Tyre with optimized roadholding

Abstract

The invention relates to a tyre (1) comprising a crown (4) having a crown reinforcement (20), a sublayer (7), and a tread (6). The sublayer (7) comprises a base layer (71), radially on the outside of the crown reinforcement (20), a cover layer (72), radially on the outside of the base layer (71), and a groove-bottom elastomeric compound layer (73) radially on the inside of the tread (6). The dynamic shear moduli G1*, G3* of the base layer (71) and of the groove-bottom elastomeric compound layer (73) satisfy G3* > 1.25xG1*; G1* and G3* being measured at 23°C under an alternating shear stress at a frequency of 10 Hz and 10% deformation.

Description

Tire optimized for road handling Field of invention [OOOlJThe present invention relates to a tire whose crown is optimized to achieve an advantageous performance compromise in road behavior and rolling resistance compared to conventional designs. Although not limited to this type of application, the invention is more particularly described with reference to a radial tire intended to be mounted on a passenger vehicle or van. Definitions

[0002] By convention, we consider a reference (O, OX, OY, OZ), whose center O coincides with the geometric center of the tire, the circumferential directions OX, axial OY, and radial OZ 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.

[0003] By radially inner, respectively radially outer, we mean closer, respectively further from the axis of rotation of the tire.

[0004] 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.

[0005] The constitution of the tire is usually described by a representation of its constituents in a meridian plane, that is to say a plane containing the axis of rotation of the tire. Such a choice is motivated by, as a first approximation, the axisymmetry of the geometry of the tire around its axis of rotation.

[0006] 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.

[0007] A radial tire further comprises a reinforcing reinforcement, consisting of a crown reinforcement, radially inside the tread, and a carcass reinforcement, radially inside the crown reinforcement.

[0008] The crown reinforcement of a radial tire comprises a superposition of crown layers extending circumferentially, radially outside the carcass reinforcement. Each crown layer is made up of reinforcements parallel to each other and coated with a polymeric material of the elastomer type or elastomeric mixture. The assembly consisting of the crown reinforcement and the tread is called the crown. [0009JThe carcass reinforcement of a radial tire concerned by the invention usually comprises at least one carcass layer consisting of metallic or textile reinforcing elements each coated in an elastomeric coating mixture. Said at least one carcass layer comprises a main part, connecting the two beads together and is wound, in each bead, around an annular reinforcing structure, which is most often a bead wire. [OOlOJBy elastomeric mixture is meant an elastomeric material obtained by mixing its various constituents. An elastomeric mixture conventionally 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. For certain applications, the elastomers considered may also comprise thermoplastics (TPE). [OOllJBy the expression "based on" composition, we mean 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.

[0012] 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 rubber composition considered.

[0013] An elastomeric mixture can be characterized mechanically, in particular after curing, by its dynamic properties, such as a dynamic shear modulus G*= (G'2+G”2)l / 2, where G' is the elastic modulus of shear stiffness and G” the viscous shear modulus, and a viscoelastic loss Tanô=G” / G'. The dynamic shear modulus G* and the viscoelastic loss Tanô are measured on a Metravib VA4000 type viscoanalyzer, according to ASTM D 5992-96. The response of a sample is recorded of vulcanized elastomeric mixture having the form of a sinusoidal stress in alternating simple shear, at the frequency of 10Hz, at a temperature of 100°C. It is carried out with a sweep in amplitude of deformation from 0.1% to 50% (forward cycle), then from 50% to 0.1% (return cycle). For the forward cycle, the maximum value of Tan(ô) observed is indicated, noted Tan(ô)max. On this same forward cycle, the value of the dynamic elastic shear modulus, G*, is indicated. The dynamic shear modulus G* is also known according to the term “complex shear modulus”, the viscoelastic loss Tan(ô) being also known according to the term “dynamic loss”. Prior art [0014JThe skilled person, a tire designer, knows that the expected functions of a tire are at least three in number. Firstly, it is a question of carrying the load, resulting from the mass of the vehicle and all the overloads linked to the dynamic movements of the vehicle as well as any aerodynamic overloads at high speed. Secondly, it is necessary to be able to guide the vehicle on the trajectories decided by the driver, and finally, it is necessary to transmit to the ground the acceleration or braking forces decided by the driver. [0015JThe crown reinforcement is an essential element that contributes decisively to the three functions of carrying, guiding, and transmitting. In usual design, said crown reinforcement with at least two crossed metal layers surrounds the carcass reinforcement to provide the tire with the necessary strength to fulfill its carrying function.

[0016] The Steering function is also known as "road handling." It refers to the responses of a vehicle / tire assembly to multiple driver inputs (steering, acceleration, braking, etc.). Handling is essential both in terms of safety for the stability of the vehicle and for driving pleasure.

[0017] The tire plays a key role in road behavior because it ensures, at the end of the chain, the transmission of forces between the vehicle and the ground in order to maintain the trajectory defined by the driver.

[0018] When cornering, to keep the vehicle on a trajectory, it is necessary to generate a force equivalent (but in the opposite direction) to the centrifugal force which tends to eject the vehicle from the trajectory. This lateral force must be generated by the vehicle's 4 tires to overcome the centrifugal force.

[0019] The deformation of the rubber blocks in contact with the ground generates a lateral force. The mechanism that allows the tire to deform the rubber blocks when cornering is drift. Drift is the angle between the direction of the wheel and the trajectory followed by the vehicle. When cornering, this angle is not zero in order to allow the tire to deform the rubber blocks of the tread and thus generate the necessary lateral forces.

