LIGHTWEIGHT TIRES WITH A LAYER OF CIRCULATION REINFORCEMENT ELEMENTS

Description

[0001] The present invention relates to a tire, with a radial carcass reinforcement and more particularly a tire intended to equip vehicles carrying heavy loads and traveling at high speed, such as, for example, trucks, tractors, trailers or road buses.

[0002] Generally speaking, in heavy goods vehicle type tires, the carcass reinforcement is anchored on both sides in the bead area and is surmounted radially by a crown reinforcement consisting of at least two layers, superimposed and formed of parallel wires or cables in each layer and crossed from one layer to the next, making angles of between 10° and 45° with the circumferential direction. Said working layers, forming the working reinforcement, may also be covered with at least one so-called protective layer and formed of advantageously metallic and extensible reinforcement elements, called elastic.It may also comprise a layer of low-extensibility metal wires or cables forming an angle of between 45° and 90° with the circumferential direction, this ply, called the triangulation ply, being radially located between the carcass reinforcement and the first crown ply, called the working ply, formed of parallel wires or cables having angles at most equal to 45° in absolute value. The triangulation ply forms with at least said working ply a triangulated reinforcement, which, under the various stresses to which it is subjected, exhibits little deformation, the triangulation ply having the essential role of absorbing the transverse compression forces to which all the reinforcing elements are subjected in the area of ​​the crown of the tire.

[0003] Cables are said to be inextensible when, under a tensile force equal to 10% of the breaking force, the said cables exhibit a relative elongation of at most 0.2%.

[0004] Cables are said to be elastic when, under a tensile force equal to the breaking load, the said cables exhibit a relative elongation of at least 3% with a maximum tangent modulus of less than 150 GPa.

[0005] Circumferential reinforcing elements are reinforcing elements which make angles with the circumferential direction in the range + 2.5°, - 2.5° around 0°.

[0006] The circumferential direction of the tire, or longitudinal direction, is the direction tangent to the periphery of the tire and defined by the rolling direction of the tire.

[0007] The transverse or axial direction of the tire is parallel to the tire's axis of rotation.

[0008] The radial direction is a direction intersecting the axis of rotation of the tire and perpendicular to it.

[0009] The tire's axis of rotation is the axis around which it rotates during normal use.

[0010] A radial or meridian plane is a plane that contains the tire's axis of rotation.

[0011] The circumferential median plane, or equatorial plane, is a plane perpendicular to the tire's axis of rotation and which divides the tire into two halves.

[0012] The "modulus of elasticity" of a rubber compound is defined as the secant modulus of extension at 10% elongation and at room temperature.

[0013] For rubber compounds, the secant modulus of elasticity at 10% elongation is the elastic modulus of the compound measured in a uniaxial tensile experiment at an elongation value of 0.1 (i.e. 10% elongation, expressed as a percentage). A constant uniaxial tensile speed is imposed on the specimen, and its elongation and stress are measured. The measurement is carried out using an INSTRON tensile testing machine at a temperature of 23°C and a relative humidity of 50% (ISO 23529 standard). The measurement conditions and the analysis of the results to determine the elongation and stress are as described in NF ISO 37: 2012-03. The stress is determined for an elongation of 0.1 and the secant modulus of elasticity is calculated at 10% elongation by dividing this stress value by the elongation value.The person skilled in the art will know how to choose and adapt the dimensions of the test piece according to the quantity of mixture accessible and available, particularly in the case of taking samples from a finished product such as a tire.

[0014] Some current tires, called "road tires", are designed to travel at high speeds and on increasingly long journeys, due to the improvement of the road network and the growth of the motorway network throughout the world. All the conditions under which such a tire is called upon to travel undoubtedly allow an increase in the number of kilometers traveled, the wear of the tire being less; on the other hand, the endurance of the latter and in particular of the crown reinforcement is penalized.

[0015] There are indeed constraints at the level of the crown reinforcement and more particularly shear constraints between the crown layers, combined with a significant increase in the operating temperature at the ends of the axially shortest crown layer, which result in the appearance and propagation of cracks in the rubber at the said ends.

[0016] In order to improve the endurance of the crown reinforcement of the type of tire studied, solutions relating to the structure and quality of the layers and / or profiles of rubber compounds which are arranged between and / or around the ends of the plies and more particularly the ends of the axially shortest ply have already been provided.

[0017] It is known in particular to introduce a layer of rubber mixture between the ends of the working layers to create a decoupling between said ends to limit the shear stresses. Such decoupling layers must however have very good cohesion. Such layers of rubber mixtures are for example described in patent application WO 2004 / 076204.

[0018] Patent FR 1 389 428, to improve the resistance to degradation of rubber compounds located near the edges of the crown reinforcement, recommends the use, in combination with a low hysteresis tread, of a rubber profile covering at least the sides and marginal edges of the crown reinforcement and consisting of a low hysteresis rubber compound.

[0019] Patent FR 2 222 232, to avoid separations between crown reinforcement plies, teaches coating the ends of the reinforcement in a rubber mattress, the Shore A hardness of which is different from that of the tread surmounting said reinforcement, and greater than the Shore A hardness of the rubber mixture profile arranged between the edges of the crown reinforcement plies and the carcass reinforcement.

[0020] The tires thus produced effectively improve performance, particularly in terms of endurance.

[0021] Furthermore, it is known to introduce a layer of circumferential reinforcing elements to produce tires with a very wide tread or to give tires of a given size greater load capacities. Patent application WO 99 / 24269 describes, for example, the presence of such a layer of circumferential reinforcing elements.

[0022] The layer of circumferential reinforcing elements is usually made up of at least one metal cable wound to form a turn whose laying angle relative to the circumferential direction is less than 2.5°.

[0023] In addition, the use of tires on heavy-duty vehicles of the "site approach" type leads to the tires being subjected to shocks when driving on stony ground. These shocks are of course detrimental to performance in terms of endurance.

[0024] It is also known to those skilled in the art to increase the number of plies constituting the crown reinforcement to improve the endurance of the tire with regard to such impacts.

[0025] Whichever solution is considered among the solutions presented above, the presence of one or more layers of additional reinforcement elements leads to a greater mass of the tire and to higher tire manufacturing costs.

[0026] Document WO 2017 / 149223 also discloses a tire whose crown reinforcement is lightened, improving the tire's endurance properties with respect to such impacts. The inventors have, however, demonstrated that when driving on extremely demanding ground for tires, the endurance performance of such tires could be degraded, for example in driving conditions combining the speed of the vehicle, the load carried by the tire and the nature of the ground, which is particularly aggressive. It is indeed possible to observe a degradation of the endurance performance, for example, when driving at relatively high speed on construction site approach type ground that is very aggressive for the tire.

