Pneumatic tyre

Abstract: A pneumatic tyre comprises a circumferential groove (10) disposed in the axial centre (Yc) of the tread portion (2), circumferentially spaced main grooves (11) disposed on both sides of the tyre equator, the main grooves (11) extending axially outwardly beyond the tread edges (Te), while inclining to one circumferential direction and increasing the inclination angle (α) of the groove centre line (N1) with respect to the circumferential direction (C(No)), auxiliary grooves (12) each disposed between the circumferentially adjacent main grooves (11) and extending from a middle portion (Yn) of one of the main grooves towards the above-mentioned circumferential direction beyond the tread edge (Te), each of the auxiliary grooves having a part (12A) equidistant from the adjacent main grooves (11) and having a length more than 30 % of the length (K) of the auxiliary groove (12). The intersecting angle (β) of the auxiliary grooves with the main grooves (11) is in the range of from 20 to 60 degrees.

Description: The present invention relates to a pneumatic tyre and more particularly to a tyre with a directional tread pattern capable of improving the steering stability, wet performance and tyre noise. In conventional pneumatic tyres for passenger cars and the like, the tread portion is provided with relatively wide circumferential grooves which extend continuously in the tyre circumferential direction to prevent aquaplaning. However, such circumferential groove generates a noise during running due to its columnar resonance whose primary mode frequency ranges from 800 to 1.2 kHz. It has been known to be effective for reducing the resonance noise to decrease the groove width, but this decreases the groove volume and as a result the aquaplaning phenomenon becomes liable to occur on wet roads. If a number of relatively narrow circumferential grooves are disposed in the tread, the drainage may be improved, but the steering stability is liable to deteriorate because it is difficult to maintain the necessary tread rigidity. These problems are especially remarkable in low-aspect-ratio high performance tyres. Other proposals in the prior art include EP-A-0 705 718 which according to the pre-amble of claim 1 discloses a tyre having no circumferential grooves and slanted main grooves extending from the pattern centre towards the tread edge having steeply slanted portions and gently slanted portions which do not cross any other groove. Each main groove is provide with at least two branched grooves which in turn extend without crossing any other groove and the points between the grooves have a tapered zone so as to cut off the point so as to provide the required properties. Another proposal is EP-A-0 565 270 which combines circumferentially extending grooves at either side of the pattern with slant-wise lateral grooves so the tyre is divided into blocks. Such a tyre in having the circumferentially extending grooves at either side of the tyre can generate noise in use. It is therefore, an object of the present invention to provide a pneumatic tyre in which the steering stability, wet performance and tyre noise are effectively improved at the same time. According to the present invention a pneumatic tyre is provided having the features as set out in Claim 1. Preferably, the junction of each of the auxiliary grooves with the main groove is spaced apart by 20 to 60 % of the axial distance SL between one of the tread edges and the centre line of the circumferential groove from the centre line. The intersecting angle β of the auxiliary grooves with the main grooves is in the range of from 20 to 60 degrees. The auxiliary grooves extend axially outwardly, while having an increasing inclination angle of the groove centre line with respect to the circumferential direction. An embodiment of the present invention will now be described in detail in conjunction with the accompanying drawings, in which: Fig.1 is a cross sectional view of an embodiment of the present invention; Fig.2 is a developed partial plan view thereof showing an example of the tread pattern; Fig.3 is an enlarged view showing a main groove and an auxiliary groove; Fig.4 is a diagram showing a foot print of the tyre pattern; and Fig.5 is a developed partial plan view of a conventional tyre. In Fig.1, the pneumatic tyre 1 according to the invention is a 235/45ZR17 radial tyre for passenger cars. The tyre 1 comprises a tread portion 2, a pair of axially spaced bead portions 4 each with a bead core 5 therein, a pair of sidewall portions 3, a carcass 6 extending between the bead portions 4, and a belt 7 disposed radially outside the carcass 6 in the tread portion 2. The carcass 6 comprises at least one ply of organic fibre cords arranged radially at an angle of from 70 to 90 degrees with respect to the tyre equator C. In this embodiment, the carcass is composed of a single ply of polyester cords arranged at 90 degrees. The belt 7 comprises at least two crossed plies of high modulus cords such as steel and the like laid at angles of 10 to 30 degrees with respect to the tyre equator C. In this embodiment, the belt is composed of a radially inner ply 7A and a radially outer ply 7B, each made of parallel cords laid at 24 degrees. A band 9 is disposed radially outside the belt to cover at least the edges of the belt 7 and comprises at least one cord ply in which the cord angle is less than 5 degrees with respect to the tyre equator C. In this embodiment, the band is composed of a pair of axially spaced edge plies 9A disposed on the axial edge parts of the belt and an axially outer full-width ply 9B extending over the belt 7, each is ply made of at least one spirally-wound organic fibre cord such as nylon and the like. The tread portion 2 is provided near or on the tyre equator C with at least one circumferential groove 10 extending continuously