Pneumatic radial tires for passenger cars

Abstract: A high-speed running pneumatic radial tire comprises a cylindrical crown portion, a pair of sidewall portions extending inward therefrom, a radial carcaas (7) extending from one of the sidewall portions through the crown portion to the other sidewall portion, and a belt (8) and a tread (9) successively superimposed on the crown portion outward in the radial direction of the tire. The tread has a plurality of rows of blocks (3) defined by a plurality of circumferential grooves (1a,1b) and by lateral grooves (2a,2b), and the belt (8) is comprised of at least two main cross-cord belt layers (8a) and an auxiliary belt layer (8b) arranged thereon. The auxiliary belt layer is formed by spirally winding a rubberized ribbon ply of heat-shrinkable cords (12) on the main belt layers (8a), in which an overlapped amount of the ribbon ply is made larger in a zone (B,C) of the tread substantially corresponding to a block row (3) than in a zone (A) of the tread substantially corresponding to a circumferential groove (1a,1b).

Description: This invention relates to a pneumatic radial tire for passenger cars such as defined in the preamble of claim 1 (see EP-A- 0 333 628), and more particularly to an improvement in the belt in a low-section profile radial tire for running at an extremely high speed. Recently, it has become possible to stably run a vehicle at an extremely high speed exceeding 150 km/hr with improved technical innovation for passenger cars, and hence low-section profile radial tires having an aspect ratio of not more than 0.6 (ratio of section height to maximum width in the tire) and sufficient performances even during running at such an extremely high speed are being developed. However, when the tire is run at the extremely high speed, separation between tread and belt, chipping of tread rubber and the like particularly become problems, and consequently it is important that durability against such failures is high or so-called high-speed durability is excellent. Considering the high-speed durability, there has been proposed a tire structure wherein a rubberized fabric of organic fiber cords is spirally wound around an outermost belt layer arranged outside the carcass under a constant tension. In general, such a tire develops sufficient performances in case of running at middle to high speeds, but does not possess sufficient performances in case of running at an extremely high speed. In this tire, the tread is provided at its surface with a plurality of circumferential grooves of relatively wide width along the circumference of the tread having regard to the drainage property and has a thickness which is thin at a position corresponding to the circumferential groove and is thick at a position corresponding to a block row defined between the adjoining circumferential grooves, so that the difference in rubber volume of the tread between the position of the circumferential groove and the position of the block row becomes large. When the tire is run at an extremely high speed, the protruding quantity of tread rubber in an outward direction of the tire produced through centrifugal force becomes largely different between the position of the circumferential groove and the position of the block row in the widthwise direction of the tread. Moreover, when the extremely high-speed running is continued heat is generated due to an excessive ground contact pressure of the block row through the centrifugal force, which exceeds the heat-resistant limit of the tread rubber to cause blowing-out or peeling of the tread rubber and hence the occurrence of so-called chunk-out, and in this case the tire life becomes very short. Further, when individual blocks in the block row are viewed during the extremely high-speed running, the protruding quantity in the central portion of the block becomes larger than that in the peripheral portion of the block and hence the ground contact pressure in the central portion of the block becomes higher than that in the peripheral portion of the block. Also, the ground contact pressures of the blocks located in the widthwise direction of the tread become non-uniform. Consequently, the steering stability is adversely affected during the extremely high-speed running and also uneven wear is caused due to such non-uniform ground contact pressures of the blocks. It is, therefore, an aim of the present invention to provide pneumatic radial tires having performances sufficiently durable for use during extremely high-speed running, having very small protruding quantity of rubber at the tread surface during the extremely high-speed running. The inventors have made various studies and found that the difference of the protruding quantity at the tread surface in the widthwise direction can be made small by changing modulus of an auxiliary belt layer disposed between a belt layer and a carcass at positions corresponding to the circumferential groove and the block row in the widthwise direction of the tread. According to the invention, there is provided a pneumatic radial tire comprising a cylindrical crown portion, a pair of sidewall portions extending from both ends of the crown portion inward in the radial direction of the tire, a radial carcass extending from one of the sidewall portions through the crown portion to the other sidewall portion to reinforce these portions, and a belt and a tread successively superimposed on the crown