Tire optimized for rolling resistance without compromising road handling

Abstract

The invention relates to a tire (1) having a crown (4) optimized for an advantageous compromise between handling 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 external to the crown reinforcement (20), a top layer (72), radially external to the base layer (71), and a groove base layer (73) radially internal to the tread (6). The thermo-viscoelastic properties of the layers of the sub-layer (7) satisfy: Tan (δ3) < 0.75xTan (δ4); and preferably 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 10% strain. Figure from the abstract: Figure 1

Description

Title of the invention: Tire optimized for rolling resistance without compromising road handling Scope of the invention

[0001] The present invention relates to a tire whose crown is optimized to achieve a favorable performance compromise in rolling resistance and road handling compared to conventional designs. Although not limited to this type of application, the invention is described more particularly with reference to a radial tire intended for mounting on passenger cars or light trucks. Definitions

[0002] By convention, we consider a frame (O, OX, OY, OZ), whose center O coincides with the geometric center of the tire, the circumferential direction OX, axial direction OY, and radial direction OZ 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 inside, respectively radially outside, we mean closer, respectively further from the axis of rotation of the tire.

[0004] By axially inside, respectively axially outside, we mean 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 construction 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, as a first approximation, by the axisymmetry of the geometry of the tire around its axis of rotation.

[0006] A tire includes a crown, intended to come into contact with a ground by means of a tread, the two axial ends of which are connected by means of two sidewalls to 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 consists of parallel reinforcements between them and coated by 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 relevant to the invention usually comprises at least one carcass layer made up of metallic or textile reinforcing elements, each coated in an elastomeric coating compound. This at least one carcass layer comprises a main portion, connecting the two beads to each other and winding, within each bead, around an annular reinforcing structure, which is most often a bead.

[0010] An elastomeric blend is defined as an elastomeric material obtained by mixing its various constituents. An elastomeric blend typically comprises an elastomeric matrix with at least one diene elastomer of the natural or synthetic rubber type, at least one reinforcing filler of the carbon black and / or silica type, a crosslinking system most often sulfur-based, and protective agents. For certain applications, the elastomers considered may also include thermoplastics (TPEs).

[0011] The expression "based on" composition means a composition comprising the mixture and / or 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 manufacturing the composition, in particular during its crosslinking or vulcanization.

[0012] The expression "part by weight per hundred parts by weight of elastomer" (or pce) is to be understood in the context of the present invention as the part, by mass per hundred parts of elastomer present in the rubber composition considered.

[0013] An elastomeric blend can be mechanically characterized, particularly after curing, by its dynamic properties, such as a dynamic shear modulus G* = (G'² + G”²)l / ², where G' is the elastic shear stiffness modulus and G” is the viscous shear modulus, and a dynamic loss Tan(θ) = G” / G'. The dynamic shear modulus G* and the dynamic loss Tan(θ) are measured on a Metravib VA4000 viscoalyzer, according to ASTM D 5992-96. The response of a vulcanized elastomeric blend sample in the form of a sinusoidal alternating simple shear load, at a frequency of 10 Hz, at a temperature of 100°C, is recorded. A strain amplitude sweep is performed 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 is indicated, denoted Tan(ô)max.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 make it possible to determine the stresses Elasticity and tensile strength are measured. Unless otherwise specified, these measurements 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) are measured at 10% elongation (denoted "MA10") and 100% elongation ("MA100") at the second elongation (i.e., after one accommodation cycle). All tensile measurements are performed under normal temperature (23±2°C) and humidity (50±5% relative humidity) conditions, according to French standard NF T 40-101 (December 1979). Tensile stresses (in MPa) and elongations at break (in %) are also measured at a temperature of 23°C. Previous technique

[0015] A person skilled in the art, a tire designer, knows that the expected functions of a tire are at least threefold. First, it must carry the load resulting from the vehicle's mass and all the additional loads related to the vehicle's dynamic movements, as well as any aerodynamic loads at high speed. Second, it must be able to guide the vehicle along the trajectories chosen by the driver, and finally, it must transmit to the road the acceleration or braking forces chosen by the driver.

