Patent Description:
In recent years, premiumisation and quality improvement of vehicles lead, from the view point of occupant's comfort and environmental considerations in particular toward an electrification of a vehicle, desire to various noise reductions.

It is known that the noise from the tires includes various components, that is a component from external noise and a component from internal noise. The internal noise includes a vibration due to excitation of a tread portion during rolling, the vibration is transmitted to a wheel rim, an axis, a suspension and a body of the vehicle then heard in an interior of the vehicle as noise.

In order to improve such noise, it is known that reduction of the vibration due to excitation of the tread portion by placing relatively soft rubber in the tread is effective. Various solutions have been proposed to improve such noise.

<CIT> discloses a tire having a reduced rigidity rubber component interposed between a tread layer and a belt reinforcement, the reduced rigidity rubber component having a hardness and a geometry such that the maximum longitudinal and lateral contact stresses on the tread layer are reduced relative to a tire without such reduced rigidity rubber component.

<CIT> discloses a pneumatic vehicle tire having a tread which is comprised in the radial direction of two layers consisting of different rubber compounds, a tread cap and a tread base, and the tread base viewed in the axial direction has at least directed radially outward, a central segment and two lateral segments, the two lateral segments consist of a rubber compound that has a lower dynamic elastic modulus E' at <NUM> and a lower hysteresis than the central segment of the tread base for a lower rolling resistance without worsening the handling behavior.

<CIT> discloses a pneumatic tire having a tread portion in a two-layered structure of a cap tread layer and an under tread layer, the under tread layer is constituted by a center region and a shoulder region extended continuously from the center region to an outer, the regions comprising rubber composition different from each other, the center region is arranged with a rubber layer where JIS-A hardness is from <NUM> to <NUM>, the shoulder region is arranged with a rubber layer where JIS-A hardness is from <NUM> to <NUM>.

<CIT> discloses a tire having two layers tread, a base rubber and a cap rubber, the base rubber is divided into a center base rubber segment located at the widthwise center and shoulder base rubber segments located on both sides outside the center base rubber segment in the width direction, a dynamic modulus of the elasticity (E') of the center base rubber segment is <NUM> to <NUM> at <NUM>, the dynamic modulus of the elasticity (E') of the shoulder base rubber segments is <NUM> to <NUM> at <NUM>.

<CIT> discloses a multi-stage tire tread having two or more wear layers comprising an outer wear layer and one or more inner wear layers arranged within the thickness of the tread below the outer wear layer. One or more outer grooves are arranged within the outer wear layer, while one or more inner grooves arranged within at least one of the one or more inner wear layers, the tread further includes a volumetric void ratio equal to approximately <NUM> to <NUM> in the unworn condition and approximately <NUM> to <NUM> in a worn condition where the outer, ground-engaging side is arranged along one of the inner wear layers in the worn condition, the tread further having a contact surface ratio equal to approximately <NUM> - <NUM> in the unworn condition and approximately <NUM> - <NUM> in the worn condition for providing improved worn tire performance, especially in wet or snow conditions, without sacrificing new tire performance.

<CIT> discloses a pneumatic tire including a tread portion including an inboard tread region between a tire equator and an inboard tread edge, and an outboard tread region between the tire equator and an outboard tread edge, the inboard tread region is provided with a first main groove extending and a second main groove each extending continuously in the tire circumferential direction, the outboard tread region is provided with a plurality of recesses that are not in communication with other grooves, the recesses include a plurality of deep bottom recesses having depths equal to or more than <NUM>, a total of opening areas of the deep bottom recesses on a ground contact surface of the tread portion is equal to or less than <NUM>% of a surface area of the outboard tread region that is obtained by filling up all grooves and recesses provided thereon for better dry performance, wet performance and wear resistance. <CIT> discloses a pneumatic tire is formed with a land segment. A tire inner cavity surface of a tread part is provided with a long sheet-like acoustic damper made of foamed material and extending in a tire peripheral direction. The land segment rim-assembled into a normal rim and having an acoustic damper overlapping segment M positioned outside of a tire radial direction of the acoustic damper at a tire meridian section including a tire rotary axis under a normal state where normal inner pressure is charged. The acoustic damper overlapping segment M is provided with dimples composed of small holes at a tread surface and/or side wall surfaces facing against sipes. <CIT> discloses a read for a tire having a mid-circumferential plane. The tread comprises a plurality of tread elements arranged in a circumferential array about the tire. The tread also comprises a plurality of transverse extending grooves. Each of the grooves separate a pair of circumferentially adjacent tread elements. An area moment of inertia is associated with each of the tread elements when viewed normal to the tread element. Each of the area moments of inertia have a respective pair of principle axes of inertia relative to the mid-circumferential plane. The angular orientation between any pair of principle axes of inertia of different tread elements is within a predetermined angular range of <NUM> degrees or less.

