Pneumatic tyre

Abstract: A pneumatic tyre with a low aspect ratio of 0.55 to 0.70 comprising a tread (5) provided with four circumferential grooves (G) disposed symmetrically about the tyre equatorial plane (C) to divide the tread into axially spaced five land portions (R), at least one of the five land portions (R1) having a set of lateral grooves (Y) crossing the land portion. A sea area ratio Ss/S of a total area Ss of grooves (G) to a total area S of a tread surface (5S) is 0.20 to 0.27. A distance La from the tyre equatorial plane (C) to the inner groove edge of the inner circumferential groove (G1) among the four circumferential grooves (G) is 0.090 to 0.125 times the tread width TW, and a distance Lb from the tyre equatorial plane (C) to the inner groove edge of the outer circumferential groove )G2) among the four circumferential grooves (G) is 2.5 to 3.0 times the distance La.

Description: The present invention relates to a pneumatic tyre capable of effectively reducing the tyre noise. Low noise and quiet running of automobiles have been keenly demanded recently, and reduction of tyre noise is needed accordingly. Generally, on the tread surface of a tyre, tread grooves comprising circumferential grooves and lateral grooves crossing them are formed for the purpose of providing wet grip performance and the like. An all season tyre improved with a more uniform handling response under different conditions having such an arrangement of such grooves in accordance with the preamble of claim 1 is known, for example, from EP-456949. While improving the water escape effect, the tread grooves generate air column resonance and increase tyre noise. The air column noise is a resonant vibration caused when the air passes through the air column formed between the tread grooves and the road surface. The air column noise is one of the main sound sources of the noise around 1000 Hz which is very irritating to the ear. Hitherto, to reduce this kind of air column resonance, it was attempted to decrease the groove area and groove volume of the tread grooves. Such casual decrease of groove area or volume not only fails to show a decrease of air column resonance effectively, but also lowers the wet grip performance. The inventor hence accumulated studies on air column resonance and wet grip performance. As a result it was found, especially in a low profile radial tyre suited to high speed running having four circumferential grooves on the tread surface, that a large vibration is generated near the tyre equatorial plane, and that this vibration induces air column resonance based on the circumferential grooves. In addition, it was found that induction of air column resonance by tyre vibration can be effectively suppressed by increasing the distance of the inner circumferential groove of the four circumferential grooves from the tyre equatorial plane, and that the sound pressure derived from the air column resonance is effectively decreased by reducing the groove area of the circumferential grooves. It is hence a primary object of the invention to present a pneumatic tyre capable of effectively reducing the tyre noise, by defining the distance of the inner circumferential groove from the tyre equator plane, in a low profile tyre having four circumferential grooves. According to the present invention, a pneumatic tyre with a low aspect ratio of 0.55 to 0.70 comprises a tread (5) provided with four circumferential grooves (G) extending in the tyre circumferential direction at symmetrical positions about the tyre equatorial plane (C) to divide the tread (5) into five axially spaced land portions, at least one of the five land portions having a set of lateral grooves (Y) crossing said one of the five land portions to divide into a plurality of blocks (B1), each of distances La from the tyre equatorial plane (C) to inner groove edges of inner circumferential grooves at the inner side of the tyre axial direction among said four circumferential grooves is 0.090 to 0.125 times a tread width TW, and each of distances Lb from the tyre equatorial plane (C) to inner groove edges of outer circumferential grooves at the outer side of the tyre axial direction among said four circumferential grooves is 2.5 to 3.0 times the distance La characterised by a sea area ratio Ss/S of a total area Ss of grooves including said circumferential grooves (G) and lateral grooves (Y) to a total area S of a tread surface (5S) being 0.20 to 0.27, and in that the set of lateral grooves (Y) comprises wide lateral grooves (YA) with a wide groove width WA and narrow lateral grooves (YB) with a groove width WB being not larger than 0.8 times the groove width WA, and the wide lateral grooves (YA) and narrow lateral grooves (YB) are arranged alternately. When a set of lateral grooves is provided in each land portion in at least one pair of adjoining land portions, a deviated amount α of axially confronting lateral grooves between one set and the other set is preferred to be 0.2 to 0.45 times the pitch length of the lateral grooves. The groove width K1 of the inner circumferential groove is preferred to be 1.5 to 2.0 times the groove width K2 of the outer circumferential groove, and sum of the groove widths K1, K1, K2, K2 is preferred to be 0.13 to 0.17 times the tread width TW. An embodiment of the present invention will now be described, by way of example, referring to the attached diagrammatic drawings, in which: Fig. 1 is a sectional view of a tyre showing an embodiment of the invention; Fig. 2 is a plan view showing an example of its tread pattern; Fig. 3 is a plan view