Agricultural vehicle tire summit

Abstract

Tire (1) for agricultural vehicle having the tread (2) radially inside a crown reinforcement (3) comprising crown layers (31, 32, 33, 34) comprising textile reinforcing elements. The tread (2) comprises a central portion and two lateral portions. The volumetric notch rate of the central portion (Pc) is between 35 and 45% and the volumetric notch rates of the lateral portions (Pl) are between 50 and 65%. The crown reinforcement (3) comprises at most 5 crown layers and the reinforcing elements of the crown layers (31, 32, 33, 34) have a tensile breaking force at least equal to 32 daN. Abstract figure: figure 3

Description

Title of the invention: Tire top for agricultural vehicle

[0001] The present invention relates to a tire for an agricultural vehicle, such as an agricultural tractor or an agro-industrial vehicle, and more particularly relates to the crown, namely the tread and the crown reinforcement, of such a tire.

[0002] Like any tire, a tire for an agricultural vehicle 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, to two beads ensuring the mechanical connection between the tire and the rim on which it is intended to be mounted.

[0003] A radial tire for an agricultural vehicle comprises a reinforcing reinforcement, consisting of a crown reinforcement, radially inside the tread, and a carcass reinforcement, radially inside the crown reinforcement and connecting two beads intended to be in contact with a rim through two sidewalls.

[0004] In the following, the circumferential, axial and radial directions respectively designate a direction tangent to the rolling surface and oriented in the direction of rotation of the tire, a direction parallel to the axis of rotation of the tire and a direction perpendicular to the axis of rotation of the tire.

[0005] The dimensional specifications and conditions of use (load, speed, pressure) of a tire for an agricultural vehicle, or agricultural tire, are defined by use. The tires are medium to large in size depending on the type of crop. The diameter of the rims for mounting agricultural tires is at least 16 inches and at most 46 inches. For agricultural tires, the minimum recommended inflation pressure corresponding to the indicated load capacity is most often at most 300 kPa, but can go down to 240 kPa for an IF tire ("Improved Flexion"), or even 160 kPa for a VF tire ("Very improved Flexion").IF tires are distinguished from standard tires known as NF ("Normal Flexion", standard flexion), i.e. not falling into the IF or VF classification according to the ETRTO 2020 standard, of the same size by a load capacity increased by 20% at constant pressure or a pressure 20% lower at equal load and respectively 40% with VF ("Very improved flexion") tires. The maximum speed of these tires is 65 Km / h corresponding to the speed index D. The load index of these tires is at least equal to 107 (975 Kg) and at most equal to 189. (10300 kg).

[0006] A tire for an agricultural vehicle is intended to run on various types of soil such as more or less compacted earth in fields, unpaved access roads to fields and paved road surfaces. Given the diversity of use, in the field and on the road, a tire for an agricultural vehicle, and in particular its tread, must offer a compromise of performance between, but not limited to, traction in the field, resistance to tearing, resistance to wear on the road, resistance to forward movement, and vibration comfort on the road. The tread is essentially composed of a polymeric material, or elastomeric mixture, or rubber mixture, comprising an elastomer, obtained by mixing.

[0007] To satisfy this set of performances, the tread of a tire for an agricultural vehicle generally comprises a plurality of raised elements, called sculpture elements, extending radially from a bottom surface and for the sculpture blocks to the rolling surface. The subject of the invention is an agricultural tire offering optimal traction in the field and therefore comprising at least on the lateral portions of its tread, sculpture blocks specific to agricultural tires called bars.

[0008] A bar generally has a generally elongated parallelepiped shape, consisting of at least one rectilinear or curvilinear portion, and is separated from the adjacent bars by grooves. A bar may consist of a succession of rectilinear portions, as described in documents US3603370, US4383567, EP795427 or have a curvilinear shape, as presented in documents US4446902, EP903249, EP1831034. The bars are such that the tire has a recommended direction of rotation.Furthermore, each lateral portion comprises a circumferential distribution of lugs, two by two separated by transverse hollows forming, with a circumferential direction (XX1) of the tire, an average angle at least equal to 40° and at most equal to 60°, the axially innermost part of each lug coming into contact with the ground before its axially outer part when the tire rolls on ground in the recommended direction of rotation. For complex lug geometries, the average profile of the lug will be drawn, which is the line of the contact surface passing through the middle of the lug, of which a linear interpolation will be made to measure the average angle. A tread therefore comprises two rows of lugs in a V or chevron pattern, the tire having a preferred direction of rotation according to the point of the chevrons.

