Pneumatic radial tires

Abstract: In a pneumatic radial tire for trucks and buses mounted on a 15° center drop rim, an outer surface portion (13) of a composite side-rubber (5) and/or 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 mountable 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. 16 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. 17. 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 are effective for 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 volume 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. 18 of the accompanying drawings, which corresponds to the preamble of claim 1. Fig. 18 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 kgf/cm 2 (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 actual running over 100,000 km. As seen from Fig. 18, 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. 18. 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. It is, therefore, an object of the invention to provide pneumatic radial tires for trucks and buses mountable 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 the invention, there is provided a pneumatic radial tire, in particular for trucks and buses mountable on an approved 15° drop center 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 one in each bead portion so as to reinforce said bead portions, the at least one carcass ply being reinforced with cords radially arranged therein and wound around the bead cores from the inside of the tire toward the outside thereof to form a turnup portion in each bead portion and a main body therebetween, each of said bead cores being a wound laminate of steel wire having a circular or tetragonal shape in cross-section, a belt superimposed on an outer periphery of the at least one carcass ply to reinforce the tread portion and comprised of two or more cross steel cord layers, and a composite side-rubber extending from the bead base of each bead portion through the outside of the bead portion to the tread rubber of the tread portion and comprising a rubber chafer and a sidewall rubber, wherein an outer surface of at least the composite side-rubber among members constituting each bead portion of the tire has a shape directed toward the outside of the tire at a region opposite to an inner curved surface of a slantly rising portion in each flange of the approved rim, characterized in that the shape of said outer surface is concavedly curved; in that the concavedly curved surface of the composite side-rubber is located between a bead base line and a first straight line passing through the radially outer surface of the steel wire located at the outermost side of the bead core in the radial direction of the tire and being parallel to the bead base line, and the curve of the concavedly curved surface of the composite side-rubber in section of the tire is comprised of plural arc segments smoothly connecting to each other and having a peak in one of said segments, and said peak is located between the first straight line and a second straight line passing through the innermost position of the turnup portion of the carcass ply around the bead core in the radial direction of the tire and parallel with the bead base line; and in that the concavedly curved surface of the composite side-rubber is formed so that the radius of curvature R 1 of the arc segment having the peak among the arc segments constituting the concavedly curved surface and the radius of curvature R of the major inner curved surface in the flange at a section of the approved rim satisfy the relationship of 0.4xR≦R 1 ≦1.6xR. 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. 19 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 kgf/cm 2 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. 20 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. 20 is obtained under a cold inflation pressure of 7.0 kgf/cm 2 , 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 load-ing 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. 20. 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 so-called 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. 18 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. 18 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 a section of the bead core. On the contrary, when the outer surface portion of at least the composite side-rubber among members constituting the bead portion of the tire is rendered into a curved shape concavedly directing toward the outside of the tire at a region opposite to an inner curved surface of a slantly rising portion in the flange of the approved rim, the composite side-rubber contacting with the flange of the rim may obtain an even contact pressure distribution to the inner curved surface of the flange under not only a high internal pressure but also a heavy loading, which controls the aforementioned creep deformation to a minimum and particularly develops the effect of effectively suppressing the shifting, deformation and shape loss of the bead core. As a result, it is possible to largely reduce the shearing strain acting to the turnup end portion of the carcass ply and the durability of the bead portion can considerably be improved. In order to further enhance the above effect, the concavedly curved surface of the composite side-rubber is located between a bead base line and a first straight line passing through a radially outer surface of a steel wire located at an outermost side of the bead core