GB2148806A - High pressure, low rolling resistance radial tire - Google Patents

Abstract

In order to minimise the reduction of the restoring torque (self-aligning torque), and to further reduce rolling resistance, in a pneumatic radial tire for passenger cars which is used at an internal pressure 25 to 50% higher than normal, the carcass has a radial profile such that when the tire is mounted on a normal rim and inflated at a working internal pressure, a ratio of radii R/R' is within a range of 0.65-0.85 and f is within a range of 5-10 mm, where R' is a radius of a standard arc passing through points B, E and C; R is a radius of curvature in a curved line of a shoulder profile of the carcass line passing through the point C, and f is a maximum distance between the standard arc and the remaining carcass line smoothly extending from the curved line of the shoulder profile to the point B and having a single inflection point. Further, to improve ride comfort and the restoring torque, the tread has a width corresponding to only 55-62% of the maximum tire width. <IMAGE>

Description

SPECIFICATION Pneumatic radial tire This invention relates to a pneumatic radial tire, and more particularly to a pneumatic radial tire for passenger cars which is used at an internal pressure higher than the ordinarily used normal internal pressure.

It is well-known that the internal pressure of a tire may be increased for the purpose of reducing the rolling resistance. However, increasing the internal pressure of an ordinary tire leads to the following drawbacks: (a) The spring constant in the radial direction of the tire is increased to adversely affect the ride comfortability against vibration; and (b) The ground contact area is decreased to reduce restoring torque (self-aligning torque) when a slip angle is given to the tire, resulting in deterioration of the steering stability such as poor reaction force in the steering during high-speed running. For this reason, tires are not used at a higher internal pressure.

It is, therefore, an aim of the present invention to provide a pneumatic radial tire for use particularly in passenger cars under a high internal pressure, wherein the radial profile of the carcass is adequately changed not only to minimize the reduction of the restoring torque due to the high internal pressure but also to further enhance the improvement of the rolling resistance. It is also envisaged that the width of the tread will be made narrower not only to improve the degradation of the ride comfortability against vibration but also to increase the restoring torque.

According to the invention, there is provided a pneumatic radial tire, comprising a pair of sidewall portions each provided at its inner end with a bead portion, a tread portion extending between the sidewall portions at their outer ends in the radial direction, a reinforcement for the said sidewall and tread portions composed of a toroidal carcass comprising at least one ply containing organic fiber cords embedded therein in a substantially radial direction of the tire and extending between two bead cores embedded in the bead portions and turned up around each of the bead cores from the inside toward the outside, and a belt comprising at least two rubberized layers, each layer containing high modulus cords embedded therein, superimposed about a crown region of the carcass and crossing with each other at a relatively small angle with respect to the mid-circumferential line of the tire, and a rubber filler disposed between the carcass and the turnup thereof to enhance the rigidity of the bead portion, wherein the carcass has such a radial profile of a carcass line in tire section that when the tire is mounted on a normal rim and infiated at a working internal pressure, a ratio of radii R/R' is within a range of 0.65-0.85 and f is within a range of 5-10 mm, where R' is a radius of a standard arc passing through points B, E and C, where point B is an intersection of the carcass line and a straight line parallel with the rotational axis of the tire passing an alienating point A of a flange of the rim to the outer surface of the bead portion, point C is an intersection of the carcass line and a line segment drawn from the intersection B perpendicular to the said straight line, point E is an intersection of a line extending through a middle point D of the line segment BC as a chord and parallel with the tire rotational axis and a line extending through a maximum width point F of the carcass line and perpendicular to the tire rotational axis and parallel with the line segment BC, R is a radius of curvature in a curved line of a shoulder profile of the carcass line passing through the point C, and f is the maximum distance between the standard arc and the remaining carcass line smoothly extending from the curved line of the shoulder profile to the point B and having a single inflection point, and wherein the tread has a width corresponding to 55-62% of the maximum tire width, and wherein the working internal pressure is higher by 25-50% than a normal internal pressure.

