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>).

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

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.

The belt of a heavy duty pneumatic tire normally includes four belt plies stacked in the radial direction. Each belt ply includes a large number of belt cords aligned with each other. Normally, a steel cord is used as each belt cord. If the belt can be composed of three belt plies, mass reduction of the tire can be achieved. The mass reduction contributes to improvement of the fuel economy performance and an increase in the carrying capacity of a vehicle.

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, while achieving mass reduction thereof.

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

A heavy duty pneumatic tire according to an aspect of the present invention has a nominal aspect ratio of not greater than <NUM>%. The tire includes: a tread having at least three circumferential grooves formed thereon; a pair of sidewalls each connected to an end of the tread; a pair of beads each located radially inward of the sidewall; a carcass located inward of the tread and the pair of sidewalls and extending on and between a first bead and a second bead out of the pair of beads; a reinforcing layer located radially inward of the tread; and a buffer layer located radially inward of the reinforcing layer and formed from a crosslinked rubber. Among the at least three circumferential grooves, a circumferential groove located on each outer side in an axial direction is a shoulder circumferential groove. The reinforcing layer includes a belt including a large number of belt cords aligned with each other, and a band including a helically wound band cord. The buffer layer is stacked on the carcass, and each end of the buffer layer is located axially outward of an end of the belt. The belt includes a first belt ply stacked on the buffer layer, a second belt ply located radially outward of the first belt ply, and a third belt ply located radially outward of the second belt ply. The buffer layer includes a sheet portion and a pair of hump portions located axially outward of the sheet portion and thicker than the sheet portion. Each of the pair of hump portions has a maximum thickness at an end of the first belt ply. The hump portion is integrated with the sheet portion.

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, 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, a ratio of a width of the buffer layer to a width of the tread is not less than <NUM> and not greater than <NUM>.

Preferably, in the heavy duty pneumatic tire, a ratio of a distance in the axial direction from a position at which a first hump portion out of the pair of hump portions has a maximum thickness to a position at which a second hump portion out of the pair of hump portions has a maximum thickness, to a width of the belt, is not less than <NUM> and not greater than <NUM>.

Preferably, in the heavy duty pneumatic tire, the band includes a full band located between the first belt ply and the second belt ply in a radial direction. Each end of the full band is located axially outward of the shoulder circumferential groove, each end of the full band is located axially inward of the end of the first belt ply, and each end of the full band is located axially inward of an end of the second belt ply.

More preferably, in the heavy duty pneumatic tire, the band further includes a pair of edge bands opposed to each other across an equator plane. Each of the pair of edge bands is located radially outward of the end of the full band.

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, while achieving mass reduction thereof, is obtained.

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

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

The standardized 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 standardized rims.

The standardized 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 standardized internal pressures.

A standardized load means a load specified in the standard on which the tire is based. The "maximum load capacity" in the JATMA standard, the "maximum value" recited in the "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" in the TRA standard, and the "LOAD CAPACITY" in the ETRTO standard are standardized loads.

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> width 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.

<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 standardized load is applied to the tire <NUM> in the standardized 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 that has 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 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 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>. The width of each circumferential groove <NUM> is represented as the shortest distance from one edge of the circumferential groove <NUM> to the other edge of the circumferential groove <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 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 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 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 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> is located axially outward of the carcass <NUM> and 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 a first bead <NUM> and a second bead <NUM> out of the pair of beads <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 the filler cords.

The reinforcing layer <NUM> is located between the tread <NUM> and the carcass <NUM> in the radial direction. The reinforcing layer <NUM> is located radially inward of the tread <NUM>. The reinforcing layer <NUM> of the tire <NUM> includes a belt <NUM> and a band <NUM>.

The belt <NUM> includes at least three belt plies <NUM>. Each belt ply <NUM> is disposed such that both ends 42e thereof are opposed to each other across the equator plane CL.

The belt <NUM> includes a first belt ply 42A, a second belt ply 42B, and a third belt ply 42C. The first belt ply 42A, the second belt ply 42B, and the third belt ply 42C are aligned in the radial direction in this order.

