Tires with a tread pattern optimized for grip on wet surfaces when worn.

Description

Technical field

[0001] The present invention relates to a tire for a motor vehicle whose longitudinal grip performance on wet ground, in the worn state, is improved. The invention is more particularly suitable for a radial tire intended to equip a passenger vehicle or a van.

[0002] A worn tyre is a tyre that has had a normal service life, reaching the legal limit of the wear indicators positioned on the sidewalls of the tyres.

[0003] The legality in question here relates to the UNECE (United Nations Economic Commission for Europe) R30 and R54 tire safety regulations. This organization includes most European countries, particularly those in the European Union, but also countries outside Europe such as Japan, Thailand, and Australia. These countries have validated a mutual recognition agreement for tire regulatory approvals. In other words, a tire that is legally approved in one UNECE country is also validated in all other countries of the organization where these regulations are in force.

[0004] Before being marketed in UNECE countries, a passenger car or van tire must comply with the safety regulatory requirements of regulations R30 and R54 and the performance threshold requirements of regulation R117. Wear indicators in the grooves or transverse grooves of the tread, as well as their identification on the sidewalls of the tires, are requirements of these regulations. The R117 performance threshold regulation requires that tires have a performance level higher than the thresholds set by the regulation with regard to rolling resistance, grip on wet and snowy surfaces, and rolling noise.

[0005] For example, a passenger car tire to be sold in UNCE countries must have a rolling resistance value less than or equal to 6 kg / t, as required by regulation R117. This same tire must have wet braking performance at least 10% higher than the control tire set by regulation R117. The methods and parameters of these tests are also defined by this regulation.

[0006] The R117 performance threshold regulation applies to new tyres for their marketing on the UNECE market, but the question arises as to the effectiveness of these requirements depending on tyre wear.

[0007] The invention here relates to the durability of wet grip performance, i.e. the ability of the tire to still comply with regulatory thresholds even for a tire close to legal wear, according to an appropriate test method. Definitions

[0008] By convention, a reference (O, XX', YY', ZZ'), whose center O coincides with the center of the tire, the circumferential XX', axial YY', and radial ZZ' directions respectively designate a direction tangent to the rolling surface of the tire according to the direction of rotation, a direction parallel to the axis of rotation of the tire, and a direction orthogonal to the axis of rotation of the tire.

[0009] By radially inner, respectively radially outer, we mean closer, respectively further from the axis of rotation of the tire.

[0010] Axially inner, respectively axially outer, means closer, respectively further from the equatorial plane of the tire, the equatorial plane of the tire being the plane passing through the middle of the tire tread and perpendicular to the axis of rotation of the tire.

[0011] A tire comprises a crown, intended to come into contact with the ground via a tread, the two axial ends of which are connected via two sidewalls with two beads ensuring the mechanical connection between the tire and the rim on which it is intended to be mounted.

[0012] Generally speaking, a person skilled in the art defines the tread of a tire primarily using the following design characteristics: the tread surface defining the total width of the tread, and the sculpture characterized by a volumetric notch rate.

[0013] The "rolling surface" of the tread means the surface that includes all the points of the tire that will come into contact with the ground under normal driving conditions. These points that will come into contact with the ground belong to the contact faces of the blocks. For a tire, the "usual driving conditions" are the conditions of use defined by the ETRTO (European Tire and Rim Technical Organization) standard. These conditions of use specify the reference inflation pressure corresponding to the load capacity of the tire indicated by its load index and its speed code. These conditions of use can also be called "nominal conditions" or "conditions of use".

[0014] The total width of the tread is the axial distance between the axial ends of the tread surface, symmetrical with respect to the equatorial plane of the tire. In practical terms, an axial end of the tread surface does not necessarily correspond to a clearly defined point. Knowing that the tread is delimited externally, on the one hand, by the tread surface and, on the other hand, by two connecting surfaces with two sidewalls connecting said tread to two beads intended to ensure the connection with a mounting rim, an axial end can then be defined mathematically as the orthogonal projection, on the tread, of a theoretical point of intersection between the tangent to the tread surface, in the axial end zone of the tread surface, and the tangent to the connecting surface, in the radially outer end zone of the connecting surface.The total tread width corresponds approximately to the axial width of the contact patch when the tire is subjected to the recommended load and pressure conditions.

[0015] An elastomeric mixture is understood to mean an elastomeric material obtained by mixing its various constituents. An elastomeric mixture typically comprises an elastomeric matrix with at least one diene elastomer of natural or synthetic rubber type, at least one reinforcing filler of carbon black type and / or silica type, a crosslinking system most often based on sulfur, and protective agents.

