TIRES WITH A TREAD WITH ORIENTED FIBERS

Description

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

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

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

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

[0005] The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the rolling direction of the tire.

[0006] The tire's axis of rotation is the axis around which it rotates during normal use.

[0007] A radial or meridian plane is a plane that contains the tire's axis of rotation.

[0008] The circumferential median plane, or equatorial plane, is a plane perpendicular to the tire's axis of rotation and which divides the tire into two halves.

[0009] The transverse or axial direction of the tire is parallel to the tire's axis of rotation. An axial distance is measured along the axial direction. The expression "axially inside, respectively axially outside" means "whose axial distance measured from the equatorial plane is less than, respectively greater than".

[0010] The radial direction is a direction intersecting the axis of rotation of the tire and perpendicular to it. A radial distance is measured in the radial direction. The expression "radially inward to, respectively radially outward to" means "whose radial distance measured from the axis of rotation of the tire is less than, respectively greater than".

[0011] Radially outside the crown reinforcement is the tread, usually made of polymeric materials intended to come into contact with the ground in the contact area between the ground and the tire.

[0012] It is known to provide the tread, that is to say the part of the tire intended to come into contact with the ground when rolling and to wear during rolling, with a sculpture formed of raised elements delimited by cutouts such as grooves, whether circumferential, transverse or oblique in orientation. The objective of such a sculpture is to give the tread good performance when rolling on dry roads and on water-covered roads, particularly in wet weather.

[0013] To improve the performance of treads without, however, excessively lowering the shear rigidities of said strips, it is known to form on the rolling surface a plurality of transversely or obliquely oriented edges in order to cut the film of water on a roadway to ensure good contact between the tread and the roadway. One means of obtaining such edges consists of providing the strip with a plurality of cutouts, these cutouts having the form of grooves or the form of incisions. In the present application, incisions are distinguished from grooves in that the incisions have a width appropriate to allow at least partial contact during rolling between the facing walls delimiting these incisions and in particular during the passage into contact with the ground, which cannot be the case for grooves under normal conditions of use of the tire.

[0014] For the purposes of the invention, a longitudinally oriented cutout is a cutout whose mean plane of at least part of the walls of said cutout forms an angle with a longitudinal plane of less than 10°. This angle formed with a longitudinal plane may be oriented in one direction or the other relative to said longitudinal plane. A longitudinally oriented cutout may also be a cutout whose walls undulate or zigzag around a mean plane as just described.

[0015] For the purposes of the invention, a transversely oriented cutout is a cutout whose mean plane of at least part of the walls of said cutout forms an angle with a radial plane of less than 35°. This angle formed with a radial plane may be oriented in one direction or the other relative to said radial plane. A transversely oriented cutout may also be a cutout whose walls undulate or zigzag around a mean plane as just described.

[0016] For the purposes of the invention, an obliquely oriented cutout is a cutout whose mean plane of at least part of the walls of said cutout forms an angle with a radial plane of between 35° and 80°. This angle formed with a radial plane may be oriented in one direction or the other relative to said radial plane. An obliquely oriented cutout may also be a cutout whose walls undulate or zigzag around a mean plane as just described.

[0017] Some current tires, called "road tires," are designed to travel at high speeds and over increasingly long distances, due to the improvement of the road network and the growth of the motorway network around the world. The set of conditions under which such a tire is called upon to travel allows for an increase in the number of kilometers traveled, with less wear on the tire.

[0018] To further increase tire life, it is common to choose polymeric tread materials with improved wear resistance properties. However, such materials are known to penalize grip properties, especially on wet surfaces.

[0019] Such tires are for example described in document US 6,247,512. This document describes the superposition of two layers of materials to form the tread, the external material coming into contact with the ground being in particular more efficient in terms of wear.

[0020] Document EP 3 031 622 A1 describes a tire comprising a colored wear indicator.

[0021] Document EP 3 103 657 A1 describes a tire sculpture combining grooves and incisions to limit irregular wear phenomena.

[0022] The article by Mortazavian Seyyedvahid et al. "Effects of fiber orientation and anisotropy on tensile strength and elastic modulus of short fiber reinforced polymer composites" published on December 8, 2014 in Science Direct- composites: Part B, pages 1-14 (XP55821290) presents a study on the effect of the presence of glass fibers on the mechanical properties of thermoplastic materials.

