Lightweight tire

Abstract

The invention relates to a tire (1) with a radial carcass reinforcement, two working crown layers (41, 42) which are present only to form the crown reinforcement (4) over at least 40% of the width L5 of the tread (5), the absolute value of the difference between the absolute values of the angles α2 and α1 being greater than 4°, α2 being greater than α1 in absolute terms, the mean angle α satisfying the relationship: 20+164*exp(-L / 100) < α < 23+164*exp(-L / 100). According to the invention, the rubber mixture constituting at least one innerliner layer of at least one working crown layer comprises a composition that comprises 60 to 80 phr of reinforcing fillers, including at least 10 phr pyrolytic carbon black.

Description

LIGHTWEIGHT PNEUMATIC

[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 said cables exhibit, under a tensile force equal to the breaking load, a relative elongation at least equal to 3% with a maximum tangent modulus 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 taking the ratio of this stress value to the elongation value. The man of the profession 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 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] The presence of one or more layers of additional reinforcing elements leads to a greater mass of the tire and higher tire manufacturing costs.

[0026] Also known from document WO 2017 / 149222 is a tire whose crown reinforcement is lightened, improving the endurance properties of the tire with respect to such impacts. The inventors have, however, demonstrated that when driving on surfaces that are extremely stressful for tires, the performance in terms of 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, especially at the shoulders of the tire. It is indeed possible to observe a degradation of performance in terms of endurance, for example, when driving at relatively high speed on construction site approach type ground which is very aggressive for the tire.

[0027] 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, whose endurance performance with regard to the impacts suffered on the tread is improved whatever the nature of the ground and the driving conditions.

[0028] 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 (a1, a2) greater than 8° with the circumferential direction, said angles a1 and a2 being oriented on either side of the circumferential direction, a layer C of rubber mixture being arranged between at least the ends of said at least two working crown layers, 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 40% of the axial width of the crown reinforcement, the reinforcing elements of the radially outermost working layer forming an angle a2 with the circumferential direction greater in absolute value than the angle al 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 a2 and al being greater than 4°, the average angle a satisfying the relationship:, 12+13 l*exp(-L / 100) < a < 20+164*exp(-L / 100), a being defined by the relation a = Arctan((tan(|al|)*tan(|a2|)) 1 / 2), L being the maximum width of the tire measured in the axial direction and expressed in mm, and the rubber mixture constituting at least one calendering layer of at least one working crown layer comprising a composition comprising 60 to 80 pce of reinforcing fillers, including at least 10 pce of pyrolysis carbon black.

[0029] For the purposes of the invention, a 15° drop center type hollow rim or wedge seat rim is a one-piece rim, as defined in the ETRTO, the seats of which intended to receive the tire beads 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 flat-base drop-center hooks whose rim seats have substantially cylindrical shapes.

[0030] The expression "part by weight per hundred parts by weight of elastomer" (or pce) means the part by mass per hundred parts by mass of elastomer or rubber, the two terms being synonymous.

[0031] In certain embodiments, the composition comprises from 60 to 80 phr of reinforcing fillers, the reinforcing fillers being pyrolysis carbon black. It must then be understood that the composition comprises pyrolysis carbon black as the only reinforcing fillers (the composition therefore does not comprise inorganic reinforcing fillers and other organic reinforcing fillers).

[0032] The reinforcing fillers may be as described below.

[0033] For the purposes of the present invention, the term "pyrolysis carbon black" means a carbon black resulting from a process for the pyrolysis of a material comprising at least one carbon polymer and one carbon black, hereinafter the material to be pyrolyzed, for example in the context of the recycling of such a material. The physical state in which the material to be pyrolyzed is present is indifferent, whether in the form of powder, granules, strips, or any other form, in a crosslinked or non-crosslinked state.

[0034] Preferably, the material to be pyrolyzed can be recovered from manufactured articles or products generated during their manufacture / production (such as by-products or offcuts); these manufactured articles can be chosen from the group consisting of tires, solid tires, conveyor belts industrial, transmission belts, rubber seals, rubber hoses, shoe soles and windshield wipers. More preferably still, the pyrolysis carbon black usable in the context of the present invention is a carbon black obtained from a pyrolysis process in which the material to be pyrolyzed comes from manufactured articles chosen from the group consisting of tires and solid tires.