The invention will be further described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a schematic view illustrating the relation between a radial profile of a tire according to the invention (a solid line) and a standard arc (a broken line); Figure 2 is a schematic sectional view of the tire of Example 1 referred to hereinafter (after inflation); Figure 3 is a schematic sectional view of the tire of Example 2 referred to hereinafter (after inflation); Figure 4 is a schematic sectional view of the tire of Example 3 referred to hereinafter (after inflation); and Figure 5 is a schematic sectional view of the tire of Comparative Example 1 referred to hereinafter (after inflation).

In Figure 1 is shown a radial profile of a carcass of a tire according to the invention obtained by adequately changing from a natural equilibrium curve. The term "natural equilibrium curve" used herein means a shape of a carcass line or a middle line in the thickness of the carcass ply in the radial section of the tire including a meridional line thereof which can withstand the effect of the internal pressure only by the tension bearing of the carcass ply without being subjected to bending force and shearing force.

That is, according to the invention, it is necessary that the curvature in a curved line of a shoulder profile of the carcass corresponding to the upper region of the sidewall portion is made large with respect to the standard arc BEC of Figure 1, i.e. the ratio of radius of curvature R in the shoulder profile to radius R' of the standard arc is within a range of 0.65-0.85, while the curvature of the carcass corresponding to the lower region of the sidewall portion is made small or into a reversed R-shape, i.e. a maximum distance f between the carcass line FB and the arc BE in Figure 1 is within a range of 5-10 mm.

The arc BEC shown in Figure lisa more standard arc and is originally different from the radial profile of the carcass based on the so-called natural equilibrium curve. However, the upper region of the sidewall portion is relatively thin and of low rigidity, so thatthe radial profile of this region based on the natural equilibrium curve becomes very approximte to a part EC of the above arc. Therefore, it will be apparent that the ratio R/R' of 0.65-0.85 will be obtained only insofar the carcass line is intentionally precluded from the natural equilibrium curve.

On the other hand, the lower region of the sidewall portion is a region having a relatively large rigidity because the carcass ply is turned up around the bead core from the inside toward the outside in the radial direction and a rubber filler is disposed between the carcass ply and the turnup thereof to strengthen the bead portion. In any case, such a lower region is generally existent inside the arc BE in the radial profile of the carcass based on the natural equilibrium curve. According to the invention, however, the f value of 5-10 mm is first given by intentionally and largely precluding the carcass line from the natural equilibrium curve, i.e. only by making the curvature of the carcass at the lower region of the sidewall portion small or into the reversed R-shape, which can be completely distinguished from the conventional natural equilibrium curve.

The reason why the rolling resistance is improved in a tire having a carcass radial profile obtained by intentionally precluding from the natural equilibrium curve is disclosed in detail in EPC Publication No.

103,984 and also the means disclosed in the same publication is required in the invention for the manufacture of such tires.

When the ratio RIR' is more than 0.85, it is not possible to obtain the effect of reducing the rolling resistance by reducing the shearing deformation in the upper region of the sidewall portion by intentionally precluding from the natural equilibrium curve as shown in the EPC Publication No. 103,984, while when the ratio R/R' is less than 0.65, the bending deformation concentrates in a relatively thick buttress portion to cancel the effect of reducing the rolling resistance resulting from the reduced shearing deformation.On the other hand, when the value off is less than 5 mm, the effect of reducing the consumed energy according to the principle disclosed in the EPC Publication No. 103,984 cannot be sufficiently achieved, while when the value off exceeds 10 mm, the tension in the carcass in the lower region of the sidewall portion when filled with the internal pressure is too high to adversely affect the durability of the tire, and as the carcass enters inside of the tire, the outer surface of the tire locates on the relatively inner side to adversely affect the fitting of the rim.

