Pneumatic tire

Abstract

To provide a pneumatic tire with a wide and small diameter capable of improving riding comfort performance.SOLUTION: Provided is a pneumatic tire 1 comprising a tread part 2 that includes a tread rubber 2G forming a ground contact surface 2s and a belt layer 7 disposed inwardly in a tire radial direction of the tread rubber 2G. The outer diameter of the pneumatic tire 1 is 350 to 600 mm, and the rim width RW of a rim is 78% to 99% of a tire cross-sectional width SW. The ground contact surface 2s is divided into a plurality of land parts 9 including a shoulder land part 9B, by circumferential grooves 8 provided in the tread part 2. The shoulder land part 9B has an inner end edge 9e located inside in a tire axial direction from an outer end part 7e of the belt layer 7. A distance LA between the inner end edge 9e and the outer end part 7e is 10% to 30% of the maximum belt width WA of the belt layer 7. The hardness Hs of the tread rubber 2G satisfies mathematical formula (1) based on the distance LA and the tire cross-sectional width SW.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a pneumatic tire having a tread portion.

Background Art

[0002] Conventionally, pneumatic tires with a small diameter, which are suitably used for small shuttle buses and the like, are known. For example, Patent Document 1 below proposes a pneumatic tire with improved durability performance of the bead portion where strain concentrates due to the reduction in diameter.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, a small-diameter pneumatic tire used for a heavy vehicle as in Patent Document 1 has a so-called wide contact surface shape with a short contact length and a wide contact width, and there is room for improvement in the riding comfort performance due to problems such as the dispersibility of road surface input, input sensitivity, and frequency.

[0005] The present invention has been devised in view of the above actual situation, and the main object thereof is to provide a wide and small-diameter pneumatic tire capable of improving the riding comfort performance.

Means for Solving the Problems

[0006] The present invention relates to a pneumatic tire having a tread portion, the tread portion including a tread rubber forming a ground contact surface and a belt layer disposed inward in the tire radial direction of the tread rubber. The outer diameter of the pneumatic tire is 350 to 600 mm, and the rim width RW of the rim on which the pneumatic tire is mounted is 78% to 99% of the tire section width SW. The ground contact surface is divided into a plurality of land portions by at least two circumferential grooves provided in the tread portion and continuously extending linearly in the tire circumferential direction. The plurality of land portions include a pair of shoulder land portions located on the outermost side in the tire axial direction. Each of the pair of shoulder land portions has an inner edge in the tire axial direction, and each of the inner edges is located inward in the tire axial direction than each of the pair of outer ends of the belt layer in the tire axial direction. The distance LA in the tire axial direction between the adjacent inner edge and the outer end in the tire axial direction is 10% to 30% of the maximum belt width WA in the tire axial direction of the belt layer. The hardness Hs of the tread rubber satisfies the following formula (1).

Number

Advantages of the Invention

[0007] By having the above-described configuration, the pneumatic tire of the present invention can improve the riding comfort performance.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Mode for Carrying Out the Invention

[0009] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is a tire meridian cross-sectional view showing the pneumatic tire 1 in the normal state of the present embodiment. Here, the "normal state" means a no-load state in which the pneumatic tire 1 is rim-mounted on a normal rim and adjusted to a normal internal pressure. Hereinafter, unless otherwise specified, the dimensions and the like of each part of the pneumatic tire 1 are values measured in this normal state.

[0010] The "normal rim" is a rim defined for each tire when the standard system including the standard on which the pneumatic tire 1 is based exists. For example, in the case of JATMA, it is the "standard rim", in the case of TRA, it is the "Design Rim", and in the case of ETRTO, it is the "Measuring Rim". The "normal rim" is a rim that can be rim-mounted and does not cause air leakage when the standard system including the standard on which the pneumatic tire 1 is based does not exist. Among them, it is the rim with the smallest rim diameter RD and the smallest rim width RW among them.

[0011] The "normal internal pressure" is the air pressure defined for each tire when the standard system including the standard on which the pneumatic tire 1 is based exists. In the case of JATMA, it is the "maximum air pressure", in the case of TRA, it is the maximum value described in the table "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES", and in the case of ETRTO, it is the "INFLATION PRESSURE". The "normal internal pressure" is the air pressure determined for each tire by the manufacturer or the like when the standard system including the standard on which the pneumatic tire 1 is based does not exist.

