PNEUMATIC TIRE TREAD AND PNEUMATIC TIRE HAVING SAID TREAD

BACKGROUND 1. Field

The present disclosure relates to a pneumatic tire tread and to a pneumatic tire having said tread, and in particular the present disclosure relates to a pneumatic tire tread having improved performance on snow and performance on ice by virtue of a reinforcing part provided on the frontal side wall of a block, and to a pneumatic tire having said tread.

2. Description of Related Art

Winter tires which are also called “studless” tires are well known as tires which can travel on winter road surfaces covered with snow or ice. Winter tires are generally provided with a plurality of narrow incisions known as sipes which open at the contact surface, and adhesion to a road surface in winter is improved by means of what is known as an “edge” effect and a water film-removal effect, and also by using a compound which is softer than that used for tires which are not for winter use.

The mechanism by which frictional force is generated with the road surface in a winter tire actually differs depending on whether the road surface is snowy or icy, so it is known that even if a soft compound is used and a large number of narrow incisions are provided in blocks which are the ground-contact elements in order to improve performance on ice, there will be a reduction in block rigidity as a result, and this will hinder any improvement in the performance on snow.

It is known that the introduction of reinforcing parts onto the side walls of blocks is effective as a means for achieving good performance on ice and good performance on snow at the same time.

For example, JP 7-047814 A (mainly FIG. 3) describes a pneumatic tire in which a balance between performance on snow and performance on ice is achieved by providing reinforcing parts employing a rubber having a JIS A hardness of 80 to 95 degrees on block side walls facing a transverse groove and an auxiliary transverse groove, in a block provided with three narrow incisions and one auxiliary groove.

Furthermore, JP 2010-105509 A (mainly FIG. 2) describes technology in which a balance between performance on snow and performance on ice is achieved by using a composition in which at least 50 parts by weight of carbon black and/or silica are combined with 100 parts by weight of a diene rubber containing 30 wt % or more of a rubber component having a glass transition temperature of −60° C. or more, and by providing reinforcing parts employing rubber having a brittleness temperature of −30° C. or less on the side walls of the blocks.

Furthermore, PCT/JP2011/079188 (WO 2013/088570 pamphlet) (mainly FIG. 1), which is a prior application, describes a pneumatic tire tread which achieves a balance between performance on snow and performance on ice by virtue of the fact that reinforcing layers (reinforcing parts) having a material modulus (elastic modulus) of 200 MPa or greater are provided to a thickness of less than 0.5 mm over a region of at least 50% of the block side walls.

As a means for simultaneously achieving good performance on ice and performance on snow, JP 2009-292229 A (mainly FIG. 3) describes a pneumatic tire which achieves a balance between performance on snow and performance on ice by virtue of the fact that blocks are inclined in the direction of rotation.

SUMMARY
Problem to be Solved

However, it is difficult to achieve a high-level balance between performance on snow and performance on ice with the pneumatic tires described in Patent Documents 1, 2, 3 and 4, and improvement in the performance on snow in particular is inadequate, and there is a need for a pneumatic tire which can achieve a higher-level balance between performance on snow and performance on ice from the point of view of safety of travel on winter road surfaces.

The present invention, in an embodiment, is intended to solve the problems of the references described above, and the aim thereof lies in providing a pneumatic tire tread provided with a reinforcing part on the side wall of a block, which can achieve a higher-level balance between performance on snow and performance on ice, and also in providing a pneumatic tire having such a tread.

Means for Solving the Problem

In order to achieve the abovementioned aim, the present invention, in an embodiment, provides a pneumatic tire tread formed by means of at least one rubber composition, characterized in that: the at least one rubber composition has an elastic modulus Et; the tread comprises: at least one circumferential main groove, a plurality of auxiliary grooves, and a plurality of blocks which are defined by the circumferential main groove and the auxiliary grooves; at least one of the blocks from among the plurality of blocks comprises: an upper surface constituting a contact surface at least partly in contact with a road surface when the tire is rolling, two frontal side walls positioned along the tire circumferential direction, and two lateral side walls positioned along the tire axial direction; the upper surface of the block has two frontal edges formed at positions intersecting the two frontal side walls; the block has a reinforcing part provided on at least one of the two frontal side walls; the reinforcing part is formed by means of a material having an elastic modulus Ef, and the elastic modulus Ef of the reinforcing part and the elastic modulus Et of the rubber composition have values obtained from the tensile test defined in the standard ASTM D882-09, and the elastic modulus Ef of the reinforcing part is at least 20 times greater than the elastic modulus Et of the rubber composition; the reinforcing part of the block has a mean thickness of between 0.1 mm and 2.0 mm, and is provided in such a way as to face at least the auxiliary groove over a region of at least 70% of the frontal side wall; and the two frontal side walls are formed in such a way that the angles thereof with respect to the upper surface form angles T1 and T2, respectively, the angles T1 and T2 both being less than 90°.

