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. 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.

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. Low-flatness tires having an aspect ratio of not greater than <NUM>% include a tire having a wide tread surface. In a low-flatness tire, a shape change tends to be larger than that in a high-flatness tire. 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.

The band cord included in the full band extends substantially in the circumferential direction. A force acts on the full band of the tire 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.

The 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. When edge bands are added in order to protect a full band, the tire mass further increases.

Further pneumatic tires are known from <CIT>, <CIT>, <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 disposed such that both ends thereof are opposed to each other across an equator plane, and a pair of edge bands each located outward of the end of the full band in the radial direction. The full band is located between the first belt ply and the second belt ply in the radial direction. The buffer layer is formed from a crosslinked rubber.

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, each end of the full band is located outward of the shoulder circumferential groove in the axial direction.

Preferably, in the heavy duty pneumatic tire, a distance in the axial direction from the end of the full band to an inner end of the edge band is not less than <NUM>.

Preferably, in the heavy duty pneumatic tire, each end of the buffer layer is located inward of the inner end of the edge band in the axial direction.

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

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 outward 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 inward of the shoulder circumferential groove <NUM> in the axial direction. From the viewpoint of preventing damage, the third belt ply 44C is disposed such that the end 44Ce of the third belt ply 44C does not overlap the shoulder circumferential groove <NUM> in the radial 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 W1 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> and a pair of edge bands <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 pair of edge bands <NUM> are disposed so as to be spaced apart from each other in the axial direction across the equator plane CL. In the tire <NUM>, the third belt ply 44C which forms a part of the belt <NUM> is located between the pair of edge bands <NUM>.

In the tire <NUM>, each edge band <NUM> is located between the tread <NUM> and the full band <NUM>. The edge band <NUM> is located outward of the end 46e of the full band <NUM> in the radial direction. In the axial direction, an inner end 48ue of the edge band <NUM> is located inward of the end 46e of the full band <NUM>. An outer end 48se of the edge band <NUM> is located outward of the end 46e of the full band <NUM> in the axial direction. The edge band <NUM> overlaps the end 46e of the full band <NUM> in the radial direction. The position of the outer end 48se of the edge band <NUM> may coincide with the position of the end 46e of the full band <NUM> in the axial direction. In this case as well, the edge band <NUM> is located outward of the end 46e of the full band <NUM> in the radial direction.

<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 50A) 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 50B). In the tire <NUM>, the belt <NUM> is formed such that the first belt cords 50A and the second belt cords 50B 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 50C) is the same as the tilt direction of the second belt cords 50B. The tilt direction of the third belt cords 50C may be opposite to the tilt direction of the second belt cords 50B.

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 50A). 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 50B). 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 50C).

In the tire <NUM>, each of the tilt angle θ1 of the first belt cords 50A, the tilt angle θ2 of the second belt cords 50B, and the tilt angle θ3 of the third belt cords 50C 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 50A is more preferably not less than <NUM>° and more preferably not greater than <NUM>°. The tilt angle θ1 of the first belt cords 50A is further preferably not greater than <NUM>°. The tilt angle θ2 of the second belt cords 50B is more preferably not less than <NUM>° and more preferably not greater than <NUM>°. The tilt angle θ2 of the second belt cords 50B is further preferably not greater than <NUM>°. The tilt angle θ3 of the third belt cords 50C 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 50A and the intersection angle of the carcass cords is large.

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, 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. In the tire <NUM>, as a band cord 54F of the full band <NUM> and band cords 54E 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 54F. The full band <NUM> has a jointless structure. In the full band <NUM>, an angle of the band cord 54F relative to the circumferential direction is preferably not greater than <NUM>° and more preferably not greater than <NUM>°. The band cord 54F extends substantially in the circumferential direction.

The density of the band cord 54F 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 54F is represented as the number of cross-sections of the band cord 54F 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 54E. The edge band <NUM> has a jointless structure. In the edge band <NUM>, an angle of the band cord 54E relative to the circumferential direction is preferably not greater than <NUM>° and more preferably not greater than <NUM>°. The band cord 54E of the edge band <NUM> extends substantially in the circumferential direction.

