Patent Application
20180186189
The present invention relates to tires, and more particularly to a tire, the grip performance of which is improved.
In general, a tire is an object exhibiting symmetry of revolution about an axis of rotation. A tire comprises two beads intended to be mounted on a rim; it also comprises two sidewalls connected to the beads, a crown comprising a tread intended to come into to contact with the ground, the crown having a first side connected to the radially exterior end of one of the two sidewalls and having a second side connected to the radially exterior end of the other of the two sidewalls.
The makeup of the tire is usually described by a representation of its constituent components in a meridian plane, which means to say a plane containing the axis of rotation of the tire. The radial, axial and circumferential directions denote the directions perpendicular to the axis of rotation of the tire, parallel to the axis of rotation of the tire and perpendicular to any meridian plane, respectively. In what follows, the expressions “radially”, “axially” and “circumferentially” respectively mean “in a radial direction”, “in the axial direction” and “in a circumferential direction” of the tire. The expressions “radially on the inside or, respectively, radially on the outside” mean “closer to or, respectively, further away from, the axis of rotation of the tire, in a radial direction, than”. The equatorial plane is a plane perpendicular to the axis of rotation of the tire, positioned axially in such a way as to intersect the surface of the tread substantially mid-way between the beads. The expressions “axially on the inside or, respectively, axially on the outside” mean “closer to or, respectively, further away from, the median plane of the tire, in the axial direction, than”.
As is known per se, the tread of a tire for application to road vehicles, whether intended to be fitted to a passenger vehicle or to a heavy goods vehicle, is provided with a tread pattern comprising tread pattern blocks delimited by various grooves, some oriented circumferentially and others oriented axially, or still others oriented obliquely. The tread pattern blocks may also comprise various finer sipes or incisions. The grooves form channels that are intended to remove water during running on wet ground and the walls of these grooves define what are referred to as the leading and trailing edges of the tread pattern elements, on the surface of the tread, and with reference to the direction of running.
In order to improve the grip of a tire, and more particularly the grip on dry ground and on wet ground for non-winter running, it is well known practice to use, for the tread, a rubber compound of low stiffness (which means to say also of low hardness, which is where the usual designation of soft compound comes from). This reduction in tread stiffness allows the latter to better conform to the surface of the ground it is running on and in particular to conform well to the roughness of the ground, and thus the actual area of contact with the ground it is running on is increased and the grip performance improved with respect to a tread of which the rubber compound is stiffer.
Such a choice performs very well for tires the tread pattern of which has few or no grooves oriented rather circumferentially, notably on the axially exterior side of the tire. An example is given in document FR 3 007 693 A1. However, in order to encourage good removal of water and make it possible to keep the tread surface of the tire in close contact with the ground during use on very wet ground, it is necessary for the tread pattern to have enough grooves oriented rather circumferentially. In this latter configuration, the use of a less stiff rubber tread compound promotes shearing of the tread pattern blocks and rocking thereof, and this generates greatly raised pressures on the leading edges of the tread pattern blocks, and these 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 compound, particularly the grip potential in an axial direction (also referred to as transverse grip).
In order to improve the transverse grip performance of tires the tread of which comprises a plurality of circumferential grooves delimited by opposing lateral faces and a groove bottom, document WO 2011/073022 A1 proposes arranging, under the carcass ply and even under the inner liner, a reinforcing ply comprising elements of the cord type, oriented substantially axially.
However, the reinforcing effect of such a structure remains limited and such a structure may prove to exhibit insufficient endurance. Document DE 10 2010 038199 A1, or its equivalent WO2012/048930 A1, gives an example of a protective reinforcing ply situated radially on the outside of two plies made up of monofilament reinforcers, forming with the carcass ply the typical triangulation of radial-carcass tires, in which example the reinforcers of the reinforcing ply make an angle of between 50° and 90° with the circumferential direction of the tire. Document EP 0 987 129 A2 gives an example of a protective reinforcing ply, situated this time again radially on the outside of two plies which, with the carcass ply, form the typical triangulation of radial-carcass tires, the purpose of which is to improve the resistance to puncturing; once again, the reinforcing ply comprises elements of the cord type, oriented at an angle close to the transverse direction. However, these teachings relate to what is known as the “plunger test” which is an objective that has nothing in common with the context of the present invention.
