EP1499507A1

EP1499507A1 - Tread with tread design elements which are formed from at least two mixtures of rubber

Info

Publication number: EP1499507A1
Application number: EP03727330A
Authority: EP
Inventors: Rémi Bruant; Didier Vasseur
Original assignee: Michelin Recherche et Technique SA Switzerland; Michelin Recherche et Technique SA France; Societe de Technologie Michelin SAS
Current assignee: Compagnie Generale des Etablissements Michelin SCA; Michelin Recherche et Technique SA France
Priority date: 2002-04-19
Filing date: 2003-04-22
Publication date: 2005-01-26
Also published as: CN1646335A; JP5201778B2; JP2005523193A; WO2003089257A1; US7311128B2; US20050109436A1; CN1326716C; AU2003233037A1

Abstract

The invention relates to a tyre tread comprising a plurality of raised elements (rib and/or block) comprising a contact surface which comes into contact with the road during rolling and side surfaces, the intersection of each side surface with the contact surface forming an edge; a plurality of cut-outs in the form of grooves or incisions, said cut-outs being defined by main surfaces facing each other, each tread design element being formed with at least one rubber mixture (base mixture), characterized in that from a cross-sectional view on a plane containing the thickness of said tread, at least one surface defining at least one cut-out is formed at least in part with a rubber mixture (covering mixture), the base mixture having a wet road adhesion lower than the covering mixture adhesion to the same road.

Description

TREAD HAVING SCULPTURE ELEMENTS FORMED FROM AT LEAST TWO RUBBER MIXTURES.

The invention relates to tires and more particularly to the tread of such tires.

In order to obtain satisfactory driving performance, especially on wet pavement, it is known practice to provide a tread for a tire with a tread formed by tread elements separated from each other by cutouts (branches of greater or equal average width at 2.5 mm and incisions of average width less than 2.5 mm), these cutouts being obtained for example by molding. The sculpture elements thus formed include a contact face intended to come into contact with the roadway during driving and lateral faces also delimiting the cutouts; the intersection of each lateral face with the contact face forms an edge facilitating the contact of the tire with the roadway and in particular when the latter is wet. More generally, an edge is defined as the geometric limit of contact of a sculpture element with the rolling ground.

By tread elements, there are elements which do not make the complete turn of the tire (blocks) and elements which make the complete turn (ribs). Furthermore, the sculpture elements may include one or more incisions to form additional edges, each incision may or may not lead to at least one lateral face of the sculpture element. By definition, an incision is the space delimited by two main faces opposite each other with a width less than 2.5 mm.

To improve the grip of the tire on a wet road, it is known that the very nature of the rubber mixture (or also called: rubber composition) making up the tread has a significant impact. Thus, a tread of a tire made of a rubber mixture having better grip on wet ground makes it possible to obtain improved performance when driving under wet ground conditions. However, in parallel with this improvement in performance in wet grip, there is a loss of performance in wear on dry road, which results in a reduced wear life and requires a more frequent change of tires on a vehicle.

Consequently, the need exists for a tire, the tread of which retains a good performance in terms of service life over wear while substantially improving the grip performance on wet surfaces. For this purpose, a rubber tread is proposed comprising:

A plurality of relief elements (rib and / or block) comprising a contact face intended to come into contact with the roadway during the rolling of a tire provided with said strip and the lateral faces, the intersection of each face lateral with the contact face forming an edge,

A plurality of cutouts in the form of antlers and / or incisions, said cutouts being delimited by opposite lateral faces,

• each tread element being formed mainly with at least a first rubber mixture (called "base mixture"), this tread being characterized in that, seen in section in a plane containing the thickness of this band, at at least one face of height H delimiting at least one cutout is covered at least in part with a second rubber mixture, called "covering mixture", this part in covering mixture extending over a height Hr at least equal to 30% from the height H of the face, at least one base mixture opens onto the contact face in new condition or at the latest after wear at most equal to 10% of the height Hr, the thickness of each mixture of covering on each covered side face being at most equal to 50% of the average distance D separating said side face from the side face of the same rubber element which it delimits and which is closest to it, the mixture of ba having a wet grip less than the adhesion of the coating mixture on the same floor.

