The present disclosure relates to a heavy-duty tire compatible with wear resistance, uneven wear resistance, and wet performance. In the present disclosure, by providing, to a tread section, a pair of center primary grooves, a pair of shoulder primary grooves, a plurality of center lateral grooves, a plurality of middle lateral grooves, and a plurality of shoulder lateral grooves, the tread section is divided into a center block, a middle block, and a shoulder block.

TECHNICAL FIELD

The present invention relates to a heavy-duty tire compatible with wear resistance, uneven wear resistance and wet performance.

BACKGROUND ART

Conventionally, the following Patent Document 1 has proposed a tread pattern for heavy-duty tires used for trucks and buses, the tread pattern being provided with a plurality of circumferentially and continuously extending main grooves and a plurality of lateral grooves each extending between the main grooves as well as between one of the main grooves and a tread edge to form a plurality of tread blocks, for example.

The heavy-duty tires are required excellent wear resistance and uneven wear resistance, in view of economics and saving maintenance. In general, in order to improve the wear resistance and uneven wear resistance, it may be effective to increase the rigidity of the tread portion while ensuring a sufficient rubber volume of the tread portion by decreasing the groove volume of the tread portion.

CITATION LIST

Patent Literature

Patent Document 1: Japanese Unexamined Patent Application Publication No. 2011-195045

Unfortunately, when reducing the groove volume of the tread portion, the drainage property is lowered, thereby deteriorating the wet performance.

SUMMARY OF INVENTION

Technical Problem

The present invention has been made in view of circumstances as described above, and has a main object to provide a heavy-duty tire compatible with wear resistance, uneven wear resistance and wet performance at a high level.

Solution to Problem

The present invention provides a heavy-duty tire including a tread portion being provided with a pair of circumferentially and continuously extending zigzag central main grooves arranged on both sides of a tire equator, a pair of circumferentially and continuously extending zigzag shoulder main grooves each arranged between one of the central main grooves and one of tread edges, a plurality of central lateral grooves connecting the pair of central main grooves and a plurality of middle lateral grooves connecting the central main groove with the shoulder main groove on both sides of the tire equator, thereby the tread portion including a central land portion in which a plurality of central blocks separated by the pair of central main grooves and the central lateral grooves are arranged in a circumferential direction of the tire, a pair of middle land portions in which a plurality of middle blocks separated by one of the central main grooves, one of the shoulder main grooves and the middle lateral grooves are arranged in a circumferential direction of the tire, and a pair of shoulder land portions separated between one of the shoulder main grooves and one of the tread edges, each of the central main grooves and the shoulder main grooves including a long side and a short side arranged alternately in a circumferential direction of the tire wherein the short side is inclined in an opposite direction to the long side and has a circumferential length shorter than that of the long side, each central lateral groove connecting the long sides of the pair of central main grooves, each middle lateral groove connecting an outer zigzag corner of the central main groove with an inner zigzag corner of the shoulder main groove, wherein the outer zigzag corner protrudes on the side of the tread edge, and wherein the inner zigzag corner protrudes on the side of the tire equator, and the middle blocks being provided with an inclined slot on a location facing one of the central lateral grooves through the central main groove, wherein the inclined slot has a depth increasing gradually toward the central main groove.

In the heavy-duty tire according to the present invention, it is preferable that the inclined slot overlaps with an opening of the central lateral groove at the central main groove in a circumferential region of from 25% to 50% of a circumferential length of the central lateral groove.

In the heavy-duty tire according to the present invention, it is preferable that the inclined slot extends from the central main groove toward the shoulder main groove and terminates without reaching the shoulder main groove, and a length of the inclined slot from its opening facing the central main groove to its end is in a range of from 55% to 65% of a width of the central main groove.

In the heavy-duty tire according to the present invention, it is preferable that the inclined slot has a depth at a deepest portion in a range of from 50% to 100% of a depth of the central main groove.

In the heavy-duty tire according to the present invention, it is preferable that the middle blocks have an axial length in a range of from 95% to 105% of an axial length of the central blocks.

In the heavy-duty tire according to the present invention, it is preferable that a plurality of shoulder lateral grooves connecting the shoulder main groove with the tread edge are provided on each shoulder land portion to form a row of circumferentially arranged plurality of shoulder blocks each of which is defined by the shoulder main groove, the tread edge and a pair of shoulder lateral grooves.

In the heavy-duty tire according to the present invention, it is preferable that the shoulder blocks have an axial length in a range of from 95% to 105% of an axial length of the central blocks.

In the heavy-duty tire according to the present invention, it is preferable that the inclined slot comprises a slop inclined radially inwardly from a ground contact surface of the middle block, and an angle formed between the slop and the ground contact surface of the middle block is in a range of from 50 to 70 degrees.

In the heavy-duty tire according to the present invention, it is preferable that the long side is inclined at an angle of from 3 to 9 degrees with respect to the circumferential direction of the tire, the middle lateral grooves are inclined at an angle with respect to an axial direction of the tire, and the central lateral grooves are inclined in an opposite direction to the middle lateral grooves.

