Patent Description:
In <CIT>, there has been proposed a pneumatic tire in which a band layer is disposed in a tread portion. And, a hybrid cord obtained by twisting a nylon fiber strand and an aramid fiber strand is used as a band cord of the band layer.

<CIT> discloses a tire having the features in the preamble of claim <NUM>. <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and <CIT> disclose further prior art.

As described above, a pneumatic tire in which a hybrid cord including materials having different elastic moduli is used as a band cord, can be expected to improve riding comfort and steering stability. In recent years, on the other hand, with the improvement of vehicle performance, pneumatic tires are required to further improve steering stability during high-speed running.

The present invention has been devised in view of the above circumstances, and
a primary object thereof is to provide a pneumatic tire in which steering stability at high speeds is improved, while maintaining good ride comfort.

The above object is solved by a pneumatic tire according to claim <NUM>. Further advantageous embodiments are disclosed in the dependent claims.

By employing the above configurations, the pneumatic tire according to the present invention can improve steering stability during high-speed running, while maintaining good ride comfort.

<FIG> shows a meridian cross-sectional view of a pneumatic tire (hereinafter sometimes simply referred to as "tire") <NUM> in the present embodiment.

<FIG> is a transverse cross-sectional view of the tire <NUM> under its normal state, including the rotational axis.

As shown in <FIG>, the tire <NUM> in the present embodiment is a pneumatic tire for passenger cars. The present invention is however, not limited to such embodiment, and may be applied to tires for heavy loads and tires for motorcycles.

The "normal state" means, in the case of a tire for which various standards have been established, that the tire is mounted on a regular rim and inflated to a regular internal pressure but loaded with no load. In the case of a tire for which various standards have not been established, the normal state means a standard usage state according to the purpose of use of the tire, which is a state in which the tire is not mounted on the vehicle and no load is applied.

The "regular rim" is a rim defined for each tire in a standard system that includes standards on which the tire is based, for example, "Standard Rim" in JATMA, "Design Rim" in TRA, and "Measuring Rim" in ETRTO.

The "regular internal pressure" is air pressure specified for each tire by each standard in the standard system including the standards on which the tire is based, and is "maximum air pressure" in JATMA, the maximum value described in the table "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" in TRA and "INFLATION PRESSURE" in ETRTO.

In this specification, unless otherwise noted, the dimensions of each part of the tire are the values measured under the normal state.

In the case of a component inside the tire that cannot be measured while maintaining its normal state, its dimensions shall be measured while maintaining its shape under the normal state.

Further, in this specification, unless otherwise noted, the composition of each part of the tire means the composition of the material taken out from a new and unused tire.

As the taking-out method at that time, a method that does not change the composition as much as possible is appropriately adopted.

The tire <NUM> in the present embodiment has a carcass <NUM>. The carcass <NUM> is composed of, for example, one carcass ply 6A. The carcass ply 6A comprises a plurality of carcass cords, and topping rubber covering them. The carcass cords are arranged, for example, at an angle of <NUM> to <NUM> degrees with respect to the tire circumferential direction.

Incidentally, "<NUM> to <NUM> degrees" means "not less than <NUM> degrees and not more than <NUM> degrees", and the same applies hereinafter in the present specification.

As the carcass cords, preferably used are organic fiber cords such as nylon, polyester or rayon, and the like.

The carcass ply 6A has a main body portion 6a and a folded portion 6b.

The main body portion 6a extends from one bead portion <NUM> to the other bead portion <NUM> through one sidewall portion <NUM>, a tread portion <NUM> and the other sidewall portion <NUM>.

The folded portion 6b continues from the main body portion 6a, and is folded around a bead core <NUM> from the inside to the outside in the tire axial direction so as to extend outward in the tire radial direction.

The tread portion <NUM> of the present embodiment is provided with a belt layer <NUM>.

The belt layer <NUM> comprises, for example, two belt plies 8A and 8B. For example, each of the belt plies 8A and 8B comprises a plurality of belt cords arranged obliquely with respect to the tire circumferential direction, and a topping rubber covering them.