[0016] The expression "based on" composition means a composition comprising the mixture and / or the reaction product of the different constituents used, some of these basic constituents being capable of, or intended to, react with each other, at least in part, during the different phases of manufacture of the composition, in particular during its crosslinking or vulcanization.

[0017] By the expression "part by weight per hundred parts by weight of elastomer" (or pce), it is meant, within the meaning of the present invention, the part, by mass per hundred parts of elastomer present in the mixture composition considered.

[0018] The choice of tread material is an essential step in tire design. Generally, it is an elastomeric compound characterized by its dynamic properties, such as its viscoelastic loss Tgδ (tangent delta) and / or its complex dynamic shear modulus G*.

[0019] The viscoelastic loss, Tgδ, and the complex shear modulus G* are generally determined when measuring the dynamic properties of the elastomeric mixture, on a viscoanalyzer (Metravib VA4000), according to the ASTM D 5992-96 standard. The measurement of the dynamic properties is carried out on a sample of vulcanized elastomeric mixture, i.e. cured to a conversion rate of at least 90%, the sample having the shape of a cylindrical test piece with a thickness equal to 2 mm and a cross-section equal to 78.5 mm 2< . The response of the elastomeric mixture sample to a sinusoidal alternating simple shear stress, having a peak-peak amplitude equal to 0.7 MPa and a frequency equal to 10 Hz, is recorded. A temperature sweep is carried out at a constant temperature rise rate of +1.5°C / min.The results used are generally the complex dynamic shear modulus G*, comprising an elastic part G' and a viscous part G'', and the dynamic loss Tgδ, equal to the ratio G'' / G'. The glass transition temperature Tg is the temperature at which the dynamic loss Tgδ reaches a maximum during the temperature scan. The value of G* measured at 23°C is representative of the rigidity of the rubber material, i.e. its resistance to elastic deformation. Prior art

[0020] As is known, the tread of a tire is provided with a sculpture comprising in particular sculpture elements or elementary blocks delimited by various main grooves, longitudinal, transverse or even oblique, the elementary blocks being able to comprise various incisions or finer sipes. The grooves constitute channels intended to evacuate water when driving on wet ground and define the edges of the sculpture elements.

[0021] As a tire wears, the tread volume decreases due to material loss due to friction with the road surface. When the tire is new, the tread is at its maximum thickness. As the tire wears, the height of the basic tread blocks decreases, and the stiffness of these blocks increases. The increase in the stiffness of the basic tread blocks results in a decrease in certain tire performance characteristics, such as wet grip, even when the tread block height is at the legal wear indicator.

[0022] It is therefore necessary not to limit ourselves to testing tires in their new condition but also to carry out performance tests in their worn condition, particularly on wet ground.

[0023] For example, patent EP2163403B1 proposes a tread comprising two layers of materials superimposed radially, and placed externally on the crown of a tire. The interface between the two layers making up the tread is corrugated. The tread pattern adapts according to the wear of the tire to reveal other cutouts.

[0024] In international application WO2004 / 096583A1, a tire with a multi-layer tread, and its manufacturing method are disclosed. The tread comprises a first inner layer, radially inside an outer layer intended to be in contact with a ground. On the one hand, the elastic modulus of extension-compression of the inner layer is greater than that of the outer layer, and on the other hand the radially inner layer of the tread has a profile with radially oriented protuberances outwardly spread over the entire width of the tread. This disclosure explains that such a tire mounted on a vehicle improves the behavior of the vehicle, particularly when taking turns.

[0025] In document DE19731525A1, a multi-layer tread of at least two superimposed layers is disclosed, in such a way that their common interface is above the minimum profile depth, so that the lower part of these layers, in a later phase of the life of the tire, comes into contact with the ground.

[0026] The lifespan of tires varies depending on their usage conditions. However, even under severe usage conditions, i.e. driving with high lateral acceleration of the vehicle, this lifespan is often greater than 10,000 km.

[0027] To limit the costs associated with obtaining a used tire, it is customary to plane the tread, i.e. to reduce the thickness of the tire tread by cutting or brushing to a height corresponding to that which would be obtained by wear during rolling of the tire. To this end, patent FR3059421 discloses a planing method for preparing tires for tests in the worn state.

