FIELD OF THE INVENTION

[0001 ] The present invention relates to the radial tyres for land vehicles and more particularly to radial tyres for passenger vehicles. The invention relates more particularly to lightweight tyres and to the arrangements suitable for increasing the resistance of these tyres to the damage to which they are exposed when they hit obstacles on the ground, such as pavements or potholes.

BACKGROUND

[0002] Research on tyres serving to reduce the energy consumption of a vehicle is presently gaining momentum. Among the promising approaches explored by tyre designers, mention may be made of reducing the rolling resistance of tyres, especially by the use of low-hysteresis materials, but also reducing the tyre weight. It has been proposed to reduce the tyre weight by reducing the thicknesses of material and the densities of the reinforcing elements (use of textile cords) or of the rubber compounds, or by using reinforcing elements which allow reduction of certain volumes of inner liner compounds, for example in the region of the bead. Such tyres are described for example in U.S. patent US 6,082,423 and in the documents cited therein.

[0003] Another way of reducing the tyre mass involves generally reducing its dimensions. Of course, such a reduction is not without consequence on the service capability of the tyre. The International standards, such as those of the ETRTO or JATMA define, for each nominal dimension, the physical dimensions of the tyre, such as its sectional height and its sectional width when fitted onto a rim of given diameter and width. At the same time, they define a "loading capacity" of the tyre, that is to say, the maximum admissible static load on a wheel of the vehicle at a given service pressure.

[0004] In such standards, the load capacities are deduced from the nominal dimensions using semi-empirical relationships. These relationships set a maximum static deflection for a tyre, normalized by its dimensions, and are based on a standard geometry of the section profiles of the tyres of the current technology. They predict that the loading capacity of tyres of course decreases when, all other things being equal, the section height or width decreases. [0005] A reduction in the dimensions of a tyre is of course not without consequence with respect to the serviceability of the tyre, its wear lifetime and the endurance of its structure for a given service load on a wheel of the vehicle.

[0006] However, these standards leave the designer with certain degrees of freedom regarding the dimensions of the section profile that it is possible to use in the context of reducing the mass and rolling resistance of a tyre. Most of the mass of a tyre and most of its rolling resistance result from the region of its crown. Reducing the width of the crown would therefore result in an almost proportional increase in the contribution of the crown to the mass and, as experience has shown, an increase in its contribution to rolling resistance.

[0007] When a tyre is rolling on the ground under normal use conditions (in terms of speed and load), it may be subjected to shocks on the tread or sidewalls, the frequency and intensity of which are often considerable. It is one of the main functions of a tyre that it should absorb these shocks and damp them without the vehicle's wheel in question being substantially affected thereby, either in its movement or in its integrity.

[0008] However, it happens that this absorption capability encounters its limits when the impact conditions are such that the sidewall of the tyre impacted comes into abutment inside the air chamber either directly with the rim, on which the tyre is mounted, or more usually on another region of the sidewall of the tyre itself, directly bearing on the wheel rim. This is especially the case when the rim has an external radial projection relative to the actual seat. Such a projection (usually called a "rim flange") is in general provided for preventing the tyre bead from coming off the rim due to the effect of axially directed stresses during wheel manoeuvres.

[0009] The impact with an obstacle can then transfer brief, but very intense loads, possibly in certain cases reaching several tons, on the abutted parts but also, beyond the rim, on the mechanical suspension attachments for the wheel assembly, or even on the body of the vehicle. They are capable of creating serious damage on the suspension members and of permanently deforming the vehicle body. Vehicle designers therefore have to provide sufficient damping systems to prevent this damage and to design vehicle bodies in accordance with normally predictable extreme situations.

[0010] Unfortunately, even when the vehicle is, strictly speaking, suitably protected, the tyre subjected to this type of incident is liable to seriously suffer the consequences of the abovementioned phenomenon. In the section impacted by the shock, the internal wall of the tyre is suddenly folded and pinched between the obstacle and the rim flange. This may cause the wall to rupture and the tyre to suddenly lose its inflation pressure, which usually means that the vehicle is immediately immobilized. However, even when the tyre withstands this impact, it is often found that its components have been damaged by the incident: indentations in the sidewalls or other signs indicate to a tyre expert that the structure of the tyre has been weakened and that its sidewall runs the risk of rupturing under the effect of its components being repeatedly flexed, in the relative long term, which would of course be incompatible with the conditions necessary for safe driving.

