Pneumatic tire for off-road motorcycle

A pneumatic tire for motorcycle for running on rough terrain comprises a tread portion 2 provided with a directional tread pattern having an intended tire rotational direction X. The tread portion 2 is provided with crown blocks 4 and middle blocks 5. The crown block 4 comprises an axially long main portion 4m and a protruding portion 4t protruding from an axial central part of the main portion 4m toward the opposite direction of the intended tire rotational direction. In a block group Y consisting of each crown block 4 and two axially adjacent middle blocks 5 disposed one on each side of the crown block 4, the center P1 of gravity of the crown block 4 is positioned on the toe-side of each middle block 5, and the center P2 of gravity of each middle block 5 is positioned on the heel-side of the crown block 4.

BACKGROUND OF THE INVENTION

The present invention relates to a pneumatic tire for off-road motorcycle, more particularly to a block tread pattern capable of improving transitional characteristics during cornering and the traction performance on rough terrain.

Japanese Patent Application Publication No. 2009-67245 (hereinafter the patent document 1) discloses a pneumatic tire for running on rough terrain which is a directional tire provided in the tread portion with crown blocks disposed on the tire equator, and middle blocks disposed on each side of the crown block in the tire axial direction. The drawings of the patent document 1 show that the middle blocks are arranged with different circumferential pitches from those of the crown blocks, and some of the crown blocks are aligned in line in the tire axial direction with some of the middle blocks.

Therefore, when the tread portion is deflected in the ground contacting patch, the aligned crown block and middle block are liable to contact with each other and to act as if one block. In such case, for example, it becomes difficult to lean the motorcycle body to initiate turning, and if the lean angle increases over a certain value, the motorcycle body is abruptly leant. Thus, there is a problem with transitional characteristics during cornering.

SUMMARY OF THE INVENTION

It is therefore, an object of the present invention to provide a pneumatic tire for motorcycle for running on rough terrain, in which transitional characteristics during cornering as well as the traction performance on rough terrain can be improved.

According to the present invention, a pneumatic tire for motorcycle for running on rough terrain comprises:

a tread portion provided with a directional tread pattern having an intended tire rotational direction,

the tread pattern comprising

crown blocks disposed on the tire equator, and

middle blocks disposed on each side in the tire axial direction of the crown blocks,

each crown block comprising

a main portion having an axially long shape such that the dimension in the tire axial direction is larger than the dimension in the tire circumferential direction, and

a protruding portion protruding from a central part in the tire axial direction of the main portion toward the opposite direction of the intended tire rotational direction, and

in a block group consisting of each crown block and two axially adjacent middle blocks disposed one on each side of the crown block,

the center of gravity of the crown block is positioned on the toe-side of each middle block, and

the center of gravity of each middle block is positioned on the heel-side of the crown block.

The pneumatic tire according to the present invention may have the following features:

(1) the heel-side edge of the tread of each crown block comprises an arc-shaped part curved convexly toward the opposite direction of the intended tire rotational direction;

(2) the tread of each crown block has two axially inner and outer side edges each extending parallel with the tire circumferential direction;

(3) the tread of each middle block has an axially long shape such that the dimension in the tire axial direction is larger than the dimension in the tire circumferential direction;

(4) the heel-side edge of the tread of each middle block comprises an arc-shaped part curved convexly toward the opposite direction of the intended tire rotational direction;

(5) the heel-side edge of the tread of each middle block is inclined to the intended tire rotational direction toward the axially outside;

(6) the tread of each middle block has two axially inner and outer side edges extending parallel with the tire circumferential direction.

In this application including specification and claims, various dimensions, positions and the like of the tire refer to those under a normally inflated unloaded condition of the tire unless otherwise noted.

The tread width TW is the width measured under the normally inflated unloaded condition, as the axial distance between the tread edges Te.

The normally inflated unloaded condition is such that the tire is mounted on a standard wheel rim and inflate to a standard pressure but loaded with no tire load.

The standard wheel rim is a wheel rim officially approved or recommended for the tire by standards organizations, i.e. JATMA (Japan and Asia), T&RA (North America), ETRTO (Europe), TRAA (Australia), STRO (Scandinavia), ALAPA (Latin America), ITTAC (India) and the like which are effective in the area where the tire is manufactured, sold or used.

