Pneumatic tyre

Abstract: A pneumatic tyre having an aspect ratio of not less than 60% comprises a pair of bead cores (5) disposed one in each bead portion (4), a carcass (6) extending between the bead portions (4) through a tread portion (2) and sidewall portions (3) and turned up around the bead cores (5) to define a pair of turnup portions (6b), a belt (7) disposed radially outside the carcass (6) in the tread portion (2) and comprising belt plies (7A,7B) of which the radially innermost ply (7A) has an axial width (BW) of from 0.8 to 1.2 times the tread width (TW), a pair of bead apexes (9) each tapering radially outwardly from each of the bead cores (5), characterised in that each of the carcass turnup portions (6b) has a radially outer end (X1) at a height (HC) of not more than 0.15 times the tyre section height (H), measured from the bead base line (BL), each of the bead apexes (9) has a radially outer end (9A) at a height (Hb) of not more than 0.25 times the tyre section height (H), measured from the bead base line (BL), a tread rubber (10) disposed in the tread portion (2) to define the ground contacting tread face having a loss tangent of not more than 0.14, a pair of short fibre reinforcing layers (11) each disposed in each of the sidewall portions (3) between the axial outer edges (X2) of the radially innermost belt ply (7A) and the radially outer ends (X1) of the turnup portions (6b) of the carcass (6), and extending along the axially outer surface of the carcass (6), each of the short fibre reinforcing layers (11) having a thickness of from 0.3 to 1.0 mm, the short fibre reinforcing layers (11) being in a compound of 100 parts by weight of a base rubber compound containing 40 to 70 parts by weight of butadiene rubber and 60 to 30 parts by weight of natural rubber and/or isoprene rubber 10 to 30 parts by weight of short fibre, and not more than 30 parts by weight of carbon black whose iodine number is in the range of from 30 to 90mg/g, the short fibre in each of the short fibre reinforcing layers (11) being oriented circumferentially of the tyre so that the direction of orientation of not less than 90% of the short fibre is in the range of from +20 to -20 degrees to the circumferential direction of the tyre, and each of the short fibre reinforcing layers (11) has a directional complex elastic modulus such that the complex elastic modulus E*a in the circumferential direction is not less than 5 times the complex elastic modulus E*b in the radial direction.

Description: The present invention relates to a pneumatic tyre, in which the rolling resistance is reduced without sacrificing steering stability and ride comfort. In recent years, it is strongly requested that fuel consumption of cars is decreased to improve the earth environment and save resources. In general, for decreasing the fuel consumption of a car, it is very important to improve the efficiency of its internal combustion engine and to reduce the running resistance. If the pneumatic tyres are increased in inner pressure and/or decreased in weight, the rolling resistance thereof is decreased and the rolling resistance of the car is decreased. However, other tyre performances, for example, steering stability, ride comfort and the like are greatly deteriorated. In a pneumatic tyre, therefore, it has been proposed to reduce energy losses during rolling such as internal friction and internal heat generation of the tread rubber by disposing a low hysteresis loss rubber compound in the tread surface which is subjected to a compressive deformation when contacting the road surface. When such a low hysteresis loss rubber compound is used it is possible to reduce the internal friction in the tread portion. However, other parts other than the tread, such as the sidewall portions and bead portions can then increase rolling resistance. As a result, a sufficient effect on reducing the rolling resistance can not be obtained only by improvement in the tread portion. In a pneumatic radial tyre disclosed by EP-A-0 652 118 and corresponding to the preamble of claim 1, which comprises a carcass having edge portions turned up one around each bead core from the axially inside to outside of the tyre and a bead apex extending radially outwardly from the bead core, it is beneficial for reducing the rolling resistance to decrease the heights of the turnup portion and bead apex. However, when these heights are decreased, tyre rigidity decreases, and steering stability is greatly decreased. On the other hand, when the