Pneumatic tyre

Abstract: A pneumatic tyre (1) comprises a pair of sidewall portions (3) each provided on the axially outer surface thereof with a side protector (11), the side protector comprising at least two concentric ribs and at least one circumferential groove (12) defined therebetween, the circumferential groove being composed of a series of deep parts (12a) and shallow parts (12b) which alternate in the tyre circumferential direction.

Description: The present invention relates to a pneumatic tyre, more particularly to a sidewall structure capable of improving the durability under high speed running on dirt roads. In pneumatic tyres used on dirt roads at high speed such as a rally, it is important to protect tyre sidewall from edged objects on the roads. One known countermeasure is shown in Fig.6 where each sidewall portion (a) is provided with a plurality of axially protruding concentric ribs (b), wherein parts (c) between the adjacent ribs (b) are formed like a constant-depth groove (hereinafter the "groove (c)"). Such a tyre, which corresponds with the preamble of claim 1, is disclosed in Japanese Patent No.2613996. However, if the bottom of the groove (c) is cut during running, the cut tends to easily grow due to the presence of the ribs because tensile stress in the tyre radial direction is concentrated at the groove bottom when the tyre is deformed. Such a cut may be avoided if the amount of axial protrusion of the ribs is increased, but an increase in the tyre weight which is especially undesirable for a racing tyre is then unavoidable. On the other hand, if the rubber thickness at the groove bottom is increased, namely, the depth of the groove (c) is decreased, whilst this part (c) may be increased in resistance to cut an increase in the tyre weight results which is unfavourable and also there is a liability for cracks to occur in the groove bottom because the tensile stress f which concentrates on the groove bottom increases as the groove depth decreases. Further, the shock absorbing performance of the tyre is deteriorated. It is therefore, an object of the present invention to provide a pneumatic tyre suitable for high speed running on dirt roads, in which the resistance to cuts is improved and the occurrence of cracks is effectively controlled without increasing the tyre weight. According to the present invention, a pneumatic tyre comprises a pair of sidewall portions each provided on the axially outer surface thereof with a side protector, the side protector comprising at least two concentric ribs and at least one circumferential groove defined therebetween, wherein the circumferential groove is composed of a series of deep parts and shallow parts which alternate in the tyre circumferential direction. As a result the shallow parts connect the adjacent ribs to each other to resist such a motion that opens the groove. Even if the deep parts are cut, as the tensile stress in the deep parts is effectively reduced by the presence of the shallow parts, growth of the cut is prevented. If the shallow parts are cut, the growth of the cut is prevented because such parts have a enough rubber thickness. An embodiment of the present invention will now be described in detail, by way of example only, in conjunction with the accompanying drawings in which: Fig.1 is a cross sectional view of a pneumatic tyre according to the present invention; Fig.2 is a side view of the tyre showing an example of the side protector thereof; Figs.3A and 3B are sectional views taken along a line A-A and a line B-B of Fig.2, respectively; Figs.4 and 5 are sectional views each showing a modification of the groove bottom shown in Fig.3A; and Fig.6 is a cross sectional view for explaining the problems in the prior art. In the drawings, a pneumatic tyre 1 according to the present invention comprises a tread portion 2, a pair of sidewall portions 3, a pair of bead portions 4 each with a bead core 5 therein, a carcass 6 extending between the bead portions 4 and a belt 7 disposed radially outside the carcass 6 in the tread portion 2. In this example, the tyre 1 is a radial tyre designed for rally use. The carcass 6 comprises at least one, in this example two plies 6A and 6B of cords arranged radially or semi-radially at an angle of from 70 to 90 degrees with respect to the tyre equator C, and extending between the bead portions 4 through the tread portion 2 and sidewall portions 3, and turned up around the bead core 5 in each bead portion from the axially inside to the axially outside of the tyre so as to form a pair of turned up portions 6b and a main portion 6a therebetween. For the carcass cords, organic fibre cords, e.g. nylon, polyester, aromatic