Radial tyre for motorcycle and method of making the same

Abstract: A radial tyre for motorcycles comprises a carcass (6) extending between bead portions (4) through a tread portion (2) and sidewall portions (3), and a belt (7) disposed radially outside the carcass (6) in the tread portion (2) and comprising two crossed breaker plies (7A,7B) of cords laid at angles of from 16 to 26 degrees with respect to the tyre equator. The method of making the radial tyre comprises vulcanising the tyre in a mould (20) which applies a variable stretch (S) to the belt during vulcanising the tyre, wherein the stretch (Sc) at the tyre equator is in a range of from 6.0 to 7.0%, and the stretch gradually decreases from the tyre equator to the axial edges (BE) of the belt. The stretch is defined as (BD2/BD1-1)X100 wherein BD1 is the inside diameter of the belt (7) before tyre vulcanisation, and BD2 is the inside diameter of the belt (7) during tyre vulcanisation.

Description: The present invention relates to a radial tyre for motorcycles and a method of making the same being capable of improving lateral force variation of the tyre. For the motorcycle, if the lateral force variation of the front tyre is large, lateral vibrations occur on the tyre and handle-bar, and stability during straight running and cornering is greatly deteriorated. Hitherto, off-centre location of tyre components such as the carcass, tread belt and the like caused during building a green tyre was regarded as the main cause of lateral force variation. Accordingly, efforts have been made to improve tyre uniformity by improving the precision of tyre components and the accuracy of assembling the tyre components. In recent years, the requirements for radial tyres for high-performance high-speed motorcycles have become more and more severe. Therefore, to meet such requirements, it becomes necessary to further improve the lateral force variation and the like. It is therefore, an object of the present invention to provide a radial tyre for motorcycles and a method of making the same, in which the lateral force variation is greatly improved together with other tyre performance. According to the present invention, a radial tyre for motorcycles comprises a carcass extending between bead portions through a tread portion and sidewall portions, and a belt disposed radially outside the carcass in the tread portion and comprising two or three plies including two crossed breaker plies of cords laid at angles of from 16 to 26 degrees with respect to the tyre equator characterised by vulcanising the tyre in a mould causing a stretch being defined at (BD2/BD1-1)X100 where BD1 is the inside diameter of the belt before tyre vulcanisation, and BD1 is the inside diameter of the belt during tyre vulcanisation in the mould, the stretch at the tyre equator being in a range of from 6.0 to 7.0%, and the stretch gradually decreases from the tyre equator to the axial edges of the belt. Embodiments of the present invention will now be described in detail in conjunction with the accompanying drawings: Fig.1 is a cross sectional view of a radial tyre of the present invention; Fig.2 shows a cross sectional view showing an exaggerated change in the tyre shape together with a tyre mould, wherein a right half shows a state during tyre vulcanisation and a left half shows a state before tyre vulcanisation; Fig.3 is a diagram showing an example of the belt stretch distribution according to the present invention; and Fig.4 is a diagram for explaining difference between the bead width and wheel rim width. In the drawings, a radial tyre 1 comprises a tread portion 2 with tread edges Te, a pair of bead portions 4 each with a bead core 5 therein, a pair of sidewall portions 3, a carcass 6 extending between the bead portions 4 and a belt 7 disposed radially outside the carcass 6 in the tread portion 2. In this embodiment, the tyre size is 120/60R17 for a motorcycle front wheel. The belt 7 is composed of two or three plies including two crossed breaker plies 7A and 7B of parallel cords laid at an angle of 16 to 26 degrees with respect to the tyre equator C. For the breaker cords, organic fibre cords, e.g. aromatic polyamide, aromatic polyester, nylon, polyester, rayon and the like can be used. Especially, high modulus organic fibre cords, e.g. aromatic polyamide, aromatic polyester are suitably used. In this example, the belt 7 is composed of a radially inner breaker ply 7A and