Pneumatic radial tires

Abstract: In a pneumatic radial tire for trucks and buses mounted on a 15° center drop rim, an envelope surface (4-13) of the bead core (4) among members constituting the bead portion (1) of the tire has a curved shape concavedly directing toward the outside of the tire at a region opposite to an inner curved surface (21S) of a slantly rising portion in a flange (21F) of an approved rim (21), whereby the durability of the bead portion and the steering stability of the tire may be considerably improved.

Description: This invention relates to pneumatic radial tires for trucks and buses mounted on 15° drop center rims (hereinafter abbreviated as 15° taper radial tire), and more particularly to a tubeless pneumatic radial tire for trucks and buses adaptable to a drop center rim having a 15° tapered bead seat (15° drop center rim). The invention is especially concerned with a 15° taper radial tire for trucks and buses having an advantageously improved durability of the bead portion thereof. As mentioned above, the 15° taper radial tire for trucks and buses is a tubeless (T/L) tire. In order to directly mount the tire of this type on a one-piece mold rim 21 having a section profile as shown in Fig. 7 of the accompanying drawings, there is applied a drop center rim (hereinafter referred to as 15° drop center rim) in which a height of a flange 21F is made considerably lower than that of a width-wide flat base rim having a section profile shown in Fig. 8. Owing to the particular section profile of this flange, the bead portion of the T/L tire for truck and bus considerably differs from the bead portion of a tubed (W/T) tire to be mounted on a width-wide flat base rim. Further, the T/L tire is characterized by having a structure that a taper of about 15° is applied to a bead base of the bead portion so as to match with a taper of 15°±1° formed on the bead seat of the rim 21 for ensuring air tightness and at the same time a given interference is applied to the bead portion. As seen from the above, although both the W/T tire and the T/L tire for truck and bus are used under the same heavy loading condition, a great difference in external force applied to the bead portion during the running under the heavy loading is naturally caused between both the tires. This is because the T/L tire having a lower height of the flange 21F becomes less suitable as regards the fixation of the tire to the rim and the fall quantity of the bead portion toward the outside of the tire becomes larger as compared with that of the W/T tire. As the fall quantity increases, it is inevitable that problems relating to the bead portion are apt to be caused. Furthermore, it is difficult to attain automation in the mounting of the W/T tire onto the rim, while it is possible to easily assemble the T/L tire on the rim by means of an automatic assembling device. As a result, the use of the T/L tire has become more widespread. Therefore, various countermeasures as mentioned below have been taken in order to improve the durability of bead portion in the T/L tire. These countermeasures are generally divided into means for improving the rigidity of the bead portion and means for rationalizing the arrangement and shape of various cord layers extended from the bead portion to the sidewall portion. As the means for improving the rigidity, there are proposed (1) increase of bead portion volume and additional arrangement of bead portion reinforcing cord layer, particularly means for the bead portion volume increase in which since the bead core plays a basic role, a bead core having a low sectional height and a wide sectional width is arranged in correspondence with the low flange height of the aforementioned rim and a hard stiffener rubber extending taperedly outward in the radial direction of the tire is arranged from an outer peripheral surface of the bead core along the main body of the carcass ply to increase the volume of said rubber occupied in the bead portion as far as possible, (2) means for rendering an inner peripheral surface of a bead core having a flat and tetragonal section or a bead core having a flat and hexagonal section as mentioned in detail below into a tapered surface substantially matched with the surface of a 15° tapered bead seat to more strengthen the engagement of the bead portion with the rim, and the like. As the means for rationalizing the arrangement and shape of the cord layer, there are proposed (3) means for adjusting the heights of a turnup end position of a radial carcass ply and an end position of a bead portion reinforcing cord layer, or means for adjusting positions of these ends in the widthwise direction of the tire, (4) means for mitigating stress at an end portion of rubber surrounding the above ends, (5) means for rationalizing a radial carcass line (a line of thickness center of the main body of the carcass ply other than the turnup portion thereof), and the like. All of the above means for are effective improving the bead portion durability. However, the above conventional means are no longer able to cope with present requirements such as more long- lasting tire performances, increase of demand for the formation of a recapped tire after the tread rubber is worn, severer demand for weight reduction and the like. Particularly, the increase of the bead portion volume and the additional arrangement of the bead portion reinforcing cord layer are inappropriate for the weight reduction. Furthermore, the excessive volume increase and additional arrangement bring about a greater amount of heat generation during the running of the tire under heavy loading and hence the inside of the bead portion reaches a high temperature. Such a high temperature brings about rubber deterioration and adhesion degradation, whereby separation failure is apt to be caused from cracking at the turnup end portion of the carcass ply or the end portion of the bead portion reinforcing cord layer. Further, when the tire is used under conditions of such a high temperature over a long period, the rationalization of the arrangement and shape of the cord layer is damaged by a large creep deformation of rubber as mentioned later and hence the