Patent Description:
The tread portion of a heavy duty pneumatic tire (hereinafter, referred to as a tire) is provided with a pair of cushion layers. The pair of cushion layers are disposed so as to be spaced apart from each other in the axial direction. Each cushion layer is located between an end of a belt and a carcass.

When the tire runs, strain occurs in a portion at each end of the belt. Various studies have been conducted for the cushion layers in order to prevent damage due to such strain (for example, <CIT>).

The pair of cushion layers are independent of each other. The positions of the cushion layers are likely to be shifted relative to the equator plane. The position shift may influence the shape balance of a ground-contact surface, causing a decrease in uneven wear resistance.

Against the backdrop of the tightening of environmental regulations and driver shortages in recent years, vehicles such as trucks and buses have increased load capacities and decreased floor heights. Accordingly, the demand for low-flatness tires has increased.

In a low-flatness tire, it is structurally more difficult to ensure a space for disposing cushion layers, than in a high-flatness tire. If cushion layers having conventional specifications are used in a low-flatness tire, each end of a belt may be raised when the tire expands. In the low-flatness tire, the internal pressure distributed to the belt is larger than in the high-flatness tire. In the low-flatness tire, a significant change of the ground-contact shape due to dimensional growth may occur. The ground-contact shape that lacks stability leads to a decrease in uneven wear resistance.

In a tire, the load acting on the tread portion is reduced by bending of each side portion. The bending allowance of a low-flatness tire is smaller than that of a high-flatness tire. In the low-flatness tire, the degree of contribution of each side portion to reduction of the load acting on the tread portion is low. In the low-flatness tire, a larger load acts on the tread portion than in the high-flatness tire. The larger load may lead to a decrease in durability or uneven wear resistance.

A heavy duty pneumatic tire known from <CIT> is described in the preamble of claim <NUM>. Further heavy duty tires are known from <CIT>, <CIT>, <CIT>, <CIT> and <CIT>.

The present invention has been made in view of such circumstances. An object of the present invention is to provide a heavy duty pneumatic tire that can achieve stabilization of a ground-contact shape and reduction of a load acting on a tread portion.

The present invention provides a heavy duty pneumatic tire as defined in claim <NUM>. Dependent claims <NUM> to <NUM> are directed to preferable embodiments thereof.

Preferably, in the heavy duty pneumatic tire, each hump portion has an apparent maximum thickness of not less than <NUM> and not greater than <NUM>.

Preferably, in the heavy duty pneumatic tire, the sheet portion has an apparent thickness of not less than <NUM> and not greater than <NUM>.

Preferably, in the heavy duty pneumatic tire, a ratio of the apparent maximum thickness of each hump portion to the apparent thickness of the sheet portion is not less than <NUM> and not greater than <NUM>.

Preferably, in the heavy duty pneumatic tire, a belt cord included in the fourth belt ply is a steel cord composed of a single elemental wire.

Preferably, in the heavy duty pneumatic tire, the belt cord included in the fourth belt ply has an outer diameter of not less than <NUM> and not greater than <NUM>.

Preferably, in the heavy duty pneumatic tire, the belt cord included in the fourth belt ply has a breaking load of not less than <NUM> N and not greater than <NUM> N.

Preferably, in the heavy duty pneumatic tire, the belt cord included in the fourth belt ply is thinner than a belt cord included in the third belt ply. A ratio of the outer diameter of the belt cord included in the fourth belt ply to an outer diameter of the belt cord included in the third belt ply is not less than <NUM> and not greater than <NUM>.

According to the present invention, a heavy duty pneumatic tire that can achieve stabilization of a ground-contact shape and reduction of a load acting on a tread portion, is obtained.

In the present disclosure, a state where a tire is fitted on a normal rim, the internal pressure of the tire is adjusted to a normal internal pressure, and no load is applied to the tire is referred to as a normal state.

In the present disclosure, unless otherwise specified, the dimensions and angles of each component of the tire are measured in the normal state.

The dimensions and angles of each component in a meridian cross-section of the tire, which cannot be measured in a state where the tire is fitted on the normal rim, are measured in a cross-section of the tire obtained by cutting the tire along a plane including a rotation axis, with the distance between right and left beads being made equal to the distance between the beads in the tire that is fitted on the normal rim.

The normal rim means a rim specified in a standard on which the tire is based. The "standard rim" in the JATMA standard, the "Design Rim" in the TRA standard, and the "Measuring Rim" in the ETRTO standard are normal rims.

The normal internal pressure means an internal pressure specified in the standard on which the tire is based. The "highest air pressure" in the JATMA standard, the "maximum value" recited in "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" in the TRA standard, and the "INFLATION PRESSURE" in the ETRTO standard are normal internal pressures.

In the present disclosure, the "nominal aspect ratio" is the "nominal aspect ratio" included in "tyre designation" specified in JIS D4202 "Automobile tyres-Designation and dimensions".

In the present disclosure, a tread portion of the tire is a portion of the tire that comes into contact with a road surface. A bead portion is a portion of the tire that is fitted to a rim. A side portion is a portion of the tire that extends between the tread portion and the bead portion. The tire includes a tread portion, a pair of bead portions, and a pair of side portions as portions thereof.

In the present disclosure, the number of cords included per <NUM> of a tire element, including aligned cords, is represented as the density of cords included in this element (unit: ends/<NUM>). Unless otherwise specified, the density of the cords is obtained in a cross-section of the element obtained by cutting the element along a plane perpendicular to the longitudinal direction of the cords.

In the present disclosure, a crosslinked rubber refers to a molded product, of a rubber composition, obtained by pressurizing and heating the rubber composition. The rubber composition is an uncrosslinked rubber obtained by mixing a base rubber and chemicals in a kneading machine such as a Banbury mixer. The crosslinked rubber is also referred to as vulcanized rubber, and the rubber composition is also referred to as unvulcanized rubber.

Examples of the base rubber include natural rubber (NR), butadiene rubber (BR), styrene-butadiene rubber (SBR), isoprene rubber (IR), ethylene-propylene rubber (EPDM), chloroprene rubber (CR), acrylonitrile-butadiene rubber (NBR), and isobutylene-isoprene-rubber (IIR). Examples of the chemicals include reinforcing agents such as carbon black and silica, plasticizers such as aromatic oil, fillers such as zinc oxide, lubricants such as stearic acid, antioxidants, processing aids, sulfur, and vulcanization accelerators. Selection of a base rubber and chemicals, the amounts of the selected chemicals, etc., are determined as appropriate according to the specifications of components, such as a tread and a sidewall, for which the rubber composition is used.

In the present disclosure, a complex elastic modulus at a temperature of <NUM>, of a component formed from a crosslinked rubber, of the components included in the tire, is measured using a viscoelasticity spectrometer ("VES" manufactured by Iwamoto Seisakusho) under the following conditions according to the standards of JIS K6394.

In this measurement, a test piece is sampled from the tire. When a test piece cannot be sampled from the tire, a test piece is sampled from a sheet-shaped crosslinked rubber (hereinafter, also referred to as a rubber sheet) obtained by pressurizing and heating a rubber composition, which is used for forming the component to be measured, at a temperature of <NUM> for <NUM> minutes.

In the present disclosure, a breaking load of a steel cord is measured in accordance with the standards of JIS G3510. The tensile speed to obtain the breaking load is set at <NUM>/min.

<FIG> shows a part of a heavy duty pneumatic tire <NUM> (hereinafter, also referred to simply as "tire <NUM>") according to an embodiment of the present invention. The tire <NUM> is mounted to a vehicle such as a truck and a bus. The nominal aspect ratio of the tire <NUM> is not greater than <NUM>%. In other words, the tire <NUM> has a nominal aspect ratio of not greater than <NUM>%. The tire <NUM> is a low-flatness tire.

<FIG> shows a part of a cross-section (hereinafter, also referred to as meridian cross-section) of the tire <NUM> along a plane including the rotation axis of the tire <NUM>. In <FIG>, the right-left direction is the axial direction of the tire <NUM>, and the up-down direction is the radial direction of the tire <NUM>. The direction perpendicular to the surface of the drawing sheet of <FIG> is the circumferential direction of the tire <NUM>. An alternate long and short dash line CL represents the equator plane of the tire <NUM>.

The tire <NUM> includes a tread <NUM>, a pair of sidewalls <NUM>, a pair of beads <NUM>, a pair of chafers <NUM>, a carcass <NUM>, an inner liner <NUM>, a pair of steel fillers <NUM>, a reinforcing layer <NUM>, and a buffer layer <NUM>.

The tread <NUM> comes into contact with a road surface at an outer surface thereof. The outer surface is a tread surface <NUM>. In <FIG>, reference character PC represents the point of intersection of the tread surface <NUM> and the equator plane CL. The point of intersection PC is the equator of the tire <NUM>. The equator PC is the radially outer end of the tire <NUM>.

