Patent Description:
From the viewpoint of drainage performance, at least three circumferential grooves are formed on the tread of a heavy duty pneumatic tire (hereinafter, referred to as tire). Among the circumferential grooves formed on the tread, the circumferential groove located on each outer side in the axial direction is a shoulder circumferential groove.

A belt and a band are provided between the tread and a carcass. The belt includes a plurality of belt plies aligned in the radial direction. Each belt ply includes a large number of belt cords aligned with each other. Normally, steel cords are used as the belt cords. The band includes a helically wound band cord. A cord formed from an organic fiber such as nylon fiber, or a steel cord is used as the band cord. The stiffness of a tread portion is controlled by adjusting the configuration of the belt or the band (for example, <CIT>).

In a tire in a running state, deformation and restoration are repeated. Accordingly, the shape of the tire is changed. The ground-contact shape of the tire is changed, so that there is a concern about a decrease in uneven wear resistance.

In a running state of the tire, a tread end portion moves actively. The stiffness of the portion where the circumferential grooves are formed is lower than the stiffness of the portion where the circumferential grooves are not formed. Low-flatness tires having an aspect ratio of not greater than <NUM>% include a tire having a wide tread surface. In the tire, each shoulder circumferential groove is located more outward in the axial direction than that in a high-flatness tire. In the tire, a shape change is large around the shoulder circumferential groove. In order to suppress a shape change, use of a full band including a helically wound band cord is considered.

The belt of a heavy duty pneumatic tire normally includes four belt plies stacked in the radial direction. When a full band is added in order to suppress a shape change, the mass of the tire increases. The tire mass influences the fuel efficiency and load capacity of a vehicle. Establishment of a technology capable of achieving suppression of a shape change due to running while taking into consideration the influence on tire mass is required.

Further pneumatic tires are known from <CIT>, <CIT> and <CIT>.

The present invention has been made in view of the above circumstances. An object of the present invention is to provide a heavy duty pneumatic tire that can achieve suppression of a mass increase and suppression of a shape change due to running.

A heavy duty pneumatic tire according to an aspect of the present invention has a nominal aspect ratio of not greater than <NUM>%. The heavy duty pneumatic tire includes: a tread having at least three circumferential grooves formed thereon; a pair of sidewalls each connected to an end of the tread and located inward of the tread in a radial direction; a pair of beads each located inward of the sidewall in the radial direction; a carcass located inward of the tread and the pair of sidewalls and extending on and between one bead and the other bead; a reinforcing layer located inward of the tread in the radial direction; and a buffer layer located between the carcass and the reinforcing layer in the radial direction. 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 belt includes a first belt ply, a second belt ply, and a third belt ply. The first belt ply, the second belt ply, and the third belt ply are aligned in the radial direction in this order. A direction in which the belt cords included in the first belt ply are tilted is opposite to a direction in which the belt cords included in the second belt ply are tilted. The band includes a full band located between the first belt ply and the second belt ply in the radial direction. Each end of the full band is located outward of the shoulder circumferential groove in the axial direction. The buffer layer is formed from a crosslinked rubber, and a ratio of a width in the axial direction of the buffer layer to a width in the axial direction of the first belt ply is not less than <NUM>%.

Preferably, the heavy duty pneumatic tire further includes a pair of cushion layers located outward of the buffer layer in the axial direction. Each of the pair of cushion layers is located between the reinforcing layer and the carcass at an end of the reinforcing layer. Each end of the buffer layer is located outward of an inner end of the cushion layer in the axial direction.

Preferably, in the heavy duty pneumatic tire, the buffer layer has a complex elastic modulus of not less than <NUM> MPa and not greater than <NUM> MPa.

Preferably, in the heavy duty pneumatic tire, the complex elastic modulus of the buffer layer is higher than a complex elastic modulus of each cushion layer.

Preferably, in the heavy duty pneumatic tire, the belt cords included in the first belt ply have a breaking load of not less than <NUM> N.

According to the present invention, a heavy duty pneumatic tire that can achieve suppression of a mass increase and suppression of a shape change due to running, 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>, a pair of cushion layers <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 also referred to as the equator of the tire <NUM>.

