TIRE WITH A RETRACTABLE TREAD

Abstract

The invention relates to a tire (1) that can be regrooved at least once and whose tread, forming a tread surface (7) centered on the median plane, has a width less than 80% of the tread width when new. According to the invention, the height (HRC) of at least one regrooved groove (3A) of said portion of the tread (6A) forming the tread surface (7) when new is greater than or equal to 70% of the groove height (HNC) of said at least one regroovable groove (3A) of said portion of the tread (6A) forming the tread surface (7) when new. Figure for the abstract: Fig 1

Description

Title of the invention: TIRE WITH A REGRESSABLE TREAD

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

[0002] Generally, 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 superimposed layers formed of parallel wires or cables in each layer and crossed from one layer to the next at angles between 10° and 45° with the circumferential direction. These working layers, forming the working reinforcement, may be further covered by at least one protective layer formed of advantageously metallic and extensible reinforcing elements, known as elastic elements.It may also include a layer of low-extensibility wires or cables forming an angle of between 45° and 90° with the circumferential direction. This layer, known as the triangulation layer, is radially positioned between the carcass reinforcement and the first crown layer, known as the working layer, which is formed of parallel wires or cables having angles of no more than 45° in absolute value. The triangulation layer, together with at least the aforementioned working layer, forms a triangulated reinforcement which exhibits little deformation under the various stresses it is subjected to. The essential role of the triangulation layer is to resist the transverse compression forces exerted on all the reinforcing elements in the crown area 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 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 of at least 3% with a maximum tangent modulus of 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 which 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 along 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 top reinforcement, there 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, the part of the tire intended to come into contact with the ground during rolling and to wear down during rolling, with a tread pattern formed of raised elements delimited by cutouts such as grooves, whether circumferential, transverse, or oblique. The objective of such a tread pattern is to give the tread good rolling performance on dry pavement and on water-covered pavement, particularly in wet weather.

[0013] To improve tread performance without excessively reducing the shear stiffness of said treads, it is known to form a plurality of transversely or obliquely oriented edges on the tread surface in order to cut the water film on a road surface and ensure good contact between the tread and the road. One means of obtaining such edges consists of providing the tread with a plurality of cutouts, these cutouts having the form of grooves or incisions. In the present application, incisions are distinguished from grooves in that the incisions have a width suitable for allowing at least partial contact between the opposing walls delimiting these incisions during rolling, particularly during contact with the ground, which would not be the case for grooves under normal tire operating conditions.

[0014] For the purposes of the invention, a longitudinally oriented cutout is a cutout in which the mean plane of at least a portion of the walls of said cutout forms an angle of less than 10° with a longitudinal plane. This angle formed with a longitudinal plane may be oriented in either direction with respect to said longitudinal plane. A longitudinally oriented cutout may also be a a cutout whose walls undulate or zigzag around an average plane such as has just been described.

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

[0016] For the purposes of this invention, an obliquely oriented cutout is a cutout in which the mean plane of at least a portion 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 either direction with respect to said radial plane. An obliquely oriented cutout may also be a cutout in which the walls undulate or zigzag around a mean plane such as has just been described.

[0017] Combined with this need to improve grip performance through the presence of edges formed by the transverse cuts, it is also required that the performance of a tread be consistent, that is to say, that satisfactory performance be achieved even after partial wear, to a greater or lesser degree. Partial wear of a tread is understood to mean a state of wear corresponding to a tread thickness no greater than the total tread thickness that can be worn before the tire needs to be changed, particularly for regulatory reasons.

[0018] The grooves generally include wear indicators, small platforms of vulcanized rubber compound covering a certain circumferential length of the bottom of these grooves. This indicator shows the minimum tread depth that must legally remain on the tread during use. Some tread patterns for heavy-duty vehicles are regroovable (an operation by which new grooves can be cut), and tires with such tread patterns bear the English word "Regroovable" or the symbol "U" on their sidewalls. Regrooving allows, on the one hand, for an extension of the grip potential of the heavy-duty tire and, on the other hand, for a significant increase in mileage.

