Tyre tread pattern

Abstract: A tyre tread pattern for a radial ply heavy vehicle tyre comprising at least two circumferentially extending continuous main treads grooves (4) providing at each edge of the tread a shoulder rib (5) characterised in that each shoulder rib (5) comprises in its shoulder edge (7) a narrow edge zone (8) having, in cross section of the tyre, a radial height at least 2mm less than the ground engaging surface of the shoulder rib (5) immediately adjacent to said edge zone (8) and a junction zone (9) between the edge zone (8) and shoulder rib (5) which is made flexible in both the transverse and circumferential directions by a series of circumferentially spaced narrow slots (10) extending across the junction zone (9) at an angle to the centre line of the tread between 20 and 60 degrees, the slots (10) having a width between 0.4 and 2.0mm and the depth in the junction zone (9) substantially the same as the main tread grooves (4).

Description: This invention relates to a tyre tread pattern and in particular to a pattern suitable for heavy vehicle radial ply tyres such as those used on trucks and buses. Modern vehicles run at high speeds often for very long distances, and radial ply truck tread patterns comprising two circumferentially extending main grooves providing in each of the tread edges a shoulder rib are prone to wearing unevenly at the very edges of the tread i.e. the shoulders. This wear begins as a narrow step or 'drop' in the shoulder but progresses across the tyre as the tyre wears so that the step becomes wider and deeper. The step however often is uneven around the tyre and this results in reduction of tyre performance and also effective ground contact patch area. The appearance of the tyre becomes progressively more untidy and this makes it particularly difficult for the user to accurately assess the state of wear of the tyre which means that the tyre is not used fully economically. Such treads may have circumferential ribs or rows of blocks between the shoulder ribs. Previous attempts to solve this uneven and unpredictable shoulder wear have included, for example in US-A-4739812, restricting the tread contact width by forming a step in the shoulder but this reduces the cost effectiveness of the tyre in respect of the useful tread area for wear. Effectively the carcass size/cost for the tyre is made larger than should really be necessary for the tread width and thus the tyre is more expensive. Another attempt in the prior art disclosed in GB-A-2118910 uses a narrow, circumferentially-extending, continuous decoupling groove adjacent to the tread edge to form a narrow rib which itself has a reduced radius to the main tread radius, so that it has a step appearance. This design, although it gives good consistent tyre appearance through the wear life of the tyre, however, has an unnecessary reduction in the work efficiency of the tread zone as it effectively makes the narrow shoulder rib a non-working rib i.e. not contributing to tread life. Accordingly, it is an object of the present invention to provide a tyre tread which avoids the uneven edge wear problem but retains from that edge region of the tyre a useful contribution to tyre performance and resulting tread life. According to one aspect of the present invention, a tyre tread pattern for a radial ply heavy vehicle tyre comprises at least two circumferentially extending continuous main tread grooves providing at each edge of the tread a shoulder rib wherein each shoulder rib comprises in its shoulder edge a narrow edge zone having, in cross section of the tyre, a radial height at least 2mm less than the ground engaging surface of the shoulder rib immediately adjacent to said edge zone and a junction zone between the edge zone and shoulder rib and extending across the junction zone a series of circumferentially spaced narrow slots, the slots having a width between 0.4 and 2.0mm and the depth in the junction zone substantially the same as the main tread grooves, characterised in that the junction zone is steeply sloping and the narrow slots extend across the junction zone and into the edge zone and the adjacent shoulder rib at an angle to the centreline of the tread of between 20 and 60 degrees so that the junction zone is made more flexible in both the transverse and circumferential directions and the narrow stepped shoulder edge zone is made more flexible from the junction zone to the tread edge. Preferably the shoulder edge zone has a width axially of the tyre between 3 and 8 percent of the tread width, also the narrow slots more preferably are at an angle of 40 to 50 degrees to the centreline of the tread. The slots may also have additional to the length at the angle between 20 and 60 degrees to the centreline of the tread two wing