EP2714433A1 - Winter tyre - Google Patents

Abstract

A winter tyre comprises a tread band in which is formed a plurality of grooves, at least one block delimited by said plurality of grooves, at least one sipe formed in said block and extending between a top (7) and a bottom (8) of said sipe and being shaped so as to define in each of the block portions that are separated by said sipe: a first surface (11) which faces the other block portion and extends from said top (7) to an intermediate depth (P) of said sipe, which first surface (11) has a substantially straight profile, and a second surface (12) which faces the other block portion, is connected to said first surface (11), extends from said intermediate depth (P) to said bottom (8), and has an undulating profile formed by a series of concave portions (13) and convex portions (14), each having a respective crest (13a; 14a) the distance of which from a reference plane (A) comprising said first surface (11) increases progressively from said intermediate depth (P) to said bottom (8).

Description

Winter tyre

Specification

The present invention relates to a winter tyre comprising a tread band whose blocks are duly provided with sipes configured to improve the performance of the tyre on wet and dry surfaces, in particular conditions of use, without thereby impairing its road-holding properties on an icy or snow-covered surface.

In a typical structural configuration, a winter tyre comprises a tread band provided with a plurality of circumferential and transverse grooves delimiting a corresponding plurality of blocks, on each of which a plurality of sipes is in turn obtained . Typically, sipes extend within the block in a radial reference plane substantially perpendicular to the tread surface of the tyre and their presence is one of the more evident features which distinguish, from a visual point of view as well, a winter tyre from conventional summer tyres.

The function of the sipes is to provide further edges to grip a snow-covered surface and internally to retain a certain quantity of snow which, as is known, has a higher level of friction with the snow on the road surface than that of the tread band itself.

Sipes may be obtained on the blocks in different configurations, possibly as a function of the performance required of the tyre, and may in particular be provided with a configuration having a straight profile or a configuration having an undulating profile, for instance of a zigzag type.

It will be appreciated that in this context the term "sipe" means a recess provided in a tread band portion and having a width of between 0.1 and 1.5 mm and a depth of between 1 and 15 mm, and the term "groove" means a recess provided in a tread band portion and having a width greater than 1.5 mm and a depth greater than 5 mm.

Moreover, in the present specification and in the appended claims, it is said that a surface has an "undulating" profile when an alternating series of concave and convex portions can be identified thereon. In this way, any section of said surface gives rise to a generically curved line formed by an alternating series of first and second sections of opposite concavity.

As a particular case of an undulating surface, said concave and convex portions may be formed by a series of differently lying incident planes, wherein the section line is represented by a polygonal chain in the form of a zigzag line comprising first and second straight sections oriented in different directions.

Moreover, in the case of a surface with an undulating profile, each first and second section line section has a "apex" understood as the most distant point from the reference plane along which the sipe extends, and each concave or convex portion has a "crest", i.e. a line formed by all the apexes of the section line sections identified as the section plane varies. In the present specification and in the appended claims, the term "substantially straight", when it refers to a sipe or to a profile of a block portion surface determined by the sipe, means a curve which may depart from a perfectly straight condition at most by a measurement equal to one twentieth of its overall longitudinal extension.

JP 11-151914 discloses a winter tyre whose blocks bear sipes shaped so as to present a substantially plane surface extending from the top of the sipe to an intermediate depth, after which the sipe is divided into two separate branches extending in an inclined manner with respect to the plane surface within one and the other block portion separated by the sipe, respectively. After a predetermined degree of wear of the tyre, the sipe leads to the definition, between the two original block portions, of an additional block portion able to increase the overall linear development of the sipe and therefore to reduce the loss of performance on snow.

The Applicant has verified that undulating sipes have undeniable advantages with respect to simple straight sipes, including the fact that, with the same longitudinal extension, they have a greater linear development than a straight configuration and therefore a gripping edge on a snow-covered surface which is more extensive and which has a greater snow retention capacity. A further advantage of the configuration with an undulating profile with respect to a straight configuration is that it has a greater resistance to shearing stresses parallel to the longitudinal direction of said sipe as a result of the reciprocal interference between the block portions separated by the sipe.