given object can not be attained. Also, the shape of the carcass line is lost to bring about degradation of the steering stability. Even when the increase of bead portion volume and the additional arrangement of bead portion reinforcing cord layer are within adequate ranges, the T/L radial tire for truck and bus mounted on the rim having a low flange height is naturally required to ensure a bead portion volune sufficiently durable to heavy loading. When a tire having such a bead portion structure was run over a considerably longer distance such as 100,000 km, it has been confirmed that the bead portion creates a large creep (plastic) deformation as a whole. The state of this deformation is partially and sectionally shown in Fig. 9 of the accompanying drawings. Fig. 9 is a section view of a main part of the bead portion when an assembly of a T/L radial tire having a tire size of 11R22.5 and a rim is inflated under an internal pressure of 7 bar (under no load), in which a portion shown by a solid line is a new tire and a portion shown by a broken line is a tire after the actual running over 100,000 km. As seen from Fig. 9, the bead portion after running over 100,000 km creates a non-restoring creep deformation directing toward the outside of the tire. In the tire after the running (portion shown by the broken line), the hexagonal bead core moves in a direction shown by arrows and hence the carcass ply and its turnup portion are largely pulled out in a direction shown by arrows therealong. Such a large pulling-out phenomenon always causes a large shearing strain represented by a lozenge shape at both sides of the turnup end portion in Fig. 9. Because the above large shearing strain under the inflation of internal pressure simultaneously acts together with shearing strain produced in the turnup end portion during the running under heavy loading, cracking is first created at the turnup end portion and grows into separation failure as the running distance becomes long. Therefore, even if it is intended to rationalize the aforementioned means for improving the durability of the bead portion, sufficient bead portion durability can not be realized. Attention is also drawn to the disclosure of DE-3924619 A, which discloses the features of the prambe of claim 1. It is, therefore, an object of the invention to provide pneumatic radial tires for trucks and buses mounted on 15° drop center rims which can maintain adequate arrangement and shape of a bead portion reinforcing cord layer inclusive of a carcass line while controlling excessive increase of bead portion volume and holding weight reduction without extra additional arrangement of the bead portion reinforcing cord layers and suppressing temperature rise in the inside of the bead portion during running under heavy loading as far as possible, and can properly control creep deformation of the bead portion to considerably improve the bead portion durability at the turnup end portion of the carcass ply or the end portion of the cord layer and separation accompanied therewith and maintain the expected good steering stability. According to a first aspect of the invention, there is provided a pneumatic radial tire, in particular for trucks and buses mountable on a 15° center drop rim, comprising a pair of bead portions, a pair of sidewall portions, a tread portion, at least one carcass ply extending between bead cores embedded in the bead portions so as to reinforce said portions and comprised of cords radially arranged therein and wound around the bead core from the inside of the tire toward the outside thereof to form a turnup portion, each of said bead cores being a wound laminate of steel wire and having an outer envelope line of a polygonal shape in section, a belt superimposed on an outer periphery of the carcass ply to reinforce the tread portion and comprised of two or more cross steel cord layers, and a composite side-rubber extending from a bead base of the bead portion through the outside of the bead portion to a tread rubber of the tread portion and comprising a rubber chafer and a sidewall rubber, wherein a segment of an envelope surface of a cross section of the bead core has a concave shape in a region of the bead core that is located opposite to a convex surface of a slantly rising portion in a flange of a standard rim; characterized in that at least a central region of the concave surface in the envelope surface for steel wire in the section of the bead core is comprised of an arc with a radius of curvature R 2 having a curvature center located on a perpendicular bisector of a line segment connecting both ends of the central region to each other so that the radius of curvature R 2 and a radius of curvature R in a convex surface of a flange in a standard rim satisfy a relationship of 0.7xR≦R 2 ≦1.2xR. The bead portion of the conventional 15° taper radial tire for trucks and buses comprises a bead base having a given interference to a bead seat of the rim for holding air tightness as T/L tire when the tire is mounted onto a 15° drop center rim and inflated under a given internal pressure, and has an outer surface portion having a given interference to the flange of the rim. These interferences serve to strongly fix the bead portion to the rim in addition to the maintenance of the air tightness. In the section of the bead portion in the conventional tire as shown in Fig. 10 of the accompanying drawings together with a section profile of the rim shown by phantom line, the latter interference is attained by forming an outer surface of a composite side-rubber located at a position of contacting and engaging with the flange with an arc convexedly extending toward the outside of the tire and having a relatively large radius of curvature. When the conventional T/L tire is mounted onto an approved rim and inflated under a high internal pressure defined in JATMA or TRA such as 8.00 bar as a cold inflation pressure for tire size of 11R22.5 (110, 120 PSI in TRA), the convex arc portion of the composite side-rubber is deformed along the inner curved surface of the flange (concave curved surface directing toward the outside of the tire) by such an inflated internal pressure and hence