TIRE WITH A RETRACTABLE TREAD

Abstract

The invention relates to a tire (1) that can be regrooved at least once and whose tread, consisting of at least one first elastomeric compound forming at least the tread surface, has at least one circumferential groove (3) that can be regrooved. According to the invention, the material to be regrooved consists of at least one first layer of a second elastomeric compound, said second elastomeric compound having a volume resistivity ρ different from that of said first elastomeric compound, forming at least the tread surface. Figure for the abstract: Fig 1

Description

Title of the invention: TYRE COMPRISING A RECROUGABLE TREAD

[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 radially surmounted 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 in the area of ​​the crown of the tire are subjected.

[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 at most equal to 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 tangent to the periphery of the tire and defined by the rolling direction of the tire.

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

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

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

[0009] The transverse or axial direction of the tire is parallel to the axis of rotation of the tire. 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 inside, respectively radially outside" 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 during rolling and to wear during rolling, with a sculpture formed of raised elements delimited by cutouts such as grooves, whether they are 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 the 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, during rolling, at least partial contact between the facing walls delimiting these incisions and in particular during the passage into contact with the ground, which could not be the case for the grooves under normal conditions of use of the tire.

[0014] For the purposes of the invention, a cutout of longitudinal or circumferential orientation is a cutout of which the 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 cutout of longitudinal orientation may also be a cutout whose walls undulate or zigzag around a medium plane as just described.

[0015] For the purposes of the invention, a transversely oriented cutout is a cutout whose mean plane of at least a portion 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] Combined with this need to improve grip performance by the presence of edges formed by the transverse cutouts, it is also required that the performance of a tread be sustainable, that is to say that satisfactory performance is achieved even after more or less advanced partial wear. Partial wear of a tread is understood to mean a state of wear corresponding to a tread thickness at most equal to the total tread thickness that can be worn before having to change the tire, in particular for regulatory reasons.

[0018] Heavy goods vehicle treads can be regrooved (an operation by which new grooves can be regrooved), and tyres with such treads carry the English wording "Regroovable" or the symbol "U" on their sidewalls. Regrooving makes it possible, on the one hand, to extend the grip potential of the heavy goods vehicle tyre and, on the other hand, to significantly increase mileage.

[0019] Regrooving heavy goods vehicle tires is a routine operation authorized for safety and the increased efficiency it brings. For example, it is explicitly authorized by the French Highway Code (Art. 4 of the decree of 24 / 10 / 94) and recommended by ETRTO (European Tires and Rim Technical Organization) and AFNOR (French Association for Standardization) with the NFR12714 standard. For this operation, manufacturers are required to provide regrooving diagrams that are binding on the technicians in charge of this regrooving.

[0020] The groove heights on new tires are measurable and the groove heights after regrooving can be deduced from these regrooving diagrams.

[0021] As known per se, the regrooving of a groove can be carried out using a blade heated rounded blade, still often handled by an operator. Said blade, connected to a frame which rests on the tread surface, can be used manually so as to follow the path of the groove on the tread surface fairly faithfully, even in the case of a groove with a non-rectilinear path.

[0022] Regrooving allows sharp edges to be restored and is usually intended to restore a tread height corresponding to that of a mid-wear heavy goods vehicle tyre. The usual recommendations aim for a height after regrooving of 5 to 6 mm composed of a regrooving of 3 to 4 mm and 2 mm of remaining height; in practice, regrooving is often carried out at the initiative of the tyre user earlier with remaining heights of between 4 and 5 mm leading to a height after regrooving of 7 to 9 mm, for tread heights of tyres in new condition of between 12 and 20 mm.

[0023] Regrooving a tire has several advantages. First of all, by restoring the tread height of the tire, regrooving helps extend the life of the tire.

[0024] Furthermore, since the regrooving is carried out when the thickness of the tread is the smallest and therefore when the tire has the lowest rolling resistance, the distance traveled is extended when the rolling resistance is the lowest.

[0025] Furthermore, document WO 2022 / 180325 has already described tires having rolling resistance performance that is further improved throughout their running by combining one or more regrooving steps with particularly low tire tread heights when new.

[0026] There is currently a demand for automation of these regrooving steps, particularly in the case of circumferential cutting. As explained above, regroovable tires are supplied with regrooving patterns. The automation of these regrooving steps requires a prior step of programming the regrooving machine to adapt it to the tire to be regrooved. This means that if tires with different tread patterns need to be regrooved, the machine requires different programming. For each different regrooving, the programming is detrimental to the productivity of the machine relative to the actual regrooving.

