Run-flat tyre

Abstract: A run-flat tyre comprises a carcass ply (6) extending between the bead portions (4) , and a sidewall reinforcing layer (10) disposed axially inside said carcass ply in each sidewall portion, wherein in a normally inflated unloaded state, a profile line from a tyre equator point (P) to a maximum tyre section width point (D) is curved substantially along an involute curve so as to have a radius of curvature decreasing continuously from the tyre equator point (P) towards the sidewall portion, the basic circle of the involute curve is an ellipse whose major axis extends in the tyre radial direction, the radial distance (h) between the tyre equator point (P) and the maximun tyre section width point (D) is more than 34 % but less than 50 % of the tyre section height (H). In a x-y co-ordinate system which is defined in a meridian section of the tyre such that: y-axis extends radially outwardly on the tyre equator; x-axis extends axially outwardly in the tyre axial direction; the x-y co-ordinates of said tyre equator point (P) is (0, B); B is a radius of curvature of the profile line at the tyre equator point (P), the elliptic function of the ellipse is ( x-a ) 2 a 2 + y 2 b 2 = 1 wherein 0 < a < b .

Description: The present invention relates to a pneumatic tyre, more particularly to a run-flat tyre having an improved tyre profile capable of improving run-flat performance of the tyre without sacrificing running performance under normal conditions. Recently, there is a strong demand for run-flat tyres capable of running safely for a certain distance at a normal speed, for example, at a speed of 80 km/h for at least 80 km distance. Hitherto, it has been widely suggested, as a means of providing the ability to run under flat conditions, to dispose a reinforcing rubber layer in each sidewall portion of a conventional type pneumatic tyre. Such tyres are disclosed in laid-open Japanese patent applications JP-A-53-18104, JP-A-1030809, corresponding to the preamble of claim 1, and JP-A-2-281289 for example. In such tyres, however, it is difficult to satisfy the above-mentioned requirements of at least 80 km distance at 80 km/h, because the sidewall reinforcing layer is inevitably increased in thickness, volume and weight. Also when such a great mass of sidewall reinforcing rubber layer is used, tyre performance under normally inflated conditions is poor. For example, the tyre weight and rolling resistance increase, and the fuel consumption of the vehicle is increased, the ride comfort is not good, and furthermore belt edge looseness is liable to occur thus decreasing the durability. It is therefore, an object of present invention to provide a run-flat tyre in which run-flat performance is improved and at the same time the above-mentioned problems can be solved. According to one aspect of the present invention, a run-flat tyre comprises a tread portion, a pair of sidewall portions, a pair of bead portions each with a bead core therein, a carcass comprising a carcass ply extending between the bead portions, and a sidewall reinforcing layer disposed axially inside the carcass ply in each of the sidewall portions, and in a normally inflated unloaded state, having a profile line from a tyre equator point in the tread portion to a maximum tyre section width point in each sidewall portion which is curved substantially along an involute curve so as to have a radius of curvature decreasing continuously from the tyre equator point towards the sidewall portion, wherein the basic circle of the involute curve is an ellipse whose major axis extends in the tyre radial direction, the radial distance between the tyre equator point and the maximum tyre section width point being more than 34 % but less than 50 % of the tyre section height. An embodiment of the present invention will now be described in detail, by way of example, in conjunction with the accompanying drawings: Fig.1 is a cross sectional view of a run-flat tyre according to the present invention; Fig.2 is an enlarged cross sectional view of the belt thereof; Fig.3 is a schematic cross sectional view of the tyre for explaining the profile line of the tyre; Fig.4 is a diagram for explaining an involute curve defining the profile line; Fig.5 is a schematic cross sectional view of the run-flat tyre under a deflated or flat condition reproduced by removing the tyre valve core and loading with a standard load; Fig.6(A) is a schematic cross sectional of a conventional tyre in its normal state; Fig.6(B) is a schematic cross sectional of the conventional tyre under a run-flat condition reproduced by the same way as in Fig.5; Fig.7 is a graph in which test tyres results are plotted; and Fig.8(A) and Fig.8(B) show axial distributions of circumferential tension of a breaker. In Figs.1-5, a run-flat tyre according to the present invention comprises a tread portion 2, a pair of sidewall portions 3, a pair of bead portions 4 and with a bead core 5 therein, a carcass 6 extending between the bead portions 