[0027] It is also known from document WO 2017 / 198952 A1 to choose rubber mixtures for the calendering layers of the working plies, of lower rigidity to improve the rolling resistance performance of the tires.

[0028] The inventors have thus set themselves the mission of providing tires for "Heavy Goods" vehicles, for example of the "construction site approach" type, whose overall mass is limited and whose endurance performance with regard to impacts suffered on the tread is improved whatever the nature of the ground and the driving conditions.

[0029] This object is achieved according to the invention by a tire intended to be mounted on a hollow rim of the 15° drop center type, comprising a radial carcass reinforcement, said tire comprising a crown reinforcement comprising two working crown layers, each formed of reinforcing elements inserted between two calendering layers of rubber mixture crossed from one layer to the other making angles (α1, α2) greater than 8° with the circumferential direction, said angles α1 and α2 being oriented on either side of the circumferential direction, and at least one layer of circumferential reinforcing elements, the crown reinforcement being capped radially with a tread, said tread being joined to two beads by means of two sidewalls,said two working crown layers and said at least one layer of circumferential reinforcing elements being alone present to constitute the crown reinforcement over at least 75% of the axial width of the crown reinforcement, the reinforcing elements of the radially outermost working layer forming an angle α2 with the circumferential direction greater in absolute value than the angle α1 formed by the reinforcing elements of the radially innermost working layer with the circumferential direction, the absolute value of the difference between the absolute values ​​of the angles α2 and α1 being greater than 7°, the average angle α satisfying the relationship: , 13 + 131 * exp − L / 100 < α < 28 + 110 * exp − L / 100 , α being defined by the relation α = Arctan((tan(|α1|)*tan(|α2|)) 1 / 2< ), L being the maximum width of the tire measured in the axial direction and expressed in mm, the secant modulus of elasticity at 10% elongation of at least one calendering layer of at least one working crown layer being less than 8.5 MPa.

[0030] For the purposes of the invention, a hollow rim of the 15° drop center type or rim with a wedged seat is a one-piece rim, as defined in the ETRTO, the seats of which intended to receive the beads of the tire have a truncated cone shape, the angle formed with the axial direction being substantially equivalent to 15°. These seats are also extended by rim hooks of reduced height compared to rim hooks with flat bases whose rim seats have substantially cylindrical shapes.

[0031] The width L is measured on a tire mounted on its nominal rim and inflated to its nominal pressure, according to ETRTO, and is expressed in millimeters.

[0032] The angles α1 and α2, expressed in degrees, are measured on a section of the tire. According to the invention, the angle measurements are carried out at the level of the circumferential median plane.

[0033] Advantageously according to the invention, the maximum value of tan(δ), noted tan(δ) max , of said at least one calendering layer, of at least one working crown layer, whose secant modulus of elasticity at 10% elongation is less than 8.5, is less than 0.100.

[0034] The loss factor tan(δ) is a dynamic property of the rubber compound layer. It is measured on a viscoanalyzer (Metravib VA4000), according to ASTM D 5992-96. The response of a sample of vulcanized compound (cylindrical specimen 2 mm thick and 78 mm 2< in cross-section) is recorded, subjected to 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 1% (return cycle). For the return cycle, the maximum value of tan(δ) observed is indicated, noted tan(δ) max .

[0035] The person skilled in the art will know how to choose and adapt the dimensions of the test piece according to the quantity of mixture accessible and available, particularly in the case of taking samples from a finished product such as a tire.

[0036] Rolling resistance is the resistance that appears when the tire rolls. It is represented by the hysteretic losses linked to the deformation of the tire during a revolution. The frequency values ​​linked to the revolution of the tire correspond to values ​​of tan(δ) measured between 30 and 100°C. The value of tan(δ) at 100°C thus corresponds to an indicator of the rolling resistance of the tire while rolling.

[0037] According to a preferred embodiment of the invention, the secant modulus of elasticity at 10% elongation of the calendering layers of said two working crown layers is less than 8.5 MPa and more preferably, the tan(δ) max value of the calendering layers of said at least two working crown layers is less than 0.100.

[0038] According to a preferred embodiment of the invention, said at least one calendering layer, of at least one working crown layer, whose secant modulus of elasticity at 10% elongation is less than 8.5 is an elastomeric mixture based on natural rubber or synthetic polyisoprene with a majority of cis-1,4 chains and optionally at least one other diene elastomer, the natural rubber or the synthetic polyisoprene in the case of blending being present at a majority rate compared to the rate of the other or other diene elastomers used and a reinforcing filler consisting of: a) either by carbon black with a BET specific surface area greater than 60 m 2 < / g, i. used at a rate of between 20 and 40 pce when the black structure index (COAN) is greater than 85, ii.used at a rate of between 20 and 60 pce when the black structure index (COAN) is less than 85, b) either by carbon black with a BET specific surface area of ​​less than 60 m 2 < / g, whatever its structure index, used at a rate of between 20 and 80 pce, and preferably between 30 and 50 pce, c) either by a white filler of silica and / or alumina type comprising SiOH and / or AlOH surface functions chosen from the group formed by precipitated or pyrogenic silicas, aluminas or aluminosilicates or even carbon blacks modified during or after synthesis with a BET specific surface area of ​​between 30 and 260 m 2 < / g used at a rate of between 20 and 80 pce, and preferably between 30 and 50 pce, d) either by a blend of carbon black described in (a) and / or carbon black of carbon described in (b) and / or a white filler described in (c), in which the overall filler rate is between 20 and 80 phr, and preferably between 40 and 60 phr.

[0039] The BET specific surface area measurement is carried out according to the method of BRUNAUER, EMMET and TELLER described in "The Journal of the American Chemical Society", vol. 60, page 309, February 1938, corresponding to the NFT 45007 standard of November 1987.

[0040] The COAN (Compressed Oil Absorption Number) black structure index is measured according to ASTM D3493.

[0041] In the case of using a clear filler or white filler, it is necessary to use a coupling and / or covering agent chosen from agents known to those skilled in the art. Examples of preferred coupling agents that may be mentioned are sulfurized alkoxysilanes of the bis-(3-trialkoxysilylpropyl) polysulfide type, and among these, in particular bis-(3-triethoxysilylpropyl) tetrasulfide marketed by the company DEGUSSA under the names Si69 for the pure liquid product and X50S for the solid product (50 / 50 blend by weight with black N330). Examples of covering agents include a fatty alcohol, an alkylalkoxysilane such as a hexadecyltrimethoxy or triethoxysilane respectively marketed by the DEGUSSA Company under the names Si116 and Si216, diphenylguanidine, a polyethylene glycol, a silicone oil possibly modified by means of OH or alkoxy functions.The covering and / or coupling agent is used in a weight ratio relative to the filler ≥ 1 / 100 and ≤ 20 / 100, and preferably between 2 / 100 and 15 / 100 when the light filler represents the entire reinforcing filler and between 1 / 100 and 20 / 100 when the reinforcing filler consists of a blend of carbon black and light filler.