portion outward in the radial direction of the tire; the tread having a plurality of rows of blocks defined by a plurality of circumferential grooves extending circumferentially of the tread and transverse grooves crossing the said circumferential grooves and extending substantially in parallel with each other; and the said belt being comprised of at least two main belt layers each containing inextensible cords arranged at a small cord angle with respect to an equator of the tire, the cords of which layers being crossed with each other, and an auxiliary belt layer formed by winding a rubberized ribbon ply containing a plurality of parallel arranged heat-shrinkable cords therein on the circumference of the main belt layers over substantially the full width while partially over-lapping said ribbon ply in the widthwise direction of the strip, characterized in that the overlapped amount of the ribbon ply of the auxiliary belt layer is made larger in a zone of the tread substantially corresponding to said row of blocks than in a zone of the tread substantially corresponding to said circumferential groove. The invention will be further described, by way of example only, with reference to the accompanying drawings, wherein: Fig. 1 is a developed view of an embodiment of the tread pattern in a first embodiment of a pneumatic radial tire according to the invention; Fig. 2 is a schematically section view of the pneumatic radial tire shown in Fig. 1; Fig. 3 is a partial perspective view of an embodiment of the ribbon ply constituting the auxiliary belt layer in the tire according to the invention; Figs. 4a and 4b are schematic views illustrating an overlapped state of the ribbon ply, respectively; and Fig. 5 is a schematically section view of a second embodiment of a pneumatic radial tire according to the invention. In Fig. 1 is shown a main part of a tread in the pneumatic radial tire according to the invention. The tread is divided into four rows in total of blocks 3 by two pairs of circumferential grooves 1a, 1b, which are arranged at both sides with respect to an equator of the tire (circumference at the center of tread width) so as to continuously extend along the circumference of the tread and substantially in parallel with each other at a certain distance, and transverse grooves 2a connecting the circumferential grooves 1a and 1b to each other and transverse grooves 2b connecting the circumferential grooves 1b to the tread edges T. Moreover, the central portion of the tread between the circumferential grooves 1a is divided into two ribs 5 by an auxiliary circumferential groove 4 located at the center of the tread and having a depth shallower than that of the circumferential groove and a width narrower than that of the circumferential groove. In the illustrated embodiment, the number of the circumferential grooves 1a, 1b is four in total, but it is preferable that the number of the circumferential grooves is 4 to 8 in total. Each of the transverse grooves 2a, 2b extends in a direction of converging toward the equator of the tire at an acute angle with respect to the circumferential groove 1a, 1b and has a width and a depth equal to or less than those of the circumferential groove. Moreover, each of the ribs 5 is provided with a notch 6 extending in a direction of converging toward the equator of the tire at a position corresponding to an extension line of the transverse groove 2a. The structure of the tire according to the invention is shown in Fig. 2. In Fig. 2, numeral 7 is a carcass, numeral 8 a belt, numeral 9 a tread, numeral 10 a bead core, and numeral 11 a bead filler composed of a hard rubber. The carcass 7 is a rubberized ply containing organic fiber cords such as polyester, rayon or nylon fiber cords arranged in a direction substantially perpendicular to the equator of the tire (radial direction), each end portion of which ply is wound around the bead core 10 from inside of the tire toward outside thereof to form a turnup portion. The belt 8 is comprised of at least two main belt layers 8a containing inextensible cords such as steel cords, aromatic polyamide fiber cords or the like arranged at a cord angle of 15-35° with respect to the equator of the tire, the cords of which layers being crossed with each other, and an auxiliary belt layer 8b arranged on the circumference of the main belt layers 8a over the full width. Moreover, a tread 9 is arranged on the belt 8. The auxiliary belt layer 8b is formed by spirally winding a ribbon ply, which is obtained by covering a plurality of heat-shrinkable cords 12 (3-14 nylon cords) arranged in parallel with each other with a rubber 13 as shown in Fig. 3, on the circumference of the main belt layers 8a over the full width while partially overlapping the ribbon ply in the widthwise direction thereof. Furthermore, the overlapped amount of the ribbon ply in the auxiliary belt layer is made larger in zones B and C of the tread 9 substantially corresponding to the rows of blocks 3 than in a zone A of the tread 9 substantially corresponding to the circumferential grooves 1a, 1b as shown, for example, in Figs. 4a and 4b, whereby the reinforcing rigidity per unit width of the auxiliary belt layer 8b is made larger in the zones B and C than in the zone A. Preferably, the rigidity in the zones B and C is larger by 20% than that in the zone A. The zone A corresponding to the circumferential grooves 1a, 1b is a zone having a width corresponding