4, and a belt 7, 9 disposed radially outside the carcass 6 in the tread portion 2. In this embodiment, the tyre is for passenger cars, and Fig.1 shows a meridian section of the tyre which is mounted on a standard rim J and inflated to a standard pressure but loaded with no tyre load. (hereinafter normally inflated unloaded state) Here, the standard rim is the "standard rim" specified in JATMA, the "Measuring Rim" in ETRTO, the "Design Rim" in TRA or the like. The standard pressure is the "maximum air pressure" in JATMA, the "Inflation Pressure" in ETRTO, the maximum pressure given in the "Tyre Load Limits at Various Cold Inflation Pressures" table in TRA or the like. However, in general if the tyre is for passenger cars, 180 kPa is used as the standard pressure. The standard load is the "maximum load capacity" in JATMA, the "Load Capacity" in ETRTO, the maximum value given in the above-mentioned table in TRA or the like. The carcass 6 comprises at least one carcass ply 6A extending between the bead portions 4 through the tread portion 2 and sidewall portions 3 and turned up around the bead core 5 from the axially inside to the outside of the tyre to form a pair of turnup portions 6b and a main portion 6a. The carcass ply 6A is composed of organic fibre cords, e.g. nylon, polyester, rayon, aromatic polyamide or like arranged radially at an angle of from 70 to 90 degrees with respect to tyre equator C and rubberised with a topping rubber. In this embodiment, the carcass 6 is composed of one ply 6A of cords arranged radially at 90 degrees. Between the turnup portion 6b and main portion 6a in each of the bead portions 4, a bead apex 8 is disposed to increase the bending rigidity. The bead apex 8 is made of hard rubber and has a shape which tapers and extends radially outwardly from the bead core 5. The above-mentioned carcass ply turnup portion 6b extends radially outwardly beyond the radially outer end of the bead apex 8. Further, in each of the sidewall portions 3, a sidewall reinforcing layer 10 is disposed axially inside the carcass 6 to increase the bending rigidity. This layer 10 is made of rubber and has a crescent shape tapering radially outwards and inwards. The maximum thickness occurs near the maximum tyre section width point D. As shown in Fig.1, the radially inner end 10B of the sidewall reinforcing layer 10 overlaps the radially outer end of the bead apex rubber 8. In order to assist this sidewall reinforcing rubber layer 10, a reinforcing layer made of rubberised cords may be disposed. Furthermore, a reinforcing cord layer may be disposed as the sidewall reinforcing layer 10 instead of the rubber layer. The above-mentioned breaker 7 comprises at least two crossed plies of parallel cords laid at an angle of 10 to 35 degrees with respect to the tyre equator. For the breaker cords, steel cords, and high modulus organic fibre cords such as aramid, rayon or like can be used. In this embodiment, the breaker is composed of a radially inner first ply 7A and a radially outer second ply 7B each made of steel cords. Meanwhile, a cause of tyre burst during run-flat is belt edge looseness, namely a separation of belt cords from the surrounding rubber. Thus, it is very important for improving run-flat performance to control the belt edge looseness. In general, as shown in Fig.8 (A) and Fig.8(B), the distribution of the circumferential tension of the breaker greatly alters between the ground contacting patch (Fig.8(B)) and other place (Fig.8(A)), and a relatively large shear stress occurs between the crossed plies in the breaker edges. Thus, belt edge looseness is caused. In the present invention, firstly, the radially outer breaker ply 7B is extended into the sidewall portions 3 beyond the ends of the radially inner breaker ply 7A. Therefore, the ends of the inner breaker ply 7A are covered by the outer breaker ply 7B, and occurrence of belt edge loose therefrom can be prevented. Preferably, the axial distance S1 between the ply ends is not less than 5 mm, in other words, the difference BW2-BW1 between the axial width BW2 of the radially outer breaker ply 7B and the axial width BW1 of the radially inner breaker ply 7A is not less than 10 mm. Further, in this example, the axial end 7e of the breaker 7 overlaps the above-mentioned carcass ply turnup portions 6b which is extended into the tread portion 2, and the radially outer end 6e of the turnup portions is secured between the breaker 7 and the carcass main portion 6a. The overlap EW of the carcass turnup portion and breaker measured in the tyre axial direction is more than 5 mm, preferably more than 10 mm, more preferably in the range of from 15 to 25 mm. Therefore, the sidewall portion 3 can be effectively reinforced by the minimum carcass ply number, which also helps to decrease the thickness or weight of the sidewall reinforcing layer 10. Furthermore, in this example, a band or bandage 9 which comprises one or two, preferably one cord ply is disposed radially outside the breaker 7. The