A heavy duty radial tyre for improved road damage

Abstract: A heavy duty radial tyre (1) such as may be used for a truck or a bus comprising a reinforced carcass ply (2) extending between two bead regions (3) through sidewall regions (4) to a tread region (5) and a reinforcing breaker (7) disposed in the tread region (5) extending circumferentially around the tyre (1) radially outside the carcass ply characterised in that the tread region (5) is substantially curved in cross-section and has a camber value C/L of between 0.25 to 0.75, the breaker (7) comprises at least two plies (7A,7B,7C) of reinforcing fabric, a narrower breaker ply (7A) which is radially innermost and a wider breaker ply (7C) which is radially outermost, and each of the breaker plies (7A,7B,7C) except the outermost ply (7C) has adjacent to each of its lateral edges a circumferentially extending support strip (8A,8B) which extends laterally outwards from the edge of the breaker ply to at least the lateral position of the edge of the widest breaker ply (7C).

Description: The present invention relates to a heavy duty radial tyre such as may be used for trucks and buses. Such tyres cause damage to road surfaces which depends on amongst other factors the structure of the road surface and the mode of operation of the vehicle. The problem has been the subject of various studies and it is now widely accepted that road damage due to the tyre increases proportionately to at least the fourth power of the vehicle axle load and in order to minimise damage it is necessary to minimise the contact pressure at the tyre/road interface by maximising the contact area between them, commonly known as the 'tyre footprint'. The world-wide trend in truck tyre sales is now towards increased usage of the steel breaker radial tyre rather than the older so-called 'cross-ply' construction. The radial tyre has been found to be more damaging than the cross-ply tyre. It is an object of the present invention to provide a radial heavy duty tyre such as may be used for trucks or buses which reduces such road damage. According to one aspect of the present invention a heavy duty radial tyre of the type for a truck or a bus comprises a reinforced carcass ply extending between two bead regions through sidewall regions to a tread region and a reinforcing breaker disposed in the tread region extending circumferentially around the tyre radially outside the carcass ply wherein the tread region is substantially curved in cross-section and has a camber value C/L of between 0.25 to 0.75, the breaker comprises at least two plies of reinforcing fabric, a narrow breaker ply which is radially innermost and a wider breaker ply which is radially outermost, and each of the breaker ply or plies except the outermost ply has adjacent to each of its lateral edges a circumferentially extending support strip which extends laterally outwards from the edge of the breaker ply to at least the lateral position of the edge of the widest breaker ply. By camber value is meant the ratio C/L between the distance C in the radial direction from the centre CP of the tread to the edge E of the tread and the distance L in the axial direction from the tread edge E to the tread centre CP. Preferably the radius of curvature of a centre part of the tread, which lies within 25% of the tread width from the circumferential centreline of the tyre, is in the range of 10% to 30% of the maximum diameter T of the tyre, and/or the width of the radially innermost breaker ply is at least 60% of the width of the radially outermost breaker ply. The edges of the tread may lie laterally outside the bead regions. Preferably the breaker plies comprise parallel cords laid at an angle of 10 to 30 degrees or more preferably at 25 degrees to the circumferential plane of the tyre. The breaker plies may be steel, aromatic polyamide or other such material known in the art. Further aspects of the present invention will become apparent from the following description of one embodiment of the invention in conjunction with the following drawings, in which:- Figure 1 shows a half cross-section of a tyre according to the present invention; Figure 2 is a schematic diagram showing the relative shapes and sizes of the contact area of a conventional front axle truck tyre over a range of loads; Figure 3 is a graph showing the variation of contact pressure with load for the conventional truck tyre of Figure 2; Figure 4 is a schematic diagram showing the relative shapes and sizes of the contact area of the tyre of the present invention shown in Figure 1 over a range of loads; and Figure 5 is a graph showing the variation of contact pressure with load for the tyre of the invention shown in Figure 1. A tyre 1 has a reinforcing carcass ply 2 extending radially between two bead regions 3 through sidewall regions 4 to a tread region 5. In each bead region 3 the carcass ply is turned up from the inside to the outside around an inextensible bead core 6. Located in the tread region 5 radially outside of the carcass ply 2 is a reinforcing breaker 7 comprising three plies of reinforcing fabric 7A,7B and 7C. The three breaker plies 7A,7B and 7C comprise cords of aromatic polyamide and the cords of each ply are laid in opposing senses to cross at an angle of 25 degrees with respect to circumferential centreline of the tread. The radially innermost ply 7A is the narrowest whilst the outermost ply 7C is the widest having a width similar to that of the tread. The width of each of the breaker plies measured parallel to the carcass ply 2 is 135mm, 170mm and 205mm respectively. Adjacent to each of the lateral edges of the inner breaker plies 7A and 7B is positioned a circumferentially extending rubber support strip 8A,8B having a thickness in the radial direction similar to that of the breaker ply to which it is in edge to edge abutment. These strips 8A and 8B extend from the edge of the respective breaker ply 7A,7B to in this case a position adjacent to the edge of the outermost breaker 7C but they may extend further towards or into the bead region 3 if desired. These rubber support strips prevent the lateral portions of the breaker immediately radially outside from moving radially inwardly towards the carcass ply 2 and thus the line of the breaker is maintained in a similar shape to the profile of the tread. Details of the geometry of the tyre are shown in the right hand side of Figure 3. The tyre overall diameter T is 640mm. The camber value C/L of the tyre is 0.56 and the radius of curvature Rc of the tread in the central region is 100mm. Illustrated in Figure 2 is the variation of the contact area of a conventional 11R22.5 size front axle radial truck tyre at the scheduled inflation pressure of 8 bar under loads ranging from 33% to 200% of the scheduled load. The arrow A indicates the contact area length in the circumferential direction of the tyre. This shows that the contact area increases with load. However the corresponding variation in the average contact pressure, shown in Figures 3, shows clearly that the increase in contact area is not sufficient to maintain a constant contact pressure. Accordingly the contact pressure and hence the road damaging capability of the conventional radial tyre construction increase with increasing tyre load. Thus it can be deduced from Figure 2 that for example operating a typical 11R22.5 radial front axle truck tyre at only 20% overload leads to a 30% increase in potential damage. In contrast Figure 4 shows the relative shapes and sizes of the contact area of the tyre of the present invention shown in Figure 1 over a range of loads. The arrow B indicates the contact area length in the tyre circumferential direction. In comparison to the corresponding shapes of the conventional truck tyre, shown in Figure 2, the tyre of the present invention displays a contact area shape in which the proportions of length and width are retained over a wide range of loads. Accordingly the variation of contact area with load in the case of the tyre of the present invention is such that the average contact pressure does not significantly increase with load. In fact Figure 5, in which the contact pressure for the tyre of Figure 1 is plotted against load, shows that contact pressure is actually decreased at higher loads and thus the potential damage to the road surface by the tyre is significantly reduced over prior art tyres. Whilst the breaker 7 has been described as comprising three plies of width 135, 170 and 205mm, the invention can also be used with a breaker comprising other numbers of plies, for example two, and also to breaker plies having greater widths suitable for tyres for high load carrying application. Whilst the invention has been illustrated by the description of a tyre of 640mm overall diameter it is also applicable to tyres of other diameters.