It is preferable that each belt cord is inclined at an angle of <NUM> to <NUM> degrees with respect to the tire circumferential direction.

The tread portion <NUM> comprises a tread rubber <NUM> forming a ground contacting surface, and a band layer <NUM> disposed radially inside the tread rubber <NUM>.

The tread rubber <NUM> is provided with at least one first circumferential groove <NUM> extending continuously in the tire circumferential direction in the ground contacting surface. The tread rubber <NUM> of the present embodiment is provided with three first circumferential grooves <NUM>.

The groove width of the first circumferential groove <NUM> is, for example, <NUM> to <NUM>.

The depth of the first circumferential groove <NUM> is, for example, <NUM> to <NUM>.

However, the groove width and groove depth of the first circumferential groove <NUM> are not particularly limited as long as a certain degree of drainage can be expected during wet running.

Further, the tread rubber <NUM> may be provided with a second circumferential groove different from the above-described first circumferential groove <NUM>.

In <FIG>, an enlarged perspective view of the band layer <NUM> is shown.

The band layer <NUM> is composed of, for example, one band ply <NUM>. The band ply <NUM> comprises band cords <NUM> arranged substantially parallel to the tire circumferential direction, and a topping rubber <NUM> covering the band cords <NUM>.

For example, the band layer <NUM> may be a plurality of superimposed band plies <NUM>, or a plurality of band plies <NUM> arranged in the tire axial direction apart from each other. In the present embodiment, the band ply <NUM> is configured as a so-called jointless band in which, for example, a single band cord is wound in the tire circumferential direction.

In another embodiment, the band ply <NUM> may include a plurality of band cords <NUM> arranged substantially parallel to the tire circumferential direction.

The expression "the band cord <NUM> extends substantially parallel to the tire circumferential direction" includes at least such a mode that the band cord <NUM> extends at an angle of <NUM> degrees or less with respect to the tire circumferential direction.

<FIG> shows an enlarged perspective view of the band cord <NUM>.

As shown in <FIG>, the band cord <NUM> is a hybrid cord in which a first filament <NUM> and a second filament <NUM> which has a smaller elastic modulus than the first filament <NUM> are twisted together. The first filament <NUM> and the second filament <NUM> are made of organic fibers, and materials which have been conventionally used as materials for tire cords such as nylon, aramid and PET, are employed. Thereby, a known hybrid cord is appropriately employed for the band cord <NUM> of the present embodiment.

In <FIG>, there is shown an enlarged view of the first circumferential groove <NUM> and the band layer <NUM>. In <FIG>, the belt layer <NUM> (shown in <FIG>) disposed radially inside the band layer <NUM> is omitted. in <FIG>, although the cross section of the band cord <NUM> is simplified and indicated by a circle, it goes without saying that the band cord <NUM> has the above-described configuration.

According to the present invention, as shown in <FIG>, in a region radially inside the first circumferential groove <NUM>, the maximum thickness t1 of the topping rubber <NUM> on the radially outside of the band cord <NUM> is not more than <NUM>. Further, the minimum distance d1 from the groove bottom of the first circumferential groove <NUM> to the radially outer surface <NUM> of the band layer <NUM> is not more than <NUM>. Furthermore, the distance d2 in the tire normal direction from a groove edge 10e of the first circumferential groove <NUM> to the radially outer surface <NUM> of the band layer <NUM> is not more than <NUM>.

The outer surface <NUM> of the band layer <NUM> means the outer surface of the topping rubber <NUM> covering the band cord <NUM>.

The tire normal direction means a direction normal to the ground contacting surface of the tread portion <NUM>.

In this specification, the ground contacting surface of the tread portion <NUM> means a surface contacting with a plane when the tire <NUM> under the normal state is contacted with the plane at a camber angle of <NUM> degrees under the normal load. Further, the groove edge means the boundary between the opening area of the groove and the ground contacting surface. The ground contacting surface and the groove edge can be imaged by a known method such as CT imaging.

