Heavy duty tyre

Abstract: A pneumatic tyre for heavy duty use improved in shoulder wear resistance and wandering performance, in which a tread profile extending between tread edges (PT) is formed by an arc having an outwardly projecting convex curvature of a radius (TR),a buttress profile extending radially inwardly from a point (PB) on the surface of each tyre sidewall is formed by an arc having an inwardly projecting concave curvature of a radius (BR),a connecting profile extending between each tread edge (PT) and said point (PB) is formed by an arc having an outwardly projecting convex curvature and intersecting the tread profile at the tread edge (PT) to form a angled corner and smoothly connected with the buttress profile at the point (PB), the ratio (t/m) of the radial distance (t) between the tread edge (PT) and the equatorial point (C) on the tread profile to the axial distance (m) therebetween is 0.09 to 0.12, the ratio (m/L) of said axial distance (m) to the axial distance (L) between the equatorial point (C) and an imaginary point (PS) at which the tread arc having the radius of curvature (TR) and the buttress arc having the radius of curvature (BR) intersect each other if extended is 0.85 to 0.95, and the inclination angle (ϑ) of the tangential line (T) to the connecting profile drawn at each tread edge (PT) is 20 to 60 degrees with respect to the radial direction of the of the tyre.

Description: The present invention relates to a pneumatic tyre for heavy duty use in which shoulder wear is reduced and wandering performance is improved. In the tyres for heavy vehicles, e.g. truck, bus and the like, a square shoulder in which, as shown in Fig.4, an outwardly curved tread face (a) and inwardly curved buttress face (b) intersect at the tread edges at about a right angle, has been used. Such a square shoulder has the merit of an even ground pressure distribution, and thereby uneven tread wear becomes less, and the wear life of the tread is excellent. In such tyres having a square shoulder, however, the tread edges are liable to be damaged during running on rough road, and further directional stability is not good. That is, wandering of the vehicle occurs during running on a road having a surface irregularly inclined in the lateral direction, for example, a rutted road. The countermeasures for such problems which have been widely employed are to use a round shoulder in which the tread edge portion (c) is rounded with an arc (R) as shown in Fig.5; or a taper shoulder in which the tread edge portion is formed by a tapered face (e) as shown in Fig.6. Such a tyre is also known from EP-A-0 267 139. However, in tyres having a round shoulder, as the ground pressure is decreased in the region near the tread edge, shoulder wear in which the tread shoulder portions are worn rapidly in comparison with the tread crown portion occurs. On the other hand, in the tyres having a taper shoulder, the improvement in wandering performance is not sufficient. It is therefore, an object of the present invention to provide a heavy duty tyre which is improved in both wandering performance and shoulder wear performance. According to one aspect of the present invention, a pneumatic tyre for heavy duty comprises a tread profile extending between tread edges (PT), a buttress profile extending radially inwardly from a point (PB) on the surface of each tyre sidewall, and a connecting profile extending between each tread edge (PT) and said point (PB), wherein in a cross section including the tyre axis, the tread profile is formed by an arc having an outwardly projecting convex curvature of a radius (TR), characterised in that each buttress profile is formed by an arc having an inwardly projecting concave curvature of a radius (BR), each connecting profile is formed by an arc having an outwardly projecting convex curvature and intersecting the tread profile at the tread edge (PT) to form a angled corner and smoothly, connected with the buttress profile at the point (PB), the ratio (t/m) of the radial distance (t) between the tread edge (PT) and the equatorial point (C) on the tread profile to the axial distance (m) therebetween is 0.09 to 0.12, the ratio (m/L) of said axial distance (m) to the axial distance (L) between the equatorial point (C) and an imaginary point (PS) at which the tread arc having the radius of curvature (TR) and the buttress arc having the radius of curvature (BR) intersect each other if extended is 0.85 to 0.95, and the inclination angle (ϑ) of the tangential line (T) to the connecting profile drawn at each tread edge (PT) is 20 to 60 degrees with respect to the radial direction of the tyre. 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 diagram showing the profile thereof; Fig.3 is a developed plan view showing the tread pattern thereof; and Figs.4-6 are schematic sectional views each showing a prior art profile. In Fig.1, heavy duty tyre 1 is a truck/bus radial tyre having a typical size of 10.00R20-14PR, and is shown in the normal inflated state, mounted on the specified rim of size 7.50VX20 and inflated to the maximum specified pressure. The tyre 1 has a tread portion 5, a pair of axially spaced bead portions 3 and a pair of sidewall portions 4 extending therebetween. The tyre 1 comprises a pair of bead cores 2 disposed one in each bead portion, a carcass 6 having at least one ply of cords extending between the bead portions and turned up around the bead cores form the axially inside to the outside thereof, a stiff belt 7 disposed radially outside the carcass