The Applicant has also verified that the road-holding of a tyre on a snow- covered road surface, all other conditions remaining the same, in particular such as the tread compound used and the configuration of the blocks on the tread band ("tread pattern"), depends on its degree of wear. With wear and the passage of time, the tread compound tends to lose partly its properties of softness and elasticity at low temperatures with an adverse impact on its road-holding ability on snow.

A solution to this drawback involves the provision of sipes of the type of those disclosed in the cited document JP 11-151914, wherein, at a predetermined depth from the tread surface, the sipe has a greater linear development than the linear development to be found at the tread surface. However, the Applicant has observed that a configuration of the type disclosed in JP 11-151914, as well as making the removal of the tyre from the mould more complicated when it is being produced, entails a further division of the block into portions thereby potentially decreasing the road- holding of the tyre when braking and cornering on a wet or dry road surface.

The Applicants have also verified that the performance of partially worn tyres having sipes with an undulating profile is, all other conditions remaining the same, better than that of sipes with a substantially straight profile in terms of lateral road-holding, braking and traction on snow.

Applicant has nevertheless verified that the provision of sipes with an undulating profile may entail problems of excessive surface abrasion of the tread band when the tyre is subject to sudden swerving at high speeds and with particularly heavy sports vehicles on wet and dry road surfaces.

Applicant has therefore realized that a sipe having a profile that varies with depth could make it possible to improve performance as a function of the degree of wear of the tyre.

Applicant has lastly found that a sipe shaped so as to present, in its radially external portion, a substantially straight profile and, in its radially internal portion, an undulating profile whose amplitude progressively increases as depth increases, makes it possible to obtain tyres able to offer both good performance on dry surfaces when the tyre is new and in cases of very high tangential stresses, preventing excessive surface abrasion, and good performance on snow-covered surfaces even when the tyre is progressively worn .

In particular, according to its first aspect, the invention relates to a tyre comprising a tread band in which is formed a plurality of grooves, at least one block delimited by said plurality of grooves, at least one sipe formed in said at least one block and extending between a top of said sipe which opens in a tread surface radially external of said tread band and a bottom of said sipe radially internal of said tread band, said at least one sipe being shaped so as to define in each of the block portions that are separated by said sipe :

- a first surface which faces the other block portion and extends from said top to an intermediate depth that is defined in said sipe between said top and said bottom, and has a substantially straight profile, and - a second surface which faces the other block portion, is connected to said first surface, extends from said intermediate depth to said bottom, and has an undulating profile,

and wherein each of said concave portions and said convex portions has a respective crest, the distance of which from a reference plane comprising said first surface increases progressively from said intermediate depth to said bottom.

The Applicants consider that sipes formed in this way reduce the loss of performance on snow-covered road surfaces even when the tread band is progressively worn, ensuring, at the same time, an acceptable ease of removal of the tyre from the production mould . The present invention, in its above-mentioned aspect, may include at least one of the preferred features described below.

For each of said concave portions and said convex portions, the distance of said crest from said reference plane is preferably zero at said intermediate depth.

In one embodiment, for each of said concave portions and said convex portions, the distance of said crest from said reference plane increases linearly from said intermediate depth to said bottom.

This facilitates the operation to remove the tyre from the mould .

Said concave portions and said convex portions are preferably arranged on opposite sides of said reference plane.

Said concave portions and said convex portions are more preferably arranged symmetrically with respect to said reference plane.

In this way, with the linear development of the undulating profile remaining the same, the smallest possible bulk in the transverse direction is obtained with respect to the reference plane of the sipe, thereby minimizing the problem of removal from the production mould .

According to a first preferred embodiment, said undulating profile defines a substantially sinusoidal line when said second surface is sectioned by an incident plane.

According to an alternative embodiment, said undulating profile defines a polygonal chain in the form of a zigzag line when said second surface is sectioned by an incident plane.

Said polygonal chain in the form of a zigzag line preferably comprises an alternating series of respective first straight portions and second straight portions which are substantially parallel, respectively, to an axial direction and to a circumferential direction that are defined in said tread band .

