Patent Description:
A pneumatic tire has a tread which is a portion that contacts a road surface. The tread includes a plurality of blocks. Each block has sipes formed therein. The sipes provide an edge effect and a water removing effect on icy and snowy roads. The sipes on the pneumatic tire are formed by using sipe blades arranged in a tire mold.

For example, <CIT> discloses a technique of forming sipes such that the amplitude of a waveform of the sipe is gradually decreased from the block surface side toward the block bottom side in the tire radial direction. In addition, <CIT> discloses a technique of forming depressions in a block surface of one of sipe walls and reducing a cross-sectional area of the depressions from the block surface side to the block bottom side in the tire radial direction.

However, the pneumatic tires disclosed in <CIT> and <CIT> have room for improvement with regard to braking performance on wet road surfaces and steering stability performance on dry road surfaces. <CIT> discloses a tire running surface that comprises cutouts which form a zigzag or wavy tread pattern and incorporate the bounding surfaces of adjacent pointed body elements whose orientation alternates from cutout to cutout. <CIT> discloses a pneumatic wherein a sipe inner wall surface of a sipe of the tire has a wavy first concave and convex row, a wavy second concave and convex row which is arranged at a distance from the first concave and convex row on a sipe bottom side, and a third concave and convex row which extends in a branched manner toward the sipe bottom side. <CIT> discloses a pneumatic vehicle tire with a profiled tread which has two shoulder block rows with profile blocks, wherein in the profile blocks uniformly step-shaped or wave-shaped incisions are formed. <CIT> discloses a pneumatic vehicle tire with a tread which is composed of profile elements, wherein at least one profile element is provided with a fine sipe which extends in the radial direction from the tread periphery into the profile element interior and which has corresponding fine sipe walls in the form of a sinusoidal wave.

In consideration of the above, an object of the present invention to provide a pneumatic tire and a tire mold that can improve braking performance on wet road surfaces and improve steering stability performance on dry road surfaces.

A pneumatic tire according to the present invention has a tread including a block in which a sipe is formed. At least one wall surface of the sipe is formed in a waveform including a plurality of curved portions extending from a block surface side to a block bottom side when viewed from a tire radial direction. The curved portions include an inwardly curved portion that bulges toward the inside of the sipe and an outwardly curved portion that bulges toward the outside of the sipe. The radii of curvature of the inwardly curved portion and the outwardly curved portion are identical on the block surface side in the tire radial direction, and the radii of curvature of the inwardly curved portion and the outwardly curved portion differ on the block bottom side.

The pneumatic tire according to the present invention can improve braking performance on wet road surfaces and improve steering stability performance on dry road surfaces.

Embodiments of the present disclosure will be described based on the following figures, wherein:.

Embodiments of the present invention will be described in detail hereinafter. In the description below, specific shapes, materials, directions, numerical values, etc. are provided as illustrations for facilitating the understanding of the present invention, and can be appropriately changed according to applications, purposes, specifications, etc..

A pneumatic tire <NUM>, which is an embodiment of the present invention, will be described with reference to <FIG>.

As shown in <FIG>, the pneumatic tire <NUM> has a tread <NUM> that includes blocks <NUM>. The blocks <NUM> have sipes <NUM> formed therein. The pneumatic tire <NUM> makes it possible to improve braking performance on wet road surfaces and improve steering stability performance on dry surfaces (described in detail below).

Components of the pneumatic tire <NUM> will be described below with reference to a tire axial direction X, tire circumferential direction Y, and tire radial direction Z. The tire width may be described by using an equator CL with respect to the tire axial direction X. The tire depth in the tire radial direction Z may be described by using words "block surface side" and "block bottom side". The block surface and the block bottom are respectively labeled as SF and BM in the figures.

The tread <NUM> is a portion of the pneumatic tire <NUM> that contacts a road surface. The tread <NUM> has a plurality of blocks <NUM> separated by major grooves <NUM> and minor grooves <NUM>. Each of the plurality of blocks <NUM> is formed to have a rectangular shape. The blocks closer to the equator CL in a plan view have a smaller length in the tire axial direction X. The blocks <NUM> are arranged into lines in the tread <NUM>. The shape of the block <NUM> is not limited to this, and the block <NUM> may have a rhombic or parallelogram shape, as long as the blocks <NUM> are separated by the major grooves <NUM> and the minor grooves <NUM>.

