Patent Description:
Patent Document <NUM> below discloses a pneumatic tire, wherein the tread portion thereof comprises a first cap rubber layer and a second cap rubber layer which have loss tangents specifically defined in order to improve rolling resistance and wet braking performance of the tire. Patent Document <NUM>: <CIT>.

A tire in accordance with the preamble of claim <NUM> is known from <CIT>. A related tire is described in <CIT>.

On the other hand, there is a demand for a tire which can exhibit good wet performance from the initial stage to the final stage of the wear life of the tread portion. But, in the case of a tire of which tread portion comprises rubber layers having different loss tangents such as the pneumatic tire disclosed in Patent Document <NUM>, there is a problem such that the wet performance tends to change suddenly in the middle stage of the wear life of the tread portion.

The present invention was made in view of the above circumstances, and a primary objective of the present invention is to provide a tire which can stably exhibit good wet performance from the initial stage to the final stage of the wear life of the tread portion.

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 provided with circumferential grooves which extend continuously in the tire circumferential direction to divide a first land portion and a second land portion which is located axially inside the first land portion,.

According to an embodiment of the invention, the tread portion comprises a base tread rubber layer disposed on the radially inside of the intermediate tread rubber layer, and the loss tangent δb of the base tread rubber layer is smaller than the loss tangent <NUM> of the cap tread rubber layer.

According to an embodiment of the invention, the loss tangent δb of the base tread rubber layer is less than <NUM>.

According to an embodiment of the invention, the base tread rubber layer extends in the tire axial direction with a substantially constant thickness.

According to an embodiment of the invention, the tread portion is provided with a tread reinforcing cord layer composed of cords coated with topping rubber and disposed radially inside the base tread rubber layer, and the thickness of the base tread rubber layer is <NUM>% to <NUM>% of the tread thickness measured from the radially outer surface of the tread portion to the radially outer surface of the tread reinforcing cord layer.

According to an embodiment of the invention, the loss tangent <NUM> of the cap tread rubber layer is <NUM> to <NUM>.

According to an embodiment of the invention, the loss tangent δ2 of the intermediate tread rubber layer is <NUM> to <NUM>.

According to an embodiment of the invention, the distance L1 is <NUM>% to <NUM>% of the distance L2.

In the tire according to the present invention, therefore, owing to the different loss tangents δ1 and δ2 and the different distances L1 and L2 to the respective intermediate tread rubber layers, good wet performance is stably exhibited from the initial stage to the final stage of the tread wear life.

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

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

<FIG> is a tire meridian cross-sectional view of a pneumatic tire <NUM> for passenger cars as an embodiment of the present invention under a standard state of the tire <NUM>.

In this application including specification and claims, various dimensions, positions and the like of the tire refer to those under the standard state unless otherwise noted.

In the case of a pneumatic tire for which various specifications are set by a standardization organization, the standard state is such that the tire is mounted on a standard rim, and inflated to a standard pressure, but loaded with no tire load.

In the case of a tire for which various specifications are not yet set, the standard state means a standard usage state of the tire according to the purpose of use of the tire, which is not mounted on the vehicle and loaded with no tire load. The standard wheel rim is a wheel rim officially approved or recommended for the tire by the standardization organization, 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 under-mentioned 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 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 tire 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.

The under-mentioned tread edges Te are the axial outermost edges of the ground contacting patch of the tire which occurs when the tire under its standard state is loaded with the standard tire load and placed on a horizontal flat surface at a camber angle of zero degree.

In the present embodiment of the invention, the carcass <NUM> is composed of two carcass plies 6A and 6B. Each of the carcass plies 6A and 6B is composed of rubberized cords arranged radially at an angle of <NUM> to <NUM> degrees with respect to the tire circumferential direction. In this example, the carcass cord is made of organic fibers.

In the present embodiment of the invention, the tread reinforcing cord layer <NUM> is composed of two plies 7A and 7B of reinforcing cords. For example, each of the reinforcing plies 7A and 7B is composed of parallel reinforcing cords arranged at an angle of <NUM> to <NUM> degrees with respect to the tire circumferential direction and rubberized with topping rubber. As the reinforcing cords, organic fiber cords or steel cords may be appropriately used.

The tread portion <NUM> is provided with circumferential grooves <NUM> extending continuously in the tire circumferential direction to axially divide a first land portion <NUM> and a second land portion <NUM> which is located on the tire equator C side of the first land portion <NUM>.

In the present embodiment of the invention, the tread portion <NUM> is provided with three circumferential grooves <NUM>, and thereby, the tread portion <NUM> is axially divided into four land portions: a pair of axially outer first land portions <NUM> and a pair of axially inner second land portions <NUM> therebetween. The present invention is however, not limited to such a four land portion arrangement.

