JP2001301421A - Pneumatic tire - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pneumatic tire effectively reducing cavity resonance sound originating in a toroidal tubular length within a tire and causing unpleasant noise within a cabin, without sacrifice of performance. SOLUTION: This pneumatic tire has a carcass 3; a belt 8 having at least two rubber coated cord layers extending aslant to a tire equator face 2 in the outer circumference of a crown part of the carcass 3 and constituting a cross belt layer 7 in which cords cross each other on the both sides of the tire equator face 2 with two rubber coated cord layers 5, 6 of these rubber coated cord layers; and a tread 11, and the width WS of the narrow rubber coated cord layer 6 of the two rubber coated cord layers 5, 6 constituting the cross belt layer 7 is 75% or less of the width WL of the wide rubber coated cord layer 5.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention sacrifice other performances due to cavity resonance caused by the length of a circular tube inside a toroidal tire and contributing to generation of unpleasant cabin noise. The present invention relates to a pneumatic tire that can be effectively reduced without using the same.

[0002]

2. Description of the Related Art A pneumatic tire is an elastic body filled with air. Due to its structure, when a vibrating force is applied from the outside, the vibrating force causes the tread portion and the sidewall portion of the tire, and furthermore, The air inside vibrates, and when the vibration of the air causes resonance (resonance), so-called cavity resonance sound is generated, which is transmitted to the vehicle interior through the axle and causes unpleasant vehicle interior noise. .

[0003] Further, since the frequency at which the cavity resonance sound is generated (hereinafter referred to as "resonance frequency") is uniquely determined from the sound velocity of the air inside the tire and the length of the pipe, unless the tire size is changed, the tire does not change. Are constant. For example, in the case of a normal passenger car tire, the resonance frequency is approximately 200 to 270 Hz based on the length of the circular tube.

[0004] Cavity resonance is a phenomenon that is inevitably generated in pneumatic tires because the cause of the resonance is air resonance. Therefore, conventionally, even if the tire structure is improved, the cavity resonance is generated. It was considered difficult to reduce.

For this reason, conventionally, for example, Japanese Patent Application Laid-open No. Sho 62-5002
As described in Japanese Patent Publication No. 03, etc., it is necessary to insert a sponge inside the tire to reduce cavity resonance sound by a method other than improving the tire structure, such as a method of improving the sound field inside the tire. There was no.

[0006]

Although the method described in the above publication has the effect of reducing cavity resonance, it is a method of mounting the tire after the tire has become a product. However, it is not preferable in terms of cost.

Therefore, the inventor has conducted various studies to develop means for reducing cavity resonance by improving the tire structure.

First, the inventor conducted a detailed investigation on the relationship between the tire vibration mode and the cavity resonance.

As a result, when the vibration mode of the tire is viewed in the tire width direction, the sectional height changes uniformly in the tire width direction with both beads being fixed ends as shown in FIG. When a tire (broken line) before vibration is set as a reference (standing wave), a vibration mode (hereinafter referred to as “2
Section mode. " ), And thereafter, as shown in FIG. 6, the mode shifts to a vibration mode (hereinafter, referred to as a “four-node mode”) that deforms to have four nodes, and further, six nodes, 8
It has been found that the mode sequentially shifts to various vibration modes deformed so as to have a knot,-.

[0010] In the case of a tire for a passenger car, the frequency band of the two-bar mode is usually approximately as shown in FIG.
In the range of 30-300 Hz, and hence the resonance frequency (200-27
0Hz) is located in the frequency band of the two-bar mode, which was found to be the cause of the increase in cavity resonance.

Next, the inventor shifted the frequency band of each vibration mode and examined the change of the cavity resonance at this time in detail. As a result, when the resonance frequency was located in the frequency band of the four-bar mode, We found that the cavity resonance sound became smaller.

[0012] An object of the present invention is to shift the frequency band of the four-bar mode to the lower frequency side and position the resonance frequency in the frequency band of the four-bar mode, thereby causing the toroidal tube inside the tire to have a toroidal shape. Accordingly, it is an object of the present invention to provide a pneumatic tire in which cavity resonance noise which is generated and contributes to generation of unpleasant vehicle interior noise is effectively reduced without sacrificing other performances.