[0023] Document WO 02 / 090094 A1 describes a tire comprising a tread consisting of several layers of radially superimposed elastomeric compounds separated locally by interlayers to facilitate regrooving of the tread.

[0024] Document US 6,472,461 B1 describes a tire whose tread comprises textile fibers to improve braking properties on wet ground or on ice.

[0025] The inventors set themselves the task of being able to provide tires that allow for ever-increasing rolling distances with improved wet grip properties.

[0026] This object has been achieved according to the invention by a tire, with a radial carcass reinforcement, comprising a crown reinforcement, itself radially capped with a tread joined to two beads by means of two sidewalls, said tread comprising grooves forming at least one tread element, said tread comprising at least one layer of elastomeric mixture forming the tread surface of the tire, said at least one layer of elastomeric mixture being reinforced by non-metallic fibers arranged substantially parallel to the outer surface of the tread, at least 40% of the non-metallic fibers being oriented in a direction making an angle of less than 30° relative to the main axis of inertia associated with the maximum moment of inertia of the surface in contact with the ground of said at least one tread element, the middle of said surface in contact with the ground,along the longitudinal direction passing through the center of gravity of said surface in contact with the ground, being located longitudinally at the center of the contact area, the tire being subjected to its nominal load and inflated to its nominal pressure on the nominal rim and the value of the longitudinal modulus of elasticity at room temperature of said at least one layer of elastomeric mixture, measured along at least one direction parallel to the rolling surface being greater than 5 times the value of the longitudinal modulus of elasticity at room temperature of said at least one layer of elastomeric mixture measured along the direction perpendicular to the rolling surface.,

[0027] Preferably according to the invention, the value of the longitudinal modulus of elasticity at room temperature of said at least one layer of elastomeric mixture, measured in at least one direction parallel to the rolling surface, is less than 20 times the value of the longitudinal modulus of elasticity at room temperature of said at least one layer of elastomeric mixture measured in the direction perpendicular to the rolling surface and preferably still less than 10 times the value of the longitudinal modulus of elasticity at room temperature of said at least one layer of elastomeric mixture measured in the direction perpendicular to the rolling surface.

[0028] For the purposes of the invention, the nominal load and pressure conditions as well as the nominal rim are defined according to ETRTO, TRA or JATMA regulations.

[0029] The longitudinal modulus of elasticity is measured according to standard NF ISO 37 of December 2005 on a type 2 dumbbell test piece by measuring the modulus of elasticity at 5% deformation at 23°C.

[0030] The measurement of the angle of the short fibers within the tread is carried out on a sample taken from the tread, preferably by removing half the width of a rib along a plane parallel to the tread surface, so as to reveal an interface containing the short fibers, then by taking a material specimen by cutting the tread along a plane parallel to the tread surface. A histogram of the orientation of the fibers relative to the longitudinal direction is established on the specimen by optical microscopy in reflection on a sample of at least 100 fibers in accordance with the recommendations found in "Orientation of short fibers in reinforced thermoplastic parts - Observation of the orientation of the fibers", Techniques de l'ingénieur, Reference AM3729, July 10, 2003, Michel VINCENT.

[0031] According to a preferred embodiment of the invention, said at least one elastomeric mixture reinforced with non-metallic fibers has a tan(δ)-20°C value strictly less than 0.50.

[0032] The dynamic property tan(δ)-20°C is measured on a viscoanalyzer (Metravib VA4000), according to ASTM D 5992-96. The response of a test piece consisting of two cylindrical pellets, each 2 mm thick and one centimeter in diameter, is recorded (the test piece is made from a tire sample taken at mid-height of the layer concerned as close as possible to the equatorial plane area in an area of sufficient thickness to constitute the test piece), subjected to a sinusoidal stress in alternating simple shear, at a frequency of 10 Hz.

[0033] For the measurement of tan(δ)-20°C, a temperature scan is carried out between -80 and +100°C, under a stress of 0.7 MPa, and the value of tan(δ) observed at -20°C is recorded. It is recalled that, in a manner well known to those skilled in the art, the value is representative of the wet grip potential: the higher the value of tan(δ)-20°C, the better the grip. An arbitrary value of 100 is given for the control composition, a result greater than 100 indicating an increase in the value of tan(δ)-20°C, corresponding to an improvement in the wet grip performance.

[0034] The grip of a tire is characterized by the force that the tire is able to transmit to the ground under braking stress for example. It is in particular represented by the hysteretic losses linked to the deformations of the tread during the application of the stress. The value of tan(δ)-20°C thus corresponds to an indicator of the grip on wet ground of a tire when braking between 80km / h and 0km / h.

