[Means for Solving the Problems] Separation at the end of the belt layer that occurs in a flat tire having an aspect ratio of 75% or less is not mainly caused by interlayer shear strain at the end of the belt layer. It was newly found that the main cause is a crack in the edge of the cord due to the large tensile stress that occurs. That is, as shown in FIG. 1, the tread rubber mainly moves to the outside in the axial direction of the tire which is released due to the contact pressure generated on the tread surface while the tire is in contact with the ground. Figure 2
As shown in the behavior of the rubber in the vicinity of the steel cord, the movement of the rubber is restricted at the adhesion point a with the peripheral surface of the steel cord of the belt layer, while the end surface b of the steel cord is not subjected to the adhesion treatment. The rubber moves at point b. Then, a cavity is created between the steel cord and the rubber,
A large tensile stress acts on the edge of the steel cord that maintains the adhesion with the rubber, and if the movement of this tread rubber is repeated, cracks occur at the edge of the cord, and heat is also generated where the ground pressure is high. Since the temperature becomes large and the temperature is high, the cracks once generated continue to grow and eventually propagate to the separation at the end portion of the belt layer.

In particular, since the movement of the tread rubber is proportional to the ground contact pressure of the tread, the movement of the tread rubber in a heavy-duty tire is naturally large, and in the tires for running on rough terrain, an unbalanced load is often applied to the tread end. The cracks at the edges of the cords grow in the circumferential direction of the tread and are connected to each other, and at the same time, they propagate along the steel cord that is subjected to shear stress in the equatorial direction of the tread, so that separation is easily achieved.

From the above findings, the first is to suppress the movement of the tread rubber at the steel cord end as much as possible, and the belt layer structure capable of following the movement of the tread rubber while additionally relaxing the tensile stress applied to the steel cord edge. By establishing
The inventors have found that it is possible to prevent the occurrence of belt layer separation, and have completed the present invention.

That is, according to the present invention, at least two layers of a carcass consisting of at least one ply in which steel cords are radially arranged and at least two cord layers consisting of steel cords arranged in parallel are provided on the outer side of the carcass in the tire radial direction. A main belt layer laminated in an intersecting arrangement and at least one cord layer composed of high-elongation steel cords arranged in parallel are provided in order to form a protective belt layer and a tread, and the aspect ratio is 75%.
In the following pneumatic radial tire, the maximum width layer of the main belt layer is composed of a central belt layer divided into three in the axial direction of the tire and an outer belt layer sandwiching the central belt layer, and an outer end of the outer belt layer. At least one of the main belt layers adjacent to the outer belt layer is 80% of the outer belt layer width and extends axially outward of the tire beyond the area of 75% of the tread width that divides the tire equator into equal points. % Or more, and further, in the main belt layers except the outer belt layer, the central belt layer and these divided belt layers, the inclination angles α _A , α _B and α _C of the steel cord with respect to the equator of the tire and the unit width steel cord axis direction (a total breaking strength) / ratio Z _a of (elongation at break), and a Z _B and Z _C is a heavy duty pneumatic radial tire, characterized by satisfying each of the following relationship for. Note α _B ≧ α _C > α _A 1.2 ≦ Z _A / Z _C ≦ 2.2 0.7 ≦ Z _B / Z _C ≦ 1.1

The tilt angles α _A , α _B and α _C are
14 ° ≤ α _A ≤ 20 °, 22 ° ≤ α _B ≤ 32 °, 22 ° ≤ α _C ≤ 26
And the total width W of the outer belt layer and the central belt layer is 3/4 TW <W with respect to the tread width TW.
<Tw is satisfied and the width L of the outer belt layer satisfies 1 / 6TW ≤ L ≤ 1 / 4TW with respect to the tread ground contact width TW,
It is advantageous for implementation.

Now, FIG. 3 shows a cross section of the heavy duty pneumatic radial tire according to the present invention for the left half. In the figure, 1 is a bead core whose only one side is shown, 2 is a carcass (1 ply in the figure) consisting of at least one ply in which steel cords are radially arranged, 3 is a bead part, 4 is a sidewall part, 5 is a tire diameter of the carcass 2. A main belt layer in which at least two layers (three layers in the figure) of steel cords arranged in parallel to the outside in the direction are arranged, and 6 is made of high elongation steel cords arranged in parallel in the tire radial direction outside of the main belt layer. At least one code layer
The layers are protective belt layers in place, and 7 is the tread.

