Tire optimized for rolling resistance without degrading road handling

Abstract

The invention relates to a tire (1) having a crown (4) optimized for an advantageous performance compromise in road behavior and rolling resistance. Said crown (4) comprises a crown reinforcement (20), a sub-layer (7), and a tread (6). The sub-layer (7) comprises a base layer (71), radially outside the crown reinforcement (20), a covering layer (72), radially outside the base layer (71) and a groove base layer (73) radially inside the tread (6). The thermo-viscoelastic properties of the layers of the sub-layer (7) satisfy: Tan (δ3) < 0.75xTan (δ4); and preferentially Tan (δ3) < 0.5xTan (δ4) with Tan (δ3), and Tan (δ4) measured at 23°C under alternating shear stress at a frequency of 10 Hz and at 10% strain. Abstract figure: Figure 1

Description

Title of the invention: Tire optimized for rolling resistance without degrading road handling Field of invention

[0001] The present invention relates to a tire whose crown is optimized to achieve an advantageous performance compromise in rolling resistance and road behavior 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), the center O of which coincides with the geometric center of the tire, the circumferential OX, axial OY, and radial OZ directions respectively designate a direction tangent to the rolling surface of the tire in 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, is meant closer, respectively further from the axis of rotation of the tire.

[0004] By axially inner, respectively axially outer, is meant 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 tread of the tire 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 parallel reinforcements between them 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.

[0009] The 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.

[0010] By 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).

[0011] By the expression "based on" composition is meant 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 dynamic loss Tan (ô) = G” / G'. The dynamic shear modulus G* and the dynamic loss Tanô are measured on a viscoa-nalyser of the Metravib VA4000 type, according to the ASTM D 5992-96 standard. The response of a sample of vulcanized elastomeric mixture in the form of a sinusoidal stress in alternating simple shear is recorded, at a frequency of 10Hz, at a temperature of 100°C. A strain amplitude sweep is carried out from 0.1% to 50% (forward cycle), then from 50% to 0.1% (return cycle). For the outward cycle, the maximum value of Tan (ô) observed, noted Tan (ô)max, is indicated.On this same forward cycle we indicate the value of the dynamic elastic shear modulus, G*.

[0014] An elastomeric mixture can also be characterized by static mechanical properties. Tensile tests make it possible to determine the stresses elasticity and breaking properties. Unless otherwise indicated, they are carried out in accordance with the French standard NFT 46-002 of September 1988. The so-called "nominal" secant moduli (or apparent stresses, in MPa) at 10% elongation (noted "MA10") and 100% elongation ("MA100") are measured in second elongation (i.e. after an accommodation cycle). All these tensile measurements are carried out under normal conditions of temperature (23±2°C) and hygrometry (50+5% relative humidity), according to the French standard NF T 40-101 (December 1979). The breaking stresses (in MPa) and the elongations at break (in %) are also measured, at a temperature of 23°C. Prior art

[0015] The person skilled in the art, a tire designer, knows that the functions expected 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.

[0016] The crown reinforcement is an essential element which 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.

[0017] The Guide function is also known as "road behavior". It involves the responses of a vehicle / tire assembly to multiple driver inputs (steering, acceleration, braking, etc.). Behavior is essential both in terms of safety for the stability of the vehicle and for driving pleasure.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] Transverse drift stiffness is the variation of the 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 Newton per degree (N / °).

[0022] For small drift angles, i.e. angles less than 4°, the transverse force, in a direction parallel to the axis of rotation of the tire, is proportional to the drift angle. The transverse drift stiffness is equal to this coefficient of proportionality.

[0023] 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.

[0024] Rolling resistance is another performance addressed in the invention. Rolling resistance is one of the forces that oppose the movement of the vehicle. The rolling resistance coefficient of a tire (Crr) is the rolling resistance force related to the load carried by the tire. The coefficient is expressed in kg / t. Rolling resistance is essentially linked to the deformation of the tire. For illustration, the beads associated with the sidewalls represent 20% to 30% of the rolling resistance of the tire, while the tread contributes 60% to 80%.

