JP2008120232A - Pneumatic tire - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve performances on snow and on ice in a well-balanced manner. <P>SOLUTION: In this pneumatic tire, a tread part 2 is provided with at least four longitudinal main grooves 3 including a pair of inner longitudinal main grooves 3a arranged on both sides of the tire equator and continuously extended in the tire circumferential direction and a pair of outer longitudinal main grooves 3b arranged in the outer side of the inner longitudinal main grooves 3a and continuously extended in the tire circumferential direction. To the inner longitudinal main grooves 3a, lateral grooves 4a, 4b are opposingly connected from the tire equator C side and the tread grounding end E side, so as to form inner groove crossing parts 7 with cross shapes. To the outer longitudinal main grooves 3b, lateral grooves 4b, 4c are opposingly connected from the tire equator C side and the tread grounding end E side, so as to form outer groove crossing parts 8 with cross shapes. The width of the lateral groove in the outer groove crossing part 8 is set larger than that in the inner groove crossing part 7. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a pneumatic tire that can improve performance on snow and performance on ice in a well-balanced manner.

  Since the use of spiked tires has been banned, there are increasing opportunities for snow-covered slippery roads, called mirrorburns, in urban areas, especially in urban areas. For example, winter tires, snow tires or studless tires with improved driving performance on icy roads and snowy roads can be improved with respect to the road surface by reducing the groove area of the tread part in order to improve the driving performance on such mirror burns. Increasing the ground contact area and, in turn, increasing the frictional force.

On the other hand, in this type of tire, a driving force on the snow is obtained by a reaction force when the snow on the road surface is pressed and hardened in the groove of the tread portion to form a snow column and sheared. Therefore, if the groove area of the tread portion is reduced in order to improve the performance on ice, there is a problem that the above-mentioned performance on snow, particularly the running performance on a deep snow road, is remarkably deteriorated. Related technologies include the following.
JP2003-63211A JP-A-2005-47397

  The present invention has been devised in view of the above-described problems. In the tread portion, an inner groove intersecting portion and an outer intersecting portion forming a cross road shape are formed by intersecting a longitudinal main groove and a transverse groove. In addition, on the basis of forming the groove width of the lateral groove at the outer groove intersection portion larger than the groove width of the lateral groove at the inner groove intersection portion, a large frictional force is obtained in the crown region having a large road surface contact length. In the shoulder region, the main object is to provide a pneumatic tire capable of improving the performance on snow and the performance on ice in a high level in a well-balanced manner by exhibiting a large snow column shear force.

  According to the first aspect of the present invention, the tread portion has a pair of longitudinal main grooves disposed on both sides of the tire equator and continuously extending in the tire circumferential direction, and disposed on the outer side thereof and in the tire circumferential direction. A pneumatic tire provided with at least four longitudinal main grooves including a pair of outer longitudinal main grooves extending continuously to each other, wherein each of the longitudinal main grooves includes a tire equator side and a tread grounding end. The lateral grooves are connected to each other from the side to form a crossing portion of the inner groove that forms a cross road, and the outer vertical main grooves have lateral grooves from the tire equator side and the tread grounding end side, respectively. An outer groove crossing portion having a cross road shape is formed by being connected to face each other, and the groove width of the lateral groove at the outer groove intersection portion is larger than the groove width of the horizontal groove at the inner groove intersection portion. Features.

  The lateral groove connected to the outer groove intersection from the tire equator side is at least 7 mm inward in the tire axial direction from the connection position to the outer groove intersection. The pneumatic tire according to claim 1, wherein the tire extends without decreasing.

  According to a third aspect of the present invention, the lateral groove connected to the inner groove intersecting portion includes a widened portion connected to the inner vertical main groove by enlarging the groove width in a trumpet shape. The pneumatic tire described.

  According to a fourth aspect of the present invention, the horizontal groove includes a middle horizontal groove that connects the inner vertical main groove and the outer vertical main groove, and the middle horizontal groove extends from the inner intersection to the outer side. The pneumatic tire according to any one of claims 1 to 3, including a gradually increasing portion in which the groove width gradually increases toward the intersecting portion.

  According to a fifth aspect of the present invention, the groove width of the outer vertical main groove is greater than 1.0 times and not greater than 1.3 times the groove width of the inner vertical main groove. The pneumatic tire according to any one of the above.

