Patent Publication Number: US-2022234392-A1

Title: Motorcycle tire and front and rear motorcycle tires

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to a motorcycle tire and a set of front and rear motorcycle tires. 
     Background Art 
     The following Patent Document 1 discloses a motorcycle tire whose tread portion is provided with main grooves having a groove width of more than 2 mm and narrow grooves having a groove width of 2 mm or less. The main grooves and the narrow grooves are inclined with respect to the tire circumferential direction.
     Patent Document 1: Japanese Patent Application Publication No. 2019-104329   

     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In the case of a bias-ply motorcycle tire whose tread portion is not provided with a belt layer for providing a hoop effect, generally, the rubber thickness of the tread portion becomes larger in tread shoulder regions, and the rigidity becomes higher in the tread shoulder regions, so there is a problem such that handling performance is liable to deteriorate. Here, the handling performance means easiness of handling operation in which the vehicle body is leant to give a camber angle to the tires when turning the motorcycle. 
     On the other hand, if a large number of grooves and sipes are provided in the tread portion, there is a problem such that grip performance is liable to deteriorate. 
     The present disclosure was made in view of the above circumstances, and a primary objective of the present disclosure is to provide a motorcycle tire capable of improving the handling performance and the grip performance. 
     According to the present disclosure, a motorcycle tire comprises: 
     a carcass having a bias-ply structure, and 
     a tread portion provided with main grooves and sipes, wherein 
     the tread portion comprises a pair of axially outer shoulder regions and a crown region therebetween, 
     the main grooves include first main grooves extending from the crown region into one of the shoulder regions, and second main grooves extending from the crown region into the other of the shoulder regions, and 
     the sipes are formed only within the shoulder regions, wherein 
     the shoulder regions are defined as extending axially inwardly from respective tread edges of the tread portion to have a developed axial width of 30% of a developed axial width of the tread portion between the tread edges. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a developed partial view of a tread portion of a motorcycle tire as an embodiment of the present disclosure. 
         FIG. 2  is a cross-sectional view of the motorcycle tire taken along line A-A of  FIG. 1 . 
         FIGS. 3-5  are developed partial views of the tread portion which are the same views but differently provided with reference numbers and signs. 
         FIG. 6  is a cross-sectional view of one of the sipes taken along the longitudinal direction of the sipe. 
         FIG. 7  is a developed partial view of a tread portion of a motorcycle tire as another embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present disclosure is suitably applied to on-road motorcycle tires for driving on well-paved roads such as dry asphalt roads although the present disclosure is not limited to such use conditions. 
     Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. 
       FIG. 1  shows a part of the developed tread portion  2  of a motorcycle tire  1  as an embodiment of the present disclosure. 
     In the present embodiment, the tire  1  is designed for on-road driving such as on dry asphalt roads. 
       FIG. 2  is a cross-sectional view taken along line A-A of  FIG. 1 . As shown in  FIG. 2 , the tread portion  2  has a tread surface  2   a  curved in an arc shape which is radially outwardly convex so that a sufficient ground contact area can be obtained even when turning with a large camber angle. 
     The tire  1  comprises a carcass  6  extending between bead portions  4 , and a band  7  disposed radially outside the carcass  6  in the tread portion  2 . 
     The carcass  6  in this embodiment is composed of two carcass plies  6 A and  6 B having a bias-ply structure. 
     Each carcass ply  6 A and  6 B is composed of carcass cords arranged at an angle of 30 to 45 degrees with respect to the tire circumferential direction. 
     The band  7  is composed of at least one, in the present embodiment, only one ply  7 A of one or more band cords wound spirally at an angle of 5 degrees or less with respect to the tire circumferential direction. 
     As shown in  FIG. 1 , the tread portion  2  has a pair of shoulder regions Sh and a crown region Cr therebetween. 
     The shoulder regions Sh extend axially inwardly from respective tread edges Te of the tread portion  2 , and each have a developed axial width Ws of 30% of the developed tread width TWe.
 
The crown region Cr is a region on which a large ground contact pressure acts mainly during traveling straight.
 
Each shoulder region Sh is a region on which a large contact pressure acts mainly during turning.
 
The developed tread width TWe is the distance in the tire axial direction measured between the tread edges Te along the tread surface in the tire meridian section ( FIG. 1 ).
 
The developed axial width is the distance in the tire axial direction measured along the tread surface in the tire meridian section ( FIG. 1 ).
 
     The tread portion  2  is provided with a plurality of main grooves  8  and a plurality of sipes  9 . 
     The tread portion  2  in the present embodiment is further provided with a circumferential groove  10  extending continuously in the tire circumferential direction.
 