[0035] The various measurements are carried out on new tires, which have not yet been driven.

[0036] The inventors were able to demonstrate that the elastomeric mixture reinforced by non-metallic fibers as described previously leads to a satisfactory compromise between the wear resistance properties and the wet grip properties of the tire.

[0037] In fact, the elastomeric mixture constituting the radially outer part of the tread has a rigidity measured in the direction perpendicular to the tread surface, giving it satisfactory wear performance.

[0038] The presence of the fibers in this elastomeric mixture in the orientation described above, giving the mixture a rigidity as described above, leads to less deformation of the tire tread when the tire is crushed in the contact area. The inventors were able to demonstrate that the presence of the non-metallic fibers as described limits the so-called transverse deformations linked to the incompressibility of the elastomeric mixture constituting the radially outer part of the tread. The inventors were also able to demonstrate that the deformations of the layers of elastomeric mixtures which form the tread of the tire induce so-called parasitic forces in the contact area. These so-called parasitic forces, described in “Effect of friction on rolling tire - pavement interaction”, H. Wang, IL Al-Qadi & I. Stanciulescu, NEXTRANS Project No.049IY02, USDOT Region V Regional University Transportation Center Final Report, reduce the amount of force that can be transmitted in the contact patch before reaching the maximum force that can be transmitted, characterized by the tire sliding on the ground. In other words, these so-called parasitic forces consume part of the grip potential. The presence of non-metallic fibers in this elastomeric compound, which limit tread deformation, thus contributes to better tire grip.

[0039] Preferably according to the invention, the non-metallic fibers have an average length of between 0.5 and 10 mm, and more preferably between 2 and 5 mm.

[0040] The measurement of the length of the short fibers is carried out according to the invention by optical microscopy using automated optical analysis according to ISO 16065 or by one of the methods described in "A REVIEW OF IMAGE ANALYSIS BASED METHODS TO EVALUATE FIBER PROPERTIES", Ulrich Hirn and Wolfgang Bauer, Lenzinger Berichte, 86 (2006) 96-105.

[0041] Preferably according to the invention, the non-metallic fibers have an average thickness of between 5 and 40 µm, and more preferably between 10 and 30 µm.

[0042] The measurement of the thickness of the short fibers is carried out according to the invention by optical microscopy using automated optical analysis according to ISO 16065 or by one of the methods described in "A REVIEW OF IMAGE ANALYSIS BASED METHODS TO EVALUATE FIBER PROPERTIES", Ulrich Hirn and Wolfgang Bauer, Lenzinger Berichte, 86 (2006) 96-105.

[0043] Advantageously according to the invention, the form factor of the non-metallic fibers, that is to say the ratio between the length and the thickness of the fibers, is between 12.5 and 2000. Advantageously again, the form factor is between 50 and 1500, and more preferably between 100 and 1000.

[0044] The non-metallic fibers can have any known cross-section, for example cubic, cylindrical, star-shaped. Preferably, according to the invention, the fibers have a cylindrical cross-section. In this case, the thickness corresponds to the diameter of the short fibers.

[0045] According to a preferred embodiment of the invention, the Young's modulus of the non-metallic fibers is between 0.5 and 500 GPa, preferably between 0.5 and 200 GPa, and more preferably between 0.5 and 50 GPa.

[0046] The Young's modulus of short fibers is measured according to the invention in accordance with ASTM D885.

[0047] According to an advantageous embodiment of the invention, the non-metallic fibers are present in the layer of elastomeric mixture forming the rolling surface of the tire with a content of between 2.5 and 10% by volume, and preferably between 5 and 7.5% by volume.

[0048] For the purposes of the invention, the fiber content of an elastomeric mixture, expressed as a volume percentage, is determined using an X-ray tomography method as described in the thesis “Multi-resolution imaging by X-ray tomography: application to local tomography in materials science”, Tao Zhang, University of Grenoble, 2012 (NNT: 2012GRENI020. tel-00876871). The person skilled in the art then knows how to use the acquisition parameters, filters and image processing adapted to identify the fibers within the matrix as well as the corresponding volume, and thus determine the volume ratio of said fibers.

[0049] Advantageously according to the invention, the non-metallic fibers are chosen from natural fibers such as cotton, linen, hemp, bamboo, etc. Such a choice has the advantage of reinforcing elements which will disappear through biodegradability when, during wear of the tire tread, they are released into nature.

