Patent Description:
The present invention relates to a method of using a tire.

Patent Document <NUM> below has proposed a pneumatic tyre that is expected to have low rolling resistance and better wet braking performance by specifying the loss tangent of a first cap layer and a second cap layer of the tread.

A method of using a tyre having the features of the preamble of claim <NUM> is known from <CIT>. Related aspects are described in <CIT>, <CIT> and <CIT>.

In general, as tread wear progresses, the volume of the grooves on the tread decreases, resulting in a gradual decrease in wet performance. In particular, the tyre of Patent Document <NUM> tends to further deteriorate wet performance when the second cap layer appears on the ground contact surface.

The smaller the reduction in wet performance due to tread wear, the more desirable it is. It is also desirable that tread wear can be confirmed immediately when the tyre is observed.

The present invention has been made in view of the above circumstances and has a major object to provide a method of using a tyre capable of suppressing the deterioration of wet performance, in particular wherein in the tyre deterioration of wet performance due to tread wear is small and the tread wear can be checked immediately.

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

According to an embodiment of the invention, the second reference internal pressure is equal to or less than <NUM>% of the first reference internal pressure.

According to an embodiment of the invention, the tread portion further comprises a third rubber layer made of a base rubber that is disposed inwardly in the tyre radial direction of the second rubber layer, wherein the base rubber has a loss tangent tanδb smaller than the loss tangent tanδ1.

According to an embodiment of the invention, a distance in the tyre radial direction from the ground contact surface to the outer surface of the first indicator is in a range from <NUM>% to <NUM>% of a distance in the tyre radial direction from the ground contact surface to the first wear line.

According to an embodiment of the invention, the tread portion further comprises a tread edge and a buttress surface extending outwardly in a tyre axial direction from the tread edge, wherein the buttress surface is provided with a mark indicating the presence of the first indicator at a same position in a tyre circumferential direction as the first indicator.

According to an embodiment of the invention, the groove portion is a circumferential groove that extends continuously in a tyre circumferential direction.

According to an embodiment of the invention, the groove portion is a sipe having an opening width equal to or less than <NUM> at the ground contact surface.

According to an embodiment of the invention, the groove portion further comprises a second indicator in which a bottom of the groove portion locally raises, wherein the second indicator has an outer surface in the tyre radial direction, and wherein the radially outer surface of the second indicator substantially coincides with a second wear line that is a wear limit of the tread portion.

One or more embodiments of the present invention will be described below with reference to the drawings.

<FIG> is a tyre meridian cross-sectional view of one embodiment of the tyre <NUM> under a normal state according to the present invention. As illustrated in <FIG>, the present invention is preferably applied, for example, to a pneumatic tyre for passenger car. However, the present invention is not limited to such an aspect, but may be applied to a heavy-duty tyre, for example.

As used herein, when a tyre is a pneumatic tyre based on a standard, the "normal state" is such that the tyre <NUM> is mounted onto a standard wheel rim with a standard pressure but loaded with no tyre load. If a tyre is not based on the standards, the normal state is a standard state of use according to the purpose of use of the tyre and means a state of no load. As used herein, unless otherwise noted, dimensions of portions of the tyre are values measured under the normal state.

As used herein, the "standard pressure" is a standard pressure officially approved for each tyre by standards organizations on which the tyre is based, wherein the standard pressure is the "maximum air pressure" in JATMA, the maximum pressure given in the "Tire Load Limits at Various Cold Inflation Pressures" table in TRA, and the "Inflation Pressure" in ETRTO, for example.

The tyre <NUM> according to the present embodiment of the invention includes tyre components, such as a carcass <NUM> and a tread reinforcing cord layer <NUM>, therein. For these tyre components, well known components may be adopted as appropriate.

The carcass <NUM> extends from one of bead portions <NUM> to the other one of the bead portions <NUM>, through a pair of sidewall portions <NUM> and a tread portion <NUM>. In the present embodiment of the invention, the carcass <NUM>, for example, is composed of two carcass plies 6A and 6B. The carcass plies 6A and 6B, for example, include carcass cords made of an organic fiber oriented at an angle of from <NUM> to <NUM> degrees with respect to the tyre circumferential direction.

The tread reinforcing layer <NUM>, for example, includes two reinforcing plies 7A and 7B. The reinforcing plies 7A and 7B, for example, each include a plurality of cords covered with a topping rubber. The cords, for example, are oriented at an angle of from <NUM> to <NUM> degrees with respect to the tyre circumferential direction. As the cords, various organic fiber cords or steel cords may be used as appropriate, for example.