sidewall portion of such tyre is reinforced with a steel cord ply, the rolling resistance increases and the ride comfort decreases. Thus, the present inventors devotedly studied how to greatly decrease rolling resistance without deteriorating steering stability and ride comfort of a pneumatic tyre which has a low-height carcass turnup portion and low-height bead apex and a low hysteresis loss tread rubber. It is therefore, an object of the present invention to provid such a pneumatic tyre, in which the rolling resistance is decreased without deteriorating the steering stability and ride comfort. According to the present invention, a pneumatic tyre having an aspect ratio of not less than 60%, comprises a pair of bead cores disposed one in each bead portion, a carcass extending between the bead portions through a tread portion and sidewall portions and turned up around the bead cores to define a pair of turnup portions, a belt disposed radially outside the carcass in the tread portion and comprising belt plies of which the radially innermost ply has an axial width of from 0.8 to 1.2 times the tread width, a pair of bead apexes each tapering radially outwardly from each of the bead cores, each of the carcass turnup portions having a radially outer end at a height of not more than 0.15 times the tyre section height H, measured from the bead base line, each of the bead apexes having a radially outer end at a height of not more than 0.25 times the tyre section height H, measured from the bead base line, characterised by a tread rubber disposed in the tread portion to define the ground contacting tread face having a loss tangent of not more than 0.14, a pair of short fibre reinforcing layers each disposed in each of the sidewall portions between the axial outer edges of the radially innermost belt ply and the radially outer ends of the turnup portions of the carcass, and extending along the axially outer surface of the carcass, each of the short fibre reinforcing layers having a thickness of from 0.3 to 1.0 mm, the short fibre reinforcing layers being in a compound of 100 parts by weight of a base rubber compound containing 40 to 70 parts by weight of butadiene rubber and 60 to 30 parts by weight of natural rubber and/or isoprene rubber 10 to 30 parts by weight of short fibre, and not more than 30 parts by weight of carbon black whose iodine number is in the range of from 30 to 90mg/g, the short fibre in each of the short fibre reinforcing layers being oriented circumferentially of the tyre so that the direction of orientation of not less than 90% of the short fibre is in the range of from +20 to -20 degrees to the circumferential direction of the tyre, and each of the short fibre reinforcing layers has a directional complex elastic modulus such that the complex elastic modulus E*a in the circumferential direction is not less than 5 times the complex elastic modulus E*b in the radial direction. An embodiment of the present invention will now be described in detail in conjunction with the accompanying drawing, in which: Fig.1 is half a tyre meridian sectional view (a right half) of an embodiment of the present invention. In Fig.1, a pneumatic tyre of the present invention comprises a tread portion 2, a pair of bead portions 4 each with a bead core 5 therein, a pair of sidewall portions 3 extending therebetween, a carcass 6 extending between the bead portions 4 through the tread portion 2 and sidewall portions 3, a belt 7 disposed radially outside the carcass 6 in the tread portion 2, and a bead apex 9 disposed in each bead portion and tapering radially outwardly from the bead core 5. The carcass 6 comprises at least one ply of cords arranged radially at an angle of from 90 to 65 degrees to the tyre equator C, which ply is turned up around the bead cores 5 in the bead portions from the inside to outside of the tyre to have a pair of turnup portions 6b and one main portion therebetween. In this embodiment, the carcass 6 consists of a single ply 6A. For the carcass cords, organic fibre cords, e.g. nylon, rayon, polyester, aromatic polyamide and the like and inorganic cords, e.g. steel and the like can be used. But organic fibre cords are preferably used in view of a tyre weight reduction. The height Hc of the radially outer end X1 of the turnup portions 6b from the bead base line BL is not more than 0.15 times, preferably 0.12 to 0.15 times the tyre section height H so as to have a so called low turnup (LTU) carcass structure. The belt 7 comprises at least two plies having