In <FIG>, reference character PE represents an end of the tread surface <NUM>. A length indicated by an arrow WT is the width of the tread surface <NUM>. The width WT of the tread surface <NUM> is the distance in the axial direction from a first end PE of the tread surface <NUM> to a second end PE of the tread surface <NUM>. The width WT of the tread surface <NUM> is also referred to as a width WT of the tread <NUM>.

In the tire <NUM>, when the ends PE of the tread surface <NUM> cannot be identified from the appearance, the positions, on the tread surface <NUM>, corresponding to the outer ends in the axial direction of a ground-contact surface obtained when the normal load is applied to the tire <NUM> in the normal state and the tire <NUM> is brought into contact with a flat surface at a camber angle of <NUM>° are defined as the ends PE of the tread surface <NUM>.

The tread <NUM> includes a base portion <NUM> and a cap portion <NUM> located radially outward of the base portion <NUM>. The base portion <NUM> is formed from a crosslinked rubber having low heat generation properties. The cap portion <NUM> is formed from a crosslinked rubber for which wear resistance and grip performance are taken into consideration. As shown in <FIG>, the base portion <NUM> of the tire <NUM> covers the entirety of the reinforcing layer <NUM>. The cap portion <NUM> covers the entirety of the base portion <NUM>.

In the tire <NUM>, at least three circumferential grooves <NUM> are formed on the tread <NUM>. On the tread <NUM> shown in <FIG>, four circumferential grooves <NUM> are formed. These circumferential grooves <NUM> are aligned in the axial direction and continuously extend in the circumferential direction.

Among the four circumferential grooves <NUM>, the circumferential groove <NUM> located on each outer side in the axial direction is a shoulder circumferential groove <NUM>. The circumferential grooves <NUM> located axially inward of the shoulder circumferential grooves <NUM> are middle circumferential grooves <NUM>. The four circumferential grooves <NUM> of the tire <NUM> include a pair of the middle circumferential grooves <NUM> and a pair of the shoulder circumferential grooves <NUM>.

In the tire <NUM>, from the viewpoint of contribution to drainage performance and traction performance, the width in the axial direction of each middle circumferential groove <NUM> is preferably not less than <NUM>% and not greater than <NUM>% of the width WT of the tread <NUM>. The depth of each middle circumferential groove <NUM> is preferably not less than <NUM> and not greater than <NUM>. The width in the axial direction of each shoulder circumferential groove <NUM> is preferably not less than <NUM>% and not greater than <NUM>% of the width WT of the tread <NUM>. The depth of each shoulder circumferential groove <NUM> is preferably not less than <NUM> and not greater than <NUM>.

As described above, at least three circumferential grooves <NUM> are formed on the tread <NUM>. Accordingly, at least four land portions <NUM> are formed in the tread <NUM>. On the tread <NUM> of the tire <NUM> shown in <FIG>, four circumferential grooves <NUM> are formed, so that five land portions <NUM> are formed in the tread <NUM>. These land portions <NUM> are aligned in the axial direction and continuously extend in the circumferential direction.

Among the five land portions <NUM>, the land portion <NUM> located on each outer side in the axial direction is a shoulder land portion <NUM>. Each shoulder land portion <NUM> is located axially outward of the shoulder circumferential groove <NUM> and includes the end PE of the tread surface <NUM>. The land portions <NUM> located axially inward of the shoulder land portions <NUM> are middle land portions <NUM>. The land portion <NUM> located axially inward of the middle land portions <NUM> is a center land portion 30c. The five land portions <NUM> of the tire <NUM> include the center land portion 30c, a pair of the middle land portions <NUM>, and a pair of the shoulder land portions <NUM>.

In the tire <NUM>, the width in the axial direction of the center land portion 30c is not less than <NUM>% and not greater than <NUM>% of the width WT of the tread <NUM>. The width in the axial direction of each middle land portion <NUM> is not less than <NUM>% and not greater than <NUM>% of the width WT of the tread <NUM>. The width in the axial direction of each shoulder land portion <NUM> is not less than <NUM>% and not greater than <NUM>% of the width WT of the tread <NUM>. The width in the axial direction of each land portion <NUM> is represented as the width in the axial direction of the top surface of the land portion <NUM> which forms a part of the tread surface <NUM>.

In the tire <NUM>, the land portion <NUM> located at the center in the axial direction among the land portions <NUM> formed in the tread <NUM>, that is, the center land portion 30c, is located on the equator plane CL. The tire <NUM> includes the tread <NUM> formed such that the land portion <NUM> is located on the equator plane CL. The tread <NUM> may be formed such that the circumferential groove <NUM> is located on the equator plane CL.

Each sidewall <NUM> is connected to an end of the tread <NUM>. The sidewall <NUM> extends radially inward from the end of the tread <NUM>. The sidewall <NUM> is located radially inward of the tread <NUM>. The sidewall <NUM> is formed from a crosslinked rubber.

Each bead <NUM> is located radially inward of the sidewall <NUM>. The bead <NUM> includes a core <NUM> and an apex <NUM>.

The core <NUM> extends in the circumferential direction. The core <NUM> includes a wound wire made of steel. The core <NUM> has a substantially hexagonal cross-sectional shape.

The apex <NUM> is located radially outward of the core <NUM>. The apex <NUM> includes an inner apex 34u and an outer apex <NUM>. The inner apex 34u extends radially outward from the core <NUM>. The outer apex <NUM> is located radially outward of the inner apex 34u. The inner apex 34u is formed from a hard crosslinked rubber. The outer apex <NUM> is formed from a crosslinked rubber that is more flexible than the inner apex 34u.

Each chafer <NUM> is located axially outward of the bead <NUM>. The chafer <NUM> is located radially inward of the sidewall <NUM>. The chafer <NUM> comes into contact with a rim (not shown). The chafer <NUM> is formed from a crosslinked rubber for which wear resistance is taken into consideration.

The carcass <NUM> is located inward of the tread <NUM>, the pair of sidewalls <NUM>, and the pair of chafers <NUM>. The carcass <NUM> extends on and between one bead <NUM> and the other bead <NUM>.

The carcass <NUM> includes at least one carcass ply <NUM>. The carcass <NUM> of the tire <NUM> is composed of one carcass ply <NUM>. The carcass ply <NUM> is turned up around each core <NUM> from the inner side toward the outer side in the axial direction. The carcass ply <NUM> has a ply body 36a which extends from a first core <NUM> to a second core <NUM>, and a pair of turned-up portions 36b which are connected to the ply body 36a and turned up around the respective cores <NUM> from the inner side toward the outer side in the axial direction.

The carcass ply <NUM> includes a large number of carcass cords aligned with each other, which are not shown in <FIG>. These carcass cords are covered with a topping rubber. Each carcass cord intersects the equator plane CL. In the tire <NUM>, an angle of the carcass cords relative to the equator plane CL (hereinafter, referred to as intersection angle of the carcass cords) is not less than <NUM>° and not greater than <NUM>°. The carcass <NUM> has a radial structure. The carcass cords of the tire <NUM> are steel cords.

The inner liner <NUM> is located inward of the carcass <NUM>. The inner liner <NUM> is joined to the inner surface of the carcass <NUM> via an insulation (not shown) formed from a crosslinked rubber. The inner liner <NUM> forms an inner surface of the tire <NUM>. The inner liner <NUM> is formed from a crosslinked rubber that has an excellent air blocking property. The inner liner <NUM> maintains the internal pressure of the tire <NUM>.

Each steel filler <NUM> is located at a bead portion. The steel filler <NUM> is turned up around the core <NUM> from the inner side toward the outer side in the axial direction along the carcass ply <NUM>. The steel filler <NUM> includes a large number of filler cords aligned with each other, which are not shown. Steel cords are used as filler cords.

The reinforcing layer <NUM> is located between the tread <NUM> and the carcass <NUM> in the radial direction. The reinforcing layer <NUM> of the tire <NUM> includes a belt <NUM> and a band <NUM>. The reinforcing layer <NUM> may be composed of only the belt <NUM>.

The belt <NUM> is located radially inward of the tread <NUM>. The belt <NUM> includes four belt plies <NUM>. These belt plies <NUM> are aligned in the radial direction. Each belt ply <NUM> is disposed such that both ends 42e thereof are opposed to each other across the equator plane CL.

The four belt plies <NUM> are a first belt ply 42A, a second belt ply 42B, a third belt ply 42C, and a fourth belt ply 42D.

Among the four belt plies <NUM>, the belt ply <NUM> located on the inner side in the radial direction is the first belt ply 42A. The second belt ply 42B is located radially outward of the first belt ply 42A. The third belt ply 42C is located radially outward of the second belt ply 42B. The fourth belt ply 42D is located radially outward of the third belt ply 42C. The fourth belt ply 42D is located on the outer side in the radial direction among the four belt plies <NUM>.

