Patent Description:
Patent Document <NUM> below discloses a pneumatic tire wherein the tread rubber thereof is composed of three layers: a radially outermost cap layer, a radially inner most base layer and a middle layer therebetween, and the loss tangents of the cap layer and middle layer are specifically defined in order to improve the rolling resistance and the braking performance of the tire under wet conditions. Patent Document <NUM>: <CIT>.

A tire in accordance with the preamble of claim <NUM> is known from <CIT>. Related tires are described in <CIT>, <CIT> and <CIT> (document according to Article <NUM>(<NUM>) EPC).

In conventional tires, wet performance is gradually deteriorated as the tread groove volume is decreased due to wear of the tread portion.

In recent years, on the other hand, vehicle tires are required to exert good wet performance even if wear of the tread portion progresses.

The present invention was made in view of the above circumstances, and
a primary objective of the present invention is to provide a tire capable of exhibiting good wet performance even if the tread portion is worn.

The object is solved by a tire having the features of claim <NUM>. Sub-claims are directed to preferable embodiments of the invention.

According to the present invention, a tire comprises
a tread portion having a pair of tread edges, a tread surface extending between the tread edges, and a pair of buttress surface extending axially outwards from the respective tread edges,.

According to an embodiment of the invention, the tread rubber includes a third rubber layer disposed on the radially inside of the second rubber layer and made of a base rubber compound, and a loss tangent δb of the base rubber compound is smaller than the loss tangent δ1 of the first cap rubber compound.

According to an embodiment of the invention, the angle of the outer lateral groove with respect to the tire axial direction is in a range from <NUM> to <NUM> degrees.

According to an embodiment of the invention, in a worn state in which the second cap rubber compound is exposed and the tread edge shifts axially outwards, an axially inner end of the outer lateral groove is located axially inwards of the tread edge.

According to an embodiment of the invention, an axially inner end of the outer lateral groove is located axially inwards of the tread edge when the tread portion is worn by <NUM>%.

According to an embodiment of the invention, in the state in which the tread portion is worn by <NUM>%, the second rubber layer is at least partially exposed in the tread surface.

According to an embodiment of the invention, the tread portion is provided with a groove which is provided therein with a tread wear indicator protruding from the bottom of the groove, and, in a worn state in which wear of the tread portion reaches to the tread wear indicator, said second rubber layer forms the tread surface.

According to an embodiment of the invention the maximum depth of the outer lateral groove is in a range from <NUM> to <NUM>.

According to an embodiment of the invention the depth of the outer lateral groove increases from the axially inner end thereof toward the axially outside of the tire.

Therefore, in the tire according to the present invention, even when the tread rubber is worn off, good wet performance is exhibited owing to the second rubber layer exposed in the tread surface and the outer lateral grooves extending across the tread edge.

The present invention can be applied to pneumatic tires for various vehicles, e.g. passenger cars, heavy duty vehicles such as trucks and buses, and the like, but suitably applied to pneumatic tires for passenger cars.

Taking a pneumatic tire for passenger cars as an example, an embodiment of the present invention will now be described in detail in conjunction with accompanying drawings.

<FIG> shows a pneumatic tire <NUM> for passenger cars as an embodiment of the present invention under its normally inflated unloaded condition.

The pneumatic tire <NUM> comprises a tread portion <NUM> whose radially outer surface defines the tread surface <NUM>, a pair of axially spaced bead portions <NUM> mounted on rim seats, a pair of sidewall portions <NUM> extending between the tread edges Te and the bead portions <NUM>, a carcass <NUM> extending between the bead portions <NUM> through the tread portion <NUM> and the sidewall portions <NUM>, and a tread reinforcing cord layer <NUM> disposed radially outside the carcass <NUM> in the tread portion <NUM>.

The normally inflated unloaded condition is such that the pneumatic 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 undermentioned 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.