a large contact pressure is applied to such a deformed portion. In Fig. 11 of the accompanying drawings is shown a distribution state of contact pressure at positions A-E of the bead portion with the rim in the conventional tire having the above tire size. The position D corresponds to a position indicating a maximum deformation quantity and is under the influence of a contact pressure considerably larger than those in the other positions. Moreover, the contact pressure distribution shown in Fig. 11 is obtained under a cold inflation pressure of 7.0 bar, so that when the tire is actually run under loading, the internal pressure is raised to about 1.2-1.4 times the cold inflation pressure by a greater quantity of heat generation and also the outward flexing of the bead portion under loading occurs, whereby the contact pressure distribution at use state is largely shifted toward the side of high contact pressure as compared with that shown in Fig. 11. When the tire is run under loading, the composite side-rubber of the bead portion in a region contacting with the flange of the rim repeats such a motion that the rubber shrinks in the radial direction of the tire and along the circumferential direction of the tire at a zone of the tread contacting with the ground and returns to its original state at a zone kicking out from the ground contact zone. Since this motion is relative to the flange of the rim, the quantity of heat generation based on hysteresis loss becomes larger at a position wherein the contact pressure becomes higher. A part of heat generated is released to the exterior through the rim, while the remaining greater quantity of heat generated is stored in the rubber portion known as a poor conductor of heat and the stored heat is gradually diffused to finally render the bead portion into a high temperature. When such a high-temperature state is held over a long period of time, creep deformation is caused in rubber. Although the region of the bead portion other than the composite side-rubber is naturally raised to a high temperature through heat generation through hysteresis loss being a viscoelastic property inherent to rubber based on amplitudes of internal strain and internal stress produced by the repetition of loading and its releasing during the running of the tire, the heat generation produced in the composite side-rubber under a high contact pressure to the flange particularly contributes to the occurrence of the creep deformation. Furthermore, the composite side-rubber located under the high contact pressure prematurely causes wearing and finally wears to a shape extending along the flange of the rim, which is socalled rim chafing phenomenon. That is, the bead portion exposed to the high temperature over a long period of time causes creep deformation to a shape shown by a broken line in Fig. 9 together with the above rim chafing. In the creep deformation, the shifting, deformation and shape loss of the bead core damage the function inherent to the bead portion that cord layers such as carcass ply and the like are closely fixed between the bead core and the bead seat of the rim. Therefore, the fixed state of cords in the carcass ply bearing the high internal pressure is slackened and finally the pulling-out of the carcass ply as previously mentioned with reference to Fig. 9 is apt to be caused. As a result, a large shearing strain is produced in the turnup end portion of the carcass ply to create a breaking nucleus in a boundary face between the cord and the rubber surrounding the cords, which progresses in the occurrence of failure from cracks to separation. At this point, it can be said that the shifting, deformation and shape loss of the bead core considerably affect the durability of the bead portion. The steel wire constituting the bead core has a circular or tetragonal shape at section. The former circular shape includes a complete circle and an oval shape near to the circle, while the latter tetragonal shape includes a complete tetragon as well as a lozenge, tetragon provided with four rounded corners, tetragon provided with a small protrusion and the like. Further, the wiring lamination of such a steel wire may be carried out by various shaping methods. As an example of these methods, a single long steel wire is spirally wound side by side to form an innermost peripheral portion and further spirally wound on the innermost peripheral portion side by side riding on each wound steel cord constituting the innermost peripheral portion and then such a procedure is repeated to form a wound laminate of the steel cord. As another example, a plurality of ring-shaped wires may be gathered side by side in horizontal and vertical directions to form a wound laminate. Moreover, an inner peripheral surface of the bead core preferably has a taper of about 15° with respect to a bead base line parallel in the same plane as a rim diameter line mentioned later viewing at a section of the bead core. The tire according to the invention has an arrangement of the bead core such that an envelop surface among plural envelop surfaces constituting the outer envelop line of the polygonal shape at the section of the bead core is located so as to be opposite to the convex surface of the flange in the standard rim. In this tire, the envelop surface for steel wires of the bead core opposite to the convex surface of the flange in the standard rim is rendered into a concave shape, whereby it is made possible to control the deformation and shape losing of the bead core through the creep deformation and also to strongly push the carcass ply between the inner peripheral surface of the bead core and the bead seat of the rim to effectively prevent the pulling-out of the carcass ply. The polygonal shape of the outer envelop line at the section of the bead core means a triangle or more according to geometrical definition but generally indicates a tetragon or hexagon. However, the shape of the bead core inside the tire is not necessarily maintained at an accurate polygonal shape, but is generally a shape somewhat shifting from a shape formed by connecting line segments of n points to each other in turn. When the bead core is made from a