Pneumatic radial tire

Abstract: The present invention provides a pneumatic radial tire which is excellent in drainage and capable of reducing air-column-tube resonance sounds in circumferentially extending grooves to decrease noises. The pneumatic radial tire of the present invention comprises a tread provided thereon with a groove extending circumferentially in the circumferential direction of the tire and a vibration buffer wall having a roughly identical cross-sectional shape provided near the sidewall of said circumferentially extending groove, wherein the height of the outside edge of said vibration buffer wall in the radial direction of the tire is lower than the height of the outside edge of said circumferentially extending groove in the radial direction of the tire.

Description: The present invention relates to a pneumatic radial tire and particularly to a pneumatic radial tire excellent in drainage with low noises for use in high-speed large trucks or automobiles. In automobiles running at high speed on a rainy day, rainwater is penetrated and wedged between the tread of a tire and the surface of a road to cause a phenomenon called hydroplaning in which the friction between the tread and the road surface is lost and the steering wheel and brake do not correctly function and the safety of driving is worsened. Conventionally, the pneumatic radial tire 1 as shown in FIG. 15 is proposed to solve the problem of hydroplaning in automobiles. This pneumatic radial tire 1 is formed in the shape of a loop as a whole with rubber layer 2, and the beads 3, 3 in the sectional terminals of the radial tire 1 are provided with a pair of bead cores 4, 4 formed into a ring around the rotating shaft of the radial tire 1. A 2-ply carcass layer 6, consisting of ply cords 5 arranged in the shape of a loop in the sectional direction of the radial tire 1, is provided across a pair of the bead cores 4, 4, and the terminals of the carcass layer 6 in the longer direction are turned outside and wound up around a pair of bead cores 4, 4. The radial outside of carcass layer 6 is provided with a 2-ply steel belt layer 8 consisting of nonstretchable steel cord 7, and on the radial outside of the steel belt layer 8, the wall-thick tread 9 is formed so as to cover the steel belt layer 8. Further, the surface of the tread 9 is provided at predetermined intervals with a plurality of continuous rib grooves circumferentially extending around the radial tire 1 to permit water penetrated between the tread 9 and the surface of a road to be discharged outside via the circumferentially extending grooves 10, thus improving the drainage of the radial tire 1. In the radial tire 1 as proposed above, however, there occur air-column tubes between the circumferentially extending grooves 10 and the surface of a road in the rolling tire, and the air in the air-column tubes causes noises by resonating with the vibration of the radial tire 1 brought about by the uneven surface of a road. If circumferential grooves 10 are enlarged to secure drainage, the noises will be raised because the air-column-tube resonance sounds are raised generally in proportion to the size of circumferential groove 10. As a means to solve the problem, there is proposed e.g. a pneumatic tire having a tread provided with circumferentially extending grooves and in the crosswise direction of the tire with laterally extending grooves with an incline to the equatorial plane of the tire, where notches are formed roughly parallel to and adjacent to the laterally extending grooves (Japanese Patent Application Laid-Open Publication No. 38412/1991). However, such a pneumatic tire is not enough effective for preventing the noise because both the laterally extending grooves and the notches come into contact with the ground and the sidewalls of both the laterally extending grooves and the notches are simultaneously vibrated. That is, the pneumatic tire disclosed in Japanese Patent Application Laid-Open Publication No. 38412/1991 has not succeeded in solving the above problem and there remains the problem to be solved. A pneumatic radial tire in which the upper edges of the sidewalls of a circumferentially extending groove are made slopes with an incline to the equatorial plane of the tire is also proposed in e.g. Japanese Patent Application Laid-Open Publication No. 117414/1995. However, this prior art pneumatic radial tire cannot relieve the lateral vibration of its circumferentially extending grooves and can thus not reduce air-column-tube resonance sounds satisfactorily. Hence, the pneumatic tire disclosed in Japanese Patent Application Laid-Open Publication No. 117414/1995 has not succeeded in solving the above problem and there remains the problems to be solved. There is further proposed a pneumatic radial tire having a tread provided with a rib and with a circumferentially extending wide groove, said rib having circumferentially extending thin grooves formed outside said circumferentially extending wide groove, wherein the edge zone between the circumferentially extending wide and thin grooves is lower