DESCRIPTION

[0058] FIG. 1 shows a tyre 1 according to the invention in the inflated configuration, in the absence of a load. The tyre 1 is classed as 195/65 R 15 and is of the radial type. The tyre 1 comprises a casing 2, belt strips 3, 4 and 5, a tread strip 6 and bead cores 7. FIG. 1 shows a shoulder 8, a sidewall 9, a bead 10 and a bead filler 11, which are located to the right of an equatorial plane 13. Those located to the left of the equatorial plane 13 are not shown, because the tyre 1 is symmetrical about this plane. The tyre 1 has an air-tight inner layer (liner) 12. The casing 2 is formed from a rubberized fabric ply, reinforced with textile cords lying in meridian (radial) planes and wrapped around bead cores 7.

[0059] The belt strips 3, 4 and 5 and the tread strip 6 are placed on the crown of the casing 2 and extend circumferentially around it. The belt strips 3 and 4 are formed from rubberized fabric plies, reinforced with corresponding metal cords, crossing each other symmetrically with respect to the equatorial plane 13. The belt strip 5, placed on top of the belt strips 3 and 4, is formed from a rubberized fabric ply, reinforced with textile cords, orientated circumferentially (0° belt). The tread strip 6 is provided with projections, blocks and grooves (not shown) which, during operation, come into contact with a road surface.

[0060] The outer profile of the tyre 1 has a maximum chord {overscore (C)} of 200.8 mm and a maximum height {overscore (H)} of 128.1 mm. The ratio {overscore (H)}/{overscore (C)} is 0.63 and lies in a range from 0.6 to 0.8.

[0061] The casing ply 2 of the tyre 1 has a meridian profile (plyline) 14, represented by a broken line, comprising a central crown portion (under-belt) 15 and two sidewall portions 16, one on the right and one on the left. FIG. 1 shows only half of the crown portion 15 and the sidewall portion 16 which are located to the right of the equatorial plane 13. Those located to the left are mirror images. Each sidewall portion 16 is delimited by two terminal points, S and K. The point S is located at one edge of the belt package 3, 4 and 5, and forms the point of separation between the crown portion 15 and the sidewall portion 16. The point K forms the point of separation between the sidewall portion 16 and a bead portion 17. The crown portion 15 of the meridian profile 14 has a radius of curvature ρ_c. The sidewall profile 16 forms an angle α_s with the axis of rotation of the tyre at the point S, and forms an angle α_k with the axis of rotation of the tyre at the point K. The tread 6 has a width l.

[0062] In the tyre 1, the angle α_s advantageously ranges from 250 to 300 as the radius of curvature ρ_c varies in a range from 406 mm to 690 mm.

[0063] The angle α_k lies in a range from 45° to 50°.

[0064] In particular, in the tyre 1 the radius of curvature ρ_c=689.8 mm; the angle α_s=29.6°; the angle α_k=45.8°; and the width l=74.6 mm.

[0065] In the tyre 1, the variations of curvature between the inflated and flattened states of the belt package 3, 4 and 5 and the tread strip 6 are minimized by making these have an extremely flat shape in the inflated configuration.

[0066] This is demonstrated by the formula discovered by the inventors, which relates the variation of equatorial curvature ΔC due to the passage through the footprint to the characteristic parameters of the tyre:

ΔC=K_r*s/(T/p)=K_r*s/(1−2*ρ_s*α_s)K_r*s/(1*(1−ρ_s/ρ_c))

[0067] where ρ_c, α_sand α_k represent the parameters indicated above;

[0068] ρ_s=radius of meridian curvature in the area of the point S;

[0069] K_r=radial rigidity of the sidewall;

[0070] s=flattening;

[0071] T=total pull on the belt package in the circumferential direction;

[0072] p=air pressure inside the tyre.

[0073] From the above relation it may be seen that ΔC decreases with a decrease in α_x and with a decrease in the ratio ρ_s/ρ_c, in other words with an increase in ρ_c.

[0074] In the tyre 1, the radius of curvature ρ_c has a much higher value than in a conventional tyre, as will be illustrated in greater detail below. The meridian profile 14 of the tyre 1 therefore has a smaller ratio ρ_s/ρ_c. Additionally, the lower area of the sidewall 16 has a more upright shape than that of a conventional tyre. When the tyre is flattened, this shape of the sidewall tends to cause the deformations due to the meridian inflection to be concentrated in the area of the sidewall lying between the maximum chord and the edge of the belt package.

[0075] The configuration adopted for the meridian profile meets the conditions of the enclosing rectangle in the inflated state and enables the casing profile to be joined to the bead filler while avoiding points of discontinuity in the sidewall portion of said profile.

[0076] FIG. 2 shows the tyre 1 in the inflated configuration 1G, with an operating pressure p=2.2 bar, and in the flattened configuration 1S, under a load Q=493 kg.

[0077] FIG. 3 shows a conventional tyre 21, having the classification 195/65 R 15 and identified by the symbol NP6.

[0078] The tyre 21 comprises a casing 22 formed by a ply folded around bead cores 27, belt strips 23, 24 and 25, and a tread strip 26. A shoulder 28, a sidewall 29, a bead 30 and a bead filler 31 of the tyre 21 are shown. An air-tight inner layer 32 is also shown.

