WO2008073098A1

WO2008073098A1 - Improved run-flat tire

Info

Publication number: WO2008073098A1
Application number: PCT/US2006/047851
Authority: WO
Inventors: Ronald Hobart Thompson
Original assignee: Michelin Recherche Et Technique S.A.; Societe De Technologie Michelin
Priority date: 2006-12-15
Filing date: 2006-12-15
Publication date: 2008-06-19

Abstract

An improved run-flat tire is provided having unique sidewall and crown portions that reduce the overall thicknesses of the materials required in these parts of the tire. The unique constructions described herein can provide for a weight reduction of run-flat tires so as to allow, for example, better economy during inflated operation while also permitting a reasonable distance of extended mobility after a loss of inflation pressure.

Description

PCT PATENT APPLICATION

TITLE

IMPROVED RUN-FLAT TIRE

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to radial pneumatic tires used for vehicles and more particularly to the unique design of the sidewall and crown portions of a run-flat tire so as to reduce the overall thicknesses of the materials required in these parts of the tire. The unique constructions described herein can provide for a weight reduction of run-flat tires so as to allow, for example, better economy during inflated operation while also permitting a reasonable distance of extended mobility after a loss of inflation pressure.

BACKGROUND OF THE INVENTION

[0002] Various proposals have been devised to allow for continued mobility after a tire has experienced a loss of inflation pressure. For example, tire constructions have been proposed for preventing substantial damage to the tire for a reasonable distance after the loss of inflation. Efforts have also been made at constructions that provide a reasonably comfortable ride during the period of extended mobility.

[0003] Typically, for a pneumatic tire, the load is mainly supported by internal inflation pressure. When the tire is punctured, support of the load can be shifted to the sides of the tire if structure of sufficient rigidity is present in the tire sidewall. Accordingly, proposed run-flat constructions have included relatively thick, crescent-shaped rubber portions that are incorporated into the sidewall to provide additional rigidity and thus support for run-flat operation. An example of such a construction is presented in Japanese Patent Application Publication No. 52-41521.

[0004] Such reinforcement of the sidewall portions of a run-flat tire can increase the load bearing ability of the sidewall while reducing or preventing undesirable buckling of the sidewall. However, such constructions typically require a significant increase in sidewall thickness. Such increase in thickness is generally less desirable for several reasons. For example, the increased thickness requires that more materials, such as rubber, must be used for a given tire, which can increase the manufacturing cost. By way of further example, the increased thickness of materials adds weight to the tire that can decrease fuel economy. Additionally, increasing the thickness of the sidewall to bear more vehicle load can lead to buckling of the crown portion. Buckling of the crown portion is a condition wherein a bending moment is created such that the tread of the tire is lifted away from the travel surface and a portion of the sidewall begins to make undesirable contact with the travel surface. Constructions have also been presented for addressing the buckling problem. For example, U.S. Patent No. 5,871,601 describes a rubber reinforcement for the sidewall that has a crescent-shaped cross-section. An auxiliary belt, with cords optimally in a range of 45 to 75 degrees, is provided at the side of the crown portion of the tire to suppress buckling. Byway of further example, U.S. Patent No. 6,561,245 indicates a rubber reinforcement for the sidewall that has a crescent-shaped cross-section. A steel cord reinforced layer is deployed radially outward of the belt structure and a cord-reinforced fabric layer is placed between the innerliner and the radially innermost ply.

[0005] Although various constructions have been proposed, it would be advantageous to provide for unique tire constructions that allow a reduction in the overall thickness of the sidewall and crown portions of the tire while still providing for a reasonable distance of extended mobility after a loss of inflation pressure. Such reductions in overall thickness could provide advantages such as economies in the amount of materials required for manufacture and in the fuel efficiency of the tire. These advantages and others are provided in embodiments of the present invention, examples of which will now be described.

SUMMARY OF THE INVENTION

[0006] Objects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention.

