WO2013088717A1

WO2013088717A1 - Pneumatic tire

Info

Publication number: WO2013088717A1
Application number: PCT/JP2012/007955
Authority: WO
Inventors: 岡田　淳一
Original assignee: 株式会社ブリヂストン
Priority date: 2011-12-12
Filing date: 2012-12-12
Publication date: 2013-06-20
Also published as: JP6118729B2; AU2012353870B2; AU2012353870A1; CN103987541B; CN103987541A; JPWO2013088717A1

Abstract

Provided is a pneumatic tire, comprising a plurality of lands, which are segmented by at least one circumferential primary groove which extends along the tire circumference in the track of the tread, and by a plurality of oblique grooves, one end of which opens in the circumferential primary groove, and which extend obliquely with respect to the tire circumference direction in plan view from the one end to another end thereof. The lands further comprise oblique wall parts on lateral wall faces which face the circumferential primary groove and form a portion of groove lateral walls of the circumferential primary groove. The incline of the oblique wall parts increases gradually in the tire circumference direction from the one end to the other end thereof.

Description

Pneumatic tire

The present invention relates to a pneumatic tire having a tread pattern that can achieve both wet performance and wear resistance performance.

The pneumatic tire has a plurality of circumferential main grooves extending along the tire traveling direction, i.e., along the tire circumferential direction, on the tread surface in order to realize stable running on not only dry road surfaces but also wet road surfaces. Is normal (see, for example, Patent Document 1), and water that has entered the tire contact surface via the circumferential main groove can be discharged out of the contact surface.

By the way, in order to improve the wet performance, it is useful to increase the groove area ratio (negative rate). However, when the groove area ratio is increased by widening the groove width of the circumferential main groove, There is a problem that rigidity is reduced in the adjacent land portion, a burden on the side edge portion on the land portion with respect to a lateral force or the like is increased, and uneven wear is increased. Therefore, the groove volume of the circumferential main groove is increased by inclining the groove side wall of the circumferential main groove with respect to the normal to the land surface so that the groove width increases toward the groove opening. On the other hand, a reduction in rigidity of the land portion adjacent to the circumferential main groove is suppressed to achieve both wet performance and uneven wear resistance (see, for example, Patent Document 2).

JP 2010-006161 A JP-A-6-001116

However, in the above-described conventional pneumatic tire in which the side wall surface of the land portion facing the circumferential main groove is inclined at a single angle and extends with the same cross-sectional shape, the inclined side wall surface is a starting point of bending. In other words, stress is likely to be concentrated when a lateral force or the like is input, uneven wear cannot be sufficiently suppressed, and further improvement in wet performance is required.

Therefore, an object of the present invention is to provide a pneumatic tire that can achieve both higher levels of wet performance and uneven wear resistance than conventional techniques.

The present invention has been made to solve the above problems, and a pneumatic tire according to the present invention includes at least one circumferential main groove extending along the tire circumferential direction on the tread surface, and the circumferential main. In a pneumatic tire provided with a plurality of land portions that are open at one end in a groove and are partitioned by a plurality of inclined grooves that are inclined and extend from the one end to the other end in a plan view. The land portion has an inclined wall surface portion on a side wall surface that faces the circumferential main groove and forms a part of the groove side wall of the circumferential main groove, and the inclination of the inclined wall surface portion is one end in the tire circumferential direction. It is characterized by gradually increasing toward the other end.

In a pneumatic tire having such a configuration, an inclined wall surface portion is provided on the side wall surface of the land portion facing the circumferential main groove, and the inclined wall surface portion is inclined from one end to the other end in the tire circumferential direction. As compared with the case where the side wall surface of the land portion facing the circumferential main groove is inclined at a single angle and extends with the same cross-sectional shape as in the prior art, the rubber volume of the land portion is increased. It is possible to increase the groove volume while suppressing the decrease in resistance, and to achieve both wet performance and uneven wear resistance at a high level. In addition, by gradually increasing the inclination of the inclined wall surface from one end to the other in the tire circumferential direction, a torsion surface is formed on the inclined wall surface portion, and when the lateral force or the like is applied to the land portion, the inclined wall surface portion Since the stress applied to the land portion is dispersed and the land portion becomes difficult to bend and deform, this point is also important in suppressing uneven wear occurring in the land portion.