[0020] Transverse drift stiffness refers to the variation in transverse forces generated in the contact patch of the moving tire crushed by the load carried, as a function of the drift angle applied to the tire. Transverse drift stiffness is expressed in Newtons per degree (N / °).

[0021] For small drift angles, i.e. angles less than 4°, the transverse force, in a direction parallel to the tire's axis of rotation, is proportional to the drift angle. The transverse drift stiffness is equal to this coefficient of proportionality.

[0022] Transverse drift stiffness is an essential mechanical quantity which connects the tire to the vehicle and which determines the quality of the vehicle's behavior on the road.

[0023] Rolling resistance is another performance covered in the invention. Rolling resistance is one of the forces that oppose the vehicle's movement. The rolling resistance coefficient of a tire (RRC) is the rolling resistance force related to the load carried by the tire. The coefficient is expressed in kg / t.

[0024] Rolling resistance is primarily related to tire deformation. For example, the beads associated with the sidewalls represent 20% to 30% of the tire's rolling resistance, while the tread contributes 60% to 80%.

[0025] Reducing greenhouse gas emissions from transport is one of the major challenges facing vehicle manufacturers today. Tires represent an important source of progress, through a reduction in rolling resistance, because this has a direct impact on the vehicle's fuel consumption. For example, a 20% reduction in the rolling resistance of a passenger car tire saves approximately 3% of fuel per 100 km in a combined cycle.

[0026] The choice of tread plays a key role in establishing a compromise between road behavior and rolling resistance. Among the tire design parameters, those skilled in the art know treads made up of a stack of sub-layers with dynamic and geometric properties. appropriate for each sub-layer. An example of a sub-layer, i.e. a layer of rubber inserted between the crown reinforcement and the tread material, is described in document FR 2 954 333.

[0027] Another example where this time the material constituting the sub-layer is flush with the bottom of the grooves of the tread is given by document EP 2 865 543. In general, sub-layer materials are used under the tread so as to improve the rolling resistance of the tire with a low hysteretic material, or to stiffen the tread in shear, but with modest rigidities so as not to oppose too much the flattening of the tire tread in the area of ​​contact with the ground.

[0028] However, the lower the stiffness, the poorer the tire's drift thrust response to vehicle steering input. In fact, schematically, the stacking of rubber layers radially outside the crown reinforcement can be considered as a succession of springs in series. This is why we avoid introducing materials with too low a dynamic shear modulus so as not to penalize the drift stiffness. However, this can be contrary to the objective of minimizing rolling resistance. Even in the highest stiffness variants, the dynamic shear modulus G* of a sub-layer material is generally much lower than 8 MPa, even when the best performance in behavior is sought.

[0029] Document WO 2015 / 170615 also discloses a tire comprising a base layer, i.e. a sub-layer, formed by two materials superimposed radially. The modulus of the tread material and the value of Tan (δ) (delta tangent) thereof are lower than the values ​​of the same parameters of the sub-layer material in contact with the tread material, i.e. of the radially outermost of the two layers. The modulus of the material of the radially inner layer of the sub-layer materials and the value of Tan (δ) thereof are lower than the values ​​of the same parameters of the sub-layer material in contact with the tread material. However, a tire produced according to this teaching does not provide progress in the balance of performance.

[0030] Document WO2019 / 145621 discloses a tread positioned radially outwardly to a sub-layer, which itself rests radially outwardly on a crown reinforcement. Said sub-layer consists of a first low-rigidity layer, i.e. with a dynamic shear modulus G* less than or equal to 0.6 MPa, laid radially outwardly on the crown reinforcement. Said sub-layer also comprises a second high-rigidity layer laid radially outwardly to said low-rigidity layer, and radially inwardly to the tread. The dynamic shear modulus of the high-rigidity layer, greater than 7 MPa, is greater than the dynamic shear modulus of the tread. Such a tread configuration gives improved results in rolling resistance and road behavior compared to a tire of conventional design.

[0031] Patent EP3031627 discloses a tread comprising circumferential grooves. These circumferential grooves are reinforced by the use of a reinforcing compound, allowing a gain in drift stiffness Dz.

[0032] The inventors set themselves the objective of identifying other levers to further improve the drift stiffness performance of state-of-the-art tires and, preferably, improve the compromise between drift stiffness and rolling resistance performance. Statement of the invention

[0033] This aim has been achieved by a tire for a motor vehicle comprising a crown comprising a crown reinforcement, a sub-layer, and a tread, the tread comprising grooves oriented essentially circumferentially; said sub-layer being provided with a base layer, radially external to the crown reinforcement, and a covering layer, radially external to the base layer and radially internal to the tread; said base layer having a dynamic shear modulus Gl* less than or equal to 1.5 MPa; the covering layer having a dynamic shear modulus G2* greater than or equal to 5 MPa; a groove bottom layer is positioned radially internal to the outer profile of the tread, and a dynamic shear modulus G3* of said groove bottom elastomeric compound layer is strictly greater than 1.25xGl*; Gl*, G2* and G3* being measured at 23°C under alternating shear stress at a frequency of 10 Hz and at 10% strain.

[0034] The "crown" of the tire is an area that includes the tread, the underlay, and the crown reinforcement, which includes the composite layers that surround the carcass reinforcement. This description is made from the outside to the inside of the tire.