[0028] The inventors set themselves the goal of creating a tire that maintains wet grip performance throughout its life without degrading other performance such as rolling resistance. Exposed of the invention

[0029] This aim was achieved by a tire comprising a crown reinforcement, an underlayer, and a multi-layer tread formed from three layers of elastomeric compounds M1, M2, M3, the radially outermost layer of which M3 is intended to come into contact with the ground via a rolling surface: the crown reinforcement being arranged radially inside the multi-layer tread; the sub-layer being arranged radially outside the crown reinforcement and radially inside the multi-layer tread, and extending over the entire axial width of the crown reinforcement; the multi-layer tread comprising a first layer M1 arranged radially outside the sub-layer and extending axially over all or part of the axial width of the sub-layer at least in sections; the tread also comprising a radially outermost layer M3 provided with a tread surface; the elastomeric mixtures of the layers respectively M1, M2, M3 having respectively a shear stiffness modulus, G'_M1, G'_M2, G'_M3, and having respectively a viscoelastic loss Tgδ_M1, Tgδ_M2, Tgδ_M3, measured at 23°C at 10 Hz and under an alternating shear deformation of 10% of the material of each layer;the multi-layer tread comprises an intermediate layer M2 arranged radially external to the first layer M1 in contact with it, and radially internally with the most radially external layer M3, extending axially at least in sections along the first layer M1; the ratio G'_M1 / G'_M2 is within the interval [7; 25]; the viscoelastic loss Tgδ_M2 of the elastomeric mixture of the intermediate layer M2, and the viscoelastic loss Tgδ_M3 of the elastomeric mixture of the radially outermost layer M3, are linked by the relation: (Tgδ_M2-Tgδ_M3) / Tgδ_M2 ≥ 20% the ratio of the radial thickness of the radially outermost layer M3 divided by the radial thickness of the intermediate layer M2, Thickness_M3 / Thickness_M2, is within the interval [3; 10], said thicknesses being measured on the axis of symmetry of a meridian plane of the tire. ;

[0030] The principle of the invention is to achieve a gain in wet grip in return for an increase in rolling resistance in controlled proportions, because the tire of the invention must comply with the regulations on performance thresholds, including rolling resistance. To this end, the addition of the intermediate layer M2 optimized for grip in place of part of the volume of the radially outermost layer M3, optimized for rolling resistance, creates a new balance of performance. After wear of the M3 layer of the tread, the tire still has a high wet grip potential with the intermediate layer M2. In fact, the tire of the invention, throughout its lifetime of use, potentially maintains wet grip performance that complies with the regulations.

[0031] According to the invention, the multi-layer tread comprises an intermediate layer M2 arranged radially external to the first layer M1 in contact with it, and radially internal to the most radially external layer M3, extending axially at least in sections along the first layer M1.

[0032] The intermediate layer M2 is optimized for wet grip with relatively high viscoelastic dissipation compared to that of the mixture of the radially outermost layer M3. For illustration, if the viscoelastic loss of the mixture of the intermediate layer M2 is 0.25, then for the radially outermost layer M3, the viscoelastic loss will be of the order of 0.17. The intermediate layer M2 is flexible in the sense that its shear stiffness is relatively low compared to the stiffnesses of the adjacent materials. The high hysteresis combined with the low stiffness of the mixture of the intermediate layer M2 tends to improve wet grip.

[0033] For example, the shear stiffness modulus of the mixture of the first layer M1 could be of the order of 25 Mpa, and that of the mixture of the intermediate layer M2 of the order of 1.7 Mpa. With such values, the mixture of the layer M1 is said to be rigid compared to that of the intermediate layer M2.

[0034] The phenomenon of tire grip on wet ground occurs in three stages. The first stage consists of drying the ground by evacuating water thanks to the tread pattern, then a phase of contact with the ground by adhesion and indentation. And it is precisely during the indentation phase that the asperities of the ground cling to the layer of mixture to oppose the movement. The greater the hysteresis of the mixture, the more the opposition to the movement is amplified.

[0035] To obtain the desired performance compromise, according to the invention, the rigidities and hysteresis of the tread compound layers M1, M2, and M3 satisfy the following relationships: a. the ratio G'_M1 / G'_M2 is in the interval [7; 25]; b. the viscoelastic loss Tgδ_M2 of the elastomeric mixture of the intermediate layer M2, and the viscoelastic loss Tgδ_M3 of the elastomeric mixture of the radially outermost layer M3, are related by the relation: (Tgδ_M2 - Tgδ_M3) / Tgδ_M2≥ 20%.