[0027] The inventors set themselves the task of improving the productivity of automatic regrooving of tires comprising regroovable circumferential cutouts.

[0028] This aim has been achieved according to the invention by a tire, regroovable at least once, comprising a crown reinforcement, itself radially capped with a tread, consisting of at least a first elastomeric mixture forming at least less the tread surface, joined to two beads by means of two sidewalls, said tread comprising at least one regroovable circumferential cutout forming at least one tread pattern element constituting the tread pattern of the tire, having a circumferential cutout height when the tire is new and consisting of material to be regrooved in at least one regrooving step, said material to be regrooved consisting of at least a first layer of a second elastomeric mixture, said second elastomeric mixture having a volume resistivity p different from that of said first elastomeric mixture.

[0029] For the purposes of the invention, the height of a circumferential cutout is measured in a meridian section of the tire and corresponds to the distance measured between the radially outer surface of the tread, forming the contact surface with the ground and extrapolated to disregard the cutouts, and the surface of the bottom of the circumferential cutout, said distance being measured in a direction normal to the radially outer surface of the tread. The bottoms of circumferential cutouts are the radially innermost points of said circumferential cutouts, disregarding the presence of elements such as wear indicators, regrooving depth indicator wells or any other element whose sum of the surfaces is less than 15% of the total surface of the circumferential cutout.

[0030] For the purposes of the invention, the height of material to be regrooved is measured between the bottom of said at least one circumferential cutout in the new state and the bottom of said circumferential cutout after the last regrooving. It can be determined from the regrooving plans associated with the tire.

[0031] The volume electrical resistivity p is measured statically according to the ASTM D 257 standard, p being expressed in ohm.cm.

[0032] The invention advantageously provides that several circumferential cutouts of the tire are regrooved according to the invention during the same step.

[0033] The production of semi-finished products intended to form the tread part of the tire made up of several elastomeric mixtures can be carried out according to usual methods known to those skilled in the art and in particular by coextrusion of the mixtures during the preparation of the semi-finished product(s) intended to constitute at least part of the tread.

[0034] Regrooving operations on an automatic machine could be carried out on tires conforming to the invention without requiring specific programming of the automatic machine. For this purpose, the machine was provided with a device for measuring electrical conductivity. Such a device is, for example, mounted on the regrooving head to come into contact with the tire before the regrooving blade. One way to proceed consists for example of starting the regrooving head from the edge of the tread and making it follow a helical movement on the surface of the tread while carrying out a continuous measurement of the electrical conductivity. When a variation in electrical conductivity is detected, the helical movement is stopped to become a circular movement and the regrooving is started while continuing the conductivity measurement until a change in conductivity is measured again.

[0035] According to a preferred embodiment of the invention, the first elastomeric mixture and the second elastomeric mixture have a difference in electrical resistivity p expressed in the form of log(p) greater than 3.

[0036] More preferably, one of the elastomeric blends is conductive and the other is non-conductive.

[0037] For the purposes of the invention, an elastomeric mixture is non-conductive when log(p) is greater than 8, the measurement being carried out on a new tire.

[0038] According to an advantageous embodiment of the invention, the first elastomeric mixture is an electrically conductive mixture and the second elastomeric mixture is an electrically non-conductive mixture.

[0039] According to this advantageous embodiment of the invention, the second elastomeric mixture has a volume resistivity p such that log(p) is greater than 8.

[0040] According to an alternative embodiment of the invention, at least one cutout being provided with at least two regrooving steps, said material to be regrooved of said at least one cutout provided with at least two regrooving steps is made up of at least two layers of radially superimposed elastomeric mixtures.Advantageously, said material to be regrooved from said at least one cutout is made up of at least three radially superimposed layers of elastomeric mixtures, a second layer of a third elastomeric mixture radially inside said first layer of elastomeric mixture and in contact with the latter, said third elastomeric mixture having a volume resistivity p different from that of said second elastomeric mixture, and a third layer of a fourth elastomeric mixture radially inside said second layer of elastomeric mixture and in contact with the latter, said fourth elastomeric mixture having a volume resistivity p different from that of said third elastomeric mixture and said fourth elastomeric mixture having a volume resistivity p different from that of said first elastomeric mixture.

[0041] Preferably according to this variant embodiment of the invention, the third elastomeric mixture and the second elastomeric mixture have a difference in electrical resistivity p expressed in the form of log(p) greater than 3.