[0013]

In order to achieve the above-mentioned object, the present invention provides at least one rubberized cord that is inclined at an angle of 70 to 90 degrees with respect to the tire equatorial plane.
A ply carcass and at least two layers of cord rubberized layers extending obliquely to the tire equatorial plane around the crown of the carcass, and two rubberized layers of the rubberized layers Is a two-layer rubberized layer that forms a cross belt layer in a pneumatic tire that has a cross belt layer in which the cords intersect each other across the equatorial plane of the tire and a pneumatic tire that has a tread portion. Among them, the pneumatic tire is characterized in that the width of the narrow rubberized layer is 75% or less of the width of the wide rubberized layer. Incidentally, the width of the wide rubberized layer is 70 times the tread width.
% Is preferable.

[0014] The tread portion has at least two circumferential grooves extending parallel to the tire equatorial plane, and two of the circumferential grooves have a central area sandwiching the tire equatorial plane. In the case where the first circumferential groove is provided, the width of the narrow rubberized layer is made wider than the distance measured along the tire width direction between the width center positions of both the first circumferential grooves. Is preferred.

Further, the tread portion has at least two circumferential grooves extending parallel to the tire equatorial plane, and two circumferential grooves of the circumferential grooves are each one in each shoulder region.
In the case of the second circumferential grooves provided one by one, the width of the narrow rubberized layer is set to be equal to or less than the distance measured along the tire width direction between the width center positions of both the second circumferential grooves. Is preferred.

Still further, the tread portion has at least four circumferential grooves extending parallel to the tire equatorial plane,
Of the circumferential grooves, two circumferential grooves are first circumferential grooves disposed in the central region with the tire equatorial plane interposed therebetween, and another two circumferential grooves are one in each of the shoulder regions. 2nd arranged by book
In the case of a circumferential groove, the width of the narrow rubberized layer,
Preferably, the distance between the width center positions of both first circumferential grooves is wider than the distance measured along the tire width direction and the distance between the width center positions of both second circumferential grooves is equal to or less than the distance similarly measured,
In addition, it is more preferable that the number of the circumferential grooves is four, that is, two first circumferential grooves and two second circumferential grooves.

When both the first circumferential groove and the second circumferential groove are provided, one of the first and second circumferential grooves may be narrow and shallow.

[0018]

FIG. 1 shows a typical cross section in the width direction of a pneumatic tire according to the present invention, in which 1 is a pneumatic tire, 2 is a tire equatorial plane, 3 is a carcass, Is a crown portion of a carcass, 5 and 6 are rubberized layers, 7 is a cross belt layer, 8 is a belt, 9a to 9d are circumferential grooves, 10a to 10e are partitioned land portions, and 11 is a tread portion.

The tire 1 shown in FIG. 1 has a carcass 3 of at least one ply (one ply in FIG. 1) obtained by rubberizing a cord inclined at an angle of 70 to 90 ° with respect to the tire equatorial plane 2. On the outer periphery of the crown portion 4 of the carcass 3,
It is composed of at least two rubber layers (two layers in FIG. 1) of a cord extending at an angle to the plane 2, and two rubber layers 5, 6 of which are arranged such that the cords A belt 8 having an intersecting belt layer 7 having an intersecting arrangement relationship, and at least one circumferential groove extending parallel to the tire equatorial plane 2, and four circumferential grooves 9 a to 9 d in FIG. And a tread portion 11 which is disposed to form a plurality of partitioned land portions 10a to 10e.

The main features of the configuration of the present invention are as follows.
Of cross belt layer 7 two layers of rubberized layers 5 and 6 that constitute the, the width W _S of the rubberized layer 6 narrow, to a more than 75% of the width W _L of the wide rubberized layer 5 is there.

Hereinafter, the process of completing the present invention will be described together with the operation of the present invention. As shown in FIG. 8, when an exciting force F _input is applied to the tire from the outside, the air inside the tire vibrates, and when the air resonates, a cavity resonance sound is generated. At a resonance frequency at which the cavity resonance sound is generated, A primary pressure distribution occurs on the circumference of the tire, and the pressure distribution becomes uneven on the circumference, and a pressure difference generates a tire axial force F _spindle , which vibrates the vehicle as an axle excitation force. , Causing vehicle interior noise.

