Motorcycle tyre and method of making the same

Abstract: A motorcycle tyre comprises a tread portion (2) curved convexly such that the maximum cross section width TW of the tyre lies between the tread edges (E), a carcass ply (6) of cords arranged radially at an angle of 70 to 90 degrees to the tyre equator, and a belt ply (7) of at least one cord laid at an angle of 0 to 35 degrees disposed radially outside the carcass ply (6) in the tread portion. When the tyre is mounted on a standard rim and inflated from 10 % of a standard inner pressure to 100 % thereof, the tread portion swells such that the amount of swelling is larger in a shoulder region than a crown region of the tread portion. The carcass cord tension Tcs in the shoulder region is 2.0 to 3.8 times the carcass cord tension Tcc in the crown region, and the belt cord tension Tbs in the shoulder region is 1.2 to 2.0 times the belt cord tension Tbc in the crown region.

Description: The present invention relates to a motorcycle tyre and a method of making the same, more particularly an improved tyre profile capable of improving the rigidity of the tread portion. Usually, motorcycles are tilted during cornering and a relatively large camber angle is given to the tyres. Therefore, if the rigidity of the tread shoulder regions which come into contact with the ground is low, the road grip decreases. Further, during straight running, especially high speed straight running, the convergence of tyre vibrations becomes slow or worse and the straight running stability is deteriorated. Hitherto, in order to increase the rigidity in the tread shoulder regions, it has been preferred to dispose a reinforcing cord layer in each shoulder region, to use a relatively rigid cord material for a reinforcing cord layer in the shoulder region or to increase the thickness of the tread rubber in the shoulder regions. However, satisfactory results can not be obtained, and further, the tyre uniformity is liable to be spoilt and there is danger of ply separation, heat generation and the like. EP-A-0 294 153 relates to a motorcycle tyre reinforced by a breaker which comprises a main reinforcing layer and an auxiliary reinforcing layer. The main reinforcing layer comprises organic cords and has a width substantially equal to the width of the tread. The width of the auxiliary reinforcing layer lies in the range of 10 to 70 % of the maximum width of the tyre to lessen stress concentration in the breaker edges. However, providing a tyre with additional reinforcement layers increases its weight. It is therefore an object of the present invention to provide a motorcycle tyre in which the rigidity of the tread portion is optimised to improve tyre performance such as straight running stability, cornering grip and the like without utilising any additional reinforcement. Further, it is another object of the present invention to provide a method of making such a motorcycle tyre. These objects are attained by a motorcycle tyre according to claim 1 and by a method of making a motorcycle tyre according to claim 4. Here, the standard rim is the 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 standard inner pressure is the maximum air pressure for the tyre officially specified by such organisation. 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 a tyre according to the present invention which is disposed in a vulcanising mould; Fig.2 is a cross sectional view of the tyre mounted on a standard rim and inflated to standard pressure; Fig.3 shows the difference in vulcanising profile between the present invention and prior art; Fig.4 is a schematic cross sectional view showing the growth of the tread portion of the tyre when inflated from 10 % to 100 % of the standard pressure; and Fig.5 is graphs showing the belt cord tension and carcass cord tension as a function of the axial position. First of all, an example of the internal structure of the motorcycle tyre according to the invention is described, according to Figs.1 and 2. The tyre comprises a tread portion 2, a pair of axially spaced bead portions 4 each with a bead core therein, a pair of sidewall portions 3 extending between the tread edges E and the bead portions 4, a carcass 6 extending between the bead portions 4 through the tread portion and sidewall portions, and a tread reinforcing belt 7 disposed radially outside the carcass and inside a tread rubber. The carcass 6 comprises at least one ply of cords arranged radially at an angle of from 70 to 90 degrees with respect to the tyre equator. In this example, the carcass 6 consists of a single ply of nylon cords arranged at 90 degrees with respect to the tyre equator C. The carcass ply is turned up around the bead core 5 in each bead portion 4 from the axially inside to the outside of the tyre to form a pair of turnup portions and one main portion therebetween. For the carcass cords, organic fibre cords, e.g. polyester, rayon and the like can be used. The turnup portion 6b of the carcass ply extends up to a height of not less than 80 % of the tread edge height TH from the bead base line BL. Between each turnup