DESCRIPTION

The present invention relates to a tyre for vehicle wheels , in particular a winter tyre .

A tyre generally comprises a carcass structure which is toroidally shaped about a rotation axis , and which comprises at least one carcass ply which has end edges which are engaged in respective annular anchoring structures , called bead cores .

In a radially external position with respect to the carcas s structure , there is provided a belt structure comprising, in the case of tyres for cars , at least two radially overlapping strips of rubberi zed fabric which is provided with reinforcement cords , which are usually made of metal and which are arranged in each strip parallel with each other but crosswise with respect to the cords of the adj acent strip, preferably symmetrically with respect to the equatorial plane of the tyre .

Preferably, the belt structure also further comprises , in a radially outer position at least on the ends of the underlying belt strips , a third layer of textile or metal cords , arranged circumferentially ( at 0 degrees ) . In tyres of the tubeless-type , there is further present a radially inner layer known as a " l iner" and which has characteristics of impermeability in order to obtain the air-tightness in the tyre itsel f .

In a radially external position with respect to the belt structure , there is applied a tread band which is made of elastomer material and on which a tread surface which is intended for contact with the road surface is defined .

In order also to obtain adequate road grip on a wet road surface , the tyres have a tread band which is provided with grooves with various shapes and geometries and the main obj ect of which is to allow the evacuate the water present between the surface of the tyre and the road surface during the mutual contact .

The grooves generally define on the tread band a plurality of blocks , each of which comprises a radially outer surface intended to contact with the road surface and which therefore forms a portion of the tread surface of the tyre . Each block is further bounded at least partially by one or more walls which are de fined by the grooves which surround it .

In some cases , a wall of a block, or a portion thereof , i s connected to the tread surface of the block by means of a chamfer, which is typically formed by a plane , suitably slanted with respect to the wall and the tread surface , and which defines an edge of the block .

In the case of winter tyres , there are generally formed on the blocks of the tread band small recesses , called " sipes" , which extend from the tread surface of the tyre towards the inside of the block . The function of the sipes is to provide additional gripping elements in the case of travel on snowy surfaces and to retain a certain quantity of snow, thereby improving the grip with respect to the road surface . The sipes can have very varied and di f ferent configurations from each other, for example , they can have a rectilinear or curvilinear or undulating progression .

The term "circumferential" direction means a direction which is generally directed in the rotation direction of the tyre or, in any case , slightly slanted ( at most by about 5 ° ) with respect to the rotation direction of the tyre .

The term "axial" direction means a direction which i s substantially parallel with the rotation axis of the tyre or, at most , slightly slanted ( at most by about 5 ° ) with respect to this rotation axis of the tyre . The axial direction is generally perpendicular to the circumferential direction .

The term "radial" direction means a direction perpendicularly intersecting the rotation axis of the tyre or at most which is slightly slanted ( for example , at most by about 5 ° ) with respect to the direction perpendicular to the rotation axis of the tyre .

The term "equatorial plane" of the tyre means an axial centreline plane which is perpendicular to the rotation axis of the tyre .

The term "groove" means a recess which is formed in a tread band portion and which extends in a main longitudinal direction and which has a width greater than or equal to 1 . 5 mm . Preferably, the groove has a depth of at least 3 mm .

The term " s ipe" means a recess which i s formed in a tread band portion, and which extends in a main longitudinal direction and which has a width less than 1 . 5 mm .

The " longitudinal development direction" of the sipe is defined by the linear progression of the sipe in the region of the radially external surface of the block .

The term "block" means a tread band portion which is intended for contact with the road surface and which is delimited over the entire periphery thereof by one or more grooves and where applicable by an axial end of the tread band .

Therefore , a block is considered to be both a tread band portion with a closed contour which i s delimited by three or more grooves and a circumferential rib which i s delimited by a pair of grooves which develop circumferentially around the tread band .

A "central block" is defined as a block which is not bounded by the axial ends of the tread band while a block which is partially delimited by one of the axial ends of the tread band is defined as a " shoulder block" .

