Pneumatic tyre

Abstract: A pneumatic tyre having a tread portion (2) provided with a pair of circumferentially extending main grooves (G) one on each side of the tyre equator (C) to define an axially outermost circumferentially extending part (SH) between each main groove and each tread edge (E), each said axially outermost circumferentially extending part (SH) being provided with lateral grooves (gs) extending from the adjacent main groove (G) to the adjacent tread edge (E) to provide a row of circumferentially spaced shoulder blocks (BS) which have at least two different circumferential lengths, characterised in that a first quotient (Lmax/Lmin) of the maximum (Lmax) of the circumferential lengths divided by the minimum (Lmin) thereof is in the range of 1.25 to 2.00, the width (W) of each main groove (G) changes in the circumferential direction of the tyre such that the larger the circumferential length of the shoulder block the larger the width of the main groove part adjacent to the respective block and a second quotient (Wmax/Wmin) of the maximum (Wmax) of the main groove width divided by the minimum (Wmin) which is in the range of 1.1 to the first quotient (Lmax/Lmin).

Description: The present invention relates to a pneumatic tyre, and more particularly to an improvement in a tread pattern, in which wet performance and low noise performance are both improved. In general, a tyre tread is provided with grooves for water drainage. However, such tread grooves especially lateral grooves generate noise during running. In order to reduce such noise, the so called "variable pitching method" has been widely used. In this method, the circumferential pitches of the lateral grooves are varied so as to change the pulse noises generated from the lateral grooves to give so-called "white noise". As a result, the circumferential lengths of the resultant blocks between successive lateral grooves vary, and some relatively long blocks are formed. However, where there are long blocks the lateral drainage at those portions is hindered, and water is liable to remain undischarged, which deteriorates the wet performance. To promote water discharge, the width of the main wide circumferential groove can be increased but then, due to resonance in the main groove, high-frequency noise is increased, which spoils the low noise performance. It is therefore, an object of the present invention to provide a pneumatic tyre in which wet performance and low noise performance are both improved. According to one aspect of the present invention, a pneumatic tyre has a tread portion provided with a pair of circumferentially extending main grooves one on each side of the tyre equator to define an axially outermost circumferentially extending part between each main groove and each tread edge, each said axially outermost circumferentially extending part being provided with lateral grooves extending from the adjacent main groove to the adjacent tread edge to provide a row of circumferentially spaced shoulder blocks having at least two different circumferential lengths, characterised in that a first quotient (Lmax/Lmin) of the maximum of the circumferential lengths divided by the minimum thereof is in the range of 1.25 to 2.00, the width of each main groove changes in the circumferential direction of the tyre such that the larger the circumferential length of the shoulder block, the larger the width of the main groove part adjacent to the respective block, and a second quotient (Wmax/Wmin) of the maximum of the main groove width divided by the minimum which is in the range of 1.1 to the first quotient (Lmax/Lmin). As a result the widened part of the main groove promotes discharge of the water between the tread and the ground. Further, the wide part can hold a larger volume of water which is thus taken from the block surfaces. Further, as the width of the main groove is varied along its length, resonance therein is prevented or damped. Accordingly, the generation of high-frequency noise is effectively prevented. Embodiments of the present invention will now be described in detail in conjunction with the accompanying drawings, in which: Fig.1 is a developed plan view of a tyre according to the invention showing the tread pattern thereof; Fig.2 is a cross sectional view; and Fig.3 is a developed plan view of another tyre according to the invention showing the tread pattern. In the figures, a pneumatic tyre 1 has a tread portion 2, a pair of axially spaced bead portions 15, and a pair of sidewalls 13 extending between the tread edges (E) and the bead portions 15. The tyre 1 also comprises a pair of bead cores 16 disposed one in each bead portion 15, a carcass 17 extending between the bead portions 15 and turned up at each edge around a bead core 16 from the axially inside to the outside thereof to be secured thereto, a belt 19 disposed radially outside the carcass 17, and a rubber tread disposed radially outside the belt 19 to provide the tread portion 2. Fig.1 