In the pneumatic tire according to the present invention, it is preferable that the other end of the inclined wall surface portion having the largest inclination is connected to the one end of the inclined groove. The water flowing through the circumferential main groove can be smoothly discharged into the inclined groove, and the wet performance can be further improved. In addition, when one land portion that is not inclined at all is used, the rigidity of the land portion is increased and the wear performance is greatly improved, but the wet performance is significantly decreased. In addition, when the inclined part is eliminated and used as a part of the circumferential groove, the water flowing through the circumferential main groove flows into the inclined groove significantly smoothly, but the rigidity of the land part is greatly reduced, and the wear performance. Will drop significantly.

Furthermore, in the pneumatic tire according to the present invention, the side wall surface of the land portion rises from the groove bottom wall of the circumferential main groove and is connected to the tire radial inner edge of the inclined wall surface portion. And an upper vertical wall surface portion that rises from the tire radial direction outer edge of the inclined wall surface portion and is connected to the tread surface of the land portion, and in this way, all the side wall surfaces of the land portion are configured by the inclined wall surface portion. Compared to the case, the rubber volume of the land portion can be secured, and the corner portion and the corner portion can be formed along the contour line on the side wall surface of the land portion. Since a reinforcing effect when a lateral force or the like is applied to the portion can be exhibited, uneven wear occurring in the land portion can be further suppressed.

Moreover, in the pneumatic tire according to the present invention, the tread surface further includes a circumferential main groove in which the other end of the inclined groove opens, and the land portion has the other end of the inclined groove opened. The inclined wall surface portion has an inclined wall surface portion on a side wall surface that faces the circumferential main groove and forms a part of the groove side wall of the circumferential main groove, and the inclined wall surface portion is inclined from one end to the other end in the tire circumferential direction. It is preferable that it gradually increases toward the surface, and in this way, both wet performance and uneven wear resistance can be achieved more reliably.

In the pneumatic tire according to the present invention, it is preferable that the inclination angle of the minimum inclination portion of the inclined wall surface portion is 0 to 45 °. In this way, the wet performance and the anti-bias resistance are more reliably ensured. It is possible to achieve both wear performance.

Furthermore, in the pneumatic tire according to the present invention, it is preferable that the inclination angle of the maximum inclination portion of the inclined wall portion is 20 to 60 °, and in this way, the wet performance and the anti-bias resistance are more reliably ensured. It is possible to achieve both wear performance.

Thus, according to the present invention, it is possible to provide a pneumatic tire capable of achieving both higher wet performance and uneven wear resistance performance than in the prior art.

It is an expanded view of the tread pattern in the pneumatic tire of 1st Embodiment according to this invention. It is a top view which expands and shows the principal part of the tread pattern of FIG. It is a perspective view which shows the principal part of the tread pattern of FIG. (A) is a sectional view taken along line AA in FIG. 2, (b) is a sectional view taken along line BB in FIG. 2, and (c) is a sectional view taken along line CC in FIG. FIG. It is the top view explaining the mode of inclination using the contour line in the inclined wall surface part of the middle land part of the tread pattern of FIG. It is an expanded view of the tread pattern in the pneumatic tire of 2nd Embodiment according to this invention. It is an expanded view of the tread pattern in the pneumatic tire (tire of the comparative example 1) for comparing with this invention. (A) is a development view of a tread pattern in a pneumatic tire (the tire of Comparative Example 2) for comparison with the present invention, and (b) is a contour line showing the state of inclination in the inclined wall surface portion of the intermediate land portion. It is the top view demonstrated using FIG.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Here, the pneumatic tire according to the embodiment of the present invention includes a conventional tire structure (not shown), for example, a pair of bead portions and a pair of sidewalls extending in the tire radial direction of the bead portions. And a tread portion straddling between the sidewall portions, the bead portion, the sidewall portion and the tread portion extend into the toroidal shape inside the tire, and the end portions of the bead portions respectively. A carcass locked to a bead core embedded in the belt, and a belt disposed on the outer side in the tire radial direction of the carcass. The tire structure is not limited to this, and a reinforcing layer such as a cap layer or a layer layer may be disposed on the outer side in the tire radial direction of the belt, and the ply constituting the carcass may be either a radial ply or a bias ply. good. The pneumatic tire has a tread pattern shown below on a tread surface (hereinafter referred to as “tread surface”) 1.

