Tire for cart

Abstract

To provide a tire 2 for a cart which can achieve enhancement of turning performance while restraining a carcass from exposing.SOLUTION: A tire 2 comprises a pair of beads 8 and a carcass 10 which includes a bias structure. External surface 2G of the tire 2 comprises tread surface 32 and a pair of side surfaces 34. The side surfaces 34 contain an upper side circular arc part 38 which includes center on reference line and is represented by circular arc which extends radially outward from maximum width position and a lower side circular arc part 40 which includes center on reference line and is represented by circular arc which extends radially inward from maximum width position. Ratio of radius Rj of circular arc representing the upper side circular arc part 38 to radius Rk of the circular arc representing the lower side circular arc part 40 is 105% or less. The carcass 10 contains a large number of carcass cords 24 in parallel which is composed of organic fiber. Total fineness of each carcass cord 24 is 3400 dtex or less.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a tire for a cart. [Background technology]

[0002] In recent years, the performance of racing karts (hereinafter, karts) has improved significantly. Accordingly, further improvements in performance are required for the tires mounted on the karts. In order to meet this demand, various studies have been conducted (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2010-42711 A Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to provide a tire for a cart which can achieve improved cornering performance. [Means for solving the problem]

[0005] A cart tire according to one aspect of the present invention includes a pair of beads and a carcass having a bias structure that spans between the pair of beads. The outer surface of the tire includes a tread surface including the equator of the tire, and a pair of side surfaces that are connected to the tread surface and include the maximum width position of the tire. In a meridian section of the tire in a reference state defined below, each of the side surfaces includes an upper arc portion that has a center on a reference line defined below and is represented by an arc extending radially outward from the maximum width position, and a lower arc portion that has a center on the reference line and is represented by an arc extending radially inward from the maximum width position. The ratio of the radius of the arc representing the upper arc portion to the radius of the arc representing the lower arc portion is 105% or less. The carcass includes a plurality of carcass plies. The carcass ply includes a large number of carcass cords made of organic fibers arranged in parallel. The total fineness of each of the carcass cords is 3400 dtex or less. Standard condition: The tire is mounted on a rim specified by the International Karting Committee, the internal pressure of the tire is adjusted to 100 kPa, and no load is applied to the tire. Reference line: A straight line that passes through the maximum width position and extends in the axial direction Effect of the Invention

[0006] According to the present invention, a tire for a cart is obtained that can achieve improved cornering performance. [Brief description of the drawings]

[0007] [Figure 1] 1 is a cross-sectional view showing a portion of a tire according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic diagram illustrating a configuration of a carcass. [Diagram 3] FIG. 2 is a cross-sectional view illustrating the contours of a tread surface and a side surface. [Figure 4] 3A to 3C are cross-sectional views illustrating a method for manufacturing a tire. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The tire of the present invention is mounted on a standard rim specified by the Committee International des Klimpses (CIK). In the present invention, the standard rim is a 5-inch rim specified by the CIK. The standard condition is when a tire is mounted on a standard rim, the internal pressure of the tire is adjusted to 100 kPa, and no load is applied to the tire. In the present invention, unless otherwise specified, the dimensions and angles of each part of the tire are measured under standard conditions. The dimensions and angles of each part in the meridian section of the tire, which cannot be measured when the tire is mounted on the rim, are measured on the cut surface of the tire obtained by cutting the tire along a plane including the axis of rotation. In this measurement, the tire is set so that the distance between the left and right beads matches the distance between the beads of the tire mounted on the rim. The tire configuration that cannot be confirmed when the tire is mounted on the rim is confirmed on the cut surface.

[0009] In the present invention, the number of cords per 5 cm width of a tire element including parallel cords is determined in a cross section of the element obtained by cutting the element on a plane perpendicular to the length direction of the cord, unless otherwise specified. The number of cords per 5 cm width of an element is also called the cord density (unit: ends / 5 cm) or end count.

[0010] In the present invention, the tread portion of a tire is the portion of the tire that comes into contact with the road surface. The bead portion is the portion of the tire that fits onto the rim. The sidewall portion is the portion of the tire that bridges between the tread portion and the bead portion. A tire has the following portions: a tread portion, a pair of bead portions, and a pair of sidewall portions. The equatorial portion of the tread is called the crown, the edge of the tread is called the shoulder, and the boundary between the tread and the sidewall is called the buttress.

[0011] [Foundations underlying the present invention] The tire includes a carcass that spans between a pair of beads and includes a carcass ply that includes a number of carcass cords arranged in parallel. The side surface of the tire has an axially outwardly convex shape. The side surface includes a maximum width position. Of the contours of the side surface defined in the meridian section of the tire, the contour near the maximum width position is represented by two arcs whose centers are on a straight line (hereinafter referred to as the reference line) that passes through the maximum width position and extends in the axial direction. One is an arc that extends radially outward from the maximum width position, and the portion represented by this arc is called the upper arc. The other is an arc that extends radially inward from the maximum width position, and the portion represented by this arc is called the lower arc. In kart tires, the radius of the arc that represents the outline of the upper arc portion is conventionally set to 120% or more of the radius of the arc that represents the outline of the lower arc portion, from the standpoint of improving cornering performance, and the outline of the side surface is designed to increase the contact width during cornering.

[0012] A tire is obtained by preparing a green tire (a tire in an unvulcanized state) and pressurizing and heating it in a mold. An expanding bladder is pressed against the green tire from the inside. The green tire is pressed against the mold while changing its shape. Kart tires are bias tires. The shape change of raw tires is greater than that of radius tires. If the radius of the arc that represents the outline of the upper arc portion is set to a large radius, the spacing of the carcass cords in the shoulder area tends to widen, and there is a concern that the rubber, including the inner liner that is provided to maintain internal pressure, will flow between the carcass cords, exposing the carcass to the inside. There is also a concern that if a large amount of rubber flows in, the risk of air leakage or punctures during driving will increase. Therefore, the inventors have reviewed the contour design of the side surface, and conducted extensive research into technology that can improve cornering performance while suppressing exposure of the carcass by controlling the change in shape of the raw tire during vulcanization molding, and have completed the invention described below.

