HEAVY DUTY PNEUMATIC RADIAL TIRES

Abstract: The uneven wear produced in a tread side rib in a heavy duty pneumatic radial tire (11) is prevented or reduced by arranging a peripheral fine groove (18) in a particular region of the side face (12b) of the tread portion (12) in a particular form. The fine groove has an opening (18a) of width corresponding to 5-75% of the width of a main groove (16) of the tread, and extends inward from a position (A) to a position defined by certain axial and radial distances (B,C).

Description: This invention relates to heavy duty pneumatic radial tires, and more particularly to a heavy duty pneumatic radial tire which effectively prevents the occurrence of uneven wear at shoulder ribs in a tread of the tire. As the conventional heavy duty pneumatic radial tire, there is, for example, a tire shown in Fig. 6. Such a heavy duty pneumatic radial tire 1 comprises a tread 2 contacting with ground, a belt 3 composed of plural belt layers containing steel cords therein for the reinforcement of the tread 2, and a carcass 6 located inside the belt 3 and extending between a pair of bead cores 5. In the tread 2 of the heavy duty pneumatic radial tire 1, both end portions 3a of the belt 3 reinforce both shoulder portions 2a of the tread 2, while the central portion 3b of the belt 3 reinforces the central portion 2b of the tread 2. However, the rigidity of the shoulder portion 2a in the tread 2 is smaller than that of the central portion 2b. Furthermore, there is a dragging phenomenon based on the difference in the size between the central portion 2b and the shoulder portion 2a in the tread 2. Therefore, as shown in Fig. 6, the shoulder portion 2a of the tread 2 is prematurely worn to generate a stepwisely uneven wear or a so-called shoulder wear. In some occasions, the outer surface of the shoulder portion 2a in the tread 2 is wavily worn in the circumferential direction (not shown), which propagates toward the central portion of the tread to generate the uneven wear called as a wavy wear. In order to prevent the occurrence of these uneven wears, there are proposed the increase of the rigidity in the island part 8 of the shoulder portion 2a of the tread 2, the enlargement of crown radius R of the tread 2, the arrangement of many sipes on full surface 8a of the island part 8 of the shoulder portion 2a in the axial direction, and the like. Among these proposals for the prevention of uneven wear, the increase of the rigidity in the island part 8 of the shoulder portion 2a is attained by increasing the width in widthwise direction of the island part 8 of the shoulder portion 2a. In this case, the effect of preventing the shoulder wear is recognized to a certain extent, but when lateral force applied to the tread end 2c becomes severer, the worn portion having a width W₀ and a stepwise difference t₀ or a so-called edging wear 9 (see Fig. 6) is produced in the tread end 2c, which undesirably progresses into the shoulder wear or wavy wear, so that the sufficient restraining effect is not obtained. On the other hand, when the crown radius R of the tread 2 is made large in order to reduce the difference in the radius between the shoulder portion 2a and the central portion 2b in the tread 2, the dragging of the shoulder portion 2a of the tread 2 is prevented during the running to prevent the occurrence of uneven wear. In this case, however, the ground contact pressure in the central portion of the tread lowers to generate the uneven wear in the central portion. Moreover, when many sipes are arranged on the full surface of the island part of the shoulder portion in the tread in the axial direction, if the tire is used under severer use conditions, particularly a heavy load condition, a small block defined between the adjoining sipes is worn before and after the sipe in the rotational direction of the tire to cause a so-called heel and toe wear. A tire according to the preamble of claim 1 is known, e.g. from DE-A-1480934. It is, therefore, an object of the invention to provide a heavy duty pneumatic radial tire having a specific tread form, particularly a specific form of both side regions of the tread which reduces the ground contact pressure and effectively prevent the occurrence of uneven wear even under severer use conditions by arranging a peripheral fine groove opening to the side face of the ground contact portion of the tread in a specific form at a specific region and further to provide a heavy duty pneumatic radial tire not causing troubles such as crack, tear and the like in the wall portion of the fine