[0035] By essentially circumferentially oriented groove, we mean a cut in the tread that is substantially circumferential over one revolution of the wheel, the distance between the walls of material that delimit it of which is greater than 2 mm and the depth of which is greater than or equal to 1 mm. In a meridian section, each axially inner and axially outer wall of a groove makes an angle with the radial direction, and extends towards the inside of the tire. A curve connecting the two radially inner ends of the two walls of a groove forms the bottom contour of the groove which is in contact with the ambient environment, radially outwards. The tread optionally includes NBS grooves, most often NB S is 3 or 4 depending on the axial width of the tread.

[0036] A tire of the invention comprises a tread having a sub-layer, which is itself composed of three layers, two of which are radially superimposed. A base layer is placed radially outwardly on the crown reinforcement, and the base layer has a dynamic shear modulus less than or equal to 1.5 MPa. The base layer is therefore a so-called "soft" layer for the present application. A covering layer is positioned radially outwardly on the base layer. This second layer has a dynamic shear modulus greater than or equal to 5 MPa and is called "rigid". The invention provides a third layer, the groove bottom elastomeric compound layer, arranged in different configurations depending on the desired performance compromise. In terms of rigidities, the dynamic shear modulus G3* of this groove bottom elastomeric compound layer is strictly greater than 1.25xGl*, which means that the elastomeric compound layer at the bottom of the groove is more rigid than the base layer, in terms of transverse shear rigidities, and thus contributes to improving the drift rigidity of the tire.

[0037] It is thus possible to design a tire comprising a relatively rigid elastomeric groove bottom compound layer while freeing itself from the constraints of a composition with high performance in terms of grip, and / or wear and / or hysteresis. Therefore, it is possible, for example and without this being limiting to the invention, to envisage a relatively rigid elastomeric groove bottom compound layer with low hysteresis which dissipates less than a tread compound, the advantage of such a solution being to provide a gain in rolling resistance at the same road behavior. It is also possible, for example and without this being limiting to the invention, to envisage a relatively rigid elastomeric groove bottom compound layer having a high capacity to resist the attack of ozone from the ambient environment, or even a groove bottom compound layer with good resistance to the risk of oxidation and attack by stones. It is also possible, for example and without this being limiting of the invention, to envisage a layer of relatively rigid elastomeric mixture at the bottom of the groove already existing among the layers used for the manufacture of the tire, for example the sidewall layers.

[0038] The main characteristics of the invention lead to the tire of the invention having an improvement in the drift rigidity of the tire, for example to improve the compromise of performance in road behavior and rolling resistance thanks to the dynamic properties of the crown layers, and thanks to an appropriate radial distribution of said layers.

[0039] Other characteristics, linked to different embodiments of the invention, contribute to further improving the performance compromise of the tire. Most often, these characteristics relate to geometric and / or dynamic properties of the layer of elastomeric mixture at the bottom of the groove.

[0040] In optional and advantageous embodiments, the top comprises for the or each groove a sub-groove volume extending radially, from the groove bottom surface to a bottom of the sub-groove volume and extending axially between each of the extensions of the walls of the or each groove, the or each sub-groove volume comprises the layer of elastomeric groove bottom mixture.

[0041] In a meridian section, each groove is delimited axially by two side walls, for example in the shape of a "V", said side walls being able to form an angle with the radial direction for example in the range of 0° to 30°, and extending towards the inside of the tire. The bottom of the groove is delimited by a surface which connects the two side walls. The groove bottom is in contact with the ambient environment, in particular the ambient air. The radial depth of the groove is the distance measured from the rolling surface to the groove bottom at its center.

[0042] In the or each groove, the under-groove volume is considered to be defined, radially externally, by the bottom of the groove and radially internally by the first interface encountered radially when moving radially inwards from the bottom of the groove, for example with the tread, or with an under-layer to the tread, or with the crown reinforcement. The under-groove volume is defined axially by the radially internal extension of the walls axially delimiting the or each groove.

[0043] Thus, in embodiments, the groove bottom elastomeric mixture layer is entirely included in the under-groove volume. In a variant of these embodiments, the under-groove volume exclusively comprises the groove bottom elastomeric mixture layer. In another variant of these embodiments, the under-groove volume comprises the groove bottom elastomeric mixture layer and another layer of another elastomeric mixture. In other embodiments, the groove bottom elastomeric mixture layer is present in the under-groove volume and outside the under-groove volume, for example axially outside the extensions of the walls axially delimiting the groove.

[0044] The preceding characteristics relating to the geometry of the groove bottom elastomeric mixture layer by defining its positioning in the top, its axial width, and its maximum radial thickness make it possible to define the volume of the groove bottom elastomeric mixture layer.

[0045] The total volume of the elastomeric groove bottom mixture corresponds to the sum of the surface area of ​​each section in the meridian plane, integrated over one wheel revolution.

[0046] Advantageously, the layer of elastomeric mixture at the bottom of the groove having a viscoelastic loss Tan (53), the tread having a viscoelastic loss Tan (54), Tan (53) is strictly less than Tan (54), preferably Tan (53) is strictly less than 0.75xTan (54) and even more preferably Tan (53) is strictly less than 0.5xTan (54), Tan (53) and Tan (54) being measured at 23°C under alternating shear stress at a frequency of 10 Hz and at 10% strain.

[0047] In addition to improving road handling, this feature allows rolling resistance to be controlled at a low level.

[0048] In optional and advantageous embodiments for improving road behavior, the tread having a dynamic shear modulus G4* measured at 23°C under alternating shear stress at a frequency of 10 Hz and at 10% deformation, G3* is greater than or equal to G4*, preferably G3* is strictly greater than G4*.

[0049] In optional and advantageous embodiments, G4* is strictly less than G2*.

[0050] In optional and advantageous embodiments, G4* is strictly greater than Gl*.

