Patent Application
20180117972
The present invention relates to tires, and more particularly to a tire, the grip performance of which is improved.
As is known, the tread of a tire, regardless of whether it is intended to be fitted on a passenger vehicle or a heavy-duty vehicle, is provided with a tread pattern comprising, notably, tread pattern elements or elementary blocks delimited by various main, longitudinal or circumferential, transverse or oblique grooves, the elementary blocks also being able to have various finer slits or sipes. The grooves form channels that are intended to evacuate water during running on wet ground and the walls of these grooves define the leading and trailing edges of the tread pattern elements, depending on the direction of the bend.
In order to improve the grip of a tire, and more particularly for grip on dry and wet ground, it is well known to reduce the stiffness or the stiffness of the constituent rubber mixture of the tread. This reduction in tread stiffness allows the latter to better match the rough surface of the running surface and thus the actual area of contact with the running surface is increased and the grip performance improved with respect to a tread of which the rubber mixture is stiffer.
However, notably in the case of transverse grip, the use of a less stiff rubber tread mixture promotes shearing of the tread pattern elements and rocking thereof, and this generates greatly raised pressures on the leading edges of the tread pattern elements, which in turn generate very significant heating.
These raised pressures and this heating can contribute towards very rapid damage to the tread of the tire and to non-optimal exploitation of the grip potential of the tread mixture.
The document EP 0 869 016 A2 discloses a tire with a tread comprising two superimposed rubber mixtures, wherein the inner mixture is less stiff than the outer mixture in order to maintain good grip of the tire after the tread has become partially worn and this inner mixture has been revealed at the surface. The documents JP201411392 A and US2015/107735 also present tires with treads comprising two different rubber mixtures.
In order to improve the grip performance of tires by stabilizing the tread pattern elements, the document EP 2 708 382 A1 proposes a tire having an axis of rotation and a median plane perpendicular to the axis of rotation, comprising two beads, two sidewalls connected to the beads, a crown connected to the ends of the two sidewalls and having a crown reinforcement, and a tread radially on the outside, the tread comprising a plurality of tread pattern elements having lateral faces and a contact face intended to come into contact with the road surface while the tire is being driven on, a plurality of circumferential grooves, each of which is delimited by lateral faces of adjacent tread pattern elements that face one another, and is delimited by a bottom, and a circumferential reinforcement made up of a rubber mixture with a stiffness greater than the stiffness of the rubber mixture of the rest of the tread.
In said tire, the circumferential reinforcement has a reinforcing element that is positioned under each circumferential groove and extends radially from the radially inner surface of the tread until it forms the entire bottom of the groove.
The reinforcement of the circumferential grooves that is thus produced makes it possible to increase the drift thrust of the tire, but the presence of a rigid mixture in the groove bottom makes it difficult to mould the wear indicators. A significant increase in the rolling resistance associated in particular with the limiting of the transverse and longitudinal flattening processes has also been observed.
The subject of the invention is a tire according to the preamble of Claim 1, characterized in that, the tire having an outer side and an inner side, the circumferential reinforcement has a reinforcing element positioned in the tread pattern elements disposed axially on the outside with respect to one of the first and second circumferential grooves of the tread from the outside to the inside and axially close to said circumferential groove, in that the reinforcing element extends radially from the radially outer surface of the crown reinforcement towards the outside of the tread with an axial width which decreases gradually and over a partial or total height of the thickness of the tread, and in that the tread pattern elements disposed axially on the inside with respect to said first circumferential groove do not have reinforcing elements disposed close to the axially inner faces of said groove.
The circumferential reinforcing element thus disposed on the trailing edge of the rib or of the most highly loaded tread pattern elements on the outer side of the tread of the tire during rapid cornering opposes, as a result of its high compressive and shear stiffness, the shearing and rocking of these tread pattern elements and thus makes it possible to maintain a large area of contact with the running surface, to limit the raised pressures on the leading edge of the rib or of the tread pattern elements and thus to limit the heating and rapid wear of the leading edge of the rib. The presence of a reinforcing element for a single groove already makes it possible to obtain a significant improvement in the transverse grip performance of vehicle tires.