The density of the band cord 54E 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 54E is represented as the number of cross-sections of the band cord 54E included per <NUM> width of the edge band <NUM> in a cross-section of the edge band <NUM> along a plane perpendicular to the direction in which the band cord 54E extends.

<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>. A plurality of 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>.

As described above, in the tire <NUM>, the full band <NUM> is disposed such that both ends 46e thereof are opposed to each other across the equator plane CL. The full band <NUM> extends in the axial direction from the equator plane CL toward each end 46e. Furthermore, in the tire <NUM>, each edge band <NUM> is located outward of the end 46e of the full band <NUM> in the radial direction.

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. 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 54F 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 54F.

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>, each edge band <NUM> holds the end 46e of the full band <NUM>. Fluctuation of the tension of the band cord 54F included in the full band <NUM> is suppressed, so that occurrence of a break of the band cord 54F 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. The edge band <NUM> is narrower than the full band <NUM>. Therefore, tension fluctuation as in the full band <NUM> is less likely to occur in the band cord 54E of the edge band <NUM>. A break is less likely to occur in the band cord 54E of the edge band <NUM>.

Furthermore, 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 54F 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 50A 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 50A, 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 50A.

In the tire <NUM>, 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> and the pair of edge bands <NUM>, that is, the 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> and the pair of edge bands <NUM> are 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 50A 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 50A. 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. In the tire <NUM>, 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>.

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

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

In the tire <NUM>, each end 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>. Since each edge band <NUM> is located outward of the end 46e of the full band <NUM>, this deformation is sufficiently suppressed. In particular, in the tire <NUM>, a shape change around each shoulder circumferential groove <NUM> is effectively suppressed. From this viewpoint, in the tire <NUM>, each end 46e of the full band <NUM> is preferably 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 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 54F 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>. Fluctuation of the tension of the band cord 54F 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>, the inner end 48ue of each edge band <NUM> is located inward of the end 46e of the full band <NUM> in the axial direction. In <FIG>, a length indicated by a double-headed arrow We is the distance in the axial direction from the end 46e of the full band <NUM> to the inner end 48ue of the edge band <NUM>.

In the tire <NUM>, the distance We in the axial direction from the end 46e of the full band <NUM> to the inner end 48ue of the edge band <NUM> is preferably not less than <NUM>. Accordingly, the edge band <NUM> effectively holds the end 46e of the full band <NUM>. Fluctuation of the tension of the band cord 54F included in the full band <NUM> is suppressed, so that 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 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 of the shoulder circumferential groove <NUM> is taken into consideration for setting the position of the inner end 48ue of the edge band <NUM>. From the viewpoint of effectively suppressing occurrence of damage starting from the bottom of the shoulder circumferential groove <NUM>, in the axial direction, the inner end 48ue of the edge band <NUM> is preferably located outward of the bottom 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 48ue of the edge band <NUM> may be located inward of the bottom of the shoulder circumferential groove <NUM> in the axial direction. In this case, the inner end 48ue of the edge band <NUM> is more preferably located further inward of the shoulder circumferential groove <NUM> in the axial direction.

In the tire <NUM>, 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 preferably 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. From this viewpoint, the ratio (WB/W1) is more preferably not less than <NUM>% and further preferably not less 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 starting 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 the axial direction. In this case, from the viewpoint of appropriately maintaining the volume of the cushion layer <NUM> and effectively suppressing the influence of the cushion layer <NUM> on the mass of the tire <NUM>, as described above, 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 more preferably set to be not less than <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 that the buffer layer <NUM> is formed with an appropriate width and the buffer layer <NUM> can effectively contribute to suppression of a mass increase, the overlap length LB is preferably not greater than <NUM> and more preferably not greater than <NUM>.

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>.

For example, as shown in <FIG>, in a zone of the tire <NUM> from the inner end 48ue of the edge band <NUM> to the end 46e of the full band <NUM>, the full band <NUM> and the edge band <NUM> overlap each other in the radial direction. In the band <NUM> of the tire <NUM>, the zone from the inner end 48ue of the edge band <NUM> to the end 46e of the full band <NUM> has high stiffness. Therefore, if the buffer layer <NUM>, which is formed from a crosslinked rubber, is disposed inward of this zone, there is a concern that the distribution of strain generated between the carcass ply <NUM> and the first belt ply 44A may become uneven and damage may occur at the position corresponding to the inner end 48ue of the edge band <NUM>.