[0013] [Overview of the embodiment of the present invention] The present invention is a cart tire comprising a pair of beads and a carcass having a bias structure spanning the pair of beads, wherein the outer surface of the tire comprises a tread surface including the equator of the tire, and a pair of side surfaces that are connected to the tread surface and include a maximum width position of the tire, and in a meridian cross section of the tire in the above-mentioned reference state, each of the side surfaces comprises an upper arc portion having a center on the above-mentioned reference line and represented by an arc extending radially outward from the maximum width position, and a lower arc portion having a center on the reference line and represented by an arc extending radially inward from the maximum width position, wherein the ratio of the radius of the arc representing the upper arc portion to the radius of the arc representing the lower arc portion is 105% or less, and the carcass comprises a plurality of carcass plies, the carcass plies including a large number of carcass cords made of organic fibers arranged in parallel, and the total fineness of each of the carcass cords is 3400 dtex or less.

[0014] The tire of the present invention can achieve improved cornering performance while suppressing exposure of the carcass. The mechanism by which such effects are achieved has not been made clear, but is presumed to be as follows.

[0015] In this cart tire, the ratio of the radius of the arc representing the upper arc portion to the radius of the arc representing the lower arc portion is smaller than that of a conventional tire. The tension of the carcass cord in the shoulder portion is reduced compared to a conventional tire. The total fineness of the carcass cord in a conventional tire is 4000 dtex or more, while the total fineness of the carcass cord is 3400 dtex or less. The carcass cord of this tire is thinner than the carcass cord of a conventional tire. The circumferential rigidity of the tread portion is reduced. The shape change during vulcanization molding is promoted in the crown portion. In contrast, the shape change in the shoulder portion is suppressed. Since the spacing between the carcass cords in the shoulder portion does not expand as in a conventional tire, the inflow of rubber between the carcass cords is suppressed. The exposure of the carcass to the inner surface of the tire is suppressed. The risk of air leakage and punctures during driving is reduced. As described above, the ratio of the radius of the arc representing the upper arc portion to the radius of the arc representing the lower arc portion is smaller than that of conventional tires. Since the outer diameter of the tire is smaller than that of conventional tires, the rigidity of the tire is increased. Meanwhile, since the carcass cord is thinner than that of conventional tires, the thickness of the tire at the buttress is reduced. Since the amount of flexure in the shoulder portion increases, the contact width is expanded. This tire can improve cornering performance. This tire can achieve improved cornering performance while suppressing exposure of the carcass.

[0016] It is preferable that, in the meridian cross section, the tread surface is represented by a plurality of arcs that are arranged in the axial direction and tangent to each other, the plurality of arcs include a center arc, a pair of shoulder arcs, and a pair of corner arcs, the center arc is an arc having a center on the equatorial plane of the tire, each of the corner arcs is an arc located axially outwardly and connected to the side surface, each of the shoulder arcs is an arc located axially inside the corner arcs and connected to the corner arcs, the corner arcs have the smallest radius among the plurality of arcs, and the ratio of the radius of the shoulder arc to the radius of the center arc is 60% or less. This ensures a sufficient radial distance between the equator, which is the intersection of the equatorial plane and the tread surface, and the portion represented by the shoulder arc on the tread surface. When the green tire is placed in the mold and pressed against the mold by the inflated bladder, the crown portion first comes into contact with the mold, and then the green tire and the mold gradually come into contact with each other toward the shoulder portion. As described above, the carcass cords of this tire are thinner than those of conventional tires. Therefore, the shape change during vulcanization is effectively promoted in the crown portion, while it is effectively suppressed in the shoulder portion. Since the spacing between the carcass cords in the shoulder portion does not expand as in conventional tires, the inflow of rubber between the carcass cords is suppressed. In this tire, the exposure of the carcass to its inner surface is suppressed.

[0017] The number of the carcass cords included in the 5 cm width of the carcass ply is preferably 48 or more. This allows the spacing between the carcass cords in the carcass ply to be maintained at an appropriate distance, and effectively prevents rubber from flowing between the carcass cords, thereby preventing the carcass from being exposed to the inner surface of the tire.

[0018] It is preferable that the angle that the carcass cord forms with respect to the equatorial plane of the tire is equal to or greater than 21 degrees and equal to or less than 41 degrees. By setting the angle at 21 degrees or more, shape changes during vulcanization are promoted in the crown area and suppressed in the shoulder area. Since the intervals between the carcass cords in the shoulder area do not expand as in conventional tires, the inflow of rubber between the carcass cords is suppressed. In this tire, the exposure of the carcass to the inner surface is suppressed. By setting the angle at 41 degrees or less, the shape of the crown portion is not changed too much during vulcanization, and the shape change of the entire tread portion is suppressed. The shape of the tread surface of the tire is prevented from becoming rounded, so a contact surface with a sufficient contact width is formed even during cornering. This tire can improve cornering performance.

[0019] [Details of the embodiment of the present invention] Hereinafter, the present invention will be described in detail based on preferred embodiments with reference to the drawings as appropriate.

[0020] FIG. 1 shows an example of a tire 2 for a cart (hereinafter, tire 2) according to one embodiment of the present invention. FIG. 1 shows a part of a cross section (hereinafter, also referred to as a meridian cross section) of the tire 2 along a plane including the rotation axis of the tire 2. The direction indicated by the double-headed arrow AD is the axial direction of the tire 2. The axial direction of the tire 2 means a direction parallel to the rotation axis of the tire 2. The direction indicated by the double-headed arrow RD is the radial direction of the tire 2. The direction perpendicular to the plane of FIG. 1 is the circumferential direction of the tire 2. The dashed dotted line CL represents the equatorial plane of the tire 2.