[001 1 ] It turns out that lightweight tyres of small dimensions are particularly vulnerable to this type of damage.

SUMMARY OF THE INVENTION

[0012] The objective of the present invention is to address these concerns and to reduce the risks incurred by a lightweight tyre when it is faced with the shock of contact with obstacles on a roadway, for example when encountering a pothole or a pavement.

[0013] This objective is achieved by a tyre having a rotation axis and comprising: two beads intended to come into contact with a mounting rim, each bead comprising at least one annular reinforcing structure, defining a mid-plane which is perpendicular to the rotation axis of the tyre and located equidistant from the annular reinforcing structures of each bead, the annular reinforcing structures having, in any radial cross section, a radially innermost point; two sidewalls extending the beads radially outwards, the two sidewalls joining in a crown comprising a crown reinforcement, having two axial ends, said crown reinforcement being surmounted by a tread; at least one carcass reinforcement extending from the beads through the sidewalls as far as the crown, the carcass reinforcement comprising a plurality of radially oriented carcass reinforcement elements and being anchored in the two beads by an upturn around the annular reinforcing structure, so as to form an main portion and a wrapped-around portion of the carcass reinforcement, wherein, when the tyre is fitted onto the mounting rim and inflated to its service pressure: the tyre has a maximum axial width SW such that the ratio TW/SW < 75% (and preferably TW/SW < 73%), where TW denotes the axial distance between the two axial ends of the crown reinforcement, the maximum axial width SW being reached at a radial distance X from the radially innermost point of the annular reinforcing structures; the axial distance RW of the two points of intersection of the axial direction passing through the radially innermost point of the annular reinforcing structures with the external surface of the tyre is such that TW/RW < 85% (and preferably TW/RW < 83%); the tyre satisfies the following three conditions: X/SH < 50%, Y/SH≥ 80% and Z/SH≥ 90%, where SH denotes the distance between the radially outermost point of the tyre and the radially innermost point of the annular reinforcing structures, Y denotes the radial distance between (i) the points on the carcass reinforcement having the same axial positions as the axial ends of the crown reinforcement and (ii) the radially innermost point of the annular reinforcing structures, and Z denotes the radial distance between the radially outermost point of the carcass reinforcement and the radially innermost point of the annular reinforcing structures; the absolute value of the angle a (alpha) between the tangent to the carcass reinforcement at the points on the carcass reinforcement having the same axial positions as the axial ends of the crown reinforcement and the axial direction is less than or equal to 22°; and at any point on the carcass reinforcement, the radius of curvature p is such that

where R _s is the radial distance between the rotation axis of the tyre and the radially outermost point of the carcass reinforcement, R _E is the radial distance between the rotation axis of the tyre and the axial position where the tyre reaches its maximum axial width SW, and R is the radial distance between the rotation axis of the tyre and the point in question on the carcass reinforcement; wherein each sidewall of the tyre comprises a first reinforcing member and a second reinforcing member, the first reinforcing member being reinforced by reinforcement elements placed parallel to one another, making an angle β (beta) of between 10° and 90° with the circumferential direction, the second reinforcing member being reinforced by reinforcement elements parallel to one another, making an angle γ (gamma) of -β with the circumferential direction; and

wherein the first and second reinforcing members extend along the carcass reinforcement, between a radially inner end and a radially outer end, the radial distance of each of the radially inner ends from the radially innermost point of the annular reinforcing structure being less than 10% of the distance SH, and the radial distance of each of the radially outer ends from the radially innermost point of the annular reinforcing structure being greater than 30% of the distance SH.

[0014] In a particular embodiment, the reinforcement elements that reinforce the first reinforcing member and the second reinforcing member may desirably be filamentary reinforcement elements, but it is also possible to use flat reinforcement elements such as, for example, strips made of thermoplastic material. In this situation, the angles are understood to mean with respect to the longitudinal direction of the strips.