In the tire <NUM>, among the at least three belt plies <NUM> included in the belt <NUM>, the belt ply <NUM> located on the innermost side in the radial direction is the first belt ply 42A, and the first belt ply 42A, the second belt ply 42B, and the third belt ply 42C are aligned in this order from the inner side. The belt <NUM> includes the first belt ply 42A, the second belt ply 42B located radially outward of the first belt ply 42A, and the third belt ply 42C located radially outward of the second belt ply 42B. In the tire <NUM>, the belt <NUM> may be formed such that the first belt ply 42A, the second belt ply 42B, and the third belt ply 42C are aligned in this order from the outer side in the radial direction.

In the tire <NUM>, the belt <NUM> may be composed of four or more belt plies <NUM>. From the viewpoint of mass reduction of the tire <NUM>, the belt <NUM> is preferably composed of three belt plies <NUM>, that is, the first belt ply 42A, the second belt ply 42B, and the third belt ply 42C.

As shown in <FIG>, an end 42Ae of the first belt ply 42A is located axially outward of the shoulder circumferential groove <NUM>. An end 42Be of the second belt ply 42B is located axially outward of the shoulder circumferential groove <NUM>. An end 42Ce of the third belt ply 42C is located axially outward of the shoulder circumferential groove <NUM>. The end 42Ce of the third belt ply 42C may be located axially inward of the shoulder circumferential groove <NUM>. From the viewpoint of preventing damage, the third belt ply 42C is disposed such that the end 42Ce of the third belt ply 42C does not overlap the shoulder circumferential groove <NUM> in the radial direction.

In <FIG>, a length indicated by an arrow W1 is the width of the first belt ply 42A. A length indicated by an arrow W2 is the width of the second belt ply 42B. A length indicated by an arrow W3 is the width of the third belt ply 42C. The width 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 width of the belt ply <NUM> is also referred to as width in the axial direction. The end 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 first belt ply 42A has the largest width W1, and the third belt ply 42C has the smallest width W3. The width W2 of the second belt ply 42B is smaller than the width W1 of the first belt ply 42A and larger than the width W3 of the third belt ply 42C. The belt <NUM> effectively increases the stiffness of a tread portion T.

In the tire <NUM>, from the viewpoint of effectively increasing the stiffness of the tread portion T, the ratio (W1/WT) of the width W1 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 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 of the third belt ply 42C to the width WT of the tread <NUM> is preferably not less than <NUM>. The width W3 of the third belt ply 42C 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>, among the at least three belt plies <NUM> included in the belt <NUM>, the belt ply <NUM> located on the innermost side in the radial direction is the first belt ply 42A having the largest width W1.

In <FIG>, a length indicated by an arrow WB is the width of the belt <NUM>. In the tire <NUM>, the width WB of the belt <NUM> is represented as the width of the belt ply <NUM> having the largest width. As described above, the first belt ply 42A has the largest width W1 in the axial direction. The width WB of the belt <NUM> of the tire <NUM> is represented as the width W1 of the first belt ply 42A. In the tire <NUM>, the end 42Ae of the first belt ply 42Ais an end 38e of the belt <NUM>. The end 38e of the belt <NUM> is also an end 18e of the reinforcing layer <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 axially inward of the end 42Ae of the first belt ply 42A and located axially inward of the end 42Be of the second belt ply 42B. Each end 44e of the full band <NUM> is located axially outward of the end 42Ce of the third belt ply 42C.

The pair of edge bands <NUM> are disposed so as to be spaced apart from each other in the axial direction. A first edge band <NUM> and a second edge band <NUM> out of the pair of edge bands <NUM> are opposed to each other across the equator plane CL. In the tire <NUM>, each of the first edge band <NUM> and the second edge band <NUM> is located radially outward of the second belt ply 42B, and the third belt ply 42C which forms a part of the belt <NUM> is located between the first edge band <NUM> and the second edge 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 opposite to 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). In the tire <NUM>, the belt <NUM> is formed such that the first belt cords 48A and the second belt cords 48B intersect each other.

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 the same as the tilt direction of the second belt cords 48B. The tilt direction of the third belt cords 48C may be opposite to the tilt direction of the second belt cords 48B.

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).

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, and the tilt angle θ3 of the third belt cords 48C is preferably not less than <NUM>° and preferably not greater than <NUM>°.

From the viewpoint of effectively restraining movement of the tread portion T 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 θ1 of the first belt cords 48A is further 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>°.

