JP2579309B2 - Radial tire - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a radial tire with improved steering stability and riding comfort.

(Prior art)

In recent years, radial tires in which cords of carcass plies are arranged at right angles in the circumferential direction of the tire and in which a strong breaker layer is provided in the circumferential direction along the tread portion have been frequently used.

However, in such a radial tire, although the breaker layer has high rigidity, and therefore has a large cornering force and can reduce rolling resistance and improve abrasion resistance, such a breaker layer having such a high rigidity is not suitable for a road surface. It is inferior in the cushioning effect of the impact force when riding over unevenness or protrusions, that is, the envelope performance is deteriorated, and the riding comfort is deteriorated.

On the other hand, in a radial tire, the carcass plies are arranged at right angles in the circumferential direction, and the lateral rigidity is reduced, so that the vehicle is likely to run out. For this reason, a so-called flat tire in which the height of the tire is reduced is used.

However, when flattening the tire, the rigidity in the vertical direction is also increased to further reduce the above-mentioned entropy performance, and the bumps and bumps on the road surface are transmitted to the vehicle via the bead portion and the rim when the vehicle passes over a protrusion. It is easy and the ride is disturbed. As described above, although the driving stability and the riding comfort performance are in conflict with each other, the tendency is sometimes remarkable in flat tires.

On the other hand, such radial tires are conventionally vulcanized and molded by a mold having a bore shape that has a natural equilibrium shape in which a carcass is not deformed even when a standard internal pressure is applied to a molded tire. Have been.

Here, the natural equilibrium shape refers to a carcass profile determined by the natural equilibrium shape theory, and the natural equilibrium shape theory is defined by Hoff Aberth (W. Hofferberth)
Autsch. Gummi (8-1955, pp. 124-130), this theory considers the breaker layer as a rigid ring that does not deform due to the filling of the internal pressure, and considers this breaker layer and the other deformation. An inextensible carcass that is arranged between the bead core that does not occur and extends from the sidewall part to the bead part,
It is intended to be preliminarily formed by a vulcanizing mold into a shape that does not deform due to the filling of the internal pressure.

The Hoff-Aberth theory is based on bias tires. However, Mr. Akasaka stated in the Journal of the Society of Composite Materials, Japan, Vol. 3.4 (1977), pp. 149-154, "About the cross-sectional shape of radial tires". It has been extended to be adaptive.

In addition, it is preferable to supplement the application of the natural equilibrium shape theory in at least the following two points.

First, even when the breaker layer is made of a metal cord or the like, it is not a completely rigid body, but is slightly deformed by internal pressure. In particular, a flat tire having a small aspect ratio pushes up the carcass by filling with internal pressure. By
The breaker layer is prone to deformation.

Second, in the vicinity of the bead portion, the rigidity is large due to the folded portion of the carcass, the bead apex, and other reinforcing layers. Therefore, the inflection point of the carcass profile, which is usually called a rim point, that is, the equivalent bead position In the range up to, it does not match the natural equilibrium shape theory, and therefore, the curve according to the theory should be considered starting from the equivalent bead position.

As described above, the natural equilibrium shape theory adopted in the conventional tire is that, as described above, the tire in the mold vulcanized and molded in the vulcanization mold is loaded on a standard rim and the standard internal pressure is reduced. Even when filled, without changing the shape of the carcass,
This is a theory for creating a curve that produces uniform tension in the carcass.In general, the clip ring width of the molded tire in the vulcanization mold, that is, the length between the outer surfaces of the bead bottom, is the width of the rim to be mounted. Is set equal to

On the other hand, a proposal for consciously deforming a carcass profile based on such a natural equilibrium shape theory in order to satisfy the above trade-off condition between steering stability and ride comfort has been proposed, for example, in Japanese Patent Publication No. Sho 61-28521 ( JP-A-58-161603)
It is made by the gazette.