However with the solutions disclosed in these documents, improvement on noise performance is not satisfactory especially due to incompressibility of rubber material. Also at the same time, degradation of handling performance is not an acceptable level. Thus there is a desire to further improvement of noise performance while maintaining reasonable level on handling performance.

Therefore, there is a need for a tread for a tire which provides further improvement on noise performance while improving or maintaining reasonable level on handling performance.

A "radial direction/orientation" is a direction/orientation perpendicular to axis of rotation of the tire. This direction/orientation corresponds to thickness orientation of the tread.

An "axial direction/orientation" is a direction/orientation parallel to axis of rotation of the tire.

A "circumferential direction/orientation" is a direction/orientation which is tangential to any circle centered on axis of rotation. This direction/orientation is perpendicular to both the axial direction/orientation and the radial direction/orientation.

A "tire" means all types of elastic tire whether or not subjected to an internal pressure.

A "tread" of a tire means a quantity of rubber material bounded by lateral surfaces and by two main surfaces one of which is intended to come into contact with ground when the tire is rolling.

A "groove" is a space between two rubber faces/sidewalls which do not contact between themselves under usual rolling condition connected by another rubber face/ bottom. A groove has a width and a depth.

A "contact patch" is a footprint of a tire mounted onto its standard rim as identified in tire standards such as ETRTO, JATMA or TRA, and inflated at its nominal pressure and under its nominal load. A "width TW" of a contact face is a maximum contact width of the contact patch along with an axis of rotation of the tire.

A "shear storage modulus G‴ is a shear storage modulus measured at <NUM>, <NUM> and <NUM>% of strain.

It is thus an object of the invention to provide a tread for a tire which provides improvement on noise performance while maintaining reasonable level on handling performance.

The present invention provides a tread for a tire according to claim <NUM>.

This arrangement provides an improvement on noise performance while maintaining reasonable level on handling performance.

Since the tread is provided with at least one compressive contact element comprising the plurality of compressive elements among the plurality of contact elements, such the compressive contact element can absorb deformation due to Poisson's effect and limit stiffening due to incompressibility of rubber material constituting the tread thanks to the plurality of compressive elements. Therefore it is possible to improve noise performance.

Since the volumetric void ratio of the at least one compressive contact element in a unit region surrounded by a center of the at least one circumferential groove and a center of the plurality of transverse grooves delimiting the compressive contact element among the plurality of contact elements is at least equal to <NUM>%, the compressive contact element can ensure enough void to absorb deformation due to Poisson's effect. Therefore it is possible to improve effectively noise performance.

If the volumetric void ratio of the at least one compressive contact element in the unit region is less than <NUM>%, there is a risk that the compressive contact element cannot ensure enough void to absorb deformation due to Poisson's effect thus stiffening finally the contact element leading insufficient improvement on noise performance. By setting this volumetric void ratio of the at least one compressive contact element in the unit region at least equal to <NUM>%, it is possible to improve effectively noise performance.

This volumetric void ratio of the at least one compressive contact element in the unit region is preferably at least equal to <NUM>%, more preferably at least equal to <NUM>%.

Since the aspect ratio defined as the ratio of the surface of the compressive contact element supposed to contact with ground divided by the sum of the surface area of the compressive contact element touching with air other than the surface of the compressive contact element supposed to contact with ground is at most equal to <NUM>%, the compressive contact element can limit structural stiffening of the tread. Therefore it is possible to improve effectively noise performance.

If this aspect ratio is more than <NUM>%, there is a risk that the compressive contact element cannot limit structural stiffening of the tread thus stiffening finally the contact element leading insufficient improvement on noise performance. By setting this aspect ratio at most equal to <NUM>%, it is possible to improve effectively noise performance.

