JP2017121906A - Pneumatic tire - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pneumatic tire which suppresses uneven wear and has excellent durability.SOLUTION: In a pneumatic tire 2, in a normal internal pressure condition, a center region between one base point Pr on a tread surface and the other base point Pr in a shaft direction has a width Wr, and a third layer 46 is formed in a shape which protrudes inward in a radial direction at a curvature radius R. When an interval between a center line L1 of a main groove 26 and a center line L2 of a main groove 28 is designated as Dg, the base point Pr is positioned further outside in the shaft direction from the center line L1 of the main groove 26 by more than one-half of the interval Dg. The base point Pr is positioned inward in the shaft direction from the center L2 of the main groove 28 to be closer to inside by more than 7 mm. The curvature radius R is set to be 670 mm or more and 900 mm or less.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a pneumatic tire mounted on a vehicle.

  Japanese Unexamined Patent Application Publication No. 2002-187410 discloses a tire that suppresses uneven wear in the shoulder region of the tread. The tire belt includes a central convex portion that is convex outward in the radial direction and an outer concave portion that is continuous with the central convex portion and has a convex arc shape inward in the radial direction. By providing this belt, when the tread surface contacts the road surface, slippage between the shoulder region and the road surface is suppressed. Thereby, the partial wear of the shoulder region is suppressed.

JP 2002-187410 A

  In the tire disclosed in Japanese Patent Laid-Open No. 2002-187410, the contact pressure in the shoulder region tends to be high. When the ground contact pressure in the shoulder region increases, distortion is likely to occur at the end of the belt. This distortion can cause peeling of the end of the belt, so-called looseness. This tire has room for improvement in terms of durability.

  In this tire, the contact pressure in the shoulder region can be relatively reduced by increasing the thickness of the tread in the center region. However, when the thickness of the tread is increased, the heat generation of the tread tends to increase. Tires with large heat generation tend to be inferior in durability. Furthermore, by increasing the thickness of the tread in the center region, slipping easily occurs in the shoulder region. In a tire with a thick tread in the center region, it is difficult to appropriately suppress the occurrence of uneven wear.

  An object of the present invention is to provide a pneumatic tire excellent in durability while suppressing uneven wear.

A pneumatic tire according to the present invention includes a tread having a tread surface that comes in contact with the road surface, a carcass that is located on the radially inner side of the tread and protrudes in a toroidal shape radially outward, and is located between the carcass and the tread. Belt. The belt includes a first layer located on the radially inner side, a second layer laminated on the radially outer side of the first layer, and a third layer laminated on the radially outer side of the second layer. ing. Each of the first layer, the second layer, and the third layer includes a belt cord and a topping rubber that covers the belt cord. The tread has a first main groove extending in the circumferential direction on the inner side in the axial direction and a pair of second main grooves extending in the circumferential direction on the outer side in the axial direction of the first main groove.
In a normal internal pressure state that is incorporated in a normal rim and filled with normal internal pressure air, the width Wr of the central region from one axial base point Pr to the other axial base point Pr on the tread surface across the equator plane is The three layers are formed so as to protrude radially inward with a radius of curvature R. When the distance between the center line of the first main groove and the center line of the second main groove is Dg, the base point Pr is ½ times the distance Dg or more axially outward from the center line of the first main groove. The base point Pr is located 7 mm or more inward in the axial direction inward from the center line of the second main groove. The curvature radius R is set to 670 mm or more and 900 mm or less.

  Preferably, the thickness Tc of the tread on the equator plane is greater than or equal to the thickness Tb at the axial end of the third layer. The difference (Tc−Tb) between the thickness Tc and the thickness Tb is set to 0 to 2.0 mm.

  Preferably, the thickness Tg of the tread at the groove bottom of the first main groove is set to 3.5 mm or more and 5.5 mm or less.

