Tyre optimised for driving performance and rolling resistance

Abstract

The invention relates to a tyre (1) provided with a crown (4) including a crown reinforcement (20), an underlayer (7), and a tread (6). The underlayer (7) comprises a first base layer (71), radially outward of the crown reinforcement (20), and a second covering layer (72), radially outward of the first base layer (71) and radially inward of the tread (6). The first base layer has a dynamic shear modulus G1* of 1.5 MPa or less, while the second covering layer has a dynamic shear modulus of 5 MPa or more. The thicknesses of the second covering layer (72), E72, and the maximum undercut thickness, ESC, are related by the relationship: E72 (mm) is greater than or equal to (0.53 * ESC + 0.1) and E72 (mm) is less than or equal to (0.47 * esc + 1.1), ESC being between [1; 3.5] mm.

Description

Tire optimized for road behavior and rolling resistance

[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.

[0002] By convention, we consider a reference (O, OX, OY, OZ), whose center O coincides with the geometric center of the tire, the circumferential directions OX, axial OY, and radial OZ respectively designate a direction tangent to the rolling surface of the tire according to the direction of rotation, a direction parallel to the axis of rotation of the tire, and a direction orthogonal to the axis of rotation of the tire.

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

[0004] Axially inner, respectively axially outer, means closer, respectively further from the equatorial plane of the tire, the equatorial plane of the tire being the plane passing through the middle of the tire tread and perpendicular to the axis of rotation of the tire.

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

[0006] A tire comprises a crown, intended to come into contact with the ground via a tread, the two axial ends of which are connected via two sidewalls with two beads ensuring the mechanical connection between the tire and the rim on which it is intended to be mounted.

[0007] A radial tire further comprises a reinforcing reinforcement, comprising 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 comprises reinforcements parallel to each other and coated by a polymeric material of the elastomer type or elastomeric mixture. The assembly comprising 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 comprising 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] An elastomeric blend is an elastomeric material obtained by mixing its various constituents. An elastomeric blend typically comprises an elastomeric matrix with at least one diene elastomer of natural or synthetic rubber type, at least one reinforcing filler of carbon black type and / or silica type, a crosslinking system most often based on sulfur, and protective agents. For certain applications, the elastomers considered may also include thermoplastics (TPE).

[0011] The expression "based on" composition means a composition comprising the mixture and / or the reaction product of the different constituents used, some of these basic constituents being capable of, or intended to, react with each other, at least in part, during the different phases of manufacture of the composition, in particular during its crosslinking or vulcanization.

[0012] By the expression "part by weight per hundred parts by weight of elastomer" (or pce), it is meant, within the meaning of the present invention, the part, by mass per hundred parts of elastomer present in the rubber composition considered.

[0013] An elastomeric mixture can be characterized mechanically, in particular after curing, by its dynamic properties, such as a dynamic shear modulus G*= (G'2+G”2)l / 2, where G' is the elastic modulus of shear stiffness and G” the viscous shear modulus, and a viscoelastic loss tô=G” / G'. The dynamic shear modulus G* and the viscoelastic loss tg ô are measured on a Metravib VA4000 type viscoanalyzer, according to ASTM D 5992-96 standard. The response of a sample of vulcanized elastomeric mixture is recorded, at a temperature of 23 °C, in the form of a sinusoidal stress in alternating simple shear, 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% to 0.1% (return cycle). For the forward cycle, the maximum value of tô observed, noted Tômax, is indicated. On this same forward cycle, the value of the dynamic shear modulus G* is indicated. Prior art

[0014] The skilled person, a tire designer, knows that the expected functions of a tire are at least three in number. Firstly, it is a question of carrying the load, resulting from the mass of the vehicle and all the overloads linked to the dynamic movements of the vehicle as well as any aerodynamic overloads at high speed. Secondly, it is necessary to be able to guide the vehicle on the trajectories decided by the driver, and finally, it is necessary to transmit to the ground the acceleration or braking forces decided by the driver.

