Pneumatic tyre

Abstract: A pneumatic tyre comprising a tread portion (T) provided with a circumferential groove (7) having a pair of side walls (7A,7B), said circumferential groove (7) being disposed within the ground contacting width (SW) of the tread in the axial direction of the tyre, where the said ground contacting width (SW) is the maximum axial width of the ground contacting area of the tread portion (T) under the standard loaded state, when the tyre is mounted on its regular rim and inflated to 70% of its maximum air pressure and loaded with 88% of its maximum load, characterised in that the axial width (Wg) of said circumferential groove (7) is in the range of 0.2 to 0.35 times the ground contacting width (SW), and at least one of said side walls (7A,7B) has a nonrectilinear configuration having a plurality of axially deviated parts, the width (Wg) of the circumferential groove (7) being periodically decreased in the longitudinal direction thereof so as to form narrow width portions (16) having a first positive circumferential length (P3), wide width portions (17) with a second positive circumferential length (P4) and intermediate portions respectively connecting each of the wide and narrow width portions, along said longitudinal direction, the width (Wm) at each said narrow width portion (16) being more than 0 but not more than 0.8 times the maximum width (WM) of the circumferential groove (7) and the maximum width (WM) of the circumferential (7) being in the range of from 30 to 70mm.

Description: The present invention relates to a pneumatic tyre in which tyre noise is reduced without deteriorating wet performance. Recently, as the noise generated by the car mechanics and body becomes reduced, the percentage of the noise generated by the tyre increases. Therefore, there is an increasing demand for a low noise tyre. For reducing the tyre noise, it is especially effective to reduce relatively high frequency noise to which the human ears are sensitive, that is, the noise whose frequency range is around 1 kHz. Air resonance noise is such a high frequency tyre noise. The air resonance noise is explained as follows. In the ground contacting patch of the tyre, an air tube is formed by the road surface and continuous circumferential grooves which are provided in the tyre tread portion. During running, this tube for air is excited by air flow, tread deformation or movement, roughness of the road surface and the like, and a noisy sound having a specific wave length which corresponds to two times the length of the air tube is generated. Most of the tyre noise whose frequency range is about 800 to 1.2k Hz generated by the tyre having a circumferential groove is such air resonance noise. The frequency of the air resonance is almost constant not depending on the running speed. Since it is known that air resonance can be prevented by decreasing the groove volume, in order to reduce air resonance noise, it has been proposed to decrease the groove volume and/or the number of circumferential grooves. However as a result, wet performance such as aquaplaning, wet grip and the like is inevitably decreased. JP-A-3 139402 shows a known tyre which discloses the features of the preamble of independent claim 1. It is therefore, an object of the present invention to provide a pneumatic tyre in which the air resonance noise is effectively reduced to decrease the over all noise level without sacrificing wet performance. According to the present invention a pneumatic tyre comprises a tread portion (T) provided with a circumferential groove having a pair of side walls, said circumferential groove being disposed within the ground contacting width of the tread in the axial direction of the tyre, where the said ground contacting width is the maximum axial width of the ground contacting area of the tread portion under the standard loaded state, when the tyre is mounted on its regular rim and inflated to 70% of its maximum air pressure and loaded with 88% of its maximum load, the axial width of said circumferential groove being in the range of 0.2 to 0.35 times the ground contacting width, and at least one of said side walls having a nonrectilinear configuration having a plurality of axially deviated parts, the width of the circumferential groove being periodically decreased in the longitudinal direction thereof, characterised in that the circumferential groove forms narrow width portions having a first positive circumferential length, wide width portions with a second positive circumferential length and intermediate portions respectively connecting each of the wide and narrow width portions, along said longitudinal direction, the width at each said narrow width portion being more than 0 but not more than 0.8 times the maximum width of the circumferential groove and the maximum width of the circumferential groove being in the range of from 30 to 70mm. Embodiments of the present invention will now be described in detail in conjunction with the accompanying drawings. Fig.1 is a cross sectional view of a pneumatic tyre according to the present invention; Fig.2 is a cross sectional view of the circumferential groove thereof; Fig.3 is a plan view showing an example configuration of the circumferential groove and an example of the tread pattern; Fig.4 is a plan view showing another example of the location of the circumferential groove of Fig.3; and Fig.5 is a plan view showing another example configuration of the circumferential groove. The present invention can be applied to various types of tyres, but is suitably applied to a low aspect ratio passenger radial tyre. Fig.1 shows an example of the tyre structure for such a low aspect ratio passenger radial tyre, in which the tyre 1 comprises a tread portion T, a pair of axially spaced bead portions B, a pair of sidewall portions S extending between the tread edges and the bead portions, a pair of bead cores 2 each