Run-flat tyre system

Abstract: A run-flat tyre system comprising a tyre (2) and a tyre core (5) disposed therein, the tyre core (5) being an annular body of an elastic closed-cell material, the radially outside and inside of the tyre core each being provided with a circumferentially extending groove (6,7), the radially inner groove (7) extending radially outwardly beyond a bead core line (CL) drawn between the centres of the bead cores (C). When the tyre is mounted on a standard rim (3), the radially outer groove (6) forms a radially outer space (9) extending circumferentially of the tyre and having a cross sectional area of from 1 to 6 % of the total sectional area of the tyre cavity, and the radially inner groove (7) forms a radially inner space (10) extending circumferentially of the tyre having a cross sectional area (Si) of from 10 to 25 % of the total sectional area (S) of the tyre cavity.

Description: The present invention relates to a run-flat tyre system, more particularly to an improved tyre core disposed in a pneumatic tyre to support a tyre load when the tyre air pressure is low. As a measure for run-flat, the inside of a pneumatic tyre could be filled with an elastic material having rigidity in some degree which is enough to support the tyre load when the tyre air pressure is decreased. In this case, due to the rigidity of the filled material or tyre core, it is difficult to move or bend the bead portions axially inward. Such action is however necessary for mounting the tyre on a wheel rim. Therefore, it is difficult to mount the tyre on a rim. Further, during running, the outer surface of the tyre core rubs against the inner surface of the tyre. As a result, temperature rise occurs especially in the tread portion, and the raised temperature becomes high enough for the plastics and synthetic resins to be melted. It is therefore, an object of the present invention to provide a run-flat tyre system, in which the temperature rise of a tyre core is suppressed and the rim mounting operation is improved without decreasing the ability to run-flat. The document NL-A-7812531 discloses a run-flat system according the preamble of claim 1. According to one aspect of the present invention, a run-flat tyre system comprises a tyre and a tyre core disposed therein, the tyre having a toroidal shape and comprising a tread portion, a pair of sidewall potions and a pair of axially spaced bead portions each with a bead core therein and, the tyre core being an annular body of an elastic closed-cell material, characterised in that the radially outside and inside of the tyre core each are provided with a circumferentially extending groove, the radially inner groove extending radially outwardly beyond a bead core line being defined as a straight axial line drawn between the centres of the bead cores characterized in that when the tyre is mounted on a standard rim, the radially outer groove forms a radially outer space extending circumferentially of the tyre and having a cross sectional area which is 1 to 6 % of the total sectional area of the tyre cavity, and the radially inner groove forms a radially inner space extending circumferentially of the tyre having a cross sectional area which is 10 to 25 % of the total sectional area S of the tyre cavity. Incidentally, the tyre cavity is a cavity surrounded by the inner surface of the tyre and the inner surface of the wheel rim when the tyre is mounted on the rim. The standard rim is the "standard rim" specified in JATMA, the "Measuring Rim" in ETRTO, the "Design Rim" in TRA or the like. As a result the contact area of the tyre core with the tyre and wheel rim is decreased by the presence of the radially outer and inner grooves. Accordingly, the heat generated during running decreases. Further, generated heat is diffused by the radially inner and outer spaces, and heat break down of the tyre core is controlled to improve durability. As the radially inner groove extends beyond the bead core line, it is relatively easy to move or bend the bead portions axially inwardly to mount the tyre on a rim even though the tyre core is inserted in the tyre. Therefore, the working efficiency is greatly improved. 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, tyre core and wheel rim showing an embodiment of the present invention; Fig.2 is a partial perspective view of the assembly; and Fig.3 is a cross sectional view of an assembly of a tyre, tyre core and wheel rim showing another embodiment of the present invention. In the drawings, the assembly 1 comprises a pneumatic tyre 2, a wheel rim 3 on which the tyre is mounted and a tyre core 5 disposed in the tyre cavity. The tyre 2 comprises a tread portion T, a pair of axially spaced bead portions B each with a bead core C therein, a pair of sidewall portions S, a carcass extending between the bead portions B and reinforced by for example an organic fibre cord ply (not shown). In this example, the tyre 2 is an off-road tyre for motorcycles whose tread portion is provided with a block type tread pattern. The wheel rim 3 is a standard rim for the tyre and comprises a pair of axially spaced bead seats 3b, a rim well 3c therebetween, and a pair of flanges extending from the axially outer ends of the bead seats. The tyre core 5 is an annular body of an elastic closed-cell material. In this example, a sponge rubber having an expansion rate of from 400 to 1500 %, 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. If