As shown in FIG. 1, this pneumatic tire has at least one, here, four circumferential

main grooves

3, 4, 5, 6 extending on the tread surface 1 along the tire circumferential direction. . For convenience of explanation, the two circumferential main grooves 3 and 4 close to the tire equator plane E are referred to as “central circumferential main grooves”, respectively, and the two circumferential

main grooves

5 and 6 far from the tire equator plane E are formed. Each will be described below as a “lateral circumferential main groove”. Further, a tread surface area sandwiched between the two lateral circumferential

main grooves

5 and 6 is defined as a central area C, and areas outside the lateral circumferential

main grooves

5 and 6 in the tire width direction are respectively lateral. Region S is assumed. Accordingly, the central circumferential main grooves 3 and 4 are located in the central region C of the tread surface 1.

Here, “contact width” refers to the contact width of the tread tread surface 1 when a pneumatic tire is assembled to a normal rim, filled with a normal internal pressure, and pressed against a flat surface with a normal load applied. The regular rim means “standard rim” defined in JATMA, “Design Rim” defined in TRA, or “Measuring Rim” defined in ETRTO. Also, the normal internal pressure means “maximum air pressure” defined by JATMA, the maximum value of “TIRE LOAD LIMITS AT VARIOUS COLD INFUREATION PRESSURES” prescribed by TRA, or “INFLATION PRESSURES” prescribed by ETRTO. The normal load means “maximum load capacity” defined by JATMA, the maximum value of “TIRE LOAD LIMITS AT VARIOUS COLD INFUREATION PRESSURES” prescribed by TRA, or “LOAD CAPACITY” prescribed by ETRTO.

Further, this pneumatic tire has one end opened on the tread tread surface 1 in the central circumferential main grooves 3 and 4 and extends incline with respect to the tire circumferential direction toward the other end in plan view. A plurality of inclined grooves (hereinafter referred to as “intermediate inclined grooves”) 8 opening in the lateral circumferential

main grooves

5 and 6 are provided. Therefore, the tread tread surface 1 is a block-shaped land whose tread surface shape forms a substantially parallelogram (substantially rhombus) by the central circumferential main grooves 3, 4, the lateral circumferential

main grooves

5, 6 and the intermediate inclined groove 8. Portion (hereinafter referred to as “intermediate land portion”) 10 is formed. In addition, the intermediate inclined groove 8 extends in a slightly curved manner so as to protrude in the inclination direction here, but may extend in a straight line. Further, the intermediate land portion 10 is formed with a narrow groove 12 that connects the intermediate inclined grooves 8 adjacent to each other in the tire circumferential direction and divides the intermediate land portion 10 into two divided

land portions

10a and 10b. However, the narrow groove 12 may not be provided.

Further, the pneumatic tire includes a plurality of side inclined grooves 14 that are open in the side circumferential

main grooves

5 and 6 and the tread end TE and extend in a direction inclined in the tire circumferential direction, A plurality of side blocks 16 formed at intervals in the tire circumferential direction by the inclined grooves 14 and the side circumferential

main grooves

5 and 6 are provided.

In addition, the pneumatic tire includes a central land portion 18 that is defined between the two main circumferential grooves 3 and 4 on the tread surface 1, and the central land portion 18 further includes a tire periphery. Two

sub-grooves

20, 21 that extend along the direction and have a narrower groove width than the central circumferential main grooves 3, 4 and the lateral circumferential

main grooves

5, 6, and the two sub-grooves 20, 21 A plurality of central blocks 25 each defined by a plurality of central inclined grooves 23 that open and extend in a direction inclined in the tire circumferential direction, and ribs defined by central circumferential main grooves 3, 4 and sub-grooves 20, 21. It consists of

land portions

26 and 27.

2 to 5, the intermediate land portion 10 faces the central circumferential main grooves 3, 4, and is one of the groove side walls of the central circumferential main grooves 3, 4. The inclined wall surface portion 29 is provided on the side wall surface 28 that forms the portion, and the inclination of the inclined wall surface portion 29 (inclination with respect to the normal line of the land portion tread) is directed from one end 29a to the other end 29b in the tire circumferential direction. It is configured to gradually increase with time. In other words, in a plan view, the contour line P of the inclined wall surface portion 29 or the extension line of the contour line intersects with each other on the one end 29a side of the inclined wall surface portion 29 as indicated by a virtual line in FIG.