If there are no standards applicable to the pneumatic tire <NUM>, a wheel rim, maximum air pressure and maximum tire load specified by the tire manufacturer are applied as the standard wheel rim, the standard pressure and the standard tire load.

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

In this embodiment of the invention, the carcass <NUM> is composed of two carcass plies 6A and 6B of carcass cords arranged radially at angles of <NUM> to <NUM> degrees with respect to the tire circumferential direction, and extending between the bead portions <NUM> through the tread portion <NUM> and the sidewall portions <NUM>.

For example, organic fiber cords are used as the carcass cords.

In this embodiment of the invention, the tread reinforcing cord layer <NUM> is composed of two plies 7A and 7B of reinforcing cords covered with topping rubber.

The reinforcing cords in each ply are arranged, for example, at an angle in a range from <NUM> to <NUM> degrees with respect to the tire circumferential direction.

As the reinforcing cords, for example, organic fiber cords and/or steel cords can be appropriately employed.

The tread portion <NUM> has a pair of tread edges Te, a tread surface <NUM> extending between the tread edges Te, and a buttress surfaces <NUM> extending axially outwards from each of the tread edges Te.

Here, the tread edges Te are the axial outermost edges of the ground contacting patch of the tire <NUM> which occurs when the tire <NUM> under the normally inflated unloaded condition is contacted with a horizontal flat road surface at a camber angle of <NUM> degrees and then loaded with <NUM>% of the standard load.

The tread surface <NUM> is a radially outer surface of the tread portion <NUM> which comes into contact with the road surfaces during running under normal load conditions.

But, when a larger load acts on the tire <NUM>, the buttress surfaces <NUM> may come into contact with the road surfaces in addition to the above-mentioned tread surface <NUM>.

The tread surface <NUM> near the tread edges Te and the buttress surfaces <NUM> are inclined such that the tread edges Te shift axially outwards as the tread portion <NUM> or tread rubber <NUM> is worn off.

Here, the expression "the tread portion <NUM> or tread rubber <NUM> is worn off" means wear occurring over the entire tread surface <NUM> of the tread portion <NUM> when the tire <NUM> is used in a normal manner.

In the present embodiment of the invention, the tread portion <NUM> is provided with
a plurality of circumferential grooves <NUM> continuously extending in the tire circumferential direction and located between the tread edges Te.

Thus, the tread portion <NUM> is axially divided by the circumferential grooves <NUM>, and
the tread portion <NUM> includes a first land portion <NUM> and a second land portion <NUM> closer to the tire equator C than the first land portion <NUM>.

In the present embodiment of the invention, the tread portion <NUM> includes a pair of second land portions <NUM> disposed one on each side of the tire equator C, and a pair of first land portions <NUM> respectively disposed axially outside the second land portions <NUM>.

In the present embodiment of the invention, the tread portion <NUM> is axially divided into four land portions by three circumferential grooves <NUM>. However, in the present invention, the tread portion <NUM> may be provided with four circumferential grooves <NUM> so as to be axially divided into five land portions.

The tread grooves disposed in the tread portion <NUM> include a plurality of lateral grooves (not shown) extending in a tire axial direction in addition to the above-mentioned circumferential grooves <NUM>.

It is desirable that the tread grooves are provided with tread wear indicators (not shown) therein.

As well known in the tire art, the tread wear indicator protrudes from the bottom of the tread groove to indicate the wear limit of the tread portion <NUM> by the radially outer surface becoming at the same level as the tread surface of the worn tread portion.

<FIG> shows an enlarged cross-sectional view of the tread portion <NUM>.

As shown, the tread portion <NUM> comprises the tread rubber <NUM> disposed on the tread reinforcing cord layer <NUM>.

The tread rubber <NUM> includes a radially outermost first rubber layer <NUM> and a radially inner second rubber layer <NUM>.

The first rubber layer <NUM> is made of a first cap rubber compound <NUM> and forms the tread surface <NUM>.