the expansion rate is less than 400 %, the shock absorption by the tyre core 5 becomes insufficient and ride comfort is not good. If the expansion rate is more than 1500 %, the load support by the tyre core 5 becomes impossible and stability during straight running and cornering is lost. The specific gravity of the tyre core 5 is preferably in the range of from 0.1 to 0.3. Further, the surface of the tyre core 5 has a JIS-C hardness of from 10 to 35 degrees, preferably 10 to 30 degrees, whereby the road grip and tyre rigidity during cornering are improved. Thus it becomes possible to increase the critical cornering speed and also to improve the handling performance. The tyre core 5 is provided on its surface with circumferentially extending hollow parts or grooves 6 and 7. The radially outer groove 6 is disposed in the radially outside 5a to form a space 9 from the tyre 2, and the radially inner groove 7 is disposed in the radially inside 5b to form a space 10 from the wheel rim 3 when the tyre 2 is mounted on the wheel rim 3. The radially outer space 9 and radially inner space 10 extend continuously in the tyre circumferential direction. Therefore, the spaces 9 and 10 function as cooling ventilation. The radially outer groove 6 has a U-shaped sectional shape. As a result, the sectional shape of the radially outer space 9 is rounded but generally rectangular as shown in the Figs.1 and 3. The radially outer space 9 has a sectional area So which is preferably in the range of from 1 to 6 %, more preferably 3 to 6 % of the total sectional area S of the tyre cavity. If the area So is less than 1% of the area S, the cooling effect and heat radiation decrease. If more than 6%, it is difficult to obtain the required rigidity for the tread portion T, and further it is difficult to support the tyre load when the tyre is punctured. Therefore, the steering stability decreases. Preferably, the radially outer space 9 has a maximum radial height Ho along the tyre equatorial plane Q which is less than 20% of the section height H of the tyre cavity 4, and a maximum axial width Wo which is less than 20% of the section width W of the tyre cavity 4. If the depth Ho is more than 20% of the height H and/or the width Wo is more than 20% of the width W, the rigidity decreases in the tread portion T and the steering stability is liable to deteriorate. The radially inner space 10 extends radially outwardly beyond a bead core line CL, the bead core line CL being a straight axial line drawn between the centres of the bead cores C. Due to the presence of the part 10a radially outward of the bead core line CL, a hindrance to the bead movements during rim mounting is removed and the mounting operation becomes easier. The maximum radial height Hi of the inner space 10 occurs along the tyre equatorial plane Q and it is in the range of from 27 to 45% of the section height H. If the height Hi is less than 27% of the height H, the rim mounting operation is liable to become difficult. If the height Hi is more than 45% the height H, the bead portions B are liable to be displaced into the rim well 3c when the tyre pressure is low. Further, the radially inner space 10 has a sectional area Si which is preferably in the range of from 10 to 35%, preferably 10 to 25% of the sectional area S of the tyre cavity 4. If the area Si is less than 10% of the area S, the rim mounting operation is liable to become difficult. If more than 25%, it becomes difficult to support the tyre load when punctured and sometimes the steering stability decreases. Usually, the radially outer and inner spaces 9 and 10 are substantially symmetrical with respect to the tyre equator Q. In Fig.1, the radially inner groove 7 has a V-shaped cross sectional shape where the two straight sides 10b extend from a point 10c on the tyre equator Q. The axial width of the inner groove 7 increases radially inwardly from the peak point 10c to the radially inner end. In this example, the radially inner ends of the two sides 10b are positioned on the inner surface of the tyre and radially outward of the bead cores C. As a result, it becomes relatively easy to move the bead portions B during mounting the tyre on the rim. The cross sectional shape of the radially inner space 10 is a generally folding-fan shape. The maximum axial width Wi of the radially inner space 10 is in the range of from 27 to 80 %, preferably 27 to 70 %, more preferably 27 to 55 % of the section width W of the tyre cavity. If the width Wi is less than 27% of the width W, the rim mounting operation is liable to become difficult. If the width Wi is more than 80% of the width W, the bead portions B are liable to be displaced into the rim well 3c when the tyre pressure is low. In Fig.3, the radially inner groove 7 has a U-shaped cross sectional shape wherein the two oppositely opposed sides are parallel with the tyre equator Q and the radially inner ends thereof are positioned on the inner surface of the rim. Thus the axial width Wi is substantially constant from the rim to the groove bottom. The axial width Wi of the radially inner space 10 is in the range of from 27 . to 70%, preferably 27 to 45% of the section width W of the tyre cavity. If the width Wi is less than 27% of the width W, the rim mounting operation is liable to become difficult. If the width Wi is more than 70% of the width W, the bead portions B are liable to be displaced into the rim well 3c when the tyre