[0016] The crown reinforcement is an essential element that contributes decisively to the three functions of supporting, guiding, and transmitting. In a typical design, said crown reinforcement, with at least two cross-laminated metal layers, encircles the carcass reinforcement to provide the tire with the necessary strength to perform its supporting function.

[0017] The Guidance function is also known as "road behavior." This refers to the responses of a vehicle / tire assembly to various driver inputs (steering, acceleration, braking, etc.). This behavior is essential both for safety, ensuring vehicle stability, 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 its trajectory, a force equivalent to (but in the opposite direction to) the centrifugal force that tends to eject the vehicle from its trajectory must be generated. This lateral force must be generated by the vehicle's four tires to overcome the centrifugal force.

[0020] The deformation of the tread blocks in contact with the ground generates a lateral force. The mechanism that allows the tire to deform the tread blocks when cornering is called 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 tread blocks and thus generate the necessary lateral forces.

[0021] Transverse drift stiffness is defined as the variation of the transverse forces generated in the contact area of ​​the moving tire compressed by the load, as a function of the drift angle applied to the tire. Transverse drift stiffness is expressed in Newtons per degree (N / °).

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

[0023] Transverse drift stiffness is an essential mechanical quantity that links the tire to the vehicle and determines the quality of the vehicle's behavior on the road.

[0024] Rolling resistance is another performance characteristic addressed in the invention. Rolling resistance is one of the forces that oppose the forward motion of the vehicle. The rolling resistance coefficient of a tire (Crr) is the rolling resistance force expressed per unit load on the tire. The coefficient is expressed in kg / t.

[0025] Rolling resistance is essentially linked to tire deformation. For example, the sidewall beads account for 20% to 30% of the tire's rolling resistance, while the tread contributes 60% to 80%.

[0026] Reducing greenhouse gas emissions from transportation is one of the major challenges facing vehicle manufacturers today. Tires represent a significant source of progress through a reduction in rolling resistance, as this has a direct impact on vehicle fuel consumption. For example, a 20% reduction in the rolling resistance of a passenger car tire can save approximately 3% of fuel per 100 km in the combined cycle.

[0027] The choice of tread pattern plays a crucial role in establishing a compromise between road handling and rolling resistance. Among the tire design parameters, those skilled in the art are familiar with tread patterns made up of a stack of sub-layers with thermomechanical and geometric properties appropriate to each sub-layer. An example of a sub-layer, i.e., a layer of rubber inserted between the crown reinforcement and the tread material, is described in document FR 2 954 333.

[0028] Another example where this time the constituent material of the sub-layer is flush The bottom of the tread grooves is given by document EP 2 865 543. In general, underlayer materials are used under the tread to improve the rolling resistance of the tire with a low hysteresis material, or to stiffen the tread in shear, but with modest stiffnesses 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 worse the tire's drift response under steering input. Schematically, the radial stacking of rubber layers outside the crown reinforcement can be considered as a series of springs. This is why materials with too low a modulus are avoided to prevent compromising 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 well below 8 MPa, even when the best handling performance is sought.

[0030] According to document WO 2015 / 170615, a tire comprising a base layer, i.e., a sub-layer, formed by two radially superimposed materials is also known. The modulus of the tread material and its tangent value (Δ) are lower than the values ​​of the same parameters of the sub-layer material in contact with the tread material, i.e., the outermost of the two radially outermost layers. The modulus of the material of the radially innermost layer of the sub-layer materials and its tangent value are lower than the values ​​of the same parameters of the sub-layer material in contact with the tread material. However, a tire manufactured according to this teaching does not provide any improvement in performance balance.

[0031] Document WO2019 / 145621 discloses a tread positioned radially externally to a sub-layer, which itself rests radially externally on a crown reinforcement. This sub-layer consists of a first "soft" layer, i.e., with a dynamic stiffness modulus G* less than or equal to 0.6 MPa, laid radially externally on the crown reinforcement. This sub-layer also includes a second "rigid" layer laid 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 provides improved rolling resistance and handling compared to a tire of a different design. usual.

