Patent Publication Number: US-2007095450-A1

Title: Pneumatic tire

Description:
TECHNICAL FIELD  
      The present invention relates to a pneumatic tire, and in particular, to the so-called side-reinforced run flat tire capable of being run for a predetermined distance even if an internal pressure thereof drops to nearly zero, due to puncture and so forth, thereby attaining enhancement in safety when the tire is being run with the internal pressure in a range of 50 to 150 kPa.  
     BACKGROUND TECHNOLOGY  
      The conventional side-reinforced run flat tire described as above with a belt-reinforcing layer (cap/layer) provided on the outer side of a belt layer, in the radial direction of the tire, includes, for example, a pneumatic radial tire disclosed in Patent Document 1. As shown in  FIG. 8 , a pneumatic radial tire  21  comprises a tread  22 , sidewalls  23  extending inward in the radial direction of the tire, from respective ends of the tread  22 , and beads  24  positioned at respective ends of the sidewalls  23 , on the inner side of the tire, in the radial direction thereof.  
      A bead core  25  used for fixing the pneumatic radial tire  21  to a rim, not shown, against an internal pressure of the pneumatic radial tire  21  is provided inside the respective beads  24 . Further, there is provided a carcass ply  26  in a toroidal form, extended up to the respective beads  24  via the tread  22 , and the respective sidewalls  23  so as to be folded back along the peripheries of the respective bead cores  25  inside the beads  24  from the inner side to the outer side, in the direction of the axis of tire rotation to be thereby cocked outward in the radial direction of the tire. A portion of each of the carcass plies  26 , extending from the inner side of the tread  22 , in the radial direction of the tire, to the bead core  25  is a body portion  26   a,  and a portion of each of the carcass plies  26 , folded back toward the external periphery of the bead core  25  is a fold-back part  26   b.  Rubber layers of the pneumatic radial tire  21  are reinforced by the respective carcass plies  26 , thereby forming a shape of the tire in whole.  
      A belt layer  27  comprised of a first belt layer  27   a,  and a second belt layer  27   b  is provided in a region on the inner side of the tread  22 , in the radial direction of the tire, and on the outer side of each of the carcass plies  26 , in the radial direction of the tire, and further, on the outer side of the belt layer  27 , in the radial direction of the tire, there are provided two belt-reinforcing layer  28   a,    28   b.  The belt-reinforcing layer  28   a,    28   b  are made up of an organic fiber cord, respectively. The belt layer  27  is not limited to two layers, but may comprise one layer, or three or more layers.  
      An inner liner  29  is stuck to the surface of each of the sidewalls  23 , on the inner side thereof, in the direction of the axis of tire rotation, and further, a reinforcing rubber layer  30  substantially crescentic in sectional shape, extending from the tread  22  to the respective beads  24  via the respective sidewalls  23  is provided in regions, on the inner sides of the respective body portions  26   a,  in the direction of the axis of tire rotation, and on the outer side of the respective inner liners  29 , in the direction of the axis of tire rotation, that is, in a portion of each of the carcass plies, sandwiched between the body portion  26   a  and the inner liner  29 .  
      For the organic fiber cord, use is made of a cord smaller in elongation than a Nylon (registered trademark) cord that is an organic fiber cord for use in a common side-reinforced run flat tire, and so forth, for example, a cord for at least part of which polyethylene-2, and 6-naphthalate fiber is used, thereby enhancing rigidity of the belt-reinforcing layers, leading to higher rigidity of a belt structure in whole, including the belt layer, and the belt-reinforcing layers, than rigidity of the common side-reinforced run flat tire. As a result of enhancement in the rigidity of the belt structure in whole, vibration of the belt structure is effectively deterred, resulting in reduction of noise in a vehicle compartment.  
      Further, as another example of the side-reinforced run flat tire provided with belt-reinforcing layers on a side of a belt layer, adjacent to the outer periphery of the tire (on the outer side of the belt layer, in the radial direction of the tire), there is disclosed a radial ply pneumatic tire in Patent Document 2. With the radial ply pneumatic tire, an overlay layer made of aramid-reinforced fabric, is provided on the outer side of a belt ply, in the radial direction of the tire. With the radial ply pneumatic tire, since aramid is effectively higher in modulus of elasticity than Nylon, it is possible to implement reinforcement of the tire, stronger than that by two Nylon layers.  
      With the side-reinforced run flat tire provided with the belt-reinforcing layers on the outer side of the belt layer, in the radial direction of the tire, either a high-modulus cord or a low-modulus cord is used according to various target performances, and so forth, at the time of running at a normal internal pressure and run-flat tire running.  
      Meanwhile, as a normal tire provided with a belt-reinforcing layer, there is included, for example, “a tire having improved running noise performance” as disclosed in Patent Document 3. This tire is provided with a carcass ply, and two belt layers disposed on the outer side of the carcass ply, in the radial direction of the tire, and a ply made up of a bi-modulus reinforcement is disposed between the carcass ply, and the one belt layer of the two belt layers, disposed on a side thereof, adjacent to the carcass ply, between the two belt layers, or on a side of the carcass ply, adjacent to the inner periphery of the tire (on the inner side of the carcass ply, in the radial direction of the tire). Herein, the bi-modulus reinforcement is formed by combining together at least one fiber made of material high in modulus of elasticity (aramid is preferable, however, use may be made of other fabric high in modulus of elasticity, made of a liquid crystal polymer such as, for example, PBO (paraphenylenebenzobisoxazole)} and material low in modulus of elasticity {Nylon is preferable, however, use may be made of PET (polyethylene terephthalate)} to be then helically twisted.  
      The tire provided with the bi-modulus reinforcement exhibits a behavior of a reinforcement low in modulus of elasticity at the time of low-speed running while reacting as a reinforcement high in modulus of elasticity at the time of high-speed running. Owing to such characteristic of the bi-modulus reinforcement as described, reduction in running noise, and enhancement in drift rigidity performance and speed resistance performance can be attained.  
      Further, as another example of the normal tire provided with the belt-reinforcing layer, there is disclosed a pneumatic tire in Patent Document 4. This pneumatic tire is provided with a belt layer disposed on the outer side of a carcass ply, in the radial direction of the tire, and a band layer disposed on the outer side of the belt layer. The band layer is formed by helically winding at least one strand of a composite cord made up by twisting together a low-modulus filament composed of Nylon fiber, and so forth, and a high-modulus filament composed of aromatic polyamide fiber and so forth, so as to be continuous in the circumferential direction of the tire.  
      With this pneumatic tire, since the composite cord has a low-modulus region, vulcanizing forming is easily executed while using the high-modulus filament, so that dilation (lifting) on the outer circumference of the tire as completed, at the time of rotation at a high speed, is decreased owing to the high-modulus filament, thereby preventing exfoliation between rubber and the carcass ply, at the ends of the belt layer, and alleviating vibration. More specifically, it becomes possible to prevent the lifting at the time of rotation at a high speed that is a drawback of the tire provided with the band layer using the low-modulus filament composed of Nylon fiber, and so forth, and also to prevent aggravation in deformation of the tire, and increase in vibration, at the time of rotation at a high speed, that is, a drawback of the tire provided with the band layer using the high-modulus filament composed of polyester, and so forth.  
      Thus, with the normal tire provided with the belt-reinforcing layer disposed on the outer side of the belt layer in the radial direction of the tire, the composite cord formed by twisting the high-modulus cord, and the low- modulus cord together is used as the belt-reinforcing layer in order to improve high-speed running performance, and so forth. 
      Patent Document 1: JP 2000-43518 A     Patent Document 2: JP 6-191244 A     Patent Document 3: JP 2001-63310 A    