showing a magnified view of a part of the tread pattern; Fig. 4 is a diagram showing the relation of the ratio La/TW, wet grip performance, and uneven wear resistance performance; Fig. 5 is a diagram showing the relation of the ratio Lb/La, steering stability, and uneven wear resistance performance; Fig. 6 is a diagram showing the relation of sea area ratio Ss/S, wet grip performance, and pass by noise performance; Fig. 7 is a diagram showing the relation of the number of lateral grooves, steering stability, and wet grip performance; and Fig. 8 is a diagram showing the relation between ratio α/P and compartment noise performance. With reference to Fig. 1, a pneumatic tyre 1 comprises a pair of beads 3 each having a bead core 2, side walls 4 extending from the bead 3 outwardly in the tyre radial direction and a tread 5 linking the radially outer ends of the side walls 4. The tyre 1 has an aspect ratio H/T of the tyre sectional height H to the tyre maximum width T in a range of 0.55 to 0.70. A low-profile radial tyre for a passenger car in a tyre size of 215/65R14 is formed in this embodiment which thus has a .65 aspect ratio. A radial carcass 6 extends from the tread 5 through the side walls 4 and is folded back from the inside to the outside around the bead core 2, and a belt layer 7 is provided above the carcass 6 and radially inwards of the tread 5. The carcass 6 is composed of one or more carcass plies of cords, two in this embodiment comprising inner and outer plies. The carcass cords are arranged at an angle of 75 to 90 degrees to the tyre equator C, and are made of organic fibre cords of nylon, rayon, polyester, etc. The belt layer 7 is composed of one or more belt plies of cords, two in this embodiment, comprising inner and outer plies 7A, 7B. The belt cords are arranged at an angle of 30 degrees or less, for example, 24 degrees to the tyre equator C, and the plies 7A, 7B are overlaid by varying the direction so that the belt cords cross between plies. The belt cords are steel or other metal cords, and the carcass 6 is tightened by its hoop effect with a large flatness and the tread 5 is reinforced at the same time. The ply width of the inner belt ply 7A is wider than the outer belt ply 7B, and the maximum ply width, that is, the belt width BW is 0.95 to 1.05 times the tread width TW. The tread width TW is the axial width of the tread surface 5S which comes in contact with the ground in the state, mounted on a regular rim, inflated to a regular internal pressure and carrying a normal load. The regular rim is the rim officially approved for the tyre by for example JATMA (Japan), TRA (USA), ETRTO (Europe) and the like; the regular internal pressure is the maximum air pressure for the tyre officially specified in Air-pressure/Max.-loads Table by for example JATMA (Japan), TRA (USA), ETRTO (Europe) and the like; and the normal load is the maximum load for the tyre officially specified in Air-pressure/Max.-loads Table by for example JATMA (Japan), TRA (USA), ETRTO (Europe) and the like. In this embodiment, the ratio TW/T of the tread width TW to the tyre maximum width T is raised to about 0.80 so as to erect or strengthen the profile or shape of the tyre side, thereby reducing drumming noise. Drumming noise is an inside sound of low frequency (40 to 80 Hz) generated in the vehicle compartment when running on a road surface having tiny waves or a cracked road surface. The tread surface 5S is divided axially into five spaced land portions R by four circumferential grooves G extending in the tyre circumferential direction at symmetrical positions about the tyre equatorial plane and tread edges e, and at least one of the five land portions is formed as a row RB of blocks. The circumferential grooves G consist of two inner circumferential grooves G1 disposed inside in the tyre axial direction, and two outer circumferential grooves G2 disposed at its axially outside. An inner land portion R1 extending on the tyre equator C is formed between the inner circumferential grooves G1 and G1, two middle land portions R2 are formed between the inner and outer circumferential grooves G1 and G2, and two outer land portions R3 are formed between the outer circumferential groove G2 and the tread edge e. The middle land portion R2 is provided with a set of lateral grooves Y1 crossing this middle land portion R2 to form row RB1 of blocks B1. The outer land portion R3 is, also, provided with a set of lateral grooves Y2 crossing this outer land portion R3 to form a row RB2 of blocks B2. In the inner land portion R1, a set of terminated lateral grooves J having one end connecting with circumferential groove G1 and the other end being terminated in this inner land portion R1 is provided on each side of the inner land portion R1, thereby forming the inner land portion R1 as a rib form continuous in the circumferential direction. Each terminated lateral groove J has a length of more than 25% of the width in the axial direction of the tyre land portion R1 so as to give water discharge capability. In the invention, one of the five land portions R can be formed in a row of blocks, but from the viewpoint of wet grip performance, it is desired to use two or, more preferably, four rows of blocks. The inner land portion R1, however, is preferably in the rib form to decrease the tyre vibration itself which may otherwise excite air column resonance and to improve the straightforward stability. A distance La between the inner groove edge in the tyre axial direction of the inner circumferential groove G1 and the tyre equator plane C is 0.090 to 0.125 times