in the radial direction of the tire and being parallel to the bead base line, and a curve of the concavedly curved surface of the composite side-rubber in a section of the tire is comprised of plural arc segments smoothly connecting to each other and having a peak in one of these segments, and the peak is located between the first straight line and a second straight line passing through an innermost position of the turnup portion of the carcass ply around the bead core in the radial direction of the tire and parallel with the bead base line. The term "bead base line" used herein means a straight line passing through an intersect between an extension line of the bead base and a straight line concaved with a line of an outer surface contour of a bead heel portion and perpendicular to a rotating axial center of the tire, and being parallel with the rotating axial center of the tire in the section of the tire. In order to control the creep deformation, the concavedly curved surface of the composite side-rubber is formed so that a radius of curvature R 1 of an arc segment having the peak among the arc segments constituting the concavedly curved surface and a radius of curvature R of a major inner curved surface in the flange at a section of the approved rim satisfy a relationship of 0.4xR≦R 1 ≦1.6xR. Further, it is practically desirable that when a hypothetical arc convexedly extending toward the outside of the tire in section of the tire is imagined so as to pass through a first intersect between the first straight line among both the straight lines and an outer contour line of the bead portion and a second intersect between the bead base line and a surface contour line of the bead heel portion perpendicular to the bead base line and smoothly connect to the outer contour line in the vicinity of the first intersect, a distance D between the peak and the hypothetical arc as measured on a normal line passing through the peak toward the hypothetical arc is within a range of 22-70% of a distance d between the hypothetical arc and a position of the turnup portion of the carcass ply located on the normal line. The tire 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. The former case may take the construction as mentioned above. In the latter case, at least one reinforcing cord layer is arranged on the outer surface of the carcass ply in the bead portion, and the distance D between the peak and the hypothetical arc as measured on the normal line passing through the peak toward the hypothetical arc is within a range of 22-70% of a distance d between the hypothetical arc and a position of an outermost bead portion reinforcing cord layer located on the normal line, 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 further enhance the effect of the concavedly curved surface of the composite side-rubber, it is effective that a convex arc segment further projecting from the hypothetical are toward the outside of the tire and having a peak is provided on an outer surface of the bead portion at at least one of the inner side and the outer side of the concavedly curved surface of the composite side-rubber in the radial direction of the tire so as to smoothly connect to the concavedly curved surface, and a distance L between the peak and the hypothetical arc as measured on a normal line passing through the peak toward the hypothetical arc is not more than 0.65 times the distance D. As another means for further enhancing the effect of the concavedly curved surface of the composite side-rubber, it is effective that a convex arc segment having a peak located inward from the hypothetical arc and projecting toward the outside of the tire is provided on an outer surface of the bead portion at at least one of the inner side and the outer side of the concavedly curved surface of the composite side-rubber in the radial direction of the tire so as to smoothly connect to the concavedly curved surface, and a distance M between the peak and the hypothetical arc as measured on a normal line passing through the peak toward the hypothetical arc is within a range of 0.1-0.5 times the distance D. The above two formations of the convex arc segment may be applied separately or used together. 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 the curved surfaces concavedly directing toward the outside of the tire on the outer surface portion of the composite side-rubber and the envelop surface for steel wire of the bead core opposite to the inner curved surface of the slantly rising portion in the flange of the approved 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 of a main part of a first embodiment of a tire according to the invention; Fig. 2 is a partly perspective view sectionally shown of a hexagonal bead core; Fig. 3 is a diagrammatic section view illustrating details of an embodiment of the tire bead portion of Fig. 1; Fig. 4 is a diagrammatic section view illustrating details of a modified embodiment of the tire bead portion shown in Fig. 3; Fig. 5 is a diagrammatic section view illustrating details of another modified embodiment of the tire bead portion shown in Fig. 3; Fig. 6 is a diagrammatic section view illustrating details of a modified embodiment of the tire bead portion shown in Fig. 5; Fig. 7 is a diagrammatic section view illustrating details of a further modified embodiment of the tire bead portion shown in Fig. 3; Fig. 8 is a diagrammatic section view illustrating details of a