Claims: A heavy duty radial tyre (1) such as may be used for a truck or a bus comprising a reinforced carcass ply (2) extending between two bead regions (3) through sidewall regions (4) to a tread region (5) and a reinforcing breaker (7) disposed in the tread region (5) extending circumferentially around the tyre (1) radially outside the carcass ply characterised in that the tread region (5) is substantially curved in cross-section and has a camber value C/L of between 0.25 to 0.75, the breaker (7) comprises at least two plies (7A,7B,7C) of reinforcing fabric, a narrower breaker ply (7A) which is radially innermost and a wider breaker ply (7C) which is radially outermost, and each of the breaker plies (7A,7B,7C) except the outermost ply (7C) has adjacent to each of its lateral edges a circumferentially extending support strip (8A,8B) which extends laterally outwards from the edge of the breaker ply to at least the lateral position of the edge of the widest breaker ply (7C). A heavy duty tyre according to claim 1 characterised in that the radius of curvature Rc of the centre part of the tread lying within 25% of the tread width from the circumferential centreline of the tyre is in the range of 10% to 30% of the maximum diameter T of the tyre. A heavy duty tyre according to either of claims 1 or 2 characterised in that the width of the radially innermost breaker ply (7A) is at least 60% of the width of the radially outermost breaker ply (7C). A heavy duty radial tyre according to any of claims 1 to 3 characterised in that the edges E of the tread are laterally outside the bead regions (3). A heavy duty tyre according to any of claims 1 to 4 characterised in that the breaker plies (7A,7B,7C) comprise parallel cords laid at an angle of 10 to 30 degrees to the circumferential plane of the tyre. A heavy duty tyre according to any of claims 1 to 4 characterised in that the breaker plies (7A,7B,7C) comprise parallel cords laid at an angle of 25 degrees to the circumferential plane of the tyre. A heavy duty tyre according to either of claims 5 or 6 characterised in that the breaker plies (7A,7B,7C) comprise cords of an aromatic polyamide. A heavy duty tyre according to either of claims 5 or 6 characterised in that the breaker plies (7A,7B,7C) comprise steel cords. A heavy duty tyre according to any of claims 1 to 8 characterised in that the support strip (8A,8B) is made of rubber.

Inventors: WATKINS DAVID ROBERT, WILLIAMS ARTHUR ROGER, WATKINS, DAVID ROBERT, WILLIAMS, ARTHUR ROGER
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1992
Main classification: B60C11
IPC codes: B60C11, B60C9