and inside a rubber tread, and a bead apex 9 disposed in each bead portion and between the carcass ply turned up portion and main portion. In this embodiment, the carcass is composed of one ply 6A of cords arranged radially at an angle of 70 to 90 degrees with respect to the tyre equator CO. For the carcass cords, steel cords are used, but organic fibre cords, e.g. nylon, polyester, rayon, aromatic polyamide and the like and inorganic fibre cords may be used. The belt 7 in this embodiment is composed of first to forth plies 7A, 7B, 7C and 7D arranged in this order from the radially inner carcass side to outer side. The radially innermost first belt ply 7A is composed of steel cords laid at an angle of 40 to 70 degrees with respect to the tyre equator CO, and the second to forth belt plies 7B-7D are each composed of steel cords laid at an angle of 10 to 30 degrees with respect to the tyre equator. With regard to the cord inclinations based on the tyre equator, in order to provide a stiff triangulated structure for the belt, the first ply 7A and the second ply 7B are inclined in the same direction, but the second ply 7B is inclined in a direction opposite to that of the third ply 7C to cross each other, and further the third ply 7C and the forth ply 7D are inclined in the same direction. The belt 7 is disposed on the radially outside of the carcass, and the major part of the belt or the innermost belt ply comes into direct contact with the carcass, but between each of the edge portions thereof and the carcass a wedge-shaped breaker cushion 10 made of soft rubber is disposed so as to provide a distance which increases gradually toward the axially outside of the tyre. Further, in order to effectively disperse and mitigate the shear stress which is liable to be concentrated in a region between the breaker cushion and the belt edge portion, the ply edges are shifted such that the second ply 7B is widest, and the first ply 7A has almost the same width as the third ply 7C but slightly narrower, and the radially outermost forth ply 7D is narrowest. Furthermore, between each edge portion of the third ply 7C and the second ply 7B, a wedge-shaped rubber strip is disposed to provide a gradually increasing distance therebetween as shown in Fig.1. In the present invention, to effectively improve wandering performance and shoulder wear resistance, the profile of the tyre is specifically defined as shown in Fig.2 which shows a cross section of the tyre including the tyre axis CO. The tyre profile comprises a tread profile 11 extending between the tread edges PT, a buttress profile 13 extending radially inwardly from a point PB on each tyre sidewall, and a connecting profile 14 extending between the point PB and each tread edge PT. The tread profile 11 is formed by an arc 20 having a single radius of curvature TR, and the center thereof is positioned on the tyre equatorial plane so that the tread profile swells radially outwardly of the tyre. The buttress profile 13 is formed by an arc 21 having a single radius of curvature BR, and the center thereof is positioned axially outward of the sidewall so that the profile swells inwardly of the tyre. The connecting profile 14 is formed by an arc 12 having a single radius of curvature R, and the center thereof is positioned axially inward of the sidewall so as to swell outwardly of the tyre. The connecting profile 14 is smoothly joined to the buttress profile 13 at the above-mentioned point PB, therefore, the point PB is a point of inflection between the arcs 12 and 13. The connecting profile 14 intersects the tread profile 11 at the tread edge PT at an angle narrower than a straight angle and wider than a right angle so as to form an angled corner. A tangential line T to the connecting profile 14 drawn at the tread edge PT to extend radially outwardly is inclined axially inwardly at an angle ϑ of 20 to 60 degrees with respect to the radial direction, whereby the tread edges are prevented form being torn off. When the angle ϑ is less than 20 degrees, the angle of the tread edge becomes too narrow, and the tread edge is liable to be torn off. When the angle ϑ is more than 60 degrees, a region of the connecting profile 14 near the tread edge contacts with the ground, and shoulder wear starts therefrom. Further, the tread camber defined by the ratio t/m of the radial distance t between the tread edge PT and the central point C on the tread profile at the tyre equator to the axial distance m therebetween is set in the range of 0.09 to 0.12. When the ratio t/m is less than 0.09, the ground pressure becomes large in the tread shoulder regions in comparison with the tread central region, and shoulder wear is caused. On the contrary, when the ratio t/m is more than 0.12, the ground pressure becomes large in the central region, and the central region wears rapidly. As the connecting profile is formed by the outwardly swelling arc 12, it becomes easy to run over protrusions on the road surface such as rut, rails and the like, and the shock when running over can be softened. As the tread edge PT is angled and the t/m ratio is 0.09 to 0.12, the ground pressure distribution becomes even, and uneven wear, e.g. shoulder wear of the tread can be reduced. Furthermore, the ratio m/L of the axial distance m between the tread edge PT and the central point C to the axial distance L between the central point C and an imaginary point PS at which the arc of the radius TR and the arc of the radius BR intersect each other if extended, is set in the range of 0.85 to 