their cords crossed, a radially inner ply 7A and a radially outer ply 7B disposed radially outside thereof. The cords of each ply 7A, 7B are laid at a small angle of not more than 30 degrees, preferably not more than 20 degrees with respect to the tyre equator to provide a hoop effect on the carcass 6 and to reinforce the tread portion. The inner ply 7A is wider than the outer ply 7B, and the axial width BW of the innermost ply 7A is in the range of from 0.8 to 1.2 times the tread width TW. The bead apex 9 is made of a hard rubber compound having a JIS A hardness of from 65 to 95 degrees. The bead apex is disposed axially between each turnup portion and the main portion of the carcass and tapers radially outwardly from the bead apex. The radially outer end 9A of the bead apex 9 is preferably disposed at a height Hb of not more than 0.25 times, preferably 0.20 to 0.25 times the tyre section height H, each measured from the bead base line BL. As the turnup portion and bead apex are set at a low height, the decreasing of the rolling resistance is facilitated. However, in order to provide a good balance between rolling resistance and steering stability, the tyre aspect ratio is set in the range of more than 60%. Further, a tread rubber 10 is disposed on the radially outer side of the belt in the tread portion to define the ground contacting surface thereof. The loss tangent of the tread rubber is usually about 0.18 to 0.28, but in the present invention, the loss tangent is set in a lower range of not more than 0.14, but not less than 0.07. If the loss tangent is more than 0.14, the internal friction of the rubber tends to increase, and the special tread portion does not decrease the rolling resistance. If the loss tangent is less than 0.07, the wear resistance decreases. By using such a low hysteresis loss rubber in the tread portion, the internal friction and internal heat generation are decreased. As a result, the rolling resistance is decreased. In this case, accordingly, a part other than the tread portion, especially the sidewall portion has much influence on the rolling resistance. Thus, to further reduce the rolling resistance with a contribution from the sidewall portion, a short fibre reinforcing layer 11 is provided. The short fibre reinforcing layer 11 is disposed within a region Y in each sidewall portion between the radially outer end X1 of the carcass turnup portion 6b and the belt edge X2 which is the axial outer edge X2 of the radially innermost belt ply 7A in this example. The short fibre reinforcing layer 11 extends continuously in the tyre circumferential direction along the axially outer surface of the carcass 6. The short fibre reinforcing layer is made of a rubber compound including a base rubber, carbon black, short fibre and the like. The base rubber is a mixture of 40 to 70 parts by weight of butadiene rubber and 60 to 30 parts by weight of (natural rubber and/or isoprene rubber). For the short fibre, for example organic fibre such as nylon, polyester, aramid, rayon, vinylon, cotton, cellulose resin, crystalline polybutadiene, and inorganic fibre such as metal fibre, glass fibre and the like can be used. It is also possible to use fibre of a mixture or plurality of different materials as well as fibre of a single material. It is preferable to use a surface treatment to improve the adhesion of the short fibre to the rubber. The short fibre content amounts to 10 to 30, preferably 10 to 20 parts by weight with respect to 100 parts by weight of the base rubber. If the short fibre content is less than 10 parts by weight, the tyre circumferential rigidity can not be maintained, and there is no effect on improving the steering stability. If the short fibre exceeds 30 parts by weight, the rigidity unfavourably increases, and ride comfort is deteriorated. The average length (L) of the short fibre is in the range of not less than 20 micron, preferably 50 to 1000 micron. The ratio (L/D) of the fibre length (L) to the fibre diameter (D) is in a range of not less than 100, more preferably, 200 to 2000. If the ratio (L/D) is less than 100, the rubber is not provided with a sufficient strength by the orientation of the short fibre described hereinafter. In the present invention, it is very important to orient at least 90% of the short fibre within plus/minus 20 degrees to the tyre circumferential direction. As a result, the rigidity in the tyre circumferential direction in improved, and both the steering stability and ride comfort are improved. If the