Radial tyre for a motorcycle

Abstract: A radial tyre for a motorcycle having its maximum cross section width (WT) between tread edges (E1,E2), comprising a pair of bead cores (5) disposed one in each bead portion (4) of the tyre, a carcass (6) extending between the bead portions (4) through sidewall portions (3) and a tread portion (2), the carcass (6) having at least one ply of organic fibre cords arranged radially at an angle of 70 to 90 degrees to the tyre equator and turned up around the bead cores (5) to form two turned up portions (6b) and a main portion (6a) therebetween, a tread (2) disposed radially outside the carcass (6) to define the tread portion, and a belt (7) disposed radially outside the carcass (6) and inside the tread (2), characterised in that the radial height Ht of the radially outer edge of each carcass turned up portion (6b) being 0.5 to 1.2 times the radial height (Hs) of the tread edge (E1,E2), both from the bead base (15), said belt (7) comprising two lateral pieces (7b,7c) and a central piece (7a) therebetween, each piece being made of at least one spirally wound cord having an elastic modulus of not less than 600 kgf/sq.mm and being formed by winding spirally a ribbon of rubber in which said at least one cord is embedded, and the width (WC) of the central piece (7a) being 0.1 to 0.6 times the tread width (WT).

Description: The present invention relates to a motorcycle radial tyre with a spiral cord belt, in which straight running performance and cornering performance are improved. Recently, a radial ply carcass has been used for motorcycle tyres. Such a motorcycle radial tyre has been made based on the techniques for four-wheeled vehicle tyres, e.g. passenger car tyres. That is, the tyre has been provided with a belt reinforcement formed as follows: a rubberised fabric (d) is cut bias at a small angle (alpha) to the cord direction as shown in Fig.7, which is usually 15 to 30 degrees; and the cut fabric (e) is wound around a carcass with connecting the ends (f) as shown in Fig.6 to form an annular belt reinforcement (a), and accordingly the laid angle of the belt cords (b) to the tyre equator becomes the same bias angle (alpha). In such a bias belt structure, however, steering stability during straight running and cornering, especially at high speed, is not good. On the other hand, Japanese Utility-Model Publication No.58-160805 discloses a belt making method, in which a belt cord is wound spirally around the radial carcass and continuously across the tread width at small angles to the circumferential direction of the tyre to form a jointless belt. When this is used in a motorcycle tyre instead of the conventional belt, steering stability during high speed straight running is improved, but the stability during cornering is not improved contrary to expectation, and further deterioration of tyre durability has sometimes been observed. Thus, it is difficult to satisfy the requirements for cornering and the requirements for straight running at the same time because of the cornering mechanism of the motorcycles which is greatly different from that of the four-wheeled vehicles. That is, the motorcycle is greatly inclined during cornering and accordingly the ground contacting region moves substantially across the tyre. It is therefore, an object of the present invention to provide a motorcycle radial tyre having a spiral cord type of belt, which is improved in cornering performance and durability as well as straight running performance. According to the present invention a radial tyre for a motorcycle having its maximum cross section width between tread edges, comprises a pair of bead cores disposed one in each bead portion of the tyre, a carcass extending between the bead portions through sidewall portions and a tread portion, the carcass having at least one ply of organic fibre cords arranged radially at an angle of 70 to 90 degrees to the tyre equator and turned up around the bead cores to form two turned up portions and a main portion therebetween, a tread disposed radially outside the carcass to define the tread portion, and a belt disposed radially outside the carcass and inside the tread, the radial height Ht of the radially outer edge of each carcass turned up portion being 0.5 to 1.2 times the radial height of the tread edge, both from the bead base, said belt comprising two lateral pieces and a central piece therebetween, each piece being made of at least one spirally wound cord having an elastic modulus of not less than 600 kgf/sq.mm and being formed by winding spirally a ribbon of rubber in which said at least one cord is embedded, and the width of the central piece being 0.1 to 0.6 times the tread width. An embodiment of the present invention will now be described in detail with reference to the accompanying drawings, in which: Fig.1 is a sectional view showing an embodiment of the present invention; Fig.2 is a developed plan view showing the arrangement of the carcass cords and the belt cords; Fig.3 is a sectional view showing a modification of the belt; Fig.4 is a perspective view showing a belt cord ribbon; Fig.5 is a sectional view showing winding of the ribbon; Figs.6 and 7 are plan views showing prior arts. In the figures, motorcycle tyre 1 has a tread portion, a pair of bead portions 4, and a pair of sidewall portions 3 extending radially inwardly of the tyre from the edges of the tread portion to the bead portions. The tread portion is curved so that the maximum cross sectional width (WT) of the tyre lies between the edges E1 and E2 of the tread, and the tread has an arched profile which is generally one third of a circle. The tyre 1 comprises: pair of bead cores 5 disposed one in each bead portion 4; a carcass 6 extending between the bead portions 4 through the sidewall portions 3 and the tread portion 2, and having at least one ply of radially arranged cords turned