[0015] The crown reinforcement is an essential element that contributes decisively to the three functions of carrying, guiding, and transmitting. In conventional design, the said crown reinforcement with at least two crossed metal layers surrounds the carcass reinforcement to provide the tire with the necessary strength to fulfill its carrying function.

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

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

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

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

[0020] Transverse drift stiffness is the variation of 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 / °).

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

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

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

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

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

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

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

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

[0029] Document WO 2015 / 170615 also discloses a tire comprising a base layer, i.e. a sub-layer, formed by two materials superimposed radially. The dynamic shear modulus of the tread material and the viscoelastic loss tg 5 (delta tangent) thereof are lower than the values ​​of the same parameters of the sub-layer material in contact with the tread material, i.e. of the radially outermost of the two layers. The dynamic shear modulus of the material of the radially inner layer of the sub-layer materials and the viscoelastic loss tg 5 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.

[0030] Document WO2019 / 145621 discloses a tread positioned radially outwardly to a sub-layer, which itself rests radially outwardly on a crown reinforcement. Said sub-layer consists of a first "soft" layer, i.e. with a dynamic shear modulus G* less than or equal to 1.5 MPa, placed radially outwardly on the crown reinforcement. Said sub-layer also comprises a second "rigid" layer placed radially outwardly to the soft layer, and radially inwardly to the tread. The dynamic shear modulus of the rigid layer, greater than 7 MPa, is greater than the shear modulus tread dynamics. Such a tread configuration provides improved results in rolling resistance and road handling compared to a conventionally designed tire.

[0031] The inventors set themselves the objective of identifying other levers, in this case linked to the geometric properties of the layers of the sub-layer, to further improve the compromise between rolling resistance and drift stiffness performance of the design of a reference tire cited above (WO2019 / 145621).

[0032] This aim has been achieved by a tire for a motor vehicle comprising a crown comprising a crown reinforcement, a sub-layer, and a tread; 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 first base layer and radially internal to the tread; said first base layer having a dynamic shear modulus Gl* less than or equal to 1.5 MPa, and the second covering layer having a dynamic shear modulus G2* greater than or equal to 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; a groove of the tread being a cutout, 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;E72 being the radial thickness of the second covering layer over at least 75% of its axial width, measured at a point on the radially outer contour of the second covering layer in a meridian plane; ESCi being the radial thickness of the elastomeric compound at the bottom of a groove i, measured in the middle of a bottom contour of said radially outer groove i, and ESC being the maximum value of ESCi, for i ranging from 1 to NBS, where the NBS is the number of grooves in the tread, said tire has a thickness E72 (in mm) greater than or equal to (0.53*ESC + 0.1) a thickness E72 (in mm) less than or equal to (0.47*ESC + 1.1), and such that ESC is between [1; 3.5] mm.;

[0033] The crown of the tire is an area that includes the tread, the underlay and the crown reinforcement, which includes the composite layers that surround the carcass reinforcement. This description is made in the direction from the outside to the inside of the tire.

[0034] By groove is meant a cut in the tread, substantially circumferential over one turn of the wheel, the distance between the walls of material which delimit it is greater than 2 mm and whose depth 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 outwardly. The tread comprises NBS grooves, most often NBS is 3 or 4 depending on the axial width of the tread.

[0035] 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 ESCi determines the radial thickness of the second covering layer.

[0036] The radial thickness of the second covering layer, E72 is measured between a first point located at the intersection of a radially outer contour of the second 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 second covering layer with said radial axis OZ. A meridian plane being a plane containing the axis of rotation of the tire.

[0037] 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 include steel or nylon reinforcements coated in an elastomeric mixture.

[0038] The ESC value represents the distance between a reinforcement of the first composite layer encountered and the external environment of the tire. This thickness of elastomeric mixture serves to protect the reinforcements of the first composite layer encountered from the external environment of the tire. Aggressions from the external environment are of several types, such as the oxidation of metal reinforcements by the attack of oxygen, or the rupture of textile reinforcements by stones which could get into the grooves.