groove even when the running distance is largely increased. This is achieved by the features of claim 1. The present invention provides a pneumatic radial tire comprising a tread having a tread portion for contacting the ground and provided with a plurality of circumferential main grooves, a fine groove having an opening with a fine-groove width and substantially continuously extending in the peripheral direction of the tire being arranged in at least one side face of said tread portion, and said fine groove starting from a starting position A located radially inward from the effective ground contact end of said tread portion along the outer profile of said side face in the radial direction and curvedly extends inward a distance B from said ground contact end in the axial direction and inward a distance C from said ground contact end in the radial direction, characterized in that: a) said fine-groove width corresponds to 5-75% of the width of said main groove; b) said starting position A corresponds to 0-150% of the depth of said main groove; c) said distance B corresponds to 0.5-10% of the ground contact width of said tread; and d) said distance C corresponds to 20-180% of the depth of said main groove. In a preferred embodiment of the invention, the groove wall of said fine groove is covered with a rubber layer having cracking resistance and ozone resistance at a thickness corresponding to a depth of at least 0.5 mm measured from the groove wall surface. In another preferred embodiment of the invention, the fine groove has at least one bent portion. The reason why the groove wall portion of the fine groove is covered with the rubber layer having a particular thickness is due to the fact that as the running distance of the heavy duty pneumatic radial tire according to the invention largely increases, troubles such as ozone cracking, tearing or the like may be caused in the groove bottom and groove wall portion of the fine groove. Further, it is preferable that the rubber layer is large in the content of antioxidant as compared with the tread rubber in the central portion of the tread. The heavy duty pneumatic radial tires according to the invention are preferable to satisfy the following: that is, the fine groove has a groove depth corresponding to 20-150%, preferably 60-100% of the depth of the main groove. Further, the bent portion of the fine groove is preferable to be a dogleg-like form in section in the depthwise direction of the fine groove. Particularly, the bent portion of the fine groove has a dogleg-like form bending so as to focus each side of the fine groove into the central portion of the tire. Moreover, the fine groove consists of an upper groove portion and a lower groove portion connecting thereto through a bent portion, wherein the length of the upper groove portion is approximately equal to that of the lower groove portion. The width of the opening in the fine groove is within a range of 5-75%, preferably 30-45%. When the opening width is less than 5%, the opposed groove wall portions of the fine groove contact with each other under a slight loading during the running and the reduction of ground contact pressure in the vicinity of the ground contact end is insufficient, while when it exceeds 75%, the movement of outer blocks in the radial direction becomes too large as compared with the fine groove and the cracking or other trouble occurs in the inside of the fine groove. The starting position of the fine groove is a position corresponding to 0-150%, preferably 25-65% of the depth of the main groove located inward from an effective ground contact end of the tread portion along an outer profile of the side face in the radial direction. When the starting position does not reach 0% or the starting position of the fine groove is a tread portion of the tread contacting with ground, the tear trouble occurs, while when it exceeds 150%, the effect of reducing the ground contact pressure at both side end parts of the tread portion is insufficient. Further, the fine groove extends inward from the ground contact end in the axial direction to a position corresponding to 0.5-10%, preferably 1.0-5.0% of a ground contact width of the tread and inward from the ground contact end in the radial direction to a position corresponding to 20-180%, preferably 70-130% of the depth of the main groove. When the extending position is less than 0.5% inward in the axial direction, the reduction of the ground contact pressure at both side end parts of the tread portion is insufficient, while when it exceeds 10%, the ground contact