[0051] In optional and advantageous embodiments, G3* is strictly less than G2*.

[0052] According to an optional and advantageous embodiment, the layer of elastomeric mixture at the bottom of the groove is formed of several axially separated sections, each section being positioned under a groove. This embodiment is shown in Figure 1. By positioned under a groove, it is understood that said section is at least partly arranged radially in line with the groove in question.

[0053] Advantageously, the axial width of at least one section of the layer of elastomeric mixture at the bottom of the groove is at least equal to 50% of the axial width of the groove of said section.

[0054] In a variant of this embodiment, the covering layer is axially continuous between two shoulders of the tire.

[0055] In an alternative variant illustrated for example in Figure 2, the covering layer is formed from several axially separated sections, said covering layer being interrupted under the grooves.

[0056] In one variant, the base layer is formed from several axially separated sections, said base layer being interrupted under the grooves. This configuration is shown in Figure 7. In certain configurations of this variant, the groove bottom elastomeric mixture layer is positioned axially between two sections of the base layer.

[0057] In a configuration of the variant in which the covering layer is formed from several axially separated sections, the groove bottom elastomeric mixture layer is positioned axially between two sections of the covering layer.

[0058] Advantageously, the tread comprising a number of NBS grooves, ESCi being the radial thickness of the elastomeric mixtures at the bottom of a groove i, measured at a first point in the middle of the radially outer contour of the bottom of said groove i, and a second point touching a reinforcement of the first composite layer encountered in the crown of the tire, radially inwardly, the first and second points being on the same radial line, ESC being the maximum value of ESCi, for i ranging from 1 to NBS, the radial thickness of said layer of elastomeric mixture at the bottom of the groove, taken in a groove at a point in the middle of its radially outer contour, is in the interval [20%; 100%] of ESC, preferably in the interval [50%; 100%] of ESC, said thickness radial being measured between said first point and a point at the intersection of the radially inner contour of said layer of elastomeric groove bottom mixture and a radial straight line passing through said first point.

[0059] We consider the radial thickness ESCi which is measured at a first point in the middle of the contour of the bottom of a groove i, and a second point touching a reinforcement of the first composite layer encountered in the crown of the tire, the two points being on the same radial line. Said composite layer encountered is made up of reinforcements parallel to each other and coated in an elastomeric mixture. Here, the first composite layer encountered is understood by going radially from the outside to the inside of the tire. Of course, the value of ESCi can be different from one groove to another, but according to the invention the maximum value of ESCi determines the radial thickness of the layer of elastomeric mixture at the bottom of the groove.

[0060] In embodiments, the first composite layer encountered in the radial direction towards the inside of the tire is a hoop with fabric reinforcements, but in other cases this composite layer may comprise steel or nylon reinforcements coated in an elastomeric mixture.

[0061] The ESC value represents the distance between a reinforcement of the first composite layer encountered and the external environment of the tire. This thickness of elastomeric mixture serves to protect the reinforcements of the first composite layer encountered from the external environment of the tire. The aggressions coming from the external environment are of several types such as the oxidation of the metal reinforcements by the attack of oxygen, or the rupture of textile reinforcements by stones which could enter the grooves.

[0062] The inventors have provided a preferred minimum value for the ESC thickness. It is preferably greater than 1.0 mm. Below 1.0 mm, ESC, which represents the thickness of a protective layer, presents a risk of not being able to combat the oxidation of the metal reinforcements, and beyond 3.5 mm, this thickness can lead to a degradation of the rolling resistance.

[0063] In order to improve rolling resistance and at the same time gain in drift rigidity, wedge-shaped elements can be used in the wearing part of the tread. According to one embodiment, shown in Figure 3, the tread comprises sculpture blocks separated by the grooves oriented essentially circumferentially, the covering layer is, axially opposite certain tread blocks, extended radially outwards by at least one reinforcing element extending radially from the radially outer surface of the covering layer towards the outside of the tread up to a radial height greater than 50% 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.

[0064] Preferably, the groove base layer is based on a chemical composition identical to that of the sidewall layers.

[0065] In order to further optimize the rolling resistance of the tire without degrading the industrial cost, it is possible to use a layer of elastomeric compound at the bottom of the groove with the same chemical composition as that of the sidewall layers. Since the layer of elastomeric compound at the bottom of the groove is not intended to be in contact with a rolling surface, its mechanical and viscoelastic properties are suitable for use in such a position. Furthermore, based on its chemical composition, the sidewall has the ability to resist external aggressions from ozone which are suitable for the bottom of the groove which is also in contact with the ambient environment. The industrial cost is preserved to the extent that standardization in manufacturing is reinforced by removing an elastomeric material reference for the bottom of the groove, when the elastomeric material of the sidewall is used.

[0066] Preferably, the elastomeric mixture of the groove bottom elastomeric mixture layer has a rubber composition based on at least one blend of natural rubber polyisoprene and polybutadiene, a crosslinking system, and a reinforcing filler at an overall rate of at most 45 phr, and comprising carbon black, at a rate of at most 5 phr, and predominantly silica at a rate of at least 20 phr and at most 40 phr.

[0067] The chemical composition defined above is deduced from that of a low hysteresis flank layer mainly loaded with silica.

[0068] Alternatively, the elastomeric mixture of the groove bottom elastomeric mixture layer has a rubber composition based on at least one blend of natural rubber polyisoprene and polybutadiene, a crosslinking system, and a reinforcing filler at an overall rate of at most 45 phr, and comprising carbon black at a rate of at least 20 phr and at most 40 phr.