short fibre is not so oriented, such effects can not obtained. Further, the ratio (E*a/E*b) of the complex elastic modulus E*a to the complex elastic modulus E*b of the short fibre reinforcing layer 11 is set in the range of not less than 5, preferably, 7 to 20, wherein, the complex elastic modulus E*a is that in the orientation direction of the short fibre, that is, the tyre circumferential direction. The complex elastic modulus E*b is that in the direction perpendicular to the orientation direction, that is, the tyre radial direction. If the ratio (E*a/E*b) is less than 5, the ride comfort may be improved, but the steering stability, especially handling response, decreases. The thickness of the short fibre reinforcing layer is set in the range of from 0.3 to 1.0 mm, preferably 0.5 to 1.0 mm. If the thickness exceeds 1.0 mm, the rigidity difference between the fibre oriented direction and the direction perpendicular to it decreases. As a result, the improvement in the steering stability is decreased, and the rolling resistance and tyre weight increase. If the thickness is less than 0.3 mm, the production is difficult. For the carbon black, a black having an iodine number of from 30 to 90 mg/g is preferably used. For example, HAF(82), FEF(43), GPF(36) and the like provided by Showa Cabot company, Mitsubishi chemical company, Tokai carbon company etc., can be used. If the iodine number is less than 30 mg/g, the reinforcement to the rubber is low and the rubber decreases in strength and cut resistance. If the iodine number is more than 90 mg/g, the rubber increases in heat generation, and the tyre rolling resistance increases. With respect to 100 parts by weight of the base rubber, the carbon black content is in the range of not more than 30 parts by weight, preferably, 10 to 25 parts by weight. If the carbon black content exceeds 30 parts by weight, a heat generation increases and also the rolling resistance increases. Thus, in the present invention, the content of the carbon black which generally increases the rigidity and heat generation of the rubber can be decreased, because the tyre circumferential rigidity can be increased by the oriented short fibre. Thus, the balance between the rolling resistance and steering stability is improved, and a tyre weight reduction can be achieved. Further, oil, antioxidant, wax, vulcanisation accelerator and the like are added to the rubber compound. By means of extruding or calendering the above-mentioned rubber compound, a sheet of rubber for forming the short fibre reinforcing layer in which at least 90% of short fibre is oriented in the specified direction is manufactured. The short fibre reinforcing layer is preferably disposed in a region between the radially outer end of the bead apex 9 and the maximum tyre section width point M to achieve both the steering stability and ride comfort. In Fig.1, the radially inner end of the short fibre reinforcing layer 11 is disposed at the substantially same height H1 as the height Hb of the radially outer edge 9A of the bead apex 9. The radially outer end is disposed at the substantially same height H2 as that of the maximum tyre sectional width point M. The height H2 from the bead base line BL is 0.45 to 0.70 times, preferably 0.45 to 0.55 times the tyre section height H. (In Fig.1, H2 = 0.52 H) It is however also possible that the short fibre reinforcing layer 11 extends from the position X1 to the position X2. The above-mentioned loss tangent and complex elastic modulus were measured with a viscoelasticity spectrometer (VESF-III model) made by Iwamotoseisakusyo Co., Ltd. The measuring conditions were as follows: temperature of 70 degrees C, frequency of 10Hz, initial strain of 10%, dynamic distortion of +-1%. (Comparison Tests) Using five compositions 1 to 5 shown in Table 1, short fibre reinforcing layers were made by means of extruding. The short fibre A and B in Table 1 were Kevlar short fibre and nylon short fibre, respectively. The short fibre A, B was mixed with natural rubber NR at the specified percentage in advance. NR in Table 1 indicates the total of NR included in the rubber compound. The iodine number of carbon black was 43 mg/g. Only the compositions 1 and 2 satisfied the limitations of the present invention. (parts by weight ) Composition 1 2 3 4 5 NR 45 45 45 45 70 BR 55 55 55 55 30 Short fibre A 15 5 15 Short fibre B 15 35 Carbon black 20 20 20 40 10 By using a short fibre reinforcing layer made of composition 1 in Table 1 (E*a/E*b = 12.3), a tyre according to