The wet grip of each mixture can be obtained under various conditions, in particular by producing tires having treads in a single rubber mixture and by evaluating the grip characteristics of several wet mixes. One can also proceed under the conditions of the test defined according to standard NF P18-578.

Preferably, and to have a perceptible effect on the new tire, the covering mixture of at least one face is disposed from the contact face in the new state.

Preferably, the thickness of the covering mixture is greater than 0.2 mm over at least 80% of the height Hr. Preferably, the thickness of the covering mixture is at least equal to 0.2 mm and at most equal to 3 mm, on at least at least 80% of the height Hr, and even more advantageously between 0.4 mm and 2 mm, over at least 80% of the height Hr. By average distance separating a lateral face covered by a mixture of overlapping of another lateral face of the same element, one understands the average distance, measured perpendicular to the lateral face considered, between this face and the lateral face delimiting with it an element of rubber having at least these two sides for lateral faces. The term “average thickness of covering mixture on one face” means the average of the maximum and minimum thicknesses of covering mixture, it being understood that these thicknesses are measured on the tire in the initial state perpendicular to the face considered.

By average distance separating a cut (groove, incision) considered from its closest cut which is substantially parallel to it, is meant the average of the maximum and minimum distances separating said cutouts.

Preferably, the covering mixture is at least 5% more adherent on wet ground than the basic mixture and even more preferably at least 10% more adherent on wet ground than the basic mixture, in particular under the conditions of the test defined according to the standard. NF P18-578. The tire according to the invention surprisingly makes it possible to obtain a good balance between the wear performance on dry ground and the grip performance on wet ground. Indeed, if it were known, to obtain a high-performance tire on snow-covered ground, to form relief elements in a first base mixture and to cover the lateral faces of these elements with a thin layer in a mixture having a average glass transition temperature (Tg) lower than that of the base mixture (see for example US Pat. No. 5,840,137), no document described or suggested the proposed solution according to which the coating mixture has a wet adhesion which is better than that of the base mixture. Regarding the wet grip of a mixture, we can refer to the following document: "Rubber friction on self-affine road tracks" paper 43, American Chemical Society, Rubber division meeting, Chicago April 13-16 1999 This document clearly states that there is a relationship between the hysteresis of a mixture and its wet grip performance. One way of increasing the adhesion on wet ground of a rubber mixture is to increase its hysteresis by choosing for example a mixture having a higher glass transition temperature Tg. For each rubber mixture, the wet grip characteristics are evaluated according to the provisions laid down by standard NF PI 8-578 intended to characterize a ground or a pavement using a portable device called "British Pendulum Tester ", marketed by Stanley Corporation. According to this standard, a road surface is characterized by its level of adhesion in wet condition, the latter being measured with the previous device using a mixture called "reference mixture" defined in the same standard NF PI 8-578 and also marketed by Stanley Corporation.

This standard has been used here to assess the relative levels of adhesion of the different rubber mixtures used. The measurements are carried out at a temperature of approximately 25 ° C. The roadway on which the assessments of said grip levels in wet conditions are carried out is an actual roadway whose "true sand height" (HSV) as defined by standard NF P 98-216-1 is between 0.4 and 1.5 mm. To establish a satisfactory characterization of each rubber mixture, it is advisable to use an actual road surface (corresponding to the road surface on which a vehicle is driven to run) rather than any abrasive track; preferably, the characterization of the grip is made on a roadway identical to that on which the driving tests with vehicle are made.

It has been mentioned above that a way of improving the adhesion performance of a wet composition of a rubber composition can be obtained by increasing the Tg of said composition. This route consists, for example and as is known, of modifying the nature of the polymer and in particular its microstructure (in particular in the case of an SBR by acting either on the level of styrene or on the rate of vinyl or on these two rates appropriately). Preferably, the difference in Tg between the base mixture and the covering mixture is at least 5 ° C.