The term "wall" of a block means the surface which generally extends towards the tread surface portion of the block from a bottom of a groove bounding the block .

The term "edge" of a block means the block region which j oins a wall of the block with the tread surface portion of the block .

The edge may be formed by a corner i f the wall intersects directly with the tread surface portion or can be formed by a connection surface which connects the wall to the tread surface portion, which is defined as a "chamfer" .

Two or more linear distances , such as , for example , two or more distances between sipes or between sipe elements , have a " substantially equal" length when the respective lengths di f fer from each other at most by an extent equal to 10% o f the greatest length .

Two or more directions or two or more elements which extend in respective directions , such as , for example , two or more sipes or portions thereof , are " substantially parallel" when they are slanted relative to each other by an angle less than 10 ° , preferably less than 5 ° .

The inclination of a first direction with respect to a second direction, for example , the direction of a sipe portion with respect to a longitudinal development direction of a sipe , is defined by the acute angle which i s formed by the first direction with the second direction . As a particular case , when the first direction is perpendicular to the second direction, the inclination is 90 ° .

Two or more directions or two or more elements which extend in respective directions , such as , for example , two or more sipes or portions thereof , are inclined in a "concordant manner" with respect to a further direction when the trend o f the two or more directions is for all increasing or for all decreasing when considered on a Cartesian plane which contains said two or more directions and the axis of the abscissas of which is parallel to the other direction . For example , two di f ferent portions of a sipe which extends in a longitudinal development direction o f the sipe are inclined in a concordant manner with respect to the longitudinal development direction of the sipe when both said portions of the sipe extend in respective directions which both increase or both decrease when considered on a Cartesian plane which has the axis of the abscissas parallel to the longitudinal development direction of the sipe .

Consequently, two or more directions or two or more elements which extend in respective directions , such as , for example , two or more sipes or portions thereof are inclined in a "discordant manner" when the trend thereof , when considered on this Cartesian plane , increases for one or more of said directions and decreases for one other or more than one of said directions .

The term "peak" which is defined on a sipe or a portion thereof means a maximum zone or a minimum zone which i s defined on a linear section of said sipe or said portion thereof when the linear section is considered on a Cartesian plane , the axis of the abscissas of which is positioned in the longitudinal development direction of the sipe . Two sipe portions , which are spaced apart from each other, are " substantially aligned" when, at least in the region o f the respective ends thereof facing each other, the longitudinal directions thereof are :

- aligned; or,

- i f the respective longitudinal directions are incident with respect to each other, then said longitudinal directions are inclined at an angle less than 10 ° , preferably less than 5 ° , or

- i f the respective longitudinal directions are parallel with each other, then said longitudinal directions are staggered by a value less than 1 . 5 mm.

Two sipes are "consecutive" when they are arranged on the tread surface of the block one following the other, considering a direction perpendicular to the longitudinal development direction of at least one of the two sipes .

The term " single-wave portion" of a sipe means a section of the linear trend of the sipe at the radially external surface of the block having a first peak and a second peak which are consecutive and opposite with respect to the longitudinal development direction of the sipe itsel f .

A sipe which is formed on a block separates this block into two block portions having respective mutually facing surfaces .

A sipe is defined to be "of the complex type" when at least one of the mutually facing surfaces of the block portions has at least one protuberance , while the other of these surfaces has a corresponding recess which is able to at least partially receive this protuberance . In other words , the respective proj ections in a plane perpendicular to the radial direction of the two block portions which are separated by the sipe are , in the region of the protuberance and the corresponding recess , at least partially overlapping .

In this manner, a relative movement of the two block portions in the radial direction is hindered by the interference between the two block portions in the region of the protuberance and the recess .

A sipe of the complex type is also known in the field as a

"three-dimensional" sipe .

A sipe i s def ined as being "of the simple type" when there are not present on the mutually facing surfaces of the block portions protuberances and/or recesses which interfere with each other during the relative movement of the two block portions in the radial direction .