[0035] Pyrolysis in the context of the present invention means any type of thermal decomposition in the absence of oxygen and whose raw material is the material to be pyrolyzed as defined above. Pyrolysis carbon blacks are therefore distinguished from so-called industrial and / or ASTM grade carbon blacks in that the carbon raw material used for pyrolysis is a material comprising at least one carbon polymer and one carbon black and not materials derived from petroleum fractions or from coal or from oils of natural origin.

[0036] The pyrolysis carbon blacks that can be used in the context of the present invention are distinguished from known carbon blacks such as industrial carbon blacks, in particular so-called “furnace” carbon blacks, in particular by a higher ash content than that of said so-called “furnace” carbon blacks. The ash content of so-called “furnace” carbon blacks is less than 1% by weight relative to the total weight of the so-called “furnace” carbon black.

[0037] Preferably, the pyrolysis carbon black usable in the context of the present invention has an ash content ranging from 5 to 30% by weight, more preferably less than 25% by weight, more preferably still less than 22% by weight, relative to the total weight of the pyrolysis carbon black.

[0038] Preferably, the pyrolysis carbon black usable in the context of the present invention has a sulfur content greater than 1.5% by weight, preferably greater than 2% by weight, and more preferably ranging from 2.5 to 5% by weight, relative to the total weight of the pyrolysis carbon black.

[0039] The sulfur content of so-called “furnace” carbon blacks is less than 1.2% by weight relative to the total weight of the so-called “furnace” carbon black.

[0040] Preferably, the pyrolysis carbon black usable in the context of the present invention has a zinc content greater than or equal to 2% by weight, preferably ranging from 2.5 to 8% by weight, relative to the total weight of the pyrolysis carbon black.

[0041] The zinc content of so-called “furnace” carbon blacks is almost zero and a fortiori less than 0.5% by weight relative to the total weight of the so-called “furnace” carbon black.

[0042] Preferably, the pyrolysis carbon black usable in the context of the present invention has a specific surface area STSA measured according to the ASTM D 6556-2021 standard within a range from 20 to 200 m 2 / g, more preferably ranging from 30 to 90 m 2 / g.

[0043] Preferably, the pyrolysis carbon black usable in the context of the present invention has a void volume measured according to standard ASTM D7854 (2018) and at a pressure of 50 MPa within a range from 30 to 60 ml / 100g, more preferably from 35 to 55 ml / 100g.

[0044] The ash content is determined by calcination in platinum capsules in a muffle furnace at 825°C according to the following protocol. A capsule is previously identified before each series of measurements and is tared to the nearest 0.1 mg and the mass is noted PO. In the capsule, 5 g of pyrolysis carbon black sample are introduced and weighed precisely to the nearest 0.1 mg; this mass is noted PL The capsule and its contents are pre-calcined using a Bunsen burner until fumes appear and the product ignites. Once the product has completely burned, the capsule and its contents are introduced into a muffle furnace heated to 825°C for 1 h. After 1 h, the capsule is removed from the furnace and immediately placed in a desiccator at room temperature. When the capsule and the ash have returned to room temperature, the capsule is weighed again to obtain the mass P2.Finally, it is possible to obtain the ash content (% ash) using the formula below:. % ash 100

[0045] The zinc content in the pyrolysis carbon black is determined after calcination of the sample, then recovery of the ashes in an acid medium and determination by ICP-AES (inductively coupled plasma atomic emission spectroscopy). The ash is obtained by carrying out the protocol above. Approximately 100 mg of ash (test sample) is taken and placed in a PFA (perfluoroalkoxy) tube for a HotBlock hot plate. 8 mL of 37% concentrated hydrochloric acid, 3 mL of 65% concentrated nitric acid and 0.5 mL of 40% hydrofluoric acid are then added. The tube is closed with its cap and heated at 130°C for 2 hours. After cooling, the contents are then transferred using ultrapure water into a 100 mL PTFE (polytetrafluoroethylene) volumetric flask already containing 2 g of boric acid (to neutralize the hydrofluoric acid). The volume is topped up with ultrapure water to the mark.The solution obtained is diluted by 100, by taking 1 mL in a 100 mL PFTE flask, previously containing 8 mL of 37% concentrated hydrochloric acid, 3 mL of 65% concentrated nitric acid, 0.5 mL of 40% hydrofluoric acid and 2 g of boric acid. This diluted solution is then filtered through a 0.45 pm GHP syringe filter before being analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES). Before analyzing the diluted solution, at least 5 standards are analyzed by ICP-AES at zinc concentrations of 0, 0.5, 1, 2 and 5 mg / L. These standards were prepared in 100 mL volumetric flasks, by diluting a certified commercial solution to a zinc concentration of 1 g / L.

