Tire optimized for road behavior and rolling resistance

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 first base layer (71), radially outside the crown reinforcement (20), and a second cover layer (72), radially outside the base layer (71) and radially inside the tread (6). The base layer has a dynamic elastic shear modulus G1* of less than 1.5 MPa, and as for the cover layer, its elastic shear modulus is greater than 5 MPa. The thickness of the cover layer (72) is in the range between [20%; 80%] of the radial thickness of the sub-layer (7). Abstract figure: Figure 2

Description

Title of the invention: Tire optimized for road behavior and rolling resistance Field of invention

[0001] The 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), 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 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.

[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 Tanô=G” / G'. The dynamic shear modulus G* and the dynamic loss Tanô are measured on a viscoanalyzer 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 10 Hz, at a temperature of 100°C. A strain amplitude sweep is carried out from 0.1% to 50% (forward cycle), then from 50% at 0.1% (return cycle). For the forward cycle, the maximum value of tan(ô) observed, noted Tan(ô)max, is indicated. On this same forward cycle, the value of the dynamic elastic shear modulus, G*, is indicated.

[0014] An elastomeric mixture can also be characterized by static mechanical properties. Tensile tests are used to determine the yield stresses and the properties at break. Unless otherwise indicated, they are carried out in accordance with 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 temperature (23±2°C) and hygrometry (50+5% relative humidity) conditions, according to French standard NF T 40-101 (December 1979). The stresses at break (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 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.

[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 (RRC) is the rolling resistance force related to the load carried by the tire. The coefficient is expressed in kg / t.

[0025] 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 material and geometric properties appropriate to each sub-layer. An example of 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.

[0028] 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 the 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 material that is not very hysteretic, or to stiffen the tread in shear, but with modest rigidities so as not to oppose too much the flattening of the tread of the tire in the area of ​​contact with the ground.

[0029] However, the lower the stiffness, the poorer the drift thrust response of the tire to vehicle steering stress. 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 tg ô (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., that of the two radially outermost layers. The modulus of the material of the radially inner layer of the sub-layer materials and the value of tg ô 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 externally to a sub-layer, which itself rests radially externally 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 externally on the crown reinforcement. Said sub-layer also comprises a second "rigid" layer placed radially externally to the soft layer, and radially internally 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 handling compared to a conventionally designed tire.

[0032] The inventors set themselves the objective of identifying other levers, in this case linked to the geometric properties of the layers of the underlayer, to further improve the compromise between rolling resistance and drift stiffness performance of the reference tire design 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* of less than 1.5 MPa, and the covering layer having a dynamic elastic shear modulus G2* of greater than 5 MPa; Gl* and G2* being measured at 23°C under an alternating shear stress at a frequency of 10 Hz and at 10% deformation; E72 being the radial thickness of the covering layer, measured at a point on the radially external contour of said covering layer (72) in a meridian plane;E71 being the radial thickness of the base layer, measured at a point on the radially outer contour of said base layer (71) in a meridian plane. Said tire (1) is characterized in that the ratio of the thickness E72 / (E71+E72) is included in the percentage interval [20; 80], over at least 75% of the axial width of said covering layer, a radial thickness of a layer being measured between a first point on the radially inner contour of a layer, and a second point at the intersection of the radially outer contour of this same layer, and a radial straight line passing through said first point. ;

[0034] The crown of the tire is defined as an area which includes the tread, the underlay 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 turn 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 contour of the bottom 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] 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 covering layer.

[0037] The radial thickness of the covering layer, E72, is measured between a first point located at the intersection of a radially outer contour of the covering layer with the radial axis OZ passing through said point, and between a second point located at the intersection of a radially inner contour of said covering layer with said radial axis OZ. A meridian plane being a plane containing the axis of rotation of the tire.