than the rib (US Patent No. 4,936,363). However, the pneumatic radial tire proposed in US Patent No. 4,936,363 was structured as above to prevent the edge of the rib from undergoing wear, and said lower edge zone has projecting and reentrance portions on the sidewalls the circumferentially extending wide groove, that is, the crosssectional shape of the edge zone is uneven, while the outside surface does also have projecting and reentrance portions. Therefore, this prior art tire may be effective for preventing wear on the rib, but can not relieve the vibration in the crosswise direction of the circumferentially extending groove of the rolling tire, so that this tire has not attained the sufficient prevention of noises resulting from air-column-tube resonance sounds etc. A tire according to the preamble of claim 1 is known e.g. from EP-A-0 313 361. The present invention was made in view of the above problem and the object is to provide a pneumatic radial tire being excellent in drainage and capable of reducing noises by decreasing air-column-tube resonance sounds. To solve the problem, the pneumatic radial tire of the present invention comprises the features of claim 1. A loop-shaped carcass layer is provided across a pair of bead cores, a belt layer and a tread arranged in this order on the outside of said carcass layer in the radial direction, and circumferentially extending grooves are provided on the surface of said tread, wherein a vibration buffer wall is formed adjacent to the sidewall of the circumferentially extending groove, and the height of the outside edge of said vibration buffer wall in the radial direction of the tire is made lower than the height of the outside edge of said circumferentially extending groove in the radial direction of the tire. Figures 1 to 6 are not covered by the claims of this invention. FIG. 1 is a perspective sectional view of a pneumatic radial tire not belonging to the present invention. FIG. 2 is a plan view of an essential part of the pneumatic radial tire shown in FIG. 1. FIG. 3 is a sectional view of the pneumatic radial tire across line X-X in FIG. 2. FIG. 4(a) and (b) show a perspective view of a circumferentially extending groove of a modification of the pneumatic radial tire shown in figs. 1 to 3. FIG. 5 show a characteristic sound-pressure level against frequency, of rib grooves in rolling tires. FIG. 6(a) and (b) show characteristic vibration levels of rib grooves in rolling tires in low- and high-frequency ranges, respectively. FIG. 7 is a sectional perspective view of a first example of the pneumatic radial tire of the present invention. FIG. 8(a), (b), and (c) show a plan view of an essential part of the first example of the pneumatic radial tire of the present invention and its sectional views across lines X-X and Y-Y, respectively. FIG. 9(a), (b), and (c) show a plan view of an essential part of a second example of the pneumatic radial tire of the present invention and its sectional views across lines X-X and Y-Y, respectively. FIG. 10(a), (b), and (c) show a plan view of an essential part of a third example of the pneumatic radial tire of the present invention and its sectional views across lines X-X and Y-Y, respectively. FIG. 11 shows a characteristic sound-pressure level against frequency, of rib grooves in rolling tires. FIG. 12(a) and (b) show a method of measuring a vibration level at the bottom of rib grooves. FIG. 13(a) and (b) show characteristic vibration levels at response points C and D in rib grooves, respectively. FIG. 14 shows a characteristic vibration level against frequency, of rib grooves in rolling tires. FIG. 15 is a sectional perspective view of a prior art pneumatic radial tire. In the drawings, 11, 11' are pneumatic radial tires; 12, rubber layer; 13, bead; 14, bead core; 15, ply cord; 16, carcass layer; 17, steel cord; 18, steel belt layer; 19, tread; 20, rib groove; 20a, 20b, 21c, sidewalls; and 21, vibration buffer wall. In a pneumatic radial tire, said vibration buffer walls are formed roughly parallel to and adjacent to the sidewalls of said circumferentially extending groove, and said vibration buffer walls have notches. In an embodiment of the invention, said vibration buffer wall is formed adjacent to a first sidewall of said circumferentially extending groove, and a second sidewall of said circumferentially extending groove is inclined at an acute angle to the tread face, at an angle of 20° to 45° to a normal line perpendicular to the tread face, and the sidewall of said vibration buffer wall at the side of the circumferentially extending groove is inclined in the same direction as another sidewall and at an angle of 20° to 45° to a normal line perpendicular to the tread face, and if necessary, the upper edge of another sidewall of said circumferentially extending groove is chamfered. Another preferable embodiment of the pneumatic radial tire of the present invention is characterized by having the following relationships (1) to (4): 0.50D≦H≦0.95D S≦0.30B 0.01B≦W≦0.30B 0.50D≦L≦1.50D wherein B is the width of said circumferentially