  Preferably, each belt cord of the first layer, the second layer, and the third layer is made of a steel cord. The belt cord of the first layer extends while being inclined at an absolute value θ1 with respect to the equator plane. The belt cord of the second layer extends with an absolute value θ2 of the angle with respect to the equator plane. The belt cord of the third layer extends while being inclined at an absolute value θ3 of the angle with respect to the equator plane. The absolute value θ1 of this angle is set to 45 ° or more and 55 ° or less. The absolute value θ2 of the angle and the absolute value θ3 of the angle are set to 15 ° to 30 °. The direction of inclination of the belt cord of the second layer and the direction of inclination of the belt cord of the third layer are reversed with respect to the equator plane.

  Preferably, a ratio (Wb / Wt) of the belt width Wb to the tread width Wt is 0.7 or more and 0.95 or less.

  The pneumatic tire according to the present invention has a shape in which the belt protrudes radially inward with a radius of curvature R in a normal internal pressure state. The third layer of the belt is shaped to protrude radially inward within the range of the width Wr across the first main groove. Thereby, the contact pressure of the tread is made uniform in the axial direction. Shoulder drop wear in the shoulder area of the tread is suppressed. The durability of the belt has been improved.

FIG. 1 is a cross-sectional view illustrating a pneumatic tire according to an embodiment of the present invention. FIG. 2 is a partially enlarged view of the tire of FIG. FIG. 3 is an explanatory view showing the ground contact shape of the tire of FIG.

  Hereinafter, the present invention will be described in detail based on preferred embodiments with appropriate reference to the drawings.

  FIG. 1 shows a pneumatic tire 2. In FIG. 1, the vertical direction is the radial direction of the tire 2, the horizontal direction is the axial direction of the tire 2, and the direction perpendicular to the paper surface is the circumferential direction of the tire 2. In FIG. 1, an alternate long and short dash line CL represents the equator plane of the tire 2. The shape of the tire 2 is symmetric with respect to the equator plane except for the tread pattern.

  The tire 2 includes a tread 4, a pair of sidewalls 6, a pair of clinch 8, a pair of beads 10, a carcass 12, a belt 14, a band 16, a pair of edge bands 18, a pair of cushion layers 19, an inner liner 20, and a pair. The chafer 22 is provided. The tire 2 is a tubeless type. The tire 2 is mounted on a light truck, for example.

  The tread 4 faces outward in the radial direction. The tread 4 forms a tread surface 24 that contacts the road surface. The tread 4 has a main groove 26 as a first main groove and a main groove 28 as a second main groove. The main groove 26 and the pair of main grooves 28 are formed by making the tread 4 go around in the circumferential direction. In the tire 2, the main groove 26 is formed at the center in the axial direction. The pair of main grooves 28 are formed outside the main grooves 26 in the axial direction. The pair of main grooves 28 are formed symmetrically with respect to the equator plane. In addition to the main groove 26 and the pair of main grooves 28, grooves extending in the axial direction may be formed. A tread pattern is formed by these grooves. The main groove 26 may be formed as a pair of main grooves formed symmetrically on the equator plane across the equator plane.

  The tread 4 has a base layer 30 and a cap layer 32. The cap layer 32 is located on the outer side in the radial direction of the base layer 30. The cap layer 32 is laminated on the base layer 30. The base layer 30 is made of a crosslinked rubber having excellent adhesiveness. A typical base rubber of the base layer 30 is natural rubber. The cap layer 32 is made of a crosslinked rubber having excellent wear resistance, heat resistance, and grip properties.

  Each sidewall 6 extends substantially inward in the radial direction from the end of the tread 4. A radially outer end of the sidewall 6 is joined to the tread 4. The radially inner end of the sidewall 6 is joined to the clinch 8. This sidewall 6 is made of a crosslinked rubber having excellent cut resistance and weather resistance. This sidewall 6 prevents the carcass 12 from being damaged.

  Each clinch 8 is located substantially inside the sidewall 6 in the radial direction. The clinch 8 is located outside the beads 10 and the carcass 12 in the axial direction. The clinch 8 is made of a crosslinked rubber having excellent wear resistance. The clinch 8 contacts the rim flange when the tire 2 is assembled into the rim.

