Patent Publication Number: US-2021162815-A1

Title: Pneumatic Tire

Description:
TECHNICAL FIELD 
     The technology relates to a pneumatic tire and particularly relates to a pneumatic tire with improved noise performance of the tire. 
     BACKGROUND ART 
     In order to improve the wet traction characteristics of a tire, a pneumatic tire for pickup trucks includes a row of blocks defined by through lug grooves in a tread portion center region. However, such a tread pattern has a problem that pass-by noise performance of the tire degrades. The technology described in Japan Unexamined Patent Publication No. H07-40712A is known as a conventional pneumatic tire which aims to improve pass-by noise performance of the tire. 
     SUMMARY 
     The technology provides a pneumatic tire with improved noise performance of the tire. 
     A pneumatic tire according to an embodiment of the technology is a pneumatic tire including: a shoulder main groove and a center main groove disposed in one region demarcated by a tire equatorial plane; and a shoulder land portion and a middle land portion defined by the shoulder main groove, the shoulder land portion being a rib that is continuous in a tire circumferential direction, the middle land portion comprising a plurality of middle lug grooves and a plurality of middle blocks defined by the plurality of middle lug grooves, a section A in the tire circumferential direction being defined along an edge portion, close to the shoulder main groove, of one of the plurality of middle blocks, and a groove width of the shoulder main groove continuously increasing or decreasing in the section A. 
     The pneumatic tire according to an embodiment of the technology provides an advantage that since the groove width of the shoulder main groove increases or decreases continuously, air column resonance generated in the shoulder main groove is reduced, and the pass-by noise performance of the tire is improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view in a tire meridian direction illustrating a pneumatic tire according to an embodiment of the technology. 
         FIG. 2  is a plan view illustrating a tread surface of the pneumatic tire illustrated in  FIG. 1 . 
         FIG. 3  is an enlarged view illustrating a tread portion center region of the pneumatic tire illustrated in  FIG. 2 . 
         FIG. 4  is an enlarged view illustrating main parts of the tread portion center region illustrated in  FIG. 3 . 
         FIG. 5  is a cross-sectional view illustrating main parts of a middle land portion. 
         FIG. 6  is an explanatory diagram illustrating a deformed example of the pneumatic tire illustrated in  FIG. 2 . 
         FIG. 7  is an enlarged view illustrating a tread portion shoulder region of the pneumatic tire illustrated in  FIG. 2 . 
         FIG. 8  is an enlarged view illustrating main parts of the tread portion shoulder region illustrated in  FIG. 7 . 
         FIG. 9  is a graph illustrating the change in a groove width of a shoulder main groove illustrated in  FIG. 8 . 
         FIGS. 10A and 10B  include a table showing the results of performance tests of pneumatic tires according to embodiments of the technology. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the technology are described in detail below with reference to the drawings. However, the technology is not limited to these embodiments. Moreover, constituents of the embodiments include elements that are substitutable while maintaining consistency of the technology, and obviously substitutable elements. Furthermore, the plurality of deformed examples described in the embodiments can be combined as desired within the scope apparent to one skilled in the art. 
     Pneumatic Tire 
       FIG. 1  is a cross-sectional view in a tire meridian direction illustrating a pneumatic tire according to an embodiment of the technology. The same drawing illustrates a cross-sectional view of a half region in the tire radial direction. Moreover, the same drawing illustrates an all-position tire for pickup trucks as an example of a pneumatic tire. 
     In reference to the same drawing, “cross section in a tire meridian direction” refers to a cross section of the tire taken along a plane that includes the tire rotation axis (not illustrated). Reference sign CL denotes the tire equatorial plane and refers to a plane normal to the tire rotation axis that passes through the center point of the tire in the tire rotation axis direction. “Tire width direction” refers to the direction parallel with the tire rotation axis. “Tire radial direction” refers to the direction perpendicular to the tire rotation axis. 
     A pneumatic tire  1  has an annular structure with the tire rotation axis as its center and includes: a pair of bead cores  11 ,  11 , a pair of bead fillers  12 ,  12 , a carcass layer  13 , a belt layer  14 , a tread rubber  15 , a pair of sidewall rubbers  16 ,  16 , and a pair of rim cushion rubbers  17 ,  17  (see  FIG. 1 ). 
     The pair of bead cores  11 ,  11  are formed by winding one or a plurality of bead wires made of steel by multiple times in an annular shape and are embedded in the bead portion to constitute a core of the left and right bead portions. The pair of bead fillers  12 ,  12  are disposed outward of the pair of bead cores  11 ,  11  in the tire radial direction and reinforce the bead portions. 
     The carcass layer  13  has a single layer structure made from one carcass ply or a multilayer structure made from stacking a plurality of carcass plies and spans between the left and right bead cores  11 ,  11  in a toroidal shape to form the framework of the tire. Additionally, both edge portions of the carcass layer  13  are turned back to the outer side in the tire width direction so as to wrap around the bead cores  11  and the bead fillers  12  and fixed. The carcass ply of the carcass layer  13  is formed by performing a rolling process on coating rubber-covered carcass cords made from steel or an organic fiber material (for example, aramid, nylon, polyester, rayon, or the like) and has a carcass angle (defined as the inclination angle in the longitudinal direction of the carcass cords with respect to the tire circumferential direction) ranging from 80° to 90° as an absolute value. 
     The belt layer  14  is formed by laminating a pair of cross belts  141 ,  142  and is disposed around the outer circumference of the carcass layer  13 . The pair of cross belts  141 ,  142  are made by performing a rolling process on coating rubber-covered belt cords made from steel or an organic fiber material. The cross belts  141 ,  142  have a belt angle, as an absolute value, ranging from 15° to 55°. Furthermore, the pair of cross belts  141 ,  142  have belt angles (defined as the inclination angle in the longitudinal direction of the belt cords with respect to the tire circumferential direction) of opposite signs and are stacked so that the longitudinal directions of the belt cords intersect each other (so-called a crossply structure). 
     The tread rubber  15  is disposed outward of the carcass layer  13  and the belt layer  14  in the tire radial direction and constitutes a tread portion of the tire. The pair of sidewall rubbers  16 ,  16  are disposed on the outer side of the carcass layer  13  in the tire width direction and constitute left and right sidewall portions. The pair of rim cushion rubbers  17 ,  17  extend from the inner sides in the tire radial direction of the turned back portions of the carcass layer  13  and the left and right bead cores  11 ,  11  toward the outer side in the tire width direction to form a rim-fitting surface of the bead portion. 
     Tread Pattern 
       FIG. 2  is a plan view illustrating a tread surface of the pneumatic tire  1  illustrated in  FIG. 1 .  FIG. 2  illustrates a tread surface of an all-position tire for pickup trucks as an example pneumatic tire  1 . In reference to  FIG. 2 , “tire circumferential direction” refers to the direction revolving about the tire rotation axis. Reference sign T denotes a tire ground contact edge, and dimension sign TW denotes a tire ground contact width. 
     As illustrated in  FIG. 2 , the pneumatic tire  1  includes a plurality of circumferential main grooves  21 ,  22  extending in the tire circumferential direction and a plurality of land portions  31  to  33  defined by the circumferential main grooves  21 ,  22 . The plurality of circumferential main grooves  21 ,  22  and the plurality of land portions  31  to  33  are provided in a tread surface. 