[0012] As shown in FIG. 1, the pneumatic tire 1 of the present embodiment has a so-called wide ground contact surface shape with a small diameter, a short ground contact length, and a wide ground contact width. When such a pneumatic tire 1 is used for a minibus, a low-floor electric vehicle, or the like, the interior space and the space inside the rim can be increased, and it is possible to cope with an improvement in transportation efficiency, a large-diameter brake, an increase in the size of an in-wheel motor, and the like.

[0013] The outer diameter SD of the pneumatic tire 1 is preferably 350 to 600 mm. By having the outer diameter SD of the pneumatic tire 1 be 350 mm or more, the space inside the rim can be secured. By having the outer diameter SD of the pneumatic tire 1 be 600 mm or less, it helps to increase the interior space of a minibus, a low-floor electric vehicle, or the like.

[0014] The rim width RW of the rim on which the pneumatic tire 1 is mounted is 78% to 99% of the tire cross-sectional width SW. Here, the rim width RW of the rim is the width between a pair of outer surfaces 4s in the tire axial direction on the inner side in the tire radial direction of the pneumatic tire 1 in a normal state.

[0015] By having the rim width RW be 78% or more of the tire cross-sectional width SW, the volume of the tire inner cavity can be ensured to be large, which helps to improve the riding comfort performance. By having the rim width RW be 99% or less of the tire cross-sectional width SW, it helps to support a high load such as that of a minibus or a low-floor electric vehicle.

[0016] The pneumatic tire 1 of the present embodiment includes a tread portion 2, a pair of sidewall portions 3, and a pair of bead portions 4. The sidewall portion 3 is, for example, a portion that extends from both sides in the tire axial direction of the tread portion 2 inward in the tire radial direction. The bead portion 4 is, for example, a portion located inward in the tire radial direction of the sidewall portion 3.

[0017] Each of the pair of bead portions 4 has, for example, an annular bead core 5 that extends in the tire circumferential direction. The bead core 5 is formed of, for example, a steel wire. Here, the outer surface 4s in the tire axial direction on the inner side in the tire radial direction of the above-described pneumatic tire 1 is the outer surface 4s in the tire axial direction of the bead portion 4.

[0018] The pneumatic tire 1 preferably includes a carcass 6 extending between a pair of bead portions 4 and a belt layer 7 disposed in the tread portion 2. The tread portion 2 of the present embodiment includes a tread rubber 2G forming a ground contact surface 2s, a belt layer 7 disposed radially inward of the tread rubber 2G in the tire radial direction, and a carcass 6 disposed radially inward of the belt layer 7 in the tire radial direction.

[0019] FIG. 2 is an enlarged cross-sectional view of the tread portion 2. As shown in FIG. 2, the ground contact surface 2s is divided into a plurality of land portions 9 by at least two, in this embodiment two, circumferential grooves 8 that continuously and linearly extend in the tire circumferential direction provided in the tread portion 2. Note that the circumferential grooves 8 are not limited to two, but it is desirable to provide 2 to 4 circumferential grooves 8 in the tread portion 2 from the viewpoint of ensuring the ground contact area of the land portion 9 for supporting a high load.

[0020] The plurality of land portions 9 include, for example, at least a crown land portion 9A located on the inner side in the tire axial direction and a pair of shoulder land portions 9B located on the outermost side in the tire axial direction. Each of the pair of shoulder land portions 9B of the present embodiment has an inner end edge 9e in the tire axial direction.

[0021] Each of the inner end edges 9e of the present embodiment is located axially inward of each of the pair of outer end portions 7e of the belt layer 7 in the tire axial direction. Such a shoulder land portion 9B is reinforced by the belt layer 7, so it can exhibit high rigidity and is suitable for supporting a high load.

[0022] As shown in FIGS. 1 and 2, the axial distance LA between the adjacent inner end edge 9e and outer end portion 7e in the tire axial direction is preferably 10% to 30% of the maximum belt width WA of the belt layer 7 in the tire axial direction.