  Each bead 10 is located inside the clinch 8 in the axial direction. The bead 10 includes a core 34 and an apex 36 that extends radially outward from the core 34. The core 34 is ring-shaped and includes a wound non-stretchable wire. A typical material for the wire is steel. The apex 36 is tapered outward in the radial direction. The apex 36 is made of a highly hard crosslinked rubber.

  The carcass 12 includes a first ply 38 and a second ply 40. The first ply 38 and the second ply 40 are stretched between the beads 10 on both sides, and extend in a toroidal shape outward in the radial direction. The carcass 12 constitutes the skeleton of the tire 2. The first ply 38 and the second ply 40 are along the tread 4 and the sidewall 6. The first ply 38 is folded around the core 34 from the inner side to the outer side in the axial direction. By this folding, the main portion 38a and the folding portion 38b are formed in the first ply 38. The second ply 40 is folded around the core 34 from the inner side in the axial direction to the outer side. By this folding, the main portion 40a and the folding portion 40b are formed in the second ply 40. The end of the folded portion 38b is located outside the end of the folded portion 40b in the radial direction.

  Each of the first ply 38 and the second ply 40 includes a large number of carcass cords and topping rubbers arranged in parallel. The absolute value of the angle formed by each carcass cord with respect to the equator plane is 75 ° to 90 °. In other words, the carcass 12 has a radial structure. These carcass cords are made of organic fibers. A polyester fiber is illustrated as a preferable organic fiber. The carcass cord may be made of nylon fiber, rayon fiber, polyethylene naphthalate fiber, and aramid fiber. The carcass 12 may be formed from a single ply.

  The belt 14 is located on the inner side in the radial direction of the tread 4. The belt 14 is laminated on the outer side in the radial direction of the carcass 12. The belt 14 reinforces the carcass 12. The belt 14 includes a first layer 42, a second layer 44, and a third layer 46. The second layer 44 is laminated on the outer side in the radial direction of the first layer 42. The third layer 46 is laminated on the outer side in the radial direction of the second layer 44. The third layer 46 is located on the outermost side of the belt 14 in the radial direction. As is clear from FIG. 1, the width of the second layer 44 is slightly larger than the width of the first layer 42 in the axial direction. The width of the first layer 42 is slightly larger than the width of the third layer.

  Each of the first layer 42, the second layer 44, and the third layer 46 is composed of a number of parallel belt cords and topping rubbers (not shown). The belt cord of the first layer 42 is inclined toward the equator plane. The absolute value θ1 of the angle inclined with respect to the equator plane is, for example, not less than 45 ° and not more than 55 °. The belt cords of the second layer 44 and the third layer 46 are inclined with respect to the equator plane. The absolute value θ2 of the angle at which the belt cord of the second layer 44 is inclined with respect to the equator plane is, for example, 10 ° or more and 30 ° or less. The absolute value θ3 of the angle at which the belt cord of the third layer 46 is inclined with respect to the equator plane is, for example, 10 ° or more and 30 ° or less. The inclination direction of the belt cord of the second layer 44 with respect to the equator plane is opposite to the inclination direction of the belt cord of the third layer 46 with respect to the equator plane. The preferred material for these belt cords is steel. Organic fibers may be used for these belt cords.

  The band 16 is located on the radially outer side of the belt 14. The band 16 is laminated on the outer side in the radial direction of the third layer 46. In the axial direction, the width of the band 16 is larger than the width of the belt 14. The band 16 covers the belt 14. The band 16 extends from one end in the axial direction to the other end and covers the belt 14. This band 16 is a so-called full band.