[0019] Regrooving heavy goods vehicle tires is a common and authorized operation due to its safety benefits and increased efficiency. For example, it is explicitly authorized by the French Highway Code (Article 4 of the decree of October 24, 1994) and recommended by the ETRTO and AFNOR (standard NFR12714). For this operation, manufacturers are required to provide regrooving diagrams that are mandatory for the technicians responsible for 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 is known per se, recutting a groove can be carried out using a heated, rounded blade, often still operated manually. This blade, attached to a frame that rests on the tread surface, can be used manually to follow the groove's path quite closely on the tread surface, even in the case of a groove with a non-straight path.

[0022] Regrooving restores sharp edges and is usually intended to restore a tread depth corresponding to that of a half-worn heavy-duty tire. Standard recommendations aim for a post-regrooving depth of 5 to 6 mm, consisting of 3 to 4 mm of regrooving and 2 mm of remaining depth; in practice, regrooving is often carried out earlier, with remaining depths of between 4 and 5 mm, resulting in a post-regrooving depth of 7 to 9 mm, for tire tread depths when new of between 12 and 20 mm.

[0023] Regrooving a tire has several advantages. First, by restoring the tread depth to the tire, regrooving extends the life of the tire.

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

[0025] Document WO 2022 / 180325 further proposes tires which can be regrooved at least once, whose tread depth in new condition is very low to allow for rolling that optimizes performance in terms of rolling resistance of the tire throughout its use.

[0026] It is also known to produce tires whose tread surface has a width less than the width of the tread to improve wear performance without significantly penalizing the tire's endurance performance.

[0027] Document FR 2278508Al describes such a solution for improving wear due to a particular tread profile while preserving endurance performance.

[0028] Document JP2010208505A further describes such an arrangement to limit irregular wear and improve endurance performance by moving the rolling surface away from the ends of the top reinforcement.

[0029] Tests carried out with such tires under normal usage conditions do not reveal any significant performance in terms of wear and the Endurance performance is not at the expected level with treads made in this way.

[0030] In addition, this type of tire design does not meet current expectations in terms of rolling resistance.

[0031] The inventors have set themselves the task of being able to provide tires with a compromise between performance in terms of wear and performance in terms of improved endurance, ensuring satisfactory performance in terms of rolling resistance and good grip properties on wet ground.

[0032] This objective has been achieved according to the invention by a tire, which can be regrooved at least once, comprising a crown reinforcement, itself radially capped by a tread, made of at least one elastomeric compound, joined to two beads by means of two sidewalls, said tread forming a tread surface, centered on the median plane, the width of which is less than 80% of the width of the tread when new, the part of the tread forming the tread surface when new comprising at least one regroovable groove having a height HN c between the bottom of said at least one regroovable groove and the tread surface when the tire is new, two axially external parts of the tread not forming the tread surface when new,each comprising at least one regroovable groove having a height HN B between the bottom of said at least one groove and the surface of the axially outer part of the tread when the tire is new, after regrooving, the height HR c of said at least one regrooved groove of said part of the tread forming the tread surface when new being greater than or equal to 70% of the height HN c of said at least one regroovable groove of said part of the tread forming the tread surface when new.

[0033] In the context of the invention, the groove height is measured in a meridional cross-section of the tire and corresponds to the distance measured between the radially outer surface of the tread, forming the contact area with the ground and extrapolated to disregard the cutouts, and the surface of the tread depths, said distance being measured in a direction normal to the radially outer surface of the tread. The tread depths are the radially innermost points of the grooved areas on the tread, disregarding the presence of elements such as wear indicators, regrooving depth indicator wells, or any other element whose sum of areas is less than 15% of the total area of ​​the cutout.

[0034] The groove height of said at least one regroovable groove of the tire in its new condition is thus measured on a new tire. The height of said at least one regrooved groove can similarly be measured when the tire is regrooved; the height of said at least one regrooved groove can also be determined as explained above from the new tire and the regrooving diagram provided by the manufacturer.

[0035] During testing, the inventors demonstrated that increasing the tread thickness, limited to a portion of the tread centered on the median plane, results in wear reductions substantially proportional to the amount of material added, but with endurance performance that falls short of expectations. The inventors believe this result is due to the increased groove depth in the area of ​​the tread that forms the contact patch when new. While the absence of ground contact at the shoulders effectively limits heating at the ends of the top reinforcement, greater groove depths in the central part of the tread contribute to significant overall tread heating.The axially outer parts of the tread thus reach temperatures which lead to performance below expectations due to the outer geometry of the tread.

