Patent Publication Number: US-2017368888-A1

Title: Pneumatic tire

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is based upon and claims the benefit of priority to Japanese Patent Application No. 2016-125165, filed Jun. 24, 2016, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a pneumatic tire having improved steering stability. 
     Description of Background Art 
     Japanese Patent Laid-Open Publication No. 2014-184828 describes a pneumatic tire in which main grooves each continuously extending in a tire circumferential direction are respectively provided on a tire equator and on both sides of the tire equator. The entire contents of this publication are incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a pneumatic tire includes a tread part having a pair of crown main grooves formed on outer sides of a tire equator and continuously extending in a tire circumferential direction respectively, a pair of shoulder main grooves formed on tire axial direction outer sides of the crown main grooves and continuously extending in the tire circumferential direction respectively, a crown land portion formed between the pair of the crown main grooves, a pair of middle land portions formed between the crown main grooves and the shoulder main grooves, and a pair of shoulder land portions positioned on tire axial direction outer sides of the shoulder main grooves. The crown land portion and the middle land portions are formed such that no grooves each having a width of 2 mm or more are formed in the crown land portion and the middle land portions, the tread part has a first tread half defined from the tire equator to a first tread ground contact edge and a second tread half defined from the tire equator to a second tread ground contact edge on an opposite side with respect to the first tread ground contact edge, the tread part has first sipes formed in the first tread half such that each first sipe has a width of less than 2 mm and is extending smoothly continuously from the crown land portion to the shoulder land portion via the crown main groove and the shoulder main groove, the tread part has second sipes formed in the second tread half such that each second sipe has a width of less than 2 mm and is extending smoothly continuously from the crown land portion to the shoulder land portion via the crown main groove and the shoulder main groove and terminated without reaching the second tread ground contact edge, and the tread part has third sipes formed in the middle land portion in the second tread half such that each third sipe has a width of less than 2 mm and is extending in the tire axial direction between a pair of the second sipes that are adjacent to each other in the tire circumferential direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a developed view of a tread part of a pneumatic tire according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view along an A-A line of the tread part of  FIG. 1 ; 
         FIG. 3  is an enlarged developed view of a first tread half in  FIG. 1 ; 
         FIG. 4  is an enlarged developed view of a second tread half in  FIG. 1 ; 
         FIG. 5  is a cross-sectional view along a B-B line of the tread part in  FIG. 1 ; and 
         FIG. 6  is a cross-sectional view along a C-C line of the tread part in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The embodiments will now be described with reference to the accompanying drawings, wherein like reference numerals designate corresponding or identical elements throughout the various drawings. 
       FIG. 1  is a developed view of a tread part  2  of a pneumatic tire (not entirely illustrated in the drawings) of the present embodiment.  FIG. 2  is a cross-sectional view along an A-A line of the tread part  2  of  FIG. 1 . The pneumatic tire of the present embodiment can be suitably used, for example, as a pneumatic tire for a passenger car, and has an asymmetrical tread pattern in which a mounting orientation for mounting the pneumatic tire on a vehicle is specified. The mounting orientation for mounting the pneumatic tire on a vehicle is displayed, for example, on a side wall part (not illustrated in the drawings) using characters or the like. 
     As illustrated in  FIG. 1 , the pneumatic tire of the present embodiment has a pair of crown main grooves ( 3 ,  4 ) continuously extending in a tire circumferential direction and a pair of shoulder main grooves ( 5 ,  6 ) continuously extending in the tire circumferential direction. The crown main grooves ( 3 ,  4 ) are respectively formed on both outer sides of a tire equator (C). The shoulder main groove  5  is formed on a tire axial direction outer side of the crown main groove  3 , and the shoulder main groove  6  is formed on a tire axial direction outer side of the crown main groove  4 . The crown main grooves ( 3 ,  4 ) and the shoulder main grooves ( 5 ,  6 ) of the present embodiment are straight grooves that linearly extend. Such crown main grooves ( 3 ,  4 ) and shoulder main grooves ( 5 ,  6 ) have excellent drainage performance and allow wet performance of the pneumatic tire to be improved. 
     Widths (W 1 , W 2 ) of the crown main grooves ( 3 ,  4 ) and widths (W 3 , W 4 ) of the shoulder main grooves ( 5 ,  6 ) may be determined in various ways. For example, in the present embodiment, in a pneumatic tire for passenger car, the widths (W 1 , W 2 , W 3 , W 4 ) are desirably each 4.0%-8.5% of a tread ground contact width (TW). When the widths (W 1 , W 2 , W 3 , W 4 ) are each less than 4.0% of the tread ground contact width (TW), there is a risk that drainage performance may be affected. On the other hand, when the widths (W 1 , W 2 , W 3 , W 4 ) each exceed 8.5% of the tread ground contact width (TW), there is a risk that a rubber volume of the tread part  2  may decrease and wear resistance may be affected. 
     The “tread ground contact width (TW)” refers to a tire axial direction distance between tread ground contact edges (Te 1 , Te 2 ) when the tire in a normal state is loaded with a normal load and is grounded on a flat surface at a camber angle of 0 degrees. 
     The “tread ground contact edges (Te 1 , Te 2 )” respectively refer to tread ground contact edges on tire axial direction outermost sides when the tire in the normal state is loaded with the normal load and is grounded on a flat surface at the camber angle of 0 degrees. The term “normal state” refers to a no-load state in which the tire is mounted to a normal rim (not illustrated in the drawings) and is filled with air at a normal internal pressure. In the following, unless otherwise specified, values of dimensions and the like of the parts of the tire are values measured in the internal state. 
     The term “normal rim” refers to a rim for which standards are set for each tire in a system of standards that includes standards on which the tire is based. For example, the “normal rim” refers to a “Standard Rim” in the JATMA standards, a “Design Rim” in the TRA standards, or a “Measuring Rim” in the ETRTO standards. 
     The term “normal internal pressure” refers to an air pressure for which standards are set for each tire in a system of standards that includes the standards on which the tire is based, and refers to a “Highest Air Pressure” in the JATMA standards, a maximum value published in the table “Tire Load Limits at Various Cold Inflation Pressures” in the TRA standards, or an “Inflation Pressure” in the ETRTO standards. When the tire is for a passenger car, the normal internal pressure is 180 kPa. 
     The term “normal load” refers to a load for which standards are set for each tire in a system of standards that includes the standards on which the tire is based, and refers to a “Maximum Load Capacity” in the JATMA standards, a maximum value published in the table “Tire Load Limits at Various Cold Inflation Pressures” in the TRA standards, or a “Load Capacity” in the ETRTO standards. When the tire is for a passenger car, the normal load is a load corresponding to 88% of the above-described load. 
     As illustrated in  FIG. 2 , depths (D 1 , D 2 ) of the crown main grooves ( 3 ,  4 ) and depths (D 3 , D 4 ) of the shoulder main grooves ( 5 ,  6 ) may be determined in various ways. In the present embodiment, when the pneumatic tire is for a passenger car, the depths (D 1 , D 2 , D 3 , D 4 ) are desirably each 5-10 mm. 
     When the depths (D 1 , D 2 , D 3 , D 4 ) are each less than 5 mm, there is a risk that the drainage performance may be affected. On the other hand, when the depths (D 1 , D 2 , D 3 , D 4 ) each exceed 10 mm, there is a risk that rigidity of the tread part  2  may be insufficient and steering stability may be affected. 
     Due to the crown main grooves ( 3 ,  4 ) and the shoulder main grooves ( 5 ,  6 ), the tread part  2  is divided into a crown land portion  10 , middle land portions ( 11 ,  12 ), and shoulder land portions ( 13 ,  14 ). The crown land portion  10  is positioned between the pair of the crown main grooves ( 3 ,  4 ). The middle land portion  11  is positioned between the crown main groove  3  and the shoulder main groove  5 , and the middle land portion  12  is positioned between the crown main groove  4  and the shoulder main groove  6 . The shoulder land portion  13  is positioned on a tire axial direction outer side of the shoulder main groove  5 , and the shoulder land portion  14  is positioned on a tire axial direction outer side of the shoulder main groove  6 . 
     The tread part  2  has a first tread half  21  from the tire equator (C) to the first tread ground contact edge (Te 1 ) and a second tread half  22  from the tire equator (C) to the second tread ground contact edge (Te 2 ). The second tread ground contact edge (Te 2 ) is positioned on an opposite side of the first tread ground contact edge (Te 1 ). 
     It is desirable that the pneumatic tire of the present embodiment be mounted such that the first tread half  21  faces an inner side of a vehicle. The first tread half  21  has the first crown main groove  3 , the first shoulder main groove  5 , the crown land portion  10 , the first middle land portion  11  and the first shoulder land portion  13 . The second tread half  22  has the second crown main groove  4 , the second shoulder main groove  6 , the crown land portion  10 , the second middle land portion  12  and the second shoulder land portion  14 . 