Here, “groove” refers to a space having a width and a depth which is constructed by connecting two opposing surfaces (wall surfaces, side walls) which do not come into contact with each other under normal usage conditions, by means of another surface (bottom surface).

Furthermore, “main groove” refers to a groove which is mainly responsible for fluid drainage and has a relatively large width among the various types of grooves formed in the tread. In many cases, “main groove” means a groove extending in a linear, zigzag or undulating manner in the tire circumferential direction, but a groove having a relatively large width which is mainly responsible for fluid drainage and extends at an angle with respect to the direction of rotation of the tire is also included.

Furthermore, grooves other than the “main groove” are referred to as “auxiliary grooves”.

Furthermore, “edge” refers to the intersection between the upper surface of a block and the frontal side wall or lateral side wall (the edge parts on the upper surface of the block or the boundary on the upper surface of the block with the frontal side wall or lateral side wall). The upper surface of the block which forms part of the contact surface is defined by edges such as these. If a bevel is formed between the upper surface and the frontal side wall or lateral side wall, the bevelled part is understood as being part of the upper surface. The intersection between the upper surface of the block and the frontal side wall in the direction of rotation is referred to as the “frontal edge”.

Furthermore, “elastic modulus” refers to the elastic modulus in tension E calculated from a tensile test curve obtained from the tensile test defined in the standard ASTM D882-09. That is to say, the elastic modulus Et of the rubber composition and the elastic modulus Ef of the reinforcing part are calculated from a tensile test curve obtained from the tensile test defined in the standard ASTM D882-09. The elastic modulus in tension E has the following relationship with the elastic shear modulus G, as described in “POLYMER PHYSICS” (Oxford, ISBN 978-0-19-852059-7, Chapter 7.7, Page 296), for example.

E=2G(1+v)

Here, v is Poisson's ratio, the Poisson's ratio of rubber material being a value very close to 0.5.

Moreover, when it is being confirmed that the elastic modulus Ef of the material forming the reinforcing part is at least 20 times greater than the elastic modulus Et of the rubber composition forming the tread, this confirmation can also be made by substituting the abovementioned elastic modulus Et and elastic modulus

Ef with the complex elastic modulus (dynamic shear modulus: G* of the material) M. The storage elastic modulus represented by G′ and the loss elastic modulus represented by G″, which are known dynamic properties, are measured by means of a viscosity analyser (viscoanalyser: Metravib VB4000) using a test piece moulded from the raw composition or a test piece which is combined with the composition after vulcanization. The test piece which is used is described in Figure X2.1 (a circular method) of the standard ASTM D 5992-96 (version published September 2006, initially approved in 1996). The diameter “d” of the test piece is 10 mm (consequently the test piece has a circular cross section of 78.5 mm²), the thickness “L” of each part of the rubber compound is 2 mm, and the ratio “d/L” (described in paragraph X2.4 of the ASTM standard, unlike the ratio “d/L” of 2 recommended in the standard ISO 2856) is 5. In the test, the response of a test piece comprising a vulcanized rubber composition subjected to a simple alternating sinusoidal shear load is measured at a frequency of 10 Hz. The maximum shear stress applied during the test is 0.7 MPa. The measurement is taken by varying the temperature from Tmin, which is a temperature lower than the glass transition temperature (Tg) of the rubber material, to a maximum temperature Tmax in the vicinity of 100° C., at a rate of 1.5° C. per minute. The test piece is stabilized for approximately 20 minutes at Tmin prior to the start of the test in order to obtain a satisfactory uniformity of temperature within the test piece. The results obtained are the storage elastic modulus (G′) and the loss elastic modulus (G″) at the prescribed temperature. The complex elastic modulus G* is defined in terms of the absolute values of the storage elastic modulus and the loss elastic modulus using the following formula:

G*=√{square root over (G′²+G″²)}

According to an embodiment of the present invention having the configuration described above, the angles T1, T2 formed by the frontal side walls and the upper surface are both less than 90°, so when the tire is travelling on a road surface for which the coefficient of friction is sufficient to cause deformation of the ground-contact elements, such as a snowy road surface, it is possible to prevent buckling deformation of the blocks by virtue of the effect of the reinforcing part provided over a region of 70% or more of at least one frontal side wall, and as a result it is possible to achieve locally high edge pressure. This makes it possible for the frontal edge to bite effectively into the snow, and as a result it is possible to improve the performance on snow.