The standard pressure and the standard tire load are the maximum air pressure and the maximum tire load for the tire specified by the same organization in the Air-pressure/Maximum-load Table or similar list. For example, the standard wheel rim is the “standard rim” specified in JATMA, the “Measuring Rim” in ETRTO, the “Design Rim” in TRA or the like. The standard pressure is the “maximum air pressure” in JATMA, the “Inflation Pressure” in ETRTO, the maximum pressure given in the “Tire Load Limits at various cold Inflation Pressures” table in TRA or the like. The standard load is the “maximum load capacity” in JATMA, the “Load Capacity” in ETRTO, the maximum value given in the above-mentioned table in TRA or the like.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will now be described in detail in conjunction with accompanying drawings.

The present invention is directed to a pneumatic tire for motorcycle.

As well known in the art, a pneumatic tire comprises a tread portion2, a pair of axially spaced bead portions, a pair of sidewall portions extending between the tread edges Te and the bead portions, a carcass extending between the bead portions through the tread portion2and the sidewall portions, and a tread reinforcing cord layer disposed radially outside the carcass in the tread portion.

As a characteristic of a motorcycle tire, the tread portion2(inclusive of the carcass, the tread reinforcing cord layer and a tread rubber thereon) is convexly curved so that the tread face between the tread edges Te is curved like an arc swelling radially outwardly, and the maximum cross sectional width of the tire occurs between the tread edges.

The pneumatic tire according to the present invention is a directional tire, and the intended tire rotational direction X is indicated by characters and/or sign in the sidewall portion for example.

As shown inFIG. 1, the tread portion2is provided with a plurality of blocks3. The blocks3include a plurality of crown blocks4disposed on the tire equator C, and a plurality of middle blocks5disposed on each side in the tire axial direction of the crown blocks4. In this embodiment, the blocks3further include a plurality of shoulder blocks6arranged along each tread edge Te.

As shown inFIG. 2, the crown block4comprises a main portion4mand a protruding portion4t.

The main portion4mhas an axially long shape such that its dimension L2in the tire axial direction is more than its dimension L1in the tire circumferential direction. Such crown block4is increased in the axial edge component, and the traction performance during straight running can be improved.

The axial dimension L2of the main portion4mis preferably set in a range of 20% to 30% of the tread width TW. If the axial dimension L2of the main portion4mis less than 20% of the tread width TW, then there is a possibility that the axial edge component becomes insufficient. If the axial dimension L2of the main portion4mis more than 30% of the tread width TW, then drainage in a tire equator side is decreased, and the force acting on the crown block4when contacting with the ground becomes large, and there is a possibility that the crown block4causes excessive leaning deformation in the tire circumferential direction.

In order to prevent the crown block4from causing leaning deformation, the circumferential dimension L1of the main portion4mis preferably not less than 25% of the axial dimension L2of the main portion4m.

However, if the area of the tread S of the crown block4is excessively increased, there is a possibility that the crown block4can not bite into the earth. Therefore, the circumferential dimension L1of the main portion4mis preferably set in a range of not more than 35% of the axial dimension L2of the main portion4m.

The protruding portion4tprotrudes from a central portion in the tire axial direction of the main portion4mtoward the opposite direction of the intended tire rotational direction X. Such protruding portion4tincreases the circumferential edge component of the crown block4, and supports the main portion4mfrom the toe-side to prevent the crown block4from causing excessive leaning deformation in the tire circumferential direction.

The dimension L3in the tire circumferential direction of the protruding portion4tof the crown block4is preferably set in a range of from 50% to 65% of the circumferential dimension L1of the main portion4m.

If the circumferential dimension L3is less than 50% of the circumferential dimension L1, it becomes difficult to prevent the leaning deformation of the crown block4. If the circumferential dimension L3is more than 65% of the circumferential dimension L1, then wear and tearing-off are be liable to occur in a toe-side part of the protruding portion4t, and thereby it becomes difficult to prevent the excessive leaning deformation in the tire circumferential direction of the crown block4.

Preferably, the dimension L4in the tire axial direction of the protruding portion4tis set in a range of from 10% to 15% of the axial dimension L2of the main portion4m.

If the axial dimension L4is less than 10% of the axial dimension L2, there is a possibility that excessive leaning deformation in the tire circumferential direction of the crown block4can not be prevented. If the axial dimension L4is more than 15% of the axial dimension L2, the rigidity of the crown block4is excessively increased, and it becomes hard to lean the motorcycle body to initiate cornering.