steel wire having a circular section, an intersect between extension lines is adopted instead of the point because the point is not existent. The shaping of the bead core from the steel wire is carried out as previously mentioned. Here, the bead core having a hexagonal shape at section is explained with reference to Fig. 2 or 3 of the accompanying drawings. In the manufacture of the bead core 4 having a hexagonal section shown in Fig. 2 or 3, a single long steel wire 4w usually coated with rubber of a very thin thickness, for example 0.01-0.02 mm in thickness, is spirally wound side by side in a shaping mold of a concave form to form an innermost peripheral portion and thereafter the spiral winding side by side riding on each wound steel wire is repeated to form a bead core of a given shape as shown in Fig. 2 or 3. An outer envelop surface of the bead core made from a group of gathered steel wires makes an envelop line of substantially an oblong hexagonal shape viewing in section of the bead core, which is generally called a hexagonal bead core. If such a bead core is subjected to shifting, deformation and shape losing, the hexagonal shape as a hexagonal bead core is no longer held. In Fig. 2 or 3, a straight line BLc is substantially parallel to the rim diameter line and bead base line, and an envelop surface line of the innermost peripheral portion of the bead core has an inclination angle α of about 15° with respect to the straight line BLc parallel with the rim diameter line. In order that the envelop surface of the bead core having the concave surface practically and effectively develops the effect of controlling the creep deformation likewise the aforementioned composite side-rubber, at least a central region of the concave surface in the envelop surface for steel wire in the section of the bead core is comprised of an arc with a radius of curvature R 2 having a curvature center located on a perpendicular bisector of a line segment connecting both ends of the central region to each other so that the radius of curvature R 2 and a radius of curvature R in a convex surface of a flange in a standard rim satisfy a relationship of 0.7xR≦R 2 ≦1.2xR. In order to more enhance the effect of controlling the creep deformation by the bead core, it is advantageous that a shape of the turnup portion of the carcass ply located along the concave surface in the envelop line for steel wire in the section of the bead core is comprised of one or more arcs having a curvature center located outside the tire. Further, it is preferable that a minimum radius of curvature R 3 among radii of curvature in the arcs forming the turnup portion of the carcass ply is within a range of 0.75-1.2 times of the radius of curvature R in the flange of the standard rim. The tire using the bead core having the concave envelop surface according to the invention includes a case of using no bead portion reinforcing cord layer and a case of using a bead portion reinforcing cord layer. In the latter case, at least one reinforcing cord layer is arranged on the outer surface of the carcass ply in the bead portion along at least the turnup portion of the carcass ply, and has an arc-shaped portion along the arcs of the turnup portion so that a minimum radius of curvature R 4 among radii of curvature forming the arc-shaped portion simultaneously satisfies relationships of R 4 ≧R 3 and R 4 ≦1.65xR 3 with respect to the aforementioned minimum radius of curvature R 3 and the radius of curvature R, whereby the occurrence of cracking and separation at the turnup end portion of the carcass ply and the end portion of the bead portion reinforcing cord layer are simultaneously controlled. In order to better control the creep deformation to further enhance the effectiveness of the tire according to the invention, it is desirable that both the composite side-rubber and the bead core are provided with concave surfaces respectively on an outer surface portion of the composite side-rubber and the envelop surface for steel wire of the bead core opposite to the convex surface of the slantly rising portion in the flange of the standard rim. In the tire according to the invention, in order to enhance the resistance to pulling-out of the carcass ply as far as possible, it is preferable that the tire is provided with a hard stiffener rubber taperedly extending outward from an outer periphery of the bead core along the main body of the carcass ply in the radial direction of the tire and a bead core wrapping rubber surrounding the bead core and a modulus at 100% elongation of the wrapping rubber E 1 is within a range of 0.75-1.20 times of a modulus at 100% elongation of the stiffener rubber E 2 . The wrapping rubber has a tendency to gather between the inner peripheral surface of the bead core and the carcass ply surrounding the bead core after the building-up of the tire through vulcanization at a high temperature under a high pressure and plays an important role for enhancing the fastening force of the carcass ply. In order that the composite side-rubber having the concavedly curved surface has large resistances to creep deformation and rim chafing, it is desirable that a modulus at 100% elongation E 3 of at least a rubber portion having the concavedly curved surface in the composite side-rubber is within a range of 0.68-1.15 times of the modulus at 100% elongation of the hard rubber stiffener E 2 . In order that the steel wire of the bead core has a large resistance to creep deformation likewise the composite side-rubber, it is desirable that the steel wire constituting the bead core is provided at its periphery with a very thin coating rubber, and a modulus at 100% elongation of the coating rubber E 4 is not less than 0.45 times of the modulus at 100% elongation of the hard stiffener rubber E 2 . The invention will be further described with reference to the accompanying drawings, wherein: Fig. 1 is a diagrammatic section view illustrating details of a bead portion in an embodiment of the tire according to the invention; Fig. 2 is a partly perspective view sectionally shown of a modified embodiment of the hexagonal bead core shown in Fig. 1; Fig. 3 is a partly perspective view