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 in enhancing icy and snowy road performance. Here, the icy and snowy road performance refers to steering stability performance on snowy road surfaces and braking performance on frozen road surfaces. <CIT> discloses a snow tire for improving steering and braking performance, the snow tire comprising a plurality of tread blocks, wherein sipes are formed in the plurality of the tread blocks in parallel each other in one direction to subdivide the tread blocks into a plurality of subblocks. <CIT> discloses a tire with a sipe having a projecting section projecting toward the circumference direction of the tire. <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.

In consideration of the above, an object of the present invention to provide a pneumatic tire and tire mold that can improve the icy and snowy road performance.

A pneumatic tire according to the present invention has a tread including a block in which a sipe is formed. In this pneumatic tire, at least one wall surface of the sipe is formed in a waveform including a plurality of curved portions 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 inwardly curved portion and the outwardly curved portion have different radius of curvature.

The pneumatic tire according to the present invention can improve the icy and snowy road performance.

Embodiments of the present invention 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> can improve icy and snowy road performance (described in detail later).

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 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, 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 with 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> when on the icy and snowy road.

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

The wall surfaces <NUM> of the sipe <NUM> are formed in a waveform 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 a cross-sectional view 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>. Furthermore, the waveform of the wall surface <NUM> of the sipe <NUM> keeps the same shape from the block surface side to the block bottom side in the tire radial direction Z. By forming the wall surfaces <NUM> of the sipes <NUM> in a waveform, it is possible to improve 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, according to the invention, 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.

The radii of curvature of the inwardly curved portion 42A and the outwardly curved portion 42B are formed to differ from each other. The radius of curvature of the inwardly curved portion 42A is formed to be larger than the radius of curvature of the outwardly curved portion 42B. Here, a larger radius of curvature indicates that the curve is gentler.

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 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 snow column shearing performance.

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 further improve the icy and snowy road performance when the wall surfaces <NUM> of the sipes <NUM> are formed in a waveform as described above.

More specifically, by forming the wall surfaces <NUM> of the sipe <NUM> in a waveform as described above, a slight gap is created between the inwardly curved portion 42A and the outwardly curved portion 42B as shown in <FIG> during driving and braking operation, and this can improve the water absorption performance and improve the snow column shearing performance. Snow column shearing force refers to the ability of the blocks <NUM> to compress snow to harden it, form columns of snow, and kick out the columns to provide a driving force. The snow column shearing force can thus improve the driving performance of the pneumatic tire <NUM> on snowy road surfaces.

Forming the wall surfaces <NUM> of the sipes <NUM> in a waveform as above also makes it possible to further increase the edge effect by the sipes <NUM> to thereby improve the braking performance on frozen 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 the icy and snowy road performance.

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. Furthermore, the waveform of the side surface <NUM> of the sipe blade <NUM> keeps the same shape from the block surface side to the block bottom side in 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.

The radii of curvature of the inwardly curved portion 62A and the outwardly curved portion 62B are formed to differ from each other. The radius of curvature of the inwardly curved portion 62A is formed to be larger than the radius of curvature 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 B1 (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 the wall surface of the sipe, the radius of curvature R1 of an inwardly curved portion, and the radius of curvature R2 of an outwardly curved portion are as follows:.

A pneumatic tire B2 was made in the same manner as Comparative Example <NUM>, except that the radius of curvature R2 of the outwardly curved portion was changed.

A pneumatic tire A1 was made in the same manner as Comparative Example <NUM>, except that the radius of curvature R2 of the outwardly curved portion was changed.

A pneumatic tire A2 was made in the same manner as Comparative Example <NUM>, except that the radius of curvature R2 of the outwardly curved portion was changed.

A pneumatic tire A3 was made in the same manner as Comparative Example <NUM>, except that the radius of curvature R2 of the outwardly curved portion was changed.

A pneumatic tire A4 was made in the same manner as Comparative Example <NUM>, except that the radius of curvature R1 of the inwardly curved portion and the radius of curvature R2 of the outwardly curved portion 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 steering stability performance on a snowy road surface and braking performance on a frozen road surface were evaluated by the following method.

The steering stability performance was evaluated based on the driver's perception when the test vehicle was driven on a snowy 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".

The above test vehicle was driven at <NUM>/h on an icy road surface and then stopped, and the braking distance was measured until the vehicle speed of <NUM>/h was reached. 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 better the braking performance on the frozen road surface.

Table <NUM> indicates that, although, in Comparative Example <NUM>, a space between top portions of the inwardly and outwardly curved portions is closed, there is a gap between straight portions, and this causes the behavior of the blocks in the longitudinal direction of the sipe when the blocks are inclined.

In Comparative Example <NUM>, the sipe inclination prevention effect is improved compared to Comparative Example <NUM>, and thus the braking performance on the frozen road surface tends to be slightly better. On the other hand, because there are no gaps in the sipes, the water absorption performance and the snow column shear effect are not achieved.

In Examples <NUM> to <NUM>, the edge effect is increased compared to Comparative Examples <NUM> and <NUM>, and the braking performance on the frozen road surface is improved. In addition, due to gaps in the sipes, the water absorption performance and the snow column shearing effect are improved, resulting in improved steering stability performance on the snowy road surface.

Although, in Example <NUM>, the force of supporting the sipe wall surfaces in the longitudinal direction to prevent inclination of the sipes is smaller, the edge effect is increased, and thus, the braking performance on the frozen road surface is equal to that in Comparative Example <NUM>. Furthermore, although, due to larger gaps in the sipes, the water absorption performance and the snow column shearing effect are improved, the sipe inclination prevention effect is reduced, and thus, the steering stability performance on the snow surface is equal to that in Example <NUM>.

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>) 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) differ from each other,
characterized in that a straight portion (<NUM>) is formed between the inwardly curved portion (42A) and the outwardly curved portion (42B).