In addition, according to an embodiment of the present invention, the angles T1, T2 formed by the frontal side walls and the upper surface are both less than 90°, so when the tire is travelling on a road surface for which the coefficient of friction is insufficient to cause deformation of the ground-contact elements, such as an icy road surface, a moment force in a direction producing a reduction in the ground-contact pressure can be generated in the vicinity of the frontal edge part of the block. Accordingly, it is possible to prevent the formation of a water film between the tread and the ice, which is well known as a cause of reducing the friction coefficient on ice, and as a result it is possible to improve the performance on ice.

According to an embodiment of the present invention, the angles T1 and T2 are both preferably 85° or less.

According to an embodiment of the present invention, the angles T1 and T2 are both preferably 60° or greater.

According to an embodiment of the present invention having the configuration described above, it is possible to ensure that the blocks are sufficiently rigid to exhibit various aspects of performance required of a tire.

According to an embodiment of the present invention, the angles T1 and T2 are both more preferably 70° or greater.

According to an embodiment of the present invention, T1 and T2 are preferably angles which are different from each other.

According to an embodiment of the present invention, the angle T2 is preferably greater than the angle T1.

According to an embodiment of the present invention having the configuration described above, when the tire is rolling, the frontal edge on the side of the angle T2, which is a relatively larger angle, makes it possible to reduce the moment force in a direction producing a reduction in the contact surface area of the block generated in the vicinity of the frontal edge on the trailing side, especially when said frontal edge lies on the trailing side, so it is possible to effectively increase the contact surface area of the block. As a result, it is possible to improve the performance on ice.

Here, the “leading-side frontal edge” refers to the frontal edge on the side of the direction of rotation of the tire as it rolls, while on the other hand, the “trailing-side frontal edge” refers to the frontal edge on the opposite side in the direction of rotation of the tire as it rolls.

According to an embodiment of the present invention, the angles T1 and T2 preferably satisfy the following relationship:

T
₂
≧T
₁+5°.

According to an embodiment of the present invention having the configuration described above, it is possible to more effectively achieve a balance between performance on ice and performance on snow.

According to an embodiment of the present invention, the angle T1 is preferably 80° or less.

According to an embodiment of the present invention having the configuration described above, when the tire is travelling on a road surface for which the coefficient of friction is sufficient to cause deformation of the ground-contact elements, such as a snowy road surface, the performance on snow is improved as a result of the fact that a high edge pressure for causing the frontal edge to bite sufficiently into the snow is produced especially if the frontal edge on the angle T1 side is the leading-side frontal edge, and when the tire is travelling on a road surface for which the coefficient of friction is insufficient to cause deformation of the ground-contact elements, such as an icy road surface, then it is possible to improve the performance on ice by preventing generation of excessively high ground-contact pressure at the frontal edge.

According to an embodiment of the present invention, the angle T2 is preferably between 75° and 85°.

According to an embodiment of the present invention having the configuration described above, when the tire is rolling it is possible to reduce the moment force in a direction producing a reduction in the contact surface area of the block generated in the vicinity of the trailing-side frontal edge, especially if the frontal edge on the angle T2 side is the trailing-side frontal edge, so it is possible to increase the contact surface area of the block and as a result the performance on ice can be improved and an improvement in the performance on snow can also be envisaged.

According to an embodiment of the present invention, the reinforcing part provided on the frontal side wall is preferably provided in such a way as to extend at least partly in the width direction of the frontal edge.

According to an embodiment of the present invention having the configuration described above, it is possible to more reliably produce a high edge pressure at the frontal edge part when the tire is travelling on a road surface for which the coefficient of friction is sufficient to cause deformation of the ground-contact elements, such as a snowy road surface, which means that the frontal edge can be made to bite more reliably into the snow, and as a result it is possible to further improve the performance on snow.

According to an embodiment of the present invention, the reinforcing part is preferably provided over a region of at least equal to 90% of the at least one frontal side wall.

According to an embodiment of the present invention having the configuration described above, it is possible to more reliably produce a high edge pressure at the frontal edge part when the tire is travelling on a road surface for which the coefficient of friction is sufficient to cause deformation of the ground-contact elements, such as a snowy road surface, which means that the frontal edge can be made to bite more reliably into the snow. As a result it is possible to further improve the performance on snow.

According to an embodiment of the present invention, the reinforcing part is more preferably provided over the whole region of the at least one frontal side wall, and even more preferably the reinforcing part is provided over the whole region of the two frontal side walls.

According to an embodiment of the present invention having the configuration described above, it is possible to more reliably improve the performance on snow.

