Pneumatic tire

Abstract

To provide a pneumatic tire capable of excellently maintaining shock burst resistance and belt folding durability, securing excellent snow performance, and highly making these performance compatible, while reducing tire weight by reducing the number of carcass layers.SOLUTION: The pneumatic tire includes one carcass layer 4, and a plurality of belt layers 7 and belt cover layers 8 on the outer peripheral side of the carcass layer 4 in a tread part 1. The glass-transition temperature Tg of the tread rubber configuring the tread portion 1 is set to - 57 °C. or lower, the total fineness of the cover cords configuring the belt cover layer 8 is set to 900dtex to 2300dtex, the breaking elongation of the carcass cords configuring the carcass layer 4 is set to 20% or greater and the total fineness is set to 4000dtex to 6000dtex, the end of the folded back portion of the carcass layer 4 is disposed between the belt layer 7 and the main body portion of the carcass layer 4, and the rubber thickness Ga under the main groove 40 is set to 2. 5mm to 4. 0mm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a pneumatic tire in which the number of carcass layers is reduced to one in order to reduce weight. [Background technology]

[0002] In recent years, there has been an increasing demand for lighter tires and reduced rolling resistance, and, for example, efforts have been made to reduce the rubber thickness of the tread portion, particularly the rubber thickness under the main grooves provided in the tread portion (under-groove rubber gauge) (see, for example, Patent Document 1). On the other hand, tires designed for running on snowy roads in winter (winter tires, snow tires, studless tires, studded tires, all-season tires, tires for low-temperature road surfaces, etc.) tend to have a larger under-groove rubber gauge to ensure running performance on snowy road surfaces (snow performance), so it has been necessary to reduce the tire weight by other means while maintaining a moderate under-groove rubber gauge.

[0003] For example, as a method for reducing the weight of a tire using factors other than rubber gauge, reducing the fineness of the belt cover layer and reducing the number of carcass layers in tires that previously used two carcass layers (see, for example, Patent Document 2) have been proposed. While these technologies could be applied to the aforementioned winter tires, simply reducing the fineness of the belt cover layer or the number of carcass layers may result in a failure to fully ensure performance equivalent to that of conventional tires. For example, there is a concern that shock burst resistance and belt breakage durability may be reduced. Therefore, there is a need for a measure that ensures snow performance while maintaining good shock burst resistance and belt breakage durability, and reduces tire weight, thereby achieving a high level of both of these performance characteristics.

[0004] The above-mentioned "shock burst resistance" refers to the durability against damage (shock burst) that occurs when a tire receives a large shock while traveling and the carcass breaks. This shock burst resistance is measured, for example, by a plunger energy test (a test in which a plunger of a predetermined size is pressed against the center of the tread to measure the breaking energy when the tire breaks). In this specification, "shock burst resistance" may be referred to as "plunger performance." [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-058515 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-137812 Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to provide a pneumatic tire that can reduce the number of carcass layers to reduce tire weight, while maintaining good shock burst resistance and belt breakage durability and ensuring good snow performance, thereby achieving a high degree of compatibility between these performances. [Means for solving the problem]

[0007] In order to achieve the above object, the pneumatic tire of the present invention includes a tread portion extending in a circumferential direction of the tire to form an annular shape, a pair of sidewall portions disposed on both sides of the tread portion, and a pair of bead portions disposed radially inward of the sidewall portions, the pneumatic tire having one carcass layer mounted between the pair of bead portions, a plurality of belt layers disposed on the outer peripheral side of the carcass layer in the tread portion, and a belt cover layer disposed on the outer peripheral side of the belt layer, wherein the glass transition temperature Tg of the tread rubber constituting the tread portion is −57° C. or lower, and the cover cords constituting the belt cover layer have a total fineness of the carcass cords constituting the carcass layer are organic fiber cords having a breaking elongation of 20% or more and a total fineness of 4000 dtex to 6000 dtex, the carcass layer comprises a main body portion extending from the tread portion through each sidewall portion to each bead portion, and a turned-up portion turned back at each bead portion and extending toward each sidewall portion, an end of the turned-up portion of the carcass layer is disposed between the belt layer and the main body portion of the carcass layer, and the rubber thickness under the circumferential main grooves formed in the tread portion is 2.5 mm to 4.0 mm. [Effects of the Invention]