the tread width TW, and a distance Lb between the inner groove edge of the outer circumferential groove G2 and the tyre equator plane C is 2.5 to 3.0 times the distance La. By setting the distance La at 0.090TW or more so as to keep the circumferential groove G1 away from the tyre equator plane C, generation of air column resonance of circumferential groove G1 by tyre vibration can be effectively suppressed. Meanwhile, when La is smaller than 0.090 TW, the air column resonance suppressing effect is inferior, and resistance to uneven wear is sacrificed, or when larger than 0.125TW, to the contrary, the wet grip performance is lowered. If the distance Lb is smaller than 2.5 times the distance La, the balance of lateral rigidity of tread pattern is worsened, the cornering force is decreased, steering stability on dry road surface is lowered, and the noise increases due to the effect of tyre vibration. If larger than 3.0 times, to the contrary, the rigidity of the outer land portion is extremely lowered, and uneven wear is induced. The groove width K1 of the inner circumferential groove G1 is larger than the groove width K2 of the outer circumferential groove G2. That is, in order to have a uniform water discharge performance in the entire tread surface, it is necessary to relatively increase the water discharge capacity at the tyre equatorial plane side, and hence the groove width ratio of the circumferential grooves K1/K2 is defined to be within 1.5 to 2.0 in this embodiment. The sum ΣK of the groove widths K1, K1, K2, K2 is 0.13 to 0.17 times the tread width TW, and by decreasing, as compared with an ordinary tyre having the value of 0.2 times or more, the sea area ratio Ss/S of the total area Ss of the grooves comprising the circumferential grooves G, crossing lateral grooves Y and terminated lateral grooves J to the total area S of the tread surface 5S is defined within 0.20 to 0.27. By thus decreasing the groove width K1, K2 of the circumferential grooves, the sound pressure itself attributable to the air column resonance can be reduced. Besides, by decreasing the groove width of the lateral grooves, when the sea area ratio Ss/S is lowered, the noise reducing effect is hardly exhibited, while the water discharge performance is largely lowered. In the embodiment, the number of the crossing lateral grooves arranged per one land portion R, that is, the number of crossing lateral grooves Y1, Y2 per one set is in the range of 90 to 110, respectively, and lowering of water discharge performance due too low sea area ratio Ss/S is prevented, and wet grip performance is maintained. Meanwhile, the number of crossing lateral grooves Y1 and the number of crossing lateral grooves Y2 may be different within the specified range (90 to 110), but it is preferred to use the same number, as in the embodiment, from the viewpoint of uniformity, and the number of terminated lateral grooves J on one side is also in a range of 90 to 110, and coincides with the number of crossing lateral grooves Y1, Y2. Incidentally, when each number of the lateral grooves is in a range of 90 to 110, primary components of the pattern noise are concentrated at 800 to 1000 Hz, nearly coinciding with the frequency band of air column resonance derived from the circumferential grooves, and the sound pressure is raised significantly, thereby increasing the noise. To avoid such increase of noise, accordingly, the crossing lateral grooves Y arranged in the row RB of blocks are formed in two types of grooves differing in groove width, and preferably they are arranged alternately in the tyre circumferential direction. Moreover, in at least one pair of land portions axially adjacent to each other, when the lateral grooves are provided on each land portion R in the pair, the lateral grooves axially confronting each other between the both side of the land portions should be preferably deviated in a specific range in the tyre circumferential direction. Herein, the lateral grooves are used as a common name for the crossing lateral grooves Y and terminated lateral grooves J. Among the sets of the lateral grooves Y1, Y2, the set of lateral grooves Y1 disposed in the row RB1 of blocks are composed of two types, that is, wide lateral grooves YA with a wide groove width WA and narrow lateral grooves YB with a narrow groove width WB being not larger than 0.8 times the groove width WA, and the lateral grooves YA, YB are alternately arranged in the tyre circumferential direction. By thus alternately arranging two types of lateral grooves differing in groove width, the apparent number of the lateral grooves YA, YB may be reduced to half of the initial number of 90 to 110, and the primary components of pattern noise generated depending on the number may be transferred to the low frequency band. Therefore, when the groove width WB is greater than 0.8 times the groove width WA, there is no effect of reducing the apparent number to half, and the peak sound of the air column resonance and pattern noise coincide with each other. Therefore, the groove width WB should be preferably 0.5 times or less of the groove width WA. To obtain the necessary water discharge capacity, the groove width WA is preferred to be in a range of 1.5 to 3.0 mm, and the groove width WB is desired to be in a range of 0.8 to 1.2 mm. A case of dislocating the mutually confronting lateral grooves in the tyre circumferential direction is described below referring to Fig. 3, representatively showing lateral grooves Y1, Y2 provided in the land portions R2, R3 axially adjacent to each other. As shown in Fig. 3, the