WO2020012279A1

WO2020012279A1 - Tyre for vehicle wheels

Info

Publication number: WO2020012279A1
Application number: PCT/IB2019/055390
Authority: WO
Inventors: Giovanni Casarotto; Diego Ettore Speziari; Roberto Bolzoni
Original assignee: Pirelli SpA; Pirelli Tyre SpA
Current assignee: Pirelli and C SpA; Pirelli Tyre SpA
Priority date: 2018-07-13
Filing date: 2019-06-26
Publication date: 2020-01-16
Anticipated expiration: 2021-01-13

Abstract

A tyre for wheels of vehicles, comprises a tread band (2), on which there are defined: - a plurality of first pairs of main grooves that are formed by a first and a second main groove (10, 20) and extend from a first and from a second shoulder region (4, 5), respectively, towards a central region (6) with an inclination that generally decreases towards the central region, a plurality of second pairs of main grooves that alternate with the first pairs of main grooves (10, 20) and are formed by a third and a fourth main groove (30, 40) extending from the first and from the second shoulder region (4), respectively, towards the central region (6) with an inclination that generally decreases towards the central region, a plurality of first pairs of secondary grooves that are formed by a first and a second secondary groove (50, 60) and extend between two first and two second main grooves (10, 20), respectively, which are mutually successive and are inclined with respect to the circumferential direction at an angle of between approximately 20° and approximately 30°, in a discordant manner with respect to the first and to the second main groove, respectively, and a plurality of second pairs of secondary grooves that are formed by a third and a fourth secondary groove (70, 80) and extend between two first and two second main grooves (10, 20), respectively, which are mutually successive and are inclined with respect to the circumferential direction at an angle of between approximately 20° and approximately 30°, in a discordant manner with respect to the first and to the second main groove, respectively.

Description

TYRE FOR VEHICLE WHEELS

Description

The present invention relates to a tyre for vehicle wheels.

A tyre generally comprises a carcass structure that is in the shape of a toroid about an axis of rotation and comprises at least one carcass ply having end flaps that engage in respective annular anchoring structures, said bead cores.

A belt structure is provided on the radial outside of the carcass structure and, in tyres for cars, comprises at least two strips that are radially overlaid by rubber textile fabric provided with reinforcing cords, usually metal cords, arranged inside each strip in parallel with one another but crossed with the cords of the adjacent strip, preferably symmetrically with respect to the equatorial plane of the tyre. Furthermore, the belt structure preferably also comprises a third layer of textile or metal cords on the radial outside, at least at the ends of the above-mentioned belt strips, which cords are arranged circumferentially (at 0 degrees).

In tubeless tyres, a radially internal layer is also provided, called a "liner", which is impermeable in order to ensure that said tyre is airtight.

A tread band is applied in radially external position with respect to the belt structure, which band is made of elastomer material and on which there is defined a tread surface that is intended to make contact with the road surface. In order to ensure adequate road holding even when the road surface is wet, the tyres comprise a tread band provided with grooves of various shapes and geometries, which delimit tread band portions that are intended to make contact with the ground, called blocks.

The main function of the grooves is to allow the water present between the surface of the tyre and the road surface to escape when these come into contact with one another, thus preventing the hydrostatic pressure originating from the impact of the water against the advancing tyre from being able to cause the tyre to lift, even in part, from the road surface and preventing the vehicle from consequently losing control.

Small notches can also be made in the blocks of the tread band, called "sipes", which extend from the tread surface of the tyre towards the inside of the block. The function of the sipes is to offer further grip elements when driving on a surface covered in snow and trapping a certain amount of snow, thereby improving the grip with the road surface. The provision of sipes on the blocks is generally adopted in winter tyres.

Furthermore, studs may be provided in the blocks of the tread band, which, thanks to a portion thereof that projects from the tread surface, improve grip on icy road surfaces.

The overall configuration of the tread band defined by all the grooves and blocks represents the tread pattern.

The Applicant has preliminarily observed that the performance of a studded tyre on icy surfaces, in particular in terms of braking and traction, is especially influenced by the arrangement of the studs on the tread band.

In particular, the Applicant has observed that, in footprint area, it would be appropriate for the studs to be offset with respect to the circumferential direction in order to prevent a stud being located in the rut of a stud that precedes it and thus not being able to apply the required grip to the road surface during braking or sharp acceleration.

In order to avoid this drawback, the Applicant has identified a plurality of "virtual" circumferential tracks on the tread band, which extend in parallel with the equatorial plane of the tyre, and has therefore positioned the studs in these tracks such that the spacing between successive studs in the same track is always greater than the circumferential dimension of the footprint area.

The Applicant has subsequently determined that, in order to be able to arrange the highest number of studs, it was necessary to increase the number of tracks that can be effectively used on the tread band to as many as possible. Furthermore, the Applicant has verified that, in order to be effectively usable, each track had to comply with certain constraints, which include being at a minimum spacing from other tracks, depending on the dimensions of the studs, and that of crossing at least a plurality of blocks in a region thereof that is at a sufficient spacing from the respective edges such that the stud can be firmly held on the tread band.

On the basis of these considerations, the Applicant has therefore observed that circumferential tracks cannot be defined in possible circumferential grooves in the tread band, and therefore the useful tread band portion on which to position the studs is, in fact, reduced.

However, the Applicant has verified that the presence of circumferential grooves makes it possible for the water present in the footprint area to be removed from the tread band both rapidly and effectively, thus also improving the performance of the tyre in terms of road holding, in particular turning.

The Applicant has therefore felt the need to provide a tyre, the tread pattern of which was configured to allow a large number of circumferential tracks to be defined and, as a result, the possibility of providing a large number of studs in the tread band, thus simultaneously offering high performance in terms of road holding.

The Applicant has lastly found that designing the tread band to have a plurality of main grooves that extend from the opposite shoulder regions up to the central region at an inclination with respect to the circumferential direction that decreases as said grooves get closer to the central region, and providing pairs of secondary grooves that extend between successive main grooves at an inclination that is opposite to that of said main grooves and limited to a narrow and specific angular range, a tyre is produced that has optimum behaviour in terms of lateral grip and, at the same time, the possibility of providing studs on a particularly large number of circumferential tracks. In particular, in a first aspect, the invention relates to a tyre for vehicle wheels, comprising a tread band.

A first shoulder region and a second shoulder region, which are axially opposite one another, and a central region arranged between said first and second shoulder region are preferably defined on the tread band.

