TIRES FOR PASSENGER CARS

Description

Field of invention

[0001] The present invention relates to a tire for a motor vehicle whose rolling resistance performance is improved without degrading the transverse drift rigidity. The invention is more particularly suitable for a radial tire intended to equip a passenger vehicle or a van. Definitions

[0002] By convention, we consider a reference frame (O, XX', YY', ZZ'), whose center O coincides with the center of the tire, the circumferential XX', axial YY', and radial ZZ' directions 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.

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

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

[0008] The crown reinforcement of a radial tire comprises a superposition of crown layers extending circumferentially, radially outside the carcass reinforcement. Each crown layer consists of reinforcements parallel to each other and coated with a polymeric material of the elastomer type or elastomeric mixture. The assembly consisting of the crown reinforcement and the tread is called the crown.

[0009] The carcass reinforcement of a radial tire usually comprises at least one carcass layer consisting of metallic or textile reinforcing elements coated in an elastomeric coating mixture. The reinforcing elements are substantially parallel to each other and form, with the circumferential direction, an angle of between 85° and 95°. The carcass layer comprises a main part, connecting the two beads together and wrapping, in each bead, around an annular reinforcing structure. The annular reinforcing structure may be a bead wire which comprises a circumferential reinforcing element, most often metallic, surrounded by at least one material, non-exhaustively elastomeric or textile. The wrapping of the carcass layer around the annular structure goes from the inside to the outside of the tire to form a turn-up, comprising an end.The turn-up, in each bead, allows the anchoring of the carcass reinforcement layer to the annular structure of the bead.

[0010] Each bead comprises a filler layer extending radially outwards the annular reinforcement structure. The filler layer consists of at least one elastomeric filler compound. The filler layer axially separates the main part and the turn-up of the carcass reinforcement.

[0011] Each bead also comprises a protective layer extending radially inwards from the sidewall and axially outwards from the turn-up. The protective layer is also at least partly in contact by its axially outward face with a hook of the rim. The protective layer is made of at least one protective elastomeric mixture.

[0012] Each bead finally includes a lateral reinforcement layer positioned between the sidewall and the turn-up of the carcass reinforcement. The external lateral reinforcement layer is made of at least one elastomeric mixture.

[0013] Each tire sidewall comprises at least one sidewall layer consisting of an elastomeric mixture and extending axially towards the inside of the tire from an outer face of the tire, in contact with atmospheric air.

[0014] By "radial cut" or "radial section" is meant here a cut or section along a plane which contains the axis of rotation of the tire.

[0015] 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).

[0016] 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.

[0017] 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 mixture composition considered.

[0018] 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)1 / 2, where G' is the elastic shear modulus and G'' the viscous shear modulus, and a dynamic loss Tanδ=G'' / G'. The dynamic shear modulus G* and the dynamic loss Tanδ are measured on a Metravib VA4000 type viscoanalyzer, according to ASTM D 5992-96 standard. The response of a sample of vulcanized elastomeric mixture in the form of a cylindrical specimen 4 mm thick and 400 mm 2< in cross-section, subjected to a sinusoidal stress in alternating simple shear, at a frequency of 10 Hz, at a temperature of 100°C, is recorded. We perform a deformation amplitude sweep from 0.1% to 50% (forward cycle), then from 50% to 0.1% (return cycle). For the forward cycle, we indicate the maximum value of tan(δ) observed, noted Tan(δ) max.

[0019] "Handling" performance corresponds to the responses of a vehicle / tire assembly to multiple driver inputs (steering, acceleration, braking, etc.). Handling is essential both in terms of safety for the stability of the vehicle and for driving pleasure.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] Transverse drift stiffness refers to the variation in transverse forces generated in the contact patch of the moving tire crushed by the load carried, as a function of the drift angle applied to the tire. Transverse drift stiffness is expressed in Newtons per degree (N / °).

[0024] For small drift angles, i.e. angles less than 10°, 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.

[0025] Transverse drift stiffness is an essential physical quantity that connects the tire to the vehicle and determines the quality of the vehicle's behavior on the road.

[0026] 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 (CR RR ) is the rolling resistance force related to the load carried by the tire. The coefficient is expressed in kg / t.

[0027] 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%.

[0028] Most often in the invention, the tire appears mounted on a rim. Said rim is chosen according to the specifications of the ETRTO (European Tire and Rim Technical Organization) standard which associates recommended rims with a given tire dimension. In general, several rim widths may be suitable for the same tire dimension. The part of the rim which interacts with the tire in the context of the invention is axisymmetric with respect to the tire's axis of rotation. To describe the rim, it is sufficient to describe the generating profile in a meridian plane.

[0029] In a meridian plane, the rim comprises at least one hook located at an axial end, and connected to a seat which is intended to receive a face of the bead located most radially inward. Between the seat and the hook, there is a rectilinear portion which connects the rim hook to the seat by fillets. The rim hook extended by the rectilinear portion axially limits the movement of the beads during inflation.