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 T of the tire <NUM>.

In the tire <NUM>, each of the end 42Ae of the first belt ply 42A and the end 42Be of the second belt ply 42B is covered with a rubber layer <NUM>. Two rubber layers <NUM> are further disposed between the end 42Ae of the first belt ply 42A and the end 42Be of the second belt ply 42B, 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 42Ae of the first belt ply 42A and the end 42Be of the second belt ply 42B. The edge member <NUM> is formed from a crosslinked rubber. The edge member <NUM> contributes to maintaining the interval between the end 42Ae of the first belt ply 42A and the end 42Be of the second belt ply 42B. In the tire <NUM>, a change of the positional relationship between the end 42Ae of the first belt ply 42A and the end 42Be of the second belt ply 42B 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 radially inward of the reinforcing layer <NUM>. The buffer layer <NUM> is stacked on the carcass <NUM> on the radially inner side of the tread <NUM>.

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.

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

In <FIG>, a position indicated by reference character SH represents a specific position on the outer surface of the buffer layer <NUM>. In the tire <NUM>, the position SH is the boundary between the sheet portion <NUM> and the hump portion <NUM>. The boundary SH is located axially outward of a bottom 28sb of the shoulder circumferential groove <NUM>.

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>.

In <FIG>, a position indicated by reference character Pa is the point of intersection of the outer surface of the sheet portion <NUM> and the equator plane CL. A length indicated by arrows Ta is the thickness of the sheet portion <NUM> at the equator plane CL. The thickness Ta 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).

In the tire <NUM>, of the buffer layer <NUM>, a portion between a first boundary SH and a second boundary SH opposed to each other across the equator plane CL is maintained with the thickness Ta, and this portion is the sheet portion <NUM>. The sheet portion <NUM> has a uniform thickness Ta.

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 arrows Tb is the thickness of the hump portion <NUM> at the position Pb. The thickness Tb is measured along the inner surface normal line NLb of the tire <NUM>.

In the tire <NUM>, the thickness of the hump portion <NUM> gradually increases from the boundary SH between the sheet portion <NUM> and the hump portion <NUM> toward the position Pb, and gradually decreases from the 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.

The thickness of the hump portion <NUM> shows the maximum at the position Pb. The hump portion <NUM> has the maximum thickness Tb at the position Pb. The position Pb is a maximum thickness position of the hump portion <NUM>.

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

In the tire <NUM>, the reinforcing layer <NUM> is stacked on the buffer layer <NUM> from the radially outer side. A radially inner portion of the reinforcing layer <NUM> is composed of the first belt ply 42A. In the tire <NUM>, the first belt ply 42A is stacked on the buffer layer <NUM>.

As shown in <FIG>, the maximum thickness position Pb of the hump portion <NUM> is located near the end 42Ae of the first belt ply 42A. The hump portion <NUM> has the maximum thickness Tb at the end 42Ae of the first belt ply 42A. As described above, the end 42Ae of the first belt ply 42A of the tire <NUM> is the end 38e of the belt <NUM>. In the tire <NUM>, the hump portion <NUM> has the maximum thickness Tb at the end 38e of the belt <NUM>.

In the present disclosure, the hump portion <NUM> having the maximum thickness Tb at the end 38e of the belt <NUM> means that the shortest distance from the end 38e of the belt <NUM> to the maximum thickness position Pb of the hump portion <NUM> in the meridian cross-section of the tire <NUM> is not greater than <NUM>.

In the tire <NUM>, the hump portion <NUM> of the buffer layer <NUM> is located between the end 42Ae of the first belt ply 42A 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 T.

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

In a conventional tire, between a belt ply corresponding to the first belt ply 42A of the tire <NUM> (hereinafter, referred to as corresponding ply) and a carcass ply, a belt ply (hereinafter, referred to as tilt ply) in which the tilt angle of belt cords is adjusted so as to be smaller than the intersection angle of carcass cords and larger than the tilt angle of belt cords of the corresponding ply is provided. The tilt ply is stacked on a carcass, and the corresponding ply is stacked on the tilt ply.