A plurality of first pairs of main grooves that are provided in regular succession over the circumferential extent of said tread band are preferably defined in the tread band.

Each first pair is preferably formed by a first main groove that extends from said first shoulder region towards said central region and from a second main groove that extends from said second shoulder region towards said central region.

The inclination of said first main groove with respect to said circumferential direction preferably generally decreases from said first shoulder region towards said central region.

The inclination of said second main groove with respect to said circumferential direction preferably generally decreases from said second shoulder region towards said central region.

A plurality of second pairs of main grooves that are provided in regular succession over the circumferential extent of said tread band are preferably defined in the tread band.

Said second pairs of main grooves are preferably made in the tread band in an alternating position with said first pairs of main grooves.

Each second pair of main grooves is preferably formed by a third main groove that extends from said first shoulder region towards said central region, and by a fourth main groove that extends from said second shoulder region towards said central region.

The inclination of said third main groove with respect to said circumferential direction preferably generally decreases from said first shoulder region towards said central region.

The inclination of said fourth main groove with respect to said circumferential direction preferably generally decreases from said second shoulder region towards said central region.

A plurality of first pairs of secondary grooves are preferably made in said tread band and are provided in regular succession over the circumferential extent of said tread band.

Each first pair of secondary grooves is preferably formed by a first secondary groove and by a second secondary groove.

Said first secondary groove preferably extends between two of said first main grooves that are mutually successive.

Said first secondary groove is preferably inclined with respect to said circumferential direction by an angle of between approximately 20° and approximately 30°.

Said first secondary groove is preferably inclined in a discordant manner with respect to said first main groove.

Said second secondary groove preferably extends between two of said second main grooves that are mutually successive.

Said second secondary groove is preferably inclined with respect to said circumferential direction by an angle of between approximately 20° and approximately 30°.

Said second secondary groove is preferably inclined in a discordant manner with respect to said second main groove.

A plurality of second pairs of secondary grooves are preferably made in said tread band and are provided in regular succession along the circumferential extent of said tread band.

Each second pair of secondary grooves is preferably formed by a third secondary groove and by a fourth secondary groove. Said third secondary groove preferably extends between two of said first main grooves that are mutually successive.

Said third secondary groove is preferably inclined with respect to said circumferential direction by an angle of between approximately 20° and approximately 30°.

Said third secondary groove is preferably inclined in a discordant manner with respect to said first main groove.

Said fourth secondary groove preferably extends between two of said second main grooves that are mutually successive.

Said fourth secondary groove is preferably inclined with respect to said circumferential direction by an angle of between approximately 20° and approximately 30°.

Said fourth secondary groove is preferably inclined in a discordant manner with respect to said second main groove.

On account of these features, the Applicant has verified that the tyre according to the invention makes it possible to define a very high number of circumferential tracks on the tread band, which are suitable for the arrangement of the studs, thus also providing optimum performance in terms of lateral grip and ability to remove the water.

This advantageous effect is attributed to the overall configuration of the main grooves and of the secondary grooves.

In fact, the provision of the secondary grooves that extend between successive main grooves at an inclination of between approximately 20° and 30° with respect to the circumferential direction makes it possible to define, in the tread band portion that said secondary grooves cross, effective circumferential tracks in which possible studs of the tyre are to be positioned, thereby still ensuring the speediness with which the water is removed, and edges that can bear possible transverse tangential stresses. Furthermore, the provision of the main grooves with their curvilinear profile, the inclination of which gradually decreases as it approaches the equatorial plane, allows for rapid discharge of water from the central region towards the opposite shoulder regions, and offers effective edges for gripping the road surface for traction and when turning.

These features advantageously make it possible to use the tread pattern as a base for forming a studded tyre, in particular a studded tyre having a large number of studs, for example more than 150, but also as a basis for forming a winter tyre.

The term "equatorial plane" of the tyre is intended to mean a plane that is perpendicular to the axis of rotation of the tyre and divides the tyre into two equal parts.

The "circumferential" direction is intended to mean a direction that is generally oriented according to the direction of rotation of the tyre or, at most, slightly inclined (no more than approximately 5°) with respect to the direction of rotation of the tyre.

The "axial" direction is intended to mean a direction that is substantially parallel to the axis of rotation of the tyre, or, at most, slightly inclined (no more than approximately 5°) with respect to said axis of rotation of the tyre. The axial direction is generally perpendicular to the circumferential direction.

The term "effective width" referring to the tread band is intended to mean the width of the radially outermost portion of the tread band (from edge to edge), which is intended to make contact with the ground.

The "central region" of the tread band is intended to mean a tread band portion that extends circumferentially about an equatorial plane of said tyre for a width that is equal to at least 30% of the effective width of the tread band, preferably for a width of between 40% and 60% of said effective width.

The central region may or may not extend symmetrically with respect to the equatorial plane.

In particular, when circumferential or almost circumferential grooves are made in the tread band, the central region can be delimited on one or both sides by one of said circumferential grooves.

The "shoulder regions" of the tread band are intended to mean the tread band portions that extend circumferentially on opposite sides of the central region on the axial outside of the tread band.

Each shoulder region preferably extends for a width that is equal to at least 10% of the effective width of the tread band.

The term "groove" is intended to mean a recess made in a tread band portion that has a width that is greater than or equal to 1.5 mm, and preferably has a depth of more than 3 mm.

A groove is said to be "circumferential" when it extends in a circumferential direction or, at most, is inclined with respect to the circumferential direction by an angle of less than 5°.

A groove is said to be "transverse" when it extends in a direction that is inclined with respect to the circumferential direction by an acute angle of more than at least 10°.

The term "sipe" is intended to mean a recess made in a tread band portion that has a width that is smaller than 1.5 mm, preferably smaller than or equal to 1 m m .

The width of the sipes and grooves is intended to be measured at a depth that is greater than or equal to 1 mm, preferably greater than or equal to 1.5 mm.

In the case in which the width of the groove or of the sipe varies along its longitudinal extent, the width is considered to be an average width, the value of which is the average of the various width values that are suitably weighed on the basis of the relative longitudinal extensions. For example, if a groove has a width of 5 mm for 80% of its longitudinal extension and a width of 3 mm for the remaining 20%, the average of the width to be taken into account will be equal to 5 x 0.8 + 3 x 0.2 = 4.6 mm.

Similarly, if the width of one groove, in particular of a transverse groove, varies along the tread band depending on the length of the pitch to which it belongs, it is considered to be the average.

