Heavy duty pneumatic tires

Abstract: A heavy duty pneumatic tire having excellent wet-running performances and wear resistance comprises a tread portion (1) provided at its surface with a plurality of main grooves (4a, 4b) arranged on both sides with respect to an equatorial plane (3) at a given interval in the circumferential direction of the tire and has a negative ratio of not more than 0.3. In this tire, the main grooves (4a, 4b) curvedly extend in a converging direction from each side end (2a, 2b) of the tread toward the equatorial plane (3) and terminate on the equatorial plane or in the vicinity thereof within 25% of a tread width (W) centering around the equatorial plane so as to meet the terminals (5a, 5b) of opposed main grooves (4a,4b) with each other or somewhat separate away from each other and have a particular inclination angle (α), and continuously projection lengths (L 1 , L 2 ) in the widthwise and circumferential directions.

Description: This invention relates to heavy duty pneumatic tires, and more particularly to a heavy duty pneumatic radial tire having a satisfactory wear resistance at a relatively small negative ratio and good wet-running performances by enhancing the arrangement efficiency of the tread grooves thereof. In general, main grooves extending circumferentially of the tire and various grooves such as auxiliary grooves extending in the widthwise direction of the tire are arranged in a tread portion of a tire, if necessary, in order to sufficiently develop wet-running performances such as traction and braking properties on wet road surfaces. In order to improve the wet-running performances, it is effective to make large a ratio of groove area occupied in a ground contact area of the tread portion or so-called negative ratio. Inversely, it is effective to make small the ratio of groove area occupied in a ground contact region of the tread portion in order to improve the wear resistance. That is, the requirements for wet-running performances and the wear resistance conflict with each other, so that it is difficult to simultaneously establish both properties. Attention is also drawn to EP-0625435A, document falling within the terms of Art. 54(3)EPC, which describes a pneumatic tire having slant main grooves 3 extending from positions adjacent to a circumferential rib 2 in a tread center zone 4 toward each tread end in opposite directions to each other at a relatively small angle with respect to the circumferential direction of the tire. Each of the slant grooves extends at an angle of not more than 40° to the circumferential direction of the tire, and slant auxiliary grooves 6 are formed so as to extend at an angle of not less than 50° with respect to the circumferential direction of the tire from the ends of the slant main grooves 3 to the tread end in each of tread side zones 5, and further the negative ratio of the tread 1 is maximized at the portion adjacent to the circumferential rib 2 and tends to be decreased towards the tread ends. It is an object of the present invention to provide a heavy duty pneumatic tire possessing sufficient wear resistance by setting the negative ratio to a relatively small value of not more than 0.3 and good wet-running performances by rationalizing the arrangement and shape of the main grooves. The present invention provides a heavy duty pneumatic tire comprising a tread portion provided at its surface with a plurality of main grooves arranged on both sides with respect to an equatorial plane at a given interval in the circumferential direction of the tire, in which the main grooves curvedly extend in a converging direction from each side end of the tread toward the equatorial plane and terminate on the equatorial plane or in the vicinity thereof so as to form a directional pattern, auxiliary grooves connecting adjacent main grooves on each side of said equatorial plane thereby forming a block pattern along said equatorial plane in which a terminal of a main groove lies within a said block, a circumferential groove arranged on each side of said equatorial plane in side regions of said tread portion, lateral grooves crossing said circumferential grooves to define in each side portion a row of blocks positioned between said circumferential groove and the side end of the tread portion, each main groove terminating at one of said blocks so as not to communicate with the side end of said tread portion, wherein a negative ratio of the tread portion is not more than 0.3, and the terminals of the main grooves are located within 25% of a tread width (W) centered around the equatorial plane, and a part of the main groove located in a zone ranging from the equatorial plane to at least 25% of the tread width (W) is such that an inclination angle (α) of a tangential line (m) drawn to said part with respect to the equatorial plane gradually increases in accordance with the separation from the equatorial plane, and a continuously projection length (L 1 ) of the main groove in the axial direction is not less than 30% of the tread width (W) and a continuously projection length (L 2 ) of the main groove in the circumferential direction is not less than 70% of the tread width (W). The term "tangential line (m) drawn to the part of the main groove" used herein means a tangential line drawn to a phantom line passing through a widthwise center of the main groove, and the terms "continuously projection lengths (L 1 and L 2 ) of the main groove in the axial direction and the circumferential directions" used herein mean lengths produced when the phantom line passing through the widthwise center of the main groove is projected in the axial direction and the circumferential direction, respectively. The invention will be further described with reference to the accompanying drawings, wherein: Fig. 1 is a partially developed view of a tread portion in a heavy duty pneumatic tire according to the invention; Fig. 2 is a graph showing braking performance at wet state plotted when the negative ratio is changed; Fig. 3 is a diagrammatic view showing a block pattern of a conventional tire; and Fig. 4 is a graph showing an index of braking performance when tires are braked at speed of 40 km/h and 60 km/h on wet road surface. In Fig. 1 is shown a part of a tread portion in a typical heavy duty pneumatic tire according to the invention, in which numeral 1 is a tread portion, numerals 2a and 2b side ends of the tread portion, numeral 3 an equatorial plane of the tire, numerals 4a and 4b main grooves, and numerals 5a and 5b terminals of the main grooves, respectively. In the tire having a tread pattern shown in Fig. 1, a plurality of main grooves 4a and 4b are arranged in the tread portion 1 on both sides with respect to the equatorial plane 3 at a given interval in the circumferential direction of the tire in which these main grooves curvedly extend in a converging direction from each side end 2a, 2b of the tread toward the equatorial plane 3 and terminate in the vicinity of the equatorial plane 3 such that the terminals 5a, 5b of opposed main grooves 4a, 4b are shifted from each other by a half pitch in the circumferential direction and are separated from each other at an equal distance from the equatorial plane 3 and form a directional pattern by gradually entering into a ground contact region of the tread portion from the terminal 5a or 5b toward the side end 2a or 2b. In addition to the illustrated embodiment, the terminals 5a and 5b of the opposed main grooves 4a and 4b may take various position relations. For example, these terminals 5a, 5b may be arranged in symmetry with each other with respect to the equatorial plane 3 without shifting them in the circumferential direction or meet with each other on the equatorial plane or in the vicinity thereof (i.e. the main grooves 4a, 4b may communicate with each other). Alternatively, the main grooves 4a, 4b may be terminated in the vicinity of the equatorial plane 3 so as to separate the terminals 5a, 5b from each other in the axial direction and/or the circumferential direction. In the tire according to the invention, the negative ratio is restricted to not more than 0.3, whereby good wear resistance can be ensured. According to the invention, the reason why the good drainage property is obtained even when the negative ratio is not more than 0.3 is based on the following. In general, when a tire is run on a wet road surface under loading, water existing on the road surface ahead the tire enters between the ground contact region of the tire and the ground to reduce the ground contact area of the tire tread directly contacting with the ground (hereinafter referred to as actual ground contact area), whereby slipping of the tire on the wet road surface is caused to degrade the wet-running performances. In tires having a conventional tread pattern such as rib pattern, block pattern or the like, water entered into the ground contact region is discharged through the grooves. In such a tire, however, the main groove are arranged along the circumferential direction of the tire, so that water entered into the ground contact region is pushed ahead of the tire along the arranging direction of the main groove and hence the pushed water is again treaded by the tire to decrease the actual ground contact area, whereby the wet-running performances are degraded and particularly the braking performance during running at higher speed is considerably degraded. In the heavy duty pneumatic tire according to the invention, however, the main grooves are formed so as to gradually enter into the ground contact region of the tire from the terminals toward the tread end and the terminals 5a, 5b of the main grooves 4a, 4b are located within 25% of the tread width W centered around the equatorial plane of the tire, and an inclination angle α of a tangential line m drawn to a part of the main groove in a zone ranging from the equatorial plane to at least 25% of the tread width W is gradually increased in accordance with the separation from the equatorial plane, preferably within a range of 5-25°, so that water entered into the ground contact region is pushed sideward from the tire along the arranging direction of the main groove and hence the pushed water is not again treaded by the tire, whereby decreasing of the actual ground contact area can be prevented to improve the wet-running