In the heavy-duty tire according to the present invention, it is preferable that the central main grooves comprises a first groove edge on the side of the tire equator and a second groove edge on the side of the tread edge, and the first groove edge comprises a first zigzag corner located nearest the tread edge, and the second groove edge comprises a second zigzag corner located nearest the tire equator, wherein the first zigzag corner is located on the side of the tire equator with respect to the second zigzag corner.

In the heavy-duty tire according to the present invention, it is preferable that a ratio of W11/TW of an axial distance W11from the first zigzag corner to the second zigzag corner relative to a tread width TW is in a range of from 0.005 to 0.02.

In the heavy-duty tire according to the present invention, it is preferable that the shoulder main grooves comprise a third groove edge on the side of the tire equator and a fourth groove edge on the side of the tread edge, and the third groove edge comprises a third zigzag corner located nearest the tread edge, and the fourth groove edge comprises a fourth zigzag corner located nearest the tire equator, wherein the third zigzag corner is located on the side of the tire equator with respect to the fourth zigzag corner.

In the heavy-duty tire according to the present invention, it is preferable that a ratio of W21/TW of an axial distance W21from the third zigzag corner to the fourth zigzag corner relative to a tread width TW is in a range of from 0.005 to 0.02.

In the heavy-duty tire according to the present invention, it is preferable that the central main grooves have an axial zigzag-amplitude W12in a range of from 10% to 18% of an axial length WA of the central blocks.

In the heavy-duty tire according to the present invention, it is preferable that the tread portion has a land ratio in a range of not less than 65%.

In the heavy-duty tire according to the present invention, it is preferable that the tread portion has a land ratio in a range of not more than 75%.

In the heavy-duty tire according to the present invention, it is preferable that the inclined slot comprises a slope in a planar shape.

In the heavy-duty tire according to the present invention, it is preferable that a maximal depth D3of the inclined slot is greater than an axial length W3of the inclined slot.

In the heavy-duty tire according to the present invention, it is preferable that a circumferential length L4of the inclined slot is greater than an axial length W3of the inclined slot.

Advantageous Effects of Invention

The heavy-duty tire according to the present invention includes the middle lateral grooves each connecting the outer zigzag corner on the side of the tread edge of the central main groove with the inner zigzag corner on the side of the tire equator of the shoulder main groove. Such a middle lateral groove may offer an excellent drainage performance among the central main groove and the shoulder main groove and the middle lateral grooves.

Furthermore, the central lateral grooves connect the long sides of a pair of the central main grooves, and the middle blocks are provided with the inclined slot on the location facing one of the central lateral grooves through the central main grooves. Since the inclined slot has the depth increasing gradually toward the central main groove, drainage performance of the middle block improves by promoting the water flow from the middle block toward the central lateral grooves. Thus, it is possible to improve the drainage performance without increasing the groove volume, and therefore the wear resistance, uneven wear resistance and wet performance can be improved at a high level.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be explained below with reference to the accompanying drawings.FIG. 1illustrates a development view of a tread portion2illustrating an embodiment of a heavy-duty tire (the whole not shown) according to the present invention.FIG. 2illustrates a cross-sectional view of the tread portion2taken along lines A-A ofFIG. 1. As illustrated inFIG. 1, the tread portion2is provided with a pair of circumferentially and continuously extending zigzag central main grooves3on both sides of the tire equator C and a pair of circumferentially and continuously extending zigzag shoulder main grooves4disposed axially outward of the central mail grooves3.

The central main grooves3include a long side3ainclined with respect to the circumferential direction of the tire and a short side3bhaving a circumferential length shorter than that of the long side3a, and which are arranged alternately in the circumferential direction of the tire. The short side3bis inclined in an opposite direction to the long side3ato form the zigzag central main grooves3.

Similarly, the shoulder main grooves4include a long side4aand a short side4bhaving a circumferential length shorter than that of the long side4a, and which are arranged alternately in the circumferential direction of the tire. The short side4bis inclined in an opposite direction to the long side4ato form the zigzag shoulder main grooves4.

The central main grooves3include a plurality of inner zigzag corners3ilocated nearest the tire equator C to protrude axially inwardly and a plurality of outer zigzag corners3olocated nearest the tread edge Te to protrude axially outwardly. Similarly, the shoulder main grooves4include a plurality of inner zigzag corners4ilocated nearest the tire equator C to protrude axially inwardly and a plurality of outer zigzag corners4olocated nearest the tread edge Te to protrude axially outwardly.

Widths W1of the central main grooves3and widths W2of the shoulder main grooves4are set according to a tread width TW. As used herein, the tread width TW is an axial distance between the tread edges Te and Te.

As used herein, the tread edges Te refer to axially outermost edges of the ground contacting patch which occurs under a standard condition with a standard tire load when the camber angle of the tire is zero. Here, the standard condition is such that the tire1is mounted on a standard wheel rim (not illustrated) with a standard pressure and is loaded with no tire load. Various dimensions, positions and the like of the tire refer to those under the standard condition of the tire unless otherwise noted.

As used herein, the standard wheel rim is a wheel rim officially approved or recommended for the tire by standards organizations, wherein the standard wheel rim is the “standard rim” specified in JATMA, the “Measuring Rim” in ETRTO, and the “Design Rim” in TRA or the like, for example.