zones each of which extend at 90 degrees to the centreline of the tyre such that these wing zones extend beyond the junction zone into the shoulder rib and edge zones respectively. Preferably the slots in the shoulder rib and edge rib zones have a depth less than the depth of the slot in the junction zone. The depth in the edge zones is preferably between 70 and 90 percent of the depth of the angled zone of the slots. The spacing between the slots around the tyre is preferably such that the distance between adjacent pairs of slots is between 2 and 4 times the length of the angled portion of each slot. Further aspects of the present invention will become apparent from the description, by way of example only, of one embodiment of the present invention:- Figure 1 is a part cross section of a truck tyre having a shoulder rib according to the present invention. Figure 2 is a scrap view in the direction of arrow II of the shoulder region of the tyre shown in figure 1. Figure 3 is a detailed view of one narrow slot shown in the previous figures and Figure 4 shows the shape of the tread mould knife blade to form the slot shown in figure 3. The tyre shown in figure 1 comprises a truck tyre having a tread region 1 reinforced by a breaker 2. The tread region 1 has adjacent to its shoulder region 3, a circumferentially extending main tread groove 4 which sub-divides the tyre tread to provide a shoulder rib 5 extending circumferentially of the tyre. A similar shoulder rib is provided at each side of the tyre although only one side of the tyre is shown in figure 1. The central part 6 of the tyre tread pattern may comprise a series of blocks or further circumferentially extending ribs, and this part of the tread formed no part of the present invention which relates to the shoulder rib 5. Each shoulder rib has at its shoulder edge 7 a narrow edge zone having a radial height a distance A which is 2mm less than the height of the ground engaging surface of the shoulder rib immediately adjacent to the said edge zone 8. The stepped edge zone 8 has a width in this example of 10mm and it has a slightly curved surface in transverse cross section of the tyre having the same radius of curvature R1 as the radius of curvature R2 of the central tread zone 5. The shoulder edge 8 has a steeply sloping junction zone 9 between the edge 8 and the shoulder rib 5. Across this junction zone 9 are positioned a series of circumferentially spaced basically transversely extending narrow slots 10. Each slot 10 has a Z or S shaped plan view as shown in figures 2 and 3 positioned so that the two ends 11 and 12 of the narrow slot 10 are parallel but are joined together by a central region 13 which then extends at an angle of 45 degrees to the direction of the end regions 11 and 12. This central region 13 is arranged to traverse the junction zone 9 at an angle of 45 degrees to the axial direction of the tyre as shown in figure 2 such that the central region 13 is equally divided by the junction zone 9. Each narrow slot 10 is formed by a blade shown in figure 4 which is mounted in the tyre mould used to form the pattern in the tread when the tyre is moulded and vulcanised. Each blade has a stepped upper region 14 to provide for the step in the tread surface and is shaped in the Z shape of figure 3 in plan. When looked at in the side view shown in figure 4 it can be seen that the centre region 15 of the blade is longer than the edge region 16. The resultant narrow slot 10 formed in the tyre has the angled central region 13 deeper into the tread surface than the two side regions 11 and 12. The spacing distance B between adjacent narrow slots 10 for a 11R22.5 truck tyre is 8 mm's so that the distance between adjacent pairs of slots is 2.7 times the length of the angled portion of each narrow slot 10. The resultant structure provides a step region 8 at the edge of the new tyre tread and in the junction region 9 the result of the angled slots is that the stiffness of the tyre in the junction zone 9 in both the circumferential and axial directions of the tyre is reduced so that the narrow stepped shoulder edge zone 8 is made more flexible from the junction zone 9 to the edge of the tyre. Thus in use of the tyre this more flexible edge zone is caused to move more upon the road surface than the remainder of the shoulder rib 5 and consequently this edge zone 8 wears more rapidly than the shoulder rib 5 which preserves the sharp line appearance at the edge of the rib 5 marked by the junction zone 9. An important feature of the present invention however, is that the spacing of the narrow slots 10 and the angled central region of these slots allows the stiffness of the junction zone 9 in both the circumferential and transverse directions to be adjusted so that the wear effect mentioned