Patent Publication Number: US-2023141057-A1

Title: Motorcycle tire

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority on U.S. Provisional Application No. 63/277,709 filed on Nov. 10, 2021, the entire content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to motorcycle tires. 
     BACKGROUND ART 
     The tread surface of a motorcycle tire has a shape projecting radially outward. During straight running, a center portion of the tread surface comes into contact with a road surface. During cornering, an axially outer portion of the tread surface comes into contact with the road surface. 
     Running of a motorcycle involves changes in the attitude of the vehicle. During cornering, the vehicle is tilted. During running on a circuit, the rider tilts the vehicle to the limit. 
     In a state where the vehicle is tilted to the limit (that is, full bank state), an end portion of the tread surface comes into contact with a road surface. When traction is applied to the tire at once in a full bank state, a disturbance such as slipping of the tire and vibration occurs. The disturbance makes it difficult to control the attitude of the vehicle. 
     For tires, studies have been conducted to improve transient characteristics and cornering stability (for example, PATENT LITERATURE 1 below). 
     CITATION LIST 
     Patent Literature 
     PATENT LITERATURE 1: Japanese Patent No. 6859821 (U.S. Pat. No. 10,946,699) 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     Attitude control is left to the rider. Since the aforementioned disturbance occurs when traction is applied to the tire at once in a full bank state, vehicle attitude control is difficult. In recent years, however, attitude control has increasingly become electronic, and a vehicle itself performs attitude control when a disturbance occurs. 
     A motorcycle generates a camber thrust to turn. A vehicle for which attitude control is made electronic can stably run even when the vehicle is tilted. As the vehicle can be tilted to a larger extent, a larger camber thrust can be generated, so that the vehicle can turn at a higher speed. 
     A tire that allows a motorcycle to be titled at a larger angle during cornering and is capable of exerting a high grip force even when the motorcycle is tilted to the limit, is desired. 
     The present invention has been made in view of such circumstances. An object of the present invention is to provide a motorcycle tire that allows a motorcycle to be titled at a larger angle during cornering and is capable of exerting a high grip force even when the motorcycle is tilted to the limit. 
     Solution to Problem 
     A motorcycle tire according to an aspect of the present invention includes a tread surface having a shape projecting radially outward. The tread surface includes a center portion including an equator, and a pair of shoulder portions including ends of the tread surface, respectively. A zone from the equator to each end of the tread surface, of a contour line of the tread surface, is divided into five equal sections, whereby the contour line is divided into ten parts. Among the ten parts, two parts including the equator are each represented by an arc, and two parts including the ends of the tread surface, respectively, are each represented by an arc. A degree of curvature of the center portion is represented by an average of radii of the arcs that represent the two parts including the equator, respectively. A degree of curvature of each shoulder portion is represented by a radius of an arc having a larger radius, out of an arc that represents a part including a first end of the tread surface and an arc that represents a part including a second end of the tread surface. An angle, with respect to an axial direction, of a tangent line that is tangent to the arc that represents the part including the end of the tread surface, at the end of the tread surface is a bank angle. An angle, with respect to the axial direction, of a line segment connecting the equator and the end of the tread surface is a camber angle. The bank angle is larger than the camber angle. A ratio of the degree of curvature of the shoulder portion to a width of the tread surface is not less than 50%. A ratio of a distance in a radial direction from the equator to the end of the tread surface to a tire cross-sectional height is not less than 50%. 
     Preferably, in the motorcycle tire, the bank angle is not less than 50°. 
     Preferably, in the motorcycle tire, a ratio of the bank angle to the camber angle is not less than 120% and not greater than 180%. 
     Preferably, in the motorcycle tire, a ratio of the degree of curvature of the center portion to the degree of curvature of the shoulder portion is not greater than 50%. 
     Preferably, in the motorcycle tire, no groove is formed in the parts including the ends of the tread surface, respectively. 
     Advantageous Effects of the Invention 
     According to the present invention, a motorcycle tire that allows a motorcycle to be titled at a larger angle during cornering and is capable of exerting a high grip force even when the motorcycle is tilted to the limit, is obtained. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross-sectional view showing a part of a motorcycle tire according to an embodiment of the present invention. 
         FIG.  2    is a development of a tread surface. 
         FIG.  3    is a cross-sectional view showing a contour line of a tire outer surface. 
         FIG.  4    is a cross-sectional view showing a part of the contour line shown in  FIG.  3   . 
     
    
    