modified embodiment of the tire bead portion shown in Fig. 7; Fig. 9 is a diagrammatic section view illustrating details of another modified embodiment of the tire bead portion shown in Fig. 1; Fig. 10 is a diagrammatic section view illustrating details of another modified embodiment of a tire bead portion; Fig. 11 is a diagrammatic section view illustrating details of a further modified embodiment of a tire bead portion; Fig. 12 is a graph showing contact pressure distribution of a bead portion engaged with a flange of a rim; Fig. 13 is a graph showing rubber gauge change of a composite side-rubber in a bead portion engaged with a flange of a rim; Fig. 14 is a graph showing change of maximum contact pressure in a bead portion engaged with a flange of a rim; Fig. 15 is a graph showing a relation between distance ratio and maximum contact pressure in a bead portion engaged with a flange of a rim; Fig. 16 is a diagrammatic section view of a 15° drop center rim; Fig. 17 is a diagrammatic section view of a width-wide flat bottom rim; Fig. 18 is a diagrammatic section view of a bead portion mounted on a rim illustrating creep deformation of the conventional tire; Fig. 19 is a diagrammatic section view of a bead portion in the conventional tire; and Fig. 20 is a graph showing contact pressure distribution of a bead portion in the conventional tire engaged with a flange of a rim. Figs. 1, 3-9, and 10-11 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. Fig. 2 sectionally shows an embodiment of the bead core used in the invention, respectively. According to the custom, the T/L radial tire shown in Figs. 1, 3-9 and 10-11 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. 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 Figs. 1 and 4. The structure and usual production method of the bead core 4 are described with reference to Fig. 2 particularly showing a hexagonal section of the bead core. In Fig. 2, 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 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 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 six flat envelop surfaces Ev 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. The T/L radial tire is provided with a curved surface 13 concavedly directed toward the outside of the tire at an outer surface portion of at least the composite side-rubber 5, particularly the rubber chafer 6 in the illustrated embodiment, among the members constituting the bead portion 1, i.e. composite side-rubber 5, turnup portion 3t and bead core 4 located opposite to a curved surface 21S of a slantly rising portion in a flange 21F of a rim 21 an inner section contour of which is shown by a phantom line in Fig. 1. The curved surface 13 may be shaped by forming in a mold during the vulcanization. Moreover, Fig. 1 shows a state just before the complete fitting of the tire to the rim 21. The contact pressure distribution of the bead portion in each of Experimental Examples A and B having different curved surfaces 13 with respect to the curved surface 21S of the flange 21F is measured to obtain results as shown in Fig. 12 together with results of a conventional example having the same tire size (11R 22.5) and containing no curved surface. As seen from Fig. 12, the contact pressure distribution of each Experimental Example is considerably uniformized as compared with that of the conventional example. After each tire of Experimental Examples A and B and the conventional example having the contact pressure distribution shown in Fig. 12 is mounted onto a truck and actually run over a distance of 100,000 km, it is demounted from the rim and a gauge changing amount of the composite side-rubber 5 (rubber chafer 6) at the contact face with the flange 21F of the rim 21 is measured as a creep deformation amount to obtain results as shown in Fig. 13, in which the creep deformation amount becomes large as a minus value becomes large. From these results, it is apparent that the contact pressure is inversely proportional to the creep deformation amount. The concavedly curved surface 13 is located between the bead base line BL and a first straight line Lp (see Figs. 1, 3 and 4) passing through a radially outer surface of a steel wire 4wo (see Figs. 1-4) located at an outermost side of the bead core 4 in the radial direction of the tire and being parallel to the bead base line BL. The concavedly curved surface 13 of the rubber chafer 6 is represented by a curve in section in each of these figures, which consists of plural arc segments smoothly connecting each other and convexedly directed toward the inside of the tire and a peak T is existent in the thus formed convex arc (see Figs. 3 and 4). This peak T is located between the first straight line Lp and a second straight line (not shown) passing through an innermost position 3i of the turnup portion 3t of the carcass ply 3 around the bead core 4 in the radial direction and being parallel to the bead base line BL. This is also confirmed by similar results obtained by using T/L radial tires of Experimental Examples And B and Conventional Example having a tire size (11/70 R22.5) different from the aforementioned tire size. As seen from results of Fig. 14 showing a change of maximum contact pressure, the optimum position of the peak T in the curved surface 13 is located between the two straight lines as mentioned above. Referring to Figs. 1 and 3, a radius of curvature R 1 of the arc segment having the peak