In <FIG>, reference character PE represents an end of the tread surface <NUM>. A double-headed arrow WT indicates the width of the tread surface <NUM>. The width WT of the tread surface <NUM> is the distance in the axial direction from one end PE of the tread surface <NUM> to the other end PE of the tread surface <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> 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> of the tire <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> formed on the tread <NUM>, the circumferential groove <NUM> located on each outer side in the axial direction is a shoulder circumferential groove <NUM>. The circumferential groove <NUM> located inward of the shoulder circumferential groove <NUM> in the axial direction is a middle circumferential groove <NUM>. In the tire <NUM>, the four circumferential grooves <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 surface <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 surface <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> formed in the tread <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 outward of the shoulder circumferential groove <NUM> in the axial direction and includes the end PE of the tread surface <NUM>. The land portions <NUM> located inward of the shoulder land portions <NUM> in the axial direction are middle land portions <NUM>. The land portion <NUM> located inward of the middle land portions <NUM> in the axial direction is a center land portion 32c. In the tire <NUM>, the five land portions <NUM> include the center land portion 32c, 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 32c is not less than <NUM>% and not greater than <NUM>% of the width WT of the tread surface <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 surface <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 surface <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 32c, 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 inward of the tread <NUM> in the radial direction. The sidewall <NUM> is formed from a crosslinked rubber.

Each bead <NUM> is located inward of the sidewall <NUM> in the radial direction. 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 outward of the core <NUM> in the radial direction. The apex <NUM> includes an inner apex 36u and an outer apex <NUM>. The inner apex 36u extends radially outward from the core <NUM>. The outer apex <NUM> is located radially outward of the inner apex 36u. The inner apex 36u 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 36u.

Each chafer <NUM> is located outward of the bead <NUM> in the axial direction. The chafer <NUM> is located inward of the sidewall <NUM> in the radial direction. 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 38a which extends from one core <NUM> to the other core <NUM>, and a pair of turned-up portions 38b which are connected to the ply body 38a 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. 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.

Each cushion layer <NUM> is located outward of the buffer layer <NUM> in the axial direction. The cushion layer <NUM> is located between the reinforcing layer <NUM> and the carcass <NUM> (specifically, the ply body 38a of the carcass ply <NUM>) at an end 20e of the reinforcing layer <NUM>. In the axial direction, an inner end 14ue of the cushion layer <NUM> is located inward of the end 20e of the reinforcing layer <NUM>. In the axial direction, an outer end 14se of the cushion layer <NUM> is located outward of the end 20e of the reinforcing layer <NUM>. In the radial direction, the outer end 14se of the cushion layer <NUM> is located inward of the end 20e of the reinforcing layer <NUM>. The cushion layer <NUM> is formed from a flexible crosslinked rubber. From the viewpoint of preventing occurrence of damage at the end 20e of the reinforcing layer <NUM>, a complex elastic modulus E*c of the cushion layer <NUM> is preferably not less than <NUM> MPa and not greater than <NUM> MPa.

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). 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 B. 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 inward of the tread <NUM> in the radial direction. The reinforcing layer <NUM> is located between the tread <NUM> and the carcass <NUM>. The reinforcing layer <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 44e thereof are opposed to each other across the equator plane CL.

The belt <NUM> includes a first belt ply 44A, a second belt ply 44B, and a third belt ply 44C. The first belt ply 44A, the second belt ply 44B, and the third belt ply 44C 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 44A, and the first belt ply 44A, the second belt ply 44B, and the third belt ply 44C are aligned in this order from the inner side. The belt <NUM> includes the first belt ply 44A, the second belt ply 44B located outward of the first belt ply 44A in the radial direction, and the third belt ply 44C located outward of the second belt ply 44B in the radial direction. In the tire <NUM>, the belt <NUM> may be formed such that the first belt ply 44A, the second belt ply 44B, and the third belt ply 44C 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 reducing the mass of the tire <NUM>, the belt <NUM> is preferably composed of three belt plies <NUM>, that is, the first belt ply 44A, the second belt ply 44B, and the third belt ply 44C.

As shown in <FIG>, an end 44Ae of the first belt ply 44A is located outward of the shoulder circumferential groove <NUM> in the axial direction. An end 44Be of the second belt ply 44B is located outward of the shoulder circumferential groove <NUM> in the axial direction. An end 44Ce of the third belt ply 44C is located inward of the shoulder circumferential groove <NUM> in the axial direction. In the tire <NUM>, the end 44Ce of the third belt ply 44C may be located outward of the shoulder circumferential groove <NUM> in the axial direction.