The resonance frequency f is uniquely determined from the equation of f = V / L, where V is the sound velocity of the air inside the tire and L is the length of the pipe, so that the resonance frequency is always constant unless the tire size is changed. It is constant.

When the relationship between the pressure and the volume inside the tire is considered by an ideal adiabatic change, the following relational expression is established. ΔP = −γ (ΔV / V) × P where γ is a specific heat ratio

From this relational expression, the pressure change inside the tire (Δ
It can be seen that P) can be reduced by reducing the volume change (ΔV) of the tire, in other words, by reducing the cross-sectional area change (ΔS), which is the volume change per unit length.

The inventor believes that if the pressure change inside the tire is reduced, the exciting force transmitted to the axle is reduced in proportion to the pressure change, and the cavity resonance is reduced accordingly. A study was conducted to reduce the change in pressure of the sample.

In the case of a normal tire, the vibration mode in which the resonance frequency is located is a two-node mode. In other words, the antinode part of the resonance of the internal air where the pressure change is maximum (the + and-parts in FIG. 8) is also a two-node mode, and the cross-sectional area (volume) per unit length in the circumferential direction depends on this mode. If the change is determined, it can be considered that the pressure change is generated correspondingly to the cross-sectional area change due to the mode configuration.

FIG. 9 shows a tire contour (dashed line) in the two-bar mode and a tire contour (dashed-dotted line) in the four-bar mode superimposed on the tire contour (solid line) before applying the exciting force. Things.

From this figure, when the amount of change in the tire cross-sectional area is determined by image processing, the tire cross-sectional areas in the two-node mode and the four-node mode are each equal to the cross-sectional area of the tire before the excitation force is applied. In comparison, when the amount of change in the cross-sectional area in the two-bar mode is set to 100, it is as small as 45 in the four-bar mode, and the pressure change inside the tire in the four-bar mode is 2 It turned out to be smaller than in knot mode.

In the case of a tire for a passenger car, the frequency band of the two-bar mode is usually about 30 to 30, as shown in FIG.
In the range of 300Hz, the resonance frequency (200-270Hz)
The inventor thought that this is a cause of increasing the cavity resonance sound because the tire is located in the frequency band of the two-node mode in which the pressure change inside the tire is large.

Then, the inventor further studied and found that the frequency band of the four-node mode was shifted to the lower frequency side, and the resonance frequency was set to be in the frequency band of the four-node mode. We found that the resonance sound became smaller.

It has also been found that as a means for shifting the frequency band of the four-bar mode to the lower frequency side, it is useful to reduce the widthwise rigidity of the tread portion.

As a component of the tire that contributes to the widthwise rigidity of the tread portion, there is an intersecting belt layer arranged on the outer periphery of the carcass crown portion. If the width of the intersecting belt layer is reduced, the width of the tread portion is reduced. Directional rigidity can be reduced.

However, if the width of the intersecting belt layer is uniformly reduced, the weight is accordingly reduced, so that the frequency band of the four-bar mode cannot be effectively shifted to the lower frequency side.

Therefore, the inventor further considered the arrangement of the cross belt layer 7 constituting the belt 8 as a cause of increasing the rigidity of the tread portion 11 in the width direction.
If only the width of one of the two rubberized layers 5 and 6 constituting the cross belt layer 7 is reduced,
It has been found that the rigidity in the width direction of the tread portion can be effectively reduced while suppressing the weight reduction rate, and accordingly, the cavity resonance can be effectively reduced.

FIG. 3 shows a wide rubberized layer having a constant width (150 m) of two rubberized layers of a cross belt layer constituting a belt of a tire having a tire size of 195 / 65R14.
m), the width (%) of the width of the narrow rubberized layer to the width of the wide rubberized layer is changed by using a narrow rubberized layer having a different width, and the cavity resonance sound at that time is changed. It is a plot.

In FIG. 3, “○” and “△” indicate that the wide rubberized layer 5 is the lower layer and the narrow rubberized layer 6 is the upper layer, as shown in FIGS. 4 (a) and 4 (b), respectively. In addition, “●” and “▲” indicate the case where the wide rubberized layer 5 is the upper layer and the narrow rubberized layer 6 is the lower layer as shown in FIGS. 4 (c) and (d), respectively. FIGS. 4 (b) and (d) show a one-layer cap composed of a rubberized layer of cords arranged on the outer periphery of the crossed belt layer so as to cover the entire width thereof, substantially in parallel with the tire equatorial plane. A layer 13 was further provided. Steel cords were used for the cords of the rubberized layers 5 and 6, and nylon cords were used for the cap layer 13.