     DETAILED DESCRIPTION 
     The following will describe in detail the present invention based on preferred embodiments with appropriate reference to the drawings. 
     In the present disclosure, a state where a tire is fitted on a normal rim, the internal pressure of the tire is adjusted to a normal internal pressure, and no load is applied to the tire is referred to as a normal state. 
     In the present disclosure, unless otherwise specified, the dimensions and angles of each component of the tire are measured in the normal state. 
     The dimensions and angles of each component in a meridian cross-section of the tire, which cannot be measured in a state where the tire is fitted on the normal rim, are measured in a cross-section of the tire obtained by cutting the tire along a plane including a rotation axis, with the distance between right and left beads being made equal to the distance between the beads in the tire that is fitted on the normal rim. 
     The normal rim means a rim specified in a standard on which the tire is based. The “standard rim” in the JATMA standard, the “Design Rim” in the TRA standard, and the “Measuring Rim” in the ETRTO standard are normal rims. 
     The normal internal pressure means an internal pressure specified in the standard on which the tire is based. The “highest air pressure” in the JATMA standard, the “maximum value” recited in “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” in the TRA standard, and the “INFLATION PRESSURE” in the ETRTO standard are normal internal pressures. 
     A normal load means a load specified in the standard on which the tire is based. The “maximum load capacity” in the JATMA standard, the “maximum value” recited in the “TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES” in the TRA standard, and the “LOAD CAPACITY” in the ETRTO standard are normal loads. 
     In the present disclosure, a tread portion of the tire is a portion of the tire that comes into contact with a road surface. A bead portion is a portion of the tire that is fitted to a rim. A side portion is a portion of the tire that extends between the tread portion and the bead portion. The tire includes a tread portion, a pair of bead portions, and a pair of side portions as portions thereof. 
       FIG.  1    shows a part of a tire  2  according to an embodiment of the present invention. The tire  2  is a motorcycle pneumatic tire. The tire  2  is mounted to the rear wheel of a motorcycle. 
       FIG.  1    shows a part of a cross-section (hereinafter, also referred to as meridian cross-section) of the tire  2  along a plane including the rotation axis of the tire  2 . In  FIG.  1   , the right-left direction is the axial direction of the tire  2 , and the up-down direction is the radial direction of the tire  2 . The direction perpendicular to the surface of the drawing sheet of  FIG.  1    is the circumferential direction of the tire  2 . An alternate long and short dash line CL represents the equator plane of the tire  2 . 
     The tire  2  is fitted onto a rim R. The rim R is a normal rim. The interior of the tire  2  is filled with air to adjust the internal pressure of the tire  2 . The tire  2  fitted on the rim R is also referred to as a tire-rim assembly. The tire-rim assembly includes the rim R and the tire  2  fitted on the rim R. 
     In  FIG.  1   , a solid line BBL extending in the axial direction is a bead base line. This bead base line is a line that defines the rim diameter (see JATMA or the like) of the rim R. 
     The tire  2  includes a tread  4 , a pair of sidewalls  6 , a pair of beads  8 , a carcass  10 , a band  12 , an insulation  14 , a pair of chafers  16 , and an inner liner  18 . 
     The tread  4  is formed from a crosslinked rubber. The tread  4  is composed of a single rubber member formed from a crosslinked rubber. The tread  4  may be composed of a plurality of rubber members aligned in the axial direction. The tread  4  may be composed of a plurality of rubber members stacked in the radial direction. In the tire  2 , the tread  4  is composed of a rubber member that is generally used as a rubber member for the tread of a motorcycle tire. 
     The tread  4  comes into contact with a road surface at an outer surface thereof. The outer surface of the tread  4  is a tread surface  20 . The tread  4  includes the tread surface  20  which comes into contact with a road surface. 
     In  FIG.  1   , reference sign PC indicates the point of intersection of the tread surface  20  and the equator plane CL. The point of intersection PC corresponds to the equator of the tire  2 . The equator PC is the radially outer end of the tire  2 . Reference sign PE indicates an end of the tread surface  20 . The tread surface  20  has a shape projecting radially outward. The end PE of the tread surface  20  is located radially inward of the equator PC. As shown in  FIG.  1   , in the tire  2 , the end PE of the tread surface  20  is an axially outer end of the tire  2 . 
     In the tread surface  20 , a portion including the equator PC is a center portion C, and a portion including the end PE of the tread surface  20  is a shoulder portion S. The tread surface  20  includes the center portion C including the equator PC, and a pair of the shoulder portions S including the ends PE of the tread surface  20 , respectively. Each shoulder portion S is located axially outward of the center portion C. 
       FIG.  2    shows a part of the tread surface  20 . In  FIG.  2   , the right-left direction is the axial direction of the tire  2 , and the up-down direction is the circumferential direction of the tire  2 . The direction perpendicular to the surface of the drawing sheet of  FIG.  2    is the radial direction of the tire  2 . 
     In  FIG.  2   , an arrow A is the rotation direction of the tire  2 . The tread surface  20  comes into contact with a road surface from the lower side to the upper side of the surface of the drawing sheet of  FIG.  2   . The upper side of the surface of the drawing sheet is a trailing side, and the lower side of the surface of the drawing sheet is a leading side. 
     Grooves  22  are formed on the tread surface  20 . Accordingly, a tread pattern is formed. The tire  2  may be a slick type tire having no groove  22  formed on the tread surface  20 . 
     On the tread surface  20  of the tire  2 , inclined grooves  24  are formed as the grooves  22 . The entirety of each inclined groove  24  of the tire  2  is located between the equator PC and the end PE of the tread surface  20 . The inclined groove  24  is inclined relative to the circumferential direction. A tailing end  26   a  of the inclined groove  24  is located on the end PE side of the tread surface  20 . A leading end  26   b  of the inclined groove  24  is located on the equator PC side. 
     Although not described in detail, the specifications such as the position, groove width, groove depth, groove length, etc., of each inclined groove  24  are set as appropriate in consideration of the specifications of the tire  2 . The grooves  22 , in other words, the tread pattern, may include grooves other than the inclined groove  24 . 
     The inclined grooves  24  include first inclined grooves  24   a  formed between the equator PC and a first end PEa of the tread surface  20 , and second inclined grooves  24   b  formed between the equator PC and a second end PEb of the tread surface  20 . 