0.95, whereby the connecting face 14 is allowed to have a width t1 and the curvature R which are appropriate for a smooth running over a protrusion in a well-balanced manner. When the ratio m/L is less than 0.85, the resultant increased radius R impairs the running-over performance, and further the ground contacting width is decreased excessively which causes deterioration in running performance. When the ratio m/L is more than 0.95, the width t1 is decreased excessively and an effective improvement in the running-over performance is not obtained. Fig.3 shows a tread pattern which is suitable for the tyre according to the present invention. The tread is provided with four circumferentially extending main grooves G to divide the tread into five ribs: one central rib 16A, two middle ribs 16B and two shoulder ribs 16C. The central rib 16A and the shoulder ribs 16C are provided with zigzag sipes 17 extending across the whole width of the respective ribs. The middle ribs 16B are provided with zigzag grooves 18 having the substantially same shape as the sipes 17 but a wider groove width. The grooves 18 extend across the whole width of the respective ribs to divide the ribs into circumferentially separated blocks. Further, to adjust the rigidity of the tread edge, the tread edge is provided with sipes or cuts 19 extending in the axially direction of the tyre from the tread face 11 to the connecting face 14. The present invention can be applied to semiradial tyres and bias tyres in addition to radial tyres. Test tyres of size 10.00R20 having specifications given in Table 1 were prepared and tested for wandering performance, shoulder wear resistance, tread life and tread edge durability. The test results are also shown in Table 1. In the wandering performance test, test tyres were mounted on the front wheels of a test vehicle, and the resultant wandering of the vehicle when running into and out of a rut formed on a rough road and the required handling operation were evaluated into four ranks A-D as the wandering performance by the driver's feeling. (A: excellent, B: good, C: passable, D: bad). In the shoulder wear test, the test tyre was run on a well paved road at a constant speed under the maximum tyre load specified in JIS, and the amount of the tread wear was measured at the tread shoulders when the depth of the remaining main grooves reached to 10 mm. (A:small, B:middle, C:large). In the tread life test, the above-mentioned constant-speed running was continued, and the total running distance until it became necessary to change the test tyre to a new tyre due to its shoulder wear was measured. The measurements are indicated by an index based on the assumption that prior art tyre 2 is 100. The larger the index, the longer the life. In the edge durability test, when the tyre reached the end of tread life in the former test, the tread edges were inspected to find whether the edges were torn off. (A:non, B:small, C:large)

Claims: A pneumatic tyre for heavy duty having a profile comprising a tread profile extending between tread edges (PT), a buttress profile extending radially inwardly from a point (PB) on the surface of each tyre sidewall, and a connecting profile extending between each tread edge (PT) and said point (PB), wherein in a cross section including the tyre axis, the tread profile is formed by an arc having an outwardly projecting convex curvature of a radius (TR), characterised in that each buttress profile is formed by an arc having an inwardly projecting concave curvature of a radius (BR), each connecting profile is formed by an arc having an outwardly projecting convex curvature and intersecting the tread profile at the tread edge (PT) to form a angled corner and smoothly connected with the buttress profile at the point (PB), the ratio (t/m) of the radial distance (t) between the tread edge (PT) and the equatorial point (C) on the tread profile to the axial distance (m) therebetween is 0.09 to 0.12, the ratio (m/L) of said axial distance (m) to the axial distance (L) between the equatorial point (C) and an imaginary point (PS) at which the tread arc having the radius of curvature (TR) and the buttress arc having the radius of curvature (BR) intersect each other if extended is 0.85 to 0.95, and the inclination angle (ϑ) of the tangential line (T) to the connecting profile drawn at each tread edge (PT) is 20 to 60 degrees with respect to the radial direction of the tyre. The tyre according to claim 1, characterised in that the tyre comprises a carcass (6) having at least one ply of cords extending between bead portions (3) and turned up around bead cores (2) and a belt (7) disposed radially outside the carcass and inside a rubber tread. The tyre according to claim 2, characterised in that said at least one carcass ply is composed of steel cords arranged radially at an angle of 70 to 90 degrees with respect to the tyre equator CO, and said belt is composed of four plies (7A, 7B, 7C and 7D) of steel cords and has a triangle cord structure. The tyre according to claim 3, characterised in that a central major part of the belt comes into direct contact with the carcass, and a wedge-shaped breaker cushion (10) made of soft rubber is disposed between each of edge portions thereof and the carcass so as to provide a distance which increases gradually toward the axially outside of the tyre.

Inventors: TAKADA YOSHIYUKI, YAMAHIRA ATSUSHI, TAKADA, YOSHIYUKI, YAMAHIRA, ATSUSHI
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1991
Main classification: B60C11
IPC codes: B60C11