disposed in each of the bead portions, a radial carcass 3 extending between the bead portions and turned up around the bead cores 2 from the axially inside to outside of the tyre, a belt 4 disposed radially outside the carcass and inside a rubber tread, and a bead apex 6 disposed radially outside each bead core 2 and between the main portion and each turned up portion of the carcass. The carcass 3 comprises at least one ply of radially arranged organic fibre cords, e.g. nylon, rayon, polyester or the like. The belt 4 comprises at least two crossed plies of high modulus cords such as steel, aromatic polyamide or the like, laid at a relatively small angle of from 15 to 30 degrees with respect to the tyre circumferential direction. The aspect ratio (height/width ratio) of the tyre 1 is in the range of from about 0.4 to 0.6. The tread portion T is provided with a convex profile. The tread profile may be a single radius curvature, but in general, the tread profile has a multi-radius curvature such that a major central portion thereof has a single radius R1 larger than the radius R2 of each of the shoulder portions, and the central portion and shoulder portions are smoothly connected without forming an angle. The tread portion T is provided with a wide circumferential groove 7 extending continuously in the tyre circumferential direction of the tyre and a plurality of axial grooves. In the present invention, the state of the tyre when it is mounted on its regular rim R and inflated to 70% of its maximum air pressure and loaded to 88% of its maximum load, is called the standard loaded state where the maximum air pressure and the maximum load are those officially specified for the tyre by for example JATMA (Japan), TRA (USA), ETRTO (Europe) and the like, and the regular rim is the rim officially specified for the tyre by the same associations for the purpose of measuring the tyre dimensions. According to the present invention, the circumferential groove 7 has a very large width than the conventional width. The width ratio Wg/Sw of the axial width Wg of the circumferential groove 7 to the ground contacting width Sw is in the range of from 0.20 to 0.35 where the ground contacting width Sw is the maximum axial width of the ground contacting area S under the above-mentioned standard loaded state. In the case of passenger radial tyres, for which the nominal section width in a metric tyre size designation is in the range of from 155 to 225, the width Wg of the circumferential groove 7 is in the range of from 30 to 70 mm, preferably, 35 to 50 mm. As shown in Fig. 2, the circumferential groove 7 has a flat bottom 7C and a pair of side walls 7A and 7B. Each of the side walls 7A and 7B is inclined towards the centre of the groove in the radially inward direction and the inclination angle (alpha) thereof is 0 to 30 degrees, preferably 5 to 20 degrees with respect to a radial line X drawn perpendicular to the tyre axis. The intersecting part between the groove bottom 7C and the side walls 7A and 7B is rounded at a small radius of curvature (r). The radius (r) is in the range of from 0.1 to 0.5 times the groove depth D. However, the intersecting part between the tread face and the side walls 7A and 7B is not radiused thereby improving road grip. The groove depth D of the circumferential groove 7 is set in the range of from 10 to 20 % of the width Wg. In the above-mentioned passenger tyres, the depth D is preferably set in the range of from 5 to 10 mm. If the depth is less than 5mm, the water drainage performance is low. If the depth is more than 10mm, it becomes necessary to increase the tread rubber thickness, which results in an increase in tyre weight, which is especially undesirable for passenger tyres. Further, the circumferential groove 7 is disposed within a tread central region which has a width of 70% of the ground contacting width Sw and is centred on the tyre equator CO. If the circumferential groove 7 protrudes outside this central region, the vibration mode of the belt edge excites the air resonance of the circumferential groove 7 and this increases the resonance noise. As explained above, by setting the groove width relatively so large, in the circumferential centre of the ground contacting part of the tread, the bottom of the circumferential groove 7 becomes convex as exaggeratedly shown in Fig.2 by the broken line. Further, the tread rubber is compressed. Accordingly, the cross sectional area of the middle groove part in the contact patch becomes smaller than parts immediately in front and behind. As a result, the resonance mode is altered and the resonance itself is largely prevented which reduces the resonance noise. If the width ratio Wg/Sw is less than 0.20, an effective deformation of the groove or air tube can not be obtained, and therefore the air resonance noise is not reduced, and further the aquaplaning phenomenon is liable to occur. If the width ratio Wg/Sw exceeds 0.35, the noise reduction effect does not increase any more, but the cornering force is decreased and the dry grip performance and steering stability are deteriorated. In the present invention, in order to eliminate the source of resonance noise, a circumferentially continuous groove which has a conventional width ranging from about 10 to 20 mm is not provided in the tread portion. Additionally a very narrow groove and a zero-width cut can be provided, but preferably, only one circumferential groove 7 is provided. By this circumferential groove 7, the tread portion T is divided into two lateral tread parts 9. Each lateral tread part 9 is provided with a plurality of axial grooves 10 so as to provide a sea/land ratio Ss/Sr in the range of 0.30 to 0.50. Here, the sea/land ratio Ss/Sr is the ratio of the sea area Ss to the land area Sr, both in the above-mentioned ground contacting area S at the standard loaded state. The sea area Ss is the total area of the grooved