     A plurality of the first inclined grooves  24   a  are arranged in the circumferential direction at a constant pitch. A first inclined groove  24   a  located on the trailing side and a first inclined groove  24   a  located on the leading side are arranged so as to be spaced apart from each other. 
     A plurality of the second inclined grooves  24   b  are arranged in the circumferential direction at a constant pitch. A second inclined groove  24   b  located on the trailing side and a second inclined groove  24   b  located on the leading side are arranged so as to be spaced apart from each other. 
     In the tire  2 , the first inclined grooves  24   a  and the second inclined grooves  24   b  are alternately arranged in the circumferential direction. A first inclined groove  24   a  and a second inclined groove  24   b  located near this first inclined groove  24   a  overlap each other in the axial direction. In the circumferential direction, between the trailing end  26   a  and the leading end  26   b  of a first inclined groove  24   a , the leading end  26   b  of the second inclined groove  24   b  on the trailing side and the trailing end  26   a  of the second inclined groove  24   b  on the leading side are located. In the circumferential direction, between the trailing end  26   a  and the leading end  26   b  of a second inclined groove  24   b , the leading end  26   b  of the first inclined groove  24   a  on the trailing side and the trailing end  26   a  of the first inclined groove  24   a  on the leading side are located. 
     Each sidewall  6  is connected to an end of the tread  4 . The sidewall  6  is located radially inward of the tread  4 . The sidewall  6  extends radially inward from the end of the tread  4 . 
     The sidewall  6  is a rubber member formed from a crosslinked rubber. In the tire  2 , the sidewall  6  is composed of a rubber member that is generally used as a rubber member for a sidewall of a motorcycle tire. 
     Each bead  8  is located radially inward of the sidewall  6 . The bead  8  includes a core  28  and an apex  30 . The core  28  includes a steel wire which is not shown. The apex  30  is located outward of the core  28  in the radial direction. The apex  30  is tapered outward. The apex  30  is formed from a crosslinked rubber that has high stiffness. 
     The carcass  10  is located inward of the tread  4  and the pair of sidewalls  6 . The carcass  10  extends on and between a first bead  8  and a second bead  8 . The carcass  10  has a radial structure. 
     The carcass  10  includes at least one carcass ply  32 . The carcass  10  of the tire  2  is composed of one carcass ply  32 . The carcass  10  may be composed of two or more carcass plies  32 . The carcass ply  32  of the tire  2  is turned up around each bead  8  from the inner side toward the outer side in the axial direction. 
     The carcass ply  32  includes a large number of carcass cords aligned with each other, which are not shown. Each carcass cord intersects the equator plane CL. Each carcass cord is a cord formed from an organic fiber. Examples of the organic fiber include nylon fibers, rayon fibers, polyester fibers, and aramid fibers. 
     The band  12  is located between the tread  4  and the carcass  10  in the radial direction. The band  12  is disposed such that both ends thereof oppose each other across the equator plane CL. The ends of the band  12  are located near the ends PE of the tread surface  20 . 
     The band  12  includes a helically wound band cord which is not shown. The band cord extends substantially in the circumferential direction. Specifically, an angle of the band cord with respect to the circumferential direction is not greater than 5°. The band  12  has a jointless structure. 
     In the tire  2 , a cord formed from an organic fiber is used as the band cord. Examples of the organic fiber include nylon fibers, rayon fibers, polyester fibers, and aramid fibers. 
     The insulation  14  is located between the band  12  and the carcass  10  in the radial direction. The insulation  14  is disposed such that both ends thereof oppose each other across the equator plane CL. Each end of the insulation  14  is located axially inward of the end of the band  12 . The width of the insulation  14  is smaller than the width of the band  12 . 
     The insulation  14  is a rubber member formed from a crosslinked rubber. The insulation  14  relieves strain generated between the band  12  and the carcass  10 . 
     The insulation  14  is not an essential element of the tire  2 . In the tire  2 , the insulation  14  does not have to be provided. In this case, the entirety of the band  12  is stacked directly on the carcass  10 . 
     Each chafer  16  is located radially inward of the bead  8 . The chafer  16  comes into contact with the rim R. The chafer  16  of the tire  2  includes a fabric and a rubber with which the fabric is impregnated. 
     The inner liner  18  is located inward of the carcass  10 . The inner liner  18  forms an inner surface of the tire  2 . The inner liner  18  is formed from a crosslinked rubber that has a low gas permeability coefficient. The inner liner  18  maintains the internal pressure of the tire  2 . 
       FIG.  3    shows a contour line of the tire  2  in the meridian cross-section. The contour line is obtained by measuring the outer surface shape of the tire  2  in the normal state, for example, by a displacement sensor. The contour line represents a virtual outer surface shape obtained on the assumption that there are no irregularities such as grooves and decorations on the outer surface of the tire  2 . 
     Of the contour line, a portion from the first end PEa of the tread surface  20  to the second end PEb thereof is a contour line of the tread surface  20 . A portion from each end PE of the tread surface  20  to a toe (position indicated by reference sign T in  FIG.  1   ) of the tire  2  corresponds to a side surface  34 , and a contour line of a part thereof is shown in  FIG.  3   . 
     The tread surface  20  and a pair of the side surfaces  34  form the outer surface of the tire  2 . The outer surface of the tire  2  is shaped by the cavity face of a mold (not shown) used for producing the tire  2 . A contour line of the cavity face is formed by smoothly connecting a plurality of arcs and straight lines. The inner surface of the tire  2  is shaped by the outer surface of an expanded bladder or a rigid core. The boundary between the outer surface and the inner surface of the tire  2  is the above-described toe T. 
     In  FIG.  3   , a length indicated by a double-headed arrow TW is the width of the tread surface  20 . The width TW of the tread surface  20  is the distance in the axial direction from the first end PEa of the tread surface  20  to the second end PEb thereof. 
     Of the contour line of the tread surface  20 , a zone from the equator PC to the first end PEa of the tread surface  20  is a first zone, and a zone from the equator PC to the second end PEb of the tread surface  20  is a second zone. In the tire  2 , by dividing each zone into five equal sections, five parts are formed in each zone. Accordingly, the contour line of the tread surface  20  is divided into ten parts. The contour line of the tread surface  20  has a shape that is symmetrical about the equator plane CL. The tread surface  20  may be formed so as to have a shape that is asymmetrical about the equator plane CL. 