[0046] These volumetric flasks first contain 8 mL of 37% concentrated hydrochloric acid, 3 mL of 65% concentrated nitric acid, 0.5 mL of 40% hydrofluoric acid and 2 g of boric acid. The standard solutions are analyzed by ICP-AES at a wavelength of XZn = 202.613 nm. For each standard concentration (c), the intensity of the zinc signal IZn is plotted on a graph IZn = f(c), which corresponds to the calibration line (of type y = ax + b). The sample solution (diluted solution) of unknown concentration is then measured under the same conditions as the standards. The measured intensity is related to the concentration using the calibration line obtained previously. This gives the concentration [c] ce ndres in % by mass directly by the software, because the test portion and volume have been previously recorded. The zinc concentration in the pyrolysis black [c] no ir in mass % is obtained by the following equation: Mnoir = Hœndres * 100 * % ashes

[0047] The determination of the sulfur content in pyrolysis carbon blacks is carried out by LECO furnace. LECO sulfur analyzers are designed to measure, in particular, the sulfur content in organic and / or inorganic materials by combustion and non-dispersive infrared detection. Before measuring the sulfur content on the sample, the boats are cleaned and the furnace calibrated. The boats for LECO furnaces are previously cleaned: this involves analyzing the empty boat, under the same conditions as the samples. The preparation of the calibration curve is done from a commercial standard called "BBOT" whose purity is greater than 99.99% and whose carbon (C), hydrogen (H), nitrogen (N), oxygen (O) and sulfur (S) content is guaranteed. This content is as follows: C%: 72.52; H% 6.09; N% 6.51; 0% 7.43 and S% 7.44. Approximately 10 ± 3, 20 ± 3 and 40 ± 3 mg of BBOT are weighed exactly in a pod.The standard / boat assembly is introduced into the combustion furnace, regulated at 1350 C under pure oxygen. The combination of the furnace temperature and the analysis flow rate causes the combustion of the sample and the release of sulfur and / or carbon in the form of SO2(g). After a time of 20 s, oxygen begins to flow through the "lance" to accelerate the combustion of difficult-to-burn materials. The sulfur and / or carbon, in the form of SCh / g), are carried by an oxygen flow through the infrared detection cells. The instrument software plots a straight line connecting the mass of standard introduced and the observed response (area) on the detector. This gives a calibration straight line. After carefully cleaning the sampling equipment, approximately 80 ± 5 mg of pyrolysis carbon black is weighed and introduced into a LECO furnace boat.

[0048] The observed SO2 peak area is related to the concentration using the calibration line. The instrument software then calculates the mass % of sulfur in the sample using the mass of the sample introduced into the basket.

[0049] Pyrolysis carbon blacks are marketed, for example, by the company BlackBear under the reference “BBCT30” or by the company Scandinavian Enviro Systems under the reference “P550”.

[0050] The maximum tire 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.

[0051] The angles a1 and a2, 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.

[0052] According to a preferred embodiment of the invention, the rubber mixture of the calendering layers of said two working crown layers comprises a composition comprising 60 to 80 phr of reinforcing fillers, including at least 10 phr of pyrolysis carbon black.

[0053] According to a preferred embodiment of the invention, said at least one calendering layer of at least one working crown layer 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.

[0054] 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.

[0055] Also preferably, said at least one calendering layer of at least one working top layer, in addition to the pyrolysis carbon black, comprises a reinforcing filler consisting of: a) either by carbon black used at a rate of between 30 and 70 pce, and preferably between 40 and 60 pce, b) or by a white filler of silica and / or alumina type comprising SiOH and / or A10H 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 30 and 70 pce, and preferably between 40 and 60 pce, c) or by a blend of carbon black described in (a) and a white filler described in (b), in which the overall filler rate is between 30 and 70 pce, and preferably between 40 and 60 pce.

[0056] The BET specific surface 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.

[0057] 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 alkoxy silanes 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 Sil 16 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 of > 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.