Examples of sipes which are configured in various manners are described, for example , in EP 3375639 , US 5198047 , US 2021 / 0107319 , EP 3867081A1 , JP 6946658 , JP 2018020689 , US 3799231 , US 20030029537 , US 2005/ 0150581 , EP 0598300 , US 5327953 and US 5591280 .

The Applicant has observed that the performance levels of a tyre on snowy road surfaces depend to a relevant extent on the quantity and the extent of the s ipes which are formed on the blocks so that , for the same tread pattern, a block on which there are formed a plurality of sipes is able to provide better behaviour on snow .

However, the Applicant has further observed that the presence of the sipes weakens the structure of the block, making it less sti f f and therefore decreasing the capacity of the block to withstand the external stresses , particularly the tangential stresses .

As a result of this weakening, in the event of braking, acceleration or travel on bends , the block may become de formed in a relevant manner, bringing about a partial raising of the block from the road surface with a resultant reduction of the contact area between the block and the road surface and consequently also of the overall friction force which is applied by the tyre to the road surface .

The Applicant has veri fied that the cause of this weakening of the block involves the fact that the sipe subdivides the block into two portions which, in the region of the sipe , can sl ide with respect to each other .

In particular, the Applicant has observed that this sliding occurs in a direction parallel with the separation surface of the two portions which can therefore always be considered to be a composition of a movement in two main directions which are defined on the separation surface : a direction which extends from the bottom of the sipe perpendicularly to the tread surface ( generally coincident with the radial direction of the tread band) and a second direction parallel with the longitudinal development direction of the sipe .

The Applicant has veri fied that the resistance of the block in the region of the sipe can be improved by blocking or limiting to the greatest possible extent the possibility of mutual sl iding in these two main directions .

The Applicant has observed that the sipes of the complex type which have a profile which is provided with protuberances and corresponding recesses , are able to oppose the mutual sliding of the block portions , particularly the movement in the first direction identi fied above . However, the provision of this type of profile is not always suf ficient to ef fectively oppose the sliding actions in the above-mentioned second direction (parallel with the longitudinal development direction of the sipe ) , particularly in the region of the tread surface .

Therefore , the Applicant has observed that , in order to oppose this type of sliding, it is generally known to configure the sipe with an undulating linear progression, for example , in a zigzag manner .

The Applicant has further observed that , in order to obtain a suf ficiently ef fective opposing action, this undulating linear progression has to extend over a large part of the longitudinal development of the sipe , providing multiple waves , with a generally symmetrical form .

However, the Applicant has veri fied that the opposing action to the tangential sliding of the block portions provided by this configuration is not always ef fective .

Furthermore , the Applicant has observed that , in the poss ible case of a sipe which is open at an edge of the block, the sti f fness of the block itsel f would be further reduced in a consistent manner in the region of this edge .

Therefore , the Applicant has perceived the need for providing a block, in which there are formed sipes with a reduced number of waves and which is further provided with good sti f fness characteristics .

In order to comply with this requirement , the Applicant has reali zed that the region of the block which is made structurally most critical by the presence o f the sipes is the edge region, in which inter alia some sipes could be open and that therefore the sliding of the block portions in a longitudinal development direction of the sipes can also be ef fectively opposed with a single sipe portion which is configured in a wave-like manner , provided that it is positioned near the edge .

Finally, the Applicant has found that a block which is provided with a pair o f sipes , where each s ipe of the pair o f sipes has only one single-wave portion interposed between linear portions and where the first sipe has the individual single-wave portion near the first edge of the block and the second sipe has the individual single-wave portion near the second edge of the block, solves the problem set out above of withstanding the tangential stresses .

In particular, in a first aspect thereof , the invention relates to a tyre for vehicle wheels comprising a tread band and a tread surface which is radially external with respect to said tread band .

Preferably, a plurality of blocks are defined on said tread band .

Preferably, at least one first s ipe and at least one second s ipe are formed on each block of said plurality of blocks .

Preferably, said at least one first sipe is consecutive with respect to said at least one second sipe .