In the case of a pneumatic tire for which various standards have been established, the "normal load" is the load defined for each tire by each standard in the standard system including the standards on which that the tire is based, and is "maximum load capacity" in JATMA, the maximum value described in the table "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" in TRA, and "LOAD CAPACITY" in ETRTO.

In the case of a tire for which various standards have not been established, the "normal load" refers to the load acting on one tire when the tire is in the standard mounting state.

The "standard mounting state" refers to a state in which the tire is mounted on a standard vehicle according to the purpose of use of the tire, and the vehicle is standing still on a flat road surface in a drivable state.

By employing the above configurations, the tire <NUM> of the present invention can improve steering stability during high-speed running while maintaining good ride comfort.

As the reason therefor, it is presumed to be the following mechanism.

In general, the band layer <NUM> greatly influences the degree of vibration absorption of the tread portion <NUM> and the degree of outer diameter increase of the tread portion <NUM>.

In other words, the band layer <NUM> has a great influence on ride comfort during normal running and steering stability during high-speed running.

In the present invention, a hybrid cord is used for the band cord <NUM> of the band layer <NUM>.

Thereby, during normal running in which the outer diameter increase of the tread portion <NUM> is small, the second filament <NUM> (shown in <FIG>, the same applies hereinafter) having a lower elastic modulus becomes dominant, and good riding comfort is maintained.

During high-speed running, the first filament <NUM> (shown in <FIG>, the same applies hereinafter) having a higher elastic modulus, suppresses the outer diameter increase of the tread portion <NUM>, and excellent steering stability is exhibited.

The developers have found, as a result of intensive research, that it is possible to further improve the steering stability by specifying the thickness of the rubber around the band layer <NUM>, and have completed the present invention.

In the present invention, as described above, the thickness t1 is defined as <NUM> or less, the distance d1 is defined as <NUM> or less, and the distance d2 is defined as <NUM> or less. Thereby, the thickness of the rubber on the radially outside of the band cords <NUM> included in the band layer <NUM>, can be set smaller, and the influence of the centrifugal force acting on the tread portion <NUM> during high-speed running can be reduced.

Therefore, the increase of the outer diameter of the tread portion <NUM> is suppressed, the tread profile can be maintained even during high-speed running, and the ground contact of the tread portion <NUM> is improved.

It is believed that, by the mechanism described above, the present invention can improve steering stability during high-speed running while maintaining good ride comfort.

More detailed configurations of the present embodiment will be described below.

εach configuration described below represents a specific aspect of the present embodiment.

Therefore, it goes without saying that the present invention can exhibit the above effects, even if it does not have the configurations described below.

Further, even if any one of the configurations described below is applied alone to the tire of the present invention having the features described above, an improvement in performance according to each configuration can be expected. Furthermore, when some of the configurations described below are applied in combination, it is possible to expect a combined improvement in performance according to the respective configurations.

As shown in <FIG>, the band cord <NUM> of the present embodiment is formed as a multi-twisted structure by twisting together a first yarn <NUM> formed by twisting first filaments <NUM> together, and a second yarn <NUM> formed by twisting second filaments <NUM> together. However, the band cord <NUM> is not limited to such a mode, and may have a single twist structure including the first filaments <NUM> and the second filaments <NUM>.

The difference in elastic modulus between the first filament <NUM> and the second filament <NUM> is determined, for example, by tensile strength specified in JIS L1017.

The conditions for measuring the tensile strength are not particularly limited as long as the tensile strength of the first filament <NUM> and the tensile strength of the second filament <NUM> are measured under the same conditions.

The total fineness of one band cord <NUM> is, for example, not more than <NUM> dtex, preferably <NUM> to <NUM> dtex, more preferably <NUM> to <NUM> dtex.

Such band cord <NUM> is helpful for improving riding comfort and steering stability in a well-balanced manner.

The stress σ1 of the band cord <NUM> when elongated by <NUM>% is, for example, not more than <NUM> (N/tex). The stress σ1 is preferably not less than <NUM> (N/tex), more preferably not less than <NUM> (N/tex), and preferably not more than <NUM> (N/tex), more preferably not more than <NUM> (N/tex).

such band cord <NUM> can exhibit excellent ride comfort, while maintaining the durability of the tread portion <NUM>.