[0009] The leading face of a tread block is, by definition, the face whose radially outer edge or leading edge first comes into contact with the ground, when the block passes through the contact surface of the tire with the ground, during the rotation of the tire. The trailing face is, by definition, the face whose radially outer edge or trailing edge last comes into contact with the ground, when the block passes through the contact surface of the tire with the ground, during the rotation of the tire. The definitions are identical when the tread blocks considered are lugs. Depending on the direction of rotation, the leading face is said to be in front of the trailing face. The average distance between the leading face and the trailing face defines the average lug thickness. For a tread element whose radially outermost face is not included in the rolling surface, the leading and trailing faces are determined by projecting that regular wear of the tire has caused the rolling surface to descend to the radially outer surface of the tread element considered.

[0010] The two rows of bars have a symmetry with respect to the equatorial plane of the tire, with most often a circumferential offset between the two rows of bars, resulting from a rotation around the axis of the tire of one half of the tread with respect to the other half of the tread. In addition, the bars can be continuous or discontinuous, and distributed circumferentially with a constant or variable pitch.

[0011] The carcass reinforcement of a radial tire for an agricultural vehicle comprises at least one carcass layer connecting the two beads together. A carcass layer comprises reinforcements, or reinforcing elements, coated with a polymeric material comprising an elastomer, obtained by mixing, or elastomeric mixture. The carcass layer reinforcements are most often made of textile polymeric materials, such as a polyester. The reinforcements of a carcass layer are substantially parallel to each other and form, with the circumferential direction, an angle of between 85° and 95°.

[0012] The crown reinforcement of a radial tire for an agricultural vehicle comprises a superposition of crown layers extending circumferentially, radially outside the carcass reinforcement. Each crown layer is made up of reinforcements coated with an elastomeric mixture and parallel to each other. When the reinforcements of a crown layer form, with the circumferential direction, an angle at most equal to 10°, they are called circumferential, or substantially circumferential, and provide a hooping function limiting the radial deformations of the tire. When the reinforcements of a crown layer form, with the circumferential direction, an angle at least equal to 10° and most often less than 40° but possibly up to 50°, they are called angle reinforcements and have a function of absorbing the circumferential and transverse forces, parallel to the axial direction, applied to the tire.The top layers are then called working layers. The reinforcements of the top layers can be made of poly materials. textile materials, such as polyester, for example, or by metallic materials, such as steel.

[0013] Regarding field operation, an important concern of the tire designer is improving the resistance of the bars and the crown to attacks, and more particularly to attacks by residual stubble or stubbles in fields after harvesting, particularly corn or cotton. A stubble is a portion of plant stem whose free end is generally sharp. The sharp free end of a stubble, which comes into contact with the leading face of a bar, is likely to perforate it locally and superficially, which results in local tearing of the elastomeric material constituting the bar. Repeated attacks on the leading faces of the bars by stubble can cause significant deterioration in the appearance of the bars, or even tearing, more particularly in the vicinity of the axially inner ends of the bars.These degradations are a potential reason for complaints from users, which may require the tire to be replaced.

[0014] Furthermore, these residual stubbles, and more particularly for cotton stubbles, can perforate the top and cause a loss of pressure.

[0015] Documents EP 2714431 BI and FR3068648 describe the tread of a tire for an agricultural vehicle with a reduced risk of attack on the leading faces of the axially outer ends of the bars by residual stubble after harvesting ("stubble") and therefore of tearing.

[0016] Thus it is important to increase the resistance to residual stubble but in a context of limited energy, this increase in resistance must not be accompanied by a deterioration in the energy efficiency index, namely rolling resistance and traction.

[0017] The inventors set themselves the objective of improving resistance to damage by stubble, using a coupled optimization of the tread and the crown reinforcement while improving rolling resistance and traction.