The circumferential reinforcing element also has the essential feature of bearing directly on the crown reinforcement of the tire. This makes it possible to have a bearing point for stiffening the crown and the tread.
It is very advantageous that the tread pattern elements disposed axially on the inside with respect to the first circumferential groove do not have reinforcing elements disposed close to the axially inner faces of this groove. This is because the presence of such reinforcing elements on the leading edge of the second rib of the tread is liable to result in deterioration of the transverse grip properties of the tire and of the vehicle on account of the high stiffness of the material of these reinforcing elements when these reinforcing elements come into contact with the running surface.
It should also be noted that the reduction in the volume of very stiff rubber causes a substantial reduction in the rolling resistance of the tire with respect to the tires disclosed in the cited document EP 2 708 382 A1.
Preferably, the circumferential reinforcement has two reinforcing elements positioned respectively in the tread pattern elements that are externally adjacent to the first and the second circumferential groove of the tread from the outside to the inside and axially close to the first and second circumferential grooves.
This enhances the favourable effect in terms of transverse grip.
Advantageously, the tread having at least three circumferential grooves, the circumferential reinforcement also has a reinforcing element positioned in the tread pattern elements that are externally adjacent to the third circumferential groove of the tread from the outside to the inside and axially close to the third circumferential groove.
The circumferential reinforcement may also advantageously have reinforcing elements positioned in all of the tread pattern elements that are externally adjacent to a circumferential groove and axially close to this circumferential groove.
According to one advantageous embodiment, the circumferential reinforcement has a reinforcing element positioned in the tread pattern elements that are internally adjacent to the circumferential groove axially closest to the inner side of the tire.
This makes it possible to stabilize the ribs or tread pattern elements on the inner side of the tire when this inner side is loaded as a leading edge when cornering. Therefore, the same anti-rocking and anti-shearing effect associated with the high compressive stiffness of the reinforcing element is found.
According to one advantageous exemplary embodiment, the tread having at least four circumferential grooves, the circumferential reinforcement has two reinforcing elements positioned respectively in the tread pattern elements that are internally adjacent to the first and the second circumferential groove of the tread from the inside to the outside and axially close to the first and second circumferential grooves.
According to another advantageous embodiment, the circumferential reinforcing elements are disposed symmetrically with respect to the median plane of the tire.
According to one particular exemplary embodiment, the tread having a circumferential groove through which the median plane passes, two circumferential reinforcing elements are disposed axially close to and on either side of the circumferential groove through which the median plane passes.
The shape of the circumferential reinforcing element has a cross section that tapers radially towards the outside. This enhances its effectiveness as a bearing point. The walls of this circumferential reinforcing element may be concave, convex or in the form of a staircase.
Preferably, the angle of the two lateral walls of the circumferential reinforcing element(s) is between 35 and 45 degrees.
Below 35 degrees, the effectiveness of the bearing point is reduced and beyond 45 degrees, the volume of the circumferential reinforcing element becomes too large.
According to a preferred embodiment, the reinforcing elements having a base with a radial height strictly less than the distance between the bottom of a circumferential groove and the radially outer surface of the crown reinforcement and a top part, the top part extends radially towards the outside to at least half the height of the lateral faces of the adjacent circumferential grooves.
This minimum height of the top parts of the circumferential reinforcing elements is useful for obtaining a stabilizing effect throughout the life of the tire.
According to one advantageous embodiment, the top part of the reinforcing elements at least partially forms the lateral face of the adjacent circumferential groove.
According to another advantageous embodiment, the top part of the reinforcing elements is disposed at an axial distance of 1 to 8 mm and preferably 2 to 5 mm from the lateral face of the adjacent circumferential groove.
This embodiment makes it possible not to disrupt the moulding of the circumferential grooves of the tread while retaining a substantial effect of improving the transverse grip performance of the tires of a vehicle.
The base of the reinforcing elements may advantageously extend axially under at least some of the bottoms of the adjacent circumferential grooves.
This embodiment has the advantage of enhancing the effectiveness of the circumferential reinforcing element(s) while retaining the mixture of the tread for the bottoms of the grooves and thus improving the moulding of the wear indicators.