One end of one groove is defined as being "blind" when it is not open to another groove.

The inclination of a transverse groove with respect to a circumferential direction identified on the tread band is defined by the acute angle formed by the groove and the circumferential direction. As a specific case, a transverse groove that extends in parallel with the axis of the tyre will have an inclination of 90° with respect to the circumferential direction.

Two (or more) transverse grooves are inclined "in a concordant manner" when the profile of both grooves increases or decreases when considered on a Cartesian plane positioned on the tread band (tangentially thereto), in which the y-axis is parallel to the circumferential direction and the x-axis is parallel to the axis of the tyre.

As a result, two transverse grooves are inclined "in a discordant manner" when the profile thereof increases for one groove and decreases for the other groove, when considered in said Cartesian plane.

A groove has an inclination that "generally decreases" from a first region towards a second region when :

- for segments equal to at least 50% of the longitudinal extension of the groove between the first and the second region, the inclination of the groove decreases as it moves from the first region towards the second region, and

- for segments equal to at least 80% of the longitudinal extension of the groove between the first and the second region, the inclination of the groove does not increase as it moves from the first region towards the second region.

Two groove segments (or two grooves) are "substantially aligned" when their longitudinal axes are offset, at least at their respective ends that face one another, by an amount that is smaller than their width (for segments having different widths, the larger width is taken into consideration).

Two grooves are "successive" when they are arranged one behind the other in the tread surface, taking into consideration the rotation of the tyre in any one of the two directions of rotation. In particular, two grooves of the same type are successive when no other grooves of this type are provided therebetween.

The "tread pattern" means the overall configuration of the tread band as defined by all of the grooves and the blocks delimited by the grooves.

A "module" of the tread pattern is defined by the minimum tread band portion, a repeated sequence of which configuration is provided along the circumferential extent of the tread band in order to form said tread pattern.

A module extends between the axial ends of the tread band.

Furthermore, while maintaining an identical basic configuration, the modules can have a circumferential dimension (said "pitch") that differs slightly for each module, for example modules having two, three or four different pitches can be used on one tread band in various combinations.

A module may be formed by two or more elementary portions (or submodules), which extend between the two axial ends of the tread band and are arranged in the same sequence inside each module.

In this case, each elementary portion can have the same basic configuration in different modules but slightly different circumferential dimensions (called "pitches"), such that each module can be formed by elementary portions having a different pitch.

The modules having two elementary portions are also called "two-pitch modules", while the modules that do not comprise at least two elementary portions (or, in other words, modules that comprise just one submodule that coincides with the module) are also called "single-pitch modules".

The "footprint area" means the tread band portion that immediately comes into contact with the road surface during the revolution of the tyre. Under real-life conditions, the footprint area is dependent on various parameters, including the tyre pressure, the load it is subjected to, the road surface and the driving conditions, for which reference values may, however, be defined.

In the above-mentioned aspect, the present invention can comprise at least one of the additional preferred features specified below.

Each of said first main grooves preferably comprises a first segment having a curvilinear profile, which extends from said first shoulder region towards an equatorial plane of said tread band, and has an inclination with respect to said circumferential direction that generally decreases from said first shoulder region towards said central region.

Each of said second main grooves preferably comprises a first segment having a curvilinear profile, which extends from said second shoulder region towards an equatorial plane of said tread band, and has an inclination with respect to said circumferential direction that generally decreases from said second shoulder region towards said central region.

Said first segment of said first main groove and said first segment of said second main groove are preferably inclined in a discordant manner with respect to said circumferential direction.

In this way, removal of the water from the central region towards the respective shoulder regions is facilitated in particular when the tyre is rotated in a predefined direction.

Said first segments of each of said first and said second main grooves preferably have a curvilinear profile that does not comprise inflection points. The inclination with respect to said circumferential direction of said first segment of each of said first and/or said second main grooves in said first shoulder region is preferably defined by a first angle of between 70° and 90°.

The inclination with respect to said circumferential direction of said first segment of each of said first and/or said second main grooves in said central region is preferably defined by an angle of between 20° and 45°.

In this way, the rapid removal of the water from the central region towards the shoulder regions is facilitated, while simultaneously maintaining a high degree of resistance of the shoulder regions to the transverse tangential stresses.

The width of each of said first main grooves preferably decreases from said first shoulder region towards said equatorial plane.

The width of each of said second main grooves preferably decreases from said second shoulder region towards said equatorial plane.

In this way, on the one hand, the removal of the water from the central region towards the respective shoulder regions is facilitated, and, on the other hand, the rigidity of the first and the second blocks in the central region is improved. Said first main groove and said second main groove are preferably axially offset in each first pair of main grooves.

Said third main groove and said fourth main groove are preferably axially offset in each second pair of main grooves.

In this way, the noise produced by the tyre rolling on the road surface is advantageously decreased, since the impacts of the edges of the respective grooves with the ground are offset with respect to one another, thereby decreasing the intensity of the overall noise generated.

Each of said third secondary grooves is preferably substantially parallel to said first secondary groove.

Each of said fourth secondary grooves is preferably substantially parallel to said second secondary groove. Said first and said second shoulder region preferably do not comprise circumferential grooves.

In this way, there are no circumferential groove in the shoulder regions, giving the possibility, at most, for one or more circumferential grooves to be provided in the central region, where it is preferable not to provide too many studs.

In a more preferred manner, said tread band does not comprise any circumferential grooves.

In this way, it is possible to position studs in every circumferential tread band portion.

Said third main grooves preferably do not intersect any other first, second, third or fourth main groove.

Said fourth main grooves preferably do not intersect any other first, second, third or fourth main groove.

In this way, blocks remain defined that are sufficiently wide so as to allow studs to be securely positioned and not too closely to the edges.

Said third main grooves and said fourth main grooves preferably have a profile that is substantially symmetrical with respect to said circumferential direction. Said third main grooves preferably have a profile that is substantially parallel to said first main grooves.

Said fourth main grooves preferably have a profile that is substantially parallel to said second main grooves.

Said second main groove preferably has an axially internal end that is connected to said first main groove.

Said first main groove preferably comprises an axially internal end segment, which is connected to a second main groove that is arranged behind said second main groove, so as to define a pair of central blocks in said central region that are delimited by said first pairs of main grooves and by said first pairs of secondary grooves. In this way, on the one hand, a grid of grooves is obtained in the central region that allows the water to be quickly removed, and, on the other hand, a pair of central blocks is defined, which improve the traction of the tyre and provide a sufficient amount of space for positioning the studs.