     The five parts included in each zone are a first part P 1 , a second part P 2 , a third part P 3 , a fourth part P 4 , and a fifth part P 5  from the equator PC toward the end PE of the tread surface  20 . 
     In  FIG.  3   , reference signs B 1 , B 2 , B 3 , and B 4  indicate the boundaries between the adjacent parts. The first part P 1  connects the equator PC and the boundary B 1 . The second part P 2  connects the boundary B 1  and the boundary B 2 . The third part P 3  connects the boundary B 2  and the boundary B 3 . The fourth part P 4  connects the boundary B 3  and the boundary B 4 . The fifth part P 5  connects the boundary B 4  and the end PE of the tread surface  20 . The first part P 1  includes the equator PC. The fifth part P 5  includes the end PE of the tread surface  20 . 
     In the tire  2 , among the ten parts included in the contour line of the tread surface  20 , the first parts P 1  of the first zone and the second zone are each represented by an arc having a radius R 1 . The fifth parts P 5 , of the first zone and the second zone, each including the end PE of the tread surface  20  are each represented by an arc having a radius R 5 . 
     In the tire  2 , the two first parts P 1  including the equator PC are each represented by an arc, and the two fifth parts P 5  each including the end PE of the tread surface  20  are each represented by an arc. 
     In the tire  2 , in each of the first zone and the second zone, the second part P 2  is represented by an arc having a radius R 2 , the third part P 3  is represented by an arc having a radius R 3 , and the fourth part P 4  is represented by an arc having a radius R 4 . In other words, the second part P 2 , the third part P 3 , and the fourth part P 4  which are located between the first part P 1  and the fifth part P 5  of each zone are each represented by an arc. In the tire  2 , all the five parts included in each zone are represented by arcs. 
     In the tire  2 , between the first part P 1  and the fifth part P 5  of each zone, the second part P 2  may be represented by a straight line, the third part P 3  may be represented by a straight line, or the fourth part P 4  may be represented by a straight line. The second part P 2  and the third part P 3  may be represented by straight lines, the third part P 3  and the fourth part P 4  may be represented by straight lines, or the fourth part P 4  and the second part P 2  may be represented by straight lines. The second part P 2 , the third part P 3 , and the fourth part P 4  may be represented by straight lines. The second part P 2 , the third part P 3 , and the fourth part P 4  which are located between the first part P 1  and the fifth part P 5  of each zone are each represented by an arc or a straight line. 
     In the present disclosure, whether a part included in the contour line of the tread surface  20  is represented by an arc or a straight line is determined by whether the part is curved with respect to the line segment connecting both ends of the part. If a part is curved with respect to the line segment connecting both ends of the part, the part is determined to be represented by an arc. In this case, the point of intersection of the part and the perpendicular bisector of the line segment connecting both ends of the part is defined as the center of the part, and an arc passing through both ends and the center of the part is used as an arc that represents this part. If a part is not curved with respect to the line segment connecting both ends of the part, this is the case where the part is represented by a straight line, and thus this part is determined to be represented by a straight line. 
     In the tire  2 , when a contour line represented by an arc or a straight line is an approximate contour line, from the viewpoint of increasing the conformity between the shape of the part and the approximate contour line, the distance from the approximate contour line to the part, measured along a normal line of the approximate contour line, is preferably not greater than 3%, and more preferably not greater than 1%, of the length of the line segment connecting both ends of the part. 
     As described above, the tread surface  20  includes the center portion C including the equator PC and the pair of the shoulder portions S each including the end PE of the tread surface  20 . 
     In the tire  2 , a degree of curvature Bc of the center portion C is represented by the average of the radii of the arcs that respectively represent the two first parts P 1  including the equator PC. Specifically, the degree of curvature Bc of the center portion C is represented by the average of the radius R 1  of the arc that represents the first part P 1  in the first zone and the radius R 1  of the arc that represents the first part P 1  in the second zone. 
     A degree of curvature Bs of each shoulder portion S is represented by the radius of the arc having a larger radius, out of the arc that represents the fifth part P 5  including the first end PEa of the tread surface  20  and the arc that represents the fifth part P 5  including the second end PEb of the tread surface  20 . In the case where the arc that represents the fifth part P 5  of the first zone and the arc that represents the fifth part P 5  of the second zone have the same radius, the degree of curvature Bs of the shoulder portion S is represented by the radius of the arc that represents any of these parts. 
       FIG.  4    shows a part of the contour line shown in  FIG.  3   . 
     In  FIG.  4   , a solid line LE is a straight line passing through the end PE of the tread surface  20  and extending in the axial direction. A solid line TL is a tangent line that is tangent to the arc that represents the fifth part P 5 , at the end PE of the tread surface  20 . An angle θb is an angle formed between the straight line LE and the tangent line TL. The angle θb is an angle, with respect to the axial direction, of the tangent line TL which is tangent to the arc that represents the fifth part P 5  including the end PE of the tread surface  20 , at the end PE of the tread surface  20 , and is a bank angle. 
     In  FIG.  4   , a solid line LC is a straight line passing through the equator PC and extending in the axial direction. A solid line CE is a straight line including the line segment connecting the equator PC and the end PE of the tread surface  20 . An angle θc is an angle formed between the straight line LC and the straight line CE. The angle θc is an angle, with respect to the axial direction, of the line segment connecting the equator PC and the end PE of the tread surface  20 , and is a camber angle. 
     In  FIG.  4   , a length indicated by a double-headed arrow d is a camber height. The camber height d is the distance in the radial direction from the equator PC to the end PE of the tread surface  20 . A length indicated by a double-headed arrow H is a tire cross-sectional height. The tire cross-sectional height H is the distance in the radial direction from the equator PC to the bead base line BBL. 
     A motorcycle (vehicle) generates a camber thrust to turn. As the vehicle can be tilted to a larger extent, a larger camber thrust can be generated, so that the vehicle can turn at a higher speed. 
     In the tire  2 , the bank angle θb is larger than the camber angle θc. A vehicle (motorcycle) on which the tire  2  is mounted can turn at a large tilt. The vehicle can generate a large camber thrust. The vehicle on which the tire  2  is mounted can turn at a high speed. When the tire  2  is mounted to a vehicle whose attitude is electronically controlled during running, a rider can turn the vehicle at a larger tilt. A larger camber thrust is generated, so that the rider can turn the vehicle at a higher speed. 