Further, in this embodiment, the other end 29b of the inclined wall surface portion 29 having the largest inclination is connected to the one end 8a that opens in the central circumferential main grooves 3 and 4 of the intermediate inclined groove 8, That is, the intermediate inclined groove 8 extends from the vicinity of the other end 29 b of the inclined side surface portion 29 and opens in the lateral circumferential

main grooves

5 and 6. Therefore, the central circumferential main grooves 3 and 4 and the intermediate inclined groove 8 communicate with each other via the other end 29 b of the inclined side surface portion 29 and one end 8 a of the intermediate inclined groove 8.

Furthermore, in this embodiment, as shown in FIG. 3, the side wall surface 29 of the intermediate land portion 10 facing the central circumferential main grooves 3, 4 rises from the bottom wall of the central circumferential main grooves 3, 4. A lower vertical wall surface portion 31 connected to the inner radial edge of the inclined wall surface portion 29 and an upper vertical wall surface 33 rising from the outer radial edge of the inclined wall surface portion 29 and connected to the tread surface of the intermediate land portion 10. The side wall surface 29 includes a corner 33 and a corner 37 extending along contour lines at a boundary between the inclined wall surface portion 29 and the upper vertical wall surface portion 33 and a boundary between the inclined wall surface portion 29 and the lower vertical wall surface portion 31. Is formed.

Further, in this embodiment, as shown in FIG. 1, the side wall surface 38 of the intermediate land portion 10 facing the side circumferential direction

main grooves

5 and 6 has the same configuration as the inclined wall surface portion 29 described above. An inclined wall surface 39 is provided. The inclined wall surface portion 39 on the side circumferential direction

main grooves

5 and 6 side has, as one end 39a, a position substantially corresponding to the other end 29b of the inclined wall surface portion 29 on the center circumferential direction main groove 3 and 4 side in the tire width direction. The inclination is gradually increased from the one end 39a toward the other end 39b. That is, the end point position (the other end 29b) of the inclined wall surface portion 29 formed on the side wall surface 28 facing the central circumferential main grooves 3 and 4 of the intermediate land portion 10, and the lateral circumferential direction of the intermediate land portion 10 The starting point position (one end 39a) of the inclined wall surface portion 39 formed on the side wall surface 38 facing the

main grooves

5 and 6 substantially coincides with the tire width direction. Further, the side wall surface 38 on the side circumferential direction

main grooves

5, 6 side of the intermediate land portion 10 rises from the bottom wall of the central circumferential direction main grooves 3, 4 in the same manner as the side wall surface 29, and It has a lower vertical wall surface portion connected to the inner edge in the tire radial direction and an upper vertical wall surface portion that rises from the tire radial direction outer edge of the inclined wall surface portion and is connected to the tread surface of the intermediate land portion. Corners and corners extending along the contour lines are formed (not shown).

In the pneumatic tire having such a configuration, an inclined wall surface portion 29 is provided on the side wall surface 28 of the intermediate land portion 10 facing the central circumferential main grooves 3 and 4, and the inclined wall surface portion 29 is inclined. Since the torsional surface is formed on the inclined wall surface portion 29 by gradually increasing from the one end 29a to the other end 29b in the circumferential direction, the stress applied to the inclined wall surface portion 29 when a lateral force or the like is applied to the intermediate land portion 10 As a result, it is difficult for the intermediate land portion 10 to bend and deform, and uneven wear that occurs in the intermediate land portion 10 can be suppressed. At the same time, as compared with the case where the side wall surface of the land portion facing the circumferential main groove is inclined at a single angle and extends with the same cross-sectional shape as in the prior art, the rubber volume of the intermediate land portion 10 is secured. However, since the groove volume can be increased, the wet performance can be improved.

Further, according to the pneumatic tire of the above embodiment, the inclination wall surface portion 29 of the intermediate land portion 10 gradually increases in inclination from one end 29a to the other end 29b, and the inclination wall surface portion 29 Since the other end 29a having the largest inclination is connected to the one end 8a of the intermediate inclined groove 8, water flowing through the central circumferential main grooves 3 and 4 can be smoothly discharged to the intermediate inclined groove 8, and more. The wet performance can be further improved.