      Patent Document 4: JP 2757940 B  
     DISCLOSURE OF THE INVENTION  
     PROBLEM TO BE SOLVED BY THE INVENTION  
      If an internal pressure of a tire drops to 50 to 150 kPa due to leakage of air, and so forth, with a vehicle in a state as-fitted with normal tires, a driver will become aware of a drop in the internal pressure owing to a change in driving comfort, and driving stability (controllability·stability), so that countermeasures, such as repair/replacement of the tire, filling the tire up with air, and so forth can be taken by the driver.  
      With the side-reinforced run flat tire, however, a longitudinal spring constant is high owing to presence of a side reinforcing rubber layer, so that a driver has difficulties in taking notice of the internal pressure dropping to 50 to 150 kPa. Accordingly, there is a high possibility that the driver keeps driving until occurrence of breakage of the tire. Hence, it is very important to secure safety when the vehicle is running with the side-reinforced run flat tire at its internal pressure in the range of 50 to 150 kPa.  
      However, with the conventional side-reinforced run flat tire provided with the belt-reinforcing layer, as disclosed in Patent Documents 1, and 2, respectively, if the belt-reinforcing layer is made up of a low-modulus cord such as a Nylon cord, and so forth, a tread is prone to deformation because the rigidity of the tread, in the circumferential direction, is low, and at the time of running with the internal pressure in the range of 50 to 150 kPa, a ground contact area of the tire undergoes expansion in the circumferential direction. As a result, the tread undergoes large deformation to thereby cause strain of the tread to increase, resulting in premature failure of the tire. Further, if the belt-reinforcing layer is made up of a high-modulus cord composed of aramid fabric, and so forth, deformation of the tread is deterred because the rigidity of the tread, in the circumferential direction, is high, and at the time of running with the internal pressure in the range of 50 to 150 kPa, large strain is caused to occur to sidewalls to support a load of the vehicle. Accordingly, premature failure of the tire results. That is, whether the belt-reinforcing layer is made up of either the low-modulus cord or the high-modulus cord, the premature failure of the tire is unavoidable. Hence, it is desirable to enhance safety of the tire at the time of running with its internal pressure in the range of 50 to 150 kPa.  
      It is therefore an object of the invention to strike a balance between a load on the tread, and a load on the sidewalls to thereby enhance durability of the side-reinforced run flat tire at the time of running with its internal pressure in the range of 50 to 150 kPa, and to enhance safety.  
      Means for Solving Problems  
      A pneumatic tire in claim  1  comprises a tread, sidewalls extending inward from respective ends of the tread, in the radial direction of the tire, beads positioned at respective ends of the sidewalls, on the inner side of the tire, in the radial direction thereof, the beads each having a bead core-provided therein, a carcass ply in a toroidal form, extended up to the respective beads via the tread, and the respective sidewalls, to be folded back along the periphery of the bead core in the respective beads from the inner side to the outer side of the bead core, in the direction of the axis of tire rotation to be thereby cocked outward in the radial direction of the tire, reinforcing rubber layers substantially crescentic in sectional shape, disposed on the inner sides of the respective sidewalls, in the direction of the axis of the tire rotation, a belt layer disposed on the inner side of the tread, in the radial direction of the tire, and on the outer side of the carcass ply, in the radial direction of the tire, and a belt-reinforcing layer disposed on the outer side of the belt layer, in the radial direction of the tire, characterized in that the belt-reinforcing layer is made up of a composite cord formed by twisting high-modulus fiber and low-modulus fiber together, a stress-strain curve thereof having a low-modulus region, and a high-modulus region.  
      With the pneumatic tire in claim  2 , in the pneumatic tire in claim  1 , it is characterized in that the belt-reinforcing layer is disposed so as to cover the belt layer in whole.  
      With the pneumatic tire in claim  3 , in the pneumatic tire in claim  2 , it is characterized in that the belt-reinforcing layer may comprise a layer disposed so as to cover the belt layer in whole, and layers disposed on the outer sides of respective ends of the layer, in the radial direction of the tire.  
      With the pneumatic tire in claim  4 , in the pneumatic tire in any of claims  1  to  3 , it is characterized in that the composite cord has characteristic wherein a stress-strain curve thereof has an inflection point between the low-modulus region, and the high-modulus region, falling in a range of 2 to 7% in strain.  