As a result of this configuration, each block portion section faces in the circumferential direction or in the axial direction, so as to exert its road- holding ability in the best possible way, when the tyre is worn, on a snow- covered road surface both when turning and when braking or accelerating . Moreover, by appropriately dimensioning the length of said first and second sections it is possible to modulate the different road-holding of the tyre in the most appropriate way to circumferential or axial shearing stresses, giving priority to one or the other depending on the intended type of use.

More preferably, said sipe extends in said block in a longitudinal direction which is inclined with respect both to an axial direction and to a circumferential direction that are defined in said tread band .

Said sipe preferably opens, at its opposite longitudinal ends, into a pair of grooves delimiting said block, so as to extend through said block from side to side.

In particular, it is preferable for said second surface to be defined, in the longitudinal direction, in a central region of said sipe and to be interposed between flat surfaces which are formed at the opposite longitudinal ends of said sipe.

Moreover, said flat surfaces are preferably coplanar with said first surface. In a preferred embodiment, said intermediate depth is spaced from said bottom by between approximately 1 mm and approximately 10 mm.

Said intermediate depth is also preferably spaced from said top by between approximately 1 mm and approximately 4 mm .

In this way the second surface of said sipe gradually emerges on the tread surface following the wear of the radially external layer of the tread band, after a period of time considered to correspond to a typical ageing time of the mixture from which the tyre is made, in which there is a loss of its properties of softness and resilience.

Said second surface preferably extends in a longitudinal direction for a distance of between approximately 50% and approximately 100% of the entire longitudinal extent of said sipe.

Other advantages and features of the invention will become clear from the following detailed description of a preferred embodiment thereof, given with reference to the appended drawings which are provided purely by way of non-limiting example and in which :

- Fig . 1 is a diagrammatic front perspective view of a significant portion of a winter tyre embodied in accordance with the present invention;

- Fig . 2a is a diagrammatic top view, on an enlarged scale and in perspective, of a block portion of the tyre of Fig . 1 bearing a sipe;

- Fig . 2b is a diagrammatic bottom view, on an enlarged scale and in perspective, of the block portion of Fig . 2a;

- Fig . 3a is a view, on an enlarged scale and in perspective, of a block portion of Fig . 2a;

- Figs. 3b and 3c are views of the block portion of Fig . 3a with increasing wear;

- Fig . 4 is a view similar to Fig . 2b of a block portion bearing a sipe obtained in accordance with a variant of the embodiment of the tyre of Fig . 1.

With reference initially to Figs. 1 to 3c, a tyre embodied in accordance with the present invention is shown overall by 1.

The tyre 1 comprises a carcass structure disposed in a toroidal manner about an axis Z, a belt structure disposed in a radially external position with respect to said carcass structure, and a tread band 2 disposed in a radially external position with respect to said belt structure and on which a tread surface 3, defined as the radially external surface of the tread band 2, is formed, which surface is designed to come into contact with a road surface on which said tyre 1 is intended to roll .

Said tread band 2 is provided with a plurality of grooves, all shown generically by 4, delimiting a plurality of blocks 5 disposed in sequence in a circumferential direction Y of said tread band 2.

One or a plurality of sipes 6 are provided on each block 5 and extend in a longitudinal direction X which, in the preferred embodiment described here, is inclined both with respect to the axial direction Z and the circumferential direction Y as defined in the said tread band 2.

It will be appreciated that alternative embodiments, in which the shape of said blocks 5 and the arrangement of the sipes 6 are configured differently on the tread band in order to meet particular operating requirements, may be readily obtained by a person skilled in the art.

Each sipe 6 preferably opens, at its opposite longitudinal ends 6a, 6b, into a pair of grooves 4 delimiting the block 5, so as to extend through said block from side to side. Each sipe 6 further extends between a top 7 of said sipe, open on the tread surface 3, and a bottom 8 of said sipe, radially internal of the tread band 2, and defines a first and a second block portion, shown by 9 and 10 respectively, separated from one another by the sipe 6 and spaced by a distance which remains substantially constant along the development of the sipe 6.