     “Main groove” refers to a groove on which a wear indicator must be provided as specified by JATMA (The Japan Automobile Tyre Manufacturers Association, Inc.) and has a groove width of 4.0 mm or greater and a groove depth of 6.5 mm or greater. In particular, in the tire for pickup trucks, the main groove has a groove depth of 10.0 mm or greater. The lug grooves described below are lateral grooves extending in the tire width direction and are open when the tire contacts the ground to function as grooves. Additionally, the sipes described below are cuts formed in the tread contact surface and are distinguished from lug grooves in terms of closing when the tire comes into contact with the ground. 
     The groove width is measured as the distance between the left and right groove walls at the groove opening portion when the tire is mounted on a specified rim, inflated to the specified internal pressure, and in an unloaded state. In a configuration in which the land portions include notch portions or chamfered portions in the edge portions thereof, the groove width is measured with reference to the intersection points between the tread contact surface and the extension lines of the groove walls as measurement points, in a cross-sectional view in which the groove length direction is a normal direction. 
     The groove depth is measured as the maximum distance from the tread contact surface to the groove bottom when the tire is mounted on a specified rim, inflated to the specified internal pressure, and in an unloaded state. Additionally, in a configuration in which the grooves include an uneven portion or sipes on the groove bottom, the groove depth is measured excluding these portions. 
     “Specified rim” refers to a “standard rim” defined by JATMA, a “Design Rim” defined by TRA (The Tire and Rim Association, Inc.), or a “Measuring Rim” defined by ETRTO (The European Tyre and Rim Technical Organisation). Additionally, “specified internal pressure” refers to a “maximum air pressure” defined by JATMA, to the maximum value in “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” defined by TRA, or to “INFLATION PRESSURES” defined by ETRTO. Additionally, “specified load” refers to a “maximum load capacity” defined by JATMA, the maximum value in “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” defined by TRA, or “LOAD CAPACITY” defined by ETRTO. However, in the case of JATMA, for a passenger vehicle tire, the specified internal pressure is an air pressure of 180 kPa, and the specified load is 88% of the maximum load capacity. 
     For example, in the configuration of  FIG. 2 , the pneumatic tire  1  has an approximately point symmetric tread pattern having a center point on the tire equatorial plane CL. However, no such limitation is intended, and, for example, the pneumatic tire  1  may have a left-right line symmetric tread pattern or a left-right asymmetric tread pattern with respect to the tire equatorial plane CL as the center and may have a tread pattern having directionality in the tire rotation direction (not illustrated). 
     Furthermore, in the configuration of  FIG. 2 , the left and right regions demarcated by the tire equatorial plane CL each have two circumferential main grooves  21 ,  22 . These circumferential main grooves  21 ,  22  are disposed in left-right symmetry with respect to the tire equatorial plane CL as the center. Five rows of land portions  31  to  33  are defined by these circumferential main grooves  21 ,  22 . In addition, one land portion  33  is disposed on the tire equatorial plane CL. 
     However, no such limitation is intended, and five or more circumferential main grooves may be disposed, or the circumferential main grooves may be arranged in left-right asymmetry with respect to the tire equatorial plane CL as the center (not illustrated). In addition, the land portion may be arranged at a position off from the tire equatorial plane CL, with one circumferential main groove being arranged on the tire equatorial plane CL (not illustrated). 
     Additionally, among the circumferential main grooves  21 ,  22  disposed in one region demarcated by the tire equatorial plane CL, the circumferential main groove  21  on the outermost side in the tire width direction is defined as the shoulder main groove, and the circumferential main groove  22  close to the tire equatorial plane CL is defined as the center main groove. 
     For example, in the configuration of  FIG. 2 , a distance D 1  from the tire equatorial plane CL to the groove center line of the left and right shoulder main grooves  21 ,  21  is in a range of from 45% or greater to 75% or smaller of the tire ground contact half-width TW/2. A distance D 2  from the tire equatorial plane CL to the groove center line of the left and right center main grooves  22 ,  22  is in a range of from 10% or greater to 30% or smaller of the tire ground contact half-width TW/2. 
     The groove center line is defined as an imaginary line connecting the midpoints of the distance between the left and right groove walls. 
     In a case where the groove center line of the main groove has a zigzag shape or a wave-like shape, the distance to the groove center line is measured using a straight line parallel to the tire circumferential direction passing through the midpoint of the maximum amplitude positions on the left and right of the groove center line as the measurement point. 
     The tire ground contact width TW is measured as the maximum linear distance in the tire axial direction of a contact surface between the tire and a flat plate when the tire is mounted on a specified rim, inflated to the specified internal pressure, placed perpendicular to the flat plate in a static state, and loaded with a load corresponding to the specified load. 
     The tire ground contact edge T is defined as the maximum width position in the tire axial direction of the contact surface between the tire and a flat plate when the tire is mounted on a specified rim, inflated to the specified internal pressure, placed perpendicular to the flat plate in a static state, and loaded with a load corresponding to the specified load. 
     The land portion  31  located on the outer side in the tire width direction defined by the outermost circumferential main groove  21  is defined as a shoulder land portion. The shoulder land portion  31  is a land portion on the outermost side in the tire width direction and is located on the tire ground contact edge T. 
     The land portion  32  located on the inner side in the tire width direction defined by the outermost circumferential main groove  21  is defined as a middle land portion. Accordingly, the middle land portion  32  is adjacent to the shoulder land portion  31  with the outermost circumferential main groove  21  disposed therebetween. 
     Furthermore, the land portion  33  located closer to the tire equatorial plane CL than the middle land portion  32  is defined as a center land portion. The center land portion  33  may be disposed on the tire equatorial plane CL (see  FIG. 2 ) or may be arranged at a position off from the tire equatorial plane CL (not illustrated). 
     Note that in a configuration including four circumferential main grooves as in  FIG. 2 , a pair of shoulder land portions  31 ,  31 , a pair of middle land portions  32 ,  32 , and a single center land portion  33  are defined. Additionally, for example, in a configuration including five or more circumferential main grooves, two or more rows of center land portions are defined (not illustrated). Moreover, in a configuration including three circumferential main grooves, the middle land portion also serves as the center land portion (not illustrated). 
     Middle Land Portion and Center Land Portion 
       FIG. 3  is an enlarged view illustrating the tread portion center region of the pneumatic tire  1  illustrated in  FIG. 2 . The same drawing illustrates a row of blocks of the middle land portion  32  and the center land portion  33 .  FIG. 4  is an enlarged view illustrating main parts of the tread portion center region illustrated in  FIG. 3 .  FIG. 5  is a cross-sectional view illustrating main parts of the middle land portion  32 . The same drawing illustrates a cross-sectional view including the notch portion  323  and a middle sipe  324  in a middle block  322 . 
     As illustrated in  FIG. 2 , the middle land portion  32  includes: a plurality of middle lug grooves  321 , a plurality of the middle blocks  322 , the notch portion  323 , and a plurality of the middle sipes  324 . Therefore, the middle land portion  32  forms a row of blocks. 