FIG. 3 shows that the ratio of the width of the narrow rubberized layer to the width of the wide rubberized layer is 75% or less regardless of the difference in the positional relationship between the narrow rubberized layer and the wide rubberized layer. It can be seen that the cavity resonance peak level has been reduced.

Therefore, in the present invention, of the two rubberized layers constituting the cross belt layer, the width of the narrow rubberized layer is 75% or less of the width of the wide rubberized layer.

Further, the width of the wide rubberized layer is preferably at least 70% of the tread width. If the width of the wide rubberized layer is less than 70% of the tread width, the tire strength tends to be insufficient.

The structure of the present invention does not affect the spring constant or block rigidity of the tire, so that the performance of other tires does not decrease.

Of course, as means for lowering the frequency band of the four-bar mode, means for changing the stiffness of other portions and means for increasing the weight of the tire can be considered.

However, it is not preferable to use a means for reducing the rigidity in a portion other than the tread portion, since the spring constant of the tire also changes, which greatly affects other performances such as maneuverability.

When the means for increasing the tire weight is used, it is difficult to increase the weight locally without increasing the rigidity. For example, even if weight is increased by increasing the thickness of the rubber layer on the side, since the thickness of the rubber layer increases, the second moment of area increases,
Since the bending rigidity also increases, the effect of increasing the weight cannot be obtained efficiently. Further, it is not desirable to increase the weight also from the viewpoint of fuel efficiency of the vehicle.

Therefore, as an appropriate means for reducing the influence on the basic characteristics of the tire and lowering the frequency band of the four-bar mode, the narrow rubberized layers 5 and 6 constituting the cross belt layer 7 are used. the width W _s of the rubberized layer 6 of width,
It can be said that to the more than 75% of the width W _L of the wide rubberized layer 5.

Another component of the tire that contributes to the widthwise rigidity of the tread portion is the thickness of the tread rubber located on the outer periphery of the belt.

In particular, when the tread center portion is locally deformed to take a four-bar mode deformation mode, the rigidity of the thinnest portion of the tread rubber, that is, the tread portion provided with the circumferential grooves is different from that of the other tread. Since it is smaller than the portion, the tread portion of the circumferential groove is greatly deformed.

Therefore, the inventor limited the ratio of the width of the narrow rubberized layer to the width of the wide rubberized layer to 75%,
Further investigation was made on the relationship with the circumferential grooves provided in the tread portion. As a result, it was found that the cavity resonance can be more effectively reduced by the following configuration.

The first circumferential groove 9b of the pair across the tire equatorial plane 2 in the central region 14 of the tread portion 11, when disposed. 9c, the width W _s of the narrow rubberized layer 6, Both first circumferential grooves 9
b, 9c between the width center position to be wider than the distance L1 measured along the tire width direction,

[0049] Both shoulder areas 15a, 15b of the tread portion 11
Each one second circumferential groove 9a in, when disposed 9d is the narrow rubberized layer 6 width W _s, both the second circumferential groove 9a, the inter-9d width center position of similarly Less than the measured distance L2,

In the tread portion 11, a pair of first circumferential grooves 9b and 9c are arranged in the central region 14 with the tire equatorial plane 2 interposed therebetween, and one second circumferential groove 9b and 9c are provided in both shoulder regions 15a and 15b. Directional groove 9a,
9d, the width W _{s of the} narrow rubberized layer 6
Was similarly measured between the width center positions of both first circumferential grooves 9b and 9c, which was wider than the distance L1 measured along the tire width direction, and between the width center positions of both second circumferential grooves 9a and 9d. Distance L2 or less.

[0051] possible to increase the width W _s than the distance L1 is advantageously to effectively reduce cavity resonance noise while maintaining the steering stability, also the width W _s is than the distance L2 It is not preferable to increase the width as well because the effect of reducing the cavity resonance sound is reduced. Therefore, if any one of the above configurations is adopted, the cavity resonance can be more effectively reduced.