extending groove, D is the depth of said circumferentially extending groove, H is the height of said buffer vibration wall from the bottom of said circumferentially extending groove, S is the width of said circumferentially extending groove, W is the distance between the sidewall of said circumferentially extending groove and said vibration buffer wall, and L is the depth of the groove between said circumferentially extending groove and said vibration buffer wall. By forming the vibration buffer walls with a roughly identical cross-sectional shape adjacent to the sidewalls of the circumferentially extending groove, the pneumatic radial tire constituted as above suppresses transmission of tread vibration from the circumferentially extending grooves in the rolling tire to the air in the circumferentially extending grooves, so that the air-column-tube resonance in the circumferentially extending grooves is suppressed and the generation of noises is thus prevented. That is, the shear strength in the crosswise direction of the circumferentially extending grooves in the rolling tire is reduced, the vibration in the crosswise direction of the circumferentially extending grooves is relieved, and air-column-tube resonance sounds at high frequency of 1.6 KHz or more is reduced. By arranging the vibration buffer walls roughly parallel to the sidewalls of said circumferentially extending groove, the capacity of the circumferentially extending grooves is secured thus permitting the circumferentially extending grooves to accommodate a sufficient amount of water to ensure drainage. By making the height of the outside edge of said vibration buffer wall lower than the height of the outside edge of the circumferentially extending groove, the vibration buffer wall receives relatively low pressure from the ground as compared with other portions of the tire. Therefore, the vibration buffer wall itself does not vibrate in such a high level as in other portions and thus it does not cause the vibration of the air in the circumferentially extending grooves. The generation of noises is thereby effectively prevented. Further, when a part or the whole of the outer edge (surface of the tread) of the circumferentially extending groove comes to have the same height as the outer edge of the vibration buffer wall as a result of the spread of wear on the tread, the outer edge of the vibration buffer wall will undergo much wear than the outer edge of the circumferentially extending grooves because the rigidity of the outer edge of the vibration buffer wall is lower than that of the outer edge of the circumferentially extending groove. Therefore, the height of the outer side edge of the vibration buffer wall is maintained always lower than the height of the outer edge of the circumferentially extending groove. Furthermore, the production of tires is facilitated by the presence of notches in the vibration buffer walls. By allowing another (i.e. a second) sidewall of said circumferentially extending groove to be inclined at an acute angle to the tread face, that is, at an angle of 20° to 45° to a normal line perpendicular to the tread face, the compressive rigidity of the tread in the rolling tire is lowered and vibration occurring on an uneven road is relieved. The vibration at the bottom of the circumferentially extending groove is thereby relieved, and reduction of the level of air-column-tube resonance sounds of about 800 Hz results. This reduction could be attained because among the air-column-tube resonance sounds in the circumferentially extending groove, sounds of about 800 Hz occur due to the vibration of the bottom of the groove in the direction of a normal line perpendicular to the tread face, while high-frequency sounds of 1.6 KHz or to the vibration in the crosswise direction of the groove wall. The inclination of another sidewall of said circumferentially extending groove to a normal line perpendicular to the tread face is limited to an angle of 20° to 45° because at a degree of less than 20° it is hard to change compressive rigidity, while at an angle of 45° or more, the strength and durability of the upper edge of the tread become weak and its manufacturing is also problematic because it is hard to release it from a mold after vulcanization. The angle of inclination of the sidewall, at the side of the circumferentially extending wall, of the vibration buffer wall is preferably set at an angle of 20° to 45° for manufacturing. The vibration buffer wall with a sidewall inclining at the side of the circumferentially extending groove will not touch the ground because the height of the outside edge of the vibration buffer wall is made lower than the height of the outside edge (surface of the tread) of the circumferentially extending grooves. Thus, the shear strength of the circumferentially extending groove in the crosswise direction is lowered in the tire on the ground, and the vibration of the circumferentially extending groove in the crosswise direction is suppressed. In this case, the height of the vibration buffer wall is made about 0.8-time the depth