     The middle lug groove  321  passes through the middle land portion  32  in the tire width direction and is open to the left and right circumferential main grooves  21 ,  22  partitioning the middle land portion  32 . Additionally the plurality of middle lug grooves  321  are disposed at a predetermined interval in the tire circumferential direction. Moreover, a maximum groove width W 21  (see  FIG. 3 ) of the middle lug groove  321  is in a range of 5.0 mm≤W 21 ≤15 mm. A maximum groove depth H 21  (not illustrated) of the middle lug groove  321  is in a range of 4.0 mm≤H 21 , and a maximum groove depth H 1  (not illustrated) of the shoulder main groove  21  has a relationship of 0.30≤H 21 /H 1 ≤1.00. 
     For example, in the configuration of  FIG. 3 , the center main groove  22  has a bent shape formed by alternately connecting an arc-shaped long portion  222  and a linear short portion  221  in the tire circumferential direction. Moreover, the groove width of the center main groove  22  takes a minimum value W 2 ′ at the connection portion (the short portion  221 ) with the first middle lug groove  321  and takes a maximum value W 2  at the connecting portion (the long portion  222 ) with the second middle lug groove  321 . In addition, the linear short portion  221  has a constant groove width. Additionally, the groove width of the arc-shaped long portion  222  increases as it is left from the tire equatorial plane CL. 
     Additionally, the middle lug groove  321  has an arc shape curved in the tire circumferential direction, and the maximum groove width W 21  of the middle lug groove  321  gradually decreases toward the tire equatorial plane CL. Additionally, adjacent middle lug grooves  321 ,  321  are curved in the same direction. The middle lug groove  321  is smoothly connected to the long portion  222  of the center main groove  22 . Therefore, the center main groove  22  appears to be formed by being alternately connected to the short portion  221  and the long portion  222  extended from the middle lug groove  321 . 
     The middle block  322  is defined by adjacent middle lug grooves  321 ,  321 . Additionally, the plurality of middle blocks  322  are arranged in a row in the tire circumferential direction. A ground contact width Wb 2  of the middle block  322  preferably has a relationship of 0.25≤Wb 2 /(TW/2)≤0.50 with respect to the tire ground contact half-width TW/2 and more preferably has a relationship of 0.30≤Wb 2 /(TW/2)≤0.50. 
     The block ground contact width is measured as the maximum linear distance in the tire axial direction on a contact surface between the block and a flat plate when the tire is mounted on a specified rim, inflated to the specified internal pressure, placed perpendicular to the flat plate in a static state, and loaded with a load corresponding to the specified load. 
     Moreover, as illustrated in  FIG. 3 , the edge portion on the inner side in the tire width direction and the edge portion on the outer side in the tire width direction of the middle block  322  have a shape convex toward the center main groove  22  and the shoulder main groove  21 . Additionally, the edge portion on the inner side in the tire width direction of the middle block  322  has a shape corresponding to the long portion  222  and the short portion  221  of the center main groove  22 , that is, a bent shape in which an arc-shaped long portion and a linear short portion are connected. Additionally, the edge portion on the outer side in the tire width direction of the middle block  322  has an arc shape formed from a single arc. 
     As illustrated in  FIG. 3 , the notch portion  323  is formed in the center portion of the edge portion, close to the tire equatorial plane CL, of the middle block  322 . Moreover, a single notch portion  323  is formed in each middle block  322 . Furthermore, the notch portion  323  is formed on an extension line of center lug grooves  331  described below and is open toward the groove opening portion of the center lug groove  331 . Due to this, the drainage properties of the tread portion center region are improved, and the tire wet traction characteristics are improved. Additionally, an opening width W 23  of the notch portion  323  preferably has a relationship of 0.20≤W 23 /W 31 ≤1.20 with respect to a maximum groove width W 31  of the center lug groove  331  described later and more preferably has a relationship of 0.80≤W 23 /W 31 ≤1.20. 
     In the configuration of  FIG. 2 , as illustrated in  FIG. 3 , the notch portion  323  of the middle block  322  is formed on an extension line of the center lug groove  331  and is open toward the groove opening portion of the center lug groove  331 . The notch portion  323  is on an extension line of the groove center line of the center lug groove  331 . Moreover, the width W 23  of the notch portion  323  is set to be approximately the same as an opening width W 31 ′, close to the center main groove  22 , of the center lug groove  331  (dimension sign is omitted in the drawing, in  FIG. 3 , W 31 =W 31 ′). 
     The notch portion is defined as a step-shaped recess portion (that is, a step portion) having a bottom surface parallel to the road contact surface of the land portion. The notch portion includes a cut groove or a short lug groove. 
     The opening width of the notch portion is measured as the maximum value of the opening width with respect to the circumferential main groove in a plan view of the block road contact surface. Additionally, in a case where the block has a chamfered portion (reference sign  325  in  FIG. 4 ) in the opening portion of the notch portion, the opening width of the notch portion is measured using the intersection point of the extension line of the edge portion of the block and the extension line of the wall surface of the notch portion as a measurement point. 
     The chamfered portion is defined as a portion that connects, with a flat surface or a curved surface, the intersection portion between the road contact surface of the land portion and the groove wall surface. 
     In  FIG. 4 , an extension length L 23  of the notch portion  323  in the tire width direction preferably has a relationship of 0.10≤L 23 /Wb 2 ≤0.90 with respect to the ground contact width Wb 2  of the middle block  322  and more preferably has a relationship of 0.10≤L 23 /Wb 2 ≤0.50. 
     The extension length of the notch portion is measured as the maximum value of the extension length of the notch portion in the block road contact surface. 
     Additionally, as illustrated in  FIG. 4 , the notch portions  323  are preferably inclined in the same direction in the tire circumferential direction with respect to the center lug grooves  331 . In addition, an inclination angle θ 23  of the notch portion  323  with respect to the tire circumferential direction is preferably in the range of 35°≤θ 23 ≤85° and more preferably in the range of 60°≤θ 23 ≤75°. The inclination angle θ 23  of the notch portion  323  preferably has a relationship of θ 31 ≤θ 23  with respect to an inclination angle θ 31  of the center lug grooves  331  with respect to the tire circumferential direction. 
     The inclination angle of the notch portion is measured as the angle between the tire circumferential direction and a straight line connecting the opening portion and the terminating end portion of the notch portion. 
     In  FIG. 5 , a depth H 23  of the notch portion  323  preferably has a relationship 0.15≤H 23 /H 2 ≤0.35 with respect to a groove depth H 2  of the center main groove  22 . Additionally, the depth H 23  of the notch portion  323  is preferably deeper than a depth H 25  of the chamfered portion  325 . Additionally, the chamfered portion  325  may be omitted (not illustrated). 
     The depth of the notch portion is measured as the maximum depth from the block road contact surface to the step portion of the notch portion. 
     The depth of the chamfered portion is measured as the maximum depth from the block road contact surface. 
     As illustrated in  FIG. 3 , the middle sipe  324  extends from the notch portion  323  in the tire width direction and is open to the edge portion on the outer side in the tire width direction of the middle block  322 . With this middle sipe  324 , the difference in rigidity between the center portion of the middle block  322  and the circumferential edge portion (the edge portion close to the middle lug groove  321 ) is mitigated, and thus, heel and toe wear of the tire is suppressed. Additionally, the drainage properties of the road contact surface of the middle blocks  322  are improved, and the wet traction characteristics of the tire is improved. In the configuration illustrated in  FIG. 3 , the middle sipe  324  has a gentle arc shape with an inclination angle with respect to the tire circumferential direction increased toward the shoulder main groove  21  and extends substantially in parallel to the middle lug groove  321 . Additionally, the middle sipe  324  may extend along the notch portion  323  to pass through the step portion of the notch portion  323  (see  FIG. 5 ) and may terminate at the connection portion with the notch portion  323  (not illustrated). 
