optimized civil engineering-type tire architecture for electric vehicles

Abstract

Radial tire (1) for a construction vehicle, with a load index of 24 tonnes and a speed rating of 30 km / h, having 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 of not more than 20 mm, including a tread and crown reinforcement. The tread has an axial width L and a radial thickness Eml. The radial thickness Eml of the tread is at least 40 mm. The axial width of the tread is at least 850 mm and at most 1200 mm. (Shortcut figure: Figure 2)

Description

Title of the invention: Optimized civil engineering tire architecture 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 vehicle of the civil engineering type are designated in accordance with the standard of the European Tyre and Rim Technical Organisation (European Tyre and Rim Technical Organisation) or ETRTO.

[0003] For example, a radial tire for heavy-duty construction equipment, as defined by the European Tyre and Rim Technical Organisation (ETRTO) standard, is intended to be mounted on a rim with a diameter of 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, particularly vehicles for transporting materials extracted from quarries or surface mines, by means of a rim with a diameter of at least 35 inches and up to 51 inches.

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

[0005] In what follows, the expressions "radially inward" and "radially outward," respectively, mean "closer" and "farthest," respectively, from the axis of rotation of the tire. "Axially inward" and "axially outward" mean "closer" and "farthest," respectively, from the equatorial plane of the tire, the equatorial plane of the tire being the plane passing through the midpoint of the tread surface and perpendicular to the axis of rotation. "An element A axially inward to an element B by an axial distance D" means that element A is closer to the equatorial plane than element B and that the axial distance between the two elements is equal to distance D. This type of sentence can be generalized to the radial and circumferential directions and the external versus internal position of either element.

[0006] Generally, a tire comprises a tread, intended to come into contact with a ground via a tread surface, the two axial ends are connected via two sidewalls to 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 heavy-duty construction vehicles typically comprises at least one carcass layer including metallic reinforcements coated with a polymeric material of the elastomeric or elastomeric type, obtained by mixing and called the coating compound. A carcass layer includes a main portion connecting the two beads and generally wrapping, within each bead, from the inside to the outside of the tire around a circumferential reinforcement element, most often metallic, called a bead, to form a inversion. The metallic reinforcements of a carcass layer are substantially parallel to each other and form an angle of between 85° and 95° with the circumferential direction.

[0009] The crown reinforcement of a radial tire for construction vehicles comprises a superposition of crown layers extending circumferentially, radially outside the carcass reinforcement. Each crown layer consists of reinforcements, generally metallic, parallel to each other 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, tensile mechanical characteristics of the metal reinforcement are deduced, such as the structural elongation As (in %), the total elongation at break At (in %), the breaking force Fm (maximum load in N), and the tensile strength Rm (in MPa), these characteristics being measured according to ASTM D 2969-04.

[0011] The total elongation At of the metal reinforcement is, by definition, the sum of its structural, elastic, and plastic elongations (At = As + Ae + Ap), 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 small tensile force. The elastic elongation Ae results from the elasticity of the metal of the individual wires constituting the metal reinforcement, the behavior of the metal following Hooke's law. The plastic elongation Ap results from the plasticity, that is, the irreversible deformation, beyond the elastic limit, of the metal of these individual wires. These different elongations, as well as their respective meanings, are well known to humankind. profession, are described, for example, in documents US5843583, WO2005 / 014925 and WO2007 / 090603.

[0012] A tensile modulus, expressed in GPa, is also defined at every point on the force-stretch curve of a metallic reinforcement. This modulus represents the slope of the line tangent to the force-stretch curve at that point. In particular, the tensile modulus of the linear elastic portion of the force-stretch curve is called the elastic tensile modulus or Young's modulus.

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

[0014] An elastic metallic reinforcement, in its rubberized state from the tire, is characterized by a structural elongation As of at least 0.3% and a total elongation at break At of at least 3%. Furthermore, an elastic metallic reinforcement has an elastic modulus in tension of at most 150 GPa, and is usually between 40 GPa and 110 GPa.

[0015] A non-extensible metallic reinforcement is characterized by a total elongation At of at most 2%. Furthermore, a non-extensible metallic reinforcement has an elastic modulus in tension usually between 150 GPa and 200 GPa.

[0016] Among the top layers, a distinction is usually made between the protective layers, which form part of the protective reinforcement and are radially furthest out, comprising elastic reinforcing elements (or reinforcements), and the working layers, which also include reinforcing elements, form part of the working reinforcement, and are radially located between the protective reinforcement and the carcass reinforcement. The angles of the reinforcing elements in the protective layers, or their relatively greater elasticity compared to that of the reinforcing elements in the working layers, are such that the protective layers absorb very little stress during rolling compared to the working layers.

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

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

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

[0020] The working reinforcement usually comprises two radially superimposed working layers formed of non-extensible metallic reinforcements, parallel to each other in each layer and crossed from one layer to the next, forming angles with the circumferential direction of at most 60°, and preferably at least 15° and at most 45°. To reduce the shear stresses on the rubber compounds, or rubber mixtures, at the axial ends of the working layers, it is common practice to axially offset the position of said ends relative to each other. The top reinforcement therefore usually comprises a working layer with a greater axial width and a working layer with a smaller axial width. The shear stresses on the rubber compounds are maximal at the ends of the stiffer top layers.Indeed, these maximum shear stresses due to displacements of the tips of the top layers are distributed over the radial thickness of the rubber compounds between the different top layers. These shear maxima are generally reduced by adding one or more decoupling gums between the tips of the top layers.