[0042] As other examples of reinforcing fillers having the morphology and SiOH and / or AlOH surface functions of the silica and / or alumina type materials previously described and which can be used according to the invention as a partial or total replacement for these, mention may be made of carbon blacks modified either during synthesis by adding a silicon and / or aluminum compound to the furnace feed oil or after synthesis by adding an acid to an aqueous suspension of carbon black in a sodium silicate and / or aluminate solution so as to at least partially cover the surface of the carbon black with SiOH and / or AlOH functions. As non-limiting examples of this type of carbon filler with SiOH and / or AlOH functions on the surface, mention may be made of the CSDP type fillers described in Conference No. 24 of the ACS Meeting, Rubber Division, Anaheim, California, May 6-9, 1997, as well as those of patent application EP-A-0 799 854.

[0043] When a clear filler is used as the sole reinforcing filler, the hysteresis and cohesion properties are obtained by using a precipitated or pyrogenic silica, or a precipitated alumina or an aluminosilicate with a BET specific surface area of ​​between 30 and 260 m 2 < / g. Non-limiting examples of this type of filler include silicas KS404 from Akzo, Ultrasil VN2 or VN3 and BV3370GR from Degussa, Zeopol 8745 from Huber, Zeosil 175MP or Zeosil 1165MP from Rhodia, HI-SIL 2000 from PPG, etc.

[0044] Among the diene elastomers that can be used in blending with natural rubber or a synthetic polyisoprene with a majority of cis-1,4 chains, we can cite a polybutadiene (BR) preferably with a majority of cis-1,4 chains, a styrene-butadiene copolymer (SBR) solution or emulsion, a butadiene-isoprene copolymer (BIR) or even a styrene-butadiene-isoprene terpolymer (SBIR). These elastomers may be elastomers modified during polymerization or after polymerization by means of branching agents such as divinylbenzene or star-forming agents such as carbonates, halotins, halosilicones or even by means of functionalization agents leading to grafting onto the chain or at the end of the chain of oxygenated carbonyl, carboxyl functions or even an amine function such as for example by the action of dimethyl or diethylamino benzophenone.In the case of blends of natural rubber or synthetic polyisoprene with a majority of cis-1,4 chains with one or more of the diene elastomers mentioned above, the natural rubber or synthetic polyisoprene is preferably used at a majority rate and more preferably at a rate greater than 70 pce.

[0045] The results obtained with tires conforming to the invention have effectively demonstrated that endurance performance can be improved, the crown reinforcement of the tire being lightened, whatever the nature of the ground and the driving conditions. The lightening of the crown reinforcement of the tire is also accompanied by a simplification of manufacturing and a reduction in manufacturing costs.

[0046] Against all expectations, the results have indeed shown that the tires according to the invention can be made lighter by reducing the number of layers constituting the crown reinforcement while improving the endurance properties of the crown of the tire, particularly with regard to impacts appearing on the tread whatever the nature of the ground and the driving conditions.

[0047] It is in fact known to those skilled in the art that to improve the endurance performance of the crown reinforcement of a tire with regard to this type of impact, it is usual to increase the number of layers of reinforcing elements as well as to increase the rigidity of the calendering compounds.

[0048] The inventors believe that these results can be interpreted because the angle formed with the circumferential direction by the reinforcing elements of the radially innermost working crown layer is smaller in absolute value than that formed by the reinforcing elements of the radially outermost working crown layer. They found that this smaller angle appears to cause a delay in the tensioning of the reinforcing elements during such an impact. Usually, during impacts comparable to those observed when driving on stony ground, the rupture of reinforcing elements, if it occurs, is observed on the radially innermost layer. These findings seem to indicate that in the face of this type of attack, the difference in angles of the reinforcing elements between the two working crown layers makes it possible to improve the endurance performance of the tire while reducing the number of layers of the crown reinforcement.

[0049] The value of the average angle α, increased compared to more usual values ​​known from usual tires for this type of application, further improves the endurance properties of the crown of the tire with regard to impacts appearing on the tread. The presence of said at least one layer of circumferential reinforcement elements allows an average angle α of the reinforcement elements of the two working crown layers greater than the average angle defined by the reinforcement elements of the two working crown layers in more usual tires.Indeed, the circumferential rigidity provided by the presence of said at least one layer of circumferential reinforcing elements makes it possible to increase the average angle formed by the reinforcing elements of each of the working crown layers with the circumferential direction, said average angle thus appearing to promote the operation of the tire and in particular its maneuverability when traveling under heavy loads or with a very large angle formed with the direction of travel. The inventors were thus able to demonstrate that the dynamic properties of the tire, in particular the drift rigidity, are preserved or even improved regardless of the use.

[0050] On the other hand, the inventors also believe that they have demonstrated that the angle formed with the circumferential direction by the reinforcing elements of the radially innermost working crown layer, which is smaller in absolute value than that formed by the reinforcing elements of the radially outermost working crown layer, leads, under extreme stresses, particularly when driving at sustained speed, to high stresses on the reinforcing elements of the radially innermost working layer, which may be detrimental to performance in terms of endurance.

[0051] The use of low-rigidity calendering compound for this radially innermost working crown layer, according to the invention, is still a factor that is not favorable for the endurance of this radially innermost working crown layer during high stresses such as rolling at high speed.

[0052] Typically, the secant moduli of elasticity at 10% elongation of the crown layer calenders are greater than 8.5 MPa and most often greater than 10 MPa. Such moduli of elasticity are required in particular to limit the compression of the reinforcing elements of the working crown layers, particularly when the vehicle follows a winding route. Indeed, the shear forces in the axial direction which occur on the tread in the area of ​​the contact surface with the ground lead to the compression of the reinforcing elements of a working crown layer.

[0053] The inventors were able to demonstrate that the layer of circumferential reinforcing elements allows choices of elasticity moduli of the rubber mixtures of the calendering layers of the lower working crown layers without harming the endurance properties of the tire due to the compression of the reinforcing elements of said working crown layers as described previously.

[0054] The inventors were also able to demonstrate that the cohesion of the calendering layers of the working crown layers, when it has a secant modulus of elasticity at 10% elongation of less than 8.5 MPa, remains satisfactory.