Advantage of the Invention

The pneumatic tire tread and pneumatic tire having such a tread according to an embodiment of the present invention make it possible to achieve a higher-level balance between performance on snow and performance on ice.

BRIEF DESCRIPTION OF THE FIGURES

[FIG. 1] is a perspective view schematically showing a pneumatic tire tread according to a first mode of embodiment of the present invention;

[FIG. 2] is an enlargement in cross section of the block of the pneumatic tire tread seen along the line II-II in FIG. 1;

[FIG. 3] is a perspective view schematically showing a pneumatic tire tread according to a second mode of embodiment of the present invention;

[FIG. 4] is an enlargement in cross section of the block of the pneumatic tire tread seen along the line IV-IV in FIG. 3; and

[FIG. 5] is an enlargement in cross section of the block of a conventional pneumatic tire tread.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The pneumatic tire tread and pneumatic tire employing said tread according to preferred modes of embodiment of the present invention will be described below with reference to the appended figures.

The pneumatic tire tread according to a first mode of embodiment of the present invention will be described first of all with the aid of FIG. 1 and FIG. 2. FIG. 1 is a perspective view schematically showing the pneumatic tire tread according to the first mode of embodiment of the present invention, and FIG. 2 is an enlargement in cross section of the block of the pneumatic tire tread seen along the line II-II in FIG. 1.

First of all, as shown in FIG. 1, the reference symbol 1 is a pneumatic tire tread according to the first mode of embodiment of the present invention. It should be noted that the size of the pneumatic tire to which the pneumatic tire tread 1 is applied in this example is 205/55R16.

The overall structure of the tread 1 will be described next with the aid of FIG. 1 and FIG. 2.

The tread 1, which comprises a rubber composition having an elastic modulus Et, has a contact surface 2 which comes into contact with the road surface when the tire is rolling, and two circumferential main grooves 3 and a plurality of auxiliary grooves 4 are formed therein. A plurality of blocks 5 are defined by the circumferential main grooves 3 and auxiliary grooves 4.

The blocks 5 comprise: an upper surface 51 forming part of the contact surface 2; two side walls (frontal side walls) 52, 53 which are positioned longitudinally along the tire circumferential direction and are formed in such a way as to face the auxiliary grooves 4; and two side walls (lateral side walls) 54, 55 which are positioned transversely along the direction of the tire axis of rotation and are formed in such a way as to face the circumferential grooves 3.

Frontal edges 521, 531 are formed on the upper surface 51 at edge parts intersecting the frontal side walls 52, 53.

Next, a reinforcing part 6 comprising a material having an elastic modulus Ef which is at least 20 times greater, and preferably at least 50 times greater than the elastic modulus Et of the rubber composition forming the tread 1 is provided on the two frontal side walls 52, 53. In this mode of embodiment, the elastic modulus Et of the rubber composition forming the tread is 5.4 MPa. The elastic modulus Et is preferably between 1.5 MPa and 15 MPa. Furthermore, in this mode of embodiment, the elastic modulus Ef of the material forming the reinforcing part 6 is 270 MPa. This means that the elastic modulus Ef of the rubber composition forming the tread 1 is formed in such a way as to be 50 times greater than the elastic modulus Et of the material forming the reinforcing part 6.

Here, the elastic modulus Et of the rubber composition forming the tread 1 and the elastic modulus Ef of the material forming the reinforcing part 6 can be calculated from a tensile test curve obtained from the tensile test defined in the standard ASTM D882-09.

The arrangement of the reinforcing parts 6 on the blocks 5 of the tread 1 will be described next.

According to this mode of embodiment, the reinforcing parts 6 are provided in such a way as to face the auxiliary grooves 4 over at least equal to 70% of the region of the frontal side walls 52, 53, and preferably over the whole region of the frontal side walls 52, 53.

Furthermore, the reinforcing parts 6 are provided in such a way that the mean thickness t thereof (shown in FIG. 2) is less than 2.0 mm and preferably less than 1.0 mm. Here, the thickness (t) of the reinforcing parts 6 constitutes the thickness in a direction perpendicular to the surface of the frontal side walls 52, 53 on which the reinforcing parts 6 are provided facing the auxiliary grooves 4, and the “mean thickness” of the reinforcing parts 6 is the mean value of the reinforcing parts 6 measured from the bottom surface side of the auxiliary grooves 4 to the upper surface 51 side of the blocks 5, in other words the mean value over essentially the whole surface of the reinforcing parts 6. According to this mode of embodiment, the reinforcing parts 6 are provided over the whole region (100%) of the frontal edges 521, 531 and the frontal side walls 52, 53, and the mean thickness t is 0.5 mm. Here, the mean thickness t of the reinforcing parts 6 is preferably at least equal to 0.2 mm.