As used herein, the standard pressure is a standard pressure officially approved or recommended for the tire by standards organizations, wherein the standard pressure is the “maximum air pressure” in JATMA, the “Inflation Pressure” in ETRTO, and the maximum pressure given in the “Tire Load Limits at Various Cold Inflation Pressures” table in TRA or the like, for example.

As used herein, the standard tire load is a tire load officially approved or recommended for the tire by standards organizations, wherein the standard tire load is the “maximum load capacity” in JATMA, the “Load Capacity” in ETRTO, and the maximum value given in the above-mentioned table in TRA or the like.

Preferably, the widths W1of the central main grooves3are in a range of from 1.5% to 5% of the tread width TW. When the widths W1are less than 1.5% of the tread width TW, drainage performance of the tread portion2may deteriorate. When the widths W1are more than 5% of the tread width TW, wear resistance and uneven wear resistance may deteriorate due to reduction of the rubber volume.

As illustrated inFIG. 2, the depths D1of the central main grooves3are preferably in a range of from 10 to 20 mm. When the depths D1are less than 10 mm, drainage performance of the tread portion2may deteriorate. When the depths D1are more than 20 mm, wear resistance and uneven wear resistance may deteriorate due to reduction of rigidity of the tread portion2.

Preferably, the widths W2of the shoulder main grooves4, for example, are in a range of from 1.5% to 5% of the tread width TW. Preferably, the depths D2of the shoulder main grooves4are in a range of from 10 to 20 mm.

The tread portion2is separated into a plurality of land portions by the central main grooves3and the shoulder main grooves4. That is, the tread portion2is separated into a central land portion5disposed between a pair of the central main grooves3and3, a pair of middle land portions6each disposed between one of the central main grooves3and one of the shoulder main grooves4, and a pair of shoulder land portions7each disposed axially outward of each shoulder main groove4.

FIG. 3illustrates an enlarged development view of the central land portion5and a pair of the central main grooves3and3. The central land portion5is provided with a plurality of central lateral grooves51connecting the central main grooves3and3. Each of the central lateral grooves51connects the long sides3aof a pair of the central main grooves3. In the preferred embodiment, each end of the central lateral grooves51, for example, is connected to a circumferential middle portion of the long side3a. Such a central lateral groove51may improve wet performance of the heavy-duty tire by offering an excellent drainage performance between the central main grooves3and3as well as edge effect.

The central land portion5is separated into a plurality of central blocks52by the central lateral grooves51. Thus, the central land portion5is formed as a row53of plurality of the central blocks52which are arranged apart from one another in the circumferential direction of the tire.

Chamfered portions54are provided on acute angle corners of the central blocks52where the central main groove3and the central lateral grooves51intersect. The acute angle corners of the central blocks52are positioned in a diagonal line of the central blocks52. The chamfered portions54may promote the water flow between the central main groove3and the central lateral grooves51. Furthermore, the chamfered portions54may relax the stress of the corners of the blocks to suppress damage such as chipping. Alternatively, a rounded corner portion may be provided on the corners instead of the chamfered portions54.

FIG. 4illustrates an enlarged view of the central main groove3, the shoulder main groove4and the middle land portion6. The middle land portion6is provided with a plurality of middle lateral grooves61each connecting between the central main groove3and the shoulder main groove4. The middle lateral grooves61are inclined in an opposite direction to the central lateral grooves51. Each of the middle lateral grooves61connects one of the outer zigzag corners3o(an intersection between the long side3aand the short side3b) of the central main groove3with one of the inner zigzag corners4i(an intersection between the long side4aand the short side4b) of the shoulder main groove4. Such a middle lateral groove61may offer an excellent drainage performance as well as edge effect on a portion between the central main groove3and the shoulder main groove4, thereby improving wet performance of the heavy-duty tire.

The middle land portion6is divided into a plurality of middle blocks62by the middle lateral grooves61. Thus, the middle land portion6is formed as a row63of a plurality of middle blocks62arranged in the circumferential direction of the tire apart from one another.

FIG. 5illustrates a partial enlarged view of the tread portion2viewed from one of the tread edges Te.FIG. 6illustrates an enlarged view of the middle blocks62viewed from on the side of the tire equator C. As illustrated inFIGS. 4 to 6, each middle block62is provided with an inclined slot64on the side of the central main grooves3, wherein the inclined slot64has a depth increasing gradually toward the central main groove3.

The inclined slot64is provided on a location facing one of the central lateral grooves51through the central main groove3. Here, the inclined slot64facing one of the central lateral grooves51should be understood to include an aspect where at least a part of the inclined slot64is provided within a projected region in which the central lateral groove51is projected in the axial direction of the tire. Such an inclined slot64may promote the water flow toward the central lateral groove51from the middle blocks62to improve drainage performance under the middle blocks62.

In this embodiment, since the chamfered portions54are provided on the corners of the central blocks52which faces the inclined slot64through the central main groove3, the water flow toward the central lateral groove51from the middle blocks62can further be promoted.

As illustrated inFIG. 4, a circumferential region65where the central lateral groove51and the inclined slot64face one another across the central main groove3is indicated by hatching. Preferably, the region65has a circumferential length L1in a range of from 25% to 50% of a circumferential length L2of the central lateral groove51. That is, the inclined slot64and the central lateral groove51which faces the inclined slot64are overlapped one another in a circumferential region of from 25% to 50% of the circumferential length L2of the central lateral groove51.