polyamide, aromatic polyester, rayon and the like, carbon fibre cords, steel cords and the like can be used. Between the turned up portion 6b and main portion 6a in each bead portion 4, there is disposed a bead apex 8 extending radially outwardly from the bead core 5. In this example, the bead portion 4 is not provided with any additional reinforcing layer. But, it is possible to dispose a reinforcing cord layer if need be. The belt 7 in this example is composed of two crossed plies, a radially inner ply 7A and a radially outer ply 7B, each made of parallel steel cords. Aside from steel cords, organic fibre cords, e.g. aromatic polyamide, aromatic polyester, nylon, polyester, rayon and the like can be used as the belt cords. According to the present invention, the sidewall portions 3 are each provided on the outer surface 10 with an axially-protruding circumferentially-extending side protector 11. The side protector 11 comprises at least two concentric ribs and at least one circumferential groove 12 defined therebetween. The circumferential groove 12 is composed of a series of deep parts 12a and shallow parts 12b which alternate in the tyre circumferential direction. Thus, the depth of the groove 12 changes cyclically in the tyre circumferential direction. As to the number of the groove 12, it is preferable that three or more grooves 12 are provided on each of the side protectors 11, and the grooves 12 (in Fig.1, six grooves) are disposed substantially equidistantly in the radial direction. The width W of each groove 12 measured in the radial direction is preferably set in a range of from 0.25 to 4 mm, more preferably from 1 to 3 mm. The side protector 11 is formed on the radially outside of a position at about one half of the tyre section height H. Preferably, as shown in Fig.1, the side protector 11 is formed to cover from the axial end (B) of a tread rubber to or near the maximum section width point of the carcass 6. From a point immediately radially inside the side protector 11 which is, in this example, near the maximum section width point of the carcass 6, the sidewall portion becomes thinnest. This thin part continues radially inwardly for a certain extent which is larger than the width W of the groove 12. The protrusion height (h) of the side protector 11 which is measured from the thin part on the radially inside of the side protector 11 is preferably set in a range of not less than 2.5 mm, more preferably in a range of from 3 to 4 mm. The radial extent S of the protector 11 is preferably set in a range of from 20 to 60 %, more preferably from 25 to 40 % of the tyre section height H. The pitch length (p) between the grooves 12 measured in the tyre radial direction from groove centre to groove centre is preferably set in a range of from 1.0 to 3 times, more preferably 1.5 to 2.5 times the groove width W. Preferably, the side protector 11 is made of rubber having a hardness of from 60 to 75, and a 100% modulus of from 3900 to 6000 kPa when measured with a durometer type-A according to Japanese Industrial Standard K-6253. In this example, the side protector 11 is made of the same rubber as the sidewall rubber 15 which is disposed axially outside the carcass in the sidewall portion 3. However, it is also possible to make it from another rubber. As mentioned above, each groove 12 changes in depth cyclically in the circumferential direction, and is made up of alternate deep parts 12a and shallow parts 12b. The number of deep parts 12a (or shallow parts 12b) per groove is preferably set in a range of from 4 to 20, more preferably from 6 to 16. The difference (D1-D2) between the maximum depth D1 in the deep parts 12a and the minimum depth D2 in the shallow parts 12b is set in a range of from 0.3 to 3.0 mm, preferably 0.5 to 2.0 mm. The maximum depth D1 is set in a range of from 0.5 to 8 mm, preferably 3 to 5 mm, and it is less than the protrusion height (h). As to the cross sectional shape of the groove 12, U-shape, V-shape, semicircle, rectangle, trapezoid and the like can be used. In this example, a U-shaped cross sectional shape is used. In Figs.3A and 3B, the bottom L of the groove 12 is waved like a sine wave. As a result, the groove 12 in this example is made up of variable-depth parts 12c extending between the deepest points and the shallowest points. Figs.4 and 5 show modifications of the groove bottom. In Fig.4, the maximum depth D1 continues for a certain circumferential length s2 to form the deep part 12a. Also, the minimum depth D2 continues for a certain circumferential length s1 to form the shallow part 12b. The deep parts 12a and shallow parts 12b are linearly bridged by variable-depth