[0039] 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 rolling resistance.

[0040] A tire of the invention comprises a tread having a sub-layer, which is itself composed of two radially superimposed layers. A first base layer is placed radially outwardly on the crown reinforcement, and said first base layer has a dynamic shear modulus less than or equal to 1.5 MPa. The first base layer is therefore a so-called "soft" layer for the present application. A second covering layer is positioned radially outwardly on the first base layer. This second covering layer has a dynamic shear modulus greater than or equal to 5 MPa and is called "rigid". The invention proposes a range of permissible thicknesses of the second covering layer to achieve an advantageous compromise of performance in road handling and rolling resistance.Radially external to the second covering layer, the tread which represents the largest volume of elastomeric mixture at the top and intended to come into contact with the ground during rotation of the tire, sees its thickness vary according to the choice made for the thickness of the second covering layer, however the total thickness of the sub-layer and the tread remains constant.

[0041] One of the objectives of the invention is to generalize the operation of the technical solution to tire ranges comprising several dimensions. The range of admissible thicknesses for the second covering layer makes it possible to cover all dimensions by adapting the thickness of the second covering layer 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 second covering layer, base layer and tread are in a homothetic ratio to make the invention work. In addition, the invention makes it possible to absorb the differences in thicknesses of the base and covering layers during manufacturing, so as to further guarantee the improvements obtained.

[0042] The inventors assume that the sum of the thicknesses of the tread, base layers and overlay is constant. Therefore, the balance of performance in road handling and rolling resistance can be controlled by a distribution of layer thicknesses and adapted thermomechanical properties.

[0043] By increasing the thickness of the second covering layer, the tread thickness is reduced accordingly. The balance of viscoelastic losses of the second covering layer and the tread shows a beneficial effect on rolling resistance: the dissipation of the second covering layer is lower than the reduction obtained in the dissipation of the tread due to its reduced thickness. This results in an overall reduction in rolling resistance for the tire, in parallel with an increase in the drift stiffness. [0044JI1 it is appropriate for each tire dimension to establish the distribution of suitable radial thicknesses between the different layers of the crown to obtain the desired performance compromise.

[0045] The main characteristics of the invention lead 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 layers of the tread.

[0046] Other characteristics, linked to different embodiments of the invention, contribute to further improving the performance compromise of the tire. Most often, these characteristics concern other geometric characteristics, but especially the mechanical and viscoelastic properties of the crown layers.

[0047] In a particular and optional embodiment, the tire comprises two sidewalls, each sidewall having a sidewall layer, advantageously, the distance EAXE from a point of the axial end of the second covering layer to the outer contour of the sidewall layer on the same side of a meridian plane is included in the interval [5%; 15%] of the nominal bead size of the tire, said nominal bead size being indicated in the designation of the tire dimension.

[0048] For example, for a tire size designated by the following name: "205 / 65 R16", the nominal section 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.

[0049] This interval [5%; 15%] is used to delimit the axial width of the second covering layer laid radially externally on the first base layer. The second The overlay 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.

[0050] In addition to the thickness seen previously, the width of the second covering layer contributes to the performance compromise between road behavior and rolling resistance. In the solutions tested, the distance from the end of the second 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 to avoid degrading the drift stiffness.

[0051]

[0052] In a first particular configuration, the dynamic shear modulus Gl* is less than or equal to 1 MPa and preferably 0.6 MPa. These are conventional values ​​for sub-layers of usual configuration, i.e. single-material and extending axially from one shoulder to the other of the tire.

[0053] In a second particular configuration and advantageously, the viscoelastic loss tg 51 measured at 23°C at 10 Hz and under an alternating shear deformation of 10% of the material of the first base layer is less than or equal to 0.15.