[0007] hi one exemplary embodiment of the present invention, a radial run-flat tire is provided having an equator and defining axial and radial directions. The run-flat tire comprises a crown portion having a tread and a shear layer disposed radially inward of the tread. The shear layer has a dynamic shear modulus. A first membrane is adhered to a radially inward extent of the shear layer and a second membrane is adhered to a radially outward extent of the shear layer. The ratio of the circumferential compressive modulus of either one of the membranes to the dynamic shear modulus of the shear layer is at least about 100 to 1. The tire includes a pair of axially spaced-apart, annular bead portions. Each bead portion has a bead core and a bead apex, wherein the radially-outermost extent of the bead apex is positioned at a predetermined distance below the equator. A pair of sidewall portions are provided with each sidewall portion extending radially between a respective axial edge of the crown portion and a respective bead portion. Each sidewall portion has a reinforcing member extending radially outward from a position contiguous with a respective bead apex to a position radially inward of the crown portion. Each reinforcing member is of a uniform thickness over that portion extending between a respective bead apex to a position adjacent an end of the first membrane. A carcass layer is disposed radially inward from the tread portion, axially inward of the pair of sidewall portions, and axially outward of each reinforcing member along a respective sidewall portion. The carcass layer extends between the pair of bead portions and is anchored in each respective bead portion. [0008] For this exemplary embodiment of the present invention, the first and second membranes may be constructed a follows. For example, the first membrane may be constructed from a first ply and a second ply. The first ply has first ply cords that are oriented at a first angle of about 20 degrees or less from the circumferential plane of the tire. The second ply has second ply cords that are oriented at a second angle of about 20 degrees or less from the circumferential plane and in a manner that is opposite to the first angle of the first ply cords such that the first ply cords and the second ply cord cross. The second membrane may be constructed from a third ply having third ply cords that are oriented substantially parallel to the circumferential plane. The third ply cords may have a compressive modulus of about 12,000 MPa., a tensile modulus of about 40,000 MPa, and/or an infinite endurance limit at a compression strain of about 1 percent or less. The third ply cords may have an equivalent homogenous thickness of about 0.30 mm² per mm width of the third ply.

[0009] Continuing with this exemplary embodiment of the present invention, the dynamic shear modulus of the shear layer may be greater than about 2 MPa but less than about 5 MPa. The shear layer may have a thickness along the radial direction of greater than about 1 mm but less than about 5 mm. As an alternative, and by way of example, the dynamic shear modulus of the shear layer may be about 2 MPa and the shear layer may have a thickness along the radial direction of about 2 mm. The Elongation at Break at 100⁰C for the shear layer may be greater than about 100 percent, and the hysteresis for the shear layer may be less than about 0.2 at strains between about 15 percent and about 30 percent.

[0010] For this exemplary embodiment of the present invention, the predetermined distance by which the radially-outermost extent of the bead apex is positioned below the equator may be between about 5 mm and about 25 mm. The thickness of each reinforcing member along each sidewall portion may be in a range of about 4 mm to about 8 mm. Each reinforcing member along each said sidewall portion may be comprised of a plurality of rubber-based portions or may be of singular construction. For each reinforcing member, the Elongation at Break at 100⁰C may be greater than about 100 percent. The hysteresis for each reinforcing member may be less than about 0.2 at strains between about 15 percent and about 30 percent. [0011] In still another exemplary embodiment of the present invention, a run-flat tire is provided that has an equator and defines axial and radial directions. The tire includes a crown portion having a tread and a shear layer disposed radially inward of the tread. The shear layer has a dynamic shear modulus. The shear layer has a radial thickness greater than about 1 mm but less than about 5 mm. A first membrane is adhered to a radially inward extent of the shear layer. A second membrane is adhered to a radially outward extent of the shear layer. A ratio of the circumferential modulus in compression of either of the membranes to the dynamic shear modulus of the shear layer is at least about 100 to 1. This embodiment includes a pair of axially spaced-apart, annular bead portions, where each bead portion has a bead core and a bead apex. The radially-outermost extent of the bead apex is positioned at a predetermined distance below the equator. A pair of sidewall portions are provided with each sidewall portion extending radially between a respective axial edge of the crown portion and a respective bead portion. Each sidewall portion has a reinforcing member that extends radially outward from a position contiguous with a respective bead apex to a position radially inward of the crown portion. Each reinforcing member is of a uniform thickness over that portion extending between a respective bead apex to a position adjacent an end of the first membrane. A carcass layer is disposed radially inward from the tread portion, axially inward of said pair of sidewall portions, and axially outward of each reinforcing member along each respective sidewall portion. The carcass layer extends between the pair of bead portions and is anchored on each side in a respective bead portion.

[0012] These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] A full and enabling disclosure of the present subject matter, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which:

[0014] Fig. 1 is a cross-sectional view along a meridian plane of an exemplary embodiment of the present invention.

[0015] Fig. 2 is a partial perspective and cross-sectional view of the exemplary embodiment of the present invention shown in Fig. 1, illustrating various components and layers of the tire as described below.