pressure of the tread shoulder portion lowers as a whole and the ground contact pressure of the ground contact end becomes relatively high. On the other hand, when the extending position of the fine groove is less than 20% inward in the radial direction, the reduction of the ground contact pressure at both side end parts of the tread portion is insufficient, while when it exceeds 180%, the rigidity of the tread shoulder portion lowers and the troubles such as tear and the like occur. The reason why the fine groove has at least one bent portion is based on the fact that the presence of the bent portion disperses strain concentrated in the bottom of the fine groove to prevent the occurrence of cracking and tearing in the groove bottom. Further, the reason why the thickness of the rubber layer covering the groove wall face of the fine groove is limited to at least 0.5 mm from the groove wall surface is due to the fact that when it is less than 0.5 mm, the effect by the arrangement of the rubber layer is lost. Preferably, the thickness of the rubber layer is within a range of 0.5-5.0 mm. Since the rubber layer is applied to the side face of the tread portion of the tread, the upper limit of the thickness is 5.0 mm from viewpoints of the improvement of the processability and the suppression of the production cost. In the heavy duty pneumatic radial tire of the invention, the peripheral fine groove opening to the side face of the tread portion is arranged in the predetermined region at given groove width, groove depth and form, so that the rigidity and ground contact pressure at both side end parts of the tread portion lower. Therefore, the stress concentration in the ground contact end is prevented and also the premature occurrence of edging wear is restrained to prevent the occurrence of uneven wear at the ground contact end. In the heavy duty pneumatic radial tire of the invention, the fine groove as defined in the first invention the groove wall face of the fine groove is covered with the particular rubber layer having the specified thickness, so that even when the repetitive strain is applied to the groove wall face of the fine groove as the running distance of the tire increases, there is caused no ozone crack or tear trouble. The invention will be described with reference to the accompanying drawings, wherein: Fig. 1 is a partially sectional view of a first embodiment of the heavy duty pneumatic radial tire according to the invention; Fig. 2 is an enlargedly sectional view of a main part of Fig. 1; Fig. 3 is a partially sectional view of a second embodiment of the heavy duty pneumatic radial tire according to the invention; Fig. 4 is a partially sectional view of a third embodiment of the heavy duty pneumatic radial tire according to the invention; Fig. 5 is an enlargedly sectional view of a main part of Fig. 4; and Fig. 6 is a partially sectional view of the conventional tire. In Fig. 1 is shown a first embodiment of the heavy duty pneumatic radial tire according to the invention, the construction of which will be described below. In Fig. 1, numeral 11 is a heavy duty pneumatic radial tire having a tire size of 11R22.5 14PR, which is used on a rim of 8.25×22.5 under an internal pressure of 7.25 kg/cm². The tire 11 comprises a tread 12 having a tread portion 12a contacting with ground, a belt 13 reinforcing the tread 12 and composed of plural belt layers containing steel cords therein, and a carcass 14 located inside the belt 13 and extending between a pair of bead cores. The tread portion 12a of the tread 12 is divided into five island portions 17 by four circumferential main grooves 16 in the axial direction of the tire. The effective ground contact width W₁₂ of the tread is 208 mm, and the width W₁₆ and depth D₁₆ of the main groove 16 are 13.5 mm and 14.7 mm, respectively. The tread 12 has a fine groove 18 having an opening 18a at each side face 12b of the tread portion 12a in the axial direction thereof and substantially continuously extending in the peripheral direction thereof, a main part of which is enlargedly shown in Fig. 2. The width W₁₈ of the opening 18a in the fine groove 18 is 4.5 mm, which is 33.3% of the width W₁₆ of the main groove 16. The fine groove 18 starts form a position A of 4 mm i.e. a distance L A corresponding to 27.2% of the depth D₁₆ of the main groove 16 located inward from an effective ground contact end 20 of the tread portion 12a in the tread 12 along an outer profile of the side face 12b in the radial direction. Further, the fine groove 18 consists of an upper groove segment 18A, a bent segment 