[0069] In this case, the chemical composition defined above is deduced from that of a low hysteresis flank layer mainly loaded with carbon black. [0070JD' other characteristics of the invention relate to the dynamic properties of the elastomeric mixtures of the layers of the tread.

[0071] Preferably, G3* is greater than or equal to 2 MPa, preferably 4 MPa.

[0072] Preferably, Gl* is less than or equal to 0.6 MPa.

[0073] Preferably, the base layer has a viscoelastic loss Tan (51) less than or equal to 0.15, Tan (51) being measured at 23°C under alternating shear stress at a frequency of 10 Hz and at 10% strain.

[0074] Advantageously, G2* is greater than or equal to 7 MPa, and preferably greater than or equal to 12 MPa.

[0075] Preferably, the covering layer has a viscoelastic loss Tan (52) less than or equal to 0.35, Tan (52) being measured at 23°C under an alternating shear stress at a frequency of 10 Hz and at 10% deformation. Preferably, Tan (52) is greater than or equal to 0.25.

[0076] Advantageously, the elastomeric mixture of the tread has a dynamic shear modulus G4* strictly less than 4.0 MPa and preferably strictly less than 2.5 MPa, G4* being measured at 23°C under an alternating shear stress at a frequency of 10 Hz and at 10% deformation.

[0077] Preferably, Tan (53) is greater than or equal to 0.15 and less than or equal to 0.25.

[0078] Preferably, Tan (54) is greater than or equal to 0.15 and less than or equal to 0.25.

[0079] According to an embodiment of the invention visible in Figure 4, the tread comprises at least one tread portion, hereinafter referred to as the tread wing, at at least one of the axial ends of the tread and a central portion of the tread arranged axially inside the tread wing, said tread wing having an axially outer contour delimited by a first point, located at the intersection of the axially and radially outer contour of the sidewall and the axially and radially outer contour of the tread, and a second point located at a curvilinear distance of between 5% and 25% of the nominal bead width of the tire dimension, said tread wing being radially inwardly in contact with the covering layer.

[0080] Preferably, the presence of the tread wings makes it possible to further improve the rolling resistance by replacing the extreme parts of the tread with an elastomeric material with lower hysteresis than the elastomeric material of the central portion of the tread. Thus, the dynamic shear modulus of the tread wing is at most equal to 80% of the dynamic shear modulus of the central portion of the tread, the dynamic shear moduli being measured at 23°C, and under an alternating shear stress at a frequency of 10 Hz and at 10% deformation.

[0081] Preferably, the viscoelastic loss Tan (ô) of the tread wing is at most equal to 80% of the viscoelastic loss of the central portion of the tread, the viscoelastic loss Tan (ô) and the viscoelastic loss of the central portion of the tread being measured at 23°C, and under an alternating shear stress at a frequency of 10 Hz and at 10% deformation. Brief description of the drawings

[0082] The present invention will be better understood upon reading the detailed description of embodiments taken as examples, in no way limiting and illustrated by the appended drawings in which: - Figure 1 shows a schematic meridian section of a tire according to a first embodiment of the invention, with a layer of elastomeric groove base mixture discontinuous in sections positioned under the grooves. The covering layer is axially continuous from one shoulder to the other; - figure 2 shows a schematic meridian section of a tire according to a second embodiment of the invention, still with a layer of elastomeric groove base mixture discontinuous in sections positioned under the grooves, but this time, the covering layer is discontinuous in sections; - Figure 3 shows a schematic meridian section of a tire according to a third embodiment of the invention, with a discontinuous groove base elastomeric compound layer in sections positioned under the grooves. The covering layer is also in sections, but with wedge-shaped reinforcing elements emerging up to the tread; - figure 4 shows a schematic representation of a tire according to a fourth embodiment of the invention; - Figure 5 is an enlargement of a portion of the crown of the tire of Figure 1 showing dimensions of the crown; - Figures 6 and 7 show schematic representations respectively of tires according to fifth and sixth embodiments of the invention; and - Figure 8 represents a state-of-the-art tire. Detailed description of the invention [0083JThe invention has been more particularly studied for a passenger car tire of standardized designation, according to the specification standard of the ETRTO (European Technical Organization for Rims and Tires), 245 / 45 RI 8 XL 100W.

[0084] In the various figures, identical or similar elements bear the same references. Given the symmetry of the tread, for the readability of the figures, the elements are in principle referenced only once on one side of a meridian plane.

[0085] Figure 1 shows a tire 1 according to a first embodiment of the invention, an equatorial plane CP, two beads 50 and two sidewalls 3 each connected to a bead 50. The tire has a crown 4, which has a crown reinforcement 20, a tread 6, and an underlayer 7. The tread 6 extends axially from one shoulder 60 to the other shoulder 60. The tread 6 comprises a rolling surface 61 intended to come into contact with the roadway during the rolling of the tire 1. The tread 6 comprises sculpture blocks 63 separated by grooves 62 oriented essentially circumferentially, here NBS=4. Each groove 62 is delimited radially inwards by a groove bottom 620 and axially by two walls 621, 622 extending from the rolling surface 61 to the groove bottom 620.The top 4 comprises for each groove 62 a sub-groove volume extending radially, from the groove bottom 620 to a bottom of the sub-groove volume and extending axially between each of the extensions of the walls 621, 622.

[0086] The bead 50 comprises at least one carcass layer, shown in Figure 1 in broken lines. The carcass layer is formed of reinforcements coated with an elastomeric mixture. The carcass layer comprises a main part 53, connecting the two beads 50 together and is wound in each bead, from the inside to the outside of the tire around a circumferential reinforcement element, most often metallic, called a bead wire 51, to form a turn-up 52. The reinforcements of a carcass layer are substantially parallel to each other and form, with the circumferential direction, an angle of between 85° and 95°.