[0016} Fig. 3 is a cross-sectional view along a meridian plane of another exemplary embodiment of the present invention.

[0017] Fig. 4 is a cross-sectional view along a meridian plane of another exemplary embodiment of the present invention.

DFTATT /F.n DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The present invention provides advantageous constructions for a run-flat tire in which uniquely constructed sidewalls are provided that have a reduced thickness compared to certain previous run-flat constructions. A novel crown portion is also provided that helps to control buckling while also bearing a portion of the vehicle load with the sidewalls during run-flat operation. More specifically, exemplary embodiments of the present invention include a shear band that assists uniquely constructed sidewall portions with carrying vehicle load for a reasonable period of run-flat operation. The shear bands of the present invention are constructed in a particular manner to achieve these results without the overall thickness required for the shear layer of previously described non-pneumatic tires. The result provides an advantageous run-fiat tire that can have less weight than certain previous run-flat tire constructions while still providing for extended mobility.

[0019] Reference will now be made in detail to embodiments of the invention, one or more examples of which are illustrated in the figures. Each example is provided by way of explanation of the invention, and not meant as a limitation of the invention. For example, features illustrated or described as part of one embodiment can be used with another embodiment to yield still a third embodiment. It is intended that the present invention include these and other modifications and variations. Repeat use of identical or similar reference characters throughout the present specification and appended drawings is intended to represent same or analogous features or elements of the invention. In each figure, arrow R represents a direction that is radially outward while arrow A indicates a direction that is axially outward.

Definitions

[0020] The following terms are defined as follows for this disclosure: [0021] "Circumferential plane" means a plane perpendicular to the tire's axis of rotation and passing through the tread.

[0022] "Compressive Modulus" (E_c) as used here for the cords or cables of a ply is determined as follows. A mold is fabricated of the following dimensions: a length of 50 mm, a width of 30 mm, and a thickness of 25 mm. The cables are positioned precisely one relative to another in a parallel orientation using two rectangular supports (beams). The cables pass through holes in these supports, which are positioned in a parallel fashion 40 mm apart from one another. The spacing between the holes ensures an accurate pace of the cables. The pace reflects the pace that will be used in the ply in the tire. The distance between two supports (40 mm) is slightly smaller than the length of the mold (50 mm), such that the supports and the cables can be placed inside the mold. The cables and their supports are then placed in a mold such that the cables are located in the center of the mold in the thickness direction. For the dimensions listed above, this means that the centerline of the cables is approximately 12.5 mm from the bottom of the mold. Liquid polyurethane is poured into the mold, filling the mold. The mold is placed in an oven for 24 hours at 110 ⁰C. After curing, the resulting sample has cables protruding from two sides in the length direction. The sample is cut with a saw such that these ends are removed along with a small thickness of polyurethane. The approximate final length is 40 mm. Cutting must be carefully controlled to ensure the cross-sections are perpendicular with the length of the cables being essentially equal to the sample width. Using the same mold, a sample is prepared that consists of polyurethane alone, with no cables. This sample is cured in the same manner as the samples containing cables, and it is cut to the same outer dimensions as the samples having cables.

[0023] Once ready for measurement, the samples are placed between two metallic plateaus in an Instron testing machine of type 44666 and compressed at a rate of 25 rnrn/min. The Instron machine is used to record force versus deflection. The measurements are taken for at least five samples. An elementary cable compression modulus is calculated by subtracting the force vs. deflection measurements of samples prepared without cables from force vs. deflection measurements of samples prepared with cables. The resulting force vs. deflection values are used to compute the effective compressive modulus of the cables using the following equations:

ε_c = compressive strain = compressive deflection / initial sample length

σ_c = compressive stress = compressive force / area

where Area = total cross sectional area of cables contained in the sample

E_c = compressive modulus (E₀) = σ_c / ε_c

[0024] "Dynamic Shear Modulus" means the shear modulus measured per ASTM D5992.

[0025] "Elongation at Break" means the tensile elongation as measured by ASTM D412-98a and conducted at 100⁰C rather than ambient.

[0026] "Equator" means the radial position corresponding to the point of maximum width or axial extent of the exterior surface of the tire.

[0027] "Equivalent homogeneous thickness" means the cross sectional area of an individual cord as used in a ply divided by the pace of such cords used in the same ply.