[0018] This objective has been achieved according to the invention by a tire for an agricultural vehicle, comprising, radially from the outside to the inside, a tread and a crown reinforcement comprising crown layers comprising textile reinforcing elements: -the tread having an axial width L and comprising tread blocks separated from each other by hollows and extending radially outwards from a bearing surface to a rolling surface, the radial distance measured at the centre of the tread from the bearing surface to the rolling surface determining the tread height being at least equal to 35 mm, - the tread comprising two lateral portions whose axial width is between 30 and 40% of the axial width L of the tread, axially external to a central portion, each lateral portion comprising a circumferential distribution of tread blocks in the form of bars, two by two separated by transverse hollows forming, with a circumferential direction (XX1) of the tire, an angle at least equal to 40° and at most equal to 60°, - the central portion comprising a circumferential distribution of tread blocks, two by two separated by transverse hollows, - the central and axial portions having volumetric notch rates, defined as the ratio between the volume VC of hollows and the total volume V of the tread assumed to be without hollows, between the bearing surface and the rolling surface, - the volumetric notch rate of the central portion being strictly lower than the volumetric notch rates of the lateral portions, - the crown reinforcement comprises at most 5 crown layers and the reinforcing elements of the crown layers having a tensile breaking force of at least 32 daN.

[0019] The solution is more advantageous for a VF (“very improved flexion”) tire.

[0020] The bearing surface is a theoretical surface constituted by the torus obtained by the rotation around the YY' axis of rotation of the tire of all the most radially inner points of the tread grooves on all the meridian planes. The maximum tread height is the maximum distance between the bearing surface which includes the most radially inner points of the grooves and the rolling surface. The distance between the most radially inner point of the grooves and the rolling surface gives an idea of ​​the tread height of an agricultural tire. It is at least equal to 35 mm in order to have good grip in the fields.

[0021] The invention is based on the observation that the problems of resistance to stubble are linked to the resistance of the center of the sculpture. It therefore consists first and foremost in having sculpture blocks that are more massive than the bars. On the lateral portions, the angle made by the bars plus the possible escape of the stubble towards the axial exterior of the tire, makes them less sensitive to aggression. Concerning the central portion of the tread, it is advantageous to increase the rigidity of the sculpture in order to limit the impact of aggression from the stubble. For the same contact material, one way of expressing the advantage of the sculpture blocks of the central portion being more massive is to consider the volumetric notch rates of the axial and central portions of the tire. Thus the volumetric notch rate, defined as the ratio between the volume VC of hollow and the total volume V of the tread assumed without hollow, included between the bearing surface and the rolling surface, of the central portion, must be strictly lower than the volumetric notch rates of the lateral portions, more particularly the volumetric notch rate of the central portion, is between 35 and 45% and the volumetric notch rates of the lateral portions, are between 50 and 65%. The volume of hollow and the total volume will be evaluated respectively for each of the central and lateral zones then the ratio will be calculated for each of the zones. However by increasing the volume of rubber mixture in the center of the tire, the volume of total rubber mixture and therefore the rolling resistance are increased.A surprising solution to find a compromise is to reduce the number of crown layers and limit it to 5 crown layers where the tires on the market include 6 crown layers but while maintaining the resistance of the crown layers to perforation by using crown layer reinforcement elements having a tensile breaking force at least equal to 32 daN, the breaking force being measured according to the standard designated D885 / D885M - 10A (2014). Such an architecture in fact reduces the flattening force by reducing the thickness of the beam that is the crown reinforcement while compensating for the reduction in resistance to stubble penetration by a constraint on the breaking strength of the crown layer reinforcement elements.

[0022] Advantageously, the crown layers comprise reinforcing elements comprising an aramid strand and a strand of a PET or nylon textile fiber, the aramid strand having a tensile breaking force of at least 30 daN. This type of hybrid reinforcement is particularly suitable for reducing the number of crown layers. Preferably, the crown layers comprise reinforcing elements comprising an aramid strand and a strand of a PET textile fiber.

[0023] An advantageous solution in terms of endurance and mass is that the reinforcing elements of the working layers are hybrid cables consisting of an aramid strand with a linear mass of between 160 and 180 g per km and a PET strand with a linear mass of between 140 and 160 g per km, the reinforcing elements of the working layers being arranged in the working layers at a pitch of between 0.8 mm and 1 mm.

[0024] In order to lighten the tire as much as possible to preserve material consumption and reduce crown thicknesses for a good balance between endurance and rolling resistance, the crown reinforcement comprises at most 4 crown layers with a linear breaking strength of at least 30 daN / mm, preferably at least 40 daN / mm.