[0008] In the present invention, when reducing the number of carcass layers to one to reduce tire weight, the structures and physical properties of the tread rubber, belt cover layer (cover cord), carcass layer (carcass cord), and rubber thickness under the circumferential main grooves are set as described above, thereby maintaining good shock burst resistance and belt breakage durability while ensuring good snow performance. In particular, snow performance can be ensured by having the glass transition temperature Tg of the tread rubber be -57°C or lower. The total fineness of the cover cords constituting the belt cover layer is 900 dtex to 2300 dtex, which allows for both a reduction in tire weight and ensuring belt breakage durability. The breaking elongation of the carcass cords constituting the carcass layer is 20% or more, which allows for shock burst resistance. The total fineness of the carcass cords constituting the carcass layer is 4000 dtex to 6000 dtex, which allows for both a reduction in tire weight and ensuring snow performance. By positioning the end of the folded-back portion of the carcass layer between the belt layer and the main body of the carcass layer, side rigidity can be ensured and snow performance can be ensured even with a single carcass layer. By setting the rubber thickness under the circumferential main grooves formed in the tread portion to 2.5 mm to 4.0 mm, belt breakage durability can be improved while suppressing an increase in tire weight. These combined effects enable a high degree of compatibility between shock burst resistance and belt breakage durability, while reducing tire weight and ensuring snow performance.

[0009] The glass transition temperature Tg of the tread rubber can be determined as the midpoint temperature of the transition region by measuring a thermogram by differential scanning calorimetry (DSC) at a temperature rise rate of 20°C / min. The breaking elongation of the carcass cord constituting the carcass layer is the elongation (%) at break of the sample cord (carcass cord) measured in accordance with JIS L1017 (2002) "Test Method for Chemical Fiber Tire Cords," in which the carcass cord is taken out from a location including the maximum tire width portion and used as a sample cord, and attached to a tensile tester so that the location corresponding to the maximum tire width portion is at the center, and a tensile test is carried out under conditions of a grip interval of 100 mm and a tensile speed of 300±20 mm / min.

[0010] In the present invention, the carcass cords constituting the carcass layer are preferably made of polyester fiber, which is advantageous in ensuring shock burst resistance and belt breakage durability.

[0011] The present invention can be suitably applied to winter tires. The present invention can also be suitably applied to extra-load tires. "Winter tires" refers to tires such as winter tires designed for driving on snowy roads in winter, snow tires, studless tires, studded tires, all-season tires, and tires for low-temperature road surfaces. "Extra-load tires" refer to tires that meet the extra-load (XL) or reinforced (Ref) standards specified by the ETRTO, and have a tire structure with enhanced pressure resistance so that they can be inflated with a higher air pressure than the standard (STD) standard. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a meridian cross-sectional view showing a pneumatic tire according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] The configuration of the present invention will be described in detail below with reference to the accompanying drawings.

[0014] As shown in Figure 1, the pneumatic tire of the present invention includes a tread portion 1, a pair of sidewall portions 2 arranged on either side of the tread portion 1, and a pair of bead portions 3 arranged radially inward of the sidewall portions 2. In Figure 1, the symbol CL indicates the tire equator. Although not depicted in Figure 1 because it is a meridian cross-section, the tread portion 1, sidewall portions 2, and bead portions 3 each extend in the tire circumferential direction and form an annular shape, thereby constituting the basic toroidal structure of the pneumatic tire. The following explanation using Figure 1 will be based basically on the meridian cross-section shape shown, but each tire constituent member also extends in the tire circumferential direction and forms an annular shape.