[0028] "Hysteresis" means the dynamic loss tangent (max tan δ). The dynamic characteristics of the materials are measured on an MTS 831 Elastomer Test System in accordance with ASTM

D5992. The response of a sample of vulcanized material (cylindrical test piece of a thickness of

4 mm and a section of 400 mm ), subjected to an alternating single sinusoidal shearing strain, at a frequency of 10 Hz and at 80 ⁰C, is recorded. Scanning is effected at an amplitude of deformation of 0.1 to 50% (outward cycle), then of 50% to 0.1% (return cycle). The maximum shear modulus G* max in MPa and the maximum value of the tangent of the loss angle tan delta (max tan δ) is determined during the outward cycle.

[0029] "Infinite endurance limit" means the referenced material can undertake at least 1 million cycles of a specified compressive strain without losing more than 10 percent of its tensile modulus, as determined by the following test method: Using a Zwick 1841, a cord or strand of the material is placed into a loop with one end of the cord anchored and the other end attached to an imposed, cyclic displacement. The loop is maintained between two sheets of TEFLON. The starting diameter of the loop is determined by placing the loop in a compressive strain ε_c of 0.67 percent , where ε_c= D / (2R)

D = diameter of the cord or strand of material

R = (0.5)(starting diameter of the loop)

The material is then subjected, in a cyclic manner, to a predetermined compressive strain at a frequency of 3 Hz. Cycling is continued until the measured force at the compressive strain of 0.67 percent falls to 90 percent of its initial value, which indicates the properties of the material are beginning to degrade. The number of cycles at which this occurs is then determined. If the material can withstand at least 1,000,000 cycles at a specified compressive strain before falling to 90 percent, then as used herein, the material has an "infinite endurance limit" at the specified compressive strain.

[0030] "Modulus" of the membranes means the compressive modulus of elasticity at 1% compression in the circumferential direction multiplied by the effective thickness of the membrane. This modulus can be calculated by Equations 1 or 2 below and is denoted with a prime C) designation. [0031] "Pace" as used herein means the distance between cords in a given ply.

DETAILED DESCRIPTION

[0032] Figs. 1 and 2 illustrate an exemplary embodiment of a run-flat tire 100 as may be constructed according to the present invention. The tire defines a circumferential plane (an example is designated as CP in Fig. 1), an equator E, an axial direction with the axially outward direction denoted by arrows A, and a radial direction with the radially outward direction denoted by arrow R. Tire 100 includes a crown portion 105 having a tread 110 that may be sculptured with grooves 115 or other features as are suitable for the intended use.

[0033] Along each side, tire 100 includes a sidewall portion 120, which may be constructed from any suitable rubber material. Each sidewall portion 120 extends between an axial outer edge 125 of the crown portion 105 and a bead portion 130. The sidewall portions 120 help protect a carcass 135 that extends between bead cores 140 located in each bead portion 130 of the tire. As shown, carcass 135 is radially inside of the tread 110 in the crown region 105 and is axially inside of the sidewall portion 120 along each side of tire 100. Carcass 135 also wraps around a respective bead core 140 and a bead apex 145 on each side of tire 100. The turn-up end 150 on each side of carcass 135 extends radially outward and eventually adjacent to carcass 135 itself. It should be understood that the present invention is not limited to a tire construction where the carcass is wrapped about the bead core; instead, the present invention includes a tire where the carcass ends, or is anchored in, the bead portion as well. Carcass 135 may be constructed from a variety of materials including, by way of example only, various textile materials. For example, carcass 135 may be constructed as a composite from polyester, nylon, or rayon. For this exemplary embodiment, tire 100 also includes an air impermeable inner liner 152 covering the inner surface of tire 100. Inner liner 152 may be constructed from any suitable material capable of retaining the tire's inflation pressure and is preferably constructed from a halobutyl rubber.

[0034] As illustrated in Fig. 1, tire 100 includes a pair of sidewall reinforcing members 155. Each member 155 is located along a side of the tire 100 at a position that is axially inward of the carcass 135 but axially outward of inner liner 152. The reinforcing members 155 extend radially from a bead portion 130 to radially inside of an annular shear band 160 in crown portion 105. Each reinforcing member 155 has a "uniform" thickness along at least a portion of its length. More specifically, each reinforcing member 155 has a uniform thickness Ti along that portion of its length Li, which is that portion extending between a position adjacent to the radially outermost extent 165 of bead apex 145 to a position radially inward of the axial end 170 of annular shear band 160. Desirably, for this exemplary embodiment of the present invention, thickness T₍ is in a range of about 4 mm to about 8 mm. As used herein within regard to the reinforcing members, "uniform" includes deviations of plus or minus 2 mm. For example, a reinforcing member 155 having a thickness of 5 mm ± 2 mm has a "uniform" thickness within the meaning of the present invention.