[0038] The radial thickness of the base layer, E71, is measured between a first point located at the intersection of a radially outer contour of the base layer with the radial axis OZ passing through said point, and between a second point located at the intersection of a radially inner contour of said base layer with said radial axis OZ. A meridian plane being a plane containing the axis of rotation of the tire.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] A tire of the invention, in addition to the characteristics of the preamble of the first claim, comprises a covering layer whose thickness can be chosen from a range of admissible thicknesses which are in proportion to the thickness of the underlayer to achieve an advantageous compromise of performance in road behavior and rolling resistance. Radially external to the covering layer, the tread which represents the largest volume of elastomeric mixture of the crown and intended to come into contact with a ground during rotation of the tire remains at iso thickness so that the total thickness of the underlayer and the tread remains constant.

[0043] One of the objectives of the invention is to generalize the operation of the technical solution to tire ranges comprising several dimensions. The value of the covering layer deduced from the maximum under-cushion thickness makes it possible to cover all dimensions by adapting the under-cushion thickness ESC to the case considered. Thus, for a small dimension such as 165 / 70R13 or a large dimension such as 245 / 45R18, the thicknesses of the covering layer, base layer and tread are in a homothetic ratio to make the invention work.

[0044] The inventors assume that the sum of the thicknesses of the base and covering layers is constant. It is therefore possible to control the balance of performance in road behavior and rolling resistance by a distribution of the layer thicknesses and adapted thermomechanical properties.

[0045] The covering layer with a dynamic shear modulus greater than or equal to 5 MPa, contributes significantly to the drift rigidities. A thickness of this layer greater than 20% of the sum of the thicknesses of the base layer and the covering layer guarantees a sufficient level of drift rigidity, but beyond 80% the rolling resistance is too penalized to retain these solutions.

[0046] For each tire dimension, it is appropriate to establish the distribution of suitable radial thicknesses between the different layers of the crown to obtain the desired performance compromise.

[0047] The main characteristic of the invention leads to the tire of the invention characterized in that it achieves a compromise of performance in road behavior and rolling resistance thanks to the thermomechanical properties, and thanks to the appropriate distribution of the thicknesses of the base and covering layers.

[0048] Other characteristics linked to different embodiments of the invention contribute to further improving the performance compromise of the tire. more often, these characteristics concern other geometric characteristics, but especially mechanical and viscoelastic properties of the top layers.

[0049] Preferably, the thickness ratio E72 / (E72+E71) is included in the percentage interval [20; 60], over at least 75% of the axial width of said covering layer (72).

[0050] Advantageously, the thicknesses of the covering layer, E72, and of the sub-hollow, ESC, verify the relationship: on the one hand the thickness E72 (in mm) is greater than or equal to (0.53*ESC + 0.1) and on the other hand, the thickness E72 (in mm) is less than or equal to (0.6*ESC + 1.1), when ESC is between [1; 3.5] mm.

[0051] Advantageously, the distance EAXE from a point on the axial end of the covering layer to the outer contour of a sidewall layer on the same side of a meridian plane is within the interval [5%; 15%] of the nominal bead size of the tire, said nominal bead size being indicated in the designation of the tire.

[0052] For example, for a tire size designated by the following name: "205 / 65 R16", the nominal flange width is 205 mm. It corresponds to the axial width of the inflated tire, mounted on a rim, and under a pressure, recommended by a specification standard such as ETRTO (European Tire and Rim Organization). Still with this example, the EAXE distance must be within the interval [10.25; 30.75] mm.

[0053] This interval [5%; 15%] is used to delimit the axial width of the covering layer laid radially externally on the base layer. The covering layer may be continuous or discontinuous in sections. When it is discontinuous, the interval [5%; 15%] determines the range of variation of the axially outermost section.

[0054] In addition to the thickness seen previously, the width of the covering layer contributes to the performance compromise between road behavior and rolling resistance. In the solutions tested, the distance from the end of the covering layer to the sidewall in the shoulder area, measured in the axial direction, must be greater than or equal to 5% of the tire's bead size to have an effect on rolling resistance, and must remain less than or equal to 15% of this same bead size so as not to degrade the drift rigidity.

[0055] Preferably, the base layer material is characterized by a dynamic shear modulus Gl*, measured at 23°C, and under alternating shear stress at a frequency of 10 Hz and at 10% strain, of less than 1.5 MPa, and preferably less than 0.6 MPa. Advantageously, the value of tg ôl measured at 23°C at 10 Hz and under alternating shear strain of 10% of the base layer material is less than 0.15.

