Pneumatic radial tires for small-size passenger cars

Abstract: In a bead portion of a pneumatic radial tire for a small-size passenger car having substantially the same structure as in a conventional tire, a bead core to be used is a monostrand bead core having a particular sectional shape obtained by spiral winding of a rubberized steel wire having a diameter of 1.2 - 2.2 mm, in which the ratio of the distance (H) between centre points of adjoining steel wires in the bead core to the diameter (D) of the steel wire is in the range of 1.01 - 1.2 and the length overlapping from the winding start point of the steel wire to the winding terminal point thereof is given by the equation: L ≧ 4.0 x 10 x πr 2 x (f/A)

Description: This invention relates to a pneumatic radial tire for a passenger car, in particular for a small-size passenger car having an improved steering stability by improving the structure of the bead portion without causing lowering of tire breaking pressure (safety factor of tire) resulting from the bead portion and capable of reducing the cost and improving the production efficiency by controlling an extra use of bead wire. In this type of pneumatic tire, the bead core located in the bead portion acts to fix an end portion of the carcass ply comprised of fiber cords or steel wire cords wound therearound and to define the size of the inner periphery of the tire itself and ensure the fitting to a wheel rim. Thus, the bead core is a very important element in a pneumatic tire, and there have been proposed various structures of the bead portion. For instance, there are known a bead core obtained by spirally winding a band-shaped rubber-wire composite containing plural wires arranged side by side and embedded therein several times into a given shape as shown in Figs. 1 and 2 of the accompanying drawings (JP-U-61-110406), a monostrand bead core for use in heavy duty pneumatic tires obtained by coating a wire with uncured rubber at a given thickness to from a bead wire and then spirally winding the bead wire in a given shape (JP-A-2-286407), and so on. In the conventional pneumatic tire, however, a relation between a length overlapping a winding start portion with a winding terminal portion in the bead core and a tire breaking pressure (safety factor of tire) is not always satisfactory, so that the tire breaking pressure in the bead portion is insufficient, and particularly the stress concentration is increased in a step portion defined by the winding start portion or the winding terminal portion to bring about lowering of the tire breaking pressure. Furthermore, the bead wire is excessively used to increase the cost and degrade the production efficiency. Moreover, the shape of the bead core made from the band-shaped rubber-wire composite or the monostrand bead core as mentioned above is largely determined in tire building and vulcanization steps, so that it is difficult to maintain the tire uniformity in circumferential and radial directions of the tire and hence the steering stability of the resulting tire product is considerably degraded. It is, therefore, an object of the present invention to overcome the aforementioned problems of the conventional bead cores and to provide a pneumatic radial tire for a small-size passenger car capable of improving the steering stability by controlling the tire uniformity in radial and circumferential directions due to the bead core without lowering of the tire breaking pressure (safety factor of tire) and controlling the use of bead wire for achieving reduction of cost and improvement of production efficiency. According to the invention, there is provided a pneumatic radial tire for a passenger car comprising a tread portion, a pair of sidewall portions extending from both side ends of the tread portion, and a pair of bead portions each including a bead core therein and each extending from an inner peripheral end of a sidewall portion, wherein each said bead core is a monostrand bead core having a compact unit structure of 3 to 10 steel wires in section and is formed by spirally winding a steel wire of circular section covered with a coating rubber and binding the steel wire windings into a bundle, and wherein said steel wire has a diameter (D) of 1.2-2.2 mm, and a the ratio of the distance between center points of adjoining steel wire windings in said bead core (H) to the diameter (D) of said steel wire is within a range of 1.01-1.2, and the length L (mm) overlapping from the winding start point of said steel wire to the winding terminal point thereof satisfies the following equation: L ≧ 4.0 x 10 x πr 2 