[0036] The tests have also shown that the tires according to the invention, in combination with improved wear performance, maintain satisfactory endurance performance and satisfactory rolling resistance performance.

[0037] Furthermore, the tires according to the invention allow for the maintenance of satisfactory wet grip properties throughout the life of the tire.

[0038] The inventors have thus been able to demonstrate that the combination of an increased tread thickness in its central part with reduced groove heights when new compared to those of more conventional tire designs allows for maintaining satisfactory rolling resistance and endurance performance, with improved wear performance.

[0039] The height of the grooves after regrooving exceeding 70% of the height of the grooves in the new condition means that the height of the grooves in the new condition is reduced compared to that of tires of more conventional design.

[0040] As stated previously, the regrooving of more conventionally designed tires results in groove heights less than half the groove height when new. According to the invention, for a given tread thickness, the tread regrooving must therefore be probably done faster, i.e. with less tread wear than in the case of a more conventionally designed tire.

[0041] The inventors have thus been able to demonstrate that this accelerated regrooving of the tire tread according to the invention makes it possible to maintain satisfactory rolling resistance and endurance performance, despite an increased tread thickness in its central part for improved wear performance.

[0042] According to a preferred embodiment of the invention, the height corresponding to the distance between the tread surface in the new condition and the bottom of said at least one groove, of said part of the tread forming the tread surface in the new condition, after the last regrooving is greater than or equal to 200% of the height HN c between the bottom of said at least one groove of said part of the tread forming the tread surface in the new condition and the tread surface when the tire is new.Preferably, the height corresponding to the distance between the tread surface in its new condition and the bottom of said at least one groove, of said part of the tread forming the tread surface in its new condition, after the last regrooving is greater than or equal to 250% of the height HN c between the bottom of said at least one groove, of said part of the tread forming the tread surface in its new condition, and the tread surface when the tire is new.

[0043] The height corresponding to the distance between the tread surface in the new condition and the bottom of the grooves after the last regrooving can be determined when the tire is regrooved or can also be determined as explained previously from the new tire and the regrooving diagram provided by the manufacturer.

[0044] According to particularly advantageous embodiments of the invention, the width of the tread surface, centered on the median plane, when new, is greater than 50% of the tread width and preferably between 60% and 75% of the tread width. Such widths of the central part of the tread significantly improve wear performance; values ​​below 50% would be less beneficial. Above 75%, rolling resistance and durability performance could be degraded.

[0045] According to preferred embodiments of the invention, the height HN c, between the bottom of said at least one groove, of said part of the tread forming the tread surface in the new state, and the tread surface when the tire is new, is between 6 and 8 mm.

[0046] Such groove depths present in the central part of the tread ensure satisfactory wet grip properties when new.

[0047] Preferably also according to the invention, the height HF c between the bottom of said at least one groove, of said part of the tread forming the tread surface in the new state, and the extrapolation of the surface of the axially external parts of the tread is less than or equal to 2 mm and preferably greater than 1 mm.

[0048] For the purposes of the invention, the extrapolation of the surface of the axially outer parts of the tread is a surface, substantially parallel to the tread surface, in the new state, which connects the radially outer surfaces of said two axially outer parts of the tread.

[0049] Preferably also according to the invention, the height HN B between the bottom of said at least one groove of each of said two axially external parts of the tread, and the surface of said at least two axially external parts of the tread when the tire is new is equal to the height HF c between the bottom of said at least one groove, of said part of the tread forming the tread surface in the new state, and the extrapolation of the surface of the axially external parts of the tread.

[0050] According to this preferred embodiment of the invention, during regrooving, all the grooves have a substantially identical depth, which can be chosen as corresponding to the wear limit to allow rolling. After this regrooving step, or just before, the width of the tread surface is substantially equal to the width of the tread band, the wear of the tire causing the extrapolated surface of the axially outer surfaces to coincide with the tread surface of the tire in its central part.

[0051] According to a first embodiment of the invention, said at least one recuttable groove of said part of the tread forming the tread surface in the new condition and said at least one recuttable groove of each of said two parts of the tread, axially external, not forming the tread surface in the new condition are circumferential.