[0058] As other examples of reinforcing fillers having the morphology and the SiOH and / or A1OH 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 the synthesis by adding a silicon and / or aluminum compound to the furnace feed oil or after the synthesis by adding, to an aqueous suspension of carbon black in a sodium silicate and / or aluminate solution, an acid so as to at least partially cover the surface of the carbon black with SiOH and / or A1OH functions. As non-limiting examples of this type of carbon fillers with SiOH and / or A1OH functions on the surface, we can cite 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.

[0059] 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.

[0060] 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.

[0061] 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, in particular with regard to impacts appearing on the edge of the tread, in particular at the shoulders of the tire, whatever the nature of the ground and the driving conditions.

[0062] 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 mixtures used.

[0063] 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 and by the presence of pyrolysis black used as a filler within at least one calendering layer of at least one working crown layer. They found that the smaller angle of the reinforcing elements of the radially innermost working crown layer appears to cause a delay in the tension taking by the reinforcing elements during such an impact.Furthermore, the inventors were able to demonstrate that the presence of pyrolysis black in the rubber mixture constituting at least one calendering layer of at least one working crown layer gives it higher elongation at break values ​​than with more usual mixtures. Usually, during impacts comparable to those observed when driving on stony ground, more specifically impacting the shoulders of the tire, the rupture of reinforcing elements, if it occurs, is observed on the radially innermost layer and more specifically at its ends.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 associated with the presence of pyrolysis black in the rubber mixture constituting at least one calendering layer of at least one working crown layer make it possible to improve the endurance performance of the tire while reducing the number of layers of the crown reinforcement.

[0064] The use of pyrolysis black in at least one calendering layer of at least one working crown layer, however, leads to a reduction in the rigidity of said at least one calendering layer of at least one working crown layer. This lower rigidity compared to more usual mixtures is a factor that is not favorable for the endurance of this radially innermost working crown layer during high stresses such as rolling at sustained speed.

[0065] The most common tire designs actually provide that the secant moduli of elasticity at 10% elongation of the calendering layers of the working crown layers are greater than 10 MPa. Such moduli of elasticity are required to limit the compression of the reinforcing elements of the working crown layers, particularly when the vehicle follows a winding route, during maneuvers in parking lots or when passing roundabouts. Indeed, the shearing in the axial direction which occurs on the tread in the area of ​​the contact surface with the ground leads to the compression of the reinforcing elements of a working crown layer.

[0066] The less rigid pyrolysis black-based mixtures of said at least one calendering layer of at least one working crown layer contribute to limiting the temperature rises generated when they are subjected to shear stresses.

[0067] 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.

[0068] The inventors have also been able to demonstrate that the cohesion of said at least one calendering layer of at least one working crown layer in accordance with the invention remains satisfactory.

[0069] 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 domain covered by the measurement is included in the temperature range -20°C and +150°C, 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 signal frequency is of the order of 10Hz on average.

[0070] The measure includes 3 parts: • Accommodation of the “PS” specimen, 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 hO = initial height of the specimen. The use of “force / displacement” acquisitions thus gives the relationship between “E” and the amplitude of the stress. • The cracking measurement, after notching the test piece “PS”. The information collected leads to determining the crack propagation speed “Vp” as a function of the imposed stress level “E”.

[0071] The inventors have in particular demonstrated that the lightening of the crown reinforcement associated with the less rigid pyrolysis black-based mixtures of said at least one calendering layer of at least one working crown layer contribute to a lesser change in the cohesion of said at least one calendering layer of at least one working crown layer. Indeed, the more usual tire designs comprising in particular calendering layers of the working crown layers with secant moduli of elasticity at 10% elongation greater than 10 MPa, lead to a change in the cohesion of said layers of rubber mixture arranged between the ends of the working crown layers, this tending to weaken.The inventors note that the lightening of the crown reinforcement associated with the less rigid pyrolysis black-based mixtures of said at least one calendering layer of at least one working crown layer limits the temperature increases and leads to a small change in the cohesion of said at least one calendering layer of at least one working crown layer. The inventors thus consider that the cohesion of layer C, weaker than what exists in more usual tire designs, is satisfactory in the design of the tire according to the invention.

[0072] The combination of the lightweight crown reinforcement as described according to the invention with mixtures of said at least one calendering layer of at least one working crown layer based on pyrolysis black and of lower rigidity 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 to interpret this result 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 mixtures of said at least one calendering layer of at least one working crown layer based on pyrolysis black, less rigid, the combination of these elements appearing to reverse the expected effects in terms of endurance.

