Pneumatic tyre

Abstract: A pneumatic tyre comprises a tread rubber layer (2), a side rubber layer (V) and a wing rubber layer (12) interposed between the tread rubber layer (2) and the side rubber layer (V). The side rubber layer (V) comprising a radially outer black sidewall rubber (15), a radially inner black sidewall rubber (16) and a white sidewall rubber (14) therebetween. The surface of the wing rubber layer (12) consists of a part (12a) contacting with the carcass (6), a part (12b) contacting with the tread rubber layer (2) and a part (12c) contacting with the radially outer black sidewall rubber (15) and the white sidewall rubber (14). The surface of the radially outer black sidewall rubber (15) consists of a part (15c) extending form the tread edge (P1) to a point (P3) on the tyre outer surface, a part (15a) contacting with the wing rubber layer (12) and extending from the tread edge (P1) to a point (P2), and a part (15b) contacting with the white sidewall rubber (14) and extending from the point (P2) to the point (P3). The radially inner black sidewall rubber (16) extends radially inwardly from a point (P4) on the tyre outer surface radially inside the point (P3) while contacting with the axially outside of the white sidewall rubber (14). The white sidewall rubber (14) is provided between the point (P3) and the point (P4) with a protruding part (17) so that the thickness (DP) of the white sidewall rubber (14) in the protruding part (17) measured from the outer surface to cords of the carcass is not less than 4 mm. The thickness (DS) of the radially outer black sidewall rubber (15) having a maximum value of not less than 3 mm at the point (P2).

Description: The present invention relates to a pneumatic tyre provided in the sidewall portion with a white rubber layer, more particularly to an improved tyre sidewall structure being capable of preventing the white rubber from change of colour due to adjacent black rubber. In pneumatic tyres for motorcycles, passenger cars and the like provided in the sidewall portions with white veneer rubber, the part of the white rubber adjacent to the black rubber is liable to change colour because chemicals, e.g. antidegradant and the like spread from the black rubber to the white rubber with time. Therefore, if the white veneer rubber on the black rubber is thin, the whole of the white rubber becomes dirty. If the thickness of the white veneer rubber is increased, this may be avoided, but the durability and steering stability of the tyre are liable to deteriorate because white rubber generally has a lower rigidity and a lower strength than the black rubber. In the case of motorcycle tyres, in comparison with tyres in other categories such as passenger car tyre and the like, the sidewall portion is relatively small because the tread portion is curved with a relatively small radius of curvature and accordingly approaches the bead portion. As a result, the sidewall space taken up by the white veneer rubber is large, and the durability and steering stability decreased since the deformation of the sidewall portion is effectively larger during cornering. Thus it is difficult to increase the occupation of the white veneer rubber on the black rubber without greatly deteriorating the durability, steering stability and the like. A tyre according to the preamble of claim 1 is known from GB 1,017,464. It is therefore, an object of the present invention to provide a pneumatic tyre, in which a white rubber layer disposed in the sidewall portion can be prevented from change of colour without decreasing durability, steering stability and the like. According to the present invention, a pneumatic tyre comprises a tread rubber layer disposed radially outside a carcass to define a part of the tyre outer surface between tread edges, a side rubber layer disposed axially outside the carcass to define a part of the tyre outer surface radially inward of each tread edge, the side rubber layer comprising a radially outer black sidewall rubber, a radially inner black sidewall rubber and a white sidewall rubber therebetween, a wing rubber layer interposed between the tread rubber layer and the side rubber layer the surface of the wing rubber layer consisting of a part contacting with the carcass a part contacting with the tread rubber layer, and a part contacting with the radially outer black sidewall rubber and the white sidewall rubber, and the surface of the radially outer black sidewall rubber consisting of a part extending form the tread edge to a point P 3 on the tyre outer surface, a part contacting with the wing rubber layer and extending from the tread edge to a point P2, and a part contacting with the white sidewall rubber and extending from the point P2 to the point P3, the radially inner black sidewall rubber extending radially inwardly from a point on the tyre outer surface radially inside the point P3 while contacting with the axially outside of the white sidewall rubber, the white sidewall rubber provided between the point P3 and the point P4 with a protruding part so that the thickness of the white sidewall rubber