In addition, there is another way in which, at the same Tg, two mixtures can have different adhesion performance on wet ground. This second way consists for example in:

replace all or part of the carbon black filler with a reinforcing inorganic filler (and in particular a highly dispersible silica);

- increase the charge rate (preferably that of the reinforcing inorganic charge).

Of course, a combination of the routes mentioned above is entirely possible. The covering material having a higher adhesion than the base mixture can be placed on the opposite faces delimiting the same cutout by covering or not covering the bottom of the cutout.

Advantageously, all the lateral faces of the elements of reliefs delimiting circumferential grooves are provided with a mixture of adhesion covering on wet ground greater than the adhesion of the base mixture in order to very significantly improve the performance under cornering conditions ( under transverse stress) on wet ground. Of course, it is possible to use different overlay mixtures in combination with a base mixture (or even several base mixes); in this case, each covering mixture has a wet grip which is better than that of the base mixture which is directly adjacent to it. Advantageously, the relief elements located towards the interior of a vehicle on which the tire fitted with the tread according to the invention is mounted may comprise a mixture of interior covering and the elements on the exterior may comprise a mixture of outer covering, these covering mixes having different grip characteristics, the highest grip being, for example, located inside with respect to the equatorial plane of the tire in order to achieve a better compromise between grip on the road dry and grip on wet pavement.

We know that for various reasons, and in particular due to the evolution of the properties of the material with use and time, the level of grip on wet ground of the tire can gradually decrease, even if the material is initially homogeneous. in the thickness of the tread. To overcome this drawback, it is advantageous, in combination with the provisions of the invention, to provide for the presence of a second base mixture which is more adherent on wet ground than the first base mixture and whose arrangement within the elements is such that this second mixture appears progressively in width on the contact face of said elements, or more generally appears progressively over the width of the running surface of the tread as and when this strip wears. According to an advantageous variant, the second mixture can be identical to the covering mixture.

Preferably, it was determined that the base mixture should have a loss factor greater than 0.20 for a peak-to-peak shear deformation of 2%. The loss factor is equal to the loss modulus of the material divided by the elastic modulus of the material, these moduli being measured according to standard ASTM D 2231-71 at a temperature of 23 ° C and at a frequency of 10 Hz. losses is commonly noted: tan delta (tanδ). Even more preferably, the loss factor of the base mixture (s) is greater than 0.23 under the same conditions (which corresponds substantially to a tan delta value greater than 0.20 at 70 ° C).

Other characteristics and advantages of the present invention emerge from the description given below with reference to the appended drawings, in which: FIG. 1 represents a partial plan view of a block sculpture of a tread; Figure 2 shows the blocks of Figure 1 in cross section along the section line II-II; - Figure 3 shows a partial plan view of a ribbed sculpture around the tire; Figure 4 shows the cross section along the TV-TV section line of the rib shown in Figure 3 in the initial state; Figure 5 shows in cross section a variant of a sculpture block according to the invention; Figure 6 shows a perspective view of a variant of a sculpture block according to the invention in the initial state; Figure 7 shows a section along an NII-NII plane parallel to the contact face of the block shown in Figure 6; - Figure 8 shows a sculpture according to the invention, the results of which will be explained below; Figure 9 shows a cross section along line LX-LX of the tread of Figure 8; Figure 10 shows an alternative embodiment of a sculpture element according to the invention; - Figure 11 shows another alternative embodiment of a sculpture element according to the invention.

In FIG. 1, a distinction is made between the contact faces 2 of two blocks 1 of a tread pattern according to the invention. Each of these blocks 1 comprises four lateral faces 13 and 14 delimited respectively by grooves of longitudinal orientation 3 and transverse 4. The intersections of the lateral faces of each block with its contact face 2 form edges 23, 24 which play an important role in driving and in particular on wet pavement. Each block 1 is of rectangular shape of width L1 and of length L2 (the direction of the length L1 of the blocks being, in the case described, coincides with the longitudinal direction of the tread or else the circumferential direction on the tire provided with said strip. On the cross section shown in Figure 2 and made along line II-II of Figure 1 and perpendicular to the contact faces 2 of the blocks 1, we see that the sides or side faces 13 limiting the grooves of longitudinal orientation 3 are covered with a substantially constant thickness El of a covering rubber mixture MR which is different from the rubber mixture MB (called base mixture in the following) in that it has wet grip properties which are better than those of the base mixture on the same soil and under the same conditions. The average thickness El of coating mixture MR of the lateral faces is less than the total half width L1 of the blocks and preferably less than 30% of L1. To obtain a sensitive and lasting effect, it is preferable that this average thickness is at least equal to 0.2 mm in the case of a tread intended for a tread of a tire for a passenger vehicle. It can be seen that the covering mixture MR is also present at the bottom of the grooves 3.