Each block portion 9, 10 in particular comprises a first surface 11 facing the other block portion and extending from the top 7 to an intermediate depth P defined in said sipe 6 between said top 7 and said bottom 8, and a second surface 12 also facing the other block portion, connected to said first surface 11 and extending from said intermediate depth P to said bottom 8 of said sipe 6.

Said first surface 11 is substantially flat and therefore has a substantially straight profile.

In contrast, said second surface 12 has an undulating profile and comprises a series of concave portions 13 and convex portions 14 arranged symmetrically on opposite sides with respect to a reference plane A comprising said first surface 11, preferably coincident with a radial plane of said tyre 1.

A respective crest 13a, 14a is defined on each concave portion 13 and convex portion 14, whose distance from the reference plane A increases progressively in a linear manner from said intermediate depth P, where this distance is zero, to said bottom 8 where this distance is at a maximum.

Said concave portions 13 and said convex portions 14 are shaped such that when said second surface 12 is sectioned by any plane incident thereto, a substantially sinusoidal line is obtained .

As a result of the progressive closing up of said concave and convex portions towards the reference plane A, the sinusoids obtained at different distances from said bottom 8 have the same wavelength but an increasingly lower amplitude as they approach said intermediate depth P where the sinusoid becomes similar to a straight line.

The highest amplitude of said sinusoid is obtained at the bottom 8 of said sipe where the distance of said crests 13a and 14a from said reference plane A is between approximately 0.5 mm and approximately 2 mm, preferably between approximately 1 mm and approximately 1.5 mm.

Said intermediate depth P is preferably spaced from said bottom 8 by between approximately 1 mm and approximately 10 mm, and more preferably by between approximately 2 mm and approximately 6 mm, and from said top 7 by between approximately 1 mm and approximately 4 mm. Said second surface 12 extends in the longitudinal direction X over a distance of between approximately 50% and approximately 100% of the entire longitudinal extent of said sipe 6, any remaining part of its extent being equally divided between said flat surfaces 16 and 17.

Said second surface 12 is preferably defined in a central region of said sipe 6 and is interposed between flat surfaces 16, 17 which are formed at the opposite longitudinal ends 6a, 6b of said sipe 6 in a manner coplanar with said first surface 11.

As shown in Figs. 3a to 3c, as the outer surface layer of the tread band 2 is gradually worn, the development of the sipe 6 on the tread surface 3 changes from straight (in the case of a new tyre, Fig. 3a), to slightly undulating (midway through its life, Fig . 3b) and then to highly undulating (towards the end of its life, Fig . 3c) thereby increasing the development of the gripping edge provided by the sipe 6 in respect of a snow-covered road surface and at least partly counterbalancing the reduced road-holding properties of the tyre due to the ageing of the mixture used for the tyre 1. Fig. 4 shows a variant of the tyre 1, shown overall by 100, in diagrammatic form.

Details similar to the embodiment described above bear the same reference numerals.

Tyre 100 differs from the tyre 1 as a result of the different shape of the undulating profile of the radially internal part of the sipe, shown overall by 106.

The sipe 106 differs from the sipe 6 in that the second surface 12 defines, when sectioned by any plane incident thereto, a polygonal chain in the form of a zigzag line which can be clearly seen in the bottom view of Fig . 4 where the profile of said second surface 12 is sectioned by a plane passing through the bottom 8 and parallel to the tread surface 3.

The second surface 12 is shaped such that the polygonal chain in the form of a zigzag line comprises an alternating series of first and second straight sections, shown by 101 and 102 respectively, which are connected together such that the polygonal chain has rounded apexes.