to 50-150% of an opening width of the circumferential groove (hereinafter referred to as groove width) around the center of the circumferential groove and extending along the circumferential groove, and the remaining zones are the zones B and C. Moreover, the reinforcing rigidity M (kgf/cm²) per unit width of the auxiliary belt layer is defined by the following equation: M = [(Fx50)/S] x n wherein F is a load (kgf) at 2% elongation of cord for the auxiliary belt layer taken out from the tire, S is a sectional area (cm²) of cord, and n is the number of cords per unit width. The reinforcing rigidity of each zone is represented by an average value of the reinforcing rigidity M calculated from the above equation in each other zones A, B and C. In the tire running at an extremely high speed, a very strong centrifugal force is applied as compared with the tire running at middle and low speeds, so that the tread protrudes outward in the radial direction of the tire. The protruding quantity is large in the blocks of the tread and small in the zone corresponding to the circumferential grooves, so that the ground contact pressure becomes unequal in the widthwise direction of the tread. According to the invention, the reinforcing rigidity per unit width of the auxiliary belt layer is changed in accordance with the above difference of the protruding quantity by adjusting the overlapped quantity of the ribbon ply arranged inside the tread in the radial direction and constituting the auxiliary belt layer, whereby the difference of the protruding quantity between block and groove is made small as far as possible. In other words, the rigidity per unit width of the auxiliary belt layer is made larger in the zones B and C corresponding to the block rows than in the zone A corresponding to the circumferential grooves, whereby a high modulus is given to the block rows to suppress the increase of protruding quantity through the centrifugal force, while the modulus of the circumferential groove having a small protruding quantity is made small. Thus, the difference of the protruding quantity between the block row and the circumferential groove is made small to make the protrusion of the tread uniform in the widthwise direction. Moreover, the protrusion of the block row through the centrifugal force tends to frequently occur in the blocks adjacent to the tread edge. In order to suppress such a protrusion, it is effective to wind the ribbon ply double at the zone C corresponding to a block row 9 adjacent to the tread edge T as shown in Fig. 5. The following examples are given in illustration of the invention and are not intended as limitations thereof. Example 1 A pneumatic radial test tire having a tire size of 255/40 ZR17 was prepared according to the tread pattern shown in Fig. 1 and the structure shown in Fig. 2. In this test tire, the circumferential groove had a width of 10 mm and a depth of 8 mm, and the auxiliary circumferential groove had a width of 5 mm and a depth of 6 mm, and the transverse groove had a width of 5 mm and a depth of 7 mm and converged at an angle of 80° toward the equator of the tire so that an angle between the circumferential groove 1a and the transverse groove 2a was 60° and an angle between the circumferential groove 1b and the transverse groove 2b was 70°. Outside the carcass were arranged two main belt layers each containing steel cords of 1x5 structure arranged at a cord angle of 20° with respect to the equator of the tire, and further the auxiliary belt layer was arranged on the circumference of the main belt layers over the full width by spirally winding a rubberized ribbon ply containing 11 nylon cords of 1260d/2 therein while partially overlapping the ribbon ply in the widthwise direction. In the formation of the auxiliary belt layer, the zone A beneath the circumferential groove had an overlapped quantity corresponding to 1/2 of the width of the ribbon ply as shown in Fig. 4a and the zones B and C corresponding to the block rows had an overlapped quantity corresponding to 2/3 of the width of the ribbon ply as shown in Fig. 4b, and consequently the reinforcing rigidity per unit width in the zone A was 4.0x10⁵ kgf/cm² and the reinforcing rigidity per unit width in the zones B and C was 8.0x10⁵ kgf/cm². Further, a comparative tire having the same tread pattern and structure as in the above test tire was prepared except that the reinforcing rigidity per unit width of the auxiliary layer was uniform (8.0x10⁵ kgf/cm²) over the full width as in the conventional technique. Tests for ground contacting property, steering stability and wear resistance were made with respect to these tires to obtain results as shown in Table 1. Moreover, the tire was mounted on a passenger car under an internal pressure of 2.5 kgf/cm² and actually run by a professional driver. These properties were evaluated by an index on the basis that the comparative tire was 100. The ground contact property was evaluated by measuring a distribution of ground contact pressure when the tire was run at a speed of 200 km/hr under a load of 500 kg. The steering stability was evaluated by measuring a lap time when the vehicle was run at a full throttle state on a circuit course of 2.5 km several times while including a feeling of the driver. The wear resistance was evaluated by measuring a worn amount after the tire was run on a test course of 6 km at a speed of 150 km/hr over a distance of 5000 km. Example 2 A