  In the pneumatic tire of the present invention, the longitudinal main groove and the lateral groove intersect with each other in the tread portion, so that an inner groove intersecting portion and an outer intersecting portion forming a cross road shape are formed. Such a crossroad-shaped groove intersection can form a more rigid snow column than a three-pronged groove intersection, and obtains a large driving force on the snow by shearing it. To help. Further, in the present invention, the groove width of the lateral groove at the outer groove intersection portion is formed larger than the groove width of the lateral groove at the inner groove intersection portion. For this reason, in the region on the tire equator side including the inner intersection, the contact area is increased and a large frictional force is ensured. On the other hand, in the region on the tread ground contact side including the outer groove intersection, the groove volume can be increased and the snow column shear force can be further increased. Thereby, the pneumatic tire of this invention can make the performance on snow and the performance on ice compatible in a high dimension.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a development view of a tread portion 2 of a studless tire for a passenger car as a pneumatic tire (not shown) as a pneumatic tire according to the present embodiment.

  The tread portion 2 is provided with a plurality of longitudinal main grooves 3 extending continuously in the tire circumferential direction and lateral grooves 4 extending in a direction intersecting with the longitudinal main grooves 3. Thus, a block pattern in which a plurality of blocks 5 surrounded by the vertical main grooves 3 and the horizontal grooves 4 (or the vertical main grooves 3, the horizontal grooves 4, and the tread grounding end E) is divided is formed in the tread portion 2. The tread rubber forming this pattern is desirably formed of a soft rubber having a JISA hardness of 55 ° or less, more preferably 50 ° or less, in order to improve the followability to the road surface on an icy road.

  The longitudinal main grooves 3 are arranged on both sides of the tire equator C and continuously extend in the tire circumferential direction, and a pair of longitudinal main grooves 3a arranged on the outer side and continuously extend in the tire circumferential direction. It consists of an outer vertical main groove 3b. In the present embodiment, a total of four vertical main grooves 3 are provided.

  Each of the longitudinal main grooves 3a and 3b forms a preferred mode extending linearly along the tire circumferential direction. Such a vertical main groove improves snow discharge performance when traveling straight on a snowy road, and can exert a large snow column shear force even when turning with a slip angle, which helps to improve steering stability. Needless to say, the vertical main groove 3 includes various embodiments such as a curved shape, a wavy shape, and / or a zigzag shape. However, in consideration of wear resistance, performance on snow, etc., the vertical main groove 3 is a zigzag shape having a linear shape or a gentle slope ( For example, the maximum inclination angle with respect to the tire circumferential direction is preferably 15 ° or less.

  The groove widths GW1 and GW2 of the inner and outer longitudinal main grooves 3a and 3b are not particularly limited. However, in order to ensure sufficient snow drainage and formation of a large snow column, preferably tread grounding It is desirable that the width TW is 3.0% or more, more preferably 4.5% or more. However, if the groove width GW1 or GW2 becomes excessively large, the contact area of the tread portion 2 may be reduced, and the running performance on an icy road may be deteriorated. From this point of view, the groove widths GW1 and GW2 are preferably 7.5% or less, more preferably 6.0% or less of the tread ground contact width TW. The depth of each longitudinal main groove 3a, 3b is preferably 7.5 mm or more, more preferably 8.5 mm or more, and preferably 11.5 mm or less, more preferably 10.0 mm or less.

  Here, the “tread contact width” TW is a tire axial distance between the tread contact ends E, E of the tread portion 2. The “tread grounding end” E is a grounding when a normal load is applied to a tire in a normal state with a normal rim assembled to a normal rim and filled with a normal internal pressure and pressed against a flat surface with a camber angle of 0 °. The end.

  The “regular rim” is a rim determined for each tire in a standard system including a standard on which the tire is based, for example, a standard rim for JATMA, “Design Rim” for TRA, For ETRTO, use “Measuring Rim”.

  The “regular internal pressure” is the air pressure defined by each standard for each tire in the standard system including the standard on which the tire is based. The maximum air pressure for JATMA and the table “TIRE LOAD for TRA” The maximum value described in “LIMITS AT VARIOUS COLD INFLATION PRESSURES” is “INFLATION PRESSURE” if it is ETRTO, but it is uniformly 180 kPa if the tire is for passenger cars.

