GB2198996A - Radial tyres - Google Patents

Abstract

In a radial tyre a breaker 7 is disposed in the circumferential direction inwardly of the tread portion 5 and outside a carcass 6. The breaker 7 has a radius of curvature R1 at the crown smaller than the radius of curvature R2 at the shoulder portions when the tyre is mounted on a rim and inflated to 25% of the standard pressure. This radial tyre is further characterised in that the breaker is provided within a curve which is inscribed in an arc (R0), Fig 2, having its centre on the equator of the tyre and passing through the intersection P where the breaker intersects the equator of the tyre and a point Q on the breaker distant from the intersection P by a distance L corresponding to the smaller value selected between 1/4 and 1/2 the width of either the breaker 7 or the ground contacting width W2 of the tread portion, at the intersection P and which passes inside the arc R0 and intersects the arc R0 at the point Q when the tyre is mounted on a rim and inflated to 25% of standard pressure. <IMAGE>

Description

RADIAL TYRE The present invention relates to a radial tyre which is capable of enhancing steering stability.

With the development of higher powered and higherspeed automobiles, tyres have been required to have improved properties especially steering stability and cornering properties. For this purpose, the rigidity of he tread portion has been progressively enhanced.

On the other hand, a radial tyre has been tried comprising a carcass with carcass cords arranged in the radial direction of the tyre and a breaker using high strength cords disposed outside the carcass so as to increase the hoop stiffness effect.

In such a radial tyre, the following efforts have been made in order to improve the rigidity of the tread portion so as to enhance the above-described cornering properties: a) to change the material of the breaker cords by replacing rayon cord with a cord having a high tensile strength such as, for example, steel cord or Keviar (trademark) cord; b) to increase the breaker size, width and number of plies and c) to change the structure of the breaker by, for example, using a folded breaker in place of a cut breaker or using a nylon band together with a cut breaker.However, an increase in the size, number of plies or width of a breaker tends to increase the weight of the tyre, thereby increasing the rolling resistance and reducing the fuel efficiency, which tendency easily leads to increased cost, deterioration of high-speed efficiency, and deterioration of ride comfort of the vehicle, etc.

Furthermore, change in the structure of the breaker tends to make the structure itself complicated, lower the productivity of the tyre, and impedes good uniformity of the tyre.

Therefore, the improvement of the steering stability by the above-described measures is limited to the range which does not lead to the above-described problems.

In a conventional radial tyre, a breaker A is formed in such a manner as to have a single radius of curvature RA with a centre on the equator of the tyre, as shown in Fig 11, or sometimes in such a manner as to provide a composite curve having a radius of curvature RA1 at the crown portion and a radius of curvature RA2 at the shoulder portions which is smaller than the radius of curvature RAl indicated by the one-dot chain line in Fig.13.

The breaker A having the above mentioned single curvature expands outwardly by a substantially uniform amount X from the crown portion to the shoulder portions, as shown in Fig. 12, when the tyre is inflated. On the other hand, with breaker A consisting of the two curvatures shown in Fig. 13, the amount of expansion X is comparatively large at the crown portion B, while the amount of expansion X is reduced in the shoulder portions C, as exaggerately shown in Fig. 14.

When the conditions of the material and size are the same in each portion of the tyre, the sum of the tensions of the breaker which are applied in the axial direction of the tyre, namely, the entire length in the widthwise direction when the tyre is inflated is equal, and the tension of the breaker A which is applied to each portion in the axial direction of the tyre is proportional to the abovedescribed X of expansion at each position, so that the strain distribution at each portion of the breaker exhibited on inflation is approximately proportional to the distribution of the amount X of expansion.

The apparent rigidity of the breaker is enhanced with the increase in the amount of strain, so that in the breaker A shown in Fig. 11, the apparent rigidity acts uniformly in the axial direction of the tyre, while in the breaker A shown in Fig.13, the apparent rigidity at the shoulder portions C is lowered in comparison with that at the crown portion B.

The cornering properties which contribute to the steering stability of the tyre becomes effective by enhancing the rigidity of the breaker at the shoulder portions C. Therefore, a conventional breaker, especially, a breaker A having the structure shown in Fig.13 reduces the steering stability.