The second rubber layer <NUM> is made of a second cap rubber compound <NUM> and disposed on the radially inside of the first rubber layer <NUM>.

In the present invention, the loss tangent δ2 of the second cap rubber compound <NUM> is set to be larger than the loss tangent δ1 of the first cap rubber compound <NUM> since rubber having a larger loss tangent can exert a larger frictional force on wet road surfaces.

The loss tangent (tanδ) refers to the value measured according to Japanese Industrial Standard (JIS) K6394, using a dynamic mechanical characteric analyzer (GABO EPLEXOR Series), under the following conditions:
Initial strain <NUM>%, Dynamic strain amplitude +/-<NUM>%, Frequency <NUM>, Temperature <NUM> degrees C, Deformation mode Tensile mode.

<FIG> is a perspective view of a part of the tire shoulder near the tread edge Te showing a part of the tread surface <NUM> and a part of buttress surface <NUM>.

Specifically, <FIG> shows the above-mentioned first land portion <NUM> in this embodiment of the invention which is divided by the axially outermost circumferential groove <NUM>.

One of the buttress surfaces <NUM>, preferably each of the buttress surfaces <NUM>, is provided with outer lateral grooves <NUM> extending in a tire axial direction as shown in <FIG> and <FIG>.

When the tire is new, namely, when the tread portion <NUM> is not worn, the outer lateral grooves <NUM> are positioned axially outwards of the tread edge Te.

When the tread edge Te shifts axially outwards due to wear of the tread portion <NUM>, at least an axially inner portion of each outer lateral groove <NUM> is located axially inwards of the shifted tread edge Te. The outer lateral grooves <NUM> are so arranged.

As a result, the tire <NUM> can exhibit good wet performance even when the tread portion <NUM> is worn as explained hereunder.

In the tire <NUM> of the present invention, the second rubber layer <NUM> is exposed in the tread surface as the wear of the tread portion <NUM> progresses.

The second cap rubber compound <NUM> forming the second rubber layer <NUM> has a larger loss tangent and can be expected to exert an excellent wet grip performance.

Therefore, the second rubber layer <NUM> can compensate for the deterioration of wet performance due to the decrease in tread groove volume resulting from the wear of the tread portion <NUM>, and sufficient wet performance can be exhibited even if the tread portion <NUM> is worn off.

Further, in the tire <NUM> of the present invention, when the wear of the tread portion <NUM> progresses, at least a portion of the outer lateral groove <NUM> is positioned axially inward of the tread edge Te which has shifted axially outwards. Thus, the outer lateral grooves <NUM> exhibit drainage ability and can maintain the wet performance.

In the present invention, therefore, even if the tread portion <NUM> is worn off, sufficient wet performance can be obtained.

It is preferable that the loss tangent δ1 of the first cap rubber compound <NUM> is not less than <NUM>, more preferably not less than <NUM>, still more preferably not less than <NUM>, but not more than <NUM>, more preferably not more than <NUM>, still more preferably not more than <NUM>.

At the start of use of the tire <NUM>, such first cap rubber compound <NUM> can exhibit steering stability on dry road surfaces and wet performance in a well-balanced manner.

In order to improve the steering stability on dry road surfaces and wet performance when the tread portion <NUM> is worn off, it is preferred that the loss tangent δ2 of the second cap rubber compound <NUM> is not less than <NUM>, more preferably not less than <NUM>, still more preferably not less than <NUM>, but not more than <NUM>, more preferably not more than <NUM>, still more preferably not more than <NUM>.

In the present embodiment of the invention, the tread rubber <NUM> further includes a radially innermost third rubber layer <NUM> made of a base rubber compound <NUM>.

The third rubber layer <NUM> is disposed on the radially inside of the second rubber layer <NUM>.

It is preferable that the loss tangent δb of the base rubber compound <NUM> is smaller than the loss tangent δ1. Specifically, the loss tangent δb is not more than <NUM>.