axial width BW3 of the band 9 is set to be larger than the width BW2 of the widest breaker ply 7B so that the ends of the widest breaker ply 7B are covered by the band 9 to prevent further the occurrence of belt edge looseness at these ends. The axial difference S2 between the ends of the band 9 and the widest breaker ply 7B is preferably not less than 10 mm. In this embodiment, the band 9 is formed by spirally winding a tape of rubber in which a single cord or parallel cords are embedded along the length thereof. Accordingly, the band 9 is composed of one or more spiral cords. It is also possible to form the band by winding a full-width strip of rubberised parallel cords. For the band cords, organic fibre cords, e.g. aramid, nylon, polyester, rayon and like, and steel cords can be used. In this embodiment, two parallel aramid cords are used. In any case, the band cord or cords or windings are arranged substantially parallel with the circumferential direction of the tyre. The belt end, in this example band end overlaps the radially outer end 10A of the above-mentioned sidewall reinforcing layer 10. In each tyre shoulder region, a rubber strip 13 having a thickness of less than 1.5 mm is disposed between the breaker 7 and band 9. Each rubber strip 13 extends from the axially inside to the axially outside of the breaker end 7e. The axial width GW thereof is preferably set in the range of from 8 to 30 % of the tyre section width SW. In case of passenger car tyres, the width Gw is more than 20 mm, preferably more than 30 mm, in this example 50 mm. Thereby, shear stress between the band 9 and breaker 7 is mitigated and a ply separation failure is further prevented. In a meridian section of the tyre under the normally inflated unloaded state, as shown in Fig.3 and Fig.4, the profile line of the tyre is symmetrical about the tyre equator C, and a profile line 2e on each side of the tyre equator C defined as extending from the tyre equator point P in the tread portion 2 to the maximum tyre section width point D in the sidewall portion 3 is formed as an involute curve G so as to decrease the radius of curvature thereof gradually and continuously from the tyre equator point P towards the sidewall portion. Incidentally, an involute curve is the locus described by one end of a fixed length thread of which other the end is fixed to the origin when the thread is wound around a basic circle without slack. In this embodiment, the base circle is an ellipse V. When a x-y co-ordinate system is defined as shown in Fig.4, the elliptic function of the ellipse V is ( x - a ) 2 a 2 + y 2 b 2 = 1 wherein 0 < a < b . In this x-y co-ordinate system, the y-axis is on the tyre equator, and the x-axis is on an axial line, and the origin O is radially inward of the tyre equator point P by a distance B. Accordingly, the minor axis (length=aX2) of the ellipse V extends on the x-axis, and the major axis (length=bX2) thereof extends in the radial direction, and the co-ordinates of the equator point P is (0, B). The radial distance (h) between the equator point P and the maximum width point D is set in the range of more than 34 % but less than 50 % of the tyre section height H. The minor axis length (a X 2) of the ellipse V is set in the range of from 53 to 89 % of the tyre section width SW. And in this example, the major axis length (b X 2) is equal to the length in the y-axis (radial) direction between the origin O and the maximum width point D. Accordingly, b = B - h. If the distance h is 34 % or less, the ride comfort and tyre weight reduction deteriorate. If the distance h is 50 % or more, camber thrust increases and the wandering performance deteriorates. As the tread portion 2 is relatively round, its effect on the vertical spring constant of the tyre under the normal state increases. As a result, the vertical spring constant is decreased to improve the ride comfort. Further, the length along the profile line 2e from the tyre equator point P to the maximum width point D becomes shorter than that of conventional tyres. Especially, the sidewall portion becomes short. As a result, the rubber volume decreases and the tyre weight is reduced. Further, the ground pressure distribution is made even. The co-ordinates (X, Y) of a point (A) on the profile line 2e can be expressed as a function of x as follows. wherein y ( x ) = b 1- ( x - a ) 2 a 2 y '( x ) = - b 2 ( x - a ) y · a 2 R ( x ) = B - s ( x ) s(x) gives a length measured along the ellipse V from the origin O to a point F(x, y). R(x) gives a radius of curvature of the profile line 2e. As explained above, the profile line 2e, that is, the contour of the outside of the tyre is defined by an involute curve. In the present invention, further, the contour of the inside of the tyre is also defined as follow. As shown in Fig.5, when the tyre is mounted on the above-mentioned standard wheel rim J, loaded with the standard load, and the air is released by removing the tyre valve core (hereinafter flat state), the inside of the tyre