In the tire of the above configuration, the tension of the carcass in the upper region of the sidewall portion becomes relatively low, which contributes to increase a pneumatic trail when being subjected to a slip angle, i.e. a distance between the center of the ground contact surface and the working point of the concering force located therebehind to thereby increase the restoring torque.

In Figure 2 is shown a first embodiment of the tire shape according to the invention, wherein a tread width TW is made narrower than a tire width SW as compared with the case of the ordinarily used radial tire. Figure 3 shows a second embodiment of the tire according to the invention having the same structure as in Figure 2 except that the tread portion has a cap and base structure, and Figure 4 shows a third embodiment of the tire according to the invention having the same structure as in Figure 3 except that the rubber filler has a composite structure of hard rubber stock and soft rubber stock. In these Figures, numeral 1 is a bead portion, numeral 2 an annular sidewall portion, numeral 3 a tread portion, numeral 4 a carcass, numeral 5 a belt, numeral 6 a rubber filler, numeral 7 a bead core, numeral 8 a rim and numeral 9 a rim flange.For the comparison, the ordinarily used radial tire is shown in Figure 5.

When the tread width TW is made narrower than the tire width SW, the length of the ground contact surface of the tire under loading becomes relatively longer as compared with the case of the ordinarily used radial tire and as a result the enveloping property against irregular road surface is enhanced even under a higher internal pressure to improve the deterioration of the ride comfortability produced under the high internal pressure, and also the restoring force when giving the slip angle is increased. Further, the volume of the tread rubber can be reduced, so that the tread rubber gauge is made uniform over the whole of the tread width to reduce the rolling resistance.

According to the invention, a ratio TW/SW of tread width to tire width is within a range of 55%-62%. When the ratio exceeds 62%, it is difficult to obtain the aforementioned effect, while when the ratio is less than 55%, the width of the ground contact surface under loading is too narrow, resulting in the occurrence of problems relating to stability.

Moreover, the reason why the tire according to the invention is used at a working internal pressure higher by 25-50% than a normal internal pressure (about 1.7 kg/cm2) results from the reduction of the rolling resistance. When the working internal pressure is not higher by 25% than the normal internal pressure, the sufficient effect for the reduction of the rolling resistance can not be obtained, while when the working internal pressure exceeds 50% higher than the normal internal pressure, the effect of reducing the rolling resistance is saturated and the durability of the tire is adversely affected.

The invention will be described below while comparing the tires shown in Figures 2-4 as Examples 1-3 with the tire of Figure 5 as Comparative Example 1 and a tire formed by intentionally precluding only the radial profile of the carcass in the ordinarily used radial tire from the natural equilibrium curve as Comparative Example 2.

The structure of each tire of Examples 1-3 and Comparative Examples 1-2 is shown in the following Table 1.

TABLE 1 Structure Structure Kind of RIR' f(mm) TWISW oftread ofbead tire rubber filler single Comparative 1.02 3.0 65% single rubber Example 1 layer stock Comparative 0.80 6.0 65% Example 2 Example 1 0.80 6.0 59% Example 2 0.80 6.0 59% two layer hard and Example 3 0.80 7.5 59% " soft rubber stocks Note: tire size ... P175/70R13 normalrim...5J-13 In these tires, the carcass 4 was a single ply containing polyester cords of 1500 d/2 arranged at 90 with respect to the equator of the tire, and the belt 5 was composed of two cord layers each containing steel cords (twisting construction :1 > < 5 5 5 x 0.25 mm) arranged at a cord angle of 20" with respect to the equator of the tire, the cords of which layers being crossed with each other.Further, the rubber of the tread portion (3, 3a) had a loss tangent of 0.110 as measured by means of a spectrometer, made by Iwamoto Seisakusho, under the conditions of 50 Hz, 1% tensile strain and 60"C, and a Shore A hardness of 57", while the under tread rubber (3b) in the cap and base structure of Figures 3 and 4 had a loss tangent of 0.06 and a Shore A hardness of 56". As the rubber of the bead filler 6 was used rubber having a Shore A hardness of 95" except for the case of Figure 4. The bead filler of Figure 4 was composed of a hard rubber stock 6a having a Shore A hardness of 95" and a soft rubber stock 6b having a Shore A hardness of 74".