[0030] The fitability of the beads on a rim during inflation is also a performance impacted by the invention. The fitability performance of the beads consists of evaluating the ability of the beads of a tire to be correctly installed on a rim during inflation. On the radially innermost face of the bead, the contact with the seat must be sufficient to prevent any leakage of the tire inflation air. In general, a contact pressure of at least 1.4 MPa is expected in this contact zone. The inflation pressure wedges the bead against the rim hook. Here again, the contact pressure on the hook must be sufficient to prevent the tire from coming off the rim, particularly during sharp turns at high speed. Observation methods, particularly radiographic methods, of the beads mounted on a rim make it possible to diagnose the quality of the fitment.

[0031] It is therefore possible to classify two tires according to their rim fitment performance. Prior art

[0032] Reducing greenhouse gas emissions from transport is one of the major challenges facing vehicle manufacturers today. Tires represent an important source of progress, through a reduction in rolling resistance, because this has a direct impact on the vehicle's fuel consumption. For example, a 20% reduction in a tire's rolling resistance saves approximately 3% of fuel per 100 km in a combined cycle.

[0033] There is still a need to reduce the rolling resistance of passenger car tires without degrading their behavior on the vehicle.

[0034] It has already been proposed to improve the rolling resistance of passenger vehicle tires by optimizing their beads. Document WO 2010 / 072736 teaches in particular the use of elastomeric compositions having low elastic shear moduli G' around 15 MPa and viscous shear moduli G'' lower by more than 20% than the elastic shear moduli to obtain a significant reduction in rolling resistance.

[0035] This document also recommends further reducing rolling resistance by optimizing the geometry of the elastomeric compound layers whose elastic and viscous shear moduli satisfy the previous relationship. This optimization leads to shorter and wider elastomeric compound layer profiles than in traditional tires, and can lead to implementation difficulties.

[0036] Document FR2994127 describes an improvement to document WO 2010 / 072736, proposing to add a reinforcing reinforcement in the beads. The reinforcing reinforcement is formed from reinforcements coated in an elastomeric mixture.

[0037] The major disadvantage of this solution is a significant deterioration in industrial production costs with the introduction of new semi-finished products into the tire manufacturing process.

[0038] The inventors set themselves the goal of creating a tire that improves the level of rolling resistance without degrading the vehicle's handling, while controlling the associated manufacturing costs. Statement of the invention

[0039] This goal was achieved by a passenger vehicle tire comprising: two beads intended to be mounted on a rim, two layers of sidewalls connected to the beads, a crown comprising a tread intended to come into contact with a ground, the crown having a first side connected to the radially outer end of one of the two layers of sidewalls and having a second side connected to the radially outer end of the other of the two layers of sidewalls; at least one carcass reinforcement extending from the two beads through the layers of sidewalls to the crown, the carcass reinforcement comprising a plurality of carcass reinforcement elements and being anchored in the two beads by an upturn around an annular reinforcement structure, so as to form in each bead a main part and an upturn;a first layer of elastomeric filler mixture occupying a volume comprised at least in part on the one hand between the main part of the carcass reinforcement, and on the other hand the radially outer portion of the annular reinforcement structure, and extending radially outwardly to an end located at a normal distance DRB to the axial line HH' tangent to the annular reinforcement structure at its radially innermost point; a second layer of elastomeric mixture forming a lateral reinforcement layer occupying a volume comprised at least in part between the sidewall layer, and the upturn of the carcass reinforcement, extending radially outwardly to an end located at a normal distance DRL to the axial line HH' tangent to the annular reinforcement structure at its radially innermost point;an elastic shear modulus and a viscoelastic loss of the elastomeric mixtures being measured according to the ASTM D 5992-96 standard, at 100°C, under 10% deformation; in each bead a rim contact curve comprising the points of the tire in contact with the rim, said rim contact curve connecting a first point M1 of the tire axially positioned most externally, and in contact with the rim, and a second point M2 of the tire also in contact with the rim and located in the middle of the rectilinear portion connecting the hook to the seat of the rim; the length of said rim contact curve being the curvilinear distance from point M1 to point M2 along the rim contact curve;two sections in a vertical meridian section of the inflated tire, mounted on a rim, and crushed on a hard flat ground by a vertical load where the load, the inflation pressure are at their nominal value of the ETRTO (European Tire and Rim Technical Organization) standard; a first section being located in the contact area, and a second section being located on the opposite side to the previous one with respect to the axis of rotation of the tire; in the first section located in the contact area, in at least one first bead, the length of the rim contact curve, LADC, being measured; in the second section located opposite the contact area with respect to the axis of rotation of the tire, in at least one second bead, the length of the rim contact curve, LCJ, being measured; the ratio of the difference in the lengths of the rim contact curves of the two sections, i.e. 100*(LADC-LCJ) / LCJ, is greater than or equal to; 30; the viscoelastic loss Tan (δ)max of the elastomeric mixture constituting the lateral reinforcement layer of at least one bead has a value less than or equal to 0.100.

[0040] The rim contact variation rate, 100*(LADC-LCJ) / LCJ of the tires of the invention greater than 30, combined with a level of hysteresis Tan(δ)max less than or equal to 0.100 of the elastomeric mixture constituting the lateral reinforcement layer, lead to a reduction in the rolling resistance of the tire without degrading the road behavior of the vehicle on which it is mounted. The bead of such a tire achieves a balance of rolling resistance and transverse drift stiffness performances thanks to the material properties and the geometric profile of the sidewall layer in the contact zone with the rim. The manufacture of such a tire does not require any particular evolution of the processes, nor the introduction of new materials, which keeps the industrial cost price unchanged compared to the state of the art.