It is an object of the present invention to provide a radial tyre which is capable of enhancing the cornering properties and improving the steering stability-by making the strain of the breaker due to inflation pressure larger at the shoulder portions, thereby increasing the belt tension and apparent rigidity at the shoulder portions.

According to one aspect of the present invention a radial tyre is provided comprising a carcass reinforced by cords disposed in the radial direction and a breaker composed of high strength cords and disposed in the circumferential direction inwardly of a tread portion of said tyre and radially outside the carcass, the said breaker having a radius of curvature R1 at the crown portion of the said tread portion smaller than the radius of curvature R2 at the shoulder portions when said tyre is mounted on a rim and inflated to 25% of the full or standard inflation pressure.

The radial tyre preferably hos the breaker inside a curve which is inscribed by an arc RO having the centre thereof on the equator of the said tyre, and passing through the intersection P where the breaker intersects the equator of the tyre, and point Q on the breaker spaced from the intersection P by a distance L corresponding to the smaller value selected between 1/4 to 1/2 width W1 of said breaker and the ground contacting width W2 of said tread region at the intersection T and which passes inside the said arc RO and intersects it at the point Q in the condition when the tyre is mounted on a rim and inflated to 25% of the standard inflation pressure.

Preferably the portions of radius of curvature R1 and R2 connect at points which are spaced from the equator of the tyre by a distance of L1 where L1 is 0.1 to 0.35 times the width W1 of the breaker. More preferably L1 is between 0.15 and 0.3.

The radius of curvature R2 is preferably in the range of 1.1 to 2.5 times the radius of curvature R1. More preferably R2 is in the range of 1.2 to 2 times R1.

Further aspects and advantages of the present invention will become apparent from the following description of some preferred embodiments thereof, taken in conjunction with the accompanying diagrammatic drawings in which: Fig.1. is a sectional view of a first embodiment of the present invention; Fig.2. shows schematically the configuration of the breaker of the embodiment shown in Fig.1; Fig.3. is an explanatory view of the effective width of the ground contacting surface; Fig.4 shows schematically the amount of expansion; Fig.5 (a) shows schematically the change in the apparent rigidity; Fig.5. (b) shows schematically the apparent rigidity seen from the ground contacting surface; Fig.6. is a explanatory view of the bending of the breaker in a plane; ; Fig.7. shows schematically the deformation of. the breaker exhibited when the tyre is cornering; Fig.8. is a graph showing the cornering force; Figs.9 & 10 shows schematically other embodiments of the present invention; Fig.11 shows schematically a conventional tyre; Fig.12. shows schematically the amount of expansion of the breaker of the tyre shown in Fig.11 when the tyre is inflated; Fig.13. shows schematically a conventional tyre; and Fig.14. shows schematically the amount of expansion of the breaker of the tyre shown in Fig.13.

Referring first to Figs. 1 and 2, which show an embodiment of the present invention inflated to 25% of the standard pressure, a radial tyre 1 is provided with two bead portions 3 each reinforced by a bead core 2, and side wall portions 4 which extend radially outwards from the bead portions 3. A tread portion 5 is connected to the upper ends of the side wall portions 4. Between the bead portions 3, and extending around the sidewall portions 4 and tread portion 5 is provided a carcass ply 6 which is turned up from the inside of the bead core 2 toward the outside. The tread portion 5 is provided with a breaker 7 outside the carcass 6. A bead apex 9 is provided between the main portion of the carcass 6 and the turned-up-portion. The tyre 1 is mounted on a rim 10 with the beads 3 against flanges 11 of the rim 10.

The radial tyre 1 in this embodiment has an aspect ratio T/W where the distance T is from the lower end of the bead portion 3 to point of maximum height a of the tread surface, which is the upper surface of the tread portion 5, and the width is the distance between the outermost positions D to which the tyre expands. Preferably the tyre is a low aspect ratio tyre is less than 70%. For the tyre described T/W is 60%.

The bead portion 3 is provided with a reinforcing layer 12 which starts from the upper portion o; the bead core 2, passes around the bead core 2, is turned up inwardly at the lower portion of the bead core 1 and extends upwardly in contact with the bead apex 9 approximately to the intermediate region between the bead core 2 and the outermost position D of the width W. On the outer surface of the bead portion 3 is provided a chafer 14 for preventing abrasion between the bead portion and a rim such that the chafer 14 surrounds the outer surface of the bead portion 3 and the upper portion of the chafer 14 is clamped between the carcass 6 and a clinch apex 13 at the outside of the tyre.