[0035] Bead apex 145 is constructed with a predetermined height relative to the size of tire 100. As shown in Fig. 1, L₂ is the distance between the radially outermost extent 165 of bead apex 145 and the equator E of tire 100. Preferably, the distance L₂ for tire 100 is preferably between about 5 mm and 25 mm. [0036] Reinforcing members 155 are desirably constructed from rubber materials having a dynamic shear modulus greater than about 2 MPa but less than about 5 MPa. It is also desirable to construct reinforcing members 155 from one or more materials having an Elongation at Break at 100⁰C of greater than about 100 percent and a hysteresis of less than about 0.2 at strains between about 15 percent and about 30 percent.

[0037] As previously referenced, tire 100 includes a shear band 160. In order to reduce the necessary thickness of sidewall reinforcing members 155, the load borne by these members 155 during run-flat operation is reduced by having at least a portion of the vehicle load carried by way of annular shear band 160, which is constructed in a manner that also helps control buckling. Continuing with Figs. 1 and 2, shear band 160 is shown to include several layers in the exemplary embodiment of tire 100, including a shear layer 175 that is positioned between a first membrane 180 and a second membrane 195. Shear layer 175 is preferably constructed from rubber materials having a dynamic shear modulus greater than about 2 MPa but less than about 5 MPa, an Elongation at Break at 100⁰C of greater than about 100 percent, and a hysteresis of less than about 0.2 at strains between about 15 percent and about 30 percent. Using a construction for second membrane 195 as described below, shear layer 175 can have a thickness of between about 1 mm and about 5 mm. Preferably, in one exemplary embodiment of the present invention, shear layer 175 has a thickness of about 2 mm and a dynamic shear modulus of about 2 MPa.

[0038] First membrane 180 comprises a first ply 185 and second ply 190. Second membrane 195 is comprised of a third ply 197. The third ply 197 is offset radially inward from the bottom of the tread grooves 115 a sufficient distance to protect the structure of the second membrane 195 from cuts and small penetrations of the tread 110. The offset distance may be increased or decreased depending on the intended use of the tire.

[0039] As illustrated in Fig. 2 for example, plies 185, 190, and 197 each comprise essentially inextensible cables or cord reinforcements 186, 191, and 196, respectively. Preferably, each cord 186, 191, and 196 is embedded in an elastomeric coating. For a tire constructed of elastomeric materials, first membrane 180 and second membrane 195 are adhered to shear layer 175 by the vulcanization of the elastomeric materials. Alternatively, first and second membranes 180 and 195 may be adhered to shear layer 175 by any suitable method of chemical or adhesive bonding or mechanical fixation.

[0040] The cord reinforcements 186 and 191 of first membrane 180 maybe any of several materials suitable for use as tire belt reinforcements in conventional tires such as monofilaments or cords of steel, aramid, or other high modulus textiles. Although the variations of the invention disclosed herein have cord reinforced layers for each of the membranes 180 and 195, any suitable material may be employed for the membranes which meets the requirements for the tensile stiffness, bending stiffness, and compressive buckling resistance properties required of the annular shear band 160 as described herein. For example, the structure of membranes 180 and 195 may be any of several alternatives such as a homogeneous material, a fiber reinforced matrix, or a layer having discrete reinforcing elements provided the mechanical properties described herein are met.

[0041] For first membrane 180, first ply 185 and second ply 190 have essentially parallel cords 186 and 191. However, cords 186 are oriented at an angle -α relative to the circumferential plane CP of tire 100 while cords 191 are oriented at an angle +α relative to the circumferential plane CP such that cords 186 and 191 have an opposite angle relative to circumferential plane CP and cross one another. Desirably, the absolute value of angle α will be about 20° or less. Cords 186 and 191 are not required to be oriented at mutually equal and opposite angles α. For example, it may be desirable for the cords of the layer pairs to be asymmetric relative to the tire circumferential plane CP.

[0042] For second membrane 195, cords 196 are oriented in a manner that is substantially parallel to the circumferential plane CP of tire 100. One of ordinary skill in the art, using the teachings disclosed herein, will understand that the process of manufacturing a ply such as that used in second membrane 195 may include wrapping cords 196. As such, cords 196 are inevitably offset from the circumferential plane CP by some extent such that slight angles of, for example, up to about 5°, are still considered substantially parallel within the scope of the present invention.