When the length L1is less than 25% of the length L2, the effect of promoting the flow of water toward the central lateral groove51from the middle blocks62by the inclined slot64may decrease. On the other hand, when the length L1is more than 50% of the length L2, the flow of water toward the center lateral grooves51from one side of the middle blocks62with respect to the tire equator C tends to excessively be strong, and such a water flow may inhibit the flow of water toward the center lateral grooves51from the other side of the middle blocks62. Accordingly, the drainage performance of the entire tread portion2may be lowered.

As illustrated inFIG. 4, each of the inclined slots64extends from the central main groove3toward the shoulder main groove4, and terminates within each middle block62without reaching the shoulder main groove4. A length of the inclined slot64from its opening64afacing the central main groove to its end64bis in a range of from 55% to 65% of the width W1of the central main groove3.

When the length L3is less than 55% of the width W1, due to insufficient capacity of the inclined slot64, the effect of promoting the flow of water directed from the middle blocks62to the central lateral grooves51may decrease. On the other hand, when the length L3is more than 65% of the width W1, due to insufficient rubber volume of the middle blocks62, wear resistance may deteriorate. Furthermore, the rigidity of the middle blocks62may be lowered, and uneven wear resistance may be decreased.

As illustrated inFIGS. 2B and 6, the depth D3of the inclined slot64at the opening64a, i.e., the depth of the deepest portion of the inclined slot64, is preferably in a range of from 50% to 100% of the depth D1of the central main groove3.

When the depth D3is less than 50% of the depth D1, the effect of promoting the water of flow toward the central lateral grooves51from the middle blocks62by the inclined slot64may decrease.

As illustrated inFIGS. 2B and 6, the inclined slot64includes a slope64con its bottom, and the slope64cis inclined radially inwardly from the ground contact surface62sof the middle block62. Preferably, the angle θ formed between the ground contact surface62sof the middle block62and the slope64cis in a range of from 50 to 70 degrees, for example.

When the angle θ is less than 50 degrees, due to insufficient volume of the inclined slot64, the effect of promoting the flow of water going toward the central lateral grooves51from the middle block62may decrease. On the other hand, when the angle θ is more than 70 degrees, due to insufficient rubber volume of the middle blocks62, wear resistance may deteriorate. Furthermore, the rigidity of the middle blocks62may be lowered, and uneven wear resistance may also be decreased.

As illustrated inFIG. 1, in this embodiment, the inclined slots64and64provided on both sides of each central lateral groove51are located in different positions from each other with respect to the circumferential direction of the tire. Such inclined slots64effectively suppress uneven wear of the middle blocks62.

As illustrated inFIGS. 4 to 6, chamfered portions66are provided on acute angle corners of the middle blocks62where the central main groove3or the shoulder main groove4intersects the middle lateral grooves61. The acute angle corners of the middle blocks62are positioned in a diagonal line of the middle blocks62. The chamfered portions66may promote the flow of water between the central main groove3and the middle lateral grooves61as well as the flow of water between the shoulder main groove4and the middle lateral grooves61. Furthermore, the chamfered portions66may relax the stress of the corners of the blocks to suppress damage such as chipping. Alternatively, a rounded corner portion may be provided on the corners instead of the chamfered portions66.

FIG. 7illustrates an enlarged view of the shoulder main groove4and the shoulder land portion7. The shoulder land portion7is provided with a plurality of shoulder lateral grooves71each communicating the shoulder main groove4with the tread edge Te. The shoulder lateral grooves71communicate between the long sides4aof the shoulder main groove4and the tread edge Te. The shoulder land portion7is divided into a plurality of shoulder blocks72by a plurality of the shoulder lateral grooves71. Thus, the shoulder land portion7is formed as a row73of a plurality of the shoulder blocks72arranged in the circumferential direction of the tire apart from one another.

The shoulder land portion7is provided with a plurality of shoulder lateral sipes74that extend from the shoulder main groove4to the tread edge Te. As used herein, a “sipe” means a cut having a width of equal to or less than 1.0 mm, and which is distinguishing from a groove for drainage. The shoulder lateral sipes74connect the outer zigzag corners4oof the shoulder main groove4with the tread edge Te. Due to edge effect of the shoulder lateral sipes74, wet performance of the heavy-duty tire can be improved.

As illustrated inFIG. 3, the central blocks52have an axial maximal width WA in a range of from 0.15 to 0.25 times of the tread width TW (shown inFIG. 1, and the same hereinafter). The central blocks52may offer an excellent wear resistance while maintaining wet performance.

As illustrated inFIGS. 3 and 4, the middle blocks62have an axial length WB in a range of from 95% to 105% of the axial length WA of the central blocks52, for example.

When the length WB is less than 95% of the above mentioned length WA, there is a possibility that uneven wear occurs on the middle blocks62due to insufficient rubber volume of the middle blocks62. On the other hand, when the length WB is more than 105% of the length WA, there is a possibility that uneven wear occurs on the central blocks52due to insufficient rubber volume of the central blocks52.