The rolling resistance was measured with respect to these tires under internal pressures of 2.5 kg/cm2 and 2.2 kg/cm2to obtain results as shown in the following Tables 2 and 3, respectively, wherein the rolling resistance was represented by an index on a basis that the value of the rolling resistance of Comparative Example 1 under an internal pressure of 1.7 kg/cm2 is 100. The larger the index value, the more the reduction of the rolling resistance.Moreover, the evaluation of the rolling resistance was performed as follows: that is, the tire was pushed against a drum of 1,707 mm in diameter and rotated up to a predetermined speed together with the drum by the driving of a motor, and then the driving of the motor was stopped to run the drum by inertia, during which the rolling resistance was calculated by a degree of deceleration during the rotation of the drum and tire.

TABLE 2 Kind of tire Comparative Comparative Comparative Example Example Example Internal Example 1 Example 1 Example 2 1 2 3 pressure (kg/cm 1.7 2.5 2.5 2.5 2.5 2.5 Speed 50 km/h 100 120 130 140 145 149 80 km/H 100 118 130 140 144 149 100 km/H 100 118 134 141 144 148 TABLE 3 Kind of tire Comparative Comparative Comparative Example Example Example Internal Example 1 Example 1 Example 2 1 2 3 pressure (kg/cm) 1.7 2.2 2.2 2.2 2.2 2.2 Speed 50 km/H 100 113 123 133 136 140 80 km/H 100 112 124 133 137 142 100 km/H 100 112 128 134 137 140 As apparent from Tables 2 and 3, the tires of Examples 1-3 have such an effect that the rolling resistance under the high internal pressure is reduced as much as 30-50% in comparison with the rolling resistance of Comparative Example 1 under the internal pressure of 1.7 kg/cm2, and show an improving effect of 20-30% compared to Comparative Example 1 under the high internal pressure (2.2 or 2.5 kg/cm2). The tires of Examples 2 and 3 give particularly excellent results.

Then, the restoring torque was measured with respect to the tires to obtain results as shown in the following Tables 4 and 5.

The evaluation of the restoring torque was made as follows: that is, the tire subjected to the predetermined internal pressure (1.7,2.2 or 2.5kg/cm2) was pushed against a drum of 1,707 mm in diameter and a slip angle given to the tire was changed while rotating the tire, during which a maximum value of the restoring torque (self-aligning torque) was measured and represented by an index on a basis that the maximum value of Comparative Example 1 under an internal pressure of 1.7 kg/cm2 is 100. The larger the index value, the better the restoring torque.

TABLE 4 Kind of tire Comparative Comparative Comparative Example Example Example Internal Example 1 Example 1 Example 2 1 2 3 pressure (kg/cm) 1.7 2.5 2.5 2.5 2.5 2.5 Item Self-aligning torque 100 72 85 97 96 98 TABLE 5 Kind of tire Comparative Comparative Comparative Example Example Example Internal Example 1 Example 1 Example 2 1 2 3 pressure (kg/cm) 1.7 2.2 2.2 2.2 2.2 2.2 Item Self-aligning torque 100 80 93 102 100 103 As apparent from Tables 4 and 5, in the tires of Examples 1-3 according to the invention, the reduction of the restoring torque is not substantially accomplished even with the increase of the internal pressure.

Further, the test for the ride comfortability was made with respect to the tires to obtain results as shown in the following Tables 6 and 7.