[0087] As known per se, the crown reinforcement 20 comprises layers of cables or monofilament reinforcements generally coated with a thin layer of rubber. The crown reinforcement 20 comprises two crossed layers 22, 23 consisting of reinforcements coated in an elastomeric mixture. The reinforcements of the crossed layers 22, 23 make an angle of between 10° and 45° with the circumferential direction. A third reinforcement layer 21 binds the two previous layers. Said layer 21 also comprises reinforcements coated in an elastomeric mixture, which make an angle of approximately ±2.5° with the circumferential direction. [0088JL4 a sub-layer 7 is arranged radially outside the crown reinforcement 20 and radially inside the tread 6. This sub-layer 7 is provided with a base layer 71, a covering layer 72, and finally a layer of groove bottom elastomeric mixture 73. The bottom of the under-groove volume is here formed by the interface between the groove bottom elastomeric mixture layer 73 and the covering layer 72.

[0089] The base layer 71 is arranged radially directly on the crown reinforcement 20. The base layer 71 is axially continuous between the two shoulders 60. In the described embodiment, the base layer 71 is arranged radially directly on the crown reinforcement 20, i.e. the base layer 71 is in contact with the radially outermost layer of the crown reinforcement 20.

[0090] The base layer 71 has a dynamic shear modulus Gl* and a viscoelastic loss Tan (51) measured at 23 °C under alternating shear stress at a frequency of 10 Hz and at 10% strain. Gl * is less than or equal to 1.5 MPa, preferably less than or equal to 0.6 MPa. Tan (51) is less than or equal to 0.15.

[0091] An example of a suitable formulation for base layer 71 having a dynamic shear modulus Gl* equal to 0.2 MPa is described in the following Table 1.

[0092] Table 1

[0093] The formulations are given in mass (pce meaning percentage of the mass of elastomer).

[0094] The covering layer 72 is arranged radially outside the base layer 71. The covering layer 72 is axially continuous between the two shoulders 60. The covering layer 72 is positioned radially inside the tread 6. In this first embodiment, the covering layer 72 is axially continuous between two shoulders 60.

[0095] The cover layer 72 has a dynamic shear modulus G2* and a viscoelastic loss Tan (52) measured at 23°C under alternating shear stress at a frequency of 10 Hz and at 10% strain. G2* is greater than or equal to 5 MPa, preferably greater than or equal to 7 MPa and even more preferably greater than or equal to 12 MPa. Tan (52) is less than or equal to 0.35 and greater than or equal to 0.25.

[0096] An example of a suitable formulation for the overlay layer having a dynamic shear modulus G2* equal to 25 MPa is described in the following Table 2.

[0097] Table 2

[0098] The formulations are given in mass (pce meaning percentage of the mass of elastomer).

[0099] The tread 6 has a dynamic shear modulus G4* and a viscoelastic loss Tan (54) measured at 23°C under alternating shear stress at a frequency of 10 Hz and at 10% deformation. G4* is strictly less than 4.0 MPa and preferably strictly less than 2.5 MPa. Tan (54) is less than or equal to 0.25 and greater than or equal to 0.15. [OOlOOJThe following Table 3 gives an example of tread layer 6 formulation. [OOlOlJTable 3

[0102] The formulations are given in mass (pce meaning percentage of the mass of elastomer) with: (a) SBR with 27% styrene, butadiene -1,2:5%, cis-1,4:15%, trans-1,4:80% Tg -48°C (b) Silica “Zeosill 165MP” 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 Company “Escorez 2173” Resin (f) Antioxidant “Santoflex 6PPD” from the company Solutia (g) “Santocure CBS” accelerator from the company Solutia.

[0103] Each under-groove volume comprises the groove bottom elastomeric mixture layer 73. The groove bottom elastomeric mixture layer 73 is formed of several axially separated sections, each section being positioned under a groove 62. The width of each section of the groove bottom elastomeric mixture layer 73 is at least equal to 50% of the axial width of the groove 62 of said section. The groove bottom elastomeric mixture layer 73 has a dynamic shear modulus G3* and a viscoelastic loss Tan (53) measured at 23°C under an alternating shear stress at a frequency of 10 Hz and at 10% strain. G3* is greater than or equal to 2 MPa, preferably greater than or equal to 4 MPa. Tan (53) is less than or equal to 0.25 and greater than or equal to 0.15.

[0104] For the groove bottom elastomeric mixture layer 73, a formulation such as that described in Table 4 is used.

[0105] Table 4

[0106] Tan (53) and Tan (54) are such that Tan (53) < Tan (54), preferably Tan (53) < 0.75xTan (54) and even more preferably Tan (53) < 0.5xTan (54). Gl* and G3* are such that G3* is strictly greater than 1.25 x Gl*. We also have G3* is greater than or equal to G4*, preferably G3* is strictly greater than G4*. We will also note that G4* is strictly less than G2*, G4* is strictly greater than Gl* and G3* is strictly less than G2*.

[0107] With reference to Figure 5, the hooping layer 21 comprises reinforcements 24 coated in an elastomeric matrix. The under-groove thicknesses ESC1 and ESC2 can be seen, which measure the radial thicknesses of the elastomeric mixtures at the bottom of the grooves 62 from a first point in the middle of the radially outer contour of the groove bottom and a second point touching a reinforcement 24 of the first composite layer encountered in the crown of the tire 1 radially inwardly. The first and second points are on the same radial line. ESC is the maximum value between ESC1 and ESC2. ESC is greater than 1.0 mm and less than 3.5 mm. Here, ESC=ESC2=2.0 mm.