[0042] Preferably also according to this variant embodiment of the invention, the fourth elastomeric mixture and the third elastomeric mixture have a difference in electrical resistivity p expressed in the form of log(p) greater than 3.

[0043] More preferably, according to this variant embodiment of the invention, the third elastomeric mixture has an electrical resistivity identical to that of the first elastomeric mixture and more preferably, the third elastomeric mixture and the first elastomeric mixture are identical.

[0044] According to this variant embodiment of the invention, the fourth elastomeric mixture has a volume resistivity p such that log(p) is greater than 8.

[0045] According to a preferred embodiment of this variant embodiment of the invention, the fourth elastomeric mixture has an electrical resistivity identical to that of the second elastomeric mixture and more preferably the fourth elastomeric mixture and the second elastomeric mixture are identical.

[0046] Preferably also according to this variant embodiment of the invention, the fourth elastomeric mixture and the first elastomeric mixture have a difference in electrical resistivity p expressed in the form of log(p) greater than 3.

[0047] Advantageously according to the invention, the second layer radially interposed between the first layer and the third layer has a thickness less than or equal to 1 mm.

[0048] For the purposes of the invention, the thickness of a layer of elastomeric mixture is measured radially between its radially outer surface and its radially inner surface.

[0049] Such a thickness of the second layer consists of using this second layer only to stop the regrooving phases.

[0050] Advantageously, the width of the second layer has an axial width less than that of the circumferential cutout. Preferably, the distance between a wall of the circumferential cutout and one end of the second layer is greater than the width of the electrical conductivity measuring device associated with the regrooving head.

[0051] Successive regroovings of a cutout intended to be regrooved at least twice were carried out on an automatic machine on tires in accordance with the invention without the need for specific programming of the automatic machine as in the case of a single regrooving. As previously, continuous measurement on the surface of the tread of the electrical conductivity makes it possible to successively trigger the different regrooving stages. When a variation in electrical conductivity is detected, regrooving is initiated while continuing the conductivity measurement until a variation in conductivity is measured again. Indeed, during the second regrooving phase, when the second layer is visible, the regrooving phase starts when a variation in conductivity is detected at the level of the third layer, the latter being apparent between the wall of a circumferential cutout and the end of the second layer. The thickness of the second layer is advantageously designed to be eliminated during this regrooving, the end of the regrooving being triggered after elimination of the third layer and detection of a new variation in conductivity upon contact again with the first elastomeric mixture appearing radially under the third layer.

[0052] The invention as just described is particularly well suited to the regrooving of tires such as those described in document WO 2022 / 180325, after regrooving, the height of said at least one regrooved circumferential cutout being greater than or equal to 70% of the height of said at least one regroovable circumferential cutout of the tire in the new state.

[0053] Advantageously, in the case of the tires described in document WO 2022 / 180325, the height corresponding to the distance between the tread surface in the new state and the bottom of the circumferential cutouts after the last regrooving of such tires is greater than or equal to 200% of the height between the bottom of the circumferential cutout and the tread surface when the tire is new.

[0054] Advantageously, the height corresponding to the distance between the tread surface in the new condition and the bottom of the circumferential groove after the last regrooving is greater than or equal to 250% of the height between the bottom of the circumferential groove and the tread surface when the tire is new.

[0055] The invention as just described is also particularly well suited to the regrooving of tires such as those described in patent application PCT / EP2023 / 057206 which, in addition to the description of the tire according to document WO 2022 / 180325, provides for the presence of at least one incision, flush with the tread surface when the tire is new and having a height when the tire is new greater than the height of a regroovable circumferential groove of the tire in the new condition and advantageously equal to the height corresponding to the distance between the tread surface in the new condition and the bottom of said at least one circumferential groove after the last regrooving.

[0056] According to a preferred embodiment of the invention, the height of said at least one incision of the tire in the new condition is greater than 90% of the height corresponding to the distance between the rolling surface in the new condition and the bottom of said at least one groove after the last regrooving.

[0057] More preferably, the height of said at least one incision of the tire in the new condition is equal to the height corresponding to the distance between the rolling surface in the new condition and the bottom of the grooves after the last regrooving.

[0058] Such tires in accordance with the invention also have the advantage of being comparison with conventional regroovable tires, improved rolling resistance performance while maintaining similar wet grip properties and for mileages until complete tire wear that are substantially identical.