18B and a lower groove segment 18C in the section of the tire which extend inward in the axial direction and inward in the radial direction, wherein the upper groove segment 18A and the lower groove segment 18C are bent in form of dogleg through the bent segment 18B. Moreover, the acute angle ϑ₁ between a center line N 18A of the upper groove Segment 18A and a line I perpendicular to the rotating axis of the tire is 60°, while the acute angle ϑ₂ between a center line N 18C of the lower groove segment 18C and the line I perpendicular to the rotating axis of the tire is -15° (provided that the outside of the line I in the axial direction is + and the inside of the line I in the axial direction is -). The angle of the bent segment 18B with respect to the line I gradually changes with the curvature of center line N 18B thereof. In this case, it is preferable that the angle ϑ₁ is relatively large and the angle ϑ₂ is relatively small. Particularly, the angle ϑ₁ is within a range of 0-140°, preferably 35-80°, while the angle ϑ₂ is within a range of +90°∼-90°, preferably -20°∼+40°. The fine groove 18 starts from the position A and extends to a position of 5.5 mm or a distance B corresponding to 2.6% of the effective ground contact width W₁₂ of the tread 12 located inward from the effective ground contact end 20 in the axial direction and to a position of 14.7 mm or a distance C corresponding to 100% of the depth D₁₆ of the main groove 16 located inward from the effective ground contact end 20 in the radial direction. The width W₁₈ of the fine groove 18 gradually reduces from the opening width of 4.5 mm as the depth of the fine groove becomes deeper, and the minimum width W 18M of the fine groove 18 is 2.0 mm above a bottom 18D thereof. The groove bottom 18D takes a flask form considering the cracking or the like. The depth D₁₈ of the fine groove 18 is a length extending from the opening to the groove bottom 18D along center lines N 18A to N 18C of the fine groove 18, which is 13.5 mm and corresponds to 92% of the depth D₁₆ of the main groove 16. Furthermore, the length of the upper groove segment 18A in the fine groove 18 is approximately equal to that of the lower groove segment 18C. As shown in Fig. 1, a concave portion 22 extending peripherally of the tire may be arranged in the side face 12b of the tread 12 inward from the fine groove 18 in the radial direction. In the illustrated tire, the fine groove 18 is arranged in each side end part 12c of the tread portion 12a during the running, so that the ground contact pressure at both side end parts reduces to prevent the stress concentration in the ground contact end 20 and suppress the occurrence of uneven wear. In Fig. 3 is shown a second embodiment of the heavy duty pneumatic radial tire according to the invention, which has the same construction as in the first embodiment except for the following point. That is, in the tire 31, the fine groove 18 consists of the upper groove segment 18A and the lower groove segment 18C curvedly extending inward in the axial and radial directions so that the angle ϑ₁ between the center line N 18A of the upper groove segment 18A and the line I perpendicular to the rotating axis (+60°) and the angle ϑ₂ between the center line N 18C of the lower groove segment 18C and the line I (+18°C) are outside of the line I in the axial direction. In Figs. 4 and 5 is shown a third embodiment of the heavy duty pneumatic radial tire according to the invention, which has the same construction as in the second embodiment except for the following point. That is, in the tire 41, the groove wall portion 18F at the groove bottom 18D and the side wall 18E of the fine groove 18 is composed of a rubber layer 43 having a thickness D₄₃ of 2 mm as measured from the groove wall surface. As shown in Figs. 4 and 5, the side face 12c of the tread portion 12a is also covered with the rubber layer 43, which is due to the restriction in the formation of the rubber layer. The rubber layer 43 contains an antioxidant for ozone resistance, an amount of which is larger than that contained in the central portion of the tread 12. In the illustrated tire 41, the groove wall portion 18F of the fine groove 18 is covered with the rubber layer 43, so that even when the groove bottom 18D and side wall 18E of the fine groove 18 are repeatedly subjected to bending as the running distance increases, there is caused no cracking through ozone, nor cracking and tear troubles due to bending fatigue. Then, the effect of the invention will be described by using the following four test tires (Test