[0026] Reducing greenhouse gas emissions from transport is one of the major challenges facing vehicle manufacturers today. Tires are an important source of progress, through a reduction in rolling resistance, because this has a direct impact on the vehicle's fuel consumption. As an illustration, a 20% reduction in the rolling resistance of a passenger car tire saves approximately 3% of fuel per 100 km in the combined cycle.

[0027] The choice of the tread plays an essential role in establishing a compromise between road behavior and rolling resistance. Among the design parameters of tires, those skilled in the art know treads consisting of a stack of sub-layers with thermomechanical and geometric properties appropriate to each sub-layer. An example of a sub-layer, that is to say a layer of rubber inserted between the crown reinforcement and the tread material, is described in document FR 2 954 333.

[0028] Another example where this time the material constituting the underlayer is flush at the bottom of the tread grooves is given by document EP 2 865 543. In general, underlay 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 contact area with the ground.

[0029] However, the lower the stiffness, the poorer the drift thrust response of the tire to vehicle steering input. Indeed, 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 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 we are looking for the best performance in behavior.

[0030] 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 made according to this teaching does not provide progress in the balance of performance.

[0031] 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 "soft" layer, i.e. with a dynamic stiffness modulus G* less than or equal to 0.6 MPa, placed radially outwardly on the crown reinforcement. Said sub-layer also comprises a second "rigid" layer placed radially outwardly to the soft layer, and radially inwardly to the tread. The dynamic stiffness modulus of the rigid layer, greater than 7 MPa, is greater than the dynamic stiffness modulus of the tread. Such a tread configuration gives improved results in rolling resistance and road behavior compared to a tire of design usual.

[0032] The inventors set themselves the objective of identifying other levers, in this case linked to the geometric properties of the layers of the sub-layer, to further improve the compromise between rolling resistance performance and drift rigidity of the design of a reference tire cited above (WO2019 / 145621). Statement of the invention

[0033] This object has been achieved by a tire for a motor vehicle comprising a crown comprising a crown reinforcement, a sub-layer, and a tread; said sub-layer being provided with a first base layer, radially external to the crown reinforcement, and a second covering layer, radially external to the base layer and radially internal to the tread; said base layer having a dynamic elastic shear modulus Gl* less than or equal to 1.5 MPa, and a viscoelastic loss Tan (ôl); the covering layer having a dynamic elastic shear modulus G2* greater than or equal to 5 MPa, and a viscoelastic loss Tan (ô2); the tread layer having a dynamic elastic shear modulus G4*, and a viscoelastic loss Tan (ô4); said tire is characterized in that a layer of elastomeric groove bottom mixture is positioned radially internally to the outer profile of the tread, in that the viscoelastic loss Tan (ô3) of said layer of elastomeric groove bottom mixture is such that Tan (ô3) < 0.75xTan (ô4), and preferably Tan (ô3) < 0.5xTan (ô4) with Gl*, G2*, G3*, and G4*, Tan (ôl), Tan (ô2), Tan (ô3), and Tan (ô4) being measured at 23°C under an alternating shear stress at a frequency of 10 Hz and at 10% deformation, G3* being the dynamic elastic shear modulus of the groove bottom layer (73).

[0034] The crown of the tire is defined as an area which includes the tread, the underlayer and the crown reinforcement comprising the composite layers which surround the carcass reinforcement. This description is made in the direction from the outside to the inside of the tire.

[0035] By groove is meant a cutout in the tread, substantially circumferential over one revolution of the wheel, the distance between the walls of material which 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 outward. The tread comprises NBS grooves, most often NBS 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 first base layer is placed radially externally on the crown reinforcement, and said 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 second covering layer is positioned radially externally 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 base layer, which is most often positioned in sections under the grooves in the axial direction, or else according to different configurations depending on the desired performance compromise.In terms of viscoelastic property, the viscoelastic loss of said groove base layer Tan (ô3) is strictly less than 0.75xTan (ô4), and preferably Tan (ô3) strictly less than 0.5xTan (ô4); which amounts to saying that the groove base layer is less hysteretic than the tread, which is sufficient to obtain a perceptible reduction in the rolling resistance of the tire.