As shown in <FIG>, each of the first land portions <NUM> and the second land portions <NUM> comprises a cap tread rubber layer <NUM>, an intermediate tread rubber layer <NUM> and a base tread rubber layer <NUM>. The cap tread rubber layer <NUM> is radially outermost, and forms the ground contacting surface <NUM>. The intermediate tread rubber layer <NUM> is disposed on the radially inside of the cap tread rubber layer <NUM>. The base tread rubber layer <NUM> is disposed on the radially inside of the intermediate tread rubber layer <NUM>. In the present embodiment of the invention, the tread rubber disposed on the radially outside of the tread reinforcing cord layer <NUM> is composed of three rubber layers: the cap tread rubber layer <NUM>, the intermediate tread rubber layer <NUM> and the base tread rubber layer <NUM>. The present invention is however, not limited to such three layer structure. Incidentally, in <FIG>, the tread rubber is hatched as if a single rubber layer for simplification.

It has been known that, in general, rubber having a large loss tangent can exert a large frictional force on a wet road surface. However, in the present invention, the loss tangent (tan δ2) of the radially inner intermediate tread rubber layer <NUM> is set to be larger than the loss tangent (tan <NUM>) of the radially outermost cap tread rubber layer <NUM>.

Here, the loss tangent (tan δ) is measured according to Japanese Industrial Standard (JIS) K6394, using a dynamic mechanical characteristic 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.

According to the present invention, the shortest distance L2 from the ground contacting surface <NUM> of the second land portion <NUM> to the radially outer surface of the intermediate tread rubber layer <NUM> of the second land portion <NUM> is set to be smaller than
the shortest distance L1 from the ground contacting surface <NUM> of the first land portion <NUM> to the radially outer surface of the intermediate tread rubber layer <NUM> of the first land portion <NUM>. Thereby, the tire <NUM> according to the present invention can stably exhibit good wet performance from the initial stage to the final stage of the wear life of the tread portion <NUM> for the following reason.

Overall, as the wear of the tread portion <NUM> progresses, the intermediate tread rubber layer <NUM> having excellent frictional force on wet road surfaces is exposed, and thereby, the deterioration of the wet performance due to the wear of the tread portion <NUM> can be compensated for by exposing the intermediate tread rubber layer <NUM>, and good wet performance is stably exhibited.

If the intermediate tread rubber layer <NUM> is exposed in the entire area or a wide area of the tread surface of the worn tread portion <NUM> at about the same time, then the change in wet performance before and after the intermediate tread rubber layer <NUM> is exposed becomes large. This is not desirable.

According to the present invention, since the distance L2 in the second land portion <NUM> is smaller than the distance L1 in the first land portion <NUM>, the intermediate tread rubber layer <NUM> of the second land portion <NUM> is exposed, and after a while, the intermediate tread rubber layer <NUM> of the first land portion <NUM> is exposed. Therefore, the change in wet performance, namely, improvement due to the exposure of the intermediate tread rubber layer <NUM> becomes gradual. On the other hand, as the wear progresses, tread grooves are gradually decreased in depth and volume, which affects the wet performance. But, the deterioration of the wet performance due to the gradual decrease in the groove depth and volume is compensated by the gradual exposure of the intermediate tread rubber layer <NUM>.

For this reason, the tire <NUM> according to the present invention can exhibit good wet performance stably from the initial stage to the final stage of the wear life of the tread portion <NUM>.

The loss tangent (tan δ1) of the cap tread rubber layer <NUM> is preferably set to be 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>. Such cap tread rubber layer <NUM> can exhibit steering stability on dry road surfaces (hereinafter, simply referred to as "steering stability") and wet performance in a well-balanced manner in the initial stage of the tread wear life.

It is preferable that the loss tangent (tan δ2) of the intermediate tread rubber layer <NUM> is set to be 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 order to improve steering stability and wet performance when the tread portion <NUM> is worn.

If the difference between the loss tangent (tan <NUM>) and the loss tangent (tan δ2) is small, it is difficult to derive the above-mentioned effect. If the difference is large, then the separation failure is likely to occur at the boundary between the cap tread rubber layer <NUM> and the intermediate tread rubber layer <NUM>. From such a viewpoint, it is preferable that the loss tangent (tan δ2) is <NUM> to <NUM> times, more preferably <NUM> to <NUM> times the loss tangent (tan δ1).

Each of the cap tread rubber layer <NUM> and the intermediate tread rubber layer <NUM> has a substantially constant thickness except for the vicinity of each circumferential groove <NUM>. Thereby, their separation at the boundary of these rubber layers can be suppressed. The above-said vicinity of the circumferential groove <NUM> may include a range of not more than <NUM>% of the groove width of the circumferential groove <NUM> at the groove top, from each of the groove walls toward the both sides of the circumferential groove <NUM>. The expression "substantially constant thickness" means that the difference between the maximum value and the minimum value of the thickness is not more than <NUM>% of the maximum value.

