AUSTRALIA Patents Act 1990 THE YOKOHAMA RUBBER CO., LTD. COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Pneumatic tire The following statement is a full description of this invention including the best method of performing it known to us:- PNEUMATIC TIRE Background of the Invention The present invention relates to a pneumatic tire preferably used for 5 off-road driving, and particularly to a pneumatic tire with which reduction of pattern noise is achieved while the off-road running performance is maintained. Conventionally, it has been required that a pneumatic tire to be mounted on a four-wheel drive vehicle be excellent in the off-road running 10 performance typified by the on-mud (muddy terrain) running performance, the on-snow running performance and the on-sand (sandy terrain) running performance. In such a pneumatic tire, the on-mud running performance and the on-snow running performance are secured by increasing the ratio of the area of grooves to that of the tread portion (hereinafter referred to as 15 the groove area ratio) while the on-sand running performance is secured by increasing the length of a block in the circumferential direction of the tire. However, the increase in the groove area ratio leads to deterioration in noise performance. Above all, the tread pattern noise is increased (see Patent Document 1, for example). 2 0 Specifically, the increase in the groove area ratio increases the impact sound at the time when a block comes into contact with the ground. In addition, the increase in the groove area ratio increases the pumping noise, which is generated when air compressed between the tread portion and the road surface flows out through lug grooves, and also increases the 25 air column resonance noise in a main groove, which is excited by the pumping noise. For this reason, it is extremely difficult to reduce the pattern noise while maintaining the off-road running performance. [Patent Document 1] Japanese Patent Application Kokai Publication No. Hei 11-245625 30 1 A 2 Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general 5 knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion 10 of any other element, integer or step, or group of elements, integers or steps. Summary of the Invention An embodiment of the present invention provides a pneumatic tire with which reduction of pattern noise is achieved while maintaining the off-road running 15 performance. According to an aspect of the invention, there is provided a pneumatic tire which includes a land portion and block rows. The land portion extends continuously in the circumferential direction of the tire along the center line of a tread portion. Each of the block rows includes a plurality of blocks defined by two 20 main grooves and a plurality of lug grooves. The two main grooves, each with a zigzag shape, extend in the tire circumferential direction on each side of the land portion. Each of the lug grooves connects these main grooves to each other. Each of the blocks of the block rows is divided into a center-side block portion and an outer-side block portion by a narrow groove. The narrow groove is inclined at an 25 inclination angle of 5* to 120 with respect to the tire circumferential direction, and has a groove width of 2% to 4% of the tread contact width. Moreover, each of the center-side block portions and the adjacent one of the outer-side block portions are disposed, in a manner that these block portions are displaced relative to each other, along a direction in which the narrow groove extends, respectively towards the 30 shorter block sides of respective block portions, which sides face the corresponding lug grooves.
3 rows is divided into the center-side block portion and the outer-side block portion by the narrow groove. The block rows are located respectively in portions, where vibration is likely to occur, on both sides of the land portion along the center line of the tread portion while the narrow groove is inclined slightly with respect to the tire 5 circumferential direction. Concurrently, each of the center-side block portions and the adjacent one of the outer-side block portions are disposed, in a manner that these block portions are displaced relative to each other along the direction in which the narrow groove extends. This configuration reduces impact sounds generated when these center-side and outer-side block portions come into contact with the ground, 10 and also makes the impact sounds be generated by respective side portions at different timings. As a result, noise due to the impact sounds can be reduced. In particular, it is possible to effectively reduce impact sounds generated when a pair of the center-side and outer-side block portions come into contact with the ground. This is because the portion that comes first into contact with the ground has a shorter 15 length of a block side than that the portion that comes subsequently into contact with the ground has. On the other hand, the groove area ratio of the tread portion is set sufficiently large with the main grooves, each of which has a zigzag shape, the lug grooves and the narrow grooves. This makes it possible to achieve a favorable off-road running performance. 