The carcass 6 in this embodiment consists of two plies, an inner ply 6A and an outer ply 6B. The upper end of the- turned-up portion of the inner ply 6A is positioned below the position D of the width W, while the upper end of the turned-up portion of the outer ply 6B is positioned above the position D. The carcass 6 having such a high turned-up structure enhances the rigidity from the bead portion 3 to the central region of the sidewall 4.

The breaker 7 is, for example, a two-layer structure consisting of a first ply 7A provided on the side of the carcass 6 and a second ply 7B provided above the first ply 7A. In both the first and second plies & and 7B, high strength cords such as metal cords are used inclined at a comparatively blunt angle relative to the equatorial surface C1 of the tyre. The cords in the first ply 7A are inclined in the opposite direction to those in the second ply 7B. Both ends of the first ply 7A extend to the vicinity of the lower portions of the edge portions b at which the side wall portions 4 intersect the tread portion 5.

The breaker 7 is provided at the end portions thereof with cushion rubbers 21 and on the upper surface including the cushion rubbers 21 with a band 22 consisting of a synthetic resin reinforced material such as nylon.

The breaker 7 is so designed that the radius of curvature R1 at the crown portion 15 is smaller than the radius of curvature R2 at the shoulder portions 16, as shown in Fig.2, in the state in which the tyre is mounted on the rim 10 and inflated to 25% of standard pressure,. thus for example for normal pressure of 2.0 kg/cm inflated to 0.5 kg/cm . '(0.4 to 0.6 kg/cm is permissible) In this embodiment, the breaker 7 is formed by an arc 24 having a radius of curvature of R1 which has its centre on the equator C1 of the tyre and extends outwardly from the intersection P with the equator C1 and arcs 25 having a radius curvature R2 which are joined to the arc 24 at contact points 27 which are spaced from the equator C1 by a distance L1.In Fig.2, the arc 24 is indicated by the broken line, while the arc 25 is indicated by the three-dot chain line. The breaker 7 is, as indicated by the solid line on the right-hand side in Fig.2, curved along the arc 24 for the distance L1 until the contact point 27 and along the arcs 25 outside of the contact points 27. Thus, the breaker 7 has a larger radius of curvature R2 at the shoulder portions 16 than the radius of curvature R1 on the crown portion 15, as described above.

The radius of curvature R2 at the shoulder portions 16 is set at 1.1 to 2.5 times, preferably about 1.2 to 2 times the radius of curvature R1 at the crown portion 15, and the distance L1 is set at 0.1 to 0.35 times, preferably 0.15 to 0.3 times the width W1 of the breaker 7.

The breaker 7 is positioned inside a curve which is inscribed by an arc RO having its centre point on the equator of the tire and passing through the intersection P where the breaker 7 intersects the equator of the tyre and a point Q on the breaker spaced from the intersection P by a distance L which corresponds to a smaller value selected between 1/4 to 1/2 of the width W1 of the breaker and the ground contact width W2 of the tread portion, at the intersection P and which passes inside the arc RO and intersects the arc RO at the point Q, as shown in Fig.2.

The distance L is determined, in principle, on the basis of the distance between the edge portions of the tread portion 5 which comes into contact with the road when the vehicle is running, namely, the ground contacting width W2. When the ground contacting width W2 is larger than the width W1 of the breaker, the distance L is set at 1/4 to 1.2 the width W1 of the breaker. The ground contacting width W2 is generally 75 to 100% of the width W1 of the breaker, in other words, the width W1 of the breaker is set to be wider than the ground contacting width W2. As shown in Fig.

3, about 1/3 of the ground contacting width W2 at the central portion is called the crown portion 30 and each 1/3 thereof on both sides is called the shoulder portion 31. If the beneficial effect of the breaker is expected over the entire grounding contacting width, the ground contacting width L is restricted to the range K included not less than 1.2 of the shoulder portions 31, as shown in Fig.3.