Similarly, as illustrated inFIGS. 3 and 7, an axial length WC of the shoulder blocks72, for example, is in a range of from 95% to 105% of the axial length WA of the central blocks52.

When the length WC is less than 95% of the length WA, there is a possibility that uneven wear occurs on the shoulder blocks72due to insufficient rubber volume of the shoulder blocks72. On the other hand, when the length WC is more than 105% of the length WA, there is a possibility that uneven wear occurs on the central blocks52due to insufficient rubber volume of the central blocks52.

Preferably, the land ratio of the tread portion2having the above-mentioned pattern is in a range of from 65% to 75%, for example.

When the land ratio of the tread portion2is less than 65%, there is a possibility that the wear resistance and the uneven wear resistance is deteriorated due to insufficient rubber volume of the tread portion2and reduction of rigidity of the tread portion2. Furthermore, chipping may be occurred on the central blocks52, the middle blocks62and the shoulder blocks72due to rigidity reduction of the tread portion2. On the other hand, when the land ratio of the tread portion2is more than 75%, drainage performance may be deteriorated due to reduction of groove volume of the tread portion2.

As described above, in the heavy-duty tire according to the present embodiment, since the middle lateral grooves61connect the outer zigzag corners30of the central main groove3to the inner zigzag corners4iof the shoulder main groove4, drainage performance of a portion among the central main groove3, the shoulder main groove4and the middle lateral grooves61can be improved.

Furthermore, the central lateral grooves51connect between the long sides3aand3aof the central main grooves3, and the middle blocks62are provided with inclined slots64on a location facing one of the central lateral grooves51through the central main groove3. Since the inclined slot64has a depth gradually increasing toward the central main groove3, the water flow directed toward the central lateral groove51from the middle blocks62can be promoted, thereby improving drainage performance around the middle blocks62. Thus, drainage performance can be improved without increasing groove volume of the tread portion2. Consequently, wear resistance, uneven wear resistance and wet performance of the heavy-duty tire can be improved in high level.

FIG. 8illustrates a developed view of the tread portion in accordance with another embodiment of the heavy-duty tire.FIG. 9illustrates a cross-sectional view of the tread portion2taken along lines A-A ofFIG. 8. In the heavy-duty tire according to the present embodiment, note that portions which are not described in the following can be embodied as the configuration of the heavy duty tire as illustrated inFIGS. 1 to 7discussed above.

InFIG. 8, angles α1of long sides3aof the central main grooves3are preferably in a range of not less than 3 degrees, more preferably in a range of not less than 5 degrees, but preferably in a range of not more than 9 degrees, more preferably in a range of not more than 7 degrees, relative to the circumferential direction of the tire. When the angles α1are less than 3 degrees, it may be difficult to obtain sufficient traction on wet condition due to insufficient axial edge components. On the other hand, when the angles α1are more than 9 degrees, it may be difficult to offer sufficient wet performance due to reduction of the drainage performance of the central main grooves3caused by a large zigzag-amplitude of the central main groove3.

As with the central main grooves3, angles α2of the long sides4aof the shoulder main grooves4are preferably in a range of from not less than 3 degrees, more preferably not less than 5 degrees, but preferably in a range of not more than 9 degrees, more preferably not more than 7 degrees, relative to the circumferential direction of the tire. The heavy-duty tire in accordance with the present embodiment exhibits an excellent wet performance since the angles α1of the long sides3aof the central main grooves3and the angles α2of the long sides4aof the shoulder main grooves4are set in a suitable range.

FIG. 10illustrates a central region of the tread portion2including a pair of the central main grooves3and3. Each central main groove3includes a first groove edge3con the side of the tire equator C and a second groove edge3don the side of the tread edge Te. The first groove edge3cincludes a plurality of first zigzag corners3jlocated nearest the tread edge Te. The second groove edge3dincludes a plurality of second zigzag corners3plocated nearest the tire equator C.

Each first zigzag corner3jis located on the side of the tire equator C with respect to each second zigzag corner3p. That is, as illustrated by hatching inFIG. 10, the central main groove3includes a straight grooved region3E that straightly extends along the circumferential direction of the tire between the first zigzag corners3jand the second zigzag corners3p. In this embodiment, since the central main groove3includes the straight grooved region3E, drainage performance of the central main groove3may be improved, thereby improving wet performance of the heavy-duty tire.

The width of the straight grooved region3E corresponds to an axial distance W11measured from one of the first zigzag corners3jto one of the second zigzag corners3p. A ratio W11/TW of the distance W11to the tread width TW, for example, is preferably in a range of not less than 0.005, more preferably not less than 0.01, and preferably not more than 0.02, more preferably not more than 0.015.

When the ratio W11/TW is less than 0.005, it may be difficult to improve drainage performance of the central main groove3sufficiently due to lack of the width of the straight grooved region3E. On the other hand, when the ratio W11/TW is more than 0.02, the wear resistance and uneven wear resistance may be deteriorated due to lack of rubber volume of the central region of the tread portion2.

FIG. 11illustrates a middle region of the tread portion2which includes one of the central main grooves3and one of the shoulder main grooves4. The shoulder main groove4includes a third groove edge4con the side of the tire equator C and a fourth groove edge4don the side of the tread edge Te. The third groove edge4cincludes a plurality of third zigzag corners4jlocated nearest the tread edge Te. The fourth groove edge4dincludes a plurality of fourth zigzag corners4plocated nearest the tire equator C.