[0043] in this exemplary embodiment of the present invention, cords 196 in third ply 197 are dispersed in a unique manner and are constructed from one or materials having certain mechanical properties. More specifically, cords 196 preferably have a compressive modulus E_c of about 12,000 MPa. Alternatively, cords 196 of a higher compressive modulus may be used provided such is at least about 12,000 MPa. Preferably, cords 196 have a tensile module E_t of about 40,000 MPa; a higher tensile modulus may be used provided such is at least about 40,000 MPa. Desirably, cords 196 have an infinite endurance limit at about 1.0 percent compressive strain. Cords 196 with an infinite endurance limit at higher compressions may also be used. When constructed with the above mechanical properties, each cord 196 preferably has a cross- sectional area of about 0.43 mm or larger and are arranged at a pace of at least about 1.4 mm. Such construction provides an equivalent homogenous thickness (area per cord/pace) of about 0.30 mm² per mm width of ply 197. Nevertheless, regardless of how third ply 197 is constructed, it is desirable that the third ply 197 be able to repeatedly bear a compressive stresses during run-flat conditions of at least about 3600 Newtons per mm of width of the third ply 197 without incurring significant damage. Along with reinforcing members 155 and shear layer 175 constructed as described above, such a construction for third ply 197 provides a run-fiat tire 100 capable of a reasonable period of extended mobility without adding unnecessary thickness and weight to tire 100.

[0044] Any suitable material meeting the mechanical requirements described above may be used for the cords 196 of third ply 197. A particularly desirable construction for the cords 196 of the present invention is described in U.S. Patent No. 7,032,637. More specifically, cords 196 may be constructed from an elongate composite element of monofilament appearance, comprising substantially symmetrical fibers that are of great lengths. The fibers are impregnated in a thermoset resin having an initial modulus of extension of at least about 2.3 GPa. The fibers are all configured as parallel to each other. The elongate composite element has an elastic deformation in compression at least equal to 2% and having in flexion a breaking stress in compression greater than the breaking stress in extension. The fiber so used to construct cord 196 may be, for example, one of various glass fibers. It is also possible to use a hybrid assembly comprising glass fibers. Preferably, the thermoset resin has a glass transition temperature T_g greater than 130° C.

[0045] The cords 186, 191, and 196 of plies 185, 190, and 197, respectively, are embedded in an elastomeric coating layer. It is preferred that the dynamic shear modulus of the coating layers IS

be greater than the dynamic shear modulus of the shear layer 120 to insure that deformation of the annular band is primarily by shear deformation within shear layer 120. For example, the elastomeric coating layer may have a dynamic shear modulus of about 20 MPa.

[0046] The relationship between the dynamic shear modulus G of the shear layer 175 and the effective circumferential modulus E'_mcπibra_ne of the first and second membranes 180 and 195 in compression controls the deformation of the annular shear band 160 under an applied load such as, for example, when tire 100 is operating under a loss of inflation pressure. For reference, a description of a shear layer as used in a non-pneumatic tire is disclosed in U.S. Patent No.

6,769,465, which is owned by applicant's assignee.

[0047] The effective circumferential modulus E'MEMBRAN_E of the first membrane 180, when in compression, using conventional tire belt materials can be estimated by the following:

(1)

_£. Y]

^MEMBRANE

JJ

Where:

E_RUBBER ⁼ compressive modulus of the coating material

P = cord pace (cord centerline spacing) measured perpendicular to the cord direction

D = cord diameter v = Poisson's ratio for the coating material α = cord angle with respect to the equatorial plane t = rubber thickness between cables in adjacent layers

Note that E'_MEMBRANE is the elastic modulus of the membrane times the effective thickness of the membrane. Equation 1 should be used for plies where the cord angle α is greater than about 10° from the circumferential plane of the tire.

[0048] The effective circumferential modulus E'MEMBRAN_E of the second membrane 195, when in compression, can be estimated by the following: (2)

E'MEMBRANE = Ec * V * t MEMBRANE Where:

Ec = compressive modulus of the cord

V = volume fraction of the cord in the membrane t _MEMBRANE ⁼ thickness of the membrane

For membranes comprising a homogeneous material or a fiber or other material reinforced matrix, the modulus is the compressive modulus of the material or matrix. Equation 1 should be used for plies where the cord angle α is less than about 10° from the circumferential plane of the tire.