sectionally shown of another modified embodiment of the hexagonal bead core shown in Fig. 1; Fig. 4 is a diagrammatic section view illustrating details of a modified embodiment of the tire bead portion shown in Fig. 1; Fig. 5 is a diagrammatic section view illustrating details of another modified embodiment of the tire bead portion shown in Fig. 1; Fig. 6 is a diagrammatic section view illustrating details of the other modified embodiment of the tire bead portion shown in Fig. 1; Fig. 7 is a diagrammatic section view of a 15° drop center rim; Fig. 8 is a diagrammatic section view of a width-wide flat bottom rim; Fig. 9 is a diagrammatic section view of a bead portion mounted on a rim illustrating creep deformation of the conventional tire; Fig. 10 is a diagrammatic section view of a bead portion in the conventional tire; and Fig. 11 is a graph showing contact pressure distribution of a bead portion in the conventional tire engaged with a flange of a rim. Figs. 1, and 4 - 6 sectionally show main parts of left sides of various embodiments of pneumatic radial tires for trucks and buses mounted on a 15° drop center rim according to the invention (hereinafter abbreviated as T/L radial tire or tire simply), respectively. Figs. 2 and 3 sectionally show various embodiments of the bead core used in the invention, respectively. According to the custom, the T/L radial tire shown in Figs. 1, and 4 - 6 comprises a pair of bead portions 1 (showing only one-side bead portion), a pair of sidewall portions 2 (showing only one-side sidewall portion), a tread portion (not shown), at least one carcass ply 3 (one ply in the illustrated embodiment) toroidally extending between a pair of bead cores 4 embedded in the bead portions 1 to reinforce the bead portion 1, the sidewall portion 2 and the tread portion and containing rubberized cords of radial arrangement, and a belt (not shown) superimposed on an outer periphery of the carcass ply 3 and comprised of two or more cross steel cord layers. Although steel cords are used in the carcass ply 3 of the illustrated embodiment, organic fiber cords may also be used. For example, polyester cords may be applied in the case of plural carcass plies, while aramide fiber cords may be applied in the case of a single carcass ply. The carcass ply 3 is wound around the bead core 4 from the inside toward the outside to form a turnup portion 3t having an upper end 3t E The bead portion 1 of the T/L radial tire is provided with a composite side-rubber 5 over a region ranging from a bead base of the bead portion along the outside of the tire toward a tread rubber (not shown) of the tread portion, and the composite side-rubber 5 consists of a rubber chafer 6 and a sidewall rubber 7. Although the rubber chafer 6 in the illustrated embodiment is located toward the outside of the tire so as to envelop the sidewall rubber 7, the inner portion of the sidewall rubber 7 in the radial direction of the tire as a whole may be located outside the rubber chafer toward the outside of the tire as a modified embodiment. Further, the rubber chafer 6 forms an inner peripheral portion of the bead portion 1 ranging from a heel region (in the vicinity of the position Q) through a bead base region 11 forcedly engaging with a bead seat of a 15° drop center rim to a toe region. The bead base region 11 forms an inner peripheral surface taperedly extending toward the inside of the tire, in which a taper angle is about 15° with respect to a bead base line BL. Further, the T/L radial tire is provided with a hard stiffener rubber 8 taperedly extending outward from the outer peripheral surface of the bead core 4 along a main body of the carcass ply 3 (portion other than the turnup portion 3t) in the radial direction, a soft filler rubber 9 joining with the outer surface of the stiffener rubber 8 and extending outward along the inside of the turnup portion and the main body of the carcass ply in the radial direction, and an innerliner 10 extending between a pair of bead toe regions (showing only one-side bead toe region) inside the main body of the carcass ply 3 and made from an air impermeable rubber. On the other hand, the bead core 4 is covered with a bead core wrapping rubber (not shown). This wrapping rubber mainly lies between the inner peripheral surface of the bead core 4 and the turnup portion of the carcass ply 3 corresponding to the position of this inner peripheral surface after the vulcanization, and such rubber is represented by numeral 12 in Fig. 1. The structure and usual production method of the bead core 4 are described with reference to Figs. 2 and 3 particularly showing a generally hexagonal section of the bead core. In Figs. 2 and 3 numeral 4w is a steel wire having substantially a circular section and a diameter of 0.9-2.2 mm. The bead core 4 is formed by wiring and laminating a single long steel wire obtained by coating the steel wire 4w with a very thin rubber having a thickness of about 0.01-0.20 mm. As an example of the a production method, a single rubberized and unvulcanized steel wire is wound side by side from a left-side or right-side end of a lowest stage in Fig. 2 or 3 by a given number of windings and thereafter the wire is shifted on a stage located above the lowest stage and wound side by side, and such a winding operation is repeated up to a given number of stages. In this case, there is used a ring-shaped two-split type or size-reducing type device having a concave sectional shape which restricts both outer sides of a stage having greatest number of windings, i.e. a fourth stage in the illustrated embodiment, and houses this stage therein, in which the steel wire 4w of an upper stage is wound between mutual steel wires 4w of a lower stage. After the completion of the winding up to the fourth stage in the illustrated embodiment, the winding operation is carried out in the order of fifth, sixth and seventh stages at an opening state of the device. Moreover, the number of upper stages and the number of lower stages bordering the stage of greatest windings are not necessarily the same as the envelop line of the section of the bead core 4 forms substantially a hexagonal shape to provide flat envelop surfaces as