[0056] As for the covering layer, preferably, its material is characterized by a dynamic shear modulus G2*, measured at 23°C, and under an alternating shear stress at a frequency of 10 Hz and at 10% deformation, greater than 5 MPa and preferably greater than 7 MPa; very advantageously, the dynamic shear modulus G2* of this material, measured at 23°C, and under an alternating shear stress at a frequency of 10 Hz and at 10% deformation, has a value greater than 12 MPa. In this way, good drift rigidity is achieved and an excellent performance compromise is obtained, reconciling a drift thrust capable of providing the tire with good dynamic response and excellent rolling resistance. Advantageously, the value of tg ô2 measured at 23°C at 10 Hz and under an alternating shear deformation of 10% of the covering layer material is less than 0.35.

[0057] According to a first embodiment of the invention illustrated by figures 1 and 2, the covering layer is axially continuous between the two shoulders.

[0058] According to a second embodiment illustrated in [Fig. 3], a covering layer is formed of several axially separated sections, said covering layer being interrupted under the grooves. At the location of the interruption, the material appearing at the bottom of the groove is the same as the material of the tread. Overall, this embodiment makes it possible to significantly improve rolling resistance at the cost of a very modest degradation of the drift rigidity.

[0059] According to a third embodiment of the invention, illustrated in [Fig.3] and very close to that illustrated by [Fig.3]: in this third embodiment, at the location of the interruption, the material appearing at the bottom of the groove is the same as the material of the base layer.

[0060] 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. In [Fig. 5], a fourth embodiment is illustrated in which a covering layer, axially at the level of certain tread blocks, is extended radially outwards by a wedge-shaped reinforcing element (seen in meridian section). This reinforcing element extends 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 is 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.The angle formed in radial section by the two side walls of said reinforcing element is preferably between 10° and 50° degrees, for example 40°, as illustrated in [Fig.3]. The tread comprises such a . reinforcing element on either side of each groove. Each reinforcing element is advantageously made of the same rubber mixture as the covering layer, which allows them to be extruded in a single operation with the covering layer. Under these conditions, the thickness of the covering layer E72 is measured under the tread blocks.

[0061] According to another embodiment of the invention, 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 width of the tire dimension.

[0062] 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, and the viscoelastic loss tg ô3 of each wing is at most equal to 80% of the viscoelastic loss of the central portion of the tread, the viscoelastic loss tg ô3 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

[0063] The present invention will be better understood on 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 conforming to a first embodiment of the invention; • [Fig.2] is a partial enlargement of [Fig.l] showing the main geometric dimensions of the invention; • [Fig.3] shows a schematic meridian section of a tire in accordance with a second embodiment of the invention; • [Fig.4] is a simplified representation of a third embodiment of the invention; • [Fig.5] shows a simplified representation of a fourth mode of realization of the invention; • [Fig.6] shows evaluation results of tires of the invention. It includes a diagram with the relative gain in rolling resistance on the abscissa and the relative gain in drift stiffness on the ordinate, between a tire of the invention and a state-of-the-art control. Detailed description of the invention

[0064] 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 NL 96W.

[0065] 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 the meridian plane.

[0066] In [Fig.l] we see 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 sub-layer (7) and a tread 6. 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 during the rolling of the tire. The tread 6 comprises sculpture blocks 63 separated by grooves 62 oriented essentially circumferentially. Each groove 62 is delimited radially inwards by a groove bottom 620.

[0067] The bead 50 comprises at least one carcass layer, shown in [Fig.l]-A 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 50, 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 metallic reinforcements of a carcass layer are substantially parallel to each other and form, with the circumferential direction, an angle of between 85° and 95°.

[0068] 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.

[0069] The crown also comprises a sub-layer 7 arranged radially outside the crown reinforcement 20 and radially inside the tread 6. This underlayer is formed of two parts: a base layer 71 and a covering layer 72.

[0070] The base layer 71 is arranged radially directly on the crown reinforcement 20. As known per se, the crown reinforcement comprises layers of monofilament cables or 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 coating with rubber.

