Tread comprising sculpted elements comprising a covering layer

Abstract

A tread made of rubber material for a snow tire having a tread surface intended to be in contact with a road surface when the tire is being driven on, and provided with a plurality of cuts delimited by walls situated facing one another and forming lateral walls of raised elements of the tread. Each lateral wall intersects the tread surface to form an edge corner. At least one lateral wall of a raised element is formed partially or fully by a cover layer extending from the edge corner associated with this lateral wall, of a material having an elastic modulus higher than that of the rubber material of the tread. The raised element has a depression in its lateral wall, delimited by the cover layer. The raised element moreover has a width W and the depth P of the depression is at least equal to 10% of the width W of the element and less than or equal to 40% of this width W. Finally the volume of the depression is less than 10% of the volume of the raised element.

Claims

1. A tread made of rubber material for a snow tire comprising: a tread surface adapted to be in contact with a road surface when the tire is being driven on, a plurality of cuts delimited by walls situated facing one another and forming lateral walls of raised elements of the tread, each of the said lateral walls intersecting the tread surface to form an edge corner, wherein at least one lateral wall of a raised element is formed partially or fully by a cover layer extending from the edge corner associated with this lateral wall, wherein the material of this cover layer has an elastic modulus higher than an elastic modulus of the rubber material of which the tread is made, wherein the raised element comprises, in its lateral wall, a depression of depth P, this depression being delimited by the cover layer, wherein with the raised element having a width W, the depth P of the depression is at least equal to 10% of the width W of the element and less than or equal to 40% of this width W, wherein the volume of the depression is less than 10% of the volume of the raised element, and wherein the cover layer does not extend beyond the depression.

2. The tread according to claim 1, wherein the depression, when viewed in cross section, has a triangular form pointing in the raised element in a direction away from the edge corner.

3. The tread according to claim 1, wherein the depression opens onto the tread surface of the tread, the cover layer delimiting this depression making an angle () of less than 90 with this tread surface.

4. The tread according to claim 1, wherein the raised element comprises two depressions opening respectively onto two opposite lateral walls of this element.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages of the invention will emerge from the following description, given by way of nonlimiting example, with reference to the attached drawings in which:

(2) FIG. 1 depicts a schematic diagram of part of a tread according to embodiments of the invention;

(3) FIG. 2 depicts, viewed in cross section, a raised element of the tread of FIG. 1 according to a first embodiment of the invention, in a state of rest;

(4) FIG. 3 depicts the raised element of FIG. 2 in contact with an icy road surface;

(5) FIG. 4 depicts a raised element according to a second embodiment;

(6) FIG. 5 depicts the raised element of FIG. 4 in contact with an icy road surface;

(7) FIG. 6 depicts a raised element according to a third embodiment;

(8) FIG. 7 depicts a raised element according to a fourth embodiment;

(9) FIG. 8 depicts a raised element according to a fifth embodiment.

(10) In the description that follows, elements that are substantially identical or similar will be denoted by identical references.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

(11) FIG. 1 depicts part of a tread 1 of a snow tire. This tread comprises a plurality of raised blocks 2 of rubber organized to form the tread pattern of this tread. The blocks of rubber 2 are delimited by grooves 4 which may extend in an axial direction Y parallel to the axis of rotation of the tire, in a circumferential direction X perpendicular to the axial direction Y, or in an oblique direction that has both a non-zero circumferential component and a non-zero axial component. Each block of rubber 2 is in this instance divided into a plurality of strips of rubber 7 separated by sipes 3.

(12) FIG. 2 is a view of a strip 7 in cross section on A-A of the tread visible in FIG. 1, according to a first embodiment of the invention. The strip 7 here comprises a lateral wall 5 formed by a cover layer 11. This layer 11 extends here from the edge corner 9 as far as the bottom of the sipe 3 that delimits the strip 7. The strip 7 also comprises a depression 12 delimited by the cover layer 11. This depression 12 is located with respect to the edge corner 9 at a distance D less than of the height H of the strip 7. As an alternative, the depression 12 is situated with respect to the edge corner 9 at a distance D of less than half this height H. In addition, it will be noted that the depth P of the depression is at least equal to 10% of the width W of the strip 7 and less than or equal to 40% of this width W. The volume of the depression is itself less than 10% of the volume of the strip 7.