[0054] In a third particular configuration, the dynamic shear modulus G2*, measured at 23°C, and under an alternating shear stress at a frequency of 10 Hz and at 10% deformation, is greater than or equal to 7 MPa and preferably 12 MPa, and very preferably 20 MPa. In this way, good drift stiffness 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.

[0055] In a fourth particular configuration and advantageously, the viscoelastic loss tg 52 measured at 23°C at 10 Hz and under an alternating shear deformation of 10% of the material of the second covering layer is less than or equal to 0.35.

[0056] According to a first embodiment illustrated by figures 1 and 2, the tire comprising two shoulders, the second covering layer is axially continuous between the two shoulders.

[0057] According to a second embodiment illustrated in Figure 3, the tread comprises grooves oriented essentially circumferentially and a second covering layer is formed of several axially separated sections, said second covering layer being interrupted under the grooves. Overall, this second embodiment makes it possible to significantly improve rolling resistance at the cost of a very modest degradation of the drift rigidity.

[0058] By "under the furrows" we mean a region bounded by two axial limits, the two axial limits being determined, in a meridian section plane, by a straight line extending the principal direction of each of the radial walls of the furrow considered. Thus we can have an interruption over the entire axial width of the region, or over a part of the axial width of the region.

[0059] According to a variant of the second embodiment, the material appearing at the bottom of the groove is the same as the material of the tread, preferably at the location of the interruption.

[0060] According to a third embodiment, illustrated in figure 4 and very close to that illustrated by figure 3: the material appearing at the bottom of the groove is the same as the material of the first base layer, preferably at the location of the interruption.

[0061] According to a fourth embodiment and 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. Figure 5 illustrates the fourth embodiment in which a second covering layer is, axially opposite certain tread blocks, extended radially outwards by at least one reinforcing element, for example wedge-shaped (seen in meridian section). This reinforcing element extends radially from the radially outer surface of the second 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 sculpture block, said axial width decreasing when moving radially upwards.

[0062] Preferably, 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°.

[0063] Preferably, the tread comprises such a reinforcing element on either side of each groove.

[0064] Preferably, each reinforcing element advantageously comprises the same rubber mixture as the second covering layer, which allows them to be extruded in a single operation with the second covering layer. Under these conditions, the thickness of the second covering layer E72 is measured under the sculpture blocks.

[0065] According to another embodiment, the tire comprising two sidewalls, each sidewall having a sidewall layer, 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 one of the or each sidewall layer 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.The presence of tread wings allows rolling resistance to be further improved by replacing the outer portions of the tread, preferably with a material of lower hysteresis than the material of the central portion of the tread.

[0066] It is thus advantageous 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.

[0067] According to another advantageous embodiment, the viscoelastic loss tg 5 of each wing of the tread is at most equal to 80% of the viscoelastic loss of the central portion of the tread, the viscoelastic loss tg 5 of the wing of the tread being measured at 23°C, and under an alternating shear stress at a frequency of 10 Hz and at 10% deformation.

[0068] 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: Figure 1 shows a schematic meridian section of a tire according to a first embodiment of the invention; Figure 2 is a partial enlargement of Figure 1 showing the main geometric dimensions; - figure 3 shows a schematic meridian section of a tire according to a second embodiment of the invention; - Figure 4 is a simplified representation of a third embodiment of the invention; - Figure 5 shows a simplified representation of a fourth embodiment of the invention; - Figure 6 shows evaluation results of tires of the invention. It includes two graphs 6-A and 6-B with respectively results of drift stiffness and rolling resistance, as a function of the thickness of the second covering layer 72.

[0069] The invention has been more particularly studied for a 1 passenger car tire of standard designation, according to the specification standard of the ETRTO (European Technical Organization for Rims and Tires), 245 / 45 RI 8 XL 100W.

[0070] In the various figures, identical or similar elements bear the same references. Given the symmetry of the tread, for the readability of the figures, the elements are referenced only once on one side of a meridian plane.