Each third zigzag corner4jis located on the side of the tire equator C with respect to each fourth zigzag corner4p. That is, as illustrated by hatching inFIG. 11, the shoulder main groove4includes a straight grooved region4E that straightly extends along the circumferential direction of the tire between the third zigzag corners4jand the fourth zigzag corners4p. In this embodiment, since the shoulder main groove4includes the straight grooved region4E, drainage performance of the shoulder main groove4may be improved, thereby improving wet performance of the heavy-duty tire.

The width of the straight grooved region4E corresponds to an axial distance W21measured from one of the third zigzag corners4jto one of the fourth zigzag corners4p. A ratio W21/TW of the distance W21to the tread width TW, for example, is preferably in a range of not less than 0.005, more preferably not less than 0.01, and preferably not more than 0.02, more preferably not more than 0.015.

When the ratio W21/TW is less than 0.005, it may be difficult to improve drainage performance of the shoulder main groove4sufficiently due to lack of the width of the straight grooved region4E. On the other hand, when the ratio W21/TW is more than 0.02, the wear resistance and uneven wear resistance may be deteriorated due to lack of rubber volume of the central region of the tread portion2.

As illustrated inFIG. 10, each of the central blocks52is formed into an octagonal shape having a pair of stepped portions55by the zigzag central main grooves3. Preferably, the central main grooves have an axial zigzag-amplitude W12in a range of not less than 10% of a maximal axial length WA of the central blocks52, more preferably not less than 13%, but preferably not more than 18%, more preferably not more than 16%.

When the zigzag-amplitude W12is less than 10% of the maximal axial length WA, it may be difficult to obtain sufficient traction on wet road due to lack of an axial edge component. On the other hand, the zigzag-amplitude W12is more than 18% of the maximal axial length WA, it may be difficult to obtain sufficient wet performance due to lack of drainage performance of the central main grooves3.

FIG. 12illustrates a partial enlarged view of the tread portion2viewed from the tread edge Te.FIG. 13illustrates an enlarged view of the shoulder main groove4and the shoulder land portion7. In this embodiment, the shoulder land portion7is formed continuously in the circumferential direction of the tire. Since the shoulder land portion7has a high rigidity, uneven wear such as shoulder wear may be suppressed. Furthermore, since the shoulder land portion7may ensure sufficient rubber volume around the tread edge Te, wear resistance and uneven wear resistance may be improved.

As illustrated inFIGS. 10 and 13, the axial maximal length WC of the shoulder land portion7, for example, is in a range of not less than 95%, more preferably not less than 98%, but preferably not more than 105%, more preferably not more than 102% of the axial maximal length WA of the central blocks52.

When the maximal length WC is less than 95% of the length WA, uneven wear may be occurred on the shoulder land portion7due to lack of rubber volume of the shoulder land portion7. On the other hand, when the maximal length WC is more than 105% of the length WA, uneven wear may be occurred on the central block52due to lack of rubber volume of the central blocks52.

In this embodiment, since the shoulder land portion7is continuous in the circumferential direction of the tire, sufficient land ratio of the tread portion2can be ensured, and wear resistance and uneven wear resistance can be improved. Preferably, the land ratio of the tread portion2is set in a range of not less than 70%.

When the land ratio of the tread portion2is less than 70%, wear resistance and uneven wear resistance may be deteriorated due to rigidity reduction of the tread portion2caused by reduction of rubber volume of the tread portion2. Furthermore, chipping may be occurred on the central blocks52and the middle blocks62due to rigidity reduction of the tread portion2.

FIG. 14illustrates a development view of the tread portion of the heavy-duty tire in accordance with another embodiment of the present embodiment.FIG. 15illustrates a cross-sectional view of the tread portion2taken along a line A-A ofFIG. 14. In the heavy-duty tire according to the present embodiment, note that portions which are not described in the following can be embodied as the configuration of the heavy duty tire as illustrated inFIGS. 1 to 13discussed above.

In this embodiment, the central lateral grooves51are connected approximately vertical to the long sides3a. The central lateral grooves51are inclined at an angle β1of from 5 to 15 degrees relative to the axial direction of the tire.

The widths W5of the central lateral grooves51, for example, are in a range of from 5.0 to 10.0 mm. Such a central lateral groove51may improve wet performance while ensuring wear resistance.

The angles β2of the respective middle lateral grooves61, for example, are in a range of from 5 to 15 degrees relative to the axial direction of the tire. The widths W6of the middle lateral grooves61, for example, are in a range of from 5.0 to 10.0 mm.

FIG. 16illustrates an enlarged view of the middle land portion6.FIG. 17illustrates an enlarged perspective view of a middle block62. Each of the middle blocks62includes a ground contact surface62sin substantially parallelogram shape. Each of the middle blocks62is provided with the inclined slot64where the edge62eis recessed.

The slope64c, for example, has an approximately trapezoidal shape in which a first edge67aon the ground contact surface62sof the middle blocks62is parallel to a second edge67bon the sidewall62tof the middle blocks62.