The distance L1 in the first land portion <NUM> is preferably <NUM>% to <NUM>%, more preferably <NUM> to <NUM>% of the tread thickness t1 measured in the first land portion <NUM> from the radially outer surface of the tread reinforcing cord layer <NUM> to the radially outer surface of the tread portion <NUM>.

The distance L2 of the second land portion <NUM> is preferably <NUM>% to <NUM>%, more preferably <NUM>% to <NUM>% of the tread thickness t2 measured in the second land portion <NUM> from the radially outer surface of the tread reinforcing cord layer <NUM> to the radially outer surface of the tread portion <NUM>.

Further, the distance L1 is preferably not less than <NUM>%, more preferably not less than <NUM>%, but preferably not more than <NUM>%, more preferably not more than <NUM>% of the distance L2. Thereby, the wet performance is more stably exhibited.

In the first land portion <NUM>, the thickness t3 of the intermediate tread rubber layer <NUM> is preferably set in a range from <NUM>% to <NUM>% of the above-said tread thickness t1. In the second land portion <NUM>, the thickness t4 of the intermediate tread rubber layer <NUM> is preferably set in a range from <NUM>% to <NUM>% of the tread thickness t2. The present invention is however, not limited to such thickness ranges.

The tread portion <NUM> includes the base tread rubber layer <NUM> disposed on the radially inside of the intermediate tread rubber layer <NUM>. The loss tangent (tan δb) of the base tread rubber layer <NUM> is smaller than the loss tangent (tan <NUM>) of the cap tread rubber layer <NUM>. Specifically, the loss tangent (tan δb) is less than <NUM>. Such base tread rubber layer <NUM> helps to suppress excessive heat generation of the tread portion <NUM>.

The base tread rubber layer has a substantially constant thickness except for the vicinity of the circumferential groove <NUM>. The meaning of the expression "substantially constant thickness" is as explained above. The thickness of the base tread rubber layer <NUM> is preferably set in a range from <NUM>% to <NUM>% of the tread thickness from the radially outer surface of the tread reinforcing cord layer <NUM> to the radially outer surface of the tread portion <NUM>.

In the present embodiment of the invention, the boundary <NUM> between the cap tread rubber layer <NUM> and the intermediate tread rubber layer <NUM> extends parallel to the ground contacting surface, except for the vicinity of each circumferential groove <NUM>. The present invention is however, not limited to such example. For example, <FIG> shows another example of the first land portion <NUM> and the second land portion <NUM>, wherein the boundary <NUM> (except for portions in the vicinities of the circumferential grooves <NUM>) does not extend parallel to the ground contacting surface <NUM> of the tread portion <NUM>. More specifically, in each of the first land portion <NUM> and the second land portion <NUM>, the boundary <NUM> comprises a radially outwardly protruding portion <NUM>, and
the radial distance from the radially outer surface of the tread portion <NUM> to the protruding portion <NUM> of the boundary <NUM> is increased toward both sides in the tire axial direction from the radially outwardly most protruding position. Accordingly, as the wear of the tread portion progresses, the intermediate tread rubber layer <NUM> is gradually increased in occupied area in the tread surface, therefore, the wet performance can be more stably exhibited.

It is preferable that the radius of curvature R2 of the boundary <NUM> in the second land portion <NUM> is smaller than the radius of curvature R1 of the boundary <NUM> in the first land portion <NUM>. As a result, it is possible to increase the amount of wear required for the intermediate tread rubber layer <NUM> to be completely exposed in the second land portion <NUM>, while suppressing the separation of the rubber layers at the boundary <NUM>.

<FIG> shows still another example of the first land portion <NUM> and the second land portion <NUM>, wherein the boundary <NUM> is undulated. Specifically, in the tire meridian cross section, the boundary <NUM> extends in the tire axial direction, while oscillating in the tire radial direction, thus, showing a wavy shape. In this example, too, the intermediate tread rubber layer <NUM> is gradually increased in occupied area in the tread surface, as the wear of the tread portion progresses, and thereby, the wet performance can be more stably exhibited. In addition, the substantial length of the boundary <NUM> is increased, which helps to suppress the separation between the rubber layers at the boundary <NUM>.

As shown in <FIG>, the amplitude A2 in the tire radial direction of the boundary <NUM> in the second land portion <NUM> is preferably larger than the amplitude A1 in the tire radial direction of the boundary <NUM> in the first land portion <NUM>. Thereby, it is possible to increase the amount of wear required for the intermediate tread rubber layer <NUM> to be completely exposed in the second land portion <NUM>.