of the circumferentially extending groove. This is because the effect the vibration buffer wall is decreased if the vibration buffer wall is too low, whereas if it is too high, it will receive vibration upon contact of the rolling tire with the ground to increase air-column-tube resonance sounds. In addition, the sidewall of the circumferentially extending groove, which inclines at an acute angle, is chamfered at its upper end whereby the durability of the sidewall is improved and its manufacturing is made easy. The embodiments of the present invention are further described by reference to the drawings. Those elements which have the same function have the same symbols in the following description of the embodiments. FIG. 1 is a sectional perspective view of a pneumatic radial tire, FIG. 2 is a plan view of an essential part of the pneumatic radial tire of Fig.1, FIG. 3 is a sectional view of the pneumatic radial tire across line X-X in FIG. 2, and FIG. 4 is a perspective view of a circumferentially extending groove of a modification of the pneumatic radial tire of figures 1 to 3. In the pneumatic radial tire 11 in FIG. 1, its tire size is PSR265/70R16 and the tire has a loop shape in section and is made of rubber layer 12 as a whole. The beads 13 at the sectional terminals of the radial tire 11 are provided with a pair of bead cores 14, 14 formed into a ring around the rotating shaft of radial tire 11. A 2-ply carcass layer 16, consisting of ply cord 15 arranged in the form of a loop in the sectional direction of radial tire 11, is provided across a pair of the bead cores 14, 14. The terminals of the carcass layer 16 in the longer direction are turned outside and wound up around a pair of the bead cores 14, 14. The radial outside of the carcass layer 16 is provided with a 2-ply steel belt layer 18 consisting of nonstretchable steel cord 17, and the wall-thick tread 19 is formed on the outside of steel belt layer 18 in the radial direction, so as to cover the steel belt layer 18. Further, the surface of the tread 19 is provided at predetermined intervals with a plurality of continuous rib grooves 20 (circumferentially extending grooves) circumferentially extending around the radial tire 11 in the crosswise direction of the radial tire. The rib grooves 20 are provided inside with a pair of wall-thin vibration buffer walls 21 having a roughly uniform cross-sectional shape and being roughly parallel to and adjacent to sidewalls 20a respectively, and the height of the outside edge of this vibration buffer wall 21 in the radial direction is set lower than the height of the outside edge (surface of the tread 19) of the rib groove 20 in the radial direction of the tire. In this case, the width (B) of rib groove 20 is 13 mm, the depth (D) of rib groove 20 is 10 mm, the height (H) of vibration buffer wall 21 from the bottom of rib groove 20 is 8 mm, the width (S) of vibration buffer wall 21 is 2 mm, the distance (W) between the sidewall 20a of rib groove 20 and the vibration buffer wall 21 is 1 mm, and the depth (L) of the groove between the rib groove 20 and the vibration buffer wall 21 is 8 mm. The rib groove 20 in the modification in FIG. 4 has a different structure from the rib groove in the first example of the pneumatic radial tire in FIG. 1. That is, in FIG. 4(a), the vibration buffer wall 21 is roughly parallel to and adjacent to only one of sidewalls 20a of rib groove 20, and in FIG. 4(b), the vibration buffer wall 21 in FIG. 4(a) is provided intermittently with notches 21a. By providing the vibration buffer wall 21 with notches 21a, the extremely thin vibration buffer wall 21 can easily vulcanized and molded. The pneumatic radial tire was evaluated by comparing it with a conventional pneumatic radial tire as the comparative example. The performance of the first example of the present pneumatic radial tire was compared that the comparative example. The measurement results are shown in Table 1. Comparative Example First Example Noise level (overall value) 0 dB (A) Δ0.5 dB(A) (Small value is good) Hydroplaning performance 100 99 (Large value is good) Partial wear 100 95 (Small value is good) These values are relative to those (as 0 dB(A), 100) of the comparative example. As can be seen from Table 1, the performance of the first example is improved: that is, the noise level is reduced, the hydroplaning performance is hardly reduced, and the wear resistance of the tire is decreased. This may be because the vibration buffer wall 21 undergoes relatively much wear work in the circumferential direction upon contact with the ground, by which wear work on other portions is relatively decreased. In this measurement, the noise level was evaluated with an on-bench noise meter drum. To evaluate drainage, a car ran straight on a pool of 10 mm in depth and the lowest speed when hydroplaning occurred was determined and evaluated. The partial wear was evaluated by examining wear work with a machine for measurement of on-bench contact pressure and sliding. It is said that among the air-column-tube resonance sounds in the rib groove 20, sounds of 