     As described above, the tread  4  is formed from a crosslinked rubber, and the tire  2  comes into contact with a road surface at the tread surface  20 . 
     In general, when stress acts on a crosslinked rubber, the crosslinked rubber becomes deformed. For example, the road surface of a circuit course is made of asphalt, and irregularities exist on the road surface. A tire supports a vehicle body and a rider. A load is applied to a tread that is in contact with a road surface. Due to the application of the load, stress acts on the tread. The tread surface becomes deformed according to the irregularities of the road surface. The area in which the tread surface is actually in contact with the ground (hereinafter, referred to as actual ground-contact area) increases. The frictional force of the tread against the road surface increases. 
     As the load applied to the crosslinked rubber is increased and the stress acting on the crosslinked rubber is increased, the crosslinked rubber becomes compressed and eventually hardens. The hardening apparently decreases the coefficient of friction of the crosslinked rubber. During cornering in which a large load is applied to the tire, the frictional force of the tread may be decreased due to this hardening. If the tread cannot maintain a high frictional force, even if vehicle attitude control is made electronic and the vehicle can stably run while being tilted to the limit, the vehicle cannot sufficiently exhibit its performance. 
     In the tire  2 , the ratio (Bs/TW) of the degree of curvature Bs of the shoulder portion S to the width TW of the tread surface  20  is not less than 50%. 
     The shoulder portion S contributes to an increase in the ground-contact area during cornering. Even when the vehicle is tilted to the limit and a large load is applied to the tread  4 , the stress acting on the tread  4  can be reduced. The progress of hardening due to application of a load is suppressed. The tire  2  can exert a high grip force even when the vehicle is tilted to the limit. From this viewpoint, the ratio (Bs/TW) is preferably not less than 60% and more preferably not less than 70%. 
     From the viewpoint that, when a running state shifts from cornering to straight running, the overturning moment required to raise the vehicle is sufficiently obtained and good lightness can be maintained, the ratio (Bs/TW) is preferably not greater than 200% and more preferably not greater than 150%. 
     In a state where the vehicle is turning while being tilted to the limit, a large impact may be momentarily applied to the tire due to the irregularities of the road surface or accelerator operation serving as a trigger, causing the tire to leap. In this case, the tire becomes separated from the road surface, so that the ground-contact area is momentarily reduced. The grip force is suddenly lost, resulting in an increase in the risk of falling over of the vehicle. 
     In the tire  2 , the ratio (d/H) of the camber height d to the tire cross-sectional height H is not less than 50%. 
     In the tire  2 , in a state where the vehicle is turning while being tilted to the limit, the tread portion having lower stiffness and a larger thickness than each side portion absorbs a momentarily applied impact. Even when an impact is momentarily applied to the tire  2  in a state where the vehicle is turning while being tilted to the limit, the tire  2  does not leap, and a state where the tread  4  is in sufficient contact with the road surface is maintained. In addition to reducing the risk of falling over of the vehicle, the tire  2  can maintain a high grip force in a state where the vehicle is tilted to the limit. From this viewpoint, the ratio (d/H) is preferably not less than 55% and more preferably not less than 60%. From the viewpoint of maintaining ride comfort during straight running, the ratio (d/H) is preferably not greater than 75% and more preferably not greater than 70%. 
     In the tire  2 , the bank angle θb is larger than the camber angle θc, the ratio (Bs/TW) of the degree of curvature Bs of the shoulder portion S to the width TW of the tread surface  20  is not less than 50%, and the ratio (d/H) of the distance d in the radial direction from the equator PC to the end PE of the tread surface  20  to the tire cross-sectional height H is not less than 50%. The tire  2  allows the motorcycle to be titled at a larger angle during cornering and is capable of exerting a high grip force even when the motorcycle is tilted to the limit. 
     In the tire  2 , the bank angle θb is preferably not less than 50°. Accordingly, the vehicle on which the tire  2  is mounted can turn at a large tilt. The vehicle can generate a large camber thrust. From this viewpoint, the bank angle θb is more preferably not less than 55° and further preferably not less than 60°. From the viewpoint of preventing interference with a road surface, the bank angle θb is preferably not greater than 75° and more preferably not greater than 70°. 
     In the tire  2 , the ratio (θb/θc) of the bank angle θb to the camber angle θc is preferably not less than 120% and not greater than 180%. 
     When the ratio (θb/θc) is set to be not less than 120%, the tread portion can effectively absorb a momentarily applied impact in a state where the vehicle is turning while being tilted to the limit. Even when an impact is momentarily applied to the tire  2  in a state where the vehicle is turning while being tilted to the limit, the tire  2  does not leap, and a state where the tread  4  is in sufficient contact with the road surface is maintained. In addition to reducing the risk of falling over of the vehicle, the tire  2  can maintain a high grip force in a state where the vehicle is tilted to the limit. From this viewpoint, the ratio (θb/θc) is more preferably not less than 130% and further preferably not less than 140%. 
     When the ratio (θb/θc) is set so as to be not greater than 180%, the contour line of the tread surface  20  is maintained in an appropriate shape, and a state where it is possible to tilt the vehicle to the limit is maintained. From this viewpoint, the ratio (θb/θc) is more preferably not greater than 170% and further preferably not greater than 160%. 
     In the tire  2 , the ratio (Bc/Bs) of the degree of curvature Bc of the center portion C to the degree of curvature Bs of the shoulder portion S is preferably not greater than 50%. In the tire  2 , the degree of curvature Bc of the center portion C is sufficiently lower than the degree of curvature Bs of the shoulder portion S. With the tire  2 , good lightness is achieved. The tire  2  can shift its state from a standing state to a full bank state at once. Since the degree of curvature Bs of the shoulder portion S is sufficiently higher than the degree of curvature Bc of the center portion C, the tire  2  can exert a high grip force in a full bank state. From this viewpoint, the ratio (Bc/Bs) is more preferably not greater than 45% and further preferably not greater than 40%. 
     From the viewpoint that a sufficient ground-contact area is obtained and good braking performance can be maintained during straight running, the ratio (Bc/Bs) is preferably not less than 10%, more preferably not less than 20%, and further preferably not less than 30%. 