in the circumferential direction. Further, on each side of the tyre equator, main grooves 11 and auxiliary grooves 12 are disposed. The above-mentioned term 'near the tyre equator' means 'inside of a tread centre region Yc' extending from the tyre equator C toward both sides thereof by a distance of 0.1 times the tread width TW, and also it means that the centre line N0 of the circumferential groove 10 is disposed within the region Yc. Here, the tread width TW is the axial width between the axial outmost edges Te of the ground contacting area under such a condition that the tyre is mounted on its standard rim, inflated to its standard pressure and loaded with its standard load. The standard rim is a rim officially approved for the tyre by standard organisation, for example JATMA (Japan), TRA (USA), ETRTO (Europe) and the like, and the standard inner pressure and the standard load are the maximum air pressure and the maximum tyre load for the tyre officially specified in Air-pressure/Maximum-load Table. Further, the tread length TL is defined as the maximum circumferential length of the ground contacting area. In this embodiment, only one circumferential groove 10 is disposed in the tread, which is a straight groove whose centre line N0 is just on the tyre equator C. However, a zigzag groove may be used instead. In that case, the amplitude of the zigzag is preferably smaller than the groove width and the groove centre line is disposed within the region Yc. The circumferential groove 10 has a U-shaped cross sectional shape so that edged corners are formed between the groove sidewalls and tread face. As shown in Fig.2, the main grooves 11 extend axially outwardly from the tread centre to beyond the tread edge Te, and all the main grooves 11 incline towards the same direction, which is reverse to the designed rolling direction F of the tyre, so as to form a directional tread pattern. The main grooves each have an axially inner end P1 spaced apart from the circumferential groove 10, and the minimum axial distance L1 therebetween is in the range of from 5 to 30 mm, more preferably 10 to 20 mm. If the distance L1 is more than 30 mm, drainage from the tread centre is insufficient and aquaplaning is liable to occur. If distance L1 less than 5 mm, the rigidity of the tread rubber near the circumferential groove 10 becomes excessively low, and the straight running stability deteriorates. The inclination of each of the main grooves 11 gradually increases from the axially inner end P1 to the outer end P2 so as to curve gently. Thus, the angle α with respect to the circumferential direction, of the groove centre line N1 is also increased. As shown in Fig.3, the angle α1 at the inner end P1 is set in the range of from 0 to 15 degrees and the angle α2 at the tread edge Te is set in the range of from 60 to 90 degrees. The curvature of the centre line N1 may have a single radius, but in this example, in order to effectively decrease the resistance to water flow and thereby further improve the drainage, a multi-radius such that the line N1 has no straight part is employed. In this case, it is preferable that the radius is decreased towards the axially outside. The width of the main grooves 11 first increases gradually from the inner end P1 towards the outer end P2, but decreases near the tread edges Te to stop the noise called pumping noise. As a result, a widest part 11A is formed in the middle Yn of the groove length as shown in Fig.3. In this embodiment, the groove width gradually decreases from the middle Yn to the tread edge. To make up a possible decrease in drainage due to the decreased width or further improve the drainage, relatively-narrow-width auxiliary grooves 12 are disposed one between each adjacent pair of the circumferential main grooves 11. Each of the auxiliary grooves 12 extends from the widest part 11A of one of the main grooves 11 in the above-mentioned reverse direction to the designed tyre rolling direction F beyond the tread edge Te, whilst inclining axially outwards. The inclination angle of the groove centre line N2 thereof with respect to the tyre circumferential direction gradually increases from the axial inner end Q1 to the outer end or tread edge so that the auxiliary grooves 12 are curved in a similar fashion to the main grooves 11. In order to even the tread pattern rigidity, each auxiliary groove 12 is provided with a part 12A equidistant from the adjacent main grooves 11, and this parallel part 12A is more than 30 % of the length K of the auxiliary groove 12 from the inner end Q1 to the tread edge Te. The parallel part 12 usually starts from the tread edge. The intersecting angle β of the auxiliary groove 12 with the main groove 11 or the angle between the centre lines N1 and N2 of these grooves at the inner end Q1 is preferable set in the rage of from 20 to 60 degrees, preferably 30 to 50 degrees, more preferably 35 to 40 degrees. If more than 60 degrees, the flow of water from the main groove to the auxiliary groove lessens, and further pumping sound is liable to occur at the junction. If less than 20 degrees, the corners 13 between the main grooves 11 and auxiliary grooves 12 are liable to wear and tear, In this embodiment, to prevent the corners 13 from being damaged, the auxiliary grooves 12 are gradually decreased in width, when approaching the inner ends Q1. The junction of the auxiliary groove 12 and the main groove 11 is spaced apart from the centre line N0 of the circumferential groove 10 by a distance Ln which is in the range of from 20 to 60 %, preferably 30 to 50 % of the distance SL between the centre line N0 and the tread edge Te. If more than 60 %, the drainage can not be effectively improved. If less than 20 %, it becomes difficult to set the angle α in the above-mentioned range. The pitches T2 of the main grooves 11, defined as the circumferential