Said end segment of said first main groove preferably comprises a first portion that extends as a continuation of said first segment towards said first shoulder region, and a second portion that extends as a continuation of said first portion towards said second shoulder region.

Said first portion and said second portion of said end segment preferably straddle an equatorial plane of said tread band.

Said second portion of said end segment preferably extends up to said second successive main groove.

The end segment thus assumes a configuration having a zigzag-shaped polygonal chain, which joins the first curvilinear segment of the first main groove to the second main groove of the subsequent pair of first main grooves.

Said first portion of said end segment preferably extends over the extension of said fourth main groove.

Said second portion of said end segment preferably extends over the extension of said third main groove.

Each of said pairs of central blocks preferably comprises a first block and a second block that are V-shaped, the concavities of which point towards one another.

Said first and second block are preferably substantially specular with respect to said circumferential direction, more preferably with respect to said equatorial plane.

On account of these features, the first and the second blocks define a central region, the behaviour of which is scarcely influenced by the direction of rotation of the tyre. This feature is especially important in that it makes it possible to obtain substantially balanced performance in terms of traction (that is of acceleration and of braking).

Each of said first secondary grooves preferably intersects one of said third main grooves in an axially internal end region thereof.

Each of said third main grooves preferably comprises a blind axially internal end. Said blind axially internal end is preferably positioned between said first secondary groove and said end segment of said first main groove, inside one of said central blocks.

Each of said first secondary grooves preferably extends between a first axially external end and a second axially internal end, said first end and said second end being open in respective successive first main grooves.

Said first secondary grooves are preferably substantially rectilinear.

Said first secondary grooves are preferably parallel to one another.

Said first secondary grooves are preferably substantially aligned in said circumferential direction.

Each of said first secondary grooves preferably comprises a first portion that extends between said first end and a third main groove, and a second portion that extends between said third main groove and said second end.

Said first and said second portion of said first secondary groove are preferably substantially rectilinear and aligned with one another.

The width of said first portion of said first secondary groove preferably increases from said first end up to said third main groove.

The ratio between the width of said first portion of said first secondary groove at said third main groove and at said first end is preferably between 1.25 and 1.5, more preferably between 1.3 and 1.4.

This maximises the snow-trapping effect during braking.

The width of said second portion of said first secondary groove preferably increases from said third main groove up to said second end. The ratio between the width of said second portion of said first secondary groove at said first end and at said third main groove is preferably between 1.25 and 1.5, more preferably between 1.3 and 1.4.

This maximises the snow-trapping effect during braking.

This configuration of the first secondary grooves promotes the grip of the tyre on a snow-covered surface, in particular for traction.

Each of said second secondary grooves preferably intersects one of said fourth main grooves at an axially internal end region thereof.

Each of said fourth main grooves preferably comprises a blind axially internal end.

Said blind axially internal end is preferably positioned between said second secondary groove and said end segment of said first main groove, inside one of said central blocks.

Each of said second secondary grooves preferably extends between a first axially external end and a second axially internal end, said first end and said second end being open in respective successive second main grooves.

Each of said second secondary grooves is preferably inclined with respect to said circumferential direction in a discordant manner with respect to said first segment of said second main groove.

Said second secondary grooves are preferably substantially rectilinear.

Said second secondary grooves are preferably substantially aligned in said circumferential direction.

Said second secondary grooves are preferably parallel to one another.

Each of said second secondary grooves preferably comprises a first portion that extends between said first end and a fourth main groove, and a second portion that extends between said fourth main groove and said second end.

Said first and said second portion of said second secondary groove are preferably substantially rectilinear and aligned with one another. The width of said first portion of said second secondary groove preferably increases from said first end up to said fourth main groove.

The ratio between the width of said first portion of said second secondary groove at said fourth main groove and at said first end is preferably between 1.25 and 1.5, more preferably between 1.3 and 1.4.

This maximises the snow-trapping effect during braking.

The width of said second portion of said second secondary groove preferably increases from said fourth main groove up to said second end.

The ratio between the width of said second portion of said second secondary groove at said second end and at said fourth main groove is preferably between 1.25 and 1.5, more preferably between 1.3 and 1.4.

This maximises the snow-trapping effect during braking.

Said first secondary grooves and said second secondary grooves are preferably substantially mutually specular with respect to an equatorial plane of said tread band.

Said third secondary grooves are preferably substantially rectilinear.

Said third secondary grooves are preferably substantially aligned in said circumferential direction.

Said third secondary grooves are preferably arranged between said first secondary grooves and a first lateral edge of said tread band that externally delimits said first shoulder region.

Each of said third secondary grooves preferably comprises a first portion that extends between a first axially external end and a third main groove, and a second portion that extends between said third main groove and a second axially internal end.

Said first and said second portion of said third secondary groove are preferably substantially rectilinear and aligned with one another.

The width of said first portion of said third secondary groove preferably increases from said first end up to said third main groove.

The width of said second portion of said third secondary groove preferably increases from said third main groove up to said second end.

Said fourth secondary grooves are preferably substantially rectilinear.

Said fourth secondary grooves are preferably substantially aligned in said circumferential direction.

Said fourth secondary grooves are preferably arranged between said second secondary grooves and a second lateral edge of said tread band that externally delimits said second shoulder region.

Each of said fourth secondary grooves preferably comprises a first portion that extends between a first axially external end and a fourth main groove, and a second portion that extends between said fourth main groove and a second axially internal end.

Said first and said second portion of said fourth secondary groove are preferably substantially rectilinear and aligned with one another.

The width of said first portion of said fourth secondary groove preferably increases from said first end up to said fourth main groove.

The width of said second portion of said fourth secondary groove preferably increases from said fourth main groove up to said second end.

Said third secondary groove and said fourth secondary groove are preferably substantially mutually specular with respect to an equatorial plane of said tread band.

Respective pluralities of sipes are preferably made in at least some blocks defined on said tread band.

Sipes extending in a substantially axial direction are preferably made in said first and second blocks.

In this way, the action of the sipes is also substantially independent of the direction of rotation of the tyre in the central region of the tread band. Sipes extending in a direction that is transverse to the main direction of extension of said blocks are preferably made on the blocks defined between said first pairs of secondary grooves and said second pairs of secondary grooves.

Sipes that extend in a direction that is substantially parallel to the main direction of extension of said blocks are preferably made in the blocks defined between said second pairs of secondary grooves and the respective lateral edges of said tread band.