This aspect ratio is preferably at most equal to <NUM>%, more preferably at most equal to <NUM>% and still more preferably at most equal to <NUM>%.

In another preferred embodiment, a width of one of the pair of shoulder regions between an axial extremity of the tread and the center of axially outermost circumferential groove closest to the axial extremity of the tread is from <NUM>% to <NUM>% of the width TW, the compressive contact element locates in at least one of the pair of shoulder regions.

If the width of one of the pair of shoulder regions is less than <NUM>% of the width TW, there is a risk that a volume of the compressive contact element available on the tread becomes insufficient thus insufficient improvement on noise performance. If the width of one of the pair of shoulder regions is more than <NUM>%, there is a risk that a volume of the compressive contact element available on the tread becomes too much thus degrading handling performance. By setting the width of one of the pair of shoulder regions between the axial extremity of the tread and the center of axially outermost circumferential groove closest to the axial extremity of the tread from <NUM>% to <NUM>% of the width TW and locating the compressive contact element in at least one of the pair of shoulder regions, it is possible to improve noise performance while maintaining reasonable level on handling performance.

In another preferred embodiment, an average of the volumetric void ratio of the plurality of contact elements including the compressive contact element at one of the pair of shoulder regions is at least equal to <NUM>%, and an average of the aspect ratio of the plurality of contact elements including the compressive contact element at one of the pair of shoulder regions is at most equal to <NUM>%.

According to this arrangement, it is possible to improve effectively noise performance while maintaining reasonable level on handling performance as one of the pair of shoulder regions can get maximum benefit of the compressive contact element available in the pair of shoulder regions.

In another preferred embodiment, all the contact elements in the pair of shoulder regions are the compressive contact element.

According to this arrangement, it is possible to improve efficiently noise performance while maintaining reasonable level on handling performance as shoulder regions can get full benefit of the compressive contact element available in the shoulder regions.

In another preferred embodiment, the plurality of compressive elements is a plurality of holes opening to the contact face and extending in radially inward of the tread.

According to this arrangement, it is possible to improve efficiently noise performance while maintaining reasonable level on handling performance as the holes can absorb efficiently deformation of the compressive contact element due to Poisson's effect.

In another preferred embodiment, the plurality of compressive elements is a plurality of incisions opening to the contact face and extending in an orientation having non-zero angle with axial orientation of the tread.

According to this arrangement, it is possible to improve noise performance in particular exterior noise performance as the incisions are not open directly to exterior of the tread.

In another preferred embodiment, radially innermost of the enlarged portion locates at a level below radially innermost of the groove.

According to this arrangement, it is possible to improve efficiently noise performance while maintaining reasonable level on handling performance as the enlarged portion can effectively support deformation of the compressive contact element.

In another preferred embodiment, the pair of shoulder regions each comprises a shoulder rubber layer of a thickness ts made of a shoulder rubber composition different from a rubber composition constituting the tread, and a shear storage modulus G' of the shoulder rubber composition is smaller than a shear storage modulus G' of the rubber composition constituting the tread.

According to this arrangement, it is possible to improve noise performance while maintaining reasonable level on handling performance since the shoulder rubber layer is comprised in the pair of shoulder regions and not the center region, degradation on handling performance is limited as the center region has a dominant impact on handling performance due to longer contact length compared to shoulder regions, and the shoulder rubber layer can effectively reduce excitation of the tread during rolling thanks to the shoulder rubber composition having the shear storage modulus G' smaller than the rubber composition constituting the tread.

In another preferred embodiment, the shear storage modulus G' of the shoulder rubber composition is less than or equal to <NUM> MPa.

If the shear storage modulus G' of the shoulder rubber composition is more than <NUM> MPa, there is a risk that an improvement on noise performance would be insufficient. By setting the shear storage modulus G' of the shoulder rubber composition less than or equal to <NUM> MPa, the shoulder rubber layer can effectively reduce excitation of the tread during rolling.

According to the arrangements described above, it is possible to provide a tread for a tire which provides further improvement on noise performance while maintaining reasonable level on handling performance.

Other characteristics and advantages of the invention arise from the description made hereafter in reference to the annexed drawings which show, as nonrestrictive examples, the embodiment of the invention.

Preferred embodiments of the present invention will be described below referring to the drawings.

A tread <NUM> for a tire according to a first embodiment of the present invention will be described referring to <FIG>.