Preferably, said first sipe is open at said tread surface so as to define a first longitudinal development direction of said first sipe .

Preferably, said first longitudinal direction intersects with a first edge and a second edge of said block . Preferably, said second sipe is open at said tread surface so as to define a second longitudinal development direction of said second sipe .

Preferably, said second longitudinal direction intersects with said first edge and said second edge of said block .

Preferably, said first sipe comprises a first linear portion which substantially extends along said first longitudinal direction towards said first edge of said block .

Preferably, said first linear portion of said first sipe i s rectilinear or curvilinear with a minimum radius o f curvature not less than 20 mm .

Preferably, said first sipe comprises a single-wave portion which extends in continuation of said first linear portion of said first sipe .

Preferably, said first sipe comprises only one single-wave portion .

Preferably, said single-wave portion of said first sipe has a distance from said first edge measured along said first longitudinal direction between 5% and 40% of the distance between said first edge and said second edge measured along said first longitudinal direction .

Preferably, said first sipe comprises a second linear portion which extends in continuation of said single-wave portion towards said second edge .

Preferably, said second linear portion of said first sipe is rectilinear or curvilinear with a minimum radius o f curvature not less than 20 mm .

Preferably, said second sipe comprises a first linear portion which substantially extends along said second longitudinal direction towards said second edge of said block .

Preferably, said first linear portion of said second sipe is rectilinear or curvilinear with a minimum radius o f curvature not less than 20 mm .

Preferably, said second sipe comprises a single-wave portion which extends in continuation of said first linear portion of said second sipe .

Preferably, said second sipe comprises only one single-wave portion .

Preferably, said single-wave portion of said second sipe has a distance from said second edge measured along said second longitudinal direction between 5% and 40% of the distance between said first edge and said second edge measured along said second longitudinal direction .

Preferably, said second sipe comprises a second linear portion which extends in continuation of said single-wave portion towards said first edge .

Preferably, said second linear portion of said second sipe is rectilinear or curvilinear with a minimum radius o f curvature not less than 20 mm .

The Applicant believes that, as a result of these characteristics , it is possible to ef fectively oppose the sliding actions of the block portions which are separated by the sipes , even with only one single-wave portion, allowing to obtain, for the same other conditions , a block which is more sti f f and which is able to provide better behaviour on a dry road surface and also on an icy road surface .

It may be noted that the distance of the single-wave portion from the first edge or from the second edge , respectively, i s measured from the centre point of the single-wave portion at the first edge and at the second edge , respectively .

The present invention, in the above-mentioned aspect , may have at least one of the additional preferred features indicated below .

In some embodiments , said first linear portion of said first sipe is open at said first edge of said block .

In some embodiments , said first linear portion of said second sipe is open at said second edge of said block .

In some embodiments , said single-wave portion of said first sipe has a distance from said first edge measured along said first longitudinal direction between 25% and 35% of the distance between said first edge and said second edge measured along said first longitudinal direction .

In some embodiments , said single-wave portion of said second sipe has a distance from said second edge measured along said second longitudinal direction between 25% and 35% of the distance between said first edge and said second edge measured along said second longitudinal direction .

In some embodiments , said single-wave portion of said first sipe comprises a first segment which is inclined with respect to said first longitudinal direction .

Preferably, said first segment extends from said first linear portion of said first sipe as far as a first peak o f said singlewave portion .

Preferably, said single-wave portion of said first sipe comprises a second segment which is inclined with respect to said first longitudinal direction .

Preferably, said second segment extends from said first peak as far as a second peak of said single-wave portion .

Preferably, said second peak is defined at the opposite side to said first peak with respect to said first longitudinal direction

Preferably, said single-wave portion of said first sipe comprises a third segment which is inclined with respect to said first longitudinal direction .

Preferably, said third segment extends from said second peak as far as said second linear portion of said first sipe .

Preferably, said second segment of said single-wave portion is inclined with respect to said first longitudinal direction at an angle greater than said first segment and said third segment of said single-wave portion .