     The crown land portion  10  is positioned on both sides of the tire equator (C). Due to such a crown land portion  10 , initial responsiveness during steering is improved and good steering stability is obtained. 
     In the present embodiment, in the first tread half  21 , grooves each having a width of 2 mm or more are not provided in the crown land portion  10  and the first middle land portion  11 . Therefore, in the crown land portion  10  and the first middle land portion  11 , there are no groove edges that tend to become starting points of uneven wear such as so-called heel-and-toe wear, and thus, uneven wear resistance is increased. 
     Similarly, in the second tread half  22 , grooves each having a width of 2 mm or more are not provided in the crown land portion  10  and the second middle land portion  12 . Therefore, in the crown land portion  10  and the second middle land portion  12 , there are no groove edges that tend to become starting points of uneven wear such as so-called heel-and-toe wear, and thus, uneven wear resistance is increased. 
     In the first tread half  21 , multiple first sipes  25  each having a width of less than 2 mm are provided. In the present embodiment, in the first tread half  21 , grooves each having a width of 2 mm or more are not provided in the crown land portion  10  and the first middle land portion  11 . Therefore, there is a risk that drainage performance of the crown land portion  10  and the first middle land portion  11  may be affected. However, due to an edge effect of the first sipes  25 , sufficient wet performance can be easily ensured. Each of the first sipes  25  extends in the tire axial direction while being curved in an arc shape. Such first sipes  25  contribute to improvement in steering stability by dispersing a load in multiple directions. 
     In the first tread half  21 , the first sipes  25  each smoothly continuously extend from the crown land portion  10  through the first middle land portion  11  to the first shoulder land portion  13  via the first crown main groove  3  and the first shoulder main groove  5 . As a result, the crown land portion  10 , the first middle land portion  11  and the first shoulder land portion  13  of the first tread half  21  deform in the same mode along the first sipes  25 . Therefore, transient characteristics during cornering, in which a center of a tread surface moves from the crown land portion  10  to the first shoulder land portion  13 , are improved, and good steering stability can be obtained. 
     Similarly, in the second tread half  22 , multiple second sipes  26  each having a width of less than 2 mm are provided. In the present embodiment, in the second tread half  22 , grooves each having a width of 2 mm or more are not provided in the crown land portion  10  and the second middle land portion  12 . Therefore, there is a risk that drainage performance of the crown land portion  10  and the second middle land portion  12  may be affected. However, due to an edge effect of the second sipes  26 , sufficient wet performance can be easily ensured. Each of the second sipes  26  extends in the tire axial direction while being curved in an arc shape. Such second sipes  26  contribute to improvement in steering stability by dispersing a load in multiple directions. 
     In the second tread half  22 , the second sipes  26  each smoothly continuously extend from the crown land portion  10  through the second middle land portion  12  to the second shoulder land portion  14  via the second crown main groove  4  and the second shoulder main groove  6 . As a result, the crown land portion  10 , the second middle land portion  12  and the second shoulder land portion  14  of the second tread half  22  deform in the same mode along the second sipes  26 . Therefore, transient characteristics during cornering, in which a center of a tread surface moves from the crown land portion  10  to the second shoulder land portion  14 , are improved, and good steering stability can be obtained. 
     The second sipes  26  are each terminated without reaching the second tread ground contact edge (Te 2 ). As a result, rigidity of the second shoulder land portion  14  of the second tread half  22  is increased, and grip performance during cornering, in which a ground contact pressure of the second shoulder land portion  14  is increased, can be improved, and good steering stability can be obtained. 
     In the second middle land portion  12  of the second tread half  22 , third sipes  38  each having a width of less than 2 mm are each provided between a pair of second sipes  26  that are adjacent to each other in the tire circumferential direction. The third sipes  38  extend in the tire axial direction along the second sipes  26 . Due to an edge effect of the third sipes  38 , grip performance of the pneumatic tire is improved, and steering stability is more easily improved. Further, due to the second sipes  26  and the third sipes  38 , a density of the sipes gradually decreases from the second middle land portion  12  of the second tread half  22  to the second tread ground contact edge (Te 2 ). As a result, rigidity of the land portions is gradually increased from the second middle land portion  12  to the second tread ground contact edge (Te 2 ), and transient characteristics and grip performance during cornering are improved, and even better steering stability can be obtained. 
     The first sipes  25  are divided into first crown sipes  31 , first middle sipes  33  and first shoulder sipes  35  by the first crown main groove  3  and the first shoulder main groove  5 . Since the first sipes  25  each smoothly continuously extend via the first crown main groove  3  and the first shoulder main groove  5 , the first middle sipes  33  are respectively formed on imaginary extension lines that are respectively formed by smoothly extending the crown sipes  31 , and the first shoulder sipes  35  are respectively formed on imaginary extension lines that are respectively formed by smoothly extending the first middle sipes  33 . 
     Similarly, the second sipes  26  are divided into second crown sipes  32 , second middle sipes  34  and second shoulder sipes  36  by the second crown main groove  4  and the second shoulder main groove  6 . Since the second sipes  26  each smoothly continuously extend via the second crown main groove  4  and the second shoulder main groove  6 , the second middle sipes  34  are respectively formed on imaginary extension lines that are respectively formed by smoothly extending the crown sipes  32 , and the second shoulder sipes  36  are respectively formed on imaginary extension lines that are respectively formed by smoothly extending the second middle sipes  34 . 
     In the crown land portion  10 , the multiple first crown sipes  31  are provided. Each of the first crown sipes  31  extends in the tire axial direction from the first crown main groove  3  toward the tire equator (C) while being curved in an arc shape. The first crown sipes  31  each have a width of less than 2 mm. Such first crown sipes  31  achieve an edge effect and improve wet performance. Further, the first crown sipes  31  are closed on a tread surface when a normal load is loaded, and thus suppress a decrease in rigidity of the crown land portion  10 . As a result, steering stability is improved. Further, groove edges of first crown sipes  31  are unlikely to become starting points of uneven wear, and thus suppress a decrease in uneven wear resistance. 
     In the first middle land portion  11 , the multiple first middle sipes  33  are provided. Each of the first middle sipes  33  connects the first crown main groove  3  and the first shoulder main groove  5  and extends in the tire axial direction while being curved in an arc shape. The first middle sipes  33  each have a width of less than 2 mm. Similar to the first crown sipes  31 , such first middle sipes  33  improve wet performance by achieving an edge effect, and suppress a decrease in rigidity of the first middle land portion  11  by being closed on a tread surface when a normal load is loaded. As a result, steering stability is improved. Further, a decrease in uneven wear resistance is suppressed. 
     In the first shoulder land portion  13 , the multiple first shoulder sipes  35  are provided. Each of the first shoulder sipes  35  extends in the tire axial direction from the first shoulder main groove  5  toward the first tread ground contact edge (Te 1 ) while being curved in an arc shape. The first shoulder sipes  35  each extend beyond the first tread ground contact edge (Te 1 ) to a tire axial direction outer side. Due to such first shoulder sipes  35 , wet performance of the pneumatic tire is further improved. 
     The first shoulder sipes  35  each have a width of less than 2 mm. Similar to the first crown sipes  31 , such first shoulder sipes  35  improve wet performance by achieving an edge effect, and suppress a decrease in rigidity of the first shoulder land portion  13  by being closed on a tread surface when a normal load is loaded. As a result, steering stability is improved. Further, a decrease in uneven wear resistance is suppressed. 
       FIG. 3  illustrated the first tread half  21 . Each of the first crown sipes  31 , each of the first middle sipes  33  and each of the first shoulder sipes  35  are respectively formed in arc shapes of curvature radii (R 1 , R 2 , R 3 ) that are convex on one side of the tire circumferential direction (lower side in  FIG. 3 ). The first crown sipes  31 , the first middle sipes  33  and the first shoulder sipes  35  each gradually increase an angle with respect to the tire circumferential direction from the tire equator (C) toward the first tread ground contact edge (Te 1 ). Due to such first crown sipes  31 , first middle sipes  33  and first shoulder sipes  35 , tire axial direction rigidity of the tread part  2  is gradually increased from the tire equator (C) toward the first tread ground contact edge (Te 1 ), which contributes to improvement in steering stability. 
     The curvature radii (R 1 , R 2 , R 3 ) satisfy the following relations: 
         R 1≦ R 2≦ R 3
 