Furthermore, according to the pneumatic tire of the above embodiment, in addition to the inclined wall surface portion 29, the inclined wall surface portion 29 rises from the groove bottom wall of the central circumferential main grooves 3, 4 on the side wall surface 28 of the intermediate land portion 10. The lower vertical wall surface portion 31 connected to the inner radial edge of the tire and the upper vertical wall surface portion 33 rising from the outer radial edge of the inclined wall surface portion 29 and connected to the tread surface of the intermediate land portion 10 are provided. Compared to the case where all of the 10 side wall surfaces 28 are configured by the inclined wall surface portion 29, the rubber volume of the intermediate land portion 10 can be secured, and the side wall surface 28 of the intermediate land portion 10 has a corner portion 35 along the contour line. And the corner portion 37 can be formed. The corner portion 35 and the corner portion 37 can exhibit a reinforcing effect when a lateral force or the like is applied to the intermediate land portion 10. Further suppress uneven wear It is possible.

Further, according to the pneumatic tire of this embodiment, the intermediate land facing the central circumferential main grooves 3 and 4 is also formed on the side wall surface 38 of the intermediate land portion 10 facing the lateral circumferential

main grooves

5 and 6. Since the inclined wall surface portion 39 similar to the inclined wall surface portion 29 formed on the side wall surface 28 of the portion 10 is provided, the drainage performance can be improved also in the side circumferential direction

main grooves

5 and 6. The wet performance can be improved. Furthermore, in this embodiment, the end point position (the other end 29b) of the inclined wall surface portion 29 formed on the side wall surface 28 facing the central circumferential main grooves 3 and 4 of the intermediate land portion 10 and the intermediate land portion 10 Since the starting point position (one end 39a) of the inclined wall surface portion 39 formed on the side wall surface 38 facing the lateral circumferential

main grooves

5 and 6 is made substantially coincident when viewed in the tire width direction, the intermediate land portion 10 The change in the cross-sectional area along the tire width direction from one end to the other end in the tire circumferential direction can be minimized, and as a result, the deformation of the intermediate land portion 10 when a lateral force or the like is applied to the intermediate land portion 10. Therefore, uneven wear is further suppressed.

Incidentally, the range of the inclination angle θ of the minimum inclined portion (in this example, one end 29a) in the inclined wall surface portion 29 is preferably 0 to 45 °. When the inclination angle θ exceeds 45 °, the groove volume increases and it may be difficult to ensure rigidity. In addition, the range of the inclination angle θ of the maximum inclination portion (the other end 29b in this example) in the inclined wall surface portion 29 is preferably 20 ° to 60 °. If the inclination angle θ is less than 20 °, there is a risk that the groove volume is insufficient and sufficient drainage performance may not be obtained. If the inclination angle θ exceeds 60 °, the groove volume increases and the rigidity is secured. May become difficult.

Although the present invention has been described based on the illustrated examples, the present invention is not limited to the above-described embodiment, and can be appropriately changed within the scope of the claims. For example, in the above-described embodiment, Although an example of a point-symmetric tread pattern with respect to the tire equatorial plane E has been shown, this invention can also be applied to a line-symmetric tread pattern or an asymmetric tread pattern.

Next, the pneumatic tires according to the present invention (the tires of Examples 1 to 19) and the comparative pneumatic tire (the tires of Comparative Examples 1 and 2) were prototyped and tested for wet performance and uneven wear resistance performance. This will be described below. The tires of Examples 1 to 19 and the tires of Comparative Examples 1 and 2 are all radial tires having a tire size of 225 / 65R17, and the test was performed on a rim having a size of 7 × 17 and an internal pressure of 210 kPa (relative pressure) was applied. Applied.

Here, the tire of Example 1 has the tread pattern shown in FIG. 1 on the tread surface, and specifically has an inclined wall surface portion on the side wall surface of the intermediate land portion facing the main groove in the central circumferential direction. The inclined wall surface portion gradually increases from one end to the other end in the tire circumferential direction. In addition, the intermediate land portion has an inclined wall surface portion on the side wall surface facing the central circumferential main groove. The inclined wall surface portion gradually increases from the one end to the other end with the substantially other end position of the inclined wall surface portion on the central circumferential main groove side as one end. Other specifications of the tire of Example 1 are as shown in Table 1.