the bead core 4. In this case, the inner peripheral surface of the bead core 4 used in the T/L radial tire is arranged at an inclination angle α of about 15° with respect to a straight line BLc passing through an inner edge of the steel wire at the right-side or left-side end (right-side end in the illustrated embodiment) of the innermost stage and parallel to the bead base line. The bead core 4 after the completion of the winding lamination may be fed to the tire building at an uncured state because the steel wires are integrally united together by uncured tackiness of the very thin rubber, or may be subjected to a precuring prior to the tire building. Fig. 1 shows a state just before the complete fitting of the T/L radial tire to a rim 21 having a section profile shown by a solid line. In the tire of Fig. 1, at least an outer surface region of the bead core 4 among the members constituting the bead portion 1, i.e. the composite side-rubber 5, the turnup portion 3t and the bead core 4 has a curved envelop surface (envelop line segment) 4-13 which has a concave shape at a position opposite to a convex surface 21S of a slantly rising portion in a flange 21F of a rim 21. In an outer envelop line for steel wire of the bead core 4, at least a central region of the above curved envelop surface 4-13 is comprised of an arc with a radius of curvature R 2 having a curvature center C 4-13 located on a perpendicular bisector HL of a line segment L 4-13 connecting both ends of the central region to each other. This arc may extend over the whole of the curved envelop surface 4-13, or may be formed by forming each end portion of the curved envelop surface 4-13 with an arc having a radius of curvature different from the radius of curvature R 2 . The radius of curvature R 2 is a value satisfying a relationship of 0.7xR≦R 2 ≦1.2xR with respect to the radius of curvature R forming the convex surface 21S of the flange 21F at the section of the rim 21. Thus, it is effective to prevent the shape losing of the bead core 4. The details of the section of the bead core 4 having the curved envelop surface 4-13 are shown in Figs. 2 and 3. A region of the curved envelop surface 4-13 is formed in the outer envelop line of the bead core 4, so that the outer envelop line in the section of the bead core as a whole becomes a somewhat deformed hexagonal shape. In Fig. 2, the deformation is slight, so that the length of the curved envelop surface 4-13 is not so long, while the degree of the deformation in Fig. 3 becomes large as compared with the case of Fig. 2 and the length of the curved envelop surface 4-13 is sufficiently long. In any case, the use of the bead core 4 shown in Figs. 2 and 3 may be determined in accordance with the tire application, use conditions and the like. In the production method of the bead core 4 having the curved envelop surface 4-13, the shaping device should be a two-split type or size-reducing type device having a concave sectional shape corresponding to an outer envelop line of a polygonal shape, a substantially hexagonal envelop line in the illustrated embodiment. The two-split type device has an unsymmetrical shape that one of side faces in each of the concave shaping split molds is the curved envelop surface 4-13 shown in Figs. 2 and 3 and the other side face thereof is a plain (straight line), while the the size-reducing type device is a monobloc shaping ring (size-reducible) having on both side surfaces a region of the curved envelop surface 4-13 and a region of the plain (straight line) in the shaping. After the shaping, the uncured bead core 4 may be applied to the tire building as it is, or may be subjected to precuring treatment when there is caused a danger of the shape losing in the bead core during the vulcanization of the uncured tire in a mold. Moreover, the innermost peripheral surface of the bead core 4 is naturally tapered at an inclination angle α of about 15° with respect to the straight line BLc parallel to the bead base line. In Figs. 4-6, the carcass ply 3 and the reinforcing steel cord layer 19 are simply represented by a single curved line, respectively. The tire shown in Figs. 4-6 is provided with the bead portion 1 shown in Fig. 1 and a reinforcing steel cord layer 19. In the bead portion 1 of Fig. 4, each of the turnup portion 3t of the carcass ply 3 and the reinforcing steel cord layer 19 is comprised of one or more arc segments (arcs) having a curvature center located outside the tire at a position corresponding to the curved envelop surface 4-13 of the bead core 4. In Fig. 4, a minimum radius of curvature R 3 among the radii of curvature in the arcs forming the turnup portion 3t is within a range of 0.75-1.2 times of the radius of curvature R in the flange 21F of the rim 21, while a minimum radius of curvature R 4 among radii of curvature in the arcs forming the reinforcing steel cord layer 19 satisfies relationships of R 4 ≧R 3 and R 4 ≦1.65R with respect to the minimum radius of curvature R 3 and the radius of curvature R in the flange 21F of the rim 21. The bead portion 1 shown in Fig. 5 has a structure having a concavedly curved surface 13 of the composite side-rubber 5 (rubber chafer 6) and the curved envelop surface 4-13 of the bead core 4 shown in Figs. 1-3. In this case, the radius of curvature R 1 of the curved surface 13, radius of curvature R 2 of the envelop surface 4-13 and radius of curvature R of the flange satisfy the relationships as previously mentioned. The bead portion 1 shown in Fig. 6 has a structure having one or more arc segments (arcs) having a curvature center located outside the tire at a position corresponding to the curved envelop surface 4-13 of the bead core 4 in each of the turnup portion 3t of the carcass ply 3 and the reinforcing steel cord layer 19 shown in Fig. 4, the concavedly curved surface 13 of the composite side-rubber 5 (rubber chafer 6) and a curved envelop surface 4-13 of the bead core 4 shown in Figs. 1-3. In this case, the radii of curvature R 1 to R 4 and radius of curvature R satisfy the relationships as previously mentioned. As seen from the above, it is possible to adopt various combinations based on the descriptions with reference to Figs. 