[0023] The distance LA between the inner edge 9e and the outer end 7e is 10% or more of the maximum belt width WA, thereby improving the rigidity of the shoulder land portion 9B and enabling support of a high load. The distance LA between the inner edge 9e and the outer end 7e is 30% or less of the maximum belt width WA, thereby suppressing an excessive increase in the rigidity of the shoulder land portion 9B and improving the riding comfort performance during turning.

[0024] The hardness Hs of the tread rubber 2G of the present embodiment satisfies the following mathematical formula (1). Here, the hardness Hs of the tread rubber 2G is the durometer hardness measured at 23°C by a durometer type A based on JIS-K6253. The tire cross-sectional width SW is the distance in the tire axial direction between a pair of tire maximum width positions P in the tire profile excluding convex portions locally protruding from the tire surface.

Equation

[0025] The hardness Hs of the tread rubber 2G has an upper limit set by the mathematical formula (1) in relation to the structural rigidity of the shoulder land portion 9B. Such tread rubber 2G enables reinforcement of the shoulder land portion 9B when the structural rigidity is low and can relieve the impact force acting on the shoulder land portion 9B when the structural rigidity is high. Therefore, the pneumatic tire 1 of the present embodiment can improve the riding comfort performance in a wide and small-diameter tire capable of supporting a high load.

[0026] As a more preferable aspect, the outer end 7e of the belt layer 7 is located outside the tread end Te in the tire axial direction. That is, it is preferable that the maximum belt width WA is larger than the tire tread width TW. Such a belt layer 7 can improve the rigidity of the entire grounding surface 2s of the tread portion 2 and is suitable for supporting a high load.

[0027] Here, the tread edge Te is the outermost grounding position in the tire axial direction when the pneumatic tire 1 under a load of 70% of the normal load with respect to the pneumatic tire 1 in the normal state is grounded on a plane at a camber angle of 0°. The tire tread width TW is the distance in the tire axial direction between a pair of tread edges Te. Note that the central position in the tire axial direction between the pair of tread edges Te is the tire equator C. In the present embodiment, the tire equator C is located at the outermost in the tire radial direction.

[0028] The "normal load" is, when there is a standard system including the standard on which the pneumatic tire 1 is based, the load determined for each tire by each standard. In the case of JATMA, it is the "maximum load capacity", in the case of TRA, it is the maximum value described in the table "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES", and in the case of ETRTO, it is the "LOAD CAPACITY". The "normal load" is, when there is no standard system including the standard on which the pneumatic tire 1 is based, the load determined for each tire by the manufacturer or the like as the maximum applicable load when using the pneumatic tire 1.

[0029] When the pneumatic tire 1 is grounded on a plane at a camber angle of 0° and a load of 50% of the normal load is applied, it is desirable that each of the pair of shoulder land portions 9B comes into contact with the ground. Such a pneumatic tire 1 can suppress uneven wear of the tread portion 2 because the shoulder land portion 9B comes into contact with the ground even when the load is relatively small as in the case of a minibus when unloaded.

[0030] The camber amount LC at the position of the outer end portion 7e in the tire axial direction is preferably 2.5% to 12.5% of the maximum belt width WA. Here, the camber amount LC at the position of the outer end portion 7e is the distance in the tire radial direction between the tire equator C and the position of the outer end portion 7e in the tire profile.

[0031] When the camber amount LC is 2.5% or more of the maximum belt width WA, the impact on the shoulder land portion 9B during grounding can be mitigated, and the riding comfort performance can be improved. When the camber amount LC is 12.5% or less of the maximum belt width WA, the shoulder land portion 9B can be grounded even when the applied load is relatively small, and uneven wear of the tread portion 2 can be suppressed.

[0032] The distance LB in the tire axial direction between the inner end edges 9e of each of the pair of shoulder land portions 9B is preferably 25% to 95% of the maximum belt width WA. When the distance LB between the inner end edges 9e is 25% or more of the maximum belt width WA, it is possible to suppress the shoulder land portion 9B from becoming excessively large and improve the riding comfort performance during turning. When the distance LB between the inner end edges 9e is 95% or less of the maximum belt width WA, the rigidity of the shoulder land portion 9B can be maintained, and it is suitable for supporting high loads.

