Pneumatic tyre and method of making the same

Abstract: A pneumatic tyre (1) and a method of making the same, in which the steering performances are improved without increasing tyre weight. The tyre comprises a toroidal carcass ply (6) composed of cords (11A,11B) arranged radially at the same angle to the tyre equator and extending continuously between the bead portions (4) under the tread portion (2) and through sidewall portions (3), and the carcass ply (6) is increased in thickness partially in at least the tyre shoulder portion (Y1). The method comprises the step of heating the raw tyre, having assembled a raw carcass ply of cords having at least two different heat-shrinkage-percentages.

Description: The present invention relates to a pneumatic tyre having an improved carcass structure capable of improving steering performances without increasing tyre weight, and also relates to a method of making the same. In general, a pneumatic tyre is provided with at least one carcass ply. Further, in order to reinforce the tread portion, a belt composed of cords arranged at different angles from the carcass cord angle is provided. Furthermore, to reinforce the bead portions and sidewall lower portions, a hard rubber bead apex and a bead reinforcing cord layer having a different cord angle from the carcass cord angle is disposed in each bead. Due to the different cord angles, an additional carcass ply can not be substituted for such a belt and bead reinforcing layer. On the other hand, recent high performance motorcars require a high performance tyre in which steering performances are improved but the tyre weight is not increased. If an additional carcass ply is used, the lateral stiffness of the tyre is increased and the steering performances can be improved. However, such an additional carcass ply is redundant in the tread region and in the bead portions and only increases tyre weight. If an additional carcass ply is not used, the tyre shoulder portion or sidewall upper portion is relatively weak. If a partial reinforcing cord layer is disposed in the shoulder portion, the edges of the layer are liable to separate since the bending deformation in the shoulder portion is greatest. GB-A-1159093 discloses a tyre in which the carcass cords are disposed in the sidewall in two radially separated contiguous layers in order to locally stiffen the structure. An object of the present invention is therefore, to provide a pneumatic tyre in which the steering performance is improved without increasing the tyre weight. Another object of the present invention is to provide a method of making such a tyre. According to the present invention, a pneumatic tyre comprises a tread portion, a pair of axially spaced bead portions each with a bead core disposed therein, a pair of sidewall portions extending therebetween, a pair of tyre shoulder portions, a toroidal carcass ply composed of cords arranged radially at the same angle to the tyre equator and extending continuously between the bead portions through the tread portion and sidewall portions, and a belt disposed radially outside the carcass in the tread portion, said carcass ply having an increased thickness partially in at least the shoulder portions, radially outward from the bead portions to the shoulder portions, and axially outward from the tyre equator to the shoulder portions, characterised in that in said increased thickness carcass ply portions, said cords in the carcass ply including inner cords and outer cords each having a diameter, the inner cords being positioned inside of the outer cords so that the distance between the centres of the inner cords and the centres of the outer cords is in the range of from 0.3 to 1.5 times the diameter, whereby the carcass ply thickness is increased. Further, according to another aspect of the present invention, a method of manufacturing a pneumatic tyre having a toroidal carcass ply composed of cords arranged radially of the tyre at the same angle to the tyre equator and extending continuously between the tyre bead portions under the tread portion and through sidewall portions of the tyre comprises assembling a raw carcass ply and other components into a raw tyre, and heating the raw tyre in a mould for vulcanisation, wherein said raw carcass ply is assembled from cords having at least two different heat-shrinkage-percentages, characterised in that the cords are materially the same organic cords and the cords having a lower heat-shrinkage-percentage and the cords having a higher heat-shrinkage-percentage are arranged alternately in the tyre circumferential direction. 