The major groove <NUM> is a groove that extends along the tire circumferential direction Y. The major groove <NUM> is formed linearly along the tire circumferential direction Y. The minor groove <NUM> is a groove that extends along the tire axial direction X. The minor groove <NUM> is formed linearly along the tire axial direction X. It should be noted that the major groove <NUM> and the minor groove <NUM> of the present embodiment are not limited to these shapes, and the major groove <NUM> may be formed at an angle to the tire circumferential direction Y, and the minor groove <NUM> may be formed at an angle to the tire axial direction X.

The sipes <NUM> will be described with reference to <FIG>.

As shown in <FIG>, the sipes <NUM> are formed in the block <NUM>. The sipe <NUM> is a groove that extends along the tire axial direction X. Wall surfaces <NUM> of the sipe <NUM> are formed in a waveform along the tire axial direction X (described in detail below). For example, three sipes <NUM> are formed at equal intervals in the tire circumferential direction Y of the block <NUM>. The depth of the groove of the sipe <NUM> is preferably <NUM> to <NUM>% of the length of the block <NUM> in the tire radial direction Z.

The number and shape of the sipes <NUM> are not limited to those in the present embodiment, and three to five sipes <NUM> may be formed in the block <NUM>. The sipes <NUM> may also be formed at an angle to the tire axial direction X. Furthermore, the sipes <NUM> may be open sipes that penetrate the sides of the block <NUM> or may be closed sipes that do not penetrate the sides of the block <NUM>.

The sipes <NUM> soften the block <NUM> to increase the area of the surface that contacts the road surface, and thereby increase friction with the road surface. The sipes <NUM> also provide an edge effect and a water removing effect on an icy and snowy road. The edge effect is the effect of increasing grip by scratching the surface of the icy and snowy road with the corners of the blocks <NUM> or the sipes <NUM>. The water removing effect is the effect of capturing water in a gap in the sipe <NUM> on the icy and snowy road.

The waveform of the sipes <NUM> will be described with reference to <FIG> and <FIG>.

As shown in <FIG>, the wall surfaces <NUM> of the sipe <NUM> are formed in a waveform from the block surface side to the block bottom side when viewed from the tire radial direction Z. In other words, the wall surfaces <NUM> of the sipe <NUM> are formed in a waveform in cross section, perpendicular to the tire radial direction Z. The wall surface <NUM> of the sipe <NUM> is a wall (surface) perpendicular to the tire circumferential direction Y of the sipe <NUM>. By forming the wall surfaces <NUM> of the sipes <NUM> in a waveform, it is possible to further increase the above edge effect.

Although, in the present embodiment, one wall surface <NUM> and the other wall surface <NUM> of the sipe <NUM> are formed in a waveform, this is not limiting. Only the one wall surface <NUM> of the sipe <NUM> may be formed in a waveform, or only the other wall surface <NUM> of the sipe <NUM> may be formed in a waveform.

More specifically, in the present embodiment, the wall surfaces <NUM> of the sipe <NUM> are formed in a waveform including a plurality of curved portions <NUM> when viewed from the tire radial direction Z. The plurality of curved portions <NUM> include inwardly curved portions 42A that bulge toward the inside of the sipe <NUM> and outwardly curved portions 42B that bulge toward the outside of the sipe <NUM>. The inwardly curved portions 42A and the outwardly curved portions 42B are arranged alternately to form the waveform. A straight portion <NUM> is also formed between the inwardly curved portion 42A and the outwardly curved portion 42B.

In other words, when a reference line RL is drawn at the center of the width of the sipe <NUM> (length in the tire circumferential direction Y) along the longitudinal direction of the sipe <NUM>, the inwardly curved portion 42A is the curved portion <NUM> that bulges toward the reference line RL, and the outwardly curved portion 42B is the curved portion <NUM> that bulges toward the opposite side of the reference line RL.

As shown in <FIG>, although the radius of curvature R1 of the inwardly curved portion 42A and the radius of curvature R2 of the outwardly curved portion 42B are identical on the block surface side in the tire radial direction Z (cross-section along line A-A in the figure), they differ from each other on the block bottom side (cross-section along line B-B in the figure). The waveform is thus continuously changed in the tire radial direction Z such that the radius of curvature R1 of the inwardly curved portion 42A and the radius of curvature R2 of the outwardly curved portion 42B that are identical to each other on the block surface side differ from each other on the block bottom side.