T among the plural arc segments forming the concavedly curved surface 13 of the composite side-rubber 5 (rubber chafer 6) in the section of the tire and a radius of curvature R mainly forming the curved line 21S of the flange 21F at the inner section contour of the rim 21 satisfy a relationship of 0.4xR≦R 1 ≦1.6xR. There is hypothesized an arc 20 convexedly extending toward the outside of the tire in section of the tire and having a radius of curvature r (see Fig. 3) and which is imagined so as to pass through a first intersect P between the first straight line Lp and an outer contour line of the bead portion 1 and a second intersect Q between the bead base line BL and a surface contour line of a small beveled round part in the bead heel portion perpendicular to the bead base line BL and which smoothly connects to the outer contour line in the vicinity of the first intersect P. Referring to Fig. 3, a distance D between the peak T and the hypothetical arc 20 as measured on an extension of a line segment connecting a curvature center C 20 of the hypothetical arc 20 to the peak T or a normal line passing through the peak T toward the hypothetical arc 20 is preferably within a range of 22-70% of a distance d between the hypothetical arc 20 and a position of the turnup portion 3t of the carcass ply located on the same normal line (or extension). This is confirmed by similar results obtained by using T/L radial tires of Experimental Examples And B and Conventional Example having the two different tire sizes as previously mentioned. As seen from results of Fig. 15 showing an index of maximum contact pressure, the value of D/d when the index of maximum contact pressure is less than 100 as compared with that of the conventional example is within a range of 0.22-0.70. The bead portion 1 shown in Figs. 4-8 has a structure that one or more reinforcing cord layers, particularly reinforcing steel cord layers 19 (one cord layer in the illustrated embodiment) are provided on the carcass ply 3. The steel cords constituting the cord layer 19 are arranged at an inclination angle of 30-80° with respect to the radial direction in the vicinity of the bead core 4. In Fig. 4, the distance D between the hypothetical arc 20 and the peak T as measured on the extension of the line segment connecting the curvature center C 20 of the hypothetical arc 20 to the peak T is preferably within a range of 22-70% of the distance d between the hypothetical arc 20 and the outermost reinforcing steel cord layer as measured on the same extension. In the embodiment of Fig. 5, a convex arc segment 15 projecting from the hypothetical arc 20 toward the outside of the tire and having a peak T 1 is provided on the outer surface of the bead portion 1 at the outer side of the concavedly curved surface 13 of the composite side-rubber 5 (rubber chafer 6) in the radial direction so as to smoothly connect to the concavedly curved surface 13. Since the arc segment shown by a radius of curvature r 15 from a center C 15 has the peak T 1 , it is preferable that a distance L between the peak T 1 and the hypothetical arc 20 as measured on a normal line passing through the peak T 1 toward the hypothetical arc 20 is not more than 0.65 times the above-defined distance D. In the embodiment of Fig. 6, a convex arc segment 16 projecting from the hypothetical arc 20 toward the outside of the tire and having a peak T 2 is provided on the outer surface of the bead portion 1 at the outer side of the concavedly curved surface 13 of the composite side-rubber 5 (rubber chafer 6) in the radial direction so as to smoothly connect to the concavedly curved surface 13. Since the arc segment shown by a radius of curvature r 16 from a center C 16 has the peak T 2 , it is preferable that a distance L between the peak T 2 and the hypothetical arc 20 as measured on a normal line passing through the peak T 2 toward the hypothetical arc 20 is not more than 0.65 times the above-defined distance D. Various experiments as Experimental Examples C and D are made by varying a ratio of distance L to distance D to obtain results as shown in Table 1 in which the contact pressure is represented by an index on the basis that the conventional example is 100. L/D Experimental Example C Experimental Example D 0.05 84 78 0.20 93 88 0.60 96 94 0.70 115 119 In the embodiment of Fig. 7, a convex arc segment 17 projecting inside the hypothetical arc 20 toward the outside of the tire and having a peak T 3 is provided on the outer surface of the bead portion 1 at the outer side of the concavedly curved surface 13 of the composite side-rubber 5 (rubber chafer 6) in the radial direction so as to smoothly connect to the concavedly curved surface 13. Since the arc segment shown by a radius of curvature r 17 from a center C 17 has the peak T 3 , it is preferable that a distance M between the peak T 3 and the hypothetical arc 20 as measured on a normal line passing through the peak T 3 toward the hypothetical arc 20 is within a range of 0.1-0.5 times the above-defined distance D. In the embodiment of Fig. 8, a convex arc segment 18 projecting inside the hypothetical arc 20 toward the outside of the tire and having a peak T 4 is provided on the outer surface of the bead portion 1 at the outer side of the concavedly curved surface 13 of the composite side-rubber 5 (rubber chafer 6) in the radial direction so as to smoothly connect to the concavedly curved surface 13. Since the arc segment