In <FIG>, a length indicated by reference character W1 is the width in the axial direction of the first belt ply 44A. A length indicated by reference character W2 is the width in the axial direction of the second belt ply 44B. A length indicated by reference character W3 is the width in the axial direction of the third belt ply 44C. The width in the axial direction of each belt ply <NUM> is the distance in the axial direction from one end 44e of the belt ply <NUM> to the other end 44e of the belt ply <NUM>.

In the tire <NUM>, the first belt ply 44A has the largest width W1 in the axial direction, and the third belt ply 44C has the smallest width W3 in the axial direction. The width W2 in the axial direction of the second belt ply 44B is smaller than the width W <NUM> in the axial direction of the first belt ply 44A and larger than the width W3 in the axial direction of the third belt ply 44C.

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 44A having the largest width W1 in the axial direction. 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 in the axial direction of the first belt ply 44A 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 44B to the width WT of the tread <NUM> is preferably not less than <NUM> and not greater than <NUM>. The width W3 in the axial direction of the third belt ply 44C is set as appropriate according to the specifications of the tire <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 first belt ply 44A has the largest width W1 in the axial direction. The width in the axial direction of the belt <NUM> of the tire <NUM> is represented as the width W1 in the axial direction of the first belt ply 44A. In the tire <NUM>, the end 44Ae of the first belt ply 44A is an end 40e of the belt <NUM>. The end 40e of the belt <NUM> is also the end 20e of the reinforcing layer <NUM>.

The band <NUM> includes a full band <NUM>. The full band <NUM> is disposed such that both ends 46e thereof are opposed to each other across the equator plane CL. In the axial direction, each end 46e of the full band <NUM> is located inward of the end 40e of the belt <NUM>.

The band <NUM> of the tire <NUM> is composed of only the full band <NUM>. Although not shown, the band <NUM> may further include a pair of edge bands disposed so as to be spaced apart from each other in the axial direction across the equator plane CL. In this case, each edge band is disposed so as to overlap the end 46e of the full band <NUM> in the radial direction.

In the tire <NUM>, from the viewpoint of mass reduction, the band <NUM> is preferably composed of only 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 44A 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 44B 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 44C 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 44A 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 44B 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 44C 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>°.

As described above, the carcass ply <NUM> includes the carcass cords, and the intersection angle of the carcass cords is not less than <NUM>° and not greater than <NUM>°. In the tire <NUM>, the belt ply <NUM> closest to the carcass ply <NUM> is the first belt ply 44A. In the tire <NUM>, the difference between the tilt angle θ1 of the first belt cords 48A and the intersection angle of the carcass cords is large.

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

In the tire <NUM>, the band cords are steel cords or cords formed from an organic fiber (hereinafter, organic fiber cords). In the case where organic fiber cords are used as the band cords, examples of the organic fiber include nylon fibers, polyester fibers, rayon fibers, and aramid fibers.

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

The density of the band cord <NUM> 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 <NUM> is represented as the number of cross-sections of the band cord <NUM> included per <NUM> width of the full band <NUM> in a cross-section of the full 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 44Ae of the first belt ply 44A and the end 44Be of the second belt ply 44B is covered with a rubber layer <NUM>. Two rubber layers <NUM> are further disposed between the end 44Ae of the first belt ply 44A and the end 44Be of the second belt ply 44B, 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 44Ae of the first belt ply 44A and the end 44Be of the second belt ply 44B. The edge member <NUM> is formed from a crosslinked rubber. The edge member <NUM> contributes to maintaining the interval between the end 44Ae of the first belt ply 44A and the end 44Be of the second belt ply 44B. In the tire <NUM>, a change of the positional relationship between the end 44Ae of the first belt ply 44A and the end 44Be of the second belt ply 44B 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 reinforcing layer <NUM> in the radial direction. The buffer layer <NUM> is disposed such that both ends 22e thereof are opposed to each other across the equator plane CL. In the axial direction, each end 22e of the buffer layer <NUM> is located inward of the end 46e of the full band <NUM>.

In <FIG>, a length indicated by reference character WB is the width in the axial direction of the buffer layer <NUM>. The width WB in the axial direction of the buffer layer <NUM> is the distance in the axial direction from one end 22e of the buffer layer <NUM> to the other end 22e of the buffer layer <NUM>.