[0108] The radial thicknesses of the layers of the sub-layer 7, namely the layers 71, 72, and 73 have the respective references E71, E72, and E73. The tread 6 has a thickness EKM. The radial thickness E73 of the groove bottom elastomeric compound layer 73 is measured between the first point and a point at the intersection of the radially inner contour of the groove bottom elastomeric compound layer 73, and a radial straight line passing through the first point described above. The radial thickness E73, taken in each groove 62 at a point in the middle of its radially outer contour, is included in the interval [20%; 100%] of ESC, preferably in the interval [50%; 100%] of ESC. Here E73=0.8 mm.

[0109] A tire according to a second embodiment will now be described with reference to Figure 2. In this figure, elements similar to those of the first embodiment are designated by identical references. The covering layer 72 is formed of several axially separated sections. The covering layer 72 is interrupted under the grooves 62. Each section of the groove bottom elastomeric compound layer 73 is axially interposed between two sections of the covering layer 72. In each groove 62, the groove bottom elastomeric compound layer 73 extends radially from the groove bottom 620 to the base layer 71 thus interrupting the covering layer 72. The bottom of the under-groove volume is here formed by the interface between the groove bottom elastomeric mixture layer 73 and the covering layer 72.

[0110] A tire according to a third embodiment will now be described with reference to Figure 3. In this figure, elements similar to those of the previous embodiments are designated by identical references. The groove bottom elastomeric mixture layer 73, like the covering layer 72, comprises axially separated sections. Each section of the groove bottom elastomeric mixture layer 73 is positioned radially under a groove 62. In addition, each section of the groove bottom elastomeric mixture layer 73 protrudes axially outside the extensions of the walls 621, 622 so that the groove bottom elastomeric mixture layer 73 is present in the under-groove volume and outside the under-groove volume. The sections of the groove bottom elastomeric mixture layer 73 are arranged axially between two sections of the covering layer 72.Upstream and downstream of each groove 62, a reinforcing element 630 extends the covering layer 72 axially opposite the tread blocks 63. The reinforcing element 630 extends from the radially outer surface of the covering layer 72 towards the outside of the tread 6 up to a radial height greater than 50% of the radial thickness of the tread 6 and here until reaching the tread surface 61. Each reinforcing element 630 is of variable axial width, from a maximum value less than 50% of the axial width of said tread block. The axial width decreases when moving radially outwards. [OOllljWe will now describe a tire according to a fourth embodiment with reference to Figure 4. In this figure, elements similar to those of the previous embodiments are designated by identical references. Unlike the second embodiment, the tire 1 according to the fourth embodiment comprises a tread 6 comprising at least one portion 90 of tread, hereinafter designated wing 90 of tread 6, at at least one of the axial ends of the tread 6 and a central portion of the tread 6 arranged axially inside each wing 90.Each wing 90 has an axially outer contour delimited by a first point, located at the intersection of the axially and radially outer contour of the sidewall 3 and the axially and radially outer contour of the tread 6, and a second point located at a curvilinear distance of between 5% and 25% of the nominal flange thickness of the. dimension of the tire 1. Each wing 90 is radially internally in contact with the covering layer 72. The dynamic shear modulus of each wing 90 of tread 6 is at most equal to 80% of the dynamic shear modulus G4* of the central portion of the tread 6. The viscoelastic loss Tan (ô) of each wing 90 is at most equal to 80% of the viscoelastic loss Tan(4) of the central portion of the tread 6.

[0112] We will now describe tires according to fifth and sixth embodiments with reference respectively to Figures 6 and 7. In these figures, elements similar to those of the previous embodiments are designated by identical references.

[0113] The tires according to the fifth and sixth embodiments are such that the groove bottom elastomeric mixture layer 73 is based on a composition identical to that of the sidewall layers 3.

[0114] The tire according to the sixth embodiment of Figure 7 is such that each base layer 71 and cover layer 72 is formed of several axially separated sections, said base layer 71 and said cover layer 72 being interrupted under the grooves 62. In addition, the groove bottom elastomeric mixture layer 73 extends radially from the groove bottom 620 to the crown reinforcement 20, thus interrupting the base layer 71 and the cover layer 72. The bottom of the under-groove volume is here formed by the interface between the groove bottom elastomeric mixture layer 73 and the crown reinforcement 20. The groove bottom elastomeric mixture layer 73 is thus positioned axially between two sections of the base layer 71 and the cover layer 72.

[0115] The state-of-the-art tire of Figure 8 has a sub-layer comprising two layers 71 and 72. The elastomeric mixture of the groove base layer is that of the tread 6. [00116JThe person skilled in the art, a tire designer, may adopt alternative embodiments in which the tread itself comprises several different materials, superimposed radially and / or juxtaposed axially. Comparative tests

[0117] Tires of the invention PI, P2, P3 and P4 were evaluated to clearly highlight the performances provided by the invention. The results of the evaluation tests are compared with those obtained for control tires T, T'.

[0118] Each tire Pl, P2, P3 and P4 according to the invention and control T, T' is a tire intended to equip a passenger vehicle, with a reference pressure of 290 kPa, with a dimension equal to 245 / 45 RI 8 XL 100W. The crown comprises a crown reinforcement, a sub-layer and a tread. The sub-layer comprises a base layer, radially outside the crown reinforcement and a covering layer, radially outside the base layer and radially inside the tread. The sub-layer also comprises a layer of elastomeric groove base compound.