[0015] A carcass layer 4 including a plurality of reinforcing cords (hereinafter referred to as carcass cords) extending in the tire radial direction is mounted between a pair of left and right bead portions 3. A bead core 5 is embedded in each bead portion 3, and a bead filler 6 having a generally triangular cross section is disposed on the outer periphery of the bead core 5. The carcass layer 4 is folded back around the bead core 5 from the inner side to the outer side in the tire width direction. As a result, the bead core 5 and the bead filler 6 are enclosed by the main body portion of the carcass layer 4 (the portion extending from the tread portion 1 through each sidewall portion 2 to each bead portion 3) and the folded-back portion (the portion folded back around the bead core 5 in each bead portion 3 and extending toward each sidewall portion 2). As described below, in the present invention, the terminals of the folded-back portions reach the tread portion 1 through each sidewall portion 2 and are always disposed between the main body portion of the carcass layer 4 and a belt layer 7 described below.

[0016] Meanwhile, multiple belt layers 7 (two layers in the illustrated example) are embedded on the outer peripheral side of the carcass layer 4 in the tread portion 1. Each belt layer 7 includes multiple reinforcing cords (hereinafter referred to as belt cords) that are inclined with respect to the tire circumferential direction, and the belt cords are arranged so as to cross each other between layers. In these belt layers 7, the inclination angle of the belt cords with respect to the tire circumferential direction is set in the range of 10° to 40°, for example. Steel cords are preferably used as the reinforcing cords of the belt layers 7.

[0017] Furthermore, a belt cover layer 8 is provided on the outer peripheral side of the belt layer 7 for the purpose of improving high-speed durability. The belt cover layer 8 includes reinforcing cords (hereinafter referred to as cover cords) oriented in the tire circumferential direction. In the belt cover layer 8, the cover cords are set at an angle of, for example, 0° to 5° with respect to the tire circumferential direction. The belt cover layer 8 may include a full cover layer 8a that covers the entire width of the belt layer 7, and a pair of edge cover layers 8b that locally cover both ends of the belt layer 7 in the tire width direction, either alone or in combination (in the illustrated example, both the full cover layer 8a and the edge cover layer 8b are provided). The belt cover layer 8 may be formed, for example, by spirally winding a strip material in the tire circumferential direction, in which at least one cover cord is aligned and covered with a coating rubber.

[0018] In the tread portion 1, a tread rubber layer 10 is disposed on the outer peripheral side of the above-mentioned tire constituent members (carcass layer 4, belt layer 7, belt cover layer 8). As shown in the figure, the tread rubber layer 10 may have a structure in which two types of rubber layers with different physical properties (a cap tread layer 11 exposed to the tread surface and an under tread layer 12 disposed on its inner peripheral side) are laminated in the tire radial direction. A side rubber layer 20 is disposed on the outer peripheral side (outside in the tire width direction) of the carcass layer 4 in the sidewall portion 2, and a rim cushion rubber layer 30 is disposed on the outer peripheral side (outside in the tire width direction) of the carcass layer 4 in the bead portion 3.

[0019] The present invention mainly relates to the above-mentioned carcass layer 4 and belt cover layer 8, the cords (carcass cords, cover cords) constituting each layer, and the tread rubber layer 10, and other basic structures of the tire are not limited to those described above.

[0020] The pneumatic tire of the present invention is a tire in which the number of carcass layers 4 is reduced in order to reduce the tire weight, and the number of carcass layers 4 is limited to one. If the number of carcass layers 4 is two or more, the tire weight cannot be sufficiently reduced.

[0021] As described above, the terminals of the turned-up portions of the carcass layer 4 reach the tread portion 1 via each sidewall portion 2, and are always disposed between the main body portion of the carcass layer 4 and the belt layer 7 described below. As a result, even if the number of layers of the carcass layer 4 is only one as described above, the carcass layer 4 essentially becomes two layers in the sidewall portion 2, which is advantageous for ensuring side rigidity and improving steering stability. In this structure, the distance in the tire width direction from the outermost end of the belt layer 7 in the tire width direction to the terminals of the turned-up portions of the carcass layer 4 (the amount of overlap between the belt layer 7 and the turned-up portions of the carcass layer 4) can be set preferably to 3 mm to 25 mm, more preferably 5 mm to 15 mm.