As shown in <FIG>, the end 42e of each belt ply <NUM> is located axially outward of the shoulder circumferential groove <NUM>. An end 42De of the fourth belt ply 42D may be located inward of the shoulder circumferential groove <NUM>. From the viewpoint of preventing damage, the fourth belt ply 42D is disposed such that the end 42De of the fourth belt ply 42D does not overlap the shoulder circumferential groove <NUM> in the radial direction.

In <FIG>, a length indicated by an arrow W1 is the width in the axial direction of the first belt ply 42A. A length indicated by an arrow W2 is the width in the axial direction of the second belt ply 42B. A length indicated by an arrow W3 is the width in the axial direction of the third belt ply 42C. A length indicated by an arrow W4 is the width in the axial direction of the fourth belt ply 42D. The width in the axial direction of each belt ply <NUM> is the distance in the axial direction from a first end 42e of the belt ply <NUM> to a second end 42e of the belt ply <NUM>. The end 42e of the belt ply <NUM> is represented by an end of a belt cord included in the belt ply <NUM> described later.

In the tire <NUM>, the second belt ply 42B has the largest width W2 in the axial direction, and the fourth belt ply 42D has the smallest width W4 in the axial direction. An end 42Ae of the first belt ply 42A and an end 42Ce of the third belt ply 42C are located between an end 42Be of the second belt ply 42B and the end 42De of the fourth belt ply 42D in the axial direction.

The width W1 in the axial direction of the first belt ply 42A and the width W3 in the axial direction of the third belt ply 42C are equal to each other. The width W1 in the axial direction of the first belt ply 42A may be larger than the width W3 in the axial direction of the third belt ply 42C, or may be smaller than the width W3 in the axial direction of the third belt ply 42C.

In the tire <NUM>, from the viewpoint of effectively increasing the stiffness of the tread portion, the ratio (W1/WT) of the width W1 in the axial direction of the first belt ply 42A to the width WT of the tread <NUM> is preferably not less than <NUM> and not greater than <NUM>. The ratio (W2/WT) of the width W2 in the axial direction of the second belt ply 42B to the width WT of the tread <NUM> is preferably not less than <NUM> and not greater than <NUM>. The ratio (W3/WT) of the width W3 in the axial direction of the third belt ply 42C to the width WT of the tread <NUM> is preferably not less than <NUM> and not greater than <NUM>. The ratio (W4/WT) of the width W4 in the axial direction of the fourth belt ply 42D to the width WT of the tread <NUM> is not less than <NUM>. The width W4 in the axial direction of the fourth belt ply 42D is set as appropriate in consideration of the specifications of the tire <NUM> (in particular, the position of each shoulder circumferential groove <NUM>).

In the tire <NUM>, the width in the axial direction of the belt <NUM> is represented as the width in the axial direction of the belt ply <NUM> having the largest width in the axial direction. As described above, the second belt ply 42B has the largest width W2 in the axial direction. The width in the axial direction of the belt <NUM> of the tire <NUM> is represented as the width W2 in the axial direction of the second belt ply 42B. In the tire <NUM>, the end 42Be of the second belt ply 42B is an end 38e of the belt <NUM>.

The band <NUM> includes a full band <NUM> and a pair of edge bands <NUM>. The full band <NUM> is disposed such that both ends 44e thereof are opposed to each other across the equator plane CL. Each end 44e of the full band <NUM> is located axially inward of the end 38e of the belt <NUM>. Each end 44e of the full band <NUM> is located between the end 42Ce of the third belt ply 42C (or the end 42Ae of the first belt ply 42A) and the end 42De of the fourth belt ply 42D in the axial direction.

The pair of edge bands <NUM> are disposed so as to be spaced apart from each other in the axial direction across the equator plane CL. In the tire <NUM>, the fourth belt ply 42D which forms a part of the belt <NUM> is located between the pair of edge bands <NUM>.

Each edge band <NUM> is located between the tread <NUM> and the full band <NUM> in the radial direction. The edge band <NUM> is located radially outward of the end 44e of the full band <NUM>. An inner end 46ue of the edge band <NUM> is located axially inward of the end 44e of the full band <NUM>. An outer end 46se of the edge band <NUM> is located axially outward of the end 44e of the full band <NUM>. The edge band <NUM> overlaps the end 44e of the full band <NUM> in the radial direction. The position of the outer end 46se of the edge band <NUM> may coincide with the position of the end 44e of the full band <NUM> in the axial direction. In this case as well, the edge band <NUM> is located radially outward of the end 44e of the full band <NUM>.

<FIG> shows the configuration of the reinforcing layer <NUM>. In <FIG>, the right-left direction is the axial direction of the tire <NUM>, and the up-down direction is the circumferential direction of the tire <NUM>. The direction perpendicular to the surface of the drawing sheet of <FIG> is the radial direction of the tire <NUM>. The front side of the drawing sheet of <FIG> is the outer side in the radial direction, and the back side of this drawing sheet is the inner side in the radial direction.

As shown in <FIG>, each belt ply <NUM> included in the belt <NUM> includes a large number of belt cords <NUM> aligned with each other. In <FIG>, for the convenience of description, the belt cords <NUM> are represented by solid lines, but the belt cords <NUM> are covered with a topping rubber <NUM>.

The belt cords <NUM> of the tire <NUM> are steel cords. The density of the belt cords <NUM> in each belt ply <NUM> is not less than <NUM> ends/<NUM> and not greater than <NUM> ends/<NUM>.

In each belt ply <NUM>, the belt cords <NUM> are tilted relative to the circumferential direction.

The direction in which the belt cords <NUM> included in the first belt ply 42A are tilted (hereinafter, referred to as tilt direction of first belt cords 48A) is the same as the direction in which the belt cords <NUM> included in the second belt ply 42B are titled (hereinafter, referred to as tilt direction of second belt cords 48B). The tilt direction of the first belt cords 48A may be opposite to the tilt direction of the second belt cords 48B.

The direction in which the belt cords <NUM> included in the third belt ply 42C are tilted (hereinafter, referred to as tilt direction of third belt cords 48C) is opposite to the tilt direction of the second belt cords 48B. In the tire <NUM>, the belt <NUM> is formed such that the tilt direction of the second belt cords 48B and the tilt direction of the third belt cords 48C are opposite to each other.

The direction in which the belt cords <NUM> included in the fourth belt ply 42D are tilted (hereinafter, referred to as tilt direction of fourth belt cords 48D) is the same as the tilt direction of the third belt cords 48C. The tilt direction of the fourth belt cords 48D may be opposite to the tilt direction of the third belt cords 48C.

In <FIG>, an angle θ1 is an angle of the belt cords <NUM> included in the first belt ply 42A relative to the equator plane CL (hereinafter, referred to as tilt angle θ1 of the first belt cords 48A). An angle θ2 is an angle of the belt cords <NUM> included in the second belt ply 42B relative to the equator plane CL (hereinafter, referred to as tilt angle θ2 of the second belt cords 48B). An angle θ3 is an angle of the belt cords <NUM> included in the third belt ply 42C relative to the equator plane CL (hereinafter, referred to as tilt angle θ3 of the third belt cords 48C). An angle θ4 is an angle of the belt cords <NUM> included in the fourth belt ply 42D relative to the equator plane CL (hereinafter, referred to as tilt angle θ4 of the fourth belt cords 48D).

In the tire <NUM>, each of the tilt angle θ1 of the first belt cords 48A, the tilt angle θ2 of the second belt cords 48B, the tilt angle θ3 of the third belt cords 48C, and the tilt angle θ4 of the fourth belt cords 48D is preferably not less than <NUM>° and preferably not greater than <NUM>°.

From the viewpoint of effectively restraining movement of the tread portion and obtaining a ground-contact surface having a stable shape, the tilt angle θ1 of the first belt cords 48A is more preferably not less than <NUM>° and more preferably not greater than <NUM>°. The tilt angle θ2 of the second belt cords 48B is more preferably not less than <NUM>° and more preferably not greater than <NUM>°. The tilt angle θ2 of the second belt cords 48B is further preferably not greater than <NUM>°. The tilt angle θ3 of the third belt cords 48C is more preferably not less than <NUM>° and more preferably not greater than <NUM>°. The tilt angle θ3 of the third belt cords 48C is further preferably not greater than <NUM>°. The tilt angle θ4 of the fourth belt cords 48D is more preferably not less than <NUM>° and more preferably not greater than <NUM>°. The tilt angle θ4 of the fourth belt cords 48D is further preferably not greater than <NUM>°.

As shown in <FIG>, the full band <NUM> and the edge bands <NUM> included in the band <NUM> each include a helically wound band cord <NUM>. In <FIG>, for the convenience of description, the band cords <NUM> are represented by solid lines, but each band cord <NUM> is covered with a topping rubber <NUM>.