       R1&lt;R3 
     That is, the curvature radii of the sipes ( 31 ,  33 ,  35 ) are increased from the tire equator (C) toward the first tread ground contact edge (Te 1 ). As a result, from the tire equator (C) toward the first tread ground contact edge (Te 1 ), a decrease in torsional rigidity of the land portions ( 10 ,  11 ,  13 ) is suppressed, and uneven wear such as so-called shoulder wear in the shoulder land portion  13  is suppressed, and at the same time, transient characteristics during cornering are improved and steering stability of the pneumatic tire is improved. 
     The first crown sipes  31  are respectively smoothly continuous to the first middle sipes  33  via the first crown main groove  3 . On the other hand, the first middle sipes  33  and the first shoulder sipes  35  are formed at positions such that the first middle sipes  33  are respectively smoothly continuous to the first shoulder sipes  35  via the first shoulder main groove  5 . As a result, along the sipes ( 31 ,  33 ,  35 ), rigidity distribution and ground contact pressure distribution of the land portions ( 10 ,  11 ,  13 ) are smooth, and uneven wear is further suppressed. Further, transient characteristics during cornering are improved and steering stability of the pneumatic tire is improved. 
     The first crown sipes  31  each extend from the crown main groove  3  of the first tread half  21  to beyond the tire equator (C) and are each terminated within the crown land portion  10  without reaching the crown main groove  4  on the other side. Such first crown sipes  31  suppress a decrease in the rigidity of the crown land portion  10  while ensuring a good edge effect in the crown land portion  10  in the first tread half  21 . As a result, steering stability is improved. Further, uneven wear resistance is further improved. 
     A length (L 1 ) of each of the first crown sipes  31  is desirably 70%-100% of the width (W 1 ) of the first crown main groove  3 . When the length (L 1 ) is less than 70% of the width (W 1 ), there is a risk that the edge effect of the first crown sipes  31  may be insufficient and wet performance cannot be sufficiently improved. On the other hand, when the length (L 1 ) exceeds 100% of the width (W 1 ), there is a risk that the rigidity of the crown land portion  10  may be insufficient and steering stability and uneven wear resistance may be affected. 
     The length (L 1 ) of each of the first crown sipes  31  is desirably less than 50% of a tire axial direction length of the crown land portion  10 . When the length (L 1 ) exceeds 50% of the tire axial direction length of the crown land portion  10 , there is a risk that the rigidity of the crown land portion  10  may be insufficient and steering stability and uneven wear resistance may be affected. 
     A length (L 2 ) of each of the first middle sipes  33  is larger than the length (L 1 ) of each of the first crown sipes  31 , and a length (L 3 ) of each of the first shoulder sipes  35  is larger than the length (L 2 ) of each of the first middle sipes  33 . Due to such first crown sipes  31 , first middle sipes  33  and first shoulder sipes  35 , rigidities of the crown land portion  10 , the first middle land portion  11  and the first shoulder land portion  13  are optimized and steering stability is improved. Further, uneven wear is further suppressed. 
     A tire axial direction length of the second middle land portion  12  is desirably equal to or larger than a tire axial direction length of the first middle land portion  11 . A tire axial direction length of the second shoulder land portion  14  is desirably equal to or larger than a tire axial direction length of the first shoulder land portion  13 . Due to such second middle land portion  12  and second shoulder land portion  14 , rubber volumes of the second middle land portion  12  and the second shoulder land portion  14  can be easily ensured and uneven wear resistance can be easily improved. 
     In the crown land portion  10 , the multiple second crown sipes  32  are provided. Each of the second crown sipes  32  extends in the tire axial direction from the second crown main groove  4  toward the tire equator (C) while being linearly inclined. It is also possible that the second crown sipes  32  are each formed in an arc shape. 
     The second crown sipes  32  and the first crown sipes  31  are alternately formed in the tire circumferential direction. As a result, rigidity distribution of the crown land portion  10  is made uniform, and steering stability and uneven wear resistance are improved. The second crown sipes  32  each have a width of less than 2 mm. Similar to the first crown sipes  31 , such second crown sipes  32  improve wet performance by achieving an edge effect, and suppress a decrease in the rigidity of the crown land portion  10  by being closed when a normal load is loaded, and thus, decreases in steering stability and uneven wear resistance are suppressed. 
     In the second middle land portion  12 , the multiple second middle sipes  34  are provided. Each of the second middle sipes  34  connects the second crown main groove  4  and the second shoulder main groove  6  and extends in the tire axial direction while being curved in an arc shape. The second middle sipes  34  each have a width of less than 2 mm. Similar to the first crown sipes  31 , such second middle sipes  34  improve wet performance by achieving an edge effect, and suppress a decrease in the rigidity of the second middle land portion  12  by being closed when a normal load is loaded, and thus, decreases in steering stability and uneven wear resistance are suppressed. 
     In the second shoulder land portion  14 , the multiple second shoulder sipes  36  are provided. Each of the second shoulder sipes  36  extends in the tire axial direction from the second shoulder main groove  6  toward the second tread ground contact edge (Te 2 ) while being curved in an arc shape. The second shoulder sipes  36  each extend from the second shoulder main groove  6  and are each terminated within the second shoulder land portion  14  without reaching the second tread ground contact edge (Te 2 ). Such second shoulder sipes  36  suppress a decrease in tire circumferential direction rigidity of the second shoulder land portion  14 , and further improve steering stability and uneven wear resistance. 
     The second shoulder sipes  36  each have a width of less than 2 mm. Similar to the first crown sipes  31 , such second shoulder sipes  36  improve wet performance by achieving an edge effect, and suppress a decrease in the rigidity of the second shoulder land portion  14  by being closed when a normal load is loaded, and thus, decreases in steering stability and uneven wear resistance are suppressed. 
       FIG. 4  illustrated the second tread half  22 . Each of the second middle sipes  34  and each of the second shoulder sipes  36  are respectively formed in arc shapes of curvature radii (R 4 , R 5 ) that are convex on the other side of the tire circumferential direction (upper side in  FIG. 4 ). The second middle sipes  34  and the second shoulder sipes  36  each gradually increase an angle with respect to the tire circumferential direction from the tire equator (C) toward the second tread ground contact edge (Te 2 ). Due to such second middle sipes  34  and second shoulder sipes  36 , the tire axial direction rigidity of the tread part  2  is gradually increased from the tire equator (C) toward the second tread ground contact edge (Te 2 ), which contributes to improvement in steering stability. 
     The curvature radii (R 4 , R 5 ) satisfy the following relation: 
       R4≦R5
 
