Pneumatic tyre

Abstract: A pneumatic tyre comprises 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.

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. A pneumatic tyre having a wide central circumferential groove according to the preamble of claim 1 is known for example from JP-A-3139402. In this known tyre the ratio of the width of the ground contracting tread portions to the total tread width is set between specific limits to improve off-road performance whilst maintaining adequate on-road performance. 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 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 30mm to 70mm, and the side walls having a nonrectilinear configuration having a plurality of axially deviated parts extending in a zigzag or wavy configuration substantially parallel with each other, characterised in that the circumferential groove satisfys the following condition: A > 0.8 X P X W/ (P 2 - W 2 ) where A is the axial distance between circumferentially adjacent peak points (a) and (b) of the centre line of the circumferential groove, P is the circumferential distance between said peak points (a) and (b), and W is the width of the circumferential groove measured at a right angle to the centre line thereof and said circumferential distance P is in the range of from 0.8 to 3 times the ground contacting length, where the ground contacting length is the maximum circumferential length of said ground contacting area. 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; Fig.4 is a plan view showing an example pattern in which the circumferential groove of Fig.3 is used; Fig.5 is a plan view showing another example tread pattern in which the circumferential groove of Fig.3 is used; Fig.6 is a plan view showing another example configuration of circumferential groove; Fig.7 is a plan view showing still another example configuration of circumferential groove and an example of the tread pattern; Fig.8 is a plan view showing still another example configuration of circumferential groove; Fig.9 is a plan view showing another example of the location of the circumferential groove of Fig.7; Fig.10 is a plan view showing another example configuration of the circumferential groove and an example of the tread pattern; Fig.11 is a plan view showing another example configuration of the circumferential groove and an example of the tread pattern; Fig.12 is a plan view showing another example of the location of the circumferential groove of Fig.11; and Fig.13 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 larger 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. Fig.3 shows such an example of this configuration of the circumferential groove 7. In this example, both the side walls 7A and 7B extend in a zigzag configuration. The zigzag of each side wall 7A, 7B consists of two different cycles having different amplitudes and arranged alternately in the tyre circumferential direction. However, the zigzag can be made up of more than two different cycles or only one cycle. Each zigzag cycle is defined by straight circumferential segments (7s1, 7s2, 7s3, 7s4) extending parallel with the tyre equator CO, and straight inclined segments (7i1, 7i2, 7i3, 7i4) extending between the adjacent circumferential segments. The straight circumferential segments (7s1, 7s2) have deviations D1 from the centre line C, which have the same smaller amplitude but in the opposite directions. The straight circumferential segments (7s3, 7s4) have deviations D2 from the centre line C, which have the same larger amplitude but in the opposite directions. All the circumferential segments 7s1 to 7s4 and the inclined segments 7i1 to 7i4 have substantially same circumferential length L. However, the circumferential lengths of the segments can be varied from one another. Further, the circumferential lengths of the zigzag cycles can be varied. The side walls 7A and 7B are parallel with each other. However, the zigzag configuration of one of the side walls 7A can be shifted from that of the other 7B in 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. If the axial width of the circumferential groove 7 varies in the tyre circumferential direction due to the deviation, the mean axial width is used for the above-mentioned axial width Wg. In practice, the circumferential groove 7 of Fig.3 is used in combination with axial grooves as shown in Figs.4 and 5. In Fig.4 which shows an example complete tread pattern, the circumferential groove 7 is centred on the tyre equator CO, and a plurality of axial grooves 10 (10a, 10b) are disposed on each side of the circumferential groove 7. In this example, the axial lengths K1 and K2 of the axial grooves 10a and 10b respectively, are varied in proportion to the axial width of the lateral tread part 9 measured from each tread edge to the circumferential groove 7, whereby the variation of tread rigidity in the tyre circumferential direction is decreased and running stability and uneven wear are improved. In Fig.5 showing another example tread pattern, the circumferential groove 7 is disposed asymmetrically about the tyre equator CO. The centre line CL of the circumferential groove 7 is set to one side from the tyre equator CO by a distance (d). Preferably, the distance (d) is less than 20 % of the ground contacting width Sw. The tyre having this asymmetrical tread pattern is used with the tread edge 13a which is further from the circumferential groove 7 is located on the outside of a car, and the nearer tread edge 13b on the inside of the car, whereby the outside tread part, which is subjected to a larger load than the inside tread part during cornering, is increased in rigidity to improve the cornering performance. In this example, the axial lengths of the axial grooves 10 disposed on each side of the circumferential groove 7 are not varied. Fig.6 shows another example configuration of the circumferential groove 7, which is a modification of the circumferential groove 7 of Fig.3. In this example, the side wall 7B is modified so as to extend straight and parallel to the tyre equator CO, but the side wall 7A is kept unmodified. The circumferential groove 7 of this example is centred on the tyre equator CO, but it can be set aside from the tyre equator CO by a distance (d). Figs.7 and 8 show other example configurations of the circumferential groove 7, in which the side walls 7A and 7B extend zigzag or wavily parallel to each other. In the example of Fig.7, each of the side walls 7A and 7B of the circumferential groove 7 extends in a zigzag and consists of alternate straight segments 7a and 7b which are inclined in opposite directions. In the example of Fig.8, each of the side walls 7A and 7B of the circumferential groove 7 extends wavily. In these examples, the amplitude of the deviation is set to satisfy the following relationship: A > 0.8 X P X W/ (P 2 - W 2 ) where A is the amplitude which is the axial distance between the circumferentially adjacent peak points (a) and (b) of the centre line CL of the circumferential groove 7, P is the pitch which is the circumferential distance between the above-mentioned peak points (a) and (b), and W is the groove width which is the width measured at a right angle to the centre line CL of the circumferential groove 7. If the amplitude (A) is outside the above-mentioned range, suppression of air resonance and the dispersion of the noise to a wide frequency range become insufficient. Preferably, the amplitude (A) is not more than 25% of the ground contacting width SW. If the amplitude (A) is more than 25%, the angle of the circumferential groove to the tyre circumferential direction becomes too large, and the rigidity of the lateral tread part is decreased which deteriorates the steering stability and causes uneven wear resistance. The pitch P is in the range of 0.8 to 3 times the ground contacting length SL, which is the circumferential length of the ground contacting area under the above-mentioned standard loaded state. If the pitch P is less than 0.8 times the ground contacting length SL, the drainage performance is decreased. If the pitch P is more than 3 times SL, the air resonance noise can not be improved. In Fig.7, the circumferential groove 7 is centred on the tyre equator CO, and a plurality of axial grooves 10 are disposed on each side thereof. The axial lengths of the axial grooves 10 are varied in proportion to the axial width of the lateral tread part 9. Fig.9 shows another example of the location for the zigzag circumferential groove 7 of Fig.7. In this example, the groove 7 is completely set aside from the tyre equator CO. Similarly to the Fig.5 example, the tyre having this asymmetrical tread pattern is used on a car with the tread edge 13a farther from the circumferential groove 7 located on the outside of a car, and the nearer tread edge 13b on the inside of the car. In Figs.7 and 9, the zigzag pitch P is constant in the circumferential direction, but it can be varied. Fig.10 shows another example configuration of the circumferential groove 7, which is a modification of the above-mentioned zigzag circumferential groove 7. In this example, the zigzag consists of alternate long straight segments 7a and short straight segments 7b which are inclined in the opposite directions. In each pitch, the above-explained relationship is satisfied. That is, in the long pitch P1 defined by the long segment 7a, A1 > 0.8 X P1 X W1/ (P1 2 - W1 2 ). In the short pitch P2 defined by the short segment 7b, A2 > 0.8 X P2 X W2/ (P2 2 - W2 2 ) . By using a plurality of different pitches, the noise dispersion effect can be enhanced. Fig.11 shows another 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.12. Fig.13 shows another example circumferential groove 7, which is a modification of the Fig.11 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 Figs.4, 5, 7, 10, and 11, 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 Figs.4 and 5, the axial grooves 10 in one of the lateral tread parts 9 are inclined in the opposite direction to the axial grooves 10 in the other lateral tread part 9. The angle of the axial groove gradually increases from the axially inner end thereof to the axial edge line Y of the ground contacting area, and then the angle gradually decreases to the axially outer end thereof, inclining in the reverse direction. Accordingly, the maximum angle of 90 degrees lies at the axial edge line Y. the minimum angle at the axially inner end is in the range of from about 50 to 70 degrees. In Figs.7, 10 and 11, 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 30mm to 70mm, and the side walls (7A,7B) having a nonrectilinear configuration having a plurality of axially deviated parts extending in a zigzag or wavy configuration substantially parallel with each other, characterised in that the circumferential groove (7) satisfys the following condition: A > 0.8 X P X W/ (P 2 - W 2 ) where A is the axial distance between circumferentially adjacent peak points (a) and (b) of the centre line of the circumferential groove (7), P is the circumferential distance between said peak points (a) and (b), and W is the width of the circumferential groove (7) measured at a right angle to the centre line thereof and said circumferential distance P is in the range of from 0.8 to 3 times the ground contacting length (SL), where the ground contacting length (SL) is the maximum circumferential length of said ground contacting area. A pneumatic tyre according to claim 1, characterised in that the peak-to-peak amplitude of axial deviation of said at least one of said side walls (7A,7B) is less 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 1, characterised in that the width (Wg) of the circumferential groove (7) is periodically decreased in the longitudinal direction thereof so as to form narrow width portions (16) 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, the maximum width (WM) is in the range of from 30 to 70 mm, and the circumferential pitches of the narrow width portions (P4) are such that at least one narrow width portion always exists in the ground contacting area. A pneumatic tyre according to claim 2 or 3, characterised in that except for said circumferential groove (7), no circumferentially continuously extending groove is provided in the tread portion. A pneumatic tyre according to claim 2 or 3, characterised in that said circumferential groove (7) is disposed within a tread central region which has a width of 70% of the ground contacting width.

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