Therefore, the breaker is effective in the range of the sum of the crown portion 30, namely, 1/3 of the ground contacting width W2 and two halves of the shoulder portions 31, namely, the ground contacting width W2 x 1/3 x 2, in other words, 2/3 of the ground contacting width W2 is effective. Since the ground contacting width W2 is 75 to 100% of the width W1 of the breaker, the lower limit of the distance L is 75% W1 x 2/3 = 50% W1. Since the distance L1 may extend to the peripheral edge c of the breaker 7, the upper limit and the lower limit of the distance L are set at 1/2 and 1.4, respectively, of the width W1 of the breaker.

In such a radial tyre having a radius of curvature R2 at the shoulder portions 16 larger than the radius of R1 at the crown portion 15, when it is inflated to a standard pressure, for example, 2 kg/cm the amount of expansion X2 at the shoulder portions 16 becomes larger than the amount of expansion X1 at the crown portion 15 and the amount X2 of expansion in a conventional tyre. As a result, the elongation of the breaker in the circumferential direction at each position in the axial direction of the tyre and, hence, the tensile strain of the breaker in the circumferential direction exhibit a similar distribution as that of the amount of expansion.

Consequently, as to the apparent rigidity of the breaker 7, as shown in Fig.5 (a), the apparent rigidity at the shoulder portion 16 is enhanced in comparison with a conventional tyre indicated by the broken line and shown in Fig.11 and a conventional tyre indicated by the one-dot chain line and shown in Fig.13, while the rigidity at the crown portion 15 is reduced. The distribution of the apparent rigidity shown from the ground contacting side is shown in Fig.5 (b). As is obvious from this drawing, the apparent rigidity of the radial tyre of the present invention at the shoulder portions is increased in comparison with that of the conventional tyres.

It is obvious that when breaker 7 having the abovedescribed structure is curved sideways in a plane, for example, in the ground contacting range indicated with the broken line in Fig.6, the higher the rigidity is at the shoulder portions 16 on both sides of the ground contacting range, the larger the bending moment becomes. Accordingly, the breaker 7 of the radial tyre of the present invention has a larger bending moment in a plane than those of the conventional tyres shown in Figs.ll and 13.

On the other hand, when the tyre is cornering, the breaker 7 at the ground contacting surface curves sideways in a plane in such a manner as to swell, as indicated by the solid line in Fig.7, and a bending moment in a plane is produced. As the reaction to the sideways bending moment is increased, the cornering force and the steering stability are enhanced.

It is therefore clear that the radial tyre of the present invention can enhance the cornering force in comparison with the conventional ones.

The reason why the radius of curvature R2 at the shoulder portions 16 is set at 1.1 to 2.5 times the radius of curvature RI is that if the radius of curvature R2 is smaller than 1.1 times the radius of curvature-R1, there is little difference between this tyre and a conventional one provided with a breaker consisting of a single curve, so that the degree of enhancement of the apparent rigidity at the shoulder portions is reduced. On the other hand, if the radius of curvature R2 is larger than 2.5 times the radius of curvature R1, the ground contacting pressure at the shoulder portions is excessively raised, which may cause local wear. The radius of curvature R1 of the crown portion 15 is set at about 1.2 to 3 times the width W of the tyre.If the radius of curvature R1 is smaller than 1.2 times the width W of the tyre, the radius of curvature of the crown portion 15 becomes so small that the local ground contacting pressure at the crown portion becomes so high that the vibrating force applied to the pattern Qf the tyre is disadvantageously increased and wear is accelerated. If the radius curvature R1 is larger than 3 times the width W of the tyre, the radius of curvature R2 at the shoulder portions becomes, too large, which may also cause local wear.

If L1 exceeds 0.35 times the width W1 of the breaker, the crown portion 15 protrudes radially outwardly, so that the grounding pressures at the central portion tends to be excessive, thereby causing local wear.

It is also possible that the radial tyre of the present invention is so constructed as to have a plurality of radius of curvature R1, R2 and R3 which subsequently increase in length from the crown portion 15 to the shoulder portions 16. This structure enables; the apparent rigidity of the breaker to be increased at a given portion.

Furthermore, it is possible that the radial tyre of the present invention is so designed as to have a parabolic or elliptical curve having the origins of the Y-axis of ordinate and the X-axis of abscissa as the vertexes.