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

The sheet portion <NUM> is formed from a crosslinked rubber. In the tire <NUM>, the sheet portion <NUM> contributes to reduction of the load acting on the tread portion T. The sheet portion <NUM> does not include belt cords, unlike the tilt ply of the conventional tire. In the tire <NUM>, the load acting on the tread portion T 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 T and uneven wear resistance.

Since the sheet portion <NUM> contributes to reduction of the load acting on the tread portion T, hump portions <NUM> smaller than later-described cushion layers provided in a conventional low-flatness tire can be used in the tire <NUM>. The compact hump portions <NUM> contribute to preventing the end 38e of the belt <NUM> from being raised. The contour (hereinafter, also referred to as case line) of the carcass <NUM> is formed 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.

As described above, the band <NUM> which forms a part of the reinforcing layer <NUM> includes the helically wound band cords <NUM>. The band <NUM> holds the tread portion T. The band <NUM> suppresses a shape change of the tire <NUM> due to running. As described above, the case line can be made appropriate by the buffer layer <NUM>. In the tire <NUM>, the buffer layer <NUM> and the band <NUM> effectively suppress a shape change due to running. In the tire <NUM>, even though the belt <NUM> which forms another part of the reinforcing layer <NUM> is composed of three belt plies <NUM>, the ground-contact shape can be stabilized, and good uneven wear resistance can be achieved. Furthermore, the belt <NUM> being composed of three belt plies <NUM> contributes to mass reduction of the tire <NUM>.

The tire <NUM> can achieve stabilization of the ground-contact shape and reduction of the load acting on the tread portion T, while achieving mass reduction thereof.

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 forming 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 CL.

In the tire <NUM>, each hump portion <NUM> corresponds to a cushion layer of a conventional tire. 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 forming the green tire is shortened. The buffer layer <NUM> also contributes to improvement of the productivity.

As described above, the tire <NUM> can achieve stabilization of the ground-contact shape and reduction of the load acting on the tread portion T, while achieving mass reduction thereof. The tire <NUM> is lighter and has better uneven wear resistance and tread durability than a conventional low-flatness tire. Furthermore, in the tire <NUM>, stabilization of the quality and improvement of the productivity are achieved.

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

When the thickness Ta is set to be not less than <NUM>, the sheet portion <NUM> can effectively contribute to alleviation of strain. In the tire <NUM>, good tread durability is achieved. From this viewpoint, the thickness Ta is more preferably not less than <NUM> and further preferably not less than <NUM>.

When the thickness Ta 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 thickness Ta is more preferably not greater than <NUM> and further preferably not greater than <NUM>.

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

When the 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 belt <NUM>. In the tire <NUM>, good tread durability is achieved. From this viewpoint, the thickness Tb is more preferably not less than <NUM> and further preferably not less than <NUM>.

When the 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 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 tread durability and uneven wear resistance, more preferably, the thickness Ta of the sheet portion <NUM> is not less than <NUM> and not greater than <NUM>, and the 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 thickness Tb of the hump portion <NUM> to the 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. In the tire <NUM>, good tread durability and uneven wear resistance are achieved. From this viewpoint, the ratio (Tb/Ta) is more preferably not less than <NUM> and further 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 T. In this case as well, in the tire <NUM>, good tread durability and uneven wear resistance are achieved. From this viewpoint, the ratio (Tb/Ta) is more preferably not greater than <NUM> and further preferably 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>. As described above, of the buffer layer <NUM>, the portion between the first boundary SH and the second boundary SH is the sheet portion <NUM> having a uniform thickness Ta. The distance between the carcass cord <NUM> and the first belt cords 48A is constant between the first boundary SH and the second boundary SH.

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 SH and the second boundary SH 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 18e of the reinforcing layer <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 belt <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 <FIG>, a length indicated by an arrow WC is the width of the buffer layer <NUM>. The width WC of the buffer layer <NUM> is the distance in the axial direction from a first end 20e of the buffer layer <NUM> to a second end 20e of the buffer layer <NUM>.

In the tire <NUM>, the ratio (WC/WT) of the width WC of the buffer layer <NUM> to the width WT of the tread <NUM> is preferably not less than <NUM> and not greater than <NUM>.

When the ratio (WC/WT) is set to be not less than <NUM>, the buffer layer <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 ratio (WC/WT) is more preferably not less than <NUM>.