[0052] According to a second embodiment of the invention, said at least one recuttable groove of said part of the tread forming the tread surface in the new condition and said at least one recuttable groove of each of said two parts of the tread, axially external, not forming the tread surface in the new condition are transverse.

[0053] According to a third embodiment of the invention, said at least one recuttable groove in said part of the tread forming the tread surface in the new condition and said at least one recuttable groove of each of said two parts of the tread, axially external, not forming the tread surface in the new condition are oblique.

[0054] According to other embodiments of the invention, the tire has a combination of circumferential and / or transverse and / or oblique regroovable grooves.

[0055] Whether they are in the same orientation or in several orientations, the invention advantageously provides that all the grooves of the tire are recut according to the invention during the same step.

[0056] According to an advantageous embodiment of the invention, after regrooving, the HR c height of said at least one regrooved groove of said part of the tread forming the tread surface in the new condition is greater than or equal to 85% of the HN c height of said at least one regroovable groove of said part of the tread forming the tread surface in the new condition and preferably even greater than 95% of the HNC height of said at least one regroovable groove of said part of the tread forming the tread surface in the new condition.

[0057] According to one embodiment of the invention, at least one groove can be recut at least twice. According to this embodiment, it may be possible to further increase rolling resistance performance by providing even smaller groove depths when new, while still meeting wet grip performance requirements.

[0058] Advantageously according to the invention, at least one regroovable groove of said part of the tread forming the tread surface in the new condition is regroovable at least twice and, preferably, said at least one groove of each of said two axially external parts of the tread not forming the tread surface in the new condition are regroovable at least twice.

[0059] According to this embodiment and in the case of several grooves provided according to the same orientation or according to several orientations, the invention advantageously provides that all the grooves of the tire are re-grooved simultaneously during each re-grooving.

[0060] Advantageously, according to this embodiment of the invention, the elastomeric compound recut during the first recut is different from the elastomeric compound recut during the second recut. For example, the innermost radially located elastomeric compound does not contain a black filler to provide color contrast when it appears during the first recut. Such an arrangement can, in particular, facilitate the recutting steps.

[0061] According to other embodiments of the invention, the tread comprises depth indicators in the form, for example, of wells or of incisions of small non-zero width placed at the bottom of the groove either parallel to the direction of said groove, or perpendicular to said direction, or both simultaneously, the means indicating the minimum and maximum depths then being the geometric shape of the bottom of the depth indicator incision.

[0062] Advantageously, according to the invention, the recuttable elastomeric compound differs from at least a portion of the elastomeric compound constituting the tread. Such an embodiment can be achieved by co-extrusion of the compounds during the preparation of the semi-finished product(s) intended to constitute at least part of the tread.

[0063] Either of the embodiments of the invention presented above can also be associated with the production of a complex tread, for example made up of at least two layers of radially superimposed elastomeric mixtures.

[0064] According to one embodiment of the invention, the crown reinforcement of the tire is formed of at least two working crown layers of inextensible reinforcing elements, crossed from one layer to the other making angles with the circumferential direction between 10° and 45°.

[0065] According to other embodiments of the invention, the top reinforcement also comprises at least one layer of circumferential reinforcing elements.

[0066] One embodiment of the invention further provides that the top reinforcement is completed radially on the outside by at least one additional layer, called a protective layer, of reinforcing elements called elastic, oriented with respect to the circumferential direction 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.

[0067] According to any one of the embodiments of the invention mentioned above, the top reinforcement can be further 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 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.

[0068] Other advantageous details and features of the invention will become apparent from the description of examples of embodiments of the invention with reference to [Fig. 1], which represents a meridian view of a diagram of a tire according to an example of an embodiment of the invention.

[0069] [Fig. 1] is not shown to scale for ease of understanding. [Fig. 1] represents only 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.

[0070] In [Fig. 1], the tire 1, size 315 / 70R22.5, comprises a radial carcass reinforcement 2 anchored in two beads around wires, not shown. The carcass reinforcement 2 is formed of a single layer of wire cords. The carcass reinforcement 2 is held in place by a crown reinforcement 5, itself capped by a tread 6. The tread has four circumferential grooves 3A, 3B forming five ribs 4. The tread has a central portion 6A and two axially outer portions 6B. When new, the central portion 6A of the tread forms the tread surface 7 of the tire. The central portion 6A has two circumferential grooves 3A and each of the axially outer portions 6B of the tread has one groove 3B.

