Optimized architecture for a tyre of civil engineering type for electric vehicles

Abstract

A radial tyre (1) for a civil engineering vehicle, having a load index of 24 tonnes and a speed index of 30 km / h, with a nominal rim diameter of 39 inches, intended to be inflated to a pressure of 6 bar, comprising a crown with a radial thickness Es at most equal to 20 mm, comprising a tread and a crown reinforcement. The tread has an axial width L, and has a radial thickness Eml. The radial thickness Eml of the tread is at least equal to 40 mm. The axial width of the tread is at least equal to 850 mm and at most equal to 1200 mm.

Description

Description Title: Optimized architecture of civil engineering type tires for electric vehicles

[0001] The present invention relates to a radial tire, intended to equip a heavy vehicle of the civil engineering type used in surface mines.

[0002] Radial tires intended to equip a heavy civil engineering vehicle are designated in accordance with the European Tire and Rim Technical Organization (ETRTO) standard.

[0003] For example, a radial tire for a heavy vehicle of the civil engineering type, within the meaning of the European Tire and Rim Technical Organization (ETRTO) standard, is intended to be mounted on a rim whose diameter is at least 25 inches. Although not limited to this type of application, the invention is described for a large radial tire intended to be mounted on an electric dumper, in particular vehicles for transporting materials extracted from quarries or surface mines, by means of a rim whose diameter is at least 35 inches and can reach 51 inches.

[0004] A tire having a geometry of revolution with respect to an axis of rotation, the geometry of the tire is generally described in a meridian plane containing the rotation axis of the tire. For a given meridian plane, the radial, axial and circumferential directions respectively denote the directions perpendicular to the rotation axis of the tire, parallel to the rotation axis of the tire and perpendicular to the meridian plane. The circumferential direction is tangent to the circumference of the tire.

[0005] In the following, the expressions "radially inward" and "radially outward" respectively mean "closer" and "further" respectively from the rotational axis of the tire. By "axially inward" and "axially outward" respectively, we mean "closer" and "further" respectively from the rotational axis of the tire. equatorial plane of the tire”, the equatorial plane of the tire being the plane passing through the middle of the rolling surface and perpendicular to the axis of rotation. By “an element A axially inside an element B by an axial distance D”, we mean that element A is closer to the equatorial plane than element B and that the axial distance between the two elements is equal to the distance D. This type of sentence can be generalized with the radial and circumferential direction and the external versus internal position of one or the other of the elements.

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

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

[0008] The carcass reinforcement of a radial tire for a heavy vehicle of the civil engineering type usually comprises at least one carcass layer comprising metal reinforcements, coated with a polymeric material of the elastomer or elastomeric type, obtained by mixing and called a coating mixture. A carcass layer comprises a main part, connecting the two beads together and generally wrapping, in each bead, from the inside to the outside of the tire around a circumferential reinforcement element, most often metallic, called a bead wire, to form a turn-up. The metal reinforcements of a carcass layer are substantially parallel to each other and form, with the circumferential direction, an angle of between 85° and 95°.

[0009] The crown reinforcement of a radial tire for a civil engineering vehicle comprises a superposition of crown layers extending circumferentially, radially outside the carcass reinforcement. Each crown layer is made up of generally metallic reinforcements, parallel between them and coated with a polymeric material of the elastomer type or coating mixture.

[0010] A metal reinforcement is mechanically characterized by a curve representing the tensile force (in N), applied to the metal reinforcement, as a function of its relative elongation (in %), called the force-elongation curve. From this force-elongation curve are deduced the mechanical tensile characteristics of the metal reinforcement, such as the structural elongation As (in %), the total elongation at break At (in %), the breaking force Fm (maximum load in N) and the breaking strength Rm (in MPa), these characteristics being measured according to the ASTM D 2969-04 standard. [OR] The total elongation At of the metal reinforcement is, by definition, the sum of its structural, elastic and plastic elongations (At = As + Ae + Ap) and particularly at break where each of the elongations is non-zero. The structural elongation As results from the relative positioning of the metal wires constituting the metal reinforcement under a low tensile force. The elastic elongation Ae results from the very elasticity of the metal of the metal wires, constituting the metal reinforcement, taken individually, the behavior of the metal following a Hooke law. The plastic elongation Ap results from the plasticity, that is to say from the irreversible deformation, beyond the elastic limit, of the metal of these metal wires taken individually. These different elongations as well as their respective meanings, well known to those skilled in the art, are described, for example, in documents US5843583, WO2005 / 014925 and WO2007 / 090603.

[0012] At any point on the force-elongation curve of a metal reinforcement, a modulus in extension, expressed in GPa, is also defined, which represents the slope of the line tangent to the force-elongation curve at that point. In particular, the elastic modulus in extension or Young's modulus is called the modulus in extension of the linear elastic part of the force-elongation curve.

[0013] Among the metal reinforcements, a distinction is usually made between elastic metal reinforcements, such as those used in protective layers, and non-extensible or inextensible metal reinforcements.

[0014] An elastic metal reinforcement, in its rubberized state, from the tire iis characterized by a structural elongation As at least equal to 0.3% and a total elongation at break At at least equal to 3%. In addition, an elastic metal reinforcement has an elastic modulus in extension at most equal to 150 GPa, and usually between 40 GPa and 110 GPa.

[0015] An inextensible metal reinforcement is characterized by a total elongation At of at most 2%. Furthermore, an inextensible metal reinforcement has an elastic modulus in extension usually between 150 GPa and 200 GPa.

[0016] Among the crown layers, a distinction is usually made between the protective layers, constituting the protective reinforcement and radially the outermost, comprising elastic reinforcing elements (or reinforcements), and the working layers comprising reinforcing elements, constituting the working reinforcement and radially between the protective reinforcement and the carcass reinforcement. The angles of the reinforcing elements of the protective layers, or their elasticity relatively greater than that of the reinforcing elements of the working layers, are such that the protective layers absorb only very little rolling force compared to the working layers.

[0017] The protective reinforcement, comprising at least one protective layer, essentially protects the working layers from mechanical or physicochemical attacks, likely to propagate through the tread radially towards the inside of the tire.

[0018] The protective reinforcement often comprises two protective layers, radially superimposed, formed of elastic metal reinforcements, parallel to each other in each layer and crossed from one layer to the next, forming, with the circumferential direction, angles at least equal to 15°. The protective layers are radially external to the other crown layers which they protect from attacks.

[0019] The working reinforcement, comprising at least two working layers, has the function of surrounding the tire and giving it rigidity and road holding. It absorbs both the mechanical stresses of inflation, generated by the tire's inflation pressure and transmitted by the carcass reinforcement, and mechanical rolling stresses, generated by the rolling of the tire on the ground and transmitted by the tread. It must also resist oxidation and impacts and perforations, thanks to its intrinsic design and that of the protective frame.