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

[0033] This goal has been achieved by a motor vehicle tire comprising a crown having a crown reinforcement, a sub-layer, and a tread; said sub-layer having 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; 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 cover 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 groove bottom layer of elastomeric compound is positioned radially internally to the outer profile of the tread, in that the viscoelastic loss Tan (ô3) of said groove bottom layer of elastomeric compound is such that Tan (ô3) < 0.75xTan (ô4), and preferably Tan (ô3) < 0.5xTan (ô4) with Gl*, G2*, G3*, and G4*, Tan (ô1), 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% strain, G3* being the dynamic elastic shear modulus of the groove bottom layer (73).

[0034] The crown of the tire is defined as an area comprising the tread, the sub-layer, and the crown reinforcement, including the composite layers that encircle the carcass reinforcement. This description is made in the direction from the outside to the inside of the tire.

[0035] A groove is defined as a cut in the tread, substantially circumferential over one wheel revolution, the distance between the material walls that delimit it being greater than 2 mm and the depth of which is greater than or equal to 1 mm. In a meridian cross-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 externally. 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 itself is composed of three layers, two of which are radially superimposed. A first base layer is applied radially outwardly to the crown reinforcement, and said base layer has a dynamic shear modulus of 1.5 MPa or less. The base layer is therefore referred to as a "soft" layer for the purposes of this application. A second overlay layer is positioned radially outwardly on the base layer. This second layer has a dynamic shear modulus of 5 MPa or more and is referred to as a "rigid" layer. The invention provides for a third layer, the tread base layer, which is most often positioned in sections under the grooves in the axial direction, or in various configurations depending on the desired performance compromise.In terms of viscoelastic properties, 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 is to say that the groove base layer is less hysteretic than the tread, which is sufficient to obtain a perceptible decrease in the rolling resistance of the tire.

[0037] The operating principle of the invention with this multilayered top layer composed of three sublayers consists of designing an elastomeric compound for the groove bottoms with low hysteresis, while having a rigidity at least comparable to that of a tread. Since the groove bottom compound layer is never in contact with the pavement, it is therefore possible to avoid the constraints of a high-performance composition in terms of adhesion and / or wear. Consequently, the guidelines for designing the groove bottom compound are: • Search for a low-hysteresis compound that dissipates less heat than a tread compound while exhibiting a dynamic elastic shear modulus close to that of said tread compound. The advantage of such a solution is to provide a gain with the same road behavior; • Look for a low-hysteresis compound that is also suitable for outdoor use, particularly due to its resistance to ozone from the ambient environment, or a compound with good resistance to oxidation and stone chipping. Such a compound will also provide a real advantage in rolling resistance and endurance, unlike a standard base coat compound. • Look for a mixture already existing in the materials used for the fa Tire construction, for example, using a sidewall design that would have low rolling resistance. The expected bonus here, in addition to the reduction in rolling resistance of the tire, is also an improvement in industrial performance by strengthening the standardization of the materials used.

[0038] The main features 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 tread layers, and thanks to an appropriate radial distribution of said layers.

[0039] Other features related to different embodiments of the invention contribute to further improving the performance compromise of the tire. Most often, these features relate to the geometric and / or thermomechanical properties of the tread base layer.

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

[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. 1].

[0042] Advantageously, the tread comprising 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 cover layer is axially continuous between the two shoulders.

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

[0045] According to a variant of this second embodiment, the tread having 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 below the grooves. This configuration is shown in [Fig. 6]-C.

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

[0047] Advantageously, the tread comprising a number of NBS grooves, ESCi being the radial thickness of the elastomeric compounds 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 inward, 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, E73, taken in a groove at a point in the middle of its radially outer contour, is within the range [20%; 100%] of ESC, preferably within the range [50%;[100%], 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 line passing through said first point. ;

[0048] 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. This composite layer consists of parallel reinforcements embedded in an elastomeric compound. Here, the first composite layer is understood as extending radially from the outside to the inside of the tire. Of course, the value of ESCi may differ from one groove to another, but according to the invention, the maximum value of ESCi determines the radial thickness of the bottom layer of the groove.