shown by a radius of curvature r 18 from a center C 18 has the peak T 4 , it is preferable that a distance M between the peak T 4 and the hypothetical arc 20 as measured on a normal line passing through the peak T 4 toward the hypothetical arc 20 is within a range of 0.1-0.5 times the above-defined distance D. The bead portion 1 shown in Fig. 9 has a structure wherein a nylon cord layer 19N is arranged on the reinforcing steel cord layer 19 so as to form two layers at the outer region of the tire and one layer at the inner region of the tire as a bead portion reinforcing cord layer. The concavedly curved surface 13 is formed on the composite side-rubber as described with reference to Figs. 1 and 3-8, and further a convex arc segment 15-18 may be provided thereon. Moreover, the nylon cords are arranged at the same inclination angle as in the steel cords of the layer 19 with respect to the radial direction, provided that the nylon cords of the layer 19N adjacent to the layer 19 are crossed with the steel cords of the layer 19 and the nylon cords of the two layers 19N are crossed with each other. In Figs. 10-11, 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. 10-11 is provided with a bead portion 1 and a reinforcing steel cord layer 19. In the bead portion 1 of Fig. 11, 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. 11, 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. 10 has a structure having the concavedly curved surface 13 of the composite side-rubber 5 (rubber chafer 6) as previously mentioned and a curved envelop surface 4-13 of the bead core 4 (which is not the subject of this invention). 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, and additionally that 0.7 x R ≦ R 2 ≦ 1.2 x R. The bead portion 1 shown in Fig. 11 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, the concavedly curved surface 13 of the composite side-rubber 5 (rubber chafer 6) as previously mentioned and the curved envelop surface 4-13 of the bead core 4. 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. 1-11 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 2, 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. 1-9, which is mounted on an approved 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 an inner curved 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 the hypothetical arc 20. The bead core 4 of the tires of Figs. 1-9 is a hexagonal bead core shown in Fig. 2 in which the steel wire has a diameter of 1.55 mm and the number of windings (number of wires appearing in the section) is 65 turns. There are provided tires of Examples 1-7 having various different values of ratios R 1 /R, D/d, L/D and M/D and the conventional tire. These values are shown in Table 3 together with respective figure number and values of ratios E 1 /E 2 , E 3 /E 2 and E 4 /E 2 . Items Convetional Example Example 1 2 3 4 5 6 7 Fig. 23 Fig. 3 Fig. 4 Fig. 5 Fig. 6 Fig. 7 Fig. 8 Fig. 9 R 1 /R - 1.18 1.20 0.49 0.63 1.02 0.98 0.90 D/d - 0.33 0.56 0.50 0.40 0.57 0.54 0.66 L/D - - - 0.23 0.38 - - - M/D - 0.26 0.25 - 0.34 0.28 0.46 0.27 E 1 /E 2 0.92 0.92 0.92 0.92 0.92 0.83 0.83 0.92 E 3 /E 2 0.83 0.83 0.83 0.75 0.75 0.83 0.83 0.75 E 4 /E 2 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 Index of long run on drum 100 133 154 122 120 117 121 115 Index of crack length 100 35 30 31 56 42 73 52 Index of CP 100 110 114 109 108 112 110 105 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 on approved rim and inflated under an internal pressure of 7.0 kgf/cm 2 . 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 5000 kgf 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 3, 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 3 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 3, 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 an approved 15° drop center 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 one in each bead portion (1) so as to reinforce said bead portions (1), the at least one carcass ply (3) being reinforced with cords radially arranged therein and wound around the bead cores (4) from the inside of the tire toward the outside thereof to form a turnup portion (3t) in each bead portion and a main body therebetween, each of said bead cores (4) being a wound laminate of steel wire (4w) having a circular or tetragonal shape in cross-section, a belt superimposed on an outer periphery of the at least one carcass ply (3) to reinforce the tread portion and comprised of two or more cross steel cord layers, and a composite side-rubber (5) extending from the bead base of each bead portion (1) through the outside of the bead portion (1) to the tread rubber of the tread portion and comprising a rubber chafer (6) and a sidewall rubber (7), wherein an outer surface (13) of at least the composite side-rubber (5) among members constituting each bead portion (1) of the tire has a shape directed toward the outside of the tire at a region opposite to an inner curved surface (21S) of a slantly rising portion in each flange (21F) of the approved rim (21), characterized in that the shape of said outer surface (13) is concavedly curved; in that the concavedly curved surface (13) of the composite side-rubber (5) is located between a bead base line (BL) and a first straight line (Lp) passing through the radially outer surface of the steel wire (4wo) located at the outermost side of the bead core (4) in the radial direction of the tire and being parallel to the bead base