is curved such that a radial line T passing an axial position corresponding to the width of the standard wheel rim J intersects the inside of the tyre at a radially inner point C1 and a radially outer point C2 twice, and the ratio L/N of the distance L between the two points C1 and C2 and the axial distance N from the radial line T to the axially outermost point Q on the inside of the tyre is more than 0.95 but less than 3.0, preferably from 1.0 to 2.0. If the ratic (L/N) is 0.95 or less, a large stress occurs on the inside of the sidewall portion, and this portion is liable to be damaged. If the ratio (L/N) is 3.0 or more, the ride comport deteriorates and such tyre tends to increase the weight. In this embodiment, further, in order to prevent bead unseating from the wheel rim, the outer surface of a lower sidewall portion radially inwards of the maximum width point D is provided with an axially protruding rib 12 so as to overhang the flange Jf of the standard rim J. * Comparison Tests Test tyres of size 185/55R14 having specifications shown in Table 1 were made and tested for run-flat performance, tyre weight, ride comfort, steering stability and durability. * Run-flat performance test The test tyres were mounted on all the four wheels of a Japanese FF passenger car, the tyre valve cores were removed, and then the test car was run on a test circuit course to measure the runable distance. Running speed: 80 km/h (straight), 60 km/h (corner) Tyre load: 300 kgf The measured runable distance is indicated in Table 1 by an index based on Ref.5 being 100. The larger the index, the better the performance. * Tyre weight The tyre weight is indicated in Table 1 by an index based on Ref.5 being 100. * Ride comfort and Steering stability tests The test tyres were mounted on all the four wheels of a Japanese FF passenger car, and the test car was run on various road surfaces under the normal state inflated to the standard inner pressure, and the ride comfort and steering stability were evaluated by the test driver's feeling. The results are indicated by an index based on Ref. 5 being 100. The larger the index, the better the performance. * Durability Test Using a tyre test drum having a diameter of 1.7 m, the test tyre was continuously run at a speed of 80 km/h to obtain the runable time until a belt edge loose was caused. The used wheel rim was the standard rim and the tyre pressure was the standard pressure. But, the tyre load was increased to 140 % of the standard load. The result are indicated in Table 1 by an index. The larger index, the better the durability. Tyre Ex.1 Ex.2 Ex.3 Ref.1 Ref.2 Ref.3 Ref.4 Ref.5 Ex.4 Ex.5 Ref.6 Band width (mm) 184 184 184 184 184 184 184 - 184 - - First breaker width (mm) 144 144 144 144 144 144 144 144 158 158 158 Second breaker width (mm) 158 158 158 158 158 158 158 158 144 144 144 Rubber strip width (mm) 50 50 50 50 50 50 50 0 50 0 50 Ellipse a (mm) 54.2 54.2 79.3 62.3 49.6 54.2 82.6 - 54.1 54.1 54.1 b (mm) 324.5 324.5 339.8 329.6 321.5 329.5 341.8 - 324 324 324 UN 0.966 2.953 1.95 0.85 0.801 3.21 0.52 0 0.964 0.964 0.843 h/H 0.496 0.496 0.347 0.446 0.526 0.496 0.326 - 0.491 0.491 0.494 Test results Run-flat performance 150 750 110 20 200 840 120 3 101 101 52 Tyre weight 97 106 107 90 88 112 125 100 97 97 88 Ride comfort 95 90 82 100 110 85 92 100 95 95 96 Steering stability 99 97 97 95 90 98 99 100 99 99 98 Durability 100 100 100 100 100 100 100 100 90 88 85 Tyre section height H: 101.75 mm Radius of curvature at point P: 375 mm (B=375) From the test results, it was confirmed that Example tyres according to the present invention were improved in the run-flat performance, resistance to the belt edge loose and the like without greatly increasing the tyre weight although the tyres could be used with a standard wheel rim. Fig.6(A) shows the normal state of Ref. 5. Fig.6(B) shows a state of Ref. 5 from which the tyre valve core was removed. Fig.7 is a graph in which the ratio (L/N) and the ratio (h/H) of the test tyres are plotted, wherein the larger the ratio (h/H), the better the weight reduction; the smaller the ratio (h/H), the better the steering stability; the smaller the ratio (L/N), the better the ride comfort; and the larger the ratio (L/N), the better the run-flat performance. As described above, in the run-flat tyre according to the present invention, due to the multiplier effect of the specific tyre profile and reduced sidewall reinforcing layer, run-flat performance can be effectively improved without sacrificing the tyre performances under normal conditions. The present invention can be suitably applied to pneumatic tyres for passenger cars, but it is also possible to apply to tyres of other categories such as RV, LT and the like. It is not always necessary that the profile line 2e completely or strictly coincides with the involute curve G. Some degree of tolerance is of course allowed. Therefore, within the allowable tolerance range, a multi-radius curve approximating to the involute curve G can be used.