As a result of this particular configuration, the Applicant believes that the single-wave portion of the first sipe confers on the block, in particular in the region of the first edge thereof , suf ficient sti f fness to withstand the tangential stresses relative to the tread surface in a particularly ef fective manner . In some embodiments , said single-wave portion of said second sipe comprises a first segment which is inclined with respect to said second longitudinal direction .

Preferably, said first segment extends from said first linear portion of said second sipe as far as a first peak o f said single-wave portion .

Preferably, said single-wave portion of said second sipe comprises a second segment which is inclined with respect to said second longitudinal direction .

Preferably, said second segment extends from said first peak as far as a second peak of said single-wave portion .

Preferably, said second peak is defined at the opposite side to said first peak with respect to said second longitudinal direction .

Preferably, said single-wave portion of said second sipe comprises a third segment which is inclined with respect to said second longitudinal direction .

Preferably, said third segment extends from said second peak as far as said second linear portion of said second sipe .

Preferably, said second segment of said single-wave portion is inclined with respect to said second longitudinal direction at a greater angle with respect to said first segment and said third segment of said single-wave portion .

As a result of this particular configuration, the Applicant believes that the single-wave portion of the second sipe confers on the block, in particular in the region of the second edge thereof, sufficient stiffness to withstand the tangential stresses relative to the tread surface in a particularly effective manner.

Therefore, the first sipe and the second sipe preferably have a similar configuration which is formed by a single-wave portion which is interposed between a first and a second linear portion.

In some embodiments, in said first sipe, said first segment of said single-wave portion is inclined with respect to said first longitudinal direction at an angle between 20° and 45°, more preferably at about 24°.

In some embodiments, in said first sipe, said third segment of said single-wave portion is inclined with respect to said first longitudinal direction at an angle between 20° and 45°, more preferably at about 24°.

In some embodiments, in said first sipe, said second segment of said single-wave portion is inclined with respect to said first longitudinal direction at an angle between 60° and 90°, preferably at about 80°.

In some embodiments, in said first sipe, the distance of said first peak from said first longitudinal direction is substantially identical to the distance of said second peak from said first longitudinal direction. Preferably, said distance is between 0.5 mm and 1.5 mm, more preferably it is about 1 mm.

In some embodiments, in said second sipe, said first segment of said single-wave portion is inclined with respect to said second longitudinal direction at an angle between 20° and 45°, more preferably at about 24°. In some embodiments, in said second sipe, said third segment of said single-wave portion is inclined with respect to said second longitudinal direction at an angle between 20° and 45°, more preferably at about 24°.

In some embodiments, in said second sipe, said second segment of said single-wave portion is inclined with respect to said second longitudinal direction at an angle between 60° and 90°, preferably at about 80°.

In some embodiments, in said second sipe, the distance of said first peak from said second longitudinal direction is substantially identical to the distance of said second peak from said second longitudinal direction. Preferably, said distance is between 0.5 mm and 1.5 mm, more preferably it is about 1 mm.

Preferably, in said first sipe or in said second sipe, said first segment and said third segment of said single-wave portion are inclined in a concordant manner with respect to each other.

Preferably, in said first sipe or in said second sipe, said second segment of said single-wave portion is inclined in a discordant manner with respect to said first segment and said third segment of said single-wave portion.

In some embodiments, in said first sipe or in said second sipe, said first segment and said third segment of said single-wave portion are substantially parallel with each other.

In some embodiments, in said first sipe or in said second sipe, said first segment of said single-wave portion has a length substantially identical to said third segment of said singlewave portion. Preferably, in said first sipe or in said second sipe , said first segment and said third segment of said single-wave portion have a length between 1 . 5 and 3 mm, more preferably of about 2 mm .

In some embodiments , in said first sipe or in said second sipe , said second linear portion has a length greater by at least 50% , preferably at least 100% , more preferably at least 200% , with respect to said first linear portion .

In this manner, the single-wave portion of the first sipe is greatly decentred with respect to the first and second portions , allowing the positioning thereof at an edge of the block, where it can perform the function thereof of sti f fening the block in an even more ef fective manner .