     As illustrated in  FIG. 3 , the middle sipe  324  is open to the center portion of the edge portion of the middle block  322  close to the shoulder main groove  21 . Specifically, the distance (dimension sign is omitted in the drawing) in the tire circumferential direction from the measurement point of a circumferential length Le 2  of the edge portion, close to the shoulder main groove  21 , of the middle block  322  to the opening portion of the middle sipe  324  is preferably in the range of from 30% or greater to 70% or smaller, with respect to the circumferential length Le 2 , and more preferably in the range of from 35% or greater to 65% or smaller. 
     In the configuration of  FIG. 3 , the middle block  322  includes a single middle sipe  324  and does not include other sipes or narrow grooves. Due to this, the rigidity of the middle block  322  can be increased. Furthermore, the ground contact area ratio of the portions of the middle blocks  322  defined by the notch portions  323  and the middle sipes  324  is preferably in the range of from 90% or greater to 110% or smaller and more preferably in the range of from 95% or greater to 105% or smaller. Due to this, the ground contact area of each portion of the middle block  322  is made uniform, and uneven wear of the middle blocks  322  is suppressed. 
     “Sipe” refers to a cut formed in a tread contact surface and has a sipe width of less than 1.5 mm and a sipe depth of 2.0 mm or greater, so that the sipe closes when the tire comes into contact with the ground. 
     The sipe width is measured as the maximum distance of the opening width of the sipe at the road contact surface of the land portion, when the tire is mounted on a specified rim, inflated to the specified internal pressure, and in an unloaded state. 
     The sipe depth is measured as the maximum distance from the tread contact surface to the sipe bottom, when the tire is mounted on a specified rim, inflated to the specified internal pressure, and in an unloaded state. Additionally, in a configuration in which a sipe includes a ridged/grooved portion on the groove bottom, the sipe depth is measured excluding this portion. 
     Additionally, as illustrated in  FIG. 4 , the middle sipe  324  is inclined in the opposite direction in the tire circumferential direction with respect to the notch portion  323 . Due to this, the connection portion between the middle sipe  324  and the notch portion  323  has a bent shape that protrudes in the tire circumferential direction. In addition, an inclination angle θ 24  of the middle sipe  324  with respect to the tire circumferential direction is preferably in the range of 10°≤θ 24 ≤80° and more preferably in the range of 55°≤θ 24 ≤75°. In addition, an angle α formed between the notch portion  323  and the middle sipe  324 , that is, the bend angle of the connection portion between the middle sipe  324  and the notch portion  323  is preferably in the range of 75°≤α≤175° and more preferably within the range of 100°≤α≤145°. 
     The inclination angle of the sipe is measured as the angle between the tire circumferential direction and a straight line passing through both end portions of the sipe in the block road contact surface. 
     In addition, a depth H 24  of the middle sipe  324  preferably has a relationship of H 24 /H 21 ≤0.95 with respect to the groove depth H 21  (not illustrated) of the middle lug groove  321  and more preferably has a relationship of H 24 /H 21 ≤0.90. Due to this, the middle sipe  324  is shallower than the middle lug groove  321 . Additionally, the depth H 24  (see  FIG. 5 ) of the middle sipe  324  preferably has a relationship of 0.30≤H 24 /H 1 ≤0.60 with respect to the groove depth H 1  (not illustrated) of the shoulder main groove  21 . 
     Note that, in the configuration of  FIG. 2 , as described above, the middle block  322  includes the middle sipe  324 . In this case, the edge portion of the middle sipe  324  may be chamfered. Furthermore, a narrow groove (not illustrated) may be disposed in place of the middle sipe  324 . In such a case, it is necessary that the groove width of the narrow grooves is in the range of from 0.9 mm or greater to 3.0 mm or smaller, and the groove depth is in the range of from 5.0 mm or greater to 10 mm or smaller. 
     In addition, in the configuration of  FIG. 2 , the middle sipe  324  has an arc shape, but no such limitation is intended, and the middle sipe  324  may have a straight shape, an S-shape, a bent shape, or the like (not illustrated). Additionally, a chamfered portion may be formed in the edge portion of the middle sipe  324  (not illustrated). 
     As illustrated in  FIG. 2 , the center land portion  33  includes: a plurality of the center lug grooves  331 , a plurality of center blocks  332 , and a plurality of center sipes  333 . 
     The center lug groove  331  passes through the center land portion  33  in the tire width direction and is open to the left and right center main grooves  22 ,  22  partitioning the center land portion  33 . Additionally, the plurality of center lug grooves  331  are arranged at a predetermined interval in the tire circumferential direction. 
     In  FIG. 3 , the maximum groove width W 31  of the center lug groove  331  is in the range of 2.5 mm≤W 31 ≤7.0 mm. Additionally, the maximum groove width W 21  of the middle lug groove  321  preferably has a relationship of 1.10≤W 21 /W 31 ≤3.50 with respect to the maximum groove width W 31  of the center lug groove  331  and more preferably has a relationship of 1.50≤W 21 /W 31 ≤2.50. 
     As illustrated in  FIG. 4 , the center lug groove  331  is inclined in the opposite direction in the tire circumferential direction with respect to the middle lug groove  321  of the middle land portion  32 . The inclination angle θ 31  of the center lug groove  331  with respect to the tire circumferential direction is preferably in the range of 35°≤θ 31 ≤85° and more preferably in the range of 65°≤θ 31 ≤80°. 
     Additionally, a groove depth H 31  (not illustrated) of the center lug groove  331  is in the range of 5.0 mm≤H 31 ≤16 mm. The groove depth H 31  of the center lug groove  331  has a relationship of 0.80≤H 31 /H 21 ≤1.20 with respect to the groove depth H 21  (not illustrated) of the middle lug groove  321 . Additionally, in the configuration of  FIG. 3 , the center lug groove  331  has a linear shape. 
     The center block  332  is defined by adjacent center lug grooves  331 ,  331 . Additionally, the plurality of center blocks  332  are arranged in a row in the tire circumferential direction. 
     In  FIG. 3 , the ground contact width Wb 3  of the center block  332  preferably has a relationship of 0.80≤Wb 3 /Wb 2 ≤1.40 with respect to the ground contact width Wb 2  of the middle block  322  and more preferably has a relationship of 0.90≤Wb 3 /Wb 2 ≤1.20. 
     As illustrated in  FIG. 3 , the edge portion in the tire width direction of the center block  332  has a shape in which two arc portions recessed with respect to the left and right center main grooves  22 ,  22  are connected. Specifically, the edge portion of the center block  332  has a shape corresponding to a pair of the long portions  222 ,  222  and one short portion  221  of the center main groove  22 , that is, a bent shape that connects a pair of arc portions and one linear portion. Additionally, the center block  332  is on the tire equatorial plane CL and has a point symmetrical shape. Additionally, the maximum ground contact width Wb 3  and a minimum ground contact width Wb 3 ′ of the center block  332  preferably have a relationship of 0.50≤Wb 3 ′/Wb 3 ≤0.90 and more preferably have a relationship of 0.60≤Wb 3 ′/Wb 3 ≤0.80. 