[0059] Indeed, in the case of such tires, the combination of a tread thickness substantially identical to that of a tire of more usual design with reduced groove heights when new compared to those of tires of more usual design makes it possible to significantly increase the performance in terms of rolling resistance.

[0060] Indeed, for tread thicknesses similar to those of tires of more usual design, the height of the grooves after regrooving greater than 70% of the height of the grooves in the new condition means that the height of the grooves in the new condition is reduced in comparison with that of tires of more usual design.

[0061] According to one or other of the embodiments of the invention presented previously, the part of the tread constituting the zones capable of forming the contact surface of the tire with the ground may be made up of at least two layers of radially superimposed elastomeric mixtures, at least a fifth elastomeric mixture forming the rolling surface of the tire after complete wear of the first elastomeric mixture. The differences in conductivity presented previously between the first elastomeric mixture and the second, third and fourth elastomeric mixtures must be understood as the differences with said at least fifth elastomeric mixture when the latter forms the rolling surface.

[0062] As mentioned previously, according to its embodiments of the invention, the production of the semi-finished products intended to form the tread part of the tire can be carried out according to usual methods known to those skilled in the art and in particular by co-extrusion of the mixtures during the preparation of the semi-finished product(s) intended to constitute at least part of the tread.

[0063] 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.

[0064] According to other embodiments of the invention, the crown reinforcement further comprises at least one layer of circumferential reinforcing elements.

[0065] 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. ential with an angle 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.

[0066] According to any one of the embodiments of the invention mentioned above, the crown reinforcement can also be completed, radially inside between the carcass reinforcement and the radially inner working layer closest to said carcass reinforcement, by a triangulation layer of inextensible metallic reinforcing elements made of steel 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.

[0067] Other advantageous details and characteristics of the invention will emerge below from the description of exemplary embodiments of the invention with reference to Figures 1 to 3 which represent: - [Fig.l], a meridian view of a diagram of a tire according to a first exemplary embodiment of the invention, - [Fig.2], a meridian view of a diagram of a tire according to a second exemplary embodiment of the invention, - [Fig.3a] to [Fig.3e], a schematic representation of the steps of regrooving a tread of a tire according to a third exemplary embodiment of the invention.

[0068] The figures are not shown to scale to simplify understanding. Figures 1 and 2 only represent a half-view of a tire which extends symmetrically with respect to the axis XX' which represents the circumferential median plane, or equatorial plane, of a tire.

[0069] In Figures 1 and 2, the tire 1, of dimension 315 / 70R22.5, comprises a radial carcass reinforcement 2 anchored in two beads, around bead wires, 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 three grooves 3 forming four ribs 4.

[0070] The lower zones and beads of the tire 1 are not shown in the figures.

[0071] In Figures 1 and 2, the crown reinforcement 5 is formed radially from the inside to the outside: - a first working layer 51 formed of inextensible metal cables, continuous over the entire width of the sheet, oriented at an angle al, - a layer of circumferential reinforcing elements 53 formed of elastic steel wire cables 21.23, with a pitch of 2 mm, and - a second working layer 52 formed of inextensible metal cables, continuous across the entire width of the sheet, oriented at an angle a2 and crossed to the metal cables of the first working layer.

[0072] The axial width L5i of the first working layer 51 is equal to 246 mm.

[0073] The axial width L52 of the second working layer 52 is equal to 228 mm.

[0074] As for the axial width L53 of the layer of circumferential reinforcing elements 53, it is equal to 200 mm.

[0075] In [Fig.l], in accordance with the invention, the grooves 3 are of the regroovable type. As illustrated in [Fig.l], the grooves 3 consist of a single layer A forming the bottom of the grooves in the new state and corresponding to a single regrooving.

[0076] The height on new HN3 tire of grooves 3 is equal to 7.5 mm.

[0077] The height HR of the grooves 3 after regrooving is equal to 9.5 mm.

[0078] This height HR corresponds to a regrooving of 7.5 mm when there remains a height of the initial groove of 2 mm, a value close to the generally authorized legal limit corresponding to the minimum height and corresponding to the wear indicators. This limit is symbolized by line 7 on [Fig.l].

[0079] The height which cannot be measured before this regrooving and which corresponds to the distance between the rolling surface in the new state and the bottom of a groove 3 after regrooving is thus equal to 15 mm.

[0080] [Fig. 2] illustrates a tire whose grooves 23 can be regrooved twice. The grooves 23 are made up, when new, of three layers B, C and D forming the bottom of the grooves when new and corresponding to these two regrooves. Layer D is provided with a width less than that of the groove and a thickness of approximately 1 mm.