  Furthermore, the “regular load” is a load determined by each standard for each tire in the standard system including the standard on which the tire is based. The maximum value described in “LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” is “LOAD CAPACITY” in the case of ETRTO.

  As described above, in order to obtain the frictional force on the icy road, a sufficient ground contact area is required. For this reason, it is desirable that the vertical main grooves 3 are composed of a total of four as in this embodiment. That is, when the total number of the longitudinal main grooves 3 provided in the tread portion 2 is 3, there is a strong tendency that the groove volume is reduced and sufficient on-snow performance cannot be obtained. On the contrary, when the total number of the longitudinal main grooves 3 is 5 or more, the ground contact area is lowered, and the performance on ice is likely to deteriorate. In particular, when the total number of the longitudinal main grooves 3 is an odd number, the longitudinal main grooves 3 are usually formed on the tire equator. Such a configuration is advantageous for performance on snow, but there is a reduction in performance on ice. There is a tendency to grow.

  Further, the arrangement position of each vertical main groove 3 is not particularly limited, but in order to prevent a large deviation in the contact area, the inner vertical main groove 3a has a tread contact width TW from the tire equator C to the outer side in the tire axial direction. 5% and 15% or less of the outer longitudinal main groove 3b, and the outer longitudinal main groove 3b has a groove center in the region of 25% or more and 39% or less of the tread contact width TW outward from the tire equator C in the tire axial direction. It is desirable to arrange so that it has.

  The transverse groove 4 traverses between a pair of inner longitudinal main grooves 3a, 3a and a center transverse groove 4a spaced in the tire circumferential direction and between the inner longitudinal main groove 3a and the outer longitudinal main groove 3b. In addition, it includes middle lateral grooves 4b that are spaced apart in the tire circumferential direction, and shoulder lateral grooves 4c that are transversely spaced between the outer vertical main grooves 3b and the tread ground contact edge E and spaced in the tire circumferential direction. In the present embodiment, each of the lateral grooves 4a to 4c is spaced apart in the tire circumferential direction at substantially the same pitch.

  Each of the lateral grooves 4a to 4c is preferably inclined at a relatively small angle θ of, for example, 45 ° or less, preferably 35 ° or less with respect to the tire axial direction. When the angle θ is increased, the driving force during straight traveling on snow may be reduced.

  The groove widths GW3, GW4, and GW5 of the center lateral groove 4a, middle lateral groove 4b, and shoulder lateral groove 4c are not particularly limited. However, if the width is too small, the driving force on the snow may be reduced. There is a risk that the area will decrease and the driving performance on icy roads will decrease. From such a viewpoint, the groove widths GW3 to GW5 are preferably 3.0 mm or more, more preferably 4.0 mm or more, and preferably 10.0 mm or less, more preferably 8.0 mm or less.

  The block 5 formed in the tread portion 2 by the vertical main groove 3 and the horizontal groove 4 as described above includes a center block 5a between the pair of inner vertical main grooves 3a and 3a, the inner vertical main groove 3a, and the outer A middle block 5b between the vertical main groove 3b and a shoulder block 5c between the outer vertical main groove 3a and the tread grounding end E are included. Each block 5 is preferably provided with a plurality of sipings S extending in the tire axial direction and spaced apart in the tire circumferential direction.

  Further, in the present embodiment, the middle block 5b includes a middle block 5bi disposed on the tire equator C side and a tread by an inclined auxiliary groove 6 that extends between the middle lateral grooves 4b and 4b with respect to the tire circumferential direction. The outer middle block 5bo arranged on the ground end E side is divided into two. The inclined auxiliary groove 6 is formed with a groove width GW6 in the tire axial direction smaller than the inner and outer vertical main grooves 3a and 3b. The groove width GW6 of the inclined auxiliary groove 6 is not particularly limited, but is preferably formed at 1.5 to 2.0% of the tread grounding width TW.

  Furthermore, although the tread pattern of the present embodiment is shown to be composed entirely of blocks 5, it goes without saying that it may include, for example, ribs.