[0049] When the ratio E'_MEMBRANE / G is relatively low, deformation of the annular band 160 under load approximates that of the homogeneous band and produces a non-uniform ground contact pressure for tire 100 operating in a run-flat mode. On the other hand, when the ratio E'M_EMBR_ANE / G is sufficiently high, deformation of the annular band 160 under load is essentially by shear deformation of shear layer 175 with little longitudinal extension or compression of the first and second membranes 180 and 195. In such case, ground contact pressure is substantially uniform for annular shear band 160 and thus tire 100 when operating under a loss of inflation pressure, i.e. run-flat mode. Additionally, when E'_MEMBRANE / G is sufficiently high, annular shear band 160 can effectively assist sidewall reinforcing members 155 with bearing vehicle load during run-flat operation. Thus, regardless of the manner in which first or second membranes 180 and 195 are constructed, the effective circumferential modulus E'MEMBRAN_E in compression for either membrane should be at least 100 times greater than the dynamic shear modulus G of shear layer 175 and, desirably, at least about 1000 times greater. This relationship insures that during run-flat operation of tire 100 under an applied load, first and second membranes 180 and 195 maintain a substantially constant length and relative displacement of these membranes occurs substantially by shear strain in shear layer 175.

[0050] A tire 300 according to another exemplary embodiment of the present invention is illustrated in Fig. 3. Tire 300 is similar to the exemplary embodiment of tire 100 shown in Fig. 1 with the exception of the annular shear band 360. As shown, instead of two plies, first membrane 380 is constructed of a single ply 397 in a manner similar to that described with regard to third ply 197 of tire 100. For example, cords 396 are oriented in a manner that is substantially parallel to the circumferential plane CP and are constructed from materials exhibiting physical properties as described with regard to cords 196. First membrane 380 should be capable of bearing compressive stresses of at least about 3600 Newtons per mm of width without significant damage to its construction. Second membrane 395 has two plies 385 and 390 that are constructed, for example, as previously described with regard to plies 185 and 190. Along with the reinforcing members 355 constructed as described with regard to members 155, such a construction for annular shear band 360 provides a run-flat tire 300 capable of a reasonable period of extended mobility without adding unnecessary thickness and weight. [00511 Sidewall reinforcing members 155 and 355 may be constructed as a single layer of material as shown in Figs. 1 and 3. Alternatively, Fig. 4 shows another exemplary embodiment of a tire 400 according to the present invention in which sidewall reinforcing members 455 are actually constructed from a plurality of strips 456. The plurality of strips 456 are arranged such that each reinforcing member 455 has a uniform thickness Ti along that portion of its length L|, which is that portion extending between a position adjacent to the radially outermost extent 465 of bead apex 445 to a position radially inward of the axial end 470 of annular shear band 460. As before, preferably thickness Ti is in a range of about 4 mm to about 8 mm. Reinforcing members 455 are desirably constructed from rubber materials having a dynamic shear modulus greater than about 2 MPa but less than about 5 MPa. It is also desirable to construct reinforcing members 455 from one or more materials having an Elongation at Break at 100⁰C of greater than about 100 percent and a hysteresis of less than about 0.2 at strains between about 15 percent and about 30 percent. The remaining components of tire 400 may be constructed as described, for example, with regard to tire 100.

[0052] It should be understood that the present invention includes various modifications that can be made to the exemplary embodiments described herein that come within the scope of the appended claims and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. A radial run-flat tire having an equator and defining axial and radial directions, the run- flat tire comprising: a crown portion having a tread; a shear layer disposed radially inward of said tread, said shear layer having a dynamic shear modulus; a first membrane adhered to a radially inward extent of said shear layer; a second membrane adhered to a radially outward extent of said shear layer, wherein a ratio of the circumferential modulus in compression of one of said membranes to said dynamic shear modulus of said shear layer is at least about 100 to 1 ; a pair of axially spaced-apart, annular bead portions, each said bead portion having a bead core and a bead apex, wherein the radially-outermost extent of said bead apex is positioned at a predetermined distance below the equator; a pair of sidewall portions with each said sidewall portion extending radially between a respective axial edge of said crown portion and a respective said bead portion, each said sidewall portion having a reinforcing member extending radially outward from a position contiguous with a respective said bead apex to a position radially inward of said crown portion, wherein each said reinforcing member is of a uniform thickness over that portion extending between a respective said bead apex to a position adjacent a respective end of said first membrane; and a carcass layer disposed radially inward from said tread portion, axially inward of said pair of sidewall portions, and axially outward of each said reinforcing member along each respective said sidewall portion, said carcass layer extending between said pair of bead portions and being anchored in said bead portions.