[0020] The working reinforcement usually comprises two radially superimposed working layers, formed of non-extensible metal reinforcements, parallel to each other in each layer and crossed from one layer to the next, forming, with the circumferential direction, angles at most equal to 60°, and preferably at least equal to 15° and at most equal to 45°. To reduce the shearing of the rubber compounds, or rubber mixtures, at the axial ends of the working layers, it is usual to axially offset the position of said ends relative to each other. The crown reinforcement therefore usually comprises a working layer of greater axial width and a working layer of smaller axial width. The shearing of the rubber compounds is maximum at the ends of the most rigid crown layers.In fact, these maximum shears due to the displacements of the end of the crown layers are distributed over the radial thickness of rubber compounds between the different crown layers. These shear maxima are generally reduced by adding one or more decoupling rubbers between the ends of the crown layers.

[0021] To reduce the mechanical stresses of inflation and rolling transmitted to the working reinforcement and the shearing of the rubber mixture which covers them, it is known to arrange, radially outside the carcass reinforcement, a hoop reinforcement. The hoop reinforcement, the function of which is to take up at least part of the mechanical stresses of inflation, improves the endurance of the crown reinforcement by stiffening the crown reinforcement. The hoop reinforcement can be positioned radially inside the working reinforcement, between the two working layers of the working reinforcement, or radially outside the working reinforcement.

[0022] In civil engineering applications, the hoop reinforcement can comprise two radially superimposed hoop layers, formed of reinforcements metallic, parallel to each other in each layer and crossed from one layer to the next, forming, with the circumferential direction, angles at most equal to 10°. To produce the hooping layers, on large diameter civil engineering tires, the winding of a layer of discontinuous reinforcing elements whose ends go from one axial edge of the layer to the other forming with the circumferential direction an angle of between 0° and 10°, may be preferred to the winding of a narrow strip of a few continuous reinforcing elements forming with the circumferential direction an angle of between 0° and 5° for productivity reasons.

[0023] In both cases, the hoop layers are of smaller axial width than the working layer of smaller axial width. Indeed, the stresses due to rolling at the ends of the hoop layers are very high in traction and increasing their widths would lead to degradation of the tire's endurance. This avoids placing reinforcement elements forming angles between 0° and 10° with the circumferential direction, at axial positions too far from the median plane.

[0024] Historically, the race for profitability in previous years led vehicle manufacturers to design increasingly heavy trucks or dumpers for surface mines, transporting as much ore as possible and requiring tires with increasingly high load indices. To carry this load, tire manufacturers had no choice but to increase the size of the tires. Thus, the most recent dumpers are equipped with 63-inch tires (rim size) such as the 59 / 80R63, which can carry 100t. These tires are equipped with a volume of rubber to be worn, which is a compromise depending on the aggressiveness of the mine, which determines a type of elastomeric mixture or rubber compound, with a given rigidity and hysteresis.The crown construction with the metal reinforcing elements and the filling rubbers have a limit temperature of use which determines the volume to be worn of the tread around a speed of 20 km / h. The more the rubber compound is resistant to aggression and generally the more hysteretic it is, the less the tread has a radial thickness Eml. important to avoid overheating of the tire. Thus these very large tires can carry up to 1.5 tons of rubber volume. These tires thus have the highest productivity index used today, the TKPH or ton transported per kilometer hour, which is obtained by multiplying the load index by the speed index while respecting a maximum operating temperature equal to 120°C. This gigantism is also linked to the use of thermal engines and diesel fuel allowing a long autonomy of the vehicles.

[0025] Like other industrial sectors, surface mines will have to decarbonize and for this one solution is to use electric motors. To avoid significant energy consumption in acceleration phases and to have batteries of reasonable size and weight, it is advisable to return to tires of more modest dimensions and to reduce rotational inertia. It would be possible to use existing tires of type 27.00R49 whose TKPH is around a third of 59 / 80R63, which seems of little interest. However, this dimension is much lighter and around 25% of the mass of a 59 / 80 R63. So if from the productivity side, these dimensions are less interesting than the largest dimensions, from the point of view of material efficiency which would be calculated as the TKPH / mass of the tire, they are largely competitive with a gain of almost 25% compared to the dimensions "productivists". This type of reasoning must nevertheless be pushed further to make the best use of the material for a service provided while preserving material resources as much as possible.

[0026] The inventors set themselves the objective of offering an optimal tire in terms of service provided in relation to material use.

[0027] This objective has been achieved, according to the invention, by a radial tire for a civil engineering vehicle, with a load index at least equal to 24 t and a speed index greater than 30 km / h, delimiting an internal cavity intended to receive an inflation gas when the tire is mounted on a nominal rim having a rim diameter at least equal to 39 inches and at most equal to 51 inches, a maximum axial width B and a sidewall height H, comprising: - a crown comprising a tread and a crown reinforcement radially inside the tread and a carcass reinforcement, radially inside the crown reinforcement, the crown being connected by means of two sidewalls to two beads intended to come into contact with the rim, - the tread intended to come into contact with a ground via a rolling surface having an axial width L, and having a radial thickness Eml measured at the median circumferential plane from the rolling surface to the most radially outer point of the crown reinforcement, and having a shoulder thickness Emle measured from the axial end of the rolling surface perpendicular to the carcass, - the crown reinforcement, with a radial thickness ES measured at the median circumferential plane and comprising at least four crown layers, comprising metal reinforcements, - a total radial thickness ETM measured at the median circumferential plane from the rolling surface to the internal cavity, - crown thicknesses, called useful, firstly at the median plane ENCM measured from the most radially outer point of the carcass layer to the rolling surface and secondly at the shoulder ENCe measured perpendicular to the carcass layer, from the axial end of the rolling surface to the most radially outer point of the carcass layer, - the radial thickness Eml of the tread being at least equal to 40 mm - metal reinforcements of at least two crown layers making angles with the circumferential direction whose absolute value is greater than 15°, - the radial thickness ES of the crown reinforcement, measured at the median circumferential plane, being at most equal to 20 mm, - the radial thickness ETM being at most equal to 125 mm, - the ratio between the height of the sidewall H and the width of the tire B being at most equal to 0.75, - the axial width L of the tread being at least equal to 850 mm and at most equal to 1200 mm.

[0028] At first glance, using a wider tire based on a 49-inch wheel rim diameter like a 40 / 65R49 doesn't really make sense. In fact, in a first estimate, the tire being wider, it will be heavier for an equivalent gain in rubber thickness. Creating such a dimension doesn't seem very interesting.

[0029] It is by varying the multiple design parameters by following new reasoning of efficiency on many essential performances such as the mass of the tire, its limit speed, its temperature of use, its efficiency in wear, its load, its pressure, endurance to aggression, material efficiency... that the inventors have determined a field of design particularly interesting for uses where large tires, like tires mounted on 63-inch rims, are no longer as interesting.