parts 12c. Thus, the groove 12 in this example is made up of the constant-depth deep parts 12a, constant-depth shallow parts 12b and variable-depth parts 12c each having a certain length. Preferably, the length s1 is substantially the same as the length s2. In Fig.5, the minimum depth D2 continues for a certain circumferential length s1 to form the shallow part 12b. In this example, each part between the adjacent shallow parts 12b is formed by a concave curved part 12d which can be said to be a variable-depth part 12c. Thus, the groove 12 in this example is made up of the constant-depth shallow parts 12b and variable-depth parts 12d. In any case, it is preferable that the phase of the cyclical depth change is circumferentially shifted between the radially adjacent grooves 12 such that, as shown typically in Figs.3A and 3B, the deep parts 12a of one groove adjoin the shallow parts 12b of the other groove, and accordingly the shallow parts 12b of one groove adjoin the deep parts 12a of the other groove. In other words, the deep parts 12a and shallow parts 12b are staggered between the radially adjacent grooves 12, whereby the sidewall portion can be improved in uniformity in both the tyre circumferential direction and radial direction. Comparative Tests Rally test tyres of size 195/65R15 for passenger cars having the structure shown in Fig.1 and specifications shown in Table 1 were prepared and tested for cut resistance and crack resistance. Cut resistance test: The test tyre was provided in the bottom of a groove on the side protector with a circumferential cut having a depth of 1 mm and a length of 1 cm. A test car provided with the test tyre was run on a dirt test course, and the runable distance of the tyre was measured as cut resistance. In Table 1, the results are indicate in kilometre. The larger the value, the higher the cut resistance. (Wheel rim size: 6JX15, Inner pressure: 240 kPa) Crack resistance test : The test car provided with the test tyres was run for 20000 km, and then cracks caused in the groove bottoms were measured for size (length X depth). The results are indicated in Table 1, using an index base on the Ref. tyre being 100. The smaller the value, the higher the crack resistance. Tyre Ref. Ex.1 Ex.2 Ex.3 Ex.4 Ex.5 Ex.6 Ex.7 Ex.8 Ex.9 Side protector h (mm) 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 S/H 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Groove No. of grooves 6 6 6 6 6 6 6 6 6 6 Bottom even Fig.3 Fig.3 Fig.3 Fig.3 Fig.3 Fig.3 Fig.3 Fig.3 Fig.3 D1 (mm) 3 3.5 3.8 4.3 3.5 3.8 4.3 3.5 3.8 4.3 D2 (mm) 3 3 2.8 2.3 3 2.8 2.3 3 2.8 2.3 D1-D2 (mm) 0 0.5 1 2 0.5 1 2 0.5 1 2 Deep parts 1 0 4 4 4 8 8 8 16 16 16 Tyre weight (kg) 12 11 11 11 11 11 11 11 11 11 Cut resistance 13 13 15 18 13 16 18 14 18 20 Crack resistance 100 100 115 138 100 123 138 107 138 153

Claims: A pneumatic tyre (1) comprising a pair of sidewall portions (3) each provided on the axially outer surface thereof with a side protector (11), the side protector comprising at least two concentric ribs and at least one circumferential groove (12) defined therebetween, characterised in that the circumferential groove (12) is composed of a series of deep parts (12a) and shallow parts (12b) which alternate in the tyre circumferential direction. A pneumatic tyre according to claim 1, characterised in that the difference (D1-D2) between the maximum depth D1 in the deep parts (12a) and the minimum depth D2 in the shallow parts (12b) is in a range of from 0.5 to 2.0 mm. A pneumatic tyre according to claim 1 or 2, characterised in that the number of the shallow parts (12b) is in a range of from 4 to 20 per groove. A pneumatic tyre according to claim 1, 2 or 3, characterised in that the circumferential groove (12) is composed of variable-depth parts having a variable depth and a certain length. A pneumatic tyre according to claim 1, 2 or 3, characterised in that the circumferential groove (12) is composed of constant-depth parts having a constant depth and a certain length, and variable-depth parts (12c) having a variable depth and a certain length. A pneumatic tyre according to claim 5, characterised in that said constant-depth parts are said deep parts and said shallow parts. A pneumatic tyre according to any of claims 1 to 6, characterised in that the side protector comprising at least two circumferential grooves (12), and the deep parts (12a) and shallow parts (12b) are staggered between the radially adjacent grooves (12).

Inventors: SUZUKI KAZUYA, SUZUKI, KAZUYA, Suzuki, Kazuya, c/o Sumitomo Rubber Industries Ltd
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 2001
Main classification: B60C13
IPC codes: B60C13