The upper surface 51 of the blocks 5 of the tread 1 will be described next.

The upper surface 51 forms part of the contact surface 2 of the tread 1 which comes into contact with the road surface when the tire is rolling, the upper surface 51 being defined as the region of the block 5 which can partly come into contact with the road surface under specific conditions. The upper surface 51 comprises two circumferential edges (frontal edges) 521, 531 in the tire circumferential direction, and that region of the upper surface 51 is bound by the circumferential edges 521, 531. In other words, the upper surface 51 comprises the two circumferential edges 521, 531 at each edge on the tire circumferential direction side.

The dimensional relationship of the reinforcing part 6 of the blocks 5 of the tread 1 and the frontal side walls 52, 53 on which the reinforcing part 6 is provided will be described next.

According to this mode of embodiment, the frontal side walls 52, 53 on which the reinforcing part 6 is provided are formed in such a way that the distance between the position on the outermost side of the reinforcing part 6 measured in the radial direction (the edge part on the outside in the radial direction of the reinforcing part 6) and the frontal edges 521, 531 is less than or equal to 2.0 mm. The reinforcing part 6 is preferably formed in such a way as to at least partly include the frontal edges 521, 531, and more preferably in such a way as to include the whole of the frontal edges 521, 531.

In the example shown in FIG. 2, the distance between the position on the outermost side of the reinforcing part 6 in the radial direction and the frontal edges 521, 531 is zero (0 mm), and the edge parts on the outermost side in the radial direction of the reinforcing part 6 provided on the frontal side walls 52, 53 are provided in such a way as to be present over the whole of the frontal edges 521, 531 in the width direction. On the other hand, the edge parts on the outermost side in the radial direction of the reinforcing part 6 provided on the frontal side walls 52, 53 may be provided in such a way as to be at least partly present on the frontal edges 521, 531 in the width direction of the frontal edges 521, 531.

Furthermore, the reinforcing part 6 is provided only in a partial region of the frontal side walls 52, 53 on the block 5 (this partial region constitutes at least equal to 70% of the region of the frontal side walls 52, 53, as described above), but the reinforcing part 6 is preferably present over the whole region of the frontal side walls 52, 53 in order to maximize the advantage thereof. This kind of reinforcing part 6 would of course be provided in such a way as to include the whole of the frontal edges 521, 531 in the same way as in this mode of embodiment.

The dimensional relationship of the upper surface 51 of the blocks 5 and the frontal side walls 52, 53 will be described next with the aid of FIG. 2.

In this mode of embodiment, the angles T1 and T2 are formed between the upper surface 51 of the block 5 and the two frontal side walls 52, 53, as shown in FIG. 2, as viewed in a cross section perpendicular to the axis of rotation of the tire. The angles T1 and T2 are both less than 90° (not including 90°). Furthermore, the angles T1 and T2 are preferably both 85° or less, preferably both 70° or greater, and preferably both 60° or greater.

In the example shown in FIG. 2, the abovementioned angles T1 and T2 are both the same at 70°.

Here, the angle T1 is measured by the angle between a straight line on the upper surface 51 joining one of the frontal edges 521 and the other frontal edge 531 in the tire circumferential direction (the straight line seen in the cross section of FIG. 2), and a straight line joining the frontal edge 521 on the front surface wall 52 and a position at a height 1.6 mm further inwards in the radial direction than the height of a tread wear indicator in a direction perpendicular to the frontal edge 521.

The angle T2 is likewise measured by the angle between a straight line on the upper surface 51 joining one of the frontal edges 521 and the other frontal edge 531 in the tire circumferential direction (the straight line seen in the cross section of FIG. 2), and a straight line joining the frontal edge 531 on the front surface wall 53 and a position at a height 1.6 mm further inwards in the radial direction than the height of a tread wear indicator in a direction perpendicular to the frontal edge 531.

It should be noted that, for example, if the frontal edges (521, 531) extend obliquely with respect to the direction of the axis of rotation of the tire (if the auxiliary grooves 4 extend obliquely with respect to the direction of the axis of rotation of the tire), a “line joining in the tire circumferential direction” constitutes a “line joining in a direction perpendicular to the direction of extension of the frontal edges”.

Furthermore, for example, the abovementioned mode of embodiment is also applicable if the frontal edges are formed on the block (5) in such a way that one or both of the frontal edges (521, 531) comprise(s) at least two sides, in which case a measurement is taken on a “line joining in the tire circumferential direction” or on a “line joining in a direction perpendicular to the mean direction of extension of the frontal edges” in the same way as with the method described above, taking as a reference the mean direction of extension of the two sides on the upper surface (51).

