Pneumatic tire having branched extension blocks

A pneumatic tire comprises a pair of bead portions, a sidewall portion extending from the bead portions outward in a radial direction of the tire, a tread portion connected to an outer side of the sidewall portion in the radial direction of the tire through a shoulder portion, and extension block groups in each of which a plurality of extension blocks to extending from at least the shoulder portion to the sidewall portion are arranged in a circumferential direction of the tire. The extension block group includes a first extension block whose cross section height from a tire equator point exceeds ½ of a tire cross section height, and a second extension block whose cross section height from the tire equator point is lower than the first extension block.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a pneumatic tire capable of reinforcing a sidewall portion and enhancing durability, steering stability and noise reduction ability.

2. Description of the Related Art

As shown inFIG. 5, a pneumatic tire usually includes a pair of annular bead portions1, sidewall portions2extending from the respective bead portions1radially outward of the tire, and tread portions4connected to the respective sidewall portions2radially outward of the tire through a shoulder portion3. A carcass ply5forming a tire frame is disposed such as to extend between the bead portions1. The sidewall portion2is largely bent during running of a vehicle, but its thickness is relatively small, and there is a tendency that the durability, the steering stability and the noise reduction ability are deteriorated. To enhance them, it is preferable that the sidewall portion2is reinforced, but even if the rubber hardness is enhanced, the obtained effect is limited.

Hence, Japanese Patent Application Laid-open No. H8-197917 discloses a pneumatic tire in which the sidewall portion is provided with a plurality of ribs extending in an inclined direction with respect to a circumferential direction of the tire. According to the pneumatic tire, the sidewall portion can be reinforced over the substantially entire region of its cross section height. However, since a design of keynote of the outer surface of the sidewall portion is different from that of the tread surface, discontinuous portion of the reinforced region is generated in the vicinity of the shoulder portion in some cases, and the durability may be deteriorated in this discontinuous portion in some cases. If the sidewall portion is reinforced, the vibration can be suppressed and noise can be reduced, but frequency band of suppressed vibration is relatively narrow, and reduction effect of road noise is poor.

Japanese Patent Application Laid-open No. H11-291718 discloses a pneumatic tire in which a concave block extending from the shoulder portion to the sidewall portion is disposed in the circumferential direction of the tire. According to such a pneumatic tire, since a cross section height of the concave block from the tire equator point is set ½ or less of the tire cross section height, it is not possible to reinforce a thin portion of the sidewall portion exceeding ½ of the tire cross section height, and the effect for enhancing the durability is poor. Further, since the cross section height of the concave block is uniform, there is a problem that an interface of the reinforced region is formed along the circumferential direction of the tire and a crack is prone to be generated along the circumferential direction of the tire due to the sidewall portion is bent. Similarly, since the frequency band of vibration to be suppressed by the reinforcing is relatively narrow, the effect for reducing the road noise is poor.

Japanese Patent Application Laid-open No. 2000-16031 discloses a pneumatic tire in which an inclined groove extending in a direction which is inclined with respect to the circumferential direction of the tire is extended to a lower region of the sidewall portion. However, this structure is for overcoming outer appearance inconvenience of the tire, and is not for reinforcing the sidewall portion.

SUMMARY OF THE INVENTION

The present invention has been accomplished in view of the above circumstances, and it is an object of the invention to provide a pneumatic tire capable of reinforcing a sidewall portion and enhancing durability and steering stability, and capable of suppressing vibration in wide frequency band and enhancing the noise reduction ability.

The object can be achieved by the present invention having the following structure. That is, the invention provides a pneumatic tire comprising a pair of bead portions, a sidewall portion extending from the bead portions outward in a radial direction of the tire, a tread portion connected to an outer side of the sidewall portion in the radial direction of the tire through a shoulder portion, and extension block groups in each of which a plurality of extension blocks extending from at least the shoulder portion to the sidewall portion are arranged in a circumferential direction of the tire, wherein the extension block group includes a first extension block whose cross section height from a tire equator point exceeds ½ of a tire cross section height, and a second extension block whose cross section height from the tire equator point is lower than the first extension block.