[0030] Thus, in the materials used in a tire, it is necessary to distinguish two types of materials: the wearing material and the material allowing the transmission of steering forces. These two materials can be represented respectively by the thickness of the rubber compound of the tread Eml and the crown thickness ES which represents the mass of the structure necessary to transmit the forces, resist punctures, transmit the steering forces in particular. The problem with dimensions such as 27.00R49, apart from their low TKPH given their load index for a usage speed of 20 km / h, is that the efficiency of their tread on wear is not optimal.

[0031] For a rim around 39 inches, tires with a width to sidewall height ratio (H / B), called series, lower than 0.75 will have a contact patch footprint whose ratio Sc of axial width to circumferential length increases considerably (from 0.75 to 1.4); the bending inertia on the edge increases the drift rigidity of the tire. Thus for the same transverse force, the angle required to put on the steering wheel decreases, but the slipped length at the exit of the tread is proportional to the angle and the circumferential length of the contact patch which also decreases. The tire can thus be much more efficient in terms of wear by around 25% provided that the crown remains flexible and therefore of a limited thickness to allow obtaining a the most homogeneous footprint possible in terms of the length of the contact patch between its center and the shoulders and not as is usual for series civil engineering tires less than 0.75, with a significantly shorter contact patch between the center and the shoulders.

[0032] Indeed, this shorter footprint shape in the center, which can be up to 50% difference in length between the center and the shoulder, is typical of civil engineering tires in these series below 0.75, hooped in the center to improve shoulder endurance. This contact area shape leads to differentiated wear between the center and the shoulder, which is less efficient than a contact area whose center and shoulders have similar lengths. For these types of series and according to the invention, the optimal axial width of the contact area is between 850 mm and 1200 mm.

[0033] In addition to this improvement in wear, an optimized contact area requires opening the angle of the reinforcement elements of the hoop layers or even removing them. This results in a radial softening of the crown block, which is useful for reducing crown attacks and therefore wear on aggression and a reduction in crown temperature.

[0034] The evaluation of the improvement in thermal and mechanical endurance was carried out using finite element calculations at the same load, speed and pressure. All of these combinations - series, crown flexibility, shape of the contact area, etc. - allow a very significant improvement in operating temperature compared to a 27.00R49, of the order of 15°C at the median circumferential plane and nearly 25°C at the shoulders. This drop in temperature is accompanied by a significant drop in deformation energy at the level of the rubber compounds decoupling the working layers.This gain would allow to increase the thickness of the tread to gain in life but already having a gain in efficiency of the tread in wear thanks to the contact area and in wear on aggression thanks to the flexibility of the crown, from a point of view of the efficiency of the materials, it is more relevant to increase the speed index of the tire to compensate for the loss of transported mass rather than the load. Obviously the tread must have a radial thickness at the meridian plane at least equal to 40 mm to offer a minimum life on wear and the total radial thickness ETM from the crown to the median circumferential plane since. the rolling surface up to the internal cavity of the tire must be at most equal to 125 mm to minimize rotational inertia and thermal peak, preferably at most equal to 110 mm and preferably at most equal to 90 mm. The tires thus designed have a maximum distance per hour at least equal to 30 km and preferably at least equal to 40 km. This allows between the effect of wear and speed to progress in TKPH by a factor of 1.8 compared to a 27.00R49 tire of the state of the art, certainly less than 60% of a 59 / 80R63 tire but for a reduced weight of 66%.The optimal balance between rolling resistance, wear, temperature, and material efficiency is generally achieved with a total radial thickness ETM from the crown to the mid-circumferential plane of less than 110 mm rather than 125 mm, which is more conducive to service life but with the efficiency of the additional millimeters of tread being less than the loss in thermal and rolling resistance. With an ETM of less than or equal to 90 mm, rolling resistance will be favored.

[0035] Reducing moving masses by reducing the number of layers and reducing the tread thickness is beneficial for reducing thermal stress, increasing productivity and energy efficiency.

[0036] If the material efficiency of such tires is assessed by dividing the productivity represented by the TKPH by the material required to obtain it, the tires according to the invention are twice as efficient as very large tires such as the 59 / 80R63.

[0037] But this efficiency does not take into account the tire's lifespan. For example, one method to improve this efficiency would be simply to reduce the radial thickness of the tread. But this gain at the expense of the duration of use is not relevant. An indicator of the efficiency of this dimension consists of multiplying this efficiency by the tire's lifespan. Given the wear performance of the contact area and the performance of the design in aggressive wear, the tire according to the invention improves this efficiency by around a factor of two compared to a 27.00R49 and 3 compared to a 59 / 80R63 which is very productive in terms of transported mass but not very efficient when its mass and the effectiveness of its contact area and its design with regard to wear are taken into account.

[0038] To avoid these lifetime calculations, the inventors proposed an indicator equal to TKPH*Eml / ((H / B)*ETM*ES), the Eml giving an idea of ​​the lifetime while taking into account the H / B the effectiveness of the contact patch, the ETM and the ES integrating the effectiveness of the material used to put this tread compound on the ground. With such an indicator, all the tires according to the invention are above 77 for the least optimized, and ranging from 90 to 123 for the most optimized in terms of both the shape of the contact patch and the crown, when the reference tires are between 40 and 75. For the inventors, this parameter, homogeneous to a flow of transported material, is relevant for classifying tires according to their productivity in relation to the material required for the latter.Thus it is advantageous that for a tire according to the invention having a productivity index of tonnes of ore moved in one hour, TKPH, the efficiency index equal to TKPH*Eml / ((H / B)*ETM*ES is at least equal to 77, the different thicknesses being in mm and the TKPH in Tkm / h. The TKPH will either preferably be extracted from the technical documentation of the tire manufacturer, the manufacturer sometimes applying corrective factors, or by multiplying the speed index by the load index.

[0039] The ENCe / ENCM criterion reflects the progressiveness of the transition of the carcass layer whose reinforcing elements move from an axial orientation under the center of the crown to a radial orientation at the sidewalls. Usually, for wear problems in particular, civil engineering tires are designed to be as close as possible to a cylindrical shell carried by a carcass reinforcement. The tire according to the invention, due to its width giving it very good drift rigidity and the flexibility of its crown and the gains obtained in thermal and wear, allows for a rounder passage of the carcass layer and closer to the pressure equilibrium curve, which is more favorable to rolling resistance because it further reduces the deformation rates at the shoulders. We try to maximize this parameter.

[0040] Advantageously, the radial thickness of the tread is substantially constant, namely that the ratio of the thickness Emle of the tread at the axial end of the tread surface to the radial thickness Eml of the tread measured at the median circumferential plane must be minimized. relatively low pressure of the solution and the flexibility of the crown allow the crown and in particular the tread to be optimized differently with a substantially constant thickness and a more curved profile than for other types of design.