In this application, the term “groove” means a groove whose a maximum groove width is more than 1.5 mm, and the term “sipe” means a very narrow groove whose a maximum groove width is not more than 1.5 inclusive of a cut having no substantial groove width.
 
     In the present application, dimensions relating to the tire refer to values measured in a normal state of the tire  1  unless otherwise noted. 
     The “normal state” is a state of the tire  1  which is mounted on a normal rim (not shown) and inflated to a normal pressure and loaded with no tire load. 
     The “normal rim” is a wheel rim specified for the tire by a standard included in a standardization system on which the tire is based, for example, the “standard rim” in JATMA, the “Design Rim” in TRA., the “Measuring Rim” in ETRTO, and the like. 
     The “normal pressure” is air pressure specified for the tire by a standard included in a standardization system on which the tire is based, for example, the “maximum air pressure” in JATMA, maximum value listed in the “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” table in TRA, and “INFLATION PRESSURE” in ETRTO. 
     The main grooves  8  include 
     first main grooves  11  extending from the crown region Cr into one of the shoulder regions Sh (left side in the figure), and
 
second main grooves  12  extending from the crown region Cr into the other of the shoulder regions Sh (right side in the figure).
 
The first main grooves  11  and the second main grooves  12  can relax the rigidity of the tread portion  2  in the shoulder regions Sh even when the rubber thickness of the tread portion  2  is larger in the shoulder regions Sh.
 
Therefore, in the tire  1  of the present embodiment, ground contact property of the shoulder regions Sh with the road surface at the time of turning is improved, and thus the handling performance is improved.
 
     The sipes  9  are formed only within the shoulder regions Sh. There are no sipes in the crown region Cr. Therefore, the rigidity of the tread portion is suppressed from being excessively lowered in the crown region Cr. As a result, excellent grip performance may be exhibited. 
     The sipes  9  include first sipes  9 A disposed in one of the shoulder regions Sh, and second sipes  9 B disposed in the other of the shoulder regions Sh. 
     The tread portion  2  in the present embodiment as shown in  FIG. 1  is suitably applied to a front tire of a motorcycle (not shown) although it can be applied to a rear tire. 
     In the present embodiment, the first main grooves  11 , the second main grooves  12 , and the sipes  9  are arranged without being connected to each other in order to suppress the rigidity of the tread portion  2  from being largely decreased. 
     In the present embodiment, the main grooves  8  further include third main grooves  13 , fourth main grooves  14 , fifth main grooves  15 , and sixth main grooves  16 . 
     The third main grooves  13  are arranged adjacently to the respective first main grooves  11  on one side in the tire circumferential direction, and extends from the crown region Cr into one of the shoulder regions Sh so as to have an axial length smaller than that of the first main grooves  11 . 
     The fourth main grooves  14  are arranged adjacently to the respective second main grooves  12  on the above-said one side in the tire circumferential direction, and extends from the crown region Cr into the other of the shoulder regions Sh so as to have an axial length smaller than that of the second main grooves  12 . 
     The fifth main grooves  15  are disposed within one of the shoulder regions Sh. 
     The sixth main grooves  16  are disposed within the other of the shoulder regions Sh. 
     The first main grooves  11  are shifted in the tire circumferential direction from the second main grooves  12 , and the shape of each first main groove  11  and the shape of each second main groove  12  are line-symmetrical with respect to the tire equator C. The descriptions of the first main grooves  11  can be applied to the second main grooves  12 , therefore, the descriptions of the second main grooves  12  may be omitted thereafter. 
     The third main grooves  13  are shifted in the tire circumferential direction from the fourth main grooves  14 , and the shape of each third main groove  13  and the shape of each fourth main groove  14  are line-symmetrical with respect to the tire equator C. The descriptions of the third main grooves  13  can be applied to the fourth main grooves  14 , therefore, the descriptions of the fourth main grooves  14  may be omitted thereafter. 
     The fifth main grooves  15  are shifted in the tire circumferential direction from the sixth main grooves  16 , and the shape of each fifth main groove  15  and the shape of each sixth main groove  16  are line-symmetrical with respect to the tire equator C. The descriptions of the fifth main grooves  15  can be applied to the sixth main grooves  16 . therefore, the descriptions of the sixth main grooves  16  may be omitted thereafter. 
     As shown in  FIG. 1 , while the main grooves  8  extend from their axially inner ends to axially outer ends, the main grooves  8  are inclined to one side in the tire circumferential direction (upper side in the figure). That is, the first main grooves  11 , the third main grooves  13 , and the fifth main grooves  15  are inclined upward to the left, and the second main grooves  12 , the fourth main grooves  14 , and the sixth main grooves  16  are inclined upward to the right. 
     Such main grooves  8  can make the flow of water in the grooves smooth by specifying the tire rotational direction, and high drainage performance can be exhibited. 
     In the present embodiment, the main grooves  8  include a widening portion  18  in which the groove width increases towards the axially outside and which is formed in one of the shoulder regions Sh. 
     In the present embodiment, each of the first main grooves  11  and the second main grooves  12  is provided with the widening portion  18 . Such widening portions  18  further improve the ground contact property of the shoulder regions Sh with the road surface at the time of turning. 
     In the present embodiment, the widening portions  18  extend from the crown region Cr to the respective shoulder regions Sh. 
     The widening portion  18  may be defined as having a difference (wb-wa) between the minimum groove width (wa) and the maximum groove width (wb) which is 3% or more of the axial length La between them. In other words, if less than 3%, such portion is regarded as a substantially constant width portion. 
     The main grooves  8  are terminated without reaching the respective tread edges Te in order to maintain the rigidity of the shoulder regions Sh high. 
     The developed axial distance Li between the axially outer end  8   e  of each main groove  8  and the tread edge Te is preferably 1 mm or more, more preferably 2 mm or more.
 