1-6 in the T/L radial tire according to the invention. Thus, the durability of the bead portion 1 can largely be improved to sufficiently achieve the object of the invention. In order to ensure the improvement of the durability in the bead portion 1, the modulus at 100% elongation E 1 in the wrapping rubber for the bead core 4, modulus at 100% elongation E 2 in the hard stiffener rubber, modulus at 100% elongation E 3 in rubber portion of the composite side-rubber 5 (rubber chafer 6) forming the concavedly curved surface 13 and modulus at 100% elongation E 4 in the coating rubber for the steel wire 4w constituting the bead core 4 are desirable to satisfy the following relationships: 0.75xE 2 ≦E 1 ≦1.20xE 2 0.68xE 2 ≦E 3 ≦1.15xE 2 0.45xE 2 ≦E 4 These relationships are defined by measuring shearing strain acting to the turnup end portion 3tE through 10 cases having given values of ratios E 1 /E 2 , E 3 /E 2 and E 4 /E 2 . The values of these ratios and the index of shearing strain on the basis that the conventional tire is 100 are shown in Table 1, in which the smaller the index value, the better the property. Case No. Value of E 1 /E 2 Value of E 3 /E 2 Value of E 4 /E 2 Index of shearing strain 1 0.92 0.83 0.45 34 2 0.83 0.83 0.45 70 3 0.75 0.83 0.45 95 4 1.05 0.83 0.45 38 5 1.25 0.83 0.45 102 6 0.92 0.75 0.45 45 7 0.92 0.68 0.45 88 8 0.75 0.68 0.45 97 9 0.83 0.83 0.25 105 10 0.83 0.83 0.40 101 The following examples are given in illustration of the invention and are not intended as limitations thereof. The T/L radial tire for truck and bus used in these examples has a tire size of 11R22.5 and a structure as shown in Figs. 1-6, which is mounted on a standard rim 21 having a rim size of 22.5x8.25 and a radius of curvature R of 12.7 mm at an arc section of a convex surface 21S of a flange 21F. The carcass ply 3 is a single ply containing radially arranged steel cords, and the belt is comprised of four steel cord cross belt layers. For comparison with these examples, there is provided a conventional example having the same size and construction as in the above example except that the bead portion has actually the outer surface corresponding to a hypothetical arc. As the bead core 4 of the tires of Figs. 1-6, Example 1 has a sectional shape shown in Fig. 2 in which the diameter of the steel wire is 1.55 mm and the number of windings is 65 turns, and Examples 2 - 4 have a sectional shape shown in Fig. 3 in which the diameter of the steel wire is 1.55 mm and the number of windings is 71 turns. There are provided tires of Examples 1-4 having various different values of ratios R 2 /R, R 3 /R, R 4 /R 3 and R 1 /R and the conventional tire. These values are shown in Table 2 together with respective figure number and values of ratios E 1 /E 2 , E 3 /E 2 and E 4 /E 2 . Items Conventional Example Example 1 2 3 4 Fig. 10 Fig. 1 Fig. 4 Fig. 5 Fig 6 R 2 /R - 0.93 0.98 0.91 0.97 R 3 /R - - 1.05 - 1.03 R 4 /R 3 - - 1.57 - 1.58 R 1 /R - - - 0.90 0.90 E 1 /E 2 0.92 1.05 1.05 0.92 0.92 E 3 /E 2 0.75 0.83 0.83 0.75 0.75 E 4 /E 2 0.45 0.45 0.45 0.45 0.45 Index of long run on drum 100 110 115 127 135 Index of crack length 100 88 80 42 33 Index of CP 100 103 102 105 109 In order to demonstrate the effect of the tire according to the invention, each of the example tires and the conventional tire is mounted onto a standard rim and inflated under an internal pressure of 7.0 bar. Thereafter, such a tire is subjected to a long run test on a drum under conditions that the tire is run at a speed of 60 km/h under a loading of 49000N until damage is caused in the bead portion, while the tire is actually run on road over a distance of 100,000 km to measure crack length created at the turnup end portion 3tE. The test results are also shown in Table 2, in which a long run index on drum and an index of crack length are evaluated on the basis that the conventional tire is 100. The larger the index value, the better the result on the long run test, while the smaller the index value, the shorter the crack length. A measure for determining the degree of creep deformation is the steering stability of the tire . In this connection, the tire is actually run on road over a distance of 20,000 km, during which a cornering power is measured with respect to the tire creating any creep deformation. The measured results are also shown in Table 2 in which the cornering power (CP) is represented by an index on the basis that the conventional tire is 100. The larger the index value, the better the cornering power. As seen from the results of Table 2, when the tires of the examples according to the invention are compared with the conventional tire in the test of long run on drum, the service life of the tire until the occurrence of damage in the bead portion or separation failure from the turnup end portion becomes longer and hence the durability of the bead portion is largely improved. Even in the actual running test, the crack length is considerably short in the tires of the examples, from which it is apparent that the durability of the bead portion is improved in practical use. Further, the index value of the cornering power is considerably large as compared with that of the conventional tire, from which it can be shown to prevent the degradation of the steering stability and hence the creep deformation. As mentioned above, according to the invention, there can be provided pneumatic radial tires for trucks and buses mounted on a 15° drop center rim capable of attaining weight reduction while controlling excessive volume increase of the bead portion and not requiring an extra and additional arrangement of the bead portion reinforcing cord layer, and controlling the temperature rise inside the bead portion during running under heavy loading as far as possible to maintain adequate arrangement and shape of cord layers inclusive of a carcass line in the bead portion and properly control the creep deformation of the bead portion to thereby considerably improve the durability of the bead portion in connection with the cracking and separation at the turnup end portion of the carcass ply and the end portion of the reinforcing cord layer and also maintain good steering stability.

