Run-flat tyre

Abstract: A run-flat tyre comprises a tread portion (2), a pair of bead portions (4), a pair of sidewall portions (3), a carcass (6) comprising at least one carcass ply (6A, 6B) extending between the bead portions (3), a belt (7) disposed radially outside the carcass (6) in the tread portion (2), and a tread reinforcing layer (10) disposed between carcass plies (6A,6B) or alternatively between the carcass (6) and belt (7), the tread reinforcing layer (10) being made of a high elastic modulus rubber compound having a complex elastic modulus E* of from 10 to 30 MPa, a thickness t of from 0.8 to 2.5 mm, and an axial width WA of 0.7 to 1.0 times the tread width TW of the tyre.

Description: The present invention relates to a run-flat tyre in which the tread portion is prevented from buckling to improve run-flat performance. In general, as shown in Fig.5, run-flat tyres have a great mass of rubber (b) along the inside of the sidewall portions (a) to support tyre loads during running under flat conditions. Thus, an increase in tyre weight is unavoidable. Further, due to the heat generated during running under flat conditions as well as normal conditions especially at a high speed, the internal temperate is liable to become very high, which sometimes decreases the durability of the sidewall portions contrary to expectation. Therefore, it is preferable to decrease the rubber volume in the sidewall portions as far as possible. However, if the rubber volume is decreased, the bending deformation of the sidewall portion (a) becomes relatively large as shown in Fig.5. Accordingly, the heat generation increases and the durability decreases again. Thus, it is difficult to improve run-flat performances and durability at the same time. It has been proposed in DE 2814937 showing the preamble of claim 1 to improve run-flat performance in motorcycle tyres and in particular the stability of the carcass when the motorcycle is banked over on cornering by further reinforcing the tread region of a tyre having sidewalls reinforced by rubber inserts in combination with the provision of a flexible zone between the top of each sidewall and the edge of the tread. In this disclosure the tread reinforcement is by means of a layer having a high modulus. In order to improve the run-flat performance and durability, the inventors discovered that when the tyre air pressure is zero the bending deformation (ya) of the sidewall portion (a) has a close relation to the buckling deformation (yt) of the tread portion (t). Fig.4 shows such a relationship, wherein the bending deformation (ya) of the sidewall portion is indicated by the clearance (ha) between the inner surfaces of the tyre measured in a radial direction at a distance (f) of 10 mm axially inwardly from the edge (e) of the ground contacting area as shown in Fig.5. Thus, the smaller the clearance, the larger the bending deformation. The buckling deformation (yt) of the tread portion is indicated by the amount of lift-up (ht) measured at the tyre equator from the road surface to the inner surface of the tyre. Thus, the larger the lift-up, the larger the buckling deformation. As shown in Fig.4, the bending deformation of the sidewall portion was found to increase as the buckling deformation of the tread portion increases. Therefore, if the buckling deformation is decreased, the bending deformation can be decreased. Further, it was also discovered that it is not effective for controlling the buckling deformation to dispose a reinforcing cord layer in the tread portion. The reinforcing cord layers such as a belt, carcass and the like do not show their rigidity until the tyre is inflated to a normal pressure when a full tension is applied to the cords. Thus, it is difficult to increase the resistance to buckling deformation under flat conditions as no pressure induced tension is present. It is therefore an object of the present invention to provide a run-flat tyre in which, by increasing the resistance to buckling deformation of the tread portion under flat conditions, deformation of the sidewall portions can be decreased without increasing the sidewall rubber volume, and thereby run-flat performances and durability are improved at the same time. According to the present invention, a run-flat tyre comprises a carcass comprising at least one carcass ply extending between a pair of bead portions through a tread portion and a pair of sidewall portions, a belt disposed radially outside the carcass in the tread portion, a tread reinforcing layer disposed between carcass