[0022] In the present invention, the carcass cords constituting the carcass layer 4 are composed of organic fiber cords formed by twisting together bundles of organic fiber filaments. The carcass cords (organic fiber cords) used in the present invention have a breaking elongation of 20% or more, preferably 22% to 30%, and more preferably 24% to 28%, and a total fineness of 4000 dtex to 6000 dtex, and preferably 4400 dtex to 5800 dtex. By using carcass cords (organic fiber cords) having such physical properties in the carcass layer 4, it is possible to reduce the tire weight while improving snow performance (particularly steering stability on snowy roads) and shock burst resistance. The breaking elongation of the organic fiber cords can be adjusted by, for example, the number of twists, conditions such as tension in the surface treatment process of the organic fiber cords, and the selection of the material (type of organic fiber) described below.

[0023] Specifically, because the carcass cord has the above-mentioned elongation characteristics (breaking elongation), the carcass cord easily conforms to local deformation, allowing it to fully tolerate deformation during a plunger energy test (when pressed by a plunger), thereby improving breaking energy. In other words, the tread portion has improved fracture durability against protrusion input during driving, thereby improving shock burst resistance. Furthermore, because the total fineness of the carcass cord is within the above range, the rigidity of the sidewall portion 2 is ensured while suppressing the weight of the carcass cord itself, thereby enabling good snow performance (especially steering stability on snowy roads). If the carcass cord breaking elongation is less than 20%, the effect of improving shock burst resistance cannot be fully achieved. If the total fineness of the carcass cord is less than 4000 dtex, the carcass cord is too thin, making it difficult to ensure sufficient rigidity in the sidewall portion 2. If the total fineness of the carcass cord exceeds 6000 dtex, the carcass cord becomes too thick, limiting the effect of reducing tire weight.

[0024] The type of organic fiber constituting the carcass cord (organic fiber cord) is not particularly limited, but examples thereof include polyester fiber, nylon fiber, and aromatic polyamide fiber (aramid fiber), with polyester fiber being particularly preferred. Examples of polyester fiber include polyethylene terephthalate fiber (PET fiber), polyethylene naphthalate fiber (PEN fiber), polybutylene terephthalate fiber (PBT), and polybutylene naphthalate fiber (PBN), with PET fiber being particularly preferred. Whichever fiber is used, the physical properties of each fiber are advantageous for improving shock burst resistance and belt break resistance. In particular, PET fiber, being inexpensive, can reduce the cost of pneumatic tires. It can also improve workability during cord manufacturing.

[0025] In the carcass layer 4, the product A=D×Ec of the exact fineness D per carcass cord (unit: dtex / cord) and the end count Ec of the carcass cord per 50 mm in the direction perpendicular to the extension direction of the carcass cord at the tire equator CL (unit: cord / 50 mm) is preferably 1.3×10 5 dtex / 50mm~2.5×10 5 dtex / 50mm, preferably 1.5×10 5 dtex / 50mm~2.0×10 5 dtex / 50mm. The above-mentioned product A is the fineness of the carcass cord per unit width in the carcass layer 4, and by satisfying the above-mentioned range, durability and snow performance (braking performance) can be improved. When the product A is 1.3×10 5 If the product A is less than 2.5×10 5 If the value exceeds dtex / 50mm, the spacing between the carcass cords will become narrower, limiting the effect of improving durability. The range of the end count Ec is not particularly limited as long as the product A satisfies the above-mentioned range, but it can be set preferably to 30 cords / 50mm to 43 cords / 50mm, and more preferably to 35 cords / 50mm to 43 cords / 50mm.