  Although not shown, the band 16 is composed of a band cord and a topping rubber. The band cord is wound spirally in the circumferential direction of the tire 2. The band 16 has a so-called jointless structure. This band cord extends substantially in the circumferential direction. The angle of the band cord with respect to the circumferential direction is 5 ° or less, and further 2 ° or less. The band 16 restrains the belt 14. The band 16 suppresses lifting of the belt 14. The band cord of the band 16 is made of an organic fiber. Examples of preferable organic fibers include nylon fibers, polyester fibers, rayon fibers, polyethylene naphthalate fibers, and aramid fibers.

  Each edge band 18 is located radially outside the belt 14 and in the vicinity of the end of the belt 14. Although not shown, the edge band 18 is made of a cord and a topping rubber. The cord is wound in a spiral. Each of the edge bands 18 has a so-called jointless structure. The band cord extends substantially in the circumferential direction. The angle of the band cord with respect to the circumferential direction is 5 ° or less, and further 2 ° or less. The end of the belt 14 is restrained by this band cord. The edge band 18 suppresses the lifting of the belt 14. The belt cord is made of an organic fiber. Examples of preferable organic fibers include nylon fibers, polyester fibers, rayon fibers, polyethylene naphthalate fibers, and aramid fibers.

  Each cushion layer 19 is laminated between the belt 14 and the carcass 12 in the vicinity of the end of the belt 14. The cushion layer 19 is made of a soft crosslinked rubber. The cushion layer 19 absorbs stress at the end of the belt 14.

  The inner liner 20 is located inside the carcass 12. The inner liner 20 is joined to the inner surface of the carcass 12. The inner liner 20 and the carcass 12 may be joined via an insulation rubber. The inner liner 20 is made of a crosslinked rubber having excellent air shielding properties. A typical base rubber of the inner liner 20 is butyl rubber or halogenated butyl rubber. The inner liner 20 forms a lumen surface 48 of the tire 2. The inner liner 18 maintains the internal pressure of the tire 2.

  Each chafer 22 is located in the vicinity of the bead 10. When the tire 2 is incorporated into the rim, the chafer 22 comes into contact with the rim. By this contact, the vicinity of the bead 10 is protected. In the tire 2, the chafer 22 is made of a cloth and rubber impregnated in the cloth. The chafer 22 may be integrated with the clinch 8. The material of the chafer 22 may be the same as the material of the clinch 8.

  A point Pt in FIG. 1 represents the tread edge. The tread end Pt is an outer end in the axial direction of the tread surface 24. As in the case of the tire 2, when the edge can be identified by a clear edge in appearance, the edge is set as the tread end Pt. When it is difficult to distinguish the appearance, the grounding end that contacts the plane on the outermost side in the axial direction when a normal load is applied to the tire of normal internal pressure and the tread 4 is grounded to the plane at a camber angle of 0 ° is the tread. The end Pt.

  The double arrow Wt represents the tread width. The tread width Wt is measured as a distance from one tread end Pt in the axial direction to the other tread end Pt. The tread width Wt is measured along the tread surface 24. A double-headed arrow Wb represents the belt width. The belt width Wb is measured as a distance from one axial end 14a of the belt 14 to the other axial end 14a. The belt width Wb is measured along the belt 14. In the tire 2, it is measured as a distance from one axial end 44a of the second layer 44 having the largest width to the other axial end 44a.

  A double-headed arrow Tc represents the thickness of the tread 4 on the equator plane. A double-headed arrow Tb represents the thickness of the tread 4 at the axial end of the third layer 46. A double-headed arrow Tg represents the thickness of the tread 4 at the groove bottom of the main groove 26. The thicknesses Tc, Tb, and Tg are measured as thicknesses outside the radially outer surface of the band 16. The thicknesses Tc and Tg are the total thickness of the base layer 30 and the cap layer 32. The thickness Tg is measured at the position where the thickness is the thinnest on the bottom surface of the main groove 26. Since the tire 2 includes the edge band 18, the thickness Tb is a total thickness of the base layer 30, the cap layer 32, and the edge band 18. In the tire 2 that does not include the edge band 18, the thickness Tb is measured as the combined thickness of the base layer 30 and the cap layer 32.