[0041] The ratio between the radial thickness ENCM from the radially outermost point of the carcass layer to the tread surface at the median plane and the total radial thickness ETM of the tire at the median plane, from the internal cavity to the tread surface, reflects the effectiveness of the design balance between the thermal properties of the tire, which largely determines its endurance and service life, which must take into account the effectiveness of the shape of the contact patch, which is reflected by the H / B ratio. We will therefore seek to maximize the (ENCM / ETM) / (H / B) ratio.

[0042] As for such a design, we are simultaneously looking for a good thermal balance, endurance, wear, a passage of the carcass layer to equilibrium and a tread optimized for this passage, therefore relatively constant and not maximized at the shoulder, an advantageous criterion for a good design of the tire according to the invention is that the design index ((ENCM / ETM)*(ENCe / ENCM) / ((H / B)*(EMLe / EML)) is at least equal to 1.5, all values ​​being in mm.

[0043] To maintain endurance with such a thin crown, limited to 20 mm thickness, particularly in crown aggression, a recurring problem with surface mines, it is worth lowering the nominal pressure by at least 1 bar compared to a 27.00R49 tire and limiting it to 7 bars. Below 5.5 bars, the load that can be carried is no longer sufficient, but the increase in air volume by widening the tread still allows these new dimensions to carry about 10% more load than a 27.00R49. At this pressure, a tire in this series with such characteristics has an aggression resistance equivalent to a 27.00R49, about 50% better than a 59 / 80R63 for a test with depression of a polar used to quantify the resistance to aggression.

[0044] Preferably, the tire comprises a filling rubber called a crown filling rubber between the axial end of the crown reinforcement and the carcass reinforcement, the crown filler rubber includes a silica filler at least equal to 30 pce, has a maximum dynamic loss tanô, measured at a temperature of 100°C less than 0.07 and an elongation at break at 100°C according to standard NF T 46-002 at least equal to 650%. The elongation at break performance prevents the propagation of cracks occurring at the end of the crown layers. This property must be associated with a low dynamic loss in order to combine this mechanical performance with equivalent thermal performance.

[0045] Likewise, for optimal operation, it is particularly interesting that the elastic modulus G' at 35% deformation, at 100°C and at 10 Hz, of the crown filling rubber comprising at least 30 pce of silica, measured according to standard ASTM D 5992 - 96 is at most equal to 1.2 MPa.

[0046] Advantageously, the thickness of the crown filler rubber is at least 20 mm. The thickness of the filler rubber is measured in a meridian plane as the distance from the axial end of the crown reinforcement and the most radially outer carcass layer. This thickness prevents cracks appearing at the end of the crown layers from passing through the filler rubber to reach the internal cavity in the case of use outside the recommendations, such as excessive speed or too low a pressure.

[0047] Advantageously, decoupling rubbers are positioned between the ends of the crown layers and the nearest crown layer, possibly the carcass layer for the radially innermost crown layer if this is not the widest working layer. They make it possible to reduce shear at the ends of the crown layers and in particular working layers whose reinforcements are crossed, which generate significant shear. They can extend from the end of the crown layers and aggregate with each other to form a large volume in the shoulder area.

[0048] Advantageously, the elongation at break at 100°C according to standard NF T 46-002 of the crown decoupling rubbers comprising at least 40 pce of silica is at less than 500%, and the maximum dynamic loss tanô of said crown decoupling rubbers, measured according to the same standard ASTM D 5992 - 96, at a temperature of 100°C and at 10 Hz, is at most equal to 0.06, so that the crown decoupling rubbers are optimized from the point of view of crown endurance, in particular cracking of the mixtures (or rubber) at the ends of the crown layers.

[0049] For further optimization, it is also advantageous that the elastic modulus G' at 35% deformation, at 100°C and at 10 Hz, of the decoupling rubbers comprising at least 40 pce of silica, measured according to the ASTM D 5992 - 96 standard, is at most equal to 2.2 MPa.

[0050] It is also advantageous if the decoupling rubbers have a radial thickness of at least 1.5 mm.

[0051] The elongation at break (or breaking) measurements are carried out at 100°C and in accordance with the French standard NF T 46-002 of September 1988. The breaking test specimens are of type H2 as described in the standard NF ISO 37 of March 1, 2012 with the exception of the size of the test specimen extracted from the tire which is a test specimen 40 mm long, 20 mm wide and 0.3 mm thick. The force to be exerted to obtain rupture is determined (breaking stress, in MPa (in N / mm)) and the elongation at break is measured (in %).

[0052] Preferably at least two crown layers have metal reinforcements having a structural elongation As at least equal to 0.5% and a total elongation at break At at least equal to 3% and an elastic modulus in extension at most equal to 120 GPa, either the protective layers or the working layers in order to have a flexible crown allowing optimal perforation performance.

[0053] Advantageously, the tire, being mounted on a nominal rim and inflated to the nominal pressure and crushed to the nominal load on a flat ground, to form a contact area of ​​an axial width L1 and a length L2, in the direction perpendicular to the axis of rotation, measured around the center of the rolling surface, in which the width L1 is at least equal to 1.3 times the length L2. To make this measurement, pressure-sensitive papers may be used, ink the tire to have an imprint on a paper or other support, by pressure on a translucent surface with a measuring means such as a camera, the person skilled in the art knowing how to make measurements of the static contact area of ​​a tire. By the axial width L1 is meant the maximum axial distance between two points belonging to the contact area thus measured and the length L2 the maximum distance between two points belonging to the contact area in the direction perpendicular to the axial direction in the contact area, namely the substantially circumferential direction. This proportionality allows the invention to have excellent bending rigidity on edge and therefore excellent drift rigidity even with a radially flexible crown, which is one of the bases of the invention.By "measured around the center" is meant that the person skilled in the art will carry out this measurement at the center of the contact area if possible, if no central groove prevents the measurement and otherwise as close to the center as possible.

[0054] Advantageously, the axial variation of the circumferential length of the contact patch is less than 10% over 90% of the central axial part of the contact patch. Civil engineering tires, and in particular mining tires, have treads cut at least by substantially circumferential grooves and often by transverse grooves, so that the ends of the contact patch are not continuous at the location of the grooves, in particular circumferential grooves. The measurement will only be carried out on the parts of the contact patch where it is possible, namely outside the circumferential grooves. Variation is understood to mean the maximum length less the minimum length as a percentage of the maximum length of the contact patch measured over 90% of the central axial part of the contact patch.

[0055] Advantageously, the reinforcing elements of the crown layers are desaturated. This characteristic gives these reinforcements good corrosion resistance, which is conducive to the endurance of this tire. A reinforcing element comprises layered strands, themselves made up of layers of wires. A reinforcement is desaturated if a layer of its strands is desaturated, and / or if the layers of wires are desaturated, that is to say that it is such that there is sufficient space between the strands and / or the wires of the strands so as to allow the passage of an elastomer composition, which means that the strands and / or the wires do not touch each other. not and that there is sufficient space between two adjacent strands or two adjacent wires to allow the passage of an elastomer composition to the inner strand or to the core of the strand.