[0015] According to a first preferred embodiment, the first reinforcing member is placed axially inside the main portion of the carcass reinforcement and the second reinforcing member is placed axially outside the main portion of the carcass reinforcement and axially inside the wrapped-around portion of the carcass reinforcement, e.g., over the height of the wrapped-around portion of the carcass reinforcement or some portion thereof. This embodiment makes it possible to minimize the thickness of the "inner liner" impermeable to the inflation gas, which covers the internal surface of the tyre. It also makes it possible to control the torsional stiffness of the tyre and, consequently, its cornering stiffness.

[0016] According to a second preferred embodiment, the first and second reinforcing members are placed axially outside the main portion of the carcass reinforcement and axially inside the wrapped-around portion of the carcass reinforcement, e.g., over the height of the wrapped-around portion of the carcass reinforcement, or some portion thereof. This embodiment makes it possible for the "bead filler" volume, that is to say the portion of rubber composition filling the volume between the main portion and the wrapped-around portion of the carcass reinforcement, to be very significantly reduced and for the stiffness of this region of the tyre to be modified by varying the angle of the reinforcement elements.

[0017] According to one particular embodiment, the radial distance of each of the radially outer ends of the reinforcing members from the radially innermost point of the annular reinforcing structure is greater than 80% of the distance SH. According to one particularly advantageous variant of this embodiment, the radially outer ends of the reinforcing members are axially inside the axial ends of the crown reinforcement. This embodiment makes it possible to control the torsional stiffness of the tyre and, consequently, its drift resistance. It also makes the tyre more robust with respect to perforations of the sidewalk

[0018] Preferably, in a particular embodiment the tyre has only a single carcass reinforcement so as to reduce its weight.

[0019] The invention also relates to an assembly formed by a mounting rim and a tyre as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 shows a tyre according to the prior art.

[0021 ] Figure 2 shows a partial perspective view of a tyre according to the prior art.

[0022] Figure 3 shows, in radial section, one half of a reference tyre.

[0023] Figure 4 shows part of the tyre of Figure 3.

[0024] Figure 5 illustrates the parameters used to describe an inflated carcass reinforcement in equilibrium.

[0025] Figures 6 to 8 show, in radial section, a portion of a tyre according to the invention.

DETAILED DESCRIPTION OF THE FIGURES

[0026] When employing the term "radial", a distinction should be made between several different uses of the word by those skilled in the art. Firstly, the expression refers to a radius of the tyre. It is in this sense that a point P1 is said to be "radially inside" a point P2 (or "radially to the inside" of point P2) if it is closer to the rotation axis of the tyre than point P2. Conversely, a point P3 is said to be "radially outside" a point P4 (or "radially to the outside" of point P4) when it is further away from the rotation axis of the tyre than point P4. The expression "radially inwardly (or outwardly)" means going towards smaller (or larger) radii. When distances are referred to as radial distances, this meaning of the term also applies.

[0027] However, a thread or a reinforcement is said to be "radial" when the thread or reinforcing elements of the reinforcement make an angle of not less than 80° but not exceeding 90° with the circumferential direction. It should be pointed out, that in the present document, the term "thread" should be understood in a very general sense and comprises threads in the form of monofilaments, multifilaments, a cord, a folded yarn or an equivalent assembly, irrespective of the material constituting the thread or the surface treatment for promoting adhesion to the rubber.

[0028] Finally, the term "radial section" or "radial cross section" is understood here to mean a section or cross section in a plane that contains the rotation axis of the tyre.

[0029] An "axial" direction is a direction parallel to the rotation axis of the tyre. A point P5 is said to be "axially inside" a point P6 (or "axially to the inside" of point P6) if it is closer to the mid-plane of the tyre than point P6. Conversely, a point P7 is said to be "axially outside" a point P8) or "axially to the outside" of point P8) if it is further away from the mid-plane of the tyre than point P8. The "mid-plane" of the tyre is the plane which is perpendicular to the rotation axis of the tyre and located equidistant from the annular reinforcing structures of each bead. When it is said that the mid-plane separates, in any radial section, the tyre into two tyre "halves", this does not mean that the mid-plane necessarily constitutes a plane of symmetry of the tyre. The expression "tyre half" has here a broader meaning and denotes a portion of the tyre having an axial width close to one half of the axial width of the tyre.