[0049] Often this first composite layer encountered in the radial direction towards the inside of the tire is a band with fabric reinforcements, but in other cases this composite layer may include steel reinforcements, or Nylon coated in an elastomeric mixture.

[0050] The ESC value represents the distance between a reinforcement of the first composite layer encountered and the external environment of the tire. This thickness of elastomeric compound serves to protect the reinforcements of the first composite layer from the external environment of the tire. The external environmental stresses are of several kinds, such as the oxidation of metallic reinforcements by oxygen attack, or the rupture of textile reinforcements by stones that could become lodged in the tread grooves.

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

[0052] In order to improve rolling resistance and at the same time gain 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 tread blocks separated by grooves oriented essentially circumferentially, characterized in that the cover layer is, axially opposite certain tread blocks, extended radially outwards by at least one reinforcing element extending radially from the radially outer surface of the cover layer outwards from the tread to a radial height greater than 50% of the radial thickness of the tread, said reinforcing element having a 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.

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

[0054] The total volume of the elastomeric compound at the bottom of the groove corresponds to the sum of the surface area of ​​each section in the meridian plane, integrated over one wheel revolution. This compound has a dynamic shear elastic modulus greater than 1.25xGl*, which is sufficient to improve the transverse drift stiffness (OY axis direction), and therefore the road handling.

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

[0056] Preferably, the material of the groove bottom elastomeric blend layer has the same chemical composition as the flank layers.

[0057] In order to further optimize the tire's rolling resistance without increasing production costs, a tread base layer with the same chemical composition as the sidewall layers can be used. Since the tread base layer is not intended to be in contact with a road surface, its mechanical and viscoelastic properties are suitable for such use. Furthermore, based on its chemical composition, the sidewall has resistance to external ozone degradation that is well-suited to the tread base, which is also in contact with the ambient environment. Production costs are preserved because manufacturing standardization is enhanced by eliminating the need for a specific material for the tread base when the same material is used for the sidewall.

[0058] Preferably, the elastomeric blend of the groove base layer has a rubber composition based on at least one polyisoprene blend of ca natural rubber, and polybutadiene, a crosslinking system, and a reinforcing filler at an overall rate not exceeding 45%, and comprising carbon black, at a rate not exceeding 5%, and predominantly silica at a rate not exceeding 20% ​​and not exceeding 40%.

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

[0060] Alternatively, the elastomeric blend of the groove base layer has a rubber composition based on at least one polyisoprene cut of natural rubber, and polybutadiene, a crosslinking system, and a reinforcing filler at an overall rate of not more than 45 parts per annum, and comprising carbon black at a rate of not less than 20 parts per annum and not more than 40 parts per annum.

[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 features of the invention relate to the dynamic properties of the elastomeric mixtures of the tread layers.

[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 10% strain, 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 strain of 10% of the base layer material is less than or equal to 0.15.

[0065] Preferably, the dynamic shear modulus G2* of the cover layer material measured at 23 °C under alternating shear stress at a frequency of 10 Hz and 10% strain, 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 strain of 10% of the material of the cover layer is less than or equal to 0.35.

[0067] According to a fourth embodiment of the invention shown in [Fig. 4], the tread comprises at least one tread portion, hereinafter referred to as the tread wing, at at least one of the axial ends of the tread, said tread wing having an axially external contour delimited by a first point, located at the intersection of the axially and radially external contour of the sidewall and the axially and radially external contour of the tread, and a second point located at a curvilinear distance of between 5% and 25% of the nominal sidewall thickness of the tire size, said tread wing being radially inwardly in contact with the covering layer.