In the tire <NUM>, the band cords <NUM> are steel cords or cords formed from an organic fiber (hereinafter, referred to as organic fiber cords). In the case where organic fiber cords are used as the band cords <NUM>, examples of the organic fiber include nylon fibers, polyester fibers, rayon fibers, and aramid fibers. As a band cord 52F of the full band <NUM> and band cords 52E of the edge bands <NUM>, the same cord may be used, or different cords may be used. The band cords <NUM> used for the full band <NUM> and the edge bands <NUM> are determined according to the specifications of the tire <NUM>.

As described above, the full band <NUM> includes the helically wound band cord 52F. The full band <NUM> has a jointless structure. In the full band <NUM>, an angle of the band cord 52F relative to the circumferential direction is preferably not greater than <NUM>° and more preferably not greater than <NUM>°. The band cord 52F extends substantially in the circumferential direction.

The density of the band cord 52F in the full band <NUM> is not less than <NUM> ends/<NUM> and not greater than <NUM> ends/<NUM>. The density of the band cord 52F is represented as the number of cross-sections of the band cord 52F included per <NUM> width of the full band <NUM> in a cross-section of the full band <NUM> included in the meridian cross-section.

As described above, each edge band <NUM> includes the helically wound band cord 52E. The edge band <NUM> has a jointless structure. In the edge band <NUM>, an angle of the band cord 52E relative to the circumferential direction is preferably not greater than <NUM>° and more preferably not greater than <NUM>°. The band cord 52E of the edge band <NUM> extends substantially in the circumferential direction.

The density of the band cord 52E in the edge band <NUM> is not less than <NUM> ends/<NUM> and not greater than <NUM> ends/<NUM>. The density of the band cord 52E is represented as the number of cross-sections of the band cord 52E included per <NUM> width of the edge band <NUM> in a cross-section of the edge band <NUM> included in the meridian cross-section.

<FIG> shows a part of the cross-section of the tire <NUM> shown in <FIG>. <FIG> shows the tread portion of the tire <NUM>.

In the tire <NUM>, each of the end 42Be of the second belt ply 42B and the end 42Ce of the third belt ply 42C is covered with a rubber layer <NUM>. A plurality of rubber layers <NUM> are further disposed between the end 42Be of the second belt ply 42B and the end 42Ce of the third belt ply 42C, each of which is covered with the rubber layer <NUM>. In the tire <NUM>, an edge member <NUM> including four rubber layers <NUM> in total is formed between the end 42Be of the second belt ply 42B and the end 42Ce of the third belt ply 42C. The edge member <NUM> is formed from a crosslinked rubber. The edge member <NUM> contributes to maintaining the interval between the end 42Be of the second belt ply 42B and the end 42Ce of the third belt ply 42C. In the tire <NUM>, a change of the positional relationship between the end 42Be of the second belt ply 42B and the end 42Ce of the third belt ply 42C due to running is suppressed. The edge member <NUM> is a part of the reinforcing layer <NUM>. The reinforcing layer <NUM> of the tire <NUM> includes a pair of edge members <NUM> in addition to the belt <NUM> and the band <NUM>.

The buffer layer <NUM> is located between the carcass <NUM> and the belt <NUM> in the radial direction. The buffer layer <NUM> is disposed such that both ends 20e thereof are opposed to each other across the equator plane CL. Each end 20e of the buffer layer <NUM> is located axially outward of the end 38e of the belt <NUM>. Each end 20e of the buffer layer <NUM> is located radially inward of the end 38e of the belt <NUM>.

The buffer layer <NUM> is formed from a crosslinked rubber. A complex elastic modulus E* of the buffer layer <NUM> is not less than <NUM> MPa and not greater than <NUM> MPa.

In the present disclosure, the thickness of the buffer layer <NUM> is measured along a normal line of the inner surface of the tire <NUM> in the meridian cross-section of the tire <NUM>.

The buffer layer <NUM> includes a sheet portion <NUM> and a pair of hump portions <NUM>.

The sheet portion <NUM> extends from the equator plane CL toward each hump portion <NUM>.

In <FIG>, reference character Pa represents the point of intersection of the outer surface of the sheet portion <NUM> and the equator plane CL. A length indicated by arrows a is the thickness of the sheet portion <NUM> at the equator plane CL. The thickness a is measured along an inner surface normal line that is normal to the inner surface of the tire <NUM> and passes through the point of intersection Pa (that is, the equator plane CL). The sheet portion <NUM> has a uniform thickness a. Of the buffer layer <NUM>, a portion having the thickness a is the sheet portion <NUM>.

Each hump portion <NUM> is located axially outward of the sheet portion <NUM>. The hump portion <NUM> is integrated with the sheet portion <NUM>.

In <FIG>, reference character Pb represents a specific position on the outer surface of the hump portion <NUM>. A solid line NLb is an inner surface normal line that is normal to the inner surface of the tire <NUM> and passes through the position Pb. A length indicated by a double-headed arrow b is the thickness of the hump portion <NUM> at the position Pb. The thickness b is measured along the inner surface normal line NLb of the tire <NUM>.

The thickness of the hump portion <NUM> shows the maximum at the above-described position Pb. In other words, the hump portion <NUM> has the maximum thickness b at the position Pb. The position Pb is a maximum thickness position.

The thickness of the hump portion <NUM> gradually increases from the boundary between the sheet portion <NUM> and the hump portion <NUM> toward the maximum thickness position Pb, and gradually decreases from the maximum thickness position Pb toward the end 20e of the buffer layer <NUM>. In the meridian cross-section of the tire <NUM>, the hump portion <NUM> has a triangular cross-sectional shape.

As shown in <FIG>, the maximum thickness position Pb of the hump portion <NUM> is located near the end 38e of the belt <NUM>. The hump portion <NUM> has the maximum thickness b at the end 38e of the belt <NUM>.

In the tire <NUM>, the thickness of the sheet portion <NUM> is represented as the thickness a at the equator plane CL. The thickness of the hump portion <NUM> is represented as the maximum thickness b. The hump portion <NUM> is thicker than the sheet portion <NUM>.

In <FIG>, a position indicated by reference character CS is the boundary between the sheet portion <NUM> and the hump portion <NUM> in the tire <NUM>. The boundary CS is located between a bottom 28sb of the shoulder circumferential groove <NUM> and the end 42Ae of the first belt ply 42A in the axial direction.

In the tire <NUM>, the hump portion <NUM> of the buffer layer <NUM> is located between the end 38e of the belt <NUM> and the carcass <NUM>. When the tire <NUM> runs, strain occurs in a portion at the end 38e of the belt <NUM>, but the thick hump portion <NUM> alleviates this strain. In the tire <NUM>, occurrence of damage due to this strain is suppressed. The hump portion <NUM> contributes to improvement of the durability of the tread portion.

The load acting on the tread portion is reduced by bending of each side portion. As described above, the tire <NUM> is a low-flatness tire. Since the bending allowance of the tire <NUM> is small, the side portion cannot contribute to reduction of the load acting on the tread portion, unlike each side portion of a high-flatness tire.

In the tire <NUM>, the sheet portion <NUM> of the buffer layer <NUM> is located between the carcass <NUM> and the belt <NUM> on the inner side of the tread <NUM>. The sheet portion <NUM> is stacked on the carcass <NUM> on the radially inner side of the belt <NUM>, and the first belt ply 42A of the belt <NUM> is stacked on the sheet portion <NUM>.

In the tire <NUM>, the sheet portion <NUM> of the buffer layer <NUM> contributes to reduction of the load acting on the tread portion. In the tire <NUM>, the load acting on the tread portion is reduced as compared to a conventional low-flatness tire, and occurrence of damage and uneven wear due to this load is suppressed. The sheet portion <NUM> contributes to improvement of the durability of the tread portion and uneven wear resistance.

Since the sheet portion <NUM> contributes to reduction of the load acting on the tread portion, smaller hump portions <NUM> can be used in the tire <NUM> than in a conventional low-flatness tire. The compact hump portions <NUM> contribute to preventing the end 38e of the belt <NUM> from being raised, and also contribute to forming the contour of the carcass (hereinafter, also referred to as case line) in an appropriate shape. In the tire <NUM>, a ground-contact shape close to the target ground-contact shape is obtained. Even though the internal pressure distributed to the belt <NUM> is larger than in a high-flatness tire, a significant change of the ground-contact shape due to dimensional growth is suppressed. The buffer layer <NUM> which includes the sheet portion <NUM> and the pair of hump portions <NUM> contributes to improvement of uneven wear resistance.

In the manufacture of a tire, a green tire (which is an unvulcanized tire and is also referred to as a raw cover) is prepared by combining components such as a tread and sidewalls. The tire is obtained by vulcanizing the green tire in a mold.

In a conventional tire, in order to alleviate strain of a belt, a pair of cushion layers are provided between ends of the belt and a carcass. The pair of cushion layers are disposed so as to be spaced apart from each other in the axial direction. At the time of shaping a green tire, each cushion layer is set in position independently, so that there is a concern that the positions of the cushion layers may be shifted relative to the equator plane.