According to the structure of the invention, since the pneumatic tire includes the extension block groups in each of which the plurality of extension blocks extending from at least the shoulder portion to the sidewall portion are arranged in the circumferential direction of the tire, it is possible to reinforce the sidewall portion without generating a discontinuous portion of reinforced region in the vicinity of the shoulder portion. Since the extension block group includes the first extension block whose cross section height from the tire equator point exceeds ½ of the tire cross section height, a thin portion of the sidewall portion exceeding ½ of the tire cross section height is reinforced, and the sidewall portion can be reinforced over the substantially entire region of the cross section height. Further, since the second extension block has a different cross section height from that of the first extension block, the interface of the reinforced region is inclined with respect to the circumferential direction of the tire and thus, a crack extending along the circumferential direction of the tire can be suppressed, and durability can be secured. Further, since the extension block group comprises a plurality of the extension blocks having different cross section heights, it is possible to suppress vibration in the direction of the cross section height of relatively wide frequency band from low frequency to high frequency, and the road noise can effectively be reduced.

In the above structure, it is preferable that land heights of the extension blocks constituting the extension block group are gradually reduced from the shoulder portion toward the sidewall portion. With this, difference in rigidity in the interface of the reinforced region is reduced, and durability and the steering stability can effectively be enhanced.

In the above structure, it is preferable that when one of the extension blocks constituting the extension block group which has the highest cross section height from the tire equator point is defined as a longest block, a ratio of cross section height of the extension blocks from an intersection point P between the tire surface and a contour extension line at the radius of curvature which appears on a groove bottom surface of the tread portion to a cross section height of the second extension block with respect to the longest block from the intersection point P is 20% or more. With this, reinforcing effect of the sidewall portion can be secured by the second extension block.

In the above structure, it is preferable that the extension block group includes an extension block whose cross section height from the tire equator point does not exceed ½ of a tire cross section height. With this, the extension block whose cross section height exceeds ½ of the tire cross section height and the extension block whose cross section height does not exceed ½ of the tire cross section height are mixed, vibration in the cross section height direction can be suppressed in relatively wide frequency band, and the road noise can be reduced.

In the above structure, it is preferable that tip ends of the extension blocks constituting the extension block group are tapered. With this, the rigidity is not excessively increased in a region where the tip end of the extension block and the bead filler are superposed in the thickness direction of the tire.

DESCRIPTION OF THE PREFERRED EXAMPLES

An embodiment of the present invention will be explained with reference to the drawings.FIG. 1is a semi-sectional view showing one example of a pneumatic tire according to the present invention.FIG. 2is a developed plan view of an outer peripheral surface of the tire in a range shown with a symbol20inFIG. 1.FIG. 1is a sectional view taken along the line A-A inFIG. 2, dotted line16shows an outer peripheral surface of the sidewall portion2where an extension block is not formed. The extension block will be described later.

InFIG. 1, the land height of extension block24is represented by the distance between the dotted line16, where no extension block is formed and the top of extension block24. As can be seen in this Figure, this land height is gradually reduced from the shoulder portion3to the sidewall portion2continuously.

As shown inFIG. 1, the pneumatic tire of the present invention includes a pair of annular bead portions1, sidewall portions2extending from respective bead portions1radially outward of the tire, and tread portions4connected to the respective sidewall portions2radially outward of the tire through a shoulder portion3. In the bead portion1, an annular bead6formed by coating a convergence body of steel wire with rubber, and a bead filler7made of hard rubber disposed outside of the bead6in the circumferential direction of the tire. The bead filler7has substantially triangular cross section.

The carcass ply5comprises ply cords arranged at a predetermined angle with respect to the circumferential direction of the tire, and is disposed such as to extend between the pair of bead portions1. Steel cord, organic fiber such as polyester, rayon, nylon and aramido is preferably used as the ply cord. An end of the carcass ply5is curled up outward such as to sandwich the bead6and a bead filler7.

An inner liner layer8is disposed on the carcass ply5on an inner peripheral side for keeping air pressure. A belt layer9is disposed on the carcass ply5on an outer peripheral side of the tire. The belt layer9comprising two belt plies which are laminated together in and out. The belt layer9reinforces the tire by means of hoop effect. Each belt ply comprises steel cords extending such as to be inclined at predetermined angle with respect to the circumferential direction of the tire, and the steel cords are disposed such as to intersect with each other in opposite directions between the plies. The material of the belt ply is not limited to the steel cords, and organic fiber such as polyester, rayon, nylon and aramido may also be employed. A reinforcing layer is disposed on the belt layer9on the outer peripheral side of the tire if necessary.