plies or alternatively between the carcass and belt, the tread reinforcing layer being made of a high elastic modulus rubber compound having a complex elastic modulus of from 10 to 30 MPa, a thickness of from 0.8 to 2.5 mm, and an axial width of 0.7 to 1.0 times the tread width. Embodiments of the present invention will now be described in detail in conjunction with the accompanying drawings, in which: Fig.1 is a cross sectional view of an embodiment of the present invention; Fig.2 is a cross sectional view of the tread portion thereof showing a tread reinforcing layer disposed between carcass plies; Fig.3 is a cross sectional view of another example of the tread portion in which a tread reinforcing layer is disposed between the carcass and belt; Fig.4 is a diagram for explaining the relationship between buckling deformation of a tread portion and bending deformation of a sidewall portion; and Fig.5 is a diagram for explaining the ground contacting state of the prior art tyre when the tyre inner pressure is zero. In the drawings, a run-flat tyre 1 comprises a tread portion 2, a pair of sidewall portions, a pair of bead portions, a carcass 6 extending between the bead portions, a belt 7 disposed radially outside the carcass in the tread portion 2, a sidewall reinforcing rubber layer 9 disposed in each of the sidewall portions, and a tread reinforcing rubber layer 10 disposed in the tread portion 2. In this embodiment, the tyre 1 is a passenger radial tyre of which tyre size is 225/60R16. The carcass 6 comprises at least one ply 6A, 6B of cords arranged radially at an angle of 75 to 90 degrees with respect to the tyre equator C. For the carcass cords, organic fibre cords, e.g. nylon, rayon, polyester and the like and steel cords can be used. The carcass 6 is composed of two plies 6A and 6B, each extending between the bead portions and being turned up around the bead core 5 in each of the bead portions from the axially inside to the outside to form a pair of turnup portions 13 and a main portion 12 therebetween. As shown in Fig.1, the turnup portions 13B of the outer carcass ply 6B terminate near the tyre maximum width point, but the turnup portions 13A of the inner carcass ply 6A terminate in the tread portion 2 so as to overlap with the axially outer edge portions 7E of the belt 7. Thus, separation failures starting from the ends of the turnup portions 13A and 13B during run-flat operation can be effectively prevented, and the sidewall rigidity is increased. The above-mentioned belt 7 comprises at least one ply of high elastic modulus cords, such as steel, aromatic polyamide and the like, laid at an angle of 10 to 30 degrees with respect to the tyre equator C. In this embodiment, the belt 7 is composed of two crossed plies 7A and 7B the cord inclinations of which differ from each other. The widest belt ply which is the radially inner belt ply 7A in this embodiment has a ply width BW of from 0.95 to 1.05 times the tread width TW so as to provide a hoop effect for the full width of the tread portion 2. Here, the tread width WT is the maximum ground contacting width between the axial outermost edges of the ground contacting region under the condition where the tyre is mounted on its standard rim and inflated to its standard pressure, and then loaded with a standard load. The standard rim is the rim officially approved for the tyre by, for example JATMA (Japan), TRA (USA), ETRTO (Europe) and the like, and the standard inner pressure and the standard load are the maximum air pressure and the maximum tyre load for the tyre officially specified in the Air-pressure/Maximum-load Table by the same associations. Further, in the tread portion 2 in this embodiment, a band 11 is disposed radially outside the belt 7 to control belt lifting during high-speed normal running. The band 11 is made of cords extending at substantially zero degrees or very small angle to the circumferential direction. The example shown is composed of a pair of axially spaced edge bands 11A covering the belt edges 7E and a full-width band 11B extending over the full width of the belt 7, and each of bands 11A and 11B is formed by spirally winding nylon cords. The above-mentioned bead portions are each provided between the main portion 12 and turnup portion 13 of the carcass 6 with a bead apex 15. The bead apex 15 is made of a hard rubber compound having a JIS-A hardness of from 60 to 80 degrees. Further, between the main portion 12 and turnup portion 13 in this embodiment, the above-mentioned sidewall portions 3 are each provided with