[0071] Figure 1 shows a tire 1, an equatorial plane CP, two beads 50 and two sidewalls 3 each connected to a bead 50. The tire 1 has a crown 4, which has a crown reinforcement 20 and a tread 6. The tread 6 extends axially from one shoulder 60 to the other shoulder. The tread 6 includes a contact face 61 intended to come into contact with the roadway during the rolling of the tire 1. The tread 6 includes tread blocks 63 separated by grooves 62 oriented essentially circumferentially. Each groove 62 is delimited radially inwards by a groove bottom 620.

[0072] The bead 50 comprises at least one carcass layer, shown in FIG. 1 in broken lines. The carcass layer is formed of reinforcements coated with an elastomeric mixture. The carcass layer comprises a main part 53, connecting the two beads 50 together and is wound in each bead, from the inside to the outside of the tire 1 around a circumferential reinforcement element, most often metallic. called bead wire 51, to form a turn-up 52. The reinforcements of a carcass layer are substantially parallel to each other and form, with the circumferential direction, an angle of between 85° and 95°.

[0073] The crown reinforcement 20 comprises two crossed layers (22, 23) comprising 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.

[0074] The crown 4 also comprises a sub-layer 7 arranged radially outside the crown reinforcement 20 and radially inside the tread 6. This sub-layer 7 is formed of two parts: a first base layer 71 and a second covering layer 72.

[0075] The first base layer 71 is arranged radially directly on the crown reinforcement 20. As known per se, the crown reinforcement 20 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 first 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.

[0076] An example of a suitable formulation for the material of the first base layer 71, with a dynamic shear modulus Gl* of 0.2 MPa, is as follows:

[0077] [Table 1]:

[0078] The formulations are given in mass (pce meaning percentage of the mass of elastomer).

[0079] An example of a suitable formulation for the material of the second covering layer 72 having a dynamic shear modulus G2* of 25 MPa is as follows

[0080] [Table 2]:

[0081] The formulations are given in mass (pce meaning percentage of the mass of elastomer).

[0082] As for the dynamic shear modulus G3* of the tread, G3* is less than G2* of the second covering layer 72. Advantageously, G3* is less than or equal to 4.0 MPa and preferably 2.5 MPa.

[0083] The following Table 3 gives an example of tread formulation 6:

[0084] [Table 3]:

[0085] The formulations are given in mass (pce meaning percentage of the mass of elastomer) with: (a) SBR with 27% styrene, butadiene -1.2:5%, cis-1.4:15%, trans-1.4:80% Tg -48°C (b) Silica “Zeosill l65MP” from Solvay with BET surface area of ​​160m2 / g (c) TESPT silane “SI69” from Evonik (d) TDAE oil "Flexon 630" from Shell (e) Exxon's "Escorez 2173" resin (f) Antioxidant "Santoflex 6PPD" from Solutia (g) Accelerator "Santocure CBS" of the company Solutia.

[0086] Figure 2 shows geometric dimensions. The thicknesses of the base and covering layers E72 and E71 respectively can be seen. The tread 6 has a thickness EKM. The distance from an axial end of the second covering layer 72 to the radially and axially outer contour of the sidewall on the same side of a meridian plane is EAXE. The tire has four grooves. Figure 2 shows the under-tread thicknesses at the bottoms of the grooves, ESC1 and ESC2. The distances ESC1, ESC2 are measured from the point in the middle of the groove bottom curve to the reinforcement 24 of the first composite layer encountered, going from the outside to the inside of the crown 4.

[0087] Figures 3 and 4 illustrate similar embodiments. The second covering layer 72 comprises several sections interrupted at the grooves, here consisting of several sections interrupted at the grooves. For the second embodiment illustrated in Figure 3, the bottoms of the grooves (62a, 62b) are made of the same material as the strip of bearing 6, whereas for a third embodiment the bottoms of the grooves (62a, 62b) are made of the same material as the first base layer 71.