[0025] Preferably, any bar-shaped sculpture block of the lateral portions of the tread has an average transverse thickness, measured between its leading face and its trailing face, at least equal to 40 mm and at most equal to 60 mm. These thicknesses have demonstrated their effectiveness in agricultural sculptures in terms of robustness, rigidity to minimize wear and transmit torque.

[0026] Advantageously, in line with the central portion, the radial distance from the bearing surface to the radially outermost crown layer is at least equal to the distance from the bearing surface to the radially outermost crown layer measured at the center of each lateral portion plus 4 mm. Indeed, it is advantageous not only to reduce the notch rate in the center but also to increase the thickness of the rubber compound between the radially outermost crown layer and the bearing surface in the central portion of the tread compared to this same thickness in the lateral portions in order to better protect the hollows from this aggression in the region most subject to this type of aggression.

[0027] the expression "in line with" means "for each meridian, radially interior substantially within the limit of the axial coordinates delimited by". Thus "the points of a working layer in line with a sculpture block" designates for each meridian, the set of points of the working layer radially interior to the sculpture blocks within the limit of the axial coordinates delimited by said block.

[0028] Advantageously, each tread block of the central portion is connected to at least one tread block of a lateral portion in the form of bars by a stubble protection bridge, the radially outer face of which comprises two oblique sections intended to separate the stubble during rolling. The regular distribution of bridges between the central portion and the lateral portions promotes circumferentially regular flattening of the tire during rolling, greatly reduces the differences in rigidity of the more massive blocks of the central portion and the bars of the lateral portions and the generation of flat spots, a form of irregular wear. The presence of bridges protects the axially inner part of the lateral portions in contact with the central portion from the aggression of the stubble."Stubble guard" is an adjective combining the suffix "pare" meaning protecting and the noun "stubble" which are the remains of mown cereals, cotton having sufficient rigidity to damage agricultural tires which would roll over them. This adjective makes it possible to differentiate the bridges or other sculpture elements according to the invention and other possible bridges or respectively other sculpture elements which could be present in the sculpture but would not have the same geometric characteristics. These bridges were created for this function, this does not presage the fact that they move all the stubble away nor that they cannot be improved.

[0029] Advantageously, the bridges between the sculpture blocks of the central portion are thatch-guard bridges having a radially outer face comprising at least two inclined sections, extending respectively from each of its lateral faces to a ridge line, and the average angle of the normal to each of the inclined sections with the radial direction is at least equal to 10° and at most equal to 20°. These angles are optimum for managing the compromise between the homogenization of the rigidities of the tread elements and the effectiveness of the bridges in spreading the thatches. For these same reasons, the most radially outer point of the ridge line of a thatch-guard bridge is at a radial distance from the bearing surface of between 70 and 80% of the tread height.

[0030] To protect the most sensitive part of the tire, the axial width of the central portion is advantageously between 20% and 25% of the axial width L of the tread for optimal protection of the center of the tread. It represents the axial width of the tread blocks of the central portion. Similarly, the axial width of each lateral portion is advantageously between 30 and 40% of the axial width L of the tread.

[0031] It is advantageous to promote the sliding of the stubble which would come to bear on the lateral faces of a sculpture block of the central portion towards the lateral face of the stubble bridge to which it is connected then along the face of the bar continuously towards the axial exterior of the tire. It is therefore advantageous to avoid a discontinuity between the lateral faces of the blocks of the central portion, of the bridges and of the lateral faces of the bars which could block the sliding of the stubble. Given that the faces of the bars have an angle optimized for traction in the fields, to promote this continuity and this technical effect, a leading face of each sculpture block of the central portion is advantageously substantially continuous with the leading face of a stubble bridge, and makes an angle with the circumferential direction (XX') at least equal to 40° and at most equal to 60°, preferably between 45 and 50°.Advantageously, the leading face of the thatch guard bridges is substantially continuous with the leading face of the bar to which it is connected, and makes an angle with the circumferential direction (XX') at least equal to 40° and at most equal to 60°, preferably between 45 and 50°.

[0032] To enable this solution to operate as efficiently as possible in endurance, particularly in thermal conditions despite the increase in the volume of rubber mixture in the central portion, it is advantageous for the tread to comprise a so-called contact material intended to come into contact with the ground, the dynamic loss tanô of which, measured according to the same standard ASTM D 5992 - 96, at a temperature of 60°C and under a deformation of 50% at 10 Hz, is less than 0.3.