  A part of the tire 2 is shown in FIG. A tire 2 in a normal internal pressure state is shown which is incorporated in a normal rim and filled with air of normal internal pressure. The tire 2 is not in contact with the road surface. In this normal internal pressure state, the central portion in the axial direction of the belt 14 has a shape protruding inward in the radial direction. The central portion in the axial direction of the third layer 46 is shaped to protrude inward in the radial direction.

  The arrow R in FIG. 2 represents the radius of curvature at which the third layer 46 protrudes inward in the radial direction. This radius of curvature R is measured along the radially outer surface of the third layer 46. A double-headed arrow Wr represents the width of the central region of the tread 4 where the third layer 46 protrudes with the curvature radius R. The point Pb represents the outer end in the axial direction where the third layer 46 protrudes with the curvature radius R. The point Pr represents the intersection of a straight line extending in the radial direction through the point Pb and the tread surface 24. This point Pr is a base point on the tread surface of the central portion where the third layer 46 protrudes with the curvature radius R. The aforementioned width Wr is measured as a width from one base point Pr in the axial direction to the other base point Pr. This width Wr is measured along the tread surface 24. This width Wr is a symmetric width with respect to the equator plane.

  The alternate long and short dash line L1 represents the center line of the main groove 26. A one-dot chain line L <b> 2 represents the center line of the main groove 28. A double-headed arrow Dg represents the distance from the center line L1 of the main groove 26 to the center line L2 of the main groove 28. This distance Dg is measured along the tread surface 24.

  FIG. 3 shows a ground contact surface of the tread surface 24 of the tire 2. This ground contact surface is obtained by filling the tire 2 with air of normal internal pressure, applying a normal load, setting the camber angle to 0 °, and grounding the tread 4 to a flat road surface. A double-headed arrow Lc represents the contact length of the tread surface 24 on the equator plane. A double-headed arrow Ws represents a width that is 0.7 times the tread width Wt. This width Ws is a width symmetrical to the equator plane. A double-headed arrow Ls represents the contact length of the tread surface 24 at the position Ps having the width Ws.

  In this ground contact surface, the contour of the circumferential ground contact boundary is a curved line that extends from the inner side in the axial direction to the outer side and extends from the outer side in the circumferential direction to the inner side. The contact length Lc is the longest in the contact length in the circumferential direction. The contact length at the tread end Pt is the shortest. The ground contact length gradually decreases from the equator to the tread edge.

  In the tire 2, the belt 14 includes a first layer 42, a second layer 44, and a third layer 46. This belt 14 reinforces the carcass. Thereby, when the air is filled, the central region of the tread 4 is suppressed from protruding outward in the radial direction. In the tire 2, the third layer 46 is easily protruded inward in the radial direction. The belt 14 is composed of three or more layers. From the viewpoint of obtaining the reinforcing effect by the belt 14, the belt cords of the first layer 42, the second layer 44, and the third layer 46 are preferably made of metal cords. A steel cord is suitable as the belt cord.

  In the tire 2, the absolute value θ2 of the belt cord angle of the second layer 44 and the absolute value θ3 of the belt cord angle of the third layer 46 are inclined closer to the circumferential direction than the axial direction. The belt cord of the second layer 44 and the belt cord of the third layer 46 intersect and extend in an inclined manner. The second layer 44 and the third layer 46 contribute to improving the rigidity of the tread 4 in combination with the carcass 12 having a radius structure. From this viewpoint, the absolute value θ2 of the angle with respect to the equator plane and the absolute value θ3 of the angle are preferably 30 ° or less. Further, the intersection of the belt cord of the second layer 44 and the belt cord of the third layer 46 contributes to the improvement of the rigidity of the tread 4. From this viewpoint, the absolute value θ2 of the angle and the absolute value θ3 of the angle are preferably 15 ° or more.