  In addition, the inner main groove 3a is connected to the center lateral groove 4a from the tire equator C side and the middle lateral groove 4b from the tread grounding end E side so as to face each other. The part 7 is repeatedly formed in the tire circumferential direction. The outer longitudinal main groove 3b is connected to the middle lateral groove 4b from the tire equator C side and the shoulder lateral groove 4c from the tread grounding end E side to face each other, thereby forming an outer groove intersection portion forming a cross road shape. 8 is repeatedly formed in the tire circumferential direction.

  Here, the lateral grooves are connected to “opposite”, for example, when the inner groove intersecting portion 7 is taken as an example, as shown in FIG. Virtual grooves 4ao and 4bo are defined by extending 4a and the middle horizontal groove 4b along the groove shape, which means that they have an overlapping portion X that overlaps at least partially.

  When the vehicle travels on a deep snow road, the snow is crushed in the shape of a cross inside the inner groove intersection 7 and the outer groove intersection 8 described above. Such a cross-shaped snow column has great rigidity. Therefore, a large driving force can be obtained in the tire by the reaction force generated when the vertical main grooves 3 or the horizontal grooves 4 shear the same. In addition, when the inner groove crossing part 7 or the outer groove crossing part 8 forms a three-way shape, the rigidity of the pressed snow column is small, and such an effect cannot be sufficiently obtained.

  In the pneumatic tire of the present invention, the groove width Wo of the lateral groove at the outer groove intersection portion 8 is formed larger than the groove width Wi of the lateral groove at the inner groove intersection portion 7.

  Here, the “groove width Wi of the lateral groove at the inner groove intersecting portion 7” is measured in the tire circumferential direction at the connection position to the inner vertical main groove 3 a as shown in an enlarged view in FIG. The average values of the groove widths GW3o and GW4i of the center lateral groove 4a and the middle lateral groove 4b are used. Similarly, the “groove width Wo of the lateral groove at the outer groove intersection 8” is measured in the tire circumferential direction at the connection position to the outer vertical main groove 3b, as shown in an enlarged manner in FIG. The average values of the groove widths GW4o and GW5i of the middle lateral grooves 4b and the shoulder lateral grooves 4c are set.

  Thus, by making the groove width Wo at the outer groove intersection portion 8 larger than the groove width Wi of the lateral groove at the inner groove intersection portion 7, the outer groove intersection portion 8 is larger when running on a snowy road. While it is possible to form a cross-shaped snow column and give priority to improving the running performance on snowy roads, at the inner groove intersection 7, the width of the horizontal groove is reduced to secure a large ground contact area, and eventually the icy road In particular, priority can be given to improving the running performance in ice-burning.

  FIG. 4 shows a ground contact shape when a normal load is applied to the normal state of the tread portion 2 of the pneumatic tire of the present embodiment. In order to improve the performance on ice, it is effective to increase the contact area of the crown region Cr near the tire equator where the contact pressure is high and the contact length in the tire circumferential direction is large in order to obtain a large frictional force on the road surface. Therefore, in the present invention, the on-ice performance can be effectively improved by making the groove width Wi of the lateral groove at the inner groove intersection portion 7 smaller than the groove width Wo of the lateral groove at the outer groove intersection portion 8. In other words, even if the groove width Wi of the lateral groove at the inner groove intersection portion 7 is larger than the groove width Wo of the lateral groove at the outer groove intersection portion 8, the shoulder area SH originally has a small ground contact area. Such improvement on ice cannot be expected.

  For example, in the inner groove intersecting portion 7, the groove width GW3o of the center lateral groove 4a connected thereto and the groove width GW4i of the middle lateral groove 4b do not have to be the same. However, if the difference between the groove widths GW3o and GW4i is too large, in the snow pillar formed at the inner groove crossing portion 7, the rigidity of the portion formed by the portion having the smaller groove width is reduced, and as a result, the shear force is reduced. It easily breaks down when received. This may reduce the driving force on the snow, and the snow is likely to be clogged in the groove. From this point of view, the ratio (GW3o / GW4i) of the groove width GW3o of the center lateral groove 4a and the groove width GW4i of the middle lateral groove 4b at the inner groove intersection 7 is preferably 0.5 to 2.0. . Similarly, the ratio (GW4o / GW5i) of the groove width GW4o of the middle lateral groove 4b and the groove width GW5i of the shoulder lateral groove 4c at the outer groove intersection 8 is preferably 0.5 to 2.0.