[0071] An example of a suitable formulation for the base layer material, with a dynamic shear modulus Gl* of 0.2 MPa, is as follows:

[0072] [Table 1]: component pce NR 100 Carbon black 4 6PPD Cl ri DPG 2.1 Stearic acid 3 ZnO 1.5 Resin 29.3 HTO 38

[0073] The formulations are given in mass (pce meaning percentage of the mass of elastomer).

[0074] An example of a suitable formulation for the covering layer material with a dynamic shear modulus G2* of 25 MPa is as follows:

[0075] [Table 2]: Component pce NR 100 Carbon Black 70 Formophenolic Resin 12 ZnO 3 Stearic Acid 6PPD 2.5 HMT 4 Sulfur 3 CBS ■7

[0076] The formulations are given in mass (pce meaning percentage of the mass of elastomer).

[0077] 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 G3* of less than 4.0 MPa and preferably less than 2.5 MPa.

[0078] The following Table 3 gives an example of a tread formulation:

[0079] [Table 3]: Component pce SBR (a) 100 Silica (b) 110 Coupling agent (c) 9 Liquid plasticizer (d) 20 Resin plasticizer (e) 50 Black 5 Zinc oxide 3 Stearic acid Antioxidant (f) Q Accelerator (g) 2 DPG 9 Sulfur 1

[0080] 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) Exxon Company “Escorez 2173” Resin (f) Antioxidant “Santoflex 6PPD” from the company Solutia (g) “Santocure CBS” accelerator from the company Solutia

[0081] In [Fig.2] we can see the thicknesses of the base and covering layers respectively E72 and E71. The tread layer has a thickness EKM. The distance from one axial end of the covering layer to the radially outer contour of the sidewall on the same side of the meridian plane is EAXE. The tire has four grooves. In [Fig.2], we see the thicknesses at the bottoms of grooves ESC1 and ESC2. ESC is the maximum value between ESC1 and ESC2.

[0082] Figures 2 and 3 illustrate similar embodiments. The covering layer 72 is made up of sections interrupted at the grooves. For the second embodiment illustrated in [Fig.2], the bottoms of the grooves (62a, 62b) are made up of the same material as the tread 6, while for a third embodiment the bottoms of the grooves (62a, 62b) are made up of the same material as the base layer 71.

[0083] [Fig.4] shows the covering layer 72 with the reinforcing elements 630, which border the radial walls of the grooves 62. Certain blocks 63 are also bordered by the reinforcing elements 630.

[0084] 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.

[0085] The tire of the invention was evaluated to clearly highlight the performance provided by the invention. The results of these evaluation tests are compared with those obtained for a control tire T.

[0086] Witness T corresponds to a state-of-the-art tire which comprises a sub-layer with a base layer and a covering layer. This witness T comprises 4 grooves symmetrical with respect to the axis (CP). The sub-layer comprises a covering layer with a thickness E72 of 0.25 mm, and the base layer has a thickness of 2.65 mm.

[0087] This witness differs from the tire of the invention in that the under-tread thicknesses are different. The following table summarizes these differences:

[0088] [Tables4] Thickness of top layer Thickness of base layer Ratio E72 / (E 71+E72) Resistance to rust (index) Drift stiffness (index) Control 0.25 mm 2.65 mm 0.09 100 100 Invention 1.1 mm 1.8 mm 0.38 101 108

[0089] It is easily verified that the tire of the invention is indeed covered by claim 1. Specifically, the thickness of the covering layer is 38% of the sum of the thicknesses of the base and covering layers.

[0090] Rolling resistance and drift stiffness results were evaluated for tires of the invention and of the state of the art.

[0091] 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 4.

[0092] A result greater than (respectively less than) 100% means an improvement (respectively a deterioration) of the performance considered.

[0093] Other results are obtained by varying the ratio of the thickness of the covering layer, E72, to the sum of the thicknesses of the base and covering layers to obtain the graph of [Fig.6].