The carcass 2 has the carcass ply arranged in a toroidal shape across the bead cores 1 and has a turn-back portion around the bead cores 1 which is wound from the inside to the outside of the tire.

The main belt layer 5 is formed by laminating at least two layers of cords in which steel cords are arranged in parallel so that the steel cords cross each other. In particular, the layer having the maximum width among the cord layers is a tire. The outer belt layer 5A is divided into three in the axial direction and the central belt layer 5B is sandwiched between these outer belt layers 5A. In the illustrated example, this divided belt layer is sandwiched between two continuous belt layers 5C. It has a layered structure.

Here, the outer edge of the outer belt layer 5A exceeds the area of 75% of the tread ground contact width TW (hereinafter simply referred to as the area of 75% of the tread width TW) with the equator O of the tire as an equal point. Of the continuous belt layer 5C adjacent to the outer belt layer 5A (the continuous belt layer 5C on the radially outer side in the illustrated example) is 80% or more of the outer belt layer width L. It is essential that they overlap with each other.

Further, in the outer belt layer 5A, the central belt layer 5B and the continuous belt layer 5C, the inclination angles α _A , α _B and α _C of the steel cord with respect to the equator O of the tire are α _B ≧ α _C > α _A Similarly, in the outer belt layer 5A, the central belt layer 5B and the continuous belt layer 5C, the ratio (total breaking strength) / (breaking elongation) ratio Z _A , Z _B and Z in the axial direction of the steel cord per unit width. _It is also important that _C follows the relationship of 1.2 ≤ Z _A / Z _C ≤2.2 0.7 ≤Z _B / Z _C ≤1.1.

The protective belt layer 6 comprises at least one cord layer composed of high elongation steel cords arranged in parallel. The steel cord used for the protective belt layer 6 is, for example, a high elongation steel cord having a breaking elongation of about 6 to 9%, which is advantageous for following the movement of the tread rubber. Further, the protective belt layer 6 has a relatively small wire diameter (for example, about 6 mm) in order to avoid peeling between the tread portion 7 and the protective belt layer 6 in a tire of this type that is frequently cut by the tread during rough running. It is preferable to roughly strike a steel cord (1.6 to 1.9 mmφ) (for example, about 6 to 9 cords / 25 mm). In this case, since the covering rubber between cords is large, the followability to the movement of the tread rubber is also improved.

[0018]

[Function] Particularly, in the separation of the end portion of the belt layer in the tire for flat heavy load where an overload or an unbalanced load is easily applied, the crack of the rubber caused by the large tensile stress repeatedly generated at the steel cord end of the belt layer is the trigger. As already mentioned.

Therefore, in the tire according to the present invention, first, in order to reduce the tensile stress at the steel cord end at the end of the belt layer, the outer belt layer occupying the maximum width layer side area of the main belt layer.
5A extends beyond the 75% area of the tread width TW to the outside of the tire in the axial direction. Ie tire size 25/65
Fig. 4 shows the amount of tire diameter growth during inflation, using a conventional tire of R25 (internal pressure: 5.25kgf / cm ² flatness ratio nominal 65%) as an example.
As shown in Fig. 5 for the contact pressure distribution on the tread surface, both the tire diameter growth and the contact pressure of tread width TW
It can be seen that the maximum is reached at a position near 75%. Therefore, increase in tread rubber movement near the belt cord steel cord end,
That is, in order to suppress the increase in the tire diameter and the increase in the contact pressure due to the growth of the diameter that causes an increase in the tensile stress at the steel cord end, the outer end of the outer belt layer 5A is a tread contact width.
It is necessary to exceed the 75% area of TW. Furthermore, in order to effectively suppress the growth of the diameter near the 75% area of TW, the outer belt layer
The circumferential tensile rigidity and cross-sectional bending rigidity of 5A are set as high as possible compared to other belt layers, that is, the central belt layer 5B and the continuous belt layer 5C, and at least the continuous main belt layer adjacent to the divided belt layers 5A and 5B. One outer layer
It is essential that the width is 80% or more of the width of 5A, and these structures can further enhance the diameter growth suppressing effect. In this case, it is natural that the steel cords of the respective layers are arranged so as to intersect each other with different inclination directions with respect to the equator. Furthermore, the circumferential tensile rigidity and cross-sectional bending rigidity of the central belt layer 5B are made equal to or less than the same rigidity as other continuous main belt layers, or suppressed to a slightly high level, so that the diameter growth of the central portion of the tread is equal to that of the conventional tire. In some cases, depending on the type of tire and the conditions of use, it may be effective to promote it.