The third rubber layer <NUM> made of such base rubber compound <NUM> suppresses excessive heat generation in the tread portion <NUM> and helps to improve the fuel efficiency.

In the present embodiment of the invention, at least in the portion of the tread rubber <NUM> corresponding to the tread surface <NUM>, in other words, the portion having a width corresponding to the tread width, the tread rubber <NUM> consists of the first rubber layer <NUM>, the second rubber layer <NUM> and the third rubber layer <NUM>, and no other rubber layer is included. However, the tread rubber <NUM> is not limited to such three layer structure, and further rubber layers may be included as appropriate.

On the radially inner side of the tread surface <NUM>, the first rubber layer <NUM>, the second rubber layer <NUM> and the third rubber layer <NUM> each have a substantially constant thickness except for the vicinity of each circumferential groove <NUM>.

In this embodiment of the invention, the maximum thickness t1 of the first rubber layer <NUM> is set in a range from <NUM>% to <NUM>% of the effective tread thickness ta.

Here, the effective tread thickness ta is generically the thickness of the tread rubber <NUM> from the tread surface <NUM> to a wear limit at which the tire <NUM> can maintain the minimum required dynamic performance. When the circumferential groove <NUM> is provided, the effective tread thickness ta is the thickness of the tread rubber <NUM> from the tread surface <NUM> to the bottom of the circumferential groove <NUM>.

As a result, when the wear of the tread portion <NUM> has progressed to a certain degree, the second cap rubber compound <NUM> capable of exerting excellent wet grip is exposed and the wet performance can be effectively maintained.

It is preferable that when the tread portion <NUM> is worn by <NUM>%, the second rubber layer <NUM> is at least partially exposed in the tread surface <NUM>.

The expression "the tread portion <NUM> is worn by <NUM>%" means a worn state in which the above-mentioned effective tread thickness ta has been reduced by <NUM>%.

Thereby, the second rubber layer <NUM> is exposed at a relatively early stage of wear, and wet performance can be effectively maintained.

From the viewpoint of reliably maintaining wet performance, it is preferable that the second rubber layer <NUM> forms the tread surface <NUM>, even when the tread rubber <NUM> is worn to the wear limit, that is, in a worn state in which the worn tread surface reaches to the tread wear indicator (not shown).

More preferably, the radially inner surface 22i of the second rubber layer <NUM> is located radially inside the groove bottom of the circumferential groove <NUM> except for the vicinity of the circumferential groove <NUM>. This ensures that wet performance is maintained.

The maximum thickness t2 of the second rubber layer <NUM> is set in a range from <NUM>% to <NUM>% of the effective tread thickness ta.

Thereby, the above effects can be obtained while maintaining the durability of the tread portion <NUM>.

The maximum thickness of the third rubber layer <NUM> can be variously determined so that the first rubber layer <NUM> and the second rubber layer <NUM> can have the above configurations. Preferably, the thickness t3 of the third rubber layer <NUM> is set in a range from <NUM>% to <NUM>% of the effective tread thickness ta. As a result, it is possible to improve the fuel consumption performance while exhibiting the above-described effects.

In the present embodiment of the invention, as shown in <FIG> and <FIG>, the tread portion <NUM> is provided with a plurality of lateral grooves <NUM> extending axially outwards from the axially outermost circumferential groove <NUM> beyond the tread edge Te.

The lateral grooves <NUM> and the outer lateral grooves <NUM> are disposed alternately in the tire circumferential direction.

In the present embodiment of the invention, as shown in <FIG>, the groove width W2 of the outer lateral groove <NUM> is set in a range from <NUM>% to <NUM>% of the groove width W1 of the lateral groove <NUM>.

In the present embodiment of the invention, the axially outer ends of the outer lateral grooves <NUM> and the axially outer ends of the lateral grooves <NUM> are positioned at the substantially same axial positions as shown in <FIG>.