[0055] For the purposes of the invention, a cohesive rubber mixture is a rubber mixture that is particularly robust to cracking. The cohesion of a mixture is thus evaluated by a fatigue cracking test carried out on a “PS” (pure shear) specimen. It consists of determining, after notching the specimen, the crack propagation speed “Vp” (nm / cycle) as a function of the energy release rate “E” (J / m 2 < ). The experimental range covered by the measurement is in the range -20°C and +150°C in temperature, with an air or nitrogen atmosphere. The stress on the specimen is an imposed dynamic displacement of amplitude between 0.1mm and 10mm in the form of a pulse-type stress (tangent “haversine” signal) with a rest time equal to the duration of the pulse; the frequency of the signal is on average around 10Hz.

[0056] The measure includes 3 parts: An accommodation of the “PS” specimen, of 1000 cycles at 27% deformation. an energy characterization to determine the law “E” = f (deformation). The energy release rate “E” is equal to W0*h0, with W0 = energy supplied to the material per cycle and per unit of volume and h0 = initial height of the specimen. The use of the “force / displacement” acquisitions thus gives the relationship between “E” and the amplitude of the stress. The cracking measurement, after notching the “PS” specimen. The information collected leads to determining the crack propagation speed “Vp” as a function of the imposed stress level “E”.

[0057] The inventors have in particular demonstrated that the presence of at least one layer of circumferential reinforcing elements contributes to a lesser change in the cohesion of the calendering layers of the working crown layers. Indeed, the most usual tire designs comprising in particular calendering layers of the working crown layers with secant moduli of elasticity at 10% elongation greater than 8.5 MPa, lead to a change in the cohesion of said calendering layers of the working crown layers, this tending to weaken. The inventors note that the presence of at least one layer of circumferential reinforcing elements which contributes to limiting the compression of the reinforcing elements of the working crown layers leads to a little change in the cohesion of the calendering layers.The inventors thus consider that the cohesion of the calendering layers of the working crown layers, weaker than what exists in more usual tire designs, is satisfactory in the design of the tire according to the invention.

[0058] In addition, the less rigid mixtures of the working crown layer calenders associated with a layer of circumferential reinforcing elements contribute to limiting the temperature rises generated when they are subjected to shear stresses.

[0059] Furthermore, the lightening of the crown reinforcement leads to a reduction in the overall thickness of the crown of the tire. The inventors were also able to demonstrate that when driving at high speed, compared to tires of more conventional design, the temperature of the crown of the tire is lower.

[0060] The combination of the lightweight crown reinforcement as described according to the invention with low-rigidity calendering compounds which appear to be able to compromise the endurance properties of the tire under extreme conditions of use actually leads to a conservation, or even an improvement, of the endurance performance of the tire. The inventors believe that this result can be interpreted by cumulative effects on the temperature of the crown of the tire, on the one hand of the lightweight design of the crown reinforcement and on the other hand of the presence of less rigid calendering compounds in the presence of a layer of circumferential reinforcing elements, the combination of these elements appearing to reverse the expected effects in terms of endurance.

[0061] Advantageously according to the invention, the utilization ratio of the rupture potential F2 / FR2 of the radially outermost working layer is less than 1 / 6, and in which: FR2 is the uniaxial extension breaking force of each of the cables in the radially outermost working layer, F2 = p 2 * Tc * tan α 1 / tan α 1 + tan α 2 / cos 2 α 2 + C F , with Tc = 0.078 * P * Rs * 1 − Rs 2 − R L 2 / 2 * Rt * Rs , P: the nominal inflation pressure of the tire according to ETRTO, C F = 0.00035 * min L − 80 / sin α 1 , L − 80 / sin α 2 , 480 − 480 , p 2: the laying pitch of the reinforcing elements of the radially outermost working crown layer, measured perpendicular to the reinforcing elements at the level of the circumferential median plane, Rs = Re − Es , Re: outer radius of the tire measured at the radially outermost point on the tire tread surface, said surface being extrapolated to fill any hollows, Es: radial distance between the radially outermost point of the tire and its orthogonal projection onto the radially outer face of a reinforcing element of the radially innermost working crown layer, RL: average of the radii of the axially outermost points of the main part of the carcass reinforcement layer on each side of the tire, Rt: the radius of the circle passing through three points located on the outer surface of the tread outside the hollows, defined from a shoulder end at respective axial distances equal to ¼, ½ and ¾ of the axial width of the tread.

[0062] The thickness Es and the pitch p 2 are measured on a section of the tire and are expressed in millimeters.

[0063] The meridian section of the tire is defined in accordance with the invention such that the barycenters of the bead wires form an axially oriented straight line, said barycenters being distant from each other by a distance equal to the width of the nominal rim J increased by 20 mm and reduced by twice the distance measured axially between a barycenter of a bead wire and a point on the outer surface of the tire.

[0064] The inventors also note that the choice of the absolute value of the difference between the absolute values ​​of the aforementioned angles α1 and α2 associated with the average angle α and the utilization ratio of the rupture potential F2 / FR2 as defined according to this advantageous embodiment of the invention can make it possible to eliminate the protective layer usually placed radially outside the other layers of the crown reinforcement. Such a layer is usually present to be sacrificed in the event of attacks on the tire such as cuts which can alter the integrity of metallic reinforcement elements by corrosion phenomena associated with the fatigue of said reinforcement elements.The inventors actually note that the reinforcing elements of the radially outermost working crown layer of a tire according to the invention are less stressed during inflation of the tire or during its use in normal driving than the reinforcing elements of a radially outermost working crown layer of a more usual tire; such a more usual tire has smaller absolute value differences in angles between the reinforcing elements of the different working layers, an angle of the reinforcing elements of the radially innermost working layer greater than or equal in absolute value to that of the reinforcing elements of the radially outermost working layer and a greater ratio of utilization of the rupture potential F2 / FR2.The reinforcing elements of the radially outermost working crown layer of a tire according to the invention thus have endurance properties much superior to those of a more usual tire; the inventors thus note that the removal of the protective layer is made possible and makes it possible to contribute to the lightening of the tire and the limitation of temperature increases in the tire crown area.

[0065] According to a preferred embodiment of the invention, the absolute value of the difference between the absolute values ​​of the angles α2 and α1 is greater than or equal to 10° and preferably greater than 14°. According to this embodiment, and in accordance with the interpretations given above, it will be possible to further improve the endurance performance of the reinforcing elements of the radially outermost working layer and / or further improve the performance of the tire with respect to impacts such as those experienced when driving on stony ground.

[0066] Preferably, the absolute value of the difference between the absolute values ​​of the angles α2 and α1 is less than 25° and more preferably less than 20°. Beyond these values, the tire would be likely to exhibit irregular wear under certain conditions of use.