Studs are preferably provided in at least some blocks that are defined on said tread band.

The features and advantages of the invention will become clearer from the detailed description of a few preferred embodiments thereof, which are illustrated by way of non-limiting example, with reference to the attached drawings, in which :

- Fig. 1 is a perspective front view of a first example of a tyre for vehicle wheels, which is formed in accordance with the present invention;

- Fig. 2 is an enlarged schematic view of a substantial portion of the tread band of the tyre in Fig. 1; and

- Fig. 3 is an enlarged schematic view of a substantial tread band portion of a variant embodiment of the tyre in Fig. 1.

With initial reference to Fig. 1 and 2, 1 indicates a tyre for vehicle wheels as a whole, which is formed in accordance with the present invention.

The tyre 1 comprises a tyre structure that is conventional per se but is not shown in the attached figures, and a tread band 2, on which a tread surface 3, located in a radially external position and intended to make contact with a road surface, is defined.

The tyre 1 has a conventional generically toroidal shape, which extends about an axis of rotation so as to define an axial direction Y on the tread surface 3 that is parallel to said axis of rotation, and which an equatorial plane X crosses that is perpendicular to the axis of rotation and defines a circumferential direction on the tread surface 3 that is parallel to said equatorial plane.

An effective width L is identified on the tread band 2, which is defined as the maximum width of the tread band intended to make contact with the ground during standard use conditions.

The tyre 1 is a directional tyre, in which a preferred direction of rotation of the tyre is defined, shown in the figures by the arrow F.

A first shoulder region 4 is also defined on the tread band 2, which is delimited on the axially external side by a first lateral edge 4a of the tread band 2, a second shoulder region 5, which is axially opposite the first shoulder region 4 and is delimited on the axially external side by a second lateral edge 5a of the tread band 2, and a central region 6, which is arranged between the first and the second shoulder region 4 and 5 and straddles the equatorial plane X.

A plurality of pairs of central blocks are provided in regular succession along the circumferential extent of the central region 6, each pair being formed by a first block 7 and by a second block 8, which are aligned in the axial direction Y at least in part. In other words, the respective projections in the axial direction Y of the first and the second block 7, 8 of each pair of central blocks partially overlap. Each first block 7 is V-shaped and comprises a vertex 7a that faces the first shoulder region 4 and a pair of arms 7b and 7c that extend towards the second shoulder region 5 in respective directions A and B so as to be inclined in the opposite direction to the circumferential direction.

The directions A and B are inclined with respect to the circumferential direction by approximately 35° and approximately 20°, respectively.

Similarly, each second block 8 is V-shaped and comprises a vertex 8a that faces the second shoulder region 5 and a pair of arms 8b and 8c that extend towards the first shoulder region 4 in the directions C and D, respectively. In this way, the arms 7b and 7c are substantially parallel to the arms 8c and 8b. The directions C and D are inclined with respect to the circumferential direction by approximately 35° and approximately 20°, respectively.

The directions C and D are preferably substantially specular with respect to the directions A and B with respect to the circumferential direction.

On account of this configuration, the respective concavities of the first block 7 and the second block 8 point towards one another.

However, said blocks are axially offset such that the arm 7c of the first block 7 can be partially received inside the concavity of the corresponding second block 8, and the arm 8b of the second block 8 can be partially received inside the concavity of the corresponding first block 7.

In this way, the first and second blocks 7 and 8 of each pair partially fit into one another, thereby increasing the mutual resistance to tangential stresses, in particular transverse tangential stresses.

All the vertexes 7a of said first blocks 7 and all the vertexes 8a of the second blocks 8 are preferably aligned in respective circumferential directions.

The tread band 2 does not comprise circumferential grooves, but various transverse grooves are defined thereon, as explained in the following.

A plurality of first pairs of main grooves are therefore defined in the tread band 2, which are provided in regular succession along the circumferential extent of the tread band 2 in order to delimit, in part, the first and the second blocks 7 and 8.

Each first pair is formed by a first main groove 10, which extends from the first shoulder region 4 towards the central region 6, and by a second main groove 20, which extends from the second shoulder region 5 towards the central region 6. Each first main groove 10 comprises a first segment 11, which extends from an axially external end 10a, which is open in the first shoulder region 4, towards the central region 6, and an axially internal end segment 12, which extends in the central region 6 as a continuation of the first segment 11 until it opens, with an axially internal end 10b thereof, into a second main groove 20, which is successive to the second main groove 20 that forms the pair of first main grooves together with the first main groove 10.

The first main groove 10, in particular along the entire first segment 11 thereof, has a curvilinear profile that does not comprise inflection points and has an inclination with respect to the circumferential direction that generally decreases from the first shoulder region 4 towards the central region 6.

The end segment 12 of each first main groove 10 comprises a polygonal chain configuration and comprises a first portion 13, which extends as a continuation of the first segment 11 towards the first shoulder region 4, and a second portion 14 that extends as a continuation of the first portion 13 towards the second shoulder region 5.

The first portion 13 and the second portion 14 straddle the equatorial plane X of the tread band 2 and are inclined in a discordant manner with respect to the circumferential direction.

Each second main groove 20 comprises a first segment 21, which extends from an axially external end 21a, which is open in the second shoulder region 5, towards the central region 6. In a similar way to the first segment 11 of the first main groove 10, the first segment 21 of the second main groove 20 also has a curvilinear profile, which does not comprise inflection points and has an inclination with respect to the circumferential direction that generally decreases from the second shoulder region 5 towards the central region 6.

On the whole, the profiles of the first and second main groove 10 and 20 of each first pair are therefore similar, comprising a main part of the respective first segments 11 and 21, which are substantially symmetrical with respect to the equatorial plane X, and comprising concavities that point towards the same side of the tread band.

However, the second main groove 20 does not comprise an end segment that is similar to the end segment 12, and said second main groove in fact coincides with the first segment 21 thereof.

Furthermore, the second main groove 20 is axially offset with respect to the first transverse groove 10. In fact, the respective ends 10a and 20a are offset by between 5 and 20 mm, preferably between 10 and 20 mm, even more preferably between 13 and 17 mm, for example by approximately 15 mm, measured in the circumferential direction.

By selecting a suitable value for the offset between said ends 10a and 20a, it is possible to further improve the performance of the tyre in terms of noise, thereby optimising entry of the relative blocks into the footprint area.

The first segment 21 extends from an axially external end 20a of the second main groove 20, which is open in the second shoulder region 5, up to the axially internal end 20b of the second main groove 20, which is open in the first segment 11 of the first main groove 10.