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 tyre according to the present invention; Fig.2 is a sectional view of an example of the carcass ply taken along the line I-I of Fig.1; Fig.3 is a sectional view of an example of the raw rubberised cord sheet for used to make the carcass ply shown in Fig.2; Fig.4(a) is a sectional view of another example of the raw rubberised cord sheet; Fig.4(b) is a sectional view of another example of the carcass ply formed with the raw rubberised cord sheet shown in Fig.4(a); Fig.5(a) is a sectional view of still another example of the raw rubberised cord sheet; Fig.5(b) is a sectional view of still another example of the carcass ply formed with the raw rubberised cord sheet shown in Fig.5(a); Figs.6 and 7 are diagrams for explaining the function of the present invention; Fig.8(a) is a diagram showing the crystal structure of regular modulus polyester; and Fig.8(b) is a diagram showing the crystal structure of high modulus polyester; In Fig.1, the pneumatic tyre 1 of the invention is a passenger radial tyre having a relatively low aspect ratio of not more than 0.60 (for example 0.5). The aspect ratio is the ratio H/W of the tyre section height H to the tyre section width W in the normal inflated state with the tyre mounted on its standard rim R and inflated to its normal pressure. Fig.1 shows the tyre 1 in this state. The tyre 1 comprises a tread portion 2, a pair of axially spaced bead portions 4, a pair of sidewall portions 3 extending therebetween, a bead core 5 disposed in each bead portion 4, a toroidal carcass 6 extending between the bead portions 4 through the sidewall portion 3 and under tread portion 2, and a belt 7 and 9 disposed radially outside the carcass 6 and inside the tread portion 2. The carcass 6 comprises at least one ply, in this embodiment only one ply 10, of radially arranged cords. The carcass ply is turned up around each bead core 5 from the axially inside to outside of the tyre to be secured thereto and to form a pair of carcass ply turn up portions and a main portion therebetween. The carcass cords 11 in each carcass ply are arranged at an angle in the range of from 70 to 90 degrees with respect to the tyre equator C. In the single carcass ply 10 of this example, the cords 11 are arranged at 90 degrees with respect to the tyre equator C. For the carcass cords, organic fibre cords, e.g. polyester, nylon, rayon or the like are used. Between the carcass ply turn up portion and main portion in each bead portions 4, a bead apex 8 is disposed. The bead apex 8 is made of a hard rubber compound preferably having a JIS-A hardness of 80 to 95 degrees and a complex elastic modulus E* of 300 to 600 kg/cm 2 . The bead apex 8 extends taperingly radially outwards from the bead core 5 to a position near the maximum tyre sectional width point (J) so as to reinforce the bead portion 4 and the sidewall lower portion. The belt comprises a breaker belt 7 disposed on the carcass crown and a band belt 9 disposed radially outside of the breaker belt 7. The breaker belt 7 comprises two crossed plies 7A and 7B of high modulus cords, e.g. steel cords, aromatic polyamide fibre cords or the like, and laid at an angle of not more than 35 degrees with respect to the tyre equator C. The cords in each ply are laid parallel with each other, but crosswise to the cords in the A next ply. The band belt 9 covering the axial edge portions (E) of the radially outer surface of the breaker belt 7 so as to prevent the breaker belt 7 from being lifted by the centrifugal force during high speed running. In this example, the band belt 9 comprises a pair of axially spaced narrow width inner plies 9B disposed one on each of the edges E of the breaker belt 7 so as to cover the edge portions, and a wide outer ply 9A disposed thereon to cover the whole width of the outer surface of the breaker belt 7. Each ply 9A, 9B is made of for example nylon cords having a diameter smaller than that of the breaker belt cords, and the cord angle is in the range of from 0 to 10 degrees with respect to the tyre equator C. Preferably, the angle is substantially zero. According to the present invention, at least one carcass ply, in this example the single carcass ply 10, is increased in thickness partially in at least a tyre shoulder portion Y1. Here, "increase" in ply thickness does not mean that of the topping rubber nor the cord diameter. As shown in Figs.2, 4(b) and 5(b), the "increase" means that some chosen or predetermined cords 11B in the ply are disposed outwards in the tyre, and some other predetermined cords 11A are displaced inwards of the tyre, and as a result the distance from the outer cords 11B to the inner