In this case, the degree of force produced on the rotating axis of the tire was measured during the rotation of the tire on a test drum provided with protrusions, from which the ride comfortability against vibration was evaluated by an index on a basis that the value of Comparative Example 1 under an internal pressure of 1.7 kg/cm2 is 100. The larger the index value, the betterthe ride comfortability against vibration.

TABLE 6

Kind of tire Comparative Comparative Comparative Example Example Example Example 1 Example 1 Example 2 1 2 3 Internal pressure (Kg/cm Con- 1.7 2.5 2.5 2.5 2.5 2.5 Measured item dition-.

Reaction in low speed 100 95 97 103 104 103 vertical directtion upon riding over protrusion high speed 100 81 84 96 100 102 Reaction in low speed 100 89 92 98 99 100 horizontal direction upon riding over protrusion high speed 100 95 106 110 107 104 TABLE 7

Kind of tire Comparative Comparative Comparative Example Example Example Example 1 Example 1 Example 2 1 2 3 Internal pressure (kg/cm) Con- 1.7 2.2 2.2 2.2 2.2 2.2 Measured Item dition Reaction in low speed 100 97 99 102 104 105 vertical direction upon riding over protrusion high speed 100 90 93 98 100 102 Reaction in low speed 100 94 97 101 98 100 horizontal direction upon riding over protrusion high speed 100 96 107 111 108 103 As apparent from Tables 6 and 7, in the tires of Examples 1-3 according to the invention, the ride comfortability against vibration under the high internal pressure is not substantially deteriorated and is approximately equal to that under the normal internal pressure (1.7 kg/cm2).

As mentioned above, according to the invention, the reduction of the rolling resistance can advantageously be realized under an internal pressure of 20-50% higher than the normal internal pressure by adequately determining the radial profile of the carcass and making the tread width narrow relative to the tire width without causing deterioration of the ride comfortability against vibration and reduction of the restoring torque, which have been produced when the ordinarily used tire is used under an internal pressure higher than the normal internal pressure.

Claims (2)

1. A pneumatic radial tire, comprising a pair of sidewall portions each provided at its inner end with a bead portion, a tread portion extending between the sidewall portions at their outer ends in the radial direction, a reinforcement for the said sidewall and tread portions composed of a toroidal carcass comprising at least one ply containing organic fiber cords embedded therein in a substantially radial direction of the tire and extending between two bead cores embedded in the bead portions and turned up around each of the bead cores from the inside toward the outside, and a belt comprising at least two rubberized layers, each layer containing high modulus cords embedded therein, superimposed about a crown region of the carcass and crossing with each other at a relatively small angle with respect to the mid-circumferential line of the tire, and a rubber filler disposed between the carcass and the turnup thereof to enhance the rigidity of the bead portion, wherein the carcass has such a radial profile of a carcass line in tire section that when the tire is mounted on a normal rim and inflated at a working internal pressure, a ratio of radii R/R' is within a range of 0.65-0.85 and f is within a range of 5-10 mm, where R' is a radius of a standard arc passing through points B, E and C, where point B is an intersection of the carcass line and a straight line parallel with the rotational axis of the tire passing an alienating point A of a flange of the rim to the outer surface of the bead portion, point C is an intersection of the carcass line and a line segment drawn from the intersection B perpendicular to the said straight line, point E is an intersection of a line extending through a middle point D of the line segment BC as a chord and parallel with the tire rotational axis and a line extending through a maximum width point F of the carcass line and perpendicular to the tire rotational axis and parallel with the line segment BC, R is a radius of curvature in a curved line of a shoulder profile of the carcass line passing through the point C, and f is the maximum distance between the standard arc and the remaining carcass line smoothly extending from the curved line of the shoulder profile to the point B and having a single inflection point, and wherein the tread has a width corresponding to 55-62% of the maximum tire width, and wherein the working internal pressure is higher by 25-50% than a normal internal pressure.

2. A pneumatic radial tire according to claim 1, substantially as herein described with reference to, and as shown in, any of Figures 1 to 4 of the accompanying drawings.