[0068] The presence of the tread wings further improves rolling resistance by replacing the extreme parts of the tread with a material having 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, under alternating shear stress at a frequency of 10 Hz and 10% strain; and the viscoelastic loss Tan(φ) of each wing is at most equal to 80% of the viscoelastic loss of the central portion of the tread, the viscoelastic loss Tan(φ) being measured at 23°C, under alternating shear stress at a frequency of 10 Hz and 10% strain. Brief description of the drawings

[0069] The present invention will be better understood upon reading the detailed description of embodiments taken by way of example, which are in no way limiting, and illustrated by the accompanying drawings in which: • Fig. 1 shows a schematic meridional cross-section of a tire conforming to a first embodiment of the invention, with a discontinuous groove base layer in sections positioned under the grooves. The top 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 cover layer is discontinuous in sections positioned under the grooves; • Figure 3 shows a schematic meridional cross-section of a tire according to a third embodiment of the invention, with a discontinuous base layer in sections positioned under the grooves. The top layer is also in sections but with wedge-shaped reinforcing elements emerging up to the tread; • Figure 4 shows a schematic representation of a fourth embodiment of the invention, incorporating embodiments 1 or 3, but with a tread wing positioned on either side of the equatorial plane. • [Fig.5] is a magnification of a portion of the top of [Fig.1] showing the main ribs involved in the invention. • [Fig.6] represents in view [Fig.6]-A apex 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 studied more particularly for a standardized passenger car tire designation, according to the ETRTO (European Technical Organisation for Wheels and Tyres) specification standard, 245 / 45 RI8 XL 100W.

[0071] In the various figures, identical or similar elements bear the same reference numerals. Given the symmetry of the tread, for the sake of readability of the figures, the elements are referenced only once on one side of a meridian plane.

[0072] Figure 1 shows a tire 1, an equatorial plane CP, two bead 50, and two sidewalls 3, each connected to a bead 50. The tire has a crown 4, which includes a crown reinforcement 20, a tread 6, and a sub-layer 7. The tread extends axially from one shoulder 60 to the other shoulder. The tread includes a contact face 61 intended to come into contact with the road surface during the rolling of the tire. The tread 6 has tread blocks 63 separated by grooves 62 oriented essentially circumferentially. Each groove 62 is radially delimited inwards by a groove bottom 620.

[0073] The bead 50 comprises at least one carcass layer, shown in [Fig. 1] in dashed lines. The carcass layer is formed of reinforcements coated with an elastomeric compound. The carcass layer comprises a main portion 53, connecting the two beads 50 to each other and wrapping around each bead, from the inside to the outside of the tire, around a circumferential reinforcement element, most often metallic, called a bead 51, to form a inversion 52. The reinforcements of a carcass layer are substantially parallel to each other and form an angle of between 85° and 95° with the circumferential direction.

[0074] The top reinforcement 20 comprises two crossed layers (22, 23) made of reinforcements embedded in an elastomeric compound. The reinforcements of the crossed layers (22, 23) form an angle of between 10° and 45° with the circumferential direction. A third reinforcement layer 23 is clamped to the two preceding layers. This layer 23 also comprises reinforcements embedded in an elastomeric compound, which form an angle of approximately ±2.5° with the circumferential direction.

[0075] The top also includes a sub-layer 7 arranged radially externally to the top reinforcement 20 and radially internally to the tread 6. This sub-layer is formed of three sub-layers: a base layer 71, a cover layer 72, and finally a groove bottom layer 73.

[0076] The base layer 71 is arranged radially directly on the top reinforcement 20. As known in itself, the top reinforcement comprises layers of monofilament cables or reinforcements generally coated with a thin layer of rubber. In the context of the present invention, by the fact that the base layer 71 is arranged radially directly on the top reinforcement 20, it is indicated that it is in contact with the cables or reinforcements, not taking into account their rubber coating.

[0077] The top layer 72 is arranged radially outward to the base layer 71. The bottom layer of the groove 73 is radially inward to 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 by mass (pce meaning percentage of the mass of elastomer).

[0081] An example of a suitable formulation for the cover 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 by mass (pce meaning percentage of the mass of elastomer).