portion and the carcass ply main portion, a hard rubber bead apex 9 extending radially outwardly from the bead core 5 is disposed to increase the rigidity of the sidewall portion. The belt 7 is made of at least one ply of at least one cord, the cord angle of which is in the range of from 0 to 35 degrees with respect to the tyre equator. In this example, the belt is made of aromatic polyamide cords which are wound spirally at a small angle of not more than 5 degrees across substantially over all the tread width. The axial width is not less than 90% of the tread width TW. The cord count is for example 50 /5cm. It is possible to wind a single cord, but preferably several cords which are embedded in topping rubber in the form of ribbon or tape are wound at the same time to improve the production efficiency. Further, in addition to or instead of the above-mentioned spiral belt, it is possible to dispose a breaker belt which comprises at least two crossed plies of parallel cords laid at a small angle for example 10 to 35 degrees with respect to the tyre equator C. When a spiral belt is used as the radially outermost belt, the tyre uniformity, high speed durability and high speed running stability can be effectively improved. Radially outside of the belt 7, a tread rubber is disposed to define the tread portion. The tread rubber has a substantially constant thickness over all the width thereof. In contrast with conventional tyres which swell evenly throughout the tread portion, the motorcycle tyre according to the present invention has a profile such that, as shown in Fig.4, the swelling of the tread portion 2 is greater in the shoulder region Sh than the crown region Cr when the tyre mounted on a standard rim R is inflated from 10% to 100% of the standard inner pressure. In Fig.4, the dotted line shows the profile under the 10% pressure, and the solid line shows the profile under the 100% pressure. The profile under the 100 % pressure is designed to be almost identical with the profile of the conventional tyre under the 100 % pressure. In this example, the tyre size is 180/55ZR17, the standard rim size is 17XMT5.50, and the standard pressure is 2.9 kgf/sq.cm. Accordingly, when the tyre is mounted on a standard rim and inflated to standard pressure, the carcass cords and belt cords have a larger tension in the shoulder region Sh than the crown region Cr. As a result, the tread rigidity increases in the shoulder regions Sh, and the high speed straight running stability, cornering stability, cornering grip and the like can be improved. The above-mentioned crown region Cr is defined as a 1/3 region extending between two positions disposed one on each side of the tyre equator and spaced apart from the tyre equator C by 1/6 of the tread face length along the tread face. The tread face length is the length measured in the tyre axial direction along the surface line of the tread portion. The shoulder regions Sh are each defined as a 1/6 region extending from one of the tread edges E to an axially inner position spaced apart from the tyre equator C by 1/3 of the tread face length along the tread face. As to the amount of swelling of the tread portion measured at the tread face normally to the tread face when the tyre inner pressure increases from 10% to 100%, the amount LC at the tyre equator C is about 0.8 mm in this example, the amount LE at the tread edges E is substantially equal to LC, and the amount LS at middle points is about 1.7 mm, the middle points are spaced apart from the tyre equator C by 2/3 of the distance between the tyre equator C and the tread edge E, along the tread surface line. The maximum swelling is occurred in the vicinity of the middle points. Usually, LC and LE are about 0.5 to 1.1 mm, and LS is about 1.1 to 2.2 mm. As the size variation of the motorcycle tyres is quite small in comparison with the tyres in the other categories, the limitations LC, LE and LS may be applied to almost all tyre sizes. The ratio (LS/LC) is preferably in the range of from 1.5 to 3.0, more preferably 1.5 to 2.5. If the ratio (LS/LC) is less than 1.5, an effective increase of the rigidity in the shoulder region Sh can not be obtained. If the ratio (LS/LC) is more than 3.0, the rigidity excessively increases in the shoulder region Sh. Therefore, as the camber angle increases, the rigidity changes abruptly to decrease the cornering stability, and a linear handling feeling can not be obtained. Fig.5 shows the distributions of the belt cord tension and carcass cord tension under 100% pressure which were obtained by a finite element method, wherein the chain line shows the distribution of the tyre according to the present invention and the solid line shows a conventional tyre which swells evenly. The numbers 1-9 enclosed with circles show the measuring points which are shown in Fig.2. As to the belt cord tension ratio Tb which is the ratio Ts/Tc between the maximum cord tension Tbs in the shoulder region Sh (between the measuring point 3 and 4) and the maximum cord tension Tbc in the crown region Cr (between the measuring points 1 and 2), Fig.5 shows