<FIG> is a schematic plan view of a tread according to a first embodiment of the present invention. <FIG> is a schematic cross sectional view taken along line II - II in <FIG>. The tread <NUM> shown in the <FIG> is a half of a portion of the tread divided with respect to a center line C-C' extending in circumferential orientation.

The tread <NUM> is a tread for a tire having dimension <NUM>/45R18 and has a contact face <NUM> of a width TW intended to come into contact with ground during rolling, provided with at least one circumferential groove <NUM> of a depth D (shown in <FIG>) extending in circumferential orientation and a plurality of transvers grooves <NUM> extending generally in axial orientation, both the at least one circumferential groove <NUM> and the plurality of transvers grooves <NUM> opening to the contact face <NUM>. The at least one circumferential groove <NUM> and the plurality of transverse grooves <NUM> delimiting a plurality of contact elements <NUM>. The tread <NUM> comprises a center region <NUM> containing the center line C-C' and a pair of shoulder regions <NUM> being positioned on two axial sides of the center region <NUM>.

As shown in <FIG>, the tread <NUM> is provide with at least one compressive contact element <NUM> among the plurality of contact elements <NUM>, the compressive contact element <NUM> comprises a plurality of compressive elements <NUM>. The plurality of compressive elements <NUM> is a plurality of holes <NUM> opening to the contact face <NUM> and extending in radially inward of the tread <NUM> (shown in <FIG>). A width of one of the pair of shoulder regions <NUM> between an axial extremity of the tread <NUM> and the center of axially outermost circumferential groove <NUM> closest to the axial extremity of the tread <NUM> is from <NUM>% to <NUM>% of the width TW, and the compressive contact element <NUM> locates in at least one of the pair of shoulder regions <NUM>. All the contact elements <NUM> in the pair of shoulder regions <NUM> are the compressive contact element <NUM>. In the present embodiment, the width of one of the pair of shoulder regions <NUM> is <NUM>% of the width TW, and a diameter of each the hole <NUM> as the compressive element <NUM> is <NUM>.

As shown in <FIG>, the at least one of the compressive element <NUM> among the plurality of compressive elements <NUM> comprises an enlarged portion <NUM> at radially innermost portion of the compressive element <NUM>. A radially innermost portion of the enlarged portion <NUM> locates at a level below radially innermost of the groove <NUM>. As same as typical radial tire construction, a carcass <NUM> (or a casing), a main body of a tire which may or may not be provided with an inner liner for inhibiting loss or air pressure, and a ply <NUM> (or a belt), one or more rubber-coated layers of metallic or textile or other material in a form of cable or wire or strings, are provided radially inward of the tread <NUM>. In the present embodiment, a width of each the enlarged portion <NUM> is <NUM>.

A volumetric void ratio of the at least one compressive contact element <NUM> in a unit region <NUM> surrounded by a center of the at least one circumferential groove <NUM> and a center of the plurality of transverse grooves <NUM> delimiting the compressive contact element <NUM> among the plurality of contact elements <NUM> is at least equal to <NUM>%, and an aspect ratio defined as a ratio of a surface of the compressive contact element <NUM> supposed to contact with ground divided by a sum of a surface area of the compressive contact element <NUM> touching with air other than the surface of the compressive contact element <NUM> supposed to contact with ground is at most equal to <NUM>%. In the present embodiment, the volumetric void ratio is <NUM>%, and the aspect ratio is <NUM>%.

An average of the volumetric void ratio of the plurality of contact elements <NUM> including the compressive contact element <NUM> at one of the pair of shoulder regions <NUM> is at least equal to <NUM>%, and an average of the aspect ratio of the plurality of contact elements <NUM> including the compressive contact element <NUM> at one of the pair of shoulder regions <NUM> is at most equal to <NUM>%.

Since the tread <NUM> is provided with at least one compressive contact element <NUM> comprising the plurality of compressive elements <NUM> among the plurality of contact elements <NUM>, such the compressive contact element <NUM> can absorb deformation due to Poisson's effect and limit stiffening due to incompressibility of rubber material constituting the tread <NUM> thanks to the plurality of compressive elements <NUM>. Therefore it is possible to improve noise performance.