According to another exemplary embodiment, the base of the reinforcing elements extends axially under the tread pattern elements on the opposite side from the adjacent circumferential grooves.
As before, this has the advantage of stabilizing the circumferential reinforcing elements.
According to another advantageous exemplary embodiment, the bases of the reinforcing elements may be axially contiguous and extend axially over at least 50% of the axial width of the tread of the tire.
Very advantageously, the bases of the axially contiguous reinforcing elements extend axially over at most the axial width of said crown reinforcement. This makes it possible to keep good flattening of the two shoulders of the tire and to limit the consequences in terms of the rolling resistance of the tire from the use of a rubber mixture of very high stiffness.
Advantageously, the rubber mixture of which the circumferential reinforcement is made has a dynamic modulus G*, measured at 60° C. at 10 Hz and under an alternating shear stress of 0.7 MPa, of greater than 20 MPa and preferably greater than 30 MPa.
Very advantageously, the rubber mixture of the tread has a dynamic modulus G*, measured at 60° C. at 10 Hz and under an alternating shear stress of 0.7 MPa, of less than or equal to 1.3 MPa and preferably less than 1.1 MPa.
The presence of the circumferential reinforcement makes it possible to make full use of the grip capabilities of such a tread mixture of very low stiffness.
This is particularly useful in the case of a tire for a passenger vehicle.
According to another advantageous embodiment, the tread comprises two different mixtures disposed axially one on top of the other. The mixture disposed radially on the inside is usually referred to as an “underlayer”. This underlayer may have more favourable hysteresis properties than the mixture in contact with the road surface, this improving the overall rolling resistance of the tire.
Alternatively, the underlayer may also be stiffer than the rubber mixture of the tread in order to stiffen the latter.
The invention relates more particularly to tires intended to equip motor vehicles of the passenger vehicle, SUV (“Sport Utility Vehicle”), two-wheel vehicle (especially motorcycle) or aircraft type, and industrial vehicles chosen from vans, heavy-duty vehicles, that is to say, underground trains, buses, heavy road transport vehicles (lorries, tractors, trailers) or off-road vehicles, such as heavy agricultural or construction plant vehicles, and other transportation or handling vehicles.
The subjects of the invention will now be described with the aid of the appended drawing, in which:
The tire 1 has an outer side E intended to be positioned towards the outside of a vehicle and an inner side I intended to be positioned towards the inside of a vehicle. This tire thus exhibits tread asymmetry.
This tire 1 has a crown 2 reinforced by a crown reinforcement or belt 6, two sidewalls 3 and two beads 4, each of these beads 4 being reinforced with a bead wire 5. The crown reinforcement 6 is surmounted radially on the outside by a rubber tread 9. A carcass reinforcement 7 is wound around the two bead wires 5 in each bead 4, the turn-up 8 of this reinforcement 7 being, for example, disposed towards the outside of the tire 1. In a manner known per se, the carcass reinforcement 7 is made up of at least one ply reinforced by what are known as “radial” cords, for example of textile or metal, that is to say that these cords are disposed virtually parallel to one another and extend from one bead to the other so as to form an angle of between 80° and 90° with the median circumferential plane EP. An airtight layer 10 extends from one bead to the other radially on the inside with respect to the carcass reinforcement 7.
The tread 9 has four grooves 11, 12, 13 and 14 from the outer side E to the inner side I. Each groove has an outer face 11.1, 12.1, 13.1 and 14.1, a groove bottom 11.2, 12.2, 13.2 and 14.2 and an inner face 11.3, 12.3, 13.3 and 14.3.
This tread 9 also has a circumferential reinforcement 20 made up of a reinforcing element 22 disposed adjacently to the outer wall 12.1 of the second groove 12. This reinforcing element 20 bears against the radially outer wall of the crown reinforcement 6 and has a substantially triangular cross section. This reinforcing element partially forms the outer wall 12.1 of the groove 12.
The circumferential reinforcement 20 opposes the rocking and shearing of the rib externally adjacent to the groove 12 during strong transverse loads on the tire that are oriented axially from the outside to the inside, for example during cornering of the vehicle on which the tire is mounted in the direction of the inner side of the tire.