cords 11A is increased. In Fig.1 showing the tyre in the normal inflated state, L1 is a first straight line drawn at a right angle to the carcass 6 from the edge E of the breaker belt 7, L2 is a second straight line drawn at a right angle to the carcass 6 from the maximum tyre sectional width point J on the sidewall portion 3, Y is the region of the carcass ply between the lines L1 and L2, Y1, Y2 and Y3 are one third parts of the region Y, and the radially outer, middle and inner parts, respectively. In general, the bending deformation of the carcass during running is greater in the above-mention region Y than the remaining regions. Especially, in the radially outer part Y1, the deformation is greatest. Therefore it is most effective for improving the steering performances to increase the bending rigidity in the radially outer part Y1 by increasing the carcass ply thickness in this part. The distance N between the centres of the inner cords 11A and the outer cords 11B in the radially outer part Y1 is in the range of from 0.3 to 1.5 times, preferably 0.3 to 0.7 times the diameter D of the carcass cords 11A and 11B. If the distance N is less than 0.3 times the cord diameter D, the above-mentioned bending rigidity is not increased. If the distance N is more than 1.5 times the cord diameter D, the inner carcass cords 11A sometimes break through the airtight inner liner 14, and further a separation failure between the carcass ply and the inner liner is liable to occur. The reason will be explained later. The thickness of the carcass ply is thus defined as D + N. Therefore, if N is zero as shown in Figs.3, 4 and 5(a), the ply thickness is D, the cord diameter. The thickness of the carcass ply turnup portion and the carcass ply main portion in the bead portion is not increased, that it is equal to the cord diameter D. Further, the thickness of the carcass ply is preferably increased towards the radially outer part Y1 from the bead portion 4. With respect to the thickness, it is preferable that the outer part Y1 > the middle part Y2 > the inner part Y3. Furthermore, the thickness of the carcass ply is preferably increased towards the radially outer part Y1 from the tyre equator C. The above-explained thickness increase can be achieved by using carcass cords having different heat-shrinkage-percentages in the vulcanising process of the tyre. Here, the vulcanising process is such that a raw tyre which is an assembly of the carcass, belt, bead core, bead apex, tread rubber, sidewall rubber, inner liner, etc., is placed in a mould, and an inflatable bladder that is an air bag or tube is inserted in to the tyre, and then the tyre is heated while pressurising the inside of the tyre by increasing the inner pressure of the bladder. Thus a conventional vulcanising process is used. The carcass cords are arranged in a line or in a plane and rubberised with topping rubber 16 to form rubberised sheet 17 as usual. However, the carcass cords 11A, 11B, 11C having different heat-shrinkage-percentages K are arranged in a specific sequence as shown in Figs.3, 4(a), 5(a). In order to achieve the above-explained distance N, the difference between the minimum heat-shrinkage-percentage K2 and the maximum heat-shrinkage-percentage K1 is set in the range of not more than 5.0%, and the minimum heat-shrinkage-percentage K2 is set in the range of not more than 5.0%, preferably 4.5 to 3.5%. The heat-shrinkage-percentage K is the percentage (y/x)X100 of the amount (y) of the shrinkage of the cord after the cord is placed in a 180 degrees C atmosphere for 20 minutes with no load to the length (x) of the cord before the shrinkage. Therefore, due to the heat in the vulcanising process, the cords 11A having a greater heat-shrinkage-percentage K1 shrink more than the cords 11B having a smaller heat-shrinkage-percentage K2. As a result, the cords 11A move towards the inside of the tyre by a greater amount than the cords 11B to produce an increase in the ply thickness. If the minimum heat-shrinkage-percentage K2 is more than 5.0%, the dimensional stability in the finished tyre is decreased, and tyre uniformity is liable to be disturbed. If the difference K1-K2 is more than 5.0%, the distance N is excessively increased, and the inner cords have decreased topping rubber thickness which leads to cord separation failure, or sometimes the cords break through the inner liner to affect the airtightness. In a tyre having a relatively low aspect ratio such as the tyre 1 shown in Fig.1, favourably the inward movement of the carcass cords due to shrinkage is liable to become greater toward the shoulder portion Y1 from the bead portion 4 