[0021] To reduce the mechanical stresses of inflation and rolling transmitted to the working reinforcement and the shear stresses of the rubber compound covering it, it is known to place a shrink-fit reinforcement radially outside the carcass reinforcement. The shrink-fit reinforcement, whose function is to absorb at least some of the mechanical stresses of inflation, improves the durability of the top reinforcement by stiffening it. The shrink-fit 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 tire reinforcement may comprise two radially superimposed layers of reinforcement, formed of parallel metallic reinforcements within each layer and intersecting from one layer to the next, forming angles of no more than 10° with the circumferential direction. For constructing the reinforcement layers on large-diameter civil engineering tires, winding a layer of discontinuous reinforcement elements, the ends of which run from one axial edge of the layer to the other, forming an angle of between 0° and 10° with the circumferential direction, may be preferred to winding a narrow strip of a few continuous reinforcement elements forming an angle of 0° to 10° with the circumferential direction. circumferential an angle between 0° and 5° for productivity reasons.

[0023] In both cases, the shrink-fit layers have a narrower axial width than the working layer, which has a narrower axial width. This is because the stresses due to rolling at the ends of the shrink-fit layers are very high in tension, and increasing their widths would degrade the tire's durability. Therefore, reinforcing elements that form angles between 0° and 10° with the circumferential direction are not placed at axial positions too far from the median plane.

[0024] Historically, the drive for profitability in previous years led vehicle manufacturers to design increasingly heavy trucks or dump trucks for surface mines, transporting as much ore as possible and requiring tires with increasingly higher load ratings. To carry this load, tire manufacturers had no choice but to increase tire size. Thus, the most recent dump trucks are equipped with 63-inch tires (rim size), such as the 59 / 80R63, capable of carrying 100 tons. These tires have a specific tread depth, which is a compromise based on the mine's aggressiveness, determining the type of elastomeric compound or rubber compound with a given rigidity and hysteresis.The tread compound, with its metal reinforcement elements and rubber filling, has a maximum operating temperature that determines the amount of tread wear at around 20 km / h. The more resistant the rubber compound is to wear and tear, and generally the more hysteresis it exhibits, the less radial tread thickness (Eml) is required to prevent overheating. Thus, these very large tires can carry up to 1.5 tons of rubber. These tires therefore have the highest productivity index used today, the TKPH (tonne transported per kilometer hour), which is obtained by multiplying the load index by the speed rating, while respecting a maximum operating temperature of 120°C. This gigantism is also linked to the use of internal combustion engines and diesel fuel, allowing for long vehicle ranges.

[0025] Like other industrial sectors, surface mining will have to decarbonize, and one solution is to use electric motors. To avoid significant energy consumption during acceleration and to have batteries of reasonable size and weight, it is necessary to return to smaller tire sizes and reduce rotational inertia. It would be possible to use existing 27.00R49 tires, whose TKPH (Total Weight Per Second) is about one-third that of 59 / 80R63 tires, which seems uninteresting. However, this size is much lighter, about 25% of the mass of a 59 / 80R63. Thus, while these sizes are less advantageous in terms of productivity than the larger sizes, they are still less efficient. From a material efficiency standpoint, calculated as the TKPH / tire mass, these load-bearing dimensions are highly competitive, offering a gain of nearly 25% compared to "productivist" sizes. This type of reasoning, however, needs to be further developed to optimize material use for the service provided while minimizing material resource consumption.

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

[0027] This objective has been achieved, according to the invention, by a radial tire for a construction vehicle, with a load index of at least 24 t and a speed rating greater than 30 km / h, defining an internal cavity 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 and a crown reinforcement radially internal to the tread and a carcass reinforcement radially internal to the crown reinforcement, the crown being connected via 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 tread surface having an axial width L, and having a radial thickness Eml measured at the median circumferential plane from the tread surface to the most radially outer point of the top reinforcement, and having a shoulder thickness Emle measured from the axial end of the tread surface perpendicular to the carcass, - the top reinforcement, with a radial thickness ES measured at the median circumferential plane and comprising at least four top layers, including metallic reinforcements, - a total radial thickness ETM measured at the median circumferential plane from the rolling surface to the internal cavity, - apex thicknesses, called useful, firstly at the median plane ENCM measured from the most radially outer point of the carcass layer to the tread surface and secondly at the shoulder ENCe measured perpendicular to the carcass layer, from the axial end of the tread 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 - metallic reinforcements of at least two top layers making angles with the circumferential direction whose absolute value is greater than 15°, - the radial thickness ES of the apex 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 sidewall height H and the tire width 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 on a 49-inch wheel rim diameter, such as a 40 / 65R49, doesn't seem particularly advantageous. Indeed, a first estimate suggests that since the tire is wider, it will be heavier for the same amount of tread thickness to be worn. Creating such a size therefore seems pointless.