[0036] The first relation (a) means that the first layer M1 must be very rigid with respect to the intermediate layer M2. The layer M1 is never in contact with the ground, on the one hand it participates in the stiffening of the tread, useful for generating sufficient drift thrusts, and on the other hand due to its high rigidity, during manufacturing it is the support for laying the intermediate layer M2 which is relatively "soft". Laying the intermediate layer M2 under these conditions allows its radially external contour to be parallel to the axial profile of the mold. The axial profile of the mold is defined so as to obtain a uniform distribution of vertical pressures in the contact area. Uniformity of contact pressures is necessary to obtain regular wear of the tread in the axial direction.The intermediate layer M2, after wear of the radially outermost layer M3, is intended to be in contact with the ground, and its radially outer profile parallel to the axial profile of the mold therefore guarantees that regular wear is obtained. Furthermore, penetrations between the layers are limited with the first layer M1 as a support for laying.

[0037] According to the invention, the ratio G'_M1 / G'_M2 is in the range [7; 25], which means that the shear stiffness of the first layer M1 is at least seven times higher than that of the intermediate layer M2 to begin to perceive the effects of the invention. However, beyond a shear stiffness ratio, G'_M1 / G'_M2 greater than 25, the effect asymptotically decreases.

[0038] As for the second relation (b) on the viscoelastic loss, it indicates the hysteresis difference necessary to perceive the gain in the adhesion potential of the mixture of the intermediate layer M2 which is intended to come into contact with the ground, with respect to the most radially outer layer M3. In other words, a relative difference of at least 20% between the viscoelastic losses of the elastomeric mixtures of the two layers is necessary to begin to perceive the effects of the invention.

[0039] The link between the rigidities and hystereses of the layers of the tread M1, M2, and M3 having been established, it is now a question of establishing the geometric relationships between these layers. Again according to the invention the ratio of the radial thickness of the most radially outer layer M3 divided by the radial thickness of the intermediate layer M2, Thickness_M3 / ThicknessM2, is included in the interval [3; 10], said thicknesses being measured on the axis of symmetry of a meridian plane of the tire.

[0040] Given the axisymmetry of the tire around its axis of rotation, the volume of a layer is directly proportional to its radial thickness in a meridian plane. The distribution of the volumes of the tread layers M1, M2, and M3 is the second lever to be combined with the mechanical and hysteretic properties of the layers to achieve the invention. The hysteresis-optimized tread layer M3 represents the largest volume. The invention works if the intermediate layer M2 has a volume greater than or equal to one tenth of the volume of the radially outermost layer M3, and less than or equal to one third of this volume.

[0041] If the volume of the intermediate layer M2 is less than one tenth of the volume of the layer M3, then the gain in wet grip is not significant in this case. Conversely, if the intermediate layer M2 is greater than one third of the volume of the radially outermost layer M3, then the degradation of rolling resistance becomes prohibitive.

[0042] The combination of the essential characteristics of the invention, described above, makes it possible to produce a tire with a multi-layer tread allowing the long-term performance of wet grip. In other words, thanks to the invention, the wet grip performance does not deteriorate when the tire wears. We can now examine the other characteristics and embodiments of the invention.

[0043] Advantageously, the ratio of the shear stiffness modulus of the elastomeric mixture of the first layer M1, divided by the shear stiffness modulus of the elastomeric mixture of the radially outermost layer M3, G'_M1 / G'_M3 is within the range [7; 25]. The M3 layer of the tread is intended to be in contact with the ground when the tire is new and has the largest volume among the three layers. The performance compromise argues for a radially outermost layer M3 with low hysteresis and a shear stiffness of the same order of magnitude as for the second intermediate layer M2.

[0044] Preferably, the ratio G'_M1 / G'_M3 is included in the interval [6; 22], likewise the ratio G'_M1 / G'_M2 is included in the interval [6; 22]. The difference in mechanical properties between the layers M2 and M3 is mainly at the level of hysteresis, but in terms of rigidities, the shear moduli are of the same order of magnitude compared to the mixture of the first layer M1.

[0045] Still preferably, the shear stiffness modulus of the elastomeric mixture of the sub-layer is less than or equal to 1.5 MPa. The multi-layer tread is connected to a so-called "soft" sub-layer, with respect to the shear stiffness of the elastomeric mixture of the first layer M1.

[0046] Advantageously, the viscoelastic loss Tgδ_M2 of the elastomeric mixture of the intermediate layer M2 is greater than or equal to 0.2, and even more advantageously, the viscoelastic loss Tgδ_M2 of the elastomeric mixture of the intermediate layer M2, and the viscoelastic loss Tgδ_M3 of the elastomeric mixture of the radially outermost layer M3, are linked by the relation: (Tgδ_M2 - Tgδ_M3) / Tgδ_M2 ≥ 30%. The relative difference of the viscoelastic losses of the elastomeric mixtures of the layers M2 and M3 are in a ratio greater than 30%.