[0073] 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 = p2 * Te * [(tan(|al|) / (tan(|al|)+tan(|a2|))) / cos 2 (|a2|) + CF], with Te = 0.078 * P * Rs* (l-(Rs 2 -RL 2 ) / (2*Rt*Rs)), P: the nominal inflation pressure of the tire according to ETRTO, CF = 0.00035 *(min((L-80) / sin(|ocl |), (L-80) / sin(|cx2|), 480)-480), P2: 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 surface of the tire tread, 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 14, 14 and 3 / 4 of the axial width of the tread.

[0074] The thickness Es and the pitch p2 are measured on a section of the tire and are expressed in millimeters.

[0075] 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.

[0076] The inventors also note that the choice of the absolute value of the difference between the absolute values ​​of the aforementioned angles a1 and a2 associated with the average angle a 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. The inventors thus note that the removal of the protective layer is made possible and helps to lighten the tire and limit temperature increases in the area of ​​the tire crown.

[0077] According to a preferred embodiment of the invention, the absolute value of the difference between the absolute values ​​of the angles a2 and a1 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.

[0078] Preferably, the absolute value of the difference between the absolute values ​​of the angles a2 and a1 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.

[0079] 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.

[0080] 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: FRI is the uniaxial extension breaking force of each of the cables in the radially innermost working layer, Fl = pi * Te * [(tan(|a2|) / (tan(|al|)+tan(|oc2|))) / cos 2(|al|) + CF], with pi: the laying pitch of the reinforcing elements of the radially innermost working crown layer, measured perpendicular to the reinforcing elements at the circumferential median plane.

[0081] 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.

[0082] According to an advantageous embodiment of the invention, the axially widest working crown layer is radially inside the other working crown layers.

[0083] According to a preferred embodiment of the invention, optimizing the thinning of the crown of the tire, the two working crown layers are alone present to constitute the crown reinforcement over the entire axial width of the crown reinforcement.

[0084] The metal elements are preferably steel cables.

[0085] According to a preferred embodiment of the invention, the reinforcing elements of the working crown layers are inextensible metal cables.

[0086] 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 figure which represents a meridian view of a diagram of a tire according to an embodiment of the invention.

[0087] 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.

[0088] In the figure, the tire 1 is of dimension 275 / 80 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 cords. 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 cords oriented at an angle equal to 18°, 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.

[0089] 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 mm.

[0090] The crown frame itself is topped with a tread 5.

[0091] The tire is inflated to a pressure of 8.5 bars.

[0092] The axial width L 41 of the first working layer 41 is equal to 220 mm.

[0093] The axial width L 42of the second working layer 42 is equal to 200 mm.

[0094] The axial width of the tread Ls is equal to 215 mm.

[0095] The maximum axial width L is equal to 302 mm.

[0096] 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.

[0097] According to the invention, the calendering layers of the working crown layers 41, 42 consist of an elastomeric mixture comprising a pyrolysis black.

[0098] 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°.

[0099] The average angle is equal to 23.4° and is well within the range of 20.4° to 28.0°.

[0100] The measured value of Re is equal to 541.7 mm.

[0101] The measured value of Es is equal to 22.3 mm.

[0102] The average RL value of the measured radii is equal to 410 mm.

[0103] The Rt value determined on the tire is equal to 900 mm.

[0104] The calculated value of Te is equal to 362 N / mm.

[0105] The calculated value of CF is equal to -0.01.

[0106] The value of Fl is equal to 514.4 N.

[0107] The value of F2 is equal to 311.2 N.

[0108] The breaking forces of the reinforcing elements of the working top layers FRI and FR2 are equal to 2600 N.

[0109] The utilization ratio of the F2 / FR2 rupture potential is equal to 12%.

[0110] The utilization ratio of the F1 / FR1 breaking potential is equal to 19.8%.

[0111] The F1 / FR1 breaking potential utilization ratio is 65% higher than the F2 / FR2 breaking potential utilization ratio.

[0112] Different tires according to the invention are compared to different reference tires of the same size.

[0113] Tires I according to the invention comprise calendering layers of the working crown layers 41, 42 made of the mixture 1.

[0114] First reference tires T1 differ from tires II according to the invention by the nature of the mixtures of the calendering layers of the working crown layers, these being made up of the mixture R.