     As a result, from the tire equator (C) toward the second tread ground contact edge (Te 2 ), a decrease in torsional rigidity of the land portion ( 12 ,  14 ) is suppressed, and steering stability is improved and uneven wear such as so-called shoulder wear in the shoulder land portion  14  is suppressed. 
     The second crown sipes  32  are respectively smoothly continuous to some of the second middle sipes  34  via the second crown main groove  4 . On the other hand, the second middle sipes  34  and the second shoulder sipes  36  are formed at positions such that the second middle sipes  34  are respectively smoothly continuous to the second shoulder sipes  36  via the second shoulder main groove  6 . As a result, along the sipes ( 32 ,  34 ,  36 ), rigidity distribution and ground contact pressure distribution of the land portions ( 10 ,  12 ,  14 ) are smooth, and uneven wear is further suppressed. Further, transient characteristics during cornering are improved and steering stability of the pneumatic tire is improved. 
     The first crown sipes  31  and the second middle sipes  34  are formed as positions such that the first crown sipes  31  are respectively smoothly continuous to some of the second middle sipes  34  via the crown land portion  10  and the second crown main groove  4 . As a result, over the entire tread part  2 , along the sipes ( 35 ,  33 ,  31 ,  34 ,  36 ), rigidity distribution and ground contact pressure distribution of the land portions ( 13 ,  11 ,  10 ,  12 ,  14 ) are smooth, and uneven wear is further suppressed. Further, transient characteristics during cornering are improved and steering stability of the pneumatic tire is improved. 
     The second crown sipes  32  each extend from the second crown main groove  3  of the second tread half  22  and are each terminated within the crown land portion  10  without reaching the tire equator (C). Such second crown sipes  32 , together with the first crown sipes  31 , ensure a good edge effect at the crown land portion  10  of the second tread half  22  and suppress a decrease in the rigidity of the crown land portion  10 , and further improve steering stability and uneven wear resistance. 
     A length (L 4 ) of each of the second crown sipes  32  is desirably 20%-40% of the width (W 2 ) of the second crown main groove  4 . When the length (L 4 ) is less than 20% of the width (W 2 ), there is a risk that the edge effect of the second crown sipes  32  may be insufficient and wet performance cannot be sufficiently improved. On the other hand, when the length (L 4 ) exceeds 40% of the width (W 2 ), there is a risk that the rigidity of the crown land portion  10  may be insufficient and steering stability and uneven wear resistance may be affected. 
     The length (L 4 ) of each of the second crown sipes  32  is desirably less than 25% of the tire axial direction length of the crown land portion  10 . When the length (L 4 ) exceeds 25% of the tire axial direction length of the crown land portion  10 , there is a risk that the rigidity of the crown land portion  10  may be insufficient and steering stability and uneven wear resistance may be affected. 
     A length (L 5 ) of each of the second middle sipes  34  is larger than the length (L 4 ) of each of the second crown sipes  32 , and a length (L 6 ) of each of the second shoulder sipes  36  is smaller than the length (L 5 ) of each of the second middle sipes  34 . Due to such second crown sipes  32 , second middle sipes  34  and second shoulder sipes  36 , rigidities of the crown land portion  10 , the second middle land portion  12  and the second shoulder land portion  14  are optimized and uneven wear is further suppressed. 
     The length (L 6 ) of each of the second shoulder sipes  36  is desirably 50-55% of the tire axial direction length of the second shoulder land portion  14 . When the length (L 6 ) is less than 50% of the tire axial direction length of the second shoulder land portion  14 , there is a risk that the edge effect due to the second shoulder sipes  36  may be insufficient and wet performance may be affected. On the other hand, when the length (L 6 ) exceeds 55% of the tire axial direction length of the second shoulder land portion  14 , there is a risk that the rigidity of the second shoulder land portion  14  may be insufficient and steering stability and uneven wear resistance may be affected. 
     As illustrated in  FIG. 3 , in the first shoulder land portion  13 , first shoulder lug grooves  41  are provided such that each extends from the first tread ground contact edge (Te 1 ) toward a tire axial direction inner side. Due to the first shoulder lug grooves  41 , drainage performance of the first shoulder land portion  13  is improved. The first shoulder lug grooves  41  each extend beyond the first tread ground contact edge (Te 1 ) to a tire axial direction outer side. It is desirable that the first shoulder lug grooves  41  each have a groove width of 2 mm or more. Due to such first shoulder lug grooves  41 , wet performance of the pneumatic tire is further improved. 
     The first shoulder lug grooves  41  are each terminated within the first shoulder land portion  13  without reaching the first shoulder main groove  5 . Such first shoulder lug grooves  41  suppress a decrease in tire circumferential direction rigidity of the first shoulder land portion  13 , and further improve steering stability and uneven wear resistance. The first shoulder lug grooves  41  and the first shoulder sipes  35  are alternately formed in the tire circumferential direction. As a result, rigidity distribution of the first shoulder land portion  13  is made uniform, and steering stability and uneven wear resistance are improved. 
     The first shoulder lug grooves  41  each have linear portion ( 41   a ) that linearly extends from the first tread ground contact edge (Te 1 ) at an angle of  85 - 95  degrees with respect to the tire circumferential direction, and a curved portion ( 41   b ) that extends in an arc shape along the first shoulder sipes  35 . Drainage performance during cornering is improved due to the linear portion ( 41   a ), and transient characteristics during cornering are improved due to the curved portion ( 41   b ). 
     As illustrated in  FIG. 4 , in the second shoulder land portion  14 , second shoulder lug grooves  42  are provided such that each extends from the second tread ground contact edge (Te 2 ) toward a tire axial direction inner side. Due to the second shoulder lug grooves  42 , drainage performance of the second shoulder land portion  14  is improved. The second shoulder lug grooves  42  each extend beyond the second tread ground contact edge (Te 2 ) to a tire axial direction outer side. It is desirable that the second shoulder lug grooves  42  each have a groove width of 2 mm or more. Due to such second shoulder lug grooves  42 , wet performance of the pneumatic tire is further improved. 
     The second shoulder lug grooves  42  are each terminated within the second shoulder land portion  14  without reaching the second shoulder main groove  6 . Such second shoulder lug grooves  42  suppress a decrease in tire circumferential direction rigidity of the second shoulder land portion  14 , and further improve steering stability and uneven wear resistance. The second shoulder lug grooves  42  and the second shoulder sipes  36  are alternately formed in the tire circumferential direction. As a result, rigidity distribution of the second shoulder land portion  14  is made uniform, and steering stability and uneven wear resistance are improved. The second shoulder sipes  36  are each terminated on a tire axial direction inner side of a tire axial direction outer side edge of a curved portion of each of the second shoulder lug grooves  42 . As a result, rigidity of the second shoulder land portion  14  near the second tread ground contact edge (Te 2 ) is increased, and steering stability and uneven wear resistance are improved. 
     The second shoulder lug grooves  42  each have linear portion ( 42   a ) that linearly extends from the second tread ground contact edge (Te 2 ) at an angle of 85-95 degrees with respect to the tire circumferential direction, and a curved portion ( 42   b ) that extends in an arc shape along the second shoulder sipes  36 . Drainage performance during cornering is improved due to the linear portion ( 42   a ), and transient characteristics during cornering are improved due to the curved portion ( 42   b ). 