Since the volumetric void ratio of the at least one compressive contact element <NUM> in a unit region <NUM> surrounded by a center of the at least one circumferential groove <NUM> and a center of the plurality of transverse grooves <NUM> delimiting the compressive contact element <NUM> among the plurality of contact elements <NUM> is at least equal to <NUM>%, the compressive contact element <NUM> can ensure enough void to absorb deformation due to Poisson's effect. Therefore it is possible to improve effectively noise performance.

If the volumetric void ratio of the at least one compressive contact element <NUM> in the unit region <NUM> is less than <NUM>%, there is a risk that the compressive contact element <NUM> cannot ensure enough void to absorb deformation due to Poisson's effect thus stiffening finally the contact element <NUM> leading insufficient improvement on noise performance. By setting this volumetric void ratio of the at least one compressive contact element <NUM> in the unit region <NUM> at least equal to <NUM>%, it is possible to improve effectively noise performance.

This volumetric void ratio of the at least one compressive contact element <NUM> in the unit region <NUM> is preferably at least equal to <NUM>%, more preferably at least equal to <NUM>%.

Since the aspect ratio defined as the ratio of the surface of the compressive contact element <NUM> supposed to contact with ground divided by the sum of the surface area of the compressive contact element <NUM> touching with air other than the surface of the compressive contact element <NUM> supposed to contact with ground is at most equal to <NUM>%, the compressive contact element <NUM> can limit structural stiffening of the tread. Therefore it is possible to improve effectively noise performance.

If this aspect ratio is more than <NUM>%, there is a risk that the compressive contact element <NUM> cannot limit structural stiffening of the tread <NUM> thus stiffening finally the contact element <NUM> leading insufficient improvement on noise performance. By setting this aspect ratio at most equal to <NUM>%, it is possible to improve effectively noise performance.

Since the width of one of the pair of shoulder regions <NUM> between an axial extremity of the tread <NUM> and the center of axially outermost circumferential groove <NUM> closest to the axial extremity of the tread <NUM> is from <NUM>% to <NUM>% of the width TW, the compressive contact element <NUM> locates in at least one of the pair of shoulder regions <NUM>, it is possible to improve noise performance while maintaining reasonable level on handling performance.

If the width of one of the pair of shoulder regions <NUM> is less than <NUM>% of the width TW, there is a risk that a volume of the compressive contact element <NUM> available on the tread <NUM> becomes insufficient thus insufficient improvement on noise performance. If the width of one of the pair of shoulder regions <NUM> is more than <NUM>%, there is a risk that a volume of the compressive contact element <NUM> available on the tread <NUM> becomes too much thus degrading handling performance.

Since the average of the volumetric void ratio of the plurality of contact elements <NUM> including the compressive contact element <NUM> at one of the pair of shoulder regions <NUM> is at least equal to <NUM>%, and an average of the aspect ratio of the plurality of contact elements <NUM> including the compressive contact element <NUM> at one of the pair of shoulder regions <NUM> is at most equal to <NUM>%, it is possible to improve effectively noise performance while maintaining reasonable level on handling performance as one of the pair of shoulder regions <NUM> can get maximum benefit of the compressive contact element <NUM> available in the pair of shoulder regions <NUM>.

Since all the contact elements <NUM> in the pair of shoulder regions <NUM> are the compressive contact element <NUM>, it is possible to improve efficiently noise performance while maintaining reasonable level on handling performance as shoulder regions <NUM> can get full benefit of the compressive contact element <NUM> available in the shoulder regions <NUM>.

Since the plurality of compressive elements <NUM> is a plurality of holes <NUM> opening to the contact face <NUM> and extending in radially inward of the tread <NUM>, it is possible to improve efficiently noise performance while maintaining reasonable level on handling performance as the holes <NUM> can absorb efficiently deformation of the compressive contact element <NUM> due to Poisson's effect.

Since at least one of the compressive element <NUM> among the plurality of compressive elements <NUM> comprises an enlarged portion <NUM> at radially innermost portion of the compressive element <NUM>, it is possible to improve efficiently noise performance while maintaining reasonable level on handling performance as the enlarged portion <NUM> can absorb efficiently deformation of the compressive contact element <NUM> due to Poisson's effect.

Since radially innermost of the enlarged portion <NUM> locates at a level below radially innermost of the groove <NUM>, it is possible to improve efficiently noise performance while maintaining reasonable level on handling performance as the enlarged portion <NUM> can effectively support deformation of the compressive contact element <NUM>.