[0026] The elongation of the carcass cord at the sidewall portion under a load of 2.0 cN / dtex is preferably 5.5% to 8.5%, more preferably 6.0% to 7.5%. If the elongation of the carcass cord at the inner circumferential side of the belt layer 7 under a load of 2.0 cN / dtex is less than 5.5%, the fatigue resistance of the carcass cord deteriorates and high-speed durability decreases. If the elongation of the carcass cord at the inner circumferential side of the belt layer 7 under a load of 2.0 cN / dtex exceeds 8.5%, the carcass cord will rise up during high-speed running and high-speed durability decreases. The "elongation under a load of 2.0 cN / dtex" of a carcass cord (polyester fiber cord) is the elongation percentage (%) of the sample cord under a load of 2.0 cN / dtex, measured in accordance with JIS L1017 (2002) "Testing Methods for Synthetic Fiber Tire Cords," by removing a carcass cord from a location that includes the widest part of the tire, attaching it to a tensile tester so that the part that corresponds to the widest part of the tire is at the center, and conducting a tensile test under conditions of a gripping distance of 100 mm and a tensile speed of 300±20 mm / min.

[0027] Furthermore, the twist factor K of the carcass cord, represented by the following formula (1), is preferably 2000 to 2500, and more preferably 2100 to 2400. Note that this twist factor K is the value of the cord after dipping. By using a cord having such a twist factor K, it is possible to improve the cord fatigue resistance and ensure excellent durability. In this case, if the twist factor K of the cord is less than 2000, the cord fatigue resistance will decrease. If the twist factor K of the cord exceeds 2500, it will be difficult to ensure cord productivity. K=T×D 1 / 2 ···(1) (In the formula, T is the number of final twists of the cord [turns / 10 cm], and D is the total fineness of the cord [dtex].)

[0028] The cover cords constituting the belt cover layer 8 are made of organic fiber cords formed by twisting together organic fiber filament bundles. The cover cords (organic fiber cords) used in the present invention have a total fineness of 900 dtex to 2300 dtex, preferably 1100 dtex to 2200 dtex. By using cover cords (organic fiber cords) having such physical properties in the belt cover layer 8, it is possible to improve the belt crease durability while reducing the tire weight. If the total fineness of the cover cords is less than 900 dtex, the cover cords are too thin, and the effect of improving the belt crease durability is limited. If the total fineness of the cover cords exceeds 2300 dtex, the cover cords are too thick, and the effect of reducing the tire weight is limited.

[0029] The type of organic fiber constituting the cover cord (organic fiber cord) is not particularly limited, but examples thereof include polyester fiber, nylon fiber, and aromatic polyamide fiber (aramid fiber), with polyester fiber being particularly preferred. Examples of polyester fiber include polyethylene terephthalate fiber (PET fiber), polyethylene naphthalate fiber (PEN fiber), polybutylene terephthalate fiber (PBT), and polybutylene naphthalate fiber (PBN), with PET fiber being particularly preferred. Whichever fiber is used, the physical properties of each fiber are advantageous for improving belt crease resistance. In particular, PET fiber, being inexpensive, can reduce the cost of pneumatic tires. It can also improve workability during cord manufacturing.

[0030] As described above, the tread rubber layer 10 is disposed on the outer peripheral side of the tire constituent members (carcass layer 4, belt layer 7, belt cover layer 8) in the tread portion 1. The glass transition temperature Tg of the tread rubber constituting this tread rubber layer 10 is set to −57°C or lower, preferably −65°C to −85°C. When the tread rubber layer 10 is composed of a cap tread layer 11 and an undertread layer 12 as in the illustrated example, the glass transition temperature Tg of the cap tread rubber constituting the cap tread layer 11, which is in contact with the road surface, satisfies the above-mentioned range. By using a rubber having such a glass transition temperature Tg for the tread rubber layer 10 (cap tread layer 11), the physical properties of the rubber in contact with the road surface are improved, thereby improving snow performance (especially steering stability on snowy roads). If the glass transition temperature Tg of the tread rubber is higher than −57°C, the effect of improving snow performance is limited. The glass transition temperature of the tread rubber can be adjusted by the types and amounts of polymers, resins, and oils blended into the rubber composition that constitutes the tread rubber. The glass transition temperature Tg is not determined solely by the blending described above, but can also be adjusted by, for example, the kneading conditions and kneading method.