     In the tire  2 , the ratio (Bc/TW) of the degree of curvature Bc of the center portion C to the width TW of the tread surface  20  is preferably not greater than 70%. 
     In the tire  2 , the center portion C is moderately rounded. The center portion C contributes to the ease of tilting the vehicle. The vehicle on which the tire  2  is mounted easily shifts from straight running to cornering. The tire  2  contributes to improvement of lightness. From this viewpoint, the ratio (Bc/TW) is more preferably not greater than 60% and further preferably not greater than 50%. 
     The ratio (Bc/TW) is preferably not less than 10%. Accordingly, the roundness of the center portion C is appropriately maintained. The center portion C contributes to the ease of raising the vehicle. The vehicle on which the tire  2  is mounted easily shifts form cornering to straight running. With the tire  2 , good lightness is maintained. From this viewpoint, the ratio (Bc/TW) is more preferably not less than 15% and further preferably not less than 20%. 
     In  FIG.  2   , each solid line LB 4  represents the boundary B 4  between the fourth part P 4  and the fifth part P 5 . As shown in  FIG.  2   , in the tire  2 , no groove  22  is formed between each end PE of the tread surface  20  and each boundary B 4 , that is, in each fifth part P 5  including the end PE of the tread surface  20 . 
     The fifth part P 5  comes into contact with a road surface in a full bank state. Since no groove  22  is formed in the fifth part P 5 , the tire  2  is in sufficient contact with the road surface in a full bank state. The fifth part P 5  contributes to exertion of a high grip force in a full bank state. From this viewpoint, in the tire  2 , preferably, no groove  22  is formed in the fifth part P 5  including the end PE of the tread surface  20 . 
     As described above, in the tire  2 , the second part P 2 , the third part P 3 , and the fourth part P 4  which are located between the first part P 1  and the fifth part P 5  of each zone in the contour line of the tread surface  20  are each represented by an arc or a straight line. In other words, the three parts located between the first part P 1  including the equator PC and the fifth part P 5  including the end PE of the tread surface  20  are each represented by an arc or a straight line. Accordingly, the tread surface  20  having a smooth contour line is formed. The tire  2  can smoothly shift from straight running to cornering or from cornering to straight running. The tire  2  has good transient characteristics. From this viewpoint, in the tire  2 , preferably, the three parts located between the first part P 1  including the equator PC and the fifth part P 5  including the end PE of the tread surface  20  are each represented by an arc or a straight line. 
     In the tire  2 , more preferably, at least one part out of the three located between the first part P 1  including the equator PC and the fifth part P 5  including the end PE of the tread surface  20  is represented by an arc. 
     Accordingly, non-linear changes in physical actions such as frictional forces and moments caused by the boundaries between the parts when the running state shifts from straight running to cornering or from cornering to straight running are suppressed. The tire  2  can smoothly shift from straight running to cornering or from cornering to straight running. The tire  2  has good transient characteristics. The good transient characteristics contribute to improvement of lightness. From this viewpoint, in the tire  2 , further preferably, as in the contour line of the tread surface  20  shown in  FIG.  3   , the three parts located between the first part P 1  including the equator PC and the fifth part P 5  including the end PE of the tread surface  20  are all represented by arcs. 
     In the contour line of the tread surface  20  shown in  FIG.  3   , the radius R 2  of the arc that represents the second part P 2  located on the end PE side of the tread surface  20  with respect to the first part P 1  is larger than the radius R 1  of the arc that represents the first part P 1  located on the equator PC side with respect to the second part P 2 . The radius R 3  of the arc that represents the third part P 3  located on the end PE side of the tread surface  20  with respect to the second part P 2  is larger than the radius R 2  of the arc that represents the second part P 2  located on the equator PC side with respect to the third part P 3 . The radius R 4  of the arc that represents the fourth part P 4  located on the end PE side of the tread surface  20  with respect to the third part P 3  is larger than the radius R 3  of the arc that represents the third part P 3  located on the equator PC side with respect to the fourth part P 4 . The radius R 5  of the arc that represents the fifth part P 5  located on the end PE side of the tread surface  20  with respect to the fourth part P 4  is larger than the radius R 4  of the arc that represents the fourth part P 4  located on the equator PC side with respect to the fifth part P 5 . 
     In the tire  2 , between the equator PC and the end PE of the tread surface  20 , the radius of the arc that represents the part located on the end PE side of the tread surface  20  is larger than the radius of the arc that represents the part located on the equator PC side. 
     In the tire  2 , between the equator PC and the end PE of the tread surface  20 , the plurality of arcs included in the contour line are formed such that the radii thereof gradually increase from the inner side toward the outer side in the axial direction. In the tire  2 , non-linear changes in physical actions caused by the boundaries between the parts when the running state shifts from straight running to cornering or from cornering to straight running are effectively suppressed. The tire  2  can smoothly shift from straight running to cornering or from cornering to straight running. The tire  2  has good transient characteristics. The good transient characteristics contribute to improvement of lightness. A large ground-contact surface is formed during cornering, so that the tire  2  can exert a high grip force. From this viewpoint, in the tire  2 , between the equator PC and the end PE of the tread surface  20 , the radius of the arc that represents the part located on the end PE side of the tread surface  20  is preferably larger than the radius of the arc that represents the part located on the equator PC side. Specifically, as for two adjacent arcs, the ratio of the radius of the arc that represents the part located on the end PE side of the tread surface  20  to the radius of the arc that represents the part located on the equator PC side is more preferably not less than 115% and not greater than 155%. 
     In the case where all the parts included in the contour line of the tread surface  20  are represented by arcs as shown in  FIG.  3   , from the viewpoint that the tire  2  can exert a high grip force during cornering without impairing transient characteristics, the ratio (R 2 /R 1 ) of the radius R 2  of the arc that represents the second part P 2 , to the radius R 1  of the arc that represents the first part P 1 , is preferably not less than 115% and not greater than 155%. The ratio (R 3 /R 2 ) of the radius R 3  of the arc that represents the third part P 3 , to the radius R 2  of the arc that represents the second part P 2 , is preferably not less than 115% and not greater than 155%. The ratio (R 4 /R 3 ) of the radius R 4  of the arc that represents the fourth part P 4 , to the radius R 3  of the arc that represents the third part P 3 , is preferably not less than 115% and not greater than 155%. The ratio (R 5 /R 4 ) of the radius R 5  of the arc that represents the fifth part P 5 , to the radius R 4  of the arc that represents the fourth part P 4 , is preferably not less than 115% and not greater than 155%. 