[0033] To resist stubble penetration and wear, it is preferred that the so-called contact material has an elastic shear modulus G' at 50% de- summit formation, measured according to ASTM D 5992 - 96, greater than 1.50 MPa.

[0034] The rolling resistance performance can be improved if the tread comprises a so-called underlay material, radially inside the so-called contact material, the dynamic loss tanô of which, measured according to the same standard ASTM D 5992 - 96, at a temperature of 60°C and under a deformation of 50% at 10 Hz, is at most equal to 0.15. Similarly, it is advantageous for the so-called underlay material to have an elastic shear modulus G' at 50% peak deformation, measured according to the standard ASTM D 5992 - 96 at least equal to 1.0 MPa. The underlay material is preferably radially inside the bearing surface.

[0035] The terms: elastic modulus, viscous modulus designate, for a person skilled in the art of elastomers, well-known dynamic properties. The phase angle ô between the force and the displacement, translated into dynamic loss tanô is equal to the ratio of the viscous and elastic moduli G" / G'.

[0036] These properties can be measured on glued specimens extracted from a tire tread. Specimens as described in ASTM D 5992-96 (version published in September 2006) can be used. The specimen used is cylindrical with a diameter of 10 mm and a height of 2 mm.

[0037] The specimen is subjected to sinusoidal stress in alternating simple shear, at a frequency of 10 Hz with imposed stress, symmetrically around its equilibrium position. An accommodation of the specimen is carried out prior to the temperature scanning measurement. For this purpose, the specimen is subjected to sinusoidal shear stress at 10 Hz, at 100% peak-peak strain at a temperature of 60°C.

[0038] The temperature scanning measurement is carried out during an increasing temperature ramp of 1.5°C per minute, starting from a minimum temperature lower than the glass transition temperature Tg of the material up to a maximum temperature. Before carrying out the measurements, the test piece is stabilized at the minimum temperature for at least 20 minutes. The glass transition temperature Tg is the temperature at which the dynamic loss tanô reaches a maximum during the temperature scanning.

[0039] It may be advantageous in particular in the case where there is an excess thickness of elastomeric mixture, but not only, for any hollow of each lateral portion to comprise a face with a radial height at least equal to 3 mm, called the stubble guard, the trailing edge of which is axially outside its leading edge, this face also making it possible to orient the stubble in this zone towards the axial outside of the tire. In order not to create a part of this face more prone to tearing, it is advantageous for the angle of the normal to the so-called stubble guard face of the lateral portion (PI) with the circumferential direction to evolve continuously from the leading edge to the trailing edge. Similarly, in order for this orientation to be compatible with the orientation of the bars, the angle of the normal to the so-called thatch-guard face advantageously makes at its axial end with the circumferential direction (XX') an angle of between 40 and 60°.

[0040] The characteristics of the invention are illustrated by schematic figures 1 to 3, not shown to scale: - [Fig.l]: portion of the sculpture of the tire according to the invention, - [Fig.2]: detail of the thatch protection bridge - [Fig.3]: diagram of a half-meridian section of the tire according to the invention.

[0041] [Fig.l][Fig.l] represents a portion of the tread 2 of a tire for an agricultural vehicle having a recommended rolling direction 12. The tread 2, of an axial width L, comprises tread elements 22 including tread blocks 221, 222 separated from each other by hollows 23, in this case bars 221 of the lateral portions PI of axial width Lp separated by hollows 231 and blocks 222 of the central portion Pc of axial width Le separated by hollows 232. The tread blocks 221, 222 extend radially outwards from a bearing surface 233 to a rolling surface 25, the bearing surface 233 of which only one line is visible, being in fact a theoretical surface constituted by the torus obtained by the rotation around the YY' axis of rotation of the tire of all the most radially inner points of the hollows 23 of the sculpture on all the meridian planes.The bars 221 of the lateral portions form, with the circumferential direction (XX1) of the tire, an angle Al at least equal to 40° and at most equal to 60° and are arranged in a chevron pattern with a circumferential offset between the bars of the two lateral portions. Each bar 221 is connected to a tread block 222 of the central portion by a bridge 224 whose average transverse thickness is equal to the average transverse thickness emt of the bar being 44.5 mm. The length Ipc of a stubble protection bridge, defined as the distance separating the tread blocks connected by said bridge 224, is equal to 27.5 mm. The volumetric notch rate, defined as the ratio between the volume VC of hollow 23 and the total volume V of the tread 2 assumed to be without hollow, between the bearing surface 233 and the rolling surface 25, of the central portion Pc, is lower than the volumetric notch rate of the lateral portions PI.The leading faces of the blocks 222 of the central portion Pc, of the bridges 224 and of the bars are continuous. The figure also represents a thatch guard face 234, the trailing edge of which is axially external to its leading edge, and continuous.