Claims: A run-flat tyre comprising a tread portion (2), a pair of sidewall portions (3), a pair of bead portions (4) each with a bead core (5) therein, a carcass (6) comprising a carcass ply (6a) extending between the bead portions (4), and a sidewall reinforcing layer (10) disposed axially inside said carcass ply in each said sidewall portion, characterised in that in a normally inflated unloaded state in which the tyre is mounted on a standard rim (J) a profile line (2e) from a tyre equator point (P) in the tread portion (2) to a maximum tyre section width point (D) in each said sidewall portion (3) is curved substantially along an involute curve (G) so as to have a radius of curvature decreasing continuously from the tyre equator point (P) towards the sidewall portion, the basic circle of the involute curve being an ellipse (V) whose major axis extends in the tyre radial direction, the radial distance (h) between the tyre equator point (P) and the maximum tyre section width point (D) is more than 34 % but less than 50 % of the tyre section height (H). A run-flat tyre according to claim 1, characterised in that in a x-y co-ordinate system which is defined in a meridian section of the tyre such that: y-axis extends radially outwardly on the tyre equator; x-axis extends axially outwardly in the tyre axial direction; and the x-y co-ordinates of said tyre equator point (P) are 0 and B, respectively, wherein B is the radius of curvature of the profile line (2e) at the tyre equator point (P), the elliptic function of said ellipse is ( x - a ) 2 a 2 + y 2 b 2 = 1 wherein 0 < a < b . A run-flat tyre according to claim 1 or 2, characterised in that in a flat state in which the tyre is mounted on the standard rim and loaded with the standard load but the pressure is zero, the inside of the tyre is curved so that a radial line (T) passing an axial position corresponding to the rim width of the standard rim intersects the inside of the tyre at a radially inner point (C1) and a radially outer point (C2), and the radial distance L between the radially inner point (C1) and outer point (C2) is more than 95 % but less than 300% of the axially distance (N) from the radial line (T) to an axially outermost point (Q) on the inside of the tyre. A run-flat tyre according to claim 1, 2 or 3, characterised by further comprising a breaker (7) disposed radially outside the carcass ply (6) in the tread portion (2), and a band (9) disposed radially outside the breaker (7), said breaker (7) comprising a radially inner ply (7a) and an radially outer ply (7b) wider than the inner ply, and said band (9) is wider than the breaker (7). A run-flat tyre according to claim 4, characterised in that said carcass (6) consists of said carcass ply which extends between the bead portions and is turned up around the bead core (5) in each bead portion (4) from the inside to the outside of the tyre to form a pair of turnup portions and a main portion therebetween, the radially outer edge (6e) of each said turnup portion overlaps one of axial edges of the breaker (7), and said band (9) is composed of spiral windings of a single aramid cord or a plurality of parallel aramid cords. A run-flat tyre according to claim 4 or 5, characterised in that a strip of rubber (13) having a thickness of less than 1.5 mm and an axial width of more than 20 mm is disposed between the breaker (7) and the band (9) so as to cover each axial edge of the breaker (7).

Inventors: ASANO KAZUO, ASANO, KAZUO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1999
Main classification: B60C17
IPC codes: B60C9, B60C11, B60C3, B60C17