[0031] In addition to having the above-mentioned glass transition temperature Tg, the tread rubber preferably has a hardness Hs at -10°C of 70 or less, more preferably 50 to 60. Using a rubber with such a hardness Hs for the tread layer 10 (cap tread layer 11) further improves the physical properties of the rubber that contacts the road surface, which is advantageous for improving snow performance (particularly steering stability on snowy roads). If the hardness Hs of the tread rubber exceeds 70, the tread rubber becomes too hard, limiting its effectiveness in improving snow performance. The hardness Hs of the tread rubber is the hardness of the rubber measured at -10°C using a Type A durometer in accordance with JIS K6253-3 (2012) "Vulcanized rubber and thermoplastic rubber - Determination of hardness - Part 3: Durometer hardness." The hardness Hs of the tread rubber can be adjusted by adjusting the amount of carbon black and resin blended into the rubber composition that constitutes the tread rubber. The hardness Hs is not determined solely by the above-mentioned composition, but can also be adjusted by, for example, the kneading conditions and the kneading method.

[0032] The pneumatic tire of the present invention uses the above-mentioned cords, while the rubber thickness Ga below the main grooves 40 is set to 2.5 mm to 4.0 mm, preferably 3.0 mm to 3.5 mm. Since the rubber thickness Ga is kept appropriately large, belt breakage can be prevented even with the above-mentioned structure, and belt breakage durability can be maintained or improved. If the rubber thickness Ga is less than 2.5 mm, belt breakage cannot be suppressed and durability cannot be sufficiently ensured. If the rubber thickness Ga exceeds 4.0 mm, tire weight cannot be reduced. The rubber thickness Ga is the thickness of the tread rubber layer 11 measured from the deepest part of the main groove 40 to the outer surface of the outermost reinforcing member (belt cover layer 8 in the illustrated example) in a direction perpendicular to the outer surface of the outermost reinforcing member. When multiple main grooves 40 are provided, the rubber thickness Ga below each main groove 40 must all satisfy the above range.

[0033] The pneumatic tire of the present invention, thanks to the cooperation of the physical properties of the above-mentioned components, can maintain good shock burst resistance and belt breakage durability and ensure good snow performance, even when the number of carcass layers 4 is reduced to one to reduce the tire weight. To achieve the effects of the present invention, at least the glass transition temperature Tg of the cap tread rubber, the total fineness of the cover cords constituting the belt cover layer 8, the breaking elongation and total fineness of the carcass cords constituting the carcass layer 4, the turn-up structure of the carcass layer 4, and the rubber thickness Ga under the main grooves 40 formed in the tread portion 1 must be satisfied. Other physical properties and structural conditions can be combined as appropriate.

[0034] As described above, the pneumatic tire of the present invention maintains good shock burst resistance and belt breakage resistance while reducing tire weight, and exhibits good snow performance, achieving a good balance of these performance characteristics, making the pneumatic tire of the present invention suitable for use as a winter tire. Furthermore, because the pneumatic tire of the present invention has excellent shock burst resistance and belt breakage resistance as described above, it can be used suitably in so-called extra load tires that can be used under high internal pressure and high load.

[0035] The present invention will be further explained below with reference to examples, but the scope of the present invention is not limited to these examples. [Example]

[0036] Pneumatic tires (test tires) of Conventional Example 1, Comparative Examples 1 to 8, and Examples 1 to 10 were manufactured, each having a tire size of 235 / 60R18 and the basic structure shown in FIG. 1 , with the number of carcass layers, the material of the carcass cord, the breaking elongation of the carcass cord, the total fineness of the carcass cord, the arrangement of the folded ends of the carcass layers, the total fineness of the cover cord, the glass transition temperature of the tread rubber, and the rubber thickness Ga under the main grooves set as shown in Tables 1 and 2.