Such a middle block62with the inclined slot64may exhibit high rigidity as compared with a block with a narrow groove which perfectly traverses the block. Thus, wear resistance of the block can be improved. In addition, since the inclined slot64includes the slope64c, rigidity of the middle blocks62around the inclined slot64may be changed gradually from a middle side of the block toward the central main groove. Accordingly, the middle blocks62may offer an excellent uneven wear resistance as compared with a block provided with the narrow groove or lug groove.

Furthermore, each inclined slot64may guide water from between the ground contact surface62sof the middle blocks62and the ground smoothly to the central main groove3when traveling on wet road. Since each inclined slot64faces each central lateral groove51one another, the water guided by the inclined slot64into the central main groove3is drained effectively outside the tire in conjunction with water in the central lateral groove51. Accordingly, the pneumatic tire of the present invention may offer an excellent wet performance.

FIG. 18Aillustrates a cross-sectional view taken along lines B-B ofFIG. 17. As illustrated inFIG. 18A, in order to further improve the advantageous effect, the angle θ of the slope64crelative to the ground contact surface is preferably in a range of not less than 45 degrees, more preferably not less than 50 degrees, but preferably in a range of not more than 70 degrees, more preferably not more than 60 degrees.

FIG. 18Billustrates an enlarged plan view of the ground contact surface62sof a middle block62ofFIG. 17. As illustrated inFIG. 5B, the inclined slot64includes an opening edge20on the ground contact surface62s. The opening edge20includes a circumferential first edge21and a pair of second edges22extending outwardly of the block from both ends21tof the first edge21. The second edges22and22are inclined in an opposite direction each other so that the circumferential width of the opening edge20increases toward outside of the block. The inclined slot64may guide the water which is pushed out by the ground contact surface of the middle block62effectively toward the central main groove when traveling on wet road.

As illustrated inFIG. 17, the inclined slot64, for example, includes a pair of slot sidewalls64eeach of which extends from each second edge22to the bottom64dof the inclined slot64between the block sidewall62tand the slope64c. The slot sidewalls64eis formed as a plane having an approximately a triangular shape, for example.

As illustrated inFIG. 18B, the circumferential length L4of the inclined slot64is preferably not less than 0.08 times of the circumferential maximal length L5of the middle block62, more preferably not less than 0.1 times, but preferably not more than 0.16 times, more preferably not more than 0.14 times. The inclined slot64may improve wear resistance of the middle blocks62while ensuring its circumferential rigidity.

In the same point of view, the length L4of the inclined slot64is preferably greater than the axial width W3of the inclined slot64. A ratio W3/L4of the width W3to the length L4of the inclined slot64is preferably not less than 0.65, more preferably not less than 0.68, but preferably not more than 0.75, more preferably not more than 0.72.

Preferably, the width W3of the inclined slot64is not less than 0.08 times of the axial width WB of the middle block62, more preferably not less than 0.11 times, but preferably not more than 0.17 times, more preferably not more than 0.14 times. Such an inclined slot64may offer an excellent steering stability while improving wet performance and wear resistance.

As illustrated inFIG. 18A, the maximal depth D3of the inclined slot64in the radial direction is preferably greater than the width W3(shown inFIG. 18B) of the inclined slot64. The maximal depth D3of the inclined slot64is preferably not less than 0.45 times of the depth D1of the central main groove3, more preferably not less than 0.48 times, but preferably not more than 0.55 times, more preferably not more than 0.52 times. Such an inclined slot64may improve wet performance and wear resistance in good balance.

FIG. 19illustrates an enlarged view of the central land portion5. Each of the central blocks52includes a first portion56formed between a pair of the short sides3band3bof the central main grooves3, a second portion57located on one side of the first portion56in the circumferential direction of the tire, and a third portion58located on the other side of the first portion56in the circumferential direction of the tire.

The first portion56has a ground surface area56sin an approximately parallelogram shape.

The second portion57and the third portion58have ground contact surface areas57sand58sin an approximately trapezoidal shape. The ground contact surface area57sof the second portion57has substantially the same shape as the ground contact surface area58of the third portion58. The second portion57is located in different position to the third portion58in the axial direction of the tire. Such a central block52may increase traction on wet and snow conditions.

FIG. 20illustrates an enlarged view of the shoulder land portion7. The shoulder lateral grooves71, for example, extend from the shoulder main groove4to the tread edge Te. The shoulder lateral grooves71, for example, are inclined in the same direction as the middle lateral grooves61(shown inFIG. 1). The angles β5of the shoulder lateral grooves71, for example, are in a range of from 5 to 15 degrees relative to the axial direction of the tire. The widths W7of the shoulder lateral grooves71are in a range of from 8 to 12 mm, for example.

As illustrated inFIG. 15, the depths D7of the shoulder lateral grooves71, for example, are in a range of from 0.15 to 0.20 times of the depth D2of the shoulder main groove4. Such a shoulder lateral groove71may offer an excellent steering stability by enhancing rigidity of the shoulder land portion7.

As illustrated inFIG. 20, the shoulder blocks72, for example, includes a first shoulder block piece75and a second shoulder block piece76which are separated by a shoulder lateral sipe74that extends straightly from the shoulder main groove4to the tread edge Te.