In the tire <NUM>, the sheet portion <NUM> extends between a first hump portion <NUM> and a second hump portion <NUM>. By setting the first hump portion <NUM> at an appropriate position, the second hump portion <NUM> is also set at an appropriate position. In the tire <NUM>, the positions of the hump portions <NUM> are less likely to be shifted relative to the equator plane CL. The buffer layer <NUM> contributes to stabilization of the quality of the tire <NUM>. Since the sheet portion <NUM> and the pair of hump portions <NUM> are simultaneously combined into a green tire, the time for shaping the green tire is shortened. The buffer layer <NUM> also contributes to improvement of the productivity.

In the tire <NUM>, stabilization of the ground-contact shape and reduction of the load acting on the tread portion are achieved. The tire <NUM> has better durability of the tread portion and better uneven wear resistance than a conventional low-flatness tire. Furthermore, in the tire <NUM>, stabilization of the quality and improvement of the productivity are achieved while the durability of the tread portion and the uneven wear resistance are improved.

In the tire <NUM>, the thickness b of each hump portion <NUM> is preferably not less than <NUM> and not greater than <NUM>.

When the thickness b of the hump portion <NUM> is set to be not less than <NUM>, the hump portion <NUM> can effectively contribute to alleviation of strain generated in the portion at the end 38e of the belt <NUM>. The tire <NUM> can improve the durability of the tread portion. The hump portion <NUM> also contributes to formation of the tread surface <NUM> having an appropriate contour. From this viewpoint, the thickness b of the hump portion <NUM> is more preferably not less than <NUM> and further preferably not less than <NUM>.

When the thickness b of the hump portion <NUM> is set to be not greater than <NUM>, the hump portion <NUM> is formed so as to be compact. The compact hump portion <NUM> contributes to formation of an appropriate case line. The tire <NUM> can stabilize the ground-contact shape. From this viewpoint, the thickness b of the hump portion <NUM> is more preferably not greater than <NUM> and further preferably not greater than <NUM>.

In the tire <NUM>, the thickness a of the sheet portion <NUM> is preferably not less than <NUM> and not greater than <NUM>.

When the thickness a of the sheet portion <NUM> is set to be not less than <NUM>, the sheet portion <NUM> can effectively contribute to reduction of the load acting on the tread portion. The tire <NUM> can improve the durability of the tread portion. From this viewpoint, the thickness a of the sheet portion <NUM> is more preferably not less than <NUM> and further preferably not less than <NUM>.

When the thickness a of the sheet portion <NUM> is set to be not greater than <NUM>, the influence of the sheet portion <NUM> on the case line is suppressed. Since an appropriate case line is formed, the tire <NUM> can stabilize the ground-contact shape. From this viewpoint, the thickness a of the sheet portion <NUM> is more preferably not greater than <NUM> and further preferably not greater than <NUM>.

In the tire <NUM>, from the viewpoint of improving the durability of the tread portion and the uneven wear resistance, more preferably, the thickness b of the hump portion <NUM> is not less than <NUM> and not greater than <NUM>, and the thickness a of the sheet portion <NUM> is not less than <NUM> and not greater than <NUM>.

In the tire <NUM>, the ratio (b/a) of the thickness b of the hump portion <NUM> to the thickness a of the sheet portion <NUM> is preferably not less than <NUM> and not greater than <NUM>.

When the ratio (b/a) is set to be not less than <NUM>, the hump portion <NUM> contributes to alleviation of strain generated in the portion at the end 38e of the belt <NUM>, and the sheet portion <NUM> contributes to stabilization of the ground-contact shape. The tire <NUM> can improve the durability of the tread portion and the uneven wear resistance. From this viewpoint, the ratio (b/a) is more preferably not less than <NUM>.

When the ratio (b/a) is set to be not greater than <NUM>, the hump portion <NUM> contributes to stabilization of the ground-contact shape, and the sheet portion <NUM> contributes to reduction of the load acting on the tread portion. In this case as well, the tire <NUM> can improve the durability of the tread portion and the uneven wear resistance. From this viewpoint, the ratio (b/a) is more preferably not greater than <NUM>.

In the tire <NUM>, from the viewpoint of further improving the durability of the tread portion and the uneven wear resistance, further preferably, the thickness b of the hump portion <NUM> is not less than <NUM> and not greater than <NUM>, the thickness a of the sheet portion <NUM> is not less than <NUM> and not greater than <NUM>, and the ratio (b/a) of the thickness b of the hump portion <NUM> to the thickness a of the sheet portion <NUM> is not less than <NUM> and not greater than <NUM>.

<FIG> shows the state of the arrangement of the buffer layer <NUM> at the equator plane CL. In <FIG>, an element indicated by reference character <NUM> is the carcass cord of the carcass ply <NUM>. An element indicated by reference character <NUM> is a topping rubber.

The buffer layer <NUM> is located between the carcass ply <NUM> and the first belt ply 42A. As shown in <FIG>, the topping rubber <NUM> of the carcass ply <NUM>, the buffer layer <NUM>, and the topping rubber <NUM> of the first belt ply 42A are located between the carcass cord <NUM> and the first belt cords 48A. A rubber component is located between the carcass cord <NUM> and the first belt cords 48A, and the rubber component includes the topping rubber <NUM>, the buffer layer <NUM>, and the topping rubber <NUM>.

In the tire <NUM>, the distance between the carcass cord <NUM> and the first belt cords 48A is controlled by adjusting the thickness of the buffer layer <NUM>. In the tire <NUM>, of the buffer layer <NUM>, a portion between a first boundary CS and a second boundary CS is the sheet portion <NUM> having a uniform thickness a. The distance between the carcass cord <NUM> and the first belt cords 48A is constant between the first boundary CS and the second boundary CS.

In the present disclosure, the distance between the carcass cord <NUM> and the first belt cords 48A is the apparent thickness of the buffer layer <NUM>. In particular, the apparent thickness of the buffer layer <NUM> between the first boundary CS and the second boundary CS is also referred to as apparent thickness of the sheet portion <NUM>.

In <FIG>, a length indicated by a double-headed arrow TA is the apparent thickness of the sheet portion <NUM> at the equator plane CL. The apparent thickness TA of the sheet portion <NUM> is measured along the equator plane CL. In the tire <NUM>, the apparent thickness of the sheet portion <NUM> is represented as the apparent thickness TA of the sheet portion <NUM> at the equator plane CL.

In the tire <NUM>, the apparent thickness TA of the sheet portion <NUM> is preferably not less than <NUM> and not greater than <NUM>.

When the apparent thickness TA is set to be not less than <NUM>, the rubber component between the carcass <NUM> and the reinforcing layer <NUM> can effectively contribute to alleviation of strain. Occurrence of damage due to this strain is prevented. From this viewpoint, the apparent thickness TA is more preferably not less than <NUM>.

When the apparent thickness TA is set to be not greater than <NUM>, the rubber component between the carcass <NUM> and the reinforcing layer <NUM> can contribute to suppression of a mass increase. From this viewpoint, the apparent thickness TA is more preferably not greater than <NUM>.

<FIG> shows the state of the arrangement of the buffer layer <NUM> at the end 38e of the belt <NUM>. The buffer layer <NUM> is stacked on the carcass ply <NUM>. As shown in <FIG>, the topping rubber <NUM> of the carcass ply <NUM> is located between the hump portion <NUM> of the buffer layer <NUM> and the carcass cord <NUM> of the carcass ply <NUM>.

In <FIG>, a length indicated by a double-headed arrow TB is the distance, from the carcass cord <NUM> to the maximum thickness position Pb of the hump portion <NUM>, measured along the above-described inner surface normal line NLb. In the present disclosure, the distance TB is the apparent maximum thickness of the hump portion <NUM>. The apparent maximum thickness TB of the hump portion <NUM> is larger than the apparent thickness TA of the sheet portion <NUM>.

In the tire <NUM>, the apparent maximum thickness TB of the hump portion <NUM> is preferably not less than <NUM> and not greater than <NUM>.

When the apparent maximum thickness TB is set to be not less than <NUM>, the hump portion <NUM> can effectively contribute to alleviation of strain generated in the portion at the end 38e of the reinforcing layer <NUM>. In the tire <NUM>, good tread durability is achieved. From this viewpoint, the apparent maximum thickness TB is more preferably not less than <NUM> and further preferably not less than <NUM>.

When the apparent maximum thickness TB is set to be not greater than <NUM>, the case line is formed in an appropriate shape. In the tire <NUM>, a ground-contact shape close to the target ground-contact shape is obtained. In the tire <NUM>, good uneven wear resistance is achieved. From this viewpoint, the apparent maximum thickness TB is more preferably not greater than <NUM> and further preferably not greater than <NUM>.