[0037] The operating principle of the invention with this multi-layer crown composed of three sub-layers consists of designing an elastomeric mixture for the groove bottoms with low hysteresis, while having a rigidity at least comparable to that of a tread. Noting that the groove bottom mixture layer is never in contact with the road surface, it is therefore possible to free oneself from the constraints of a composition with high performance in grip, and / or wear. Therefore, the guidelines for designing the groove bottom mixture are: • Look for a low hysteresis mixture that dissipates less than a tread mixture while having a dynamic elastic shear modulus close to that of the said tread. The advantage of such a solution is to provide a gain in road handling; • Look for a low hysteresis mix that is also compatible with external stations, particularly due to its ability to resist ozone attack from the ambient environment, or a mix with good resistance to the risk of oxidation and stone attack. Such a mix will also provide a gain in rolling resistance, a real advantage in endurance, unlike a mix of a conventionally designed undercoat. • Look for a mixture already existing in the materials used for the fa tire construction, for example a sidewall type that would have low rolling resistance. The expected bonus effect here is not only the reduction in the rolling resistance of the tire, but also an improvement in industrial performance by strengthening the standardization of the materials used.

[0038] The main characteristics of the invention lead to the tire of the invention characterized in that it achieves a compromise of rolling resistance performance without degrading road behavior thanks to the thermomechanical properties of the layers of the tread, 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 thermomechanical properties of the groove base layer.

[0040] Advantageously, the dynamic shear stiffness modulus G3* of the groove base layer is greater than 1.25xGl*. This characteristic guarantees that a tire of the invention not only has a reduction in rolling resistance, but also a gain in road behavior.

[0041] According to a first embodiment, the tread comprising grooves oriented essentially circumferentially, said tire is characterized in that the groove base layer is formed of several axially separated sections, each section being positioned under a groove. This embodiment is shown in [Fig.l].

[0042] Advantageously, the tread comprises a number of NBS of grooves, in which the axial width of at least one section of the groove base layer is at least equal to 50% of the axial width of the groove of said section.

[0043] In this first embodiment, the covering layer is axially continuous between the two shoulders.

[0044] Alternatively, according to a second embodiment indicated in [Fig.2], the tread comprising grooves oriented essentially circumferentially, said tire is characterized in that the covering layer is formed of several axially separated sections, said covering layer being interrupted under the grooves.

[0045] According to a variant of this second embodiment, the tread comprising grooves oriented essentially circumferentially, said tire is characterized in that the base layer is formed of several axially separated sections, said base layer being interrupted under the grooves. This configuration is shown in [Fig.6]-C.

[0046] In the second and third embodiments, the groove bottom layer is positioned axially between two sections of the covering layer.

[0047] 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 at the top of the tire, radially internally, the two points being on the same radial line, ESC being the maximum value of the ESCi, for i ranging from 1 to NBS, said tire is characterized in that the radial thickness of said groove bottom layer, E73, 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%], E73 being measured between said first point and a second point at the intersection of the radially inner contour of said groove bottom layer, and a radial straight line passing through said first point. ;

[0048] 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 at the top 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 the ESCi determines the radial thickness of the groove bottom layer.

[0049] Often this 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 can comprise steel or nylon reinforcements coated in an elastomeric mixture.

[0050] The value of ESC represents the distance between a reinforcement of the first composite layer encountered and the external environment of the tire. This thickness of elastomeric mixture has the function of protecting the reinforcements of the first composite layer encountered from the external environment of the tire. The attacks coming from the external environment are of several kinds 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.

[0051] The inventors have provided a minimum value for the ESC thickness. It must be greater than 1 mm. Below 1 mm, ESC, which represents a protective layer, is insufficient to combat the oxidation of the metal reinforcements, and beyond 3.5 mm, this thickness can lead to a degradation of the rolling resistance.

[0052] 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 a third embodiment, shown in [Fig.3], the tread comprises sculpture blocks separated by grooves oriented essentially circumferentially, characterized in that the covering layer is, axially opposite certain sculpture 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 sculpture block, said axial width decreasing when moving radially upwards.