The developed axial distance Li is preferably 30% or less, more preferably 25% or less of the developed axial width Ws of the shoulder region Sh.
 
     In this embodiment, the sipes  9  are also terminated without reaching the respective tread edges Te. 
     Each of the main grooves  8  has a pair of groove edges  8   s  and  8   t  extending in the longitudinal direction of the groove, wherein the groove edge  8   s  is positioned on the axially inner side of the groove edge  8   t.    
     Each of the main grooves  8  comprises 
     a first decreasing-width portion  19  in which the groove width decreases toward the axially inner end  8   i,  
 
a second decreasing-width portion  20  in which the groove width decreases toward the axially outer end  8   e , and
 
a main portion  21  connecting between the first decreasing-width portion  19  and the second decreasing-width portion  20 .
 
The first decreasing-width portion  19  includes the axially inner end  8   i.  
 
The second decreasing-width portion  20  includes the axially outer end  8   e.  
 
The main portion  21  includes a maximum width position  21   a  at which the maximum groove width wb of the main groove  8  occurs.
 
The first decreasing-width portion  19  and the second decreasing-width portion  20  maintain the rubber volume of the tread portion  2  large, and suppress an excessive decrease in rigidity of the tread portion  2 .
 
     In the present embodiment, the first decreasing-width portions  19  is formed by a portion where one of the groove edges  8   s  and  8   t  extends at a smaller angle with respect to the tire circumferential direction than the other of the groove edges  8   s  and  8   t.    
     In the present embodiment, the angle with respect to the tire circumferential direction of the groove edge  8   s  positioned on the axially inner side is smaller than that of the groove edge  8   t  positioned on the axially outer side.
 
In the present embodiment, the groove width in the first decreasing-width portion  19  is continuously decreased from the main portion  21 .
 
     In order to improve grip performance and drainage performance, it is preferable that the rotation direction N of the tire is specified such that the groove edge  8   s  and the groove edge  8   t  are on the toe side and heel side, respectively, in the rotation direction N. 
     The length Lc of the first decreasing-width portion  19  of each of the first, second, third and fourth main grooves  11 - 14  is preferably 5% or more, more preferably 10% or more, but preferably 30% or less, more preferably 25% or less of the developed axial width We of the crown region Cr. 
     The length Lc of the first decreasing-width portion  19  of each of the fifth and sixth main grooves  15  and  16  is preferably 3% or more, more preferably 5% or more, but 17% or less, more preferably 15% or less of the developed axial width Ws of the shoulder region Sh.
 
Here, the length Lc of the first decreasing-width portion  19  is the length along the widthwise center line.
 
Thereby, the handling performance and grip performance are enhanced. Further, the first decreasing-width portion  19  suppresses the occurrence of groove bottom cracks.
 
     In each of the second decreasing-width portions  20 , one groove edge  8   s  comprises an arc-shaped portion  22  connecting between the main portion  21  and the second decreasing-width portion  20  with a smooth arc, and 
     the other groove edge  8   t  comprises a crank-shaped portion  23  bent in a direction to reduce the groove width.
 
Each of the second decreasing-width portions  20  is a portion in which the groove width is abruptly reduced as compared with the main portion  21 .
 