Examples of raw material rubbers for the rubber layer and the like are natural rubber, styrene-butadiene rubber (SBR), butadiene rubber (BR), isoprene rubber (IR), butyl rubber (IIR) and the like. These rubbers are used alone or a combination thereof. These rubbers are reinforced using filler such as carbon black and silica, and cure, accelerator, plasticizer, antioxidant or the like is appropriately mixed

A tread surface10is formed with a tread pattern, and is provided with a plurality of blocks. In this embodiment, as shown inFIGS. 2 and 3, extension blocks11to14are disposed outside of the tread portion4in the widthwise direction, and the extension blocks11to14extends from the shoulder portion3to the sidewall portion2beyond a tread edge E. The extension blocks11to14are branched and extended from the shoulder portion3toward the sidewall portion2. For the sake of convenience of explanation, portions of the extension blocks11to14from finally branched portions to their tip ends are called extension blocks21to28, respectively.

Each of extension block groups15comprises the extension blocks21to28. In this embodiment, as shown inFIG. 4, each of the extension block groups15is continuously formed along the circumferential direction of the tire, but a region where the extension block group15is not formed may be provided. The extension blocks21to28constituting the extension block group15may not be arranged at equal distances from one another.

The extension blocks21to28are arranged in the circumferential direction of the tire such that cross section heights thereof from the tire equator point CP are different from one another. Here, the tire equator point CP is an intersection between the tire equator C and the tread contour in the tire meridian cross section, and when a groove is formed on the tire equator C, a tread contour when the groove is filled is used. Of the extension blocks constituting the extension block group15, the extension block24has the highest cross section height from the tire equator point CP, and the extension block24corresponds to the longest block. As can be found fromFIG. 1, cross section height h1of the extension block24as measured from the tire equator point CP is more than ½ of the tire cross section height H, and the extension block24also corresponds to the first extension block. The second extension block is determined relative to the relation with respect to other extension blocks, but if the extension block24is supposed to be the first extension block, remaining extension blocks21to23and25to28correspond to the second extension blocks.

Since the extension block group15has the extension block24, the extension block group15exceeds ½ of the tire cross section height H from the tire equator point CP and reinforces the thin portion of the sidewall portion2, and can reinforce the sidewall portion2over the substantially entire region of the cross section height. In this embodiment, the cross section height h1of the extension block24from the tire equator point CP reaches a height of the tip end of the bead filler7and with this, the sidewall portion2can be reinforced more effectively. The extension block24is tapered and its tip end does not have excessively high rigidity in a region where the bead filler7is superposed with the tip end in the thickness direction of the tire.

As described above, the extension blocks21to28constituting the extension block group15have different cross section heights from the tire equator point CP, and the interface of the reinforced region is inclined in the circumferential direction of the tire. Thus, a crack caused by distortion when the sidewall portion2is bent can be suppressed, and the durability can be enhanced effectively. Since the extension block group15comprises the plurality of extension blocks21to28having different cross section heights, it is possible to suppress vibration in the direction of the cross section height of relatively wide frequency band from low frequency to high frequency, and the road noise can effectively be reduced.

To obtain the above-described effect, it is preferable that the extension blocks constituting the extension block group15includes both the extension block having the cross section height from the tire equator point CP which exceeds ½ of the tire cross section height H and the extension block having the cross section height from the tire equator point CP which does not exceed ½ of the tire cross section height H. The extension blocks constituting the extension block group15may include the same cross section heights but it is preferable that such extension blocks are not adjacent to each other.

It is preferable that land heights of the extension blocks21to28constituting the extension block group15are gradually reduced from the shoulder portion3toward the sidewall portion2like the extension block24shown inFIG. 1. With this, rigidity difference in the interface of the reinforced region can be reduced, and durability and steering stability can effectively be enhanced.