[0115] The different mixtures used are listed below, expressing for each the secant modulus of elasticity at 10% elongation, the elongation at break.

[0116] The values ​​of the constituents are expressed in pce (parts by weight per hundred parts of elastomers).

[0117] Pyrolysis carbon black, RCB black, contains 20% ash, 1.8% sulfur and 4.5% zinc.

[0118] Carbon black N347 contains 0.5% ash, 1% sulfur and 0% zinc.

[0119] The contents of the various constituents, other than carbon black, are adapted according to the knowledge of those skilled in the art in mixture II to obtain similar temperature and curing time conditions for the different tires and to be able to compare the properties of the tires.

[0120] Second reference tires T2 differ from reference tires T1 by a crown reinforcement formed radially from the inside to the outside: a triangulation layer, consisting of two half plies, formed of non-fretted inextensible metal cables 9.28, oriented at an angle equal to 65°, a first working layer formed of metal cables oriented at an angle equal to 26°, 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.

[0121] 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 mm.

[0122] The T2 reference tire is inflated to a pressure of 8.5 bars.

[0123] The overall axial width of the triangulation layer is equal to 180 mm, each of the half layers having a width equal to 60 mm.

[0124] The axial width of the first working layer is 220 mm.

[0125] The axial width of the second working layer is equal to 200 mm.

[0126] The axial width of the protective layer is 136 mm.

[0127] The cumulative mass of the working layers, the protective layer and the triangulation layer of the T2 reference tires, including the mass of the metal cords and the calendering compounds, amounts to 10.0 kg.

[0128] The mass of the reference tire is 62.9 kg

[0129] 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 8°.

[0130] The average angle is equal to 21.7°.

[0131] The value of Fl is equal to 320 N.

[0132] The value of F2 is equal to 392 N.

[0133] The Fl 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.

[0134] The breaking forces of the reinforcing elements of the working top layers FRI and FR2 are equal to 2600 N.

[0135] The utilization ratio of the F2 / FR2 rupture potential is equal to 15.1%.

[0136] The utilization ratio of the breaking potential F 1 / FR1 is equal to 12.3%.

[0137] The F1 / FR1 breaking potential utilization ratio is 22.7% lower than the F2 / FR2 breaking potential utilization ratio.

[0138] 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.

[0139] 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.

[0140] 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.

[0141] 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. The trajectory of the tire is adjusted so that the axis of the obstacle corresponds to the position of one of the axially outermost ribs on the tread. Breaking strength is characterized by the critical height of the indenter, i.e. the maximum height of the indenter resulting in a total rupture of the crown reinforcement, i.e. the rupture of all the crown layers. The values ​​express the energy required to obtain rupture of the crown block. The values ​​are expressed from a base of 100 corresponding to the value measured for the reference tire T2.

[0142] 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 tires II and 12 according to the invention is greater than that of tire T2 and also greater than that of tire TL.

[0143] 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.

[0144] 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.

[0145] The low-speed rolling phase on a stony track is intended to penalize endurance following repeated impacts on the tread.

[0146] 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.

[0147] 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 assigned to the most damaged tire.

[0148] At the end of the rolling, the tires according to the invention II and 12 show less extensive damage than the reference tires T1 and T2.

[0149] 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.

[0150] These results show that the tires according to the invention actually have lower temperatures than the reference tires.