In the tire <NUM>, from the viewpoint of improving the tread durability and the uneven wear resistance, more preferably, the apparent thickness TA of the sheet portion <NUM> is not less than <NUM> and not greater than <NUM>, and the apparent maximum thickness TB of the hump portion <NUM> is not less than <NUM> and not greater than <NUM>.

In the tire <NUM>, the ratio (TB/TA) of the apparent maximum thickness TB of the hump portion <NUM> to the apparent thickness TA of the sheet portion <NUM> is preferably not less than <NUM> and not greater than <NUM>.

When the ratio (TB/TA) is set to be not less than <NUM>, the hump portion <NUM> contributes to alleviation of strain generated in the portion at the end 38e of the belt <NUM>, and the sheet portion <NUM> contributes to stabilization of the ground-contact shape. The tire <NUM> can improve the durability of the tread portion and the uneven wear resistance. From this viewpoint, the ratio (TB/TA) is more preferably not less than <NUM>, further preferably not less than <NUM>, and particularly preferably not less than <NUM>.

When the ratio (TB/TA) is set to be not greater than <NUM>, the hump portion <NUM> contributes to stabilization of the ground-contact shape, and the sheet portion <NUM> contributes to reduction of the load acting on the tread portion. In this case as well, the tire <NUM> can improve the durability of the tread portion and the uneven wear resistance. From this viewpoint, the ratio (TB/TA) is more preferably not greater than <NUM>.

In the tire <NUM>, from the viewpoint of further improving the durability of the tread portion and the uneven wear resistance, further preferably, the apparent maximum thickness TB of the hump portion <NUM> is not less than <NUM> and not greater than <NUM>, the apparent thickness TA of the sheet portion <NUM> is not less than <NUM> and not greater than <NUM>, and the ratio (TB/TA) of the apparent maximum thickness TB of the hump portion <NUM> to the apparent thickness TA of the sheet portion <NUM> is not less than <NUM> and not greater than <NUM>.

In the tire <NUM>, the first belt ply 42A is located between the buffer layer <NUM> and the second belt ply 42B having the largest width in the axial direction among the belt plies <NUM> included in the belt <NUM>. As described above, the tilt angle θ1 of the first belt cords 48A is more preferably not less than <NUM>° and not greater than <NUM>°, and the tilt angle θ2 of the second belt cords 48B is more preferably not less than <NUM>° and not greater than <NUM>°. In this case, the tilt angle θ1 of the first belt cords 48A included in the first belt ply 42A which is stacked on the buffer layer <NUM> is larger than the tilt angle θ2 of the second belt cords 48B included in the second belt ply 42B which is stacked on the first belt ply 42A.

In the tire <NUM>, the load acting on the tread portion is reduced stepwise as compared to the case where a belt ply <NUM> in which the tilt angle of belt cords <NUM> is small is stacked on the buffer layer <NUM>. Specific strain is prevented from being generated in the buffer layer <NUM>, so that the buffer layer <NUM> can stably exhibit a load reduction function. The buffer layer <NUM> can effectively contribute to reduction of the load acting on the tread portion. From this viewpoint, the tilt angle θ1 of the first belt cords 48A included in the first belt ply 42A which is stacked on the buffer layer <NUM> is preferably larger than the tilt angle θ2 of the second belt cords 48B included in the second belt ply 42B which is stacked on the first belt ply 42A.

<FIG> shows an example of a cross-section of the belt cord <NUM> included in the belt <NUM> of the tire <NUM>. The cross-section shown in <FIG> is a cross-section of the third belt cord 48C.

The third belt cord 48C is formed by twisting together a plurality of elemental wires <NUM> (also referred to as steel filaments) made of steel. The third belt cord 48C is a twisted wire.

The third belt cord 48C shown in <FIG> is a single-strand cord having a <NUM> × <NUM> structure and obtained by twisting together four elemental wires 68c. In the tire <NUM>, for example, a multi-strand cord having a <NUM> × <NUM> structure or a layered strand cord having a <NUM> + <NUM> structure may be used as each third belt cord 48C. In the tire <NUM>, a cord that is the same as the third belt cords 48C is used as each of the first belt cords 48A and the second belt cords 48B.

When a tire runs on a rough road, cracks may occur due to stones coming into contact with the tread or due to stones being bitten into circumferential grooves. In this case, water penetrates through the cracks into the interior of the tread portion. In the case where each belt cord is a twisted wire, the gap between elemental wires is filled with a topping rubber, but it is difficult to fill the entire gap with the topping rubber. Therefore, the water that has entered through the cracks may enter the gap between the elemental wires. In this case, rust may form on the belt cord. The rust promotes peeling of the topping rubber from the belt cord. Water penetrates through the peeled portion into the interior of the belt cord.

In the tire <NUM>, if rust forms in the fourth belt ply 42D which is located closest to the tread <NUM>, the risk of formation of rust in the third belt ply 42C is increased. If rust forms in the third belt ply 42C, the risk of formation of rust in the second belt ply 42B is increased.

If rust forms in the third belt ply 42C and the second belt ply 42B which are important for the belt <NUM> to maintain its holding force, damage (hereinafter, referred to as separation) due to peeling may occur.

<FIG> shows a cross-section of the fourth belt cord 48D included in the fourth belt ply 42D of the tire <NUM>. As shown in <FIG>, the fourth belt cord 48D is a steel cord composed of a single elemental wire 68d.

In the tire <NUM>, each of the belt cords <NUM> included in the three belt plies <NUM> located radially inward of the fourth belt ply 42D is the above-described twisted wire. Meanwhile, each fourth belt cord 48D is not a twisted wire. In the fourth belt cord 48D, there is no gap between elemental wires that exist in the belt cord <NUM> composed of a twisted wire. Since water does not accumulate in the fourth belt cord 48D, formation of rust on the fourth belt cord 48D is suppressed. Since formation of rust in the fourth belt ply 42D is suppressed, formation of rust is also suppressed in the third belt ply 42C which is located radially inward of the fourth belt ply 42D and in the second belt ply 42B which is located radially inward of the third belt ply 42C. In the tire <NUM>, occurrence of separation is prevented. The fourth belt ply 42D contributes to improvement of the durability of the tread portion. From this viewpoint, in the tire <NUM>, among the plurality of belt plies <NUM> included in the belt <NUM>, each fourth belt cord 48D included in the belt ply <NUM> located on the radially outer side, that is, in the fourth belt ply 42D, is preferably a steel cord composed of the single elemental wire 68d. In this case, from the viewpoint of forming the fourth belt ply 42D having appropriate stiffness, the breaking load of the fourth belt cord 48D is more preferably not less than <NUM> N and not greater than <NUM> N.

In <FIG>, a length indicated by a double-headed arrow D4 is the outer diameter of the fourth belt cord 48D.

In the tire <NUM>, the outer diameter D4 of the fourth belt cord 48D is preferably not less than <NUM> and not greater than <NUM>.

When the outer diameter D4 of the fourth belt cord 48D is set to be not less than <NUM>, the fourth belt ply 42D is formed so as to have required stiffness. The belt <NUM> including the fourth belt ply 42D contributes to improvement of the durability of the tread portion. From this viewpoint, the outer diameter D4 is more preferably not less than <NUM>.

When the outer diameter D4 of the fourth belt cord 48D is set to be not greater than <NUM>, the fourth belt cord 48D is disposed at an appropriate interval from the third belt cord 48C. In the tire <NUM>, as for formation of rust, the fourth belt ply 42D effectively functions as a barrier for the third belt ply 42C. The fourth belt cord 48D contributes to reduction of the mass of the tire <NUM>. From this viewpoint, the outer diameter D4 is more preferably not greater than <NUM>.

In the tire <NUM>, from the viewpoint that the fourth belt ply 42D can effectively contribute to prevention of occurrence of separation, further preferably, each fourth belt cord 48D is a steel cord composed of the single elemental wire 68d, the outer diameter D4 of the fourth belt cord 48D is not less than <NUM> and not greater than <NUM>, and the breaking load of the fourth belt cord 48D is not less than <NUM> N and not greater than <NUM> N.

In <FIG>, a length indicated by a double-headed arrow D3 is the outer diameter of the third belt cord 48C. The outer diameter D3 is represented as the diameter of the circumscribed circle of the four elemental wires 68c included in the third belt cord 48C.

In the tire <NUM>, each fourth belt cord 48D is thinner than each third belt cord 48C. Specifically, the ratio (D4/D3) of the outer diameter D4 of the fourth belt cord 48D to the outer diameter D3 of the third belt cord 48C is preferably not less than <NUM> and not greater than <NUM>.

When the ratio (D4/D3) is set to be not less than <NUM>, the fourth belt ply 42D is formed so as to have required stiffness. The belt <NUM> including the fourth belt ply 42D contributes to improvement of the durability of the tread portion. From this viewpoint, the ratio (D4/D3) is more preferably not less than <NUM> and further preferably not less than <NUM>.