     In each of the main grooves  8 , the maximum groove width we in the second decreasing-width portion  20  is preferably 15% or more, more preferably 20% or more, but preferably 50% or less, more preferably 40% or less of the maximum groove width wb of the main groove  8 . 
     In each of the main grooves  8 , the axial length Ld of the second decreasing-width portion  20  is preferably 15% or more, more preferably 20% or more, but preferably 30% or less, more preferably 25% or less of the developed axial width Ws of the shoulder region Sh. 
     In each of the main grooves  8 , the maximum depth da in the second decreasing-width portion  20  is preferably 15% or more, more preferably 20% or more, but preferably 50% or less, more preferably 40% or less of the maximum groove depth D 1  (shown in  FIG. 2 ) of the main groove  8 . 
     Such second decreasing-width portion  20  enhances the grip performance and handling performance, and suppresses the occurrence of groove bottom cracks. 
     The maximum groove depth D 1  of each of the main grooves  8  is preferably 3.8 mm or more, more preferably 4.0 mm or more, but preferably 8.0 mm or less, more preferably 7.2 mm or less. 
     In the tire circumferential direction, each of the first main grooves  11  overlaps with the circumferentially adjacent two of the first main grooves  11 , forming circumferential overlap portions  25 . 
     Such first main grooves  11  further relax the rigidity of the tread portion  2 , enhances the ground contact property with the road surface, and improves the handling performance. 
     The circumferential length Le of the circumferential overlap portion  25  is preferably 5% or more, more preferably 8% or more, but preferably 15% or less, more preferably 12% or less of the circumferential length LI of the first main groove  11 . 
     As shown in  FIG. 4 , the groove width of the third main groove  13  becomes smaller in the middle in the longitudinal direction, forming a constricted portion  26 . 
     The main portion  21  of the third main groove  13  includes: 
     the constricted portion  26 ; 
     a first constant-width portion  27  having a constant groove width and connecting between the constricted portion  26  and the first decreasing-width portion  19 ; and 
     a second constant-width portion  28  having a constant groove width and connecting between the constricted portion  26  and the second decreasing-width portion  20 . 
     In the constricted portion  26 , 
     one groove edge  8   s  is formed by an arc portion  31  extending in an arc shape which is convex toward the groove center line, and the other groove edge  8   t  is formed by a crank shape portion  32  extends in a crank shape bent in a direction to reduce the groove width. 
       FIG. 2  shows a groove depth distribution of the fourth main groove  14 . As shown, the minimum groove depth dc in the constricted portion  26  is preferably 10% or more, more preferably 20% or more, but preferably 50% or less, more preferably 40% or less of the maximum groove depth D 1  of the fourth main groove  14 . 
     In both end portions of the constricted portion  26 , the groove depth gradually smoothly increases toward the both ends. 
     As shown in  FIG. 4 , the minimum groove width wf in the constricted portion  26  is preferably 10% or more, more preferably 20% or more, but preferably 50% or less, more preferably 40% or less of the maximum groove width wb of the third main groove  13 . 
     Such constricted portion  26  suppresses the occurrence of groove bottom cracks. 
     The fifth main groove  15  comprises a third constant-width portion  35  having a constant groove width, and disposed between the first decreasing-width portion  19  and the second decreasing-width portion  20 . 
     As shown in  FIG. 5 , the developed axial length LA of the first main groove  11  is preferably 30% or more, more preferably 35% or more, but preferably 50% or less, more preferably 45% or less of the developed tread width TWe. 
     The developed axial length LB of the third main groove  13  is preferably 25% or more, more preferably 30% or more, but preferably 45% or less, more preferably 40% or less of the developed tread width TWe. 
     The developed axial length LC of the fifth main groove  15  is preferably 25% or more, more preferably 30% or more, but preferably 45% or less, more preferably 40% or less of the developed tread width TWe. 
     As shown in  FIG. 1 , while the sipes  9  extend from their axially inner ends to axially outer ends, the sipes  9  inclined to one side in the tire circumferential direction (upper side in the figure). 
     The first sipes  9 A are inclined in the same direction as the first main grooves  11 . The second sipes  9 B are inclined in the same direction as the second main grooves  12   
       FIG. 6  shows a depth distribution of the sipe  9 . 
     It is preferable that the maximum depth de of the sipe  9  is the same as the maximum groove depth D 1  of the main groove  8  (in this example, the first and second main grooves  11  and  12 ). As a result, the rigidity of the shoulder regions Sh is relaxed, the ground contact property of the shoulder regions Sh with the road surface at the time of turning is improved, and the handling performance is greatly improved. 
     The bottom  9   s  of the sipe  9  is provided with at least one protruding portion  38 . Such protruding portion  38  appropriately increases the rigidity of the sipe bottom  9   s , and suppresses the occurrence of sipe bottom cracks. Further, the protruding portion  38  can improve the handling performance without impairing the grip performance. 
     In the present embodiment, as shown in  FIG. 6 , the protruding portion  38  is formed in an arc shape convex toward the radially outside. In the protruding portion  38 , the protruding height h is gradually increased from both ends in the longitudinal direction toward the intermediate position  38   c . Such protruding portion  38  suppresses an excessive decrease in the rigidity of the shoulder region Sh. 
     In the present embodiment, the sipe  9  is provided with only one protruding portion  38  as shown in  FIG. 5 . 
     In this case, the protruding portion  38  is arranged at the intermediate position  9   c  (shown in  FIG. 5 ) of the sipe  9  in the longitudinal direction.
 