Here, assuming that the length of the sipe depth (from the block surface SF to the block bottom BM) is <NUM>, the block surface side in the tire radial direction Z is preferably within a range of distance of <NUM> from the block surface SF. In addition, assuming that the length of the sipe depth is <NUM>, the block bottom side in the tire radial direction Z is preferably within a range of distance of <NUM> from the block bottom BM.

On the block bottom side, the radius of curvature R1 of the inwardly curved portion 42A is formed to be larger than the radius of curvature R2 of the outwardly curved portion 42B. Here, a larger radius of curvature indicates that the curve is gentler.

More specifically, on the block bottom side, assuming that the distance between the center of the inwardly curved portion 42A and the center of the outwardly curved portion 42B (pitch of the waveform) is P (mm), the thickness of the sipe <NUM> (length in the tire circumferential direction) is t (mm), and that the radius of curvature of the inwardly curved portion 42A is R1 (mm), the following expression holds: <MAT>.

Here, if the radius of curvature R1 of the inwardly curved portion 42A is larger than <NUM> * P, the waveform becomes smaller and the binding force at the wall surfaces <NUM> of the sipe <NUM> becomes smaller, resulting in reduction in sipe inclination prevention performance. In addition, if the radius of curvature R1 of the inwardly curved portion 42A is smaller than <NUM> * P - <NUM> * t, a gap between the inwardly curved portion 42A and the outwardly curved portion 42B becomes smaller, resulting in reduction in braking performance on a wet road surface (described below).

Assuming that the radius of curvature of the outwardly curved portion 42B is R2 (mm), the radius of curvature R2 is within the range of the following expression: <MAT> Furthermore, the radius of curvature R2 is preferably within the range of the following expression: <MAT>.

Here, if the gap between the inwardly curved portion 42A and the outwardly curved portion 42B becomes too large, the binding force at the wall surfaces <NUM> of the sipe <NUM> becomes small.

The pneumatic tire <NUM> can improve braking performance on wet road surfaces and improve steering stability performance on dry road surfaces. More specifically, in the sipe <NUM> of the pneumatic tire <NUM>, the radius of curvature R1 of the inwardly curved portion 42A and the radius of curvature R2 of the outwardly curved portion 42B of the waveform are formed to be identical to each other on the block surface side, and this makes it possible to increase stiffness (surface stiffness) of the block surface SF. On the other hand, the radius of curvature R1 of the inwardly curved portion 42A and the radius of curvature R2 of the outwardly curved portion 42B of the waveform are formed to differ from each other on the block bottom side of the sipe <NUM>. This makes it possible to generate a gap in the sipe <NUM> to reduce the in-plane stiffness.

The increase in surface stiffness allows for increase in ground pressure per unit area of the block <NUM>, thereby enhancing the driving force. On the other hand, the reduction in in-plane stiffness allows for increase in ground contact area of the block <NUM>, removal of water from the wet road surface, and increase in coefficient of friction between the block <NUM> and the road surface, resulting in improvement in the braking performance of the pneumatic tire <NUM>. As such, the pneumatic tire <NUM> can increase the surface stiffness and reduce the in-plane stiffness, thereby improving the braking performance on wet road surfaces and improving the steering stability performance on dry road surfaces.

A tire mold <NUM>, which is an embodiment of the present invention, will be described with reference to <FIG>.

The mold <NUM> is a mold for forming the above pneumatic tire <NUM>. The pneumatic tire <NUM> has the tread <NUM> including the blocks <NUM>, in which the sipes <NUM> are formed as described above, and sidewalls (not shown) forming side surfaces. The mold <NUM> can be used to mold the pneumatic tire <NUM> that can improve braking performance on wet road surfaces and improve steering stability performance on dry surfaces.

Components of the mold <NUM> will be described below with reference to the tire axial direction X, the tire circumferential direction Y, and the tire radial direction Z of the above pneumatic tire <NUM> formed by the mold <NUM>.