[0071] The lower areas and ridges of the tire 1 are in particular not shown in [Fig.1].

[0072] The axial width of the tread L6 is equal to 270 mm.

[0073] The axial width of the rolling surface of the L6A tire is equal to 194 mm and therefore represents 72% of the axial width L6 of the tread.

[0074] Each of the axially external parts has an axial width L6B equal to 38 mm.

[0075] Figure 1 illustrates a tire whose grooves 3A and 3B can be regrooved twice. The grooves 3A and 3B are made up in their new state of two layers I and J forming the bottom of the grooves in their new state and corresponding to these two regroovings.

[0076] The height on new HN tires of grooves 3 A is equal to 8 mm.

[0077] The HFC height between the bottom of the grooves 3A and the line 71, represented in dashed lines on the [Fig.1], corresponding to the extrapolation of the surface of the axially external parts of the tread is equal to 2 mm.

[0078] The HR height of the grooves 3A after the first re-grooving is equal to 8 mm and therefore represents 100% of HN c.

[0079] This HR height corresponds to a 6 mm regrooving when the remaining height of the initial groove is 2 mm, a value close to the generally permitted legal limit corresponding to the minimum height and the wear indicators. This limit is symbolized by line 71, shown as dashed lines in [Fig. 1], chosen to coincide with the extrapolation of the surface area of ​​the axially outer parts of the tread according to our embodiment.

[0080] The height not measurable before a first re-grooving and corresponding to the distance between the bearing surface in the new state and the bottom of the groove after the first re-grooving is thus equal to 14 mm.

[0081] The HRCi height, measured after the first regrooving, and the height not measurable before a first regrooving and corresponding to the distance between the tread surface in the new condition and the bottom of the groove 3A after the first regrooving can also be determined on new tires from the regrooving diagram provided by the manufacturer, as explained previously.

[0082] The height on new HNB tire of grooves 3B is equal to 2 mm.

[0083] The height HRBides grooves 3B after the first re-grooving is equal to 8 mm.

[0084] This HRBi height corresponds to a re-excavation of 6 mm.

[0085] The HR c 2 height of the grooves 3A after the second recutting is equal to 8 mm and therefore represents 100% of HN c. And the HR c 2 height of the grooves 3A after the second recutting represents 100% of HR c i-

[0086] This height HR c 2 corresponds to a recut of 6 mm when there remains a groove height of 2 mm, a value close to the generally permitted legal limit corresponding to the minimum height. This limit is symbolized by line 72, shown in dashed lines, in [Fig. 1].

[0087] The unmeasurable height before the two regrooving operations, corresponding to the distance between the new running surface and the bottom of the grooves 3A after the second regrooving operation, is thus equal to 20 mm. The ratio of this unmeasurable height of 20 mm to the height HN c is equal to 2.5 and therefore well greater than or equal to 200%.

[0088] The HR c 2- height measured after the second regrooving, and the height not measurable before the two regroovings and corresponding to the distance between the tread surface in the new condition and the bottom of the grooves after the second regrooving can also be determined on new tires from the regrooving diagram provided by the manufacturer, as explained previously.

[0089] The HRB2 height of the 3B grooves after the second re-grooving is equal to 8 mm.

[0090] This height HRB2 corresponds to a 6 mm recutting when a height remains of the 2 mm groove, a value close to the generally permitted legal limit corresponding to the minimum height.

[0091] The tire shown in [Fig. 1] incorporates two regrooving points at different stages of tire wear in grooves 3A and 3B. Advantageously, according to the invention, grooves 3B are provided with a depth of 2 mm and grooves 3A are provided to reach the 2 mm wear limit when the tread surface widens; that is, when tire wear causes the surfaces of the axially outer parts of the tread to coincide with the surface bearing. Grooves 3A and 3B can thus be recut in the same way and simultaneously at each recutting stage.

[0092] According to other embodiments of the invention, the regrooving of the different grooves of a tire can be arranged to take place at staggered intervals. According to these embodiments, the tire may, for example, have grooves 3B that can be regrooved only once and grooves 3A that can be regrooved several times.