a sidewall reinforcing layer 9. On the other hand, between the main portions of the two carcass plies in the sidewall portions, there is no rubber layer except for topping rubber. On the inside of the carcass, only a thin inner liner for air sealing is disposed. The sidewall reinforcing layer 9 is made of a hard rubber compound having a JIS-A hardness of from 60 to 80 degrees, which is however, slightly softer than the bead apex rubber 15. The bead apex 15 extends radially outwardly from the bead core 5 to a radial height between the maximum tyre section width point and the height of the flange of the rim. The sidewall reinforcing layer 9 extends from a position near the radially outer end of the rim flange to a position below the belt edge in the tread portion along the axially outer side of the bead apex 15. The radially outer edge part of the bead apex 15 and the radially inner edge part 9C of the layer 9 are tapered to splice to each other. The radially outer edge part 9B of the layer 9 is also tapered, and terminates near the end of the turnup portions 13A, and thus overlaps with the belt edge 7E. The sidewall reinforcing layer 9 has a substantially constant thickness T in its central part 9A around the tyre maximum section width point. In this embodiment, the thickness T is set in the range of from 3 to 5 mm to minimise the rubber volume. As explained above, the sidewall reinforcing layer 9 and bead apex rubber 15 are united in one body and wrapped up in the carcass main portion and turnup portion to form a shell-like structure. Thus, the sidewall rigidity under flat conditions is effectively improved. It is preferable to dispose the sidewall reinforcing layer between the carcass main portion and turnup portion. However, it may be disposed axially inside of the carcass 6. The above-mentioned tread reinforcing layer 10 is disposed between cord plies in the tread portion 2. Preferably it is disposed between the radially adjacent carcass plies 6A and 6B as shown in Fig.2. In this connection, the carcass 6 is composed of at least two plies. The tread reinforcing layer 10 is made of a high elastic modulus rubber compound having a complex elastic modulus E* of from 10 to 30 MPa. On the other hand, the topping rubbers for the carcass plies and belt plies have a complex elastic modulus E* in the range of not more than 7.8 MPa, usually 4 to 7 MPa. Accordingly, the tread reinforcing layer 10 is greatly increased in complex elastic modulus E* from the adjacent topping rubber. The complex elastic modulus E* was measured with a viscoelastic spectrometer of Iwamoto Seisakusyo make under the following conditions: temperature = 70 degrees C, frequency = 10Hz, and dynamic distortion = 2%. The thickness t of the tread reinforcing layer 10 is in the range of from 0.8 to 2.5 mm, preferably 1.2 to 2.5 mm. The axial width WA thereof is in the range of from 0.7 to 1.0 times, preferably 0.7 to 0.95 times the tread width TW. It is preferable to provide a certain space between the axial edges of the tread reinforcing layer 10 and the radially outer edges of the sidewall reinforcing layers 9 so they do not overlap each other. The hardness of the tread reinforcing layer 10 is set to be more than the above-mentioned topping rubber which usually has a JIS-A hardness of 60 to 75 degrees, but substantially the same as the sidewall reinforcing layer 9. Therefore, the tread reinforcing layer 10 together with the adjacent cord plies greatly improves the bending rigidity of the tread portion to prevent the tread portion 2 from buckling. If the complex elastic modulus E* is less than 10 MPa, the thickness t is less than 0.8 mm, and/or the width WA is less than 0.7 times the tread width TW, the buckling deformation controlling effect decreases, and the durability is not effectively improved. If the complex elastic modulus E* is more than 30 MPa, and/or the thickness t is more than 2.5 mm, the ride comfort and noise during normal running deteriorate. If the width WA is more than 1.0 times the tread width TW and particularly when the tread reinforcing layer 10 overlaps the sidewall reinforcing layer 9, the rigidity balance is lost and the buckling deformation is increased. The tread reinforcing layer 10 can be disposed between the carcass 6 and the belt 7 as shown in Fig.3. It is however not preferable to dispose the layer between the belt plies 7A and 7B. If it is disposed between the belt plies, as the crossing cords of the adjacent plies are separated thereby, the rigidity of the belt as a whole decreases. Therefore, the