The mold <NUM> has a tread mold <NUM> for molding a surface of the tread <NUM> of the pneumatic tire <NUM> and a pair of side molds <NUM> for molding surfaces of the sidewalls.

The tread mold <NUM> has a body <NUM> having a tread forming surface <NUM>, projections <NUM> protruding from the tread forming surface <NUM>, and sipe blades <NUM> protruding from the tread forming surface <NUM> and provided between the projections <NUM>.

The body <NUM> is made of a metallic material, such as aluminum alloy. The aluminum alloy preferably includes AC4 or AC7 aluminum, for example. The projections <NUM> are portions for molding the major grooves <NUM> in the pneumatic tire <NUM>. The projections <NUM> are made of the same material as the body <NUM>.

The sipe blade <NUM>, which is an embodiment of the present invention, will be described with reference to <FIG>.

The sipe blades <NUM> are provided for molding the sipes <NUM> in the pneumatic tire <NUM>. The sipe blades <NUM> protrude in the tire radial direction Z from the tread forming surface <NUM> between the protrusions <NUM>. The sipe blade <NUM> may be flat and made of a metallic material, such as stainless steel. The stainless steel preferably includes, for example, SUS303, SUS304, SUS630, and SUS631. When a three-dimensional molding machine is used, <NUM>-4PH which is equivalent to SUS304L and SUS630 may be preferably used.

In general, a method of processing the sipe blade <NUM> includes forming the shape of the sipe blade <NUM> by using a press molding machine. To cause a change in shape of the sipe blade <NUM> in the thickness direction as in the sipe blade <NUM> of the present embodiment, the method may include cutting the sipe blade by machining. A three-dimensional molding machine may be used to make a complicated shape that is difficult to achieve by machining.

A side surface <NUM> of the sipe blade <NUM> is formed in a waveform when viewed from the tire radial direction Z. More specifically, the side surface <NUM> of the sipe blade <NUM> is formed in a waveform including a plurality of curved portions <NUM> when viewed from the tire radial direction Z. Here, the plurality of curved portions <NUM> include inwardly curved portions 62A that bulge toward the inside of the sipe blade <NUM> and outwardly curved portions 62B that bulge toward the outside of the sipe blade <NUM>. The inwardly curved portions 62A and the outwardly curved portions 62B are arranged alternately to form the waveform.

Although the radius of curvature R1 of the inwardly curved portion 62A and the radius of curvature R2 of the outwardly curved portion 62B are identical on the block surface side in the tire radial direction Z (cross-section along line A-A in the figure), they differ from each other on the block bottom side (cross-section along line B-B in the figure). The waveform is thus continuously changed in the tire radial direction Z such that the radius of curvature R1 of the inwardly curved portion 62A and the radius of curvature R2 of the outwardly curved portion 62B that are identical to each other on the block surface side differ from each other on the block bottom side.

On the block bottom side, the radius of curvature R1 of the inwardly curved portion 62A is formed to be larger than the radius of curvature R2 of the outwardly curved portion 62B.

The present invention will be further described by using examples, but the present invention is not limited to these examples.

A pneumatic tire A1 (tire size: <NUM>/55R16, rim size: <NUM> * <NUM> JJ) with sipes was made such that each sipe has curved portions on its wall surfaces as shown in <FIG>. The thickness t of a sipe, the pitch P of a waveform of a wall surface of the sipe, the radius of curvature of an inwardly curved portion on the block surface side RSF<NUM>, the radius of curvature of an outwardly curved portion on the block surface side RSF<NUM>, the radius of curvature of the inwardly curved portion on the block bottom side RBM<NUM>, and the radius of curvature of the outwardly curved portion on the block bottom side RBM<NUM>, are as follows:.

A pneumatic tire B2 was made in the same manner as Comparative Example <NUM>, except that the radii of curvature of the inwardly curved portion on the block surface side RSF<NUM> and the outwardly curved portion on the block surface side RSF<NUM> were changed.

A pneumatic tire A1 was made in the same manner as Comparative Example <NUM>, except that the radius of curvature of the outwardly curved portion on the block surface side RSF<NUM> was changed.

A pneumatic tire A2 was made in the same manner as Comparative Example <NUM>, except that the radii of curvature of the inwardly curved portion on the block surface side RSF<NUM>, the outwardly curved portion on the block surface side RSF<NUM>, and the outwardly curved portion on the block bottom side RBM<NUM> were changed.