When the ratio (D4/D3) is set to be not greater than <NUM>, the fourth belt cord 48D is disposed at an appropriate interval from the third belt cord 48C. In the tire <NUM>, as for formation of rust, the fourth belt ply 42D effectively functions as a barrier for the third belt ply 42C. The fourth belt ply 42D contributes to reduction of the mass of the tire <NUM>. From this viewpoint, the ratio (D4/D3) is more preferably not greater than <NUM> and further preferably not greater than <NUM>.

As described above, when the tire <NUM> runs on a rough road, cracks may occur in the bottoms of the circumferential grooves <NUM> due to stones being bitten into the circumferential grooves <NUM>. In this case, water may penetrate through the cracks into the interior of the tread portion, causing the belt cord <NUM> to rust.

In the belt <NUM> shown in <FIG>, the end 42De of the fourth belt ply 42D is located axially outward of the shoulder circumferential groove <NUM>. In the tire <NUM>, the fourth belt ply 42D is located radially inward of all the circumferential grooves <NUM> formed on the tread <NUM>. Accordingly, even if a stone is bitten into any of the circumferential grooves <NUM>, which are formed on the tread <NUM>, to cause a crack in the bottom of the circumferential groove <NUM>, the fourth belt ply 42D effectively functions as a barrier for the third belt ply 42C as for formation of rust. In the tire <NUM>, occurrence of separation is prevented. The fourth belt ply 42D contributes to improvement of the durability of the tread portion. From this viewpoint, in the tire <NUM>, in the case where each fourth belt cord 48D is a steel cord composed of the single elemental wire 68d, more preferably, the fourth belt ply 42D is disposed such that both ends 42De of the fourth belt ply 42D are opposed to each other across the equator plane CL, and each end 42De of the fourth belt ply 42D is located axially outward of the shoulder circumferential groove <NUM>.

As described above, in the tire <NUM>, the full band <NUM> is disposed such that both ends 44e thereof are opposed to each other across the equator plane CL. The full band <NUM> extends from the equator plane CL toward each end 44e in the axial direction. In the tire <NUM>, furthermore, each edge band <NUM> is located radially outward of the end 44e of the full band <NUM>.

Although the tire <NUM> is a low-flatness tire, the full band <NUM> and the pair of edge bands <NUM> effectively suppress deformation of the tread portion. Since a change of the case line is suppressed, a change of the ground-contact shape is suppressed.

Since the edge bands <NUM> hold the ends 44e of the full band <NUM>, fluctuation of the tension of the band cord 52F included in the full band <NUM> is suppressed. Occurrence of a break of the band cord 52F due to this tension fluctuation is prevented, so that the full band <NUM> can stably exhibit the function of suppressing a shape change.

In the tire <NUM>, the ground-contact shape is stabilized. From this viewpoint, preferably, the tire <NUM> includes the full band <NUM> disposed such that both ends 44e thereof are opposed to each other across the equator plane CL, and each edge band <NUM> is located radially outward of the end 44e of the full band <NUM>.

As described above, in the tire <NUM>, each end 44e of the full band <NUM> is located axially inward of the end 38e of the belt <NUM>. The belt <NUM> is wider than the full band <NUM>. The belt <NUM> holds each end 44e of the full band <NUM>. The belt <NUM> contributes to suppression of fluctuation of the tension of the band cord 52F included in the full band <NUM>. Since occurrence of a break of the band cord 52F due to the tension fluctuation is prevented, the full band <NUM> can stably exhibit the function of suppressing a shape change. From this viewpoint, each end 44e of the full band <NUM> is preferably located axially inward of the end 38e of the belt <NUM>.

In the tire <NUM>, furthermore, the full band <NUM> is located between the second belt ply 42B and the third belt ply 42C in the radial direction.

The second belt ply 42B and the third belt ply 42C reduce the force acting on the full band <NUM>. In particular, since the belt cords <NUM> included in the second belt ply 42B and the belt cords <NUM> included in the third belt ply 42C intersect each other, the force acting on the full band <NUM> is effectively reduced. Since fluctuation of the tension of the band cord 52F included in the full band <NUM> is suppressed, occurrence of a break of the band cord 52F due to this tension fluctuation is prevented. The full band <NUM> of the tire <NUM> can stably exhibit the function of suppressing a shape change. From this viewpoint, in the tire <NUM>, the full band <NUM> is preferably located between the second belt ply 42B and the third belt ply 42C in the radial direction.

As shown in <FIG>, in the axial direction, the end 44e of the full band <NUM> is located inward of the end 42Be of the second belt ply 42B, and the end 44e of the full band <NUM> is located inward of the end 42Ce of the third belt ply 42C. In other words, the second belt ply 42B is wider than the full band <NUM>. The third belt ply 42C is also wider than the full band <NUM>. In the tire <NUM>, the full band <NUM> is interposed between the second belt ply 42B and the third belt ply 42C which are wider than the full band <NUM>. Since fluctuation of the tension of the band cord 52F included in the full band <NUM> is more effectively suppressed, a break is less likely to occur in the band cord 52F of the full band <NUM>. The full band <NUM> of the tire <NUM> can stably exhibit the function of suppressing a shape change. From this viewpoint, in the tire <NUM>, preferably, the full band <NUM> is located between the second belt ply 42B and the third belt ply 42C in the radial direction, each end 44e of the full band <NUM> is located axially inward of the end 42Be of the second belt ply 42B, and each end 44e of the full band <NUM> is located axially inward of the end 42Ce of the third belt ply 42C.

In a running state of a tire, a tread end portion moves actively. The stiffness of the portion where circumferential grooves are formed is lower than that of the portion where the circumferential grooves are not formed.

As described above, the tire <NUM> is a low-flatness tire. In the case where the tire <NUM> has the wide tread surface <NUM>, each shoulder circumferential groove <NUM> of the tire <NUM> is located further outward in the axial direction as compared to that of a high-flatness tire. In this case, there is a concern that a shape change around the shoulder circumferential groove <NUM> may be increased.

In the tire <NUM>, each end 44e of the full band <NUM> is located axially outward of the shoulder circumferential groove <NUM>. The full band <NUM> is located radially inward of each shoulder circumferential groove <NUM>.

Although the tire <NUM> is a low-flatness tire, the full band <NUM> effectively suppresses deformation around each shoulder circumferential groove <NUM>. Since each edge band <NUM> is located outward of the end 44e of the full band <NUM>, this deformation is sufficiently suppressed. In particular, in the tire <NUM>, a shape change around each shoulder circumferential groove <NUM> is effectively suppressed. From this viewpoint, in the tire <NUM>, each end 44e of the full band <NUM> is preferably located axially outward of the shoulder circumferential groove <NUM>.

In <FIG>, a double-headed arrow SF indicates the distance in the axial direction from the shoulder circumferential groove <NUM>, specifically, the outer edge of the shoulder circumferential groove <NUM>, to the end 44e of the full band <NUM>. A double-headed arrow WS indicates the width in the axial direction of the shoulder land portion <NUM>. The width WS in the axial direction is represented as the distance in the axial direction from the inner end of the top surface of the shoulder land portion <NUM> (that is, the outer edge of the shoulder circumferential groove <NUM>) to the outer end of the top surface (in the tire <NUM>, the end PE of the tread surface <NUM>).

In the tire <NUM>, the ratio (SF/WS) of the distance SF in the axial direction from the shoulder circumferential groove <NUM> to the end 44e of the full band <NUM>, to the width WS in the axial direction of the shoulder land portion <NUM>, is preferably not greater than <NUM>%. Accordingly, the end 44e of the full band <NUM> is located away from the end portion of the tread <NUM> which moves actively in a running state, so that fluctuation of the tension of the band cord <NUM> is suppressed. In the tire <NUM>, occurrence of a break of the band cord <NUM> is suppressed. The full band <NUM> of the tire <NUM> contributes to suppression of a shape change. From this viewpoint, the ratio (SF/WS) is more preferably not greater than <NUM>% and further preferably not greater than <NUM>%.

When the ratio (SF/WS) is set to be not less than <NUM>%, the end 44e of the full band <NUM> is located at an appropriate interval from the shoulder circumferential groove <NUM>, specifically, the bottom of the shoulder circumferential groove <NUM>. In the tire <NUM>, occurrence of damage starting from the bottom of the shoulder circumferential groove <NUM> is suppressed. Since the width of the full band <NUM> is ensured, the full band <NUM> contributes to suppression of a shape change of the tire <NUM>. From this viewpoint, the ratio (SF/WS) is more preferably not less than <NUM>%.

As described above, in the tire <NUM>, the inner end 46ue of each edge band <NUM> is located inward of the end 44e of the full band <NUM> in the axial direction. In <FIG>, a length indicated by a double-headed arrow We is the distance in the axial direction from the end 44e of the full band <NUM> to the inner end 46ue of the edge band <NUM>.

