Radial tyre for motorcycle

Abstract: A radial tyre for motorcycles which comprises at least one ply (6) of radially arranged organic fibre cords turned up around the bead cores (5) to form two turned up portions (6b) and a main portion (6a) therebetween; a tread disposed radially outside the carcass to define the tread portion (2), sidewalls (13) disposed axially outside the carcass to define the sidewall portions, a belt (7) disposed radially outside the carcass and inside the tread, 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 (5), the radial height (Ht) of the radially outer edge of each carcass turned up portion (6a) being 0.3 to 0.5 times the radial height (Hs) of the tread edge, both measured from the bead base (15), the radial height (Ha) of the radially outer edge of each bead apex from the bead base (15) being 0.3 to 0.5 times said radial height (Hs) of the tread edge, the belt (7) composed of at least one spirally wound cord having an elastic modulus of not less than 600 kgf/sq.mm, and formed by winding spirally a ribbon of rubber in which at least one cord is embedded, and each sidewall (13) composed of an axially inner hard rubber layer (13a) having a JIS A hardness of 65 to 75 and an axially outer soft rubber layer (13b) disposed on the axially outside of the inner layer and having a JIS A hardness of not more than 60.

Description: The present invention relates to a radial motorcycle tyre improved in high speed steering stability and high speed durability. Recently, motorcycle tyres have come to use a radial ply carcass. Such radial motorcycle tyres have been based on techniques developed 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 at a small bias angle (α) to the cord direction as shown in Figure 8, which is usually 15 to 30 degrees; and the cut fabric (e) is wound around a carcass with the ends (f) connected as shown in Figure 7 to form an annular belt reinforcement (a). Accordingly, the laid angle of the belt cords (b) to the tyre equator becomes the same bias angle (α). In such a bias belt structure, however, steering stability during straight running and cornering, especially at high speed, is not good. Alternatively, Japanese Utility Model Publication No 58-160805 discloses a belt making method, in which a belt cord is wound spirally at a generally zero angle to the circumferential direction of the tyre to form a jointless belt. When this is used in the motorcycle tyre instead of the conventional belt, high speed steering stability is improved, but the stiffness of the tyre is reduced and bead durability is lost. A tyre according to the preamble of claim 1 is known for example from patent publication US-A-4 773 462. It is therefore an object of the present invention to provide a radial motorcycle tyre, in which straight running performance and cornering performance during high speed running are improved, and at the same time durability of the tread portion is improved. According to one aspect of the present invention a radial tyre for motorcycles having its maximum cross section width lying 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 of the tyre, the carcass having at least one ply of organic fibre cords arranged radially at an angle of 60 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, sidewalls disposed axially outside the carcass to define the sidewall portions, a belt disposed radially outside the carcass and inside the tread, and a bead apex disposed between the carcass main portion and each carcass turned up portion and extending radially outwardly and taperingly from the bead core, wherein the radial height of the radially outer edge of each carcass turned up portion is 0.3 to 0.5 times the radial height of the tread edge, both measured from the bead base, the radial height of the radially outer edge of each bead apex from the bead base is 0.3 to 0.5 times said radial height of the tread edge, the belt comprising at least one spirally wound cord having an elastic modulus of not less than 4903 MPa (500 kgf/sq.mm), and formed as a spiral winding of a ribbon of rubber in which said at least one cord is embedded, and each sidewall comprises of an axially inner hard rubber layer having a hardness of 67 to 78 Shore A (65 to 75 JIS A) and an axially outer soft rubber layer disposed on the axially outside of the inner layer. An embodiment of the present invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 is a sectional view showing an embodiment of the present invention; Figure 2 is a perspective view showing a belt cord ribbon; Figure 3 is a sectional view showing winding of the ribbon; Figure 4 is an enlarged view thereof; Figure 5 is a plan view showing an example of termination of the ribbon. Figure 6 is a developed plan view showing the arrangement of the carcass and belt; and Figure 7 and 8 are plan views showing prior art belts. 