  Furthermore, the absolute value θ1 of the belt cord angle of the first layer 42 is set to an inclination angle closer to the axial direction than to the circumferential direction. In the first layer 42, the tread 4 contributes to improvement in axial rigidity. From this viewpoint, the absolute value θ1 of the angle with respect to the equator plane is preferably 45 ° or more. The absolute value θ1 of the angle is preferably 55 ° or less. By laminating the first layer 42 with the second layer 44 and the third layer 46, the first layer 42 further contributes to improving the rigidity of the tread 4. Since the first layer 42 is laminated radially inward, the third layer 36 is easily protruded inward in the radial direction.

  In the normal internal pressure state of the tire 2, the third layer protrudes inward in the radial direction with the width Wr of the central region on the axially inner side of the main groove 28. When the tread surface 24 of the tire 2 comes in contact with the road surface, the shoulder region of the tread 4 comes in contact before the center region. The peripheral portion of the main groove 28 is grounded before the peripheral portion of the main groove 26. Since the peripheral portion of the main groove 28 is grounded first, the central portion of the third layer 46 protruding inward in the radial direction protrudes outward in the radial direction. The center area of the third layer 46 protrudes the center area of the tread 4 outward in the radial direction. The center region of the tread 4 is pressed against the road surface by the belt 14. In the axial direction, the entire tread surface 24 from one tread end Pt to the other tread end Pt can be grounded. In the tire 2, the center region of the tread 4 can be appropriately grounded without increasing the thickness Tc of the tread 4.

  In the tire 2, the third layer 46 protrudes inward in the radial direction on the inner side in the axial direction of the main groove 28. Thereby, when the tread 4 contacts the road surface, it is suppressed that the periphery of the main groove 28 protrudes. From this point of view, it is preferable that the main groove extending around the circumferential direction is not formed between the main groove 28 and the main groove 26. Thereby, the center area | region of the tread 4 protrudes appropriately. In the tire 2, the contact pressure of the tread 4 is made uniform in the axial direction. From this viewpoint, the base point Pr on the tread surface 24 is located 7 mm or more inward in the axial direction from the center line L2 of the main groove 28.

  Further, from the viewpoint of equalizing the contact pressure of the tread 4 in the axial direction, it is preferable that the width Wr of the central region is large. From this point of view, the base point Pr is located on the axially outer side from the center line L1 of the main groove 26 by ½ times the distance Dg or more.

  In the tire 2, a main groove 26 is formed in the range of the width Wr. By forming the main groove 26, the central portion of the third layer 46 is easily protruded radially outward. The main groove 26 contributes to the uniform contact pressure of the tread 4 in the axial direction.

  By reducing the thickness Tg of the main groove 26, the central portion of the third layer 46 is easily protruded outward in the radial direction. From this viewpoint, the thickness Tg is preferably 5.5 mm or less. On the other hand, in the tire 2 having a large thickness Tg, occurrence of cracks and the like at the groove bottom of the main groove 26 is suppressed. The tire 2 having a large thickness Tg is excellent in durability. From this viewpoint, the thickness Tg is preferably 3.5 mm or more.

  In the tire 2, the curvature radius R is set to 650 mm or more from the viewpoint of uniformizing the contact pressure of the tread 4. The curvature radius R is set to 900 mm or less. Moreover, the contact pressure of the tread 4 can be made more uniform by combining this curvature radius R and the difference (Tc−Tb) between the appropriate thickness Tc and thickness Tb of the tread 4. From this viewpoint, the difference (Tc−Tb) is preferably 0 mm or more. This difference (Tc−Tb) is preferably 2.0 mm or less.

  The tire 2 having a large belt width Wb can easily control the ground contact pressure of the entire tread 4 in the axial direction. From this viewpoint, the ratio (Wb / Wt) of the belt width Wb to the tread width Wt is preferably 0.7 or more, more preferably 0.75 or more, and particularly preferably 0.8 or more. On the other hand, in the tire 2 in which the belt width Wb is too large, the contact pressure in the shoulder region of the tread 4 is large. The tire 2 in which the belt width Wb is too large is likely to be inferior in durability. In the tire 2, uneven wear of the shoulder region of the tread 4 is likely to occur. From these viewpoints, the ratio (Wb / Wt) is preferably 0.95 or less, more preferably 0.9 or less, and particularly preferably 0.85 or less.