While detailed description has been made of preferable embodiments of the present invention, the present invention can be embodied in various forms within the scope of the appended claims.

Based on the structure shown in <FIG>, pneumatic tires of size <NUM>/65R16C were experimentally manufactured as test tires including working example Ex. <NUM>, and comparative examples Ref.<NUM> and Ref.<NUM>.

Specifications of the test tires are shown in Table <NUM>.

In the comparative example Ref.<NUM>, the loss tangent (tan <NUM>) of the cap tread rubber layer was larger than the loss tangent (tan δ2) of the intermediate tread rubber layer, and the distance L1 was equal to the distance L2.

In the comparative example Ref.<NUM>, the loss tangent (tan <NUM>) of the cap tread rubber layer was smaller than the loss tangent (tan δ2) of the intermediate tread rubber layer, and the distance L1 was equal to the distance L2.

The tread thickness t1 in the first land portion and the tread thickness t2 in the second land portion were substantially the same in each test tire and also for all test tires.

All test tires had substantially the same structures except for the loss tangents, distances and thicknesses shown in Table <NUM>.

For each test tire, the relationship between the amount of wear (AW) of the tread portion and the wet braking distance (WBD) was investigated.

The test tire was mounted on a standard rim (size 16x7.0J) and attached to a test vehicle (3000cc rear-wheel-drive commercial vehicle), and the wet braking distance (WBD) was measured using the test vehicle.

The braking distance was measured by applying full brake when the test vehicle was running at a speed of <NUM>/h on an asphalt road surface of a tire test course, covered with <NUM> depth water.

Such measurement was carried out many times from a state in which the amount of wear = <NUM>% (that is, a new tire state) to a state in which the amount of wear = <NUM>% (that is, the tread wear reached to the wear indicator).

The results are shown in <FIG> as a graph, wherein the horizontal axis is the amount of wear (AW) of the tread portion, and the vertical axis is the wet braking distance (WBD). Needless to say, the smaller wet braking distance is better.

In <FIG>, the graph G1 shows the WBD-AW relationship of the comparative example Ref.<NUM>, the graph G2 shows the WBD-AW relationship of the comparative example Ref.<NUM> and the graph G3 shows the WBD-AW relationship of the working example Ex. As shown in <FIG>, in each test tire, as the amount of wear (AW) of the tread portion increased, the drainage property of the circumferential grooves was decreased and the wet braking distance (WBD) was increased.

In the comparative example Ref.<NUM>, the wet braking distance (WBD) was small in the initial stage of tread wear. However, when the tread portion was worn and the intermediate tread rubber layer having the smaller loss tangent was exposed, the wet braking distance (WBD) changed abruptly and increased. Then, the wet braking distance (WBD) was gradually increased as the amount of wear (AW) increased.

In the comparative example Ref.<NUM>, the large abrupt change of the wet braking distance was suppressed as compared with the comparative example Ref.<NUM>, but a smaller abrupt change still existed because the intermediate tread rubber layer appeared in the entire tread surface within a short period of time.

In the working example Ex. <NUM>, the wet braking distance (WBD) was changed linearly with respect to the change in the amount of wear (AW) of the tread portion without the abrupt changes as in the comparative examples.

Claim 1:
A tire (<NUM>) comprising a tread portion (<NUM>) provided with circumferential grooves (<NUM>) which extend continuously in the tire circumferential direction to divide a first land portion (<NUM>) and a second land portion (<NUM>) which is located axially inside the first land portion (<NUM>),
wherein
each of the first land portion (<NUM>) and the second land portion (<NUM>) comprises a radially outermost cap tread rubber layer (<NUM>) forming a ground contacting surface (<NUM>), and an intermediate tread rubber layer (<NUM>) arranged on the radially inside of the cap tread rubber layer (<NUM>); and
the shortest distance L2 from the ground contacting surface (<NUM>) of the second land portion (<NUM>) to the radially outer surface of the intermediate tread rubber layer (<NUM>) of the second land portion (<NUM>) is less than the shortest distance L1 from the ground contacting surface of the first land portion (<NUM>) to the radially outer surface of the intermediate tread rubber layer (<NUM>) of the first land portion
characterized in that
the loss tangent δ2 of the intermediate tread rubber layer (<NUM>) is larger than the loss tangent <NUM> of the cap tread rubber layer (<NUM>),
the loss tangent δ is measured according to Japanese Industrial Standard (JIS) K6394 at an initial strain of <NUM>%, at a dynamic strain amplitude of +/-<NUM>%, at a frequency of <NUM>, at a temperature of <NUM>° Celsius, under tensile deformation.