[0093] Tires have been made on the basis of the elastomeric mixture described below as the mixture constituting the tread. Mixture NR 80 SR 20 N234 48 6PPD - 1.3DIMETHYL BUTYL PHENYL PARAPHENYLENE-DIAMSNE 3 ZNO 3 SULFUR 1.5 Accelerator (CBS) 0.9

[0094] The values ​​of the constituents are expressed in pce (parts by weight per hundred parts of elastomers).

[0095] A tire according to the invention thus made, and conforming to the representation of [Fig.1], was compared to different reference tires.

[0096] A reference tire R0 of identical dimensions to the tire according to the invention has four non-regroovable grooves with heights of 14 mm. The reference tire R0 has, in its new condition, a tread surface whose width is identical to that of the tread.

[0097] A reference tire RI, similar to the reference tire R0 and therefore with a tread width when new identical to that of the tread, has four grooves that can be regrooved twice; the initial height of the grooves on a new tire is 6 mm. After a first regrooving of 2 mm, the height of the grooves is 4 mm, and after a second regrooving of 2 mm, the height of the grooves is 4 mm. The regrooving steps are carried out when the groove depth is 2 mm after tire wear. The height not measurable on a new tire between the bottom of the grooves after the second regrooving and the rolling surface in the new condition is thus equal to 14 mm and therefore equal to the height of the grooves of the reference RO tire in the new condition.

[0098] A reference R2 tire is identical to the RO tire with non-regroovable grooves whose heights are equal to 20 mm.

[0099] A reference tire R3, similar to the reference tire R2 and therefore with a tread width when new identical to that of the tread, has four grooves that can be regrooved twice; the initial height of the grooves on a new tire is 8 mm. After a first regrooving of 6 mm, the groove height is 8 mm, and after a second regrooving of 6 mm, the groove height is 8 mm. The regrooving steps are carried out when the groove depth is 2 mm after tire wear. The unmeasurable height on a new tire between the bottom of the grooves after the second regrooving and the tread surface when new is thus 20 mm, and therefore equal to the groove height of the reference tire R2 when new.

[0100] A reference R4 tire of identical dimensions to the tire according to the invention has, like the tire according to the invention, a tread area smaller than the width of the tread. The reference R4 tire has two grooves with a height of 20 mm in the portion of the tread forming the tread area when new and one groove with a height of 14 mm in each of the axially outer portions of the tread, said axially outer portions of the tread becoming parts of the tread area after 6 mm of wear on the central portion of the tread.

[0101] For each of the reference tires and for the tire according to the invention, the wear life is estimated by considering the volume of rubber material in the tread to be worn during the tire's life, taking into account the regrooving phases for the reference tires RI and R3 and the tire according to the invention. The data are given as relative values, with a value of 100 being assigned to the R0 tire. Higher values ​​indicate a greater volume to be worn.

[0102] Rolling resistance measurements were carried out on each of the tires under identical driving conditions in accordance with Regulation No. 117 of the United Nations Economic Commission for Europe (UNECE). The measurement results are presented in the following table, with a value of 100 assigned to tire R0. A value of 90 means that the rolling resistance coefficient is reduced by 10% and corresponds to superior rolling resistance performance. The measurements were carried out on new tires.

[0103] To estimate the durability of the reference tires and the tire according to the invention, the temperature at the edge of the tread of the tires is measured by inserting a thermocouple in the middle of the line connecting the ends of the cords of the working tread layers and rolling the tires on a wheel with a circumference of 8.56 meters at 80 km / h, the temperature being taken after 120 km of rolling. The table shows the temperature difference compared to the measurement on the RO variant. The colder the temperature, the better the durability performance.

[0104] All values ​​are shown in the following table: Bearing resistance Temperature difference Volume to be worn R0 100 0 100 RI 90 0.5 100 R2 125 10 150 R3 H0 11 150 R4 H0 0 135 Invention 100 0.5 135

[0105] The values ​​clearly show that the tire according to the invention makes it possible to increase the volume of rubber material to be worn without increasing rolling resistance compared to the reference tire R0 and without reducing its endurance, the operating temperature remaining the same.

[0106] Wet grip tests were also carried out on each of the reference R0 tires and on tires according to the invention, under identical driving conditions in accordance with ISO 15222. The results obtained show that the wet grip properties are similar for the tire according to the invention compared to the reference R0 tire, regardless of the wear condition of the tire, including the regrooving steps of the tire according to the invention.