[0029] By varying the multiple design parameters following new efficiency reasoning on many essential performance factors such as tire mass, speed limit, operating temperature, wear efficiency, load, pressure, resistance to aggression, material efficiency... the inventors have determined a design domain that is particularly interesting for uses where large tires, such as 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 between two types of materials: the wear material and the material that transmits steering forces. These two materials can be represented respectively by the tread compound thickness Eml and the crown thickness ES, which represents the mass of the structure necessary to transmit forces, resist punctures, and transmit steering forces, among other things. The problem with sizes such as 27.00R49, besides their low TKPH (Total Tire Performance) given their load index for an operating speed of 20 km / h, is that their tread wear efficiency is not optimal.

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

[0032] Indeed, this shorter contact patch shape, with up to a 50% difference in length between the center and the shoulder, is typical of construction tires in these series with a width smaller than 0.75, which are center-retained to improve shoulder durability. This contact patch shape results in less efficient wear between the center and the shoulder than a contact patch where the center and shoulders are of similar lengths. For these types of tires and according to the invention, the optimal axial width of the contact patch is between 850 mm and 1200 mm.

[0033] In addition to this improved wear resistance, such an optimized contact area requires opening the angle of the reinforcement elements in the shrink-fit layers, or even eliminating them altogether. This results in a radial softening of the top block, which is beneficial for reducing apex stresses and therefore wear on stress, as well as lowering the apex 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, apex flexibility, contact area shape, etc.—result in a very significant improvement in operating temperature compared to a 27.00R49, on the order of 15°C at the median circumferential plane and nearly 25°C at the shoulders. This temperature reduction is accompanied by a significant decrease in the strain energy of the rubber compounds decoupling the working layers.This improvement would allow for an increase in tread thickness to extend lifespan. However, given the already improved tread efficiency in terms of wear resistance due to the contact patch and wear resistance due to the flexibility of the crown, from a materials efficiency perspective, it is more relevant to increase the tire's speed rating to compensate for the loss of transported mass rather than the load. Obviously, the tread must have a radial thickness at the meridian plane of at least 40 mm to offer a minimum wear lifespan, and the total radial thickness (TMR) from the crown to the median circumferential plane, from the tread surface to the internal cavity of the tire, must be no more than 125 mm to minimize rotational inertia and crown thermals, preferably no more than 110 mm and preferably no more than 90 mm.Tires designed in this way have a maximum distance per hour of at least 30 km and preferably at least 40 km. This allows, between the effects of wear and speed, a 1.8-fold improvement in TKPH compared to a state-of-the-art 27.00R49 tire, admittedly less than 60% of a 59 / 80R63 tire, but with a 66% weight reduction. The optimal balance between rolling resistance, wear, temperature, and material efficiency is generally achieved with a total radial thickness ETM from the apex to the median circumferential plane in. A tread depth of less than 110 mm, rather than 125 mm, is preferable as it improves lifespan, but the added tread depth is less than the loss in heat and rolling resistance. With a tread depth of 90 mm or less, rolling resistance is improved.

[0035] Reducing moving masses through reducing the number of layers and reducing tread thickness is advantageous for reducing heat, increasing productivity and energy efficiency.

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

[0037] However, this efficiency does not take into account the tire's lifespan. For example, one method to improve this efficiency would simply be to reduce the radial thickness of the tread. But this gain at the expense of service life 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 patch and the design's wear resistance, the tire according to the invention improves this efficiency by a factor of approximately two compared to a 27.00R49 and three compared to a 59 / 80R63, which is very productive in terms of transported mass but not very efficient when its mass and the wear resistance of its contact patch and design are taken into account.

[0038] To avoid these lifespan calculations, the inventors proposed an indicator equal to TKPH*Eml / ((H / B)*ETM*ES), where Eml provides an idea of ​​lifespan while H / B takes into account the efficiency of the contact patch, and ETM and ES incorporate the efficiency of the material used to apply the tread compound to the ground. With such an indicator, all the tires according to the invention score above 77 for the least optimized, and from 90 to 123 for the most optimized in terms of both contact patch shape and crown, whereas the reference tires score between 40 and 75. For the inventors, this parameter, which is homogeneous with a flow of transported material, is relevant for classifying tires according to their productivity relative to the material required for that productivity.Thus, it is advantageous that for a tire according to the invention having a productivity index of tons 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 correction factors, or by multiplying the speed index by the load index.

[0039] The ENCe / ENCM criterion reflects the progressive nature of the transition from the carcass layer whose reinforcing elements transition from an axial orientation under the center of the crown to a radial orientation at the sidewalls. Typically, due to wear issues, construction tires are designed to closely resemble a cylindrical shell supported by a carcass reinforcement. The tire according to the invention, thanks to its width, which provides excellent lateral rigidity, and the flexibility of its crown, along with the gains in thermal performance and wear, allows for a smoother carcass layer profile, closer to the equilibrium curve under pressure. This is more favorable to rolling resistance because it further reduces shoulder deformation. The aim is to maximize this parameter.

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

[0041] The ratio between the radial thickness ENCM from the outermost radial 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 inner cavity to the tread surface, reflects the effectiveness of the design balance between the tire's thermal performance, which largely determines its durability and lifespan, and the effectiveness of the contact patch shape, which is represented by the ratio H / B. Therefore, the aim will be to maximize the ratio (ENCM / ETM) / (H / B).