[Problems to be solved by the invention]

As described above, the carcass profile based on the conventional natural equilibrium shape theory cannot satisfy the above two conditions. The tire proposed in Japanese Patent Publication No. Sho 61-28521 (hereinafter referred to as a modified tire 1A in the present specification) is a fourth tire.
As shown by the broken line in the figure, the radius of curvature R1A is smaller in the upper part of the carcass profile than in the curve 1B (indicated by a chain line) based on the natural equilibrium shape theory. Than the profile
It bulges outward and is formed in an upper bulge shape as a whole. The above-mentioned proposal asserts that in order to obtain such a carcass profile, it is essential to use a mold in which the clip ring width is larger than the rim width.

However, in such a modified tire 1A, the total breaker tension increases upon filling with the internal pressure, but the tension distribution shows the same tendency as that of the conventional tire.

In consideration of the deformation of the tire profile due to the internal pressure filling, for example, Japanese Patent Application Laid-Open No. 61-157402 is intended to increase lateral rigidity and longitudinal rigidity. Japanese Patent Application Laid-Open No. 54-13 / 1979 with the intention of preventing early failure due to initial distortion centered on
Japanese Patent Publication No. 6001 has been proposed.

However, the former guides the maximum overhanging deformation due to the internal pressure filling outward in the tire radial direction near the maximum width of the carcass, and is basically different from the present application in which the carcass is deformed in a downwardly bulging shape. In the latter, the carcass is a balanced state that receives a force other than the pressure generated in the belt layer and the internal pressure when filling the internal pressure above the intersection where the radius line passing through the rim flange surface intersects the bead portion of the carcass, and Although the lower part of the intersection is to be inverted, this is mainly based on using a rim flange having a height as low as the upper end height of the bead wire.
It can be said that it is based on a different technical idea.

The present invention brings the maximum width position PM of the carcass closer to the rim baseline and appropriately sets the radius of curvature of the upper portion and the lower portion of the maximum width position to increase the tension at the shoulder portion of the breaker layer and reduce the total breaker tension. It is an object of the present invention to provide a radial tire that can improve the ride comfort in addition to the steering stability by reducing it, and can meet the above demand.

[Means for solving the problem]

The present invention provides a tread portion of a tire, a carcass that passes through a sidewall portion and is folded at both ends by a bead core of a bead portion, and a carcass arranged between a main portion of the carcass and a folded portion thereof and a carcass. A bead apex made of a tapered and rubber material extending along the same, and a breaker layer made of a low-stretch cord disposed inside the tread portion and outside the carcass, and attached to a regular rim and filled with a standard internal pressure. At the maximum width position PM of the carcass from the rim baseline of the regular rim.
The height ratio h / H to the radial height H from the rim baseline to the tread surface highest position is:
0.4 to 0.5, and passes through the maximum width position PM and has a center on a line Y parallel to the rim base line, respectively, and the rim base extends from an end of a region where at least two layers of the breaker layer overlap. The radius of curvature R1 of the arc passing through the first intersection point P1 at which the perpendicular X extending perpendicular to the line intersects the carcass at the tread portion and the maximum width position PM is the second intersection P2 at which the perpendicular X intersects the carcass at the bead portion. The radius of curvature of the arc passing through the maximum width position PM is larger than the radius of curvature R2, and the carcass is located outside the maximum width position PM in a range between the maximum width position PM and the second intersection P2. An upper curve portion of the bulge and a lower curve portion of the inner bulge which is closer to the second intersection P2 and smoothly continues from the upper curve portion to the inflection point B, wherein the radius of curvature of the upper curve portion is the maximum width. position
A radial tire characterized by a gradual decrease from PM to inflection point B.