above is obtained but retaining substantial location of the shoulder region with respect to the shoulder rib 5 so that the shoulder region 8 when it contacts the road on the outer edge of the tyre as fitted to a vehicle and the vehicle is cornered, has still sufficient stiffness, in the transverse direction, to contribute to the cornering power of the tyre on the vehicle. Thus sufficient flexibility is given in the junction zone 9 to give a good wear appearance for the tyre but there is still sufficient connection between the narrow edge region 8 and the main shoulder rib 5 for the tread to give good vehicle handling properties. The slots in the tyre are between 0.4 and 2mm wide so that they have substantially no width but do provide decoupling to weaken the required region of the tread. This width is also one which is conventionally moulded into the tyre tread by means of blades. The angle of the central region 13 of each narrow slot 10 may be varied between 20 and 60 degrees to the axial direction of the tyre dependent upon the particular transverse and circumferential stiffness's required of the tyre. The difference in depth of the narrow slots 10 between the end tones 11 and 12 and the central zone 13 also provide for further control of the relevant stiffness's. The invention is particularly applicable to tyres where the stepped edge region 8 has a width axially of the tyre tread between 3 and 8%. It should also be understood that the Z shaped slots may be simplified to straight slots which lie at a constant angle between 20 and 60 degrees to the centre line of the tread. The depth of the narrow slots 10 may be the same as the depths of the main tread grooves 4 but more preferably the slots in the shoulder rib and edge rib zones have a depth slightly less than the depth of the slot of the junction zone. Finally alternative spacings between adjacent pairs of slots may be used dependent upon the ratios of transverse and longitudinal stiffness's required in the junction zone 9 to give the desired combination of tread wear properties and transverse stiffness's to give vehicle handling.

Claims: A tyre tread pattern for a radial ply heavy vehicle tyre comprising at least two circumferentially extending continuous main tread grooves (4) providing at each edge of the tread a shoulder rib (5) wherein each shoulder rib (5) comprises in its shoulder edge (7) a narrow edge zone (8) having, in cross section of the tyre, a radial height at least 2mm less than the ground engaging surface of the shoulder rib (5) immediately adjacent to said edge zone (8) and a junction zone (9) between the edge zone (8) and shoulder rib (5) and extending across the junction zone (9) a series of circumferentially spaced narrow slots (10), the slots (10) having a width between 0.4 and 2.0mm and the depth in the junction zone (9) substantially the same as the main tread grooves (4), characterised in that the junction zone (9) is steeply sloping and the narrow slots (10) extend across the junction zone (9) and into the edge zone (8) and the adjacent shoulder rib (5) at an angle to the centreline of the tread of between 20 and 60 degrees so that the junction zone (9) is made more flexible in both the transverse and circumferential directions and the narrow stepped shoulder edge zone (8) is made more flexible from the junction zone (9) to the tread edge. A tyre tread according to claim 1 characterised in that the narrow edge zone (8) has a width axially of the tread (1) in the range of 3 to 8 percent of the total tread width. A tyre tread according to claim 1 or 2 characterised in that the narrow slots (10) are at an angle of between 40 and 50 degrees to the centreline of the tread. A tyre tread according to any one of claims 1, 2 or 3 characterised in that each slot (10) has in addition to its portion (13) at an angle between 20 and 60 degrees to the centreline of the tread (1) two wing zones (11,12) each of which extend at 90 degrees to the centreline of the tread (1). A tyre tread according to claim 4 characterised in that the wing zones (11,12) are between 50 and 100 percent of the length of the angled regions (13) of the slots (10). A tyre tread according to claims 4 or 5 characterised in that the wing zones (11,12) of the slots have a depth between 50 and 90 percent of the depth of the angled slot region (13). A tyre tread according to any one of claims 1 to 6 characterised in that the slots (10) are spaced circumferentially around the tyre such that the distance (B) between each adjacent pair of slots (10) is between two and four times the length of the angled portion of the slots (13).

Inventors: CORNER MICHAEL RAYMOND, CORNER, MICHAEL RAYMOND
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1989
Main classification: B60C11
IPC codes: B60C11