20 It is preferable that the amount of the relative displacement of the center-side and outer-side block portions be 20% to 40% of the length of a block side facing the narrow groove. Thereby, it is possible to achieve noise reduction effect by dispersing impact sounds, while preventing uneven wear from occurring. In addition, it is also preferable that the length of the shorter block side Ls 25 of each of the center-side and outer-side block portions be 65% to 85% of the length of the longer block side Ll of the corresponding block portions, the shorter and longer block sides facing the corresponding lug grooves, respectively. With this configuration, it is possible to achieve the noise reduction effect by reducing impact sounds, while preventing uneven wear from occurring. 30 Moreover, it is also preferable that at least three sipes be provided to each of the center-side and outer-side block portions. By providing the sipes, it is 4 possible to improve the on-snow running performance and the on-ice running performance. Furthermore, it is also preferable that each of the center-side block portions and the adjacent one of the outer-side block portions be formed so as to be 5 symmetric to each other about the middle point of the corresponding narrow groove. In the case of employing a symmetric shape for each pair of the center-side and outer-side block portions, it is possible to effectively improve the noise performance, regardless of the direction in which the tire rotates. In an embodiment of the present invention, the groove width of each of the 10 narrow grooves is set at 2% to 4% of the tread contact width. In addition to that, it is preferable that the groove width of each of the main grooves be 4.5% to 8.0% of the tread contact width, and that the groove width of each of the lug grooves be 4.0% to 10.0% of the tread contact width. Moreover, it is also preferable that the groove width of each of the sipes be not more than 1.5 mm. Here, the tread contact 15 width means the width, in the axial direction of the tire, of the portion, where the tire is in contact with a flat surface. The width is measured when the tread portion of the tire is brought into contact with the flat surface under a condition for measuring a static loaded radius defined by a standard, which is applied to tires, of JATMA, TRA or ETRTO. 20 Brief Description of the Drawings Fig. I is an expanded view showing a tread pattern of a pneumatic tire according to an embodiment of the present invention. Fig. 2 is a plan view showing a block row between main grooves in the 25 tread pattern of Fig. 1, in an enlarged manner. Detailed Description of the Preferred Embodiments Descriptions will be given below in detail of the configuration of the present invention with reference to the accompanying drawings. 30 Fig. I shows a tread pattern of a pneumatic tire according to an embodiment of the present invention. As shown in Fig. 1, a rib-like land portion 2 is disposed in a tread portion 1, and continuously extends in the circumferential direction of the tire along the center line CL of the tread portion 1. An inner main groove 3 and an outer main groove 4 are provided 5 on each side of the land portion 2. The inner and outer main grooves 3 and 4, each with a zigzag shape, extend in the tire circumferential direction. Between the inner and outer main grooves 3 and 4 on each side, a plurality of lug grooves 5 are provided in a manner that each of the lug grooves 5 connects the main grooves 3 and 4 to each other. A block row 60, which is 10 formed of a plurality of blocks 6, is defined by the main grooves 3 and 4 and the lug grooves 5 on each side. In addition, a plurality of lug grooves 7 are provided to a region on a shoulder side of the outer main groove 4 on each side. A block row 80 is formed of a plurality of blocks 8. Each of the blocks 8 are defined by the main groove 4 and the lug grooves 7. It should 15 be noted that each of the lug grooves 5 is inclined in one direction with respect to the tire circumferential direction on both sides of the land portion 2. Concurrently, each of the lug grooves 7 is inclined in another direction with respect to the tire circumferential direction on both sides of the land portion 2. That is, in the pneumatic tire, an asymmetric pattern is formed, 20 which does not determine the direction in which the tire is mounted. Each of the blocks 6 of the block rows 60 is divided into a center-side block portion 6a and an outer-side block portion 6b by a narrow groove 9. The narrow groove 9 is inclined at an inclination angle a of 5* to 120 with respect to the tire circumferential direction, and has a groove width W of 2% 25 to 4% of the tread contact width TCW (see Fig. 2). Fig. 2 shows the block row between the main grooves in an enlarged manner. As shown in Fig. 2, each of the narrow grooves 9 is inclined at the inclination angle a. Accordingly, in each of the center-side and outer-side block portions 6a and 6b, one of two block sides that face the corresponding 3 0 lug grooves 5 is longer than the other one of the two block sides. That is, 5 the length Ls of the shorter block side is shorter than the length Li of the longer block side. In addition, each of the center-side block portions 6a and the adjacent one of the outer-side block portions 6b are disposed, in a manner that these block portions are displaced relative to each other, along 