[0047] Preferably, the shear stiffness modulus G'_M1 of the elastomeric mixture of the first layer M1 is greater than 7 MPa, and even more preferably, the shear stiffness modulus G'_M1 of the elastomeric mixture of the first layer M1 is greater than or equal to 12 MPa.

[0048] For the proper functioning of the invention, the rigidities of the layers of the tread are correlated. The first layer M1 has the greatest rigidity, with a value of at least 7 Mpa, and preferably greater than or equal to 12 Mpa.

[0049] Advantageously, the planed profile of the tread being by convention, defined such that the residual sculpture height after planing is 2 mm over the entire axial width of the tread, the maximum deviation Emax being the radial distance between the planed profile of the tread, and the radially outer profile of the first layer M1 characterized in that the maximum deviation Emax is constant at ± 5 mm over at least a central portion of the tread.

[0050] In a first embodiment, the grooves of the tread being the deepest grooves of the sculpture, the material appearing at the bottom of the groove is the same as the material constituting the most radially outer layer M3 of the tread.

[0051] In a second embodiment, the intermediate layer M2 arranged radially external to the first layer M1, and in radially internal contact with the most radially external layer M3, extends axially continuously along the first layer M1.

[0052] In a third embodiment, the tread comprising tread blocks separated by grooves oriented essentially circumferentially, the first layer M1 is, axially opposite certain tread blocks, extended radially outwards by at least one reinforcing element extending radially from the radially outer surface of the first layer M1 towards the outside of the tread up to a radial height greater than 75% of the radial thickness of the tread, said reinforcing element being of variable axial width, from a maximum value less than 50% of the axial width of said tread block, said axial width decreasing when moving radially upwards. Description of Figures

[0053] The present invention will be better understood upon reading the detailed description of embodiments taken as non-limiting examples and illustrated by the appended drawings in which: [ Fig 1 ] There figure 1 shows a schematic meridian section of a tire according to a first embodiment of the invention. Fig 2 ] There figure 2 shows on a meridian section, the wear kinetics of the tire where the Figures 2-A, 2-B, 2-C represent different states of wear of the tire, according to the first embodiment of the invention. Fig 3 ] There figure 3 shows a schematic meridian section of a tire according to a second embodiment of the invention. Fig 4 ] There figure 4 shows a schematic meridian section of a tire according to a third embodiment of the invention. Detailed description of the invention

[0054] On the figure 1, a general reference tire 1 is shown comprising a radially inner crown reinforcement 50 with a carcass reinforcement 60, a sub-layer 30, and a multi-layer tread 20 formed of layers M1, M2, M3, the most radially outer layer M3 of which is intended to come into contact with the ground via a rolling surface 10: the radially innermost carcass reinforcement 60 comprises at least one carcass layer, shown in the figure 1in broken lines. The carcass layer is formed of reinforcements coated with an elastomeric mixture. The carcass layer 60 comprises a main part 61, connecting the two beads 65 together and winding in each bead, from the inside to the outside of the tire around a circumferential reinforcement element, most often metallic, called a bead wire 63, to form a turn-up 62. The metallic reinforcements of a carcass layer are substantially parallel to each other and form, with the circumferential direction, an angle of between 85° and 95°. the crown reinforcement 50 is arranged radially outside the carcass reinforcement 60 and radially inside the multi-layer tread 20, and is formed in particular of two crown layers (51, 52) consisting of metallic reinforcements coated in an elastomeric mixture.the sub-layer 30 is arranged radially outside the crown reinforcement 50 and radially inside the multi-layer tread 20, and extends over the entire axial width of the crown reinforcement 50. The sub-layer 30 is shown in the . figure 1 , by a pattern formed of vertical hatching in white and black.

[0055] More specifically, the three layers of compounds M1, M2, and M3 of the tread 20 are radially superimposed on the underlayer 30, and oriented radially outwards. The first layer of elastomeric compound M1 is shown in the figure 1 by a black background, and is placed most radially inwardly on the sub-layer 30. The second intermediate layer M2 is radially outwardly placed on the first layer M1. On the figure 1, the intermediate layer M2 is represented by a wave-shaped pattern. Finally, the third, most radially outer layer M3 placed radially outwardly on the intermediate layer M2 is represented by a light-background pattern with black dots. This layer, the most radially outer M3, comprises the tread surface 10 and is intended to be in contact with a ground when the tire is in new condition.