The first shoulder block piece75has an approximately trapezoidal ground contact surface75s. The axial width W9of the first shoulder block piece75, for example, is in a range of from 0.12 to 0.18 times of the tread width TW.

The second shoulder block piece76has an approximately pentagonal ground contact surface76sthat includes an axially inner edge77protruding axially inwardly. The axial width W10of the second shoulder block piece76is greater than the width W9of the first shoulder block piece75. Such a second shoulder block piece76may improve steering stability by enhancing axial rigidity of the shoulder blocks72.

In order to further improve the advantageous effect, a ratio W9/W10of the width W9of the first shoulder block piece75to the width W10of the second shoulder block piece76is preferably not less than 0.85, more preferably not less than 0.87, but preferably not more than 0.95, more preferably not more than 0.93.

The shoulder lateral sipes74, for example, are inclined in the same direction as the shoulder lateral grooves71. The angles β6of the shoulder lateral sipes74are in a range of 5 to 15 degrees relative to the axial direction of the tire, for example. Such a shoulder lateral sipe74may suppress uneven wear on the shoulder blocks72by uniformizing the ground contact pressure acted hereon.

While the embodiments in accordance with the present invention have been described in detail, the present invention is not limited to the illustrated embodiments, but can be modified and carried out in various aspects.

EXAMPLE

Heavy-duty tires having a tire size of 215/75R17.5 and a basic structure illustrated inFIG. 1were manufactured based on the detail shown in Table 1. Then, wet performance and uneven wear resistance were tested. The test procedures are as follows.

Each test tire was installed to all the wheels of a truck having maximum carrying capacity of 4 tons (2-D). Then, the test truck was driven on a circular course of an asphalt road covered with water of 1.4 to 1.6 mm depth, and the average lateral acceleration for the speed range of from 70 to 90 km/h was measured while increasing the speed of the truck in stage. The results are indicated using an index based on Ex. 1 being 100. The larger the value, the better the wet performance is.

After the truck traveled for 10,000 km at a constant load, remaining groove depths of the central main grooves and the shoulder main grooves were measured. Then uneven wear amount of the tread portion was calculated based on these remaining depths of the main grooves. The results are indicated using an index based on Ex. 1 being 100. The larger the value, the better the uneven wear resistance is.

As it is clear from Table 1, it is confirmed that the heavy-duty tires in accordance with the examples improve wet performance effectively while improving wear resistance and uneven wear resistance as compared with comparative examples.

Heavy-duty tires having a tire size of 215/75R17.5 and a basic structure illustrated inFIG. 8were manufactured based on the detail shown in Table 2. Then, wet performance and uneven wear resistance were tested. The test procedures are as follows.

Each test tire was installed to the rear wheels of a truck loaded with a half load of maximum carrying capacity of 4 tons (2-D) using a rim of 17.5×6.00 with an inner pressure of 700 kPa. Then, a test driver started the truck using the second gear position by engaging its clutch at the timing of a 1,500 rpm engine speed on a wet asphalt road covered with water of 5 mm deep, and measured the time for traveling to 10 m distance. The test results were evaluated as the reciprocal of the time and were indicated using an index based on Ex. 1 being 100. The larger the value, the better the wet performance is.

The tire of Ex. 1 was installed in one side of the rear wheels of the above mentioned truck, and the other test tire was installed in the other side of the rear wheels, and then the test truck was traveled on a standard road until either one of the tires wears down at 50%. After traveling, condition of uneven wear was checked by naked eyes. The test results are indicated using a score based on Ex. 1 being 5. The larger the value, the better the uneven wear resistance is.

As it is clear from Table 2, it has been confirmed that the heavy-duty tires in accordance with the examples improve wet performance effectively without deteriorating uneven wear resistance as compared with comparative examples.

Heavy-duty tires having a tire size of 215/75R17.5 and a basic structure illustrated inFIG. 14were manufactured based on the detail shown in Table 3. As Ref. 1, the tire having middle blocks each of which is provided with a middle narrow-groove extending across the block, as illustrated inFIG. 21, was manufactured. As Ref. 2, the tire having middle blocks each of which is not provided any slots, as illustrated inFIG. 22, was manufactured. Then, wet performance and wear resistance of each tire was tested. The common specifications of tires and test procedures are as follows.Rim: 6.0×17.5Tire inner pressure: 700 kPaTire installing position: Rear wheels (drive wheels)Test vehicle: Truck loaded with a half load of maximum carrying capacity of 4 tons at front side of platform

Wear Resistance

The test truck was traveled on a standard road for a certain distance. After traveling, remaining groove depths of the central main grooves were measured. The results are indicated using an index based on Ref. 1 being 100. The larger the value, the better the wear resistance is.

Wet Performance

Using the test vehicle, passing time when the vehicle passes through the test course of full length 10 m under the following conditions was measured. The test results were evaluated as the reciprocal of the time and were indicated using an index based on Ref. 1 being 100. The larger the value, the better the wet performance is. Here, the road condition is an asphalt road covered with water of 5 mm deep, and the test vehicle was started to run by connecting clutch in the second gear with 1500 rpm engine speed fixed.

Test results are shown in Table 3.

From the test results, it is confirmed that the example tires offer an excellent wet performance and wear resistance.

REFERENCE SIGNS LIST