  Moreover, if the difference between the groove width Wi of the lateral groove at the inner groove intersection 7 and the groove width Wo of the lateral groove at the outer groove intersection 8 becomes excessively large, the balance between the performance on ice and the performance on snow will be lost, and the two It tends to be difficult to balance performance. In addition, when the difference between the groove width Wi of the lateral groove at the inner groove intersecting portion 7 and the groove width Wo of the lateral groove at the outer groove intersecting portion 8 is approximate, there is a possibility that the performance on ice may not be sufficiently improved. From such a viewpoint, the ratio (Wo / Wi) of the groove width Wi of the groove width at the inner groove intersection 7 and the groove width Wo of the lateral groove at the outer groove intersection 8 is preferably 1.2 or more, While more preferably 1.3 or more, it is preferably 1.6 or less, more preferably 1.5 or less.

  3B, the middle lateral groove 4b connected to the outer groove intersection 8 from the tire equator C side is at least inward in the tire axial direction from the connection position J to the outer groove intersection 8. It is desirable to include a uniform width portion 9 that extends without decreasing in the groove width GW4o at the connection position J in the intersection vicinity region L of 7 mm, more preferably mm. Thereby, the rigidity of the cross-shaped snow column formed in the outer groove crossing portion 8 is increased, and the performance on snow is further improved.

  From the same viewpoint, the shoulder lateral groove 4c connected to the outer groove intersection 8 from the tread grounding end E side is also at least 7 mm, more preferably mm, inward in the tire axial direction from the connection position K to the outer groove intersection 8. In the intersection vicinity region L, it is desirable that the groove width GW5i at the connection position K extends without decreasing. Thereby, the rigidity of the cross-shaped snow column formed by the outer groove crossing portion 8 is further increased, and the performance on snow is further improved. Note that the width of the shoulder lateral groove 4c may be constant, or may be gradually increased toward the tread grounding end E side.

  Further, as shown in FIG. 3A, the center lateral groove 4a and middle lateral groove 4b connected to the inner groove intersecting portion 7 are in a trumpet shape, that is, the groove width is locally expanded to increase the length of the inner main groove 4a. It is desirable to include the widened portion 10 connected to the groove 3a. Such a trumpet-shaped widened portion 10 can effectively increase the rigidity of the crossing portion of the cross-shaped snow column formed by the inner groove crossing portion 7, and thus can improve the performance on snow without deteriorating the performance on ice. . However, when the length Lr of the trumpet-shaped widened portion 10 in the tire axial direction is increased, the contact area in the center region of the tread is reduced, and there is a concern that the performance on ice may be deteriorated. From this point of view, the length Lr of the widened portion 10 is preferably 10 mm or less. In order to increase the snow column strength, the groove width expansion ratio of the trumpet-shaped widened portion 10 (ratio between the maximum width and the minimum width in the widened portion) is preferably about 1.2 to 2.0.

  Further, as shown in FIG. 1, the middle lateral groove 4 b of the present embodiment has an inner groove intersecting portion 8 and an outer groove intersecting portion 8 between the trumpet-shaped widened portion 10 and the equal width portion 9. The gradual increase part 11 which the groove | channel width increases gradually toward is included. In other words, the middle lateral groove of the present embodiment includes a trumpet-shaped widened portion 10 in which the groove width is locally reduced from the inner groove intersecting portion 7 toward the outer groove intersecting portion 8, and the trumpet-shaped widened portion 10. And a gradually increasing portion 11 in which the groove width gradually increases toward the outer groove intersecting portion 8 side, and a substantially constant groove width (maximum width of the middle transverse groove) extending to the outer groove intersecting portion 8 and continuing to the outer groove intersecting portion 8 ) And an equal width portion 9 extending. Such a middle lateral groove 4b, except for the trumpet-shaped widened portion on the inner side in the tire axial direction, smoothly changes the groove width, so that it forms a snow column that is not easily broken inside and helps to improve the performance on snow. .

