Pneumatic radial tires provided with a side portion reinforcing layer

Abstract: A pneumatic radial tire or run flat tire comprises a pair of bead cores (1), a radial carcass (2) of a rubberized cord ply, a belt (3) comprised of rubberized belt layers, a tread (4) disposed outside the belt in the radial direction, and a side portion reinforcing layer (5) extending along an inner surface of the carcass over the full region of the side portion and having a crescent shape in a radial section thereof, in which when the peripheral length of the side portion reinforcing layer (5) is divided into three equal parts along an inner surface of the tire, a thickness (T) of the side portion reinforcing layer (5) in a section in a radial direction of the tire is thickest in the vicinity of a 1/3 point (C) located outward in the radial direction.

Description: This invention relates to a pneumatic radial tire, and more particularly to a pneumatic radial tire comprising a pair of bead portions each embedding a bead core therein, a radial carcass of a rubberized cord ply extending between both bead portions through a crown portion and a pair of side portions and wound around each of the bead cores from the inside toward the outside thereof, a belt superimposed outward on a crown portion of the carcass in a radial direction and comprised of rubberized belt layers containing substantially non-extensible cords therein, a tread disposed outside the belt in the radial direction, and a side portion reinforcing layer extending along an inner surface of the carcass over the full region of the side portion and having a crescent shape in a radial section thereof, being a so-called run-flat tire capable of continuously running even if the internal pressure of the tire is lowered by a puncture or the like. The conventional run flat tire is provided with a side portion reinforcing layer extending along an inner surface of a carcass ply over the full region of each of both side portions and having a crescent shape in a radial section so as to have an approximately uniform thickness of the side portion as a whole, whereby a uniform rigidity is applied to the whole of the side portion and hence it is possible to continuously run the tire by bearing a load through the rigidity of the reinforced side portion even if the internal pressure of the tire is lowered by a puncture or the like. In such a run flat tire provided with the side portion reinforcing layer, it is most important how far the tire can continuously be run after the lowering of the internal pressure by a puncture or the like, that is the running durability of the tire after a puncture is a most important requirement. In order to improve the running durability after a puncture, it is effective to increase an elastomer constituting the side portion reinforcing layer to increase the thickness of the side portion reinforcing layer. In this case, however, there are caused an increase of tire weight and deterioration of steering stability and ride comfortability. Recently, it is strongly demanded to reduce the tire weight from a viewpoint of resource-saving and energy-saving. For this purpose, it is attempted to decrease the number of the carcass cord plies from two plies to one ply. In this case, however, the side rigidity of the tire is lacking and hence the running durability after a puncture is undesirably degraded. Attention is also drawn to the disclosure of EP-A-0 456 437, corresponding to the preamble of claim 1, of FR-A-2344413. It is, therefore, an object of the present invention to solve the aforementioned problems of the conventional technique and to provide a pneumatic radial tire or run flat tire provided with a side portion reinforcing layer having an improved running durability after a puncture without increasing tire weight and degrading basic performances required for the tire such as steering stability, ride comfortability and the like. According to the invention, there is provided a pneumatic radial tire comprising a pair of bead portions each embedding a bead core therein, a radial carcass of a rubberized cord ply extending between both bead portions through a crown portion and a pair of side portions and wound around each of the bead cores from the inside toward the outside thereof, a belt superimposed outward on a crown portion of the carcass in a radial direction and comprised of rubberized belt layers containing substantially non-extensible cords therein, a tread disposed outside the belt in the radial direction, and a side portion reinforcing layer extending along an inner surface of the carcass over the full region of the side portion and having a crescent shape in a radial section thereof, in which when a peripheral length of the side portion reinforcing layer is divided into three equal parts along an inner surface of the tire, a thickness of the side portion reinforcing layer in a radial cross-section of the tire is thickest in the vicinity of a 1/3 point located