The central region of the tread portion refers to a tread region within a range of 50% or less of the tread width TW around the tire equatorial plane, and the shoulder region refers to a tread region other than the central region.

Also, the number of circumferential grooves to be provided is two first grooves.
It is preferable that there are a total of four circumferential grooves 9b, 9c and two second circumferential grooves 9a, 9d in order to locally deform the widthwise rigidity of the tread portion.

In addition, when it is necessary to increase the local widthwise rigidity of the central region or the shoulder region of the tread portion, the first circumferential grooves 9b, 9b, It is preferable that 9c or the second circumferential grooves 9a and 9d be narrower and shallower than the other circumferential grooves. Figure 2
The second circumferential grooves 9a, 9d, which are respectively provided in both shoulder regions 15a, 15b of the tread portion, are provided in two first grooves provided in the central region 14.
This shows an example in the case where the width is smaller and the bottom is shallower than the circumferential grooves 9b and 9c.

The above is only an example of the embodiment of the present invention, and various changes can be made within the scope of the claims. For example, the cords are arranged substantially parallel to the tire equatorial plane so as to cover both ends of the cross belt layer or the cap layer on the outer circumference of the cross belt layer or on the outer circumference of the cap layer covering the cross belt layer. A pair of layer layers composed of a narrow rubberized layer of the cord may be provided.

[0056]

Next, a pneumatic tire according to the present invention was prototyped and its performance was evaluated, and will be described below. Example The tire of the example has a tire width direction cross section shown in FIG. 1, has a tire size of 195 / 65R14, and has a tread 11
, Four circumferential grooves 9a to 9d extending along the tire circumferential direction
(Width: 8 mm, depth: 8 mm), the distance L1 between the first circumferential grooves 9b and 9c and the distance between the second circumferential grooves 9a and 9d among the circumferential grooves 9a to 9d. L2 was 27 mm and 87 mm, respectively. The belt 8 is an intersecting belt 7 (cord material: steel, cord arrangement angle: ± 24 °) in which two cord rubberized layers 5 and 6 are laminated so that the cords intersect, and the narrow rubberized layer 6 is formed. the width W _s 110 mm, the width W _L of the wide rubberized layer 5 is 150 mm. The carcass 3 is a one-ply rubberized cord (cord material: PET, cord angle: 90
°). For other tire structures,
The structure was almost the same as a normal passenger car tire.

[0057] of Comparative Example 1 Comparative Example 1 tire, 140m the width W _s of the narrow-width rubberized layer 6
and m, except that a 93% ratio of the width W _L of the width 5 of the wide rubberized layer width W _s of the narrow rubberized layer 6 has a similar construction as the tire of Example.

[0058] of Comparative Example 2 Comparative Example 2 tire, 120m the width W _s of the narrow-width rubberized layer 6
m and the ratio of the width W _L of the wide rubberized layer 5 to the width W _L of the wide rubberized layer 6 was set to 80%, except that the width W _s of the narrow rubberized layer 6 was set to 80%.

(Performance evaluation) For each of the above test tires,
The tire mode frequency and the vehicle interior noise were measured, and the steering stability was also evaluated. The tire mode frequency was determined by assembling each of the test tires above on a standard aluminum rim (14-6JJ), setting the tire wheel with an air pressure of 200 kPa on an impact tester, and applying an exciting force to the tread of the tire wheel. Was measured.

The vehicle interior noise was measured by mounting the tire wheels on a 2000cc class passenger car (tire load: equivalent to two passengers).
The vehicle was run on a rougher asphalt road (test course) at a vehicle speed of 50 km / h. At this time, the measurement was performed by a measuring instrument installed at a position near the driver's ear.

The steering stability was evaluated by a sensory evaluation by a driver on a test course.

Table 1 shows the results of these measurements. Table 1
The frequency of the middle tire mode is indicated by the amount of reduction (Hz) with respect to the conventional example, and the vehicle interior noise is indicated by the amount of reduction (dB) of the noise level with respect to the comparative example 1. A case where the reduction amount (dB) was 1 dB or more was regarded as a pass level. Further, the steering stability was 100% in Comparative Example 1.
The larger the value, the better the steering stability.

[0063]

[Table 1]

From the results shown in Table 1, the noise level of the example was lower than that of the comparative examples 1 and 2, and the steering stability was almost the same level as the comparative example 1.