[0067] Advantageously still according to the invention, the utilization ratio of the rupture potential F2 / FR2 of the radially outermost working layer is less than 1 / 8. Such a utilization ratio of the rupture potential F2 / FR2 further contributes to improving the endurance performance of the reinforcing elements of the radially outermost working layer during use of the tire.

[0068] Preferably according to the invention, the utilization ratio of the rupture potential F1 / FR1 of the radially innermost working layer is less than 1 / 3, in which: FR1 is the uniaxial extension breaking force of each of the cables in the radially innermost working layer, F1 = p 1 * Tc * tan α 2 / tan α 1 + tan α 2 / cos 2 α 1 + C F , with p 1: the laying pitch of the reinforcing elements of the radially innermost working crown layer, measured perpendicular to the reinforcing elements at the level of the circumferential median plane.

[0069] More preferably, the utilization ratio of the rupture potential F1 / FR1 of the radially innermost working layer is at least 30% greater than the utilization ratio of the rupture potential F2 / FR2 of the radially outermost working layer.

[0070] According to an advantageous embodiment of the invention, the axially widest working crown layer is radially inside the other working crown layers.

[0071] According to a preferred embodiment of the invention, optimizing the thinning of the crown of the tire, the two working crown layers and said at least one layer of circumferential reinforcing elements are alone present to constitute the crown reinforcement over the entire axial width of the crown reinforcement.

[0072] According to an advantageous embodiment of the invention, the layer of circumferential reinforcing elements has an axial width greater than 0.5xS.

[0073] S is the maximum axial width of the tire, when the latter is mounted on its service rim and inflated to its recommended pressure.

[0074] The axial widths of the reinforcing element layers are measured on a cross-section of a tire, the tire being in an uninflated state.

[0075] According to a preferred embodiment of the invention, the layer of circumferential reinforcing elements is radially arranged between two working crown layers.

[0076] According to this embodiment of the invention, the layer of circumferential reinforcing elements makes it possible to limit the compression of the reinforcing elements of the carcass reinforcement to a greater extent than a similar layer placed radially outside the working layers. It is preferably radially separated from the carcass reinforcement by at least one working layer so as to limit the stresses on said reinforcing elements and not fatigue them excessively.

[0077] Advantageously still according to the invention, the axial widths of the working crown layers radially adjacent to said at least one layer of circumferential reinforcing elements are greater than the axial width of said at least one layer of circumferential reinforcing elements and preferably, said working crown layers adjacent to said at least one layer of circumferential reinforcing elements are on either side of the equatorial plane and in the immediate axial extension of said at least one layer of circumferential reinforcing elements coupled over an axial width, to then be decoupled by a layer of rubber mixture at least over the remainder of the width common to said two working layers.

[0078] For the purposes of the invention, working crown layers are said to be coupled if the respective reinforcing elements of each of the layers are separated radially by a distance less than the average diameter of the circle circumscribed by the reinforcing elements, said rubber thickness being measured radially between the respectively radially upper and lower generatrices of said reinforcing elements.

[0079] The average diameter of the circle circumscribed by the reinforcing elements is defined as the average diameter of the circles circumscribed by the reinforcing elements of each of the working crown layers.

[0080] The presence of such couplings between the working crown layers adjacent to the layer of circumferential reinforcing elements allows the reduction of tensile stresses acting on the axially outermost circumferential elements located closest to the coupling.

[0081] The thickness of the decoupling profiles between the working crown layers, axially external to the coupling zone, measured at the ends of the narrowest working layer, will be at least 2 mm, and preferably greater than 2.5 mm.

[0082] According to an advantageous embodiment of the invention, the reinforcing elements of at least one layer of circumferential reinforcing elements are metallic reinforcing elements having a secant modulus at 0.7% elongation of between 10 and 120 GPa and a maximum tangent modulus of less than 150 GPa.

[0083] According to a preferred embodiment, the secant modulus of the reinforcing elements at 0.7% elongation is less than 100 GPa and greater than 20 GPa, preferably between 30 and 90 GPa and more preferably less than 80 GPa.

[0084] Also preferably, the maximum tangent modulus of the reinforcing elements is less than 130 GPa and more preferably less than 120 GPa.

[0085] The moduli expressed above are measured on a tensile stress versus elongation curve determined with a prestress of 20 MPa, the tensile stress corresponding to a measured tension related to the metal section of the reinforcing element. The measurements are carried out on cables extracted from the tire on a part of the layer of circumferential reinforcing elements extending from an axial end of said layer over an axial width of 50 mm towards the inside of said layer.

[0086] The moduli of the same reinforcing elements can be measured on a tensile stress versus elongation curve determined with a prestress of 10 MPa, the tensile stress corresponding to a measured tension reduced to the overall section of the reinforcing element. The overall section of the reinforcing element is the section of a composite element made of metal and rubber, the latter having notably penetrated the reinforcing element during the curing phase of the tire.

[0087] According to this formulation relating to the overall section of the reinforcing element, the reinforcing elements of the axially outer parts and of the central part of at least one layer of circumferential reinforcing elements are metallic reinforcing elements having a secant modulus at 0.7% elongation of between 5 and 60 GPa and a maximum tangent modulus of less than 75 GPa.

[0088] According to a preferred embodiment, the secant modulus of the reinforcing elements at 0.7% elongation is less than 50 GPa and greater than 10 GPa, preferably between 15 and 45 GPa and more preferably less than 40 GPa.

[0089] Also preferably, the maximum tangent modulus of the reinforcing elements is less than 65 GPa and more preferably less than 60 GPa.

[0090] According to a preferred embodiment, the reinforcing elements of at least one layer of circumferential reinforcing elements are metallic reinforcing elements having a tensile stress curve as a function of relative elongation having low slopes for low elongations and a substantially constant and steep slope for higher elongations.

[0091] The various characteristics of the reinforcement elements stated above are measured on reinforcement elements taken from tires.

[0092] Reinforcing elements more particularly suitable for producing at least one layer of circumferential reinforcing elements according to the invention are, for example, assemblies of formula 21.23, the construction of which is 3x(0.26+6x0.23) 4.8 / 7.5 SS; this stranded cable is made up of 21 elementary wires of formula 3 x (1+6), with 3 twisted strands each made up of 7 wires, one wire forming a central core with a diameter equal to 26 / 100 mm and 6 wound wires with a diameter equal to 23 / 100 mm. Such a cable has a secant modulus at 0.7% equal to 45 GPa and a maximum tangent modulus equal to 98 GPa, measured on a tensile stress versus elongation curve determined with a prestress of 20 MPa, the tensile stress corresponding to a measured tension related to the metal section of the reinforcing element.On a tensile stress versus elongation curve determined with a prestress of 10 MPa, the tensile stress corresponding to a measured tension reduced to the overall section of the reinforcing element, this cable of formula 21.23 has a secant modulus at 0.7% equal to 23 GPa and a maximum tangent modulus equal to 49 GPa.