In particular, the second main groove 20 combines with the first main groove 10 shortly after having crossed the equatorial plane X.

An internal end region 15 therefore remains defined in the first segment 11 of each first main groove 10, which region is delimited by the end 20b of the second main groove 20 and by the start of the end segment 12.

Furthermore, an internal end region 22 remains defined in the first segment 21 of each second main groove 20, which region is delimited by the ends 20b and 10b of the first main groove 10.

Therefore, in this way a zigzag-shaped polygonal chain is formed, which is defined by the internal end region 22 of the second main groove 20, by the internal end region 15 of the first main groove 10, by the first portion 13 of the end segment 12, and by the second portion 14 of the end segment 12.

This polygonal chain extends continuously along the entire circumferential extension of the equatorial plane X and every subsequent portion thereof preferably crosses said plane.

This polygonal chain also separates the first and the second blocks 7 and 8 on the axially internal side.

A plurality of second pairs of main grooves are also defined in the tread band 2, which are provided in regular succession along the circumferential extent of the tread band 2 in an alternating position with the first pairs of main grooves 10 and 20.

Each second pair is formed by a third main groove 30, which extends from the first shoulder region 4 towards the central region 6, and by a fourth main groove 40, which extends from the second shoulder region 5 towards the central region 6.

In particular, each third main groove 30 extends between an axially external end 30a that is open in the first shoulder region 4 and a blind axially internal end 30b inside the first block 7.

Similarly, each fourth main groove 40 extends between an axially external end 40a that is open in the second shoulder region 5 and a blind axially internal end 40b inside the second block 8.

None of the third and fourth main grooves 30, 40 intersects another first, second, third or fourth main groove.

The third and the fourth main grooves 30 and 40 have a similar curvilinear profile that is substantially parallel to the first and the second main grooves 10 and 20.

Furthermore, the third and the fourth main grooves 30 and 40 are substantially symmetrical with respect to the equatorial plane X, even if they are axially offset with respect to one another. In particular, the respective ends 30a and 40a are offset by between 5 and 20 mm, preferably between 10 and 20 mm, even more preferably between 13 and 17 mm, for example by approximately 15 mm, measured in the circumferential direction. By selecting a suitable value for the offset between said ends 30a and 40a, it is possible to further improve the performance of the tyre in terms of noise, thereby optimising the entry of the relative blocks into the footprint area.

The first portion 13 of the end segment 12 is advantageously located on the theoretical extension of the fourth main groove 34, even if there are no points of contact between these grooves.

Similarly, the second portion 14 of the end segment 12 is also located on the theoretical extension of the third main groove 30.

All the first, second, third and fourth main grooves 10, 20, 30 and 40 have a depth of approximately 9 mm and a variable width that decreases from the shoulder regions towards the central region.

In particular, the width of these main grooves varies from approximately 9-12 mm in the relevant shoulder region up to approximately 2.5-4 mm in the central region 6.

The depth of the first main grooves 10 is reduced to approximately 4 mm in large segments of the end segment 12, as well as in the internal end region 22 of the second main groove 20 and in the internal end region 15 of the first main groove 10.

In this way, the first and second blocks 7 and 8 are connected to one another to a greater degree, thereby increasing their resistance to tangential stresses, in particular transverse tangential stresses.

A plurality of first pairs of secondary grooves are also made in the tread band 2, which are provided in regular succession along the circumferential extent of the tread band 2.

The first pairs of secondary grooves preferably partially delimit the first and the second blocks 7, 8 on the axially external side thereof.

Each first pair of secondary grooves is formed by a first secondary groove 50, which extends between a first axially external end 51 and a second axially internal end 52, which are open in two first main grooves 10 that are mutually successive, and by a second secondary groove 60 that extends between a first axially external end 61 and a second axially internal end 62, which are open in two second main grooves 20 that are mutually successive.

Each first secondary groove 50 intersects a third main groove 30 in an axially internal end region thereof, which is near to its end 30b. In this way, a first portion 53, which extends between the first end 51 and the third main groove 30, and a second portion 54, which extends between the third main groove 30 and the second end 52, remain defined on each first secondary groove 50.

The first and the second portion 53 and 54 of each first secondary groove are aligned with one another in a direction that is substantially rectilinear and is inclined with respect to the circumferential direction by an angle of approximately 25°, in a discordant manner with respect to the inclination of the first segments 11 of the first main grooves 10 between which it extends.

The width of the first portion 53 of each first secondary groove 50 increases from the first end 51, where it measures approximately 3 mm, up to the third main groove 30, where it measures approximately 4 mm.

In the same way, the width of the second portion 54 of each first secondary groove 50 increases from the third main groove 30, where it measures approximately 3 mm, up to the second end 52, where it measures approximately 4 mm.

All of the first secondary grooves 50 are substantially parallel to one another and the first and second ends 51 and 52 thereof are also substantially aligned in respective circumferential directions.

The second secondary grooves 60 are substantially specular with respect to the first secondary grooves 50 with respect to the equatorial plane X.

In particular, each second secondary groove 60 intersects a fourth main groove 40 in an axially internal end region thereof, which is near to its end 40b. In this way, a first portion 63, which extends between the first end 61 and the fourth main groove 40, and a second portion 64, which extends between the fourth main groove 40 and the second end 62, remain defined on each second secondary groove 60.

The first and the second portion 63 and 64 of each second secondary groove are aligned with one another in a substantially rectilinear direction, which is inclined with respect to the circumferential direction by an angle of approximately 25°, in a discordant manner with respect to the inclination of the first segments 21 of the second main grooves 20 between which it extends.

The width of the first portion 63 of each second secondary groove 60 increases from the first end 61, where it measures approximately 3 mm, up to the fourth main groove 40, where it measures approximately 4 mm.

In the same way, the width of the second portion 64 of each second secondary groove 60 increases from the fourth main groove 40, where it measures approximately 3 mm, up to the second end 62, were it measures approximately 4 mm.

All of the second secondary grooves 60 are substantially parallel to one another and their first and second ends 61 and 62 are also substantially aligned in respective circumferential directions.

The depth of the first and the second secondary grooves 50 and 60 varies between approximately 7 mm in the groove segments having a smaller width and approximately 4 mm in the groove segments having a larger width.

This further increases the resistance of the first and the second blocks 7 and 8 to transverse stresses.

A plurality of second pairs of secondary grooves are also made in the tread band 2, which are provided in regular succession along the circumferential extent of the tread band 2.