In the tire <NUM>, the buffer layer <NUM> is located between the carcass ply <NUM> and the first belt ply 44A in the radial direction. The buffer layer <NUM> is located between the pair of cushion layers <NUM> in the axial direction. The buffer layer <NUM> of the tire <NUM> is formed from a crosslinked rubber different from the crosslinked rubber for the cushion layer <NUM>.

The full band <NUM> of the tire <NUM> extends in the axial direction from the equator plane CL toward each end 46e. Each end 46e of the full band <NUM> is located outward of the shoulder circumferential groove <NUM> in the axial direction. 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>. A shape change of the tire <NUM>, for example, a change of the contour (hereinafter, also referred to as case line) of the carcass <NUM>, is suppressed, so that a change of the ground-contact shape of the tire <NUM> is suppressed.

As described above, the band cord <NUM> included in the full band <NUM> extends substantially in the circumferential direction. A force acts on the full band <NUM> of the tire <NUM> in a running state so as to spread from the inner side toward the outer side in the radial direction. This force increases the tension of the band cord <NUM>.

A tire bends when coming into contact with a road surface. Accordingly, the force acting on the full band is decreased, so that the tension of the band cord is decreased. When the tire becomes separated from the road surface to be restored, the force acting on the full band is increased, so that the tension of the band cord is increased. In the band cord of the tire in a running state, fluctuation of the tension is repeated. There is a concern that a break may occur in the band cord depending on the degree of fluctuation of the tension. When the band cord becomes broken, the holding force thereof is decreased. In this case, the full band may no longer be able to contribute to suppression of a shape change.

In the tire <NUM>, the full band <NUM> is located between the first belt ply 44A and the second belt ply 44B in the radial direction. The first belt ply 44A and the second belt ply 44B reduce the force acting on the full band <NUM>. In particular, since the belt cords <NUM> included in the first belt ply 44A and the belt cords <NUM> included in the second belt ply 44B intersect each other, the force acting on the full band <NUM> is effectively reduced. Since fluctuation of the tension of the band cord <NUM> included in the full band <NUM> is suppressed, occurrence of a break of the band cord <NUM> due to this tension fluctuation is suppressed. The full band <NUM> of the tire <NUM> can stably exhibit the function of suppressing a shape change.

As described above, in the tire <NUM>, among the belt plies <NUM> included in the belt <NUM>, the first belt ply 44A is closest to the ply body 38a of the carcass ply <NUM>, and the difference between the tilt angle θ1 of the first belt cords 48A included in the first belt ply 44A and the intersection angle of the carcass cords in the ply body 38a is large.

When the tire <NUM> is fitted onto a rim (not shown) and the inside of the tire <NUM> is filled with air, the tire <NUM> expands. At this time, due to the difference between the intersection angle of the carcass cords and the tilt angle θ1 of the first belt cords 48A, strain is generated between the carcass ply <NUM> and the first belt ply 44A. There is a possibility that, depending on the degree of the strain, damage occurs between the carcass ply <NUM> and the first belt ply 44A and the belt <NUM> cannot sufficiently hold the carcass <NUM>.

In a conventional tire, to alleviate strain, between a belt ply corresponding to the first belt ply 44A 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. In the conventional tire, to ensure a holding force and alleviate strain, a belt is composed of at least four belt plies.

In the tire <NUM>, the buffer layer <NUM> is provided between the carcass <NUM> and the reinforcing layer <NUM>. Specifically, the buffer layer <NUM> is provided between the carcass ply <NUM> and the first belt ply 44A.

The buffer layer <NUM> is formed from a crosslinked rubber. In the buffer layer <NUM>, cords are not included as in the carcass ply <NUM> and the first belt ply 44A. The buffer layer <NUM> is more flexible than the carcass ply <NUM> and the first belt ply 44A.

The buffer layer <NUM> contributes to alleviation of strain generated between the carcass ply <NUM> and the first belt ply 44A due to the difference between the intersection angle of the carcass cords and the tilt angle θ1 of the first belt cords 48A.