Moreover, the tread wear indicator is a device for showing the tire wear limit.

The action and effect afforded by the pneumatic tire tread according to the abovementioned first mode of embodiment of the present invention will be described next.

When a vertical load is applied to the blocks (5) while the tire is rolling, buckling deformation such as to reduce the edge pressure exerted on the frontal edges (521, 531), which is undesirable for improving the performance on snow, is produced on the blocks (5). This phenomenon is more pronounced when the tire is travelling on a road surface for which the coefficient of friction is sufficient to cause deformation of the ground-contact elements, such as a snowy road surface, due to the drive force generated in the tire direction of rotation or braking force.

In the light of this phenomenon, the reinforcing part 6 is provided over most of the region of the frontal side walls 52, 53 of the tread 1 according to this mode of embodiment, so it is possible to prevent the generation of buckling deformation of the blocks 5 on a snowy road surface, while it is also possible to generate high edge pressure at the frontal edges 521, 531, and as a result it is possible to improve the performance on snow.

At the same time, when the tire is travelling on a road surface for which the coefficient of friction is insufficient to cause deformation of the ground-contact elements, such as an icy road surface, then if a drive force generated in the direction of rotation of the tire or braking force is applied, there is a restriction on a high edge pressure being generated at the frontal edges 521, 531 even though the reinforcing part 6 is provided over most of the region of the frontal side wall 52, 53, as in this mode of embodiment. In addition, the angles T1 and T2 are formed in such a way as to be less than 90°, so when the tire is travelling on a road surface for which the coefficient of friction is insufficient to cause deformation of the ground-contact elements, such as an icy road surface, a moment force acting in a direction producing a geometric reduction in the edge force at the frontal edge parts 521, 531 is generated on the blocks 5. Accordingly, with the tread 1 according to this mode of embodiment, it is possible to prevent the formation of a water film between the tread and the ice, which is well known as a cause of reducing the friction coefficient on ice, and as a result it is possible to improve the performance on ice.

A variant example of the pneumatic tire tread according to the first mode of embodiment of the present invention will be described next.

The mean thickness t of the reinforcing part 6 is less than 2.0 mm, preferably 1.0 mm or less, and more preferably 0.5 mm or less. The mean thickness t of the reinforcing parts 6 may be different for the frontal side walls 52, 53 on the same block 5.

Furthermore, the reinforcing parts 6 provided in a region of at least equal to 70% of the frontal side walls 52, 53 as described above may be provided over different proportions of regions on the frontal side walls 52, 53 on the same block 5.

In addition to the abovementioned material based on natural resin (including rubber material) it is equally possible to use, as the material of the reinforcing part 6, a material in which fibers are mixed or impregnated with a material based on natural resin, thermoplastic resins, or lamination or mixture thereof. It is also possible to use the abovementioned materials in combination with a woven fabric or nonwoven fabric etc. impregnated with a material based on natural resin with the aim of improving adhesion with the blocks 5 or providing further reinforcement of the blocks 5. These fiber materials such as woven fabric or nonwoven fabric etc. impregnated with a material based on natural resin may be used alone as the reinforcing part 6. Furthermore, different materials may be used for the frontal side walls 52, 53 on the same block 5.

Furthermore, the bottom surface of the auxiliary grooves 4 is not covered by the reinforcing part 6 in this mode of embodiment, but it is equally possible to adopt an arrangement in which the edge part of the reinforcing part 6 on the inside of the tire in the radial direction is extended so that the reinforcing part 6 covers part or all of the bottom surface of the grooves 3, 4, with the aim of improving producibility etc. when the reinforcing part 6 is provided.

Furthermore, the reinforcing parts 6 are provided only on the frontal side walls 52, 53 of the blocks facing the auxiliary grooves 4 in this mode of embodiment, but the reinforcing parts 6 may also be provided on the side walls (lateral side walls) 54, 55 of the blocks facing the circumferential main grooves 3 in the same way. This mainly makes it possible to improve the effect of performance on snow in the tire width direction afforded by the reinforcing parts 6 provided on the lateral side walls 54, 55, and in particular makes it possible to improve the steering performance.

A pneumatic tire tread according to the second mode of embodiment of the present invention will be described next with the aid of FIGS. 3 and 4. FIG. 3 is a perspective view schematically showing a pneumatic tire tread according to the second mode of embodiment of the present invention; and FIG. 4 is an enlargement in cross section of the block of the pneumatic tire tread seen along the line IV-IV in FIG. 3.