The volumetric void ratio may be able to satisfy by using a foamed rubber composition. Preferably the rubber composition constituting the tread <NUM> is a non-foamed rubber composition.

The hole <NUM> as the compressive element <NUM> may extend in straight, curved or zig-zagged manner toward radially inward of the tread <NUM>, with or without an angle relative to radial orientation.

The diameter or a depth of each the hole <NUM> may vary from one another, including with or without the enlarged portion <NUM>. The shape of each the hole <NUM> on the contact face <NUM> may also vary from one another and may be any shape, for example circular, triangular, oblong or polygonal, or those may be the same within one compressive contact element <NUM>.

A content of the compressive element <NUM> in one single compressive contact element <NUM> may vary from one compressive contact element <NUM> to another compressive contact element <NUM>.

A tread <NUM> according to a second embodiment of the present invention will be described referring to <FIG> is a schematic plan view of a tread according to a second embodiment of the present invention. <FIG> is a schematic cross sectional view taken along line IV - IV in <FIG>. The construction of this second embodiment is similar to that of the first embodiment other than the arrangement shown in <FIG>, thus description will be made referring to <FIG>.

As shown in <FIG>, a tread <NUM> has a contact face <NUM> of a width TW (not shown), and at least one circumferential groove <NUM> of a depth D (shown in <FIG>) extending in circumferential orientation and a plurality of transvers grooves <NUM> extending generally in axial orientation, both the at least one circumferential groove <NUM> and the plurality of transvers grooves <NUM> opening to the contact face <NUM>. The at least one circumferential groove <NUM> and the plurality of transverse grooves <NUM> delimiting a plurality of contact elements <NUM>. The tread <NUM> comprises a center region <NUM> and a pair of shoulder regions <NUM> being positioned on two axial sides of the center region <NUM>.

As shown in <FIG>, the tread <NUM> is provided with at least one compressive contact element <NUM> among the plurality of contact elements <NUM>. The compressive contact element <NUM> comprises a plurality of compressive elements <NUM>. The plurality of compressive elements <NUM> is a plurality of incisions <NUM> opening to the contact face <NUM> and extending in an orientation having non-zero angle with axial orientation of the tread <NUM>. In the present embodiment, the non-zero angle is <NUM> degrees, and a width of each the incision <NUM> is <NUM>.

As shown in <FIG>, some of the incisions <NUM> as the compressive elements <NUM> are comprise an enlarged portion <NUM> at radially innermost portion of the compressive element <NUM>. A radially innermost of some of the incisions <NUM> or the enlarged portions <NUM> locates at a level below radially innermost of the groove <NUM>. In the present embodiment, a width of the enlarged portion <NUM> is <NUM>.

As shown in <FIG>, the pair of shoulder regions <NUM> each comprises a shoulder rubber layer <NUM> of a thickness ts made of a shoulder rubber composition different from a rubber composition constituting the tread <NUM>, and a shear storage modulus G' of the shoulder rubber composition is less than or equal to <NUM> MPa. In the present embodiment, the shear storage modulus G' of the shoulder rubber composition is <NUM> MPa, and a shear storage modulus G' of the rubber composition constituting the tread <NUM> is <NUM> MPa.

Since the plurality of compressive elements <NUM> is a plurality of incisions <NUM> opening to the contact face <NUM> and extending in an orientation having non-zero angle with axial orientation of the tread <NUM>, it is possible to improve noise performance in particular exterior noise performance as the incisions <NUM> are not open directly to exterior of the tread <NUM>.

The non-zero angle of the incisions <NUM> relative to axial orientation of the tread <NUM> is preferably more than <NUM> degrees, more preferably more than <NUM> degrees and still more preferably more than <NUM> degrees.

Since the pair of shoulder regions <NUM> each comprising a shoulder rubber layer <NUM> of a thickness ts made of a shoulder rubber composition different from a rubber composition constituting the tread <NUM>, and a shear storage modulus G' of the shoulder rubber composition is less than or equal to <NUM> MPa, it is possible to improve noise performance while maintaining reasonable level on handling performance since the shoulder rubber layer <NUM> is comprised in the pair of shoulder regions <NUM> and not the center region <NUM>, degradation on handling performance is limited as the center region <NUM> has a dominant impact on handling performance due to longer contact length compared to shoulder regions <NUM>, and the shoulder rubber layer <NUM> can effectively reduce excitation of the tread <NUM> during rolling thanks to the shoulder rubber composition having the shear storage modulus G' smaller than the shear storage modulus G' of the rubber composition constituting the tread <NUM>.