in the protruding part measured from the outer surface to cords of the carcass is not less than 4 mm, and the thickness of the radially outer black sidewall rubber having a maximum value of not less than 3 mm at the point P2. An embodiment 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 pneumatic tyre showing an embodiment of the present invention; and Fig.2 is an enlarged cross sectional view of the sidewall portion thereof. In Figs.1 and 2, the pneumatic tyre 1 according to the present invention is for motorcycles, and is shown as mounted on a standard wheel rim R and inflated to standard inner pressure but loaded with no tyre load. Here, the standard rim is the "standard rim" specified in JATMA, the "Measuring Rim" in ETRTO, the "Design Rim" in TRA or the like. The standard pressure is the "maximum air pressure" in JATMA, the "Inflation Pressure" in ETRTO, the maximum pressure given in the "Tire Load Limits at Various Cold Inflation Pressures" table in TRA or the like. In case of passenger car tyres, however, as the standard pressure, 180 kPa is used. Further, a standard load is the "maximum load capacity" in JATMA, the "Load Capacity" in ETRTO, the maximum value given in the above-mentioned table in TRA or the like. The pneumatic tyre 1 comprises a tread portion 2, a pair of axially spaced bead portions 4 each with a bead core 5 therein, a pair of sidewall portions 3 extending between the tread edges P1 and the bead portions 4, and a carcass 7 extending between the bead portions 4. The carcass 7 comprises at least one ply of cords 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 from the axially inside to the outside of the tyre to form a pair of turnup portions 6b and a main portion 6a therebetween. In this example, the carcass 7 is composed of an inner ply 6A, a middle ply 6B and an outer ply 6C. In the tread portion 2, a belt 9 is disposed radially outside the carcass 7. Further, a tread rubber 10 is disposed radially outside of the belt 9. The tread rubber 10 extends between the tread edges P1 to define the tread face 2A. In a tyre meridian section including the tyre axis, the tread face 2A is curved convexly. As the tyre in this embodiment is a motorcycle tyre, the tread width TW is the maximum tyre width or substantially equal to the maximum tyre width. Here, the tread width TW is the axial distance between the tread edges P1 under a standard condition in which the tyre is mounted on the standard rim and inflated to the standard load. The tread rubber 10 has a side face 10a extending radially inwardly and axially inwardly from each tread edge P1, the inner end (b) of the side face 10a is located on the outer surface of the carcass 7. In a tyre side region S radially inward of each tread edge P1, a side rubber V is disposed axially outside the carcass 7. The side rubber layer V comprises a radially outer black sidewall rubber 15, a radially inner black sidewall rubber 16 and a middle white sidewall rubber 14 therebetween. Further, a wing rubber layer 12 is interposed between the tread rubber 2 and the side rubber layer V. The surface of the wing rubber layer 12 consists of: a part 12b abutting on the side face 10a of the tread rubber 10 and extending from the tread edge P1 to a point (b) on the outer surface of the carcass 7; a part 12a abutting on the outer surface of the carcass 7 and extending from the point (b) to a radially inner point (c) on the carcass outer surface; and a part 12c extending from the tread edge P1 to the point (c). In this example, the wing rubber layer 12 has a substantially triangular cross sectional shape The surface of the radially outer black sidewall rubber 15 consists of: a part 15a abutting on the surface part 12c of the wing rubber layer 12 and extending from the tread edge P1 to a point P2 about the middle of the surface part 12c; a part 15b abutting on the axially outer surface of 14c of the white sidewall rubber layer 14 and extending from the point P2 to a point P3 on the tyre outer surface; and a part 15c extending from the tread edge P1 to the point P3 to define a part of the tyre outer surface. In this example, the radially outer black sidewall rubber 15 has a substantially triangular cross sectional shape. The surface of the radially inner black sidewall rubber 16 consists of: a part 16b1 extending radially inwardly from a point P4 to a point (e) both on the tyre outer surface to define a part of the tyre outer surface, wherein the tyre outer surface contact with the flange of the wheel rim at the point (e); a part 16b2 extending axially and radially inwardly from the point (e) to a point (f) near the bead bottom 4a to define a part of the tyre outer surface which contacts with the wheel rim flange; a part 16a extending from the point P4 to the point (f) and abutting on the axially outer surface 14c of the white sidewall rubber 14 from the point P4 to a point (d) between the points P4 and (f). The point (f) or the radially inner end of the radially inner black sidewall rubber 16 is set in a range of not more than the radial height of the rim flange. The white sidewall rubber 14 extends