[0053] The preceding characteristics relating to the geometry of the groove bottom layer by defining its positioning in the crown, its axial width, and its maximum radial thickness make it possible to define the volume of the groove bottom layer.

[0054] 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. This mixture has a dynamic shear elastic modulus greater than 1.25xGl*, which is sufficient to improve the transverse drift rigidity (OY axis direction), and therefore the road behavior.

[0055] This volume of elastomeric mixture of the groove base layer is sufficient for the reduction in rolling resistance to be perceptible.

[0056] Preferably, the material of the groove bottom elastomeric mixture layer has a chemical composition identical to that of the sidewall layers.

[0057] With a view to further optimizing the rolling resistance of the tire without degrading the industrial cost price, it is possible to use a groove base layer of the same chemical composition as that of the sidewall layers. Since the groove base layer 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, according to its chemical composition, the sidewall has abilities to resist external aggressions from ozone which are suitable for the groove base 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 a material reference for the groove base, when the sidewall material is used.

[0058] Preferably, the elastomeric mixture of the groove base layer has a rubber composition based on at least one polyisoprene blend of ca natural cabbage, and polybutadiene, a crosslinking system, and a reinforcing filler at an overall rate of no more than 45 pce, and comprising carbon black, at a rate of no more than 5 pce, and mainly silica at a rate of at least 20 pce and no more than 40 pce.

[0059] The chemical composition defined above is deduced from that of a low hysteresis flank layer mainly loaded with silica.

[0060] Alternatively, the elastomeric mixture of the groove base layer has a rubber composition based on at least one blend of polyisoprene, natural rubber, 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.

[0061] In this case, the chemical composition defined above is deduced from that of a low hysteresis flank layer mainly loaded with carbon black.

[0062] Other characteristics of the invention relate to the dynamic properties of the elastomeric mixtures of the layers of the tread.

[0063] Preferably, the dynamic shear modulus Gl* of the base layer material measured at 23°C, and under alternating shear stress at a frequency of 10 Hz and at 10% deformation, is less than or equal to 1.5 MPa, and preferably less than or equal to 0.6 MPa.

[0064] Preferably, the value of Tan (ô) measured at 23°C at 10 Hz and under an alternating shear deformation of 10% of the base layer material is less than or equal to 0.15.

[0065] Preferably, the dynamic shear modulus G2* of the material of the covering layer measured at 23°C under an alternating shear stress at a frequency of 10 Hz and at 10% deformation, is greater than or equal to 7 MPa, and even more preferably greater than or equal to 12 MPa.

[0066] Preferably, the value of Tan(ô2) measured at 23°C at 10 Hz and under an alternating shear deformation of 10% of the material of the covering layer is less than or equal to 0.35.

[0067] According to a fourth embodiment of the invention visible in [Fig.4], the tread comprises at least one tread portion, hereinafter referred to as a tread wing, at at least one of the axial ends of the tread, 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 size of the tire dimension, said tread wing being radially internally in contact with the covering layer.

[0068] The presence of the tread wings makes it possible to further improve the rolling resistance by replacing the extreme parts of the tread with a material of lower hysteresis than the 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; and that the viscoelastic loss Tan (ô) of each wing is at most equal to 80% of the viscoelastic loss of the central portion of the tread, the viscoelastic loss Tan (ô) being measured at 23°C, and under alternating shear stress at a frequency of 10 Hz and at 10% deformation. Brief description of the drawings

[0069] 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: • [Fig.l] shows a schematic meridian section of a tire according to a first embodiment of the invention, with a discontinuous groove base layer in sections positioned under the grooves. The covering layer is axially continuous from one shoulder to the other; • [Fig.2] shows a schematic meridian section of a tire according to a second embodiment of the invention, still with a discontinuous groove base layer in sections positioned under the grooves, but this time, the covering layer is discontinuous in sections positioned under the grooves; • [Fig. 3] shows a schematic meridian section of a tire according to a third embodiment of the invention, with a discontinuous groove base 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; • [Fig.4] shows a schematic representation of a fourth embodiment of the invention, repeating embodiments 1 or 3, but with a tread wing positioned on either side of the equatorial plane • [Fig.5] is an enlargement of a portion of the top of [Fig.l] showing the main ribs involved in the invention. • [Fig.6] represents in view [Fig.6]-A a vertex of a tire of the state of the art, and in figures 6-B and 6-C two embodiments of the invention. Detailed description of the invention

[0070] The 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 RI8 XL 100W.

