Improvements to tyres

Abstract: A pneumatic radial tyre having in axial cross-section a highly curved tread reinforced by a breaker assembly with improved resistance to breaker edge looseness in disclosed. The breaker assembly includes one breaker ply which extends between two bead regions and has its edges disposed between a carcass main portion and a carcass turn-up portion. An improved single-stage method of building such a tyre is also disclosed.

Description: The present invention relates to pneumatic tyres having a radial carcass structure and particularly to such tyres having ground contacting tread regions which are substantially curved in axial cross-section. The invention also relates to improvements in the manufacture of such radial tyres. Radial motorcycle tyres are one such type of tyre having a highly curved tread. These tyres have relatively short sidewalls extending radially and axially outward from beads with the maximum tyre width being located between the edges of the tread region. The highly curved tread allows the contact area between the tyre and the road surface to be maintained as the motorcycle is banked over for cornering. In addition to tyres for motorcycles there has recently been proposed in co-pending European Patent Application 94309359.1, published as EP-065 8450 a radial tyre for a passenger car. Unlike conventional flat-treaded car tyres this new tyre has a tread region having substantial curvature in transverse cross-section. In straight-ahead running the contact area between the tyre tread and the road is relatively narrow in comparison to that of the conventional tyre. In tests this new tyre has shown improved characteristics which are believed to be due in part to the ability of the contact area to move across the tread and/or increase in axial width in response to changes in load and in cornering. In common with conventional flat-tread radial tyres the above-described highly curved treaded tyres have their ground contacting tread regions reinforced by a breaker or belt which extends circumferentially around the tyre radially outward and adjacent to the tyre carcass. Conventionally such breakers comprise plural plies of tyre fabric reinforced by parallel cords disposed at an angle to the circumferential direction, the cords of one ply being crossed with respect to the cords of an adjacent ply. A common problem with such belted tyres and particularly with curved treaded tyres is so called breaker edge looseness wherein the bond between the cord reinforcement and its surrounding rubber breaks down at the edge of the breaker leading to premature failure of the tyre. In Japanese Publication No. JP 58073405 there is disclosed a tyre for a motorcycle in which a breaker ply extends between the two bead regions and has its edges disposed between the carcass main portion and the respective turnup portion of the carcass in the bead region axially inwards of an apex intended to improve the lateral rigidity of the tyre by providing an additional reinforcing layer on the outside of the carcass. It is therefore a first object of the present invention to improve the properties of radial tyres of the above-mentioned highly curved tread type including improving the resistance to breaker edge looseness. Radial tyre manufacture has conventionally employed a two-stage process. In the first-stage a cylindrical tyre carcass is built on the cylindrical surface of a drum, the body comprising an assembly of inner liner and one or more carcass plies anchored to and interconnecting axially spaced apart annular bead cores or hoops. On completion of the first stage assembly the cylindrical carcass is removed from the drum and transferred to a second-stage machine. In the second stage of manufacture the shape of the carcass is changed from cylindrical to toroidal and then the individual breaker plies are applied sequentially to the crown of the toroid followed finally by the rubber tread strips. Such a two-stage manufacturing operation is expensive in respect of equipment, factory space, labour and time. Single-stage manufacture of radial tyres has been proposed previously. For example GB 1 569 640 discloses the single-stage assembly of a conventional flat-treaded radial tyre having an additional zero degree cap band overlaying the breaker or belt. However the single-stage or so-called 'flat building' of a radial tyre assembly necessitates a considerable increase in the circumferential length of the breaker plies when the shape of the final assembly is changed from cylindrical to toroidal. This increase in circumferential length of the breaker plies is accomplished by a complex combination of stretching of the plies and trellising of the cords. In the prior art it has been found that due to the adhesion of the breaker cords to other components of the assembly, the complex movement of the breaker plies has resulted in uncontrollable and uneven movement of the cords, distortion of the carcass ply and consequent malformation of the tyre. For this reason the single-stage manufacture of a radial tyre has not been adopted. US Patent 3 730 246 discloses a single stage method of building a radial tyre which uses reinforcing cords of predetermined stretchability for manufacture of a conventional flat treaded tyre. It is therefore another object of the invention to provide an improved single-stage method by which such an improved radial tyre may be efficiently built. According to one aspect of the invention a pneumatic tyre is provided as set out in claim 1. By camber value is meant the ratio C/L between the radial distance C from the centre to the edge of the tyre tread and the axial distance L between the centre and edge of the tread. By normally inflated and fitted state is meant the state in which the tyre is mounted on its recommended wheelrim and inflated to its scheduled pressure. According to another aspect of the invention a method of fabricating the tyre of claim 1 is set out in claim 9. The steps of forming a cylindrical-shaped carcass, fitting annular bead hoops onto the radially outer surface of a carcass ply, mounting bead apexes onto the carcass, assembling a plurality of breaker plies centrally onto the radially outer surface of the carcass ply, turning each of the carcass ply edge portions lying axially outwardly of the bead hoops radially outwardly and axially inwardly around the bead hoops, assembling onto the cylindrical assembly of carcass, beads and breaker the remaining components of tyre such as a centrally disposed rubber tread flanked at either side by rubber side walls to form a cylindrical green tyre assembly, shaping the resultant cylindrical green tyre assembly into a toroid and