By configuring all the main belt layers as described above, the diameter growth at the 75% position of the tread width TW is suppressed, the ground contact pressure is reduced, and the diameter growth at the center of the tread is kept at or above that of conventional tires. It is achieved that the pressure is increased and the contact pressure distribution is made uniform over the entire tread. Under operating conditions where tire overload and uneven load are frequently applied, the contact pressure distribution is high in the tread center area under the specified internal pressure and load.
It is desirable to lower it at the 75% position. Thus, the tensile stress generated on the outer end of the steel cord of the outer belt layer 5A can be reduced and the timing of crack occurrence can be significantly delayed, but the maximum width layer of the main belt layer is divided into three parts in a butt shape in the tire axial direction. By doing so, since the outer belt layer 5A can follow the movement of the tread rubber to some extent, the movement of the rubber in contact with the steel cord edge tends to be relaxed, and the tensile stress generated on the outside of the steel cord is also somewhat relaxed. Side effects are also expected.

Furthermore, the contact pressure distribution is made uniform or the tread width is
By making the contact pressure near 75% of TW lower than that in the center of the tread, the temperature near the outer edge of the outer belt layer 5A is lower than that of conventional tires, and the degree of deterioration of the physical properties of rubber due to high temperature is reduced. Since it is mitigated, it is convenient for preventing the occurrence and development of the cracks. It is also advantageous for uniform wear of the tread surface, which is strongly affected by the ground contact pressure.

That is, the outer belt layer 5A and the central belt layer 5B
Angle α of steel cord in continuous and continuous belt layer 5C
_{When A} , α _B and α _C are set to α _B ≧ α _C > α _A , the rattling force of the outer belt layer 5A is made higher than the other portions, and the outer belt layer
The area corresponding to 5A, that is, it contributes to the reduction of the ground pressure in the vicinity of 75% of the tread ground width TW. Specifically, α _A : 14-20
°, α _B : 22-32 ° and α _C : 22-26 °,
At the same time it is advantageous in obtaining the desired ground contact pressure distribution and suppressing cracks, and at the same time, various features of steel radial tires, such as high wear resistance, low heat generation, and shock burst avoidance during rough terrain travel. Are preferred because they can be simultaneously achieved. That is, if α _A is less than 14 °, the tension load at the time of inflation becomes too large, which may cause the steel cord to break due to the impact projection input, and the rigidity difference between the adjacent main belt layers is large. As a result, the interlaminar shear stress also becomes high, and the risk of cracking and separation occurring before progressing increases, while exceeding 20 °,
The tension load is insufficient, and it becomes difficult to obtain the desired diameter growth suppression and contact pressure distribution.

If α _B is less than 22 °, the rigidity becomes too high due to an increase in the tension load at the central portion of the belt, and the diameter growth becomes small as compared with the vicinity of the TW75% area, and it becomes difficult to obtain a uniform tread contact pressure distribution. On the other hand, if the angle exceeds 32 °, the tension load in the central part of the belt is significantly reduced, the diameter growth becomes excessive in the central part of the tread, and the ground contact pressure becomes significantly high in the central part. Wear is promoted, the wear life is shortened due to the so-called center wear tendency, and the life balance of the tire body is disturbed, which is not preferable.

Next, α _C is a value relating to the rigidity of the continuous main belt, and since it is not divided, it is important in adjusting the rigidity of the entire main belt layer as a steel radial tire.
That is, when the angle is less than 22 °, the rigidity of the entire main belt layer is increased, which not only results in reducing the above-mentioned objective effect obtained by dividing the main belt layer into three, but also as a problem to be solved in the present invention. There is a problem that the separation due to the interlaminar shear stress at the end portion of the main belt layer, which is not necessary, cannot be ignored. Furthermore, there is a problem such as a risk of shock burst due to shocking projection input, which is an unavoidable problem when traveling on rough ground. On the other hand, if it exceeds 26 °, the necessary rigidity as a whole of the main belt layer cannot be obtained, and the various characteristics of the steel radial tire, such as high wear resistance and low heat buildup, are deteriorated. If the temperature of the layer rises, cracking and development of the steel cord edge at the outer end of the outer belt layer 5A is promoted, which is not preferable.