1 kHz or thereabout occur due to the vibration of the bottom of the groove, while high-frequency sounds of 2 kHz or more occur due to the vibration of the groove walls. The described example has effectively reduced high-frequency resonance sounds because the vibration level in the first example is lower by 1.5 to 2.0 dB (A) in each point ranging from 2 to 3.2 kHz than that of the comparative example, as shown in the characteristics of sound pressure level vs. frequency in FIG. 5 and the characteristics of vibration level vs. frequency in FIG. 6 with respect to the circumferentially extending grooves in the rolling tire. In this measurement of vibration level, an acceleration sensor was attached to the rib groove wall in the comparative example and to the vibration buffer wall 21 in the described example. The pneumatic radial tire of the present invention is described by reference to first to third examples. FIG. 7 is a sectional perspective view of a first example of the pneumatic radial tire of the present invention; FIG. 8(a), (b) and (c) show a plan view of an essential part of the first example of the pneumatic radial tire and sectional views of its circumferentially extending groove (referred to hereinafter as rib groove), respectively; FIG. 9(a) to (c) show a plan view of an essential part of a second example of the pneumatic radial tire of the present invention and sectional views of its rib groove, respectively; and FIG. 10(a) to (c) show a plan view of an essential part of a third example of the pneumatic radial tire of the present invention and sectional views of its rib groove, respectively. Because the pneumatic radial tire 11' in FIG. 7 is the same as the tire shown in FIG. 1 except for the tire size of TBR265/60R22.5 and the structure of rib groove 20, the description of portions with the same constitution will be omitted. A plurality of rib grooves 20 continuously extending in the circumferential direction of the radial tire 11 are provided on the surface of tread 19 at predetermined intervals in the crosswise direction of the radial tire 11. A vibration buffer wall 21 having a roughly uniform cross-sectional shape is formed within a plurality of rib grooves 20 such that it is roughly parallel to and adjacent to sidewall 20a which is one of the sidewalls of the rib groove 20, and the height of the outside edge of the vibration buffer wall 21 in the radial direction is set lower than the height of the outside edge (surface of the tread 19) of the rib groove 20 in the radial direction of the tire. Further, another sidewall 20b of rib groove 20 and the sidewall 21a facing the inside of the rib groove 20 of the vibration buffer wall 21 are formed such that their upper edges incline at an angle of 20° to 45° to a normal line perpendicular to the tread face and in the direction toward the equatorial plane of the tire, and the upper end of another sidewall 20b of the rib groove 20 is formed at an acute angle. In this case, the width (B) of rib groove 20 is 12 mm, the depth (D) of rib groove 20 is 10 mm, the angle of inclination () of the sidewall 20b of rib groove 20 and the sidewall 21a of vibration buffer wall 21 is 30°, the height (H) of vibration buffer wall 21 from the bottom of rib groove 20 is 8 mm, the width (S) of the upper end of vibration buffer wall 21 is 2 mm, the distance (W) between the sidewall 20a of rib groove 20 and the vibration buffer wall 21 is 1 mm, and the depth (L) of the groove between the rib groove 20 and the vibration buffer wall 21 is 8 mm (see FIG. 8(a) to (c)). In the pneumatic radial tire 11' as the second example in FIG. 9(a) to (c), the upper end of the sidewall 20b is chamfered in order to improve the durability of the sidewall 20b of rib groove 20 and to facilitate the production of rib groove 20. In this case, the width (R) of the chamfered part 20c is 2 mm. In the pneumatic radial tire 22 in FIG. 10(a) to (c), the sidewall 20b of rib groove 20 and the sidewall 21a of vibration buffer wall 21 incline at an angle of 20° to 45° to a normal line perpendicular to the tread face, and their upper edges incline toward the opposite side of the equatorial plane of the tire. The present pneumatic radial tire was evaluated by comparing it with the conventional pneumatic radial tire. The performance of the third to fifth examples of the present pneumatic radial tire was compared with the conventional one. The measurement results are shown in Table 2. Comparative Example 3rd Ex. 4th Ex. 5th Ex. Noise level (overall value) 0 dB (A) Δ1.1 Δ0.9 Δ0.7 Noise level (800 to 1 KHz value) 0 dB (A) Δ1.6 Δ1.4 Δ1.3 Hydroplaning performance 100 101 100 98 Partial wear 100 93 93 90 In the table, the first example is formed such that, as shown in FIG. 8(a) to (c), the sidewall 21b of rib groove 20 and the sidewall 21a of vibration buffer wall 21 incline at an angle of 20° to 45° to a normal line perpendicular to the tread face, so as to form their upper edges as slopes declining toward the equatorial plane of the tire, and the upper end of the sidewall 20b is not chamfered; the second example is formed such