distances between the corresponding inner ends P1, are preferably in the range of from 30 to 70 %, more preferably 40 to 60 % of the tread length TL, whereby the pattern rigidity and drainage are improved. If less than 30 %, the tread rigidity greatly decreases in the vicinity of the tyre equator C. If more than 70 %, the drainage becomes insufficient. The pitches T2 may be a constant value, but preferably several different values are used therefor to reduce the pattern noise. Further, between the right side and left side of the tyre equator C the tread pattern is circumferentially shifted by 20 to 50 % of the average T2a of the pitches T2, thereby decreasing the pattern noise. In the case of passenger radial tyres, the above-mentioned tread grooves 10, 11 and 12 usually have the same depth in the range of from 6.0 to 9.0 mm, and the widths thereof are varied from 3.0 to 12.0 mm. The above-explained tread pattern is suitably combined with a tyre aspect ratio of not more than 50 %. Comparison Tests Test tyres having the structure shown in Fig.1 and specifications given in Table 1 were made by way of test and tested for steering stability, aquaplaning, noise and wear resistance. The tests were carried out, using a 3000cc FR sports car, as follows. (Tyre size: 235/45ZR17, Rim size: 17X8JJ, Inner pressure: front=2.3, rear=2.5 kgf/sq.cm) Steering Stability Test On a dry asphalt test course, steering stability (response, rigidity, grip, etc.) were evaluated by the test driver's feelings. The results are indicated in Table 1 by an index based on the conventional tyre being 100, wherein the larger the index, the better the performance. Aquaplaning Test (Straight Running) On a straight wet asphalt test course provided with a 10 mm deep water pool, a wheel lock brake was applied to all the tyres at running speeds of 60 km/h, 80 km/h and 100 km/h, and the running speed corresponding to a deceleration of 0.15G was measured. The obtained speeds are indicated in Table 1 by an index based on the conventional tyre being 100, wherein the larger the value, the better the performance. Aquaplaning Test (Cornering) On a 100 m radius asphalt test course provided with a 5 mm deep 20 m long water pool, the speed of entering the water pool was increased in steps and the lateral acceleration was measured on the front tyre to obtain the average from 50 to 80 km/h. The results are indicated in Table 1 by an index based on the conventional tyre being 100, wherein the larger the value, the better the performance. Noise test On a smooth asphalt paved road surface, the pattern noise heard in the test car in a wide speed rage of from 20 to 140 km/h was evaluated by the test driver's feelings. The results are indicated in Table 1 by an index based on the conventional tyre being 100, wherein the larger the index, the smaller the noise. Wear Resistance Test After running a test circuit course the tread rubber wear was measured. The results are indicated in Table 1 by an index based on the conventional tyre being 100, wherein the larger the index, the smaller the wear.

Claims: A pneumatic tyre comprising a tread portion (2) defining a tread (2) having axial edges (Te) and a directional pattern comprising circumferentially spaced main grooves (11) disposed on both sides of the tyre equator (C(No)), the main grooves (11) extending axially outwardly beyond the tread edges (Te), while inclining to one circumferential direction and increasing in inclination angle (α) of its groove centre line (N1) with respect to the circumferential direction such that the inclination angle (α1) at the axially inner end (P1) of the main groove (11) is 0 to 15 degrees, and the inclination angle (α2) at the tread edge (Te) is 60 to 90 degrees, and auxiliary grooves (12) each disposed between the circumferentially adjacent main grooves (11) and extending from a middle portion (Yn) of one of said main grooves (11) towards said one circumferential direction beyond the tread edge (Te), characterised in .that at least one circumferential groove (10) is disposed in the axial centre region (Yc) of the tread portion (2), each said main groove (11) is not connected with said at least one circumferential groove (10), each said main groove (11) has a single branch (12) which is one of said auxiliary grooves (12), and the width of each said main groove (11) gradually increases from an axially inner end (P1) thereof and also from the tread edge (Te) so as to form a widened part in said middle portion (Yn). A pneumatic tyre according to claim 1, characterised in that the junction (Q1) of the auxiliary groove (12) with the main groove (11) is spaced apart from the centre line (No) of the adjacent circumferential groove (10) by a distance (Ln) which is 20 to 60% of the axial distance (SL) between one of the tread edges (Te) and a centre line (No)of the adjacent circumferential groove (10). A pneumatic tyre according to claim 1 or 2, characterised in that the intersecting angle (β) of the auxiliary grooves (12) with the main grooves (11) is in the range of from 20 to 60 degrees. A pneumatic tyre according to claim 1, 2, or 3, characterised in that each said auxiliary groove (12) is provided with a part (12A) equidistant from the adjacent main grooves (11), and this part (12A) is more than 30 % of the length (K) of the auxiliary groove (12) measured from the inner end (Q1) to the tread edge (Te). A pneumatic tyre according to claim 1, characterised in that at the junction (Q1) of the auxiliary groove (12) with the main groove (11) the auxiliary groove (12) is narrower in width than the main groove (11), and the width of each said auxiliary groove (12) gradually decreases when approaching the axially inner end (Q1).

Inventors: YAMAKAGE ATSUSHI, YAMAKAGE, ATSUSHI
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1998
Main classification: B60C11
IPC codes: B60C11