   In the tire 2, the protrusion of the tread 4 having the width Wr in the central region is controlled mainly by the combination of the belt 14 and the shape of the tread 4 in the normal internal pressure state. In particular, in the tire 2 having a low flatness ratio, the protrusion of the tread 4 can be easily suppressed by the combination of the belt 14 and the shape of the tread 4. From this viewpoint, the flatness of the tire 2 is preferably 75% or less, more preferably 70% or less, and particularly preferably 65% or less.

  In the tire 2, the contact pressure L in the shoulder region is suppressed from being increased by increasing the contact length Lc in the center region from the contact length Ls in the shoulder region. Thereby, the fall of durability and the partial wear of the shoulder area | region of the tread 4 are suppressed. From this viewpoint, the ratio (Lc / Ls) is preferably 1.01 or more, more preferably 1.05 or more, and particularly preferably 1.10 or more. On the other hand, if the ground contact length Lc of the center region becomes too large, slippage between the tread surface 24 and the road surface occurs in the shoulder region. This slip results in uneven wear in the shoulder region. From the viewpoint of suppressing this uneven wear, the ratio (Lc / Ls) is preferably 1.20 or less.

  In the present invention, the size and angle of each member of the tire 2 are measured in a state where the tire 2 is incorporated in a regular rim and the tire 2 is filled with air so as to have a regular internal pressure. Unless otherwise specified, no load is applied to the tire 2 during measurement. In the present specification, the normal rim means a rim defined in a standard on which the tire 2 depends. “Standard rim” in the JATMA standard, “Design Rim” in the TRA standard, and “Measuring Rim” in the ETRTO standard are regular rims. In the present specification, the normal internal pressure means an internal pressure defined in a standard on which the tire 2 relies. “Maximum air pressure” in JATMA standard, “maximum value” published in “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” in TRA standard, and “INFLATION PRESSURE” in ETRTO standard are normal internal pressures. In the present specification, the normal load means a load defined in a standard on which the tire 2 depends. “Maximum value” published in “Maximum load capacity” in the JATMA standard, “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” in the TRA standard, and “LOAD CAPACITY” in the ETRTO standard are normal loads.

  Hereinafter, the effects of the present invention will be clarified by examples. However, the present invention should not be construed in a limited manner based on the description of the examples.

[Example 1]
A tire having the structure of FIG. 1 was prepared. The tire size was 185 / 65R15. In this tire, the radius of curvature R of the third layer of the belt was 740 mm. The difference (Tc−Tb) between the thickness Tc and the thickness Tb of the tread and the thickness Tg of the groove bottom of the main groove were as shown in Table 1.

[Comparative Example 1-5]
A tire was obtained in the same manner as in Example 1 except that the radius of curvature R, the difference (Tc−Tb), and the thickness Tg were as shown in Table 1, respectively.

[Example 2-6]
A tire was obtained in the same manner as in Example 1 except that the radius of curvature R, the difference (Tc−Tb), and the thickness Tg were as shown in Table 2, respectively.

[Tread surface contact shape]
The contact shape was confirmed when a normal load was applied to the tire in the normal internal pressure state, the camber angle was set to 0 °, and the tread surface was contacted with the flat road surface. As shown in FIG. 3, the contact length Lc of the tread surface at the equator plane and the contact length Ls of the tread surface at the position Ps of the width Ws were measured. The ratio of the contact length Lc to the contact length Ls (Lc / Ls) was obtained, and the results are shown in Tables 1 and 2.