finally molding in a heated tyre mold to form the tread pattern in the tread and vulcanize the whole assembly, are already known from US 3,730,246. However, in this document the breaker plies are disposed between the carcass plies, and no breaker ply is extended axially to a position in the bead region where it overlaps the radially outer end of the bead apex. Further aspects of the present invention will become apparent from the following description by way of example only of embodiments of the invention in conjunction with the following diagrams in which:- Figure 1 shows an axial split cross-section of an embodiment, the left-half showing the tyre construction and the right-half showing details of the tread surface curvature; Figure 2 shows the configuration of a tyre bead region according to the invention; and Figures 3a) to 3g) are schematic diagrams showing the sequence of building of the tyre of Figure 1. The tyre is a motorcycle tyre comprising a convex tread region 1 extending between tread edges connected to sidewalls and terminating in bead regions. Each bead region is reinforced by an inextensible annular bead core. The tyre when normally fitted has a camber value C/L, defined as the ratio of the radial distance C and the axial distance L between the tread centre and the tread edge, of 0.6. Extending through the tread region 1, radially inward of the tread rubber, and between the bead regions is a main carcass ply 23. In each bead region the main carcass ply is anchored by being turned around the respective bead core from the axially inside to outside to form carcass ply turn-ups 25. These ply turns-ups 25 extend to a radial height TH lower than the height WH of the maximum tyre width TW. The tyre has three breaker plies 21,22 and 23, the radially innermost 23 being the widest and extending radially inward into the bead and terminating between the carcass main portion 24 and the carcass turn-up 25. The tyre bead also has an apex 26. Each of the three breaker plies 21,22 and 23 comprises aramid cords. The cords of plies 21 and 23 are disposed at an angle of 20° to the tyre circumferential direction. The cords of ply 23 are disposed at an angle of 38° in the central crown portion, and the cords of each of the three plies are crossed with respect to adjacent plies. The tyre has no tread edge in the conventional sense. However the camber value C/L of the tyre can be defined as the ratio of the radial and axial distances C and L between the tread contact edge point TCE and the maximum tyre diameter at the tread centreline. In this embodiment the camber value is 0.36. The right-hand half of Figure 1 shows details of the curvature of the outer surface of the tyre. When normally fitted the tyre outer surface has a continuously decreasing radius of curvature Rc from the point P to the tread contact edge TCE where the point P is at a distance of SP from the tyre circumferential centreline C/L equal to 20% of the distance L from the tread centreline C/L to the tread contact edge TCE. Furthermore the tread surface of the tyre axially outward of point P is a curve lying within two curves defined by the locus of a point with polar co-ordinates R'  where R' = R ± 4%R wherein R = (92.46304 + 50.02951 x  - 109.1216 x 2 + 43.74487 x  3 + 7.385639 x  4 - 4.776894 x  5 ) x (SW/194) The tyre of the present invention may be preferably manufactured using a single-stage assembly process. Such a single-stage manufacturing process will now be described with reference to the series of sequential schematic diagrams of Figure 3 which depict the upper section of the right-hand side of a cylindrical green tyre in the various stages of assembly. Accordingly as shown in Figure 3a) firstly a carcass ply 40 comprising axially extending reinforcing cords embedded in rubber is formed into a cylinder. Shown in Figures 3b) and 3c) a narrow breaker ply 41 is assembled centrally onto the radially outer surface of the cylindrical carcass ply followed by a wider breaker ply 42 which is fitted over the narrower breaker ply 41. An annular bead hoop 43 is then fitted around the carcass ply 40 axially inward of the axial outer edge 44 of the ply cylinder 40 and adjacent to the edge of the wide breaker ply 42. As shown in Figure 3d) the portion of the ply cylinder lying axially outward of the annular bead hoop 43 is then turned radially outward and axially inwardly to wrap around the bead hoop 43 to form a ply turn-up portion 45 overlying the edge portion of the wide breaker 42. Also a bead apex, although not shown in Figure 3, is fitted prior to wrapping the ply around the bead hoop. Remaining components of the tyre are then fitted onto the cylindrical assembly including for example a sidewall rubber 46 and a rubber tread strip 47 as shown in Figures 3e) and 3f). Finally, as shown in Figure 3g) the cylindrical shape of the assembly is changed into a toroidal shape by simultaneously moving the bead region axially inward and the central crown portion radially outward as indicated by the arrows. The toroidal shaped assembly is then heated under pressure in a mould to form the tyre tread pattern and vulcanise the completed assembly. In manufacturing the tyre of the present invention by a single stage process the inventors have found that the configuration of the wide breaker ply extending into the bead region controls the stretching and trellising of the breaker plies and also influences the distortion of the ply in an unexpected and beneficial manner. Thus whilst the angle of the cords of the breaker plies reduces from for example 30° to the circumferential direction in the cylindrical state to for example 20° in the finished toroidal state the finished angle of the ply cords also changes. The magnitude of the angle change of the ply cord is small when the narrowed breaker ply is immediately adjacent to the carcass ply and greater when the wide breaker ply is immediately adjacent when radial bias angle may change progressively from 90° in the bead regions to approximately 78° or even as low as 70° in the central crown region. However, regardless of the magnitude of the angle changes of the ply cords, a most surprising effect is that the movement of the ply cords is in the opposed sense to the movement of the cords of the immediately adjacent breaker. The overall effect is therefore for the ply cords to move to increasingly cross the adjacent breaker cords and so enlarge the included angle therebetween. This effect is thought to benefit the tyre by improving the breaker reinforcing characteristics, particularly where only two breaker plies are present, due to increasing the triangulation between the cords of the two breaker plies and those of the ply.