and the tyre equator. In Fig.3, the cords 11A having a greater heat-shrinkage-percentage K1 and the cords 11B having a smaller heat-shrinkage-percentage K2 are alternately arranged. Accordingly, as shown in Fig.2, in the carcass ply 10 in the finished tyre, the cords 11A are located inside and the cords 11B are located outside of the tyre. This part therefore, functions as a two-ply structure (inner ply 12 + outer ply 13). In this case, the cord count of the inner ply 12 is the same as the outer ply 13. Fig.4 (a, b) shows another example of the carcass 10. In this example, one cord 11A having the greater heat-shrinkage-percentage K1 and two cords 11B having the smaller heat-shrinkage-percentage K2 are arranged by turns in the raw state as shown in Fig.4(a). Therefore, in the finished tyre, as shown in Fig.4(b), a two-ply structure in which the cord count of the outer ply 13 is twice the inner ply 12 is formed. Fig.5 (a, b) shows still another example of the carcass 10. In this example, three different heat-shrinkage-percentages are used. As shown in Fig.5(a), in the raw state, the cords 11A having the maximum percentage K1, the cords 11B having the minimum percentage K2 and cords 11C having a medium percentage K3 are arranged as follows: K1-K3-K2-K3-K1-K3-K2-K3- --- Between the "K3" cord, "K1" and "K2" appear alternately. Therefore, in the finished tyre, as shown in Fig.5(b), a three-ply structure (outer ply 13, inner ply 12, and middle ply 19) is formed. If the carcass ply thickness is not increased as shown in Fig.6, the bending neutral line (X) when the carcass ply (A) is bent by a bending moment (M), substantially coincides with the thickness centre line of the carcass ply (A), and only the bending rigidity of the cords (B) themselves serves as the bending rigidity of the carcass ply. Accordingly, the bending rigidity is very low. However, if the carcass ply thickness is increased as explained above and as shown in Fig.7, the tensile and compressive elastic moduli of the carcass cords serves as the bending rigidity of the carcass ply in addition to the bending rigidity of the cords themselves. As a result, the bending rigidity is effectively increased. As the bending rigidity is increased in the outer part Y1, the steering performance is effectively improved. The different heat-shrinkage-percentages are preferably achieved by using cords which are materially the same but different in the cord treatment. For example, before assembly into a raw tyre, the lower-percentage cord is heat shrunk, but the higher-percentage cord is not heat shrunk. Or, both the cords are heat shrunk, but the shrinkage is stopped at different shrinkage by differing the heating time and/or temperature. Further, the heat-shrinkage-percentages can be differed by using cords which are materially the same but different in crystal structure. Fig.8(a) is a diagram showing the crystal structure of a so called regular modulus polyester of which the heat-shrinkage-percentage is higher. Fig.8(b) is a diagram showing the crystal structure of a high modulus polyester of which the heat-shrinkage-percentage is lower. In Fig.8(a, b), 20 is the crystal region, and 21 is the amorphous region. Between the regular modulus polyester and high modulus polyester, the size, percentage, orientation of such regions are differed. Furthermore, the heat-shrinkage-percentages may be differed by using materially different cords. For example, a nylon fibre cord used for the high heat-shrinkage-percentage cord, and a polyester fibre cord used for the low heat-shrinkage-percentage cord. In the present invention, the carcass 6 can include a ply composed of cords having the same heat-shrinkage-percentage in addition to the carcass ply 11 composed of the different cords (11A, 11B, 11C). Such an additional carcass ply is preferably disposed outside of the carcass ply 11. Test tyres having a size 225/50R16 and the structure shown in Fig.1 were made and tested. In the tests, the tyres mounted on a standard rim of 16X7J and inflated to normal inner pressure of 2.2kg/cm 2 were fitted on a 3000cc FR passenger car. The test car was run on a test circuit course with a tyre load of 360kg, and the test driver evaluated the cornering performance, braking performance, straight running performance, response, and controllability in a dry condition, as the steering performances thereof. The test results together with the tyre specifications are shown in Table 1. The greater the value, the better the performance. The present invention may be applied to all kinds of pneumatic tyres, but preferably is applied to the passenger car tyres. Tyre Ex 1 Ex 2 Ref 1 Ref 2 Carcass Number of ply 1 1 1 1 Cord polyester 1500d/2 polyester 1500d/2 polyester 1500d/2 polyester 1500d/2 Shrinkage-% K1 7.5 6.5 7.5 7.5 Shrinkage-% K2 4.5 3.5 7.5 7.5 Cord angle (deg) 90 90 90 90 Cord count/10cm 53 53 53 53 Distance N 0.5D 0.3D 0 0 Test result Cornering 115 108 100 118 Braking 105 102 100 107 Straight run 105 103 100 105 Response 110 106 100 110 Controllability 115 108 100 110 Circuit time 2′7″05 2′8″91 2′8″76 2′7″01 Tyre weight (kg) 10.6 10.6 10.5 11.3