When n protruding portions  38  are provided, it is desirable that the length of the sipe  9  is divided into n equal parts, and the n protruding portions  38  are respectively disposed at intermediate positions in the longitudinal direction of the respective n equal parts (not shown).
 
     In each of the shoulder regions Sh of the tread portion  2  in the present embodiment, the sipes  9  are respectively arranged adjacently to the main grooves  8  (in this example, the first/second main grooves  11 / 12 ) on one side in the tire circumferential direction, and 
     the sipes  9  overlap with the main grooves  8  in the tire axial direction to form overlap portions  39  as shown in  FIG. 5 . 
     It is desirable that, when measured on a circumferential line n passing through the overlap portion  39  in parallel to the tire circumferential direction, the circumferentially adjacent sipe  9  and main groove  8  have the same depth. As a result, the rigidity of the shoulder region Sh is appropriately reduced, and the handling performance and the grip performance are enhanced. 
     In the present embodiment, when measured at any position in the tire axial direction of the overlap portion  39 , the sipe  9  and the first main groove  11  have the same depth. 
     In the present embodiment, the overlap portion  39  has the same length Lg in the tire axial direction as the sipe  9 . 
     In each of the shoulder regions Sh, the sipes  9  are disposed axially outside the intermediate position Si in the axial direction of the shoulder region Sh. Thereby, the rigidity of the shoulder region Sh where the rubber thickness is relatively large is reduced in its axially outer portion, and it becomes possible to improve the handling performance. 
     In the present embodiment, the axially inner ends  9   i  of the sipes  9  are located axially outside the intermediate position Si in the tire axial direction of the shoulder region Sh. 
     The circumferential groove  10  extends circumferentially of the tire along the tire equator C in order to enhance drainage performance. 
     In the present embodiment, the circumferential groove  10  is composed of alternately arranged first portions  10 A and second portions  10 B. While the circumferential groove  10  extends in a tire circumferential direction, the first portions  10 A are inclined to one side in the tire axial direction, and the second portions  10 B are inclined to the other side in the tire axial direction. Thus, the circumferential groove  10  has a zigzag shape. The circumferential groove  10  repeatedly crosses the tire equator C while extending in the tire circumferential direction, therefore, the circumferential groove  10  has axially outwardly protruding portions formed alternately along the tire equator as shown in  FIG. 5 . 
     Accordingly, each of the axially outwardly protruding portions has an axially outer groove edge portion  10   s  and an axially inner groove edge portion  10   t.    
     The circumferential groove  10  in this example is provided with an axially outwardly protruding notch  40  at each of intersecting positions  10   k  of the first portions  10 A and the second portions  10 B in order to enhances drainage performance and uneven wear resistance. 
     The notch  40  is formed at the axially outer groove edge portion  10   s.  
 
The notch  40  has a first edge  40   a  merging into the axially outer groove edge portion  10   s , and a second edge  40   b  intersecting the axially outer groove edge portion  10   s  at an angle.
 
With respect to the tire circumferential direction, the second edge  40   b  is inclined at a larger angle than the first edge  40   a.  
 
The first edge  40   a  suppresses an excessive decrease in rigidity of the tread portion  2  near the tire equator C.
 
When the second edge  40   b  is positioned on the toe side in the rotation direction N and the first edge  40   a  is positioned on the heel side in the rotation direction N, the second edge  40   b  is useful for discharging water existing between the tread surface  2   a  and the road surface toward the outside of the circumferential groove  10 .
 
       FIG. 7  is a developed partial view of a tread portion  2  of a motorcycle tire  1  as another embodiment of the present disclosure. The same components as those of the former embodiment may be designated by the same reference numerals and the descriptions may be omitted. The tread portion  2  (tread pattern) of this embodiment is suitably used for a tire for a rear wheel of a motorcycle although it may be used for a front tire. 
     As shown in  FIG. 7 , the tread portion  2  of this embodiment is provided with the main grooves  8  and the sipes  9 , but no circumferential groove is provided. The main grooves  8  of this embodiment are the first to sixth main grooves  11 - 16 . 
     In this application, also in this embodiment, unless otherwise noted, the descriptions of the first main grooves  11 , the third main grooves  13  and the fifth main grooves  15  also serve as the descriptions of the second main grooves  12 , the fourth main grooves  14  and the sixth main grooves  16 , respectively. 
     The first main grooves  11  extend from the shoulder region Sh into the crown region Cr without intersecting the tire equator C. 
     In this embodiment, the axially inner ends  11   i  of the first main grooves  11  are positioned closer to the tire equator C as compared with the first main grooves  11  of the former embodiment shown in  FIG. 1 .
 