The thickness El of the coating mixture MR can be, as is the case here, substantially uniform over the entire height of the face H and between 0.2 mm and 3 mm and even more preferably between 0.4 mm and 2 mm for a passenger car tire. The example described with the presence of a more adherent mixture on two of the faces of the blocks only can be easily extended to the other two faces or even to all the lateral faces. It may also be advantageous to have separate mixtures depending on the sides or side faces covered (similarly, the average thicknesses may be different depending on the face considered). In another interesting variant, the thickness of the covering mixture can vary in the direction of the depth of the grooves (this is for example the case with the variant shown in FIG. 5). The average thickness of the covering mixture is the average between the minimum thickness and the maximum thickness, these thicknesses being measured perpendicular to the face considered. In the case of a thickness variation, the difference between the maximum thickness and the minimum thickness is at most equal to twice the average thickness. Preferably, the maximum thickness is closer to the contact face of the relief elements in the initial state (corresponding to the new tire). FIG. 3 shows a plan view of the contact face 20 of width L10 of a rib 10 of a tread delimited by two grooves of circumferential or longitudinal orientation 31 and 32. This rib 10 is provided with a plurality of incisions 33, 34 opening alternately on only one of the lateral faces 131, 132. these incisions are inclined at an angle substantially equal to 45 ° relative to the longitudinal direction. These incisions 33, 34 have substantially the same depth as the grooves delimiting the rib. The average pitch noted P between these incisions is substantially the same for the two series of incisions. The rib 10 is formed in a base mixture MB and covered on these lateral faces with a covering mixture MR having a wet grip greater than that of the base mixture.

In FIG. 4, representing a section in a plane perpendicular to the contact face 20 and the trace of which is marked by the line IN-IN in FIG. 3, the presence on the lateral faces 131, 132 of the rib 10 is distinguished. and on the faces 330 and 331 vis-à-vis delimiting each incision 33 the presence of overlapping mixtures MR1 and MR2 respectively whose thicknesses are increasing with the depth whatever the face considered (thus the durability of the performance of grip is further enhanced). These average thicknesses are here less than 30% of the minimum of the two values L10 (average width of the rib) and of the pitch P between the incisions 33 and 34. In this variant, it can be seen that the covering mixture MR1 also covers the faces in opposite 330, 331 delimiting the incision 33. One means of obtaining such a rib consists, for example, in covering a tread blank produced in a base mixture with a layer of covering mixture of appropriate thickness before molding the tread and grooves and incisions. After molding, the covering mixture on the contact face of the rib can be removed by mechanical means (in particular by grinding). This means is also applicable to the blocks shown in Figures 1 and 2.

FIG. 5 shows, in section, a variant of a relief element 100 of a tread according to the invention for which, in addition to the covering mixture MR provided on the lateral faces 300 of said element, it is provided at the 'inside the element the presence of a first base mixture MB 1 and a second base mixture MB2. This second basic mixture MB2 has the shape of a triangle whose apex SI is close to the running surface in the initial state (without however leading to said surface in the new state). This second base mixture MB2 is chosen to have intermediate adhesion characteristics between the first base mixture MB 1 and the covering mixture MR. With the wear of the tread, this second basic mixture MB2 gradually appears on the new face of contact and its level of grip higher than that of the first basic mixture makes it possible to maintain the grip level of the tire despite wear. It can also be seen in this variant that the covering mixture MR has a maximum thickness close to the contact surface in the initial state and that this thickness decreases to vanish near the bottom of the grooves 30 delimiting the element 100.