Moreover, the ratio (WB/W1) of the width WB in the axial direction of the buffer layer <NUM> to the width W1 in the axial direction of the first belt ply 44A is not less than <NUM>%. The buffer layer <NUM> is sufficiently joined to the first belt ply 44A and the carcass ply <NUM>. In the tire <NUM>, strain generated between the carcass ply <NUM> and the first belt ply 44A is sufficiently alleviated. The integrity of the carcass <NUM> and the belt <NUM> is maintained, so that the belt <NUM> can sufficiently hold the carcass <NUM>. In the tire <NUM>, the full band <NUM> can sufficiently exhibit the function of suppressing a shape change. The buffer layer <NUM> contributes to suppression of a shape change.

Furthermore, since the buffer layer <NUM> does not include any cord, the buffer layer <NUM> is lighter than the tilt ply provided in the conventional tire in order to alleviate strain. In the tire <NUM>, although the full band <NUM> is provided in order to suppress a shape change, an increase in mass is suppressed.

In the conventional tire, since the tilt angle of the belt cords included in the tilt ply is larger than the tilt angle of the belt cords included in the corresponding ply, the degree of contribution of the tilt ply to ensuring the holding force of the belt is originally low. Although the belt <NUM> of the tire <NUM> is composed of at least three belt plies <NUM>, the belt <NUM> exhibits the same level of holding force as the conventional belt composed of at least four belt plies. In the tire <NUM>, a shape change is suppressed to the same extent as in the case where the full band <NUM> is combined with the conventional belt.

In the tire <NUM>, suppression of a mass increase and suppression of a shape change due to running are achieved. The tire <NUM> can achieve improvement of uneven wear resistance while suppressing an increase in rolling resistance.

<FIG> shows a cross-section of the tire <NUM> shown in <FIG>, taken along the equator plane CL. <FIG> shows a cross-section of the tread portion T. In <FIG>, a length indicated by a double-headed arrow TC is the distance between the carcass <NUM> and the reinforcing layer <NUM>. Specifically, the distance TC is the distance between the carcass ply <NUM> and the first belt ply 44A.

The distance TC is represented as the distance (inter-cord distance) between a carcass cord <NUM> included in the carcass ply <NUM> and the first belt cord 48A included in the first belt ply 44A on the equator plane CL. The distance TC is the thickness of a rubber component located between the carcass cord <NUM> and the first belt cord 48A. The buffer layer <NUM> is included in this rubber component. The distance TC is mainly controlled by adjusting the thickness of the buffer layer <NUM>.

In the tire <NUM>, the distance TC between the carcass <NUM> and the reinforcing layer <NUM> is preferably not less than <NUM> and not greater than <NUM>.

When the distance TC 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 the rubber component is prevented. From this viewpoint, the distance TC is more preferably not less than <NUM>.

When the distance TC 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 distance TC is more preferably not greater than <NUM> and further preferably not greater than <NUM>.

In the tire <NUM>, a complex elastic modulus E*b of the buffer layer <NUM> is preferably not less than <NUM> MPa and not greater than <NUM> MPa.

When the complex elastic modulus E*b of the buffer layer <NUM> is set to be not less than <NUM> MPa, the buffer layer <NUM> having an appropriate thickness can be formed in the tire <NUM>. The buffer layer <NUM> can contribute to suppression of a mass increase. From this viewpoint, the complex elastic modulus E*b of the buffer layer <NUM> is more preferably not less than <NUM> MPa and further preferably not less than <NUM> MPa.

When the complex elastic modulus E*b of the buffer layer <NUM> is set to be not greater than <NUM> MPa, the buffer layer <NUM> can effectively contribute to alleviation of strain. In the tire <NUM>, occurrence of damage due to the buffer layer <NUM> is prevented. From this viewpoint, the complex elastic modulus E*b of the buffer layer <NUM> is more preferably not greater than <NUM> MPa and further preferably not greater than <NUM> MPa.

In <FIG>, a length indicated by reference character TB is the thickness of the buffer layer <NUM>. This thickness TB is measured along the equator plane CL.

In the tire <NUM>, the thickness TB of the buffer layer <NUM> is preferably not less than <NUM> and not greater than <NUM>.

When the thickness TB of the buffer layer <NUM> is set to be not less than <NUM>, the buffer layer <NUM> can effectively contribute to alleviation of strain. Occurrence of damage due to the buffer layer <NUM> is prevented. From this viewpoint, the thickness TB of the buffer layer <NUM> is more preferably not less than <NUM>.

When the thickness TB of the buffer layer <NUM> is set to be not greater than <NUM>, the buffer layer <NUM> can contribute to suppression of a mass increase. From this viewpoint, the thickness TB of the buffer layer <NUM> is more preferably not greater than <NUM> and further preferably not greater than <NUM>.