When the ratio (WC/WT) is set to be not greater than <NUM>, the bending allowance of the side portion S is appropriately maintained. In the tire <NUM>, occurrence of damage (Cord Broken Up: CBU) with a break of the carcass cord <NUM> is suppressed. From this viewpoint, the ratio (WC/WT) is more preferably not greater than <NUM>.

In <FIG>, a length indicated by an arrow WH is the distance in the axial direction from the position Pb at which the first hump portion <NUM> has the maximum thickness Tb to the position Pb at which the second hump portion <NUM> has the maximum thickness Tb. The distance WH in the axial direction is also referred to as inter-hump distance.

As described above, the maximum thickness position Pb of the hump portion <NUM> is located near the end 42Ae of the first belt ply 42A. Since the end 42Ae of the first belt ply 42A of the tire <NUM> is the end 38e of the belt <NUM>, the maximum thickness position Pb of the hump portion <NUM> is located near the end 38e of the belt <NUM>. Accordingly, 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, in the tire <NUM>, the ratio (WH/WB) of the inter-hump distance WH to the width WB of the belt <NUM> is preferably not less than <NUM> and not greater than <NUM>. The ratio (WH/WB) is more preferably not less than <NUM>, and more preferably not greater than <NUM>.

In <FIG>, a length indicated by an arrow WD is the distance in the axial direction from the boundary SH on the first hump portion <NUM> side between this hump portion <NUM> and the sheet portion <NUM> to the boundary SH on the second hump portion <NUM> side between this hump portion <NUM> and the sheet portion <NUM>. The distance WD in the axial direction is also referred to as inter-boundary distance.

In the present disclosure, the boundary SH 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>.

In the tire <NUM>, the ratio (WD/WH) of the inter-boundary distance WD to the inter-hump distance WH is preferably not less than <NUM> and not greater than <NUM>.

When the ratio (WD/WH) is set to be not less than <NUM>, 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 ratio (WD/WH) is more preferably not less than <NUM>.

When the ratio (WD/WH) is set to be not greater 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>. In the tire <NUM>, good tread durability is achieved. From this viewpoint, the ratio (WD/WH) is more preferably not greater than <NUM>.

As described above, the band <NUM> which forms a part of the reinforcing layer <NUM> of the tire <NUM> includes the full band <NUM>. The full band <NUM> extends in the axial direction from the equator plane CL toward each end 44e. The full band <NUM> includes the helically wound band cord 52F. The full band <NUM> holds the tread portion T. The full band <NUM> suppresses a shape change of the tire <NUM> due to running.

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 the stiffness of the portion where the circumferential grooves are not formed. A low-flatness tire has a wider tread surface than a high-flatness tire. In the low-flatness tire, each shoulder circumferential groove is located more axially outward than that in the high-flatness tire. In the low-flatness tire, a shape change is large around each shoulder circumferential groove.

In the tire <NUM>, the full band <NUM> is formed such that each end 44e of the full band <NUM> is located axially outward of the shoulder circumferential groove <NUM>. In the tire <NUM>, the full band <NUM> is located radially inward of the 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>. In the tire <NUM>, a shape change is effectively suppressed around the shoulder circumferential groove <NUM>. 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 of the shoulder land portion <NUM>. The width WS 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 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 28sb of the shoulder circumferential groove <NUM>. In the tire <NUM>, occurrence of damage starting from the bottom 28sb 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>, 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 particular, from the viewpoint of effectively suppressing a shape change around each shoulder circumferential groove <NUM>, in the tire <NUM>, each end 44e of the full band <NUM> is more preferably located between the shoulder circumferential groove <NUM> and the end 38e of the belt <NUM> in the axial direction.

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

The first belt ply 42A and the second belt ply 42B reduce the force acting on the full band <NUM>. In particular, since the belt cords <NUM> included in the first belt ply 42A and the belt cords <NUM> included in the second belt ply 42B 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 first belt ply 42A and the second belt ply 42B in the radial direction. In particular, from the viewpoint of effectively suppressing a shape change around each shoulder circumferential groove <NUM>, in the tire <NUM>, more preferably, the full band <NUM> is located between the first belt ply 42A and the second belt ply 42B in the radial direction, and each end 44e of the full band <NUM> is located between the shoulder circumferential groove <NUM> and the end 38e of the belt <NUM> in the axial direction.