 An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 1, which shows a cross section of the right half of the rim Rm assembled and filled with a standard internal pressure, a radial tire 1 has bead portions 3 on both sides through which a bead core 2 passes, and radially outside the bead portions 3. The bead part 3, the side wall part 4, and the tread part 5 are turned around the bead core 2 from the inside to the outside, while the side wall part 4 extending in the direction and the tread part 5 joining the upper end thereof are provided. The main body of the carcass 6 is laid. In the tread portion 5, a breaker layer 7 is arranged outside the carcass 6, and a bead apex 9 is provided between the body portion of the carcass 6 and its folded portion.
The bead portion 3 is fitted to the flanges 11 and 11 of the rim Rm to be mounted on the rim Rm.

The carcass 6 is a so-called radial cord arrangement in which cords are arranged at an angle of about 80 ° to 90 ° with respect to the tire equatorial plane C, and nylon, polyester, rayon, aromatic polyamide fibers, and steel are used as cords. Is done. As the carcass 6, a carcass ply having one to three layers is used.

In addition, the breaker layer 7 is provided on the first carcass 6 side.
The first and second plies 7A and 7B are each made of a metal cord and have a relatively shallow angle with respect to the tire equatorial plane C. And inclined in the opposite direction. The first ply 7A is wider than the second ply 7B, and both ends of the first ply 7A extend below the intersection of the sidewall portion 4 and the tread portion 5. In the bead part 3,
A reinforcing layer 9 is provided extending from the lower end of the bead core 2 along the folded portion of the carcass 6 and extending beyond the upper end of the bead apex 9. The reinforcing layer 9 is formed of one layer or a plurality of appropriate organic fiber cords in addition to the metal cord.

In the present embodiment, the radial tire 1 has the lower end of the bead portion 3, that is, the rim baseline RmB at the time of standard internal pressure filling.
From the tire tread surface 12, which is the upper surface of the tread portion 5, to the maximum height point a, that is, the ratio H / W of the tire total height H to the tire width W is smaller than 1, for example, a flattened shape of 0.7 or less. It is formed as a tire.

Further, the radial tire 1 of the present invention has a height h in the radial direction from the rim base line RmB of the rim Rm to the maximum width position PM of the carcass, and a height ratio h / H of the total height H of the tire.
Is set in the range of 0.4 to 0.5.

Further, an arc passing through a first intersection P1 where a perpendicular X extending perpendicularly to the rim base line RmB from the end b of the region where the breaker layers 7A and 7B overlap with the carcass 6 at the tread portion 5 and the maximum width position PM. The radius of curvature R1 of 20 is the perpendicular X
Intersection P2 at which bead 3 intersects carcass 6
And the radius of curvature R2 of the arc 21 passing through the maximum width position PM. In addition, the arcs 20 and 21 pass through the maximum width position PM and are parallel to the rim base line RmB.
Each has a center on top. In the present specification, a range between the first intersection P1 and the maximum width position PM is referred to as an upper portion of the carcass 6, and a range between the maximum width position PM and the second intersection is referred to as a lower portion. .

By determining the height h and the radii of curvature R1 and R2 in this way, the deformed tire 1A can be compared with the curve based on the natural equilibrium shape theory, as shown by the broken line in FIG. In contrast, the tire 1 of the present invention has a so-called mump-like shape with a lower bulge bulging below the maximum width position PM, as shown by a solid line in FIG. Becomes The height ratio h / H is
The fact that it is smaller than 0.5 and the radius of curvature R1 is larger than the radius of curvature R2 is a requirement for making the carcass profile lower bulge and different from the curve based on the natural equilibrium shape theory, When the height ratio h / H is 0.4 or less, the carcass line in the bead portion 3 is excessively different from the curve 1B based on the natural equilibrium shape theory, causing a sharp change in curvature in the bead portion 3, and Stress concentration is likely to occur, and the durability tends to be impaired, and manufacturing becomes difficult.

The curve 1B according to the natural equilibrium shape theory is obtained by the following equation.