The tread portion <NUM> includes a ground contact surface <NUM> and a pair of buttress surfaces <NUM>. The ground contact surface <NUM> is the area that is in contact with the ground during normal driving. The buttress surfaces <NUM> are the outer surfaces located outwardly in the tyre axial direction of the ground contact surface <NUM>. The boundaries between the ground contact surface <NUM> and the respective buttress surfaces <NUM> are the tread edges Te.

As used herein, the tread edges Te are the axial outermost edges of the ground contact surface <NUM> of the tyre <NUM> which occurs under the condition such that the tyre <NUM> under the normal state is grounded on a plane with <NUM>% of the standard tyre load by zero camber angles.

As used herein, when a tyre is a pneumatic tyre based on a standard, the "standard tyre load" is a tyre load officially approved for each tyre by the standards organization in which the tyre is based, wherein the standard tyre load is the "maximum load capacity" in JATMA, the maximum value given in the above-mentioned table in TRA, and the "Load Capacity" in ETRTO, for example. If a tyre for which no standards is specified, the "standard tyre load" is the maximum load that can be applied to the tyre according to the above-mentioned standards.

<FIG> illustrates a partial enlarged cross-sectional view of the tread portion <NUM>. As illustrated in <FIG>, the tread portion <NUM> includes a first rubber layer <NUM> and a second rubber layer <NUM>. The first rubber layer <NUM> is made of a first cap rubber <NUM> and forms at least a part of the ground contact surface <NUM>. In the present embodiment of the invention, the first rubber layer <NUM> also forms a pair of buttress surfaces <NUM> (shown in <FIG>). The second rubber layer <NUM> is made of a second cap rubber <NUM> and is arranged inwardly in the tyre radial direction of the first rubber layer <NUM>. The second rubber layer <NUM> is in direct contact with the first rubber layer <NUM>.

In general, rubber with a large loss tangent is known to exert a large frictional force on wet road surfaces. Based on such technical matters, in the present invention, a loss tangent tanδ2 of the second cap rubber <NUM> is greater than a loss tangent tanδ1 of the first cap rubber <NUM>. As used herein, a loss tangent tanδ is a value measured using a dynamic viscoelasticity measuring device (Xplexer series) manufactured by GABO under the following conditions in accordance with the provisions of JIS-K6394.

The tread portion <NUM> further includes at least one groove portion <NUM> opening to the ground contact surface <NUM>. The groove portion <NUM> means a cut regardless of the opening width or depth, such as a normal groove provided to improve drainage, a sipe with an opening width of <NUM> or less on the outer surface of the tyre, and a recess recessed from the ground contact surface <NUM> locally. The tread portion <NUM> according to the present embodiment of the invention is provided with a plurality of circumferential grooves <NUM> extending continuously in the tyre circumferential direction as the groove portion <NUM>. In addition to the circumferential grooves, the tread portion <NUM> is preferably provided with one or more lateral grooves extending in the tyre axial direction (not illustrated).

As illustrated in <FIG>, the tread portion <NUM> includes first land portions <NUM> and second land portions <NUM> sectioned by the above-mentioned circumferential grooves <NUM>. The second land portions <NUM> are arranged closer to the tyre equator C than the first land portions <NUM>. In the present embodiment of the invention, the tread portion <NUM> is configured to includes two second land portions <NUM> arranged closer to the tyre equator C and two first land portions <NUM> arranged to sandwich them. As a result, the tyre <NUM> according to the present embodiment of the invention is configured as a so-called <NUM>-rib tyre. However, the tyre <NUM> according to the present invention is not limited to such a configuration, and the tread portion <NUM> may be configured as a so-called <NUM>-rib tyre in which the tread portion <NUM> is divided into five land portions by four circumferential grooves <NUM>, for example.

<FIG> shows an enlarged perspective view of one of the circumferential grooves <NUM> as an example of the groove portion <NUM>. As illustrated in <FIG>, the groove portion <NUM> includes a first indicator <NUM> in which a bottom of the groove portion locally raises. Note that in <FIG>, the boundary between the above-mentioned rubber layers is omitted. The first indicator <NUM> has a larger height in the tyre radial direction than conventional indicators that indicate the wear limit of the tread portion <NUM> (hereinafter, may be referred to as "second indicator"). The groove portion <NUM> according to the present embodiment of the invention also includes the second indicator <NUM> (shown in <FIG>). The second indicator <NUM>, like the first indicator <NUM>, is a portion in which a bottom of the groove portion <NUM> locally raises. For example, the second indicator <NUM> may be configured as a "slip sign" having a height of <NUM> from the groove bottoms of the circumferential grooves <NUM>.