[0042] [Fig.2][Fig.2] represents the section of the thatch protection bridge along the line AA mentioned in [Fig.l]. The bridge 224 is connected to a block 222 of the central portion whose radially outer surface is included in the rolling surface 25. The bridge 224 has two substantially radial lateral faces 2244, 2245 and a ra- radially outer, connecting the two lateral faces 2244, 2245 and radially inner to the rolling surface 25. The radially outer face comprising at least two inclined faces 2242, 2243 extending respectively from each of its lateral faces 2244, 2245 to a ridge line 2241. Face 2244 is the leading face of the bridge and face 2245 its trailing face. The average angle Ap of the normal to the inclined plane considered with the radial direction (ZZ') at each inclined face is respectively equal to 13° for the inclined face 2242 and equal to 14° for the inclined face 2243.

[0043] [Fig. 3] [Fig. 3] represents a portion of the crown of the tire according to the invention in a meridian plane YZ passing through the axis of rotation YY' of the tire. The tire 1 for an agricultural vehicle comprises a crown reinforcement 3 radially inside a tread 2 and radially outside a carcass reinforcement 4. The crown reinforcement 3 comprises 4 crown layers 31, 32, 33, 34, each comprising textile reinforcement elements coated in an elastomeric material. The tread 2 comprises hollows 23, bars 221 in the lateral portion of the tread 2, and blocks 222 in the central portion of the tread, the two being connected by a bridge 224. The tread is composed of a contact material 211 and a so-called underlayer material 212.The tread has an axial half-width L / 2 and comprises a central portion of an axial half-width Lc / 2 and two axially outer portions, only one of which is shown, its axial width being equal to Lp. The radial distance from the bearing surface 233 to the rolling surface 25, measured at the center of the tread, determines the tread height Hs, being at least equal to 35 mm. A face 234 with a radial height at least equal to 3 mm, called a stubble guard, the trailing edge 2342 of which is axially outside its leading edge 2341 is present in the hollow 23. [Fig.3] also shows the thickness of rubber mixture hc equal to the radial distance from the bearing surface 233 to the radially outermost crown layer 31 and the distance hl from the bearing surface 233 to the radially outermost crown layer 31 measured at the center of each lateral portion PI.

[0044] The invention has been more particularly implemented for an agricultural tire of dimension 380 / 90R46. The tire according to the state of the art is a Michelin Spraybib © of this dimension whose tread has an axial width equal to 327 mm. Its sculpture is composed of continuous bars of 33 mm radial height in the center of the tread and making an angle of 47° with the circumferential direction, with an average transverse thickness emt of 46 mm and an axial width representing 56% of the total width of the tread without having any bridging or excess thickness in the central portion of the tread between the most radially outer crown layer and the comparative bearing surface tively to the side portions.

[0045] The volumetric notch rate of the tire according to the state of the art, defined as the ratio between the volume VC of hollow 23 and the total volume V of the tread 2 assumed to be without hollow, between the bearing surface 233 and the rolling surface 25, of the central portion Pc, is equal to 46.2% and the volumetric notch rate of the lateral portions PI is equal to 54.3%. As it is not possible to determine the central portion and lateral portions for the control tire, we took for this evaluation a ratio Lc / Lt identical to that of the invention.

[0046] The tire according to the state of the art comprises 6 crown layers whose reinforcing elements are made up of 3 rayon strands with a linear mass of 240 g per km, the reinforcing elements having a breaking force of 28 daN and being arranged at a pitch of 1.27 mm, for a breaking force of the crown layer of 22 daN / mm.