The stress σ2 of the band cord <NUM> when elongated by <NUM>% is, for example, not less than <NUM> (N/tex). The stress σ2 is preferably not less than <NUM> (N/tex), more preferably not less than <NUM> (N/tex), and preferably not more than <NUM> (N/tex),more preferably not more than <NUM> (N/tex).

such band cord <NUM> is helpful for improving ride comfort and steering stability at high speeds in a well-balanced manner.

The ratio σ1/σ2 (%) between the stress σ1 and the stress θ2 is preferably not less than <NUM>%, more preferably not less than <NUM>%, and preferably not more than <NUM>%, more preferably not more than <NUM>%.

The stress σ1 and the stress σ2 described above are measured, for example, by the method of measuring load under constant extension specified in JIS L1017.

As shown in <FIG>, the tread rubber <NUM> is provided with a plurality of first circumferential grooves <NUM>. Further, the tread rubber <NUM> is provided with a plurality of lateral grooves (not shown) extending in the tire axial direction in the ground contacting surface.

From the viewpoint of achieving both ride comfort and steering stability, the sea ratio of the ground contacting surface of the tread portion <NUM> is preferably not less than <NUM>%, more preferably not less than <NUM>%, and preferably not more than <NUM>%, more preferably not more than <NUM>%, still more preferably not more than <NUM>%.

Incidentally, the sea ratio is the ratio of the sum of the opening areas of all the grooves to the total area of the virtual ground contacting surface in such a state that all the grooves in the ground contacting surface of the tread rubber <NUM> are filled up.

It is preferable that the ratio σ1/σ2 (%) of the stress σ1 (N/tex) when the band cord <NUM> is stretched by <NUM>% to the stress σ2 (N/tex) when the band cord <NUM> is stretched by <NUM>% is larger than the sea ratio (%) of the ground contacting surface of the tread portion.

specifically, the ratio σ1/σ2 (%) is preferably not less than <NUM> times, more preferably not less than <NUM> times, and preferably not more than <NUM> times, more preferably not more than <NUM> times the sea ratio (%).

Thereby, the ratio σ1/σ2 is optimized, and excellent steering stability is exhibited both during low-to-medium speed running and high speed running.

The tread rubber <NUM> includes, for example, a rubber compound having a complex elastic modulus E*<NUM> of <NUM> to <NUM> (MPa) at <NUM> degrees C.

The complex elastic modulus E*<NUM> is more preferably <NUM> to <NUM> (MPa).

On the other hand, in the case of a tire in which riding comfort is important, the complex elastic modulus E*<NUM> at <NUM> degrees C. of the tread rubber <NUM> may be not more than <NUM> (MPa).

The complex elastic modulus E*<NUM> is a value measured using a "viscoelastic spectrometer" manufactured by Iwamoto Seisakusho Co. under the following conditions in accordance with JIS-K6394.

As a test sample for measurement, there is used, for example, a rubber piece obtained from the tread rubber <NUM> so that the long side is in the tire circumferential direction, the short side is in the tire axial direction, and the thickness is in the tire radial direction. for example, the size of the rubber piece is <NUM> long side x <NUM> short side x <NUM> thick.

The developers have found, as a result of various experiments, that specifying the complex elastic modulus E*<NUM> of the tread rubber <NUM> according to the sea ratio, contributes to both ride comfort and steering stability. specifically, the product of the complex elastic modulus E*<NUM> (MPa) and the sea ratio (%) is preferably not less than <NUM> (MPa), more preferably not less than <NUM> (MPa), and preferably not more than <NUM> (MPa), more preferably not more than <NUM> (MPa).

Thereby, the amount of deformation of the tread portion <NUM> during running is optimized, and ride comfort and steering stability can be improved in a well-balanced manner.

