Run-flat tyre system for motorcycles

Abstract: A run-flat tyre system for a motorcycle characterised by a tyre comprising a tread portion (T), a pair of sidewall portions (9) and a pair of bead portions (B) defining a tyre cavity (4) when mounted on a wheel rim (3), the tyre cavity (4) having a maximum section width (Wi) when the tyre is inflated to standard pressure, the tyre cavity (4) having a maximum diameter (dd) and a sectional area (Sd) when the tyre is not inflated, a buffer (6) comprising an annular body of an elastic closed-cell material disposed in the tyre cavity (4), said buffer (6) being provided in a radially inner surface (6i) with a circumferentially extending groove (6a), a tube (5) disposed in said circumferentially extending groove (6a), the outside diameter (D) of the buffer (6) in its free state being in the range of from 0.90 to 1.0 times said maximum diameter (dd), the maximum section width (W) of the buffer (6) in the free state being in the range of from 1.05 to 1.2 times said maximum section width (Wi), and the gross sectional area (S) of the buffer (6) in the free state inclusive of the sectional area of the groove (6a) being in the range of from 1.0 to 1.15 times said sectional area (Sd).

Description: The present invention relates to a run-flat tyre system for motorcycles, more particularly to a buffer inserted between a tyre and a tube, capable of improving the resistance to puncture and the run-flat performance. On motorcycles particularly sports bikes such as off-road bikes or motocross bikes, tyres are used under a relatively low pressure in comparison with road going bikes. As a result, the engaging force between the beads of the tyre and the bead seats of the wheel rim is decreased, and air leakage is liable to occur. Therefore, the tyres are usually used together with a tube. When such a tyre undergoes an excessive tyre load, tyre deformation becomes excessively large due to the low pressure, and the tread rubber sometimes collides with the rim through the tube. Therefore, if sharp or pointed objects such as stones, rocks, stubs and the like exist, the tread rubber is liable to be cut therewith, and sometimes the tube is punctured. As countermeasures it has been proposed to 1) increasing the tread rubber thickness, 2) increase the tube thickness, or 3) to fill the inside of the tyre with poly-urethane foam instead of a tube. However, measures 1) and 2) greatly deteriorate the steering stability and ride comfort, and it is difficult to completely prevent punctures. In measure 3), as the poly-urethane is pressed into the tyre, it is very difficult to mount the tyre on the rim, and further, during running the urethane generates heat and is liable to melt down. The above-mentioned conventional tube is hereinafter called standard tube to distinguish from a tube used in this invention. For the purpose of solving those problems, in laid-open Japanese patent applications JP-A-3-231008, JP-A-3-292206 and JP-A-8-164719, which corresponds to the preamble of claim 1, we have proposed to use sponge rubber as a buffer and to wrap the tube therein. On the basis of those propositions, the present invention was made in order to further improve the run-flat performance and the durability of a buffer. It is therefore an object of the present invention to provide a run-flat tyre system for motorcycles, in which the durability of a buffer and the run-flat performance are improved. Accordingly the present invention provides a run-flat tyre system for a motorcycle as set out in claim 1. Further features of the run-flat tyre system are disclosed in claims 2 to 6. The invention also includes buffer for the run-flat tyre system set out above. 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 assembly of a tyre, wheel rim, buffer and tube under an inflated state showing an embodiment of the present invention; and Fig.2 shows the buffer under its free state, an assembly of the tyre, rim and standard tube under a standard inflated state, and the same assembly under a non-inflated state, overlapping each other. The run-flat tyre system of Fig.1 comprises a tyre 2, a buffer 6 and a tube 5. The tyre 2 is a pneumatic tyre for motorcycles, which comprises a tread portion T, a pair of bead portions B and a pair of sidewall portions S, and of which the maximum cross sectional width lies between the tread edges. The tyre has a carcass reinforced by at least one ply of organic fibre cords for example. (not shown) In this example, the tread portion T consists of blocks (b) to provide a block pattern which is suitable for off-the-road running. Hereinafter a closed annular space 4 surrounded by the inner surface of the tyre 2 and the inner surface of a wheel rim 3 when the tyre is mounted on the rim 3 is called the tyre cavity. The wheel rim 3 is a standard rim which comprises a pair of flanges 3b, a pair of bead seats 3a, and a rim well 3c between the bead seats 3a. Here, the standard rim is the "standard rim" specified in JATMA, the "Measuring Rim" in ETRTO, the "Design Rim" in TRA or the like. The standard pressure which is referred to later is the "maximum air pressure" in JATMA, the "Inflation Pressure" in ETRTO, the maximum pressure given in the "Tyre Load Limits at Various Cold Inflation Pressures" table in TRA or the like. The buffer 6 is an annular body made of an elastic closed-cell material which preferably has a specific gravity of 0.1 to 0.35, and the surface has a JIS C hardness (HS) of 10 to 35 degrees, preferably 10 to 30 degrees. As the elastic closed-cell material, preferably an expanded rubber whose expansion rate is in the range of from 400 to 1500 %, more preferably 400 to 1100 % is used. For the rubber material therefor, preferably, butyl rubber compounds such as butyl rubber, brominated butyl rubber, halogenated butyl rubber and the like are used. The closed-cells are formed by using a forming agent together with other additives, reinforce agents, and the like during vulcanising. If the expansion rate of the buffer 6 is less than 400 %, the shock absorbing power decreases and the ride comfort is deteriorated. If the expansion rate is more than 1500 %, the buffer 6 becomes too soft and the tyre rigidity decreases. Therefore, the running stability is liable to be lost and further heat generation is liable to increase. The buffer 6 is provided on its radially inner surface 6i with a wide and deep groove 6a extending continuously in the circumferential direction to form a hollow in which the tube 5 is inserted. The groove 6a has a U-shaped sectional shape and the buffer 6 has a toroidal shape symmetrical about the equator C. The annular buffer 6 may be made for example by moulding a material as a one-piece annular body, bending a linear material and connecting the ends thereof each other into a ring, and connecting a plurality of segments into a annular body. The tube 5 is made of a gas-impermeable rubber compound with a butyl rubber base for example. The sizes, diameter and sectional area are considerably smaller in comparison with the standard tube for the tyre size. As shown in Fig.1, the buffer 6 is pushed into the tyre 2; the tube 5 is put in the circumferential groove 6a of the buffer 6; and the tyre 2 is mounted on the wheel rim 3. Then the tube 5 is inflated to the service pressure (about 0.7 to 1.5 kg/sq.cm for example). The inflated tube 5 presses the surface of the groove 6a to pressure the buffer 6, and the outside of the buffer 6 fits to and presses the inside of the tyre 2, and so the tyre 2 is inflated to its normal service condition. As explained above, the buffer 6 is to be inserted in the tyre cavity 4, but as shown in Fig.2 the outward form thereof is deliberately differed from the inward form of the tyre and rim assembly or tyre cavity as shown in Fig.2. In Fig.2, for easy comparison, the buffer 6 in its free state is shown, overlapping the tyre and rim assembly with a standard tube. As to the assembly, shown on the right hand side of the tyre equator C is a sectional shape under a non-inflated state in which the air valve of the standard tube is released, and on the left hand side of the tyre equator C, the sectional shape under an inflated state in which the standard tube is inflated to a standard pressure. The maximum outside diameter D of the buffer 6 under the free state is in the range of from 0.90 to 1.0 times, preferably 0.90 to 0.97 times the maximum diameter dd of the tyre cavity 4 of the assembly under a non-inflated state (right hand side in Fig.2). That is, D/dd is 0.90 to 1.0, preferably 0.90 to 0.97. When D/dd is less than 0.90, the contact pressure between the buffer 6 and the tyre 2 in the tread portion T is decreased if the tube 5 is punctured and the heat generation by friction therebetween increases to rise the temperature and the durability greatly decreases. When D/dd is more than 1.0, the heat generation by inner friction of the buffer insert increases and the durability tends to decrease. The maximum section width W of the buffer 6 in the free state is in the range of from 1.05 