Here, as shown in FIG. 3, D: at least two layers of the breaker layer 7 extend in the radial direction from the end b of the overlapping area, and are perpendicular to the axle Z (Z axis), and thus parallel to the rim baseline RmB. Perpendicular X to the line Y
(R-axis) intersects the carcass 6 (the first intersection P1
Equivalent).

C: The position of the maximum width of the carcass (corresponding to the maximum width position PM).

r: Height in the tire radial direction from the Z axis.

rC: the point C on the carcass from the Z axis (the maximum width position
Radial height up to PM).

rD: The intersection D of the carcass 6 (the first intersection
(Equivalent to P1) in the radial direction.

ΦD: an angle between the normal Y1 to the carcass 6 at the intersection D (corresponding to the first intersection P1) and the Z axis.

In the expression (1), the carcass 6 is determined to form an arc at least in the vicinity of the end b of the belt layer 7. Equation (1) is obtained by using the point of intersection G between the r-axis passing through the intersection D (corresponding to the first intersection P1) and the Z-axis as the origin. Giving the amount of deviation in the horizontal direction from the r-axis,
That is, the Z value can be calculated, and a curve based on the natural equilibrium shape theory can be obtained.

Further, as described above, the bead portion 3 has a relatively large bending stiffness. Therefore, the upper end of the range where the stiffness is large, that is, from the equivalent bead position B to the point C,
, The natural equilibrium shape theory is applied in a region reaching the intersection D. In the vicinity of the equivalent bead position B, the lower portion is formed as an inwardly convex arc above the position B and smoothly connected to the outwardly convex arc determined by the natural equilibrium shape theory. B forms an inflection point, that is, a rim point.

As described above, in the natural equilibrium shape theory, the position of the heights rC and rD and the angle φ
Given D, the curve is determined. When the Z value of the point C is given in advance, the angle φD and the height rC
By giving one of the two, the other can be determined.

When the equation (1) of the resource equilibrium shape theory is converted as represented by the above-mentioned r value and the radius of curvature R of each part of the carcass profile, the curve based on the natural equilibrium shape theory assumes that r · R = constant. Can be displayed.

Accordingly, the curve of the carcass 6 according to the natural equilibrium shape theory is a curve in which the radius r decreases gradually, that is, the radius of curvature R gradually increases from the first intersection P1 to the equivalent beat position B. On the other hand, Japanese Patent Publication No. 28521/1986 (Japanese Patent Laid-Open No.
The deformed tire 1A disclosed in Japanese Patent Application Laid-Open No. 161603/1990 makes the tendency remarkable, while the radial tire 1 includes a carcass profile that reduces the radius of curvature from the first intersection P1 to the equivalent bead position B. It is.

Further, in the present example, a curve approximating a profile obtained by the natural equilibrium shape theory is employed from the first intersection point P1 to the maximum width position PM in the radial tire 1 as well. The curve according to the natural equilibrium shape theory used here is the height h up to the maximum width position PM in the equation (1).
Is defined in advance as 0.4 times or more and less than 0.5 times the total height H of the tire, and the curve obtained by the above equation (1) is referred to. The curve obtained by calculating the entire carcass profile based on the natural equilibrium shape theory is as follows. Naturally different.

Further, a range between the maximum width position PM and the second intersection P2,
That is, in the lower part, the radius of curvature is smaller than the arc (indicated by a thin line in FIG. 2) at the intersection P2 than the arc through which the radius end of the radius of curvature R2 passes.
Toward the inflection point, ie, the equivalent bead position B, gradually deviates inward and gradually reduces the radius to form an upper curve portion of the outer bulge reaching the inflection point B. B, the center point of the radius of curvature is located outside the carcass 6, and
And the lower part of the carcass 6 has upper and lower curved parts outside and inside as shown in FIG. Such a carcass profile extends a curve based on the natural equilibrium shape theory forming an upper portion from the first intersection point P1 to the maximum width position PM to a lower portion, as compared with a broken line 1Bd shown in FIG. The lower part of the tire 1 is largely recessed inward of the tire (in FIG. 4, the first intersection point P1 and the lower end point (rim point) are compared while the heights are aligned. In FIG. 2, the lower part of the curve according to the natural equilibrium shape theory is indicated by a broken line
d)).