up around the bead cores 5 from the axially inside to outside thereof to form two turned up portions 6b and a main portion 6a therebetween; a tread 2 disposed radially outside the carcass to define the tread portion; a belt 7 disposed radially outside the carcass 6 and inside the tread; and a bead apex 9 disposed between the carcass main portion 6a and each carcass turned up portion 6b and extending radially outwardly and taperingly from the bead core. The carcass 6 has one ply of cords arranged radially at an angle of 70 to 90 degrees with respect to the tyre equator C. For the carcass cords, organic fibre cords, e.g. nylon, rayon, polyester, aromatic polyamide or the like can be used. Each turned up portion 6b is extended radially outwardly to a position in the sidewall portion or tread portion. In this example, this is extended into the tread portion and sandwiched between the carcass and the belt edge portion. The radial height Ht of the radially outer edge of the turned up portion is 0.5 to 1.2 times the radial height Hs of the tread edge, both measured from the bead base line 15. When the height Ht is less than 0.5 times the height Hs, the reinforcement becomes insufficient for the bead portion to endure bending deformation, and the durability is deteriorated. When the height Ht is more than 0.5 times the height Hs, the stiffness of the tyre becomes excessive, and ride comfort is deteriorated. The bead apex 9 is made of hard rubber having a JIS A hardness of not less than 60. The radial height Ha of the radially outer edge thereof from the bead base line 15 is 0.3 to 0.7 times the radial height Hs of the tread edge from the bead base line 15, and the height Ha is lower than the height Ht. The belt 7 is made of spirally wound cords, and the belt width Wb measured along the curved belt is 0.7 to 1.0 times the tread width Wt measured between the tread edges E1 and E2 along the tread face. The belt is composed of a three-piece ply made up of a central piece 7a and two lateral pieces 7b and 7c as shown in Figs.1 and 2. The central piece 7a is centred on the tyre equator C, and the width WC measured between the edges thereof along the curved shape is 0.1 to 0.6 times the tread width WT measured between the tread edges E1 and E2 along the curved tread face. In Fig.1, the left piece 7b is extended from a position near the left tread edge E1 to the left edge of the central piece 7a, and the right piece 7c is extended from a position near the right tread edge E2 to the right edge of the central piece 7a. When the width WC is less than 0.1 times the tread width WT, the central piece 7a becomes too narrow in width, and the lateral piece having a different rigidity is in the ground contacting region in straight running, and therefore, stability is deteriorated. On the other hand, when the width WC is more than 0.6 times the tread width WT, the central piece remains in the ground contacting region in cornering, and cornering performance is deteriorated. In each ply piece, at least one belt cord 11 is wound spirally and continuously from one edge to the other edge thereof at zero angle or a small angle with respect to the tyre equator C. For the belt cord 11, organic fibre cords, for example polyfluoroethylene (TEFLON), aromatic polyamide, polyester or the like, or steel cords, having a high modulus of elasticity of not less than 600 kgf/mm², are used. If the elastic modulus is less than 600 kgf/mm², the tread portion has an insufficient rigidity, and the directional stability and cornering performance at high speed and the durability are deteriorated. Preferably, aromatic polyamide fibre cords having a high modulus of the same level as steel are used. In Fig.1 and 2, the edges of the axially adjacent pieces are butt jointed and not overlapped. However, as shown in Fig.3, the pieces 7a, 7b and 7c can overlap each other by a certain width. Each of the above-mentioned belt ply pieces 7a, 7b and 7c is formed by winding a ribbon 10 around the circumference of the carcass 6. The ribbon 10 is, as shown in Fig.4, a strip of rubber 12 in which a cord or a plurality of parallel cords, in this example, two parallel cords 11 are embedded. In order to make the three-piece belt 7, three ribbons 10 are used, and they are simultaneously wound around the carcass on a revolving drum as follows: the ribbon for the left piece 7b is wound from its axially outer edge to inner edge; the ribbon for the right piece 7c is wound from its axially outer edge to inner edge; the ribbon for the central piece 7a is wound from its one edge to the other edge, for example from the left edge to the right edge. Thus, the winding time decreases to about one third of the time when the ribbon is wound continuously from one edge to the other edge of the belt. The ribbons are fed towards the drum as the drum revolves, and by changing the feed directions of the ribbons, the belt cord inclinations or arrangement can be changed. If the ribbons for the left piece and central piece are supplied from the same direction and the ribbon for the right piece is supplied from an opposite direction, then the cords in the three pieces 7a, 7b and 7c are inclined in the same direction at small angles to the tyre equator. If the ribbons for the right piece and central piece are supplied from the same direction and the ribbon for the left piece is supplied from an opposite direction, then the cords in the right and left pieces 7b and 7c are inclined in the same direction, but the cords in the central piece 7a are inclined in an opposite direction with respect to the tyre equator. If the three ribbons are supplied from the same direction, then the cords in the right and left pieces 7b and 7c are inclined in opposite directions with respect to the tyre equator. Thus, in the last case, the right and left pieces are symmetrical with respect to the tyre equator, but in the former