[0081] The height on new tire HN23 of the grooves 23 is equal to 6 mm.

[0082] The height HRides grooves 23 after the first regrooving and removal of layer B is equal to 7 mm. Layer D and partially layer C are then visible.

[0083] This height HRi corresponds to a regrooving of 5 mm when there remains a height of the initial groove of 2 mm, a value close to the generally authorized legal limit corresponding to the minimum height and corresponding to the wear indicators. This limit is symbolized by line 71 on [Fig.2].

[0084] The height which cannot be measured before a first regrooving and which corresponds to the distance between the rolling surface in new condition and the bottom of the groove after the first regrooving is thus equal to 11 mm.

[0085] The height HR2 of the grooves 23 after the second regrooving and removal of layers D and C is equal to 6 mm.

[0086] This height HR2 corresponds to a regrooving of 4 mm when there remains a groove height of 2 mm, a value close to the generally authorized legal limit corresponding to the minimum height. This limit is symbolized by line 72 on the [Fig.2].

[0087] The height which cannot be measured before the two regrooves and which corresponds to the distance between the rolling surface in the new state and the bottom of the grooves 23 after the second regroove is thus equal to 15 mm.

[0088] The tire thus presented in [Fig.2] provides two regrooves at different stages of tire wear. Advantageously according to the invention the grooves are regrooved simultaneously at each regrooving stage.

[0089] According to other alternative embodiments of the invention, the regrooving of the different grooves of a tire can be arranged to be done in a time-shifted manner. According to these alternative embodiments, the tire can comprise a portion of these grooves that can be regrooved only once and other grooves that can be regrooved several times. A tire could thus comprise a combination of grooves such as those illustrated in [Fig. 1] and grooves such as those illustrated in [Fig. 2].

[0090] In the case of [Fig.3a], which illustrates the tread 36 of a tire in new condition, the central groove 331 can be regrooved once and the grooves 332 can be regrooved twice.

[0091] The four ribs 34 forming the part of the tread intended to come into contact with the ground are made of the first elastomeric mixture.

[0092] The layer A' forming the bottom of the regroovable circumferential groove 331, identical to that described in the case of [Fig.l], is made up of the second elastomeric mixture.

[0093] Layers B' and C' forming the largest regroovable volumes of the circumferential grooves 332, similar to those described in [Fig.2], are constituted respectively by the second and fourth mixtures chosen in this embodiment which are identical and identical to the elastomeric mixture constituting layer A'.

[0094] According to one embodiment of the invention, the layers B' and C' are radially separated by a layer D' consisting of a third elastomeric mixture and having a thickness of the order of a millimeter and a width less than that of the circumferential grooves 332. The third elastomeric mixture is chosen to be identical to the first elastomeric mixture forming the ribs 34.

[0095] Tires were produced in accordance with [Fig.3a] on the basis of the elastomeric mixtures described below. First Mixture Second Mixture NR 80 100 BR 20 N234 48 NonN550 17.5 Stearic acid 1 6PPD- 1.3DIMETHYL 8UTYL PHENYL PARAPHEITr'LENE-DfAMINE 2.5 ZNO 3 5 Sulfur 1.5 4 Accelerator (CBS).... 0.9 1.5 IggCpj <4.6 >9

[0096] The values ​​of the constituents are expressed in pce (parts by weight per hundred parts of elastomers).

[0097] The first mixture therefore constitutes the ribs 34 and the layer D'.

[0098] The second mixture constitutes layers A', B' and C'.

[0099] [Fig.3b] illustrates the tread 36 after a first wear and before the first regrooving, the depth of the circumferential grooves 332 being approximately 2 mm.

[0100] [Fig.3c] illustrates the tread 36 after a first regrooving. This first regrooving consisted of removing the layer A' from the circumferential groove 331 and the layers B' in the grooves 332, leaving the layers D' and part of the layers C' visible in these circumferential grooves 332.

[0101] In the case of regrooving on an automatic machine, as explained previously, the regrooving head, associated with a device for continuously measuring electrical conductivity, starts from an edge of the tread 36 and follows a helical trajectory around the tire until it encounters a variation in electrical conductivity upon reaching the level of the second mixture of a layer B' of a circumferential groove 332. The regrooving head is then given a circumferential movement and activated to regroove the tire and remove the layer B' of a circumferential groove 332. The regrooving stops when a variation in electrical conductivity is again measured at the level of the layer D'. The helical movement then resumes in the same way as previously until the measurement of the electrical conductivity of the layer A' of the circumferential groove 331, consisting of the second mixture.Similarly, the regrooving head is then activated to remove layer A' from the circumferential groove 331. The regrooving stops when the conductivity of the first mixture is measured again. since this appears at the bottom of the circumferential groove 331 after the disappearance of layer A'. The last stage of this regrooving then starts according to the same principle as previously to eliminate layer B' from the second circumferential groove 332.