Further, the ratio (total breaking strength) / (breaking elongation) Z _A , Z _B and Z _C in the axial direction of the steel cord per unit width in each of the same belt layers is 1.2 ≦ Z _A / Z _C
Restrict to the relationship of ≦ 2.2 and 0.7 ≦ Z _B / Z _C ≦ 1.1.
The fracture elongation (%) of the steel cord was introduced in the denominator here, as demonstrated by the experiments of the comparative examples described below that it is not possible to suppress the crack generation at the steel cord end only by the total fracture strength in the axial direction of the steel cord. Because it was obtained, elongation at break 1%
This is because it has been found that the above-mentioned total breaking strength per hit is directly related to the separation durability of the belt that starts from the crack generation. The continuous main belt layer was a denominator Z _C in terms of arranging good Preferred stiffness balance of the steel radial tire of the entire main belt layer, the point is appropriate for use as a reference, the same as the alpha _C is there.

When Z _A / Z _C is less than 1.2, a preferable balance of rigidity between the outer belt layer 5A and the continuous belt layer,
That is, it is impossible to keep the diameter growth near 75% of TW within a preferable range, which leads to a high ground contact pressure, and eventually it is easy to cause cracking of the cord edge. Meanwhile Z _A / Z _C is 2.
If it exceeds 2, the rigidity of the outer belt layer 5A becomes excessively high, the balance of rigidity in the central portion and both side portions of all the main belt layers is largely lost, and the diameter growth in the central portion of the tread becomes larger than both side portions of the tread. Although the crown radius of the tread becomes smaller, when a flat tire for heavy-duty vehicles is subjected to an overload or an unbalanced load, the entire tread, including the belt layer, exhibits a kind of buckling phenomenon in the cross-sectional direction at the time of contact with the ground. The ground contact pressure on both sides of the tread including the vicinity of the 75% area increases, and the effect of the present invention is diminished.

Even if Z _B / Z _C is less than 0.7, the same phenomenon as in the case where Z _A / Z _C exceeds 2.2 occurs.
On the other hand, when Z _B / Z _C exceeds 1.1, the continuous main belt layer directly adjacent to the three-part main belt layer straddles the outer belt layer 5A and the central belt layer 5B.
The difference in rigidity between the A and 5B regions is reduced, and the desired relative suppression of diameter growth and reduction in ground contact pressure in the vicinity of the 75% area of the tread width TW, which is the object of the present invention, cannot be obtained. Incidentally, the suppression of radial growth in the present invention means not only the absolute value of growth but also the relative value.

In addition, the outer belt layer 5A and the central belt layer 5B
The total width W of the tread width TW is 3/4 TW <W <TW,
And the width L of the outer belt layer 5A is 1/6 TW ≦ L ≦ with respect to the above TW.
Setting it to 1/4 TW is essential for maintaining the effect of the present invention.

Because, when W is 3/4 TW or less, the tread width
This is because it is not possible to suppress the diameter growth near the 75% area of TW and to reduce the contact pressure, and also the width of the entire main belt layer is narrowed and the characteristics of the steel radial tire cannot be exhibited. Further, when W is TW or more, the radially outer portion of the sidewall portion 4 (the inner portion of the joint with the tread 7) is subjected to bending deformation at the time of grounding, so that compressive stress is applied to the outer end portion of the outer belt layer 5A, This is because a crack is generated in the outer end portion of the outer belt layer 5A, which leads to separation. Furthermore, L is 1/6 TW
If it is less than 1, the diameter growth suppressing effect of the outer belt layer 5A is weakened, while if it exceeds 1/4 TW, the diameter growth suppressing effect extends to the central portion of the tread, and relative diameter growth suppression in the vicinity of 75% of the tread width TW is impaired. This is because it causes pressure.

[0030]

[Example] Example 1 Tire size 45 / 65R45 L5 (tread width: 1030mm)
The pneumatic tire for a construction vehicle of No. 3 was prototyped according to the structure shown in FIG. 3 and the belt layer structure whose main parts are illustrated in FIGS. 6 (a) to 6 (c). In the figure, the solid line indicates the main belt layer and the dotted line indicates the protective belt layer. In each of the test tires, the carcass 2 had a one-ply (indentation number: 14.8 / 5 cm) radial arrangement structure composed of a steel cord having a breaking strength of 555 Kgf / line. The test tires are pneumatic: at 5.25kgf / cm ^2, the bucket capacity: attached to 10.5 m ³ loader, average wasteland working path of about 0.99 m in one way rate: running a load-and-carry condition 9km / h Then, the service life (engine hour) until the belt edge separation occurred was investigated.
The results are shown in Table 1 together with the specifications of the test tires. The structures of the comparative example and the conventional example in the table are different in the underlined items in the table, and the other specifications are the same as the conforming example shown in the table. From the table, it was confirmed that the durability of the belt end portion was enhanced to extend the specified life in each case according to the present invention. The required life at this work site was 4000 hours.