[0084] As for the tread material, according to the invention, its rigidity is less than the rigidity of the top layer. Advantageously, the rubber compound 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]: 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 2 Antioxidant (t) 2 Accelerator (g) -> DPG 2 Sulfur 1

[0087] The formulations are given by mass (pc 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) "Zeosilll65MP" silica from the Solvay company, surface area BET 160m2 / g (c) Silane TESPT “SI69” from Evonik (d) TDAE oil “Flexon 630” from the Shell company (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 base layer 73, a formulation derived from that of a low hysteresis flank layer is perfectly suitable:

[0089] Table [4]: Compositions E-stomie iXatin ' Eiastsmère BR Filler Carbon black Filler cepi-CMs? Sice Coupling Agent Protective Agent Plasticizing Agent Mixture of Fhnc 50 50 3 £0 0 4 53 10

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

[0091] Figure 2 shows a schematic meridional cross-section of a tire according to a second embodiment of the invention comprising two parts symmetrical with respect to the equatorial plane CP. The tread base layer 73 is in sections positioned in each groove 62. The topcoat layer 72 is also in interrupted sections below the grooves 62. Each section of the tread base layer is intercalated between two sections of the topcoat layer 72. In each groove 62, the volume of the tread base mixture layer 73 is the surface extending from the radially outer contour of the tread base to the contour of the tread 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 layers 71, 72, and 73, are arranged on the top reinforcement 20 formed of the working and shrink-fit layers 21, 22, and 23 respectively.The tread 6 comprises 60 sculpted blocks, with 62 grooves arranged between the blocks. The groove base layer 73 is flush with the groove bottoms 620.

[0092] Figure 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, as well as the top layer 72, are in non-contiguous sections. Each section of layer 73 is positioned radially under a groove 62. Sections of the groove base layer 73 are arranged axially between two sections of the cover layer. The three layers of the sub-layer 7, namely layers 71, 72, and 73, are arranged on the top reinforcement 20 formed by the working and reinforcement layers 21, 22, and 23, respectively. The tread 6 comprises carving blocks 60, with grooves 62 arranged between the blocks. Upstream and downstream of each groove, a reinforcing element 630 extends from the cover layer 72 to reach the wearing surface 61. The groove base layer 73 is flush with the bottoms of grooves 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] Figure 5 is a magnification of a portion of the top of Figure 1, showing the principal ribs involved in the invention. The radial thicknesses of the sub-layer layers 7, namely layers 71, 72, and 73, with respective radial thicknesses E71, E72, and E73, can be seen. The tread 6 has a thickness EKM. There are also the sub-groove thicknesses ESC1 and ESC2, which measure the thicknesses of the compound layers from a point in the middle of the radially outer contour of the groove bottom to the first reinforcement 24 of a composite layer, proceeding radially from the outside to the inside of the tire 1. The top layers 21, 22, and 23, forming the top reinforcement 20, are reinforcements 24 embedded in an elastomeric matrix.

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

[0096] A person skilled in the art, a tire designer, may adopt variant 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 to those obtained for a reference tire T shown in [Fig. 6]-A.

[0098] The T indicator is a state-of-the-art tire intended to equip a vehicle This tire is designed for road use, with a reference pressure of 290 kPa and a size of 245 / 45 RI8 XL 100W. Its top layer comprises a crown reinforcement, a sub-layer, and a tread. The sub-layer consists of a first base layer, radially external to the crown reinforcement, and a second top layer, radially external to the base layer and radially internal to the tread. The base layer has a dynamic shear modulus Gl* of 0.5 MPa, and the top layer has a shear modulus G2* of 18 MPa.