Each second pair of secondary grooves is formed by a third secondary groove 70, which extends in the first shoulder region 4 between a first axially external end 71 and a second axially internal end 72, which are open in two first main grooves 10 that are mutually successive, and by a fourth secondary groove 80, which extends in the second shoulder region 5 between a first axially external end 81 and a second axially internal end 82, which are open in two second main grooves 20 that are mutually successive.

The third secondary grooves 70 are substantially parallel to the first secondary grooves 50 and are arranged between said first secondary grooves and the first lateral edge 4a.

In a similar way to the first secondary grooves 50, the third secondary grooves 70 intersect a third main groove 30 such that a first portion 73, which extends between the first end 71 and the third main groove 30, and a second portion 74, which extends between the third main groove 30 and the second end 72, remain defined in each third secondary groove 70.

The first and the second portion 73 and 74 of each third secondary groove 70 are aligned with one another and have a variable width in a very similar way to the first and the second portion 53, 54 of each first secondary groove 50.

The fourth secondary grooves 80 are substantially parallel to the second secondary grooves 60 and are arranged between said second secondary grooves and the second lateral edge 5a.

In a similar way to the second secondary grooves 60, the fourth secondary grooves 80 intersect a fourth main groove 40 such that a first portion 83, which extends between the first end 81 and the fourth main groove 40, and a second portion 84, which extends between the fourth main groove 40 and the second end 82, remain defined in each fourth secondary groove 80.

The first and the second portion 83 and 84 of each fourth secondary groove 80 are aligned with one another and have a variable width in a very similar way to the first and the second portion 63, 64 of each second secondary groove 60. In addition to the first and the second blocks 7 and 8, shoulder blocks 9a and 9b remain defined on the tread band 2 by all the grooves defined above.

The shoulder blocks 9a are delimited by the first and second pairs of main grooves (10, 30 or 20 and 40) and by the first and second pairs of secondary grooves (50, 70 or 60 and 80), while the shoulder blocks 9b are delimited by the first and second pairs of main grooves (10, 30 or 20 and 40), by the second pairs of secondary grooves (70 or 80) and by the lateral edges of the tread band (4a or 5a).

Sipes are preferably made in all the blocks described above, which sipes support the performance of the tyre on snow-covered surfaces.

In particular, sipes 90 are made in the first and second blocks 7 and 8 and extend substantially in the axial direction Y, sipes 91 are made in the shoulder blocks 9a and extend in a direction that is transverse to the main direction of extension of the blocks 9a (defined by the direction of the first or second main grooves), and sipes 92 are made in the shoulder blocks 9b and extend in a direction that is substantially parallel to the main direction of extension of the blocks 9b (also defined by the direction of the first or second main grooves).

The blocks and the grooves identified above define, as a whole, the tread pattern of the studded tyre 1.

The tread pattern is formed by the continuous repeated sequence of a single module M that is formed by the tread band portion located between two first successive pairs of main grooves 10 and 20.

Each module M is in turn formed by a first elementary portion, which extends from a first pair of main grooves 10 and 20 up to the second pair of main grooves 30 and 40, and by a second elementary portion, which extends from the second pair of main grooves 30 and 40 up to the first successive pair of main grooves 10 and 20.

Each first elementary portion and each second elementary portion can have a circumferential dimension that corresponds to a long pitch or the size of which corresponds to a short pitch, that is shorter than the long pitch.

In particular, the long pitch is approximately 30% greater than the short pitch. Each module M can be formed by a first and a second elementary portion having a short pitch or a long pitch that are combined in various ways such that modules are provided on the tread pattern in which both the first and the second elementary portion have a long pitch, or modules in which both the first and the second elementary portion have a short pitch, or modules in which the first elementary portion has a short pitch and the second elementary portion has a long pitch or, lastly, modules in which the first elementary portion has a long pitch and the second elementary portion has a short pitch.

The tyre 1, formed as described above, performs well on snow-covered and icy surfaces, in particular in terms of lateral grip and ability to remove the water. Furthermore, the tyre 1 makes it possible to define a very large number of circumferential tracks on the tread band 2 that are suitable for the arrangement of the studs.

In particular, 12 circumferential tracks can be defined in each first and second region 4 and 5, which can be effectively used to provide a number, even particularly high, of studs in the blocks of the tread band 2.

The circumferential tracks are parallel to the equatorial plane X and are defined in the first and in the second region 4 and 5 so as to be symmetrical thereto.

The spacing between one circumferential track and another circumferential track is between 5 and 15 mm, preferably between 7 and 12 mm, and the spacing between the equatorial plane X of the circumferential track closest thereto is at least 5 mm, preferably at least 9 mm, more preferably at least 15 mm.

This minimises wear of the road surface caused by the action of the studs when the studded tyre is used on a road surface that is only partially covered in ice or when there is no ice, in compliance with the regulation that provides an upper limit for the quantity of asphalt removed by the studs during rolling on a road surface that is only partially covered in ice or when there is no ice.

The provision of a minimum spacing of the tracks, and therefore of the studs, from the equatorial plane makes it possible to minimise the quantity of asphalt removed by the studs during rolling on a road surface that is only partially covered in ice or when there is no ice. In fact, it has been verified that the studs closest to the equatorial plane tend to erode the asphalt to a greater extent than the studs closest to the shoulders.

As is clearer to see in Fig. 3, the following are defined for each first or second region 4, 5:

- 2 circumferential tracks comprising the central blocks 7 or 8,

- 5 circumferential tracks comprising the shoulder blocks 9a, and

- 5 circumferential tracks comprising the shoulder blocks 9b.

On account of these features, 190 studs can be suitably provided in a 205/55 R16 tyre compared with the 130 studs provided on a similar studded tyre of equal dimensions that is produced and sold by the Applicant.