[0042] As with such a design, we simultaneously seek a good thermal balance, endurance, wear, a transition of the carcass layer to equilibrium and a tread optimized for this transition, 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 durability with such a thin crown, limited to 20 mm in thickness, particularly in crown erosion—a recurring problem in surface mining—it is advantageous to lower the nominal pressure by at least 1 bar compared to a 27.00R49 tire and limit it to 7 bar. Below 5.5 bar, the load that can be carried is no longer sufficient, but the increased air volume due to the wider tread still allows these new tires to perform adequately. dimensions to carry approximately 10% more load than a 27.00R49. At this pressure, a tire of this series with such characteristics has a resistance to aggression equivalent to a 27.00R49, better by about 50% than a 59 / 80R63 for a test with indentation of a polar used to quantify resistance to aggression.

[0044] Preferably, the tire comprises a crown sealant located between the axial end of the crown reinforcement and the carcass reinforcement. The crown sealant contains a silica filler of at least 30 parts per cubic centimeter, has a maximum dynamic loss tanô, measured at a temperature of 100°C, of ​​less than 0.07, and an elongation at break at 100°C, according to standard NF T 46-002, of at least 650%. The elongation at break performance prevents the propagation of cracks forming at the ends of the crown layers. This property must be combined with a low dynamic loss to ensure that this mechanical performance is matched by equivalent thermal performance.

[0045] Similarly, for optimal operation, it is particularly important that the elastic modulus G' at 35% deformation, at 100°C and at 10 Hz, of the top packing rubber comprising at least 30 parts per annum of silica, measured according to ASTM D 5992 - 96, be at most equal to 1.2 MPa.

[0046] Advantageously, the thickness of the top packing rubber is at least 20 mm. The thickness of the packing rubber is measured in a meridian plane as the distance from the axial end of the top reinforcement to the outermost radial layer of the carcass. This thickness prevents cracks appearing at the ends of the top layers from penetrating the packing rubber and reaching the internal cavity in the event of use outside the recommended specifications, such as excessive speed or insufficient pressure.

[0047] Advantageously, decoupling rubbers are positioned between the ends of the top layers and the nearest top layer, or possibly the carcass layer for the innermost radially positioned top layer if this is not the widest working layer. These rubbers reduce shear stress at the ends of the top layers, particularly working layers with cross-bracing, which generate significant shear stress. They can extend from the ends of the top layers and aggregate with each other to form a substantial volume in the shoulder area.

[0048] Advantageously, the elongation at break at 100°C according to standard NF T 46-002 of the apex decoupling gums comprising at least 40 parts per cent of silica is at least equal to 500%, and the maximum dynamic loss tanô of said apex decoupling gums, 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 apex decoupling gums are optimized from the point of view of apex endurance, in particular fis- saturation of the mixtures (or gum) at the ends of the top 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 decoupling gums comprising at least 40 parts per cent of silica, measured according to ASTM D 5992-96, be at most equal to 2.2 MPa.

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

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

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

[0053] Advantageously, the tire, being mounted on a nominal rim and inflated to the nominal pressure and compressed to the nominal load on a flat surface, to form a contact area 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 equal to 1.3 times the length L2. To make this measurement, one may use pressure-sensitive paper, ink the tire to obtain an imprint on a paper or other support, by pressure on a translucent surface with a measuring means such as a camera, a person skilled in the art knowing how to measure the static contact area of ​​a tire.The axial width L1 refers to the maximum axial distance between two points within the contact area as measured, and the length L2 to the maximum distance between two points within the contact area in the direction perpendicular to the axial direction within the contact area, namely the substantially circumferential direction. This proportionality allows the invention to have excellent edge bending rigidity and therefore excellent drift rigidity even with a radially flexible apex, which is one of the fundamental aspects of the invention. "Measured around the center" means that a person skilled in the art will perform this measurement at the center of the contact area if possible, provided no central groove obstructs 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 portion of the contact patch. Civil engineering tires, particularly those used in mining, have treads with grooves that are at least substantially circumferential and often transverse, such that the ends of the contact patch are not continuous at the location of the circumferential grooves. The measurement will only be carried out on those parts of the contact patch where it is feasible, namely outside of the circumferential grooves. By variation, we mean the maximum length minus the minimum length as a percentage of the maximum length of the contact patch measured over 90% of the central axial portion of the contact patch.

[0055] Advantageously, the reinforcement elements of the top layers are desaturated. This characteristic gives these reinforcements good corrosion resistance, contributing to the durability of this tire. A reinforcement element comprises layered strands, themselves made up of layers of wire. A reinforcement is desaturated if one layer of its strands is desaturated, and / or if the wire layers are desaturated, that is to say, if there is sufficient space between the strands and / or the wires of the strands to allow the passage of an elastomeric composition. This means that the strands and / or wires do not touch and that there is sufficient space between two adjacent strands or two adjacent wires to allow the passage of an elastomeric composition to the inner strand or to the core of the strand.