[0041] The rim contact variation rate of the tires of the invention is much higher than that observed on state-of-the-art tires.

[0042] The rim contact curve represents all the points of the tire that are in contact with the rim at a given time. For each of the beads, said rim contact curve extends from a first point M1 of the tire axially positioned most externally, and in contact with the rim, and a second point M2 of the tire also in contact with the rim and located in the middle of the rectilinear portion connecting the hook to the seat of the rim. The length of said rim contact curve is the curvilinear distance from point M1 to point M2 along the rim contact curve.

[0043] When the inflated tire, mounted on a rim, is crushed by a load, the points of the tire in contact with the rim may vary from one meridian to another. It follows that the length of the rim contact curve as defined above also varies from one meridian to another.

[0044] The tire is designed so that the rim contact curve is as long as possible in the contact patch, compared to state-of-the-art tires, and more precisely in the meridian at the center of the contact patch. Under these conditions, the inventors estimate that the contribution of the rim contact to the drift stiffness is maximum.

[0045] In a meridian section of an inflated tire, mounted on a rim, and crushed by the load carried, we can see a first section of the tire which passes through the center of the contact patch. The contact patch is understood to mean all the points of the tire, at a given instant which are in contact with the crushing ground. The center of the contact patch is called the point of the contact air located on the vertical axis ZZ'. We can also see opposite the contact patch with respect to the axis of rotation YY' of the tire another section of the tire which globally defines a deformed state similar to the state of axisymmetric inflation.

[0046] The rim contact variation rate corresponds to the maximum value of the change in rim contact lengths per wheel revolution.

[0047] According to the inventors, an essential step in the design of the tire of the invention consists of modifying its external profile in the area of contact with the rim. Various solutions are possible, such as, for example, increasing the axial thickness of the sidewall layer at the junction with the protective layer. Other solutions consist of modifying the external profile so as to obtain a profile in the contact area with the same curvature as the rim hook. Yet another solution consists of inserting a compound cushion in the area at the junction of the sidewall and protective layers, at the rim hook. This compound cushion may preferably be made of the same compound as that of the sidewall layer so as to maintain the industrial cost price. The expectation with regard to this elastomeric compound cushion is above all its elastic shear modulus which could advantageously be, for example, equal to that of the sidewall layer.

[0048] According to the invention, the viscoelastic loss Tan (δ)max of the elastomeric mixture constituting the lateral reinforcement layer of at least one bead has a value less than or equal to 0.100.

[0049] The invention proposes the use of a lateral reinforcement layer with a viscoelastic loss less than or equal to 0.100 in order to improve rolling resistance. This lateral reinforcement layer can be associated with the first layer of elastomeric filling mixture which can also have a low viscoelastic loss, or be rigid, or even be flexible.

[0050] The function of the side reinforcement layer in the bead is to reinforce the first layer of elastomeric filling compound in terms of shear stiffness. The transmission of vehicle torque to the wheel requires a bead with a sufficient level of stiffness to be effective. The side reinforcement layer contributes by collaborating with the first layer of elastomeric filling compound to the stiffness of the bead.

[0051] The reduction of the rolling resistance of the bead consists of reducing the hysteresis of the elastomeric compounds of greater volume, and undergoing strong deformations. The first and second layers of the bead compound positioned respectively axially internally at the turn-over of the carcass reinforcement, and axially externally, are those occupying the greatest volume and undergoing strong deformations of bending, extension-compression and shear.

[0052] In a first advantageous embodiment, the lateral reinforcing layer of at least one bead has an elastic shear modulus in the range [1.5; 10] MPa, preferably in the range [1.5; 7] MPa.

[0053] This embodiment aims to operate the bead with a flexible reinforcing lateral layer while usually the reinforcing lateral layer has an elastic shear modulus between 20 MPa and 50 MPa. This embodiment has the advantage of using an elastomeric mixture with both a low viscoelastic loss with Tan (δ) max less than 0.1, and at the same time with an elastic shear modulus in the range [1.5; 10] MPa. Such a mixture does not present any particular difficulties to be produced given the consistency of these material properties.

[0054] In a second advantageous embodiment of the invention, the viscoelastic loss Tan (δ)max of the elastomeric mixture constituting the first layer of elastomeric mixture for stuffing at least one bead has a value less than or equal to 0.100.

[0055] In this embodiment, the performance compromise leans towards rolling resistance. The volume of this first layer of elastomeric filler compound is the largest in the bead. The reduction in its hysteresis combined with a lateral layer of loss-pass reinforcement results in a significant reduction in the bead's rolling resistance.

[0056] In other words, this embodiment is favorable to the reduction of the rolling resistance of the bead and therefore of the tire while having a level of road behavior on the vehicle comparable to a state-of-the-art tire thanks to the rate of variation of the rim contact which remains at a level above 30.

[0057] According to a third embodiment, the elastic shear modulus of the elastomeric mixture constituting the first layer of elastomeric mixture for filling at least one bead is in the range [1.5; 10] MPa, preferably in the range [1.5; 7] MPa.