As shown in <FIG>, the end 22e of the buffer layer <NUM> is located outward of the inner end 14ue of the cushion layer <NUM> in the axial direction. In the tire <NUM>, a portion of the buffer layer <NUM> at the end 22e and a portion of the cushion layer <NUM> at the inner end 14ue are overlaid and joined to each other. No gap is formed between the buffer layer <NUM> and the cushion layer <NUM>. In the tire <NUM>, occurrence of damage stating from the boundary between the buffer layer <NUM> and the cushion layer <NUM> is effectively suppressed. From this viewpoint, the end 22e of the buffer layer <NUM> is preferably located outward of the inner end 14ue of the cushion layer <NUM>.

In <FIG>, a double-headed arrow LB indicates the length of the overlap portion between the buffer layer <NUM> and the cushion layer <NUM> (hereinafter, referred to as overlap length). The overlap length LB is represented as the shortest distance from the inner end 14ue of the cushion layer <NUM> to the end 22e of the buffer layer <NUM>. When the position of the end 22e of the buffer layer <NUM> coincides with the position of the inner end 14ue of the cushion layer <NUM> in the axial direction, the overlap length LB is <NUM>.

In the tire <NUM>, from the viewpoint of stably maintaining the state where the portion of the buffer layer <NUM> at the end 22e and the portion of the cushion layer <NUM> at the inner end 14ue are joined to each other, the overlap length LB is preferably not less than <NUM> and more preferably not less than <NUM>. From the viewpoint of maintaining a joined state, it is preferable that the overlap length LB is longer, so that a preferable upper limit of the overlap length LB is not set. The upper limit of the overlap length LB is set as appropriate in consideration of the influence on mass.

In the tire <NUM>, the buffer layer <NUM> is joined at the portion at the end 22e thereof to a part of the cushion layer <NUM>. As described above, the cushion layer <NUM> is formed from a flexible crosslinked rubber. If, similar to the cushion layer <NUM>, the buffer layer <NUM> is formed from a flexible crosslinked rubber, it is necessary to increase the thickness TB of the buffer layer <NUM> in order to alleviate strain. The thick buffer layer <NUM> increases the mass of the tire.

From the viewpoint that the buffer layer <NUM> having an appropriate thickness TB is formed and the buffer layer <NUM> can effectively contribute to suppression of a mass increase, the buffer layer <NUM> is preferably formed from a crosslinked rubber that is harder than the crosslinked rubber for the cushion layer <NUM>. In other words, the complex elastic modulus E*b of the buffer layer <NUM> is preferably higher than the complex elastic modulus E*c of the cushion layer <NUM>. Specifically, the ratio (E*b/E*c) of the complex elastic modulus E*b of the buffer layer <NUM> to the complex elastic modulus E*c of the cushion layer <NUM> is preferably not less than <NUM>, more preferably not less than <NUM>, and further preferably not less than <NUM>. The ratio (E*b/E*c) is preferably not greater than <NUM>, more preferably not greater than <NUM>, and further preferably not greater than <NUM>.

As described above, in the tire <NUM>, each end 46e of the full band <NUM> is located inward of the end 40e of the belt <NUM> in the axial direction. The belt <NUM> is wider than the full band <NUM>. The belt <NUM> holds each end 46e of the full band <NUM>. The belt <NUM> contributes to suppression of fluctuation of the tension of the band cord <NUM> included in the full band <NUM>. Occurrence of a break of the band cord <NUM> due to the fluctuation of the tension is suppressed, so that the full band <NUM> can stably exhibit the function of suppressing a shape change. From this viewpoint, each end 46e of the full band <NUM> is preferably located inward of the end 40e of the belt <NUM> in the axial direction.

In the tire <NUM>, in the radial direction, the first belt ply 44A is located inward of the full band <NUM>, and the second belt ply 44B is located outward of the full band <NUM>. The full band <NUM> is interposed between the first belt ply 44A and the second belt ply 44B.