The first and second straight sections 101, 102 extend in an almost parallel manner with respect to the axial direction Z and the circumferential direction Y as defined in the tyre. Moreover, all said first sections 101 have a substantially constant extension, for instance of between approximately 1.5 mm and approximately 10 mm, preferably between approximately 2 mm and approximately 6 mm, in the same way as said second sections 102. In this way, the sipe 6 extends in a straight longitudinal direction X inclined by approximately 45° with respect to the circumferential direction Y and the axial direction Z. By varying the ratio between the measurements of the first and second sections 101, 102 in a different manner, sipes are obtained which extend in longitudinal directions having different degrees of inclination with respect to the axial direction Z and the circumferential direction Y.

Claims

A winter tyre comprising a tread band (2) in which is formed a plurality of grooves (4), at least one block (5) delimited by said plurality of grooves, at least one sipe (6) formed in said at least one block and extending between a top (7) of said sipe which opens in a tread surface (3) radially external of said tread band and a bottom (8) of said sipe radially internal of said tread band, said at least one sipe being shaped so as to define, in each of the block portions (9, 10) that are separated by said sipe :

- a first surface (11) which faces the other block portion and extends from said top (7) to an intermediate depth (P) that is defined in said sipe between said top and said bottom (8), and has a substantially straight profile, and

- a second surface (12) which faces the other block portion, is connected to said first surface (11), extends from said intermediate depth (P) to said bottom (8), and comprises a series of concave portions (13) and convex portions (14) so as to have an undulating profile,

and wherein each of said concave portions (13) and said convex portions (14) has a respective crest (13a; 14a) the distance of which from a reference plane (A) comprising said first surface (11) increases progressively from said intermediate depth (P) to said bottom (8).

A winter tyre according to claim 1, wherein, for each of said concave portions (13) and of said convex portions (14), the distance of said crest (13a; 14a) from said reference plane (A) is zero at said intermediate depth (P).

3. A winter tyre according to claim 1 or claim 2, wherein, for each of said concave portions (13) and of said convex portions (14), the distance of said crest (13a; 14a) from said reference plane (A) increases linearly from said intermediate depth (P) to said bottom (8).

4. A winter tyre according to any one of the preceding claims, wherein said concave portions (13) and said convex portions (14) are arranged on opposite sides of said reference plane (A).

5. A winter tyre according to claim 4, wherein said concave portions (13) and said convex portions (14) are arranged symmetrically with respect to said reference plane (A).

6. A winter tyre according to any one of the preceding claims, wherein said undulating profile defines a substantially sinusoidal line when said second surface (12) is sectioned by an incident plane.

7. A winter tyre according to any one of claims 1 to 5, wherein said undulating profile defines a polygonal chain in form of a zigzag line when said second surface (12) is sectioned by an incident plane.

8. A winter tyre according to claim 7, wherein said polygonal chain in the form of a zigzag line comprises an alternating series of respective first straight portions and second straight portions which are substantially parallel, respectively, to an axial direction (Z) and to a circumferential direction (Y) that are defined in said tread band (2).

9. A winter tyre according to any one of the preceding claims, wherein said sipe (6) extends in said block in a longitudinal direction (X) which is inclined with respect both to an axial direction (Z) and to a circumferential direction (Y) that are defined in said tread band (2).

10. A winter tyre according to any one of the preceding claims, wherein said sipe (6) opens, at its opposite longitudinal ends (6a, 6b), into a pair of grooves delimiting said block, so as to extend through said block from side to side.

11. A winter tyre according to any one of the preceding claims, wherein said second surface (12) is defined, in the longitudinal direction, in a central region of said sipe (6) and is interposed between flat surfaces (16, 17) which are formed at the opposite longitudinal ends (6a, 6b) of said sipe.

12. A winter tyre according to claim 11, wherein said flat surfaces (16, 17) are coplanar with said first surface (11).

13. A winter tyre according to any one of the preceding claims, wherein said intermediate depth (P) is spaced from said bottom (8) by between approximately 1 mm and approximately 10 mm.

14. A winter tyre according to any one of the preceding claims, wherein said intermediate depth (P) is spaced from said top (7) by between approximately 1 mm and approximately 4 mm.

15. A winter tyre according to claim 1, wherein said second surface (12) extends in a longitudinal direction (X) for a distance of between approximately 50% and approximately 100% of the entire longitudinal extent of said sipe (6).