[0021] In FIG. 1, a tire 2 is mounted on a rim R. The rim R is a regular rim. More specifically, the rim R is a 5-inch rim that complies with the CIK regulations. The inside of the tire 2 is filled with air, and the internal pressure of the tire 2 is adjusted.

[0022] 1, a solid line BBL extending in the axial direction is a bead baseline. This bead baseline is a line that defines the rim diameter of the rim R.

[0023] 1, the position indicated by the symbol PT is the toe of the tire 2. The toe PT is the boundary between the outer surface 2G and the inner surface 2N of the tire 2. The outer surface 2G is shaped by a mold. The inner surface 2N is shaped by a bladder.

[0024] 1, the position indicated by the symbol Eq is the intersection point between the outer surface (more specifically, the tread surface described later) and the equatorial plane of the tire 2. This intersection point Eq is the equator of the tire 2. The equator Eq is also the outer end of the tire 2 in the radial direction. The length indicated by the double arrow HS is the radial distance from the bead base line to the equator Eq. The radial distance HS is the cross-sectional height of the tire 2.

[0025] In Fig. 1, the position indicated by the symbol PW is the axial outer end of the tire 2. If the outer surface 2G has decorations such as patterns or letters, the outer end PW is identified based on a virtual outer surface obtained by assuming that there is no decoration. The axial distance from one outer end PW to the other outer end PW is the maximum width of the tire 2. The outer end PW is the position where the tire 2 shows its maximum width (hereinafter, the maximum width position). In this tire 2, the maximum width position PW is identified in a reference state.

[0026] 1, the length indicated by the double-headed arrow HW is the radial distance from the bead base line to the maximum width position PW. The radial distance HW is also called the maximum width height. In the tire 2, the ratio HW / HS of the maximum width height HW to the section height HS is, for example, not less than 0.40 and not more than 0.50. The ratio HW / HS is preferably not less than 0.42 and not more than 0.46.

[0027] The tire 2 includes a tread 4 , a pair of sidewalls 6 , a pair of beads 8 , a carcass 10 , and an inner liner 12 .

[0028] The tread 4 is located radially outward of the carcass 10. The tread 4 comes into contact with the road surface. The tread 4 is made of crosslinked rubber in consideration of wear resistance and grip. The outer surface of the tread 4 has a shape that is convex radially outward. No grooves are formed in the tread 4. The tire 2 is a slick tire. The tread 4 may have grooves to form a tread pattern.

[0029] Each sidewall 6 is continuous with the tread 4. The sidewall 6 is located radially inward of the tread 4. The sidewall 6 is located axially outward of the carcass 10. The sidewall 6 extends radially along the carcass 10. The sidewall 6 is made of crosslinked rubber in consideration of cut resistance.

[0030] Each bead 8 is located radially inward from the sidewall 6. The bead 8 includes a core 14 and an apex 16. Although not shown, the core 14 includes a steel wire. The apex 16 is located radially outward from the core 14. The apex 16 tapers radially outward. The apex 18 is made of crosslinked rubber having high rigidity.

[0031] The carcass 10 is located inside the tread 4 and the sidewall 6. The carcass 10 spans between a pair of beads 8. The carcass 10 has a bias structure. The tire 2 is a bias tire.

[0032] The carcass 10 includes a plurality of carcass plies 18. The carcass 10 of the tire 2 is composed of two carcass plies 18. Of the two carcass plies 18, the carcass ply 18 located on the inner side in the radial direction of the tread 4 is called a first ply 20. The carcass ply 18 located on the outer side is called a second ply 22.

[0033] The first ply 20 includes a first ply body 20a and a pair of first turned-up portions 20b. The first ply body 20a spans between one core 14 and the other core 14. Each of the first turned-up portions 20b is continuous with the first ply body 20a and is turned up around each of the cores 14 from the inside toward the outside in the axial direction.

[0034] The second ply 22 includes a second ply body 22a and a pair of second turned-up portions 22b. The second ply body 22a spans between one core 14 and the other core 14. Each of the second turned-up portions 22b is connected to the second ply body 22a and is turned up around each of the cores 14 from the inside toward the outside in the axial direction.

[0035] An end of the second turned-up portion 22b is located radially outward of the outer end of the apex 16. An end of the first turned-up portion 20b is located radially outward of the end of the second turned-up portion 22b. The end of the second turned-up portion 22b is covered by the first turned-up portion 20b from the outside in the axial direction.

[0036] Fig. 2 shows the configuration of the carcass 10 in the tire 2. The direction indicated by the double-headed arrow AD is the axial direction of the tire 2. The direction indicated by the double-headed arrow CD is the circumferential direction of the tire 2. The direction perpendicular to the plane of Fig. 2 is the radial direction of the tire 2.

[0037] In the tire 2, the first ply 20 and the second ply 22 each include a large number of carcass cords 24 arranged in parallel. For convenience of explanation, the carcass cords 24 are represented by solid lines in Fig. 2, and the carcass cords 24 are covered with topping rubber 26. The first ply 20 and the second ply 22 each include a large number of carcass cords 24 and the topping rubber 26 that covers these carcass cords 24.

[0038] In the tire 2, the carcass cords 24 are inclined with respect to the equatorial plane. As shown in Fig. 2, the inclination direction of the carcass cords 24 included in the first ply 20 (hereinafter, referred to as first carcass cords 28) is opposite to the inclination direction of the carcass cords 24 included in the second ply 22 (hereinafter, referred to as second carcass cords 30).