  Further, in order to increase the ground contact area in the vicinity of the inner groove intersection portion 7 and to enlarge the groove volume at the outer groove intersection portion 8, the groove width GW2 of the outer vertical main groove 3b is set to It is desirable that the groove width GW1 of the main groove 3a is at least 0.9 times, more preferably more than 1.0 times, and even more preferably 1.1 times or more. On the other hand, if the groove width GW2 of the outer vertical main groove 3b is too large, the frictional force in the vicinity of the outer groove intersecting portion 8 may be reduced on ice. Therefore, the groove width of the inner vertical main groove 3a is preferable. The GW1 is preferably 1.3 times or less, more preferably 1.2 times or less.

  Although the embodiments of the present invention have been described above, it is needless to say that the present invention is not limited to the specific embodiments illustrated and can be implemented in various forms.

Studless tires for passenger cars (size 195 / 65R15) having the tread pattern shown in FIG. 1 were manufactured based on the specifications shown in Table 1, and tested for on-ice performance and on-snow performance. The main common specifications and test methods are as follows.
<Common specifications>
Tread contact width TW: 168mm
Inner vertical main groove depth: 9.0mm
Outside vertical main groove depth: 9.0mm
Center lateral groove depth: 9.0mm
Middle lateral groove depth: 9.0mm
Shoulder lateral groove depth: 9.0mm
<Ability on ice>
Each test tire is fitted on all wheels of a 2500cc domestic FR passenger car under the conditions of a rim of 15 x 6 J and an internal pressure of 230 kPa. It was conducted. In the braking test, the vehicle is run at a speed of 30 km / h on the test road surface, sudden braking with four wheels locked is applied, the braking distance until the car stops is measured three times for each tire, and the average value is calculated. did. The evaluation is indicated by an index with the average braking distance of Comparative Example 1 being 100. The larger the value, the shorter the braking distance and the better the performance.

<Snow performance>
Running on a deep snow test course with the above vehicle equipped with each test tire, the straight running stability and steering stability on the snow were evaluated with a score of Comparative Example 1 as 100 based on the driver's sensuality. The larger the value, the better. The test results are shown in Table 1.

As a result of the test, it was confirmed that the tires of the examples had both high performance on snow and high performance on ice.

It is an expanded view of the tread part which shows one Embodiment of this invention. FIG. (A) is an enlarged view of an inner groove crossing portion, and (b) is an enlarged view of an outer groove crossing portion. It is a top view which shows the contact shape of a tread part.

Explanation of symbols

2 Tread portion 3 Vertical main groove 3b in the vertical main groove 3b Outside vertical main groove 4 Horizontal groove 4a Center lateral groove 4b Middle lateral groove 4c Shoulder lateral groove 5 Groove intersection 8 in the block 7 Outside groove intersection C Tire equator E Tread grounding end

Claims (5)

A pair of inner longitudinal main grooves that are arranged on both sides of the tire equator in the tread portion and extend continuously in the tire circumferential direction, and a pair of outer longitudinal main grooves that are arranged on the outer side and extend continuously in the tire circumferential direction. A pneumatic tire provided with at least four longitudinal main grooves including:
Each of the longitudinal main grooves in each of the above is formed with a crossing portion of the groove that forms a cross road by connecting the lateral grooves facing each other from the tire equator side and the tread grounding end side,
Each outer vertical main groove is formed with an outer groove intersecting portion forming a cross road shape by connecting the lateral grooves facing each other from the tire equator side and the tread grounding end side,
Moreover, the pneumatic tire is characterized in that the groove width of the lateral groove at the outer groove intersection portion is larger than the groove width of the lateral groove at the inner groove intersection portion.   The lateral groove connected to the outer groove intersection from the tire equator side extends at least 7 mm inward in the tire axial direction from the connection position to the outer groove intersection without extending the groove width. The pneumatic tire according to 1.   3. The pneumatic tire according to claim 1, wherein the lateral groove connected to the inner groove intersecting portion includes a widened portion connected to the inner vertical main groove by enlarging the groove width in a trumpet shape. The transverse groove includes a middle transverse groove that connects between the inner longitudinal main groove and the outer longitudinal main groove,
The pneumatic tire according to any one of claims 1 to 3, wherein the middle lateral groove includes a gradually increasing portion in which a groove width gradually increases from an inner intersecting portion toward an outer intersecting portion.   The pneumatic tire according to any one of claims 1 to 4, wherein a groove width of the outer vertical main groove is larger than 1.0 times and 1.3 times or smaller than a groove width of the inner vertical main groove.