[0030] A "circumferential" direction is a direction perpendicular both to a radius of the tyre and to the axial direction.

[0031 ] In the context of this document, the expression "rubber compound" denotes a rubber compound comprising at least one elastomer and at least one filler.

[0032] The "external surface" of the tyre denotes here the surface of the tyre which is intended to be in contact with the atmosphere (or with the mounting rim), as opposite to its internal surface, which is intended to be in contact with the inflation gas. [0033] Figure 1 shows schematically a tyre 10 according to the prior art. The tyre 10 comprises a crown, having a crown reinforcement (not visible in Figure 1 ) surmounted by a tread 40, two sidewalls 30 extending the crown radially inwards, and two beads 20 radially inside the sidewalls 30.

[0034] Figure 2 shows schematically a partial perspective view of a tyre 10 according to the prior art and illustrates the various components of such a tyre. The tyre 10 comprises a carcass reinforcement 60 consisting of threads 61 embedded in a rubber composition, and two beads 20 each comprising annular reinforcing structures 70 which hold the tyre 10 on the rim of the wheel (not shown). The carcass reinforcement 60 is anchored in each of the beads 20 by an upturn. The tyre 10 further includes a crown reinforcement comprising two plies 80 and 90. Each of the plies 80 and 90 is reinforced by filamentary reinforcing elements 81 and 91 which are parallel in each ply and crossed from one layer to the next, making angles of between 10° and 70° to the circumferential direction. The tyre further includes a hooping reinforcement 100 placed radially outside the crown reinforcement, this hooping reinforcement being formed from circumferentially oriented reinforcing elements 101 wound in a spiral. A tread 40 is placed on the hooping reinforcement; it is via this tread 40 that the tyre 10 comes into contact with the road. The tyre 10 shown is a "tubeless" tyre: it includes an "inner liner" 50 made of a butyl-based rubber composition impermeable to the inflation gas and covering the internal surface of the tyre.

[0035] Figure 3 shows, in radial cross section, one half of a reference tyre. This tyre has a rotation axis (not shown) and comprises two beads 20 intended to come into contact with a mounting rim 5. Each bead has an annular reinforcing structure, in this case a bead wire 70. Here, the two bead wires 70 have the same diameter and there are two points 71 corresponding to the radially innermost point of the bead wires 70.

[0036] The tyre has two sidewalls 30 extending the beads radially towards the outside, the two sidewalls 30 joining in a crown having a crown reinforcement formed by the plies 80 and 90. The crown reinforcement has two axial ends 189 and 289. In the illustrated embodiment, these ends coincide with the axial ends of the radially inner ply 80, but it is also perfectly possible to provide a radially outer ply 90 that extends axially beyond the inner ply, on only one side of the mid-plane 130, or on each side of this plane, without departing from the scope of the invention. The crown reinforcement is surmounted by a tread 40. In principle, it would be possible also to provide a hooping reinforcement, such as the hooping reinforcement 100 of the tyre shown in Figure 2, but in the present case the aim was to minimize the weight of the tyre by not providing a hooping reinforcement.

[0037] The tyre comprises a single radial carcass reinforcement 60 extending from the beads 20 through the sidewalls 30 to the crown, the carcass reinforcement 60 comprising a plurality of carcass reinforcing elements. It is anchored in the two beads 20 by an upturn around the bead wire 70, so as to form a main portion 62 and a wrapped-around portion 63.

[0038] When the tyre is fitted onto the mounting rim 5 and inflated to its service pressure, it meets several criteria.