     In the tire  2 , the difference (Hc−Hs) between a hardness Hc of the center portion C and a hardness Hs of the shoulder portion S is preferably not less than −5 and not greater than 5. 
     In the tire  2 , the center portion C and the shoulder portion S have substantially the same hardness. In the tire  2 , non-linear changes in physical actions caused by the boundaries between the parts when the running state shifts from straight running to cornering or from cornering to straight running are effectively suppressed. The tire  2  can smoothly shift from straight running to cornering or from cornering to straight running. The tire  2  has good transient characteristics. The good transient characteristics contribute to improvement of lightness. From this viewpoint, the difference (Hc−Hs) between the hardness Hc of the center portion C and the hardness Hs of the shoulder portion S is more preferably not less than −3 and not greater than 3. In this case, from the viewpoint of obtaining better transient characteristics and lightness, the hardness difference between the adjacent parts is preferably not less than −5 and not greater than 5 and more preferably not less than −3 and not greater than 3. 
     In the present disclosure, the hardness of each part is measured according to the standards of JIS K6253 under a temperature condition of 23° C. by pressing a type A durometer against the center in the width direction of the part. 
     As described above, according to the present invention, the motorcycle tire  2  that allows a motorcycle to be titled at a larger angle during cornering and is capable of exerting a high grip force even when the motorcycle is tilted to the limit, is obtained. 
     EXAMPLES 
     The following will describe the present invention in further detail by means of examples, etc., but the present invention is not limited to these examples. 
     Example 1 
     A motorcycle pneumatic tire (tire size=190/55ZR17) having the basic structure shown in  FIG.  1    and having specifications shown in Table 1 below was obtained. 
     In Example 1, the ratio (d/H) of the camber height d to the tire cross-sectional height H was 64%. The bank angle θb was 57°, and the ratio (θb/θc) of the bank angle θb to the camber angle θc was 154%. The ratio (Bs/TW) of the degree of curvature Bs of the shoulder portion to the width TW of the tread surface was 90%. The ratio (Bc/Bs) of the degree of curvature Bc of the center portion to the degree of curvature Bs of the shoulder portion was 35%. 
     As shown in the cell for Pattern in Table 1, grooves were formed on the tread surface, and the tread pattern having the specifications shown in  FIG.  2    was formed. 
     Comparative Example 1 
     A tire of Comparative Example 1 was obtained in the same manner as Example 1, except that the configuration of the contour line of the tread surface was changed such that the ratio (d/H), the angle θb, the ratio (θb/θc), the ratio (Bs/TW), and the ratio (Bc/Bs) were set as shown in Table 1 below. 
     Example 2 
     A tire of the Example 2 was obtained in the same manner as Example 1, except that no groove was formed on the tread surface. 
     Example 3 
     A tire of Example 3 was obtained in the same manner as Example 1, except that the configuration of the contour line of the tread surface was changed such that the ratio (Bs/TW) and the ratio (Bc/Bs) were set as shown in Table 1 below. 
     Comparative Example 2 
     A tire of Comparative Example 2 was obtained in the same manner as Example 1, except that the configuration of the contour line of the tread surface was changed such that the angle θb, the ratio (θb/θc), the ratio (Bs/TW), and the ratio (Bc/Bs) were set as shown in Table 1 below. 
     Example 4 
     A tire of Example 4 was obtained in the same manner as Example 1, except that the configuration of the contour line of the tread surface was changed such that the ratio (Bs/TW) and the ratio (Bc/Bs) were set as shown in Table 1 below. 
     Example 5 
     A tire of Example 5 was obtained in the same manner as Example 1, except that the configuration of the contour line of the tread surface was changed such that the angle θb, the ratio (Bs/TW), and the ratio (Bc/Bs) were set as shown in Table 1 below. 
     [Performance Evaluation (1)] 
     Each test tire was fitted onto a normal rim and inflated with air to adjust the internal pressure of the tire to a normal internal pressure. The tire was mounted to the rear wheel of a motorcycle having an engine displacement of 999 cc. A commercially available tire (size: 120/70ZR17) was mounted to the front wheel. A normal rim was used as the rim, and the internal pressure was adjusted to a normal internal pressure. The motorcycle was caused to run on a circuit course having an asphalt road surface, and sensory evaluation was made by the rider. The evaluation items are lightness, transient characteristics, and grip force in a full bank state. The results are shown as indexes in Table 1 below. A higher value indicates a better result. 
     [Performance Evaluation (2)] 
     Each test tire was fitted onto a normal rim and inflated with air to adjust the internal pressure of the tire to a normal internal pressure. The tire was mounted to the rear wheel of a motorcycle having an engine displacement of 999 cc. A commercially available tire (size: 120/70ZR17) was mounted to the front wheel. A normal rim was used as the rim, and the internal pressure was adjusted to a normal internal pressure. The motorcycle was caused to run on a circuit course having an asphalt road surface, and lap times were measured. A commercially available tire was used as a reference tire, and the time shortened from the lap time when the reference tire was used was obtained. The results are shown as an index in the cell for “Section time” in Table 1 below. A higher value indicates a better result. 
     [Performance Evaluation (3)] 
     Each test tire was fitted onto a normal rim and inflated with air to adjust the internal pressure of the tire to a normal internal pressure. The tire was mounted to the rear wheel of a motorcycle having an engine displacement of 999 cc. A commercially available tire (size: 120/70ZR17) was mounted to the front wheel. A normal rim was used as the rim, and the internal pressure was adjusted to a normal internal pressure. The motorcycle was caused to run on a circuit course having an asphalt road surface, and the tilt angle (maximum tilt angle) of the vehicle with respect to the road surface in a full bank state was measured using a measurement instrument having a built-in gyro sensor. The results are shown as an index in the cell for “Tilt angle” in Table 1 below. A higher value indicates that the maximum tilt angle is larger, which is better. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Comparative 
                 Example 
                 Example 
                 Example 
                 Example 
                 Example 
                 Comparative 
               