[0093] Similarly, another example of reinforcing elements is an assembly of formula 21.28, whose construction is 3x(0.32+6x0.28) 5.6 / 9.3 SS. This cable has a 0.7% secant modulus equal to 56 GPa and a maximum tangent modulus equal to 102 GPa, measured on a tensile stress versus elongation curve determined with a prestress of 20 MPa, the tensile stress corresponding to a measured tension reduced to the metal section of the reinforcing element. On a tensile stress versus elongation curve determined with a prestress of 10 MPa, the tensile stress corresponding to a measured tension reduced to the overall section of the reinforcing element, this cable of formula 21.28 has a 0.7% secant modulus equal to 27 GPa and a maximum tangent modulus equal to 49 GPa.

[0094] The use of such reinforcing elements in at least one layer of circumferential reinforcing elements makes it possible in particular to maintain satisfactory layer rigidities even after the shaping and curing steps in standard manufacturing processes.

[0095] According to a second embodiment of the invention, the circumferential reinforcing elements may be formed from inextensible metal elements and cut so as to form sections of length much less than the circumference of the shortest layer, but preferably greater than 0.1 times said circumference, the cuts between sections being axially offset from each other. More preferably, the tensile modulus of elasticity per unit width of the additional layer is less than the tensile modulus of elasticity, measured under the same conditions, of the most extensible working crown layer.Such an embodiment makes it possible to confer, in a simple manner, on the layer of circumferential reinforcing elements a modulus which can easily be adjusted (by the choice of the intervals between sections of the same row), but in all cases, lower than the modulus of the layer made up of the same metallic but continuous elements, the modulus of the additional layer being measured on a vulcanized layer of cut elements, taken from the tire.

[0096] According to a third embodiment of the invention, the circumferential reinforcing elements are corrugated metal elements, the ratio a / λ of the corrugation amplitude over the wavelength being at most equal to 0.09. Preferably, the tensile modulus of elasticity per unit width of the additional layer is lower than the tensile modulus of elasticity, measured under the same conditions, of the most extensible working crown layer.

[0097] The metal elements are preferably steel cables.

[0098] According to a preferred embodiment of the invention, the reinforcing elements of the working crown layers are inextensible metal cables.

[0099] Other details and advantageous characteristics of the invention will emerge below from the description of the exemplary embodiments of the invention with reference to the figure which represents a meridian view of a diagram of a tire according to an embodiment of the invention.

[0100] The figure is not drawn to scale to simplify understanding. The figure represents only a half-view of a tire that extends symmetrically about the XX' axis, which represents the circumferential median plane, or equatorial plane, of a tire.

[0101] In the figure, the tire 1 is of dimension 315 / 70 R 22.5. Said tire 1 comprises a radial carcass reinforcement 2 anchored in two beads, not shown in the figure. The carcass reinforcement is formed of a single layer of metal cables. This carcass reinforcement 2 is hooped by a crown reinforcement 4, formed radially from the inside to the outside: of a first working layer 41 formed of metal cables oriented at an angle equal to 18°, of a layer of circumferential reinforcing elements 43 formed of 21x23 steel metal cables, of a second working layer 42 formed of metal cables oriented at an angle equal to 30° and crossed with the metal cables of the layer 41, the cables of each of the working layers 41, 42 being oriented on either side of the circumferential direction.

[0102] The metal cables constituting the reinforcing elements of the two working layers are cables of formula 9.35. They are distributed in each of the working layers with a distance between the reinforcing elements, measured along the normal to the direction of the cable's mean line, equal to 2.2 mm.

[0103] The crown frame itself is topped with a tread 5.

[0104] The tire is inflated to a pressure of 9 bars.

[0105] The axial width L 41 of the first working layer 41 is equal to 252 mm.

[0106] The axial width L 42 of the second working layer 42 is equal to 232 mm.

[0107] The axial width L 43 of the layer of circumferential reinforcing elements 43 is equal to 194 mm.

[0108] The axial width of the tread L 5 is equal to 266 mm.

[0109] The maximum axial width L is equal to 315.9 mm.

[0110] The cumulative mass of the two working layers 41, 42 and the layer of circumferential reinforcing elements 43, including the mass of the metal cables and the calendering mixtures, thus amounts to 10.1 kg.

[0111] The difference between the angles formed by the cables of the first working crown layer with the circumferential direction and those of the cables of the second working crown layer is equal to 12°.

[0112] The average angle is equal to 23.4° and is well within the range of 18.6° to 32.7°.

[0113] The measured value of Re is equal to 508.5 mm.

[0114] The measured value of Es is equal to 22.5 mm.

[0115] The average RL value of the measured radii is equal to 401.3 mm.

[0116] The Rt value determined on the tire is equal to 792 mm.

[0117] The calculated value of Tc is equal to 308 N / mm.

[0118] The calculated value of CF is equal to -0.0029.

[0119] The value of F1 is equal to 477.2 N.

[0120] The value of F2 is equal to 323.3 N.

[0121] The breaking forces of the reinforcing elements of the working crown layers FR1 and FR2 are equal to 2950 N.

[0122] The utilization ratio of the F2 / FR2 rupture potential is equal to 11%.

[0123] The utilization ratio of the F1 / FR1 breaking potential is equal to 16.2%.

[0124] The F1 / FR1 breaking potential utilization ratio is 48% higher than the F2 / FR2 breaking potential utilization ratio.

[0125] Different tires I1, I2, I3, I4 and I5 according to the invention are compared with different reference tires of the same size. Tires I1, I2, I3, I4 and I5 comprise calendering layers consisting respectively of the compounds 1, 2, 3, 4 and 5 described below.

[0126] First reference tires T1 differ from the tires according to the invention by the nature of the calendering mixtures of the working crown layers, these being made up of the mixture R1 described below.

[0127] The different mixtures used are listed below, expressing for each the secant modulus of elasticity at 10% elongation as well as the max tan(δ) values. R1 Mix Mix 1 Mix 2 Mix 3 Mix 4 Mix 5 NR 100 100 100 100 100 100 Black N347 52 33 Black N683 44 30 Black N326 47 Silica 165G 46 Antioxidant (6PPD) 1 1.5 1 2 1 1 Stearic acid 0.65 0.9 0.65 1 0.65 0.65 Zinc oxide 9.3 7.5 9.3 8 9.3 9.3 Cobalt salt (AcacCo) 1.12 1.12 1.12 1.1 1.12 1.12 Cobalt salt (AbietateCo) Silane on black 8.3 sulfur 6.1 4.5 6.1 4.8 6.1 6.1 DCBS Accelerator 0.93 0.8 0.93 0.93 0.93 TBBS Accelerator 1.01 DPG Co-accelerator 1.1 CTP PVI retarder) 0.25 0.15 0.25 0.2 0.25 0.25 MA 10 (MPa) 10.4 5.99 5.56 7.25 6.16 4.4 tan(δ) max 0.130 0.099 0.074 0.063 0.056 0.030

[0128] The values ​​of the constituents are expressed in pce (parts by weight per hundred parts of elastomers).