[0058] Another advantage of the invention related to this variant is that by carrying out rim fitability tests comparing tires of the state of the art and those of the invention, the inventors noted that the fitability of the tires of the invention is more efficient than certain tires comprising rigid beads. Indeed, in the tires of the state of the art, the mixture of the first layer of elastomeric filler mixture has an elastic shear modulus generally between 15 Mpa and 50 Mpa. The inventors hypothesize that the relative flexibility of the bead of the tires of the invention makes it easier to fit due to their deformability which promotes better installation on the seat, and against the rim hook. In addition, the modification of the profile of the sidewall layer radially internally, associated with a rigid bead, results in a cant in the fitability on the rim.The use of flexible elastomeric mixtures (G' <lOMPa) permet de rattraper ce dévers et d'obtenir un niveau de montabilité convenable.

[0059] Preferably, the ratio of the difference in the lengths of the rim contact curves of the two sections, i.e. 100*(LADC-LCJ) / LCJ, is greater than or equal to 40, preferably greater than or equal to 50, even more preferably greater than or equal to 70.

[0060] The inventors have found that the transverse drift stiffness of the tire of the invention increases in the same direction as the rim contact variation rate. For such rim contact variation rates, modifying the outer profile of the sidewall layer facilitates bead mounting, but rates that are too high beyond 100 , could hinder mountability.

[0061] In addition to the main characteristics of the invention, the inventors have identified levers linked to the geometry of the bead compound layers to better manage the tire performance compromise with improved rolling resistance while having good road behavior.

[0062] Advantageously, the radial distance DRB of the first layer of elastomeric filling compound between the main part of the carcass reinforcement and its turn-up is less than or equal to 50% of the radial height H of the tire.

[0063] The height H of the tire is the normal distance between a first straight line parallel to the axis of rotation of the tire and tangent to the radially innermost point of the annular reinforcement structure, and between a second straight line also parallel to the axis of rotation of the tire and passing through the radially outermost point of the tread. The radial height H is measured on the tire mounted on a rim and inflated with a set pressure conforming to the ETRTO (European Rim and Tire Organization) specifications.

[0064] Advantageously, the radial distance DRI being the radial height of a radially innermost end of the lateral reinforcement layer, positioned between the sidewall layer and the turn-up of the carcass reinforcement, said radial distance DRI is included in the interval [5%; 25%] of the radial height H of the tire.

[0065] Advantageously, the distance DRL being the distance from the radially outer end of the lateral reinforcement layer positioned between the sidewall layer and the turn-up of the carcass reinforcement, said distance DRL is greater than or equal to 25% of the radial height H of the tire.

[0066] It is recalled that the distance DRL is the normal distance from the radially outermost end of the end of the lateral reinforcement layer to the axial line (HH') tangent to the annular reinforcement structure at its radially innermost point; The lateral reinforcement layer between the sidewall and the turn-up of the carcass reinforcement contributes to the rigidity of the bead in reinforcement to the first layer of elastomeric filling compound. According to the inventors, its positioning is adjusted by the dimensions DRI and DRL so as to resist the bending, extension-compression stresses of the bead when passing through the contact area.

[0067] In an advantageous embodiment of the invention, the upturn of the carcass reinforcement is in contact with the main part of the carcass reinforcement radially outwardly along said upturn.

[0068] As mentioned above, the carcass reinforcement is formed of reinforcements coated between two layers of elastomeric compounds. The upturn of the carcass reinforcement is pressed against the main part of the carcass reinforcement, meaning that the upturn is in contact with the main branch of the carcass reinforcement. The contact is made along an axially external surface of the coating of the carcass reinforcement.

[0069] In this configuration, the volume of the first layer of elastomeric filler compound is limited to the strict minimum around the annular reinforcement structure. This configuration is very advantageous for reducing the rolling resistance of the bead.

[0070] In another embodiment, a bead reinforcement reinforcement is introduced axially between the turn-up of the carasse reinforcement, and the lateral reinforcement layer axially inside the sidewall.

[0071] The reinforcement of the bead is made up of parallel reinforcements, and coated between two layers of elastomeric mixtures. The addition of this semi-finished product results in an additional manufacturing cost which must be compensated.

[0072] In order to limit the impact on the cost price of such a solution, this embodiment can be combined with the plating of the carcass reinforcement turn-over against the main part of the carcass reinforcement.

[0073] Advantageously, at least one layer of the first layer of elastomeric filler mixture and of the lateral reinforcement layer has a composition based on 100% natural rubber polyisoprene, or a blend of natural rubber and polybutadiene, a crosslinking system, a reinforcing filler, such as Carbon Black N550, at an overall rate of between 50 and 75 pce.

[0074] Preferably, the elastomeric mixture constituting the first layer of elastomeric mixture for stuffing at least one bead has the same composition as the elastomeric mixture constituting the outer lateral reinforcement layer of said bead.

[0075] The rubber composition is preferably based on at least one diene elastomer, a reinforcing filler and a crosslinking system.

[0076] By "diene" elastomer (or indistinctly rubber) is meant, in a known manner, an elastomer derived at least in part (i.e., a homopolymer or a copolymer) from diene monomers, i.e. monomers carrying two carbon-carbon double bonds, conjugated or not. The diene elastomer used is preferably chosen from the group consisting of polybutadienes (BR), natural rubber (NR), synthetic polyisoprenes (IR), butadiene-styrene copolymers (SBR), isoprene-butadiene copolymers (BIR), isoprene-styrene copolymers (SIR), butadiene-styrene-isoprene copolymers (SBIR) and the compositions of these elastomers.