Therefore, the radius of curvature R2 of the lower circular arc 21 is smaller than the radius of curvature R1 of the upper circular arc 20.

In the radial tire 1, the bead apex 9 having a large flexural hardness and size is used to intentionally deform and hold the lower portion inwardly as compared with the curve 1B based on the natural equilibrium shape theory. .

The bead apex 9 is a tapered rubber body extending from the bead core 2 to a height of 9 h beyond the inflection point B,
Hard rubber having a rubber hardness of 85 to 98 °, preferably 90 to 95 degrees (Shore A hardness) is used. Further, the inflection point B
The thickness 9t in the tire axial direction is 0.06 to 0.06 of the height h.
The height is about 0.09 times, preferably about 0.07 to 0.09 times, and the height 9h is 0.75 to 0.90 times the above h, so that the shape is larger than that of the conventional one.

As described above, the carcass profile based on the natural equilibrium shape theory is a curve in which the shape of the carcass profile does not change even when the internal pressure is charged.

On the other hand, in the radial tire 1 of the present invention, after filling with the internal pressure, as described above, the radial tire 1 swells downward as compared to the curve 1B based on the natural equilibrium shape theory, and
1A becomes the upper bulge.

On the other hand, the steering performance and the riding comfort performance of the radial tire are, as described above, the conditions of two trade-offs, and the steering stability performance is improved by increasing the rigidity of the breaker layer, and the riding comfort performance is reduced by the rigidity of the breaker layer. Can be improved.

On the other hand, the rigidity of the breaker layer is such that, in addition to the rigidity of the cord itself of the breaker layer, the tension acting on the breaker layer 7 due to the internal pressure filling becomes a resistance when the breaker layer 7 is deformed. Acts as an apparent stiffness which increases the stiffness of the layer.

On the other hand, even in a tire having a carcass profile of the natural equilibrium shape theory, the crown portion, that is, the central portion of the tread surface 12 exhibits a tension distribution in which the breaker layer tension is larger than that of a shoulder with the filling of the internal pressure.

This is because, as described above, the breaker layer 7 is not completely rigid, and the carcass 6 is actually extended to some extent by the internal pressure filling. As a result, the carcass 6 having the carcass cord layer substantially balanced with the tire axial direction pushes up the lower surface of the breaker layer 7 as the carcass 6 expands.
Increase the radius of At this image diameter, the central portion of the breaker layer 7 is most likely to bulge outward, so that the circumferential tension exhibits a tension distribution in which the crown portion is the largest and gradually decreases as it reaches the shoulder portion (natural equilibrium shape). The tension distribution on the curve in theory is shown by the dashed line in FIG. 5). Further, the tension distribution of the radial tire 1 of the present invention is shown by a solid line.

In contrast to the tension distribution on the curve 1B in the natural equilibrium shape theory described above, in the case of the radial tire 1 of the present invention, while the tension increases at the shoulder portion, the tension decreases at the center portion, and as a whole, The total tension of has decreased.

As described above, this is the radial tire 1 of the present invention.
Is due to having a carcass profile that deviates from the curve based on the natural equilibrium shape theory. Therefore, at the time of filling the internal pressure, the stress acting on the carcass 6 together with the breaker layer 7 is deformed so as to be balanced with the internal pressure, and the deformation approaches the curve 1B based on the natural equilibrium shape theory. Note that the deformed tire 1A and the radial tire 1 have different shapes even after this deformation from the carcass profile based on the natural equilibrium shape theory, as described above.