shows the tread pattern which is provided with a plurality of circumferentially extending main grooves (in this example, only two grooves G) so as to divide it into a pair of axially outermost circumferential parts (shoulder parts SH) and at least one circumferential part therebetween. The tread portion 2 is further provided with lateral grooves (gc) and (gs). The lateral grooves (gs) extend from the axially outermost main groove (G) to the tread edge (E) across the whole width of the axially outermost circumferential part (SH) so as to divide this part into a row of shoulder blocks (BS). The lateral grooves (gc) extend from one of the main grooves (G) to the adjacent main groove (G) across the whole width of the inner circumferential part so as to divide this part into a row of inner blocks (BC). The shoulder blocks (BS) have at least two different circumferential lengths (L), in this example, three lengths. The longest block (BS1) has a length (L1), the middle-length block (BS2) has a length (L2), and the shortest block (BS3) has a length (L3). As can be seen L1 > L2 > L1. Here, also the quotient ( Lmax/Lmin=L1/L3 ) of the maximum block length ( Lmax=L1 ) of the longest block (BS1) divided by the minimum block length ( Lmin=L3 ) of the shortest block (BS3) is in the range of 1.25 to 2.00. If the quotient is less than 1.25, the noise reducing effect is too low. If the quotient is more than 2.00, then steering stability especially during cornering is impaired, and further uneven wear and/or heel-and-toe wear of the blocks is liable to occur. Each of the shoulder blocks (BS) has two axial edges and two circumferential edges. The axially outer edge is straight and forms a part of the tread edge (E). The axially inner edge in this embodiment is made as a straight line parallel to the tyre circumferential direction. However, the axially inner edge may be made as a zigzag line consisting of one or more segments parallel with the tyre circumferential direction and one or more segments parallel to the tyre axial direction, or a curved line which is generally parallel with the tyre circumferential direction. The circumferential edges in this embodiment are parallel with each other and are straight. However, it is possible to use a slightly curved line. The shape of each shoulder block in this embodiment is that of a parallelogram. The groove width (W) of the axially outermost main groove (G) changes in its longitudinal direction, which is the circumferential direction of the tyre, according to the circumferential lengths of the adjacent shoulder blocks such that if the shoulder block is relatively longer, the main groove part adjacent to the same block has a relatively wider width, and where the shoulder block is relatively shorter, the main groove part adjacent to the same block has a relatively narrower width. In this embodiment, the groove part axially inwards of each of the longest blocks (BS1) has the widest width (W1), the groove part axially inwards of each of the shortest blocks (BS3) has the narrowest width (W3), and the groove part axially inwards of each of the middle-length blocks (BS2) has the middle width (W2). The quotient ( Wmax/Wmin=W1/W3 ) of the maximum width ( Wmax=W1 ) divided by the minimum width ( Wmin=W3 ) is in the range of 1.1 to the above-mentioned quotient (Lmax/Lmin). If the quotient Wmax/Wmin is less than the lower limit of 1.1, the noise damping effect is not be obtained. If Wmax/Wmin is more than the upper limit i.e. the quotient Lmax/Lmin, partial wear and uneven wear are liable to occur. Further, in this embodiment, the above-mentioned main groove width change is made by changing only the axial widths of the shoulder blocks (BS) or by changing the axial positions of the axially inner edges of the shoulder blocks (BS) such that the longer the shoulder block, the further outwards is the edge position in the axial direction. That is, the axially outward groove wall of the main groove is repositioned but the axially inward groove wall thereof is not moved. The axially inward groove wall is straight and parallel to the tyre circumferential direction. That is, the axially outer edges of the inner blocks (BC) are straight and mutually aligned. The main groove is generally straight. In this invention, however, the axially inward groove wall may be changed in its axial position either alone or in combination with the axially outward groove wall change. Further, the axially outer edge of each inner block may comprise a zigzag line consisting of one or more segments parallel with the tyre circumferential direction and one or more segments parallel with the tyre axial direction, or be in the form of a slightly curved line which is generally parallel with the tyre circumferential direction. The axial widths (l) of the shoulder blocks (BS) are such that the larger the circumferential length (L), the smaller the axial width (l). Because, L1 > L2 > L3, l1 < L2 < l3, where l1, l2 and l3 are the axial widths