The axial distance Lh from the axially inner end  11   i  to the tire equator C is preferably 3% or less of the developed tread width TWe.
 
     In this embodiment, the widening portion  18  of each of the first main grooves  11  extends to the axially outer end  11   e  of the first main groove  11   
     In other words, the first main groove  11  is not provided with the second decreasing-width portion  20 . Such first main groove  11  enhances the drainage performance. 
     In this embodiment, the fifth main grooves  15  extend from the crown region Cr into the shoulder region Sh. Such fifth main grooves  15  appropriately reduce the rigidity from the crown region Cr to the shoulder region Sh. 
     The motorcycle tire  1  having the tread portion  2  shown in  FIG. 1  as a front tire, and the motorcycle tire  1  having the tread portion  2  shown in  FIG. 7  as a rear tire constitute a set of front and rear motorcycle tires T. 
     The developed tread width TWe of the rear tire is larger than that of the front tire. As a result, the rigidity of the shoulder regions Sh of the rear tire becomes higher than the front tire. Thus, it is desirable that the shoulder regions Sh of the rear tire are provided with more sipes  9  than the front tire in order to improve the ground contact property of the rear tire and improve the handling performance. 
     In the rear tire, as shown in  FIG. 7 , there are provided two sipes  9  per one pitch P 1 , namely, between every two of the first main grooves  11 . In the front tire, as shown in  FIG. 1 , there is provided one sipe  9  per one pitch P 1 , namely, between every two of the first main grooves  11 . It is desirable that the one pitch P 1  of the first main grooves  11  of the rear tire is equal to the one pitch P 1  of the first main grooves  11  of the front tire. 
     It is desirable that the tread portion  2  of the rear tire has a land ratio of 80% or more. Thereby, an excessive decrease in the rigidity of the tread portion  2  of the rear tire is suppressed, and the grip performance is maintained. 
     The land ratio means a ratio (SB/SA) between the actual ground contacting area SB of the tread surface  2   a  of the tread portion  2  and the overall surface area SA of the tread portion  2  when it is assumed that neither groove nor sipe is provided. 
     The rear tire is not significantly affected by the rider&#39;s handle operation. 
     For the rear tire, grip performance is more important than handling performance. Therefore, in the tread portion  2  of the rear tire, it is preferable that the land ratio of the crown region Cr is not less than, more preferably more than the land ratio of each of the shoulder regions Sh. 
     The land ratios used herein have similar meanings to the land ratio of the tread portion, namely, the actual ground contacting area relative to the overall surface area. 
     The front tire is greatly affected by the rider&#39;s handle operation. For the front tire, handling performance is more important than grip performance. 
     Therefore, in the tread portion  2  of the front tire, it is preferable that the land ratio of each of the shoulder regions Sh is not less than, more preferably more than the land ratio of the crown region Cr. 
     In order to effectively exert the above-described advantageous effects, it is preferred that the land ratio of the tread portion  2  of the front tire is smaller than the land ratio of the tread portion  2  of the rear tire. 
     Each tire  1  is manufactured by using a vulcanization mold comprising a pair of dies (not shown). 
     The pair of dies is split in the tire axial direction, and their split position extends in the tire circumferential direction through the tread portion. In the vulcanization mold for the front tire, the split position is separated from the circumferential groove toward one side in the tire axial direction. In the vulcanization mold for the rear tire, the split position is positioned at the tire equator C. 
     While detailed description has been made of especially preferable embodiments of the present disclosure, the present disclosure can be embodied in various forms without being limited to the illustrated embodiments. 
     Comparison Tests 
     Based on the tread patterns shown in  FIG. 1  and  FIG. 7 , motorcycle tires were experimentally manufactured as test tires (Working examples Ex.1-Ex.7 and Comparative examples Ref.1-Ref.2), and tested for the handling performance, grip performance, drainage performance, uneven wear resistance performance, and high-speed durability performance. 
     The common specifications of the test tires and test methods are as follows. 
     &lt;Handling Performance, Grip Performance, Uneven Wear Resistance Performance, Durability Performance&gt; 
     The test tires were mounted on the front and rear wheels of a 1500cc street motorcycle under the following conditions. 
     The test tires for the front wheel had tread patterns based on  FIG. 1 .
 
The test tires for the rear wheel had tread patterns based on  FIG. 7 .
 
During the motorcycle was running on a dry asphalt road of a test course, the test rider evaluated the handling performance and grip performance. Further, after running, the occurrence of uneven wear and the occurrence of cracks at the groove bottoms and the sipe bottoms were visually checked.
 