The above-mentioned middle block5has an axially long shape such that the dimension L6in the tire axial direction is larger than the dimension L5in the tire circumferential direction. Such middle block5is increased in the axial edge component, and the traction performance can be improved.

For example, the axial dimension L6of the middle block5is preferably set in a range of from 15% to 25% of the tread width TW.

If the axial dimension L6of the middle block is less than 15% of the tread width TW, there is a possibility that the axial edge component becomes insufficient. If the axial dimension L6of the middle block5is more than 25% of the tread width TW, there is a possibility that drainage in a tire equator side of the tread portion2is decreased.

For preventing the leaning deformation of the middle block5, it is preferred that the circumferential dimension L5of the middle block5is set in a range of not less than 75% of the axial dimension L6of the middle block5.

However, if the area of the tread s of the middle block5is excessively increased, there is a possibility that the middle block5can not bite into the earth. Therefore, the circumferential dimension L5of the middle block5is preferably set in a range of not more than 85% of the axial dimension L6of the middle block5.

If all of the crown blocks4and middle blocks5in the tread portion2are grouped into a plurality of block groups Y each consisting of one crown block4and two axially adjacent middle blocks5positioned one on each side of the crown block4, then, in each of the block groups Y,

the center P1of gravity of the crown block4is positioned on the toe-side in the intended tire rotational direction X of each of the two middle blocks5, and

the center P2of gravity of each of the two middle blocks5is positioned on the heel-side in the intended tire rotational direction X of the crown block4.

Therefore, it can be avoided that, when the tread portion is deflected in the ground contacting patch, the crown block4contacts with the axially adjacent middle block5and they act as if one block since the crown block4is circumferentially shifted from each middle block5.
Further, at least part of soil not trodden down by the middle blocks5is lead to on the heel-side of the crown block4, and trodden down by the crown block4whose ground pressure is relatively high. As a result, good traction performance can be obtained.
Accordingly, the tire in this embodiment can be improved in the transitional characteristics during cornering while improving the traction performance on rough terrain.

In order to effectively derive this advantageous effect, in each block group Y in this embodiment, each of the two middle blocks5is shifted in the tire circumferential direction from the crown block4so that they are not overlapped with each other in the tire circumferential direction.

Thereby, it can be certainly avoided that the crown block4and the middle block5act as if one block when the tread portion is deflected. Consequently, it is possible to further improve the transitional characteristics during cornering.

Preferably, in each of the block groups Y, the circumferential distance P3between the center P2of gravity of the middle block5and an intersecting point of the heel-side edge4aof the crown block4with the tire equator C is set in a range of from 13% to 30% of the arrangement pitch P of the crown blocks4. The arrangement pitch P is given by the circumferential distance between the above-mentioned intersecting points, for example.

In this embodiment, the crown block4is not overlapped with each of the two middle block5in the tire axial direction, and a space CW is formed therebetween. Such space CW ensures the prevention of the crown block4and the middle block5from acting as if one block. Further, the spaces CW expedite drainage of water and mud toward the tire circumferential direction, and helps to improve the wet/mud performance. In order to effectively derive such effects, the dimension in the tire axial direction of the space CW is preferably set in a range of from 4% to 10% of the tread width TW.

As shown inFIG. 1, the heel-side edge4aof the tread s of the crown block4and the heel-side edge5aof the tread s of each middle block5each have an arc-shaped part7curved convexly toward the opposite direction of the intended tire rotational direction X. Preferably, the arc-shaped part7extends over the entire length of the heel-side edge (4a,5a). Such arc-shaped parts7scratch up soil on the heel-side of the arc-shaped parts7and pack together so that the packed soil has an increased shearing force. This helps to further improve the traction performance.

Preferably, the heel-side edge5aof the tread s of each middle block5is inclined to the intended tire rotational direction toward the axially outside.

In each block group Y, such middle blocks5can lead soil along their heel-side edges5atoward the toe-side or toward the crown block4. This helps to further improve the traction performance.

The tread S of each middle block5has two axially inner and outer side edges5bextending parallel with the tire circumferential direction. The tread S of the crown block4has two side edges4bextending parallel with the tire circumferential direction.