As shown in FIG. 3, the tread 1 according to the second mode of embodiment comprises the contact surface 2 which comes into contact with the road surface when the tire is rolling, and the two circumferential main grooves 3 and the plurality of auxiliary grooves 4 are formed therein in the same way as in the first mode of embodiment described above. The plurality of blocks 5 are defined by the circumferential main grooves and auxiliary grooves. The blocks 5 comprise: the upper surface 51 forming part of the contact surface 2; the two side walls (frontal side walls) 52, 53 which are separated in the longitudinal direction corresponding to the tire circumferential direction; and the two side walls (lateral side walls) 54, 55 which are separated in the transverse direction corresponding to the tire axial direction. The upper surface 51 intersects the frontal side walls 52, 53 and the frontal edges 521, 531 are formed at the intersections.

Furthermore, a narrow incision 7 which opens at the upper surface 51 and extends in the tire width direction while also extending in the tire radial direction (or essentially in the radial direction) is formed in the block 5 of the tread 1 according to the second mode of embodiment. The narrow incision 7 also opens at the lateral side walls 54, 55. It should be noted that the narrow incision 7 may extend over a predetermined angle with respect to the radial direction in a range such that various functions are exhibited. Here, the “tire width direction” means a direction perpendicular to the tire circumferential direction in this mode of embodiment, but also includes a direction extending obliquely through a predetermined angle with respect to the tire circumferential direction. The reinforcing parts 6 are provided on the two frontal side walls 52, 53.

According to this mode of embodiment, the reinforcing parts 6 are provided in such a way as to face the auxiliary grooves 4 over a region of at least equal to 70% and preferably at least equal to 90% of the frontal side walls 52, 53. Furthermore, the mean thickness t of the reinforcing parts 6 (see FIG. 4) is provided in such a way as to be less than 2.0 mm and preferably less than 1.0 mm. The frontal side walls 52, 53, on which the reinforcing parts 6 are provided, are provided in such a way that the distance between the position at the outermost side of the reinforcing parts measured in the radial direction and the frontal edges 521, 531 is less than or equal to 2.0 mm. In the example shown in FIG. 3, a reinforcing part 6 is provided over the whole region, i.e. a region of 100%, of one of the frontal side walls 52, and is provided over a region of 90% of the other frontal side wall 53. The mean thickness t thereof is 0.7 mm on one of the frontal side walls 52 and 0.5 mm on the other frontal side wall 53.

The frontal edges 521, 531 are formed on the upper surface 51 of the blocks 5 in the same way as in the first mode of embodiment described above, and the range of the circumferential region of the upper surface 51 is restricted by the two circumferential edges (frontal edges) 521, 531 in the tire circumferential direction. That is to say, the upper surface 51 comprises the two circumferential edges 521, 531 at the edge parts thereof on the tire circumferential direction side.

According to this mode of embodiment, as shown in FIG. 4, the blocks 5 are formed in the same way as in the first mode of embodiment, in such a way that the angles T1 and T2 are formed between the upper surface 51 of the blocks 5 and the two frontal side walls 52, 53, when viewed in a cross section perpendicular to the axis of rotation of the tire. The angles T1 and T2 can be measured as the angles which are formed by a “straight line joining the two frontal edges 521, 531” and a straight line joining “the frontal edge 521 or 531, and a point on the frontal side walls 52, 53 which is 1.6 mm further inwards in the radial direction than the height of a tread wear indicator”.

In this mode of embodiment, the angles T1 and T2 are both less than 90° (not including 90°) and formed in such a way as to be different from each other. In the example shown in FIG. 4, the angle T1 is 65° and the angle T2 is 80°.

According to this mode of embodiment, the angles T1 and T2 are thus different from each other, the angle T2 preferably being greater than the angle T1. Furthermore, the angle T1 is preferably 80° or less and the angle T2 is preferably between 75° and 85°.

Furthermore, the angles T1 and T2 preferably satisfy the following relationship:

T
₂
>T
₁+5°.

It should be noted that the symbol “°” denotes the units of the angle, namely “degrees”.

The action and effect afforded by the pneumatic tire tread according to the abovementioned second mode of embodiment of the present invention will be described next.