As a result, in the radial tire 1 of the present invention near the end b of the breaker layer 7, the deformation approaching the natural equilibrium shape theory is caused by the carcass 6 being deformed upward as shown in FIG. It acts as if pushing up the breaker layer 7. By pushing up the end b of the breaker layer 7, the tension at the shoulder-side portion of the breaker layer 7 can be increased, and the apparent rigidity of the shoulder-side portion can be improved. On the other hand, the curve of the upper bulge of the deformed tire 1A (1A
) Is the result of deformation approaching the natural equilibrium shape theory, rather it is not possible to push down the breaker layer 7 and increase the tension at the shoulder side.

Further, the total breaker tension, that is, the total circumferential tension acting on the entire width of the breaker layer 7 is obtained by the following equation with reference to FIG.

T = (1/2) × Dr · P (WB−2Ra · sin θ) (2) where T is the total breaker tension, and Dr is the first of the breaker layer 7.
, WB is the width of the breaker layer, Ra is the first
Is the radius of curvature of the carcass at the intersection P1, and θ is the first intersection
The angle between the tangent at P1 and a line parallel to the rim baseline RmB, P is the internal pressure.

As can be seen from equation (2), increasing Ra and θ decreases the total tension T acting on the breaker layer 7.

Therefore, since the point of the maximum width position is located radially outward as compared with the others, as shown in FIG. 4 with reference to FIG. 7, both the curvature radius R1A of the upper portion and the angle θ are small. In the deformed tire 1A, the total tension T is a curve according to the natural equilibrium shape theory
1B, the radius of curvature R1 is larger than that of the curve 1B.
In the radial tire 1 of the present invention in which the angle θ is larger than in the case of the above, the total breaker tension T can be reduced.

As described above, in the radial tire 1, the tension at the portion of the breaker layer 7 on the shoulder side is large, and the total breaker tension T decreases.

Increasing the apparent rigidity at the shoulder side can improve the steering stability.

In the radial tire 1, as described above, the apparent rigidity of the edge d (shown in FIG. 6 (a)) on the ground contact surface is improved by increasing the tension at the shoulder side portion. On the other hand,
As shown in FIG. 6 (b), the deformation of the breaker layer 7 at the time of cornering of the tire is curved as if it swells laterally in the plane at the time of cornering, and an in-plane bending moment is generated. Therefore, as in the tire 1 of the present invention, by increasing the rigidity at the side edge d of the ground contact surface, that is, at the shoulder side portion, the reaction force against a large bending moment at the side edge d and the in-plane bending rigidity can be effectively increased. Can be enhanced,
The steering stability can be improved by improving the cornering force.

Further, reducing the total tension of the breaker layer 7 in the radial tire 1 reduces the bending rigidity of the breaker layer over the entire breaker layer 7 in the tire radial direction, that is, the out-of-plane bending rigidity. Therefore, it is possible to effectively buffer shocks when the vehicle rides over bumps and bumps on the road surface and improve riding comfort.

The carcass profile is a line connecting the center of the thickness of the carcass 6, and the standard internal pressure is an internal pressure charged in the tire in a standard use state or a pressure in a range of 10% above and below.

〔Example〕

The tire shown in FIG. 1 having a tire size of 185 / 60R14 was prototyped according to the specifications shown in Table 1.

Further, a tire having a carcass profile 1B based on the natural equilibrium shape theory shown in FIG. 4 was prototyped as a comparative example. These tires differ only in the carcass profile, and have the same structure as that shown in FIG. These tires were assembled on a regular rim of 51/2 JJ × 14, filled with a standard internal pressure of 2.0 kg / cm ² , and the dimensions of each part were measured. The results are shown in Table 1.

Further, Table 1 also shows the results obtained by measuring the cornering force and the reaction force acting on the tire axle at the time of passing over the protrusion with an indoor drum tester, with the index value of Comparative Example 1 being 100. The larger the number, the better the result. It can be seen that the product of the example is superior to the product of the comparative example in the cornering force and the overshoot of the protrusion. 1.3 more FF
The actual vehicle was evaluated by car. The results are shown in Table 1 with similar exponential notations. The products of the examples are excellent in both steering stability and ride comfort.