      With the pneumatic tire in claim  5 , in the pneumatic tire in any of claims  1  to  4 , it is characterized in that the belt-reinforcing layer is disposed at a position where the respective ends thereof overlap respective ends of the reinforcing rubber layers, on respective sides thereof, adjacent to the tread, in the direction of the axis of the tire rotation, and a ratio of a length of an overlapping portion between the belt-reinforcing layer, and the reinforcing rubber layer to a length from an equatorial plane to the end of the belt-reinforcing layer is set to 10 to 30%,  
      According to the pneumatic tire in claims  1  to  3 , in a region of the composite cord, smaller in strain, deformation of the tread is more tolerated than in the case of using the low-modulus cord, and deflection of the sidewall is more deterred than in the case of using the high-modulus cord. Further, when the strain of the composite cord reaches the high-modulus region, the composite cord exhibits modulus of elasticity higher than that in the case of using the low-modulus cord, so that the tread does not undergo such excessive deformation as will occur in the case of using the low-modulus cord.  
      According to the pneumatic tire in claim  4 , it is possible to prevent an increase in the deflection of the sidewall due to a decrease in tolerance in the deformation of the tread, and an increase in the deformation of the tread before the strain reaches the high-modulus region.  
      According to the pneumatic tire in claim  5 , it is possible to prevent deterioration in high-speed running performance, and durability, due to an increase in stored heat quantity at the time of running, and occurrence of a large difference in rigidity level at a boundary part between the end of the reinforcing rubber layer, on the side thereof, adjacent to the tread, and the end of the belt-reinforcing layer, thereby preventing premature failure of the tire from starting at the boundary part.  
     EFFECT OF THE INVENTION  
      With the pneumatic tire according to the present invention, in the region of the composite cord, smaller in strain, the deformation of the tread is more tolerated than in the case of using the low-modulus cord, and the deflection of the sidewall is more deterred than in the case of using the high-modulus cord. Further, when the strain of the composite cord reaches the high-modulus region, the composite cord exhibits modulus of elasticity higher than that in the case of using the low-modulus cord, so that the tread does not undergo the excessive deformation as in the case of using the low-modulus cord. As a result, there will occur enhancement in center ground-contact length as well a-s durability of the tire in the case of an internal pressure thereof being kept in a range of 50 to 100 kPa, resulting in enhanced safety at the time of running with the internal pressure kept in the range of 50 to 100 kPa. That is, by striking a balance between a load on the tread, and a load on the sidewalls, the durability of the tire at the time of running with the internal pressure kept in the range of 50 to 100 kPa is enhanced to thereby enable the safety to be enhanced 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a sectional view along the meridian line, showing the left half of one embodiment of a pneumatic tire according to the present invention, in the axial direction thereof.  
       FIG. 2  is a schematic representation showing positional relationship between a belt-reinforcing layer, and reinforcing rubber layers, according to the embodiment of the invention, in the direction of the axis of tire rotation.  
       FIG. 3  is a view showing another disposition example of the belt-reinforcing layer according to the embodiment of the present invention.  
       FIG. 4  is a view showing still another disposition example of the belt-reinforcing layer according to the embodiment of the present invention.  
       FIG. 5  is a schematic representation showing makeup of a composite cord according to the embodiment of the invention.  
       FIG. 6  is a view showing stress-strain curves of a high-modulus cord, low-modulus cord, and the composite cord, respectively.  
       FIG. 7  is a schematic representation showing respective side view images of side-reinforced run flat tires using the low-modulus cord, the high-modulus cord, and the composite cord according to the present invention, respectively, as the belt-reinforcing layer, at the time of running with an internal pressure of the tire kept in a range of 50 to 100 kPa.  
       FIG. 8  is a sectional view along the meridian line, showing the left half of a conventional pneumatic tire, in the axial direction thereof. 
    