[0071] 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 referenced only once on one side of a meridian plane.

[0072] [Fig.l] shows a tire 1, 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 extends axially from one shoulder 60 to the other shoulder. The tread comprises a contact face 61 intended to come into contact with the roadway while the tire is rolling. The tread 6 comprises sculpture blocks 63 separated by grooves 62 oriented essentially circumferentially. Each groove 62 is delimited radially inwardly by a groove bottom 620.

[0073] The bead 50 comprises at least one carcass layer, shown in [Fig.l] 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°.

[0074] 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 23 binds the two previous layers. Said layer 23 also comprises reinforcements coated in an elastomeric mixture, which make an angle of approximately ±2.5° with the circumferential direction.

[0075] The crown also comprises a sub-layer 7 arranged radially external to the crown reinforcement 20 and radially internal to the tread 6. This sub-layer is formed of three sub-layers: a base layer 71, a covering layer 72, and finally a groove bottom layer 73.

[0076] The base layer 71 is arranged radially directly on the crown reinforcement 20. As known per se, the crown reinforcement comprises layers of cables or monofilament reinforcements generally coated with a thin layer of rubber. In the context of the present invention, by specifying that the base layer 71 is arranged radially directly on the crown reinforcement 20, it is indicated that it is in contact with the cables or reinforcements, without taking into account their rubber coating.

[0077] The covering layer 72 is arranged radially outside the base layer 71. The groove bottom base layer 73 is radially inside the tread 6.

[0078] An example of a suitable formulation for the base layer material, with a dynamic shear modulus Gl* of 0.2 MPa, is as follows:

[0079] [Table 1]: component pce NR 100 Carbon black 4 6PPD 2 2 DPG 2.1 Stearic acid 3 ZnO 1.5 Resin 29.3 HTO 38

[0080] The formulations are given in mass (pce meaning percentage of the mass of elastomer).

[0081] An example of a suitable formulation for the covering layer material with a dynamic shear modulus G2* of 25 MPa is as follows:

[0082] [Table 2]: Component pce NR 100 Carbon Black 70 Formophenolic Resin 12 ZnO 3 Stearic Acid 2 6PPD 2.5 HMT 4 Sulfur 3 CBS 2

[0083] The formulations are given in mass (pce meaning percentage of the mass of elastomer).

[0084] As for the tread material, according to the invention, its rigidity is lower than the rigidity of the covering layer. Advantageously, the rubber mixture has a dynamic shear modulus G4* of less than 4.0 MPa and preferably less than 2.5 MPa.

[0085] The following Table 3 gives an example of a tread formulation:

[0086] [Table 3]: SBR pce component (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 (t) 2 Accelerator (g) -> DPG 2 Sulphur 1

[0087] 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 “Zeosilll65MP” from Solvay with a BET surface area of ​​160m2 / g (c) Silane TESPT “SI69” from Evonik (d) TDAE “Flexon 630” oil from Shell (e) “Escorez 2173” resin from Exxon (f) Antioxidant “Santoflex 6PPD” from the company Solutia (g) “Santocure CBS” accelerator from the company Solutia.

[0088] For the groove bottom layer 73, a formulation derived from that of a low hysteresis flank layer is perfectly suitable:

[0089] Table [4]: Compositions Electrostatic Ixatin ' Eisastsmère BR Filler Carbon black Filler cepi-CMs? Sice Coupling agent Protective agent Vnicanissîion agent Plasticizer Mixture of Fhnc 50 50 3 £0 0 4 53 10

[0090] The composition of the sides leads to a low hysteresis measured by a tan (ô) value less than or equal to 0.1. The plasticizers to facilitate industrial implementation are at a level of 10 pce of oil.