Further, the product σ2·E*<NUM> of the stress σ2 (N/tex) at <NUM>% elongation of the band cord and the complex elastic modulus E*<NUM> (MPa) is preferably not less than <NUM> (N· MPa/tex), more preferably not less than <NUM> (N·MPa/tex), and preferably not more than <NUM> (N·MPa/tex), more preferably not more than <NUM> (N·MPa/tex).

Thereby, the amount of deformation of the tread portion <NUM> and the band layer <NUM> during high-speed running is optimized, and ride comfort and steering stability are improved in a well-balanced manner.

As shown in <FIG>, the thickness t1 is preferably not less than <NUM>, more preferably not less than <NUM>, and preferably not more than <NUM>, more preferably not more than <NUM>. Thereby, the above-mentioned effect can be exhibited while ensuring the durability of the band layer.

The distance d1 is preferably not less than <NUM>, more preferably not less than <NUM>, and preferably not more than <NUM>, more preferably not more than <NUM>. Thereby, at the groove bottom of the first circumferential groove <NUM>, the thickness of the rubber is optimized, and the above-described effects are exhibited more reliably.

The distance d2 is preferably not less than <NUM>, more preferably not less than <NUM>, and preferably not more than <NUM>, more preferably not more than <NUM>. Thereby, the thickness of the tread rubber <NUM> is optimized.

The developers have found that the above-described effect can be further improved by specifying the stress σ1 of the band cord <NUM> when elongated by <NUM>% and the distance d2 in association with each other.

specifically, the product σ1·d2 of the stress σ1 (N/tex) and the distance d2 (mm) is preferably <NUM> to <NUM> (N·mm/tex), more preferably <NUM> to <NUM>.

Thereby, the amount of deformation of the tread portion <NUM> during medium to low speed running is optimized, and ride comfort can be further improved.

Similarly, it was found that the steering stability is further improved by specifying the stress σ2 of the band cord <NUM> when elongated by <NUM>% and the distance d2 in association with each other.

specifically, the product σ2·d2 of the stress σ2 (N/tex) and the distance d2 (mm) is preferably <NUM> to <NUM> (N·mm/tex), more preferably, <NUM> to <NUM>.

Thereby, the amount of deformation of the tread portion <NUM> during high-speed running is optimized, and steering stability during high-speed running is further improved.

A ratio t1/d1 (%) between the thickness t1 and the distance d1 is preferably not less than <NUM>%, more preferably not less than <NUM>%, and preferably not more than <NUM>%, more preferably not more than <NUM>%.

Further, it is preferable that the ratio t1/d1 (%) between the thickness t1 and the distance d1 is smaller than the sea ratio (%). Specifically, the ratio t1/d1 (%) is preferably not less than <NUM> times, more preferably not less than <NUM> times, and preferably not more than <NUM> times, more preferably not more than <NUM> times the sea ratio (%). Thereby, the thickness t1 is optimized, and ride comfort is improved while maintaining the durability of the band layer <NUM>.

In addition, it was found that the distance d1 and the complex elastic modulus E*<NUM> have a large effect on ride comfort and steering stability, and it is preferable for improving these performances in a well-balanced manner, to define the distance d1 and the complex elastic modulus E*<NUM> in association with each other.

specifically, the product of the distance d1 (mm) and the complex elastic modulus E*<NUM> (MPa) is preferably not less than <NUM> (MPa·mm), more preferably not less than <NUM> (MPa·mm), and preferably not more than <NUM> (MPa·mm), more preferably not more than <NUM> (mPa·mm).

Thereby, deformation of the rubber near the groove bottom of the first circumferential groove <NUM> is optimized, and steering stability during high-speed running can be improved while maintaining good ride comfort.

As shown in <FIG>, the tread rubber <NUM> of the present embodiment is provided with three first circumferential grooves <NUM>, and the above-described configuration is satisfied by at least one of the first circumferential grooves <NUM>.

In a more desirable mode, all of the plurality of first circumferential grooves <NUM> disposed in the tread rubber <NUM> satisfy the above-described configuration. Thereby, the above-described effects can be reliably exhibited.

while detailed description has been made of an especially preferable embodiment of the present invention, the present invention can be embodied in various modes without being limited to the illustrated embodiment.