It is preferable that a ratio of cross section height of the extension blocks21to23and25to28from an intersection point P between the tire surface and a contour extension line L at the radius of curvature R which appears on a groove bottom surface of the tread portion4to a cross section height h2of the extension block24which is the longest block from the intersection point P is 20% or more. With this, the reinforcing effect of the sidewall portion2obtained by the extension blocks21to23and25to28can be secured.

The cross section height of the extension block and the tire cross section height H are measured in a state in which a pneumatic tire is mounted on a regular rim, regular internal pressure is charged into the pneumatic tire, and no load is applied. Here, regular rim is a standard rim defined in JATMA in correspondence with tire size. The regular internal pressure is maximum air pressure based on JATMA, but when the tire is for a passenger vehicle, the regular internal pressure is 180 KPa.

According to the pneumatic tire of the present invention, it is only necessary to provide a tire mold with recesses for forming the extension block groups15, and other producing procedure of the tire is the same as that of the conventional procedure.

The tread portion formed in the pneumatic tire of the present invention is not limited to the embodiment. For example, the extension blocks constituting the extension block group may extend without branching off, or may incline with respect to a radial direction of the tire. The extension block need not be tapered, and may spread toward its tip end.

EXAMPLES

An example tire which concretely shows the structure and effect of the present invention will be explained.

Durability

A test of regular time was carried out with a regular load in conformity with conditions as durability test in Federal Motor Vehicle Safety Standard FMVSS109, and running time required until a trouble was found in the sidewall portion was measured. When no trouble was found, drum running was continued with 140% load, and the running time was measured until trouble was found. Results are shown with indices while showing a comparative example 1 as 100. As the numerical value is greater, the running time is longer, i.e., durability is more excellent.

Steering Stability and Noise Reduction Ability

Tires were assembled to rims of a vehicle (5.6 L truck for export, two passengers rode therein) having wheel size of 17×9.5JJ, the vehicle was allowed to run on an excellent road and a poor road, and sensory evaluation was made by feeling test. In the feeling test, the passengers were not informed of kinds of test tires, the vehicle run with internal pressure of 200 kPa, and the passengers evaluated. Results are shown with indices while showing the comparative example 1 as 100. As the numerical value is greater, the steering stability and noise reduction ability are more excellent.

Comparative Example 1

A pneumatic tire (tire size: LT285/70R17) formed with no blocks extending from the shoulder portion to the sidewall portion was produced as the comparative example 1.

Comparative Example 2

A pneumatic tire (tire size: LT285/70R17) formed with no blocks extending from the shoulder portion to the sidewall portion but formed with a plurality of rims extending from a portion in the vicinity of the shoulder portion to a lower region of the sidewall portion in the circumferential direction of the tire was produced as the comparative example 2. The rims are not extended from the shoulder portion to the sidewall portion but have discontinuous portion.

Comparative Example 3

A pneumatic tire (tire size: LT285/70R17) formed with blocks extending from the shoulder portion to the sidewall portion was produced as the comparative example 3. The cross section heights of the extension blocks from the tire equator point are ½ of the tire cross section height.

Example Tire Of The Present Invention

The pneumatic tire (tire size: LT285/70R17) shown in the embodiment in which extension blocks extending from the shoulder portion to the sidewall portion and cross section heights of the extension blocks from the tire equator point are different from each other was prepared as the example tire of the invention. Lengths of the extension blocks21to28are 26.92, 31.82, 59.52, 71.22, 74.12, 80.92, 93.62, 123.61 (unit is mm) in the increasing order of the cross section height from an intersection point P. Results of the evaluation are shown in Table 1.

According to the results shown in Table 1, it can be found that the example tire of the present invention has most excellent durability, steering stability and noise reduction ability, the sidewall portion is effectively reinforced by the extension block group, and the noise reduction ability is enhanced. Concerning the comparative example 1, since the sidewall portion is not reinforced, the numeric value in each item is the lowest. In the comparative example 2, it is conceived that the strength was deteriorated in the discontinuous portion. In the comparative example 3, it is conceived that strength was deteriorated in the thin portion of the sidewall portion exceeding ½ of the tire cross section height of the sidewall portion. In the comparative examples 2 and 3, it is conceived that since the cross section heights of the rims or the extension blocks from the tire equator point are the same, frequency band of vibration to be suppressed is relatively narrow, and the road noise reduction efficiency was poor.