Claims

Demands

1. A tire (1), regroovable at least once, comprising a crown reinforcement (5), itself radially capped by a tread (6), made of at least one elastomeric compound, joined to two beads by means of two sidewalls, said tread forming a tread surface (7), centered on the median plane, the width of which is less than 80% of the width of the tread (6) when new, the portion of the tread (6A) forming the tread surface (7) when new having at least one regroovable groove (3A) having a height HNC between the bottom of said at least one regroovable groove (3A) and the tread surface (7) when the tire is new, two portions of the tread (6B), axially external,not forming the tread surface (7) in the new condition, each comprising at least one regroovable groove (3B) having a height HNB between the bottom of said at least one regroovable groove (3B) and the surface of the axially outer part of the tread when the tire is new, characterized in that, after regrooving, the height (HRC) of said at least one regrooved groove (3A) of said part of the tread (6A) forming the tread surface (7) in the new condition is greater than or equal to 70% of the groove height (HNC) of said at least one regroovable groove (3A) of said part of the tread (6A) forming the tread surface (7) in the new condition.

2. Tire (1) according to claim 1, characterized in that the height corresponding to the distance between the tread surface (7) in the new condition and the bottom of said at least one groove (3A) of said part of the tread (6A) forming the tread surface (7) in the new condition after the last regrooving is greater than or equal to 200% of the height (HNC) between the bottom of said at least one groove (3A) of said part of the tread (6A) forming the tread surface (7) in the new condition and the tread surface when the tire (7) is new.

3. A tire (1) according to claim 1 or 2, characterized in that the width of the tread surface (7), centered on the median plane, when new is greater than 50% of the width of the tread (6) and preferably between 60 and 75% of the tread width (6).

4. Tire (1) according to any one of claims 1 to 3, characterized in that the height (HNC), between the bottom of said at least one groove (3A), of said part of the tread (6A) forming the tread surface (7) in the new condition, and the tread surface (7) when the tire is new, is between 6 and 8 mm.

5. A tire (1) according to any one of claims 1 to 4, characterized in that the height (HFC) between the bottom of said at least one groove (3A), of said part of the tread (6A) forming the tread surface (7) in the new condition, and the extrapolation of the surface of the axially outer parts of the tread is less than or equal to 2 mm and preferably greater than 1 mm.

6. Tire (1) according to claim 5, characterized in that the height (HNB) between the bottom of said at least one groove (3B) of each of said two axially external parts of the tread (6B) and the surface of said at least two axially external parts of the tread (6B) when the tire is new is equal to the height (HFC) between the bottom of said at least one groove (3A) of said part of the tread (6A) forming the tread surface (7) in the new condition and the extrapolation of the surface of the axially external parts of the tread.

7. Tire (1) according to any one of the preceding claims, characterized in that said at least one regroovable groove (3A) of said part of the tread (6A) forming the tread surface (7) in the new condition and said at least one regroovable groove (3B) of each of said two parts of the tread (6B), axially external, not forming the tread surface in the new condition are circumferential.

8. Tire (1) according to any one of the preceding claims, characterized in that said at least one regroovable groove (3A) of said part of the tread (6A) forming the tread surface (7) in the new condition and said at least one regroovable groove (3B) of each of said two parts of the tread (6B), axially external, not forming the tread surface in the new condition are transverse.

9. Tire (1) according to any one of the preceding claims, characterized in that said at least one regroovable groove (3A) of said part of the tread (6A) forming the tread surface (7) in the new condition and said at least one regroovable groove (3B) of each of said two axially external parts of the tread not forming the tread surface in the new condition are oblique.

10. A tire (1) according to any one of the preceding claims, characterized in that at least one regroovable groove (3A) of said part of the tread (6A) forming the tread surface (7) in the new condition is regroovable at least twice and, preferably, said at least one groove (3B) of each of said two axially external parts of the tread (6B) not forming the tread surface in the new condition are regroovable at least twice.

11. Pneumatic (1) according to claim 10, characterized in that the elastomeric mixture re-grooved during the first re-grooving is different from the elastomeric mixture re-grooved during the second re-grooving.

12. Pneumatic (1) according to any one of the preceding claims, characterized in that the regroovable elastomeric compound is different from at least a part of the elastomeric compound constituting the tread.

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

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