     Additionally, in the configuration of  FIG. 3 , the center main groove  22  has a see-through-less structure in which the edge portion of the middle block  322  and the edge portion of the center block  332  overlap in the tire width direction. In other words, the measurement point of the ground contact width Wb 2  of the middle land portion  32  on the center main groove  22  side is closer to the tire equatorial plane CL than the measurement point of the ground contact width Wb 3  of the center land portion  33 . Due to this, the ground contact area ratio of the tread portion center region is enhanced. 
     As illustrated in  FIG. 3 , the center sipe  333  passes through the center block  332  in the tire width direction and is open to the left and right center main grooves  22 ,  22 . The center sipe  333  has a step shape with two bent portions and extends substantially in parallel to the center lug groove  331  at the left and right long portions (reference sign is omitted in the drawing) connected to the center main grooves  22 ,  22 . 
     Additionally, in  FIG. 4 , an inclination angle θ 33  of the center sipe  333  with respect to the tire circumferential direction is preferably in the range of 50°≤θ 33 ≤130°. Additionally, a depth H 33  (not illustrated) of the center sipe  333  preferably has a relationship of 1.05≤H 33 /H 31  with respect to the depth H 31  of the center lug groove  331  of the center land portion  33  and more preferably has a relationship of 1.10≤H 33 /H 31 . Therefore, the center sipe  333  is deeper than the center lug groove  331 . Therefore, the depth relationship between the sipe  333  and the lug groove  331  in the center land portion  33  has an opposite tendency with respect to the depth relationship between the sipe  324  and the lug groove  321  in the middle land portion  32 . Note that the upper limit of the depth H 33  of the center sipe  333  is not particularly limited, but is restricted by the range of the ratio H 33 /H 31  described above. 
     Deformed Examples 
       FIG. 6  is an explanatory diagram illustrating a deformed example of the pneumatic tire  1  illustrated in  FIG. 2 . The same drawing illustrates the relationship between the notch portion  323  and the middle sipe  324  of the middle block  322 . 
     In the configuration of  FIG. 2 , as illustrated in  FIGS. 3 and 4 , the middle sipe  324  is connected to and open to the notch portion  323 , and the middle sipe  324  extends from the opening position and is open to the edge portion of the middle block  322  close to the shoulder main groove  21 . Such a configuration is preferable in that the drainage function of the middle sipe  324  is improved. 
     However, no such limitation is intended, and as illustrated in  FIG. 6 , the middle sipe  324  may terminate at or near the notch portion  323  without being connected to the notch portion  323 , and the middle sipe  324  may extend from this termination position and be open to the edge portion of the middle block  322  close to the shoulder main groove  21 . In this case, if a distance Dg between the middle sipe  324  and the notch portion  323  is in the range of Dg≤1.0 mm, it can be said that the middle sipe  324  extends from the notch portion  323 . Such a configuration is preferable in that, during tire vulcanization molding, since a small gap can be formed between the molding blade of the middle sipe  324  and the molding blade of the notch portion  323  in the tire mold (not illustrated), vulcanization defects due to dead air space can be reduced. The lower limit of the distance Dg is not particularly limited, but if the distance Dg is 0.3 mm or greater, the air flow path is ensured, and the effect of reducing vulcanization defects is ensured. 
     Shoulder Land Portion and Second Land Portion 
       FIG. 7  is an enlarged view illustrating the tread portion shoulder region of the pneumatic tire  1  illustrated in  FIG. 2 . The same drawing illustrates a row of blocks of the shoulder land portion  31  and the middle land portion  32 .  FIG. 8  is an enlarged view illustrating main parts of the tread portion shoulder region illustrated in  FIG. 7 . 
     As illustrated in  FIG. 2 , the shoulder land portion  31  includes only a plurality of first and second shoulder sipes  311 ,  312  and does not include a through lug groove. Due to this, the shoulder land portion  31  is ribs that are continuous in the tire circumferential direction. 
     The shoulder sipes  311 ,  312  pass through the shoulder land portion  31  in the tire width direction and are open to the shoulder main groove  21  and the tire ground contact edge T, respectively. Additionally, the first and second shoulder sipes  311 ,  312  are disposed alternately in the tire circumferential direction. Because these shoulder sipes  311 ,  312  are closed when the tire comes into contact with the ground, the shoulder land portions  31  are not divided in the tire circumferential direction. Additionally, the shoulder sipes  311 ,  312  are inclined in mutually opposite directions in the tire circumferential direction with respect to the middle sipes  324  of the middle land portion  32 . 
     As illustrated in  FIG. 7 , the first shoulder sipe  311  is open to the connection portion of an arc portion  313  (see  FIG. 7 ) to be described later, and the second shoulder sipe  312  is open to the maximum protruding position of the arc portion  313 . Additionally, the first and second shoulder sipes  311 ,  312  have a gentle arc shape with an inclination angle with respect to the tire circumferential direction increasing toward the tire ground contact edge T. The first and second shoulder sipes  311 ,  312  are inclined in the same direction with respect to the tire circumferential direction. Additionally, the first and second shoulder sipes  311 ,  312  are inclined in the opposite direction in the tire circumferential direction with respect to the middle sipes  324  of the middle blocks  322 . 
     As illustrated in  FIG. 7 , the edge portion on the inner side in the tire width direction of the shoulder land portion  31  has a continuous arc shape formed by connecting a plurality of the arc portions  313 . In other words, the edge portion of the shoulder land portion  31  includes the plurality of arc portions  313  protruding toward the shoulder main groove  21 , and these arc portions  313  are continuously provided in the tire circumferential direction to form a continuous arc shape. Due to this, uneven wear of the edge portion of the shoulder land portion  31  is suppressed. 
     As described above, the middle land portion  32  includes the plurality of middle blocks  322  defined by the plurality of middle lug grooves  321 , and the edge portion on the outer side in the tire width direction of the middle blocks  322  has an arc shape formed by a single arc portion  326 . Due to this, the continuous arc shape of the edge portion of the shoulder land portion  31  and the arc shape of the edge portion of the middle land portion  32  face each other across the shoulder main groove  21 . 
     One arc portion is defined as a portion defined by adjacent inflection points on a contour line, which is obtained by extracting a contour line of the edge portion on the main groove side of the land portion in a tread plan view. 
     Additionally, the radius of curvature of the arc portions  313 ,  326  is preferably in the range of from 40 mm or greater to 150 mm or smaller. 
     Additionally, as illustrated in  FIG. 7 , the maximum protruding position (reference sign is omitted in the drawing) of the arc portion  313  of the shoulder land portion  31  and the maximum protruding position of the arc portion  326  of the middle block  322  are arranged in a staggered manner in the tire circumferential direction. Due to this, the arc portion  313  of the shoulder land portion  31  faces the opening portion of the middle lug groove  321  of the middle land portion  32 , and the arc portion  326  of the middle block  322  faces the connection portion (in  FIG. 7 , the opening portion of the first shoulder sipe  311 ) of the adjacent arc portions  313  of the shoulder land portion  31 . 