[0091] [Fig.2] shows a schematic meridian section of a tire according to a second embodiment of the invention comprising two parts symmetrical with respect to the equatorial plane CP. The groove base layer 73 is in sections positioned in each groove 62. The covering layer 72 is also in interrupted sections under the grooves 62. Each section of the groove base layer is interposed between two sections of the covering layer 72. In each groove 62, the volume of the groove base mixture layer 73 is the surface which goes from the radially outer contour of the groove base, to the contour of the groove base in contact with the base layer, integrated over one wheel revolution, in the circumferential direction. The three layers of the sub-layer 7, namely the layers 71, 72, and 73 are arranged on the crown reinforcement 20 formed of the working and hooping layers respectively 21, 22, and 23.The tread 6 comprises tread blocks 60, with grooves 62 arranged between the blocks. The groove base layer 73 is flush with the groove bottoms 620.

[0092] [Fig. 3] represents the third embodiment of a tire according to the invention, comprising two parts symmetrical with respect to the equatorial plane CP. The groove base layer 73, like the covering layer 72, are in non-contiguous sections. Each section of the layer 73 is positioned radially under a groove 62. The sections of the groove base layer 73 are arranged axially between two sections of the covering layer. The three layers of the sub-layer 7, namely the layers 71, 72, and 73 are arranged on the crown reinforcement 20 formed of the working and hooping layers respectively 21, 22, and 23. The tread 6 comprises sculpture blocks 60, with grooves 62 arranged between the blocks. Upstream and downstream of each groove, a reinforcing element 630 extends from the covering layer 72 until it reaches the running surface 61. The groove base layer 73 is flush with the groove bottoms 62a, 62b, 62c, and 62d.

[0093] [Fig.4] is identical to [Fig.3], except for the presence of the tread wings 90. This figure is a fourth embodiment of the invention.

[0094] [Fig. 5] is an enlargement of a portion of the top of [Fig. 1] showing the main dimensions involved in the invention. The radial thicknesses of the layers of the sub-layer 7 can be seen, namely the layers 71, 72, and 73, with respective radial thicknesses E71, E72, and E73. The tread 6 has a thickness EKM. There are also the under-groove thicknesses ESC1, and ESC2 which measure the thicknesses of the layers of mixtures from a point in the middle of the radially outer contour of the groove bottom to the first reinforcement 24 of a composite layer touched radially from the outside to the inside of the tire 1. The crown layers 21, 22, 23 forming the crown reinforcement 20 are reinforcements 24 coated in an elastomeric matrix.

[0095] Finally, [Fig.6] comprises 3 views [Fig.6]-A, 6-B, and 6-C. [Fig.6]-A is the top of a state-of-the-art tire close to the tires of the invention. The tire of [Fig.6]-A has a sub-layer with two layers 71, and 72. The elastomeric mixture at the bottom of the grooves 62 is that of the tread. [Fig.6]-B comprises three layers 71, 72, and 73; the mixture of the groove bottom layer 73 is identical to that of the sidewalls 3 as shown in [Fig.6]-B. [Fig.6]-C represents a configuration where the groove bottom layer 73 is positioned not only under the grooves 62, but also at the shoulders so as to further optimize the reduction in rolling resistance.

[0096] The person skilled in the art, a tire designer, will be able to adopt alternative embodiments in which the tread itself comprises several different materials, superimposed radially and / or juxtaposed axially.

[0097] Tires of the invention, PI shown in [Fig.6]-B, and P2 shown in [Fig.6]-C were evaluated to clearly highlight the performance provided by the invention. The results of the evaluation tests are compared with those obtained for a control tire T shown in [Fig.6]-A.

[0098] Witness T is a state-of-the-art tire intended to equip a vehicle passenger car, with a reference pressure of 290 kPa, with a dimension equal to 245 / 45 RI8 XL 100W. Its top comprises a crown reinforcement, a sub-layer, and a tread. The sub-layer comprises a first base layer, radially external to the crown reinforcement, and a second cover layer, radially external to the base layer and radially internal to the tread. The base layer has a dynamic elastic shear modulus Gl* equal to 0.5 MPa, and as for the cover layer, its elastic shear modulus G2* is equal to 18 MPa.