of the shoulder blocks (BS1), (BS2) and (BS31), respectively. By changing the shoulder block width as mentioned above, the water path from the main groove to the tread edge becomes shorter at the longer block positions. Accordingly, the axially outward drainage is improved. Therefore, it is preferable to change only the axially outer wall of the main groove. The axially outermost lateral grooves (gs) in each row are inclined in the same direction at substantially the same angle with respect to the tyre equator (C). Further, the widths (WS) of the axially outermost lateral grooves (gs) in each row are changed according to the lengths (L) of the shoulder blocks (BS) such that if the shoulder block is relatively longer, one of the adjacent lateral grooves has a relatively wider width, or if the shoulder block is relatively shorter, one of the adjacent lateral grooves has a relatively narrower width. The relation therebetween is always towards one circumferential direction within each block row. In Fig.1, within the right side shoulder block row, it is downwards. That is, according to the length of the shoulder block the width of its under lateral groove is changed. On the other hand, within the left side shoulder block row, it is upward. That is, according to the length of the shoulder block the width of its upper lateral groove is changed. In the right side shoulder block row, the lateral groove adjacent to the lower circumferential edge of the longest shoulder block (BS1) has a largest width (WS1), the lateral groove adjacent to the lower circumferential edge of the middle-length shoulder block (BS2) has a middle width (WS2), and the lateral groove adjacent to the lower circumferential edge of the shortest shoulder block (BS3) has a smallest width (WS3). The quotient ( WSmax/WSmin=WS1/WS3 ) of the maximum width ( WSmax=WS1 ) divided by the minimum width ( WSmin=WS3 ) is in the range of 0.6 times to 0.9 times the quotient (Lmax/Lmin). As a result, the variation of the lateral groove widths becomes smaller than the variation of the block lengths, and the pulse noise therefrom is lessened. Further, the inner lateral grooves (gc) are extended across the tyre equator C and inclined at the same angle and in the same direction as the axially adjacent lateral grooves or the axially outermost lateral grooves in this embodiment. Therefore, in Fig.1, all the lateral grooves (gs) and (gc) have right-side-upward inclinations. In this embodiment, the inner lateral grooves are arranged at regular intervals or pitches in the circumferential direction. In the present invention, for each of the block rows inclusive of the inner block row, any known or unknown "variable pitching method" for changing pulse noise generated from each lateral groove to so called "white noise" as a whole, can be employed. In Fig.1, the shoulder block arrangement is That is, the circumferential length sequence is Long Middle Short Long Middle Short -------- and the lateral groove width sequence is Wide Middle Narrow Wide Middle Narrow ------ The following is another example of the shoulder blockarrangement. This can be however, used as the inner block arrangement. The lateral groove widths (WS11, WS12, WS21, WS22, WS31, WS32) are as follows: WS11 = WS12 > WS21 = WS22 > WS31 = WS32 WS max = WS11 WS min = WS32 but the following relation can be used. WS11 > WS12 > WS21 > WS22 > WS31 > WS32 The above-explained two examples are called "three pitch variation" as three different lengths are used. In the present invention, two pitch, four pitch, five or more pitch variations can be used. Fig.3 shows a modification of the tread pattern shown in Fig.1, wherein the inner lateral groove pitching is modified, and similar to the shoulder block rows in the former example shown in Fig.1, the lateral groove widths are changed according to the block lengths. Similar to the shoulder blocks, the central blocks (BC) have at least two different circumferential lengths, in this embodiment, three lengths. The longest block (BC1), middle-length block (BC2), and shortest block (BC3) have lengths (L1), (L2) and (L3), respectively. The widths (WC) of the central lateral grooves (gc) in one row are changed in the same manner as above according to the lengths (L) of the central blocks (BC). That is, if the central block is relatively longer, one of the adjacent lateral grooves has a relatively wider width, or if the central block is relatively shorter, one of the adjacent lateral grooves has a relatively narrower width. The quotient ( Lmax/Lmin=L1/L3 ) of the maximum block length ( Lmax=L1 ) of the longest block (BC1) divided by the minimum block length ( Lmin=L3 ) of the shortest block (BC3) is in the range of 1.25 to 2.00. The quotient ( WCmax/WCmin=WC1/WC3 ) of the maximum width ( WCmax=WC1 ) divided by the minimum width ( WCmin=WC3 ) is in the range of 0.6 times to 0.9 times the quotient (Lmax/Lmin). As mentioned above, any known or unknown "variable pitching method" or block arrangement