Additionally, the radial tyre 1 of the present invention can be moulded without changing the other conditions by using a vulcanised mould having the curve of the breaker and configuration of the tread surface in parallel with each other, or can be moulded by using a conventional tyre mould by gradually changing the distance between the tread surface and the breaker, namely, the gauge thickness.

A detailed tyre will now be explained comprising the structure shown in Figs. 1 and 2. The tyre was 205/60R15 in size and was experimentally produced so as to have the specification shown in Table 1. The tyre was mounted on a rim 6JJ x 15 and was inflated to a pressure of 2.2 kg/cm . Such tyres were mounted on a Japanese FR car of a displacement of 2.0 litres and the steering ability in cornering, limit of grip, and stability were evaluated by a sensory examination to evaluate the steering stability. Furthermore, the cornering force was tested and the result is shown in Fig. 8. A conventional tyre was produced as a comparative example in accordance with the specification shown in Table 1 and similar tests were carried out..In the sensory test, the results are shown by the figures on the assumption that the value of each item in comparative example is 100, and the larger the figure, the better the result. It is clear from Fig. 8 that the tyre in example has a higher cornering force than the tyre in the comparative example.

TABLE 1

Example Comparative Example Curvature radius R1 430 850 at crown portion (mm) Curvature radius R2 at shoulder portion 600 180 (mm) Distance to contact L1 (mm) 35 -.44 Width of tread contact-164 ::164 t (mm > Steering ability 110 1~1~00 Limit o grjp;i,ng o~f~grppin~g 115 100 Stability As described above, the radial tyre of the present invention has a breaker having a larger radius of curvature in the shoulder portions than the radius of curvature at the crown portion, so an increase is achieved in the apparent rigidity of the breaker at the shoulder portions on inflation. Thus, the cornering force is enhanced and the steering stability increased.

The present invention can also be widely used not only for passenger car tyres but also for truck and bus tyres.

While there has been described what are at present considered to be preferred embodiments of the invention-=, it will be understood that various modifications may be made thereto, and it is -intended that the appended claims cover all such modifications as fall within the true spirit- and scope of the invention.

Claims (7)

1. A radial tyre comprising a carcass reinforced by cords disposed in the radial direction and a breaker composed of high strain cords and disposed in the circumferential direction inwardly of a tread portion of said tyre and radially outside the carcass, the said breaker having a radius of curvature R1 at the crown portion of said tread portion smaller than the radius of curvature R2 at the shoulder portions when said tyre is mounted on a rim and inflated to 25% of the full or standard inflation pressure.

2. A radial tyre comprising a carcass reinforced by cords disposed in the radial direction and a breaker composed of high strain cords and disposed in the circumferential direction inwardly of a tread portion of said tyre and outside the said carcass, said breaker being so designed as to have a radius of curvature R1 at the crown portion of said tread portion smaller than the radius of curvature R2 at the shoulder portions by positioning the said breakers inside a curve which is inscribed by an arc RO having the centre thereof on the equator of said tyre and passing the intersection P where said breaker intersects said equator of the tyre and the point Q on said breaker spaced from said intersection P by the distance L corresponding to the smaller value selected between 1/4 to 1/2 the width W1 of said breaker and the ground contacting width W2 of said tread portion, at said intersection P and which passes inside said arc RO and intersects said arc RO at the point Q in the condition when the said tyre is mounted on a rim inflated to 25% of the standard inflation pressure.

3. A radial tyre according to claims 1 or 2 when the portions of radius of curvature R1 and R2 are joined at points which are spaced from the equator of the tyre by a distance L1 where L1 is 0.1 to 0.35 times the width W1 of the breaker.

4. A radial tyre according to claims 1 or 2 when the portions of radius of curvature R1 and R2 are joined at points which are spaced from the equator of the tyre by a distance Li. where L1 is 0.15 to 0.3 times the width W1 of the breaker.

5. A radial tyre according to any one of claims 1 to 4 wherein the radius of curvature R2 is 1.1 to 2.5 times the radius of curvature R1

6. A radial tyre according to any one of claims 1 to 4 wherein the radius of curvature R2 is 1.2 to 2 times the radius of curvature R1.

7. A radial tyre constructed and arranged substantially as described herein and illustrated in Figs.1 of the accompanying drawings. .