[0050] According to one embodiment of the invention, the elastomeric mixture forming the tread surface of the tire comprises non-metallic fibers, in two main orientations substantially perpendicular to each other so that the values of longitudinal modulus of elasticity at room temperature measured in at least two directions parallel to the tread surface are greater than 5 times the value of longitudinal modulus of elasticity at room temperature measured in the direction perpendicular to the tread surface.

[0051] According to this embodiment of the invention, to determine the orientation of the fibers, a succession of cuts is made parallel to the rolling surface, so as to reveal the fibers and establish the histogram of fiber orientation in these planes parallel to the rolling surface relative to the longitudinal direction by optical microscopy in reflection on a sample of at least 100 fibers in accordance with the recommendations found in "Orientation of short fibers in reinforced thermoplastic parts - Observation of the orientation of the fibers", Techniques de l'ingénieur, Reference AM3729, July 10, 2003, Michel VINCENT.

[0052] According to this embodiment, to carry out the measurements of the longitudinal modulus of elasticity, at least two test pieces 1 mm thick are extracted from the tread, in a plane parallel to the rolling surface, along said two main orientations substantially perpendicular to each other, as well as a test piece 1 mm thick in a plane perpendicular to the rolling surface and in a direction perpendicular to the rolling surface.

[0053] According to one embodiment of the invention, the crown reinforcement of the tire is formed from at least two working crown layers of inextensible reinforcing elements, crossed from one layer to the other making angles of between 10° and 45° with the circumferential direction.

[0054] According to other embodiments of the invention, the crown reinforcement further comprises at least one layer of circumferential reinforcing elements.

[0055] An embodiment of the invention also provides that the crown reinforcement is completed radially on the outside by at least one additional layer, called a protective layer, of so-called elastic reinforcing elements, oriented relative to the circumferential direction with an angle of between 10° and 45° and in the same direction as the angle formed by the inextensible elements of the working layer which is radially adjacent to it.

[0056] According to any of the embodiments of the invention mentioned above, the crown reinforcement can also be supplemented, radially inside between the carcass reinforcement and the radially inner working layer closest to said carcass reinforcement, by a triangulation layer of inextensible metallic steel reinforcing elements making, with the circumferential direction, an angle greater than 60° and in the same direction as that of the angle formed by the reinforcing elements of the layer radially closest to the carcass reinforcement.

[0057] Other details and advantageous characteristics of the invention will emerge below from the description of exemplary embodiments of the invention with reference to figures 1 to 4 which represent: Figure 1 , a meridian view of a diagram of a tire according to an embodiment of the invention, Figure 2, a view of a schematic representation of the contact patch of a tire, Figure 3 , a partial view of a schematic representation of the contact patch of a tire illustrating the directions associated with the maximum moment of inertia and the minimum moment of inertia of a tread block of a tire, Figure 4 , a partial view of a schematic representation of the contact patch of a tire illustrating the directions associated with the maximum moment of inertia and the minimum moment of inertia of a rib of a tire.

[0058] Figures are not drawn to scale to simplify understanding.

[0059] On the Figure 1, the tire 1, of dimension 315 / 70R22.5, comprises a radial carcass reinforcement 2 anchored in two beads, around rods, not shown. The carcass reinforcement 2 is formed of a single layer of metal cables. The carcass reinforcement 2 is hooped by a crown reinforcement 5, itself capped with a tread 6. The tread comprises four grooves 3 forming five ribs 4.

[0060] The lower areas and beads of the tire 1 are not shown in the figures.

[0061] On the Figure 1 , the crown reinforcement 5 is formed radially from the inside to the outside: of a first working layer 51 formed of inextensible metal cables 11.35 not hooped, continuous over the entire width of the sheet, oriented at an angle equal to 18°, of a second working layer 52 formed of inextensible metal cables 11.35 not hooped, continuous over the entire width of the sheet, oriented at an angle equal to 26° and crossed with the metal cables of the first working layer, of a protective layer 53 formed of elastic metal cables 6.35 not hooped, continuous over the entire width of the sheet, oriented at an angle equal to 26° in the same direction as the metal cables of the working layer 52.

[0062] According to the invention, the tread 6 is made up of a layer of elastomeric mixture in which fibers are embedded.