    
     EXPLANATION OF LETTERS OR NUMERALS  
     
         
           1  pneumatic tire,  2  tread,  3  sidewall,  4  bead,  5  bead core,  6  carcass ply,  7  belt layer,  8  belt-reinforcing layer,  10  reinforcing rubber layer  
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      Embodiments of the invention are described hereinafter with reference to the accompanying drawings.  
       FIG. 1  is a sectional view along the meridian line, showing the left half of one embodiment of a pneumatic tire according to the present invention, in the axial direction thereof. As shown in  FIG. 1 , a pneumatic tire  1  according to the present embodiment, in whole, is formed of rubber layers, and comprises a tread  2  that is part thereof, in contact with a road surface, sidewalls  3  extending inward from respective ends of the tread  2 , in the radial direction of the pneumatic tire  1 , and beads  4  positioned at respective ends of the sidewalls  3 , on the inner side of the tire, in the radial direction thereof.  
      A bead core  5  used for fixing the pneumatic tire  1  to a rim, not shown, against an internal pressure is provided inside the respective beads  4 . Further, there is provided a carcass ply  6  in a toroidal form, extended up to the respective beads  4  via the tread  2 , and the respective sidewalls  3  to be folded back along the peripheries of the respective bead cores  5  in the beads  4  from the inner side to the outer side, in the direction of the axis of tire rotation so as to be thereby cocked outward in the radial direction of the tire. A portion of the carcass ply  6 , extending from the inner side of the tread  2 , in the radial direction of the tire, to the bead core  5  is a body portion  6   a,  and a portion of the carcass ply  6 , folded back toward the external periphery of the bead core  5  is a fold-back part  6   b.  The rubber layers of the pneumatic tire  1  are reinforced by the carcass ply  6 , and a shape of the tire in its entirety is thereby formed.  
      A belt layer  7  comprised of a first belt layer  7   a,  and a second belt layer  7   b  is provided in a region on the inner side of the tread  2 , in the radial direction of the tire, and on the outer side of the carcass ply  6 , in the radial direction of the tire, and further, in the radial direction of the tire, there is provided a belt-reinforcing layer  8   a.  The belt-reinforcing layer  8   a  is called a cap. The belt layer  7  is not limited to two layers, but may comprise one layer, or three or more layers.  
      An inner liner  9  is stuck to the surface of the sidewall  3 , on the inner side thereof, in the direction of the axis of tire rotation, and further, a reinforcing rubber layer  10  substantially crescentic in sectional shape, extending from the tread  2  to the bead  4  via the sidewall  3  is provided in a region, on the inner side of the body portion  6   a,  in the direction of the axis of the tire rotation, and on the outer side of the inner liner  9 , in the direction of the axis of the tire rotation, that is, in a portion of the carcass ply sandwiched between the body portion  6   a  and the inner liner  9 .  
      The belt-reinforcing layer  8   a  is disposed so as to cover the whole width of the belt layer  7 , in the direction of the axis of the tire rotation, respective ends of the belt-reinforcing layer  8   a  being extended beyond respective ends of the belt layer  7 . Further, as to positional relationship between the belt-reinforcing layer  8   a,  and the reinforcing rubber layers  10 , in the direction of the axis of the tire rotation, the belt-reinforcing layer  8   a  is disposed such that the respective ends thereof overlap respective ends of the reinforcing rubber layers  10 , on respective sides thereof, adjacent to the tread  2 . More specifically, assuming that a length (belt reinforcement width) from an equatorial plane C to one end of the belt-reinforcing layer  8   a  is designated as A, and a length (overlapping length) of an overlap (overlapped portion) between the belt-reinforcing layer  8   a,  and the reinforcing rubber layer  10  is designated as B, a ratio of B to A (B/A) is preferably set to 10 to 30%, as shown in  FIG. 2 . That is because if B/A exceeds 30%, a thickness at the end of the belt-reinforcing layer  8   a  becomes excessive, so that there occurs an increase in stored heat quantity at the time of running, thereby causing deterioration in high-speed running performance, and durability while if B/A becomes less than 10%, this causes occurrence of a large difference in rigidity level at a boundary part between the end of the reinforcing rubber layer  10 , on the side thereof, adjacent to the tread  2 , and the end of the belt-reinforcing layer  8   a,  thereby causing premature failure of the tire to start from the boundary part.  