[0039] Firstly, it has a maximum axial width SW such that the ratio TW/SW < 75%, where TW denotes the axial width of the crown reinforcement, i.e. the axial distance between the two axial ends 189 and 289 of the crown reinforcement. In the embodiment illustrated, TW/SW = 73%, and in a more particular embodiment, TW/SW < 73%. The maximum axial width SW is reached at a radial distance X from the radially innermost point of the annular reinforcing structures. It should be pointed out that when determining the width SW, no account is taken of excrescences such as the protective bead 140.

[0040] Secondly, the axial distance RW of the two points of intersection 201 and 202 of the axial direction A1 passing through the radially innermost point(s) 71 of the bead wires 70 with the external surface of the tyre is such that TW/RW < 85%. In the embodiment illustrated TW/RW = 83%, and in a more particular embodiment, TW/RW < 83%.

[0041 ] Thirdly, X/SH < 50% (and preferably, X/SH < 45%), where SH denotes the distance between the radially outermost point 41 of the tyre, and the radially innermost point 71 of the annular reinforcing structures 70. In the embodiment illustrated, X/SH = 50%.

[0042] Fourthly, Y/SH ≥ 80%, where Y denotes the radial distance between (i) the points 160 and 260 of the carcass reinforcement 60 having the same axial positions as the axial ends 189 and 289 of the crown reinforcement and (ii) the radially innermost point 71 of the annular reinforcing structures 70, SH being defined as above. In the embodiment illustrated, Y/SH = 80%. When the carcass reinforcement 60 has a significant width, Y should be taken at the neutral fibre of the reinforcement elements 61 constituting it. In a more particular embodiment, Y/SH≥ 90%. [0043] Fifthly, Z/SH≥ 90%, where Z denotes the radial distance between the radially outermost point 360 of the carcass reinforcement 60 and the radially innermost point 71 of the annular reinforcing structures 70, SH being defined as above. In embodiment illustrated, Z/SH = 90%.

[0044] Sixthly, the absolute value of the angle a (alpha) - indicated in Figure 4, between the tangent T to the carcass reinforcement 60 at the points 160 and 260 of the carcass reinforcement 60 having the same axial positions as the axial end points 189 and 289 of the crown reinforcement and the axial direction, is less than or equal to 22°.

[0045] Finally, at any point on the carcass reinforcement 60, the radius of curvature p is such that

where R _s is the radial distance between the rotation axis of the tyre and the radially outermost point 360 of the carcass reinforcement 60, R _E is the radial distance between the rotation axis of the tyre and the axial position where the tyre reaches its maximum axial width SR, and R is the radial distance between the rotation axis of the tyre and the point in question on the carcass reinforcement. These values are indicated in Figure 5, together with the radius of curvature p for a radial position R=R0. The reference 2 indicates here the rotation axis of the tyre 10.

[0046] As is well known to those skilled in the art, the latter criterion corresponds to the equilibrium condition for an inflated radial carcass reinforcement. It serves in particular to differentiate the invention from fortuitous prior art representing uninflated tyres for which some of the criteria listed above would be fulfilled in the uninflated state, but which would no longer be fulfilled if the tyre were to be inflated and the carcass reinforcement were to be considered in the equilibrium state. An example of this is shown in Figure 1 of document WO 1999/022952 which shows a tyre that is manifestly not in equilibrium, as the fold in the carcass reinforcement close to the ends of the crown reinforcement shows.

[0047] Such a tyre makes it possible, for a given nominal tyre size, when mounted on a given mounting rim, under a given service pressure, to best benefit from the design of the geometry of the tyre to lighten it and to reduce its rolling resistance, while still maintaining its main performance characteristics, in particular its loading capacity and its capability of not loosening. [0048] However, the Applicant has found that such a tyre may prove to have a certain vulnerability to shocks caused by obstacles that it encounters on a roadway, such as a pothole or a pavement curb.

[0049] One of the objectives of embodiments of the tyre disclosed herein is to solve this difficulty and to define a tyre which, apart from the abovementioned advantages, is more resistant to such shocks.