pressure is low. In this example, the bead displacement is effectively prevented, and the tyre rigidity is increased to improve the steering stability. In any case, the surface of the tyre core 5 except for the grooved part contacts the inner surface of the tyre 2 and the inner surface of the rim 3 (in case of Fig.3) . Therefore, a lubricant is preferably applied to the surface of the tyre core 5. For the lubricant, for example, silicon grease whose starting torque is 1000 to 1200 gf·cm and running torque is 200 to 400 gf·cm measured at -60 degrees C according to JIS K 2220 is preferably used. The above-mentioned tyre core 5 is made of a single rubber compound. However, it is possible to make it as a plurality of layers of different rubber compounds. Comparison Test Various tyre cores were made and assemblies of a tyre (size: 3.00-10 2PR) for motor scooters and a standard rim (size: 10X2.15) and the tyre cores were tested for durability, rim mounting, steering stability and bead displacement. Durability Test The durability was evaluated according to the Japanese Industrial Standard K-6366. In Table 1, tyres cleared the test without any damage are indicated as "pass". Rim Mounting Test A skilled worker mounted the tyre with each tyre core therein on the rim by hand using a tyre lever, and the difficulty was evaluated into five ranks, wherein the higher the rank, the easier the rim-mounting operation. Steering Stability Test Running a 90cc scooter in a test course, total running performance during cornering, acceleration and braking and at the time of quick braking was evaluated into five ranks by the test rider's feelings, wherein the higher the rank, the better the performance. Further, the tyres were checked if the bead portions were displaced during test running. Ex.1 Ex.2 Ex.3 Ex.4 Ex.5 Ex.6 Ex.7 Ref.1 Ref.2 Tyre core Type Fig.1 Fig.1 Fig.1 Fig.3 Fig.3 Fig.3 Fig.3 Fig.3 - Outer space None So/S (%) 2.5 4.1 5.8 2.5 2.5 4.7 6.1 0.76 Ho/H (%) 15.3 18.6 20.3 15.3 15.3 19.2 21.8 7.7 Wo/W (%) 12.8 18.6 20.3 12.8 12.8 19.2 21.8 7.7 Inner space None Si/S (%) 33.2 30.5 24.3 11.5 28.5 11.5 23 8.4 Hi/H (%) 36.3 31.7 28.1 28.2 38.5 28.2 42.3 25.7 Wi/W (%) 58.1 50 43.7 32.1 57.7 32.1 42.3 25.7 Part outward of bead core line exist exist exist exist exist exist exist None Test Results Durability pass pass pass pass pass pass pass Fail fail Rim mounting 4.5 4.3 4 3.5 4 3.5 3.8 2.5 1 Steering stability 3 3.3 3.4 4 3.1 3.7 3 4.2 4.5 Displacement occurred none none none occurred none none none None Preferably, the embodiment shown in Fig.1 is applied to a pneumatic tyre used with a low pressure of less than 1 kgf/sq.cm. The embodiment shown in Fig.3 is preferably applied to a pneumatic tyre used with a air pressure of not less than 1 kgf/sq.cm. As mentioned above, the present invention is suitably applied to a off-the-road motorcycle tyre, but it is also possible to apply a pneumatic tyres for four-wheel vehicles.

Claims: A run-flat tyre system comprising a tyre (2) and a tyre core (5) disposed therein, the tyre having a toroidal shape and comprising a tread portion (T), a pair of sidewall potions (S) and a pair of axially spaced bead portions (B) each with a bead core therein and the tyre core (5) being an annular body of an elastic closed-cell material, the radially outside and inside of the tyre core each being provided with a circumferentially extending groove (6,7), the radially inner groove (7) extending radially outwardly beyond a bead core line (CL), the bead core line (CL) being defined as a straight axial line drawn between the centres of the bead cores (C), characterised in that when the tyre is mounted on a standard rim, the radially outer groove (6) forms a radially outer space (9) extending circumferentially of the tyre and having a cross sectional area of from 1 to 6 % of the total sectional area of the tyre cavity, and the radially inner groove (7) forms a radially inner space (10) extending circumferentially of the tyre having a cross sectional area (Si) of from 10 to 25 % of the total sectional area (S) of the tyre cavity. A run-flat tyre system according to claim 1, characterised in that the radially outer space (9) has a section height (Ho) being less than 20 % of the section height (H) of the tyre cavity, and the radially inner space (10) has a section height (Hi) being 27 to 45 % of the section height (H). A run-flat tyre system according to claim 1 or 2, characterised in that the radially outer groove (6) has a substantially U-shaped cross section. A run-flat tyre system according to claim 1, 2 or 3, characterised in that the radially inner groove (7) has a substantially V-shaped cross section. A run-flat tyre system according to claim 1, 2 or 3, characterised in that the radially inner groove (7) has a substantially U-shaped cross section. A run-flat tyre system according to any of the preceding claims, characterised in that the tyre is a motorcycle tyre. A run-flat tyre system according to any of the preceding claims, characterised in that the tyre core (5) is made of a butyl rubber compound whose expansion rate is in the range of from 400 to 1500 %. A run-flat tyre system according to any of the preceding claims, characterised in that the surface of the tyre core (5) has a JIS-C hardness of from 10 to 35 degrees.

Inventors: YOSHIDA SHU, YOSHIDA, SHU
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1998
Main classification: B60C17
IPC codes: B60C17, B60C5, B60C7