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 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 a 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, sidewalls 13 disposed axially outside the carcass to define the sidewall portions, 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 60 to 90 degrees, preferably 70 to 90 degrees, with respect to the tyre equator C. For the carcass cords, organic fibre cords, such as nylon, rayon, polyester or aromatic polyamide can be used. Each turned up portion 6b is extended radially outwardly to a position in the sidewall portion, and the radial height Ht of the radially outer edge of the turned up portion is 0.3 to 0.5 times the radial height Hs of the tread edge, both measured from the bead base line 15. When the height Ht is less than 0.3 times the height Hs, the reinforcement becomes insufficient for the bead portion to endure bending deformation, and the durability is reduced. When the height Ht is is more than 0.5 times the height Hs, the rigidity of the sidewall portion as well as the bead portion is increased excessively, and ride comfort deteriorates. 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 L thereof from the bead base line 15 is 0.3 to 0.5 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. When the height Ha is less than 0.3 times the height Hs, the rigidity of the bead portion against bending deformation is low, and the bead durability is reduced. When the height Ha is more than 0.5 times the height Hs, the rigidity of the sidewall portion and the bead portion are increased excessively and ride comfort deteriorates. Each sidewall 13 is composed of two rubber layers: an axially inner sidewall 13a and an axialy outer sidewall 13b both extending from the tread edge to the bead portion. The inner sidewall 13a is made of hard rubber, having a hardness of 67 to 78 Shore A (65 to 75 JIS A), to reinforce the sidewall portion, but the outer sidewall 13b is made of softer rubber preferably having a hardness of not more than 62 Shore A (60 JIS A). By setting the hardness of the outer sidewall to be not more than 62 Shore A, cracking on the outer surface of the sidewall is prevented. When the hardness of the inner sidewall is less than 67 Shore A (65 JIS A), the bending deformation of the sidewall becomes large, and separation failure is apt to be caused at the carcass turned up edge. When the hardness is more than 78 Shore A (75 JIS A), ride comfort is impaired. The inner hard rubber sidewall 13a is further extended to the bead base along the outside of the carcass main portion and turned up portion and terminated at the bead heel so as to form the axially outer face of the bead portion which comes into contact with the flange of the wheel rim. The radially inner edge of the outer sidewall 13b is however terminated at a point radially outward of the radially outer edge of the rim flange. The belt 7 is composed of at least one spirally wound cord, 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. When the width Wb is less than 0.7 times the tread width Wt the rigidity of tread shoulder regions is reduced, and stability in rapid turning is lost. When the width Wb is more than 1.0 times the tread width Wt, ride comfort deteriorates due to excessively increased sidewall rigidity. In this embodiment, the belt 7 is composed of a two-piece ply 7 made up of a piece 7a and a piece 7b as shown in Figures 1 and 6. Each ply piece is extended from a portion near the tread edge E1,E2 to the tyre equator, and 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, aromatic polyamide, polyester and the like, or steel cords, having a high modulus of elasticity of not less than 5884 MPa (600 kgf/sq.mm) are used. When the elastic modulus is less than 5884 MPa (600 kgf/sq.mm), the tread portion has an insufficient rigidity, and the directional stability and cornering performance at high speed and durability are reduced. Preferably aromatic polyamide fibre cords having a high modulus of the same level as steel are used. The belt 7 is formed by winding a ribbon 10 around the circumference of the carcass 6. The ribbon 10 is, as shown in Figure 2, a strip of rubber 12 in which a cord or a plurality of parallel cords, in this embodiment two parallel cords 11, are embedded. In this embodiment, as shown in Figure 2, the cross sectional shape of the ribbon 10 is a flat rectangle. The distance N measured from the edge 10a of the ribbon to the centre of the adjacent cord, that is the outermost cord, is set to be not more than 1/2 of the cord pitch P. In order to make the above-mentioned two-piece ply, as shown in Figure 3, two ribbons 10 are simultaneously wound towards the tyre equator C from the respective axially outer edges F1 and F2. As indicated in Figure 3 by a chain line, the ribbons to be wound are supplied from two different or opposite directions. Therefore, the ribbon in the ply piece 7a and the ribbon in the ply piece 7b are inclined in the same direction at the same small angle to the tyre equator. Preferably, the terminal ends G1 and G2 of the ribbons of the ply piece 7a and 7b are butt jointed on the tyre equator as shown in Figure 5 and fixed by means of adhesive tape or adhesive agent. When the ribbon 10 is being wound, the adjacent edges 10a