[0129] Second reference tires T2 differ from reference tires T1 by a crown reinforcement formed radially from the inside to the outside: of a first working layer formed of metal cables oriented at an angle equal to 22°, on the same side as the cables of the triangulation layer with respect to the circumferential direction, of a layer of circumferential reinforcing elements formed of steel metal cables 21.23, , of a second working layer formed of metal cables oriented at an angle equal to 18° and crossed with the metal cables of the first working layer, the cables of each of the working layers being oriented on either side of the circumferential direction, of a protective layer formed of elastic metal cables 6.35, the distance between the reinforcing elements, measured along the normal to the direction of the center line of the cable, is equal to 2.5 mm, oriented at an angle equal to 18°, on the same side as the cables of the second working layer.

[0130] The metal cables of the two working layers are cables of formula 9.35. They are distributed in each of the working layers with a distance between the reinforcement elements, measured along the normal to the direction of the cable's average line, equal to 2.5 mm.

[0131] The T2 reference tire is inflated to a pressure of 9 bars.

[0132] The axial width of the first working layer is equal to 252 mm.

[0133] The axial width of the second working layer is 232 mm.

[0134] The axial width of the layer of circumferential reinforcing elements 53 is equal to 194 mm.

[0135] The axial width of the protective layer is 188 mm.

[0136] The cumulative mass of the working layers, the protective layer and the layer of circumferential reinforcing elements of the T2 reference tires, including the mass of the metal cords and the calendering compounds, amounts to 12.2 kg.

[0137] The absolute value of the difference between the absolute values ​​of the angles formed by the cables of the first working crown layer with the circumferential direction and those of the cables of the second working crown layer is equal to 4°.

[0138] The average angle is equal to 19.9°.

[0139] The value of F1 is equal to 410 N.

[0140] The value of F2 is equal to 424 N.

[0141] The F1 and F2 values ​​are obtained by a finite element simulation, the high number of reinforcement plies in the crown not allowing the use of a simple analytical model.

[0142] The breaking forces of the reinforcing elements of the working crown layers FR1 and FR2 are equal to 2600 N.

[0143] The F2 / FR2 rupture potential utilization ratio is equal to 16.3%.

[0144] The utilization ratio of the F1 / FR1 breaking potential is equal to 15.8%.

[0145] The F1 / FR1 breaking potential utilization ratio is 3% lower than the F2 / FR2 breaking potential utilization ratio.

[0146] Initial endurance tests, particularly thermally demanding, were carried out on a test machine requiring each of the tires to run in a straight line at a speed equal to the maximum speed index prescribed for the said tire (speed index) under an initial load of 4000 kg, gradually increased to reduce the duration of the test.

[0147] Other endurance tests, particularly demanding mechanically, were carried out on a test machine cyclically imposing a transverse force and a dynamic overload on the tires. The tests were carried out for the tires according to the invention with conditions identical to those applied to the reference tires.

[0148] The tests thus carried out showed that the distances covered during each of these tests are substantially identical for the tires according to the invention and the reference tires.

[0149] Tests to characterize the breaking strength of a tire crown reinforcement subjected to impacts were also carried out. These tests consist of rolling a tire, inflated to a recommended pressure and subjected to a recommended load, over a cylindrical obstacle or indenter with a diameter equal to 1.5 inches, or 38.1 mm, with a hemispherical head, and of a determined height, resting on the center of the tread. The breaking strength is characterized by the critical height of the indenter, that is to say the maximum height of the indenter causing a total rupture of the crown reinforcement, that is to say the rupture of all the crown layers. The values ​​express the energy necessary to obtain rupture of the crown block. The values ​​are expressed from a base 100 corresponding to the value measured for the reference tire T2. 11, 12, 13, 14,15 130 Reference T1 125 Reference T2 100

[0150] These results show that despite a reduction in the weight of the tire, in particular by a reduction in the mass of its crown reinforcement, the breaking energy during an impact on the surface of the tread of the tires according to the invention is greater than that of the T2 tire and slightly greater than that of the T1 tire.

[0151] Final endurance tests aimed at reproducing driving conditions combining vehicle speed and particularly aggressive ground conditions have been carried out. These tests thus reproduce extreme conditions, particularly for heavy goods vehicles of the "site approach" type.

[0152] These latest tests consist of reproducing twenty-five times a 2-hour driving phase at 100 km / h on a circuit under the load and pressure conditions indicated on the tire followed by a 12-minute driving phase at 35 km / h on a stony track.

[0153] The low-speed rolling phase on a stony track is intended to penalize endurance following repeated impacts on the tread.

[0154] The purpose of running at high speed on a circuit is to increase the temperature of the tire. This makes the tire more sensitive to the effects of repeated impacts on the tread and promotes the propagation of cracks initiated during running on stony tracks.

[0155] At the end of the run, the tires are inspected using shearography and dissected to analyze any damage. This is a visual analysis that allows for comparison of any cracks and their propagation. The tires are rated and compared with each other. A score above 100 corresponds to a less damaged tire. A value of 100 is given to the most damaged tire.

[0156] Only tires according to the invention I1 and I4 were tested. Note I4 120 I1 115 Reference T1 105 Reference T2 100

[0157] At the end of the rolling, the tires according to the invention I1 and I4 show less extensive damage than the reference tires T1 and T2.

[0158] During these last endurance tests, the temperature of the tires at the ends of the crown block was measured following the first phase of running for 2 hours at 100 km / h on the circuit.

[0159] During these last tests, T3 reference tires which differ from T2 reference tires by the nature of the calendering compounds of the working crown layers, these being made up of mixture 4, were also measured. Temperature I4 90°C I1 91°C Reference T1 94°C Reference T3 95°C Reference T2 99°C

[0160] These results show that the tires according to the invention actually have lower temperatures than the reference tires.