      Further, in addition to the belt-reinforcing layer  8   a  in one layer, disposed so as to cover the whole width of the belt layer  7 , in the direction of the axis of the tire rotation, the respective ends thereof being extended beyond respective ends of the belt layer  7 , as shown in  FIG. 1 , a belt-reinforcing layer  8   b  in one layer may be provided at positions on the sides of the respective ends (respective shoulder sides) of the belt-reinforcing layers  8   a,  and on the outer side thereof, in the radial direction of the tire, so as to enable the belt-reinforcing layers  8   b  to overlap the second belt layers  7   b,  respectively, as shown in  FIG. 3 . The belt-reinforcing layer  8   b  is called a layer. Further, as shown in  FIG. 4 , a belt-reinforcing layer  8   c  in one layer may be provided at positions on the end sides of the respective belt-reinforcing layers  8   a,  on the outer sides thereof, in the radial direction of the tire, not to overlap the respective ends of the second belt layer  7   b.  The belt-reinforcing layer  8   c  is called a belt-reinforcing layer of an end-winding structure.  
      As shown in  FIG. 5 , the belt-reinforcing layers  8   a,    8   b,    8   c  are each formed with the use of a composite cord  13  made up by twisting together a high-elasticity cord  11  composed of a high-modulus fiber and a low-modulus cord  12  composed of a low-modulus fiber. Further, the belt-reinforcing layers  8   a,    8   b,    8   c  are each formed by winding a single strand of the composite cord  13 , or a plurality of strands of the composite cords  13  in a bundle, on the outer side of the belt layer  7 , in the radial direction of the tire, in such a way as to be in the form of a single helix or a plurality of helices.  
      The composite cord  13  is preferably formed by use of two strands of cords of 1670 dtex, composed of, for example, Kevler (registered trademark) fiber for the high-modulus cord, and one strand of a cord of 940 dtex, composed of, for example, Nylon fiber for the low-modulus cord to be twisted together to serve as a dip cord such that an inflection point of a stress-strain curve is in a region where magnitude of strain falls in a region of 2 to 7%.  
       FIG. 6  is a view showing stress-strain curves of the high-modulus cord composed of Kevler fiber, the low-modulus cord composed of Nylon fiber, and the composite cord, respectively. In the figure, the vertical axis indicates stress, and the horizontal axis indicates strain. Curves a, b, c denote the respective stress-strain curves of the high-modulus cord, the low-modulus cord, and the composite cord.  
      Now, the stress-strain curve of the composite cord  13  is described hereinafter with reference to  FIG. 6 . When stress (tensile stress) is applied to the composite cord  13 , the composite cord  13  undergoes elongation while being untwisted. Further, the elongation of the composite cord  13  causes a twist pitch thereof to become longer. The stress-strain curve has a low-modulus region relatively large in elongation, and a high-modulus region relatively small in elongation. An inflection point cl of the stress-strain curve c of the composite cord  13  is an intersection point where a perpendicular passing through an intersection point X of a tangent line to the curve c with strain at 0 and a tangent line to the curve c at a rupture point (point where strain is at the maximum) crosses the curve c.  
      With the pneumatic tire made up as above, the composite cord  13  has the low-modulus region, so that deformation of the tread  2 , in a region of the composite cord, smaller in strain, is more tolerated than in the case of using the high-modulus cord (the curve a in  FIG. 6 ), and deflection of the sidewall  3  is more deterred than in the case of using the high-modulus cord. Further, when the strain of the composite cord  13  reaches the high-modulus region, the composite cord  13  exhibits modulus of elasticity higher than that for the low-modulus cord (the curve b in  FIG. 6 ), so that the tread  2  does not undergo such excessive deformation as will occur in the case of using the nylon cord.  
      As shown in  FIG. 6 , the inflection point cl of the stress-strain curve c of the composite cord  13  is preferably set to fall in a range of 2 to 7% in strain. If the inflection point c 1  falls in a range less than 2% in strain, tolerance in the deformation of the tread  2  becomes smaller, so that the deflection of the sidewall  3  becomes greater. Conversely, if the inflection point c 1  falls in a range exceeding 7% in strain, the deformation of the tread  2  becomes too great before the strain reaches the high-modulus region. Hence, the range from 2 to 7% is preferable, and a range from 2 to 4% is more preferable.  
       FIG. 7  is a schematic representation showing side view appearances of the side-reinforced run flat tires using the low-modulus cord, the high-modulus cord, and the composite cord according to the present invention, as the belt-reinforcing layer, respectively, at the time of running with the internal pressure kept in the range of 50 to 100 kPa.  