[0033] The belt layer 7 includes at least one sheet, preferably two or more sheets, and in this embodiment, two belt plies 7A and 7B. The two belt plies 7A and 7B include, for example, a first belt ply 7A located on the inner side in the tire radial direction and a second belt ply 7B located on the outer side in the tire radial direction of the first belt ply 7A. The outer end portion 7e of the belt layer 7 of this embodiment is the outer end portion 7e in the tire axial direction of the first belt ply 7A. Such a belt layer 7 can improve the rigidity of the tread portion 2 and is suitable for supporting high loads.

[0034] The belt plies 7A and 7B include, for example, a belt cord (not shown) made of a steel cord. The belt cord may be, for example, a single steel wire or a twisted wire formed by twisting a plurality of steel filaments.

[0035] The belt cord is arranged, for example, at an angle of 10 to 30° with respect to the tire circumferential direction. The belt cord of the first belt ply 7A and the belt cord of the second belt ply 7B are preferably inclined in opposite directions with respect to the tire circumferential direction.

[0036] Such a belt layer 7 can enhance the rigidity of the tread portion 2 in a well-balanced manner, achieving both the rigidity capable of supporting high loads and the riding comfort performance. Here, the angle of the belt cord is the angle in the pneumatic tire 1 in the normal state, and can be confirmed, for example, by partially peeling the tread portion 2.

[0037] The belt layer 7 includes the outermost belt ply arranged on the outermost side in the tire radial direction. The outermost belt ply of the present embodiment is the second belt ply 7B. The belt layer 7, for example, when there is only one belt ply 7A of the first belt ply 7A, the first belt ply 7A constitutes the outermost belt ply.

[0038] The belt width WB in the tire axial direction of the outermost belt ply is preferably 80% to 100% of the maximum belt width WA. Such a belt layer 7 can achieve both the rigidity of the tread portion 2 and weight reduction, and is useful for improving the riding comfort performance in a wide and small-diameter tire capable of supporting high loads.

[0039] As shown in FIG. 1, the carcass 6 includes at least one, in this embodiment, two carcass plies 6A and 6B. The two carcass plies 6A and 6B include, for example, in the tread portion 2, the first carcass ply 6A located on the inner side in the tire radial direction and the second carcass ply 6B located on the outer side in the tire radial direction of the first carcass ply 6A.

[0040] It is desirable that at least one of the carcass plies 6A and 6B includes a main body portion 6a extending from the tread portion 2 through the sidewall portion 3 to the bead core 5 of the bead portion 4, and a folded-back portion 6b connected to the main body portion 6a and folded back around the bead core 5 from the inner side to the outer side in the tire axial direction. The first carcass ply 6A and the second carcass ply 6B of the present embodiment each include a main body portion 6a and a folded-back portion 6b.

[0041] It is desirable that at least one of the carcass plies 6A and 6B has an outer end 6e in the tire radial direction of the folded portion 6b located radially outside the tire maximum width position P where the tire cross-sectional width SW is maximum.

[0042] In the present embodiment, the outer end 6e of the folded portion 6b of the first carcass ply 6A is located radially outside the tire maximum width position P, and the outer end 6e of the folded portion 6b of the second carcass ply 6B is located radially inside the tire maximum width position P. Such a carcass 6 can achieve both the rigidity and weight reduction of the sidewall portion 3, and is useful for improving the riding comfort performance in a wide and small-diameter tire capable of supporting a high load.

[0043] The carcass plies 6A and 6B include, for example, a carcass cord (not shown) made of an organic fiber cord. Examples of the organic fiber cord include a cord of one kind alone or a hybrid fiber of two or more kinds selected from the group consisting of polyethylene terephthalate fiber, polyethylene naphthalate fiber, nylon fiber, aramid fiber, and rayon fiber.

[0044] The carcass cord is preferably arranged at an angle of 25 to 90° with respect to the tire circumferential direction. Here, the angle of the carcass cord is the angle in the normal state of the inflated tire 1 and can be confirmed, for example, by partially peeling the tread portion 2.

[0045] When the carcass cord is inclined at an angle less than 90° with respect to the tire circumferential direction, it is desirable that the carcass cord of the first carcass ply 6A and the carcass cord of the second carcass ply 6B be inclined in opposite directions with respect to the tire circumferential direction. For example, a bias structure may be adopted for the carcass 6.

