Tread for a pneumatic tyre

Abstract: A tyre having a ground contacting tread, the tread having a directional tread pattern comprises at least two substantially circumferentially extending grooves (ILG,OLG) disposed one each side of the tread circumferential centreline (C) and a plurality of circumferentially spaced apart and substantially axially extending shoulder grooves (OAG) extending axially outwardly from each circumferential groove to the tread edge (TE) to define a row of tread blocks (OB) in each axially outer tread shoulder part, the axially extending grooves (OAG) having two opposing inclined sidewalls which provide ramp support to the adjacent tread block edge, characterised in that each of the axially extending shoulder grooves (OAG) has a groove depth which reduces from the axially inward part to towards the axially outer part, and each groove (OAG) comprises two groove main portions (OAG2,OAG1) connected end to end, each groove main portion having a varying asymmetric cross-section along its length, each groove main portion having a first groove sidewall (OFT) having a constant first angle of inclination and an opposing second groove sidewall (OTR) having a varying second inclination angle, the constant first angle being greater than the maximum value of the second inclination angle, the first groove sidewall (OFT) of each groove main portion being disposed on the same side of the axial groove.

Description: The present invention relates to a block-type tread for a pneumatic tyre and particularly but not exclusively to a tread for a heavy duty tyre such as used on trucks, trailers or buses. A problem occurring with block-type tyre tread patterns and particularly in heavy duty tyres is so-called 'heel and toe' irregular wear. This occurs when the leading front portion of the tread block wears more rapidly than the rear trailing edge as the tread block passes through the contact region with the road surface, and is manifest as a sawtooth profile to rows of blocks around the tread circumference. Whilst such irregular wear is apt to occur across the whole tread it is found to be worse in the axially outer shoulder regions. The effects of such heel and toe irregular wear are to disfigure the pattern appearance, reduce the performance of the tread design and shorten the life of the tyre. Whilst the problem of such irregular wear may be partially mitigated by stripping the tyre off the wheelrim and refitting with the reverse orientation such practice is unsatisfactory particularly in the case of a directional tread pattern which is specifically designed to be advantageously run only in one orientation. It is known from US 5 297 604, which corresponds with the features of the preamble of claim 1, to provide a block-type tyre tread pattern having constant and symmetrical buttressing of the blocks in the central region whilst having variable and asymmetrical buttressing to the blocks in the two axially outer median parts. According to US 5 297 604 the values of inclination of the groove sidewalls forming the leading and trailing edges of the blocks in the outer median parts is not critical in reducing irregular wear, it is necessary only that there is asymmetry of the values. The Applicants have investigated the proposal of the prior art and found that it does not solve the problem of irregular heel and toe wear. It is an object of the present invention to mitigate the effects of heel and toe irregular wear of a block tread pattern across the entire width of the tread and thus provide a tyre having an improved block tread pattern. According to the invention there is provided a tyre with the features of claim 1. Further aspects of the invention will become apparent from the following description, by way of example only, of one embodiment of the invention in conjunction with the following drawings in which: Figure 1 shows a schematic plan view of a tread pattern of a tyre in accordance with the invention; Figure 2 shows a schematic scrap section of the tyre tread of Figure 1 taken along section "A"-"A"; Figure 3 shows details of one of the axial grooves of the tyre of Figure 1; Figure 4 shows details of the groove sidewalls of one of the axial grooves of the tyre of Figure 1; and Figure 5 and 5a-51 show details of the cross-sectional shape of the axial tread grooves of the tyre of Figure 1. In the following by groove cross-sectional width is meant the linear distance at the tread surface measured perpendicularly to the groove longitudinal centreline, and groove sidewall inclination at a point on a groove means the inclination of the sidewall at the tread surface with respect of a line normal to the surface as shown in Figures 5a-51. Further by groove base portion (GBP) is meant the radiused portion connecting the opposing groove sidewalls at their radially innermost part as shown in Figure 5d. Shown in Figures 1 and 2 is the tread region of a heavy duty vehicle tyre of size 295/80R22.5. The tyre has a radially extending carcass having ends wrapped around inextensible bead coils disposed one in each wheelrim contacting bead region (not shown). The tread region is reinforced by a breaker assembly 2 comprising plural plies of metal cords which are crossed with respect to the cords of adjacent plies. The tyre had a tread width (TW) measured between the axially outer edges (TF) of the tread of 255mm. The tread pattern formed on the radially outer part of the tread region comprises a central longitudinal groove (CLG) extending continuously circumferentially centred on the tyre circumferential centreline C-C. To either side of the circumferential centre there are a further two longitudinal circumferentially extending grooves; an intermediate longitudinal groove (ILG) axially adjacent to the central longitudinal groove (CLG) and an outer longitudinal groove (OLG) axially outside the intermediate longitudinal groove. These further two longitudinal grooves together with the central longitudinal groove and the tread edge (TE) divide each half of the tread axially into central, intermediate and outer regions (CR,IR,OR) respectively. These regions are of approximately equal axial width being 32%, 36% and 32% respectively of the half tread width. Each of the three tread regions (CR,IR and OR) is further provided with a series of axial grooves (CAG,IAG and OAG) respectively which are spaced apart in the circumferential direction and extend fully across their respective region to divide the region into a row of individual tread blocks (CB,IB and OB) respectively. Each of circumferential grooves and the axial grooves (CAG and OAG) has a generally asymmetric "V"-shaped cross-section with a radiused groove base and it is the opposing inclined sidewalls of these grooves which provide ramp support or buttressing of the leading and trailing axially extending edges of individual tread blocks. The axial groove (OAG) has a profile which changes from V-shaped to flat bottomed proceeding axially outward as illustrated in Figures 5a-5c. As shown in Figure 1 both the width W1, W2, W3 of the axial tread grooves and their inclination to the circumferential direction increases with increasing axial displacement from the circumferential centreline C. In this embodiment each of the axial grooves are slightly curved rather than being straight grooves. It is also evident that the tread pattern is directional as the arrangement of grooves and blocks on one side of the centreline C is a reflection of that on the other side. To ensure the pattern is used in the correct direction of rotation an arrow or other marker may be provided on the sidewall or the directional appearance of the tread used to set the fitting direction. In this particular embodiment there is also a circumferential displacement of approximately half a pattern pitch length between the two halves of the pattern. The tread pattern of the present embodiment is thus intended to be used in an orientation providing forward rotation in the direction of arrow R shown in Figure 1. Accordingly the leading edges (CLE,ILE and OLE) of the tread blocks of the respective central intermediate and outer regions (CR,IR and OR) are as shown in Figure 1 on the upper edge of the blocks whilst the complimentary trailing edges (CTE,ITE and OTE) are at lower edges of the block. In this embodiment each of the axial grooves (CAG,IAG and OAG) comprises three portions. Thus each axial groove has two main groove portions, shown as GP1 and GP2 in Figure 3, which shows details of an intermediate axial groove (IAG) by way of illustration of some features of all axial grooves. Each of the two main groove portions are substantially equal in width and length and extend almost halfway across the respective region. In the middle of the region the two main groove portions are connected together by a short groove portion shown as CP. In the radially inner region of their groove base, the three groove portions are formed in mutual alignment as indicated by the pairs of broken lines shown in Figure 3. Thus the groove base extends in a smooth continuous curve along the groove. However in their radially outer regions the two main groove portions are formed in a non-aligned configuration. Thus in the middle region a small step or circumferential dislocation S between the two main groove portions is introduced via the connecting short groove portion CP. The magnitude of this circumferential dislocation increases with the distance from the groove base and it has a maximum value Sℓ at the tread surface of between 10 and 30% of the maximum width of the axial groove. Thus each of the main groove portions extend substantially in the same direction excepting the small circumferential displacement in their radially outer parts. As said previously each of the axial grooves (CAG,IAG,OAG) is slightly curved in its longitudinal direction. In this embodiment each has a radius of curvature (R) of 250mm. As shown in Figure 3, each of the tread block corners defined by the junction of an axial groove and a circumferential groove has a truncated portion (TP) which avoids a sharp corner and serves to avoid excessive wearing. With the exception of the region of the truncated portion (TP) at the junction of the axial and circumferential grooves, for each axial groove the cross-sectional width of the two axial groove main portions GP1 and GP2 are equal and constant along their length. In this embodiment the width of the main groove portions of the central, intermediate and outer grooves are respectively 8, 11 and 14mm which represent 0.75, 1.03 and 1.3% of the maximum tyre outer diameter D measured at the tyre circumferential centreline. The axial grooves (CAG and IAG) of the central and intermediate region are substantially full depth along their length. Further details of the configuration of grooves (CAG and IAG) are shown in Figure 4. The aforementioned difference in the alignment of the groove base and the two groove main portions determines a configuration in which each of the two sidewalls of the groove consists of three faces. Thus each consists of two generally axially extending faces, marked as TF1, TF2 and LF1, LF2 in Figure 4, connected by a smaller face (TCF,LCF) extending circumferentially and normally to the tread surface. Each of the axially extending faces is inclined with respect of the tread surface and curved in the axial direction. Whilst the radius groove base extends in a continuous smooth curve, shown in Figure 4 by the pair of broken lines (GBP), neither of the radially outward parts of the two main groove portions is aligned in parallel configuration to their respective parts of the groove base. Furthermore the groove base is substantially displaced from the centreline of both of the main groove portions towards the trailing edge (ITE) of the adjacent tread block. Accordingly the axial groove has an asymmetric cross-section varying along its length. The general angle of inclination to the tread surface of the groove sidewall faces LF1 and LF2 is substantially greater than that of the opposing faces TF1 and TF2. Furthermore regarding each axial groove main portion of grooves CAG and IAG, whereas the angle of sidewall inclination of faces (TF1,TF2) at the block trailing edge decrease from the axially inner end to the other end of the groove portion for the opposing groove sidewall the angle increase. Accordingly this configuration provides greater ramp support or buttressing to the leading edge (LE) of the tread block than that provided