[0063] The elastomeric mixture has a longitudinal modulus of elasticity at room temperature, measured in the direction perpendicular to the rolling surface, equal to 5.2 MPa at 5% deformation.

[0064] The elastomeric mixture has a tan(δ)-20°C value equal to 0.35.

[0065] The fibers are PET fibers with an average length of 4 mm and an average diameter of 23 microns.

[0066] The volume percentage of fibers in the elastomeric mixture layer is 5%.

[0067] The tread is made by superimposing layers of elastomeric mixture reinforced with non-metallic fibers 2 mm thick made according to the method described in patent WO2017 / 109336. The layers of reinforced elastomeric mixture are oriented so that the preferred direction of the fibers is the axial direction, and the tread is made by stacking a number of layers to achieve the desired thickness.

[0068] The longitudinal modulus of elasticity at room temperature at 5% strain of the non-metallic fiber-reinforced elastomeric mixture, measured along the fiber direction, is 49 MPa.

[0069] There Figure 2schematically illustrates the contact area 21 of the surface of a tire 22 in contact with the ground, the tire being mounted on its nominal rim, inflated to its nominal pressure and subjected to its nominal load. On this surface of the tire 22, it can be seen that the sculpture is composed in the central part in an axial direction of loaves 23 between two circumferential ribs or ribs 24 on the axially external parts of the surface of the tire 22.

[0070] On this Figure 2 axis 25 is also illustrated, oriented in the axial direction, longitudinally in the center of the contact area.

[0071] There Figure 3schematically represents a sculpture block 23 whose middle of the surface in contact with the ground, in the longitudinal direction passing through the center of gravity of the surface in contact with the ground, is positioned longitudinally at the center of the contact area and therefore on the axis 25. The two directions associated with the maximum moment of inertia (straight 26) and the minimum moment of inertia (straight 27) of the surface of the block 23 in contact with the ground are also represented on this Figure 3 . Around the direction associated with the maximum moment of inertia 26, the orientation range of the non-metallic fibers according to the invention is represented between the two straight lines 28, 29 placed respectively at 30° on either side of the direction 26.

[0072] There Figure 4schematically represents a rib or rib 24 of sculpture. The rib or rib 24 being continuous circumferentially, the middle of the surface in contact with the ground, following the longitudinal direction passing through the center of gravity of the surface in contact with the ground, is continuously positioned longitudinally at the center of the contact area and therefore on the axis 25. The two directions associated with the maximum moment of inertia (straight 36) and the minimum moment of inertia (straight 37) of the surface of the rib or rib 24 in contact with the ground are represented on this Figure 4 . Around the direction associated with the maximum moment of inertia 36, the orientation range of the non-metallic fibers according to the invention is represented between the two straight lines 38, 39 placed respectively at 30° on either side of the direction 36.

[0073] In the case of the tire according to the invention as shown in the Figure 1, the non-metallic fibers are oriented in the axial direction and therefore form a substantially zero angle with the direction associated with the maximum moment of inertia of the surface of the ribs 4 in contact with the ground.

[0074] In the case of a tire whose tread pattern is similar to that of the Figure 2 , the invention may provide that the orientation of the non-metallic fibers varies according to the axial positioning of the measurement. In other words, the orientation of the non-metallic fibers may be provided differently depending on whether one is in a bread 23 or in a rib 24 depending on the Figure 2 .

[0075] Tires were produced on the basis of the elastomeric compound A described below with its properties. Mixture A NR 70 BR 10 SBR 20 N234 50 Silica 9 Silane 2 6PPD - 1.3DIMETHYL BUTYL PHENYL PARAPHENYLENE-DIAMINE 3 ZNO 2.6 SULFUR 1.5 Accelerator (CBS) 1.15 Longitudinal extension modulus at 5% strain and room temperature (MPa) 4 tan(δ)-20°C 0.35

[0076] The values of the constituents are expressed in pce (parts by weight per hundred parts of elastomers).

[0077] A reference tire R made according to a configuration corresponding to usual designs and not including fibers in the layer of elastomeric mixture forming the tread surface. The tread is made with mixture A.

[0078] A tire T conforming to the invention as shown in the Figure 1 combines, to form the tread, the mixture A which forms the layer of elastomeric mixture forming the rolling surface in which the fibers are inserted as described previously.