outward in the radial direction, and the thickness of the side portion reinforcing layer in the radial section at the 1/3 point located outward in the radial direction is 1.8-2.8 times the thickness of the side portion reinforcing layer in the radial section at a 1/3 point located inward in the radial direction. In a preferable embodiment of the invention, the side portion reinforcing layer is made from rubber having a hardness of 75-95 degrees. Throughout the specification, the rubber hardness means a value of an International Rubber Hardness measured by hardness testing methods for vulcanized rubber according to JIS K6253-1993. The invention will be further described with reference to the accompanying drawing, wherein: Fig. 1 is a diagrammatic left-half radial section view of a pneumatic radial tire according to the invention. As previously mentioned, in the conventional run flat tire provided with the side portion reinforcing layer, it was possible to continuously run the tire even if the internal pressure of the tire was lowered by a puncture or the like because the load applied to the tire was born by the rigidity of the side portion reinforced with the side portion reinforcing layer extending along the inner surface of the carcass ply over the full region of each of the side portions and having the crescent shape in the radial section so as to have approximately a uniform thickness of the side portion as a whole. However, when the deformation behaviour of such a tire is closely investigated during the running at a punctured state, it has been found that the degree of the bending deformation in the side portion of the tire is not uniform as a whole in the side portion and becomes large at a position near to the tread, and problems during the running after a puncture concentrate in the vicinity of a 1/3 point located outward in the radial direction when the peripheral length of the side portion reinforcing layer is divided into three equal parts along the inner surface of the tire. In the pneumatic radial tire provided with the side portion reinforcing layer according to the invention, when the peripheral length of the side portion reinforcing layer is divided into three equal parts along the inner surface of the tire, the thickness of the side portion reinforc-ing layer in the radial section is thickest in the vicinity of the 1/3 point located outward in the radial direction. Also, the thickness of the side portion reinforcing layer in the radial section at the 1/3 point located outward in the radial direction is 1.8-2.8 times the thickness of the side portion reinforcing layer in the radial section at a 1/3 point located inward in the radial direction. Thus, problems hardly occur even when the tire is continuously run after the lowering of the internal pressure by a puncture or the like and hence there is obtained a run flat tire having an improved running durability after a puncture. In the pneumatic radial tire according to the invention, the side portion reinforcing layer is preferably made from rubber having a hardness of 75-95 degrees. When the hardness of rubber forming the side portion reinforcing layer is less than 75 degrees, the rigidity of the side portion reinforcing layer is lacking and the longitudinal deflection becomes large and hence when the tire is continuously run after the lowering of the internal pressure by a puncture or the like, a large strain is created in the side portion reinforcing layer to degrade the running durability after the puncture. When said rubber hardness exceeds 95 degrees, the rigidity of the side portion reinforcing layer is too large and hence the ride comfortability is considerably degraded in usual use at a state of running under a given internal pressure. An embodiment of the pneumatic radial tire provided with the side portion reinforcing layer according to the invention will be described with reference to the accompanying drawing together with a comparative example of a run flat tire provided with a conventional side portion reinforcing layer. These tires have a tire size of 275/40R17. A pneumatic radial tire or run flat tire shown in Fig. 1 as Example 1 of the invention comprises a pair of bead portions each embedding a bead core 1 therein, a radial carcass 2 of two rubberized cord plies extending between both bead portions through a crown portion and a pair of side portions and wound around each of the bead cores 1 from the inside toward the outside thereof, a belt 3 superimposed outward on a crown portion of the carcass 2 in a radial direction and comprised of rubberized belt layers containing substantially non-extensible cords therein, and a tread 4 disposed outside the belt 3 in the radial direction. Further, a side portion reinforcing layer 5 is extended along an inner surface of the carcass 2 over the full region of the side portion from a point A to a point D and has a crescent shape in a radial section thereof. When the peripheral length of the side