     In the present embodiment, the third sipes  38  include third inner side sipes ( 38   i ) and third outer side sipes ( 38   o ). That is, in the second middle land portion  12 , the third inner side sipes ( 38   i ) that each extend from the second crown main groove  4  to a tire axial direction outer side and the third outer side sipes ( 38   o ) that each extend from the second shoulder main groove  6  to a tire axial direction inner side are formed. The third inner side sipes ( 38   i ) and the third outer side sipes ( 38   o ) each have a width of less than 2 mm. The third inner side sipes ( 38   i ) and the third outer side sipes ( 38   o ) achieve an edge effect and improve wet performance of the pneumatic tire. 
     The third inner side sipes ( 38   i ) are each terminated within the second middle land portion  12  without reaching the second shoulder main groove  6 . The third outer side sipes ( 38   o ) are each terminated within the second middle land portion  12  without reaching the second crown main groove  4 . A tire axial direction length of each of the third inner side sipes ( 38   i ) and the third outer side sipes ( 38   o ) is desirably 50%-80% of a width (tire axial direction length) of the second middle land portion  12 . The third inner side sipes ( 38   i ) and the third outer side sipes ( 38   o ) are alternately formed in the tire circumferential direction with the second middle sipes  34  respectively interposed therebetween. Due to such third inner side sipes ( 38   i ) and third outer side sipes ( 38   o ), a sufficient edge effect is obtained and a decrease in the rigidity of the second middle land portion  12  is suppressed, and steering stability and uneven wear resistance are improved. 
     A ratio (W 4 /W 2 ) of the width (W 4 ) of the second shoulder main groove  6  to the width (W 2 ) of the second crown main groove  4  is desirably 0.10-0.60. When the ratio (W 4 /W 2 ) is less than 0.10, there is a risk that drainage performance of the second shoulder main groove  6  may be affected. Further, there is a risk that the rubber volumes of the crown land portion  10  and the second middle land portion  12  may be insufficient and uneven wear resistance may be affected. When the ratio (W 4 /W 2 ) exceeds 0.60, there is a risk that drainage performance of the second crown main groove  4  may be affected. Further, there is a risk that the rubber volumes of the second middle land portion  12  and the second shoulder land portion  14  may be insufficient and uneven wear resistance may be affected. 
     A ratio (W 3 /W 1 ) of the width (W 3 ) of the first shoulder main groove  5  to the width (W 1 ) of the first crown main groove  3  is desirably 0.80-1.00. When the ratio (W 3 /W 1 ) is less than 0.80, there is a risk that drainage performance of the first shoulder main groove  5  may be affected. Further, there is a risk that the rubber volumes of the crown land portion  10  and the first middle land portion  11  may be insufficient and uneven wear resistance may be affected. When the ratio (W 3 /W 1 ) exceeds 1.00, there is a risk that drainage performance of the first crown main groove  3  may be affected. Further, there is a risk that the rubber volumes of the first middle land portion  11  and the first shoulder land portion  13  may be insufficient and uneven wear resistance may be affected. 
     A ratio (W 3 /W 4 ) of the width (W 3 ) of the first shoulder main groove  5  to the width (W 4 ) of the second shoulder main groove  6  is desirably 1.40-1.60. When the ratio (W 3 /W 4 ) is less than 1.40, there is a risk that drainage performance of the first shoulder main groove  5  may be affected. Further, there is a risk that the rubber volumes of the second middle land portion  12  and the second shoulder land portion  14  may be insufficient and uneven wear resistance may be affected. When the ratio (W 3 /W 4 ) exceeds 1.60, there is a risk that drainage performance of the second shoulder main groove  6  may be affected. Further, there is a risk that the rubber volumes of the first middle land portion  11  and the first shoulder land portion  13  may be insufficient and uneven wear resistance may be affected. 
     A difference (R 5 −R 4 ) between the curvature radius (R 5 ) of the second shoulder sipes  36  and the curvature radius (R 4 ) of the second middle sipes  34  is desirably smaller than a difference (R 3 −R 2 ) between the curvature radius (R 3 ) of the first shoulder sipes  35  and the curvature radius (R 2 ) of the first middle sipes  33 . In the second tread half  22  where a large load is applied during cornering, by setting the curvature radius difference (R 5 −R 4 ) small, transient characteristics during cornering are improved and steering stability is improved. 
     As illustrated in  FIG. 3 , an angle (θ 1 ) of each of the first middle sipes  33  with respect to the tire circumferential direction at an end on the first shoulder main groove  5  side is desirably 71-78 degrees. When the angle (θ 1 ) is less than 71 degrees, there is a risk that tire axial direction rigidity of the first middle land portion  11  may be insufficient and steering stability may be affected. When the angle (θ 1 ) exceeds 78 degrees, there is a risk that heel-and-toe wear may occur at the first middle land portion  11 . Similarly, as illustrated in  FIG. 4 , an angle (θ 2 ) of each of the second middle sipes  34  with respect to the tire circumferential direction at an end on the second shoulder main groove  6  side is desirably 73-80 degrees. Further, in order to suppress the heel-and-toe wear at the first middle land portion  11 , the angle (θ 1 ) is desirably set to be smaller than the angle (θ 2 ). 
     As illustrated in  FIG. 3 , an angle (θ 3 ) of each of the first shoulder sipes  35  with respect to the tire circumferential direction at an end on the first shoulder main groove  5  side is desirably 71-78 degrees. When the angle (θ 3 ) is less than 71 degrees, there is a risk that tire axial direction rigidity of the first shoulder land portion  13  may be insufficient and steering stability may be affected. When the angle (θ 3 ) exceeds 78 degrees, there is a risk that heel-and-toe wear may occur at the first shoulder land portion  13 . Similarly, as illustrated in  FIG. 4 , an angle (θ 4 ) of each of the second shoulder sipes  36  with respect to the tire circumferential direction at an end on the second shoulder main groove  6  side is desirably 73-80 degrees. Further, in order to suppress the heel-and-toe wear at the first shoulder land portion  13 , the angle (θ 3 ) is desirably set to be smaller than the angle (θ 4 ). 
     As illustrated in  FIGS. 3 and 4 , it is desirable that first crown shallow grooves  51  and second crown shallow grooves  52  each having a width of less than 2 mm be provided in the crown land portion  10 . It is desirable that the first crown shallow grooves  51  and the second crown shallow grooves  52  each have a depth of less than 2 mm. The first crown shallow grooves  51  respectively partially include the first crown sipes  31  and are respectively formed along the first crown sipes  31 . In other words, the first crown sipes  31  are respectively formed from groove bottoms of the first crown shallow grooves  51  in a thickness direction of the tread part  2 . Relationship between the second crown shallow grooves  52  and the second crown sipes  32  is similar to the relationship between the first crown shallow grooves  51  and the first crown sipes  31 . 
     By providing the first crown shallow grooves  51  and the second crown shallow grooves  52  in the crown land portion  10 , drainage performance of the crown land portion  10  is improved and wet performance is improved. Further, since the width and the depth of each of the first crown shallow grooves  51  and the second crown shallow grooves  52  are less than 2 mm, influence on uneven wear resistance is limited. The above-described edge effects of the first crown sipes  31  and the second crown sipes  32  are obtained after the first crown shallow grooves  51  and the second crown shallow grooves  52  disappear as the crown land portion  10  wears out. 
     It is desirable that first middle shallow grooves  53  each having a width of less than 2 mm be provided in the first middle land portion  11 , and second middle shallow grooves  54 , third middle shallow grooves ( 58   i ) and fourth middle shallow grooves ( 58   o ), each having a width of less than 2 mm, be provided in the second middle land portion  12 . It is desirable that the first middle shallow grooves  53 , and the second middle shallow grooves  54 , the third middle shallow grooves ( 58   i ) and the fourth middle shallow grooves ( 58   o ), each have a depth of less than 2 mm. The first middle shallow grooves  53  respectively partially include the first middle sipes  33  and are respectively formed along the first middle sipes  33 . In other words, the first middle sipes  33  are respectively farmed from groove bottoms of the first middle shallow grooves  53  in the thickness direction of the tread part  2 . Relationship between the second middle shallow grooves  54  and the second middle sipes  34 , relationship between the third middle shallow grooves ( 58   i ) and the third inner side sipes ( 38   i ) and relationship between the fourth middle shallow grooves ( 58   o ) and the third outer side sipes ( 38   o ) are similar to the relationship between the first middle shallow grooves  53  and the first middle sipes  33 . 