[0039] 2, the angle indicated by the symbol α1 is the angle that the first carcass cord 28 makes with respect to the equatorial plane (hereinafter, referred to as the first inclination angle). The angle indicated by the symbol α2 is the angle that the second carcass cord 30 makes with respect to the equatorial plane (hereinafter, referred to as the second inclination angle). The first inclination angle α1 and the second inclination angle α2 may be the same or different. When the first inclination angle α1 is below 0 degrees or exceeds 90 degrees, the inclination direction of the first carcass cord 28 changes. The lower limit of the first inclination angle α1 is 0 degrees, and the upper limit is 90 degrees. The same is true for the second inclination angle α2, which has a lower limit of 0 degrees and an upper limit of 90 degrees.

[0040] In the tire 2, cords made of organic fibers are used as the carcass cords 24. The carcass ply 18 includes a large number of carcass cords 24 made of organic fibers and arranged in parallel. Examples of organic fibers include nylon fibers, rayon fibers, polyester fibers, and aramid fibers. Of these, the carcass cords 24 are preferably cords made of polyester fibers.

[0041] The carcass cord 24 is formed by twisting together filaments made of organic fibers. In the tire 2, a single thread formed by twisting together a bundle of many filaments may be used as the carcass cord 24, or a cord formed by twisting together a plurality of single threads formed by twisting together a plurality of filaments may be used as the carcass cord 24.

[0042] In the present invention, the thickness of the carcass cord is represented by the total fineness (dtex) of the carcass cord specified by the cord structure defined in JIS L1017. For example, when the carcass cord 24 is composed of only single yarns having a fineness of 1700 dtex, that is, when the cord structure of the carcass cord 24 is 1700 dtex / 1, the total fineness of this carcass cord 24 is 1700 dtex. When the carcass cord 24 is composed of two single yarns having a fineness of 1700 dtex twisted together, that is, when the cord structure of the carcass cord 24 is 1700 dtex / 2, the total fineness of this carcass cord 24 is 3400 dtex. In addition, when the cord structure of the carcass cord 24 is unknown, the total fineness of the carcass cord 24 is expressed by the correct fineness defined in JIS L1017. When the carcass cord 24 is inside the tire 2, the correct fineness of the carcass cord 24 sampled from the tire 2 is measured in accordance with JIS L1017, and the total fineness of the carcass cord 24 is expressed by this correct fineness.

[0043] The inner liner 12 is positioned inside the carcass 10. The inner liner 12 constitutes the inner surface 2N of the tire 2. The inner liner 12 is made of crosslinked rubber that is difficult for gases such as air and nitrogen to permeate. The inner liner 12 maintains the internal pressure of the tire 2.

[0044] Fig. 3 shows the outer surface 2G of the tire 2 in the meridian cross section shown in Fig. 1. The outer surface 2G of the tire 2 in the meridian cross section represents the contour of the outer surface 2G. In the meridian cross section, the outer surface 2G has a contour that is symmetrical with respect to the equatorial plane. The outer surface 2G shown in FIG. 3 is obtained by measuring the shape of the tire 2 in a reference state using, for example, a displacement sensor.

[0045] In the present invention, the contour of the outer surface 2G is specified by the outer surface 2G in a meridian cross section in a reference state. If grooves or the like are formed on the outer surface 2G of the tire 2, the outer surface 2G of the tire 2 is represented by a virtual outer surface obtained by assuming that the grooves or the like are not formed.

[0046] The outer surface 2G of the tire 2 includes a tread surface 32 and a pair of side surfaces 34. The tread surface 32 includes an equator Eq. Each side surface 34 is continuous with the tread surface 32 and includes a maximum width position PW of the tire 2. The position indicated by the symbol PB in FIG. 3 is the boundary between the tread surface 32 and the side surface 34.

[0047] The tread surface 32 includes a plurality of parts 36 arranged in the axial direction. Each of the plurality of parts 36 is represented by a circular arc in a meridian cross section. In adjacent parts 36, the circular arc representing one part 36 contacts the circular arc representing the other part 36 at the boundary between them. In the meridian cross section, the tread surface 32 is represented by a plurality of circular arcs that are aligned in the axial direction and tangent to each other.

[0048] The tread surface 32 shown in Fig. 3 is composed of seven parts 36. The seven parts 36 are a center arc portion 36c, a pair of middle arc portions 36m, a pair of shoulder arc portions 36s, and a pair of corner arc portions 36r.

[0049] The center arc portion 36c is located at the center of the seven parts 36. An arrow Rc in Fig. 3 indicates the radius of the arc representing the center arc portion 36c (hereinafter, referred to as the center arc). Although not shown, the center arc is an arc having a center on the equatorial plane of the tire 2.

[0050] Each corner arc portion 36r is located at the outermost position in the axial direction among the seven parts 36. The above-mentioned side surface 34 is continuous with the corner arc portion 36r. The arc representing the corner arc portion 36r (hereinafter, the corner arc) is an arc located at the outermost position in the axial direction and continuous with the side surface 34. The above-mentioned boundary PB is the boundary between the corner arc portion 36r and the side surface 34. The boundary PB is the point of contact between the corner arc and the contour of the side surface 34. 3 indicates the radius of the corner arc. The corner arc has the smallest radius Rr among the multiple arcs that define the contour of the tread surface 32.

[0051] Each shoulder arc portion 36s is located axially inside the corner arc portion 36r. The shoulder arc portion 36s is located axially outside the center arc portion 36c. The arrow Rs in FIG. 3 is the radius of the arc representing the shoulder arc portion 36s (hereinafter, shoulder arc). The shoulder arc is located axially inside the corner arc and is continuous with the corner arc. The position indicated by the symbol SR is the boundary between the shoulder arc portion 36s and the corner arc portion 36r. The boundary SR is the point of contact between the shoulder arc and the corner arc.