part, and the land area Sr is the total area of the remaining part or the actual ground contacting area. If the sea/land ratio Ss/Sr is less than 0.30, the wet grip performance especially during cornering is deteriorated. If the sea/land ratio Ss/Sr is more than 0.50, the cornering force is decreased to deteriorate steering stability. As explained above, air resonance noise is effectively reduced by setting the width of the circumferential groove larger than the conventional width range. However, it was discovered that a further noise reduction can be achieved by changing the resonance mode for every moment during running. To achieve this, at least one of the side walls 7A and 7B is formed with a nonrectilinear configuration, e.g. zigzag, wavy, stepped or the like so as to form portions having axial deviation along the tyre circumferential direction. Preferably, the peak-to-peak amplitude DM of deviation is smaller than 30%, more preferably smaller than 20% of the above-mentioned ground contacting width Sw. The peak-to-peak amplitude is the axial distance between the circumferentially adjacent peaks of opposite deviations. If the peak-to-peak amplitude DM exceeds 30%, the drainage of the circumferential groove becomes decreased, and the resultant protruding part of tread rubber into the groove is liable to wear earlier, and uneven wear starts therefrom. Fig.3 shows an example circumferential groove 7. In this example, the circumferential groove 7 extends generally straight in the tyre circumferential direction, but the axial width thereof is periodically decreased along the length thereof to form a plurality of narrow width portions 16. The axial width Wm of the circumferential groove 7 at each of the narrow width portions 16 is in the range of more than 0 but not more than 0.8 times the maximum axial width WM of the circumferential groove 7. Preferably, the width Wm is in the range of from 0.1 to 0.8 times, more preferably in the range of from 0.2 to 0.6 times the maximum width WM. The maximum width WM is set in the range of 30 to 70 mm for the passenger car tyre as explained above. The circumferential length P3 of the narrow width portion 16 is very small in comparison with the circumferential length P4 of the remaining wider portion 17. Therefore, the pitch of the one repeating cycle, which is the total of the length P3, the length P4 and the circumferential length of the transitional part between the wide portion 17 and the narrow portion 16, is set smaller than the ground contacting length so that during running at least one narrow width portion 16 is always in the ground contacting area S. In this example, in order to form the narrow width portions 16, the side walls 7A and 7B are provided at the same circumferential positions with inwardly protruding parts 16. As a result, the circumferential groove 7 is symmetrical about the centre line thereof. The circumferential groove 7 is centred on the tyre equator CO, but it can be set aside from the tyre equator CO as shown in Fig.4. Fig.5 shows another example circumferential groove 7, which is a modification of the Fig.3 groove. In this example, the protruding parts 16 of the side wall 7A and those of the other side wall 7B are staggered in the circumferential direction. As a result, the circumferential groove 7 is asymmetrical about the centre line CL thereof. The widths Wm and WM are also satisfy the above-explained conditions. In Fig.3 the axially outer ends of the axial grooves 10 extend to the tread edges. The axially inner ends thereof are not extended to the circumferential groove 7, and they are terminated in respective lateral tread portions 9, whereby the rigidity of the lateral tread parts 9 is increased to improve steering stability and uneven wear resistance. The width of the axial grooves 10 is in the range of 5 to 15 % of the axial width Wg of the circumferential groove 7. Preferably, the width of the axial groove is gradually increased from the inner end to the outer end thereof. Preferably, the number of the axial grooves is set such that during running two to four axial grooves always exist within the ground contacting area S on each side of the circumferential groove 7. If the number is less than two, the drainage performance becomes insufficient. If more than four, the tread rigidity is decreased, and steering stability is lost. Each of the axial grooves 10 is curved such that the angle of the groove centre line with respect to the tyre circumferential direction is gradually increased from the inner end toward the outer end thereof. In each lateral tread part 9, the axial grooves 10 are inclined in the same direction. In Fig.3 the axial grooves 10 in one of the lateral tread parts 9 are inclined in the same direction as the axial grooves 10 in the other lateral tread part 9. The angle of the axial groove is gradually increased from the axially inner end to the outer end thereof. The maximum angle at the axially outer end is about 50 to 70 degrees. The minimum angle at the axially inner end is about 40 to 50 degrees. The angle at the middle point therebetween is less than 60 degrees. In addition to the above-mentioned axial grooves 10, in order to improve further the wet grip and/or adjust the tread rigidity, sipes can be provided in the tread portion, wherein a sipe is a cut whose width is in the range of from 0.2 to 0.7 mm. Various sipes, e.g. a curved sipe, a straight sipe, a sipe connected with the circumferential groove 7, a sipe disconnected with the circumferential groove 7, etc. may be used. In order to prevent possible excessive deformation of the bottom of the circumferential groove 7, the circumferential groove 7 can be provided at the bottom with protrusions spaced apart from each other in the longitudinal direction of the groove. The height of such protrusions is preferably in the range of from 50 to 15 % of the depth D of the circumferential groove 7.