The tread 30 in
The additional circumferential reinforcing element 36 is disposed adjacently to the outer wall 11.1 of the first groove 11. Through its presence, it opposes the shearing and rocking of the tread pattern elements externally adjacent to the first groove 11 and thus cooperates with the action of the circumferential reinforcing element 34 during strong transverse loads on the tire.
The circumferential reinforcement 42 of the tread 40 in
In
In
In
The axial width of the axially contiguous bases 81 covers at least half the axial width of the tread and at most the axial width W of the crown reinforcement 6. The fact that the bases are continuous enhances the resistance to rocking of the entire crown block during transverse loads and the fact that they do not extend beyond the axial width of the crown reinforcement 6 promotes the flattening the shoulders and limits the rolling resistance of the tire.
The shape of the circumferential reinforcing elements depicted is triangular, but this shape may vary and the lateral walls may be concave, convex or in the form of a staircase, notably without departing from the scope of this invention.
In the examples depicted, the angle α made by these two lateral walls is around 40 degrees, i.e. between 35 and 45 degrees.
The radial height of the circumferential reinforcing elements may reach the contact face of the tread pattern elements when the tire is new, but may also be smaller. It should not be less than half the height of the tread pattern elements in order to be able to act throughout the life of the tire.
In this
Depending on the objective of the tire designer, the mixture of this underlayer may be of low hysteresis and thus improve the rolling resistance of the tire or be stiffer than the other mixture that forms the tread; in this case the underlayer has a stiffening action on the crown of the tire. All the particular reinforcement features cited above are compatible with the use of this underlayer. This underlayer is situated above the base of the reinforcing elements when the base exists, such that the reinforcement bears directly and primarily on the crown reinforcement. That is to say on the skim layer of the radially outermost ply of the crown architecture.
The tread 120 of
The circumferential reinforcing elements should serve as a bearing point for opposing the shearing and rocking of the tread pattern elements which contain them. For this purpose, the mixture of which these circumferential reinforcing elements are made is preferably very substantially stiffer than that of the tread. Preferably, the dynamic modulus G*, measured at 60° C. at 10 Hz and under an alternating shear stress of 0.7 MPa, is greater than 20 MPa and very preferentially greater than 30 MPa.
Such mixtures are described in particular in the Applicants' application WO 2011/045342 A1.
Table 1 below gives an example of such a formulation.
This formulation makes it possible to obtain mixtures of high stiffness, in particular by virtue of the combined action of an epoxy resin and an amine-comprising curing agent. The shear modulus G* measured under an alternating shear stress of 0.7 MPa at 10 Hz and 60 degrees Celsius is 30.3 MPa.
This very stiff material for circumferential reinforcements is preferably used in treads of low stiffness with dynamic moduli G* of less than 1.3 MPa and preferably less than or equal to 1.1 MPa.
The following Table 2 gives an example of a suitable formulation:
The dynamic modulus after vulcanization is 0.9 MPa.
A person skilled in the art, who is a tire designer, should be able to adapt the number and the position of the circumferential reinforcing elements in order to obtain optimum resistance to the rocking and shearing of the ribs and tread pattern elements, specifically for tires which are asymmetrical or not.
The rubber mixtures are characterized as follows.
The dynamic mechanical properties are well known to those skilled in the art. These properties are measured on a viscosity analyser (Metravib VA4000) with test specimens moulded from uncured mixtures or test specimens bonded together from vulcanized mixtures. The test specimens used are described in the standard ASTM D 5992-96 (the version published in September 2006 but initially approved in 1996 is used) in Figure X2.1 (circular test specimens). The diameter “d” of the test specimens is 10 mm (the circular cross section is thus 78.5 mm2), the thickness “L” of each portion of mixture is 2 mm, giving a “d/L” ratio of 5 (as opposed to the standard ISO 2856, mentioned in paragraph X2.4 of the ASTM standard, which recommends a d/L value of 2).
The response of a sample of vulcanized composition subjected to a simple alternating sinusoidal shear stress at a frequency of 10 Hz is recorded. The maximum shear stress imposed is 0.7 MPa.
