Studded tire

1. A tire adapted to run along ground which may be covered with ice, comprising:

a tread comprising a first part in contact with the ground and at least one second part placed radially to the inside of the first part, the first part of the tread being formed from a first rubber composition and the second part of the tread being formed from a second rubber composition different from the first rubber composition; and

at least one stud comprising:

a head intended to anchor the stud in the tread, the head having a maximum dimension DT;

a protuberance intended to come into contact with the ground, or ice or both;

a body joining the head and the protuberance, the body having an axis of symmetry, an assembly formed by the head and the body of the stud having a radial height HA; and

a longitudinal axis passing through the axis of symmetry of the body,

wherein at least one stud is placed in the tread so that:

its longitudinal axis is approximately parallel to a radial direction;

the protuberance of the stud protrudes from the tread when the stud is not in contact with the ground;

the body of the stud is at least partially in contact with the first part of the tread; and

the head of the stud is entirely anchored in the second part of the tread,

wherein at least one second part of the tread surrounds the head of said at least one stud and extends:

radially to the inside of the head of the stud over a distance DI from the radially innermost point of the stud, the distance DI being equal to or greater than 40% of the maximum dimension DT of the head of the stud (DI≧0.4DT/2);

radially to the outside of the head of the stud over a distance DE from the radially innermost point of the stud, the distance DE being equal to or greater than 40% of said radial height HA; and

in all directions perpendicular to the longitudinal axis of the stud over a distance DP from the longitudinal axis of the stud, the distance DP being equal to or greater than the maximum dimension DT of the head of the stud,

wherein a skin may optionally be formed from the first rubber composition and placed between said at least one stud and said at least one second part of the tread, wherein the thickness of any skin ( 2010 ), measured perpendicularly to the surface of the stud, is less than or equal to 1 mm;

wherein the complex modulus G*(0° C.) of the first rubber composition is less than 1.5 MPa; and

wherein the complex modulus G* of the second rubber composition varies with temperature such that G*(5° C.) is equal to or greater than 5 MPa and G*(20° C.) is less than or equal to 0.5 G*(5° C.).

2. The tire according to claim 1 , wherein the tire comprises a plurality of studs, wherein the tread comprises a plurality of second parts formed from the second rubber composition and wherein the head of each stud is entirely anchored in a second, separate part of the tread.

3. The tire according to claim 1 , wherein the tire comprises a plurality of studs, in which tire the tread comprises a single second part formed from the second rubber composition, and wherein the heads of all the studs are anchored in this second part.

4. The tire according to claim 1 , wherein the radial height HA is equal to or greater than 9 mm but less than or equal to 10 mm.

5. The tire according to claim 1 , wherein the distance DE is equal to or greater than 60% of said radial height HA.

6. The tire according to claim 1 , wherein the maximum dimension DT of the head of the stud is equal to or greater than 8 mm but less than or equal to 10 mm.

7. The tire according to claim 1 , wherein the distance DI is equal to or greater than 4 mm but less than or equal to 4.5 mm.

8. The tire according to claim 1 , wherein the distance DP is equal to or greater than 1.5DT but less than or equal to 2.5DT (1.5DT≦DP≦2.5DT).

9. The tire according to claim 8 , wherein the distance DP is equal to twice the maximum distance DT of the head of the stud (DP=2DT).

10. The tire according to claim 1 , wherein at least one stud is made of aluminium or copper.