As described above, in the axial direction, each end 44e of the full band <NUM> is located inward of the end 42Ae of the first belt ply 42A, and each end 44e of the full band <NUM> is located inward of the end 42Be of the second belt ply 42B. In other words, the first belt ply 42A and the second belt ply 42B are wider than the full band <NUM>. In the tire <NUM>, the full band <NUM> is interposed between the first belt ply 42A and the second belt ply 42B 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 first belt ply 42A and the second belt ply 42B in the radial direction, each end 44e of the full band <NUM> is located axially inward of the end 42Ae of the first belt ply 42A, and each end 44e of the full band <NUM> is located axially inward of the end 42Be of the second belt ply 42B. In particular, from the viewpoint of effectively suppressing a shape change around each shoulder circumferential groove <NUM>, in the tire <NUM>, more preferably, the full band <NUM> is located between the first belt ply 42A and the second belt ply 42B in the radial direction, each end 44e of the full band <NUM> is located axially inward of the end 42Ae of the first belt ply 42A, each end 44e of the full band <NUM> is located axially inward of the end 42Be of the second belt ply 42B, and further each end 44e of the full band <NUM> is located between the shoulder circumferential groove <NUM> and the end 38e of the belt <NUM> in the axial direction.

As described above, the band <NUM> which forms a part of the reinforcing layer <NUM> of the tire <NUM> includes the pair of edge bands <NUM> in addition to the full band <NUM>.

In the tire <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.

In the tire <NUM>, 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>, and the edge band <NUM> overlaps the end 44e of the full band <NUM> in the radial direction.

In the tire <NUM>, each edge band <NUM> is located 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 T. 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 the 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 the edge band <NUM> is located radially outward of each end 44e of the full band <NUM>.

As described above, in the tire <NUM>, the inner end 46ue of each edge band <NUM> is located axially inward of the end 44e of the full band <NUM>. 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 SH 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 SH between the sheet portion <NUM> and the hump portion <NUM> is preferably located axially outward of the end 44e of the full band <NUM>.

As shown in <FIG>, in the tire <NUM>, the boundary SH 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 SH 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 SH 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 SH 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 T, while achieving mass reduction thereof, 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>%.

Claim 1:
A heavy duty pneumatic tire (<NUM>) comprising:
a tread (<NUM>) having at least three circumferential grooves (<NUM>, <NUM>, <NUM>) formed thereon;
a pair of sidewalls (<NUM>) each connected to an end 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 a first bead (<NUM>) and a second bead (<NUM>) out of the pair of beads (<NUM>);
a reinforcing layer (<NUM>) located radially inward of the tread (<NUM>); and
a buffer layer (<NUM>) located radially inward of the reinforcing layer (<NUM>) and formed from a crosslinked rubber, wherein
among the at least three circumferential grooves (<NUM>, <NUM>, <NUM>), a circumferential groove (<NUM>) located on each outer side in an axial direction is a shoulder circumferential groove (<NUM>),
the reinforcing layer (<NUM>) includes a belt (<NUM>) including a large number of belt cords (<NUM>, 48A, 48B, 48C) aligned with each other,
the buffer layer (<NUM>) is stacked on the carcass (<NUM>),
each end (20e) of the buffer layer (<NUM>) is located axially outward of an end (38e) of the belt (<NUM>),
the belt (<NUM>) includes a first belt ply (42A) stacked on the buffer layer (<NUM>), a second belt ply (42B) located radially outward of the first belt ply (42A), and a third belt ply (42C) located radially outward of the second belt ply (42B),
the buffer layer (<NUM>) includes a sheet portion (<NUM>) and a pair of hump portions (<NUM>) located axially outward of the sheet portion (<NUM>) and thicker than the sheet portion (<NUM>), and
each of the pair of hump portions (<NUM>) has a maximum thickness (Tb) at an end (42Ae) of the first belt ply (42A),
characterized in that
a nominal aspect ratio of the heavy duty pneumatic tire (<NUM>) is not greater than <NUM>%,
the reinforcing layer (<NUM>) further includes a band (<NUM>) including a helically wound band cord (<NUM>, 52E, 52F), and
the hump portion (<NUM>) is integrated with the sheet portion (<NUM>).