a wider outer breaker ply 7B. The belt edge means the axial edge BE of the widest breaker ply 7B. The carcass 6 comprises at least one ply of cords arranged radially at an angle of from 75 to 90 degrees with respect to the tyre equator C and extending between the bead portions 4 through the tread portion 2 and sidewall portions 3 and turned up around the bead core 5 in each bead portion 4 from the axially inside to the axially outside of the tyre to form a pair of turnup portions 6b and a main portion 6a therebetween. For the carcass cords, organic fibre cords, e.g. nylon, polyester, rayon and the like can be used. Especially, nylon or rayon fibre cords are suitably used. In this example, the carcass is composed of a single ply 6A of cords arranged at 90 degrees. The turnup portions 6b are extended to a certain radial height which is substantially same as the radial height h1 of the axial edges BE of the belt 7, that is, they are extended into a radial height range K of from h1-6% to h1+6% (6% of h1) when measured from the bead base line LB. If the carcass 6 is composed of a plurality of plies, at least one of them is preferably has such high turnup portions 6b. In each of the bead portion 4, a rubber bead apex 8 is disposed between the carcass main portion 6a and turnup portion 6b and tapers radially outward from the bead core 5. In each of the sidewall portions 3, a rubber sidewall 3G is disposed axially outside the carcass 6. The rubber sidewall 3G extends radially outward from the bead portion 4 into the vicinity of the tread edge Te. In this example, the sidewall 3G extends beyond the belt edge BE or the edge BE of the radially outer breaker ply 7B, and terminates at the axial edge AE of the inner breaker ply 7A so that the radially outer end portion 3E thereof is interposed between the carcass 6 and the radially outer breaker ply 7B. The radially inner surface of the inner breaker ply 7A substantially comes into contact with the carcass 6. In the tread portion 2, a rubber tread 2G is disposed on the belt 7. In this example, an axial edge portion of the tread 2G beyond the belt edge BE sticks on the outer surface of the sidewall 3G. Therefore, the axial edges AE of the inner breaker ply 7A are covered with the outer breaker ply 7B, and the radially inside of the axial edges BE of the outer breaker ply 7B is supported by the softer sidewall rubber 3G. As a result, damage, separation and the like starting from the belt edges can be prevented. In view of this preventing effect and for the tyre lateral stiffness, it is preferable that the carcass turnup portions 6b extended into a radial height range of from h1-6% to h1, more preferably h1-3% to h1. The tread portion 2 which is a complex of the carcass 6, belt 7, tread rubber 2G, etc. is convexly curved so that the maximum tyre section width lies between the tread edges Te as shown in Fig.1. This convex profile is given to the tyre by a tyre mould 20 during vulcanising the tyre. In Fig.2, the left side (A) shows a green tyre put in the mould but not yet inflated to prepare for vulcanisation. The right side (B) shows the tyre having been vulcanised but not yet taken out of the mould 20. This shape is substantially the same as that of the finished tyre. In the green tyre state, the radius of curvature of the tread portion 2 is relatively large (flat). The green tyre put in the mould 20 is first inflated using a bladder to press the outer surface thereof onto the inside of the mould, and the tyre is heated to be vulcanised. In this state, the tread portion 2 curves along the inside of the mould and the radius of curvature thereof decreases. In the present invention, the stretch S of the belt 7 is specifically limited as follows. Here, the belt stretch S is (BD2/BD1-1)X100 wherein BD1 is the inside diameter of the belt 7 in the state of green tyre before vulcanisation, and BD2 is the inside diameter of the belt 7 during tyre vulcanisation. In order to improve the lateral force variation, steering stability during straight running and cornering performance, the stretch S(Sc) at the tyre equator C is set in a range of from 6.0 to 7.0%, and the stretch S is gradually decreased from the tyre equator C to the axial edges BE of the belt 7, as shown in Fig.3. The stretch S(Se) at the belt edges BE is set in a range of more than 0% but not more than 2.0%, preferably from 0.5 to 2.0%, more preferably from 1.0 to 2.0%. Table 1 is a test result