[0088] Figure 5 shows the second covering layer 72 with the reinforcing elements 630, which border the radial walls of the grooves 62. Some blocks 63 are also bordered by the reinforcing elements 630.

[0089] The person skilled in the art, a tire designer, will be able to adopt alternative embodiments in which the tread 6 itself comprises several different materials, superimposed radially and / or juxtaposed axially.

[0090] A tire 1 of the invention, PI, was evaluated to clearly highlight the performance provided by the invention. The results of the evaluation tests are compared with those obtained for a control tire T.

[0091] Witness T is a state-of-the-art tire intended to equip a passenger vehicle, with a reference pressure of 250 kPa. Its crown comprises a crown reinforcement, a sub-layer, and a tread. The sub-layer comprises a first base layer, radially external to the crown reinforcement, and a second covering layer, radially external to the first base layer and radially internal to the tread. The first base layer has a dynamic shear modulus Gl* equal to 0.2 MPa, and as for the second covering layer, its dynamic shear modulus G2* is equal to 25 MPa.

[0092] Witness T has 4 grooves symmetrical with respect to the axis (CP). For each groove, the under-dip thickness, ESC, is 2.5 mm. The undercoat includes a second covering layer with a thickness of E72 of 0.8 mm, and the first base layer has a thickness of 2 mm.

[0093] This witness T is compared to a tire P of the invention also intended to equip a passenger vehicle. The tire P differs from the witness T, in that the undercut thickness is 1.7 mm, and the covering layer has a thickness of 1.3 mm. In addition, the first base layer of the tire P is 1 mm. The following table summarizes the differences between the witness T, and the tire P of the invention:

[0094] [Table 4]

[0095] It is easily verified that the tire P of the invention is indeed covered by claim 1.

[0096] Rolling resistance and drift stiffness results were evaluated for tires of the invention and of the state of the art according to the two configurations.

[0097] Rolling resistance tests were carried out according to ISO 28580. For a tested tire, the result is the rolling resistance coefficient which represents the ratio of the force resisting the vehicle's forward movement by tire hysteresis divided by the load carried.

[0098] The transverse drift stiffness measurements were measured on dedicated measuring machines such as those marketed by the company MTS.

[0099] Results are also accessible by simulation means, for example by the finite element method.

[0100] A result above (or below) 100% means an improvement (or deterioration) in the performance considered.

[0101] Graph 6-A represents the evolution of the deviation in the drift stiffness (DZ) between the tire of the invention and the control as a function of the thickness E72 of the second covering layer 72. On the abscissa we have the thickness of the second covering layer 72 and on the ordinate the deviation in drift stiffness. The drift stiffness is linked to the road behavior of the vehicle. In the conditions considered here, the road behavior of the vehicle is all the better as the drift stiffness is high.

[0102] Graph 6-B represents the evolution of the difference in the rolling resistance coefficient RRT between the tire of the invention and the control as a function of the thickness of the second covering layer 72. On the abscissa we have the thickness of the second covering layer 72 and on the ordinate the difference in the rolling resistance coefficients. The rolling resistance of the tire of the invention is improved.

[0103] This example confirms that the claimed relationship between the thickness of the second covering layer 72 and the under-tread thickness leads to good operation of the tire of the invention with improved rolling resistance and drift stiffness. Against all expectations, the increase in the thickness of the second covering layer 72 does not result in a degradation of the rolling resistance, even with a dynamic shear modulus G2* equal to 25 MPa for the second covering layer 72, as can be seen in graphs 6-B. Concomitantly, the drift stiffness D(Z) increases with the increase in this same graphic thickness 6-A.