that, as shown in FIG. 9(a) to (c), the sidewalls 20b and 21a incline at an angle of 20° to 45° to a normal line perpendicular to the tread face, so as to form their upper edges as slopes declining toward the equatorial plane of the tire, and the upper end of the sidewall 20b is chamfered; and the third example is formed such that, as shown in FIG. 10(a) to (c), the sidewalls 20b and 21a incline at an angle of 20° to 45° to a normal line perpendicular to the tread face, so as to form their upper edges as slopes declining toward the opposite side of the equatorial plane of the tire, and the upper end of the sidewall 20b is not chamfered. The measurement values for the first to third examples are index values relative to those (as 0 dB (A), 100) of the comparative example, where a smaller value is good for noise level and partial wear and a higher value is good for hydroplaning performance. As can be seen from Table 2, the noise level is decreased and the hydroplaning performance is hardly reduced in the third to third examples. The partial wear, as determined in term of wear work done on the part of tread which is most susceptible to wear due to its contact with the ground, shows less partial wear in first to third examples, indicating that their resistance to partial wear is improved. This may be because the vibration buffer wall 21 undergoes relatively much wear work in the circumferential direction upon contact with the ground, by which wear work on other portions is relatively decreased. FIG. 11 (characteristics of noise level of rib grooves vs. frequency in the rolling tire) shows an on-bench noise spectrum of the first example. As can be seen in this graph, the first example demonstrates a lower noise level in the frequency range of 500 Hz to 2 KHz. The reduction in noise in this range may be attributable to the incline of the sidewall 20b of the rib groove 20 for the noises of 500 Hz to 1.25 KHz and to the provision of the vibration buffer wall 21 for the noises of 1.25 KHz to 2 KHz. The noise level was evaluated using an on-bench noise meter drum, and the drainage was evaluated by determining the lowest speed at which hydroplaning occurs in a car running straight on a pool of 10 mm in depth. The partial wear was evaluated by examining wear work with a machine for measurement of on-bench contact pressure and sliding. FIG. 12(a) and (b) show a method of examining the vibration of the groove bottom of rib grooves 10 and 12 caused by vibration on the surface of a road. A vibrating apparatus 23 was used at one position on the ground to vibrate the vibration points 1 to 7 of tread portions 9, 19 in contact with the ground, and the vibration transmission level A/F was measured with an acceleration sensor 24 attached to the response points A to D at the bottom of the rib grooves 10 and 20. The results are shown in FIG. 13(a) and (b). As can be seen from the graphs, the vibration transmission levels of the response point C to the vibrating points 2 and 3 and of the response point D to the vibrating points 5 and 6 in the first to third examples are lower than those of the comparative example, indicating that the vibration at the bottom of the grooves of the rolling tire in contact with the surface of a road is effectively relieved. FIG. 14 shows the result of the measurement of the vibration of the groove walls of rib grooves 10 and 20 in the rolling tire. The vibration level was measured with the acceleration sensor 24 attached to the walls of rib groove 10 in the comparative example and to the vibration buffer wall 21 in the first to third examples. The first to third examples show a lower vibration level in the high-frequency range of 2 KHz or more, where the level in the range of 2 KHz to 3.2 KHz is lower by a degree of 1.5 dB (A) to 2.0 dB (A) than in the comparative example. According to the first to third examples, the vibration in the crosswise direction of the circumferentially extending grooves in the rolling tire is relieved while the vibration at the bottom of the circumferentially extending grooves is also relieved as described above, so that air-column-tube resonance sounds of about 800 Hz and high-frequency air-column-tube resonance sounds of 1.6 KHz or more can be lowered. Therefore, the noise of the tire can be reduced. Although the first to third examples have been described above in detail as the embodiments of the pneumatic radial tire of the present invention, the present invention is not limited to the radial tires described in such examples, and these can be modified in design without departing from the scope of the invention described in the appended claims. For example, a description of the straight and continuously extending straight rib grooves was made for the the present pneumatic radial tire, but the grooves may be zigzag-shaped or partially interrupted. As can be understood from the above description, the pneumatic radial tire of the present invention can suppress air-column-tube resonance in circumferentially extending grooves to prevent the occurrence of noises because a vibration buffer wall with