Examples 1-3 and Comparative Example). The tire used in Test Example 1 was the same as shown in Fig. 1, and the tire used in Test Example 2 was the same as shown in Fig. 3, and the tire used in Test Example 3 was the same as shown in Fig. 4, while the tire used in Comparative Example was a tire having no fine groove 18 and concave portion 22, which corresponded to the conventional tire shown in Fig. 6. Each of these tires was mounted on a normal rim and inflated under a normal internal pressure, which was then mounted on a test vehicle and run on general road under a normal load over a distance of 200,000 km. In this case, the occurrence state of uneven wear in the tread and the occurrence state of crack or tear trouble on the surface of the fine groove were observed after the running over a distance of 100,000 km or 200,000 km. Moreover, the occurrence state of uneven wear was evaluated by measuring the edging wear produced at the tread end as a width W₀ and a stepwise difference t₀ of the uneven wear as shown in Fig. 6. The measured results are shown in the following Table 1, wherein the upper part of Table 1 shows the results after the running over a distance of 100,000 km and the lower part thereof shows the results after the running over a distance of 200,000 km. As shown in Table 1, in the measurement after the running over 100,000 km, the uneven wear was generated at a width W₀ of 1.0-3.0 mm and a stepwise difference t₀ of 2-3 mm in the tire of Comparative Example, while no uneven wear was generated in the tires of Test Examples 1-3 (each of the width W₀ and stepwise difference t₀ was zero). Further, the crack or tear trouble on the surface of the fine groove was not observed in the tires of Test Examples 1-3. On the other hand, in the measurement after the running over 200,000 km, the uneven wear in the tire of Comparative Example had a width W₀ of 1.8-4.2 mm and a stepwise difference t₀ of 5-6 mm, while the occurrence of uneven wear was not observed in the tires of Test Examples 1-3. Moreover, the crack or tear trouble on the surface of the fine groove was observed in the tires of Test Examples 1 and 2, but it was not observed in the tire of Test Example 3. As mentioned above, according to the invention, the peripheral fine groove opening to the side face of the tread portion in the tread is arranged on the particular region of the side face in a particular form, so that the ground contact pressure is reduced at the tread end part and the occurrence of uneven wear at both end parts of the tread can effectively be prevented even under severer use conditions. Further, according to the embodiment of the invention according to Fig. 4 and 5, even when the running distance is increased, the occurrence of ozone cracking and tear troubles on the surface of the fine groove can be prevented.

Claims: A pneumatic radial tire (11) comprising a tread (12) having a tread portion (12a) for contacting the ground and provided with a plurality of circumferential main grooves (16), a fine groove (18) having an opening (18a) with a fine-groove width (W₁₈) and substantially continuously extending in the peripheral direction of the tire being arranged in at least one side face (12b) of said tread portion, and said fine groove (18) starting from a starting position A located radially inward from the effective ground contact end (20) of said tread portion along the outer profile of said side face in the radial direction and curvedly extends inward a distance B from said ground contact end (20) in the axial direction and inward a distance C from said ground contact end (20) in the radial direction, characterized in that: a) said fine-groove width (W₁₈) corresponds to 5-75% of the width (W₁₆) of said main groove (16); b) said starting position A corresponds to 0-150% of the depth (D₁₆) of said main groove (16); c) said distance B corresponds to 0.5-10% of the ground contact width (W₁₂) of said tread; and d) said distance C corresponds to 20-180% of the depth (D₁₆) of said main groove (16). A tire as claimed in claim 1, characterized in that said fine groove (18) has at least one bent portion (18B). A tire as claimed in claim 1 or 2, characterized in that the groove wall of said fine groove (18) is covered with a rubber layer (43) having cracking resistance and ozone resistance at a thickness corresponding to a depth of at least 0.5 mm measured from the groove wall surface.

Inventors: ENOKI, YASUTAKA, MAKINO, SHIGEO, MURASE, SHINJI
Applicants: BRIDGESTONE CORPORATION
Application year: 1988
Main classification: B60C11
IPC codes: B60C11, B60C9