     The test results are indicated in Table 1 by an index based on Comparative Example Ref.1 being 100, wherein the higher the numerical value, the better the performance (passing line is 95 or more). 
     Front tire specifications (tire size, rim, pressure): 110/70-13M/C, 13×3.00MT, 200 kPa. 
     Rear wheel tire specifications (tire size, rim, pressure): 130/70-13M/C, 13×3.50MT, 220 kPa. &lt;Drainage performance &gt; 
     During the above-mentioned motorcycle was running on a wet asphalt road of the test course, the test rider evaluated the easiness of running. 
     The test results are indicated in Table 1 by an index based on Comparative Example Ref.1 being 100, wherein the higher the numerical value, the better the performance (passing line is 95 or more). 
     The symbols used in Table 1 have the following meanings:
 
“A”: The first main grooves and the second main grooves extended from the crown region into the shoulder regions.
 
“B”: The first main grooves and the second main grooves extended only within the shoulder regions.
 
“C”: The sipes were formed only within the shoulder regions.
 
“D”: No sipe was provided.
 
“E”: The first main grooves, the second main grooves and the sipes were not communicated with each other.
 
“F”: The first main grooves, the second main grooves and the sipes were communicated with each other.
 
“G”: The main grooves and the sipes did not reach to the tread edges.
 
“H”: The main grooves reached to the respective tread edges.
 
“I”: Number of front tire sipes &lt;Number of rear tire sipes
 
“J”: Number of front tire sipes &gt;Number of rear tire sipes
 
“K”: First main groove depth=Sipe depth when measured on circumference direction line passing through overlap portion
 
“L”: First main groove depth &gt;Sipe depth when measured on circumference direction line passing through overlap portion
 
“M”: first main groove had constant-width portion instead of widening portion.
 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                 Tire 
                 Ref.1 
                 Ref.2 
                 Ex.1 
                 Ex.2 
                 Ex.3 
               
               
                   
               
               
                 first and second main grooves 
                 B 
                 A 
                 A 
                 A 
                 A 
               
               
                 sipes 
                 C 
                 D 
                 C 
                 C 
                 C 
               
               
                 arrangement of first and 
                 E 
                 E 
                 E 
                 F 
                 E 
               
               
                 second main grooves and sipes 
                   
                   
                   
                   
                   
               
               
                 tread edges 
                 G 
                 G 
                 G 
                 G 
                 H 
               
               
                 rear tire 
                   
                   
                   
                   
                   
               
               
                 crown region&#39;s land ratio 
                 86 
                 86 
                 86 
                 86 
                 86 
               
               
                 shoulder regions&#39; land ratio 
                 84 
                 84 
                 84 
                 84 
                 84 
               
               
                 front and rear tires&#39; sipes 
                 I 
                 I 
                 I 
                 I 
                 I 
               
               
                 first main groove depth 
                 K 
                 K 
                 K 
                 K 
                 K 
               
               
                 and sipe depth 
                   
                   
                   
                   
                   
               
               
                 first main groove&#39;s 
                 present 
                 present 
                 present 
                 present 
                 present 
               
               
                 widening portion 
                   
                   
                   
                   
                   
               
               
                 handling performance 
                 100 
                 105 
                 120 
                 125 
                 125 
               
               
                 grip performance 
                 100 
                 90 
                 110 
                 105 
                 105 
               
               
                 uneven wear resistance 
                 100 
                 100 
                 105 
                 100 
                 100 
               
               
                 high-speed durability 
                 100 
                 100 
                 100 
                 95 
                 95 
               
               
                 drainage performance 
                 100 
                 110 
                 110 
                 115 
                 110 
               
               
                   
               
               
                 Tire 
                 Ex.4 
                 Ex.5 
                 Ex.6 
                 Ex.7 
                   
               
               
                   
               
               
                 first and second main grooves 
                 A 
                 A 
                 A 
                 A 
                   
               
               
                 sipes 
                 C 
                 C 
                 C 
                 C 
                   
               
               
                 arrangement of first and 
                 E 
                 E 
                 E 
                 E 
                   
               
               
                 second main grooves and sipes 
                   
                   
                   
                   
                   
               
               
                 tread edges 
                 G 
                 G 
                 G 
                 G 
                   
               
               
                 rear tire 
                   
                   
                   
                   
                   
               
               
                 crown region&#39;s land ratio 
                 86 
                 86 
                 86 
                 86 
                   
               
               
                 shoulder regions&#39; land ratio 
                 88 
                 84 
                 84 
                 84 
                   
               
               
                 front and rear tires&#39; sipes 
                 I 
                 J 
                 I 
                 I 
                   