performances. Furthermore, the continuously projection length L 1 of the main groove in the axial direction is made not less than 30% of the tread width W, and the continuously projection length L 2 of the main groove in the circumferential direction is made not less than 70% of the tread width W, whereby water entered into the ground contact region is efficiently discharged. When the continuously projection length L 1 is less than 30% and the continuously projection length L 2 is less than 70%, the drainage property is insufficient. Moreover, the negative ratio is preferably within a range of 0.25-0.29 whereby the wear resistance and the wet-running performances are most balanced. In Fig. 1, the tread portion 1 is provided with auxiliary grooves 6a-6e opening to the two main grooves and two to three blind sipes 7 formed in an end portion of a block portion facing the main groove. These auxiliary grooves and sipes may be provided, as necessary, because the arrangement of the auxiliary grooves 6a-6e is to improve the traction and braking performances at wet state and the arrangement of the blind sipes 7 is to make uniform the ground contact pressure of the block portion. Since the arrangement and shape of the main grooves in the tire according to the invention are rationalized as mentioned above, if the wet-running performances are equal to those of the conventional tire, the negative ratio can be not more than 0.3 as compared with that of the conventional tire, so that satisfactory wear resistance can be obtained. When the negative ratio is not more than 0.3, as seen from Fig. 2, the wet-running performances of a tire having a conventional block pattern considerably lower, while the degree of lowering the wet-running performances is small in the tire having the tread pattern according to the invention, so that the invention tire is recognized to be superior to the conventional tire. The following examples are given in illustration of the invention and are not intended as limitations thereof. · Invention tire A heavy duty pneumatic tire used in this example has a tread pattern shown in Fig. 1 and a tire size of 11R22.5, in which a plurality of main grooves 4a and 4b are arranged in a tread portion 1 on both sides with respect to an equatorial plane 3 at a given interval in the circumferential direction of the tire so as to curvedly extend in a converging direction from each side end 2a, 2b of the tread toward the equatorial plane 3 and terminate in the vicinity of the equatorial plane 3 such that the terminals 5a, 5b of opposed main grooves 4a, 4b are shifted from each other by a half pitch (50 mm) in the circumferential direction and are separated from each other at an equal distance (8 mm) from the equatorial plane 3 and form a directional pattern by gradually entering into a ground contact region of the tread portion from the terminal 5a or 5b toward the side end 2a or 2b. In this case, the negative ratio is 0.27. When a part of the main groove 4a, 4b is located in a zone ranging from the equatorial plane 3 to 25% of the tread width W, an inclination angle α of a tangential line m drawn to this part with respect to the equatorial plane 3 is gradually increased in accordance with the separation from the equatorial plane 3 so as to be 5° at the positions of the terminals 5a, 5b and 25° at the position corresponding to 25% of the tread width. Further, the continuously projection length L 1 of the main groove 4a, 4b in the axial direction is 48% of the tread width W, and the continuously projection length L 2 of the main groove 4a, 4b in the circumferential direction is 105% of the tread width W. The main groove has a groove depth of 16.5 mm and a groove width of 2.5 mm at a position of the terminal 5a or 5b and 7 mm at a position of the tread end 2a or 2b. Furthermore, the tread portion 1 is provided with auxiliary grooves 6a-6e opening to the two main grooves and two to three blind sipes 7 formed in an end portion of a block portion facing the main groove. Moreover, a carcass is comprised of a single rubberized radial cord ply containing steel cords therein, and a belt is comprised of four rubberized cord layers in which a cord angle with respect to the circumferential direction of the tire is 50° upward to the right in a first layer, 18° upward to the right in a second layer, 18° upward to the left in a third layer and 18° upward to the left in a fourth layer and cords of the second and third layers are crossed with each other. The other tire structures are substantially the same as in the conventional tire. · Comparative tire A tire used in this comparative example is substantially the same as in the above example except that a part of the main groove 4a, 4b located in a zone ranging from the equatorial plane 3 to 25% of the tread width W is extended straight from the equatorial plane 3 at a certain inclination angle such that the inclination angle α of the tangential line m drawn to this part with respect to the equatorial plane 3 is 15° at the positions of the terminals 5a, 5b and 15° at the position corresponding to 25% of the tread width. · Conventional tire A tire used in this conventional example is a tire having the