In FIG. 6, another variant of a block 1000 according to the invention is shown according to which at least two lateral faces 1300 of said block are covered by a plurality of strips 1310 made of a covering mixture MR whose characteristic of adhesion to wet soil is greater than that of the base mixture MB on the same soil. On each face, the strips are all of the same dimensions and are oriented perpendicular to the contact face 2000. The configuration shown corresponds to the new state and it is clearly distinguished that the strips partially cover the contact face 2000. In this case, it is not necessarily necessary to remove, for example by grinding, the strips on the contact face since there is sufficient basic mixture on this face in the initial state and only after partial wear the mixture cover MR on said face is worn.

Figure 7 shows a section along a plane VII-NII parallel to the contact surface and at a depth corresponding substantially to half the thickness of the block of Figure 6. On this section we see on the side faces 1300 the different 1310 MR overlay mix strips. The case presented with the support of FIGS. 6 and 7 can easily be modified with an identical technical effect, for example by arranging the strips so that they are inclined at a non-zero angle with a perpendicular to the contact face. , or by arranging strips in different mixtures or in different thicknesses.

FIG. 8 shows a plan view of a sculpture of a tread according to the invention. This sculpture includes three main grooves RI, R2, R3 of circumferential orientation and substantially identical depths delimiting ribs Νl and Ν2 and shoulder parts Ei and Ex. The equatorial plane identified by direction XX 'in Figure 8 cuts the rib NI in two parts. The shoulder part Ei being intended to be placed on the interior side of a vehicle and is further provided with transverse grooves Ti delimiting blocks Bi. The rib N2 situated axially between the equatorial plane and the row of blocks Bi is provided with oblique incisions Oi opening out only in the groove R3, this groove R3 being situated between said rib N2 and said row of blocks Bi. On the other shoulder part Ex intended to be on the outside of a vehicle, a notch Ce is made with a longitudinal orientation and a width less than the circumferential grooves RI, R2, R3 and transverse grooves Te from this notch axially. towards the outside of the shoulder Ex to delimit blocksBe. The tread is made up of two different base mixes MBi and MBe, the separation between these mixes taking place radially under the groove RI axially closest to the shoulder part Ex as can be seen in FIG. 9 showing a cross section according to the line IX-LX of the tread shown in FIG. 8. The two base mixes have different reinforcement loads: the base mixture MBe of the shoulder part Ex (towards the outside of the vehicle) is reinforced by loads made of 50% silica and 50% carbon black. The reinforcing filler of the base mixture MBi (forming the ribs NI, N2 and the shoulder Ei) consists of 90% silica and 10% carbon black. The faces delimiting the grooves RI, R2, R3 and those of the transverse grooves Oi and Ti are covered with a covering mixture MR having a wet grip greater than 12% than the wet grip of the base mixture Mb (this same MR coating mixture has a wet adhesion 18% higher than that of the base mixture. The average thickness of the MR coating mixture is 0.5 mm. Each basic composition or mixture of Tg substantially equal to -26 ° C is formed from a mixture of two S.SBRs having Tg of ~ 50 ° C and -30 ° C respectively; the composition used as a covering mixture is formed from an S.SBR whose Tg is -18 ° C.

The coating mixture has a glass transition temperature Tg 8 ° C higher than that of the base mixtures MB1 and MB2 (the glass transition temperature Tg is measured according to standard ASTM D 3418-82). The three mixtures MB1, MB2 and MR used have the same Shore A hardness equal to 68 at 20 ° C (the Shore A hardness is measured according to standard ASTM D 2240-86). More generally, and to maintain good performance on wet ground for temperatures between 20 ° C and 0 ° C, the hardnesses of the mixtures in this same temperature zone are preferably less than 90 and more preferentially, between 50 and 90.

Preferably, the difference between the Shore A hardnesses of the base and overlay mixtures is at most equal to 30 and even more preferably at most equal to 10.