Claims: A pneumatic radial tire, in particular for trucks and buses mountable on a 15° center drop rim, comprising a pair of bead portions (1), a pair of sidewall portions (2), a tread portion, at least one carcass ply (3) extending between bead cores (4) embedded in the bead portions (1) so as to reinforce said portions and comprised of cords radially arranged therein and wound around the bead core (4) from the inside of the tire toward the outside thereof to form a turnup portion (3t), each of said bead cores (4) being a wound laminate of steel wire (4w) and having an outer envelope line of a polygonal shape in section, a belt superimposed on an outer periphery of the carcass ply to reinforce the tread portion and comprised of two or more cross steel cord layers, and a composite side-rubber (5) extending from a bead base of the bead portion (1) through the outside of the bead portion to a tread rubber of the tread portion and comprising a rubber chafer (6) and a sidewall rubber (7), wherein a segment of an envelope surface (4-13) of a cross section of the bead core (4) has a concave shape in a region of the bead core that is located opposite to a convex surface (2 IS) of a slantly rising portion in a flange (2 IF) of a standard rim (21); characterized in that at least a central region of the concave surface in the envelope surface (4-13) for steel wire in the section of the bead core is comprised of an arc with a radius of curvature R 2 having a curvature center (C 4-13 ) located on a perpendicular bisector (HL) of a line segment (L 4-13 ) connecting both ends of the central region to each other so that the radius of curvature R 2 and a radius of curvature R in a convex surface (21S) of a flange (21F) in a standard rim (21) satisfy a relationship of 0.7xR≦R 2 ≦1.2xR. A pneumatic radial tire as claimed in claim 1, characterized in that a shape of the turnup portion (3t) of the carcass ply (3) located along the concave surface in the envelop surface (4-13) for steel wire in the section of the bead core (4) is comprised of one or more arcs having a curvature center located outside the tire. A pneumatic radial tire as claimed in claim 2, characterized in that a minimum radius of curvature R 3 among radii of curvature in the arcs forming the turnup portion (3t) of the carcass ply (3) is within a range of 0.75-1.2 times of the radius of curvature R in the flange (21F) of the standard rim (21). A pneumatic radial tire as claimed in claim 3, characterized in that at least one reinforcing cord layer (19) is arranged on the outer surface of the carcass ply (3) in the bead portion (1) along at least the turnup portion (3t) of the carcass ply (3), and has an arc-shaped portion along the arcs of the turnup portion so that a minimum radius of curvature R 4 among radii of curvature forming the arc-shaped portion simultaneously satisfies relationships of R 4 ≧R 3 and R 4 ≦1.65xR 3 with respect to the minimum radius of curvature R 3 and the radius of curvature R. A pneumatic radial tire as claimed in any of claims 1 to 4, characterized in that both the composite side-rubber (5) and the bead core (4) are provided with concave surfaces respectively on an outer surface portion (13) of the composite side-rubber (5) and the envelope surface (4-13) for steel wire (4w) of the bead core (4) opposite to the convex surface (21S) of the slantly rising portion in the flange (21F) of the standard rim (21). A pneumatic radial tire as claimed in any of claims 1 to 5, characterized in that the tire is provided with a hard stiffener rubber (8) taperedly extending outward from an outer periphery of the bead core (4) along the main body of the carcass ply (3) in the radial direction of the tire and a bead core wrapping rubber surrounding the bead core (4), and in that a modulus at 100% elongation (E 1 ) of the wrapping rubber is within a range of 0.75-1.20 times of a modulus at 100% elongation (E 2 ) of the stiffener rubber (8). A pneumatic radial tire as claimed in claim 6, characterized in that the steel wire (4w) constituting the bead cord (4) is provided at its periphery with a very thin coating rubber, and in that a modulus at 100% elongation (E 4 ) of the coating rubber is not less than 0.45 times of the modulus at 100% elongation (E 2 ) of the hard stiffener rubber (8).

Inventors: KENSHIRO KATO, KENSHIRO, KATO
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1996
Main classification: B60C15
IPC codes: B60C15