[0056] One embodiment of the invention is that the crown reinforcement comprises two elastic protective layers and at least two working layers whose reinforcing elements make an angle with the circumferential direction whose absolute value is at least equal to 20° guaranteeing the flexibility of the crown. These reinforcing elements of the working layers are made up of 85 elementary wires with a diameter of between 21 and 25 hundredths of a millimeter arranged at a pitch of between 3.5 mm and 4.9 mm. The reinforcing elements of the working layers have a structural elongation AS of less than 0.2% and a total elongation At at break of less than 2.5% for a breaking force greater than 9000 N, preferably greater than 10500 N.These reinforcing elements of the working layers of the invention compared to the reinforcing elements of the working layers of tires according to the state of the art aim to densify the reinforcing layers rather than to thicken them in order to maintain the flexibility of the crown essential to the invention. The protective layers are preferably elastic, consisting of 24 elementary wires with a diameter between 23 and 30 hundredths of a millimeter for a structural elongation As greater than 0.5% and a total elongation At at break greater than 3%. They allow optimal protection for this dimension while remaining within the low thickness of the crown reinforcement of the invention.For a crown with better crown impact and endurance performance, the crown reinforcement comprises three working layers, two of which the reinforcing elements make an angle with the circumferential direction whose absolute value is at most equal to 45° and the third of which the reinforcing elements make an angle with the circumferential direction whose absolute value is at least equal to 50°. The structural and breaking elongations are given for cables taken from the tire. All of the crown reinforcing elements form an angle with the circumferential direction whose absolute value is at least equal to 20° to maintain the flexibility of the crown which is the basis of the invention, which excludes the presence of a hooping layer whose elements. reinforcement of the summit form an angle with the circumferential direction whose absolute value is at most equal to 15°

[0057] Another preferred embodiment of the invention is that the crown reinforcement comprises two elastic protective layers and at least four working layers whose reinforcing elements make an angle with the circumferential direction whose absolute value is at least equal to 20° ensuring the flexibility of the crown. The reinforcing elements of the working layers are made up of 26 elementary wires with a diameter of between 25 and 35 hundredths of a millimeter arranged at a pitch of between 3.0 mm and 4.0 mm. These reinforcing elements of the working layers of the invention compared to the reinforcing elements of the working layers of tires according to the state of the art also aim to densify the reinforcing layers rather than thicken them in order to maintain the flexibility of the crown essential to the invention.The protective layers are preferably elastic, consisting of 24 elementary wires with a diameter between 23 and 30 hundredths of a millimeter for a structural elongation As greater than 0.5% and a total elongation At at break greater than 3%. They provide optimal protection for this dimension while remaining within the low thickness of the crown reinforcement of the invention. All of the crown reinforcement elements form an angle with the circumferential direction whose absolute value is at least equal to 20° to maintain the flexibility of the crown which is the basis of the invention, which excludes the presence of a hooping layer whose crown reinforcement elements form an angle with the circumferential direction whose absolute value is at most equal to 15°.

[0058] It is interesting that the angle formed by the reinforcing elements of the radially innermost protective layer with the circumferential direction (XX') is of the same sign as the angle formed at the median plane by the reinforcing elements of the radially outermost working layer with the circumferential direction (XX'). Indeed, it happens that in the event of aggression; the crown mixtures from the rubber mixtures of the tread to the coating mixtures of the protective layers crack, bringing water from the contact patch into the radially innermost protective layer. If the reinforcing elements of the radially outermost protective layer inner are crossed with those of the most radially outer working layer, whether at the median plane for a composite working layer or not, the water passing from the contact area towards the reinforcing element of the protective layer, and following this reinforcing element is likely to cause corrosion of many consecutive reinforcing elements of the working layer that it crosses. Corrosion of several reinforcing elements close to a working layer greatly weakens the crown. Orienting the reinforcing elements of the most radially outer working layer at the median plane and the reinforcing elements of the most radially inner protective layer with the same sign reduces this risk. Furthermore, this arrangement prevents shearing of the rubber compositions between the most radially outer working layer and the most radially inner protective layer.

[0059] It is also preferred that the angle formed by the reinforcing elements of the radially innermost protective layer with the circumferential direction (XX') be of opposite sign to the angle formed by the reinforcing elements of the radially outermost protective layer with the circumferential direction (XX') in order to homogenize in all directions the resistance of the crown to attacks and thus improve the resistance of the crown to impacts.

[0060] The characteristics of the invention are illustrated by schematic figures 1 to 6, not shown to scale. Figures 1 to 3 represent a half-meridian section of a tire crown. Figures 4 to 6 represent a contact patch.

[0061] In Figure 1, a meridian section of the crown of a tire 1 according to the state of the art for the 59 / 80R63 for heavy vehicles of the civil engineering type is shown, comprising a crown reinforcement 3, radially inside a tread 2 and radially outside a carcass reinforcement 4. The crown reinforcement 3 comprises, radially from the outside to the inside, a protective reinforcement 31, a working reinforcement 32, a hoop reinforcement 33. The protective reinforcement 31 comprises two protective layers 311, 312, comprising elastic metal reinforcements coated in an elastomeric material, parallel to each other. The working reinforcement 32 comprises in the occurrence two working layers 321, 322, the respective metal reinforcements of which, inelastic, coated in an elastomeric material, parallel to each other, the absolute values ​​of the angles with the circumferential direction of which are at least equal to 15°. The hoop reinforcement 33 comprises in this case two hoop layers 331, 332, the respective metal reinforcements of which, coated in an elastomeric material, parallel to each other, the absolute values ​​of the angles with the circumferential direction of which are at most equal to 15°. The axial widths of the hoop crown layers are significantly less than the widths of the working layers to avoid cleavage. The figure also represents the crown filling rubber 6 decoupling the carcass layer and the wider working layer.Figure 1 also represents the radial thickness of the tread at the median plane Eml, the useful radial thickness at the median plane ENCM from the most radially outer point of the carcass layer 4 to the running surface 21, of an axial width L, measured from one axial end El of the running surface to the other. Figure 1 represents the radial thickness Es at the median plane of the crown layers, and the useful thickness at the shoulder ENCe measured perpendicular to the carcass layer, from the axial end El of the running surface 21 to the most radially outer point of the carcass layer 4.