from the above-mentioned point (b) to the point (d). In the range from the point (c) to the point (d), the axially inner surface 14a of the white sidewall rubber 14 abuts on the outer surface of the carcass 7. A part 14b of the surface from the point (c) to the point P2 abuts on the surface part 12c of the wing rubber layer 12. The black sidewall rubber 15, 16 is a rubber compound containing at least one kind of polymer whose principal chain has no double bond such as chlorobutyl, EPDM and the like. The total content of at least one kind of polymer is in a range of from 35 to 75, preferably 40 to 60 parts by weight with respect to 100 parts by weight of the rubber polymer (for example, 100 parts by weight of the rubber polymer is 25 parts by weight of chlorobutyl and 25 parts by weight of EPDM and 50 parts by weight of natural rubber NR). An antidegradant which causes an unfavourable colour change to the white rubber layer 14 is not used. In general, the antidegradant includes antiozonant, antioxidant, wax and the like, and a widely used antiozonant includes amine which causes colour change. Therefore, such antiozonant is not used. As the above-mentioned wing rubber layer 12 is interposed between the side rubber V and tread rubber 2 to secure the connection therebetween, the wing rubber layer 12 is made of a rubber compound which has a good bonding property with the tread rubber 10, radially outer black sidewall rubber 15, white rubber layer 14 and the topping rubber of the carcass 7. As the wing rubber layer 12 does not appear on the tyre outer surface, it is not always necessary to add an antiozonant, but it is also possible to add one. The radial height of the point (d) at which the radially inner end of the white sidewall rubber 14 is disposed is set in a range of from 50 to 150 %, preferably 70 to 130 % (in this embodiment not more than 100%) of the height of the flange of the wheel rim. Here, a radial height is measured from the bead base line BL under a standard condition in which the tyre is mounted on the standard rim and inflated to the standard load. As to the radially outer black sidewall rubber 15, the thickness measured normally to the axially outer surface 15c becomes a maximum at the above-mentioned point P2, and the maximum thickness DS is set in a range of not less than 3 mm, preferably 3.5 to 4.5 mm, more preferably 4.0 to 4.5 mm. From the point P2, the thickness gradually decreases towards the radially outer end and inner end at P1 and P3, respectively. As a result, the radially outer black sidewall rubber 15 has a substantially triangular cross sectional shape. Further, the radially outer end portion of the white sidewall rubber 14 is tapered towards the point P2. In a range between the above-mentioned points P3 and P4, that is, between the edges of the radially outer black sidewall rubber 15 and the radially inner black sidewall rubber 16, the white rubber layer 14 appears on the outside of the tyre and protrudes axially outwards of the tyre. The protruding portion 17 increases the thickness DP of the white rubber layer 14 to increase the sidewall rigidity. The surface of the protruding portion 17 consists of: a radially outer concave part 14c1 extending from the point P3 to a point P5 radially inward and axially outward of P3; a radially inner concave part 14c2 extending from the point P4 to a point P6 radially and axially outward of P4; and a generally convex part 14c3 between the points P5 and P6. In this embodiment, the convex part 14c3 is substantially parallel to a curved line drawn tangentially to the part 15c at point P3 and further tangentially to the part 16b1 at point P4. The thickness DP measured from the surface 14c3 to the cords of the carcass 7 normally to the surface 14c3 is set in the range of not less than 4 mm, preferably 4.5 to 5.5 mm, more preferably 5.0 to 5.5 mm. Further, the radial distance between the points P3 and P4 is preferably in a range of not more than 35 mm. The protruding portion 17 in this embodiment extends continuously in the tyre circumferential direction. But, it is also possible to break the protruding portion 17 regularly or irregularly in the tyre circumferential direction as far as the rotational balance of the tyre is maintained. Of course, letters, marks and the like can be provided on the surface of the white sidewall rubber 14. As the thickness DP of the white rubber 14 is increased in the protruding portion 17, change of colour by the antidegradant of the topping rubber of the carcass 7 can be prevented. Further, the rigidity is appropriately increased to improve the steering stability and durability. In this embodiment, further, the radially outer ends of the turnup portions of the carcass plies 6A, 6B and 6C are disposed at different radial heights. In this example, the heights are gradually decreased from the axially outmost turnup portion to the axially innermost turnup portion. In other words, the radial heights are gradually decreased in the following order: the end 6A1 of the turnup portion of the inner ply 6A; the end 6B1 of the turnup portion of the middle ply 6B; and