lateral grooves Y1, Y2 are opened to the circumferential groove G2 disposed between the land portions R2, R3. The opening centre in the circumferential groove G2 of the lateral groove Y1a arranged at one land portion R2 is remote from the opening centre of the lateral groove Y2a disposed at the other land portion R3 and confronting the lateral groove Y1a, by a deviated amount α in the tyre circumferential direction. The deviated amount α is in a range 0.2 to 0.45 times the pitch length P1a in the tyre circumferential direction between the lateral groove Y1a and the lateral grocve Y1b circumferentially adjoining the lateral groove Y1a in the spacing direction. The deviated amount α is similarly in a range of 0.2 to 0.45 times of the pitch length P2a in the tyre circumferential direction between the lateral groove Y2a and the lateral groove Y2b circumferentially adjoining to the lateral groove Y2a in the spacing direction. Herein, the lateral groove Y2a confronting the lateral groove Y1a refers to the lateral groove Y2a at the closer side to the lateral groove Yla among the lateral grooves Y2a, Y2b enclosing this lateral groove Y1a. The relations 0.2P1a <= α <= 0.45P1a 0.2P2a <= α <= 0.45P2a should be satisfied in each one of the confronting lateral grooves. By thus deviating the confronting lateral grooves in a specific range in the tyre circumferential direction, the frequency of pattern noise generated from each land portion is dispersed, and may be transformed into so-called white noise. Incidentally, if the deviated amount α is less than 0.2 times the pitch length P1 or P2, the dispersing effect is not enough, or if it exceeds 0.45 which is close to 0.5 times, then a peak noise of double the frequency of the peak frequency of the noise generated by the number of the lateral grooves occurs, and the noise performance is similarly impaired. Therefore, preferably, the deviated amount α is in a range of 0.25 to 0.40 times the pitch length. As far as satisfying the conditions (1) and (2), the pitch lengths L1, L2 may be changed according to the general pitch variation method, and the average pitch length at this time is about 18 to 24 mm. More preferably, between the lateral grooves provided in the land portions R1, R2 axially adjacent to each other, that is, between the terminated grooves J and crossing lateral grooves Y1, a deviation in the tyre circumferential direction satisfying the conditions (1) and (2) is formed similarly, and the noise relating to the lateral grooves is dispersed. Trial tyres in the structure as shown in Fig. 1, tread pattern as shown in Fig. 2 and Fig. 3, and tyre size of 215/65 R15 91H were installed on a passenger car (3.3 litres, FF vehicle) each in a regular rim (6 1/2 x 15) and at regular internal pressure (front wheels: 2.1 kg/cm 2 , rear wheels: 2.0 kg/cm 2 ), and presented to tests 1 to 5 as specified below. The test results as shown in Figs. 4 to 8 were obtained. Test 1 The tread width TW (= 164 mm) and ratio Lb/La (= 2.80) were fixed, and the ratio La/TW of the distance La of inner circumferential grooves to the tread width TW was varied, and effects on resistance to uneven wear and wet grip performance were measured. The results are shown in Fig. 4. Test 2 The tread width TW (= 164 mm) and distance La of inner circumferential grooves (= 16.4 mm) were fixed, and the distance Lb of outer circumferential grooves was varied, and effects on the ratio Lb/La, steering stability (dry road surface), and resistance to uneven wear were measured. The results are shown in Fig. 5. Test 3 Only the width of each circumferential groove was varied to change the sea area ratio Ss/S, and effects on pass by noise performance and wet grip performance were measured. The results are shown in Fig. 6. Test 4 Only the number of lateral grooves was varied, and effects on steering stability and wet grip performance were measured. The results are shown in Fig. 7. Test 5 In the tyre conforming to the specification of the embodiment in Table 1, the pitch length P of each lateral groove (= 16 mm) was fixed, and effects of the ratio α/P of the deviated amount α to the pitch length P on the compartment noise performance were measured. The results are shown in Fig. 8. In the pass by noise test, a microphone was set at a position 7.5 m to the side of the vehicle running in the engine off state, and the noise was measured according to JISO Rule C-606. In the wet grip test the vehicle was driven on a course have a water pool of 5 mm in depth and 20 m in length on a road surface with a curvature of 100 m. The vehicle was driven at increasing speeds in steps, and the transverse acceleration (transverse G) was measured, and the mean transverse G at speeds of 70 to 90 km/h was evaluated. In the steering stability test (dry road surface), it was evaluated by the feel of the driver by driving on a dry paved course. In the uneven wear resistance performance test, the appearance of wear of the tread surface was visually evaluated after the steering stability performance test. In the compartment noise test, the compartment noise in steering stability performance test was evaluated by the feeling of the driver. Besides, according to the specification in Table 1, trial tyres were fabricated, and evaluated by the pass by noise test, wet grip test, steering stability test (dry road surface), uneven wear resistance performance test, and compartment noise test, and the performances were compared. Each performance is expressed by the index with that of the tyre of the embodiment as 100, and the greater figure means the superior performance.