[0062] The person skilled in the art knows how to measure these angles on a tire either by non-destructive testing means or by cutting the tire and accessing the different crown layers. The same applies to the measurement of the axial widths and radial thicknesses of the different components of the tire, usually on a meridian section. The angle measurements are made at the center of the crown layer in question. Determining the axial end E1 of the tread surface 21 is obvious if the tread ends in an angle. In this case, the end is the apex of the angle. If the end ends in a rounding of average radius r on an angle a, the point at the median angle of the rounding will be taken.

[0063] Figure 2 represents a section of the crown of a tire according to one of the specific embodiments of the invention with a crown reinforcement without hoop reinforcement but a protective reinforcement comprising 2 layers of protection 311, 312, having elastic reinforcing elements making an angle with the circumferential direction respectively equal to 33°, a working reinforcement comprising 3 working layers 321, 322, 323, the radially outermost working layers 322, 323, having reinforcing elements making an angle with the circumferential direction respectively equal to 24° and the radially innermost working layer 321, having reinforcing elements making an angle with the circumferential direction equal to 65°. Figure 2 also represents the total thickness ETM of the tire at the median plane, radial thickness measured from the tread surface 21 to the internal cavity 5 and the thickness of the tread at the end of the tread Emle, measured from the axial end (El) of the tread surface 21 perpendicular to the carcass layer (4).Tread compounds also present in the median plane or near the end of the tread surface will be taken into account and in no case a crown filler rubber will be used.

[0064] Figure 3 represents a section of the crown of a tire according to one of the specific embodiments of the invention with a crown reinforcement without hoop reinforcement but a protective reinforcement comprising 2 protective layers 311, 312, having elastic reinforcing elements making an angle with the circumferential direction respectively equal to 33°, a working reinforcement comprising 4 working layers 321, 322, 323, 324, the most radially outer working layers having inelastic reinforcing elements making an angle with the circumferential direction respectively equal to 24°.

[0065] Figure 4 represents the outer contour of a contact patch of a tire according to the state of the art such as a 27.00R49 whose axial width of the contact patch L1 is less than the width L2.

[0066] Figure 5 represents the outline of a contact patch of a tire according to the invention whose axial width of the contact patch L1 is greater than the width L2 and the variation in length L2 is less than 10%. Visible therein are the substantially transverse 7 and longitudinal 8 grooves which can take different forms. This type of measurement can be obtained using a sheet of paper on which the previously inked tire is pressed, a photo through a window on which the tire is crushed at its nominal load and at its nominal pressure. From such a measurement, it will be possible to evaluate the axial width L1 of the contact patch near the longitudinal center of the contact patch, avoiding an axial groove if necessary. It will also be possible to evaluate the variation in the length L2 of the contact patch over 90% of the central axial part of the contact patch to avoid edge effects. The variation in the length of the contact patch in Figure 5 is of the order of 5%, which allows for more regular wear than a tire with a variation greater than 10%.

[0067] Figure 6 represents the outer contour of a contact patch of a tire whose axial width of the contact patch L1 is greater than the width L2 but whose crown rigidity and the presence of hoop layers in the center do not allow for a controlled variation in length L2. The variation in the length of the contact patch in Figure 6 is of the order of 17%, which is penalizing in terms of wear.

[0068] The invention was tested on two tires of size 40 / 65R49 having a load index of 30 tonnes to compare with the 27.00R49, operating at low pressure, namely a nominal pressure (before rolling) of 6b0 or 7b0 when hot once the tire is stabilized in temperature at a speed of 48.7 km / h. The axial width L of the tread is equal to 905 mm. The crown reinforcement comprises two elastic protective layers with reinforcing elements composed of 24 unit wires of 26 hundredths of a millimeter with a structural elongation As equal to 0.8% and a total elongation at break At equal to 3.75% with a pitch of 2.5 mm. The reinforcing elements make an angle with the circumferential direction equal to 33° and are crossed from one layer to the other. The tires have a radial tread thickness Eml equal to 75.0 mm.

[0069] The tire II according to the invention the crown reinforcement comprises 2 working layers whose reinforcing elements make an angle with the circumferential direction equal, from the most radially inner working layer towards the most radially outer working layer at -24° and 24°. The reinforcing elements are made up of 85 elementary wires of 23 hundredths of a millimeter arranged at a pitch equal to 3.7 mm. The reinforcing elements of the 3 crown layers have a structural elongation As equal to 0.05% and a total elongation at break At equal to 1.9%. The crown reinforcement therefore comprises 3 working layers and two protective layers, no hoop layer. The radial thickness ES of the crown reinforcement, measured at the median circumferential plane, is equal to 13.1 mm and a radial thickness of the crown ETM measured at the median circumferential plane equal to 102.9 mm. The radial thickness Eml measured at the median circumferential plane is equal to 75.2 mm and the thickness at the shoulder Emle to 88 mm. The useful crown thickness at the median plane ENCM is equal to 96.7 mm and the useful crown thickness at the shoulder ENCe is equal to 130 mm for a design index equal to 1.66. A measurement of the dimensions of the contact area of ​​II at nominal pressure under nominal load was made, the width L1 of the contact area is equal to 1.38 times the length L2 of the contact area, for a variation equal to 5.1% of the maximum length of the contact area.

[0070] The tire 12 according to the invention, the crown reinforcement comprises four working layers whose reinforcing elements make an angle with the circumferential direction equal, from the radially innermost working layer towards the radially outermost working layer, at 24°, at -24°, at 24° and -24°. The reinforcing elements are made up of 26 elementary cords of 30 hundredths of a millimeter arranged at a pitch equal to 3.1 mm. The reinforcing elements of the 4 crown layers have a structural elongation As less than 0.1% and a total elongation at break At equal to 2.0%. The crown reinforcement therefore comprises 4 working layers and two protective layers, no hooping layer. The radial thickness ES of the crown reinforcement, measured at the median circumferential plane, is equal to 16.8 mm and a radial thickness of the crown ETM measured at the median circumferential plane equal to 102.9 mm.The radial thickness Eml measured at the median circumferential plane is equal to 74.6 mm and the thickness at the shoulder Emle to 97.3 mm. The useful crown thickness at the median plane ENCM is equal to 96.7 and the useful crown thickness at the shoulder ENCe is equal to 138.1 mm for a design index equal to 1.58. A measurement of the contact area dimensions of 12 at nominal pressure under nominal load, the width L2 of the contact area, was made. is equal to 1.52 times the length L2 of the contact area for a variation equal to 5.3% of the maximum length of the contact area.

[0071] The safety coefficients of the crown or carcass reinforcement elements of the II or 12 tires are equivalent to those of the reference tires existing on the market, either the closest in terms of size, i.e. 27.00R49, or the most productive ones fitted to large dumpers, such as the 59 / 80R63. The endurance of the reinforcement elements and their expected longevity is therefore the same for the tires according to the invention and the reference tires on the vehicles for which they are respectively sized. There is no major difference between the performances of the two embodiments of the invention.