[0056] In the embodiment of the figure 1, the tread 20 also comprises grooves 80 oriented mainly circumferentially and determining successive tread blocks in the axial direction from the shoulder 15G to the shoulder 15D. Each groove 80 is delimited radially inwards by a groove bottom. The most radially outer layer M3 extends continuously over the entire axial width of the tread passing through the bottom of the grooves, while the layers M2 and M3 are discontinuous. In certain groove bottoms, there is an indicator 70 of the state of wear of the tread. This indicator extends radially outwards from the bottom of the groove over a height of 1.6 mm.

[0057] There figure 2 represents successive states of wear of the tire, 2-A, 2-B and 2-C. The Figure 2-A shows the new tire with the tread made up of the three layers of elastomeric compound M1, M2, M3. On the Figure 2-B, the radially outermost layer M3 is completely worn, and the intermediate layer M2 comes into direct contact with the ground. The tire still has wear potential, the wear indicators 70 having a radial height lower than that of the intermediate layer M2. On the Figure 2-C , the tire is worn. The tread height is at the wear indicator 70.

[0058] There figure 3represents the diagram of a tire of a second embodiment of the invention in a meridian plane where the first layer M1 of the tread extends radially outwardly in the form of a trapezoid 27 on the tread surface 10 so as to axially border the grooves 80 on either side. When traveling along the tread from the end 15G towards the end 15D, each tread block delimited by the grooves 80 are bordered by the extensions of the first layer M1 in the form of a trapezoid 27 on either side. The blocks located at the ends 15G and 15D are bordered only on the side oriented towards the center of the tread.

[0059] There figure 4 shows the third embodiment of the invention, which is characterized by the use of three layers M1, M2 and M3 continuous over the entire width of the top.

[0060] In other embodiments, not shown here, the tire may contain a hoop reinforcement comprising one or two hoop layers, radially externally superimposed on the crown reinforcement 50. Each hoop layer is formed of reinforcements parallel to each other, making angles at most equal to 2.5° with the circumferential direction. Tests

[0061] The invention has been more particularly studied for a passenger car tire with a standardized designation, according to ETRTO (European Technical Organization for Rims and Tires), 245 / 45 R18 100W. For this dimension a version conforming to the first embodiment of the invention has been produced, compared to a conventional state-of-the-art tire with a single-layer tread.

[0062] The M1, M2, and M3 layers of the tread are characterized by their dynamic and geometric properties grouped in the following table: [Table 1] Radial thickness (mm) G' one Mpa Tgδ Volume in cm3 Undercoat 1.6 0.2 0,06 785 Mix layer M1 1.3 25 0,27 342 M2 layer mix 1.3 1.7 0,25 330 M3 layer mix 4.5 1.4 0,17 1331

[0063] The underlayer laid radially outside the crown reinforcement has a radial thickness of 1.6 mm, measured on the axis of symmetry of a meridian plane of the tire. This thickness corresponds to a volume per wheel revolution of 785 cm3.

[0064] An example of a suitable composition for the undercoat is given in the following table: [Table 2] Undercoat Component Pce NR (a)< 100 Carbon black 4 6-PPD (b)< 2.2 DPG (c)< 2.1 ZnO (d)< 1.5 Stearic acid 3 Resin 29.3 HTO (e)< 38 CBS (f)< 1,4 Sulfur 1,6 With : a. NR: Natural Rubber b. 6-PPD: N-1,3-dimethylbutyl-N'-phenyl-p-phenylenediamine ("Santoflex 6-PPD" from Flexsys) - antioxidant c. DPG: Diphenylguanidine (“Perkacit” DPG from Flexsys) - vulcanization activator d. ZnO: Zinc Oxide, Vulcanization e. HTO: Sunflower oil with 85% oleic acid by weight, “Lubrirob Tod 1880” from Novance - plasticizer f. CBS: N-cyclohexyl-2-benzothiazyl-sulfenamide (Santocure CBS from Flexsys) - vulcanization accelerator

[0065] With such a composition, the underlayer has a shear stiffness modulus of 0.45 Mpa, and a viscoelastic loss of 0.06. The underlayer is optimized in hysteresis to contribute to achieving the rolling resistance performance target.

[0066] The first layer M1 of the tread laid radially externally on the sub-layer has a thickness of 1.3 mm, measured on the axis of symmetry of a meridian plane of the tire, which represents a volume of 342 cm3.