     By providing the first middle shallow grooves  53  in the first middle land portion  11  and providing the second middle shallow grooves  54 , the third middle shallow grooves ( 58   i ) and the fourth middle shallow grooves ( 58   o ) in the second middle land portion  12 , drainage performance of the first middle land portion  11  and the second middle land portion  12  is improved, and wet performance is improved. Further, since the width and the depth of each of the first middle shallow grooves  53 , the second middle shallow grooves  54 , the third middle shallow grooves ( 58   i ) and the fourth middle shallow grooves ( 58   o ) are less than 2 mm, influence on uneven wear resistance is limited. The above-described edge effects of the first middle sipes  33 , the second middle sipes  34 , the third inner side sipes ( 38   i ) and the third outer side sipes ( 38   o ) are obtained after the first middle shallow grooves  53 , the second middle shallow grooves  54 , the third middle shallow grooves ( 58   i ) and the fourth middle shallow grooves ( 58   o ) disappear as the first middle land portion  11  and the second middle land portion  12  wear out. 
     It is desirable that first shoulder shallow grooves  55  each having a width of less than 2 mm be provided in the first shoulder land portion  13 , and second shoulder shallow grooves  56  each having a width of less than 2 mm be provided in the second shoulder land portion  14 . It is desirable that the first shoulder shallow grooves  55  and the second shoulder shallow grooves  56  each have a depth of less than 2 mm. The first shoulder shallow grooves  55  respectively partially include the first shoulder sipes  35  and are respectively formed along the first shoulder sipes  35 . In other words, the first shoulder sipes  35  are respectively formed from groove bottoms of the first shoulder shallow grooves  55  in the thickness direction of the tread part  2 . Relationship between the second shoulder shallow grooves  56  and the second shoulder sipes  36  is similar to the relationship between the first shoulder shallow grooves  55  and the first shoulder sipes  35 . 
     By providing the first shoulder shallow grooves  55  in the first shoulder land portion  13  and providing the second shoulder shallow grooves  56  in the second shoulder land portion  14 , drainage performance of the first shoulder land portion  13  and the second shoulder land portion  14  is improved, and wet performance is improved. Further, since the width and the depth of each of the first shoulder shallow grooves  55  and the second shoulder shallow grooves  56  are less than 2 mm, influence on uneven wear resistance is limited. The above-described edge effects of the first shoulder sipes  35  and the second shoulder sipes  36  are obtained after the first shoulder shallow grooves  55  and second shoulder shallow grooves  56  disappear as the first shoulder land portion  13  and the second shoulder land portion  14  wear out. 
     As illustrated in  FIG. 3 , chamfered portions  61  each having an arc shape are respectively formed at front end portions of the crown land portion  10  where the first crown shallow grooves  51  are communicatively connected to the first crown main groove  3 . The chamfered portions  61  are respectively formed at places where groove edges of the first crown shallow grooves  51  intersect a groove edge of the first crown main groove  3  at an acute angle. Due to the chamfered portions  61 , drainage performance of the first crown shallow grooves  51  is improved. 
     Chamfered portions  63  each having an arc shape are respectively formed at front end portions of the first middle land portion  11  where the first middle shallow grooves  53  are communicatively connected to the first crown main groove  3 . Chamfered portions  65  each having an arc shape are respectively formed at front end portions of the first middle land portion  11  where the first middle shallow grooves  53  are communicatively connected to the first shoulder main groove  5 . The chamfered portions  63  are respectively formed at places where groove edges of the first middle shallow grooves  53  intersect a groove edge of the first crown main groove  3  at an acute angle. The chamfered portions  65  are respectively formed at places where groove edges of the first middle shallow grooves  53  intersect a groove edge of the first shoulder main groove  5  at an acute angle. Due to the chamfered portions ( 63 ,  65 ), drainage performance of the first middle shallow grooves  53  is improved. 
     Chamfered portions  67  each having an arc shape are respectively formed at front end portions of the first shoulder land portion  13  where the first shoulder shallow grooves  55  are communicatively connected to the first shoulder main groove  5 . The chamfered portions  67  are respectively formed at places where groove edges of the first shoulder shallow grooves  55  intersect a groove edge of the first shoulder main groove  5  at an acute angle. Due to the chamfered portions  67 , drainage performance of the first shoulder shallow grooves  55  is improved. 
     As illustrated in  FIG. 4 , chamfered portions  72  each having an arc shape are respectively formed at front end portions of the second middle land portion  12  where the second middle shallow grooves  54  are communicatively connected to the second crown main groove  4 . Chamfered portions  74  each having an arc shape are respectively formed at front end portions of the second middle land portion  12  where the second middle shallow grooves  54  are communicatively connected to the second shoulder main groove  6 . The chamfered portions  72  are respectively formed at places where groove edges of the second middle shallow grooves  54  intersect a groove edge of the second crown main groove  4  at an acute angle. The chamfered portions  74  are respectively formed at places where groove edges of the second middle shallow grooves  54  intersect a groove edge of the second shoulder main groove  6  at an acute angle. Due to the chamfered portions ( 72 ,  74 ), drainage performance of the second middle shallow grooves  54  is improved. 
     Chamfered portions ( 78   i ) each having an arc shape are respectively formed at front end portions of the second middle land portion  12  where the third middle shallow grooves ( 58   i ) are communicatively connected to the second crown main groove  4 . Chamfered portions ( 78   o ) each having an arc shape are respectively formed at front end portions of the second middle land portion  12  where the fourth middle shallow grooves ( 58   o ) are communicatively connected to the second shoulder main groove  6 . The chamfered portions ( 78   i ) are respectively formed at places where groove edges of the third middle shallow grooves ( 58   i ) intersect a groove edge of the second crown main groove  4  at an acute angle. The chamfered portions ( 78   o ) are respectively formed at places where groove edges of the fourth middle shallow grooves ( 58   o ) intersect a groove edge of the second shoulder main groove  6  at an acute angle. Due to the chamfered portions ( 78   i ,  78   o ), drainage performance of the third middle shallow grooves ( 58   i ) and the fourth middle shallow grooves ( 58   o ) is improved. 
     Chamfered portions  76  each having an arc shape are respectively formed at front end portions of the second shoulder land portion  14  where the second shoulder shallow grooves  56  are communicatively connected to the second shoulder main groove  6 . The chamfered portions  76  are respectively formed at places where groove edges of the second shoulder shallow grooves  56  intersect a groove edge of the second shoulder main groove  6  at an acute angle. Due to the chamfered portions  76 , drainage performance of the second shoulder shallow grooves  56  is improved. 
       FIG. 5  is a cross-sectional view along a B-B line of the tread part  2  in  FIG. 1 . The first crown sipes  31 , the first middle sipes  33  and the first shoulder sipes  35  each have a depth that varies along the tire axial direction. 