[0119] The elastomeric compound of the groove bottom elastomeric compound layer of the control tire T is that of the tread while the elastomeric compound of the groove bottom elastomeric compound layer of the control tire T' is that of the base layer. The groove bottom elastomeric compound layer of the control tire T extends radially from the groove bottom to the base layer, thus interrupting the overlay, while the groove bottom elastomeric compound layer of the control tire T' extends radially from the groove bottom to the crown reinforcement, thus interrupting the base layer and the overlay.

[0120] The tires P1 and P4 according to the invention comprise a layer of groove bottom elastomeric mixture 73 extending radially from the groove bottom 620 to the base layer 71, thus interrupting the covering layer 72. The tires P2 and P3 according to the invention comprise a layer of groove bottom elastomeric mixture 73 extending radially from the groove bottom 620 to the crown reinforcement 20, thus interrupting the base layer 71 and the covering layer 72.

[0121] The dynamic characteristics of the tires PI to P4 according to the invention and controls T, T' are gathered in table 5.

[0122] The rolling resistance RRT and the drift stiffness Dz were simulated by the finite element method for the tires PI to P4 according to the invention and controls T, T'. The results are shown in Table 5 taking as base 100 the performance of the control tire T.

[0123] In Table 5, a value below 100 means a degradation of the performance concerned. Conversely, a value above 100 means an improvement of the performance concerned.

[0124] Table 5

[0125] These results confirm that PI to P4 tires with a groove base elastomeric compound layer with the claimed dynamic properties exhibit improved performance in Dz drift stiffness and, in addition, an improved compromise between Dz drift stiffness and RRT rolling resistance compared to the control tire T. The invention is not limited to the embodiments previously described.

Claims

Claims

1. A tire (1) for a motor vehicle comprising a crown (4) comprising a crown reinforcement (20), a sub-layer (7), and a tread (6), the tread (6) comprising grooves (62) oriented essentially circumferentially; said sub-layer (7) being provided with a base layer (71), radially external to the crown reinforcement (20), and a covering layer (72), radially external to the base layer (71) and radially internal to the tread (6); said base layer (71) having a dynamic shear modulus Gl* less than or equal to 1.5 MPa; the covering layer (72) having a dynamic shear modulus G2* greater than or equal to 5 MPa; said tire (1) being characterized in that a layer of groove bottom elastomeric mixture (73) is positioned radially inside the outer profile of the tread (6) in that a dynamic shear modulus G3* of said layer of groove bottom elastomeric mixture (73) is strictly greater than 1.25 x Gl*; Gl*, G2* and G3*; being measured at 23°C under an alternating shear stress at a frequency of 10 Hz and at 10% deformation.

2. Tire (1) according to the preceding claim, in which the groove bottom elastomeric mixture layer (73) having a viscoelastic loss Tan (53), the tread (6) having a viscoelastic loss Tan (54), Tan (53) is strictly less than Tan (54), preferably Tan (53) is strictly less than 0.75xTan (54) and even more preferably Tan (53) is strictly less than 0.5xTan (54), Tan (53) and Tan (54) being measured at 23°C under an alternating shear stress at a frequency of 10 Hz and at 10% deformation.

3. A tire (1) according to any one of the preceding claims, wherein the groove bottom elastomeric compound layer (73) is formed from several axially separated sections, each section being positioned under a groove (62).

4. Tire (1) according to the preceding claim, in which the axial width of at least one section of the groove bottom elastomeric mixture layer (73) is at least equal to 50% of the axial width of the groove (62) of said section.

5. A tire (1) according to any one of claims 1 to 4, wherein the covering layer (72) is axially continuous between two shoulders (60) of the tire (1).

6. A tire (1) according to any one of claims 1 to 4, wherein the covering layer (72) is formed from several axially separated sections, said covering layer (72) being interrupted under the grooves (62).

7. A tire (1) according to any preceding claim, wherein the base layer (71) is formed from a plurality of axially separated sections, said base layer (71) being interrupted under the grooves (62).

8. A tire (1) according to any one of the preceding claims, wherein, the tread comprising a number of NB S grooves, ESCi being the radial thickness of the elastomeric mixtures at the bottom of a groove i, measured at a first point in the middle of the radially outer contour of the bottom of said groove i, and a second point touching a reinforcement of the first composite layer encountered in the crown of the tire, radially inwardly, the first and second points being on the same radial line, ESC being the maximum value of ESCi, for i ranging from 1 to NBS, the radial thickness (E73) of said layer of groove bottom elastomeric mixture (73), taken in a groove (62) at a point in the middle of its radially outer contour, is within the range [20%; 100%] of ESC, preferably within the range [50%;100%] of ESC, said radial thickness (E73) being measured between said first point and a point at the intersection of the radially inner contour of said layer of elastomeric groove bottom mixture (73), and a radial straight line passing through said first point.;

9. A tire (1) according to any one of the preceding claims, wherein the tread (6) comprises tread blocks (63) separated by the grooves (62) oriented essentially circumferentially, the covering layer (72) is, axially opposite certain tread blocks (63), extended radially outwards by at least one reinforcing element (630) extending radially from the radially outer surface of the covering layer (72) towards the outside of the tread (6) up to a radial height greater than 50% of the radial thickness of the tread (6), said reinforcing element (630) 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 outwards.

10. A tire (1) according to any preceding claim, wherein the groove base layer (73) is based on a chemical composition identical to that of the sidewall layers (3).

Images