[0037] In the "Carcass Cord Material" column in Tables 1 and 2, when the organic fiber cord constituting the carcass cord is made of nylon fiber, it is labeled "N66," and when it is made of polyethylene terephthalate fiber, it is labeled "PET." The "Carcass Cord Breaking Elongation" is the elongation (%) of the sample cord (carcass cord) at break, and was measured in accordance with JIS L1017 (2002) "Test Method for Chemical Fiber Tire Cords." The carcass cord was taken from a location including the tire's widest portion and used as the sample cord. The sample cord was attached to a tensile tester so that the portion corresponding to the tire's widest portion was the center. A tensile test was conducted under conditions of a grip distance of 100 mm and a tensile speed of 300 ± 20 mm / min. The "Position of Carcass Layer Fold-Up End" column indicated that the end of the carcass layer's folded-up portion was located in the sidewall portion, and indicated that the end was located between the main body of the carcass layer and the belt layer, as "Side."

[0038] The glass transition temperature Tg of the rubber constituting the tread rubber layer of each test tire was measured using differential scanning calorimetry (DSC) in the "Tread Rubber Glass Transition Temperature Tg" column of Tables 1 and 2. Note that since the tire in Figure 1 includes two layers, a cap tread layer and an under tread layer, the tread rubber referred to here refers to the rubber constituting the cap tread layer.

[0039] These test tires were evaluated for tire mass, snow performance, shock burst resistance, and belt breakage durability using the following evaluation methods, and the results are shown in Tables 1 and 2.

[0040] Tire mass The mass of each test tire was measured. The evaluation results were shown as the amount of change (unit: g) relative to the measured value of Conventional Example 1. Note that when the weight decreased, it was shown as a negative value.

[0041] Snow performance Each test tire was mounted on a wheel with a rim size of 18 x 7.5J, inflated to 250 kPa, and mounted on a test vehicle (four-wheel drive) with an engine displacement of 2000 cc. With two passengers on board, the test driver conducted a sensory evaluation of handling stability on a test course consisting of a snowy road surface. The evaluation results were evaluated on a 5-point scale in 0.5 increments from 0 to 5, with Conventional Example 1 being given a score of "3.0 (standard)." A higher score indicates better snow performance (handling stability on snowy road surfaces).

[0042] Shock burst resistance Each test tire was mounted on a wheel with a rim size of 18 x 7.5J, the air pressure was set to 250 kPa, and a tire destruction test (plunger destruction test) was conducted in accordance with JIS D4230:1998 "Automobile tires" in which a plunger with a plunger diameter of 19 ± 1.6 mm was pressed against the center of the tread at a load speed (plunger pushing speed) of 50.0 ± 2.5 mm / min, and the tire strength (tire destruction energy) was measured. The evaluation results were expressed as an index, with the measurement value for Conventional Example 1 being 100. A higher value means a higher destruction energy (plunger energy) and better shock burst resistance.

[0043] Belt break resistance Each test tire was mounted on a wheel with a rim size of 18 x 7.5J, and the internal pressure of the test tire was set to 250 kPa. Using a drum testing machine equipped with a smooth steel drum surface and a diameter of 1707 mm, the ambient temperature was set to 38±3°C, the load was set to 88% of the JATMA maximum load, and the running speed was accelerated from 120 km / h by 10 km / h every 30 minutes until the tire failed. After the test, 100 mm cut samples were cut from two points around the circumference of the tire and the length of the bend in the belt cord (belt bend length) was measured. The evaluation results were ranked on a five-point scale as follows: 5: Belt crease length is 0 mm (belt crease does not occur) 4: The total length of the folded belt is over 0 mm and 5 mm or less 3: The total length of the folded belt is more than 5 mm and less than 15 mm 2: The total length of the folded belt is over 15mm and less than 25mm 1: The total length of the folded belt is over 25 mm

[0044] [Table 1]

[0045] [Table 2]

[0046] As can be seen from Table 1, the tires of Examples 1 to 10, compared to Conventional Example 1, reduced tire mass, improved snow performance and shock burst resistance, and exhibited belt breakage durability equal to or greater than that of Conventional Example 1, achieving a good balance of these performances. On the other hand, in Comparative Example 1, the end of the folded-up portion of the carcass layer did not reach under the belt, resulting in reduced snow performance. In Comparative Example 2, the total fineness of the carcass cord was low, so the effect of improving snow performance was not sufficiently achieved. In Comparative Example 3, the shock burst resistance was reduced because the breaking elongation of the carcass cord was small. In Comparative Example 4, the total fineness of the cover cord was small, so the belt breakage durability was reduced. In Comparative Example 5, the total fineness of the cover cord was large, so the tire mass increased. In Comparative Example 6, the glass transition temperature of the tread rubber was high, so the effect of improving snow performance was not sufficiently achieved. In Comparative Example 7, the thickness of the rubber under the grooves was small, so the belt breakage durability was reduced. In Comparative Example 8, the thickness of the rubber under the grooves was large, so the tire mass increased.