[0056] One embodiment of the invention is that the apex reinforcement comprises two elastic protective layers and at least two working layers whose reinforcing elements form an angle with the circumferential direction whose absolute value is at least 20°, ensuring the flexibility of the apex. These reinforcing elements of the working layers consist of 85 elementary wires with a diameter between 0.21 and 0.25 millimeters arranged at a pitch 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 reinforcement elements for the working layers of the invention, compared to the prior art reinforcement elements for tire working layers, aim to densify the reinforcement 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 diameters between 0.23 and 0.30 millimeters, 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 required low thickness. The crown reinforcement of the invention. For a crown with improved impact resistance and endurance, the crown reinforcement comprises three working layers: two whose reinforcing elements form an angle with the circumferential direction of no more than 45°, and a third whose reinforcing elements form an angle with the circumferential direction of at least 50°. Structural and breaking elongations are given for cables taken from the tire. All the crown reinforcement elements form an angle with the circumferential direction of at least 20° to maintain the crown flexibility that is the basis of the invention. This precludes the use of a reinforcement layer whose crown reinforcement elements form an angle with the circumferential direction of no more than 15°.

[0057] Another preferred embodiment of the invention is that the crown reinforcement comprises two elastic protective layers and at least four working layers, the reinforcing elements of which form an angle with the circumferential direction whose absolute value is at least 20°, ensuring the crown's flexibility. The reinforcing elements of the working layers consist of 26 elementary wires with a diameter between 0.25 and 0.35 millimeters arranged at a pitch 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 prior art, also aim to densify the reinforcing layers rather than thicken them in order to maintain the crown's flexibility, which is essential to the invention.The protective layers are preferably elastic, consisting of 24 elementary wires with diameters between 0.23 and 0.30 millimeters, providing a structural elongation (As) greater than 0.5% and a total elongation (At) at break greater than 3%. They allow for optimal protection at this dimension while remaining within the small thickness of the top reinforcement of the invention. All the top reinforcement elements form an angle with the circumferential direction with an absolute value of at least 20° to maintain the flexibility of the top, which is the basis of the invention. This precludes the use of a reinforcement layer whose top reinforcement elements form an angle with the circumferential direction with an absolute value of at most 15°.

[0058] It is interesting that the angle formed by the reinforcement elements of the innermost radially protective layer with the circumferential direction (XX') has the same sign as the angle formed at the median plane by the reinforcement elements of the outermost radially working layer with the circumferential direction (XX'). Indeed, it sometimes happens that in the event of aggression; the top layers, from the rubber compounds of the tread to the coating compounds of the Protective layers crack, allowing water to penetrate from the contact area into the innermost radially protective layer. If the reinforcing elements of the innermost radially protective layer intersect those of the outermost radially protective layer, whether at the mid-plane for a composite or non-composite working layer, the water flowing from the contact area to the reinforcing element of the protective layer, and along this reinforcing element, is likely to cause corrosion of numerous consecutive reinforcing elements of the working layer that it intersects. Corrosion of several reinforcing elements close to a working layer significantly weakens the top layer.Orienting the reinforcement elements of the outermost radially exposed working layer and the reinforcement elements of the innermost radially exposed protective layer with the same sign reduces this risk. Furthermore, this arrangement prevents shearing of the rubber compounds between the outermost radially exposed working layer and the innermost radially exposed protective layer.

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

[0060] The features of the invention are illustrated by the schematic figures 1 to 6, which are not drawn to scale. Figures 1 to 3 represent a meridional half-section of a tire apex. Figures 4 to 6 represent a contact patch.

[0061] Figure 1 shows a meridian cross-section of the crown of a tire 1 according to the state of the art for the 59 / 80R63 for heavy-duty vehicles of the construction equipment type, comprising a crown reinforcement 3, radially internal to a tread 2 and radially external to a carcass reinforcement 4. The crown reinforcement 3 comprises, radially from the outside in, a protective reinforcement 31, a working reinforcement 32, a reinforcing reinforcement 33. The protective reinforcement 31 comprises two protective layers 311, 312, comprising elastic metal reinforcements embedded in an elastomeric material, parallel to each other. The working reinforcement 32 comprises in this case two working layers 321, 322, whose respective inelastic metallic reinforcements are embedded in an elastomeric material, parallel to each other, and whose absolute values ​​of the angles with the circumferential direction are at least equal to 15°.The shrink-fit reinforcement 33 comprises in this case two shrink-fit layers 331, 332, whose respective metallic reinforcements, embedded in an elastomeric material, are parallel to each other, of which the . The absolute values ​​of the angles with the circumferential direction are at most equal to 15°. The axial widths of the crown layers of the compression layers are significantly less than the widths of the working layers to avoid cleavage. The figure also shows the crown packing rubber 6 decoupling the carcass layer and the wider working layer. [Fig. 1] also shows the radial thickness of the tread at the median plane Eml, the effective radial thickness at the median plane ENCM from the outermost radial point of the carcass layer 4 to the tread surface 21, of an axial width L, measured from one axial end El of the tread surface to the other. [Fig.l] represents the radial thickness Es at the median plane of the top layers, and the effective thickness at the shoulder ENCe measured perpendicular to the carcass layer, from the axial end El of the rolling surface 21 to the outermost radial point of the carcass layer 4. .