The measurements are made with a temperature change of 1.5° C. per minute, from a minimum temperature lower than the glass transition temperature (Tg) of the mixture or rubber to a maximum temperature greater than 100° C. Before the test begins, the test specimen is conditioned at the minimum temperature for 20 minutes to ensure good homogeneity of temperature in the test specimen.
The result used is notably the value of the dynamic modulus G* at a temperature of 60° C.
The performance of the tires according to the subjects of the invention were measured during the following tests:
The tread 110 has four grooves 11, 12, 13 and 14. Two mixtures make up the tread, the mixture 113 radially on the outside and the underlayer 115. It also has a circumferential reinforcement 112 comprising five circumferential reinforcing elements 114, 116, 117, 118 and 119. The circumferential reinforcing elements 114, 116 and 118 are each disposed adjacently to an outer face of one of the three ribs disposed furthest towards the outside. The circumferential reinforcing elements 119 and 120 are for their part disposed adjacently to an inner face of one of the two ribs disposed furthest towards the inside. The third rib is thus reinforced by two circumferential reinforcing elements. Each circumferential reinforcing element has a substantially triangular shape and is intended to be in direct contact with the radially outer surface of the architecture of the crown of the tire of which the tread is intended to form part, and one of its lateral walls partially forms a lateral face of a rib. The underlayer is interrupted by the circumferential reinforcing elements. In the present case, the underlayer has a dynamic alternating shear modulus at 60° C. of around 7 MPa.
The tread 110 of the test tires was produced in a hand-made manner. A length profile corresponding to a multiple of the perimeter of a test tire of the two mixtures of which the tread 113 and the underlayer 115 are made was obtained by coextrusion. This profile had four grooves.
Profiles of the same length corresponding to the four circumferential reinforcing elements were also produced by extrusion.
Then, four mixture volumes, each corresponding to the volume and shape of a circumferential reinforcing element, were removed from the coextruded profile of the two mixtures of the tread with a heated chisel and the four circumferential reinforcing elements were placed manually in the four volumes thus prepared.
The treads thus assembled were then placed on the crown of a tire in a manner well known to a person skilled in the art to complete it. The complete tires were then vulcanized as usual in a curing press.
The reference tires are Michelin tires of the Pilot Sport 3 type, size 225/45 R17, pressure 2.3 bar at the front and 2.7 bar at the rear, and the test vehicle is a Renault Clio Cup.
These reference tires R1 have a tread with a mixture having a dynamic shear modulus G* at 60° C. of 1.4 MPa.
Other reference tires R2 were also produced. The tread of these tires is identical to that of
The tread mixture of the reference tires R2 has a G* value at 60° C. of 0.9 MPa.
The test tires E1 have a tread mixture with a G* value of 0.9 MPa and the circumferential reinforcing elements are produced with a mixture with a G* value of 30 MPa. These tires E1 have a circumferential reinforcement corresponding to that of
Other tires E2 according to the invention were produced with a tread and a circumferential reinforcement such as E1, but additionally an underlayer with a dynamic modulus G* equal to 5 MPa. This underlayer is interrupted by the four circumferential reinforcing elements as indicated in
The circumferential reinforcing elements have an angle of 40 degrees between their lateral walls.
The use of a tread of lower stiffness normally reduces the cornering stiffness of the tire and improves the braking performance on wet ground.
The tire tested according to the invention makes it possible to obtain a gain of 10 points in the braking performance on wet ground while having a cornering stiffness comparable to that of the control R1.
A gain is considered significant starting from 0.3 s on this circuit.
It can be seen that the use of a tread with a much less stiff mixture results in only a barely significant gain whereas the results obtained with the tires having circumferential reinforcements according to the invention are very marked.
The presence of the circumferential reinforcements in the tread thus makes it possible to make full use of the grip potential of tread mixtures of lower stiffness.
By combining the choice of mixture of the tread, the choice of mixture of the underlayer and the circumferential reinforcements, it is then possible for the tire designer to offset the compromises between grip and, respectively, behaviour and rolling resistance, this not being attainable through the choice of a single material of the tread.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1553798 | Apr 2015 | FR | national |
| 1560388 | Oct 2015 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/059429 | 4/27/2016 | WO | 00 |