the present invention (EX.1) whose tyre size was 205/65R15 and which had the construction shown Fig.1 was experimentally made. Also reference tyres (Ref.1 and 2) having no short fibre reinforcing layer were made. Each tyre was tested for rolling resistance, steering stability and ride comfort as follows. (1) Rolling resistance Each tyre was mounted on a 6JJx15 rim, and the rolling resistance was measured with a rolling resistance tester under an inner pressure of 2.0 kg/sq.cm, a speed of 80 km/h and a load of 400 kgf. The resistance is indicated by an index based on the reference tyre 1 being set at 100. The smaller the index, the better the performance. (2) Steering stability A test car (a 3000cc FF car) provided on the four wheels with test tyres was run on a dry asphalt road surface on a test course, and the handling response, rigidity impression, grip and the like were evaluated by a skilled driver's feeling. The results are indicated by an index based on the reference tyre 1 being set at 100. The larger the index, the better the stability. (3) Ride comfort Similarly to the above-mentioned test, a driver's feeling test regarding ruggedness impression, push up, dumping, was carried out on a stepped dry asphalt road surface, a stone paved road surface and a pebble scattered road surface. The results are indicated by an index based on the reference tyre 1 being 100. The larger the index, the better the ride comfort. Tyre Ex. 1 Ref. 1 Ref. 2 Short fibre reinforcing layer non non Rubber composition 1 - - Thickness (mm) 0.7 - - Fibre orientation C - - Carcass Hc/H ratio 0.15 0.15 0.30 Bead apex Hb/H ratio 0.25 0.35 0.25 Test result Rolling resistance 90 100 105 Steering stability 105 100 105 Ride comfort 105 100 95 It was confirmed from the test results that the Ex.1 tyre was improved in the steering stability and ride comfort and the rolling resistance was greatly decreased. (about 10% lower than Ref.1) Next, changing the composition, thickness and complex elastic modulus ratio (E*a/E*b) of the short fibres reinforcing layer of the Ex.1 tyre, a test similar to the above-mentioned test was carried out. The test results are shown in Table 3. Tyre Ex. 1 Ex. 2 Ex. 3 Short fibre reinforcing layer Rubber composition 1 1 2 Thickness (mm) 0.7 0.5 0.7 Fibre orientation C C C E*a/E*b 12.3 15.1 6.9 Carcass Hc/H ratio 0.15 0.15 0.15 Bead apex Hb/H ratio 0.25 0.25 0.25 Test result Rolling resistance 90 90 92 Steering stability 105 107 102 Ride comfort 105 103 103 In the composition 1, when the thickness of the short fibre reinforcing layer was decreased, the ride comfort was slightly lowered, but the steering stability was improved. On the other hand, Table 4 shows the results of an evaluation test when the short fibre reinforcing layer is outside the limitations of the present invention. Tyre Ex. 3 Ref. 4 Ref. 5 Ref. 6 Ref. 7 Short fibre reinforcing layer Composition 1 1 3 4 5 Thickness (mm) 1.5 0.7 0.7 0.7 0.7 Fibre orientation C R C C C E*a/E*b 4.8 12.3 2.1 11.6 10.1 Carcass Hc/H 0.15 0.15 0.15 0.15 0.15 Bead apex Hb/H 0.25 0.25 0.25 0.25 0.25 Test result Rolling resistance 98 90 90 99 99 Steering stability 96 95 98 106 102 Ride comfort 102 95 100 102 99 In Ref.3 tyre, as the thickness of the short fibre reinforcing layer was increased, the value E*a/E*b did not reach to 5, and also the orientation effect of the short fibre was not obtained. As a result, the steering stability was remarkably deteriorated. In Ref.4 tyre, as the short fibre was oriented in the tyre radial direction, the steering stability and ride comfort were deteriorated. In Ref.5 tyre, as the short fibre content in the composition 3 was less than 10 parts by weight, the effect of the orientation of the short fibre was not obtained, and the complex elastic modulus ratio (E*/E*b) did not reached to 5. As a result, the steering stability was decreased. Similarly, in Ref.6 tyre having a high carbon black content, the heat generation from the rubber of the short fibre reinforcing layer was remarkably increased, and the rolling resistance was not decreased. Further, the same result was obtained in Ref. 7 tyre in which the short fibre content was too high. It was also confirmed that the above-mentioned benefits of the present invention can be obtained in other tyre sizes, but the present invention may be suitably applied to a passenger car tyre. As explained above, in the pneumatic tyre of the present invention, the rolling resistance is greatly reduced without sacrificing steering stability and ride comfort.