showing the correlation between the lateral force variation LFV and belt stretch. In this test, tyres of size 120/60R17 (Rim size: MT3.5X17) were made with different belt stretches. However, the belt stretch in each tyre was substantially constant in its widthwise direction. The LFV was measured according to JASO-C607 under an inner pressure of 200 kPa and a tyre load of 137 kgf. Tyre A1 A2 A3 A4 A5 Belt stretch (%) 4 5 6 7 8 Belt ply width before vulcanisation inner ply (mm) 115 115 115 115 115 outer ply (mm) 130 130 130 130 130 after vulcanisation inner ply (mm) 115 110 107 105 98 outer ply (mm) 130 129 126 123 115 LFV average (kgf) 4 3.8 3 2.9 2.8 standard deviation (kgf) 1.2 1.1 0.9 0.9 0.8 As shown in Table 1, the average LFV and standard deviation decrease as the belt stretch increases. However, in case the belt stretch is substantially constant across the belt width, the belt width in the finished tyre decreases as the belt stretch increases. As a result, high speed performance and cornering performance deteriorate. Further, the ground contacting length has a tendency to decrease as the ground contacting part shifts towards the tread edge during cornering. Thus, it is difficult to improve cornering performance. In the present invention, therefore, the belt stretch Sc at the tyre equator C is set in a specific range of from 6.0 to 7.0% and the belt stretch S is set to continuously decrease from the tyre equator to the belt edges, whereby even if the bank angle increases during cornering, a decrease in the ground contacting length is controlled, and the grip performance is improved. Further, the lateral force variation and the steering stability during straight running are also improved. If the stretch (Se) is more than 2.0%, a decrease in the ground contacting length occurs, and the cornering performance is deteriorated. If the stretch (Sc) is more than 7.0%, a decrease in the belt ply width occurs in the vulcanised tyre. If the above-mentioned belt cord angles are less than 16 degrees, it is difficult to achieve the above-mentioned variable stretch. If the belt cord angles are more than 26 degrees, high-speed durability decreases. In this embodiment, as shown in Fig.4, the bead width BW of the mould 20, which is the axial distance between the bead shaping faces 4S' thereof and which corresponds to the axial distance between the axially outer surfaces 4S of the bead portions 4 of the demoulded vulcanised tyre, is set to be wider than the rim width RW of the standard wheel rim J by 15 to 30 mm. On mounting such a tyre on the wheel rim J, the belt cord tension in the tread edge portions is decreased, and the decrease in the ground contacting length during cornering is again controlled to improve cornering performance. Further, the lateral force variation can be further improved as shown in Table 2. If the difference is less than 15 mm, the lateral force variation hardly improves. If the difference exceeds 30 mm, it becomes difficult to mount the tyre on the wheel rim. Incidentally, the standard wheel rim is a wheel rim officially approved for the tyre by, for example JATMA (Japan and Asia), T&RA (North America), ETRTO (Europe), STRO (Scandinavia) and the like. The following Table 2 is a test result showing the correlation between the lateral force variation LFV and the difference BW-RW between the bead width BW and rim width RW. In this test, tyres of size 120/60R17 (Rim size: MT3.5X17) were made changing the bead width BW, and the LFV was measured according to JASO-C607 under an inner pressure of 200 kPa and a tyre load of 137 kgf. Further, the rim mounting operation was also checked. Tyre B1 B2 B3 B4 Belt stretch Sc (%) 6 6 6 6 Se (%) 0.5 0.5 0.5 0.5 Bead width BW (mm) 89 101 119 134 Rim width RW (mm) 89 89 89 89 BW-RW (mm) 0 12 30 45 Average LFV (kgf) 3 2.7 2.5 2.5 Rim mounting good good poor impossible As described above, in the present invention, the belt stretch during vulcanising the tyre is set in the specifically limited narrow range of from 6.0 to 7.0% at the tyre equator and gradually decreased from the tyre equator to the belt edges. Therefore, the lateral force variation and steering stability during straight running can be improved. Even if the ground contacting part shifts toward the tread edge during cornering, the ground contacting length is maintained and cornering performance can be improved. The present invention can be suitably applied to tyres for front wheels.