[0062] Those skilled in the art know how to measure these angles on a tire either by non-destructive testing methods or by cutting the tire and accessing the different crown layers. The same applies to measuring the axial widths and radial thicknesses of the various tire components, usually on a meridian cross-section. The angle measurements are taken at the center of the crown layer in question. Determining the axial end E1 of the tread surface 21 is straightforward if the tread ends at an angle. In this case, the end is the vertex of the angle. If the end terminates with a rounded corner of mean radius r at an angle α, the measurement is taken at the median angle of the rounded corner.

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

[0064] Fig. 3 represents a cross-section of the top of a tire according to one of the specific embodiments of the invention with a top reinforcement without a shrink-fit 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 outermost radially working layers having inelastic reinforcing elements making an angle with the circumferential direction respectively equal to 24°.

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

[0066] Figure 5 represents the contour of a contact patch of a tire according to the invention, where the axial width of the contact patch L1 is greater than the width L2 and the variation in length L2 is less than 10%. The substantially transverse grooves 7 and longitudinal grooves 8, which can take different forms, are visible. This type of measurement can be obtained by pressing the previously inked tire onto a sheet of paper, or by taking a photograph through a window onto which the tire is pressed at its nominal load and pressure. From such a measurement, the axial width L1 of the contact patch can be evaluated near the longitudinal center of the contact patch, avoiding an axial groove if necessary. The variation in length L2 of the contact patch can also be evaluated over 90% of the central axial portion of the contact patch to avoid edge effects.The variation in the contact length of [Fig.5] is on the order of 5%, which allows for more even 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 L1 is greater than its width L2, but whose apex rigidity and the presence of central reinforcement layers prevent a controlled variation in length L2. The variation in the contact patch length of Figure 6 is on the order of 17%, which is detrimental to wear.

[0068] The invention was tested on two 40 / 65R49 tires with a load index of 30 tonnes for comparison with the 27.00R49, operating at low pressure, namely a nominal pressure (before driving) of 6 bar or 7 bar when hot, once the tire has reached operating temperature at a speed of 48.7 km / h. The axial width L of the tread is 905 mm. The crown reinforcement comprises two elastic protective layers with reinforcing elements composed of 24 individual wires of 0.26 mm diameter, with a structural elongation As of 0.8% and a total elongation at break At of 3.75% with a pitch 2.5 mm thick. The reinforcing elements are at an angle of 33° to the circumferential direction and are crossed from one layer to the next. The tires have a radial tread thickness (Eml) of 75.0 mm.

[0069] The upper reinforcement of the tire II according to the invention comprises two working layers whose reinforcing elements form equal angles with the circumferential direction, from the innermost radially inner working layer to the outermost radially outer working layer, at -24° and 24°. The reinforcing elements consist of 85 elementary wires, each 0.23 mm in diameter, arranged at a pitch of 3.7 mm. The reinforcing elements of the three upper layers have a structural elongation As of 0.05% and a total elongation at break At of 1.9%. The upper reinforcement therefore comprises three working layers and two protective layers, with no reinforcement layer. The radial thickness ES of the upper reinforcement, measured at the median circumferential plane, is 13.1 mm, and the radial thickness ETM of the upper layer, measured at the median circumferential plane, is 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 is 88 mm. The effective apex thickness at the median plane ENCM is equal to 96.7 mm and the effective apex thickness at the shoulder ENCe is equal to 130 mm for a design index equal to 1.66. A measurement was made of the dimensions of the contact area of ​​II at nominal pressure under nominal load, 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 has a crown reinforcement comprising four working layers whose reinforcing elements form equal angles with the circumferential direction, from the innermost radially inner working layer to the outermost radially outer working layer, at 24°, -24°, 24°, and -24°. The reinforcing elements consist of 26 elementary wires, each 0.30 mm in diameter, arranged at a pitch of 3.1 mm. The reinforcing elements of the four crown layers have a structural elongation As of less than 0.1% and a total elongation at break At of 2.0%. The crown reinforcement therefore comprises four working layers and two protective layers, with no reinforcement layer. The radial thickness ES of the apex reinforcement, measured at the median circumferential plane, is equal to 16.8 mm and the radial thickness of the apex ETM measured at the median circumferential plane is equal to 102.9 mm.The radial thickness Eml measured at the median circumferential plane is 74.6 mm and the thickness at the shoulder Emle is 97.3 mm. The effective apex thickness at the median plane ENCM is 96.7 mm and the effective apex thickness at the shoulder ENCe is 138.1 mm for a design index of 1.58. A measurement of the dimensions of the contact area was taken at nominal pressure under nominal load; the width L2 of the contact area 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 factors of the crown or carcass reinforcement elements of the II or 12 tires are equivalent to those of existing reference tires on the market, either the closest in terms of size, i.e., 27.00R49, or the most productive tires used on large dump trucks, such as the 59 / 80R63. The durability of the reinforcement elements and their expected lifespan are therefore the same for the tires according to the invention and the reference tires on the vehicles for which they are respectively designed. There is no major difference between the performance of the two embodiments of the invention.