Claims

CLAIMS

1. A tire (1) for a motor vehicle comprising a crown (4) comprising a crown reinforcement (20), a sub-layer (7), and a tread (6); said sub-layer (7) being provided with a first base layer (71), radially external to the crown reinforcement (20), and a second covering layer (72), radially external to the first base layer (71) and radially internal to the tread (6); said first base layer (71) having a dynamic shear modulus Gl* less than or equal to 1.5 MPa, and the second covering layer (72) having a dynamic shear modulus G2* greater than or equal to 5 MPa; Gl* and G2* being measured at 23°C under alternating shear stress at a frequency of 10 Hz and at 10% deformation;a groove (62) of the tread (6) being a cutout, the distance between the walls of material which delimit it being greater than 2 mm and the depth of which is greater than or equal to 1 mm; E72 being the radial thickness of the second covering layer (72) over at least 75% of its axial width, measured at a point on the radially outer contour of the second covering layer (72) in a meridian plane; ESCi being the radial thickness of the elastomeric mixture at the bottom of a groove i, measured in the middle of a bottom contour of said groove i radially outer, 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 (mm) is greater than or equal to (0.53*ESC + 0.1), and in that F thickness E72 (mm) is less than or equal to (0.47*Esc + 1.1) and in that ESC is between [1; 3.5] mm.;

2. A tire (1) according to the preceding claim, comprising two sidewalls, each sidewall having a sidewall layer (3), wherein the distance EAXE from a point of the axial end of the second covering layer (72) to the outer contour of the sidewall layer (3) on the same side of a meridian plane is within 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).

3. Tire (1) according to any one of the preceding claims, in which the dynamic shear modulus Gl* is less than or equal to 1 MPa, and preferably 0.6 MPa.

4. A tire (1) according to any preceding claim, wherein the viscoelastic loss tg 51, measured at 23°C at 10 Hz and under an alternating shear deformation of 10% of the material, of the first base layer (71) is less than or equal to 0.

15.

5. Tire (1) according to any one of the preceding claims, in which the dynamic shear modulus G2* is greater than or equal to 7 MPa, preferably 12 MPa, and very preferably 20 MPa.

6. Tire (1) according to the preceding claim, in which the viscoelastic loss tg 52, measured at 23°C at 10 Hz and under an alternating shear deformation of 10% of the material, of the second covering layer (72) is less than or equal to 0.

35.

7. A tire (1) according to any one of claims 1 to 6, comprising two shoulders (60), wherein the second covering layer (72) is axially continuous between the two shoulders (60).

8. A tire (1) according to any one of claims 1 to 6, wherein the tread (6) comprises grooves (62) oriented essentially circumferentially, and wherein the second covering layer (72) is formed of several axially separated sections, said second covering layer (72) being interrupted under the grooves (62).

9. A tire (1) according to any one of claims 1 to 8, wherein a material appearing at the bottom of the groove is the same material as the material of the tread (6).

10. A tire (1) according to any one of claims 1 to 8, wherein the material appearing at the bottom of the groove is the same material as the material of the first base layer (71).

11. 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, and wherein the second 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 second covering layer (72) towards the outside of the tread (6) up to a radial height greater than 50% of the radial thickness of the tread (6), said reinforcing element (630) being of width variable axial, from a maximum value less than 50% of the axial width of said sculpture block, said axial width decreasing when moving radially upwards.

12. A tire (1) according to any one of claims 2 to 11, comprising two sidewalls, each sidewall having a sidewall layer (3), the tread (6) comprising at least one tread portion, hereinafter referred to as a tread wing, at at least one of the axial ends of the tread (6), 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 one of the or each sidewall layer (3) and the axially and radially outer contour of the tread (6), and a second point located at a curvilinear distance of between 5% and 25% of the nominal bead width of the tire dimension.

13. Tire (1) according to the preceding claim, in which 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 alternating shear stress at a frequency of 10 Hz and at 10% deformation.

14. A tire (1) according to claim 12 or 13, wherein the viscoelastic loss tg 5 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 5 of the tread wing being measured at 23°C, and under alternating shear stress at a frequency of 10 Hz and at 10% deformation.

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