conventional block pattern as shown in Fig 3, in which the negative ratio is 0.32%. ·Test Method and Results The wet-running performances and wear resistance are evaluated with respect to the above tires as follows. Each of the tires to be tested is run on a wet road surface and braked at a speed of 40 km/h or 60 km/h, during which a running distance until stopping of a vehicle is measured. The wet-running performances are evaluated by a reciprocal of the running distance as an index of braking performance at wet state on the basis that the comparative tire is 100, in which the larger the index value, the better the wet-running performances. The measured results are shown in Fig. 4. As seen from Fig. 4, the invention tire has improved wet-running performances at both brake-starting speeds as compared with the comparative tire. Then, each of the tires to be tested is actually run on a road over a distance of 80,000 km and thereafter a worn amount of the tread rubber is measured, from which a running distance (km) per 1 mm of worn amount is calculated. The wear resistance is evaluated by an index of such a running distance on the basis that the conventional tire is 100, in which the larger the index value, the better the wear resistance. As a result, it was found that the index value of the comparative tire is 120 and that of the invention tire is 133, from which it is apparent that the invention tire has considerably improved wear resistance. As mentioned above, according to the invention, water entered into the ground contact region is pushed sideward from the tire along the arranging direction of the main grooves and hence the pushed water is not again treaded by the tire, in contrast to the conventional tire, whereby the decrease of the actual ground contact area can be prevented to improve the wet-running performances. Furthermore, even when the negative ratio is made sufficiently small as compared with the conventional tire, sufficient wet-running performances are obtained together with good wear resistance. Thus, the invention can provide heavy duty pneumatic tires simultaneously satisfying wet-running performances and wear resistance, which have never previously been attained in the tires having the conventional tread pattern.

Claims: A heavy duty pneumatic tire comprising a tread portion (1) provided at its surface with a plurality of main grooves (4a,4b) arranged on both sides with respect to an equatorial plane (3) at a given interval in the circumferential direction of the tire, in which the main grooves (4a,4b) curvedly extend in a converging direction from each side end (2a,2b) of the tread toward the equatorial plane (3) and terminate on the equatorial plane or in the vicinity thereof so as to form a directional pattern, auxiliary grooves (6) connecting adjacent main grooves (4a,4b) on each side of said equatorial plane (3) thereby forming a block pattern along said equatorial plane in which a terminal (5a,5b) of a main groove (4a,4b) lies within a said block, a circumferential groove arranged on each side of said equatorial plane in side regions of said tread portion (1), lateral grooves crossing said circumferential grooves to define in each side portion a row of blocks positioned between said circumferential groove and the side end (2a,2b) of the tread portion, each main groove (4a,4b) terminating at one of said blocks so as not to communicate with the side end (2a,2b) of said tread portion, wherein a negative ratio of the tread portion is not more than 0.3, and the terminals (5a,5b) of the main grooves (4a,4b) are located within 25% of a tread width (W) centered around the equatorial plane (3), and a part of the main groove (4a,4b) located in a zone ranging from the equatorial plane (3) to at least 25% of the tread width (W) is such that an inclination angle (α) of a tangential line (m) drawn to said part with respect to the equatorial plane gradually increases in accordance with the separation from the equatorial plane, and a continuously projection length (L 1 ) of the main groove (4a,4b) in the axial direction is not less than 30% of the tread width (W), and a continuously projection length (L 2 ) of the main groove (4a,4b) in the circumferential direction is not less than 70% of the tread width (W). A pneumatic tire as claimed in claim 1, characterized in that the negative ratio is within a range of 0.25- 0.29. A pneumatic tire as claimed in claim 1 or 2, characterized in that the inclination angle (α) is within a range of 5 - 25%. A pneumatic radial tire as claimed in any of claims 1 to 3, characterized in that said main grooves (4a,4b) terminate in the vicinity of said equatorial plane (3) and end points thereof are circumferentially staggered on opposite sides of said equatorial plane. A pneumatic tire as claimed in claim 4, characterized in that said end points are shifted by a half pitch in said circumferential direction. A pneumatic radial tire as claimed in claim 4, characterized in that said end points are at equal distances from said equatorial plane.

Inventors: ICHIKI YASUFUMI, ICHIKI, YASUFUMI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1995
Main classification: B60C11
IPC codes: B60C11