[0072] The comparison is made on a commercial reference tire RI 27.00R49 loaded to 24 tonnes to describe a cyclic use at 30 tonnes and operating at a higher pressure than the invention, namely a nominal pressure (before rolling) of 7b5 or 9b when hot once the tire is stabilized in temperature at a speed of 24.2 km / h. The axial width L of the tread is equal to 648 mm. The crown reinforcement comprises two elastic protective layers with reinforcing elements composed of 24 unitary wires of 26 hundredths of a millimeter with a structural elongation As equal to 0.7% and a total elongation at break At equal to 3.7% with a pitch of 2.5 mm. The reinforcing elements of the protective layers form an angle with the circumferential direction equal to 24°, crossed from one layer to the other. The 27.00R49 tires have a radial tread thickness Eml equal to 107.4 mm.The crown reinforcement comprises three working layers whose reinforcing elements make an angle with the circumferential direction equal, from the radially innermost working layer to the radially outermost working layer, at -65°, at -33° and 19°. The reinforcing elements of the working layers consist of 68 elementary wires of 23 to 26 hundredths of a millimeter arranged at a pitch equal to 2.9 mm. The radially innermost working layer is not complete and mainly allows compression to be taken up at the shoulder to avoid compression of the carcass. The elements of. The 3 crown layers have a structural elongation As equal to 0.05% and a total elongation at break At equal to 2.0%. The crown reinforcement therefore comprises 3 working layers and two protective layers, no hoop layer. The radial thickness ES of the crown reinforcement, measured at the median circumferential plane, is equal to 12.1 mm and a radial thickness of the crown ETM measured at the median circumferential plane is equal to 132.9 mm. The radial thickness Eml measured at the median circumferential plane is equal to 107.4 mm and the thickness at the shoulder Emle is equal to 125.3 mm. The useful crown thickness at the median plane ENCM is equal to 126.8 and the useful crown thickness at the shoulder ENCe is equal to 166.7 mm for a design index equal to 1.08. A measurement of the dimensions of the contact area of ​​RI at nominal pressure under nominal load was made, the width L1 of the contact area is equal to 0.9 times the length L2 of the contact area, for a variation equal to 11.2% of the maximum length of the contact patch.

[0073] The second point of comparison was made on a commercial reference R2 59.80R63 tire loaded to 100 tonnes operating at a higher pressure than the invention, namely a nominal pressure (before rolling) of 7b5 or 9b6 when hot once the tire is stabilized in temperature at a speed of 28.0 km / h. The axial width L of the tread is equal to 1214 mm. The crown reinforcement comprises two elastic protective layers with elastic reinforcing elements. The radially innermost protective layer comprises reinforcing elements of 24 unitary wires of 26 hundredths of a millimeter with a structural elongation As equal to 0.8% and a total elongation at break At equal to 3.75% with a pitch of 2.5 mm. The radially outermost protective layer comprises reinforcing elements of 52 unit wires of 26 hundredths of a millimeter with a structural elongation As equal to 1.1% and a total elongation at break At equal to 4.2% with a pitch of 3.7 mm. R2 tires have a radial tread thickness Eml equal to 130.5 mm. The crown reinforcement consists of 6 crown layers, 2 hoop layers, 2 working layers and 2 protective layers. The reinforcement elements of the hoop layers make an angle with the circumferential direction equal, from the radially innermost hoop layer to the radially outermost hoop layer, at -8° and 8°. The elements of. reinforcement of the hoop layers, are made up of 77 elementary wires of 35 hundredths of a millimeter arranged in a pitch equal to 5.5 mm. The reinforcing elements of the working layers make an angle with the circumferential direction equal, from the most radially inner working layer to the most radially outer working layer, at -33° and 24°. The reinforcing elements of the working layers, are made up of 77 elementary wires of 35 hundredths of a millimeter arranged in a pitch equal to 5.5 mm. The reinforcing elements of the working layers have a structural elongation As equal to 0.05% and a total elongation at break At equal to 2.1%. The radial thickness ES of the crown reinforcement, measured at the median circumferential plane, equal to 27.9 mm and a radial thickness of the crown ETM measured at the median circumferential plane equal to 176.5 mm. The Emle shoulder thickness is 145.7 mm.The useful crown thickness at the median plane ENCM is equal to 162 mm and the useful crown thickness at the shoulder ENCe is equal to 212.2 mm for a design index equal to 1.35. A measurement of the dimensions of the contact area of ​​RI at nominal pressure under nominal load was made, the width L1 of the contact area is equal to 1.03 times the length L2 of the contact area, for a variation equal to 19.8% of the maximum length of the contact area.

[0074] The R2 tire is not very suitable for equipping electric vehicles. Its mass of 5500 kg compared to 1400 kg for the RI tire and around 1500 kg for the tires according to the invention have an inertia such that the electric motors and the batteries necessary to start them would be of a size such that the vehicle would have little interest. Nevertheless, this R2 tire remains a reference in terms of efficiency per ton transported per hour TKPH with values ​​around 2240 compared to less than a third - 624 - for the RI tire whose mass and diameter are more compatible with low-carbon vehicles. On this performance, due to the gain obtained in particular in thermal which makes it possible to increase the speed of the vehicles, the TKPH offered by the tires according to the invention II or 12 is double that of RI around 1200.Furthermore, with digital tools, it is possible to make calculations of expected deviations in service life, the tires according to the invention are particularly relevant in terms of tread efficiency, in particular due to the wide shape of. the footprint, very effective for transmitting forces, particularly drifting, with very low wear impact. In fact, drift rigidity is increased by 50% compared to a state-of-the-art RI tire. This effect is increased by the choice of operating at low pressure, which increases the contact surface by almost 10% and lowers the pressure by the same amount, which reduces the shearing of the tread in the contact area and wear by the same amount. For an average lifespan of 6000h of a 27.00R49, the 40 / 65R49 should reach 8100h where the 59 / 80R63 should do 5700h neglecting the wear aggression linked to the rigidity of the crown block despite the fact that the mass of the tread compound of an R2 59 / 80R63 tire is almost three times that of an RI 27.00R49 tire, or of a tire according to the invention II or 12 40 / 65R49.The TKPH is a good measure of productivity but not a relevant measure of the efficiency of the material used in a dimension. For tires intended for the same types of mines in terms of aggressiveness and therefore having similar tread compounds, a method to evaluate the most balanced tire in terms of productivity and resource consumption is rather the best tire in terms of TKPH divided by the mass of the tire, to create a material efficiency index by use. With this criterion, the tires according to the invention are clearly better than the tires according to the state of the art. On a base of 100 for the R2 reference, a smaller tire such as the RI reference is 30% more relevant but the tire according to the invention is around 100% more relevant, proof that for this type of use, the invention is a break from the existing system.