x (f/A) wherein r is one radius (mm) of the steel wire, f is the tensile strength (kgf/mm 2 ) of the steel wire and A is the drawing force (kgf) of the steel wire from the coating rubber. In preferable embodiments of the invention, the tire has a tire size of not more than 13 inches (330 mm); and the diameter (D) of the steel wire is 1.25-1.55 mm; and the bead core has a hexagonal shape in section having a compact unit structure of 7 steel wires over not less than 80% of full circumference. The invention will be further described with reference to the accompanying drawings, wherein: Fig. 1 is a perspective view of an embodiment of the strand structure in a conventional bead core; Fig. 2 is a sectional view taken along a line II-II of Fig. 1; Fig. 3 is a schematic view illustrating a relation between the distance between center points of adjoining steel wire windings in a bead core and the diameter of the steel wire; Fig. 4 is a partial side view of a monostrand bead core illustrating a length (L) overlapping from the winding start point of a steel wire to the winding terminal point thereof; and Figs. 5 to 8 are sectional views of various embodiments of bead cores of tires according to the invention taken along an axial direction of a tire, respectively. In the pneumatic radial tire for a small-size passenger car according to the invention, the bead core is formed as a monostrand bead core having a compact unit structure of 3 to 10 steel wires in section, whereby the unbalance level of the tire is improved to bring about an improvement of tire uniformity and a rise of tenacity utilization. When the number of steel wires in the section of the bead core is less than 3, the compact unit structure can not be formed, while when the number of steel wires exceeds 10, there are caused problems such as increase of weight, difficulty of rim assembling and the like. As shown in Fig.3, the steel wire used in the bead core has a diameter (D) of 1.2-2.2 mm, preferably 1.25-1.55 mm, while the ratio of the distance between center points of adjoining steel wire windings in the bead core (H) to the diameter (D) of the steel wire is within a range of 1.01-1.2, preferably 1.05-1.15. When the diameter D of the steel wire is less than 1.2 mm, sufficient rigidity and strength of the bead core can not be obtained, while when the steel wire diameter exceeds 2.2 mm, the weight is undesirably increased and the rigidity of the bead core is excessively raised to degrade the rim assembling property or rim fitness. On the other hand, when the ratio H/D is less than 1.01, the rigidity of the bead core having a ring shape becomes too large and the rim assembling property is degraded, while when said ratio exceeds 1.2, the deformation of the bead core becomes undesirably large in the tire-building and vulcanization steps to degrade the tire uniformity. In the monostrand bead core of the tire according to the invention, the overlapping length (L) between the winding start point of the steel wire and the winding terminal point thereof is a length shown in Fig.4 and satisfies the above equation, preferably the following equation: 1.0 x 10 2 x πr 2 x (f/A) ≧ L ≧ 5.0 x 10 x πr 2 x (f/A). In this equation, the drawing force A (kgf) of the steel wire from the coating rubber is a value obtained by drawing the steel wire from a rubberized sample of 50 mm in length at a drawing rate of 200 mm/min according to ASTM-D2229. When the length L is less than 4.0 x 10 x πr 2 x (f/A) , the winding terminal portion is drawn out due to the peeling from the coating rubber, so that the winding terminal portion of the steel wire can not contribute to the total tenacity of the bead core and hence the tenacity utilization of the bead core is decreased to lower the safety factor of the tire. When the winding number of the steel wire is increased in order to prevent the lowering of the safety factor, the weight of the bead portion is undesirably increased. On the other hand, when the length L exceeds 1.0 x 10 2 x πr 2 x (f/A) , the weight and hence the cost is undesirably increased without changing the tenacity utilization of the bead core, and also the tire uniformity is degraded. According to the invention, the tire size of the pneumatic radial tire is preferably not more than 13 inches (330 mm). If the tire size exceeds 13 inches, the strength becomes insufficient in the structure of the bead core of the tire according to the invention. Furthermore, the sectional shape of the bead core is preferably rendered into a hexagonal shape having a compact unit structure of 7 steel wires over not less than 80% of