Claims: A pneumatic tyre comprising a tread (1) reinforced by a breaker assembly comprising at least two breaker plies (21,21,23), one of which extends between the two bead regions and overlaps the radially outer end of the bead apex (26) having its edges disposed between the carcass main portion and the respective turn-up portion in the bead regions, a reinforcing carcass ply of cords inside the breaker and extending between two bead regions forming a carcass main portion (24) and wrapped in each bead region axially outwardly around an annular bead core (11) to form carcass ply turn-up portions (25) extending radially outwardly and terminating radially inward of the point of maximum tyre width and each bead region is provided with an annular bead apex (26) extending radially taperingly outward from the bead core (11) between the carcass main portion (26) and the carcass turn-up portion (25) characterised in that in the shape of its normally inflated fitted condition the tyre has a camber value (C/L) of between 0.3 and 0.7, the cords of the reinforcing carcass are disposed at an angle of 0° to 20° to the tyre radial plane and the edges of the breaker ply (23) which extends between the two bead regions are adjacent to the carcass turn-up portion (25). A tyre according to claim 1, characterised in that the breaker assembly comprises only two breaker plies (22,23). A tyre according to claim 1 or 2, characterised in that the breaker ply extending between the bead regions is the radially outermost breaker ply. A tyre according to any of claims 1 to 3, characterised in that the cords of the breaker plies in the central region of the breaker assembly are disposed at an angle of between 15° and 30° to the tyre circumferential direction. A tyre according to claim 4, characterised in that the cords of the breaker plies are disposed at an angle of 20° to the tyre circumferential direction. A tyre according to any of claims 1 to 5, characterised in that the cords of the carcass ply are oppositely inclined to the cords of the immediately adjacent breaker ply. A tyre according to any of claims 1 to 8, characterised in that the camber value C/L is between 0.5 and 0.7. A tyre according to any of claims 1 to 7, characterised in that the tyre tread has a curvature which has a continuously decreasing radius from a point P which is at an axial distance SP from the tyre centreline equal to 20% of the distance from the tread centre to the adjacent tread contact edge. A method of fabricating the tyre of Claim 1 which has a bead apex (26) extending radially taperingly outward from a bead core (11), a carcass ply turn-up portion (25) extending radially outwardly and terminating radially inward of the point of maximum tyre width and in the shape of its normally inflated fitted condition the tyre has a camber value (C/L) of between 0.3 and 0.7, the cords of the reinforcing carcass being disposed at an angle of 0° to 20° to the tyre radial plane, comprising forming a cylindrical-shaped carcass comprising axially extending carcass reinforcing cords, fitting annular bead hoops onto the radially outer surface of the carcass ply and axially inward of each of the ply edges, mounting bead apexes onto the carcass, assembling a plurality of breaker plies centrally onto the radially outer surface of the carcass ply and co-cylindrically with it including one wide breaker ply which extends axially to a position in the bead region where it overlaps the, in the finished tyre, radially outer end of the bead apex, turning each of the carcass ply edge portions lying axially outwardly of the bead hoops radially outwardly and axially inwardly around the bead hoops to overlie the axial edges of the wide breaker ply, assembling onto the cylindrical assembly of carcass, beads and breaker, the remaining components of the tyre such as a centrally disposed rubber tread flanked at either side by rubber sidewalls to form a cylindrical green tyre assembly, shaping the resultant cylindrical green tyre assembly into a toroid and finally moulding in a heated tyre mould to form the tread pattern in the tread and vulcanise the whole assembly.

Inventors: CORNER MICHAEL RAYMOND, WATKINS DAVID ROBERT, CORNER, MICHAEL RAYMOND, WATKINS, DAVID ROBERT
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1998
Main classification: B60C9
IPC codes: B60C15, B60C9, B60C11, B60C3