that the ratio reaches to about 1.47 and thus the tension is effectively increased in the shoulder region, whereas the ratio of the conventional tyre is about 1.1 by which ratio an increase in the shoulder rigidity can not be obtained. As to the carcass cord tension ratio Tc which is the ratio Tcs/Tcc between the maximum cord tension Tcs in the shoulder region Sh (between the measuring points 7 and 8) and the maximum cord tension Tcc in the crown region Cr (between the measuring points 5 and 6), Fig.5 shows that the ratio reaches to about 3.75, whereas the ratio of the conventional tyre decreases to about 0.7 which means the tension becomes lower in the shoulder region than the crown region. The carcass cord tension ratio Tc is preferably in the range of from 2.0 to 3.8, more preferably 2.0 to 3.5. The belt cord tension ratio Tb is preferably in the range of from 1.2 to 2.0. If the ratio Tc is less than 2.0 and/or the ratio Tb is less than 1.2, the effect on increasing the rigidity of the shoulder region Sh has a tendency to decrease. If the ratio Tc is more than 3.8 and/or the ratio Tb is more than 2.0, the rigidity increases excessively in the shoulder region Sh. Therefore, as the camber angle increases, the rigidity changes abruptly to decrease the cornering stability, and a linear handling feeling can not be obtained. The above-mentioned peculiar profile change during inflation from 10% to 100% can be obtained by vulcanising the tyre in a mould 10 having a peculiar profile. According to the present invention, an intentional difference is provided between the profile when the tyre is mounted on the standard rim R and inflated to 100% of the standard pressure, and the profile when the tyre is put in a vulcanising mould 10. Fig.1 shows an example of the mould 10, which is a segmental mould having a tread shaping face 10b for shaping the tread portion 2, a pair of axially opposite sidewall shaping faces 10c for shaping the sidewall portions 3, and a pair of bead shaping faces 10a for shaping the bead portions 4. Incidentally, the tyre on removal from the vulcanising mould and not mounted on a rim has substantially the same profile as in the mould. Therefore, the profile of the tyre not mounted on a rim and the profile of the mould 10 are described together, referring to these profiles as the vulcanising profile and the above-explained profile of the tyre when mounted on the standard rim and inflated to 100% of the standard pressure as the inflated profile hereinafter for convenience' sake. Thus Fig.1 shows the vulcanising profile and Fig.2 shows the inflated profile. Fig.3 shows the vulcanising profile of the invention with a dotted line, the vulcanising profile of a conventional tyre with a chain line, and the inflated profile of these tyres with a solid line. In both of the vulcanising and inflated profiles, the tread portion 2 is curved convexly so that the maximum section width TW lies between the tread edges E or the axial edges of the tread portion 2. Thus, the maximum section width TW of the tyre is equal to the tread width. In the vulcanising profile the bead width BW' which is the axial distance between the axially outer surfaces of the bead portions 4, and the corresponding bead clip width CW which is the axial distance between the axially inner surfaces of the bead shaping faces 10a are 15 to 45 mm (or 8 to 24 % of the width TW) wider than the rim width RW of the standard rim; the axial width TW' between the axially outer edges E' of the tread portion 2 or the tread shaping face 10b is 5 to 25 mm (or 2 to 14 % of the width TW) wider than the width TW; the radial height TH' of the radially outer edges E' of the sidewall portions 3 or the sidewall shaping faces 10c is 3 to 15 mm (or 1 to 8 % of the width TW) higher than the radial height TH of the edge E, each measured from the bead base line BL; and the distance between the edge E' and the bead heel point H' is substantially the same as the distance between the edge E and the bead heel point H of the inflated profile. In the conventional tyre, the widths BW' and TW' of the vulcanising profile are substantially equal to the rim width RW and the width TW of the inflated profile. Further, the tread edge height TH' is slightly lower than the tread edge height TH. Therefore, the shoulder region Sh is hardly deformed when the tyre is mounted on a rim. When the tyre is inflated to a normal pressure, the tread portion 2 swells evenly and the cord tension becomes substantially even. In the vulcanising profile of the present invention, the tread portion curves continuously so that the maximum section width TW' lie between the tread edges, and the bead width BW', tread width TW' and tread edge height TH' are set larger than the standard rim width RW, the tread width TW and tread edge height TH under 100% pressure, respectively as explained above. Accordingly, when the tyre is mounted on a standard rim, the bead width becomes narrow, and as a result the shoulder region Sh is pulled radially inwardly. By such deformation, a large tension is produced in the shoulder region Sh and by the increase