Claims: A pneumatic tyre comprising a tread portion (T) provided with a circumferential groove (7) having a pair of side walls (7A,7B), said circumferential groove (7) being disposed within the ground contacting width (SW) of the tread in the axial direction of the tyre, where the said ground contacting width (SW) is the maximum axial width of the ground contacting area of the tread portion (T) under the standard loaded state, when the tyre is mounted on its regular rim and inflated to 70% of its maximum air pressure and loaded with 88% of its maximum load, the axial width (Wg) of said circumferential groove (7) being in the range of 0.2 to 0.35 times the ground contacting width (SW), and at least one of said side walls (7A,7B) having a nonrectilinear configuration having a plurality of axially deviated parts, the width (Wg) of the circumferential groove (7) being periodically decreased in the longitudinal direction thereof, characterised in that the circumferential groove forms narrow width portions (16) having a first positive circumferential length (P3), wide width portions (17) with a second positive circumferential length (P4) and intermediate portions respectively connecting each of the wide and narrow width portions, along said longitudinal direction, the width (Wm) at each said narrow width portion (16) being more than 0 but not more than 0.8 times the maximum width (WM) of the circumferential groove (7) and the maximum width (WM) of the circumferential groove (7) being in the range of from 30 to 70mm. A pneumatic tyre according to claim 1, characterised in that each of said sidewalls (7A,7B) has a nonrectilinear configuration, and the deviated parts of one of the sidewalls are disposed at the same circumferential positions as those of the other sidewall, thereby the configuration of the circumferential groove (7) is symmetrical about the centre line thereof. A pneumatic tyre according to claim 2, characterised in that the circumferential groove (7) is centred on the tyre equator. A pneumatic tyre according to claim 2, characterised in that the circumferential groove (7) is set to one side of the tyre equator. A pneumatic tyre according to claim 1, characterised in that each of said sidewalls (7A,7B) has a nonrectilinear configuration, and the deviated parts of one of the sidewalls are circumferentially shifted from those of the other sidewall, whereby the configuration of the circumferential groove (7) is not symmetrical about the centre line thereof. A pneumatic tyre according to any of claims 1 to 5, characterised in that the peak-to-peak amplitude (DM) of the axial deviation is smaller than 30% of the ground contacting width (SW), and the sea/land ratio (Ss/Sr) of said ground contacting area is in the range of from 0.30 to 0.50. A pneumatic tyre according to claim 6, characterised in that said tread portion (T) consists of a single said circumferential groove (7). A pneumatic tyre according to claim 6, characterised in that said tread portion (T) consists of a single said circumferential groove (7) and a plurality of axial grooves disposed on each side of the circumferential groove (7), each said axial grooves (10) having an axially outer end and an axially inner end, the axially inner end terminated before the circumferential groove (7), the axially outer end extending beyond the axial edge of the ground contacting area. A pneumatic tyre according to claim 1, characterised in that said circumferential groove (7) is disposed within a tread central region which region has a width of 70% of the ground contacting width (SW). A pneumatic tyre according to claim 1, characterised in that said width (Wm) at each said narrow width portion (16) is in the range of from 0.1 to 0.8 times the maximum width (WM) of the circumferential groove (7). A pneumatic tyre according to claim 1, characterised in that said width (Wm) at each said narrow width portion (16) is in the range of from 0.2 to 0.6 times the maximum width (WM) of the circumferential groove (7). A pneumatic tyre according to claim 1, characterised in that the second positive circumferential length (P4) of each of the plurality of wide width portions (17) is greater than the first positive circumferential length (P3) of each of the plurality of narrow width portions (16). A pneumatic tyre according to claim 1, characterised in that at least one of the narrow width portions (16) always exists in the ground contacting area (S) for preventing the formation of standing wave from an air rube occurring between the ground and the tread portion in a section of the circumferential groove (7) within the ground contacting area.

Inventors: SHIBATA KOUJI, UEMURA YOSHIAKI, SHIBATA, KOUJI, UEMURA, YOSHIAKI
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1993
Main classification: B60C11
IPC codes: B60C11