In some embodiments , said first longitudinal direction is substantially parallel with said second longitudinal direction .

Preferably, in said first sipe or in said second sipe , said first linear portion has a length between 2 and 5 mm .

Preferably, in said first sipe or in said second sipe , the depth in the region of said first linear portion is less than the depth in the region of said single-wave portion .

In this manner, the sti f fening e f fect of the block in the region of the first linear portion is advantageously increased .

In some embodiments , said second linear portion of said first sipe is open at said second edge of said block .

In some embodiments , said second linear portion of said second sipe is open at said first edge of said block . Preferably, in said first sipe or in said second sipe , said second linear portion has a length between 4 and 25 mm .

In some embodiments , in said f irst sipe , said second linear portion is substantially aligned with said first linear portion along said first longitudinal direction .

In some embodiments , in said second sipe , said second linear portion is substantially aligned with said first linear portion along said second longitudinal direction .

In some embodiments , said first peak of said first sipe and said first peak of said second sipe are transversely of fset at the same side with respect to said f irst longitudinal direction o f said first sipe and said second longitudinal direction of said second sipe , respectively .

In other words , said first segment , second segment and third segment of said first sipe are inclined, with respect to said first longitudinal direction or said second longitudinal direction, in a discordant manner relative to said first segment , second segment and third segment of said second sipe , respectively .

In this manner, the respective second segments of the first and second sipes which are intended to provide the greatest opposing action to the longitudinal sliding of the block portions , are inclined in a discordant manner relative to each other so as to make the sti f fness of the block substantially homogeneous when it is subj ected to tangential stresses parallel with the first or second longitudinal direction which are orientated in opposite directions . In some embodiments , there are formed on said block at least three mutually consecutive sipes , said at least three sipes being formed by an alternating succession of one of said first sipes and one of said second sipes .

In some embodiments , there are formed on said tread band central blocks which are delimited only by one or more grooves .

In some embodiments , each central block of said tread band comprises at least one o f said first sipes and at least one of said second sipes .

In some embodiments , at least 80% of the sipes , more preferably all of the sipes , which are formed on the central blocks o f said tread band having a length greater than a minimum predefined value , are one of said first sipes or one of said second sipes .

Preferably, said minimum predefined value is between 5 mm and 15 mm, for example , about 10 mm .

In this manner, the sipe extends suf ficiently far to be able to have the configuration with a single-wave portion interposed between two linear portions with suitable dimensions .

In some embodiments , said at least one first sipe and said at least one second sipe are of the complex type .

The features and advantages of the invention will be better appreciated from the detailed description of a number o f preferred embodiments thereof which are illustrated by way o f non-limiting example with reference to the appended drawings , in which :

- Figure 1 is a schematic front view of a tread band portion of a tyre for vehicle wheels made according to the present invention; - Figure 2 is a schematic front view, drawn to an enlarged scale , of a first sipe and a second sipe which are formed on a block of the tyre of Figure 1 .

With reference to the appended Figures , there is generally designated 1 a tyre for vehicle wheels (not completely illustrated) , which is made according to the present invention .

The tyre 1 has a conventional generally toroidal form which develops about a rotation axis and which defines an axial direction Y of the tyre and which is passed through by an equatorial plane X which is perpendicular to the rotation axis .

The tyre 1 compri ses a tread band 2 , on which there is def ined a tread surface 3 which is arranged in a radially external position with respect to the tread band 2 and which is intended for contact with a road surface .

The tyre 1 has , for example , a width with a nominal section of about 205 mm with a rim diameter of 16 inches .

There remain defined on the tread band 2 a central region which extends circumferentially and symmetrically around the equatorial plane X and a pair o f shoulder regions which extend at the axially opposite sides o f the central region, respectively, as far as the respective axial ends 2a of the tread band 2 .