[0046] The tire section width SW is preferably 145 to 285 mm. When the tire section width SW is 145 mm or more, a large volume of the tire inner cavity can be ensured, and the riding comfort performance can be improved. When the tire section width SW is 285 mm or less, an excessive increase in the tire weight can be suppressed, and the riding comfort performance can be improved.

[0047] The ratio SH / SW of the tire section height SH to the tire section width SW is preferably 35% to 75%. When the ratio SH / SW is 35% or more, a large volume of the tire inner cavity can be ensured, and the riding comfort performance can be improved. When the ratio SH / SW is 75% or less, an excessive increase in the tire weight can be suppressed, and the riding comfort performance can be improved. Here, the tire section height SH is the height in the tire radial direction from the bead base line BL to the tire equator C. Also, the bead base line BL is a virtual line corresponding to the rim diameter RD of the rim. Note that the ratio SH / SW is a value generally known as the aspect ratio.

[0048] The rim width RW of the rim on which the pneumatic tire 1 is mounted is preferably 9 inches or less. Such a pneumatic tire 1 helps to suppress an excessive increase in the rim size and an excessive increase in the rim weight, and improves the riding comfort performance.

[0049] The rim diameter RD of the rim on which the pneumatic tire 1 is mounted is preferably 10 to 18 inches. When the rim diameter RD is 10 inches or more, a space inside the rim can be ensured. When the rim diameter RD is 18 inches or less, an excessive increase in the rim weight and the tire weight can be suppressed, and the riding comfort performance can be improved.

[0050] As described above, the particularly preferred embodiments of the present invention have been described in detail. However, the present invention is not limited to the above-described embodiments and can be implemented in various forms.

Example

[0051] A pneumatic tire having the basic structure shown in FIG. 1 was prototyped based on the specifications in Table 1. Using the prototyped pneumatic tire, the ride comfort performance was evaluated. The main common specifications and test methods are as follows.

[0052] <Common Specifications> Tire size: 205 / 40R15 Rim size: 15×7.0J Air pressure: 350 kPa Test vehicle: Minibus

[0053] <Grip Performance> One test driver got on a test vehicle equipped with the prototyped tire and drove on the paved road surface of the test course. The ride comfort performance was evaluated by the driver's sensory evaluation. The results are shown as an index with Comparative Example 1 being 100, indicating that the larger the numerical value, the better the ride comfort performance.

[0054] The test results are shown in Table 1.

Table 1

[0055] As a result of the test, it was confirmed that the pneumatic tire of the example has excellent ride comfort performance compared to the comparative example, and it is possible to improve the ride comfort performance in a wide-width and small-diameter tire capable of supporting a high load.

[0056] [Appendix] The present invention is as follows.

[0057] [Invention 1] A pneumatic tire having a tread portion, The tread portion includes a tread rubber forming a ground contact surface and a belt layer disposed inward in the tire radial direction of the tread rubber, The outer diameter of the pneumatic tire is 350 to 600 mm, The rim width RW of the rim on which the pneumatic tire is mounted is 78% to 99% of the tire section width SW. The ground contact surface is divided into a plurality of land portions by at least two circumferential grooves that are provided in the tread portion and extend linearly and continuously in the tire circumferential direction. The plurality of land portions include a pair of shoulder land portions located at the outermost side in the tire axial direction. Each of the pair of shoulder land portions has an inner edge in the tire axial direction. Each of the inner edges is located on the inner side in the tire axial direction than each of the pair of outer ends of the belt layer in the tire axial direction. The distance LA in the tire axial direction between the inner edge and the outer end adjacent in the tire axial direction is 10% to 30% of the maximum belt width WA in the tire axial direction of the belt layer. The hardness Hs of the tread rubber satisfies the following formula (1). Pneumatic tire. [Number] Here, LA: Distance in the tire axial direction between the inner edge and the outer end (mm) SW: Tire section width (mm)

[0058] [Invention 2] The pneumatic tire according to Invention 1, wherein 2 to 4 circumferential grooves are provided in the tread portion.

[0059] [Invention 3] The pneumatic tire according to Invention 1 or 2, wherein the tire section width SW is 145 to 285 mm.