               
                   
                 Example 1 
                 2 
                 1 
                 3 
                 4 
                 5 
                 Example 2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Pattern 
                 FIG. 2 
                 — 
                 FIG. 2 
                 FIG. 2 
                 FIG. 2 
                 FIG. 2 
                 FIG. 2 
               
               
                 d/H [%] 
                 54 
                 64 
                 64 
                 64 
                 64 
                 64 
                 64 
               
               
                 θb [°] 
                 50 
                 57 
                 57 
                 57 
                 57 
                 40 
                 40 
               
               
                 θb/θc [%] 
                 155 
                 154 
                 154 
                 154 
                 154 
                 154 
                 88 
               
               
                 Bs/TW [%] 
                 42 
                 90 
                 90 
                 117 
                 117 
                 117 
                 117 
               
               
                 Bc/Bs [%] 
                 60 
                 35 
                 35 
                 50 
                 60 
                 50 
                 32 
               
               
                 Section time 
                 100 
                 130 
                 120 
                 110 
                 105 
                 105 
                 80 
               
               
                 Tilt angle 
                 100 
                 120 
                 120 
                 110 
                 105 
                 105 
                 100 
               
               
                 Lightness 
                 100 
                 110 
                 110 
                 100 
                 100 
                 100 
                 80 
               
               
                 Transient 
                 100 
                 110 
                 110 
                 100 
                 100 
                 100 
                 80 
               
               
                 characteristics 
               
               
                 Grip 
                 100 
                 130 
                 120 
                 120 
                 115 
                 115 
                 80 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, it is confirmed that, in each Example, the motorcycle can be titled at a larger angle during cornering, and a high grip force can be exerted even when the motorcycle is tilted to the limit. From the evaluation results, advantages of the present invention are clear. 
     INDUSTRIAL APPLICABILITY 
     The above-described technology that allows a motorcycle to be titled at a larger angle during cornering and is capable of exerting a high grip force even when the motorcycle is tilted to the limit can also be applied to various tires. 
     REFERENCE SIGNS LIST 
     
         
         
           
               2  tire 
               4  tread 
               20  tread surface 
               22  groove 
               24  inclined groove 
             C center portion 
             S shoulder portion