[0099] Witness T, like the tires of the invention, has 4 grooves symmetrical with respect to the axis (CP).

[0100] This witness T is compared to the tires PI and P2 of the invention, also intended to equip a passenger vehicle. The witness tire T has a sub-layer 7 comprising two layers 71 and 72, and the groove base compound is that of the tread 6. The tire PI has a third groove base layer comprising a dynamic elastic shear modulus G3* of 4.2 MPa, which therefore satisfies the characteristics of claim 1. Furthermore, the chemical composition of the groove base layer 73 is identical to that of the sidewalls 3. The tire P2 differs from PI by the presence of the groove base layer in sections positioned under the grooves between two portions of the covering layer 72; said sections of the groove base layer 73 extend radially inwardly, until they touch a crown layer.The following table summarizes the differences between the T witness, the PI tires, and P2 of the invention: .

[0101] [Tables5] Gl* MPa G2* MPa G3* MPa G4* MPa Tan (ôl) Tan (ô2) Tan (ô3) Tan (ô4) Crr (%) DZ (%) Fig. 6-A Witness n 0.5 18 SO 2.1 0.06 0.3 SO 0.19 100 100 Fig. 6-B PI 0.5 18 0.9 2.1 0.06 0.3 0.08 0.19 102 99 Fig. 6-C P2 0.7 18 0.7 2.1 0.03 0.3 0.03 0.19 103 108.5

[0102] The abbreviation Crr is the coefficient of resistance, positioned at a base of 100 for the witness. For the PI, P2 tires, the variation is evaluated in relation to the base of 100 of the witness. The same is done for the transverse drift rigidity. SO means "not applicable" since the witness tire does not have a third layer of furrow bottom mixture.

[0103] It is easily verified that the tires of the invention P1 and P2 are indeed covered by claim 1.

[0104] Rolling resistance and drift stiffness were simulated by the finite element method for tires of the invention and of the state of the art according to the two configurations. The results are shown in Table 5.

[0105] A result greater than (respectively less than) 100% means an improvement (respectively a deterioration) of the performance considered.

[0106] This example confirms that the tires of the invention PI, and P2 with a third groove base layer provided with the claimed thermomechanical properties have good operation with improved rolling resistance and drift rigidity compared to the tires of the state of the art.

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); said sub-layer (7) being provided with a first base layer (71), radially external to the crown reinforcement (20), and a second covering layer (72), radially external to the base layer (71) and radially internal to the tread (6); said base layer (71) having a dynamic elastic shear modulus Gl* less than or equal to 1.5 MPa, and a viscoelastic loss Tan (ôl); the covering layer (72) having a dynamic elastic shear modulus G2* greater than or equal to 5 MPa, and a viscoelastic loss Tan (ô2); the tread layer (6) having a dynamic elastic shear modulus G4*, and a viscoelastic loss Tan (ô4); said tire(l) is characterized in that a layer of elastomeric groove bottom mixture (73) is positioned radially internally to the outer profile of the tread (6), in that the viscoelastic loss Tan (ô3) of said layer of elastomeric groove bottom mixture is such that Tan (ô3) < 0.75xTan (ô4), and preferably Tan (ô3) < 0.5xTan (ô4) with Gl*, G2*, G3*, and G4*, Tan (ôi), Tan (ô2), Tan (ô3), and Tan (ô4) being measured at 23°C under an alternating shear stress at a frequency of 10 Hz and at 10% deformation, G3* being the dynamic elastic shear modulus of the groove bottom layer (73).

2. A tire (1) according to claim 1, wherein the dynamic elastic shear modulus G3* of the groove bottom elastomeric compound layer is greater than 1.25 x Gl*.

3. Tire (1) according to one of the preceding claims in which the tread (6) comprises grooves (62) oriented essentially circumferentially, said tire is characterized in that the groove base layer (73) is formed of several axially separated sections, each section being positioned under a groove (62).