[0060] [Invention 4] The pneumatic tire according to any one of Inventions 1 to 3, wherein the rim width RW is 9 inches or less.

[0061] [Invention 5] The pneumatic tire according to any one of Inventions 1 to 4, wherein the rim diameter RD of the rim is 10 to 18 inches.

[0062] [The present invention 6] For the pneumatic tire according to any one of the first to fifth aspects of the present invention, the ratio SH / SW of the tire section height SH to the tire section width SW is 35% to 75%.

[0063] [The present invention 7] For the pneumatic tire according to any one of the first to sixth aspects of the present invention, the distance LB in the tire axial direction between the inner edges of the pair of shoulder lands is 25% to 95% of the maximum belt width WA.

[0064] [The present invention 8] The belt layer includes the outermost belt ply arranged at the outermost in the tire radial direction, For the pneumatic tire according to any one of the first to seventh aspects of the present invention, the belt width WB in the tire axial direction of the outermost belt ply is 80% to 100% of the maximum belt width WA.

[0065] [The present invention 9] For the pneumatic tire according to any one of the first to eighth aspects of the present invention, the camber amount LC at the position of the outer end is 2.5% to 12.5% of the maximum belt width WA.

[0066] [The present invention 10] For the pneumatic tire according to any one of the first to ninth aspects of the present invention, each of the pair of shoulder lands comes into contact with the ground when 50% of the normal load is applied in a state of being in contact with the ground at a camber angle of 0°.

Explanation of symbols

[0067] 1 Pneumatic tire 2 Tread portion 2G Tread rubber 2s Contact surface 7 Belt layer 7e Outer end 8 Circumferential groove 9 Land portion 9B Shoulder land 9e Inner edge

Claims

1. A pneumatic tire having a tread portion, wherein the tread portion includes a tread rubber forming a ground contact surface and a belt layer disposed inward in the tire radial direction of the tread rubber, the outer diameter of the pneumatic tire is 350 to 600 mm, the rim width RW of a rim on which the pneumatic tire is mounted is 78% to 99% of the tire section width SW, the ground contact surface is divided into a plurality of land portions by at least two circumferential grooves provided in the tread portion and continuously extending linearly in the tire circumferential direction, the plurality of land portions include a pair of shoulder land portions located on the outermost side in the tire axial direction, each of the pair of shoulder land portions has an inner edge in the tire axial direction, each of the inner edges is located inward in the tire axial direction than each of a pair of outer ends of the belt layer in the tire axial direction, the tire axial distance LA between the inner edge and the outer end adjacent in the tire axial direction is 10% to 30% of the maximum belt width WA of the belt layer in the tire axial direction, the hardness Hs of the tread rubber satisfies the following formula (1), Pneumatic tire. 【Number 1】 Here, LA: the tire axial distance (mm) between the inner edge and the outer end SW: tire section width (mm)

2. The pneumatic tire according to claim 1, wherein 2 to 4 circumferential grooves are provided in the tread portion.

3. The pneumatic tire according to claim 1 or 2, wherein the tire section width SW is 145 to 285 mm.

4. The pneumatic tire according to claim 1 or 2, wherein the rim width RW is 9 inches or less.

5. The pneumatic tire according to claim 1 or 2, wherein the rim diameter RD of the rim is 10 to 18 inches.

6. The pneumatic tire according to claim 1 or 2, wherein the ratio SH / SW of the tire section height SH to the tire section width SW is 35% to 75%.

7. The pneumatic tire according to claim 1 or 2, wherein the tire axial distance LB between the inner edges of each of the pair of shoulder land portions is 25% to 95% of the maximum belt width WA.

8. The belt layer includes an outermost belt ply disposed on the outermost side in the tire radial direction, the belt width WB of the outermost belt ply in the tire axial direction is 80% to 100% of the maximum belt width WA,

9. The camber amount LC at the position of the outer end portion is 2.5% to 12.5% of the maximum belt width WA. The pneumatic tire according to claim 1 or 2.

10. The pneumatic tire according to claim 1 or 2, wherein when 50% of the normal load is applied in a state where the camber angle is 0° and the tire is in contact with the ground on a flat surface, each of the pair of shoulder land portions comes into contact with the ground.

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