full circumference, whereby the shape stability in the tire building and vulcanization steps are considerably improved. As a result, an improvement of tire uniformity and an increase of tenacity utilization are obtained. By rationalizing the formation of compact hexagonal structure of monostrand and the overlapping length L between the winding starting point and the winding terminal point as mentioned above, the strength loss can be controlled to approximately zero and also a weight reduction of the tire and a reduction of the production cost can be attained. The following examples are given in illustration of the invention and are not intended as limitations thereof. Various test tires having tire sizes of 155/65R12 and 175/70R13 are prepared by adopting structures shown in Figs. 2, 5, 6, 7 and 8 as a sectional shape of the bead core and varying the diameter D, tensile strength f, drawing force A and overlapping length L of the steel wire, and the ratio H/D of distance between centers of adjoining steel wires to wire diameter as shown in Table 1. Then, the rim assembling property of the bead core, steering stability and tire breaking pressure Pmax are measured with respect to these tires to obtain results as shown in Table 1. Rim assembling property Ten tires of each of these test tires are assembled onto rims of 12 x 4.00B or 5J-13, respectively, to measure an average rim-assembling time t (sec) per one tire. When rim-assembling time in the tire of Comparative Example 1 is t 0 , the rim assembling property of each tire is evaluated by an equation of (t 0 /t) x 100 and represented by an index value. The larger the index value, the better the property. Moreover, there is caused no problem when the index value is not less than 80. Steering stability The steering stability is evaluated by a feeling test of a test driver when the tire is mounted on a small-size passenger car and actually run on a road. It is represented by an index on the basis that the tire of Comparative Example 1 is 100. The larger the index value, the better the result. Tire breaking pressure Each of the test tires having a tire size of 175/70R13 is pressurized through the supply of water to measure a pressure Pmax in the breaking of the bead portion. As mentioned above, in the pneumatic tire for the a small-size passenger car according to the invention, the steering stability can be improved by rendering the bead core in the bead portion into a monostrand bead core of a special structure without causing lowering of tire breaking pressure (safety factor of tire) and degrading the rim assembling property. Furthermore, the loss of the strength in the bead core can be controlled to approximately zero, whereby extra use of the steel wire can be controlled and weight reduction of the tire and reduction of cost can be attained and production efficiency can be improved.

Claims: A pneumatic radial tire for a passenger car comprising a tread portion, a pair of sidewall portions extending from both side ends of the tread portion, and a pair of bead portions each including a bead core therein and each extending from an inner peripheral end of a sidewall portion, wherein each said bead core is a monostrand bead core having a compact unit structure of 3 to 10 steel wires in section and is formed by spirally winding a steel wire of circular section covered with a coating rubber and binding the steel wire windings into a bundle, and wherein said steel wire has a diameter (D) of 1.2-2.2 mm, and the ratio of the distance between center points of adjoining steel wire windings in said bead core (H) to the diameter (D) of said steel wire is within a range of 1.01-1.2, and the length L (mm) overlapping from the winding start point of said steel wire to the winding terminal point thereof satisfies the following equation: L ≧ 4.0 x 10 x πr 2 x (f/A) wherein r is the radius (mm) of the steel wire, f is the tensile strength (kgf/mm 2 ) of the steel wire and A is the drawing force (kgf) of the steel wire from the coating rubber. A pneumatic radial tire according to claim 1, wherein the tire has a tire size of not more than 13 inches (330 mm). A pneumatic radial tire according to claim 1 or 2, wherein the diameter (D) of the steel wire is 1.25-1.55 mm. A pneumatic radial tire according to any of claims 1 to 3, wherein the bead core has a hexagonal shape in section having a compact unit structure of 7 steel wires over not less than 80% of full circumference.

Inventors: YAMAMOTO MASAHIKO, YAMAMOTO, MASAHIKO
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1995
Main classification: B60C15
IPC codes: B60C15