In the tire <NUM>, the end 22e of the buffer layer <NUM> is located inward of the inner end 48ue of the edge band <NUM> in the axial direction. Since the buffer layer <NUM> is not disposed within the zone from the inner end 48ue of the edge band <NUM> to the end 46e of the full band <NUM>, occurrence of damage due to strain generated between the carcass ply <NUM> and the first belt ply 44A is effectively suppressed in the tire <NUM>. From this viewpoint, the end 22e of the buffer layer <NUM> is preferably located inward of the inner end 48ue of the edge band <NUM> in the axial direction.

In <FIG>, a length indicated by a double-headed arrow DB is the distance in the axial direction from the end 22e of the buffer layer <NUM> to the inner end 48ue of the edge band <NUM>.

From the viewpoint of effectively suppressing occurrence of damage, the distance DB in the axial direction is preferably not less than <NUM> and more preferably not less than <NUM>. In the tire <NUM>, as described above, from the viewpoint of alleviating strain generated between the carcass ply <NUM> and the first belt ply 44A, 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 preferably set to be not less than <NUM>%. Therefore, a preferable upper limit of the distance DB in the axial direction is not set.

In the tire <NUM>, a breaking load of each first belt cord 50A 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 50A 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 50B and the third belt cords 50C 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 band was composed of a full band and a pair of edge bands. The fact that the band includes the full band is indicated as "Y" in the cell for FB in Table <NUM>, and the fact that the band includes the edge bands is indicated as "Y" in the cell for EB in Table <NUM>. The band cords of the full band and the edge bands were steel cords.

The full band was provided between the first belt ply and the second belt ply, and each edge band was disposed radially outward of the end of the full band.

The distance We in the axial direction from the end of the full band to the inner end of the edge band was <NUM>.

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 distance DB in the axial direction from the end of the buffer layer to the inner end of the edge band 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 buffer layer was replaced by a tilt ply. 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 belt cords of the tilt ply were steel cords. 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 no edge band was provided.

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 the complex elastic modulus E*b of the buffer layer was set as shown in Table <NUM> below.

Tires of Examples <NUM> and <NUM> were obtained in the same manner as Example <NUM>, except that the position of the inner end of each edge band was changed such that the distance We in the axial direction and the distance DB in the axial direction 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 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. In this evaluation, the index is required to be <NUM> or higher.

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 mounted to a drive shaft of a test vehicle (tractor head). A trailer loaded with luggage was towed by the test vehicle, and the test vehicle was caused to run on a test course composed of an asphalt road surface, until the wear rate of the tire reached <NUM>% in terms of mass. After running, the tire was inspected by shearography or X-ray to confirm the presence/absence of internal damage. When internal damage was confirmed, the tire was disassembled and it was confirmed whether this internal damage was a break of the band cord of the full band. The results are represented in Tables <NUM> and <NUM> below according to the following ratings. The result is required not to correspond to the rating D.

The mass of each test tire was measured. The results are represented in Tables <NUM> and <NUM> below as indexes with the result of Example <NUM> being regarded as <NUM>. A lower value indicates that the tire is lighter. If the index indicating the result is <NUM> or lower, it is determined that an increase in mass is suppressed, and the mass is acceptable.

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 Example <NUM> being regarded as <NUM>. The higher the value is, the better the tire strength is. The index indicating the result is required to be <NUM> or higher.

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 (50B) included in the second belt ply (44B) are tilted,
the band (<NUM>) includes a full band (<NUM>) disposed such that both ends (46e) thereof are opposed to each other across an equator plane (CL), and a pair of edge bands (<NUM>) each located outward of the end (46e) of the full band (<NUM>) in the radial direction,
the full band (<NUM>) is located between the first belt ply (44A) and the second belt ply (44B) in the radial direction, and
the buffer layer (<NUM>) is formed from a crosslinked rubber.