pneumatic radial test tire having a tire size of 255/40 ZR17 was prepared according to the tread pattern shown in Fig. 1 and the structure shown in Fig. 5. In this test tire, the circumferential groove had a width of 14 mm and a depth of 8.5 mm, and the auxiliary circumferential groove had a width of 5 mm and a depth of 6 mm, and the transverse groove had a width of 4 mm and a depth of 7 mm. The other structure was the same as in Example 1 except for the auxiliary belt layer. The auxiliary belt layer was arranged on the circumference of the main belt layers over the full width by spirally winding a rubberized ribbon ply containing 11 nylon cords of 1260d/2 therein while partially overlapping the ribbon ply in the widthwise direction and partially winding double. In the formation of the auxiliary belt layer, the zone A beneath the circumferential groove had an overlapped quantity corresponding to 1/2 of the width of the ribbon ply as shown in Fig. 4a and the zones B and C corresponding to the block rows had an overlapped quantity corresponding to 2/3 of the width of the ribbon ply as shown in Fig. 4b. Moreover, the zone C had a double winding structure as shown In Fig. 5. Therefore, the reinforcing rigidity per unit width in the zone A was 4.0x10⁵ kgf/cm², the reinforcing rigidity per unit width in the zone B was 8.0x10⁵ kgf/cm² and the reinforcing rigidity per unit width in the zone C was 16.0x10⁵ kgf/cm². Further, a comparative tire having the same tread pattern and structure as in Figs. 1 and 2 was prepared except that the reinforcing rigidity per unit width of the auxiliary layer was uniform (8.0x10⁵ kgf/cm²) over the full width as in the conventional technique. Each of these tires was subjected to an internal pressure of 2.5 kgf/cm² and pushed onto a high-speed rotating drum under a load of 500 kg and run at an initial speed of 150 km/hr for 10 minutes and then the running speed was raised at a rate of 10 km/hr every 20 minutes until the tire was broken. As a result, the test tire was not broken even at the rising rate of 40 km/hr after the breakage of the comparative tire. Furthermore, the tire was mounted on a passenger car and run on a test course of 6 km at a speed of 150 km/hr over a distance of 5000 km. After the running, the difference of worn amount between central portion and end portion of the same block in the widthwise direction of the tread was measured. As a result, the wear difference was 2.5 mm in the comparative tire and 1.9 mm in the test tire, respectively. As mentioned above, according to the invention, the difference of protruding amount of the tread in widthwise direction can be made small as far as possible during the extremely high-speed running, so that the invention can provide pneumatic radial tires exhibiting a uniform ground contact pressure during the extremely high-speed running and having excellent steering stability and wear resistance.

Claims: A pneumatic radial tire comprising a cylindrical crown portion, a pair of sidewall portions extending from both ends of the crown portion inward in the radial direction of the tire, a radial carcass (7) extending from one of the sidewall portions through the crown portion to the other sidewall portion to reinforce the said portions, and a belt (8) and a tread (9) successively superimposed on the crown portion outward in the radial direction of the tire; the tread (9) having a plurality of rows of blocks (3) defined by a plurality of circumferential grooves (1a, 1b) extending circumferentially of the tread and transverse grooves (2a, 2b) crossing the said circumferential grooves and extending substantially in parallel with each other; and the said belt (8) being comprised of at least two main belt layers (8a) each containing inextensible cords arranged at a small cord angle with respect to an equator of the tire, the cords of which layers being crossed with each other, and an auxiliary belt layer (8b) formed by winding a rubberized ribbon ply containing a plurality of parallel arranged heat-shrinkable cords (12) therein on the circumference of the main belt layers (8a) over substantially the full width while partially overlapping said ribbon ply in the widthwise direction of the strip, characterized in that the overlapped amount of the ribbon ply of the auxiliary belt layer (8b) is made larger in a zone (B,C) of the tread substantially corresponding to said row of blocks (3) than in a zone (A) of the tread substantially corresponding to said circumferential groove (1a, 1b). A pneumatic radial tire as claimed in claim 1, characterized in that said transverse grooves (2a, 2b) extend to converge toward the equator of the tire at an acute angle with respect to said circumferential groove (1a, 1b). A pneumatic radial tire as claimed in claim 1 or 2, characterized in that said ribbon ply contains 3-14 nylon cords. A pneumatic radial tire as claimed in any of claims 1 to 3, characterized in that said zone (A) of the tread corresponding to said circumferential groove (1a, 1b) extends along the circumferential groove at a width corresponding to 50-150% of an opening width of the circumferential groove around the center thereof.

Inventors: KAWABATA MISAO, MATSUNAGA HIROO, TANI KATSUTOSHI, USHIKUBO HISAO, YAMAGUCHI YUTAKA, KAWABATA, MISAO, MATSUNAGA, HIROO, TANI, KATSUTOSHI, USHIKUBO, HISAO, YAMAGUCHI, YUTAKA
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1991
Main classification: B60C9
IPC codes: B60C11, B60C9