[0052] Each middle arc portion 36m is located between the center arc portion 36c and the shoulder arc portion 36s in the axial direction. The arrow Rm in FIG. 3 indicates the radius of the arc representing the middle arc portion 36m (hereinafter, referred to as the middle arc). The middle arc bridges between the center arc and the shoulder arc. The position indicated by the symbol CM is the boundary between the center arc portion 36c and the middle arc portion 36m. The boundary CM is the tangent point between the center arc and the middle arc. The position indicated by the symbol MS is the boundary between the middle arc portion 36m and the shoulder arc portion 36s. The boundary MS is the tangent point between the middle arc and the shoulder arc.

[0053] 3, the solid line LSR is a tangent to the corner arc portion 36r at the boundary SR, in other words, the inner end SR of the corner arc portion 36r, and the solid line LPB is a tangent to the corner arc portion 36r at the boundary PB, in other words, the outer end PB of the corner arc portion 36r. The position indicated by the symbol TE is the intersection point between the tangent line LSR and the tangent line LPB. In the present invention, this intersection point TE is called the tread reference end.

[0054] In FIG. 3, the length indicated by the double-headed arrow WT is the axial distance from the equatorial plane to the tread reference edge TE. The axial distance WT is also called the tread reference width. The length indicated by the double-headed arrow WA is the axial distance from the equatorial plane to the maximum width position PW. The axial distance WA is also called the reference section width. The reference section width WA is half the section width of this tire 2.

[0055] In the tire 2, for example, the ratio WT / WA of the reference tread width WT to the reference section width WA is equal to or greater than 0.80 and is equal to or less than 0.90.

[0056] 3, the length indicated by the double arrow WC is the axial distance from the equatorial plane to the boundary CM, the length indicated by the double arrow WM is the axial distance from the equatorial plane to the boundary MS, and the length indicated by the double arrow WR is the axial distance from the equatorial plane to the boundary SR.

[0057] In the tire 2, for example, the ratio WC / WT of the axial distance WC to the reference tread width WT is 0.25 or more and 0.35 or less, the ratio WM / WT of the axial distance WM to the reference tread width WT is 0.75 or more and 0.85 or less, and the ratio WR / WT of the axial distance WR to the reference tread width WT is 0.90 or more and 0.95 or less.

[0058] 3, a solid line LPW is a straight line that passes through the maximum width position PW and extends in the axial direction. In the present invention, this straight line LPW is called a reference line.

[0059] The side surface 34 includes an upper circular arc portion 38 and a lower circular arc portion 40. In the meridian cross section, the upper circular arc portion 38 and the lower circular arc portion 40 are each represented by a circular arc.

[0060] The upper arc portion 38 is represented by an arc (hereinafter, upper arc) having a center on the reference line LPW and extending radially outward from the maximum width position PW. An arrow Rj in Fig. 3 indicates the radius of the upper arc. The upper arc portion 38 of the tire 2 is directly connected to the tread surface 32. A boundary PB is a boundary between the upper arc portion 38 and the tread surface 32. One end of the upper arc portion 38 is the maximum width position PW, and the other end is the boundary PB. The upper arc portion 38 and the tread surface 32 may be connected via another part. In this case, the contour of the other part is represented by an arc or a straight line.

[0061] The lower arc portion 40 is represented by an arc having a center on the reference line LPW and extending radially inward from the maximum width position PW (hereinafter, referred to as the lower arc). An arrow Rk in Fig. 3 indicates the radius of the lower arc. 3, the position indicated by the symbol PF is the radially outer end of the contact surface between the tire 2 and the rim R. The position on the side surface 34 corresponding to the radially outer end PF is also called the outer contact end. In the tire 2, the portion of the side surface 34 from the maximum width position PW to the outer contact end PF is the lower arc portion 40.

[0062] The manufacturing method of this tire 2 will be described with reference to FIG. In the manufacturing method of this tire 2, elements such as the tread 4 are molded and combined to prepare a raw tire 2r (a tire in an unvulcanized state) for the tire 2. The raw tire 2r is placed in a mold 44. The raw tire 2r is pressurized and heated in the mold 44 to obtain the tire 2. The manufacturing method of this tire 2 includes a step of preparing the raw tire 2r and a step of pressurizing and heating the raw tire 2r in the mold 44. Pressurizing and heating the raw tire 2r in the mold 44 is also called vulcanization molding, and the process of pressurizing and heating the raw tire 2r is also called a vulcanization process.

[0063] In the vulcanization process, a vulcanizer 42 shown in Fig. 4 is used. This vulcanizer 42 is a general vulcanizer used for manufacturing kart tires. The vulcanizer 42 includes a mold 44 and a bladder 46.

[0064] The mold 44 has an inner surface thereof that is a cavity surface 48. The cavity surface 48 abuts against the outer surface of the green tire 2r and forms the outer surface 2G of the tire 2. The mold 44 is a split mold. The mold 44 includes, as its constituent members, a tread ring 50, a pair of side plates 52, and a pair of bead rings 54. The tread ring 50 has a plurality of segments 56 arranged in the circumferential direction. Usually, the number of segments 56 constituting the tread ring 50 is 8 or more and 19 or less. Combining these components constitutes the aforementioned cavity surface 48. The mold 44 in Fig. 4 is in a state in which these components are combined, in other words, in a closed state.

[0065] The tread ring 50 forms a tread portion of the tire 2. Each side plate 52 forms a sidewall portion of the tire 2. Each bead ring 54 forms a bead portion of the tire 2.

[0066] The bladder 46 is located inside the mold 44. The bladder 46 is made of crosslinked rubber. A heating medium such as steam is filled inside the bladder 46. This causes the bladder 46 to expand. The bladder 46 shown in FIG. 4 is in an expanded state filled with a heating medium. The bladder 46 abuts against the inner surface of the raw tire 2r and shapes the inner surface 2N of the tire 2. A metal rigid core (not shown) may be used instead of the bladder 46. The rigid core has a toroidal outer surface. This outer surface is approximated to the shape of the inner surface 2N of the tire 2 in a state in which air is filled and the internal pressure is maintained at 5% of the normal internal pressure.