[0031]

[Table 1]

Example 2 Tire size 25 / 65R25 L3 (tread width: 526 mm)
The pneumatic tire for a construction vehicle of No. 3 was prototyped according to the structure shown in FIG. 3 and the belt layer structure whose main parts are illustrated in FIGS. 7 (a) to 7 (c). In each of the test tires, the carcass 2 had a one-ply (12: 5 cm) radial arrangement structure made of steel cord having a breaking strength of 400 Kgf / piece. The test tire has an air pressure of 5.25 kgf / cm ² and a bucket capacity of:
It is mounted on a 2.9 m ³ loader, runs one way on a wasteland work road of about 80 m under load and carry conditions with an average speed of 6 km / h, and investigates the service life (engine hour) until belt end separation occurs. did. The results are shown in Table 2 together with the specifications of the test tires. The structures of the comparative example and the conventional example in the table are different in the underlined items in the table, and the other specifications are the same as the conforming example shown in the table. From the table, it was confirmed that the durability of the belt end portion was enhanced to extend the specified life in each case according to the present invention. The required life at this work site was 2500 hours.

[0033]

[Table 2]

[0034]

According to the present invention, when the flatness of the heavy load tire is reduced to 75% or less, the required separation of the belt end portion can be suppressed, so that the heavy load tire is economical and has excellent durability. Can be advantageously achieved.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing movement of a tread rubber.

FIG. 2 is a schematic diagram showing movement of a tread rubber around a steel cord.

FIG. 3 is a sectional view of a tire according to the present invention.

FIG. 4 is a graph showing tire diameter growth.

FIG. 5 is a graph showing a contact pressure distribution on a tread surface.

FIG. 6 is a schematic view showing a structure of a belt layer.

FIG. 7 is a schematic view showing a structure of a belt layer.

[Explanation of symbols]

1 bead core 2 carcass 3 bead part 4 Sidewall part 5 Main belt layer 5A outer belt layer 5B central belt layer 5C continuous belt layer 6 Protective belt layer 7 tread section

Claims (3)

[Claims] 1. A carcass consisting of at least one ply in which steel cords are radially arranged, and at least two layers of cords consisting of steel cords arranged in parallel are arranged outside the carcass in the tire radial direction so that the steel cords intersect each other. It is a pneumatic radial tire with a flatness ratio of 75% or less, in which a main belt layer and a cord layer composed of high-elongation steel cords arranged in parallel are arranged in order at least one protective belt layer and a tread. The maximum width layer of the main belt layer is composed of a central belt layer divided into three parts in the axial direction of the tire and an outer belt layer sandwiching the central belt layer, and the outer end of the outer belt layer divides the tire equator equally. The main belt that extends axially outward of the tire beyond the area of 75% of the tread width at the point and is adjacent to the outer belt layer. At least one of the outer belt layers overlaps by 80% or more of the width of the outer belt layer, and the inclination of the steel cord with respect to the equator of the tire in the continuous main belt layer excluding the outer belt layer, the central belt layer and these divided belt layers Angle α _A , α _B
And α _C and the ratio (total breaking strength) / (breaking elongation) Z _A , Z _B and Z _{C in the} axial direction of the steel cord per unit width satisfy the following relationships, respectively, for heavy loads Pneumatic radial tire. Note α _B ≧ α _C > α _A 1.2 ≦ Z _A / Z _C ≦ 2.2 0.7 ≦ Z _B / Z _C ≦ 1.1 2. The inclination angles α _A , α _B and α _C further satisfy the relationship of 14 ° ≦ α _A ≦ 20 ° 22 ° ≦ α _B ≦ 32 ° 22 ° ≦ α _C ≦ 26 °. The tire described in. 3. The total width W of the outer belt layer and the central belt layer satisfies 3 / 4TW <W <TW with respect to the tread width TW,
And the width L of the outer belt layer is related to the tread ground contact width TW.
The tire according to claim 1, which satisfies 1 / 6TW ≤ L ≤ 1 / 4TW.