     A distance Dp in the tire circumferential direction between the maximum protruding position of the arc portion  313  of the shoulder land portion  31  and the maximum protruding position of the arc portion  326  of the middle block  322  preferably has a relationship of 0.20≤Dp/P 1 ≤0.60 with respect to a pitch length P 1  of the arc portion  313  of the shoulder land portion  31  and more preferably has a relationship of 0.35≤Dp/P 1 ≤0.50. Accordingly, the arc portions  313 ,  326  of the shoulder land portion  31  and the middle land portion  32  are arranged in a staggered manner in the tire circumferential direction. 
     Additionally, a ground contact width Wb 1  of the shoulder land portion  31  and the ground contact width Wb 2  of the middle land portion  32  preferably have a relationship of 0.80≤Wb 2 /Wb 1 ≤1.20 and more preferably have a relationship of 0.90≤Wb 2 /Wb 1 ≤1.10. 
     As illustrated in  FIG. 7 , the shoulder main groove  21  has a see-through structure in the tire circumferential direction. That is, the edge portions of the left and right land portions  31 ,  32  do not overlap in the tire width direction when projected in the tire circumferential direction. Additionally, a see-through width Dt and a maximum groove width W 1  of the shoulder main groove  21  preferably have a relationship of 0.60≤Dt/W 1 ≤0.90 and more preferably have a relationship of 0.70≤Dt/W 1 ≤0.80. Due to this, the pass-by noise performance and the wet performance of the tire are improved. 
     The see-through width Dt is measured as the distance Dt in the tire width direction at the maximum width position of the left and right land portions. 
     Additionally, in  FIG. 8 , a circumferential length La 1  and the pitch length P 1  of the arc portion  313  of the shoulder land portion  31  preferably have a relationship of 0.80≤La 1 /P 1 ≤1.00 and more preferably have a relationship of 0.90≤La 1 /P 1 ≤1.00. Accordingly, the circumferential length La 1  of one arc portion  313  is substantially equal to the pitch length P 1  (see  FIG. 7 ) of the arc portion  313 , and adjacent arc portions  313  are disposed adjacent to each other to form the edge portion of the shoulder land portion  31 . 
     The circumferential length of the arc portion is measured as the extension length in the tire circumferential direction of a chord connecting both end portions of the arc. 
     In addition, a circumferential length La 2  of the arc portion  326  of the middle block  322  and the circumferential length Le 2  of the edge portion thereof preferably have a relationship of 0.80≤La 2 /Le 2 ≤1.00 and more preferably have a relationship of 0.85≤La 2 /Le 2 ≤1.00. Due to this, the circumferential length La 2  of the arc portion  326  is substantially equal to the circumferential length Le 2  of the edge portion, and one arc portion  326  extends across the entire edge portion of the middle block  322  close to the shoulder main groove  21 . 
     The circumferential length of the edge portion of a block is measured as the distance in the tire circumferential direction at each intersection point between the groove wall of the circumferential main groove and the groove walls of a pair of lug grooves, partitioning the block. When the corner portion of the block has a chamfered portion, the intersection point of the groove walls described above is defined by the extension line of the groove wall. 
     Additionally, as illustrated in  FIG. 7 , the distance (dimension sign is omitted in the drawing) in the tire circumferential direction from the end portion of the arc portion  313  of the shoulder land portion  31  to the maximum protruding position thereof is preferably in the range of 40% or greater to 60% or smaller, with respect to the circumferential length La 1  of the arc portion  313 , and more preferably in the range of from 45% or greater to 55% or smaller. Due to this, the maximum protruding position is located in the center of the arc portion  313 . Similarly, the distance (dimension sign is omitted in the drawing) in the tire circumferential direction from the end portion of the arc portion  326  of the middle block  322  to the maximum protruding position thereof is preferably in the range of from 40% or greater to 60% or smaller, with respect to the circumferential length La 2  of the arc portion  326 , and more preferably in the range of from 45% or greater to 55% or smaller. 
     For example, the edge portion of the middle land portion  32  is recessed at a position where the edge portion of the shoulder land portion  31  projects, and conversely, the edge portion of the middle land portion  32  projects at a position where the edge portion of the shoulder land portion  31  is recessed. Additionally, the maximum protruding position of the arc portion  313  of the shoulder land portion  31  is substantially at the same position in the tire circumferential direction with respect to the maximum protruding position of the arc portion  326  of the middle land portion  32 , and the ratio Dp/P 1  in  FIG. 7  is approximately 50%. Due to this, the distance between the edge portion of the shoulder land portion  31  and the edge portion of the middle land portion  32 , that is, the groove width of the shoulder main groove  21 , increases or decreases periodically and continuously in the tire circumferential direction. Due to this, the drainage properties of the shoulder main groove  21  are improved, and the tire wet traction characteristics are improved. 
     Moreover, the maximum groove width W 1  of the shoulder main groove  21  is greater than the maximum groove width W 2  (see  FIG. 3 ) of the center main groove  22 . Due to this, wet performance can be enhanced while ensuring the ground contact area of the tread portion center region. Additionally, the maximum groove width W 1  of the shoulder main groove  21  preferably has a relationship of 0.70≤W 2 /W 1 ≤0.95 with respect to the maximum groove width W 2  of the center main groove  22  and more preferably has a relationship of 0.70≤W 2 /W 1 ≤0.80. 
     Change in Groove Width of Shoulder Main Groove 
       FIG. 9  is a graph illustrating the change in groove width of the shoulder main groove illustrated in  FIG. 8 . The same drawing illustrates the groove width of the shoulder main groove  21  continuously increasing or decreasing in a predetermined section. 
     In  FIG. 8 , a section A in the tire circumferential direction along the edge portion close to the shoulder main groove  21 , of one middle block  322  is defined. Specifically, the section A is defined as a section in the tire circumferential direction from one end point C 1  to the other end point C 5  of the edge portion of the middle block  322  close to the shoulder main groove  21 . In this section A, the edge portion of the shoulder land portion  31  and the edge portion of the middle land portion  32  face each other, and the groove width of the shoulder main groove  21  can be measured. Furthermore, the section A does not include the opening position of the middle lug groove  321 . Additionally, a plurality of sets of the middle blocks  322  and the middle grooves  321  are arranged at a predetermined pitch in the tire circumferential direction, whereby a plurality of the sections A are arranged at a predetermined pitch in the tire circumferential direction. 
     In this case, as illustrated in  FIGS. 8 and 9 , the groove width of the shoulder main groove  21  continuously increases or decreases in the section A. Specifically, the groove width of the shoulder main groove  21  gradually decreases from the one end point C 1  of the arc portion  326  of the middle block  322  to take the minimum value at a point C 2 , and then gradually increases to take the maximum value at a point C 3 . Next, the groove width of the shoulder main groove  21  gradually decreases from the point C 3  to take the minimum value at a point C 4 , and then gradually increases toward the other end point C 5  of the arc portion  326 . Due to this, as illustrated in  FIG. 9 , the groove width of the shoulder main groove  21  increases or decreases in a wave-like manner and smoothly in the section A. Additionally, the groove width of the shoulder main grooves  21  cannot be measured at the opening portion of the middle lug groove  321 . Then, the change in groove width is repeated periodically in the tire circumferential direction. 