[0067] An example of composition for the first layer M1 is in the following table: [Table 3] M1 layer Component Pce NR (a)< 100 Carbon black 70 Resin Formophenolic 12 6-PPD (b)< 2.5 Sulfur 3 ZnO (c)< 3 Stearic acid 2 CBS (d)< 2 HTMT (e)< 4 Sulfur 3 With : a. NR: Natural Rubber b. 6-PPD: N-1,3-dimethylbutyl-N'-phenyl-p-phenylenediamine ("Santoflex 6-PPD" from Flexsys) - antioxidant c. ZnO: Zinc Oxide, Vulcanization d. CBS: N-cyclohexyl-2-benzothiazyl-sulfenamide (Santocure CBS from Flexsys) - vulcanization accelerator e. HTMT: Resin

[0068] The shear stiffness modulus of the elastomeric mixture of the first layer M1 is 25 Mpa, for a viscoelastic loss measured by a Tgδ value of 0.27.

[0069] The intermediate layer M2 of the tread laid radially outwardly to the first layer M1 has a radial thickness of 1.3 mm, measured on the axis of symmetry of a meridian plane. The volume of the intermediate layer M2 is 330 mm3. An example of the composition for this layer is: [Table 4] M2 layer Component pc SBR (a) 100 Silica (b) 110 Coupling agent (c) 9 Liquid plasticizer (d) 20 Resin plasticizer (e) 50 Black 5 Zinc oxide 3 Stearic acid 2 Antioxidant (f) 2 Accelerator (g) 2 DPG 2 Sulfur 1 With : (a) SBR with 27% styrene, butadiene -1,2:5%, cis-1,4:15%, trans-1,4:80% Tg - 48°C (b) Silica “Zeosil1165MP” from Solvay with a BET surface area of ​​160m2 / g (c) Silane TESPT “SI69” from Evonik (d) TDAE “Flexon 630” oil from Shell (e) Exxon's "Escorez 2173" resin (f) Antioxidant "Santoflex 6PPD" from Solutia (g) Accelerator "Santocure CBS" of the company Solutia

[0070] With such a composition, the shear stiffness modulus is 1.7 Mpa, and the viscoelastic loss is Tgδ of 0.25.

[0071] The shear modulus of the first layer M1 is about 15 times that of the intermediate layer M2, and the viscoelastic losses are of the same order of magnitude. The intermediate layer M2 is intended to come into contact with the ground, a relatively soft mixture, and a high hysteresis are expected for good wet grip performance.

[0072] Finally, the radially outermost layer M3 of the tread of the tire of the invention is radially externally placed on the intermediate layer M2 has a radial thickness of 4.5 mm, measured on the axis of symmetry of a meridian plane. Its volume at the wheel turn of 131 cm3. Its shear stiffness modulus is 1.4 Mpa, and the viscoelastic loss Tgδ_M3 is 17%. An example of composition for this layer is: [Table 5] Layer M3 Component Pce SBR (a) 100 Silica (b) 70 Coupling agent (c) 6 Liquid plasticizer (d) 30 Resin plasticizer (e) 0 Carbon black 5 With : (a) SBR with 27% styrene, butadiene -1.2 :5%, cis-1.4 :15%, trans -1.4: 80% Tg - 48°C (b) Silica "Zeosil1165MP" of the company Solvay of BET area 160m2 / g (c) TESPT silane “SI69” from Evonik (d) TDAE oil "Flexon 630" from Shell (e) Exxon's "Escorez 2173" resin

[0073] It is easy to check that the tested tire complies with the main characteristics of the invention: the ratio G'_M1 / G'_M2 is in the range [7; 25]; the ratio of the thickness of the layer (M3) divided by the thickness of the layer (M2), Thickness _M3 / Thickness_M2, is in the range [3; 10]; the viscoelastic loss Tgδ_M2 of the material of the layer (M2), and the viscoelastic loss Tgδ_M3 of the material of the layer (M3), are related by the relation: (Tgδ_M2 - Tgδ_M3) / Tgδ_M2 ≥ 20%.

[0074] The tread with the three layers M1, M2, and M3 was obtained by a volume coextrusion process that allows the three layers to be extruded simultaneously. Such a process is described in patent WO2018087467A1.

[0075] The inventors observed a gain in wet grip at the end of the tire's life improved by 5% against a loss in resistance of 0.06Kg / t. The simulation of the wear of the control tires and of the invention was carried out by planing the tread of the test and control tires as described in patent FR3059421A1.