       FIG. 7A  is a view showing the case of using the low-modulus cord. In this case, since the tread is prone to deformation because the rigidity of the tread, in the circumferential direction, is low, as previously described, and the ground contact area of the tire undergoes expansion in the circumferential direction. As a result, the tread undergoes large deformation to thereby cause strain of the tread to increase.  
       FIG. 7B  is a view showing the case of using the high-modulus cord. In this case, the deformation of the tread is deterred because the rigidity of the tread, in the circumferential direction, is high, as previously described, to thereby cause large strain to occur to the sidewall.  
       FIG. 7C  is a view showing the case of using the composite cord according to the present invention. In this case, in the region of the composite cord, smaller in strain, the deformation of the tread is more tolerated than in the case of using the high-modulus cord, and the deflection of the sidewall is more deterred than in the case of using the high-modulus cord, as previously described. Further, when the strain reaches the high-modulus region, the tread does not undergo such excessive deformation as will occur in the case of using the low-modulus cord. That is, both the deformation of the tread, and the deflection of the sidewall are moderate in magnitude, thereby keeping the tire in such a shape as well-balanced.  
     EXAMPLES  
      Examples 1 to 6, and comparative examples 1, 2 were prepared, and measurements were taken on respective center ground-contact lengths as well as durabilities thereof in the case of an internal pressure being kept at 75 kPa as an example of the case of the internal pressure being kept in the range of 50 to 100 kPa. Results of the measurements are shown in Tables 1 and 2. Herein, the center ground-contact lengths were obtained by measuring the ground-contact length of a central part of each of tires on conditions of a tire size: 215/45R17, rim: 7JX17, internal pressure: 75 kPa, and load: 4.0 kN, and by indexing the respective ground-contact lengths on the basis of the center ground-contact length of comparative example 1, designated as 100 (index). Further, the durabilities were obtained by fitting all 4 wheels with each of tires prepared on conditions of the tire size: 215/45R17, rim: 7JX17, internal pressure: 75 kPa, and a load corresponding to a vehicle and 2 persons therein, and by measuring a distance (endurance distance) until the tire blows out or fails to thereby index the respective durabilities on the basis of the durability of comparative example 1, designated as 100 (index). In the rows for a belt-reinforcing structure, shown in Tables 1 and 2, respectively, “1 Cap” refers to a structure provided with the cap in one layer (corresponding to  FIG. 1 ), “C/L” refers to a structure provided with a cap in one layer, and a layer in one layer (corresponding to  FIG. 3 ), and “1 Cap+end-winding” refers to a structure provided with a cap in one layer, and an end-winding structure in one layer (corresponding to  FIG. 4 ).  
      Herein, a load refers to a maximum load (maximum load capacity) of a single wheel in applicable size as set out in the following specification, an internal pressure refers to an air pressure corresponding to a maximum load (maximum load capacity) in applicable size as set out in the following specification, and a rim refers to the standard rim (“approved rim” or “recommended rim”) in applicable size as set out in the following specification. Further, the specification is determined on the basis of respective industrial specifications effective in regions where tires are in production, or in use. For example, in the U. S. A., the specification is based on “Year Book of The Tire and Rim Association Inc.”, in Europe, on “Standards Manual of The European Tire and Rim Technical Organization”, and in Japan, on “JATMA Year Book” issued by The Japan Automobile Tire Manufacturers Association, Inc.  
                                   TABLE 1                                   Example   Example   Example   Example           1   2   3   4                                                        belt-reinforcing   (Kevler               (Kevler       material   1670 dtex ×           1670 dtex +           2 +           Nylon           Nylon 940           940 dtex)           dtex)/3           /2       inflection point   2.5%   2.5%   2.5%   5.0%       of low/high       moduli       belt-reinforcing   1 Cap   C/L   1 Cap +   C/L       structure           end-winding       overlap of    15%    20%    20%    25%       belt-reinforcing       layer and       side-reinforcing       rubber layer       center ground-    75    73    73    82       contact length       durability   200   220   215   175                  
 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
               