in the inner pressure thereafter the cord tension in the shoulder region Sh is further increased. If the difference between the width BW'(=CW) and the rim width RW is less than 15 mm, the deformation of the tyre in the shoulder region Sh by rim mounting becomes insufficient for increasing the cord tension. Thus the rigidity of the shoulder region Sh becomes lower. If the difference is more than 45 mm, the cord tension in the shoulder region Sh is excessively increased. On the premise that the tyre is mounted on a standard rim R, the difference of the bead width BW' from the standard rim width RW is set in the range of from 15 to 30 mm (or 8 to 17 % of the width TW). However, if the tyre is mounted on a rim whose rim width is a half inch larger than the standard rim, it is preferable that the bead width BW' is set in the range of from 27 to 42 mm (or 14 to 23 % of the width TW) larger than the standard rim width RW. If the tyre is mounted on a rim whose rim width is an inch larger than the standard rim, it is preferable that the bead width BW' is about 45 mm (or about 24 % of the width TW) larger than the standard rim width RW. That is, the bead width BW' is preferably about 1.1 to 1.35 times the rim width of the intended rim. Further, in the vulcanising profile, the radius of curvature Rs' of the shoulder region Sh is larger than the radius of curvature Rc' of the crown region Cr. The ratio (Rs' /Rc' ) is preferably 1.9 to 4.0. Furthermore, the radius of curvature Rs' of the shoulder region Sh of the vulcanising profile is preferably 2.0 to 4.0 times the radius of curvature Rs of the shoulder region Sh of the inflated profile. In this example, the tread profile under the 100% pressure has substantially a single radius of curvature R1, which is smaller than the radius RS'. In the present invention, the tyre is made by vulcanising a raw tyre, using the mould 10 having the above-mentioned vulcanising profile. The raw tyre is put in the mould 10, and during heating of the tyre, the inside of the tyre is pressurised by inflating a bladder inserted in the tyre so as to press the outside of the tyre onto the inside of the tyre mould so that the tyre is formed with the vulcanising profile. As for other manufacturing processes and apparatuses, conventional processes and apparatuses can be used. Comparison Tests Test tyres having the same internal structure shown in Figs.1 and 2 were made by way of test and tested for straight running stability, cornering stability, tyre rigidity, cornering grip and vertical spring coefficient. The test tyres included Example tyres 1 to 3 having the vulcanising profile shown in Fig.3 with a dotted line, and Reference tyres 1 and 2 having the vulcanising profile shown in Fig.3 with a chain line. The vertical spring coefficient was measured under the following conditions, changing the camber angle from 0 to 40 degrees. Tyre size 180/55R17 Rim size 17XMT5.50 (standard rim) Inner pressure 2.5 kgf/sq.cm Tyre load 155 kgf The other performance was evaluated into five ranks by a test rider during running a 750cc four-cycle-engine motorcycle on a dry asphalt test course. The test tyres were mounted on the rear wheel. Tyre size 180/55R17 Rim size 17XMT5.50 (standard rim) Inner pressure 2.9 kgf/sq.cm Carcass 1260d/2 nylon cord, Cord count 38 /5cm Belt a single ply of spiral windings of 1500d/2 aromatic polyamide cords Cord count 50 /5cm Cord angle 0 degree to the tyre equator Ply width 205 mm On the front wheel, the same tyre was used. Tyre size 120/70R17 Rim size 17XMT3.50 Inner pressure 2.5 kgf/sq.cm Carcass: 2 plies of 1260d/2 nylon cords Cord count 41 /5cm Cord angle 88 degrees to the tyre equator Belt 2 cross plies of 1500d/2 aromatic polyamide cords Cord count 35 /5cm Cord angle 17 degrees to the tyre equator The test results are shown in Table 1. Tyre Ref.1 Ref.2 Ex.1 Ex.2 Ex.3 Carcass Number of ply 1 2 1 2 1 Cord angle (deg) 90 88 90 88 90 Vulcanising profile Bead width BW' (mm) 140 140 165 165 170 Tread width TW' (mm) 183 183 202 202 202 Tread edge height TH' (mm) 40 40 47 47 46 Inflated profile Bead width BW (mm) 140 140 140 140 140 Tread width TW (mm) 185 185 185 185 185 Tread edge height TH (mm) 45 45 45 45 44 Tread radius (mm) 100 100 100 100 100 Stability Straight running 4 4 4.3 4.5 4.4 Cornering 3.7 4 4.5 4.5 4.6 Rigidity 3.5 4 4.3 4.5 4.4 Cornering grip 3.7 4 4.5 4.5 4.6 Vertical spring coefficient camber angle 0 deg 16.84 17.4 16.96 18.21 17.01 camber angle 20 deg 17.16 19.34 18.93 20.25 18.96 camber angle 30 deg 16.19 17.94 18.39 20.19 19.01 camber angle 40 deg 17 18.71 22.29 24.46 23.42 From the tests, it was confirmed that Example tyres were improved in straight running stability and cornering grip in particular. Even if the camber angle became relatively large, Example tyres were superior to Reference tyres in the vertical spring coefficient and the rigidity of the shoulder region. The present invention is suitably applied to a rear tyre for motorcycles the tyre aspect ratio of which is 40 to 60%. It is however possible to apply to a front tyre and a tyre having an aspect ratio outside this range.