<FIG> shows an enlarged plan view of the first indicator <NUM> when the tyre is new, and <FIG> shows an enlarged plan view of the first indicator <NUM> when a radially outer part of the tread tubber is worn out. In <FIG>, the openings of the groove portion <NUM> is indicated by dots. As illustrated in <FIG>, when the tread portion <NUM> wears, the outer surface 15o of the first indicator <NUM> appears on a worn ground contact surface to be continuous to the worn ground contact surface. Thus, the first indicator <NUM> makes it possible to check whether the tread portion <NUM> has worn out beyond a certain amount.

As illustrated in <FIG>, in the present invention, when the tyre <NUM> is new, the radially outer surface 15o of the first indicator <NUM> (dotted in <FIG> for ease of understanding) is parallel to the ground contact surface <NUM> and substantially coincides with a first wear line <NUM> that passes through a radially outer surface of the second rubber layer <NUM>. Note that the first wear line <NUM> overlaps the outer surface of the second rubber layer <NUM>, but in <FIG> the first wear line <NUM> is shown as a two-pointed line passing slightly above the outer surface 22o in the tyre radial direction of the second rubber layer <NUM>. By adopting the above configuration, the tyre <NUM> according to the present invention has little deterioration in wet performance due to wear of the tread portion <NUM>, and wear of the tread portion <NUM> can be checked immediately. The reason is that when the wear of the tread portion <NUM> progresses, the second rubber layer <NUM>, which can be expected to have high wet grip, is exposed and suppresses the deterioration of wet performance. Also, the wear can be checked by the appearance of the outer surface 15o of the first indicator <NUM> in the tyre radial direction on the ground contact surface <NUM> (shown in <FIG>).

On the other hand, with conventional tyres, as tread wear progresses, a worn ground contact surface <NUM> tends to become flat, and wet performance (especially hydroplaning resistance performance) tends to decrease. In order to deal with such problems, the tyre <NUM> according to the present invention has the above-described configuration, so that the above-described tendency can be alleviated by the following method of using the tyre <NUM>. The inventive method of using the tyre <NUM> includes running the tyre <NUM> under a first standard internal pressure until the first indicator <NUM> appears on a worn ground contact surface, and running the tyre <NUM> under a second reference internal pressure greater than the first reference internal pressure after the outer surface 15o of the first indicator <NUM> appears on the worn ground contact surface.

In such a method of using the tyre <NUM> of the present invention, when the wear of the tread portion <NUM> progresses and the ground contact surface becomes flat, the internal pressure of the tyre <NUM> can be increased to bring the ground contact surface <NUM> to an appropriate round shape, thus maintaining the hydroplaning resistance performance. In this method, the first standard internal pressure is the internal pressure that is applied to the tyre <NUM> under the normal state of use, for example, the standard pressure mentioned above is adopted. In order to maintain the balance of various performances of the tyre, the second standard internal pressure is preferably equal to or less than <NUM>% of the first standard internal pressure.

In order to make it easier for users of the tyre <NUM> to identify the position of the first indicator <NUM>, at least one of the buttress surfaces <NUM> preferably includes a mark indicating the presence of the first indicator <NUM> at the same position in the tyre circumferential direction as the first indicator <NUM>.

Hereinafter, a more detailed configuration of the present embodiment of the invention will be described. Note that each configuration described below shows a specific aspect of the present embodiment of the invention. Thus, the present invention can exert the above-mentioned effects even if the tyre does not include the configuration described below. Further, if any one of the configurations described below is applied independently to the tyre of the present invention having the above-mentioned characteristics, the performance improvement according to each additional configuration can be expected. Furthermore, when some of the configurations described below are applied in combination, it is expected that the performance of the additional configurations will be improved.

As illustrated in <FIG>, "the radially outer surface 15o of the first indicator <NUM> substantially coincides with the first wear line <NUM>" shall include, at a minimum, an aspect where a tyre radial distance L2 from the ground contact surface <NUM> to the outer surface 15o of the first indicator <NUM> is <NUM>% to <NUM>% of a tyre radial distance L1 from the ground contact surface <NUM> to the first wear line <NUM>. In <FIG>, the above distances L1 and L2 are shown slightly differently, but in some preferred embodiments of the invention they are identical with each other.

The first cap rubber <NUM> has a loss tangent tanδ1 preferably equal to or more than <NUM>, more preferably equal to or more than <NUM>, still further preferably equal to or more than <NUM>, but preferably equal to or less than <NUM>, more preferably equal to or less than <NUM>, still further preferably equal to or less than <NUM>. Such a first cap rubber <NUM>, at the beginning of use, can exert well-balanced steering stability on dry roads (hereinafter simply referred to as "steering stability") and wet performance.