[0072] The comparison is made on a commercially available RI 27.00R49 reference tire loaded to 24 tonnes to describe cyclic use at 30 tonnes and operating at a higher pressure than the invention, namely a nominal pressure (before rolling) of 7.5 bar, or 9 bar when hot once the tire has reached a stable temperature at a speed of 24.2 km / h. The axial width L of the tread is 648 mm. The crown reinforcement comprises two elastic protective layers with reinforcing elements made of 24 individual wires, each 0.26 mm in diameter, with a structural elongation As of 0.7% and a total elongation at break At of 3.7% with a pitch of 2.5 mm. The reinforcing elements of the protective layers form an angle of 24° with the circumferential direction, crossing from one layer to the other. The 27.00R49 tires have a radial tread thickness Eml of 107.4 mm.The top reinforcement comprises three working layers whose reinforcing elements form equal angles with the circumferential direction, from the innermost radially working layer to the outermost radially working layer, at -65°, -33°, and 19°. The reinforcing elements of the working layers consist of 68 individual wires, 0.23 to 0.26 mm in diameter, arranged at a 2.9 mm pitch. The innermost radially working layer is not fully formed and primarily serves to absorb compression at the shoulder to prevent compression of the carcass. The reinforcing elements of the three top layers have a structural elongation (As) of 0.05% and a total elongation at break (At) of 2.0%. The top reinforcement therefore comprises three working layers and two protective layers, with no reinforcement layer. The radial thickness (ES) of the top reinforcement, measured at the median circumferential plane, is 12.1 mm and a radial thickness of the apex ETM measured at the median circumferential plane 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 125.3 mm. The useful apex thickness at the median plane ENCM is equal to 126.8 mm and the useful apex 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, 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 area.

[0073] The second point of comparison was made on a commercially available R2 59.80R63 reference tire loaded to 100 tonnes operating at a higher pressure than the invention, namely a nominal pressure (before rolling) of 7 bar, or 9 bar when hot once the tire has reached temperature stability at a speed of 28.0 km / h. The axial width L of the tread is 1214 mm. The crown reinforcement comprises two elastic protection layers with elastic reinforcing elements. The innermost radial protection layer comprises reinforcing elements of 24 individual wires, each 0.26 mm in diameter, with a structural elongation As of 0.8% and a total elongation at break At of 3.75% with a pitch of 2.5 mm. The outermost radially 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) of 4.2% with a pitch of 3.7 mm. R2 tires have a radial tread depth (Eml) of 130.5 mm. The crown reinforcement comprises 6 crown layers, 2 reinforcement layers, 2 working layers, and 2 protective layers. The reinforcement elements of the reinforcement layers form equal angles with the circumferential direction, from the innermost radially reinforced layer to the outermost radially reinforced layer, at -8° and 8°. The reinforcement elements of the reinforcement layers consist of 77 individual wires, each 0.35 mm in diameter, arranged at a pitch of 5.5 mm. The reinforcement elements of the working layers form equal angles with the circumferential direction, from the innermost radially reinforced layer to the outermost radially reinforced layer, at -33° and 24°.The reinforcement elements of the working layers consist of 77 individual wires, each 0.35 mm in diameter, arranged at a 5.5 mm spacing. These reinforcement elements have a structural elongation (As) of 0.05% and a total elongation at break (At) of 2.1%. The radial thickness (ES) of the top reinforcement, measured at the median circumferential plane, is 27.9 mm, and the radial thickness of the top reinforcement (ETM), also measured at the median circumferential plane, is 176.5 mm. The thickness at the shoulder (Emle) is 145.7 mm. The effective top thickness at the median plane (ENCM) is 162 mm, and the effective top thickness at the shoulder (ENCe) is 212.2 mm, resulting in a design index of 1.35. A measurement was made of the dimensions of the RI contact area at nominal pressure under nominal load, 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 poorly suited 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, has such inertia that the electric motors and batteries required to power them would be so large that the vehicle would be of little practical use. Nevertheless, this R2 tire remains a benchmark 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. Based on this performance, due to the gains obtained, particularly in thermal efficiency which allows for increased vehicle speed, the TKPH offered by the tires according to invention II or 12 is double that of the RI, at around 1200.Furthermore, with digital tools, it is possible to calculate expected deviations in lifespan. The tires according to the invention are particularly relevant in terms of tread efficiency, notably due to the wide contact patch, which is very effective at absorbing stresses, particularly drift, with very low wear impact. Indeed, drift stiffness is increased by 50% compared to a state-of-the-art RI tire. This effect is enhanced by the choice of low-pressure operation, which increases the contact area by nearly 10% and reduces the pressure accordingly, thus decreasing tread shear in the contact area and wear by the same amount. This results in an average lifespan of 6000 hours for a 27-inch tire.The 00R49, the 40 / 65R49 should reach 8100 hours, whereas the 59 / 80R63 should reach 5700 hours, neglecting wear and tear related to the rigidity of the crown block, despite the fact that the tread compound mass of an R2 59 / 80R63 tire is nearly three times that of an RI 27.00R49 tire, or a tire according to invention II or 12 40 / 65R49. TKPH (Total Tire Life Per Hour) is a good measure of productivity but not a relevant measure of the material efficiency of a given size. 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 tire mass, to create a material efficiency index per use.Based on this criterion, the tires according to the invention are clearly better than 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, proving that for this type of use, the invention represents a breakthrough compared to existing solutions.