Claims

1. Tyre (1) having a crown reinforcement (50), an underlayer (30), and a multilayer tread (20) formed by three layers of elastomeric compounds (M1, M2, M3), the radially outermost layer (M3) of which is intended to come into contact with the ground via a tread surface (10): - the crown reinforcement (50) being disposed radially on the inside of the multilayer tread (20); - the underlayer (30) being disposed radially on the outside of the crown reinforcement (50) and radially on the inside of the multilayer tread (20), and extending over the entire axial width of the crown reinforcement (50); - the multilayer tread (20) comprising a first layer (M1) disposed radially on the outside of the underlayer (30) and extending axially over all or part of the axial width of the underlayer (30), at least in portions; - the tread also comprising a radially outermost layer (M3) provided with a tread surface (10); - the multilayer tread (20) has an intermediate layer (M2) disposed radially on the outside of the first layer (M1) in contact with it, and radially on the inside in contact with the radially outermost layer (M3), extending axially along the first layer (M1) at least in portions, - the elastomeric compounds of the first layer (M1), the intermediate layer (M2), the radially outermost layers (M3) having respectively a modulus of shear stiffness G'(M1), G'(M2), G'(M3), and having respectively a viscoelastic loss Tgδ(M1), Tgδ(M2), Tgδ(M3), measured at 23°C at 10 Hz and under an alternating shear stress of 10% of the material of each layer, characterized in that the ratio G'(M1) / G'(M2) is within the range [7; 25]; The viscoelastic loss Tgδ (M2) of the elastomeric compound of the intermediate layer (M2), and the viscoelastic loss Tgδ (M3) of the elastomeric compound of the radially outermost layer (M3), are linked by the following relationship: (Tgδ (M2)-Tgδ (M3)) / Tgδ (M2) ≥ 20%; the ratio of the radial thickness of the radially outermost layer (M3) divided by the radial thickness of the intermediate layer (M2), Thickness (M3) / Thickness (M2), is within the range [3; 10], said thicknesses being measured along the axis of symmetry of a meridian plane of the tyre.

2. Tyre (1) according to Claim 1, characterized in that the ratio of the modulus of shear stiffness of the elastomeric compound of the first layer (M1), divided by the modulus of shear stiffness of the elastomeric compound of the radially outermost layer (M3), G'(M1) / G'(M3) is within the range [7; 25].

3. Tyre (1) according to the preceding claim, wherein the modulus of shear stiffness of the elastomeric compound of the underlayer (30) is less than or equal to 1.5 MPa.

4. Tyre (1) according to one of the preceding claims, characterized in that the viscoelastic loss Tgδ(M2) of the elastomeric compound of the intermediate layer (M2) is greater than or equal to 0.2.

5. Tyre (1) according to one of the preceding claims, characterized in that the viscoelastic loss Tgδ(M2) of the elastomeric compound of the intermediate layer (M2), and the viscoelastic loss Tgδ(M3) of the elastomeric compound of the radially outermost layer M3, are linked by the following relationship: (Tgδ(M2) - Tgδ(M3)) / Tgδ(M2) ≥ 30%.

6. Tyre (1) according to Claim 1, characterized in that the modulus of shear stiffness G'(M1) of the elastomeric compound of the first layer (M1) is greater than 7 MPa.

7. Tyre (1) according to the preceding claim, characterized in that the modulus of shear stiffness G'(M1) of the elastomeric compound of the first layer (M1) is greater than or equal to 12 MPa.

8. Tyre (1) according to one of the preceding claims, the planed-down profile of the tread being by convention, defined such that the tread pattern height remaining after planing-down is 2 mm over the entire axial width of the tread, the maximum difference Emax being the radial distance between the planed-down profile of the tread, and the radially outer profile of the first layer (M1), characterized in that the maximum difference Emax is constant, ± 5 mm, over at least a central portion of the tread.

9. Tyre (1) according to one of the preceding claims, the furrows (80) of the tread being the deepest grooves of the tread pattern, wherein the material appearing at the bottom of the furrows is the same material as the material of which the radially outermost layer (M3) of the tread is made.

10. Tyre (1) according to one of the preceding claims, characterized in that the intermediate layer (M2), which is disposed radially on the outside of the first layer (M1) and is in contact with the radially outermost layer (M3), extends axially continuously along the first layer (M1).

11. Tyre (1) according to one of the preceding claims, the tread (10) having tread pattern blocks (25) that are separated by furrows (80) oriented substantially circumferentially, characterized in that the first layer (M1), axially next to some tread pattern blocks (25), is extended radially outwards by at least one reinforcing element (27) extending radially from the radially outer surface of the layer (M1) towards the outside of the tread (10) to a radial height greater than 75% of the radial thickness of the tread, said reinforcing element (27) having a variable axial width, from a maximum value less than 50% of the axial width of said tread pattern block (25), said axial width decreasing radially upwardly.