Tests carried out with tires. Reference tires were compared, the tread of which comprises the two basic mixtures, without the presence of a covering mixture with tires according to the invention having, in addition to the same basic mixes as those of the reference tires, a covering mixture as described with the example shown in Figure 8; all these tires have the same tread pattern (corresponding to that in Figure 8). The comparison is carried out by a pilot who assigns, after several passages, an overall rating of appreciation on taxiing on the same circuit covered with a water height on average equal to 1 mm. The tires tested are 225/40 ZR 18 mounted on the front and rear of a BMW M3 vehicle; the cold inflation pressures are 2.2 bars at the front and 2.6 bars at the rear.

With tires according to the invention as described above, there is obtained, in their favor, a difference of 1 point on average on the rating which is established on a scale ranging from 1 to 10. In a wear test, the tire according to the invention shows no deterioration in wear compared to the reference tire.

The industrial production of a tread according to the invention may consist in depositing, on the unvulcanized tire blank provided with a tread in an unvulcanized base mixture, strips of mixture different from the base mixture, as described in patent document EP 0510550 (the strips can be deposited in the circumferential and / or transverse direction on the tread). Another way can consist in producing the tread by co-extrusion of two (or more) mixtures at the time of the extrusion of the tread.

If in the invention, an essential element resides in the presence of a mixture of grip on wet ground on at least one face of the elements of relief, it is reasonable to include in this invention tread patterns, of which in new condition some of the contact surfaces with the ground are also partially or completely covered with a covering mixture of the same kind. An example has already been shown and described with Figureδ. Two other variants are given by way of examples with FIGS. 10 and 11. FIG. 10 shows a section of a relief element 5 of a tread according to the invention: in this example, the thickness of the mixture covering present on the side faces 51 of the element 5 extends over the entire contact surface 52 of said element. In this configuration, the tire has, when new, an excellent performance on wet ground but quickly the wear will lead to the elimination of the major part of the layer of covering mixture on the contact surface, thus putting the base mixture in contact with the pavement to allow a reduction in the wear rate while retaining a good performance in grip on wet ground by the presence of covering mixture on the faces of the element of relief. In FIG. 11, the covering mixture present on the lateral faces 51 of an element 5 extends partially over the contact surface 52 of said element.

The invention is not limited to the examples described and shown and various modifications can be made thereto without departing from its scope. In particular, in the case of a tire intended primarily for winter type use and after partial wear on all-weather rolling, it is known to use two basic mixtures with different characteristics (see for example US 4,619,300); the implementation of the invention for this type of tire makes it possible to further improve the grip performance while retaining a good wear performance. For this, it is possible to form a tread having for a first part of wear (winter use with the new tire) a first basic mixture specific to winter driving, this first basic mixture being covered on the faces with the patterns of relief with a mixture of adhesion covering on wet ground greater than that of said first mixture as proposed above. Under this first basic mixture there is a second specific basic mixture of an all-weather run. This second basic mixture is located inside the tread relative to the tread surface in new condition, so as to appear after appropriate wear (for example from the mid-wear of the tread) . The relief patterns may or may not be extended in this second base mixture and may or may not be coated on all or parts of their side walls with a covering mixture. The wet grip characteristic of the overlay mixture is greater than the wet grip characteristic of the all weather base mix.

Claims

1 - Rubber tread in particular for tires comprising: • a plurality of relief elements (rib and / or block) comprising a contact face intended to come into contact with the roadway during the running of a tire provided with said strip and lateral faces, the limit of contact of the contact face with the ground forming at least one edge,

A plurality of cutouts in the form of a groove and / or incision, said cutouts being delimited by lateral faces facing each other,

• each tread element being formed with at least a first rubber mixture (called "base mixture"), this tread being characterized in that, seen in section in a plane containing the thickness of this band, at least a face delimiting at least one cutout is covered at least in part with a second rubber mixture, called "covering mixture", this part in covering mixture extending in new condition over a height Hr at least equal to 30 % of the height H of the face, at least a first base mixture emerges on the contact face in new condition or at the latest after wear at most equal to 10% of the height Hr, the average thickness of covering mixture on each covered side face being at most equal to 50% of the average distance D separating said side face from the side face of the same rubber element which it defines and which is closest to it, said mixture of ba having a wet grip less than the adhesion of the coating mixture on the same floor.