[0047] The tire according to the state of the art comprises a so-called contact material 211 intended to come into contact with the ground, the dynamic loss tanô of which, measured according to the same standard ASTM D 5992 - 96, at a temperature of 60°C and under a deformation of 50% at 10 Hz, is equal to 0.3 and an elastic shear modulus G' at 50% peak deformation, at 1.5 MPa. The tire according to the state of the art does not comprise an underlayer material.

[0048] The tire according to the invention has a tread with an axial width equal to 327 mm. Its tread pattern is composed of central tread blocks with a radial height of 39 mm, an axial width equal to 22% of the tread width, connected by bridges of 27.5 mm length Ipc to continuous bars making an angle Al of 47.3° with the circumferential direction, with a mean transverse thickness emt equal to 43.8 mm. The axial widths of the lateral portions represent 44% of the total width of the tread. The tread has an excess thickness between the radially outermost crown layer 31 and the bearing surface 233 in the central portion of the tread of 5 mm relative to the thickness of the tread at the centers of the lateral portions PI (hc-hl=5 mm). This extra thickness results in the presence of a face with a radial height of 4mm, called the thatch guard.This face is continuous and its trailing edge is tangent to the bar. The volumetric notch rate of the central portion Pc is equal to 43.2% and the volumetric notch rate of the lateral portions PI is equal to 57%.

[0049] The bridges between the bars have the width of the bars and their radially outer faces comprise at least two inclined sections 2242, 2243, extending respectively from each of its lateral faces 2244, 2245 to a ridge line 2241 at a radial height of 31 mm from the bearing surface. The radial heights of the leading and trailing edges of the bearing surface are equal to 26 mm. The average angle Ap of the normal to the inclined plane 2242 with the radial direction (ZZ') is equal to 13° and the average angle Ap of the normal to the inclined plane 2243 with the radial direction (ZZ') is equal to 14°.

[0050] The tire according to the invention comprises 4 crown layers whose reinforcing elements are cables made up of an aramid strand having a linear mass equal to 167 g per km and a strand of a PET textile fiber with a linear mass equal to 144 g per km, the reinforcing elements of the working layers being arranged in the working layers at a pitch equal to 0.86 mm. The reinforcing elements have a breaking force of 37 daN and the crown layers a linear breaking force of 43 daN / mm.

[0051] The tire according to the invention comprises a so-called contact material intended to come into contact with the ground, the dynamic loss tanô of which, measured according to the same standard ASTM D 5992 - 96, at a temperature of 60°C and under a deformation of 50% at 10 Hz, is equal to 0.25. and the elastic shear modulus G' of which at 50% of crown deformation is equal to 1.55 MPa. The tire according to the invention comprises a sub-layer material the dynamic loss tanô of which, measured according to the standard ASTM D 5992 - 96, at a temperature of 60°C and under a deformation of 50% at 10 Hz, is equal to 0.11 and the elastic shear modulus G' of which at 50% of crown deformation is equal to 1.19 MPa.

[0052] The tire according to the invention was simulated by finite elements for rolling reproducing use measured by position and force sensors on a vehicle in a real work situation and travel from the farm to the fields. The calculations show an improved wear performance of at least 35% for an increase in the height of the sculpture of 16%, 7% of which is due to the performance brought about by the geometry of the sculpture. The bridging allows in particular very good homogenization of the rolling forces and regular wear between the bars and the central blocks.

[0053] The tire was also tested for traction. The tires are mounted on a commercially available sprayer. The tires are inflated to the pressure and load of the vehicle in the field for cyclic use well known to users. The hubs are equipped with force sensors capable of measuring slippage. The sprayer is driven in a plowed field with an average slope of 11%. The measurement is made uphill. The bridging allows for an increase in the footprint and an improvement in the "traction" performance of 30% by reducing the slippage rate from 50% to 35%.

[0054] The tire was also tested for traction, i.e. the ability to pull a tool. The pressures and loads are identical to the previous type. The vehicle equipped with the tires tested machines pull a mass on wheels in a field. For a slip rate of 15%, the force developed is improved by 15%, again thanks to the improvement in the footprint linked to the invention.

[0055] Since the performance of resistance to stubble attacks is difficult to measure outside of a full-scale test over long use, an initial estimate is based on expert knowledge of technicians advising users. These experts expect an improvement in resistance to stubble attacks due to the sculpture's ability to evacuate or deflect them, of the order of 20 to 30%. This performance is assessed on the presence and number of visible stubble impacts after rolling in a field after mowing.