     In the configuration described above, the groove width of the shoulder main grooves  21  increases or decreases gradually, whereby the air column resonance generated in the shoulder main groove  21  is reduced and the pass-by noise performance of the tire is improved. 
     Additionally, a minimum value Ld_min of a distance Ld (see  FIG. 9 ) in the tire circumferential direction between the maximum point C 3  and the minimum points C 2 , C 4  of the groove width of the shoulder main groove  21  in the section A preferably has a relationship of 0.20≤Ld_min/P 2  with respect to a pitch length P 2  of the middle lug groove and more preferably has a relationship of 0.25≤Ld_min/P 2 . Furthermore, the minimum value Ld_min of the distance Ld is preferably in the range of 10 mm≤Ld_min. Due to this, the wavelength of the increase or decrease in groove width becomes gentle, and the drainage properties of the shoulder main groove  21  are ensured. 
     A maximum value Ld_max of the distance Ld (see  FIG. 9 ) in the tire circumferential direction between the maximum point C 3  and the minimum points C 2 , C 4  of the groove width of the shoulder main groove  21  in the section A preferably has a relationship of Ld_max/P 2 ≤0.35 with respect to the pitch length P 2  of the middle lug groove and more preferably has a relationship of Ld_max/P 2 ≤0.30. The maximum value Ld_max of the distance Ld is preferably in the range of Ld_max≤20 mm. Due to this, the effect of reducing the air column resonance, due to the increase or decrease in groove width, is ensured. 
     Additionally, the number of extreme points of the groove width of the shoulder main groove  21  in the section A is preferably three or more. The effect of reducing the air column resonance, due to the increase or decrease in groove width, is ensured. The upper limit of the number of extreme points is not particularly limited, but is restricted by the distance Ld range described above. For example, in the configuration of  FIG. 8 , the arc portion  313  of the shoulder land portion  31  and the arc portion  326  of the middle block  322  are arranged in a staggered manner in the tire circumferential direction, whereby as illustrated in  FIG. 9 , the groove width of the shoulder main groove  21  has one maximum point and two minimum points in the section A. Moreover, the minimum value at the point C 2  and the minimum value at the point C 4  have an identical value W 1   b.  However, no such limitation is intended, and two minimum values W 1   b  may have different values. 
     Furthermore, a maximum value W 1   a  and the minimum value W 1   b  of the groove width of the shoulder main groove  21  in the section A preferably have a relationship of 1.05≤W 1   a/ W 1   b≤ 1.50 and more preferably have a relationship of 1.10≤W 1   a/ W 1   b≤ 1.25. Due to this, the oscillation of the groove width is made appropriate. Note that the maximum value W 1   a  and the minimum value W 1   b  of the groove width have a relationship of W 1   a ≤W 1  and W 1 ′≤W 1   b , with respect to the maximum value W 1  and a minimum value W 1 ′ of the groove width in the entire circumference of the tire. 
     Effects 
     As described above, the pneumatic tire  1  includes: the shoulder main groove  21  and the center main groove  22  disposed in one region demarcated by the tire equatorial plane CL; and the shoulder land portion  31  and the middle land portion  32  defined by the shoulder main groove  21  (see  FIG. 2 ). Additionally, the shoulder land portion  31  is a rib that is continuous in the tire circumferential direction, and the middle land portion  32  includes the plurality of middle lug grooves  321  and the plurality of middle blocks  322  defined by the middle lug grooves  321 . Moreover, when the section A in the tire circumferential direction along the edge portion, close to the shoulder main groove  21 , of one middle block  322  is defined, the groove width of the shoulder main groove  21  continuously increases or decreases in the section A (see  FIGS. 8 and 9 ). 
     Such a configuration provides an advantage that since the groove width of the shoulder main grooves  21  increases or decreases continuously, the air column resonance generated in the shoulder main grooves  21  is reduced, and the pass-by noise performance of the tire is improved. Additionally, since the groove width of the shoulder main groove  21  increases or decreases continuously, there is an advantage that the pass-by noise performance can be improved as compared to a configuration in which the groove width of the center main groove continuously increases or decreases. In particular, in a tire for commercial vehicles, a tread pattern in which a wide lug groove is provided in a shoulder land portion is widely adopted to improve the traction performance and the wet performance. However, in such a tread pattern, the pass-by noise performance tends to degrade due to the wide lug grooves. In this regard, the configuration described above is beneficial in that the pass-by noise performance, the traction performance, and the wet performance can be achieved in a compatible manner. 
     Additionally, in the pneumatic tire  1 , the minimum value Ld_min of the distance Ld (see  FIG. 9 ) in the tire circumferential direction between the maximum point C 3  and the minimum point C 2  of the groove width of the shoulder main groove  21  in the section A has a relationship of 0.20≤Ld_min/P 2  with respect to the pitch length P 2  of the middle lug groove  321 . Due to this, there is an advantage that the wavelength of the increase or decrease in groove width becomes gentle, and the drainage properties of the shoulder main grooves  21  are ensured. 
     Additionally, in the pneumatic tire  1 , the minimum value Ld_min of the distance Ld (see  FIG. 9 ) in the tire circumferential direction between the maximum point C 3  and the minimum point C 2  of the groove width of the shoulder main groove  21  in the section A is in the range of 10 mm≤Ld_min. Due to this, there is an advantage that the wavelength of the increase or decrease in groove width becomes gentle, and the drainage properties of the shoulder main grooves  21  are ensured. 
     Additionally, in the pneumatic tire  1 , the number of extreme points of the groove width of the shoulder main groove  21  in the section A is three or more (see  FIG. 9 ). Due to this, there is an advantage that the effect of reducing the air column resonance, due to the increase or decrease in groove width, is ensured. 
     Additionally, in the pneumatic tire  1 , the maximum value W 1   a  and the minimum value W 1   b  of the groove width of the shoulder main groove  21  in the section A have a relationship of 1.05≤W 1   a/ W 1   b≤ 1.50. Due to this, there is an advantage that the oscillation of the groove width is made appropriate and the pass-by noise performance and the wet performance of the tire are improved. In other words, with the lower limit described above, the oscillation of the groove width is ensured, and the effect of reducing the air column resonance, due to the increase or decrease in groove width, is ensured. With the upper limit described above, the drainage properties of the shoulder main groove  21  are ensured, and the occurrence of hydroplaning is suppressed. 
     Additionally, in the pneumatic tire  1 , the see-through width Dt (see  FIG. 7 ) and the maximum groove width W 1  of the shoulder main groove  21  have a relationship of 0.60≤Dt/W 1 ≤0.90. Due to this, there is an advantage that the pass-by noise performance and the wet performance of the tire are improved. In other words, with the lower limit described above, the drainage properties of the shoulder main grooves  21  are ensured. With the upper limit described above, the effect of reducing the air column resonance, due to the increase or decrease in groove width, is ensured. 
     Additionally, in the pneumatic tire  1 , the edge portion of the shoulder land portion  31  has a shape formed by connecting the plurality of arc portions  313  projecting toward the shoulder main grooves  21  (see  FIG. 7 ). In addition, the edge portion of the middle block  322  has a shape formed by a single arc portion  326  that projects toward the shoulder main groove  21 . Due to this, there is an advantage that the increase or decrease in the groove width of the shoulder main groove  21  (see  FIG. 9 ) can be efficiently formed. Additionally, as compared to a configuration in which the edge portion of the land portion has a zigzag shape (not illustrated), there is an advantage that the ground contact area of the land portion is increased and the wear durability of the land portion is improved. 