Claims

1. -Tyre (1), intended to be mounted on a drop-centre rim of the 15° drop-centre type, comprising a radial carcass reinforcement (2), said tyre comprising a crown reinforcement (4) comprising two working crown layers of reinforcing elements (41, 42) which are inserted between two skim coat layers of rubber compound and that are crossed from one layer to the next and make angles (α1, α2) of greater than 8° with the circumferential direction, said angles α1 and α2 being oriented one on each side of the circumferential direction, and at least one layer of circumferential reinforcing elements (43), the crown reinforcement (4) being capped radially by a tread (5), said tread being connected to two beads (3) by two sidewalls, wherein: - said two working crown layers (41, 42) and said at least one layer of circumferential reinforcing elements (43) are the only ones present to form the crown reinforcement (4) over at least 75% of the axial width of the crown reinforcement (5), - the reinforcing elements of the radially outermost working crown layer (42) forming an angle (α2) with the circumferential direction that is greater in terms of absolute value than the angle (α1) formed by the reinforcing elements of the radially innermost working crown layer (41) with the circumferential direction, - the absolute value of the difference between the absolute values of the angles (α2) and (α1) is greater than 7°, - the mean angle α satisfies the relationship: 13 + 131 * exp − L / 100 < α < 28 + 110 * exp − L / 100 , α being defined by the relationship α = Arctan((tan(|α1|)*tan(|α2|))1 / 2), L being the maximum width of the tyre measured in the axial direction and expressed in mm, characterized in that the secant elastic modulus at 10% elongation, at a temperature of 23°C, of at least one skim coat layer of at least one working crown layer is less than 8.5 MPa.

2. - Tyre according to Claim 1, characterized in that the maximum value of tan(δ), denoted tan(δ)max, of said at least one skim coat layer of at least one working crown layer is less than 0.100.

3. - Tyre according to Claim 1or 2, characterized in that said at least one skim coat layer of at least one working crown layer is an elastomer compound based on natural rubber or on synthetic polyisoprene with a predominance of cis-1,4- linkages and optionally on at least one other diene elastomer, the natural rubber or synthetic polyisoprene in the case of a blend being present at a predominant content relative to the content of the other diene elastomer(s) used, and on a reinforcing filler composed: a) either of carbon black with a BET specific surface area of greater than 60 m2 / g, i. used at a content of between 20 and 40 phr when the compressed oil absorption number (COAN) of the carbon black is greater than 85, ii. used at a content of between 20 and 60 phr when the compressed oil absorption number (COAN) of the carbon black is less than 85, b) or of carbon black with a BET specific surface area of less than 60 m2 / g, whatever its compressed oil absorption number, used at a content of between 20 and 80 phr and preferably between 30 and 50 phr, c) or of a white filler of silica and / or alumina type comprising SiOH and / or AlOH surface functional groups, selected from the group consisting of precipitated or fumed silicas, aluminas and aluminosilicates, or alternatively carbon blacks modified during or after the synthesis having a BET specific surface area of between 30 and 260 m2 / g, used at a content of between 20 and 80 phr and preferably between 30 and 50 phr, d) or of a blend of carbon black described in (a) and / or of carbon black described in (b) and / or a white filler described in (c), in which the overall content of filler is between 20 and 80 phr and preferably between 40 and 60 phr.

4. - Tyre (1) according to one of Claims 1 to 3, characterized in that the rupture potential index F2 / FR2 of the radially outermost working crown layer (42) is less than 1 / 6, where: FR2 is the breaking force in uniaxial extension of each of the cords of the radially outermost working crown layer, F2 = p 2 * Tc * tan α 1 / tan α 1 + tan α 2 / cos 2 α 2 + C F , where Tc = 0.078 * P * Rs * 1 − Rs 2 − RL 2 / 2 * Rt * Rs , P is the nominal inflation pressure of the tyre according to the ETRTO, C F = 0.00035 * min L − 80 / sin α 1 , L − 80 / sin α 2 , 480 − 480 , p2 is the pitch at which the reinforcing elements of the radially outermost working crown layer are laid, measured perpendicularly to the reinforcing elements at the circumferential median, Rs = Re − Es , Re is the external radius of the tyre, measured at the radially outermost point on the tread surface of the tyre, said surface being extrapolated in order to fill any voids there might be, Es is the radial distance between the radially outermost point of the tyre and the orthogonal projection thereof onto the radially exterior face of a reinforcing element of the radially innermost working crown layer, RL is the mean of the radii of the axially outermost points on each side of the tyre, Rt is the radius of the circle passing through three points situated on the outer surface of the tread outside of the voids, defined from a shoulder end at respective axial distances equal to ¼, ½ and ¾ of the width of the tread.

5. - Tyre (1) according to Claim 4, characterized in that the rupture potential index F2 / FR2 of the radially outermost working crown layer (42) is less than 1 / 8.

6. - Tyre (1) according to one of Claims 4 and 5 characterized in that the rupture potential index F1 / FR1 of the radially innermost working crown layer (41) is less than 1 / 3, where: FR1 is the breaking force in uniaxial extension of each of the cords of the radially innermost working layer, F1 = p 1 * Tc * tan α 2 / tan α 1 + tan α 2 / cos 2 α 1 + C F , where p1 is the pitch at which the reinforcing elements of the radially innermost working crown layer are laid, measured perpendicularly to the reinforcing elements at the circumferential median plane.

7. - Tyre (1) according to Claim 6, characterized in that the rupture potential index F1 / FR1 of the radially innermost working layer (41) is at least 30% higher than the rupture potential index F2 / FR2 of the radially outermost working layer (42).

8. - Tyre (1) according to one of the preceding claims, characterized in that the two working crown layers (41, 42) and said at least one layer of circumferential reinforcing elements (43) are the only ones present to form the crown reinforcement over the entirety of the axial width of the crown reinforcement (4).

9. - Tyre (1) according to one of the preceding claims, characterized in that the axial widths of the two working crown layers (41, 42) are greater than the axial width of said at least one layer of circumferential reinforcing elements (43).

10. - Tyre (1) according to one of the preceding claims, characterized in that said at least one layer of circumferential reinforcing elements (43) is positioned radially between the two working crown layers (41, 42).

11. - Tyre (1) according to Claim 9 or 10, characterized in that the working crown layers (41, 42) are, on each side of the equatorial plane and in the immediate axial continuation of said at least one layer of circumferential reinforcing elements (43), coupled over an axial width and then decoupled by profiled elements of rubber compound at least over the remainder of the width that said two working layers (41, 42) have in common.

12. - Tyre (1) according to one of the preceding claims, characterized in that the reinforcing elements of said at least one layer of circumferential reinforcing elements (43) are metal reinforcing elements having a secant modulus at 0.7% elongation of between 10 and 120 GPa and a maximum tangent modulus of less than 150 GPa.

13. - Tyre (1) according to one of the preceding claims, characterized in that the reinforcing elements of the working crown layers (41, 42) are inextensible.