2. A radial run-flat tire according to claim 1, the tire defining a circumferential plane, wherein said first membrane comprises a first ply having first ply cords that are oriented at a first angle of about 20 degrees or less from the circumferential plane; a second ply having second ply cords that are oriented at a second angle of about 20 degrees or less from the circumferential plane and in a manner that is opposite to said first angle such that said first ply cords and said second ply cord cross, and said second membrane comprises a third ply having third ply cords that are oriented substantially parallel to the circumferential plane.

3. A radial run-flat tire according to claim 2, wherein said third ply cords have a compressive modulus of about 12,000 MPa.

4. A radial run-flat tire according to claim 3, wherein said third ply cords have a tensile modulus of about 40,000 MPa.

5. A radial run-flat tire according to claim 4, wherein said third ply cords have an infinite endurance limit at a compressive strain of about 1 percent or less.

6. A radial run-flat tire according to claim 1, wherein said third-ply cords have an equivalent homogenous thickness of about 0.30 mm² per mm width of said third ply.

7. A radial run-flat tire according to claim 1, wherein said dynamic shear modulus of said shear layer is greater than about 2 MPa but less than about 5 MPa.

8. A radial run-flat tire according to claim 1, wherein said shear layer has a thickness along the radial direction of greater than about 1 mm but less than about 5 mm.

9. A radial run-flat tire according to claim 1, wherein said dynamic shear modulus of said shear layer is about 2 MPa and wherein said shear layer has a thickness along the radial direction ofabout 2 mm.

10. A radial run-flat tire according to claim 1 , wherein said predetermined distance by which the radially-outermost extent of said bead apex is positioned below the equator is between about 5 mm and about 25 mm.

11. A radial run-flat tire according to claim 1, wherein each said reinforcing member along each said sidewall portion has a uniform thickness in a range of about 4 mm to about 8 mm.

12. A radial run-flat tire according to claim 11, wherein each said reinforcing member along each said sidewall portion is comprised of a plurality of rubber-based portions.

13. A radial run-flat tire according to claim 1 , wherein the Elongation at Break at 100⁰C for each said reinforcing member is greater than about 100 percent.

14. A radial run-flat tire according to claim 13, wherein the hysteresis for each said reinforcing member is less than about 0.2 at strains between about 15 percent and about 30 percent.

15. A radial run-fiat tire according to claim 1 , wherein the Elongation at Break at 100⁰C for said shear layer is greater than about 100 percent.

16. A radial run-flat tire according to claim 1, wherein the hysteresis for said shear layer is less than about 0.2 at strains between about 15 percent and about 30 percent.

17. A run-flat tire, the tire having an equator and defining axial and radial directions, the tire comprising: a crown portion having a tread; a shear layer disposed radially inward of said tread, said shear layer having a dynamic shear modulus and having a radial thickness greater than about 1 mm but less than about 5 mm; a first membrane adhered to a radially inward extent of said shear layer; a second membrane adhered to a radially outward extent of said shear layer, wherein a ratio of the circumferential modulus in compression of either said membranes to said dynamic shear modulus of said shear layer is at least about 100 to 1; a pair of axially spaced-apart, annular bead portions, each said bead portion having a bead core and a bead apex, wherein the radially-outermost extent of said bead apex is positioned at a predetermined distance below the equator; a pair of sidewall portions with each said sidewall portion extending radially between a respective axial edge of said crown portion and a respective said bead portion, each said sidewall portion having a reinforcing member extending radially outward from a position contiguous with a respective said bead apex to a position radially inward of said crown portion, wherein each said reinforcing member is of a uniform thickness over that portion extending between a respective said bead apex to a position adjacent an end of said first membrane; and a carcass layer disposed radially inward from said tread portion, axially inward of said pair of sidewall portions, and axially outward of each said reinforcing member along each respective said sidewall portion, said carcass layer extending between said pair of bead portions and being anchored in said bead portions.

18. A run-flat tire as in claim 17, wherein said second membrane comprises reinforcing elements having an infinite endurance limit at a compressive strain of 1 percent or less.

19. A run-flat tire as in claim 17, wherein said second membrane is constructed from reinforcing elements arranged within said second membrane so as to have an equivalent homogenous thickness of up to about 0.30 mm² per mm width of said second membrane.

20. A run-flat tire as in claim 17, wherein said reinforcing elements comprise cords constructed from substantially symmetrical fibers impregnated in a thermoset resin.