[0047] The present disclosure encompasses the following inventions. Invention [1] A pneumatic tire comprising a tread portion extending in the tire circumferential direction and forming an annular shape, a pair of sidewall portions disposed on both sides of the tread portion, and a pair of bead portions disposed on the tire radially inward side of these sidewall portions, and having one carcass layer mounted between the pair of bead portions, multiple belt layers disposed on the outer peripheral side of the carcass layer in the tread portion, and a belt cover layer disposed on the outer peripheral side of the belt layer, The glass transition temperature Tg of the tread rubber constituting the tread portion is −57° C. or lower, The cover cord constituting the belt cover layer is an organic fiber cord having a total fineness of 900 dtex to 2300 dtex, The carcass cord constituting the carcass layer is an organic fiber cord having a breaking elongation of 20% or more and a total fineness of 4000 dtex to 6000 dtex, the carcass layer comprises a main body portion extending from the tread portion through each sidewall portion to each bead portion, and a turned-up portion turned back at each bead portion and extending toward each sidewall portion, and an end of the turned-up portion of the carcass layer is disposed between the belt layer and the main body portion of the carcass layer, The pneumatic tire is characterized in that the rubber thickness under the circumferential main grooves formed in the tread portion is 2.5 mm to 4.0 mm. Invention [2] The pneumatic tire according to invention [1], characterized in that the carcass cords constituting the carcass layer are made of polyester fibers. Invention [3] The pneumatic tire according to invention [1] or [2], characterized in that it is a winter tire. Invention [4] The pneumatic tire according to any one of inventions [1] to [3], which is an extra load tire. [Explanation of symbols]

[0048] 1 Tread section 2 Sidewall 3 Bead section 4 carcass layers 5 bead core 6 Bead filler 7 Belt Layer 8 Belt cover layer 10 Tread rubber layer 11 Cap tread layer 12 Undertread layer 20 Side rubber layer 30 Rim cushion rubber layer 40 Main groove CL Tire Equator

Claims

1. A pneumatic tire comprising a tread portion extending in a circumferential direction of the tire and forming an annular shape, a pair of sidewall portions disposed on both sides of the tread portion, and a pair of bead portions disposed on the radially inner side of the sidewall portions, the pneumatic tire having one carcass layer mounted between the pair of bead portions, a plurality of belt layers disposed on the outer peripheral side of the carcass layer in the tread portion, and a belt cover layer disposed on the outer peripheral side of the belt layer, The glass transition temperature Tg of the tread rubber constituting the tread portion is −57° C. or lower, the cover cord constituting the belt cover layer is an organic fiber cord having a total fineness of 900 dtex to 2300 dtex, the carcass cords constituting the carcass layer are organic fiber cords having a breaking elongation of 20% or more and a total fineness of 4000 dtex to 6000 dtex; the carcass layer comprises a main body portion extending from the tread portion through each sidewall portion to each bead portion, and a turned-up portion turned back at each bead portion and extending toward each sidewall portion, and an end of the turned-up portion of the carcass layer is disposed between the belt layer and the main body portion of the carcass layer, The pneumatic tire is characterized in that the rubber thickness under the circumferential main grooves formed in the tread portion is 2.5 mm to 4.0 mm.

2. 2. The pneumatic tire according to claim 1, wherein the carcass cords constituting the carcass layer are made of polyester fibers.

3. 3. The pneumatic tire according to claim 1, wherein the pneumatic tire is a winter tire.

4. 3. The pneumatic tire according to claim 1, wherein the pneumatic tire is an extra load tire.

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