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

[0100] This test tire T is compared to the PI and P2 tires of the invention, which are also intended to equip a passenger vehicle. The test tire T has a sublayer 7 comprising two layers 71 and 72, and the tread base compound is that of the tread 6. The PI tire has a third tread base layer having a dynamic elastic shear modulus G3* of 4.2 MPa, which therefore satisfies the characteristics of claim 1. Furthermore, the chemical composition of the tread base layer 73 is identical to that of the sidewalls 3. The P2 tire differs from PI by the presence of the tread base layer in sections positioned under the grooves between two portions of the top layer 72; said sections of the tread base layer 73 extend radially inwardly, until they touch a crown layer.The following table summarizes the differences between the T test, 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 resistance coefficient, based on 100 for the control tire. For PI and P2 tires, the variation is evaluated relative to the control tire's base of 100. The same is done for the transverse drift stiffness. SO means "not applicable" since the control tire does not have a third layer. of bottom-of-groove mixing.

[0103] It is easily verified that the tires of the invention PI, 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 the prior 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 degradation) of the performance considered.

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

Claims

Demands

1. A motor vehicle tire (1) comprising a crown (4) having a crown reinforcement (20), a sub-layer (7), and a tread (6); said sub-layer (7) having a first base layer (71), radially external to the crown reinforcement (20), and a second cover 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 cover 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 groove bottom elastomeric compound layer (73) is positioned radially internally to the outer profile of the tread (6), in that the viscoelastic loss Tan (ô3) of said groove bottom elastomeric compound layer is such that Tan (ô3) < 0.75xTan (ô4), and preferably Tan (ô3) < 0.5xTan (ô4) with Gl*, G2*, G3*, and G4*, Tan (ô1), 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% strain, G3* being the dynamic elastic shear modulus of the groove bottom layer (73).

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

3. Tire (1) according to any one of the preceding claims in which the tread (6) has 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. Pneumatic (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. Pneumatic (1) according to any one of claims 1 to 3, characterized in that that the cover layer (72) is axially continuous between the two shoulders (60).

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

7. Tire (1) according to any one of the preceding claims in which the tread (6) has 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 below the grooves (62).

8. Tire (1) according to any one of the preceding claims, the tread comprising a number of NBS grooves, ESCi being the radial thickness of the elastomeric compounds 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 inward, 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 within the range [20%; 100%] of ESC, and preferably E73 within the range [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 line passing through said first point.;

9. A tire (1) according to any one of the preceding claims, wherein the tread (6) comprises tread blocks (63) separated by grooves (62) oriented essentially circumferentially, characterized in that the cover 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 cover layer (72) outwards from 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. Pneumatic (1) according to any one of the preceding claims wherein the material of the groove bottom elastomeric blend layer (73) is of the same chemical composition as the sidewall layers (3).

11. Pneumatic (1) according to any one of claims 1 to 4, wherein the elastomeric blend of the groove base layer (73) has a rubber composition based on at least one polyisoprene cut of natural rubber, and polybutadiene, a crosslinking system, and a reinforcing filler at an overall rate not exceeding 45%, and comprising carbon black, at a rate not exceeding 5%, and predominantly silica, at a rate not exceeding 20% ​​and not exceeding 40%.

12. Pneumatic (1) according to any one of claims 1 to 4, wherein the groove bottom elastomeric compound (73) has a rubber composition based on at least one polyisoprene cut of natural rubber, and polybutadiene, a crosslinking system, and a reinforcing filler at an overall rate not exceeding 45%, and comprising carbon black at a rate not exceeding 20% ​​and not exceeding 40%.

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

14. Pneumatic (1) according to any one of the preceding claims, wherein the value of Tan (λ) measured at 23°C at 10 Hz and under an alternating shear strain of 10% of the base layer material (71) is less than or equal to 0.

15.

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

16. Pneumatic (1) according to any 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 strain of 10% of the material of the cover layer (72) is less than or equal to 0.

35.

17. Tire (1) according to any one of the preceding claims, wherein the tread (6) comprises at least one tread portion (80), hereinafter referred to as tread wing (80), at at least one of the axial ends of the tread, said tread wing (80) having an axially external contour delimited by a first point, located at the intersection of the axially and radially external contour of the sidewall and the axially and radially external 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.

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 alternating shear stress at a frequency of 10 Hz and at 10% strain.

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% strain.

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