Claims: A pneumatic tyre comprising a tread portion (2), a pair of axially spaced bead portions (4) each with a bead core (5) disposed therein, a pair of sidewall portions (3) extending therebetween, a pair of tyre shoulder portions (Y), a toroidal carcass ply (6) composed of cords (11) arranged radially at the same angle to the tyre equator and extending continuously between the bead portions (4) through the tread portion (2) and sidewall portions (3), and a belt (7,9) disposed radially outside the carcass in the tread portion, said carcass ply (6) having an increased thickness partially in at least the shoulder portions (Y), radially outward from the bead portions (4) to the shoulder portions, and axially outward from the tyre equator to the shoulder portions (Y), characterised in that in said increased thickness carcass ply portions, said cords (11) in the carcass ply (6) including inner cords (11A) and outer cords (11B) each having a diameter (D), the inner cords (11A) being positioned inside of the outer cords (11B) so that the distance (N) between the centres of the inner cords (11A) and the centres of the outer cords (11B) is in the range of from 0.3 to 1.5 times the diameter (D), whereby the carcass ply thickness is increased. A pneumatic tyre according to claim 1, characterised in that said distance (N) is in the range of 0.3 to 0.7 times the diameter (D). A pneumatic tyre according to claim 1 or 2, characterised in that said carcass ply (6) consists of the materially same organic cords having at least two different heat-shrinkage-percentages (K), and the heat-shrinkage-percentage of said inner cords (11A) is higher than that of said outer cords (11B). A pneumatic tyre according to any of claims 1 to 3, characterised in that the carcass ply (6) in an unvulcanised state of the tyre is made of a sheet formed by arranging the cords having different heat-shrinkage-percentages (K) in a plane in a regular sequence and rubberising the cords with topping rubber, and the difference between the maximum (K1) and the minimum (K2) of the heat-shrinkage-percentage is not more than 5.0% and the minimum (K2) of the heat-shrinkage-percentage is not more than 5.0%. A pneumatic tyre according to claim 4, characterised in that the minimum heat-shrinkage-percentage (K2) is 3.5 to 4.5%. A pneumatic tyre according to claim 3, 4 or 5, characterised in that said different heat-shrinkage-percentages (K) are provided through different preheat shrinking processes for the cords. A pneumatic tyre according to claim 3, 4 or 5, characterised in that said different heat-kage-percentages (K) are provided by different crystal structures of the material of the cords. A pneumatic tyre according to claim 7, characterised in that said outer carcass cords (11B) are high-modulus polyester cords having a lower heat-shrinkage-percentage, and said inner carcass ply cords (11A) are regular modulus polyester cords having a higher heat-shrinkage-percentage. A method of making a pneumatic tyre having a toroidal carcass ply (6) composed of cords arranged radially of the tyre at the same angle to the tyre equator and extending continuously between the tyre bead portions (4) under the tread portion (2) and through sidewall portions (3) of the tyre, comprising assembling a raw carcass ply and other components into a raw tyre, and heating the raw tyre in a mould for vulcanisation, wherein said raw carcass ply is assembled from cords (11A,11B) having at least too different heat-shrinkage-percentages, characterised in that the cords are materially the same organic cords and the cords (11B) having a lower heat-shrinkage-percentage and the cords (11A) having a higher heat-shrinkage-percentage are arranged alternately in the tyre circumferential direction. A method according to claim 9, characterised in that the difference of the maximum heat-shrinkage-percentage from the minimum heat-shrinkage-percentage is not more than 5.0%, and the minimum heat-shrinkage-percentage is not more than 5.0%.

Inventors: ONO RYO, SUZUKI KAZUYA, ONO, RYO, SUZUKI, KAZUYA
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1995
Main classification: B60C9
IPC codes: B29D30, B60C9