The objective of the present invention is to propose a better alternative for producing a tire capable of far better performance in terms of transverse grip.
The invention relates to a tire comprising a belt reinforcement arranged in the crown, usually radially on the outside of a carcass reinforcement. In the radial direction, the belt reinforcement is a stack of at least two plies coupled with a carcass ply; the said at least two plies are usually made up of metal cords, the said metal cords typically forming an angle with the circumferential direction of between +10° and +40° for one of these plies and of between −10° and −40° for the other of these plies.
A subject of the invention is a tire comprising:
What is meant by “rigid in terms of compression” is elements of which the Young's modulus in compression in the direction of slenderness of the filamentary elements is greater than 1 GPa, and preferably 10 GPa. By way of example, the filamentary reinforcing elements are made of metal cords. Many steel cords give the stiffening ply a suitable stiffness. The filamentary reinforcing elements may also be monofilaments. They may also be formed by compressively rigid assembly of organic fibres, or alternatively of inorganic fibres such as glass fibres, carbon fibres, fibres of the aramid type, embedded in a sufficiently rigid matrix. The cross section of the monofilaments may be circular or flattened, for example of elliptical or rectangular cross section.
In order to offer the compression reinforcement performance afforded by the invention, advantageously it is appropriate for the filamentary reinforcing elements to be spaced apart in the stiffening ply by a spacing at most equal to three times the longest dimension of their cross section. The stiffening ply thus, because of its great compressive stiffness, opposes the flexing of the crown outside of its plane and thus opposes the rocking of the tread pattern elements and thus makes it possible to maintain a large area of contact with the ground on which it is running; it makes it possible to limit the raised pressures on the leading edge of the tread pattern blocks and thus limit the heating.
Thus, the stiffening ply makes it possible to use, for the tread, compounds that are considerably softer than are used in tires for passenger vehicles, and to do so without significantly prejudicing their longevity in terms of distance covered. Advantageously, the elastomeric material of which the tread is made has a dynamic shear modulus G*, at 60° C. and under a 10 Hz alternating stress loading of 0.7 MPa, of less than 1.25 MPa, namely is a material which, prior to the present invention, was confined to competition applications only.
The subjects of the invention will now be described with the aid of the appended drawing, in which:
Each bead comprises a bead wire 40; a carcass ply 41 (also visible in
The crown 2 comprises (see
The tread 5 comprises a plurality of tread pattern blocks 50. Two axially adjacent tread pattern blocks 50 are separated by a groove 7 extending at least partially circumferentially; each groove 7 is delimited radially towards the inside by a groove bottom 70.
The belt reinforcement of the tire 1 comprises a hooping ply 64 made up of hoop reinforcers formed of organic or aromatic polyamide fibres or of aramid fibres or are formed of hybrid cords containing aramid fibres, the said hoop reinforcers forming, with the circumferential direction, an angle at most equal to 5°. The hooping ply 64 comprises a single layer of hoop reinforcers. As an alternative form of embodiment, the hooping ply is made up of several layers of hoop reinforcers.
According to the invention, the tire 1 comprises a stiffening ply 8 that can be seen in
The filamentary reinforcing elements of the said stiffening ply 8 (in this instance the cords) are oriented essentially radially: experimentation carried out by the applicant shows that the stiffening effect according to the invention is substantially obtained with an angle of between 50° and 90°, and preferably an angle greater than 85°. The stiffening ply as proposed by the invention indeed has two main functions:
The range of angles given hereinabove thus makes it possible to limit bending in the plane and outside of the plane of the crown of the tire under transverse loading by, on the one hand, increasing the meridian bending stiffness of the crown (outside the plane), thereby limiting the rocking of the tread pattern blocks as a result of bending, and, on the other hand, increasing the bending stiffness on the edge of the crown (in the radial plane).