(13) The cover layer 11 here has an elastic modulus higher than the elastic modulus of the rubber material of which the tread 1 is made. Such a material is, for example, an elastomeric material of which the dynamic shear modulus G.star-solid. subjected to a maximum alternating stress of 0.7 MPa, at a frequency of 10 Hz and at a temperature of 10, is higher than 200 MPa and preferably higher than 300 MPa. In this document, the terms elastic modulus G and viscous modulus G refer to dynamic properties well known to those skilled in the art. These properties are measured on a Metravib VA4000 viscoanalyzer using test specimens moulded from uncured compositions. Test specimens such as those described in the standard ASTM D 5992-96 (the version published in September 2006, initially approved in 1996), Figure X2.1 (circular embodiment) are used. The diameter of the test specimen is 10 mm (it therefore has a circular cross section of 78.5 mm.sup.2), the thickness of each of the portions of rubber composition is 2 mm, giving a diameter-to-thickness ratio of 5 (in contrast with standard ISO 2856 mentioned in paragraph X2.4 of the ASTM standard, which recommends a d/L value of 2). The response of a test specimen of vulcanized rubber compound subjected to simple alternating sinusoidal stresses at a frequency of 10 Hz is recorded. The test specimen is loaded in sinusoidal shear at 10 Hz, with the applied load (0.7 MPa) applied symmetrically about its position of equilibrium. Measurements are taken over a temperature gradient with the temperature increasing by 1.5 C. per minute, from a temperature Tmin below the glass transition temperature (Tg) of the material up to a temperature Tmax which may correspond to the rubber platen of the material. Before the sweep is begun, the test specimen is stabilized at the temperature Tmin for 20 minutes in order to achieve a uniform temperature within the test specimen. The result exploited is the dynamic shear elastic modulus (G) and the viscous shear modulus (G) at the chosen temperatures (in this instance 0, 5 and 20 C.). The complex modulus G.star-solid. is defined as the absolute value of the complex sum of the elastic modulus G and the viscous modulus G: G*={square root over ((G.sup.2+G.sup.2))}.

(14) In an alternative form of embodiment, the elastomeric material of the cover layer contains a composition based on at least one diene elastomer with a very high sulphur content, such as ebonite.

(15) In another alternative form of embodiment, the cover layer contains a collection of fibers, for example a three-dimensional collection of fibers forming a felt. The fibers of this felt may be selected from the group consisting of textile fibers, mineral fibers and mixtures thereof. It will also be noted that the fibers in this felt may be selected from textile fibers of natural origin, for example from the group consisting of silk, cotton, bamboo, cellulose and wool fibers and mixtures thereof.

(16) In another alternative form of embodiment, the elastomeric material of the cover layer contains a composition based on at least one thermoplastic polymer, such as polyethylene terephthalate (PET). Such a polymer may have a Young's modulus higher than 1 GPa.

(17) FIG. 3 depicts the strip 7 of FIG. 2 when this strip is loaded and in contact with an icy road surface 8. In this state, the depression 12 partially or completely closes. The extent to which this depression closes up depends on the load applied to the strip 7, on the material used in the layer 11, and on the type of road surface, snowy or icy, with which the strip 7 comes into contact.

(18) The distribution of load applied to the strip 7 is also indicated. This is the example in which the strip 7 is moving past the road surface in a direction of rotation R with the vehicle accelerating, i.e. with the edge corner 9 of the strip 7 being first to come into contact with the road surface 8. The pressure applied by the road surface 8 on the strip 7 is at its maximum near the edge corner 9 and this maximum pressure extends over a width L1.

(19) FIG. 4 is a view of the strip 7 in cross section according to a second embodiment of the invention. In this embodiment, the cover layer 11 does not extend beyond the depression 12, i.e. the layer 11 is not present in the bottom of the sipe 3 that delimits the strip 7.

(20) FIG. 5 depicts the strip 7 of FIG. 4 when this strip is loaded and in contact with the icy road surface 8. The inventors then discovered that the pressure applied by the road surface 8 on the strip 7 is at its maximum near the edge corner 9 and this maximum pressure extends over a width L2 greater than the width L1 of FIG. 3. Thus the grip of the tire on the icy road surface 8 is improved. Again, this is the example in which the strip 7 is moving past the road surface in a direction of rotation R with the vehicle accelerating, i.e. it is the edge corner 9 of the strip 7 that is first to come into contact with the road surface 8.

(21) FIG. 6 depicts a third embodiment of the invention, in which the depression 12 has a triangular shape with a point 13 pointing away from the edge corner 9.

(22) FIG. 7 depicts a fourth embodiment of the invention, in which the depression 12 opens onto the tread surface 15 of the tread. The cover layer 11 then makes an angle of less than 90 with this tread surface.

(23) FIG. 8 depicts a fifth embodiment of the invention, in which the strip 7 has two depressions 12 opening respectively onto two opposite lateral walls 5 of the strip 7.

(24) The invention is not restricted to the examples described and depicted and various modifications can be made thereto without departing from its scope.