Since the shear storage modulus G' of the shoulder rubber composition is less than or equal to <NUM> MPa, the shoulder rubber layer <NUM> can effectively reduce excitation of the tread <NUM> during rolling.

If the shear storage modulus G' of the shoulder rubber composition is more than <NUM> MPa, there is a risk that an improvement on noise performance would be insufficient.

The width, the depth of the incision <NUM> with or without enlarged portion <NUM> may vary from one another, or may be the same within one compressive contact element <NUM>.

The shoulder rubber layer <NUM> may be invisible on the contact face <NUM> when the tread <NUM> being brand new. The shoulder rubber layer <NUM> may be provided radially below a level of <NUM>% of the depth D of the at least one groove <NUM>.

The center region may comprise a center rubber layer of a thickness tf made of a center rubber composition different from both the shoulder rubber composition and the rubber composition constituting the tread <NUM>, and a shear storage modulus G' of the center rubber composition may be higher than the shear storage modulus G' of the rubber composition constituting the tread <NUM>. The thickness tf of the center rubber layer may be thinner than the thickness ts of the shoulder rubber layer <NUM>.

In case the center region comprises the center rubber layer, the shear storage modulus G' of the center rubber composition is preferably from <NUM> to <NUM> times of the rubber composition constituting the tread <NUM> for handling performance improvement, as if the shear storage modulus G' of the center rubber composition is less than <NUM> times of the shear storage modulus G' of the rubber composition constituting the tread <NUM>, there is a risk that improvement on handling performance becomes insufficient due to relatively insufficient modulus of the center rubber composition, and if the shear storage modulus G' of the center rubber composition is more than <NUM> times of the shear storage modulus G' of the rubber composition constituting the tread <NUM>, there is a risk that improvement on handling performance becomes insufficient either as too hard center rubber composition may make flattening of the tire difficult which leads contact patch length shorter resulting generating less lateral force.

The shear storage modulus G' of the rubber composition constituting the tread <NUM> other than the center rubber layer or the shoulder rubber layer <NUM> may be less than or equal to <NUM> MPa for noise performance improvement while improving or at least maintaining reasonable level on handling performance, as if the shear storage modulus G' of the rubber composition constituting the tread <NUM> other than the center rubber layer or the shoulder rubber layer <NUM> is more than <NUM> MPa, there is a risk that improvement on noise performance becomes insufficient, as the tread <NUM> itself becomes too hard even provided with the shoulder rubber layer <NUM>. The shear storage modulus G' of the rubber composition constituting the tread <NUM> other than the center rubber layer or the shoulder rubber layer <NUM> is preferably less than or equal to <NUM> MPa, more preferably less than or equal to <NUM> MPa.

The invention is not limited to the examples described and represented and various modifications can be made there without leaving its framework.

<FIG> is a schematic plan view of a tread according to prior art. A tread <NUM> has a contact face <NUM>, at least one circumferential groove <NUM> and a plurality of transverse grooves <NUM> both opening to the contact face <NUM>. The at least one circumferential groove <NUM> and the plurality of transverse grooves <NUM> delimiting a plurality of contact elements <NUM>. A unit region <NUM> is defined as a region surrounded by a center of the at least one circumferential groove <NUM> and a center of the plurality of transverse groove <NUM> delimiting the contact element <NUM>. In this prior art, a volumetric void ratio of the contact element <NUM> in the unit region <NUM> is <NUM>%, an aspect ratio defined as a ratio of a surface of the contact element <NUM> supposed to contact with ground divided by a sum of a surface area of the contact element <NUM> touching with air other than the surface of the contact element <NUM> supposed to contact with ground is <NUM>%.