Claims: A pneumatic tyre having an aspect ratio of not less than 60% comprises a pair of bead cores (5) disposed one in each bead portion (4), a carcass (6) extending between the bead portions (4) through a tread portion (2) and sidewall portions (3) and turned up around the bead cores (5) to define a pair of turnup portions (6b), a belt (7) disposed radially outside the carcass (6) in the tread portion (2) and comprising belt plies (7A,7B) of which the radially innermost ply (7A) has an axial width (BW) of from 0.8 to 1.2 times the tread width (TW), a pair of bead apexes (9) each tapering radially outwardly from each of the bead cores (5), each of the carcass turnup portions (6b) having a radially outer end (X1) at a height (HC) of not more than 0.15 times the tyre section height (H), measured from the bead base line (BL), each of the bead apexes (9) having a radially outer end (9A) at a height (Hb) of not more than 0.25 times the tyre section height (H), measured from the bead base line (BL), characterised by a tread rubber (10) disposed in the tread portion (2) to define the ground contacting tread face having a loss tangent of not more than 0.14, a pair of short fibre reinforcing layers (11) each disposed in each of the sidewall portions (3) between the axial outer edges (X2) of the radially innermost belt ply (7A) and the radially outer ends (X1) of the turnup portions (6b) of the carcass (6), and extending along the axially outer surface of the carcass (6), each of the short fibre reinforcing layers (11) having a thickness of from 0.3 to 1.0 mm, the short fibre reinforcing layers (11) being in a compound of 100 parts by weight of a base rubber compound containing 40 to 70 parts by weight of butadiene rubber and 60 to 30 parts by weight of natural rubber and/or isoprene rubber 10 to 30 parts by weight of short fibre, and not more than 30 parts by weight of carbon black whose iodine number is in the range of from 30 to 90mg/g, the short fibre in each of the short fibre reinforcing layers (11) being oriented circumferentially of the tyre so that the direction of orientation of not less than 90% of the short fibre is in the range of from +20 to -20 degrees to the circumferential direction of the tyre, and each of the short fibre reinforcing layers (11) has a directional complex elastic modulus such that the complex elastic modulus E*a in the circumferential direction is not less than 5 times the complex elastic modulus E*b in the radial direction. A pneumatic tyre according to claim 1, characterised in that the short fibre reinforcing layer (11) extends between the radially outer end (9A) of the bead apex (9) and the maximum tyre section width point (M). A pneumatic tyre according to claim 1 or 2, characterised in that the height (Hc) of the radially outer ends (X1) of the carcass turnup portions (6b) is in the range of from 0.12 to 0.15 times the tyre section height (H), and the height (Hb) of the radially outer ends (9A) of the bead apexes (9) is in the range of from 0.20 to 0.25 times the tyre section height (H).

Inventors: KIKUCHI NAOHIKO, MATSUMOTO TADAO, OKADA MAIKO, KIKUCHI, NAOHIKO, MATSUMOTO, TADAO, OKADA, MAIKO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1995
Main classification: B60C13
IPC codes: B60C1, B60C11, C08K3, B60C15, B60C13, C08L21, C08K7, C08L7, C08L9