[0077] A preferred embodiment consists of using an “isoprenic” elastomer, that is to say a homopolymer or a copolymer of isoprene, in other words a diene elastomer chosen from the group consisting of natural rubber (NR), synthetic polyisoprenes (IR), the different copolymers of isoprene and the compositions of these elastomers.

[0078] The isoprene elastomer is preferably natural rubber or a synthetic polyisoprene of the cis-1,4 type. Among these synthetic polyisoprenes, polyisoprenes having a rate (mol%) of cis-1,4 bonds greater than 90%, more preferably still greater than 98%, are preferably used. According to other preferred embodiments, the diene elastomer may consist, in whole or in part, of another diene elastomer such as, for example, an SBR elastomer (E-SBR or S-SBR) used in blend or not with another elastomer, for example of the BR type.

[0079] The rubber composition may also comprise all or part of the additives normally used in rubber matrices intended for the manufacture of tires, such as, for example, reinforcing fillers such as carbon black or inorganic fillers such as silica, coupling agents for inorganic filler, anti-aging agents, antioxidants, plasticizing agents or extender oils, whether the latter are aromatic or non-aromatic (in particular very low or non-aromatic oils, for example of the naphthenic or paraffinic type, with high or preferably low viscosity, MES or TDAE oils, plasticizing resins with a high Tg greater than 30°C), agents facilitating the implementation (processability) of the compositions in the raw state, tackifying resins, a crosslinking system based either on sulfur or on sulfur and / or peroxide donors, accelerators,vulcanization activators or retarders, antireversion agents, methylene acceptors and donors such as, for example, HMT (hexamethylenetetramine) or H3M (hexamethoxymethylmelamine), reinforcing resins (such as resorcinol or bismaleimide), known adhesion promoter systems of the metal salt type, for example, in particular cobalt or nickel salts.,

[0080] The compositions are manufactured in suitable mixers, using two successive preparation phases well known to those skilled in the art: a first phase of thermomechanical working or mixing (so-called "non-productive" phase) at high temperature, up to a maximum temperature of between 110°C and 190°C, preferably between 130°C and 180°C, followed by a second phase of mechanical working (so-called "productive" phase) down to a lower temperature, typically below 110°C, a finishing phase during which the crosslinking system is incorporated.

[0081] For example, the non-productive phase is carried out in a single thermomechanical step of a few minutes (for example between 2 and 10 min) during which all the necessary basic constituents and other additives, with the exception of the crosslinking or vulcanization system, are introduced into a suitable mixer such as a conventional internal mixer. After cooling the composition thus obtained, the vulcanization system is then incorporated into an external mixer such as a cylinder mixer, maintained at a low temperature (for example between 30°C and 100°C). The whole is then mixed (productive phase) for a few minutes (for example between 5 and 15 min).

[0082] The final composition thus obtained is then calendered, for example in the form of a sheet or plate for characterization, or even extruded, to form the outer strip used in a tire according to the invention.

[0083] Vulcanization (or curing) can then be carried out in a known manner at a temperature generally between 130°C and 200°C, preferably under pressure, for a sufficient time which can vary for example between 5 and 90 min depending in particular on the curing temperature, the vulcanization system adopted and the vulcanization kinetics of the composition considered. Brief description of the drawings

[0084] Other advantageous details and characteristics of the invention will emerge below from the description of the exemplary embodiments of the invention with reference to the figures which represent meridian views of diagrams of a tire according to an embodiment of the invention. The figures are not shown to scale to simplify understanding. There figure 1represents a meridian section of the inflated tire, mounted on a rim and crushed by the load carried. We see a first section in the contact patch and a second section opposite the contact patch with respect to the axis (YY'). The Figures 2-A and 2-B show modifications to the outer profile of the tire to facilitate contact with the rim. The Figure 2-C represents an enlargement of a bead of a tire of the invention installed on a rim. The Figure 3-A illustrates the determination of the height H of a tire. The Figure 3-B represents the visualization of the main ribs of the bead in relation to the invention. Detailed description of the invention

[0085] The invention was implemented on a passenger car tire of size 245 / 45R18, according to the ETRTO (European Rim and Tire Technical Organization) specification standard. Such a tire can carry a load of 800 kilos, inflated to a pressure of 250 kPa.

[0086] On the figure 1, the tire 1 comprises a carcass reinforcement 90 consisting of reinforcements coated with rubber composition, and two beads 50 each comprising annular reinforcement structures 51 which hold the tire 1 on the rim 100. The carcass reinforcement 90 is anchored in each of the beads 50. The tire 1 further comprises a crown reinforcement 20 comprising two working layers 21, 22, and a hooping layer 23. Each of the working layers 21 and 22 is reinforced by wire reinforcement elements which are parallel in each layer and crossed from one layer to the other, making angles of between 10° and 70° with the circumferential direction. The hooping layer 23, arranged radially outside the crown reinforcement 20, this hooping layer 23 being formed of circumferentially oriented and spirally wound reinforcing elements.A tread 10 is placed radially on the hooping layer 23; it is this tread 10 which ensures the contact of the tire 1 with a ground 200. The tire 1 shown is a “tubeless” tire: it comprises an “inner rubber” 95 made of rubber composition impermeable to the inflation gas, covering the inner surface of the tire. The tire is crushed on a ground 200 by a vertical load 250. Each bead 50 comprises a layer of elastomeric mixture 80 positioned radially the innermost and intended to be in contact with the rim 100, a first layer of elastomeric filling mixture 70, positioned at least in part between the main part 52 of the carcass reinforcement 90 and the upturn 53. The bead 50 also comprises a lateral reinforcement layer 60 axially outside the upturn 53 and axially inside the sidewall 30.