               
               
                 first main groove depth 
                 K 
                 K 
                 L 
                 K 
                   
               
               
                 and sipe depth 
                   
                   
                   
                   
                   
               
               
                 first main groove&#39;s 
                 present 
                 present 
                 present 
                 M 
                   
               
               
                 widening portion 
                   
                   
                   
                   
                   
               
               
                 handling performance 
                 120 
                 115 
                 125 
                 115 
                   
               
               
                 rip performance 
                 100 
                 110 
                 100 
                 105 
                   
               
               
                 uneven wear resistance 
                 100 
                 100 
                 95 
                 105 
                   
               
               
                 high-speed durability 
                 100 
                 100 
                 95 
                 100 
                   
               
               
                 drainage performance 
                 100 
                 100 
                 115 
                 110 
               
               
                   
               
            
           
         
       
     
     From the test results, it was confirmed that the motorcycle tires according to the present disclosure were superior in the handling performance and grip performance to the comparative example tires. 
     Statement of the Present Disclosure 
     The present disclosure is as follows:--
 
Disclosure 1: A motorcycle tire comprising: a carcass having a bias-ply structure; and a tread portion provided with main grooves and sipes, wherein the tread portion comprises a pair of axially outer shoulder regions and a crown region therebetween, the main grooves include first main grooves extending from the crown region into one of the shoulder regions, and second main grooves extending from the crown region into the other of the shoulder regions, and the sipes are formed only within the shoulder regions, wherein the shoulder regions are defined as extending axially inwardly from respective tread edges of the tread portion to have a developed axial width of 30% of a developed axial width of the tread portion.
 
Disclosure 2: The motorcycle tire according to Disclosure 1, wherein the main grooves have a maximum depth, and the sipes has a maximum depth equal to the maximum depth of the main grooves.
 
Disclosure 3: The motorcycle tire according to Disclosure 1 or 2, wherein each of the sipes is arranged adjacently to one of the main grooves in the tire circumferential direction so that the sipe overlaps with the main groove in the tire axial direction to define an overlap portion, and the sipe&#39;s depth is equal to the main groove&#39;s depth when measured on a circumferential line passing through the overlap portion in parallel with the tire circumferential direction.
 
Disclosure 4: The motorcycle tire according to Disclosure 1, 2 or 3, wherein the first main grooves, the second main grooves and the sipes are arranged without being connected to each other.
 
Disclosure 5: The motorcycle tire according to Disclosure 1, 2, 3, or 4, wherein the main grooves are each terminated without reaching the respective tread edges.
 
Disclosure 6: The motorcycle tire according to any one of Disclosures 1 to 4, wherein the main grooves comprise widening portions which are positioned in the shoulder regions, and in which the groove widths increase axially outward.
 
Disclosure 7: The motorcycle tire according to Disclosure 6, wherein the widening portions extend to the axially outer ends of the respective main grooves.
 
Disclosure 8: The motorcycle tire according to any one of Disclosures 1 to 7, wherein each of the sipes is provided, in its bottom, with a protruding portion where the bottom is locally raised.
 
Disclosure 9: The motorcycle tire according to Disclosure 8, wherein the protruding portion has a radially outwardly convex arc shape in its cross section taken along the longitudinal direction of the sipe.
 
Disclosure 10: The motorcycle tire according to any one of Disclosures 1 to 9, wherein in each the shoulder regions, the sipes are offset to the axially outside from an intermediate position in the tire axial direction of the shoulder region.
 
Disclosure 11: A combination of a front tire and a rear tire for motorcycle, wherein the front tire is the motorcycle tire according to any one of Disclosures 1 to 10, the rear tire is the motorcycle tire according to any one of Disclosures 1 to 10, and a number of the sipes of the rear tire is larger than a number of the sipes of the front tire.
 
Disclosure 12: The combination according to Disclosure 11, wherein the land ratio of the tread portion of the rear tire is 80% or more.
 
Disclosure 13: The combination according to Disclosure 11 or 12, wherein in the tread portion of the rear tire, the land ratio of the crown region is not less than the land ratio of the shoulder regions.
 
Disclosure 14: The combination according to Disclosure 11, 12 or 13, wherein the land ratio of the tread portion of the front tire is smaller than the land ratio of the tread portion of the rear tire.
 
Disclosure 15: The combination according to any one of Disclosures 11 to 14, wherein in the tread portion of the front tire, the land ratio of the shoulder regions is not less than the land ratio of the crown region.
 
     DESCRIPTION OF THE REFERENCE SIGNS 
     
         
         
           
               1  motorcycle tire 
               2  tread portion 
               8  main groove 
               9  sipe 
               11  first main groove 
               12  second main groove 
             Cr crown region 
             Sh shoulder region