<FIG> is a schematic plan view of a tread according to another prior art. A tread <NUM> has a contact face <NUM>, at least one circumferential groove <NUM> and a plurality of transverse grooves <NUM> both opening to the contact face <NUM>. The at least one circumferential groove <NUM> and the plurality of transverse grooves <NUM> delimiting a plurality of contact elements <NUM>. In an axially outermost region of the tread <NUM>, a plurality of incisions <NUM> is provided without the plurality of transverse grooves <NUM>. In this prior art, a volumetric void ratio of the contact element <NUM> at the axially outermost region of the tread <NUM> is <NUM>%, an aspect ratio defined as a ratio of a surface of the axially outermost region of the tread <NUM> supposed to contact with ground divided by a sum of a surface area of the axially outermost region of the tread <NUM> touching with air other than the surface of the axially outermost region of the tread <NUM> supposed to contact with ground is <NUM>%.

In order to confirm the effect of the present invention, one type of tire of Example to which the present invention is applied and other types of tire of Reference and Comparative Example were prepared.

The Example was a tire provided with a tread as described in the above first embodiment; a width of one shoulder region was <NUM>% of a width of the tread TW, a shear storage modulus G' of a rubber composition constituting the tread was <NUM> MPa. The volumetric void ratio of the compressive contact element was <NUM>%, the aspect ratio of the compressive contact element was <NUM>%. The Reference was a tire provided with a tread as shown in <FIG>. The Comparative Example was a tire provided with a tread as shown in <FIG>.

Unused test tires were mounted onto all four wheels of a <NUM>,<NUM> cc rear-wheel drive vehicle. On a straight path weathered asphalt, the vehicle was driven at a constant speed of <NUM> kph. An interior noise was measured using a microphone positioned at a passenger's ear close to window. An A-weighted sound pressure level between <NUM> and <NUM> was calculated.

The results are shown in table <NUM>. In this table <NUM>, results are represented by difference in dB(A) against Reference, lower the value indicates better the performance. Improvement of more than <NUM> dB(A) could be considered as "improvement".

A cornering power of unused test tires mounted onto a standard rim and inflated to nominal internal pressure was measured using a flat belt tire tester. A load of <NUM> was applied while tires driven at a constant speed of <NUM> kph, lateral force at a slip angle ±<NUM>° was measured, and the lateral forces measured at +<NUM>° and at -<NUM>° in absolute value were averaged.

The results are also shown in table <NUM>. In this table <NUM>, results are represented by an index of <NUM> for Reference, higher the number indicates better the performance. Within <NUM> points difference on this comparison could be considered as reasonable.

The same comparisons were made by changing tire internal construction as to have the shoulder rubber layer as shown in <FIG> with <NUM> kinds of G' for the shoulder rubber composition. The results are shown in tables <NUM> and <NUM>.

Claim 1:
A tread (<NUM>) for a tire having a contact face (<NUM>) of a width TW intended to come into contact with ground during rolling, the tread (<NUM>) being provided with at least one circumferential groove (<NUM>) of a depth D extending in circumferential orientation and a plurality of transvers grooves (<NUM>) extending generally in axial orientation, both the at least one circumferential groove (<NUM>) and the plurality of transvers grooves (<NUM>) opening to the contact face (<NUM>), the at least one circumferential groove (<NUM>) and the plurality of transverse grooves (<NUM>) delimiting a plurality of contact elements (<NUM>), the tread (<NUM>) comprising a center region (<NUM>) and a pair of shoulder regions (<NUM>) being positioned on two axial sides of the center region (<NUM>), the tread (<NUM>) being provided with at least one compressive contact element (<NUM>) among the plurality of contact elements (<NUM>), the compressive contact element (<NUM>) comprises a plurality of compressive elements (<NUM>), and a volumetric void ratio of the at least one compressive contact element (<NUM>) in a unit region (<NUM>) surrounded by a center of the at least one circumferential groove (<NUM>) and a center of the plurality of transverse grooves (<NUM>) delimiting the compressive contact element (<NUM>) among the plurality of contact elements (<NUM>) being at least equal to <NUM>%, and an aspect ratio defined as a ratio of a surface of the compressive contact element (<NUM>) supposed to contact with ground divided by a sum of a surface area of the compressive contact element (<NUM>) touching with air other than the surface of the compressive contact element (<NUM>) supposed to contact with ground being at most equal to <NUM>%,
the tread (<NUM>) being characterized in that at least one of the compressive element (<NUM>) among the plurality of compressive elements (<NUM>) comprises an enlarged portion (<NUM>) at radially innermost portion of the compressive element (<NUM>).