Claims: A method of making a radial tyre for motorcycles, the radial tyre (1) comprising a carcass (6) extending between bead portions (4) through a tread portion (2) and sidewall portions (3), a belt (7) disposed radially outside the carcass (6) in the tread portion, and the belt (7) comprising two or three plies including two crossed breaker plies (7A,7B) of cords laid at angles of from 16 to 26 degrees with respect to the tyre equator, characterised by vulcanising the tyre (1) in a mould (20) causing a stretch (S) to the belt (7) during vulcanising the tyre, the stretch (S) being defined as (BD2/BD1-1)X100 where BD1 is the inside diameter of the belt before tyre vulcanisation, and BD2 is the inside diameter of the belt during tyre vulcanisation in the mould, the stretch (Sc)at the tyre equator (C) being in a range of from 6.0 to 7.0%, and the stretch (Sc) gradually decreases from the tyre equator (C) to the axial edges (BE) of the belt. A method according to claim 1, characterised in that the stretch (Se) at the belt edges is not more than 2.0%. A method according to claim 1 or 2, characterised in that the bead width (BW) of the mould (20) is wider than the rim width (RW) of a wheel rim for the tyre by 15 to 30 mm. A method according to claim 1, 2 or 3, characterised in that said two crossed breaker plies are a radially inner ply (7A) and a radially outer wider ply (7B) disposed on the inner ply (7A), the carcass (6) comprises a high-turnup ply having turnup portions (6b) extended radially outwardly to a radial height range of from h1-6% to h1+6%, where h1 is a radial height of axial edges (BE) of the belt (7) measured from a bead base line (LB), a rubber sidewall (3G) is disposed axially outside the carcass (6) in each of the sidewall portions (3), and the sidewall (3G) extends radially outwardly beyond the axial edge (BE) of the radially outer breaker ply (7B), passing between the radially outer breaker ply (78) and the carcass (6), and terminates at the axial edge (AE) of the radially inner breaker ply (7A). A radial tyre for motorcycles comprising a carcass (6) extending between bead portions (4) through a tread portion (2) and sidewall portions (3), a belt (7) disposed radially outside the carcass (6) in the tread portion (2) and composed of two or three plies (7A, 7B) including two crossed breaker plies of cords laid at angles of from 16 to 26 degrees with respect to the tyre equator (C), characterised in that said tyre is manufactured by the method of claims 1 to 4. A radial tyre according to claim 5, characterised in that the bead width (BW) of the tyre is wider than the rim width (RW) of a wheel rim for the tyre by 15 to 30 mm. A radial tyre according to claim 5 or 6, characterised in that said two crossed breaker plies are a radially inner ply (7A) and a radially outer wider ply (7B) disposed on the inner ply (7A), the carcass (6) comprises a high-turnup ply having turnup portions (6b) extended radially outwardly to a radial height in the range of from h1-6% to h1+6%,wherein h1 is a radial height of axial edges (BE) of the belt (7) measured from a bead base line (LB), a rubber sidewall (3G) is disposed axially outside the carcass (6) in each of the sidewall portions (3), and the sidewall (3G) extends radially outwardly beyond the axial edge (BE) of the radially outer breaker ply (7B), passing between the radially outer breaker ply (7B) and the carcass (6), and terminates at the axial edge (AE) of the radially inner breaker ply (7A).

Inventors: FUNAHARA KIYOSHI, FUNAHARA, KIYOSHI, Funahara, Kiyoshi, c/o Sumitomo Rubber Ind. Ltd.
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 2000
Main classification: B29D30
IPC codes: B60C15, B29D30, B60C9, B29C35, B60C13, B29C55, B29L30