are overlapped as shown in Figure 4. Accordingly, the wound ribbon is prevented from being loosened, and as a result belt edge separation during running is prevented. By winding in this way, the belt has an asymmetrical structure by supplying two ribbons from the same direction contrary to the arrangement shown in Figure 3. Thus, the winding direction of the cords in one belt ply piece is different from that in the other piece. Further, in each of the pieces and /or between the pieces, the inclination of the cords can be changed. Furthermore, the belt 7 can be formed by winding a ribbon 10 spirally around the carcass continuously from one edge to the other edge. If an axial gap of not more than the cord pitch P is formed between the axially inner edges of the ply pieces 7a and 7b, the gap can remain as it is. However, to reinforce the joint part, such a gap can be covered by a reinforcing strip disposed over the joint part. Test tyres of size 170/60VR17 including a working example tyre and a reference tyre having specifications given in Table 1 were made and tested for high speed cornering stability, high speed straight running stability and high speed durability. The wording example tyre had the construction shown in Figure 1, and the reference tyre had a conventional belt structure explained with Figures 7 and 8 and single-layer sidewalls. In the measurement of high speed straight running stability and high speed cornering stability, the test tyre was installed on a motorcycle and run at 260 km/h on a straight course and at 220 km/h on a 400m radius circular course. Then, using an index based on the assumption that the working example tyre is 100, the stabilities were evaluated by rider's feeling. The larger the value, the better the performance. In the determination of high speed durability, the test tyre, mounted in its recommended rim and inflated to 0.294 Pa (3.0 kgf/sq.cm) and loaded with 355 kg, was run on a test drum, and the running speed was increased in steps of 10 km/h every 10 minutes from the initial speed of 250 km/h, and the total running time until any failure occurred in the tread portion was recorded as its high speed durability. The durability is indicated by an index based on the assumption that the working example tyre is 100. The larger the value, the better the performance. As described above, in the radial motorcycle tyre according to the present invention, the stability in high speed running and durability are improved. Carcass Example Tyre Reference Tyre Cord material nylon 1260d/2 nylon 1260d/2 Cord angle (deg) 90 88 Ht (mm) 24 27 Ht/Hs 0.48 0.54 Belt Cord material aramid 1500d/2 nylon 1890d/2 Cord angle (deg) 0 16 Modulus (MPa) 6178 5687 (kgf/sq.mm) (630) (580) Wb/Wt 0.80 0.65 Bead apex Ha (mm) 22 26 Ha/Hs 0.44 0.65 Sidewall 2 layers 1 layer Hardness Shore A 60 (outer) 60 75 (inner) (JIS A) (58) (outer) (58) (72) (inner) Test Result Straight 100 75 Cornering 100 80 Durability 100 75

Claims: A radial tyre for motorcycles having its maximum cross section width (Wt) lying 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) of the tyre, the carcass (6) having at least one ply of organic fibre cords arranged radially at an angle of 60 to 90 degrees to the tyre equator (C) and turned up around the bead cores (5) to form two turned up portions (6b) and a main portion (6a) therebetween, a tread disposed radially outside the carcass to define the tread portion (2), sidewalls (13) disposed axially outside the carcass (6) to define the sidewall portions, 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 (5), wherein the radial height (Ht) of the radially outer edge of each carcass turned up portion (6b) is 0.3 to 0.5 times the radial height (Hs) of the tread edge, both measured from the bead base (15), the radial height (Ha) of the radially outer edge of each bead apex (9) from the bead base (15) is 0.3 to 0.5 times said radial height (Hs) of the tread edges, characterised in that the belt (7) comprises at least one spirally wound cord having an elastic modulus of not less than 4903 MPa (500 kgf/sq.mm), and formed as a spiral winding of a ribbon of rubber in which said at least one cord is embedded, and each sidewall (13) comprises of an axially inner hard rubber layer (13a) having a hardness of 67 to 78 Shore A (65 to 75 JIS A) and an axially outer soft rubber layer (13b) disposed on the axially outside of the inner layer (13a). A tyre according to claim 1, characterised in that the axially outer soft rubber layer (13b) has a hardness of not more than 62 Shore A (60 JIS A). A tyre according to claim 2 characterised in that the axially inner hard rubber layer (13a) is extended from the tread edge (E1,E2) to the bead base (15) along the axially outer side of the carcass (6), and the axially outer soft rubber carcass (6), and the axially outer soft rubber layer (13b) is extended from the tread edge (E1,E2) to a position radially outward of the radially outer edge of the flange of the recommend wheel rim for the tyre.

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, B60C13