Claims: A pneumatic tyre with a low aspect ratio of 0.55 to 0.70 comprising a tread (5) provided with four circumferential grooves (G) extending in the tyre circumferential direction at symmetrical positions about the tyre equatorial plane (C) to divide the tread (5) into five axially spaced land portions, at least one of the five land portions having a set of lateral grooves (Y) crossing said one of the five land portions to divide into a plurality of blocks (B1), each of distances La from the tyre equatorial plane (C) to inner groove edges of inner circumferential grooves at the inner side of the tyre axial direction among said four circumferential grooves being 0.090 to 0.125 times a tread width TW, and each of distances Lb from the tyre equatorial plane (C) to inner groove edges of outer circumferential grooves at the outer side of the tyre axial direction among said four circumferential grooves being 2.5 to 3.0 times the distance La, characterised in that a sea area ratio Ss/S of a total area Ss of grooves including said circumferential grooves (G) and lateral grooves (Y) to a total area S of a tread surface (5S) is 0.20 to 0.27, and in that the set of lateral grooves (Y) comprises wide lateral grooves (YA) with a wide groove width WA and narrow lateral grooves (YB) with a groove width WB being not larger than 0.8 times the groove width WA, and the wide lateral grooves (YA) and narrow lateral grooves (YB) are arranged alternately. A pneumatic tyre according to claim 1, characterised in that the number of the lateral grooves (Y) in said set of lateral grooves is 90 to 110. A pneumatic tyre according to claim 1 or 2, characterised by at least one pair of land portions axially adjacent to each other, each land portion in said one pair is provided with said set of the lateral grooves crossing the land portion or a set of lateral grooves connecting with the circumferential groove between the land portions in said one pair and terminated in the land portion, the number of the lateral grooves of one set disposed in one land portion is the same as the number of the lateral grooves of the other set disposed in the other land portion, said one set deviates in the tyre circumferential direction to said the other set, and the deviated amount α of the axially confronting lateral grooves to each other between said one set and said the other set is in a range of 0.2 to 0.45 times the pitch length P1 in the tyre circumferential direction between the circumferentially adjoining lateral grooves in said one set enclosing one of said confronting lateral grooves in said the other set, and in a range of 0.2 to 0.45 times the pitch length P2 in the tyre circumferential direction between the circumferentially adjoining lateral grooves in said the other set enclosing one of said confronting lateral grooves in said one set. A pneumatic tyre according to any of claims 1 to 3, characterised by groove widths K1 of said inner circumferential grooves being in a range of 1.5 to 2.0 times the groove widths K2 of said outer circumferential grooves. A pneumatic tyre according to claim 4, characterised in that the sum of said groove widths K1, K1, K2, K2 is 0.13 to 0.17 times the tread width TW. A pneumatic tyre according to any of claims 1 to 5, characterised in that among the five land portions (R1), the land portion (R1) on the tyre equatorial plane (C) is in the form of a continuous rib, and the other land portions are in the form of a row of blocks.

Inventors: HASEGAWA HIROTSUGU, HAYASHI MOTOO, HASEGAWA, HIROTSUGU, HAYASHI, MOTOO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1995
Main classification: B60C11
IPC codes: B60C11