[0067] In the vulcanization process, when the green tire 2r is placed in the mold 44, the green tire 2r is pressed against the cavity surface 48 of the mold 44 by the bladder 46 that expands from inside the green tire 2r. The vulcanization process is accompanied by a change in the shape of the green tire 2r.

[0068] In kart tires, the radius of the arc that represents the outline of the upper arc portion is conventionally set to 120% or more of the radius of the arc that represents the outline of the lower arc portion, from the standpoint of improving cornering performance, and the outline of the side surface is designed to increase the contact width during cornering.

[0069] Kart tires are bias tires. The shape change of raw tires is greater than that of radius tires. If the radius of the arc that represents the outline of the upper arc portion is set to a large radius, the spacing of the carcass cords in the shoulder area tends to widen, and there is a concern that the rubber, including the inner liner that is provided to maintain internal pressure, will flow between the carcass cords, exposing the carcass to the inside. There is also a concern that if a large amount of rubber flows in, the risk of air leakage or punctures during driving will increase.

[0070] In this tire 2, the ratio (Rj / Rk) of the radius Rj of the arc that represents the upper arc portion 38 to the radius Rk of the arc that represents the lower arc portion 40 is 105% or less. The ratio (Rj / Rk) in this tire 2 is smaller than that of a conventional tire. In this tire 2, the tension of the carcass cord 24 in the shoulder portion is reduced compared to a conventional tire.

[0071] In conventional tires, organic fiber cords having a total fineness of 4000 dtex or more are used as carcass cords in order to ensure rigidity. In contrast, the total fineness of the carcass cords 24 of this tire 2 is 3400 dtex or less. The carcass cords 24 of this tire 2 are thinner than the carcass cords of conventional tires. In this tire 2, the circumferential rigidity of the tread portion is reduced compared to conventional tires. Shape changes during vulcanization are promoted in the crown portion and suppressed in the shoulder portion. Since the intervals between the carcass cords 24 in the shoulder portion do not expand as in conventional tires, the inflow of rubber between the carcass cords 24 is suppressed. Exposure of the carcass 10 to the inner surface 2N of the tire 2 is suppressed. In this tire 2, the risk of air leakage and punctures during driving is reduced.

[0072] As described above, the ratio (Rj / Rk) is smaller than that of conventional tires. Since the outer diameter of the tire 2 is smaller than that of conventional tires, the rigidity of the tire 2 is increased. Meanwhile, since the carcass cord 24 is thinner than that of conventional tires, the thickness of the tire 2 at the buttress is reduced. Since the amount of flexure in the shoulder portion increases, the contact width is expanded. This tire 2 can improve cornering performance. The tire 2 can achieve improved cornering performance while suppressing exposure of the carcass 10.

[0073] As described above, the ratio (Rj / Rk) is 105% or less. From the viewpoint of enabling the tire 2 to achieve improved cornering performance while suppressing exposure of the carcass 10, the ratio (Rj / Rk) is preferably 103% or less. If this ratio (Rj / Rk) is low, the rigidity of the tire increases. From the viewpoint of allowing the tire 2 to bend to a necessary degree, this ratio (Rj / Rk) is preferably 87% or more.

[0074] In this tire 2, from the viewpoint of effectively suppressing exposure of the carcass 10 to the inner surface 2N, the radius Rc of the center arc and the radius Rs of the shoulder arc are adjusted among the multiple arcs that define the contour of the tread surface 32. Specifically, the ratio (Rs / Rc) of the radius Rs of the shoulder arc to the radius Rc of the center arc is preferably 60% or less.

[0075] By setting the ratio (Rs / Rc) to 60% or less, the radial distance between the equator Eq and the shoulder arc portion 36s is sufficiently secured on the tread surface 32. When the raw tire 2r is put into the mold 44 and pressed against the cavity surface 48 of the mold 44 by the expanded bladder 46, the crown portion first comes into contact with the mold 44, and then the raw tire 2r and the mold 44 gradually come into contact with each other toward the shoulder portion. As described above, the carcass cords 24 of this tire 2 are thinner than those of the conventional tire. Therefore, the shape change during vulcanization is effectively promoted in the crown portion, whereas it is effectively suppressed in the shoulder portion. Since the intervals between the carcass cords 24 in the shoulder portion do not expand as in the conventional tire, the inflow of rubber between the carcass cords 24 is suppressed. In this tire 2, the exposure of the carcass 10 to its inner surface 2N is suppressed. From this viewpoint, it is more preferable that the ratio (Rs / Rc) is 55% or less.

[0076] In this tire 2, the ratio (Rs / Rc) is preferably equal to or greater than 45%. This prevents the shape of the tread surface 32 of the tire 2 from becoming rounded, and a contact patch having a sufficient contact width is formed even during cornering. The cornering performance of this tire 2 can be improved. From this viewpoint, the ratio (Rs / Rc) is more preferably equal to or greater than 50%.

[0077] As described above, the total fineness of the carcass cords 24 of the tire 2 is 3400 dtex or less. From the viewpoint of enabling the tire 2 to achieve improved cornering performance while suppressing exposure of the carcass 10, the total fineness of the carcass cords 24 is preferably 3340 dtex or less. If the carcass cords 24 are too thin, there is a concern that the rigidity of the tire 2 will be insufficient. From the viewpoint of ensuring the necessary rigidity of the tire, the total fineness of the carcass cords 24 is preferably 2200 dtex or more.

[0078] As described above, the carcass ply 18 includes a large number of parallel carcass cords 24. In the tire 2, the number of carcass cords 24 included in a 5 cm width of the carcass ply 18 (hereinafter, referred to as the "count") is preferably 48 or more. This maintains appropriate spacing between the carcass cords 24 in the carcass ply 18. Since the inflow of rubber between the carcass cords 24 is effectively suppressed, exposure of the carcass 10 to the inner surface 2N of the tire 2 is suppressed. From this viewpoint, it is more preferable that the number of threads be 50 or more. If the number of threads per inch is too high, there is a concern that the amount of rubber between the carcass cords 24 will be insufficient, making the carcass 10 more susceptible to damage. From the standpoint of ensuring that the tire maintains good durability, it is preferable that the number of threads per inch be 59 or less.