two cases the right and left pieces are asymmetrical. In the case of Figs.1 and 2, preferably, the ends of the ribbons of the adjacent ply pieces are butt jointed and fixed by means of adhesive tape or adhesive agent. While the ribbon 10 is being wound, the adjacent edges 10a can be overlapped as shown in Fig.5, whereby the wound ribbon is prevented from being loosened, and as a result belt edge separation during running is prevented. In Figs.4 and 5, the cross sectional shape of the ribbon 10 is a flat rectangle. However, in order to make the overlap smooth, the edges of the ribbon are preferably tapered. Thus, the cross sectional shape may be a flat hexagon, rhombus, trapezoid and the like. If an axial gap is formed between the axially adjacent edges of the ply pieces, the gap can be remained as it is. However, to reinforce the joint part, such a gap can be covered by a reinforcing strip disposed over the joint part. Further, as mentioned above, the edges can be overlapped each other. In order to provide a difference in rigidity between a tread central portion and a tread lateral portion, the central piece 7a is differed from at least one of the lateral pieces 7b and 7c in respect of at least one of the cord specification, e.g. material, thickness, twist, structure and the like, and the cord arrangement, e.g. inclination, pitch, direction and the like. For example, when the twist of the cord in the central piece 7a is less than that in the lateral pieces 7b and 7c, the tread central portion is provided with rigidity and the tread lateral portions are contrary provided with suppleness, thereby improving stability during high speed straight running and road grip during cornering. Further, the two lateral pieces 7b and 7c can be differed from each other to provide a difference in rigidity or suppleness between the tread lateral regions, which is effective if there is a difference in conditions between the turning directions, for example the radius of left turn is generally smaller than that of right turn. Test tyres of size 170/60VR17 including working example tyres 1-5 and reference tyres 1-2 having specifications given in Table 1 were made and tested for high speed cornering stability, high speed straight running stability and controlling feel. The working example tyres had the construction shown in Fig.1, and the reference tyre 1 had a conventional belt structure as explained with Figs.6 and 7, and the reference tyre 2 had a one-piece belt formed by winding a cord ribbon. In the test, the tyre was installed on a motorcycle and run on a test road at 260 km/h (in a straight course) and 220 km/h (in a 400m radius circular course). Then, using a ten point method, the tyre performance was evaluated by rider's feeling. The larger the value, the better the performance.

Claims: A radial tyre for a motorcycle having its maximum cross section width (WT) between tread edges (E1,E2), comprising a pair of bead cores (5) disposed one in each bead portion (4) of the tyre, a carcass (6) extending between the bead portions (4) through sidewall portions (3) and a tread portion (2), the carcass (6) having at least one ply of organic fibre cords arranged radially at an angle of 70 to 90 degrees to the tyre equator and turned up around the bead cores (5) to form two turned up portions (6b) and a main portion (6a) therebetween, a tread (2) disposed radially outside the carcass (6) to define the tread portion, and a belt (7) disposed radially outside the carcass (6) and inside the tread (2), the radial height Ht of the radially outer edge of each carcass turned up portion (6b) being 0.5 to 1.2 times the radial height (Hs) of the tread edge (E1,E2), both from the bead base (15), characterised by said belt (7) comprising two lateral pieces (7b,7c) and a central piece (7a) therebetween, each piece being made of at least one spirally wound cord having an elastic modulus of not less than 600 kgf/mm² and being formed by winding spirally a ribbon of rubber in which said at least one cord is embedded, and the width (WC) of the central piece (7a) being 0.1 to 0.6 times the tread width (WT). A radial tyre for motorcycles according to claim 1, characterised in that the central piece (7a) has a different cord specification to at least one of the other lateral pieces (7b or 7c). A radial tyre for motorcycles according to claim 1 or 2 characterised in that the central piece (7a) is different from at least one of the lateral pieces (7b or 7c) in respect of cord arrangement. A radial tyre for motorcycles according to claim 2 or 3 characterised in that the central piece (7a) is different from at least one of the lateral pieces (7b or 7c) in respect of cord material. A radial tyre for motorcycles according to any of claims 1 to 4 characterised in that the central piece (7a) is different from at least one of the lateral pieces (7b or 7c) in respect of cord thickness. A radial tyre for motorcycles according to claims 2 to 5 characterised in that the central piece (7a) is different from at least one of the lateral pieces (7b or 7c) in respect of cord twist. A radial tyre for motorcycles according to any of claims 1 to 6 characterised in that the central piece (7a) is different from at least one of the lateral pieces (7b or 7c) in respect of the cord pitches. A radial tyre for motorcycles according to any of claims 1 to 7 characterised in that the central piece (7a) is different from at least one of the lateral pieces (7b or 7c) in respect of cord direction. A radial tyre for motorcycles according to any of claims 1 to 8 characterised in that the central piece is different from at least one of the lateral pieces in respect of cord angle.

Inventors: NAITO SADAAKI, SUZUKI SHIGEHIKO, YASUI ITSUO, NAITO, SADAAKI, SUZUKI, SHIGEHIKO, YASUI, ITSUO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1991
Main classification: B60C9
IPC codes: B60C9, B60C15