               
                   
                 Compara. 
                 Compara. 
                 Example 
                 Example 
               
               
                   
                 Example 1 
                 Example 2 
                 5 
                 6 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 belt-reinforcing 
                 Nylon 1400 
                 Kevler 1670 
                 (Kevler 
                 (Kevler 
               
               
                 material 
                 dtex)/2 
                 dtex/2 
                 1100 dtex × 
                 1670 dtex × 
               
               
                   
                   
                   
                 2 + Nylon 
                 2 + Nylon 
               
               
                   
                   
                   
                 940 dtex)/3 
                 940 dtex)/3 
               
               
                 inflection 
                 only in a 
                 only in a 
                 3.5% 
                 2.5% 
               
               
                 point of 
                 low-modulus 
                 hi-modulus 
               
               
                 low/high 
                 region 
                 region 
               
               
                 moduli 
               
               
                 belt-reinforcing 
                 C/L 
                 C/L 
                 1 Cap 
                 1 Cap 
               
               
                 structure 
               
               
                 overlap of 
                  15% 
                 15%  
                  20% 
                     5% 
               
               
                 belt-reinforcing 
               
               
                 layer and 
               
               
                 side-reinforcing 
               
               
                 rubber layer 
               
               
                 center ground- 
                 100 
                 56 
                  80 
                  75 
               
               
                 contact length 
               
               
                 durability 
                 100 
                 80 
                 185 
                 130 
               
               
                   
               
            
           
         
       
     
      Based on Tables 1 and 2, it is evident that with any of the examples 1 to 6, the center ground-contact length is longer than that for the comparative example  2  using the Kevler cord only, but shorter than that for the comparative example 1 using the Nylon cord only. Further, with any of the examples 1 to 6, results show that durability significantly exceeds that for the comparative example 1 and 2, respectively.  
     INDUSTRIAL APPLICABILITY  
      The present invention is effective for application to a side-reinforced run flat tire high in safety at the time of running with the internal pressure kept in the range of 50 to 100 kPa.