[0079] As described above, the carcass cords 24 of the tire 2 are inclined with respect to the equatorial plane. In the tire 2, the inclination angle α1 of the carcass cord 24 included in the first ply 20, i.e., the first carcass cord 28, and the inclination angle α2 of the carcass cord 24 included in the second ply 22, i.e., the second carcass cord 30, are set to the same angle. The inclination angle α1 and the inclination angle α2 may be set to different angles.

[0080] In the tire 2, the inclination angle α1 of the first carcass cord 28 is preferably equal to or greater than 21° and equal to or less than 41°. By setting the inclination angle α1 to 21 degrees or more, shape changes during vulcanization are promoted in the crown portion and suppressed in the shoulder portion. Since the intervals between the first carcass cords 28 in the shoulder portion do not expand as in conventional tires, the inflow of rubber between the first carcass cords 28 is suppressed. In the tire 2, exposure of the carcass 10 to the inner surface is suppressed. From this viewpoint, it is more preferable that the inclination angle α1 is 26 degrees or more. By setting the inclination angle α1 to 41 degrees or less, the shape change of the entire tread portion is suppressed so that the shape change of the crown portion during vulcanization molding is not too large. Since the shape of the tread surface 32 of the tire 2 is suppressed from becoming rounded, a contact surface with a sufficient contact width is formed even during cornering. The cornering performance of this tire 2 can be improved. From this viewpoint, it is more preferable that the inclination angle α1 is 36 degrees or less.

[0081] In the tire 2, the inclination angle α2 of the second carcass cord 30 is preferably equal to or greater than 21° and equal to or less than 41°. By setting the inclination angle α2 to 21 degrees or more, shape changes during vulcanization are promoted in the crown portion and suppressed in the shoulder portion. Since the intervals between the second carcass cords 30 in the shoulder portion do not expand as in conventional tires, the inflow of rubber between the second carcass cords 30 is suppressed. In the tire 2, exposure of the carcass 10 to the inner surface is suppressed. From this viewpoint, it is more preferable that the inclination angle α2 is 26 degrees or more. By setting the inclination angle α1 to 41 degrees or less, the shape change of the entire tread portion is suppressed so that the shape change of the crown portion during vulcanization molding is not too large. Since the shape of the tread surface 32 of the tire 2 is suppressed from becoming rounded, a contact surface with a sufficient contact width is formed even during cornering. The cornering performance of this tire 2 can be improved. From this viewpoint, it is more preferable that the inclination angle α2 is 36 degrees or less.

[0082] As described above, the tread surface 32 shown in FIG. 3 is represented by seven arcs consisting of a center arc, a pair of middle arcs, a pair of shoulder arcs, and a pair of corner arcs. The tread surface 32 may be configured such that the center arc is directly connected to the shoulder arc. In this case, the tread surface 32 is represented by five arcs including the center arc, a pair of shoulder arcs, and a pair of corner arcs.

[0083] When the green tire 2r is put into the mold 44 and pressed against the cavity surface 48 of the mold 44 by the expanded bladder 46, the crown portion is first brought into contact with the mold 44, and then the green tire 2r and the mold 44 are gradually brought into contact with each other toward the shoulder portion. From the viewpoint of effectively suppressing the widening of the interval between the carcass cords 24 in the shoulder portion, as shown in Fig. 3, it is preferable that the tread surface 32 is represented by seven arcs consisting of a center arc, a pair of middle arcs, a pair of shoulder arcs, and a pair of corner arcs. In this case, it is preferable that the middle arc has a radius Rm larger than the radius Rc of the center arc. Specifically, the ratio (Rm / Rc) of the radius Rm of the middle arc to the radius Rc of the center arc is preferably 110% or more and 130% or less, and more preferably 115% or more and 125% or less.

[0084] As is clear from the above description, according to the present invention, a kart tire can be obtained that can achieve improved cornering performance while suppressing exposure of the carcass. EXAMPLES

[0085] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0086] [Example 1] A tire set for a kart was obtained, which was made up of front and rear tires having the basic configuration shown in Figure 1-3 and the specifications shown in Table 1 below. The tire size of the front tire was "10 x 4.50-5". The tire size of the rear tire was "11 x 7.10-5".

[0087] The ratio (Rj / Rk) of the radius Rj of the upper circular arc to the radius Rk of the lower circular arc, the ratio (Rs / Rc) of the radius Rs of the shoulder circular arc to the radius Rc of the center circular arc, the total fineness of the carcass cord, the end count of the carcass cord, and the inclination angle of the carcass cord are as shown in Table 1 below. Cords made of polyester fiber were used for the carcass cord. The cord structure of the carcass cord was 1700 dtex / 2.

[0088] [Comparative Example 1] Comparative Example 1 is a conventional tire set. The ratio (Rj / Rk), ratio (Rs / Rc), total fineness of the carcass cord, end count of the carcass cord, and inclination angle of the carcass cord are as shown in Table 1 below. The cord structure of the carcass cord used in Comparative Example 1 was 2200 dtex / 2.

[0089] [Comparative Example 2] A tire set of Comparative Example 2 was obtained in the same manner as in Example 1, except that the ratio (Rj / Rk) and the ratio (Rs / Rc) were set as shown in Table 1 below.