It has been seen that the material of which the tread 5 is made is an elastomeric material with a dynamic shear modulus G*, at 60° C. and under a 10 Hz alternating stress loading of 0.7 MPa, of less than 1.25 MPa. What is meant by “material of which the tread is made” is the material which comprises at least 50% of the volume comprised between the surface for contact with the ground when new and a level corresponding to 1.6 mm towards the outside from the groove 7 bottom 70 (1.6 mm being the usual height of the tread wear indicators).
In order not to increase the volume of the tire, the stiffening ply according to the invention can advantageously be used as a substitute for the materials generally present at the base of the tread. A layer of an elastomeric material measuring of the order of 2 mm is generally found under the groove bottom and affords the belt reinforcement protection against the attack experienced by the tire during use. The stiffening ply of the present invention may afford sufficient protection against this attack, making it possible to reduce the thickness of the said layer of elastomeric material mentioned hereinabove, for example to reduce it to 1.5 mm at most rather than 2 mm, or even to go down to a thickness of at least 0.5 mm. This reduction in thickness also makes it possible to increase the shear stiffness of the tread and thus contribute to improving the transverse roadholding of the vehicle.
It should be noted that the stiffening ply is generally a semi-finished product containing not only steel cords but also rubber referred to as skim. This is generally the case with all the plies, whether these are the carcass ply or plies, the belt plies, the hooping ply: they are made up of filamentary, monofilamentary or corded reinforcing elements coated in skim rubber which binds them together to form a semi-finished manufactured product; the rubber skim has a composition generally chosen both to meet the requirements of the manufacturing process and to give appropriate properties to the tire as a finished product. Let us recall in passing that there are manufacturing processes in which the belt ply cords are laid down thread by thread on a green tire as it is being built, without passing via the step of manufacturing a semi-finished product such as a skimmed ply; the invention is compatible with such a process. All of these considerations are nothing more than a reminder of technical data that are well known to those skilled in the art and do not in themselves form part of the invention.
The stiffening ply 8 extends axially over a width WR. By way of example, the width WR measures 140 mm for a tire of size 225/45 R 17, namely represents 70% of the width W of the crown reinforcement. Advantageously, the stiffening ply 8 is close to the mechanical neutral axis of the structural assembly formed by the crown 2 of the tire. In another embodiment, at least 70% of the said width WR of the stiffening ply is arranged on one side of the median plane EP, namely on the side intended to be mounted towards the outboard side of the vehicle, which is to say the side that is most heavily loaded in bends taken at speed. In certain implementations of the invention, the tire is therefore asymmetric. However, this is not in any way imperative, it being highly possible for the tire according to the invention to be axially symmetrical. For preference, the stiffening ply extends over a width WR representing at minimum 50% of the axial width W of the belt reinforcement and preferably at least 70%. Advantageously, the stiffening ply extends over a width WR representing at most 100% of the axial width W of the belt reinforcement.
The invention finds a quite particularly advantageous application when the grooves 7, or some of them, extend circumferentially. They may very well be grooves which are not oriented exactly circumferentially but which may be oblique with respect to the equatorial plane EP; because the means of the invention make it possible effectively to combat the tendency of tread pattern blocks to rock and the tendency of the crown to flex when the tread is made of soft rubber in order to encourage grip, and the flexing of the crown and the rocking of the tread pattern is particularly prejudicial when the tire is loaded in the transverse direction, the invention finds a quite particularly beneficial application when there are grooves that are at least partially oriented circumferentially. Of course, there may also be grooves oriented mainly axially, and in that case the invention makes it possible to combat the rocking of the tread pattern blocks 50 in the event of longitudinal loading, originating from a high torque, for example under emergency braking. The stiffening ply 8 forms part of the belt reinforcement of the tire 1; the stiffening ply 8 is directly connected with the belt plies 62, 63 and with the hooping ply 64. That makes it possible to stiffen the crown 2 and effectively limits, or even prevents, the rocking of the tread pattern blocks 50 of the tread 5.
