Car tyre

1 . A car tyre having a tread comprising a central portion located across an equatorial plane, a first shoulder portion located towards an outer side of the tyre and a second shoulder portion located towards an inner side of the tyre, the central portion being separated from said shoulder portions by two first circumferential grooves having a respective minimum width of about 5 mm, wherein:

said first shoulder portion and said second shoulder portion comprise a plurality of first transverse grooves having a first end located substantially at a respective edge of the tread, having a width greater than or equal to about 4 mm and an axial extension equal to at least 50% of the width of the shoulder portion in which they are located;

said two first circumferential grooves are a first circumferential groove facing towards the outer side of the tyre and a first circumferential groove facing towards the inner side of the tyre; the number of said first transverse grooves of the first shoulder portion is lower than the number of said first transverse grooves of the second shoulder portion;

said central portion has two circumferential ribs, a first and a second circumferential rib respectively, comprising a plurality of second transverse grooves;

said central portion comprises one second circumferential groove so as to define said first and second circumferential ribs;

said first circumferential rib being axially delimited by the first circumferential groove facing towards the outer side of the tyre and the second circumferential groove, said second circumferential rib being axially delimited by the first circumferential groove facing towards the inner side of the tyre and the second circumferential groove;

said second transverse grooves have a maximum width smaller than or equal to about 3 mm, define in at least one of said circumferential rib a void-to-rubber ratio smaller than 0.06, and extend at most over 80% of a width of the circumferential rib in which they are located;

each circumferential rib of the central portion has its own void-to-rubber ratio; said void-to-rubber ratio is greater in the circumferential rib closest to the second shoulder portion; and

said second transverse grooves of the first circumferential rib extend from the second circumferential groove towards the equatorial plane.

2 . The tyre according to claim 1 , wherein said tread comprises a module which is cyclically repeated along the circumferential development of the tyre, and wherein for each module of the tread the number of first transverse grooves of the second shoulder portion is about twice the number of first transverse grooves of the first shoulder portion.

3 . The tyre according to claim 1 , wherein the first circumferential groove facing towards the outer side of the tyre having a width smaller than the width of the first circumferential groove facing towards the inner side of the tyre.

4 . The tyre according to claim 1 , wherein said second circumferential groove has a width greater than 12 mm.

5 . The tyre according to claim 1 , wherein said first and/or second circumferential grooves have a maximum depth greater than 5 mm.

6 . The tyre according to claim 1 , wherein said second transverse grooves extend at most over 60% of the width of the circumferential rib in which they are located.

7 . The tyre according to claim 1 , wherein each second transverse groove of the first and second circumferential rib has a centerline having an inclination α>45°, relative to the equatorial plane X-X.

8 . The tyre according to claim 7 , wherein each second transverse groove of the first and second circumferential rib has a centerline having an inclination α>60°, relative to the equatorial plane X-X.

9 . The tyre according to claim 1 , wherein in each circumferential rib, the second transverse grooves are located at a distance from each other between 25 mm and 80 mm in circumferential direction.

10 . The tyre according to claim 9 , wherein said distance in the rib of the central portion closest to the inner side of the tyre is smaller than said distance in the rib of the central portion closest to the outer side of the tyre.

11 . The tyre according to claim 1 , wherein the second transverse grooves of the first circumferential rib are positioned so as to have substantially the same distance from each other.

12 . The tyre according to claim 1 , wherein the second transverse grooves of the first circumferential rib are substantially parallel one another at least along a portion thereof.

13 . The tyre according to claim 1 , wherein each second transverse groove of the second circumferential rib is counter-inclined with respect to the second transverse groove adjacent in a circumferential direction.

14 . The tyre according to claim 1 , wherein each second transverse groove of the second circumferential rib extends from a first or a second circumferential groove.

15 . The tyre according to claim 1 , further comprising a third circumferential groove located on the first shoulder portion, the third circumferential groove having a width smaller than the width of the first circumferential groove facing towards the outer side of the tyre.

16 . The tyre according to claim 15 , wherein said third circumferential groove has a width smaller than 4 mm.

17 . The tyre according to claim 15 , wherein said third circumferential groove has a maximum depth smaller than 4 mm.

18 . The tyre according to claim 15 , wherein the plurality of first transverse grooves of the first shoulder portion traverses the third circumferential groove.