can be employed in the present invention. In the above-explained tread patterns, with respect to one circumferential direction or rotational direction, the sequence of the right side shoulder blocks is reversed to that of the left side shoulder blocks. Further, to avoid at least coincidence of the small size blocks on one side with the small size blocks on the other side, the blocks on one side are shifted from those on the other side in the circumferential direction as shown in the figures, whereby the variation of the tread rigidity in the circumferential direction is reduced, and the low noise performance can be further improved. Tyres of size 195/65R15 for a passenger car were made and tested for noise and wet performance. The test tyres all had the same construction, the structure shown in Fig.2, and the tread pattern shown in Fig.3, except for the shoulder blocks. The specifications thereof and test results are given in Table 1. A) Noise Test Running a passenger car on a dry test course at a speed of 60km/h, which was provided with the test tyre mounted on its regular rim and inflated to its regular pressure, the generated noise was evaluated into five ranks by the driver. The larger the value, the better the low noise performance. B) Wet performance test a) Cornering test Running a passenger car provided with the test tyre mounted on its regular rim and inflated to its regular pressure, on a wet test course at a predetermined radius, the critical cornering speed was measured. b) Straight running test Here the aquaplaning generating speed was measured. The measured results are indicated in Table 1 by an index based on the reference tyre 1 being set at 100. The larger the index, the better is the performance.

Claims: A pneumatic tyre having a tread portion (2) provided with a pair of circumferentially extending main grooves (G) one on each side of the tyre equator (C) to define an axially outermost circumferentially extending part (SH) between each main groove and each tread edge (E), each said axially outermost circumferentially extending part (SH) being provided with lateral grooves (gs) extending from the adjacent main groove (G) to the adjacent tread edge (E) to provide a row of circumferentially spaced shoulder blocks (BS) which have at least two different circumferential lengths, characterised in that a first quotient (Lmax/Lmin) of the maximum (Lmax) of the circumferential lengths divided by the minimum (Lmin) thereof is in the range of 1.25 to 2.00, the width (W) of each main groove (G) changes in the circumferential direction of the tyre such that the larger the circumferential length of the shoulder block the larger the width of the main groove part adjacent to the respective block and a second quotient (Wmax/Wmin) of the maximum (Wmax) of the main groove width divided by the minimum (Wmin) which is in the range of 1.1 to the first quotient (Lmax/Lmin). A tyre according to claim 1, characterised in that within each said axially outermost part (SH), the widths (WS) of the lateral grooves (gs) depend upon the circumferential lengths (L) of the adjacent shoulder blocks (BS) such that the lateral groove adjacent to one of the circumferential edges of the longer lateral block is wider than the lateral groove adjacent to the same side circumferential edge of the shorter lateral block as said edge of the longer lateral groove, and a third quotient (WSmax/WSmin) of the width (WSmax) of the widest lateral groove divided by the width (WSmin) of the narrowest lateral groove is in the range of 0.6 times to 0.9 times the first quotient (Lmax/Lmin). A tyre according to claim 2, characterised in that the widths (WS) of the lateral grooves (gs) in each said axially outermost circumferential part (SH) vary such that the larger the circumferential length of the shoulder block (BS), the larger the width of one of the two lateral grooves adjacent to this block. A tyre according to claim 1, 2 or 3, characterised in that the tread portion (2) is provided between said a pair of main grooves (G) with at least one circumferential row of inner blocks (BC) divided circumferentially by inner lateral grooves (gc), said inner blocks (BC) have at least two different circumferential lengths, a fourth quotient (Lmax/Lmin) of the maximum (Lmax) of the circumferential lengths of the inner blocks (BC) divided by the minimum (Lmin) thereof is in the range of 1.25 to 2.00, the widths of the inner lateral grooves vary such that the larger the circumferential length of the inner block, the larger the width of one of the two lateral grooves adjacent to this inner block, and a fifth quotient (WCmax/WCmin) of the width (WCmax) of the widest inner lateral groove divided by the width (WCmin) of the narrowest inner lateral groove is in the range of 0.6 times to 0.9 times the fourth quotient (Lmax/Lmin).

Inventors: SHIBATA KOUJI, SHIBATA, KOUJI
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1992
Main classification: B60C11
IPC codes: B60C11