[0090] [Exposing the carcass] The inner surface of the prototype tires (especially the shoulder area) was visually inspected to check whether the carcass was exposed. The results are shown in Table 1 below, with the following rankings. The inner liner remains in good condition and the carcass is not exposed. 3 points The inner liner remains, but the twist of the carcass cord can be confirmed. · Carcass is exposed. · 1 point

[0091] [Turning performance] The front tire was mounted on a rim (size = 4.5 x 5.0) and filled with air to adjust the internal pressure of the tire to 80 Pa. This front tire was attached to the front wheel of a test vehicle (a racing kart vehicle with an engine displacement of 125 cc). The rear tire was mounted on a rim (size = 8.0 x 5.0) and filled with air to adjust the internal pressure of the tire to 80 Pa. This rear tire was mounted on the rear wheel of the test vehicle. A running test was conducted by having the driver run the aforementioned test vehicle 10 laps on a dedicated racing kart course with a lap length of 1300m. During this running test, the driver was asked to evaluate the cornering performance (sensory evaluation). The results are shown in Table 1 below with the following rankings. Better than Comparative Example 1. 4 points Equivalent to Comparative Example 1. 3 points · Inferior to Comparative Example 1. · 2 points Even worse than Comparative Example 1. 1 point ·Not drivable ···0 points

[0092] [Table 1]

[0093] As shown in Table 1, in the examples, the cornering performance was improved while suppressing the exposure of the carcass. From these evaluation results, the superiority of the present invention is clear. [Industrial Applicability]

[0094] The technology described above that can improve cornering performance while suppressing exposure of the carcass can be applied to various tires.

[0095] [Note] The present invention includes the following aspects.

[0096] [1] A cart tire comprising a pair of beads and a carcass having a bias structure spanning the pair of beads, The outer surface of the tire includes a tread surface including an equator of the tire, and a pair of side surfaces that are continuous with the tread surface and include a maximum width position of the tire, In a meridian section of the tire in a reference state defined below, each of the side surfaces includes an upper arc portion having a center on a reference line defined below and represented by an arc extending radially outward from the maximum width position, and a lower arc portion having a center on the reference line and represented by an arc extending radially inward from the maximum width position, a ratio of a radius of the arc representing the upper arc portion to a radius of the arc representing the lower arc portion is 105% or less; The carcass includes a plurality of carcass plies, Each of the carcass plies includes a number of parallel carcass cords made of organic fibers, The total fineness of each of the carcass cords is 3400 dtex or less. Kart tires. Standard condition: The tire is mounted on a regular rim specified by the International Karting Committee, the internal pressure of the tire is adjusted to 100 kPa, and no load is applied to the tire. Reference line: A straight line that passes through the maximum width position and extends in the axial direction [2] In the meridian cross section, the tread surface is represented by a plurality of circular arcs arranged in an axial direction and tangent to each other, The plurality of arcs include a center arc, a pair of shoulder arcs, and a pair of corner arcs; the center arc is an arc having a center on the equatorial plane of the tire, Each corner arc is an arc located axially outermost and connected to the side surface, Each shoulder arc is an arc located axially inside the corner arc and continuous with the corner arc, the corner arc has a smallest radius among the plurality of arcs; The cart tire according to the above-mentioned [1], wherein a ratio of a radius of the shoulder arc to a radius of the center arc is 60% or less. [3] The kart tire according to the above-mentioned [1] or [2], wherein the number of the carcass cords included in the 5 cm width of the carcass ply is 48 or more. [4] The cart tire according to any one of [1] to [3] above, wherein the angle that the carcass cord forms with respect to the equatorial plane of the tire is greater than or equal to 21 degrees and less than or equal to 41 degrees. [Explanation of symbols]

[0097] 2. Tires 2G: Outer surface of tire 2 4. Tread 6. Sidewall 8...Bead 10. Carcass 12. Inner liner 18···Carcass ply 24···Carcass cord 32 Tread surface 34 Side surface 36c Center arc section 36m Middle arc section 36s... Shoulder arc section 36r Corner arc section 38 Upper arc section 40 Lower arc section 44 Mold 46 Bladder 48 Cavity surface

Claims

1. A cart tire comprising a pair of beads and a carcass having a bias structure extending between the pair of beads, The outer surface of the tire includes a tread surface including an equator of the tire, and a pair of side surfaces that are continuous with the tread surface and include a maximum width position of the tire, In a meridian section of the tire in a reference state defined below, each of the side surfaces includes an upper arc portion having a center on a reference line defined below and represented by an arc extending radially outward from the maximum width position, and a lower arc portion having a center on the reference line and represented by an arc extending radially inward from the maximum width position, a ratio of a radius of the arc representing the upper arc portion to a radius of the arc representing the lower arc portion is 105% or less; The carcass includes a plurality of carcass plies, Each of the carcass plies includes a number of parallel carcass cords made of organic fibers, The total fineness of each of the carcass cords is 3400 dtex or less. Kart tires. Standard condition: The tire is mounted on a regular rim specified by the International Karting Committee, the internal pressure of the tire is adjusted to 100 kPa, and no load is applied to the tire. Reference line: A straight line that passes through the maximum width position and extends in the axial direction

2. In the meridian cross section, the tread surface is represented by a plurality of circular arcs arranged in an axial direction and tangent to each other, The plurality of arcs include a center arc, a pair of shoulder arcs, and a pair of corner arcs; the center arc is an arc having a center on the equatorial plane of the tire, Each corner arc is an arc located axially outermost and connected to the side surface, Each shoulder arc is an arc located axially inside the corner arc and continuous with the corner arc, the corner arc has a smallest radius among the plurality of arcs; A ratio of a radius of the shoulder arc to a radius of the center arc is 60% or less.

2. The tire for a cart according to claim 1.

3. The number of the carcass cords included in the 5 cm width of the carcass ply is 48 or more.

2. The tire for a cart according to claim 1.

4. The cart tire according to claim 1 , wherein an angle of the carcass cord with respect to an equatorial plane of the tire is equal to or greater than 21 degrees and is equal to or less than 41 degrees.

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