[0056] Despite an increase in the volume of tread material by increasing the tread height by 16%, thanks to the coupling of the tread and tread materials, the tire according to the invention is better in rolling resistance by more than 10%. Furthermore, thanks to the lightening of the crown, the total mass of the tire remained constant and despite greater stress on each of the crown layers, the maximum temperature at the crown was reduced by 20°C in use, which is coupled with better endurance.

Claims

Claims

1. A tire (1) for an agricultural vehicle, comprising, radially from the outside to the inside, a tread (2) and a crown reinforcement (3) comprising crown layers (31, 32, 33, 34) comprising textile reinforcing elements: -the tread (2) having an axial width L and comprising tread blocks (221, 222) separated from each other by hollows (23) and extending radially outwards from a bearing surface (233) to a rolling surface (25), the radial distance measured at the centre of the tread from the bearing surface to the rolling surface determining the tread height (Hs) being at least equal to 35 mm, - the tread (2) comprising two lateral portions (PI) whose axial width is between 30 and 40% of the axial width L of the tread, axially external to a central portion, each lateral portion comprising a circumferential distribution of sculpture blocks (221) in the form of bars, two by two separated by transverse hollows (231) forming, with a circumferential direction (XX1) of the tire, an angle (Al) at least equal to 40° and at most equal to 60°, - the central portion (Pc) comprising a circumferential distribution of sculpture blocks (222), two by two separated by transverse hollows (232), - the central and axial portions having volumetric notch rates, defined as the ratio between the volume VC of hollow (23) and the total volume V of the tread (2) assumed to be without hollow, between the bearing surface (233) and the rolling surface (25), characterized in that the volumetric notch rate of the central portion (Pc) is between 35 and 45% and the volumetric notch rates of the lateral portions (PI) are between 50 and 65%, and in that the crown reinforcement (3) comprises at most 5 crown layers and the reinforcing elements of the crown layers (31, 32, 33, 34) have a tensile breaking force at least equal to 32 daN.

2. A tire according to claim 1, wherein, vertically above the central portion (Pc), the radial distance (hc) from the bearing surface (233) to the radially outermost crown layer (31) is at least equal to the distance (hl) from the bearing surface (233) to the crown layer (31). radially outermost vertex (31) measured at the center of each lateral portion (PI) plus 4 mm.

3. A tire according to any one of claims 1 or 2, wherein the crown layers (31, 32, 33, 34) comprise reinforcing elements comprising an aramid strand and a strand of a PET or nylon textile fiber, the aramid strand having a tensile breaking force of at least 30 daN.

4. A tire according to any preceding claim, wherein the crown layers (31, 32, 33, 34) comprise reinforcing elements comprising an aramid strand and a strand of a PET textile fiber.

5. A tire according to any one of the preceding claims, in which the reinforcing elements of the working layers (31, 32, 33, 34) are hybrid cables consisting of an aramid strand with a linear mass of between 160 and 180 g per km and a PET strand with a linear mass of between 140 and 160 g per km, the reinforcing elements of the working layers (31, 32, 33, 34) being arranged in the working layers at a pitch of between 0.8 mm and 1 mm.

6. A tire according to any one of the preceding claims, in which the crown reinforcement (3) comprises at most 4 crown layers with a linear breaking strength of at least 30 daN / mm, preferably at least 40 daN / mm.

7. A tire according to any one of the preceding claims, in which the tread (2) comprises a so-called contact material (211) intended to come into contact with the ground, the dynamic loss tanô of which, measured according to the same standard ASTM D 5992 - 96, at a temperature of 60°C and under a deformation of 50% at 10 Hz, is less than 0.30 and the elastic shear modulus G' of which at 50% crown deformation (5), measured according to the standard ASTM D 5992 - 96, is greater than 1.50 MPa.

8. A tire according to any one of claims 6 or 7, in which the tread (2) comprises a so-called underlayer material (212), radially inside the so-called contact material (211) whose dynamic loss tanô, measured according to the same standard ASTM D 5992 - 96, at a temperature of 60°C and under a deformation of 50% at 10 Hz, is at most equal to 0.

15.

9. A tire according to claim 8, wherein the so-called underlayer material (212) has an elastic shear modulus G' at 50% de- summit formation (5), measured according to ASTM D 5992 - 96, at least equal to 1.0 MPa.

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