In the tire <NUM>, the distance We in the axial direction from the end 44e of the full band <NUM> to the inner end 46ue of the edge band <NUM> is preferably not less than <NUM>. Accordingly, the edge band <NUM> effectively holds the end 44e of the full band <NUM>. Fluctuation of the tension of the band cord 52F included in the full band <NUM> is suppressed, so that occurrence of a break of the band cord 52F due to this tension fluctuation is suppressed. The full band <NUM> of the tire <NUM> can more stably exhibit the function of suppressing a shape change. From this viewpoint, the distance We in the axial direction is preferably not less than <NUM>. From the viewpoint of suppressing the influence of the edge band <NUM> on the mass of the tire <NUM>, the distance We in the axial direction is preferably not greater than <NUM>.

In the tire <NUM>, involvement in occurrence of damage starting from the bottom 28sb of the shoulder circumferential groove <NUM> is taken into consideration for setting the position of the inner end 46ue of the edge band <NUM>. From the viewpoint of effectively suppressing occurrence of damage starting from the bottom 28sb of the shoulder circumferential groove <NUM>, in the axial direction, the inner end 46ue of the edge band <NUM> is preferably located outward of the bottom 28sb of the shoulder circumferential groove <NUM>, and is more preferably located further outward of the shoulder circumferential groove <NUM>. In the tire <NUM>, the inner end 46ue of the edge band <NUM> may be located inward of the bottom 28sb of the shoulder circumferential groove <NUM> in the axial direction. In this case, the inner end 46ue of the edge band <NUM> is more preferably located further inward of the shoulder circumferential groove <NUM> in the axial direction.

As shown in <FIG>, in the tire <NUM>, the boundary CS between the sheet portion <NUM> and the hump portion <NUM> is located axially outward of the end 44e of the full band <NUM>. Accordingly, the hump portion <NUM> is made compact, and a case line having an appropriate shape is formed. A ground-contact shape close to the target ground-contact shape is obtained. In the tire <NUM>, good uneven wear resistance is achieved. From this viewpoint, the boundary CS between the sheet portion <NUM> and the hump portion <NUM> is preferably located axially outward of the end 44e of the full band <NUM>.

In the present disclosure, the boundary CS between the sheet portion <NUM> and the hump portion <NUM> is represented as a position at which the apparent thickness of the buffer layer <NUM> is <NUM> times the apparent thickness TA of the sheet portion <NUM>.

As shown in <FIG>, in the tire <NUM>, the boundary CS between the sheet portion <NUM> and the hump portion <NUM> is located axially inward of the outer end 46se of the edge band <NUM>. Accordingly, concentration of strain on a portion at the boundary CS between the sheet portion <NUM> and the hump portion <NUM> is suppressed. The hump portion <NUM> can effectively contribute to alleviation of strain generated in the portion at the end 38e of the belt <NUM>. In the tire <NUM>, good tread durability is achieved. From this viewpoint, the boundary CS between the sheet portion <NUM> and the hump portion <NUM> is preferably located axially inward of the outer end 46se of the edge band <NUM>.

In the tire <NUM>, from the viewpoint of achieving good uneven wear resistance and tread durability, the boundary CS between the sheet portion <NUM> and the hump portion <NUM> is more preferably located between the end 44e of the full band <NUM> and the outer end 46se of the edge band <NUM> in the axial direction.

As is obvious from the above description, according to the present invention, the heavy duty pneumatic tire <NUM> that can achieve stabilization of the ground-contact shape and reduction of the load acting on the tread portion, is obtained. The present invention exhibits a remarkable effect in the low-flatness heavy duty pneumatic tire <NUM> having a nominal aspect ratio of not greater than <NUM>%.

Hereinafter, the present invention will be described in further detail by means of examples, etc..

A heavy duty pneumatic tire (tire size = <NUM>/45R22. <NUM>) having the basic structure shown in <FIG> and having specifications shown in Table <NUM> below was obtained.

In Example <NUM>, the apparent thickness TA of the sheet portion of the buffer layer was <NUM>, and the apparent maximum thickness TB of each hump portion was <NUM>. The complex elastic modulus of the buffer layer was <NUM> MPa.

A steel cord having the configuration shown in <FIG> was used as each fourth belt cord. The outer diameter D4 of the fourth belt cord was <NUM>, and the breaking load of the fourth belt cord was <NUM> N.

A steel cord having the configuration shown in <FIG> was used as each of the first belt cords, the second belt cords, and the third belt cords.

The ratio (D4/D3) of the outer diameter D4 of the fourth belt cord to the outer diameter D3 of the third belt cord was <NUM>.

A tire of Comparative Example <NUM> was obtained in the same manner as Example <NUM>, except that a pair of independent hump portions formed a buffer layer without providing a sheet portion, the apparent maximum thickness TB of each hump portion was set as shown in Table <NUM> below, and each fourth belt cord was composed of a steel cord that was the same as the third belt cords. The tire of Comparative Example <NUM> is a conventional tire.

Tires of Examples <NUM>, <NUM>, and <NUM> and Comparative Example <NUM> were obtained in the same manner as Example <NUM>, except that the apparent maximum thickness TB was changed such that the ratio (TB/TA) was set as shown in Tables <NUM> and <NUM> below.

Tires of Examples <NUM> to <NUM> and <NUM> were obtained in the same manner as Example <NUM>, except that the apparent thickness TA was changed such that the ratio (TB/TA) was set as shown in Tables <NUM> and <NUM> below.

A test tire was fitted onto a rim (size = <NUM>×<NUM>) and inflated with air to adjust the internal pressure thereof to <NUM> kPa. On a drum running tester (drum diameter = <NUM>), the tire was caused to run while increasing the speed from <NUM>/h in steps of <NUM>/h every two hours. The time until damage (tread loose cracking (TLC)) occurred was measured. The vertical load was set to <NUM> kN. The results are represented as indexes in the cells for durability in Tables <NUM> and <NUM> below. A higher value represents that the durability of the tread portion is better.

A test tire was fitted onto a rim (size = <NUM>×<NUM>) and inflated with air to adjust the internal pressure thereof to a normal internal pressure. The tire was mounted to a drive shaft of a test vehicle (tractor head). A trailer loaded with luggage was towed by the test vehicle, and the test vehicle was caused to run on general roads. When the wear rate of the tire reached <NUM>% in terms of mass, the difference between the wear amount of the shoulder land portion and the wear amount of the middle land portion of the test tire was calculated. The results are shown as indexes in the cells for uneven wear resistance in Tables <NUM> and <NUM> below. A higher value represents that the difference in wear amount is smaller and the uneven wear resistance is better. That is, a higher value represents that a change of the ground-contact shape is smaller and the ground-contact shape is made more stable.

The total of the indexes obtained in the respective evaluations was calculated. The results are represented in the cells for overall evaluation in Tables <NUM> and <NUM> below. A higher value represents that the respective characteristics are better balanced and the result is better.

As shown in Tables <NUM> and <NUM>, in each Example, the ground-contact shape is stabilized, the durability of the tread portion is improved, and stabilization of the ground-contact shape and reduction of the load acting on the tread portion are achieved. From the evaluation results, advantages of the present invention are clear.

Claim 1:
A heavy duty pneumatic tire (<NUM>) having a nominal aspect ratio of not greater than <NUM>%, the heavy duty pneumatic tire (<NUM>) comprising:
a tread (<NUM>);
a pair of sidewalls (<NUM>) each connected to an end of the tread (<NUM>) and located radially inward of the tread (<NUM>);
a pair of beads (<NUM>) each located radially inward of the sidewall (<NUM>);
a carcass (<NUM>) located inward of the tread (<NUM>) and the pair of sidewalls (<NUM>) and extending on and between one bead (<NUM>) and the other bead (<NUM>);
a belt (<NUM>) located radially inward of the tread (<NUM>) and including a large number of belt cords (<NUM>) aligned with each other; and
a buffer layer (<NUM>) located between the carcass (<NUM>) and the belt (<NUM>) in a radial direction and formed from a crosslinked rubber, wherein
the buffer layer (<NUM>) includes a sheet portion (<NUM>) and a pair of hump portions (<NUM>) located axially outward of the sheet portion (<NUM>), thicker than the sheet portion (<NUM>), and integrated with the sheet portion (<NUM>),
the belt (<NUM>) includes a first belt ply (42A), a second belt ply (42B) located radially outward of the first belt ply (42A), a third belt ply (42C) located radially outward of the second belt ply (42B), and a fourth belt ply (42D) located radially outward of the third belt ply (42C),
the sheet portion (<NUM>) is stacked on the carcass (<NUM>), and
the first belt ply (42A) is stacked on the sheet portion (<NUM>),
characterized in that
each of the pair of hump portions (<NUM>) has a maximum thickness (b) at an end (38e) of the belt (<NUM>), wherein the thickness (b) is measured along an inner surface normal line (NLb) of the tire (<NUM>) and
the tread (<NUM>) has at least three circumferential grooves (<NUM>, <NUM>, <NUM>) formed thereon.