the end 6C1 of the turnup portion of the outer ply 6C. Furthermore, the ends 6A1, 6B1 and 6C1 of all the turnup portions are disposed in a radial height range between the points P5 and P6, whereby the rigidity in the protruding portion 17 is further improved, and so called cord-grooving -- a circumferentially extending dent caused on the tyre outer surface along the edge of the turnup portion -- is effectively prevented. Each bead portion 4 is provided with a reinforcing layer 23 made of organic fibre cords extending between the bead toe 20 and the bead heel 21 and extending radially outwards from the bead toe 20 and bead heel 21. The axially outer edge of the reinforcing layer 23 reaches to the radially inner end (point d) of the white sidewall rubber 14 and is disposed between the radially inner black sidewall rubber 16 and the carcass 6. The axially inner edge of the reinforcing layer 23 is also extended to the almost same radial height as the outer edge. In this embodiment, the radially inner black sidewall rubber 16 and the radially outer black sidewall rubber 15 are the same rubber compound, but it is possible to increase the hardness of the radially inner black sidewall rubber 16 to improve the resistance to rim chafing. Comparison Tests Test tyre of size 150/90-15 having the same structure shown in Figs.1 and 2 except for the thickness DP of the white rubber layer and rubber compounds for the black sidewall rubbers were made. The carcass was composed of three plies of nylon fibre cords arranged at 30 degrees with respect to the tyre equator . The carcass ply turnup portions' heights were 60, 50 and 40 mm (from the axially outside to the inside). The belt was composed of two plies of nylon fibre cords laid at 30 degrees with respect to the tyre equator. The test tyres mounted on a standard wheel rim of size 15XMT4.00 and inflated to an inner pressure of 280 kPa were tested as follows. (1) Colour change test Under an accelerated condition, the change of colour when exposed outdoors for one week, two weeks, one month, three months, six months, one year and two years, was observed. (2) Handling stability test A Japanese motorcycle having a 1200cc 4-cycle engine provided on the rear wheel with a test tyre was run on a dry asphalt road in a test course, and the handling stability was evaluated by the test rider into five ranks. The higher the rank, the better the handling stability. (3) Durability test Using a drum tester, the test tyre was run for 200 hours at a speed of 65 km/h under a tyre load of 419 kgf at a room temperature of 25 deg. C, and the outside of the tyre was checked and then the inside was checked by a cut-open inspection. The test results and rubber compounds are shown in Tables 1 and 2. Tyre Ex.1 Ex.2 Ex.3 Ex.4 Ref.1 Ref.2 Rubber compound Tread rubber A A A A A A Wing rubber B B B B B B Side rubber Outer black rubber C C C C B D Thickness DS (mm) 3.2 3 3 3 3 3 Middle white rubber E E E E E E Thickness DP (mm) 2 3 4 5 4.5 4.5 Inner black rubber C C C C B D Colour change of White rubber 1 week A A A A A A 2 weeks A A A A A A 1 month A A A A A A 3 months A A A A B B 6 months B A A A B B 1 year B B A A C C 2 years C C A A C C Handling stability B B A A B B Durability A A A A B B A: good B: acceptable C: not acceptable (parts by weight) Rubber compound A B C D E Polymer Natural rubber 24 40 50 50 Isoprene rubber 16 Butadiene rubber 60 Chlorobutyl rubber 25 50 EPDM 25 SBR 100 BR 60 Carbon N550 50 N660 40 60 N351 90 Accelerator 1.5 1 2 1.5 2 Antidegradant 2.5 3 4 2 Oil 20 15 10 15 Sulfur 1.5 1 0.5 1 1 From the test results, it was confirmed that the example tyres according to the present invention were improved in colour change, handling stability and durability results. As described above, in the present invention, the white sidewall rubber extends between a position near the tread edge to the bead portion so as to form the major part of the tyre sidewall. The thin part of the radially outer black sidewall rubber on the axially outside of the white sidewall rubber which extends from the point P3 towards the point P2 functions as a veneer rubber, and the thick part axially outside the point P2 has a function to secure the radially outer end of the white rubber deep in the tyre sidewall. The wing rubber has a function to improve the bond between the white rubber and the tread rubber in co-operation with the radially outer black rubber. The radially inner black sidewall rubber has functions to protect the white rubber from the rim flange and to reinforce and veneer the radially inner part thereof. In the present invention, thus, the white rubber is disposed on the axially outside of the carcass instead of the usual black sidewall rubber and the radially outer and inner black sidewall rubber is used for the above explained supplemental purposes. The present invention is suitably applied to a motorcycle tyre of which aspect ratio is not less than 70% independently of a carcass structure - bias or radial - and belted or not. However, it is also possible to apply the present invention to other vehicle tyres such as passenger car tyres.