Claims

1. Tyre for wheels of vehicles, comprising a tread band (2), on which there are defined :

- a first shoulder region and a second shoulder region (4, 5) that are axially opposite;

- a central region (6) that is interposed between said first and second shoulder regions (4, 5);

- a plurality of first pairs of main grooves (10, 20) that are provided in regular succession over the circumferential extent of said tread band, each first pair being formed by:

i. a first main groove (10) that extends from said first shoulder region (4) towards said central region (6) with an inclination with respect to said circumferential direction that generally decreases from said first shoulder region towards said central region, and by

ii. a second main groove (20) that extends from said second shoulder region (5) towards said central region (6) with an inclination with respect to said circumferential direction that generally decreases from said second shoulder region towards said central region;

- a plurality of second pairs of main grooves (30, 40) that are provided in regular succession over the circumferential extent of said tread band in an alternating position with said first pairs of main grooves (10, 20), each second pair being formed by:

i. a third main groove (30) that extends from said first shoulder region (4) towards said central region (6) with an inclination with respect to said circumferential direction that generally decreases from said first shoulder region towards said central region, and by

ii. a fourth main groove (40) that extends from said second shoulder region (5) towards said central region (6) with an inclination with respect to said circumferential direction that generally decreases from said second shoulder region towards said central region,

- a plurality of first pairs of secondary grooves (50, 60), which are provided in regular succession over the circumferential extent of said tread band, each first pair of secondary grooves being formed by:

i. a first secondary groove (50) that extends between two of said first main grooves (10), which are mutually successive, and which is inclined with respect to said circumferential direction at an angle of between approximately 20° and approximately 30°, in a discordant manner with respect to said first main groove, and by

ii. a second secondary groove (60) that extends between two of said second main grooves (20), which are mutually successive, and which is inclined with respect to said circumferential direction at an angle of between approximately 20° and approximately 30°, in a discordant manner with respect to said second main groove,

- a plurality of second pairs of secondary grooves (70, 80) that are provided in regular succession over the circumferential extent of said tread band, each second pair of secondary grooves being formed by:

i. a third secondary groove (70) that extends in said first shoulder region (4) between two of said first main grooves (10), which are mutually successive, and which is inclined with respect to said circumferential direction at an angle of between approximately 20° and approximately 30°, in a discordant manner with respect to said first main groove, and by ii. a fourth secondary groove (80) that extends in said second shoulder region (5) between two of said second main grooves (20), which are mutually successive, and which is inclined with respect to said circumferential direction at an angle of between approximately 20° and approximately 30°, in a discordant manner with respect to said second main groove.

2. Tyre according to claim 1, wherein each of said third secondary grooves (70) is substantially parallel to said first secondary groove (50).

3. Tyre according to either claim 1 or claim 2, wherein each of said fourth secondary grooves (80) is substantially parallel to said second secondary groove

(60).

4. Tyre according to any one of the preceding claims, wherein said first and said second shoulder regions (4, 5) are devoid of any circumferential grooves.

5. Tyre according to any one of the preceding claims, wherein said tread band (2) is devoid of any circumferential grooves.

6. Tyre according to any one of the preceding claims, wherein each of said first secondary grooves (50) comprises a first portion (53) that extends between a first end (51) of said first secondary groove and a third main groove (30), and a second portion (54) that extends between said third main groove (30) and a second end (52) of said first secondary groove.

7. Tyre according to claim 6, wherein said first portion (53) of said first secondary groove (50) has an increasing width from said first end (51) up to said third main groove (30) and wherein the ratio between the width of said first portion (53) of said first secondary groove at said third main groove and at said first end is between 1.25 and 1.5, more preferably between 1.3 and 1.4.

8. Tyre according to either claim 6 or claim 7, wherein said second portion (54) of said first secondary groove has an increasing width from said third main groove (30) up to said second end (52) and wherein the ratio between the width of said second portion (54) of said first secondary groove at said second end and at said third main groove is between 1.25 and 1.5, more preferably between 1.3 and 1.4.

9. Tyre according to any one of the preceding claims, wherein said first secondary grooves (50) and said second secondary grooves (60) are substantially mutually specular with respect to an equatorial plane (X) of said tread band.

10. Tyre according to any one of the preceding claims, wherein each of said second secondary grooves (60) comprises a first portion (63) that extends between a first end (61) of said second secondary groove and a fourth main groove (40), and a second portion (64) that extends between said fourth main groove (40) and a second end (52) of said second secondary groove.

11. Tyre according to claim 10, wherein said first portion (63) of said second secondary groove has an increasing width from said first end (61) up to said fourth main groove (40) and wherein the ratio between the width of said first portion (63) of said second secondary groove at said fourth main groove and at said first end is between 1.25 and 1.5, more preferably between 1.3 and 1.4.

12. Tyre according to either claim 10 or claim 11, wherein said second portion (64) of said second secondary groove has an increasing width from said fourth main groove (40) up to said second end (62) and wherein the ratio between the width of said second portion (64) of said second secondary groove at said second end and at said fourth main groove is between 1.25 and 1.5, more preferably between 1.3 and 1.4.

13. Tyre according to any one of the preceding claims, wherein each of said third secondary grooves (70) comprises a first portion (73) that extends between a first end (71) of said third secondary groove and a third main groove (30), and a second portion (74) that extends between said third main groove (30) and a second end (72) of said third secondary groove.

14. Tyre according to claim 13, wherein said first portion (73) of said third secondary groove has an increasing width from said first end (71) up to said third main groove (30) and wherein the ratio between the width of said first portion (73) of said third secondary groove at said third main groove and at said first end is between 1.25 and 1.5, more preferably between 1.3 and 1.4.

15. Tyre according to either claim 13 or claim 14, wherein said second portion (74) of said third secondary groove has an increasing width from said third main groove (30) up to said second end (72) and wherein the ratio between the width of said second portion (74) of said third secondary groove at said second end and at said third main groove is between 1.25 and 1.5, more preferably between 1.3 and 1.4.

16. Tyre according to any one of the preceding claims, wherein each of said fourth secondary grooves (80) comprises a first portion (83) that extends between a first end (81) of said fourth secondary groove and a fourth main groove (40), and a second portion (84) that extends between said fourth main groove (40) and a second end (82) of said fourth secondary groove.

17. Tyre according to claim 16, wherein said first portion (83) of said fourth secondary groove has an increasing width from said first end (81) up to said fourth main groove (40) and wherein the ratio between the width of said first portion (83) of said fourth secondary groove at said fourth main groove and at said first end is between 1.25 and 1.5, more preferably between 1.3 and 1.4.

18. Tyre according to either claim 16 or claim 17, wherein said second portion (84) of said fourth secondary groove has an increasing width from said fourth main groove (40) up to said second end (82) and wherein the ratio between the width of said second portion (84) of said fourth secondary groove at said second end and at said fourth main groove is between 1.25 and 1.5, more preferably between 1.3 and 1.4.

19. Tyre according to any one of the preceding claims, wherein said third secondary grooves (70) and said fourth secondary grooves (80) are substantially mutually specular with respect to an equatorial plane (X) of said tread band.