The outer lateral grooves <NUM> are however, not limited to such arrangement.

In the present embodiment of the invention, the outer lateral grooves <NUM> extend parallel to the tire axial direction. But, the outer lateral grooves <NUM> may be inclined with respect to the tire axial direction. In this case, the angle of the outer lateral groove <NUM> (the angle of the widthwise groove center line) with respect to the tire axial direction is preferably <NUM> to <NUM> degrees, more preferably <NUM> to <NUM> degrees. Such outer lateral grooves <NUM> can exhibit excellent drainage performance by utilizing the tire rolling.

It is preferable that, in a worn state where the second cap rubber compound <NUM> is at least partially exposed, the axially inner ends of the outer lateral grooves <NUM> are located axially inward of the tread edge Te.

It is more preferable that, in the above-mentioned state where the tread portion <NUM> is worn by <NUM>%, the axially inner ends of the outer lateral grooves <NUM> are located axially inward of the tread edge Te.

As a result, it is possible to exhibit sufficient wet performance even if the tread portion <NUM> is worn off.

The axial distance L1 from the tread edge Te to the axial inner ends 25i of the outer lateral grooves <NUM> when the tire is new, can be appropriately determined so that the above effects can be exhibited, for example, in a range from <NUM> to <NUM>.

As a result, the above effects can be reliably exhibited while maintaining the steering stability on dry road surfaces when the tire is new.

<FIG> is a cross-sectional view taken along line A-A of <FIG> showing the cross-sectional shape of the outer lateral groove <NUM> along the length direction thereof.

As shown, the outer lateral grooves <NUM> has a maximum depth d1 which is, for example, in a range from <NUM> to <NUM>.

The depth of the outer lateral groove <NUM> is gradually decreased toward the axially inner end and toward the axially outer end from a middle position at which the maximum depth d1 occurs.

As a result, the cross-sectional shape of the outer lateral groove <NUM> is generally an obtuse triangle with the obtuse-angled vertex at the middle position with the maximum depth d1 although the side opposite to the obtuse-angled vertex is slightly curved in this example.

As a result, the volume of the tread rubber in the buttress portion (a portion defining the buttress surface <NUM>) is maintained, and the durability of this portion can be maintained while obtaining the above effects.

Claim 1:
A tire (<NUM>) comprising:
a tread portion (<NUM>) having a pair of tread edges (Te), a tread surface (<NUM>) extending between the tread edges (Te) and a pair of buttress surfaces (<NUM>) extending axially outwardly from the respective tread edges (Te),
the tread portion (<NUM>) comprising a tread rubber (<NUM>) including a radially outermost first rubber layer (<NUM>) forming the tread surface (<NUM>) and made of a first cap rubber compound (<NUM>), and a second rubber layer (<NUM>) disposed on the radially inside of the first rubber layer (<NUM>) and made of a second cap rubber compound (<NUM>),
wherein
the buttress surfaces (<NUM>) have inclinations such that the tread edges (Te) shift axially outwards as the tread rubber (<NUM>) is worn off,
characterized in that
a loss tangent δ2 of the second cap rubber (<NUM>) compound is larger than a loss tangent δ1 of the first cap rubber compound (<NUM>),
one of, or alternatively, each of the buttress surfaces (<NUM>) is provided with an outer lateral groove (<NUM>) extending in a tire axial direction, and
the outer lateral groove (<NUM>) is arranged axially outside the tread edge (Te) from which the buttress surface (<NUM>) extends axially outwards so that, when the tread edge (Te) shifts axially outwards due to wear of the tread rubber (<NUM>), the outer lateral groove (<NUM>) is at least partially positioned axially inside the tread edge (Te),
with the loss tangent values (tanδ) measured according to JIS K6394 under tensile deformation
at an initial strain of <NUM>%,
with a dynamic strain amplitude of +/-<NUM>%,
at a frequency of <NUM>,
at a temperature of <NUM>° Celsius.