[0087] Still on the figure 1, the mounting rim 100 extends axially on either side of the vertical axis OZ and comprises at least one profile 105 comprising on at least one part delimited by the axis OZ, a seat 110 intended to receive the bead of the tire, said seat 110 is connected to a rectilinear portion 130 of the rim, rectilinear portion 130 which is itself connected to the hook 120.

[0088] On the Figure 2-A , the external profiles of a bead 50 of a tire 1 of the invention are shown in comparison with that of a tire of the state of the art. The bead 50 is shown in a section opposite the contact area. The two profiles differ in an area at the rim hook 120. Reference 30 indicates the profile of a tire of the state of the art, and reference 35 shows the modification of the profile made on the tire of the invention to facilitate contact with the rim 100.

[0089] On the Figure 2-B, we have the same representation as on the Figure 2-A , but except that the profiles are shown in the center of the contact patch with the ground. The tire is in contact with the entire rim hook 120 unlike the Figure 2-A The rim contact variation rate reflects this evolution of rim contact.

[0090] In the embodiment shown in the Figure 2-C, there is an elastomeric mixture cushion 40 (modification located at the radially inner end of the sidewall 30), intended to be in contact with the rim hook 120. The mixture cushion 40 is delimited radially internally by a curve which matches the profile of the rim hook 120. A first side of the elastomeric mixture cushion 40 has an appropriate geometric shape which anticipates contact with the curvature of the rim hook so as to match the shape of the rim hook 120 upon contact, a second side of the elastomeric mixture cushion extends an outer side of a sidewall in contact with the ambient air, a third side of the elastomeric mixture cushion 40 is in contact with the radially inner end of the sidewall and finally a fourth side of the elastomeric mixture cushion is in contact with the protective layer 80.

[0091] On the Figure 2-Cthe rim contact curve extends from a first point M1 of the tire axially positioned most externally, and in contact with the rim, and a second point M2 of the tire also in contact with the rim and located in the middle of the rectilinear portion connecting the hook 120 to the seat 110 of the rim. The length of said rim contact curve is the curvilinear distance from point M1 to point M2 along the rim contact curve.

[0092] On the Figure 3-A, the determination of the height H is illustrated. The height H of the tire is the normal distance between a first straight line parallel to the axis of rotation of the tire and tangent to the radially innermost point of the annular reinforcement structure, and between a second straight line also parallel to the axis of rotation of the tire and passing through the radially outermost point of the tread. The radial height H is measured on the tire mounted on a rim and inflated with a set pressure conforming to the ETRTO (European Rim and Tire Organization) specifications.

[0093] On the Figure 3-B the geometric parameters of the bead in connection with the invention have been represented. The heights are defined from the line HH', which is tangent to the rod 51 at its radially innermost point: DRI is the radial distance from HH' of the radially inner end of the lateral reinforcement layer 60. The radial distance DRI is less than or equal to 20% of the radial height H of the tire, and is 5 mm in the example presented here; DRL is the radial distance from the straight line HH' of the radially outer end of the lateral reinforcement layer 60. The radial distance DRL is greater than or equal to 25% of the radial height H of the tire and is 38 mm in the example presented here; DRR is the radial distance from HH' of the end of the turn-up of the carcass reinforcement 90. The radial distance DRR being greater than or equal to 10% of the radial height H of the tire and is 20 mm in the example presented here; DRB is the radial distance from HH' of the radially outer end of the first layer of elastomeric filler mixture 70, and is 28 mm in the example presented here.

[0094] Table 1 below gives the compositions of elastomeric mixtures of a bead concerned by the invention. The main mixtures used are listed, expressing for each the main ingredients expressed in pce (part by weight per hundred parts by weight of elastomer): [Table 1] NR Elastomer (Natural Rubber) Carbon black reinforcing filler Antioxidant agent Sulfur Accelerator Reinforcing resin Hardener M1 100 75 (N326) 1.5 8.5 0.95 12 4.18 M2 100 75 (N326) 2 7.5 0.97 12 6.8 M3 100 55 (N550) 1.3 9.0 0.68 0 0

[0095] The mixtures of the invention used in this example are based on a natural rubber elastomer, reinforced with carbon black. Plasticizers (reinforcing resin) are included in the composition to facilitate the processability of the mixtures. The mixtures also include vulcanizing agents, sulfur, accelerator, and protective agents. The associated mechanical and viscoelastic properties, measured at 23°C under a deformation amplitude of 10% are summarized in Table 2: [Table 2] G' G" Tan (δ)max M1 46 7 0.2 M2 48 8 0.2 M3 3 0.3 0.1

[0096] In the context of the invention, the elastomeric mixtures M1 and M2 with an elastic shear modulus of 46 MPa and 48 MPa respectively are said to be rigid. The mixture M3 with a viscoelastic loss equal to 0.1 is said to be of low hysteresis.