     Additionally, in the pneumatic tire  1 , the radius of curvature of the arc portion  313 ;  326  of the shoulder land portion  31  or the middle block  322  is in the range of from 40 mm or greater to 150 mm or smaller. There is an advantage that due to the lower limit described above, the drainage properties of the shoulder main grooves  21  are ensured and the occurrence of hydroplaning is suppressed, and that due to the upper limit described above, the effect of reducing the air column resonance, due to the increase or decrease in groove width, is ensured. 
     Additionally, in the pneumatic tire  1 , a distance (not illustrated) in the tire circumferential direction from the end portion of the arc portion  313 ;  326  of the shoulder land portion  31  or the middle block  322  to the maximum protruding position thereof is in the range of from 40% or greater to 60% or smaller with respect to the circumferential length La 1 ; La 2  of the arc portion  313 ;  326  (see  FIG. 9 ). Such a configuration provides an advantage that since the maximum protruding position of the arc portion  313 ;  326  is disposed in the center portion of the arc portion  313 ;  326 , the increase or decrease (see  FIG. 9 ) in the groove width of the shoulder main groove  21  can be efficiently formed. 
     Additionally, in the pneumatic tire  1 , the distance Dp (see  FIG. 7 ) in the tire circumferential direction between the maximum protruding position of the arc portion  313  of the shoulder land portion  31  and the maximum protruding position of the arc portion  326  of the middle block  322  has a relationship of 0.20≤Dp/P 1 ≤0.60 with respect to the pitch length P 1  of the arc portion  313  of the shoulder land portion  31 . Such a configuration provides an advantage that since the arc portions  313 ,  326  of the shoulder land portion  31  and the middle land portion  32  are arranged in a staggered manner in the tire circumferential direction, the increase or decrease (see  FIG. 9 ) in the groove width of the shoulder main grooves  21  can be efficiently formed. 
     Additionally, in the pneumatic tire  1 , the circumferential length La 1  and the pitch length P 1  of the arc portion  313  of the shoulder land portion  31  have a relationship of 0.80≤La 1 /P 1 ≤1.00 (see  FIG. 8 ). Due to this, there is an advantage that the increase or decrease in the groove width of the shoulder main groove  21  (see  FIG. 9 ) can be efficiently formed. 
     Additionally, in the pneumatic tire  1 , the circumferential length La 2  of the arc portion  326  of the middle block  322  and the circumferential length Le 2  of the edge portion have a relationship of 0.80≤to≤1.00 (see  FIG. 8 ). Due to this, there is an advantage that the increase or decrease in the groove width of the shoulder main groove  21  (see  FIG. 9 ) can be efficiently formed. 
     Additionally, in the pneumatic tire, the shoulder land portion  31  includes the sipe  311  (see  FIG. 7 ) or a narrow groove (not illustrated) that is open to the connection portion of the adjacent arc portions  313 ,  313 . Due to this, there is an advantage that the traction performance can be improved while maintaining the wet performance. 
     Additionally, in the pneumatic tire  1 , the shoulder land portion  31  includes the shoulder sipe  312  that is open to the maximum protruding position of the arc portion  313  (see  FIG. 7 ). Due to this, there is an advantage that the traction performance can be improved while maintaining the wet performance. 
     In addition, in the pneumatic tire  1 , the middle land portion  32  includes the middle sipe  324  that is open to the maximum protruding position of the arc portion  326  (see  FIG. 7 ). Due to this, there is an advantage that the traction performance can be improved while maintaining the wet performance. 
     EXAMPLE 
       FIGS. 10A and 10B  include a table showing the results of performance tests of pneumatic tires according to embodiments of the technology. 
     In the performance tests, (1) noise performance, (2) wet performance, and (3) uneven wear resistance performance are evaluated for a plurality of test tires. Test tires having a tire size of 225/70R19.5 are assembled on rims specified by JATMA, and an air pressure and a load specified by JATMA are applied to the test tires. The test tires are mounted on the front wheels of 2-D (pickup truck) which is a test vehicle. 
     (1) In the evaluation of noise performance, the pass-by noise of the vehicle is measured under the test conditions of the ECE (Economic Commission for Europe) R117-02 and evaluations are performed. Results of the evaluation are expressed as index values and evaluated with the Conventional Example being assigned as the reference (100). In this evaluation, larger values are preferable. 
     (2) In the evaluation of wet performance, the test vehicle is driven on an asphalt road surface sprayed with a water depth of 1 mm, and an acceleration in a speed range of from 5 to 20 km/h is measured. Evaluation was carried out by expressing the measurement results as index values with the results of the Conventional Example being defined as the reference (100). In the evaluation, larger values are preferable. 
     (3) In the evaluation of uneven wear resistance performance, the test vehicle is driven for 40000 km on a paved road, and then heel and toe wear generated in the blocks is observed, and evaluations are performed. Results of the evaluation are expressed as index values and evaluated with the Conventional Example being assigned as the reference (100). In this evaluation, larger values are preferable. Moreover, if the evaluation is  98  or higher, it is deemed that the performance is properly ensured. 
     The test tires of Examples 1 to 16 have the configuration illustrated in  FIGS. 1 and 2 , wherein the shoulder land portion  31  is a rib continuous in the tire circumferential direction, and the middle land portion  32  and the center land portion  33  are rows of blocks defined by lug grooves. Additionally, the tire ground contact width TW is 190 mm, the distances D 1 , D 2  of the shoulder main groove  21  and the center main groove  22  are 55.5 mm and 16.3 mm, respectively. Additionally, the maximum groove width W 1  of the shoulder main groove  21  is 12.5 mm, and the maximum groove width W 2  of the center main groove  22  is 3.5 mm. Additionally, the ground contact widths Wb 1 , Wb 2  of the shoulder land portion  31  and the middle land portion  32  are 36.0 mm and 33.5 mm, respectively. The pitch length P 1  of the arc portion  313  of the shoulder land portion  31  is 43.0 mm or greater and 46.0 mm or smaller, and the pitch length P 2  of the middle lug groove  321  of the middle land portion  32  is 43.0 mm or greater and 46.0 mm or smaller. 
     The test tire of Conventional Example has the configuration of Example 1 wherein the center land portion  33  is a rib that is continuous in the tire circumferential direction and does not include lug grooves. Additionally, the shoulder main grooves  21  have a straight shape since the edge portions of the shoulder land portions  31  and the middle blocks  322  close to the shoulder main grooves  21  have a straight shape. Moreover, the edge portions of the center land portion  33  and the middle blocks  322  close to the center main groove  22  have a continuous arc shape and a single arc shape similarly to the edge portions of the shoulder land portions  31  and the middle blocks  322  close to the shoulder main grooves  21 . Due to this, the groove width of the center main groove  22  continuously increases or decreases in the tire circumferential direction. Note that in the table of  FIGS. 10A and 10B , for comparison, the numerical values of the center main grooves of Conventional Example are described as numerical values of the shoulder main grooves. 
     As can be seen from the test results, the noise performance, the wet performance, and the uneven wear resistance performance of the tire are improved in the test tires of Examples 1 to 16.