A pneumatic tire A3 was made in the same manner as Comparative Example <NUM>, except that the radii of curvature of the inwardly curved portion on the block surface side RSF<NUM>, the outwardly curved portion on the block surface side RSF<NUM>, and the outwardly curved portion on the block bottom side RBM<NUM> were changed.

A pneumatic tire A4 was made in the same manner as Comparative Example <NUM>, except that the radii of curvature of the inwardly curved portion on the block surface side RSF<NUM>, the outwardly curved portion on the block surface side RSF<NUM>, the inwardly curved portion on the block bottom side RBM<NUM>, and the outwardly curved portion on the block bottom side RBM<NUM> were changed.

The pneumatic tires A1 to A4 and B1 and B2 were installed on all wheels of a test vehicle (front-wheel drive vehicle, displacement 2000cc) with an air pressure of <NUM> kPa, and braking performance on wet road surfaces and steering stability performance on dry road surfaces were evaluated by the following method.

The above test vehicle was run on a wet road surface (asphalt road surface on which water was sprinkled), and the braking distance from an initial speed of <NUM>/h was measured. In the results, numerical values obtained by dividing the braking distances of Comparative Example <NUM> and Examples by the braking distance of Comparative Example <NUM> and multiplying the reciprocals of the obtained values by <NUM> are indicated as indexes. The larger the index, the shorter the braking distance and the better the braking performance on the wet road surface.

The steering stability performance was evaluated based on the driver's perception when the test vehicle was driven on a dry road surface. The results were compared with steering stability performance of Comparative Example <NUM> and evaluated in seven levels: "much worse", "worse", "somewhat worse", "equal", "somewhat better", "better", and "much better".

As shown in Table <NUM>, Comparative Example <NUM> adopted a general waveform sipe.

In Comparative Example <NUM>, on the block surface side of the sipe, the radii of curvature of the inwardly curved portion and the outwardly curved portion of the waveform differ from each other, and the sipe does not close, resulting in reduced surface stiffness and thus reduced steering stability performance on the dry road surface.

In Example <NUM>, on the block surface side of the sipe, the radii of curvature of the inwardly curved portion and the outwardly curved portion of the waveform are identical to each other, and on the block bottom side, the radii of curvature of the inwardly curved portion and the outwardly curved portion of the waveform are identical to those in Comparative Example <NUM>. This increases the surface pressure when the sipes are inclined and increases the surface stiffness, resulting in improved steering stability performance on the dry road surface.

In Examples <NUM> and <NUM>, the radii of curvature of the inwardly and outwardly curved portions on the block surface side are smaller than those in Example <NUM>, and this allows for higher surface stiffness and thus improved steering stability performance on the dry road surface. Further, in Examples <NUM> and <NUM>, the size relationship between the radii of curvature of the inwardly curved portion and the outwardly curved portion on the block bottom side is opposite to those in Comparative Examples <NUM> and <NUM> and Example <NUM>. This reduces the in-plane stiffness and increases the ground contact area, resulting in improved braking performance on the wet road surface.

Although Example <NUM> has an effect similar to those in Examples <NUM> and <NUM>, the in-plane stiffness is lower than Examples <NUM> and <NUM>, and thus the steering stability performance on the dry road surface is reduced.

Claim 1:
A pneumatic tire (<NUM>) having a tread (<NUM>) comprising a block (<NUM>) in which a sipe (<NUM>) is formed, wherein
at least one wall surface (<NUM>) of the sipe (<NUM>) is formed in a waveform including a plurality of curved portions (<NUM>) extending from a block surface side to a block bottom side when viewed from a tire radial direction (Z),
the curved portions (<NUM>) include an inwardly curved portion (42A) that bulges toward the inside of the sipe (<NUM>) and an outwardly curved portion (42B) that bulges toward the outside of the sipe (<NUM>), and
the radii of curvature of the inwardly curved portion (42A) and the outwardly curved portion (42B) are identical on the block surface side in the tire radial direction (Z), characterized in that the radii of curvature of the inwardly curved portion (42A) and the outwardly curved portion (42B) differ on the block bottom side in the tire radial direction (Z).