2 - Tread according to claim 1 characterized in that the covering mixture of at least one face is disposed from the contact face in new condition.

3 - Tread according to claim 1 or claim 2 characterized in that the thickness of the covering mixture is greater than 0.2 mm over at least 80% of the height Hr and at most equal to 30% of the average distance D .

4 - Tread according to one of claims 1 to 3 characterized in that the thickness of the covering mixture is, over at least 80% of the height Hr, between 0.2 mm and

3 mm. 5 - Tread according to claim 4 characterized in that the thickness of the covering mixture is, over at least 80% of the height Hr, between 0.4 mm and 2 mm.

6 - Tread according to one of claims 1 to 5 characterized in that the covering mixture is at least 5% more adherent on wet ground than each base mixture. 7 - Tread according to claim 6 characterized in that the covering mixture is at least 10% more adherent on wet ground than each base mixture.

8 - Tread according to one of Claims 1 to 7, characterized in that the difference between the Shore A hardnesses of the base and covering mixtures is at most equal to 30.

9 - Tread according to claim 8 characterized in that the difference between the Shore A hardnesses of the base and covering mixtures is at most equal to 10.

10 - Tread according to one of claims 1 to 9 characterized in that at least certain relief elements comprising a covering mixture on at least one face comprise two basic mixes, the first of said basic mixes having a characteristic wet grip which is less than the wet grip characteristic of the second base mixture, these two base mixes being arranged in these elements so that the contact surface of the second base mixture gradually increases with the wear of the tread while the surface of the first basic mixture decreases in return.

11 - Tread according to one of claims 1 to 10 characterized in that the covering mixture of at least one face is arranged on said face in the form of a plurality of strips.

12 - Tread according to one of claims 1 to 11 characterized in that each base mixture has a loss factor greater than 0.20 measured at a temperature of 23 ° C and under a peak to peak deformation of 2%. 13 - Tread according to claim 12 characterized in that each base mixture has a loss factor greater than 0.23 measured at a temperature of 23 ° C and under a peak to peak deformation of 2%.

14 - Tread according to claim 10 characterized in that the second base mixture is identical to the covering mixture. 15 - Tread according to claim 1 characterized in that the base mixture emerging on the contact face in new condition or at the latest, after wear at most equal to 10% of the height Hr, is a mixture rubber suitable for driving in conditions winter, said tread further comprising another base mixture intended to appear on the tread surface after partial wear, this other mixture being suitable for running in all weather conditions.

16 - Tire tread comprising a plurality of circumferential orientation grooves delimiting tread elements, these elements comprising, in addition to a contact face intended to come into contact with the roadway during driving, at least two sides or side faces of general circumferential orientation, said tread elements being formed with at least one first rubber mixture (called "base mixture"), this tread being characterized in that for at least one of said tread elements, seen in section in a transverse plane containing the thickness of this strip and substantially perpendicular to said elements, the lateral faces of generally circumferential orientation are covered at least in part with a mixture, known as covering mixture, this part in covering mixture extending to from the contact face over at least 30% of the height of the side faces and over a thickness at most equal to 30% of the average distance separating the lateral faces of generally circumferential orientation of said element, the base mixture having a wet grip less than the grip of each covering mixture on the same floor .

17 - Tire tread according to claim 16 characterized in that it comprises, in the transverse direction, at least two different basic mixtures and in that it also comprises a means for indicating the mounting direction of a tire provided with said strip on a vehicle, the base mixture intended to be placed on the vehicle interior side having better grip on wet ground compared to the base mixture intended to be placed on the vehicle exterior side

18 - Tire tread according to claim 16 or according to claim 17 characterized in that the mixtures of covering of the tread elements on one part of the running surface are different from the mixtures of covering of the tread elements of the other part of the running surface, the covering mixes located axially towards the vehicle interior having a greater wet grip than the covering mixes of the other part located axially towards the vehicle interior.