There are formed on the tread band 2 a plurality of grooves which are all designated 4 and which delimit a respective plurality of central blocks 5 which are formed in the central region of the tread band 2 and a plurality of shoulder blocks 6 which are formed in the shoulder regions o f the tread band 2 and which are delimited in the region of the axially outer side thereof by an axial end 2a of the tread band 2 . In the embodiment described here , the grooves 4 have a depth between 3 mm and 9 mm and a width between 2 . 5 mm and 9 . 5 mm .

By way of example , Figure 1 schematically illustrates a central block 5 and a shoulder block 6 , it being understood that the same considerations also apply s imilarly to all the other central blocks and the other shoulder blocks 6 which are formed in the tread band 2 .

There is defined on the central block 5 a tread surface portion 3 which is defined on the radially external surface thereof and which is intended for contact with the road surface and a first edge 8 and a second edge 9 , which is opposite the first edge 8 , which j oin the tread surface portion 3 of the central block 5 to the walls of the respective grooves 4 .

The first edge 8 and the second edge 9 together delimit the tread surface portion 3 of the central block 5 and can be chamfered completely or partially .

A plurality of first sipes 10 and a plurality o f second sipes 10a, which alternate with each other, are formed on each central block 5 .

The first sipes 10 extend in respective first longitudinal directions A while the second sipes 10a extend in respective second longitudinal directions B . All the first and second longitudinal directions A and B are substantially parallel with each other .

In the embodiment of a central block 5 illustrated in the Figures , the first longitudinal direction A and the second longitudinal direction B are inclined at about 60 ° with respect to the equatorial plane X, but there is provision for these directions to be able to have a di f ferent inclination, and to be parallel with the axial direction Y .

The first blocks 10 and the second blocks 10a are formed on the central block 5 in an alternating manner with each other and, furthermore , are preferably equidistant , for example , by an extent between about 5 mm and about 6 mm .

The first sipes 10 and the second sipes 10a are consecutive one after the other and have a similar formation, as better explained in detail below .

As can better be seen in Figure 2 , each first sipe 10 extends in the first longitudinal direction A in a manner passing over the central block 5 so as to be open at the first edge 8 and the second edge 9 .

Each first sipe 10 comprises a first linear portion 11 which is open at the first edge 8 , a single-wave portion 12 which extends in continuation of the first linear portion 11 and a second linear portion 13 which extends in continuation of the singlewave portion 12 and opens at the second edge 9 .

Both the first linear portion 11 and the second linear portion 13 are substantially rectilinear and aligned in the first longitudinal direction A.

The second linear portion 13 may have a length greater than the first linear portion 11 , preferably it is greater by at least 50% , more preferably by at least 100% , even more preferably by at least 200% of the first linear portion 11 .

The first linear portion 11 has a length between 2 and 5 mm while the second linear portion 13 has a length between 4 and 25 mm .

The single-wave portion 12 comprises :

- a first substantially rectilinear segment 14 which extends from the first l inear portion 11 as far as a first peak 15 and which is inclined with respect to the first longitudinal direction A;

- a second substantially rectilinear segment 16 which extends from the first peak 15 as far as a second peak 17 , which i s defined at the opposite side to the first peak 15 with respect to the first longitudinal direction A; and

- a third substantially rectilinear segment 18 which extends from the second peak 17 as far as the second linear portion 13 and which is inclined with respect to the first longitudinal direction A.

The first segment 14 and the third segment 18 are substantially parallel with each other and are inclined with respect to the first longitudinal direction A at about 24 ° . Furthermore , the first segment 14 and the third segment 18 have a substantially equal length of about 2 mm .

The second segment 16 is inclined by about 80 ° with respect to the first longitudinal direction A in a discordant manner with respect to the first segment 14 and the third segment 18 with respect to the first longitudinal direction A.

The di stances o f the first peak 15 and the second peak 17 from the first longitudinal direction A are substantially identical and equal to about 1 mm, respectively .

The single-wave portion 12 is therefore substantially symmetrical with respect to the point where the second segment 16 intersects the first longitudinal direction A which is de fined by the first and second linear portions 11 and 13.