[0079] The casing ply 2 of the tyre 21 has a meridian profile (plyline) 34 comprising a central crown portion (under-belt) 35 and two sidewall portions 36, one on the right and one on the left.

[0080] The tyre 21 has a radius of curvature ρ_c=357.1 mm; angle α_s=31.4°; angle α_k=42.2°; and width l=75.8 mm.

[0081] When FIG. 1 is compared with FIG. 3, it is clear that the belt package 3, 4 and 5 and the tread strip 6 of the tyre 1 have a flatter shape than the belt package 23, 24 and 25 and the tread strip 26 of the tyre 21 so that their deformation in the meridian plane is smaller when they pass through the footprint.

[0082] FIG. 4 shows the tyre 21 in the inflated configuration 21G, with an operating pressure p=2.2 bar, and in the flattened configuration 21S, under a load Q=493 kg.

[0083] When FIG. 4 is compared with FIG. 2 it may be noted that the upper mid-point of the sidewall 9 of the tyre 1 (near the tread strip 6) is more markedly deformed in the flattened state than the lower mid-point of the sidewall (near the bead 10), while the upper and lower areas of the sidewall 29 of the tyre 21 are deformed in a more uniform way in the flattened state.

[0084] In FIG. 5, the flattened configuration 1S of the tyre 1 according to the invention and the flattened configurations 21S of the conventional tyre 21, found with the same load of 493 kg, are compared.

[0085] The graph in FIG. 6 shows the variation in the flattening with a variation in the load for the tyre 1 (line a) and for the tyre 21 (line b). The tyre 1 is found to undergo a greater flattening than the tyre 21 for the same load. This is due to the fact that the upper area of the sidewall is more deformable than that of the tyre 21.

[0086] Two tyres having mixtures with the following characteristics were used to evaluate the rolling resistance of the tyre 1 with respect to the tyre 21: 1

[0087] Tests were carried out with a vertical load of 493 kg, a speed of 100 km/hr and a mixture temperature of 70° C.

[0088] The following values were found for the tyre 1: 2

[0089] R.R. indicates the coefficient of rolling resistance, expressed in thousandths of the value of the ratio between the vertical load applied to the tyre and the tractive force required to move the tyre. The power dissipated is expressed in N*mm/s. The “Ratio” column shows the ratio between the power dissipated in the tread mixture and the power dissipated in the sidewall mixture.

[0090] The following values were found for the comparative tyre 21: 3

[0091] It is found that the ratio between the power dissipated in the tread mixture and the power dissipated in the sidewall mixture is smaller in the tyre 1 than in the tyre 21. The tyre 1 therefore has a power dissipation distribution which is more balanced between its parts than that of the tyre 21.

[0092] FIG. 7 shows a vulcanization mould 40 of the centripetal type, suitable for tyre production. The mould 40 comprises a central crown 41, formed by a plurality of radially movable sectors 44, for moulding the pattern of the tread strip and the shoulders of the tyre. The projections and grooves of the sectors 44 are not shown. The mould 40 also comprises two cheeks 42 and 43 which are mirror images of each other and are axially opposed, for moulding the sidewalls and beads of the tyre, and an inner bladder, not shown, which can be inflated by means of a pressurized fluid. The sectors 44 are moved radially in both directions (centripetal and centrifugal), perpendicularly to the longitudinal axis of the mould, by an operating ring which is not shown. The cheeks 42 and 43 are movable axially and reciprocally with respect to each other.

[0093] In the plane of the meridian section, the mould 40 has an inner profile 45, formed centrally by a base profile 46 of a sector and laterally by two profiles 47 and 48 of the cheeks. The base profile 46 has a central portion 46c flanked by two connecting portions 46r which in turn are flanked by two shoulder portions 46s. Each of the cheek profiles 47 and 48 is formed by two portions, 47s, 47i and 48s, 48i respectively.

[0094] FIG. 7 shows the following dimensional parameters of the mould 40: C=chord of tyre; L_s=width of sectors; C_b=chord of tread; R=radius of tread on crown; R₁=radius of crown-shoulder joint; R_s=radius of shoulder; α°=angle of shoulder with respect to a longitudinal axis 50; R_fs=radius of upper sidewall; R_fi=radius of lower sidewall; f=camber of tread; H_s=height of sectors; H=height of section; H_g=height of cheeks; H_lc=height of maximum chord point; E=width of mould rim.

[0095] In the mould 40, the angle α° lies in a range from 42° to 43°, and the radius of meridian curvature R lies in a range from 500 mm to 619 mm. In turn, the radii of curvature R_fs and R_fi of the portions 47s, 47i and 48s, 48i of the cheek profiles have a ratio R_fs/R_fi equal to approximately 0.5 and the centres of the radii of curvature R_fs and R_fi are located at a height H_lc which is equal to approximately 2/3 of the height of the cheek H_g.

[0096] In particular, the mould 40 for producing the tyre 1 has the following dimensions:

[0097] C=209.4 mm; L_s=183.0 mm; C_b=142.6 mm; R=619.0 mm; R₁=129.0 mm; R_s=30.0 mm; α°=42.6°; R_fs=47.4 mm; R_fi=105.0 mm; f=7.5 mm; H_s=29.5 mm; H=127.75 mm; H_g=98.25 mm; H_lc=65.46 mm; E=174.0 mm.