[Effect of the Invention] As described above, the radial tire of the present invention has a maximum width position of the carcass and a height ratio of the maximum width position of 0.4 to 0.5, and a radius of curvature R1 of an upper portion of the maximum width position is a radius of curvature of a lower portion. Since the inflection point is larger than R2, and the inflection point is located in the lower part, the upper curve part is bulged outward, and the lower curve part is bulged inward, so that the curvature change of the carcass line, especially the carcass line of the upper part, is Can be effectively pushed up, the apparent rigidity of the shoulder portion can be increased without increasing the total tension, and a radial lier that can satisfy both steering stability and ride comfort can be provided.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing an embodiment of the present invention in which internal pressure is charged, FIG. 2 is a diagram explaining the carcass profile, FIG. 3 is a diagram explaining the natural equilibrium shape theory,
FIG. 4 is an exaggerated diagram showing each of the carcass profiles of the radial tire of the present invention, the tire based on the natural equilibrium shape theory, and the deformed tire after the internal pressure filling, and FIG. 5 is a breaker layer formed by the internal pressure filling. FIG. 6 (a) is a diagram illustrating the tension distribution on the ground contact surface, FIG. 6 (b) is a diagram illustrating the cornering force, and FIG. 7 is a calculation of the total tension. FIG. 4 is a diagram illustrating an equation. 2 ... bead core, 3 ... bead part, 4 ... side wall part, 5 ... tread part, 6 ... carcass, 7 ... breaker layer, 8 ... reinforcing layer, 9 ... bead apex, 20, 21 ……arc.

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Kazuo Asano 1-1-1 Hiyodoridai, Kita-ku, Kobe-shi, Hyogo (56) References JP-A-61-157402 (JP, A) JP-A-54-136001 (JP) , A) JP-A-58-161603 (JP, A)

Claims (3)

(57) [Claims] 1. A carcass which passes through a tread portion and a side wall portion of a tire and has both ends turned back by a bead core of a bead portion, a carcass arranged in a radial direction, a carcass disposed between a main portion of the carcass and the turned back portion. A radial tire having a tapered and bead apex made of a rubber material extending along with a breaker layer made of a low-stretch cord disposed inside the tread portion and outside the carcass, and is mounted on a regular rim and has a standard internal pressure. In the filled state, the height ratio of the height h in the radial direction from the rim baseline of the regular rim to the maximum width position PM of the carcass and the height H in the radial direction from the rim baseline to the highest position on the tread surface. h / H is 0.4 to 0.5, and each has a center on a line Y passing through the maximum width position PM and parallel to the rim baseline, and A radius of curvature R1 of an arc passing through a first intersection point P1 at which a perpendicular X extending perpendicularly to the rim base line from the end of an area where at least two layers of the laser layers overlap with the rim base line intersects the carcass at the tread portion and the maximum width position PM. Is larger than a radius of curvature R2 of an arc passing through a second intersection point P2 where the perpendicular X intersects the carcass at the bead portion and the maximum width position PM, and the carcass is positioned between the maximum width position PM and the Intersection P2 of 2
And the lower curve portion of the inner bulge near the maximum width position PM and the inner bulge near the second intersection P2 and smoothly connected to the upper curve portion and the inflection point B. A radial tire characterized in that the radius of curvature of the upper curved portion gradually decreases from the maximum width position PM toward the inflection point B. 2. The bead apex extends from the bead core beyond the inflection point and the height 9h from the bead bottom to the tip of the bead apex is 0.75 to 0.75 of the height h.
2. The radial tire according to claim 1, wherein the height is 0.90 times, and the pressure 9t in the tire axial direction at the position of the inflection point B is 0.07 to 0.09 times the height h. 3. The radial tire according to claim 1, wherein the bead apex is made of a hard rubber having a Shore A hardness of 85 to 98 °.