5 a direction in which the narrow groove extends, respectively towards the shorter block sides of the corresponding block. In the above-described pneumatic tire, each of the blocks 6 of the block rows 60 is divided into the center-side block portion 6a and the outer-side block portion 6b by the narrow groove. The block rows 60 are 10 located respectively in portions where vibration is likely to occur, and the narrow groove 9 is slightly inclined with respect to the tire circumferential direction. In addition, each one of the center-side block portion 6a and the adjacent one of the outer-side block portion 6b are disposed, in a manner that these block portions are displaced relative to each other along the 15 direction in which the narrow groove 9 extends. Thereby, it is possible to reduce impact sounds generated when these center-side block portion 6a and outer-side block portion 6b come into contact with the ground. At the same time, it is also possible to make the impact sounds be generated at different timings. As a result, noise due to the impact sounds can be 20 reduced. In particular, it is possible to effectively reduce impact sounds when a pair of the center-side block portion 6a and the outer-side block portion 6b comes into contact with the ground. This is because, the length Ls of the block side in a portion which first comes into contact with the ground is relatively small, while the length Llof the block side in a portion 2 5 which subsequently comes into contact with the ground is relatively large. On the other hand, the groove area ratio of the tread portion 1 is set sufficiently large with the main grooves 3 and 4, each having a zigzag shape, the lug grooves 5 and 7, and the narrow grooves 9. As a result, it is possible to achieve a favorable off-road running performance. 3 0 Here, if the inclination angle a of each narrow groove 9 is less than 6 5, an effective noise reduction effect is not obtained since the difference between the block lengths Ls and Ll becomes small. On the other hand, if the inclination angle a is more than 12*, the on-sand running performance is deteriorated. In addition, if the groove width W of each narrow groove 9 5 is less than 2% of the tread contact width TCW, the narrow grooves 9 is likely to be clogged with mud, or the like. Accordingly, the favorable off-road running performance cannot be obtained. On the other hand, if the groove is more than 4% of the tread contact width TCW, the noise reduction performance would be adversely affected. 10 As shown in Fig. 2, it is preferable that the amount S of the relative displacement of the center-side and outer-side block portions 6a and 6b be 20% to 40% of the length L of a block side of these block portions, which side faces the narrow groove 9. If the amount S of the relative displacement is less than 20% of the length L of the block side, the noise reduction effect 15 achieved by dispersing impact sounds is deteriorated. On the other hand, if the amount S of the relative displacement is more than 40% of the length L, uneven wear is likely to occur. It is preferable that the length Ls of the shorter block side, which faces the lug groove 5, be 65% to 80% of the length Llof the longer block side, 20 in each of the center-side block portions 6a and the outer-side block portions 6b. If the length Ls is less than 65% of the length L, heel-and-toe wear is likely to occur. On the other hand, if the length Ls is more than 85% of the length L, the noise reduction effect is deteriorated. It is desirable that each of the center-side and outer-side block 25 portions 6a and 6b be provided with at least three sipes 10. By providing the sipes 10, the on-snow running performance and the on-ice running performance of the tire are improved. Especially, the three or more sipes 10 of each of the block portion 6a and 6b preferably include one that is open to the main groove 3 or the main groove 4, one that is open to the lug groove 30 5 and one that is open to the narrow groove 9. With the sipes 10 provided 7 so as to open to the corresponding grooves which surround a corresponding one of the center-side and outer-side block portions 6a and 6b in this manner, mud or snow becomes unlikely to clog in each groove. It should be noted that each of the blocks 8 of the block row 80 on each shoulder side is 5 preferably provided with a plurality of sipes 11 as well. Moreover, it is preferable that each of the center-side block portions 6a and the adjacent one of the outer-side block portions 6b be shaped so as to be symmetric to each other about the middle point A in the corresponding narrow groove 9. In the case of employing a symmetric shape for the 10 center-side and outer-side block portions 6a and 6b, it is possible to effectively improve the noise performance, regardless of the direction in which the tire rotates. Needless to say, it is preferable that the sipes 10 in each of the center-side block portions 6a and the sipes 10 in the adjacent one of the outer-side block portions 6b be also shaped so as to be symmetric to 15 each other about the middle point A in the corresponding narrow groove 9. Although the embodiment of the present invention has been described in detail above, it should be understood that various modifications, substitutions and alternations are