portion reinforcing layer 5 is divided into three equal parts along the inner surface of the tire, the thickness of the side portion reinforcing layer 5 in the radial section is thickest in the vicinity of a 1/3 point (C) located outward in the radial direction. The thickness (T) of the side portion reinforcing layer 5 in the radial section at the 1/3 point (C) located outward in the radial direction is 2.0 times the thickness (t) of the side portion reinforcing layer 5 in the radial section at a 1/3 point (B) located inward in the radial direction. The side portion reinforcing layer 5 is made from rubber having a hardness of 84 degrees. A run flat tire of Example 2 is substantially the same as the run flat tire of Example 1 except that when the peripheral length of the side portion reinforcing layer 5 is divided into three equal parts along the inner surface of the tire, the thickness (T) of the side portion reinforcing layer 5 in the radial section at the 1/3 point (C) located outward in the radial direction is 2.5 times the thickness (t) of the side portion reinforcing layer 5 in the radial section at the 1/3 point (B) located inward in the radial direction. A run flat tire of Comparative Example 1 is substantially the same as the run flat tire of Example 1 except that when the peripheral length of the side portion reinforcing layer 5 is divided into three equal parts along the inner surface of the tire, the thickness (T) of the side portion reinforcing layer 5 in the radial section at the 1/3 point (C) located outward in the radial direction is 1.5 times the thickness (t) of the side portion reinforcing layer 5 in the radial section at the 1/3 point (B) located inward in the radial direction. A run flat tire of Comparative Example 2 is substantially the same as the run flat tire of Example 1 except that when the peripheral length of the side portion reinforcing layer 5 is divided into three equal parts along the inner surface of the tire, the thickness (T) of the side portion reinforcing layer 5 in the radial section at the 1/3 point (C) located outward in the radial direction is 3.0 times the thickness (t) of the side portion reinforcing layer 5 in the radial section at the 1/3 point (B) located inward in the radial direction. A test for measuring a continuous running distance after the lowering of the internal pressure by a puncture or evaluating the running durability after the puncture is made with respect to the run flat tires of Examples 1 and 2 and Comparative Examples 1 and 2. Assuming that the internal pressure is rendered into zero by the puncture of the tire, the running distance until the occurrence of problems at a state of opening a valve is measured. When the running distance is represented by an index on the basis that the tire of Comparative Example 1 is 100, the index value of the running distance is 98 in the tire of Comparative Example 2, 120 in the tire of Example 1 and 125 in the tire of Example 2, respectively. The larger the index value, the better the running durability after the puncture. As seen from the above results, the pneumatic radial tires according to the invention are superior to the comparative pneumatic radial tire in the running durability after the puncture.

Claims: A pneumatic radial tire comprising a pair of bead portions each embedding a bead core (1) therein, a radial carcass (2) of a rubberized cord ply extending between both bead portions through a crown portion and a pair of side portions and wound around each of the bead cores (1) from the inside toward the outside thereof, a belt (3) superimposed outward on a crown portion of the carcass (2) in a radial direction and comprised of rubberized belt layers containing substantially non-extensible cords therein, a tread (4) disposed outside the belt (3) in the radial direction, and a side portion reinforcing layer (5) extending along an inner surface of the carcass (2) over the full region of the side portion and having a crescent shape in a radial section thereof, characterized in that when a peripheral length of the side portion reinforcing layer (5) is divided into three equal parts along an inner surface of the tire, a thickness (T) of the side portion reinforcing layer (5) in a radial cross-section of the tire is thickest in the vicinity of a 1/3 point (C) located outward in the radial direction, and the thickness (T) of the side portion reinforcing layer (5) in the radial section at the 1/3 point (C) located outward in the radial direction is 1.8-2.8 times the thickness (t) of the side portion reinforcing layer (5) in the radial section at a 1/3 point (B) located inward in the radial direction. A pneumatic radial tire as claimed in claim 1, characterized in that the side portion reinforcing layer (5) is made from rubber having a hardness of 75-95 degrees.

Inventors: NISHIGATA HIROSHI, NISHIGATA, HIROSHI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1997
Main classification: B60C13
IPC codes: B60C17, B60C13