Claims: A pneumatic tyre comprising a tread rubber layer (2) disposed radially outside a carcass (6) to define a part of the tyre outer surface between tread edges (P1), a side rubber layer (V) disposed axially outside the carcass (6) to define a part of the tyre outer surface radially inward of each said tread edge (P1), and comprising a radially outer black sidewall rubber (15), a radially inner black sidewall rubber (16) and a white sidewall rubber (14) therebetween, characterised in that said side rubber layer (V) further comprises a wing rubber layer (12) interposed between the tread rubber layer (2) and the side rubber layer (V), the surface of said wing rubber layer (12) consisting of a part (12a) contacting with the carcass (6), a part (12b) contacting with the tread rubber layer (2), and a part (12c) contacting with the radially outer black sidewall rubber (15) and the white sidewall rubber (14), and the surface of the radially outer black sidewall rubber (15) consisting of a part (15c) extending form the tread edge (P1) to a point (P3) on the tyre outer surface, a part (15a) contacting with the wing rubber layer (12) and extending from the tread edge (P1) to a point (P2), and a part (15b) contacting with the white sidewall rubber (14) and extending from the point (P2) to the point (P3), the radially inner black sidewall rubber (16) extending radially inwardly from a point (P4) on the tyre outer surface radially inside said point (P3) while contacting with the axially outside of the white sidewall rubber (14), the white sidewall rubber (14) provided between the point (P3) and the point (P4) with a protruding part (17) so that the thickness (DP) of the white sidewall rubber (14) in the protruding part (17) measured from the outer surface to cords of the carcass is not less than 4 mm, and the thickness (DS) of the radially outer black sidewall rubber (15) having a maximum value of not less than 3 mm at the point (P2). A pneumatic tyre as set forth in claim 1, characterised in that each of the radially outer black sidewall rubber (15) and radially inner black sidewall rubber (16) contains a polymer whose main chain has no double bond. A pneumatic tyre as set forth in claim 1 or 2, characterised in that the carcass (6) comprises three plies which are turned up in each bead portion (4) from the inside to the outside of the tyre to form three turnup portions (6a,6b,6c), the radially outer ends (6A1,6B1,6C1) of which are disposed axially inside said protruding part (17), and the radial heights of the radially outer ends are gradually decreased from the axially outer turnup portion (6a) to the axially inner turnup portion (6c). A pneumatic tyre according to any of claims 1 to 3, characterised in that the thickness (DP) of the white sidewall rubber is in the range of 4.5 to 5.5 mm. A pneumatic tyre according to any of claims 1 to 3, characterised in that the thickness (DP) of the white sidewall rubber is in the range of 5.0 to 5.5 mm. A pneumatic tyre according to any of claims 1 to 5 characterised in that the thickness (DS) is in the range of 3.5 to 4.5 mm. A pneumatic tyre according to any of claims 1 to 5 characterised in that the thickness (DS) is in the range of 4.0 to 4.5 mm.

Inventors: NIIZATO KAZUYUKI, NIIZATO, KAZUYUKI
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 2000
Main classification: B60C13
IPC codes: C08L21, B60C11, B60C1, B60C15, B60C13