Claims: A motorcycle tyre comprising a tread portion (2) with a pair of tread edges (E), a pair of sidewall portions (3), a pair of bead portions (4), a carcass ply (6) of cords arranged radially at an angle of 70 to 90 degrees to the tyre equator, a belt ply (7) of at least one cord laid at an angle of 0 to 35 degrees disposed radially outside the carcass ply in the tread portion (2), characterised by a vulcanising profile, the vulcanising profile having a bead width BW' which is 15 to 45 mm wider than the rim width of the standard rim, and a tread width TW' which is 5 to 25 mm wider tan the tread width TW under 100% of the standard pressure, a tread edge height TH' which is 3 to 15 mm larger than the tread edge height TH under 100% of the standard pressure and in that, when the tyre is mounted on a standard rim and inflated from 10 % of a standard inner pressure to 100 % thereof, the tread portion swells such that the amount of swelling is larger in a shoulder region than a crown region of the tread portion. A motorcycle tyre according to claim 1, characterised in that the amount LS of swelling at a position spaced apart from the tyre equator by 1/3 of the tread face length measured between the tread edges (E) along the tread face is in the range of 1.5 to 3.0 times the amount LC of swelling measured at the tyre equator. A motorcycle tyre according to claim 1 or 2, characterised in that the carcass cord tension Tcs in the shoulder region is 2.0 to 3.8 times the carcass cord tension Tcc in the crown region, and the belt cord tension Tbs in the shoulder region is 1.2 to 2.0 times the belt cord tension Tbc in the crown region. A method of making a motorcycle tyre as set forth in claim 1 comprising the steps of putting a raw tyre in a mould (10) and vulcanising the tyre in the mould (10), said mould comprising a tread shaping face (10b) for shaping the tread portion (2), a pair of sidewall shaping faces (10c) for shaping the sidewall portions (3), and a pair of bead shaping faces (10a) for shaping the bead portions (4), wherein the said mould having a vulcanising profile such that the axial width (BW') between the bead shaping faces is 15 to 45 mm wider than the rim width (RW) of the standard rim, the axial width (TW') of the tread shaping face is 5 to 25 mm wider than the width (TW) of the tread portion under 100% of the standard pressure, and the radial height (TH') of the axial edges of the tread shaping face is 3 to 15 mm higher than the radial height (TH) of the tread edges (E) under 100% of the standard inner pressure. A vulcanising mould for a motorcycle tyre as set forth in claim 1, characterised in that the mould comprises a tread shaping face (10b) for shaping the tread portion, a pair of sidewall shaping faces (10c) for shaping the sidewall portions, and a pair of bead shaping faces (10a) for shaping the bead portions, the axial width (RW') between the bead shaping faces being 15 to 45 mm wider than the rim width (RW) of the standard rim, the axial width (TW') of the tread shaping face being 5 to 25 mm wider than the width (TW) of the tread portion under 100% of the standard pressure, and the radial height (TH') of the axial edges of the tread shaping face being 3 to 15 mm higher than the radial height (TH) of the tread edges (E) under 100% of the standard inner pressure.

Inventors: ISHIKAWA TAKESHI, ISHIKAWA, TAKESHI
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1998
Main classification: B60C3
IPC codes: B29C33, B29D30, B60C9, B60C3, B60C11, B29L30, B29C35, B60C19