[0079] Wet grip measurements were carried out on each of the tyres under identical driving conditions in accordance with ISO 15222. The measurement results are presented in the following table, with a value of 100 being assigned to tyre R. Values above 100 express superior grip performance. R pneumatic Pneumatic T 100 103

[0080] These values highlight the interest of the presence of non-metallic fibers in the elastomeric mixture forming the rolling surface to limit the deformations of the tread when passing through the contact area inducing parasitic forces in contact with the ground, and thus contribute to better grip of the tire on wet ground.

[0081] Wear tests were carried out on a track simulating a motorway-type circuit. T tyres conforming to the invention are compared with reference R tyres.

[0082] The driving conditions are identical for all tires; an inflation pressure of 9 bars and the tires are subjected to a load of 3750 kg, the tires equipping the two positions of a steering axle of a heavy goods vehicle. The average driving speeds are equal to 70 km / h.

[0083] In the wear test carried out on the track, the average weight loss is measured after a given mileage. The tests carried out showed that for an identical weight loss, the tire T according to the invention covered a mileage equivalent to the mileage of the reference tire R.

Claims

1. - Tyre (1), with radial carcass reinforcement (2), comprising a crown reinforcement (5), itself capped radially with a tread (6) joined to two beads via two sidewalls, said tread (6) comprising grooves forming at least one tread pattern element, said tread comprising at least one layer of elastomeric compound forming the tread surface of the tyre, said at least one layer of elastomeric compound being reinforced with non-metallic fibres arranged substantially parallel to the outer surface of the tread, characterized in that at least 40% of the non-metallic fibres are oriented in a direction forming an angle of less than 30° with respect to the main axis of inertia associated with the maximum moment of inertia of the surface in contact with the ground of said at least one tread pattern element, the centre of said surface in contact with the ground, in the longitudinal direction passing through the centre of gravity of said surface in contact with the ground, being located longitudinally at the centre of the contact patch, the tyre being subjected to its nominal load and inflated to its nominal pressure on the nominal rim, and in that the value of the longitudinal modulus of elasticity at ambient temperature of said at least one layer of elastomeric compound, measured in at least one direction parallel to the tread surface, is greater than 5 times the value of the longitudinal modulus of elasticity at ambient temperature of said at least one layer of elastomeric compound measured in the direction perpendicular to the tread surface.

2. - Tyre (1) according to Claim 1, characterized in that the value of the longitudinal modulus of elasticity at ambient temperature of said at least one layer of elastomeric compound, measured in at least one direction parallel to the tread surface, is less than 20 times, and preferably less than 10 times, the value of the longitudinal modulus of elasticity at ambient temperature of said at least one layer of elastomeric compound measured in the direction perpendicular to the tread surface.

3. - Tyre (1) according to one of Claims 1 or 2, characterized in that the elastomeric compound reinforced with non-metallic fibres of the layer forming the tread surface has a value of tan(δ)-20°C strictly less than 0.50.

4. - Tyre (1) according to one of Claims 1 to 3, characterized in that the non-metallic fibres have a length between 0.5 and 10 mm, and preferably between 2 and 5 mm.

5. - Tyre (1) according to one of Claims 1 to 4, characterized in that the non-metallic fibres have an average thickness between 5 and 40 µm, and preferably between 10 and 30 µm.

6. - Tyre (1) according to one of Claims 1 to 5, characterized in that the form factor of the non-metallic fibres, defined by the ratio between the length and the thickness of the fibres is between 12.5 and 2000 and preferably between 100 and 1000.

7. - Tyre (1) according to one of Claims 1 to 6, characterized in that the Young's modulus of the non-metallic fibres is between 0.5 and 500 GPa, and preferably between 0.5 and 50 GPa.

8. - Tyre (1) according to one of the preceding claims, characterized in that the non-metallic fibres are present in the layer of elastomeric compound forming the tread surface of the tyre with a content of between 2.5 and 10% by volume, and preferably of between 5 and 7.5% by volume.

9. - Tyre (1) according to one of the preceding claims, characterized in that the textile fibres are chosen from natural fibres such as cotton, linen, hemp, or bamboo.

10. - Tyre (1) according to one of the preceding claims, characterized in that said at least one layer of elastomeric compound comprises non-metallic fibres, in two main orientations substantially perpendicular to each other, and in that the values of the longitudinal modulus of elasticity at ambient temperature of said at least one layer of elastomeric compound measured in at least two directions parallel to the tread surface are greater than 5 times the value of the longitudinal modulus of elasticity at ambient temperature of said at least one layer of elastomeric compound measured in the direction perpendicular to the tread surface.