Further, the tires of Examples 2 to 19 have the tread pattern shown in FIG. 6 on the tread surface, and specifically, the inclined wall surface portion is provided only on the side wall surface of the intermediate land portion facing the central circumferential main groove. The inclined wall surface portion gradually increases from one end to the other end in the tire circumferential direction. Other specifications of the tires of Examples 2 to 19 are as shown in Table 1.

Further, the tire of Comparative Example 1 has the tread pattern shown in FIG. 7 on the tread surface, and specifically, the side wall surface of the intermediate land portion facing the central circumferential main groove is the central circumferential main groove. The side wall surface that is perpendicular to the groove bottom surface and faces the lateral circumferential main groove of the intermediate land portion is perpendicular to the groove bottom surface of the central circumferential main groove. Other specifications of the tire of Comparative Example 1 are as shown in Table 1.

Further, the tire of Comparative Example 2 has the tread pattern shown in FIG. 8 on the tread tread surface. Specifically, the side wall surface of the intermediate land portion facing the central circumferential main groove has a uniform inclination angle. The side wall surface of the intermediate land portion facing the lateral circumferential main groove is perpendicular to the groove bottom surface of the central circumferential main groove. Other specifications of the tire of Comparative Example 2 are as shown in Table 1.

And, the wet performance was evaluated based on the measured critical speed by measuring the critical speed at which a hydroplaning phenomenon occurred on a wet road surface with a water depth of 10 mm. The evaluation results are shown in Table 2. The evaluation in Table 2 is expressed as an index for the tires of Examples 1 to 19 and the tire of Comparative Example 2 with the result of Comparative Example 1 being 100, and the larger the value, the better the wet performance.

In addition, the uneven wear resistance performance is the amount of wear (step amount) at the side edge of the intermediate land adjacent to the central circumferential main groove when traveling on a general road in a dry state in various travel modes and traveling at 10,000 km. Was measured and evaluated from the measured wear amount. The evaluation results are shown in Table 2. The evaluation in Table 2 is the index of the tires of Examples 1 to 19 and the tire of Comparative Example 1 with the result of the tire of Comparative Example 2 being 100. The larger the value, the better the uneven wear resistance. It shows that.

Thus, according to the present invention, it is possible to provide a pneumatic tire that can achieve higher levels of both wet performance and uneven wear resistance performance than the prior art.

1 Tread tread 3, 4 Central circumferential main groove (circumferential main groove)
5,6 Side circumferential main groove (circumferential main groove)
8 Intermediate inclined groove (inclined groove)
10 Middle land (land)
28 Side wall surface 29 Inclined wall surface portion 31 Lower vertical wall surface portion 33 Upper vertical wall surface portion 35 Corner portion 37 Corner portion 38 Side wall surface 39 Inclined wall surface portion

Claims

At least one circumferential main groove extending along the tire circumferential direction on the tread surface, and one end opened in the circumferential main groove, and the tire circumferential direction from the one end to the other end in plan view In a pneumatic tire including a plurality of land portions partitioned by a plurality of inclined grooves extending in an inclined manner,
The land portion has an inclined wall surface portion on a side wall surface that faces the circumferential main groove and forms a part of the groove side wall of the circumferential main groove,
The pneumatic tire is characterized in that the inclination of the inclined wall surface portion gradually increases from one end to the other end in the tire circumferential direction.
The pneumatic tire according to claim 1, wherein the other end of the inclined wall surface portion having the largest inclination is connected to the one end of the inclined groove.
The side wall surface of the land portion rises from a groove bottom wall of the circumferential main groove and rises from a lower side wall surface portion connected to a tire radial inner edge of the inclined wall surface portion and a tire radial outer edge of the inclined wall surface portion. The pneumatic tire according to claim 1, further comprising an upper vertical wall surface portion connected to the tread surface of the land portion.
The tread surface further includes a circumferential main groove in which the other end of the inclined groove opens, and the land portion faces the circumferential main groove in which the other end of the inclined groove opens. It has an inclined wall surface portion on the side wall surface forming a part of the groove side wall of the main groove,
The pneumatic tire according to any one of claims 1 to 3, wherein the inclination of the inclined wall surface portion gradually increases from one end to the other end in the tire circumferential direction.