to the trailing edge (TE) of the adjacent block. In the particular embodiment illustrated in the Figures the angle of inclination of the groove sidewall buttressing the block trailing edge reduces to zero at the axially outer part of each groove main portion. The asymmetry of the V-shaped axial grooves CAG and IAG is illustrated in Figure 5 wherein Figures 5s-5g and 5h-51 show the cross-sectional shape of the grooves at specific indicated points. Turning now to the outer axial groove (OAG) which is formed in the outer shoulder region of the tyre. This groove whilst having a general configuration similar to the other two axial grooves is different in that it has a depth which decreases axially outwardly accompanying a change in cross-sectional shape from V-shaped to flat bottomed. However of particular importance to the present invention is the groove sidewall on the side of the leading block edge, which comprises two main groove portions, having unlike the other groove portions a constant inclination to the tread surface. Specifically the axially inner portion of this sidewall, indicated by OTF2 in Figure 5, has an inclination of 22° shown in Figures 5a and 5b whilst the axially outer portion (OTF1) has an inclination of 14° shown in Figure 5c. This combination of constant angle of sidewall inclination and reducing groove depth in the outer axial groove provides the necessary buttressing to the leading edge of the tread blocks in the shoulder to prevent heel and toe irregular wear. With regard to the circumferential grooves ILG and OLG these have a configuration wherein the groove base part extends circumferentially straight but the radially outer part comprises portions which are inclined to the circumferential direction and which also vary in width. Thus these grooves whilst being straight circumferential grooves at their base have a zigzag configuration of varying width in their radially outer part comprising alternative portions of 6.0 mm and 8.5 mm width. The tyre of the invention may also have other tread pattern features common in the art such as shallow grooves such as those marked SG in Figure 1 or knife cuts or sipes.

Claims: A tyre having a ground contacting tread, the tread having a directional tread pattern comprising at least two substantially circumferentially extending grooves (ILG,OLG) disposed one each side of the tread circumferential centreline (C) and a plurality of circumferentially spaced apart and substantially axially extending shoulder grooves (OAG) extending axially outwardly from each circumferential groove to the tread edge (TE) to define a row of tread blocks (OB) in each axially outer tread shoulder part, the axially extending grooves (OAG) having two opposing inclined sidewalls which provide ramp support to the adjacent tread block edge, characterised in that each of the axially extending shoulder grooves (OAG) has a groove depth which reduces from the axially inward part to towards the axially outer part, and comprises two groove main portions (OAG2,OAG1) connected end to end; each of said groove main portions has a varying asymmetric cross-section along its length, and a first groove sidewall (OFT) having a constant first angle of inclination and an opposing second groove sidewall (OTR) having a varying second inclination angle, the constant first angle being greater than the maximum value of the second inclination angle the first groove sidewall OTF of each groove main portion being disposed on the same side of the axial groove. A tyre according to claim 1, characterised in that the variable second angle of inclination of the first and second groove main portions (OAG2, OAG1) decreases from the axially inner to the axially outer part of the groove portion. A tyre according to claim 1 or 2, characterised in that the second angle varies within the range of 0° to 10°. A tyre according to any of claims 1 to 3, characterised in that the second angle varies from 0° to 6°. A tyre according to any of claims 1 to 4, characterised in that the constant first angle is in the range of 10°-25°. A tyre according to any of claims 1 to 5, characterised in that the depth of the axial shoulder grooves (OAG) reduces progressively from the axially inner part to the outer part. A tyre according to any of claims 1 to 6, characterised in that the depth of the axial shoulder grooves (OAG) varies between 100% and 50% of the full tread depth of the circumferentially extending grooves. A tyre according to any of claims 1 to 7, characterised in that each of said groove main portions (OAG2, OAG1) is curved in its longitudinal direction. A tyre according to any of claims 1 to 8, characterised in that the at least two groove main portions (OAG2,OAG1) extend substantially in the same direction but are circumferentially dislocated from each other by a distance less than their widths. A tyre according to any of claims 1 to 9, characterised in that the tread further comprises axial grooves (IAG,CAG) defining tread blocks in the central region between the shoulder regions, the axial grooves (IAG,CAG|) in the central region comprising at least two groove main portions connected end to end, each groove main portion having a substantially constant depth and varying asymmetric cross-section along its length and each having a first groove sidewall of varying first inclination angle and a second groove sidewall of varying second inclination angle, the first inclination varying angle being greater than the second varying inclination angle and the first groove sidewall of each groove main portion being disposed on the same side of the axial groove. A tyre according to claim 10, characterised in that the first groove sidewall of the groove main portions of the central axial grooves has an inclination angle which increases from the axially inward part to the axially outward part. A tyre according to claim 10 or 11, characterised in that the second groove sidewall of the groove main portions of the central axial grooves has an inclination angle which decreases from the axially inward part to the axially outward part.

Inventors: ASHMORE STEPHEN MICHAEL, ASHMORE, STEPHEN MICHAEL
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 2000
Main classification: B60C11
IPC codes: B60C11