Pneumatic tires of size <NUM>/60R16 satisfying the requirements of the present invention were experimentally manufactured based on the specifications shown in Tables <NUM>-<NUM>. As Comparative Example <NUM>, a pneumatic tire was experimentally manufactured, wherein a band cord was a hybrid cord, and a thickness t1, a distance d1 and a distance d2 did not satisfy the requirements of the present invention.

As Comparative Example <NUM>, a pneumatic tire was experimentally manufactured, wherein a band cord is made of nylon material only, and a thickness t1, a distance d1 and a distance d2 did not satisfy the requirements of the present invention.

εxcept for the specifications shown in Tables <NUM>-<NUM>, the tires of Comparative Examples <NUM> and <NUM> had configurations substantially the same as those of the tires of Embodiments.

εach test tire was tested for ride comfort and steering stability at high speeds.

The common specifications of the test tires and test methods are as follows.

Ride comfort when the test vehicle was run on a general road was evaluated by the following method.

εach of <NUM> test drivers conducted a test drive, and scored the ride comfort on a scale of <NUM> to <NUM> points (higher number is better), and the total score of the <NUM> test drivers was calculated.

The results are indicated by a score based on the sum of the above-mentioned scoring result of Comparative Example <NUM> being <NUM>, wherein the larger the value, the better the riding comfort.

steering stability when the test vehicle was run at high speed, was evaluated by the following method.

εach of the <NUM> test drivers conducted a test drive, and scored the steering stability on a scale of <NUM> to <NUM> points (higher number is better), and the total score of the <NUM> test drivers was calculated.

The results are indicated by a score based on the sum of the above-mentioned scoring result of Comparative Example <NUM> being <NUM>, wherein the larger the value, the better the steering stability during high-speed running.

Comprehensive performance including the above-mentioned ride comfort and steering stability at high speeds, was evaluated. The results are the sum of the scores for ride comfort and steering stability during high-speed running, wherein the larger the number, the better the overall performance.

The test results are shown in Tables <NUM>-<NUM>.

In Tables <NUM> to <NUM>, with respect to the ride comfort, when the score is higher than <NUM> of Comparative Example <NUM>, it can be evaluated as "good ride comfort is maintained. " Further, with respect to the steering stability during high-speed running, when the score is higher than <NUM> of Comparative Example <NUM>, it can be evaluated as the steering stability is improved.

Claim 1:
A pneumatic tire (<NUM>) having a tread portion (<NUM>), wherein
the tread portion (<NUM>) comprises a tread rubber (<NUM>) forming a ground contacting surface, and a band layer (<NUM>) disposed radially inside the tread rubber (<NUM>),
the tread rubber (<NUM>) is provided with a first circumferential groove (<NUM>) extending continuously in the tire circumferential direction in the ground contacting surface,
the band layer (<NUM>) comprises band cords (<NUM>) arranged substantially parallel to the tire circumferential direction, and a topping rubber (<NUM>) covering the band cords (<NUM>), and
the band cords (<NUM>) are a hybrid cord in which a first filament (<NUM>) and a second filament (<NUM>) having an elastic modulus lower than that of the first filament (<NUM>) are twisted together,
characterized in that
in a region radially inside the first circumferential groove (<NUM>), a maximum thickness t1 of the topping rubber (<NUM>) on the radially outer side of the band cord (<NUM>) is not more than <NUM>,
a minimum distance d1 from the bottom of the first circumferential groove (<NUM>) to the radially outer surface (<NUM>) of the band layer (<NUM>) is not more than <NUM>,
a distance d2 in a tire normal direction from a groove edge (10e) of the first circumferential groove (<NUM>) to the radially outer surface (<NUM>) of the band layer (<NUM>) is not more than <NUM>, and
a sea ratio of the ground contacting surface of the tread portion (<NUM>) is <NUM>% to <NUM>%, wherein the sea ratio is the ratio of the sum of the opening areas of all the grooves to the total area of the virtual ground contacting surface in such a state that all the grooves in the ground contacting surface of the tread rubber are filled up.