[0097] A P1 configuration of the tire of the invention was tested to clearly highlight the performance provided by the invention. The results of these tires are compared to those of controls T1, T2, and T3

[0098] Witness T1 corresponds to a tire of usual design comprising a first layer of elastomeric filler compound located between the main part of the carcass reinforcement and its turn-up. This first layer of elastomeric filler compound is made up of the M1 compound. This tire also comprises a lateral reinforcement layer positioned between the sidewall layer and the turn-up of the carcass reinforcement. This lateral reinforcement layer is provided with the M2 elastomeric compound. The profile of the sidewall layer has not been modified to facilitate contact with the rim as is the case for the tires of the invention.

[0099] The rate of variation of the rim contact on the control tire T1 is conventionally positioned at 100, that is to say that said rate of variation of contact with the rim has a deviation compared to a tire of the invention less than 30 .

[0100] For the T2 configuration, the filler layer and the lateral reinforcement layer are made of the same M3 elastomeric mixture, but the sidewall profile, unlike the tires of the invention, has not been modified to facilitate contact with the rim.

[0101] As for the second variant T3, the first layer of elastomeric filling compound is made of the M1 compound, and the side reinforcement layer is equipped with the M2 elastomeric compound. The sidewall profile, this time, has been modified to facilitate contact with the rim, in accordance with the Figures 2-A and 2-B .

[0102] A variant of the P1 tire conforming to the invention uses the specifications of the T1 witness, with the replacement of the mixture of the second filling layer by the M3 mixture. The rim contact variation rate is 167after a partial modification of the profile of the sidewall layer in the area of contact with the rim, as shown in the Figures 2-A and 2-B .

[0103] The control tires and the configuration in accordance with the invention were tested for rolling resistance and transverse drift stiffness. These same tires were also evaluated in the rim fitability test.

[0104] Table No. 3 below summarizes the configurations considered: [Table 3] First layer of stuffing Reinforcing side layer Changing the Rim Contact Profile T1 M1 M2 nok (1)< T2 M3 M3 nok (1)< T3 M1 M2 ok P1 M1 M3 ok (1) The qualifier Nok means that the profile of the sidewall layer has not been modified to facilitate contact with the rim and ok means that said profile has been modified so as to obtain a rim contact variation rate greater than 30.

[0105] THE Rolling resistance test was 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.

[0106] The transverse drift stiffness measurements were made on dedicated measuring machines such as those marketed by the company MTS.

[0107] THE Rim fitability test consists of giving a result on the overall fitability from the breakdown of an assembly into elementary operations which include in particular: the passage of the rim hooks, the pressure measurement, the crossing of the humps of the rim, the installation of the bead by compression, the sealing under the rim seat, the bead breaking and the disassembly. To carry out this test, means are necessary such as for example a semi-automatic assembly machine, or even radiographic means.

[0108] The results obtained are summarized in table no. 4 below, which also shows the rim contact variation rate for each variant: [Table 4] Rolling resistance Transverse drift stiffness Rim contact variation rate Rim Fitment T1 100 100 100 100 T2 108 97 112 110 T3 96 103 160 95 P1 105 99 167 105

[0109] In observing the results of the tires the principle of using a bead with soft materials, such as for example the M3 mixture, to reduce the rolling resistance of the tire is confirmed (T2). On the other hand, a concomitant decrease in the drift stiffness is also observable.

[0110] Conversely, these same results teach us that when we use a rigid bead, that is to say a bead with a first layer of elastomeric filling mixture and a lateral reinforcement layer made respectively with the elastomeric mixtures (M1, M2), the rolling resistance deteriorates significantly.

[0111] Witness T3 teaches us that modifying the profile of the sidewall layer to facilitate rim contact combined with a rigid bead improves transverse drift rigidity, but degrades rim mountability.

[0112] Variant P1 shows a gain in rolling resistance. The transverse drift stiffness is affected downwards. The rim contact variation rate is at a level that limits the decrease in transverse drift stiffness. The relative flexibility of the bead linked to the decrease in the elastic shear modulus of the M3 mixture of the first layer of elastomeric filler mixture facilitates rim fitment.

[0113] The P1 variant has a transverse drift stiffness level almost identical to the control. By numerical simulations, it is confirmed that the road behavior of these variants gives results identical to the control.

[0114] All the variants presented are produced without any change in the processes and remain with an industrial cost price that is little different from the usual costs.

[0115] Furthermore, the invention can be generalized to other bead architectures than those described here, such as for example a bead having a first layer of elastomeric filling mixture, and a lateral reinforcing layer, even though the carcass reinforcement does not include a turn-up.