Each first sipe 10 has a width of about 0.5 mm and a variable depth between 2 mm, at the end regions thereof, in particular at the first linear portion 11, and about 7 mm, at the single-wave portion 12 and the centre region of the first sipe 10.

Each first sipe 10 is further preferably of the complex type, having along the radial profile thereof one or more protuberances and corresponding recesses which impede the free sliding in the radial direction of the portions of the central block 5, which are separated by the same first sipe 10.

As set out above, the second sipes 10a are similar to the first sipes 10. In fact, in each second sipe 10a there can be identified a first linear portion 11 which is followed by a single-wave portion 12 and therefore by a second linear portion 13, which is longer than the first linear portion 11.

Each second sipe 10a differs from any first sipe 10 in that in the second sipe 10a the first linear portion 11 is open at the second edge 9 (and at the first edge 8) and the second linear portion 13 is open at the first edge 8 (and at the second edge 9) .

As a result of the fact that the first sipes 10 are arranged in an alternating manner on the second sipes 10a, the single-wave portions 12 of the first sipes 10 and the second sipes 10a are positioned in an alternating manner near the first edge 8 and near the second edge 9 so as to stiffen both the edges of the central block 5 in a substantially homogeneous manner.

Furthermore, the first peak 15 of each first sipe 10 and the first peak 15 of each second sipe 10a are offset transversely at the same side with respect to the respective longitudinal direction A and B .

This configuration is obtained by configuring the sipes 10 and 10a in such a manner that the first segment 14 , the second segment 16 and the third segment 18 of the first sipe 10 are inclined, with respect to the first longitudinal direction A, in a discordant manner with respect to the first segment 14 , the second segment 16 and third segment 18 of the second sipe 10a .

Preferably, at least 80% of the sipes which are formed on a central block 5 are first sipes 10 or second sipes 10a, provided that the overall length thereof is greater than a minimum value of about 10 mm .

Where there is provided on the central block 5 a sipe with smaller dimensions , as in the case of the corner regions illustrated in Figure 1 , where these sipes are designated 19 , the sipe may have a di f ferent configuration, for example , rectilinear or undulating with a single peak .

There are formed on the shoulder block 6 a plurality of third sipes 20 which extend parallel with each other in a respective third longitudinal development direction A' which is generally di f ferent from the first and second longitudinal directions A and B .

The third sipes 20 are similar to the first sipes 10 and the second sipes 10a which are formed on the central block 5 , but , unlike the first sipes 10 and the second sipes 10a which have only one single-wave portion 12 , the third sipes 20 have two single-wave portions .

Therefore , each third sipe 20 comprises a first linear portion 21 which is open at a first edge 7 of the shoulder block 6 , a first single-wave portion 22 , which extends in continuation of the first linear portion 21 , a second linear portion 23 , which extends in continuation of the f irst single-wave portion 22 , a second single-wave portion 24 which extends in continuation of the second linear portion 23 and a third linear portion 25 which extends in continuation of the second single-wave portion 24 as far as the axial end 2a of the tread band 2 .

It may be noted that the third linear portion 25 may be considered to be the first linear portion of the second singlewave portion 24 , the second linear portion 23 being common to both the single-wave portions 22 and 24 .

The single-wave portions 22 and 24 of the third sipes 20 are entirely similar to the single-wave portion 12 which is described in detail above with reference to the first and second sipes 10 , 10a .

The Applicant has veri fied that a central block 5 which is provided with a plurality of first sipes and second sipes as described above has optimum resistance to the tangential stresses which are directed in the longitudinal development direction, improving the performance levels on icy roads and on dry roads .

In particular, the central block 5 has optimum characteristics of sti f fness and uni formity of behaviour in the region of the edges 8 and 9 .

Naturally, in order to comply with speci fic and contingent application requirements , a person skilled in the art may apply to the above-described invention additional modi fications and variants which are still included within the scope of protection as defined by the appended claims.

For example, the number of first and/or second sipes which are present on a central block may be different from the number described above.