[0065]

According to the present invention, the cavity resonance generated due to the length of the circular tube inside the toroidal tire and contributing to the generation of unpleasant cabin noise can be reduced without sacrificing other performances. It has become possible to provide a pneumatic tire that has been effectively reduced.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view in the width direction of a pneumatic tire according to the present invention.

FIG. 2 is an enlarged view of a main part of a tire tread part of FIG.

FIG. 3 is a diagram plotting a cavity resonance peak level with respect to a width ratio (%) of a narrow rubberized layer and a wide rubberized layer when a tire is vibrated.

FIGS. 4A to 4D are conceptual diagrams of tires having different belt configurations.

FIG. 5 is a diagram for explaining a deformed state of the tire in the two-bar mode.

FIG. 6 is a diagram for explaining a deformed state of the tire in the four-bar mode.

FIG. 7 is a diagram illustrating an example of plotting a force transmission rate with respect to a frequency.

FIG. 8 is a diagram for explaining a force F _spindle transmitted to an axle force when an exciting force F _input is applied to a tire tread portion by an impact tester.

FIG. 9 shows an outline (solid line) 2
Tire outline (broken line) and 4 when deformed in knot mode
Outline of tire when deformed in knot mode (dashed line)
FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Pneumatic tire 2 Tire equatorial plane 3 Carcass 4 Crown part of carcass 3 5,6 Rubberized layer 7 Cross belt layer 8 Belt 9a-9d Circumferential groove 10a-10d Land row 11 Tread part 12a-12d Circumferential groove Arrangement position 13 Cap layer 14 Central area 15a, 15b Shoulder area W _s Narrow rubberized layer width W _L Wide rubberized layer width TW Tread width L1 Distance between both first circumferential groove centers L2 Both second circumferential direction Groove center distance

Claims (8)

[Claims] 1. A carcass of at least one ply obtained by rubberizing cords that are arranged at an angle of 70 to 90 ° with respect to the tire equatorial plane, and at the outer periphery of a crown portion of the carcass,
At least two rubberized layers of cords extending at an angle to the tire equatorial plane, and two rubberized layers of the rubberized layers are arranged such that the cords cross each other across the tire equatorial plane. In a pneumatic tire comprising a belt comprising a crossed belt layer and a tread portion, the width of a narrow rubberized layer among the two rubberized layers constituting the crossed belt layer is as follows. A pneumatic tire, wherein the width is 75% or less of the width of the wide rubberized layer. 2. The pneumatic tire according to claim 1, wherein the width of the wide rubberized layer is 70% or more of the tread width. 3. The tread portion has at least two circumferential grooves extending parallel to the tire equatorial plane, and two of the circumferential grooves have a tire equatorial plane in a central region. 2. The first circumferential groove disposed between the first circumferential grooves, wherein the width of the narrow rubberized layer is wider than the distance measured along the tire width direction between the width center positions of the first circumferential grooves. 3. Or the pneumatic tire described in 2. 4. The tread portion has at least two circumferential grooves extending parallel to the tire equatorial plane, and two of the circumferential grooves have one circumferential groove in each of the shoulder regions.
2. The second circumferential grooves provided one by one, wherein the width of the narrow rubberized layer is equal to or less than the distance measured along the tire width direction between the width center positions of both the second circumferential grooves. 3. Or the pneumatic tire described in 2. 5. The tread portion has at least four circumferential grooves extending parallel to the tire equatorial plane, and two of the circumferential grooves have a tire equatorial plane in a central region. The other two circumferential grooves are second circumferential grooves provided one each in both shoulder regions, and the second circumferential groove is provided with the narrow rubberized layer. The width is wider than the distance measured along the tire width direction between the width center positions of both first circumferential grooves, and is equal to or less than the distance similarly measured between the width center positions of both second circumferential grooves. The pneumatic tire according to 1 or 2. 6. The pneumatic tire according to claim 4, wherein the number of circumferential grooves is four, that is, two first circumferential grooves and two second circumferential grooves. 7. The pneumatic tire according to claim 4, wherein the first circumferential groove is narrower and shallower than the second circumferential groove. 8. The pneumatic tire according to claim 4, wherein the second circumferential groove is narrower and shallower than the first circumferential groove.