[0050] Figures 6 and 7 show, in radial section, a portion of one embodiment of a tyre according to the invention. The particular feature of such a tyre lies in the fact that each sidewall comprises two reinforcing members 171 and 172, each being reinforced by reinforcement elements, in this case parallel filamentary elements. The reinforcing members 171 and 172 are crossed: the reinforcement elements of the first reinforcing member 171 make an angle β (beta) of between 10° and 90° to the circumferential direction and the second reinforcing member 172 makes an angle γ (gamma) of - β. In this case, β = 45° and γ = - 45°. [0051 ] In particular embodiments, filamentary reinforcement elements may be made of textile materials such as PET, nylon, rayon, aramid or metal.

[0052] In the embodiment shown in Figure 6, the first reinforcing member 171 and the second reinforcing member 172 extend along the carcass reinforcement 60 between a radially inner end 171 1 and 1721 and a radially outer end 1712 and 1722, the radial distance of each of the radially inner ends from the radially innermost point 71 of the annular reinforcing structure 70 being less than 10% of the distance SH and the radial distance of each of the radially outer ends from the radially innermost point 71 of the annular reinforcing structure 70 being greater than 30% of the distance SH. Thus, these reinforcing numbers extend over the entire region that risks becoming jammed between the rim flange and the radially outer portion of the tyre. For the tyre shown in Figure 6, DI 1/SH = 7%, DI2/SH = 5%, DE1/SH = 45% and DE2/SH = 39%.

[0053] In the embodiment shown in Figure 6, the first reinforcing member 171 is placed axially inside the main portion 62 of the carcass reinforcement 60 and the second reinforcing member 172 is placed axially outside the main portion 62 of the carcass reinforcement and, over the height of the wrapped-around portion 63 of the carcass reinforcement 60, axially to the inside thereof. [0054] Another embodiment is shown in Figure 7. Here, the first reinforcing member 171 and the second reinforcing member 172 are placed axially outside the main portion 62 of the carcass reinforcement and, over the height of the wrapped- around portion 63 of the carcass reinforcement, axially to the inside thereof. For the tyre shown in Figure 7, DI 1/SH = 5%, DI2/SH = 7%, DE1/SH = 38% and DE2/SH = 44%.

[0055] In the tyres according to the invention shown in Figures 6 and 7, the reinforcing members 171 and 172 do not rise beyond the mid-height of the tyre, but while this is illustrative, it is not necessarily required. Figure 8 shows another tyre according to the invention in which these reinforcing members extend radially further to the outside, in this case right to the crown reinforcement formed by the plies 80 and 90. Here, the radial distance of each of the radially outer ends 1712 and 1722 of the reinforcing members 171 and 172 from the radially innermost point 71 is equal to 86% of the distance SH, and the radially outer ends 1712 and 1722 of the reinforcing members 171 and 172 are axially inside the axial end 189 of the crown reinforcement formed by the plies.

[0056] Those skilled in the art will understand that the reinforcing members may extend right to the crown, even when one of these reinforcing members is placed axially inside the carcass reinforcement.

[0057] A tyre according to the invention, of 205/55 R 16 size, corresponding to the embodiment of Figure 7, was compared with a commercial "Michelin Energy Saver" tyre of the same size. The following table gives the essential geometric parameters:

Table I

[0058] The tyre according to the invention is 1 .8 kg lighter than the reference tyre (weighing 6.2 kg instead of 8.0 kg), but its rolling resistance at 90 km/h is 1 .96 kg/T lower and its main performance characteristics are equivalent, in particular its load capacity corresponding to an index of 91 (603 daN) and its ability not to unwedge.

[0059] A comparison between this tyre according to the invention and a reference tyre according to Figure 4 shows that the tyre according to the invention has a much better resistance to the shocks from obstacles on a roadway, for example when it encounters a pothole or a pavement. A reference tyre and a tyre according to the invention, when inflated to 80% of the ETRTO service pressure and loaded to 80% of the ETRTO load, were made to strike a pavement (height below rim flange +10 mm) at a speed limited to 70 km/h and at an angle of 30° to the straight-line path. It turns out that, unlike the reference tyre, the tyre according to the invention does not show any fracture of sidewall reinforcement elements, even at the maximum speed.