As shown in <FIG>, in the axial direction, the end 46e of the full band <NUM> is located inward of the end 44Ae of the first belt ply 44A, and the end 46e of the full band <NUM> is located inward of the end 44Be of the second belt ply 44B. In other words, the first belt ply 44A is wider than the full band <NUM>. The second belt ply 44B is also wider than the full band <NUM>. In the tire <NUM>, the full band <NUM> is interposed between the first belt ply 44A and the second belt ply 44B which are wider than the full band <NUM>. In the tire <NUM>, fluctuation of the tension of the band cord <NUM> included in the full band <NUM> is more effectively suppressed, so that a break is less likely to occur in the band cord <NUM> 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 44A and the second belt ply 44B in the radial direction, and in the axial direction, the end 46e of the full band <NUM> is located inward of the end 44Ae of the first belt ply 44A, and the end 46e of the full band <NUM> is located inward of the end 44Be of the second belt ply 44B.

As described above, in the tire <NUM>, each end 46e of the full band <NUM> is located outward of the shoulder circumferential groove <NUM> in the axial direction.

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 46e 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 46e 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 46e 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 46e 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>, each end 22e of the buffer layer <NUM> is located inward of the end 46e of the full band <NUM> in the axial direction. The buffer layer <NUM> can effectively contribute to alleviation of strain while contributing to suppression of a mass increase. In the tire <NUM>, the full band <NUM> can stably exhibit the function of suppressing a shape change. From this viewpoint, each end 22e of the buffer layer <NUM> is preferably located inward of the end 46e of the full band <NUM> in the axial direction.

In the tire <NUM>, a breaking load of each first belt cord 48A is preferably not less than <NUM> N. The number of belt plies <NUM> included in the belt <NUM> of the tire <NUM> is smaller than that of the belt of the conventional tire, but the belt <NUM> still effectively contributes to maintaining the strength of the tire <NUM>.

Since the breaking load of each first belt cord 48A is not less than <NUM> N, conventional belt cords having a breaking load of not less than <NUM> N can be used as the second belt cords 48B and the third belt cords 48C in the tire <NUM>. The belt <NUM> contributes to maintaining strength and reducing mass.

The tire <NUM> can suppress a mass increase while maintaining strength.

As is obvious from the above description, according to the present invention, the heavy duty pneumatic tire <NUM> that can achieve suppression of a mass increase and suppression of a shape change due to running, 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., but the present invention is not limited to these examples.

A heavy duty pneumatic tire (tire size = <NUM>/50R22. <NUM>) having the basic structure shown in <FIG> and having specifications shown in Table <NUM> below was obtained.

In Example <NUM>, the belt was composed of three belt plies including a first belt ply, a second belt ply, and a third belt ply. The belt cords of each belt ply were steel cords.

Each of the tilt angle θ1 of the first belt cords, the tilt angle θ2 of the second belt cords, and the tilt angle θ3 of the third belt cords was set to <NUM>° (degrees).

The tilt direction of the first belt cords was opposite to the tilt direction of the second belt cords, and the tilt direction of the second belt cords was the same as the tilt direction of the third belt cords.

The breaking load of each first belt cord was <NUM> N. This is indicated as "Y" in the cell for breaking load in Table <NUM>. Although not described in Table <NUM>, conventional belt cords having a breaking load of <NUM> N were used as the second belt cords and the third belt cords.

A full band was provided between the first belt ply and the second belt ply. This is indicated as "Y" in the cell for FB in Table <NUM>. The band cord of the full band was a steel cord.

A buffer layer was provided between the first belt ply and the carcass ply. This is indicated as "Y" in the cell for buffer layer in Table <NUM>. The complex elastic modulus E*b of the buffer layer was <NUM> MPa. The distance TC between the carcass and the reinforcing layer was <NUM>.

The ratio (WB/W1) of the width WB in the axial direction of the buffer layer to the width W1 in the axial direction of the first belt ply was <NUM>%.

The overlap length LB between the buffer layer and the cushion layer was <NUM>.

The complex elastic modulus E*c of the cushion layer was <NUM> MPa.

A tire of Comparative Example <NUM> was obtained in the same manner as Example <NUM>, except that the full band was removed, the buffer layer was replaced by a tilt ply, and belt cords having a breaking load of <NUM> N were used as the first belt cords. The tire of Comparative Example <NUM> is a conventional tire.

The belt of Comparative Example <NUM> was composed of four belt plies. In Comparative Example <NUM>, a tilt angle θt of the belt cords of the tilt ply was set to <NUM>°. The tilt direction of the belt cords of the tilt ply was the same as the tilt direction of the first belt cords. The breaking load of each belt cord of the tilt ply was <NUM> N. The belt was formed such that the tilt ply had a width in the axial direction equal to the width in the axial direction of the second belt ply.