a roughly identical cross-sectional shape in section is formed near the sidewalls of the circumferentially extending grooves. Hence, the problem of generation of noises from the automobiles can be prevented. In addition, the adequate capacity of the circumferentially extending grooves is secured by arranging the vibration buffer wall roughly parallel to a sidewall of the circumferentially extending grooves, thereby preventing drainage from lowering. Further, the vibration buffer wall itself does not vibrate because the height of the outside edge of the vibration buffer wall is designed to be lower than the height of the outside edge of the circumferentially extending groove. Therefore, noises can be effectively prevented, the shear strength in the crosswise direction of the tire in contact with the ground can be reduced, the vibration in the crosswise direction of the circumferentially extending grooves in the tire can be relieved, and high-frequency air-column-tube resonance sounds of 1.6 KHz or more can be decreased. The vibration buffer wall with notches are easy to mold, so it is possible to manufacture an extremely thin vibration buffer wall with good workablity. Furthermore, another sidewall of the circumferentially extending groove is inclined at an acute angle of 20° to 45° to a normal line perpendicular to the tread face so that the vibration at the bottom of the circumferentially extending grooves is relieved and air-column-tube resonance sounds of 800 Hz or thereabout can be reduced. Therefore, the problem of generation of noises from the automobiles can be prevented. Finally, the upper end of the second sidewall of the circumferentially extending groove is chamfered so that the durability of the sidewall can be improved and simultaneously the molding of such sidewall is made easy, leading to the simplification of tire manufacturing.

Claims: A pneumatic radial tire (11) comprising: a radial carcass layer (16), a pair of bead cores (14), a belt layer (18) radially outside said carcass, and a tread (19) arranged outside of said belt layer in the radial direction of the tire, a circumferentially extending groove (20) provided on the surface of said tread and having a first sidewall (20a) and a second sidewall (20b), and one vibration buffer wall (21) the outside edge of which having a width S and said circumferentially extending groove (20) having a width B and said vibration buffer wall (21) is formed adjacent to said first sidewall (20a) of said circumferentially extending groove (20), said first sidewall (20a) of said circumferentially extending groove (20) being substantially perpendicular to the tread face, wherein the height of the outside edge of the vibration buffer wall (21) in the radial direction of the tire is lower than the height of the outside edge of said circumferentially extending groove (20) in the radial direction of the tire, and said vibration buffer wall (21) and said first sidewall (20a) of said circumferentially extending groove (20) define a groove having a substantially constant cross-sectional shape, characterized in that said second sidewall (20b) of said circumferentially extending groove (20) is inclined at an acute angle to the tread face and at an angle (Θ) of 20° to 45° to a normal line perpendicular to the tread face, the sidewall (21a) of the vibration buffer wall (21) facing the inside of the circumferentially extending groove (20) is inclined with respect to the normal line in the same direction as that of said second sidewall (20b) of the circumferentially extending groove (20), and in that the width B is such that S≤0.30B . A pneumatic radial tire as claimed in claim 1, characterized in that the sidewall (21a) of the said vibration buffer wall (21) facing the inside of the circumferentially extending groove (20) is inclined at an angle of 20° to 45° to a normal line perpendicular to the tread face. A pneumatic radial tire as claimed in claim 1 or 2, characterized in that the upper end of said second sidewall (20b) of said circumferentially extending groove (20) is chamfered. A pneumatic radial tire as claimed in any of claims 1 to 3, characterized in that the following relationship is satisfied: 0.50D≤H≤0.95D 0.01B≤W≤0.30B 0.50D≤L≤1.50D wherein D is the depth of said circumferentially extending groove (20), H is the height of said vibration buffer wall (21) from the bottom of said circumferentially extending groove (20), W is the width of said groove between the sidewall (20a) of said circumferentially extending groove (20) and said vibration buffer wall (21), and L is the depth of said groove between the sidewall (20a) of said circumferentially extending groove (20) and said vibration buffer wall (21). A pneumatic radial tire as claimed in any of claims 1 to 4, characterized in that said vibration buffer wall (21) has a face extending perpendicular to the tread face.

Inventors: YOKOTA HIDETOSHI, YOKOTA, HIDETOSHI, YOKOTA, HIDETOSHI, C/O BRIDGESTONE CORP. TECH.CTR.
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1997
Main classification: B60C11
IPC codes: B60C11, B60C9