     The first crown sipes  31  each have a deep portion ( 31   a ) at a central portion of the crown land portion  10  and a shallow portion ( 31   b ) on the first crown main groove  3  side. The depth of each of the first crown sipes  31  linearly varies between the deep portion ( 31   a ) and the shallow portion ( 31   b ). Due to the shallow portion ( 31   b ), tire circumferential direction rigidity of the crown land portion  10  is increased, and uneven wear such as heel-and-toe wear is suppressed. On the other hand, the deep portion ( 31   a ) remains in the central portion of the crown land portion  10  from an intermediate stage to a terminal stage of wear and allows excellent wet performance to be maintained. 
     The first middle sipes  33  each have a deep portion ( 33   a ) at a central portion of the first middle land portion  11 , a shallow portion ( 33   b ) on the first crown main groove  3  side, and a shallow portion ( 33   c ) on the first shoulder main groove  5  side. The depth of each of the first middle sipes  33  linearly varies between the deep portion ( 33   a ) and the shallow portions ( 33   b ,  33   c ). Due to the shallow portions ( 33   b,    33   c ), tire circumferential direction rigidity of the first middle land portion  11  is increased, and uneven wear such as heel-and-toe wear is suppressed. On the other hand, the deep portion ( 33   a ) remains in the central portion of the first middle land portion  11  from an intermediate stage to a terminal stage of wear and allows excellent wet performance to be maintained. 
     The first shoulder sipes  35  each have a deep portion ( 35   a ) at a central portion of the first shoulder land portion  13 , a shallow portion ( 35   b ) on the first shoulder main groove  5  side, and a shallow portion ( 35   c ) on a tire axial direction outer side of the first tread ground contact edge (Te 1 . The depth of each of the first shoulder sipes  35  linearly varies between the deep portion ( 35   a ) and the shallow portions ( 35   b,    35   c ). Due to the shallow portions ( 35   b,    35   c ), tire circumferential direction rigidity of the first shoulder land portion  13  is increased, and uneven wear such as heel-and-toe wear is suppressed. On the other hand, the deep portion ( 35   a ) remains in the central portion of the first shoulder land portion  13  from an intermediate stage to a terminal stage of wear and allows excellent wet performance to be maintained. 
       FIG. 6  is a cross-sectional view along a C-C line of the tread part  2  in  FIG. 1 . The second middle sipes  34  and the second shoulder sipes  36  each have a depth that varies along the tire axial direction. 
     The second middle sipes  34  each have a deep portion ( 34   a ) at a central portion of the second middle land portion  12 , a shallow portion ( 34   b ) on the second crown main groove  4  side, and shallow portions ( 34   c,    34   d ) on the second shoulder main groove  6  side. The shallow portion ( 34   c ) has a depth larger than that of the shallow portion ( 34   d ). The depth of each of the second middle sipes  34  linearly varies between the deep portion ( 34   a ) and the shallow portions ( 34   b,    34   c ) and between the shallow portion ( 34   c ) and the shallow portion ( 34   d ). Due to the shallow portions ( 34   b,    34   c,    34   d ), tire circumferential direction rigidity of the second middle land portion  12  is increased, and uneven wear such as heel-and-toe wear is suppressed. Further, by providing the shallow portion ( 34   c ) on the second shoulder main groove  6  side, the depth of each of the second middle sipes  34  varies stepwise. Therefore, tire circumferential direction rigidity distribution of the second middle land portion  12  varies stepwise, and transient characteristics during cornering are improved. On the other hand, the deep portion ( 34   a ) remains in the central portion of the second middle land portion  12  from an intermediate stage to a terminal stage of wear and allows excellent wet performance to be maintained. 
     The second shoulder sipes  36  each have a deep portion ( 36   a ) at a central portion of the second shoulder land portion  14  and a shallow portion ( 36   b ) on the second shoulder main groove  6  side. The depth of each of the second shoulder sipes  36  linearly varies between the deep portion ( 36   a ) and the shallow portion ( 36   b ). Due to the shallow portion ( 36   b ), tire circumferential direction rigidity of the second shoulder land portion  14  is increased, and uneven wear such as heel-and-toe wear is suppressed. On the other hand, the deep portion ( 36   a ) remains in the central portion of the second shoulder land portion  14  from an intermediate stage to a terminal stage of wear and allows excellent wet performance to be maintained. 
     As illustrated in  FIG. 5 , the third inner side sipes ( 38   i ) each have a depth that varies along the tire axial direction. The third inner side sipes ( 38   i ) each have a deep portion ( 38   a ) at a central portion of the second middle land portion  12  and a shallow portion ( 38   b ) on the second crown main groove  4  side. The depth of each of the third inner side sipes ( 38   i ) linearly varies between the deep portion ( 38   a ) and the shallow portion ( 38   b ). Due to the shallow portion ( 38   b ), tire circumferential direction rigidity of the second middle land portion  12  is increased, and uneven wear such as heel-and-toe wear is suppressed. On the other hand, the deep portion ( 38   a ) remains in the central portion of the second middle land portion  12  from an intermediate stage to a terminal stage of wear and allows excellent wet performance to be maintained. 
     As illustrated in  FIG. 2 , the third outer side sipes ( 38   o ) each have a depth that varies along the tire axial direction. The third outer side sipes ( 38   o ) each have a deep portion ( 38   c ) at a central portion of the second middle land portion  12  and a shallow portion ( 38   d ) on the second shoulder main groove  6  side. The depth of each of the third outer side sipes ( 38   o ) linearly varies between the deep portion ( 38   c ) and the shallow portion ( 38   d ). Due to the shallow portion ( 38   d ), tire circumferential direction rigidity of the second middle land portion  12  is increased, and uneven wear such as heel-and-toe wear is suppressed. On the other hand, the deep portion ( 38   c ) remains in the central portion of the second middle land portion  12  from an intermediate stage to a terminal stage of wear and allows excellent wet performance to be maintained. 
     As illustrated in  FIGS. 1, 3 and 5 , it is desirable that a distance from the tire equator (C) to a center line of the first crown main groove  3  be smaller than a distance from the tire equator (C) to a center line of the second crown main groove  4 , and a distance from the tire equator (C) to a center line of the first shoulder main groove  5  be smaller than a distance from the tire equator (C) to a center line of the second shoulder main groove  6 . Along with this, a center line of the crown land portion  10  is positioned in the second tread half  22 , and, coupled with first crown shallow grooves  51 , water on the tread surface of the crown land portion  10  during traveling on a wet road surface is likely to be discharged to the first crown main groove  3 . 
     In the above, a pneumatic tire according to an embodiment of the present invention is described in detail. However, without being limited to the above-described specific embodiment, the present invention can also be embodied in various modified forms. 
     EXAMPLES 
     Pneumatic tires each having a size of 215/60R16 and a basic tread pattern of  FIG. 1  are prototyped based on specifications shown in Table 1 and steering stability is tested. A test method is as follows. 
     Steering Stability 
     The prototyped tires mounted on a rim of 16×7.0J are mounted on all wheels of a passenger FR car having a displacement of 2500 cc under a condition of an internal pressure of 250 kPa, and the FR car is driven by one driver on a test course of a dry asphalt road surface, and characteristics about grip performance, steering response, and responsiveness are evaluated based on sensory evaluation by the driver. The result is a score with a result of Example 1 as 100. A larger score indicates a better steering stability. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Comparative 
                 Comparative 
                 Comparative 
                 Comparative 
                   