Claims

CLAIMS 1 - Tire (1), intended to be mounted on a hollow rim of the 15° drop center type, comprising a radial carcass reinforcement (2), said tire comprising a crown reinforcement (4), comprising two working crown layers of reinforcing elements (41, 42) inserted between two calendering layers of rubber mixture, crossed from one layer to the other making angles (a1, a2) greater than 8° with the circumferential direction, said angles a1 and a2 being oriented on either side of the circumferential direction, a layer C of rubber mixture being arranged between at least the ends of said two working crown layers, the crown reinforcement (4) being radially capped with a tread (5), said tread being joined to two beads (3) by means of two sidewalls, in which: - said two working crown layers (41, 42) being the only ones present to constitute the crown reinforcement (4) over at least 40% of the axial width of the crown reinforcement (5), - the reinforcing elements of the radially outermost working crown layer (42) forming an angle (a2) with the circumferential direction greater in absolute value than the angle (al) 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 angles (a2) and (al) is greater than 4°, - the mean angle satisfied the relation: 12+13 l*exp(-L / 100) < a < 20+164*exp(-L / 100), a being defined by the relation a = Arctan((tan(|al|)*tan(|a2|)) 1 / 2), L being the maximum width of the tire measured in the axial direction and expressed in mm, characterized in that the rubber mixture constituting at least one calendering layer of at least one working crown layer comprises a composition comprising 60 to 80 pce of reinforcing fillers, including at least 10 pce of pyrolysis carbon black. 2 - Tire according to claim 1, characterized in that the pyrolysis carbon black has an ash content ranging from 5 to 30% by weight, preferably less than 25% by weight, more preferably less than 22% by weight, relative to the total weight of the pyrolysis carbon black, the ash content being determined by calcination in platinum capsules in a muffle furnace at 825°C according to the protocol presented in the description. 3 - Tire according to claim 1 or 2, characterized in that the pyrolysis carbon black has a sulfur content greater than 1.5% by weight, preferably greater than 2% by weight, and preferably ranging from 2.5 to 5% by weight, relative to the total weight of the pyrolysis carbon black, the determination of the sulfur content in the pyrolysis carbon blacks being carried out by LECO oven according to the method presented in the description. 4 - Tire (1) according to one of claims 1 to 3, characterized in that the pyrolysis carbon black has a zinc content greater than or equal to 2% by weight, preferably ranging from 2.5 to 8% by weight, relative to the total weight of the pyrolysis carbon black, the determination of the zinc content being carried out after calcination of the sample, then recovery of the ashes in an acid medium and dosage by ICP-AES (inductively coupled plasma atomic emission spectroscopy) according to the method presented in the description. 5 - Tire according to one of the preceding claims, characterized in that said at least one calendering layer of at least one working crown layer 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. 6 - Tire (1) according to one of the preceding claims, characterized in that said at least one calendering layer of at least one working crown layer of rubber mixture, in addition to the pyrolysis carbon black, comprises a reinforcing filler consisting of: a) either carbon black used at a rate of between 30 and 70 pce, and preferably between 40 and 60 pce, b) or a white filler of silica and / or alumina type comprising SiOH and / or A10H 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 between 30 and 70 pce, and preferably between 40 and 60 pce, c) either by a blend of carbon black described in (a) and a white filler described in (b), in which the overall filler rate is between 30 and 70 pce, and preferably between 40 and 60 pce. 7 - Tire (1) according to one of the preceding claims, characterized in that the utilization ratio of the rupture potential F2 / FR2 of the radially outermost working crown layer (42) is less than 1 / 6, in which: FR2 is the uniaxial extension breaking force of each of the cables of the radially outermost working crown layer, F2 = p2 * Te * [(tan(|al|) / ((tan(|al|)+tan(|a2|))) / cos 2 (|a2|) + CF], with Te = 0.078 * P * Rs* ( l-(Rs 2 -RL 2 ) / (2*Rt*Rs) ), P: the nominal inflation pressure of the tire according to ETRTO, CF = 0.00035 *(min((L-80) / sin(|ocl |), (L-80) / sin(|cx2|), 480)-480), P2: 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, Rs = Re - Es, Re: outer radius of the tire measured at the radially outermost point on the surface of the tire tread, 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 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 14, 14 and 3 / 4 of the tread width. 8 - Tire (1) according to claim 7, characterized in that the utilization ratio of the rupture potential F2 / FR2 of the radially outermost working crown layer (42) is less than 1 / 8. 9 - Tire (1) according to one of claims 7 or 8, characterized in that the utilization ratio of the rupture potential F1 / FR1 of the radially innermost working crown layer (41) is less than 1 / 3, in which: FRI is the uniaxial extension breaking force of each of the cables in the radially innermost working layer, Fl = pi * Te * [(tan(|a2|) / ((tan(|al|)+tan(|a2|))) / cos 2(|al|) + CF], with pi: the laying pitch of the reinforcing elements of the radially innermost working crown layer, measured perpendicular to the reinforcing elements at the circumferential median plane. 10 - Tire (1) according to claim 9, characterized in that the utilization ratio of the rupture potential F1 / FR1 of the radially innermost working layer (41) is at least 30% greater than the utilization ratio of the rupture potential F2 / FR2 of the radially outermost working layer (42). 11 - Tire (1) according to one of the preceding claims, characterized in that the two working crown layers (41, 42) are alone present to constitute the crown reinforcement over the entire axial width of the crown reinforcement (4).

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