to 1.2 times the maximum section width Wi of the tyre cavity 4 of the assembly under the inflated state (left hand side in Fig.2). That is, W/Wi is 1.05 to 1.2. When W/Wi is less than 1.05, compression of the buffer 6 becomes insufficient in the sidewall portions S if the tube is punctured. As a result, it is difficult to provide a sufficient support for the sidewall portions, which decreases the load capacity, shock absorbing power and the durability. When W/Wi is more than 1.2, heat generation is liable to increase and the durability is decreased. The gross sectional area S of the buffer 6 under the free state is in the range of from 1.0 to 1.15 times the sectional area Sd of the tyre cavity 4 of the assembly under the non-inflated state. That is, S/Sd is 1.0 to 1.15. When S/Sd is less than 1.0 or more than 1.15, the durability of the buffer 6 decreases. Here, the gross sectional area S is the total of the actual sectional area of the buffer 6 and that of the groove 6a, which is defined as the area surrounded by the outline of the buffer 6 and a straight line L drawn between the radially innermost points 6b as shown in Fig.2. Further, the inside diameter of the buffer 6 in the free state measured at the radially inner ends 6b is set to be smaller than the diameter at the bead base line BL of the tyre or the rim diameter, thereby preventing the bead portions B from being chafed against the rim. The following table 1 shows the results of a durability test. The durability test was made using a tyre test drum. The test tyre was run at a speed of 65 km/hr under a tyre load of 100 kgf and the running distance was measured until abnormal vibration occurred. Test tyre: 110/90-19 motorcycle tyre Wheel rim: 19X2.15 standard rim Tyre cavity Non-inflated Inflated Max. Diameter (mm) dd = 635 di = 645 Section width (mm) Wd = 100 Wi = 91 Ex.1 Ex.2 Ex.3 Ref.1 Ref.2 Ref.3 Buffer D/dd 1 0.98 0.98 1.02 1 1 W/Wi 1.1 1.1 1.18 1.1 0.84 1.2 S/Sd 1.05 1.05 1.1 1.07 1 1.2 Drum durability Tube pressure 1.0 kgf/sq.cm 3000km 3000km 3000km 3000km 3000km 3000km A A A A A A 0 kgf/sq.cm 2000km 2000km 2000km 300km 200km 170km A A A B B B From the test, it was confirmed that Examples according to the present invention could be improved in run-flat performance and durability.

Claims: A run-flat tyre system for a motorcycle tyre comprising a tread portion (T), a pair of sidewall portions (9) and a pair of bead portions (B) defining a tyre cavity (4) when mounted on a wheel rim (3), the tyre cavity (4) having a maximum section width (Wi) when the tyre is inflated to standard pressure, the tyre cavity (4) having a maximum diameter (dd) and a sectional area (Sd) when the tyre is not inflated, a buffer (6) comprising an annular body of an elastic closed-cell material disposed in the tyre cavity (4), said buffer (6) being provided in a radially inner surface (6i) with a circumferentially extending groove (6a), a tube (5) disposed in said circumferentially extending groove (6a), the outside diameter (D) of the buffer (6) in its free state being in the range of from 0.90 to 1.0 times said maximum diameter (dd), the maximum section width (W) of the buffer (6) in the free state being in the range of from 1.05 to 1.2 times said maximum section width (Wi), characterised in that the gross sectional area (S) of the buffer (6) in the free state inclusive of the sectional area of the groove (6a) is in the range of from 1.0 to 1.15 times said sectional area (Sd), the inside diameter of the buffer (6) in the free state is smaller than the diameter at the bead base line of the tyre. A run-flat tyre according to claim 1, characterised in that the outward form of the buffer is differed from the inward form of the tyre. A run-flat tyre according to claim 2, characterised in that the outward form comprises a radially outer surface coming to a point at the centreline of the buffer. A run-flat tyre system according to claim 1, 2 or 3, characterised in that the buffer (6) has a specific gravity of 0.1 to 0.35, and the surface thereof has a JIS C hardness (HS) of 10 to 35 degrees. A run-flat tyre system according to claim 1, 2, 3 or 4, characterised in that the elastic closed-cell material is an expanded butyl rubber whose expansion rate is in the range of from 400 to 1500 %. A buffer (6) used in the run-flat tyre system as set forth in any of the preceding claims.

Inventors: IGARASHI YASUO, IGARASHI, YASUO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1998
Main classification: B60C17
IPC codes: B32B5, B60C5, B60C17, B60C19