4. Tire (1) according to claim 2, the tread (6) comprising a number of NBS of grooves, in which the axial width of at least one section of the groove base layer (73) is at least equal to 50% of the width of the groove (62) of said section.

5. Tire (1) according to one of claims 1 to 3, characterized in that that the covering layer (72) is axially continuous between the two shoulders (60).

6. Tire (1) according to one of the preceding claims in which the tread (6) comprises grooves (62) oriented essentially circumferentially, said tire is characterized in that the covering layer (72) is formed of several axially separated sections, said covering layer (72) being interrupted under the grooves (62).

7. Tire (1) according to one of the preceding claims in which the tread (6) comprises grooves (62) oriented essentially circumferentially, said tire is characterized in that the base layer (71) is formed of several axially separated sections, said base layer (71) being interrupted under the grooves (62).

8. Tire (1) according to one of the preceding claims, 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 at the top of the tire, radially inwardly, the two points being on the same radial line, ESC being the maximum value of ESCi, for i ranging from 1, to NBS, said tire is characterized in that the radial thickness of said groove bottom layer (73), taken in a groove (62) at a point in the middle of its radially outer contour, E73, is included in the interval [20%; 100%] of ESC, and preferably E73 in the interval [50%;100%], E73 being measured between said first point and a second point at the intersection of the radially inner contour of said groove bottom layer (73), and a radial straight line passing through said first point.;

9. A tire (1) according to one of the preceding claims, in which the tread (6) comprises tread blocks (63) separated by grooves (62) oriented essentially circumferentially, characterized in that 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, 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 upwards.

10. Tire (1) according to one of the preceding claims in which the material of the groove bottom elastomeric mixture layer (73) is of a chemical composition identical to that of the sidewall layers (3).

11. Tire (1) according to one of claims 1 to 4, in which the elastomeric mixture of the groove base layer (73) has a rubber composition based on at least one blend of polyisoprene, natural rubber, 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.

12. Tire (1) according to one of claims 1 to 4, in which the groove bottom elastomeric mixture (73) has a rubber composition based on at least one blend of polyisoprene, natural rubber, and polybutadiene, a crosslinking system, and a reinforcing filler at an overall rate of at most 45 pce, and comprising carbon black at a rate of at least 20 pce and at most 40 pce.

13. Tire (1) according to one of the preceding claims, in which the dynamic shear modulus Gl* of the material of the base layer (71) measured at 23°C, and under an alternating shear stress at a frequency of 10 Hz and at 10% deformation, is less than or equal to 1.5 MPa, and preferably less than or equal to 0.6 MPa

14. A tire (1) according to any preceding claim, wherein the value of Tan (ôl) measured at 23°C at 10 Hz and under an alternating shear deformation of 10% of the base layer material (71) is less than or equal to 0.

15.

15. Tire (1) according to one of the preceding claims, characterized in that the dynamic shear modulus G2* of the material of the covering layer (72) measured at 23°C under alternating shear stress at a frequency of 10 Hz and at 10% deformation, is greater than or equal to 7 MPa, and even more preferably greater than or equal to 12 MPa.

16. Tire (1) according to one of the preceding claims, characterized in that the value of Tan (ô2) measured at 23°C at 10 Hz and under an alternating shear deformation of 10% of the material of the covering layer (72) is less than or equal to 0.

35.

17. A tire (1) according to one of the preceding claims, wherein the tread (6) comprises at least one tread portion (80), hereinafter referred to as a tread wing (80), at at least one of the axial ends of the tread, said tread wing (80) 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.

18. Tire (1) according to the preceding claim, characterized in that the dynamic shear modulus of the tread wing (80) 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.

19. Tire (1) according to claims 12 and / or 13 wherein the viscoelastic loss Tan (ô) of each tread wing (80) is at most equal to 80% of the viscoelastic loss of the central portion of the tread, the viscoelastic loss Tan (ô) being measured at 23°C, and under alternating shear stress at a frequency of 10 Hz and at 10% deformation.

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