line (BL), and the curve of the concavedly curved surface (13) of the composite side-rubber (5) in section of the tire is comprised of plural arc segments smoothly connecting to each other and having a peak (T) in one of said segments, and said peak (T) is located between the first straight line (Lp) and a second straight line passing through the innermost position (3i) of the turnup portion (3t) of the carcass ply (3) around the bead core (4) in the radial direction of the tire and parallel with the bead base line (BL); and in that the concavedly curved surface (13) of the composite side-rubber (5) is formed so that the radius of curvature R 1 of the arc segment having the peak (T) among the arc segments constituting the concavedly curved surface (13) and the radius of curvature R of the major inner curved surface in the flange (21F) at a section of the approved rim (21) satisfy the relationship of 0.4xR≦R 1 ≦1.6xR. A pneumatic radial tire as claimed in claim 1, characterized in that when a hypothetical arc (20) convexedly extending toward the outside of the tire in section of the tire is imagined so as to pass through a first intersect (P) between the first straight line (Lp) among both the straight lines and an outer contour line of the bead portion (1) and a second intersect (Q) between the bead base line (BL) and a surface contour line of the bead heel portion perpendicular to the bead base line and smoothly connect to the outer contour line in the vicinity of the first intersect (P), the distance D between the peak (T) and the hypothetical arc (20) as measured on a normal line passing through the peak (T) toward the arc (20) is within a range of 22-70% of the distance (d) between the arc (20) and a position of the turnup portion (3t) of the carcass ply located on the said normal line. A pneumatic radial tire as claimed in claim 1, 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), and when a hypothetical arc (20) convexedly extending toward the outside of the tire in section of the tire is imagined so as to pass through a first intersect (P) between the first straight line (Lp) among both the straight lines and an outer contour line of the bead portion (1) and a second intersect (Q) between the bead base line (BL) and a surface contour line of the bead heel portion perpendicular to the bead base line and smoothly connect to the outer contour line in the vicinity of the first intersect (P), the distance D between the peak (T) and the hypothetical arc (20) as measured on the normal line passing through the peak (T) toward the arc (20) is within a range of 22-70% of the distance (d) between the arc (20) and a position of the outermost bead portion reinforcing cord layer located on the said normal line. A pneumatic radial tire as claimed in claim 2 or 3, characterized in that a convex arc segment (15,16) further projecting from the hypothetical arc (20) toward the outside of the tire and having a peak (T 1 , T 2 ) is provided on the outer surface of the bead portion (1) at at least one of the inner side and the outer side of the concavedly curved surface (13) of the composite side-rubber (5) in the radial direction of the tire so as to smoothly connect to the concavedly curved surface (13), and the distance L between the peak (T 1 , T 2 ) and the arc (20) as measured on a normal line passing through the peak (T 1 , T 2 ) toward the arc (20) is not more than 0.65 times the distance D. A pneumatic radial tire as claimed in any of claims 2 to 4, characterized in that a convex arc segment (17,18) having a peak (T 3 ,T 4 ) located inward from the hypothetical arc (20) and projecting toward the outside of the tire is provided on the outer surface of the bead portion (1) at at least one of the inner side and the outer side of the concavedly curved surface (13) of the composite side-rubber (5) in the radial direction of the tire so as to smoothly connect to the concavedly curved surface (13), and the distance M between the peak (T 3 ,T 4 ) and the arc (20) as measured on a normal line passing through the peak (T 3 ,T 4 ) toward the arc (20) is within a range of 0.1-0.5 times the distance D. A pneumatic radial tire as claimed in any of claims 1 to 5, characterized in that both the composite side-rubber (5) and the bead core (4) are provided with curved surfaces concavedly directed toward the outside of the tire respectively on the outer surface (13) of the composite side-rubber (5) and an envelope surface (4-13) for steel wire (4w) of the bead core (4) opposite to the inner curved surface (21S) of the slantly rising portion in each flange (21F) of the approved rim (21). A pneumatic radial tire as claimed in any of claims 1 to 6, characterized in that the tire is provided with a hard stiffener rubber (8) taperedly extending outward from the 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 7, characterized in that a modulus at 100% elongation (E 3 ) of at least a rubber portion having the concavedly curved surface (13) in the composite side-rubber (5) is within a range of 0.68-1.15 times of the modulus at 100% elongation (E 2 ) of the hard rubber stiffener (8). A pneumatic radial tire as claimed in claim 7 or 8, characterized in that the steel wire (4w) constituting the bead core (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: KATO KENSHIRO, KATO, KENSHIRO
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1996
Main classification: B60C15
IPC codes: B60C9, B60C15, B60C3