                   
               
               
                   
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 4 
                 Example 1 
                 Example 2 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Width (mm) of  
                 0.5 
                 0.5 
                 0.5 
                 2.0 
                 0.5 
                 1.0 
               
               
                 first sipes 
                   
                   
                   
                   
                   
                   
               
               
                 Width (mm) of  
                 0.5 
                 0.5 
                 0.5 
                 2.0 
                 0.5 
                 1.0 
               
               
                 second sipes 
                   
                   
                   
                   
                   
                   
               
               
                 Shape of first  
                 Discontinuous 
                 Discontinuous 
                 Smoothly 
                 Smoothly 
                 Smoothly 
                 Smoothly 
               
               
                 sipes 
                   
                   
                 continuous 
                 continuous 
                 continuous 
                 continuous 
               
               
                 Shape of second  
                 Discontinuous 
                 Discontinuous 
                 Smoothly 
                 Smoothly 
                 Smoothly 
                 Smoothly 
               
               
                 sipes 
                   
                   
                 continuous 
                 continuous 
                 continuous 
                 continuous 
               
               
                 Terminating  
                 Second tread 
                 Second 
                 Second tread 
                 Second 
                 Second 
                 Second 
               
               
                 position of  
                 ground 
                 shoulder land 
                 ground 
                 shoulder land 
                 shoulder land 
                 shoulder land 
               
               
                 second sipes 
                 contact edge 
                 portion 
                 contact edge 
                 portion 
                 portion 
                 portion 
               
               
                 Third sipes 
                 None 
                 Yes 
                 None 
                 Yes 
                 Yes 
                 Yes 
               
               
                 Steering stability 
                 80 
                 90 
                 90 
                 75 
                 100 
                 95 
               
               
                 (score) 
                   
                   
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     As is apparent from Table 1, it can be confirmed that the pneumatic tires of the examples allow steering stability to be significantly improved as compared to the comparative examples. 
     For a purpose of improving steering stability, Japanese Patent Laid-Open Publication No. 2014-184828 describes a pneumatic tire in which main grooves each continuously extending in a tire circumferential direction are respectively provided on a tire equator and on both sides of the tire equator. In this pneumatic tire, grooves each having a large width are not provided in middle land portions, and thereby, rigidity of the middle land portions is increased and steering stability is improved. 
     In the above-described pneumatic tire, due to arc-shaped sipes provided in the middle land portions and shoulder land portions, a decrease in pattern rigidity is suppressed and a load is dispersed in multiple directions, and steering stability is improved. However, the sipes provided in the middle land portions and the sipes provided in the shoulder land portions have different inclinations with respect to the tire circumferential direction and are discontinuous to each other. Therefore, on a tread surface, the middle land portions and the shoulder land portions deform in different modes. Therefore, there is a risk that transient characteristics during cornering, in which a center of the tread surface (a place where a ground contact pressure is the highest) moves from a central portion of the tread toward a shoulder land portion, may be affected. 
     A pneumatic tire according to an embodiment of the present invention improves transient characteristics during cornering and provides good steering stability. 
     A pneumatic tire according to an embodiment of the present invention includes, in a tread part: a pair of crown main grooves that are respectively formed on both outer sides of a tire equator and each continuously extend in a tire circumferential direction; a pair of shoulder main grooves that are respectively formed on tire axial direction outer sides of the crown main grooves and each continuously extend in the tire circumferential direction; a crown land portion between the pair of the crown main grooves; a pair of middle land portions between the crown main grooves and the shoulder main grooves; a pair of shoulder land portions that are respectively positioned on tire axial direction outer sides of the shoulder main grooves a first tread half from the tire equator to a first tread ground contact edge; and a second tread half from the tire equator to a second tread ground contact edge, which is a tread ground contact edge on an opposite side of the first tread ground contact edge. Grooves each having a width of 2 mm or more are not provided in the crown land portion and the middle land portions. In the first tread half, multiple first sipes each having a width of less than 2 mm are provided. In the first tread half, the first sipes each smoothly continuously extend from the crown land portion to the shoulder land portion via the crown main groove and the shoulder main groove. In the second tread half, multiple second sipes each having a width of less than 2 mm are provided. In the second tread half, the second sipes each smoothly continuously extend from the crown land portion to the shoulder land portion via the crown main groove and the shoulder main groove and are each terminated without reaching the second tread ground contact edge. Further, in the middle land portion of the second tread half, third sipes that each have a width of less than 2 mm and each extend in the tire axial direction are each provided between a pair of second sipes that are adjacent to each other in the tire circumferential direction. 
     In a pneumatic tire according to an embodiment of the present invention, it is desirable that the third sipes include: third inner side sipes that each extend from the crown main groove of the second tread half toward a tire axial direction outer side and are each terminated without reaching the shoulder main groove; and third outer side sipes that each extend from the shoulder main groove of the second tread half toward a tire axial direction inner side and are each terminated without reaching the crown main groove. 
     In a pneumatic tire according to an embodiment of the present invention, it is desirable that the first sipes include first crown sipes provided in the crown land portion, the second sipes include second crown sipes provided in the crown land portion, and a length of each of the first crown sipes is larger than a length of each of the second crown sipes. 
     In a pneumatic tire according to an embodiment of the present invention, it is desirable that the first crown sipes each extend from the crown main groove of the first tread half to beyond the tire equator and are each terminated without reaching the crown main groove on the other side. 
     In a pneumatic tire according to an embodiment of the present invention, it is desirable that the second crown sipes each extend from the crown main groove of the second tread half and are each terminated without reaching the tire equator. 
     In a pneumatic tire according to an embodiment of the present invention, it is desirable that a tire axial direction length of each of the third sipes is 60%-80% of a tire axial direction length of the middle land portion in the second tread half. 
     In a pneumatic tire according to an embodiment of the present invention, it is desirable that, in each of the shoulder land portions, shoulder lug grooves are provided that each extend from the tread ground contact edge toward a tire axial direction inner side and are each terminated without reaching the shoulder main groove, and the shoulder lug grooves each have a linear portion that linearly extends from the tread ground contact edge at an angle of 85-95 degrees with respect to the tire circumferential direction, and a curved portion that is formed on a tire axial direction inner side of the linear portion and extends in an arc shape along the shoulder sipes. 
     In a pneumatic tire according to an embodiment of the present invention, it is desirable that the second sipes include second shoulder sipes that are provided in the shoulder land portion, and the second shoulder sipes are each terminated on a tire axial direction inner side of a tire axial direction outer side edge of the curved portion of each of the shoulder lug grooves in the second tread half. 
     In a pneumatic tire according to an embodiment of the present invention, it is desirable that the first sipes include first middle sipes that are provided in the middle land portion of the first tread half, and first shoulder sipes that are provided in the shoulder land portion of the first tread half, and the first middle sipes and the first shoulder sipes each have a deep portion and a shallow portion that has a depth smaller than that of the deep portion. 
     In a pneumatic tire according to an embodiment of the present invention, it is desirable that the second sipes include second middle sipes that are provided in the middle land portion of the second tread half, and second shoulder sipes that are provided in the shoulder land portion of the second tread half, the second middle sipes and the second shoulder sipes each have a deep portion and a shallow portion that has a depth smaller than that of the deep portion. 
     In a pneumatic tire according to an embodiment of the present invention, in the first tread half, the first sipes each smoothly continuously extend from the crown land portion to the shoulder land portion via the crown main groove and the shoulder main groove. As a result, the crown land portion, the middle land portion and the shoulder land portion of the first tread half deform in the same mode along the first sipes. Similarly, in the second tread half, the second sipes each smoothly continuously extend from the crown land portion to the shoulder land portion via the crown main groove and the shoulder main groove. As a result, the crown land portion, the middle land portion and the shoulder land portion of the second tread half deform in the same mode along the second sipes. Therefore, transient characteristics during cornering, in which a center of a tread surface moves from the crown land portion to the shoulder land portion, can be improved, and good steering stability can be obtained. 
     The second sipes are each terminated without reaching the second tread ground contact edge. As a result, rigidity of the shoulder land portion of the second tread half is increased, and grip performance during cornering, in which a ground contact pressure of the shoulder land portion of the second tread half is increased, can be improved, and good steering stability can be obtained. 
     Further, due to an edge effect achieved by the third sipes provided in the middle land portion of the second tread half, grip performance is improved and steering stability is more easily improved. Further, due to the second sipes and the third sipes, a density of the sipes gradually decreases from the middle land portion of the second tread half to the second tread ground contact edge. As a result, rigidity of the land portions is gradually increased from the middle land portion of the second tread half to the second tread ground contact edge, and transient characteristics and grip performance during cornering can be improved, and even better steering stability can be obtained. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.