The addition of the stiffening ply 8 contributes to strong mechanical coupling with the belt plies 62 and 63, forming a non-deformable triangle, particularly at the centre of the tire, when the coupling is well established. Thus, when the tire is spun, the centre of the tire in the vicinity of the equatorial plane EP does not deform, whereas the shoulders experience deformation (or radial extension) under the effect of the inertial forces. This difference in radial extension under the effect of spinning therefore causes a certain weakness of the tire at high speeds and may cause premature damage to the tire. As a result, in one quite particularly advantageous implementation, the invention comprises a hoop particularly suited to preventing any significant radial extension according to the maximum speed level set for the tire.
One way, in concrete terms, of adjusting the design of a tire according to a given performance specification is for the hooping ply to extend axially beyond the edge of the widest (62) of the belt plies by at least 3 mm and preferably at least 5 mm, so as to improve the relative extension stiffness of the tire at the shoulders. This aspect will be revisited in the description of the fifth alternative form of embodiment hereinbelow. Another way is to use, for the materials of which the hooping ply is made, materials that are far stiffer in terms of extension, such as aramid or hybrid cords like aramid. Yet another way is for this hooping ply to be made up of several layers of reinforcers at the shoulder, more numerous than in the centre of the tire, where coupling with the stiffening ply 8 is already performing this role. Finally, further mention is made of another way which consists in the hooping ply at the shoulders being preloaded, either because it has been laid on a tire building drum at a smaller radius at the shoulders than at the centre, before the tire is shaped in the vulcanizing press, or because it has been laid on a tire building drum under tension with greater tension at the shoulders than at the centre.
The invention was tested by creating tires of the size 225/45 R 17. The reference tire is a MICHELIN Pilot Sport 3 tire. The test tires are all derived from the MICHELIN Pilot Sport 3 tire in respect of elements not specific to the present invention, and for which no indication is given hereinbelow.
The table above in each instance gives the timing with and without the stiffening ply. The reference tire achieves a timing of 2 minutes 18.7 seconds. The difference is considered significant in this test when a 0.3 second improvement is made.
In order to create an elastomeric material of which the tread 5 is made that has a dynamic shear modulus G* of 0.9 MPa, reference may be made for example to the formulation below:
Dry time trials were held on the Charade circuit near Clermont-Ferrand, using a Renault® Clio® Cup vehicle (front (AV) pressure 2.3 b rear (ARR) pressure 2.7 b). Several tires were manufactured using materials of different stiffnesses for the tread: the dynamic shear modulus G*, at 60° C. and under 10 Hz alternating stress loading of 0.7 MPa is at the levels of 1.4 MPa, 1.1 MPa, 0.9 MPa, and 0.4 MPa. A person skilled in the art will know how to adapt the formulation in order to vary the dynamic shear modulus G* in the range indicated hereinbelow, notably the levels at 0.9 MPa, for example, by increasing the proportion of plasticizer. To summarize the aspect of the invention relating to the dynamic shear modulus G*, at 60° C. and under 10 Hz alternating stress loading of 0.7 MPa of the material of which the tread is made, this is preferably less than or equal to 1.1 MPa and advantageously less than or equal to 0.9 MPa.
This solution makes it possible to limit the coupling of the stiffening ply with the belt plies at the centre of the tire, making it possible to limit the stiffening at the centre of the tire, something which results in better casing integrity at limiting speed and a reduced impact on external noise. This alternative form allows a 15 km/h improvement on the limiting speed of the casing and a 0.7 dB reduction in external noise referred to as “coast-by” noise according to the relevant standard, for timings identical to the alternative form comprising just one continuous ply.
This alternative form allows a weight saving in the tire and also allows an improvement in rolling resistance through the positioning of the stiffening ply at the place where the radius of curvature of the crown of the tire is the greatest, thus preventing the meridian flexing from being constrained by the stiffening ply.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1555491 | Jun 2015 | FR | national |
| 1559996 | Oct 2015 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/063374 | 6/10/2016 | WO | 00 |