possible without departing from the spirit and scope of the present invention defined by the scope of the 20 appended claims. Examples Tires of Comparative Examples 1 to 3 and Examples 1 to 7 are prepared in the following manner. A pneumatic tire used for each example 25 has a tire size of 285/60R18, and is provided with a land portion and block rows. The land portion extends continuously in the tire circumferential direction along the center line of the tread portion. Each of the block rows includes a plurality of blocks defined by two main grooves and a plurality of lug grooves. The main groove with a zigzag shape extends in the tire 3 0 circumferential direction on each side of the land portion. Each of the 8 plurality of lug grooves connects these main grooves to each other. In each pneumatic tire, each of the blocks of the block rows is divided into a center-side block portion and an outer-side block portion by a narrow groove, which extends in the tire circumferential direction. Each of the center-side 5 block portions and the adjacent one of the outer-side block portions are disposed, in a manner that these block portions are displaced relative to each other, along a direction in which the narrow groove extends, respectively towards the shorter block sides of the block portions, which sides face the corresponding lug grooves. As shown in Table 1, the tires of 10 Comparative Examples 1 to 3 and Examples 1 to 7 differ from one another, in the ratio (%) of the amount S of relative displacement to the length L of the block side, in the inclination angle a (*) of each narrow groove, in the ratio (%) of the groove width W of each narrow groove to the tread contact width TCW, and in the ratio (%) of the length Ls of the shorter block side to 1 5 the length Ll of the longer block side. It should be noted that each of the center-side block portions and the adjacent one of the outer-side block portions are formed so as to be symmetric to each other about the middle point in the corresponding narrow groove. Moreover, as a conventional example, a tire in which a block row between main grooves on each side is 20 not divided by a narrow groove is prepared. These test tires are evaluated on the off-road running performance, noise performance and the amount of uneven wear by the following testing method. The evaluation results are shown together in Fig. 1 Off-road running performance: 25 Each test tire was mounted on a wheel having a rim size of 18x8JJ. Then, the wheel with the test tire was mounted with an air pressure of 230 kPa on a four-wheel drive vehicle. On each test tire, a sensory test of the running performance on the off-road was performed by a test driver. The evaluation results were indicated by indices, where the evaluation result of 30 the conventional example was taken as 100. The larger the index is, the 9 more excellent the test tire is in the driving performance on the off-road. Noise Performance: Each test tire was mounted on a wheel having a rim size of 18x8JJ. Then, the wheel with the test tire was mounted with an air pressure of 230 5 kPa on a four-wheel drive vehicle. After the load (two passengers + a measuring device + the weight of the vehicle) was adjusted on each test tire, the vehicle was driven on the paved road at a speed of 100 km/h. A vehicle interior noise due to pattern noises was measured by using a microphone placed near the side glass on the driver's seat side. The evaluation results 10 were indicated by using reciprocals of measurement values, and by indices where the evaluation result of the conventional example was taken as 100. The larger the index is, the more excellent the test tire is in the noise performance with less pattern noises. Amount of Uneven Wear: 15 Each test tire was mounted on a wheel having a rim size of 18x8JJ. Then, the wheel with the test tire was mounted with an air pressure of 230 kPa on a four-wheel drive vehicle. After driving the vehicle for a driving distance of 8000 km (mountain road: 59 %, and paved road: 41 %), the amount of heel-and-toe wear (mm) in the block row between the main 20 grooves was measured. [Table 1] 10 00 (n d> 0 0. w E l) 0 0 o C> 0 n > 00 Nt 0r 0C $. 0 0> E 0 C:) . C> 00)n C C> 0 E -j 00 m- C 4C) tn 00 In 0 0> cc 0n- O Ew K 0 0l C 4 .2) -C> 22: CL E a ) n 0 0 0 0b0< u .0 C) C o 0 C)0 C0 cq0 0 z As is clear from Table 1, in the case of each of the tires of Examples 1 to 7, the noise performance relating to pattern noises was improved while the off-road running performance was maintained, in comparison with the conventional example. In addition, uneven wear was hardly observed. 5 On the other hand, in the case of the tire of Comparative Example 1, the inclination angle a of each narrow groove was 00, and also the ratio of the amount S of relative displacement of the blocks to the length L of the corresponding side of each block was 0%. Accordingly, the noise performance was not improved. In addition, in the case of the tire of 10 Comparative Example 2, the inclination angle a of each narrow groove was too large. Accordingly, the off-road running performance was deteriorated. Moreover, in the case of the tire of Comparative Example 3, the groove width W of each narrow groove was too small. Accordingly, the off-road running performance was deteriorated. 15 12