[0094] The graph in [Fig.6] represents a diagram where the y-axis is the gain in drift stiffness expressed as a percentage, and the x-axis is the gain in rolling resistance expressed as a percentage. At each point on the curve, there corresponds a ratio of the thickness of the covering layer to the sum of the thicknesses of the base and covering layers. When the ratio E72 / (E72+E71) is close to 0.3, the gain in rolling resistance is maximum, and when the ratio is close to 0.8, conversely, the drift stiffness is optimal.

[0095] In the example of table (4), the ratio, E72 / (E72+E71), is 0.38, which gives an improvement in drift stiffness between 5% and 10%, and an improvement in rolling resistance of 1%.

[0096] The invention ensures that thicknesses of the covering layer within the claimed range provide good operation of the tire with minimal rolling resistance and maximum drift stiffness.

Claims

1. Claims 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* of less than 1.5 MPa, and the covering layer having a dynamic elastic shear modulus G2* of greater than 5 MPa; Gl* and G2* being measured at 23°C under alternating shear stress at a frequency of 10 Hz and at 10% deformation; E72 being the radial thickness of the covering layer (72),measured at a point on the radially outer contour of the covering layer (72) in a meridian plane; E71 being the radial thickness of the base layer (71), measured at a point on the radially outer contour of the base layer (71) in a meridian plane, the thickness ratio E72 / (E72+E71) is included in the percentage interval [20; 80], over at least 75% of the axial width of said covering layer (72), a radial thickness of a layer being measured between a first point on the radially inner contour of a layer, and a second point at the intersection of the radially outer contour of this same layer, and a radial straight line passing through said first point, ESCi being the radial thickness of the elastomeric mixture at the bottom of a groove i, 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, and ESC being the maximum value of ESCi, for i ranging from 1 to NBS, where NBS is the number of grooves in the tread, said tire (1) is characterized in that the thickness E72 (in mm) is greater than or equal to (0.53*ESC + 0.1) and in that the thickness E72 (in mm) is less than or equal to (0.60*ESC + 1.1), when ESC is between [1; 3.5] mm.,

2. Tire (1) according to claim 1, wherein the thickness ratio E72 / (E72+E71) is within the percentage interval [20; 60], over at least 75% of the axial width of said covering layer (72).

3. Tire (1) according to one of the preceding claims, in which the distance EAXE from a point of the axial end of the covering layer (72) to the outer contour of a sidewall layer (3) on the same side of a meridian plane is included in the interval [5%; 15%] of the nominal bead size of the tire (1), said nominal bead size being indicated in the designation of the dimension of the tire (1).

4. Tire (1) according to one of the preceding claims, characterized in that 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 0.6 MPa.

5. Tire (1) according to one of the preceding claims, characterized in that the value of tg ôl measured at 23°C at 10 Hz and under an alternating shear deformation of 10% of the material of the base layer (71) is less than 0.

15.

6. 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, and under an alternating shear stress at a frequency of 10 Hz and at 10% deformation, is greater than 7 MPa, and preferably greater than 12 MPa.

7. Tire (1) according to one of the preceding claims, characterized in that the value of tg 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 0.

35.

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

9. Tire (1) according to one of claims 1 to 8, in which the tread (6) comprises grooves (62) oriented essentially circumferentially, characterized in that the covering layer (72) is formed of several sections axially separated, said covering layer (72) being interrupted under the grooves (62).

10. Tire (1) according to one of the preceding claims, in which the material appearing at the bottom of the groove is the same material as the material constituting the tread (6).

11. A tire (1) according to claim 10, wherein the material appearing at the bottom of the groove is the same material as the material constituting the base layer (71).

12. A tire (1) according to one of claims 1 to 9, wherein 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.

13. A tire (1) according to one of the preceding claims, wherein the tread (6) 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.

14. Tire (1) according to the preceding claim, characterized in that 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% strain.

15. Tire (1) according to claims 12 and / or 13 wherein the viscoelastic loss tg ô3 of each tread wing is at most equal to 80% of the viscoelastic loss of the central portion of the tread, the viscoelastic loss tg ô3 being measured at 23°C, and under alternating shear stress at a frequency of 10 Hz and at 10% deformation.

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