According to this mode of embodiment, the angles T1 and T2 formed by the upper surface 51 and the frontal side walls 52, 53 have different angles, the angle T2 being greater than the angle T1. As a result, according to the second mode of embodiment, in addition to the action and effect of the first mode of embodiment described above, it is possible, for example, to reduce the moment force in a direction which produces a reduction in the contact surface area of the block 5 generated in the vicinity of the frontal edge 521 when the frontal edge 521 on the angle T2 side, which is a relatively larger angle, constitutes the trailing-side frontal edge while the tire is rolling, and therefore it is possible to increase the contact surface area of the block 5. As a result, when the tire is travelling on a road surface for which the coefficient of friction is sufficiently high to cause deformation of the block 5, such as a snowy road surface, it is possible to achieve a locally high edge pressure at the frontal edge 521 of the block 5 due to the effect of the reinforcing part 6, while at the same time, when the tire is travelling on a road surface for which the coefficient of friction is insufficient to cause deformation of the block 5, such as an icy road surface, it is possible to increase the contact surface area of the upper surface 51 of the block 5, so the performance on ice and performance on snow can be improved more effectively.

For this reason, the angles T1 and T2 formed by the upper surface 51 and the frontal side walls 52, 53 are preferably such that the side which is intended as the trailing-side frontal edge has a larger angle.

Preferred modes of embodiment of the present invention have been described above, but the present invention is not limited to the modes of embodiment shown in the figures and a number of variant modes may be implemented.

Moreover, FIG. 5 is an enlargement in cross section schematically showing the block of a conventional pneumatic tire tread. A block 105 of this conventional pneumatic tire tread 101 comprises an upper surface 151 forming part of a contact surface 102, and frontal edges 1521, 1531 are formed at the intersections of frontal side walls 152, 153. A narrow incision 107 which opens at the upper surface 151 and extends transversely and radially inside the tire is formed in the block 105. Reinforcing parts 106 are provided on the two frontal side walls 152, 153 in such a way as to include the whole of the frontal edges 1521, 1531. The mean thickness t of the reinforcing parts 106 is 0.5 mm, and the reinforcing parts 106 are provided in such a way as to face auxiliary grooves 104 over a region of 84% of the frontal side walls 152, 153.

Exemplary Embodiments

The results of tests carried out using a simulation (finite element method) employing commercially-available computer software will be described next in order to clarify the advantage of embodiments of the present invention, the tests involving blocks of a pneumatic tire tread according to a conventional example provided with reinforcing parts having a known form, and blocks of three types of pneumatic tire tread according to exemplary embodiments of the present invention.

Exemplary Embodiment 1 relates to a block model provided with the reinforcing parts according to the first mode of embodiment, and Exemplary Embodiments 2 and 3 relate to block models provided with the reinforcing parts according to the second mode of embodiment, the angle formed between the upper surface of the blocks and the frontal side walls being a combination of three different values.

The four types of block model size in the conventional example and the exemplary embodiments were in each case a cuboid block having a short side of length 10 mm, a long side of length 20 mm and a height of 10 mm, formed using the same rubber-based material (elastic modulus 5.4 MPa), the narrow incisions each having a width of 0.4 mm and depth of 7 mm and opening at the upper surface of the block. The reinforcing parts were formed from the same material (elastic modulus 270 MPa) and the reinforcing parts were provided over the whole region of the frontal side walls with a mean thickness of 0.5 mm, and the elastic modulus of the material of the reinforcing parts was 50 times the elastic modulus of the rubber-based material of the blocks.

With suitable loading applied to the block models set in this way, the maximum ground-contact pressure generated by the ground-contact elements under road surface conditions corresponding to a snowy road surface, and the coefficient of friction under road surface conditions corresponding to an icy road surface were obtained. The calculation results are shown in table 1. In table 1, the calculated values are shown as an index taking the conventional example as 100, and higher numerical values are more favourable.

TABLE 1

Exemplary
Exemplary
Exemplary
Conven-

Embodi-
Embodi-
Embodi-
tional

ment 1
ment 2
ment 3
Example

Angle T1 (degrees)
80
80
70
90

Angle T2 (degrees)
80
85
85
90

Maximum ground-
175
174
187
100

contact pressure

on snow (index)

Coefficient of
102
104
105
100

friction on ice

(index)

As shown in table 1, it can be confirmed that the pneumatic tire treads according to Exemplary Embodiments 1 to 3 made it possible to effectively achieve improved performance on snow and performance on ice.

KEY TO SYMBOLS

1 Pneumatic tire tread

2 Contact surface

3 Circumferential main groove

4 Auxiliary groove

5 Block

51 Block upper surface (part of which includes the contact surface 2)

52,53 Side wall on circumferential direction side, frontal side wall

521,531 Frontal edge

54,55 Side wall on tire width direction side, side surface side wall

6 Reinforcing part

7 Narrow incision (sipe)

PNEUMATIC TIRE TREAD AND PNEUMATIC TIRE HAVING SAID TREAD

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