[0102] [Fig.3d] illustrates the tread 36 after a second wear, the depth of the circumferential grooves 332 being approximately 2 mm.

[0103] [Fig. 3e] illustrates the tread 36 after the second regrooving. This second regrooving consists of removing the layers D' and C' in the grooves 332.

[0104] On the same principle as the first regrooving, the second regrooving begins with a helical movement of the regrooving head until the electrical conductivity of the layer C' of a circumferential groove 332, consisting of the second mixture, is measured, which triggers the second regrooving of a circumferential groove 332 and the elimination of the layers C' and D' of this circumferential groove 332. The narrower width of the layer D' compared to that of the circumferential groove 332 allows the electrical conductivity of the layer C' to be measured, which triggers the regrooving. The regrooving stops when the conductivity of the first mixture is measured again since it appears at the bottom of the circumferential groove 332 after the disappearance of the layers D' and C'. The last step of this regrooving then starts according to the same principle as previously to eliminate the layers D' and C' of the second circumferential groove 332.It should be noted that when the regrooving head, associated with the electrical conductivity measuring device, passes along a helical trajectory at the central groove 331, no variation in electrical conductivity is detected since the only elastomeric mixture measured is the first mixture. No regrooving is therefore triggered when passing at the central groove 331.

Claims

Claims

1. A tire (1), regroovable at least once, comprising a crown reinforcement (5), itself radially capped with a tread (6), consisting of at least a first elastomeric mixture forming at least the tread surface, joined to two beads by means of two sidewalls, said tread (6) comprising at least one regroovable circumferential cutout (3) forming at least one tread pattern element constituting the tread pattern of the tire, having a circumferential cutout (3) height (HN3) when the tire is new and consisting of material to be regrooved in at least one regrooving step, characterized in that said material to be regrooved consists of at least a first layer (A, A', B, B') of a second elastomeric mixture and in that said second elastomeric mixture has a volume resistivity p different from that of said first elastomeric mixture.

2. A tire (1) according to claim 1, characterized in that at least one regroovable circumferential groove (3) can be regrooved at least twice.

3. Tire (1) according to one of claims 1 or 2, characterized in that the first elastomeric mixture and the second elastomeric mixture have a difference in electrical resistivity p expressed in the form of log(p) greater than 3.

4. Tire (1) according to one of the preceding claims, characterized in that the first elastomeric mixture is an electrically conductive mixture and the second elastomeric mixture is an electrically non-conductive mixture.

5. Tyre (1) according to one of the preceding claims, characterized in that, after regrooving, the height (HR) of said at least one regrooved circumferential cutout (3) is greater than or equal to 70% of the groove height (HN) of said at least one regroovable circumferential cutout (3) of the tyre in the new condition.

6. Tyre (1) according to one of the preceding claims, characterized in that the height corresponding to the distance between the running surface in new condition and the bottom of the circumferential cutouts (3) after the last regrooving of such tyres is greater than or equal to 200% of the height (HN) between the bottom of the circumferential cutout (3) and the running surface when the tyre is new.

7. Tire (1) according to one of the preceding claims, said tread (6) comprising at least one incision, flush with the tread surface when the tire is new, having a height between the bottom of said at least one incision and the tread surface when the tire is new, characterized in that the height of said at least one incision of the tire in the new state is greater than the height of said at least one regroovable circumferential cutout (3) of the tire in the new state.

8. Tyre (1) according to claim 7, characterized in that the height of said at least one incision of the tyre in the new condition is greater than 90% of the height corresponding to the distance between the running surface in the new condition and the bottom of said at least one circumferential cutout (3) after the last regrooving.

9. Tyre (1) according to claim 7 or 8, characterized in that the height of said at least one incision of the tyre in the new condition is equal to the height corresponding to the distance between the running surface in the new condition and the bottom of said at least one circumferential cutout (3) after the last regrooving.

10. Tire (1) according to one of the preceding claims, characterized in that said tread comprises, at least locally, at least two layers of elastomeric mixtures radially superimposed in the tread.

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