rigidity increased by the tread reinforcing layer 10 is cancelled, and the buckling deformation controlling effect can not be obtained. Further, a short-fibre reinforced rubber may be used as the tread reinforcing layer 10. Test tyres of size 225/60R16 having the structure shown in Fig.1 and the specifications given in Table 1 were made and tested for run-flat performance as follows. A test car, a FR (front engine, rear drive) passenger car provided on the right front wheel (rim size = 16X71/2JJ) with a test tyre (air pressure = zero) was run at a speed of 80 km/h until the tyre was broken, and the running distance (km) was measured as the durability. The tyre load was 585 kg. Also, deformations of the tyre were measured. As the buckling deformation of the tread portion, the distance ht shown in Fig.5 was measured. As the bending deformation of the sidewall portion, the clearance ha shown in Fig.5 was measured. It was confirmed from the test that Example tyres 1 to 4 having a tread reinforcing layer disposed between the carcass plies or between the carcass and belt were greatly improved in the durability in comparison with Reference tyre 1 having no tread reinforcing layer and Reference tyre 2 having no tread reinforcing layer between belt plies. As described above, in the present invention, the buckling deformation of the tread portion is effectively decreased, and as a result the bending deformation of the sidewall portions is decreased. Therefore, the run-flat performances can be improved without increasing the tyre weight.

Claims: A run-flat tyre comprising a tread portion (2), a pair of bead portions, a pair of sidewall portions, a carcass (6) comprising at least one carcass ply (6A) extending between the bead portions, a belt (7) disposed radially outside the carcass (6) in the tread portion (2), a tread reinforcing layer (10) disposed between carcass plies or alternatively between the carcass (6) and the belt (7), the tread reinforcing layer (10) having a thickness (t) of from 0.8 to 2.5 mm and an axial width (WA) of 0.7 to 1.0 times the tread width TW of the tyre, characterised in that the tread reinforcing layer (10) is made of a high elastic modulus rubber compound having a complex elastic modulus e* of from 10 to 30 MPa when measured under a temperature of 70 degrees C, a frequency of 10Hz and a dynamic distortion of 2%. A run-flat tyre according to claim 1, characterised in that the sidewall portions are each provided with a sidewall reinforcing layer (9) made of a rubber compound adjoining a carcass main portion and comprising a tapered radially outer part and a tapered radially inner part. A run-flat tyre according to claim 2, characterised in that said sidewall reinforcing layer (9) has a radially outer end spaced apart from one of the axially outer ends of the tread reinforcing layer. A run-flat tyre according to claim 1, 2 or 3, characterised in that said carcass (6) comprises two carcass plies (6A,6B) each extending between the bead portions and turned up around a bead core (5) in each of the bead portions from the axially inside to outside of the tyre so that each of the two carcass plies (6A,6B) comprises a pair of turnup portions and a main portion therebetween, said tread reinforcing layer (10) is disposed between said two carcass plies (6A, 6B), the turnup portions of at least one of said carcass plies extend from the bead portions to the tread portion so that the edges thereof are secured under the belt (7), said sidewall portions are each provided with a sidewall reinforcing layer (9) disposed between one of said turnup portions extending to the tread portion and the axially inwardly adjacent carcass ply main portion, said sidewall reinforcing layer (9) is made of a hard rubber compound having a JIS-A hardness of from 60 to 80 degrees, and said sidewall reinforcing layer (9) tapers radially inwardly and outwardly and the maximum thickness thereof is in the range of from 3 to 5 mm. A run-flat tyre according to claim 4, characterised in that said sidewall portions are provided on the axially inside of the carcass (6) with only a thin inner liner for air sealing, and said sidewall portions are provided between the main portions (6A,6B) of said two carcass plies with no rubber layer other than topping rubber for the carcass plies.

Inventors: OMOTEDA MASANORI, TAGASHIRA KENJI, YOSHIOKA TETSUHIKO, OMOTEDA, MASANORI, TAGASHIRA, KENJI, YOSHIOKA, TETSUHIKO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1998
Main classification: B60C17
IPC codes: B60C17, B60C13, B60C9, B60C15