In order to improve rigidity balance of each block, it is preferred that the toe-side edge5cof the tread S of each middle block5comprises an arc-shaped part7, and the toe-side edge4cof the tread s of the crown block4comprises an arc-shaped part7(in this embodiment, on each side of the protruding portion4t).

If all of the crown blocks4, middle blocks5and shoulder blocks6in the tread portion2are grouped into a plurality of block groups Y′ each consisting of the above-mentioned one crown block4and two axially adjacent middle blocks5and further two shoulder blocks6adjacent to the two middle blocks5, respectively,

then it is preferable that the one crown block4, two middle blocks5and two shoulder blocks6in each block group Y′ are arranged on a circular arc whose center is positioned on the tire equator C and on the heel-side of the same crown block4. Such block group Y′ scratches up soil and leads the soil along the circular arc toward the toe-side or toward the crown block4. Consequently, a larger shearing force can be obtained.

The shoulder block6has a circumferentially long shape such that the dimension L7in the tire circumferential direction is larger than the dimension L8in the tire axial direction. Such shoulder block6has a long circumferential edge component, and the traction performance during cornering can be improved.

The axial dimension L8of the shoulder block6is preferably set in a range of from 5% to 15% of the tread width TW.

If the axial dimension L8of the shoulder block6is less than 5% of the tread width TW, it becomes difficult to scratch up the soil. If the axial dimension L8of the shoulder block6is more than 15% the tread width TW, drainage in a tread edge side of the tread portion2is decreased.

The circumferential dimension L7of the shoulder block6is preferably set in a range of from 155% to 175% of the axial dimension L8of the shoulder block6.

If the circumferential dimension L7is less than 155% of the axial dimension L8, there is a possibility that the traction performance during cornering can not be improved. If the circumferential dimension L7is more than 175% of the axial dimension L8, drainage in the tread edge side is decreased.

In this embodiment, each middle block5is not overlapped with the adjacent shoulder block6in the tire axial direction, and there is a space SW therebetween. Such space SW prevents the middle block5and the shoulder block6from contacting with each other and acting as if one block. Further, the spaces SW expedite drainage of water and mud toward the tire circumferential direction, and helps to improve the wet/mud performance.

In order to effectively derive such effects, the dimension in the tire axial direction of the space SW is preferably set in a range of from 4% to 10% of the tread width TW.

It is preferable that the heel-side edge6aof the tread S of the shoulder block6comprises an arc-shaped part7curved convexly toward the opposite direction of the intended tire rotational direction X. Preferably, the arc-shaped part7extends over the entire length of the heel-side edge6a. Such arc-shaped part7scratches up soil on the heel-side of the arc-shaped parts7and pack together when running on rough terrain. Thus, the heel-side edge6ahelps to further improve the traction performance.

Preferably, the arc-shaped part7of the tread S of the shoulder block6has a radius of curvature smaller than that of the crown block4and that of the middle block5. Such shoulder block6can pack the soil together on the heel-side of the heel-side edge6ain spite of a circumferentially long shape.

Even if the shoulder block6is subjected to a large force when packing the soil together by the heel-side edge6a, excessive leaning deformation in the tire circumferential direction of the shoulder block6can be prevented since it has a circumferentially long shape.

Preferably, the axially inner side edge6bof the tread S of the shoulder block6is inclined to the axially outside toward the intended tire rotational direction X. Therefore, water and mud are led toward the opposite direction of the intended tire rotational direction X. Each block group Y′ scratches up soil and leads the soil along the side edges6btoward the toe-side or toward the middle blocks5. Such side edges6bhelp to improve the wet performance and the traction performance.

While detailed description has been made of a preferable embodiment of the present invention, the specific embodiment should not be construed as to limit the scope of the present invention; the present invention may be embodied in various forms.

Comparison Tests

Test tires for front wheel (tire size: 80/100-21, rim size: 21×1.85) and rear wheel (tire size: 120/80-19, rim size: 19×2.15) were experimentally manufactured. The test tires were mounted on the front wheel and rear wheel of a 450 cc motocross bike, and both tires were inflated to 80 kPa, a relatively low inflation pressure.

Running the bike in a test course, a professional rider evaluated transitional characteristics during cornering and traction performance into ten ranks. The higher rank number is better.

The test results and specifications of the test tires are shown in Table 1.

From the test results, it was confirmed that Embodiment tires were improved in the transitional characteristics during cornering and the traction performance.

REFERENCE SIGNS LIST