Claims

1. Tyre (1) for a passenger vehicle, comprising: two beads (50) intended to be mounted on a rim, two sidewall layers (30) connected to the beads (50), and a crown (20) having a tread (10) intended to come into contact with the ground (200), the crown (20) having a first side connected to the radially outer end of one of the two sidewall layers (30) and a second side connected to the radially outer end of the other one of the two sidewall layers (30); at least one carcass reinforcement (90) extending from the two beads (50) through the sidewall layers (30) as far as the crown (20), the carcass reinforcement (90) having a plurality of carcass reinforcing elements and being anchored in the two beads (50) by way of a turn-up around the annular reinforcing structure (51), so as to form a main part (52) and a turn-up (53) in each bead; a first layer (70) of elastomeric filler compound taking up a volume which is comprised at least partially on the one hand between the main part of the carcass reinforcement (52), and on the other hand the radially outer portion of the annular reinforcing structure (51), and extending radially outwards to an end located at a normal distance DRB from the axial straight line (HH') which is tangent to the annular reinforcing structure at its radially innermost point; a second layer (60) of elastomeric compound forming a lateral reinforcing layer (60) taking up a volume comprised at least partially between the sidewall layer (30) and the turn-up of the carcass reinforcement (53), and extending radially outwards to an end located at a normal distance DRL from the axial straight line HH' which is tangent to the annular reinforcing structure (51) at its radially innermost point; the dynamic shear stiffness moduli and the viscoelastic loss Tanδ of the elastomeric compounds being measured in accordance with the standard ASTM D 5992-96, comprising the response of a sample of vulcanized elastomeric compound in the form of a cylindrical test specimen with a thickness of 4 mm and a cross section of 400 mm2, subjected to a simple alternating sinusoidal shear stress, at a frequency of 10 Hz and at a temperature of 100°C and under 10% strain carried out from an amplitude sweep from 0.1% to 50% from the outward cycle; in each bead (50), a rim contact curve comprising the points on the tyre (1) that are in contact with the rim (100); this rim contact curve connecting a first point M1 on the tyre that is positioned axially furthest on the outside, and in contact with the rim, and a second point M2 on the tyre that is also in contact with the rim and is located in the middle of the rectilinear portion connecting the flange to the seat of the rim; the length of this rim contact curve being the curvilinear distance from the point M1 to the point M2 along the contact curve; two sections in a vertical meridian plane of the inflated tyre mounted on a rim and compressed against a hard flat ground, by a vertical load (250), wherein the load and the inflation pressure are at their nominal values from the ETRTO (European Tyre and Rim Technical Organization) standard; a first section being located in the contact patch, and a second section being located on the opposite side to the first in relation to the axis of rotation of the tyre; in the first section located in the contact patch, in at least a first bead, the length of the rim contact curve, LADC, being measured; in the second section located opposite the contact patch in relation to the axis of rotation of the tyre, in at least a second bead, the length of the rim contact curve, LCJ, being measured, characterized in that the ratio of the difference in the lengths of the rim contact curves of the two sections, i.e. 100*(LADC-LCJ) / LCJ, is greater than or equal to 30, and in that the viscoelastic loss Tan(δ)max of the elastomeric compound making up the second lateral reinforcing layer (60) of at least one bead (50) has a value less than or equal to 0.100.

2. Tyre (1) according to Claim 1, wherein the second lateral reinforcing layer (60) of at least one bead (50) has an elastic shear stiffness modulus that lies within the range [1.5 ; 10] MPa, preferably within the range [1.5 ; 7].

3. Tyre (1) according to one of the preceding claims, wherein the ratio of the difference in the lengths of the rim contact curves of the two sections, i.e. 100*(LADC-LCJ) / LCJ, is greater than or equal to 40, preferably greater than or equal to 50%, more preferably greater than or equal to 70.

4. Tyre (1) according to one of the preceding claims, wherein the radial distance DRB of the first filler layer (70) comprised between the main part (52) of the carcass reinforcement (90) and its turn-up (53) is less than or equal to 50% of the radial height H of the tyre (1).

5. Tyre (1) according to one of the preceding claims, the radial distance DRI being the radial height of a radially innermost end of the lateral reinforcing layer (60) positioned between the sidewall layer (30) and the turn-up (53) of the carcass reinforcement (90), wherein the radial distance DRI lies within the range [5% ; 20%] of the radial height H of the tyre (1).

6. Tyre (1) according to one of the preceding claims, the distance DRL being the distance of the radially outer end of the lateral reinforcing layer (60) positioned between the sidewall layer (30) and the turn-up (53) of the carcass reinforcement (90), wherein said distance DRL is greater than or equal to 25% of the radial height H of the tyre (1).

7. Tyre (1) according to one of the preceding claims, wherein the turn-up (53) of the carcass reinforcement (90) is in contact with the main part (52) of the carcass reinforcement (90) radially on the outside along said turn-up (53).

8. Tyre (1) according to one of the preceding claims, wherein a reinforcement of the bead (50) is introduced axially on the outside between the turn-up (53) of the carcass reinforcement (90) and the lateral reinforcing layer (60), axially on the inside of the sidewall (30).

9. Tyre (1) according to one of the preceding claims, wherein the elastomeric compound making up at least one layer of the first and the second layer (60, 70) of at least one bead (50) has a composition on the basis of 100% polyisoprene natural rubber, or else a blend of natural rubber and polybutadiene, a crosslinking system, a reinforcing filler of carbon black N550 type, at an overall content of between 50 and 75 phr.

10. Tyre (1) according to one of the preceding claims, wherein the elastomeric compound making up the filler layer (70) of at least one bead (50) has the same composition as the elastomeric.