[Uneven wear]
A tire was incorporated into a regular rim, and the tire was filled with air to adjust the internal pressure to 600 kPa. This tire was mounted on the front of the light truck. After running this light truck for 8000 km, the appearance of the tire was inspected. The presence or absence of uneven wear in the shoulder region, so-called shoulder shoulder wear, was examined. The results are shown in Tables 1 and 2 below as indices. The numerical value 3 indicates that no shoulder wear was observed. A numerical value of 2 indicates that there is a small amount of shoulder wear but is not noticeable. The numerical value 1 indicates that shoulder wear has occurred.

[durability]
A tire was incorporated into a regular rim, and the tire was filled with air to adjust the internal pressure to 600 kPa. This tire was mounted on a drum-type running test machine, and a longitudinal load of 12 kN was applied to the tire. This tire was run on a drum having a radius of 1.7 m at a speed of 80 km / h. The distance traveled until the tire broke was measured. The results are shown in Tables 1 and 2 below as indices. A larger numerical value is preferable.

  As shown in Tables 1 and 2, in the tires of the examples, uneven wear is suppressed as compared with the tires of the comparative examples. The tires of the examples are also excellent in durability. From this evaluation result, the superiority of the present invention is clear.

  The tire described above can be widely applied to tires mounted on vehicles. This tire is particularly suitable for a tire loaded with a large load such as a light truck.

2 ... Tire 4 ... Tread 6 ... Sidewall 8 ... Clinch 10 ... Bead 12 ... Carcass 14 ... Belt 24 ... Tread surface 26, 28 ... Main groove 42 ... 1st layer 44 ... 2nd layer 46 ... 3rd layer

Claims (5)

A tread having a tread surface that comes in contact with the road surface, a carcass projecting in a toroidal shape radially inwardly of the tread, and a belt positioned between the carcass and the tread.
A first layer on which the belt is positioned radially inward; a second layer stacked on the radially outer side of the first layer; and a third layer stacked on the radially outer side of the second layer. And
Each of the first layer, the second layer, and the third layer includes a belt cord and a topping rubber that covers the belt cord,
The tread is engraved with a first main groove extending in the circumferential direction on the inner side in the axial direction and a pair of second main grooves extending in the circumferential direction on the outer side in the axial direction of the first main groove,
In a normal internal pressure state that is incorporated in a normal rim and filled with normal internal pressure air, the width Wr of the central region from one axial base point Pr to the other axial base point Pr on the tread surface across the equator plane is The three layers are shaped to protrude radially inward with a radius of curvature R,
When the distance between the center line of the first main groove and the center line of the second main groove is Dg, the base point Pr is ½ times the distance Dg or more axially outward from the center line of the first main groove. The base point Pr is located 7 mm or more inward in the axially inward direction from the center line of the second main groove,
A pneumatic tire in which the radius of curvature R is 670 mm or more and 900 mm or less.   The thickness Tc at the equator plane of the tread is set to be greater than or equal to the thickness Tb at the axial end of the third layer, and the difference (Tc−Tb) between the thickness Tc and the thickness Tb is 0 mm or more. The tire according to claim 1, wherein the tire is 2.0 mm or less.   The tire according to claim 1 or 2, wherein a thickness Tg of a tread at a groove bottom of the first main groove is set to 3.5 mm or more and 5.5 mm or less. Each belt cord of the first layer, second layer and third layer is made of steel cord,
The first layer belt cord extends at an angle of θ1 with respect to the equator plane, and the second layer belt cord extends at an angle of θ2 with respect to the equator plane. The belt cord of the third layer extends at an angle of θ3 with respect to the equator plane,
The absolute value θ1 of the angle is 45 ° or more and 55 ° or less, the absolute value θ2 of the angle and the absolute value θ3 of the angle are 15 ° or more and 30 ° or less, and the inclination of the belt cord of the second layer The tire according to any one of claims 1 to 3, wherein the direction and the inclination direction of the belt cord of the third layer are opposite to the equator plane.   The tire according to any one of claims 1 to 4, wherein a ratio (Wb / Wt) of the belt width Wb to the tread width Wt is 0.7 or more and 0.95 or less.

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