A tire of Comparative Example <NUM> was obtained in the same manner as Example <NUM>, except that the buffer layer was replaced by a tilt ply, and belt cords having a breaking load of <NUM> N were used as the first belt cords.

Comparative Example <NUM> was configured by adding the full band of Example <NUM> to the tire of Comparative Example <NUM>.

A tire of Comparative Example <NUM> was obtained in the same manner as Example <NUM>, except that no full band was provided, and the position of the inner end of each cushion layer was adjusted such that the overlap length LB was <NUM>.

A tire of Example <NUM> was obtained in the same manner as Example <NUM>, except that the distance TC and the complex elastic modulus E*b were set as shown in Table <NUM> below.

A tire of Example <NUM> was obtained in the same manner as Example <NUM>, except that the position of the inner end of each cushion layer was adjusted such that the overlap length LB was <NUM>.

A tire of Example <NUM> was obtained in the same manner as Example <NUM>, except that the distance TC was set as shown in Table <NUM> below.

A tire of Example <NUM> was obtained in the same manner as Example <NUM>, except that belt cords having a breaking load of <NUM> N were used as the first belt cords.

A test tire was fitted onto a rim (<NUM>×<NUM>) and inflated with air to adjust the internal pressure of the tire to a normal internal pressure. The tire was caused to run on a drum tester at a speed of <NUM>/h for <NUM>, and a profile of the case line on the inner side of the shoulder circumferential groove was obtained. The profile of the case line was compared with the profile of the case line before running to confirm a change in profile before and after running. The results are represented as indexes according to the following ratings in Tables <NUM> and <NUM> below. A higher value represents that a change in profile is reduced. In the running test, a normal load was applied to the tire.

The mass of each test tire was measured. The results are represented in Tables <NUM> and <NUM> below as indexes with the result of Comparative Example <NUM> being regarded as <NUM>. A lower value indicates that the tire is lighter.

Using a plunger testing machine, in accordance with Section <NUM> "Tire strength (breaking energy) test" in JIS D4230, a conical weight (plunger diameter: <NUM>, height: <NUM>, weight: <NUM>) was dropped from above the tread portion, and the dropping height when the tire became broken was converted into energy. The results are represented in Tables <NUM> and <NUM> below as indexes with the result of Comparative Example <NUM> being regarded as <NUM>. The higher the value is, the better the tire strength is.

As shown in Tables <NUM> and <NUM>, in the Examples, suppression of a mass increase and suppression of a shape change due to running 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>) having at least three circumferential grooves (<NUM>) formed thereon;
a pair of sidewalls (<NUM>) each connected to an end of the tread (<NUM>) and located inward of the tread (<NUM>) in a radial direction;
a pair of beads (<NUM>) each located inward of the sidewall (<NUM>) in the radial direction;
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 reinforcing layer (<NUM>) located inward of the tread (<NUM>) in the radial direction; and
a buffer layer (<NUM>) located between the carcass (<NUM>) and the reinforcing layer (<NUM>) in the radial direction, wherein
among the at least three circumferential grooves (<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>) aligned with each other, and a band (<NUM>) including a helically wound band cord (<NUM>),
the belt (<NUM>) includes a first belt ply (44A), a second belt ply (44B), and a third belt ply (44C), wherein the belt ply located on the innermost side in the radial direction is the first belt ply (44A), and
the first belt ply (44A), the second belt ply (44B), and the third belt ply (44C) are aligned in the radial direction in this order,
a direction in which the belt cords (<NUM>) included in the first belt ply (44A) are tilted is opposite to a direction in which the belt cords (<NUM>) included in the second belt ply (44B) are tilted,
the band (<NUM>) includes a full band (<NUM>) located between the first belt ply (44A) and the second belt ply (44B) in the radial direction,
each end (46e) of the full band (<NUM>) is located outward of the shoulder circumferential groove (<NUM>) in the axial direction,
the buffer layer (<NUM>) is formed from a crosslinked rubber, and
a ratio (WB/W1) of a width (WB) in the axial direction of the buffer layer (<NUM>) to a width (W1) in the axial direction of the first belt ply (44A) is not less than <NUM>%.