[0075] This ranking remains the same if, for comparison purposes, the tread compound specification is neutralized by multiplying by the service life estimated by the numerical tools. Indeed, depending on the type of soil, the tread compounds are The tread thickness varies, and the tires themselves vary in weight. It is possible to neutralize the effect of the tread compound thickness to compare, if necessary, tires designed for use on surfaces of varying aggressiveness. By multiplying the material efficiency index per application by the lifespan estimated through finite element calculations, it is confirmed that, across all applications, the tires according to the invention are the most efficient in terms of productivity relative to the means used to achieve it. The reference tire, R2, remains the most material-intensive in terms of tread compound, given its mass and the efficiency of the tread compound use on a very rigid crown at high pressure. It is used as a reference on a base of 100.The smaller diameter RI tires are between 125 and 150 on this new criterion (TKPH*life / tread mass) depending on the compounds used, while the tires according to the invention, due to the high efficiency of the contact area, are between 130 and 240 points above the R2 tires.

[0076] For the efficacy indicator equal to TKPH*Eml / ((H / B)*ETM*ES), the TKPH of II is equal to 1302 and therefore the efficacy indicator is 111.7. The TKPH of 12 is equal to 1168 and therefore the efficacy indicator is 77.5. The TKPH of RI is equal to 624 and therefore the efficacy indicator is 41.6. The TKPH of R2 is equal to 2240 and therefore the efficacy indicator is 74.2. With such an indicator, the set of tires according to the invention is above 77 for the least optimized, going up to 111.7 for the most optimized in terms of both the shape of the contact area and the apex, while 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 relative to the material required for the latter.

[0077] It should also be noted that measurements of the tread flexibility were taken between the different reference tires and the tires according to the invention. This flexibility determines the tread's ability to withstand the stresses caused by rocks located on the mine floor. The R2 tire, designed to carry a load and operate at higher pressures, is the most affected. The RI tire and the tires according to the invention have 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 will also be on par with the best reference tires with regard to resistance to stress.

[0078] The optimization of the 40 / 65R49 tire according to the invention makes it possible to achieve a rolling resistance of 3.5 kg / T, whereas the reference RI 27.00R49, of the same diameter, has a rolling resistance of 4.6 kg / T for the same tread compound. This provides an additional advantage 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 is considerably reduced compared to very large tires.

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

Claims

1. Demands Radial tire (1) for a construction vehicle, with a load index of at least 24 t and a speed rating greater than 30 km / h, defining 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 internal to the tread (2) and a carcass reinforcement (4), radially internal to the crown reinforcement (3), the crown being connected via two sidewalls to two beads intended to come into contact with the rim, - the tread (2) intended to come into contact with a ground by means of a tread surface (21) having an axial width L, and having a radial thickness Eml measured at the median circumferential plane from the tread surface (21) to the most radially outer point of the top reinforcement (3), and having a shoulder thickness Emle measured from the axial end (El) of the tread surface (21) perpendicular to the carcass (4), - the top reinforcement (3), of a radial thickness ES measured at the median circumferential plane and comprising at least four top layers (311, 312, 321, 322, 331, 332), including metallic reinforcements, - a total radial thickness ETM measured at the median circumferential plane from the tread surface (21) to the internal cavity (5), - apex thicknesses, called useful, firstly at the median plane ENCM measured from the most radially outer point of the carcass layer (4) to the tread surface (21) and secondly at the shoulder ENCe measured perpendicular to the carcass layer (4), from the axial end (El) of the tread 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 metallic reinforcements of at least two top 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 sidewall height H and the tire width 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. A pneumatic according to claim 1 having a productivity index of tonne 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. Pneumatic according to any one of claims 1 or 2 wherein the design index ((ENCM / ETM)*(ENCe / ENCM) / ((H / B)*(EMLe / EML)) is at least equal to 1.5, all values ​​being in mm.

4. Pneumatic according to any one of the preceding claims wherein the nominal pressure of the pneumatic is between 5.5 and 7 bar.

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

6. Pneumatic according to any one of the preceding claims wherein at least two layers of the top reinforcement (4) have metallic reinforcements having a structural elongation As of at least 0.5% and a total elongation at break At of at least 3% and an elastic modulus in tension not more than 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 flattened to the nominal load on a flat surface, to form a contact area of ​​axial width L1 and length L2, in the direction perpendicular to the axis of rotation, measured around the center of the rolling surface, wherein the width L1 is at least equal to 1.3 times the length L2.

8. Pneumatic according to claim 7 wherein the axial variation of the circumferential length of the contact area is less than 10% over 90% of the central axial part of the contact area.

9. Pneumatic according to any one of the preceding claims in which the top 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 whose absolute value is at least equal to 20° and are made up of 26 elementary wires of diameter between 25 and 35 hundredths of a millimeter arranged in a pitch between 3.0 mm and 4.0 mm.

10. Pneumatic according to any one of the preceding claims in which the top 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 whose absolute value is at least equal to 20° and are made up of 85 elementary wires of diameter between 21 and 25 hundredths of a millimeter arranged in a pitch between 3.5 mm and 4.9 mm.

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