To improve steering stability and wet performance when the tread portion <NUM> wears, the loss tangent tanδ2 of the second cap rubber <NUM> is preferably equal to or more than <NUM>, more preferably equal to or more than <NUM>, still further preferably equal to or more than <NUM>, but preferably equal to or less than <NUM>, more preferably equal to or less than <NUM>, still further preferably equal to or less than <NUM>.

In some preferred embodiments of the invention, the tread portion <NUM> may further include a third rubber layer <NUM> made of a base rubber <NUM> that is disposed inwardly in the tyre radial direction of the second rubber layer <NUM>. Preferably, the base rubber <NUM> has a loss tangent tanδb smaller than the loss tangent tanδ1. Specifically, the loss tangent tanδb is equal to or less than <NUM>. The third rubber layer <NUM>, which consists of the base rubber <NUM>, can help to suppress excessive heat generation in the tread portion <NUM>.

The tread rubber <NUM> according to the present embodiment of the invention consists of only the first rubber layer <NUM>, the second rubber layer <NUM> and the third rubber layer <NUM> described above at least in the area that constitutes the ground contact surface <NUM>, and no rubber layers other than these are provided. However, the present invention is not limited to such an embodiment of the invention, and other rubber layers may be arranged as appropriate. The first rubber layer <NUM>, the second rubber layer <NUM> and the third rubber layer <NUM> according to the present embodiment of the invention extend to have a substantially constant thickness on the inner side of the ground contact surface <NUM> in the tyre radial direction except around the circumferential grooves <NUM>.

In the present embodiment of the invention, a thickness t1 of the first rubber layer <NUM> is preferably in a range from <NUM>% to <NUM>%, more preferably from <NUM>% to <NUM>%, of an effective tread thickness ta. As a result, when the wear of the tread portion <NUM> progresses moderately, the second cap rubber <NUM>, which can be expected to have high wet grip, is exposed, and wet performance can be effectively maintained. Note that the effective tread thickness ta means the thickness of the tread rubber <NUM> from the ground contact surface <NUM> to the bottom of the groove portion <NUM> (e.g., the circumferential groove <NUM>).

From the viewpoint of reliably maintaining wet performance, it is preferable that the second rubber layer <NUM> constitutes the ground contact surface <NUM> even when the tread rubber <NUM> is worn out to the limit. In other words, when the tread portion <NUM> wears beyond the second wear line, which is the wear limit, and the radially outer surface of the second indicator <NUM> appears on a worn ground contact surface, the second rubber layer <NUM> preferably constitutes the worn ground contact surface. In some more preferred embodiments of the invention, the inner surface of the second rubber layer <NUM> in the tyre radial direction is located inwardly in the tyre radial direction of the groove bottoms of the circumferential grooves <NUM>. Thus, wet performance can be maintained for sure.

A thickness t2 of the second rubber layer <NUM> is preferably in a range from <NUM>% to <NUM>% of the effective tread thickness ta. This makes it possible to obtain the above effects while maintaining the durability of the tread portion <NUM>.

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

Claim 1:
A method of using a tyre (<NUM>), the tyre (<NUM>) comprising:
a tread portion (<NUM>), wherein
the tread portion (<NUM>) comprises
a ground contact surface (<NUM>),
a first rubber layer made (<NUM>) of a first cap rubber (<NUM>) that forms at least a part of the ground contact surface,
a second rubber layer made (<NUM>) of a second cap rubber (<NUM>) that is disposed radially inwardly of the first rubber layer (<NUM>), and
at least one groove portion (<NUM>) opening to the ground contact surface,
the first cap rubber (<NUM>) has a loss tangent tanδ1,
the second cap rubber (<NUM>) has a loss tangent tanδ2 greater than the loss tangent tanδ1,
wherein the loss tangent is a value measured using a dynamic viscoelasticity measuring device under the conditions of <NUM>% initial strain, +/- <NUM>% amplitude of dynamic strain, a frequency of <NUM>, stretch deformation mode and a measurement temperature of <NUM>,
the groove portion (<NUM>) comprises a first indicator (<NUM>) in which a bottom of the groove portion locally raises, the first indicator (<NUM>) having an outer surface (15o) in a tyre radial direction, and
the radially outer surface (15o) of the first indicator (<NUM>) substantially coincides with a first wear line (<NUM>) that is parallel to the ground contact surface (<NUM>) and passes through a radially outer surface (22o) of the second rubber layer (<NUM>),
wherein the method comprises running the tyre (<NUM>) under a first reference internal pressure,
characterized in that
the method comprising:
running the tyre (<NUM>) under a first reference internal pressure until the first indicator (<NUM>) appears on a worn ground contact surface (<NUM>); and
running the tyre (<NUM>) under a second reference internal pressure greater than the first reference internal pressure after the outer surface (15o) of the first indicator (<NUM>) appears on the worn ground contact surface (<NUM>).