[0075] This ranking remains the same if, for comparison, we neutralize the tread compound specification by multiplying by the lifespan estimated by numerical tools. Indeed, depending on the type of soil, the tread compounds are more or less thick and the tires more or less heavy. It is possible to neutralize the effect of the tread compound thickness to compare whether tires designed for uses on soils of different aggressiveness were needed. By multiplying the material efficiency index per use by the lifespan estimated by finite element calculations, we confirm that, for all uses combined, the tires according to the invention are the most relevant in terms of productivity compared to the means used to obtain it. The R2 reference tire remains the most consumer of tread material, given its mass and the efficiency of the use of tread compounds on a very rigid crown at high pressure. It is taken as a reference on a base of 100. The RI tires of smaller diameters being between 125 and 150 on this new criterion (TKPH*life / tread mass) depending on the compounds used when the tires according to the invention due to the high efficiency of the contact patch are between 130 and 240 points above the R2 tires.

[0076] For the efficiency indicator equal to TKPH*Eml / ((H / B)*ETM*ES), the TKPH of II is equal to 1302 and therefore the efficiency indicator is worth 111.7. The TKPH of 12 is equal to 1168 and therefore the efficiency indicator is worth 77.5. The TKPH of RI is equal to 624 and therefore the efficiency indicator is worth 41.6. The TKPH of R2 is equal to 2240 and therefore the efficiency indicator is worth 74.2. With such an indicator, all the tires according to the invention are above 77 for the least optimized going to 111.7 for the most optimized both in contact area and crown shape, when the reference tires are between 40 and 75. For the inventors, this parameter, homogeneous to a flow of transported material, is relevant for classifying the tires according to their productivity in relation to the material necessary for the latter.

[0077] It should also be noted that measurements of the crown flexibility were made between the different reference tires and those according to the invention. This flexibility determines the crown's capacity to resist aggression due to rocks located on the mine floor. The R2 tire, designed to carry the load and at higher pressure, is the most penalized. The RI tire and the tires according to the invention are at the same level of flexibility. We therefore expect that the tires according to the invention will not only provide a real gain in material efficiency but also that they will be at the level of the best reference tires in terms of resistance to aggression.

[0078] The optimization of the 40 / 65R49 tire according to the invention makes it possible to achieve a rolling resistance of 3.5kg / T while the RI 27.00R49 witness, likewise diameter, is 4.6kg / T for the same tread composition. This gives additional interest for electric vehicles.

[0079] This material efficiency also makes it possible to reduce energy consumption during acceleration phases when using electric vehicles, the rotating mass being considerably reduced compared to very large tires.

[0080] All of these performances demonstrate the interest of the invention.

Claims

Claims 1. Radial tire (1) for a civil engineering vehicle, with a load index of at least 24 t and a speed index greater than 30 km / h, delimiting an internal cavity (5) intended to receive an inflation gas when the tire is mounted on a nominal rim having a rim diameter of at least 39 inches and at most 51 inches, a maximum axial width B and a sidewall height H, comprising: - a crown comprising a tread (2) and a crown reinforcement (3) radially inside the tread (2) and a carcass reinforcement (4), radially inside the crown reinforcement (3), the crown being connected by means of two flanks to two beads intended to come into contact with the rim, - the tread (2) intended to come into contact with a ground via a rolling surface (21) having an axial width L, and having a radial thickness Eml measured at the median circumferential plane from the rolling surface (21) to the most radially outer point of the crown reinforcement (3), and having a shoulder thickness Emle measured from the axial end (El) of the rolling surface (21) perpendicular to the carcass (4), - the crown reinforcement (3), of a radial thickness ES measured at the median circumferential plane and comprising at least four crown layers (311,312,321,322,331,332), comprising metal reinforcements, - a total radial thickness ETM measured at the median circumferential plane from the rolling surface (21) to the internal cavity (5), - crown thicknesses, called useful, firstly at the median plane ENCM measured from the most radially outer point of the carcass layer (4) to the rolling surface (21) and secondly at the shoulder ENCe measured perpendicular to the carcass layer (4), from the axial end (El) of the rolling surface (21) to the most radially outer point of the carcass layer, characterized in that the radial thickness Eml of the tread (2) is at least equal to 40 mm, in that the metal reinforcements of at least two crown layers make angles with the circumferential direction whose absolute value is greater than 15°, in that the radial thickness ES of the crown reinforcement, measured at the median circumferential plane, is at most equal to 20 mm, in that the radial thickness ETM is at most equal to 125 mm, in that the ratio between the height of the sidewall H and the width of the tire B is at most equal to 0.75, in that the axial width L of the tread (2) is at least equal to 850 mm and at most equal to 1200 mm.

2. Tire according to claim 1 having a productivity index of tonnes of ore moved in one hour, TKPH, in which the efficiency index equal to TKPH*Eml / ((H / B)*ETM*ES is at least equal to 77, the different thicknesses being in mm and the TKPH in Tkm / h.

3. Tire according to any one of claims 1 or 2 in which the design index ((ENCM / ETM)*(ENCe / ENCM) / ((H / B)*(EMLe / EML)) is at least equal to 1.5, all values ​​being in mm.

4. Tire according to any one of the preceding claims in which the nominal pressure of the tire is between 5.5 and 7 bars.

5. Tire according to any one of the preceding claims in which the radial thickness ETM is at most equal to 110 mm, preferably at most equal to 90 mm.

6. Tire according to any one of the preceding claims in which at least two layers of the crown reinforcement (4) have metal reinforcements having a structural elongation As at least equal to 0.5% and a total elongation at break At at least equal to 3% and an elastic modulus in extension at most equal to 120 GPa.

7. A tire according to any one of the preceding claims, the tire being mounted on a nominal rim and inflated to the nominal pressure and crushed to the nominal load on level ground, to form a contact patch of axial width L1 and length L2, in the direction perpendicular to the axis of rotation, measured around the center of the rolling surface, in which the width L1 is at least 1.3 times the length L2.

8. A tire according to claim 7 wherein the axial variation of the circumferential length of the contact patch is less than 10% over 90% of the central axial portion of the contact patch.

9. A tire according to any one of the preceding claims, in which the crown reinforcement comprises two protective layers (311, 312) with elastic reinforcing elements and at least four working layers (321, 322, 323, 324) whose reinforcing elements make an angle with the circumferential direction, the absolute value of which is at least equal to 20° and are made up of 26 elementary wires with a diameter of between 25 and 35 hundredths of a millimeter arranged at a pitch of between 3.0 mm and 4.0 mm.

10. A tire according to any one of the preceding claims, in which the crown reinforcement (3) comprises two elastic protective layers (31) and at least two working layers (32) whose reinforcing elements make an angle with the circumferential direction, the absolute value of which is at least equal to 20° and are made up of 85 elementary wires with a diameter of between 21 and 25 hundredths of a millimeter arranged at a pitch of between 3.5 mm and 4.9 mm.

Images