GB2224703A

GB2224703A - Tyre for commercial vehicles

Info

Publication number: GB2224703A
Application number: GB8924977A
Authority: GB
Inventors: Shouichi Fujii
Original assignee: Sumitomo Rubber Industries Ltd
Current assignee: Sumitomo Rubber Industries Ltd
Priority date: 1988-11-11
Filing date: 1989-11-06
Publication date: 1990-05-16
Anticipated expiration: 2009-11-06
Also published as: GB8924977D0; GB2224703B

Abstract

A tyre for commercial vehicles comprises a carcass of organic fibre core and a belt composed of metallic (for example, steel) cords, the initial modulus of elasticity of the organic fibre being in a range from 300 to 900 kgf/sq.mm, the sectional area of the steel cords being more than 0.2 sq.mm and less than 0.35 sq.mm and the cords being laid at an angle of 22 degrees to 26 degrees with respect to the equator of the tyre, the tread rubber compound having a JIS A hardness of more than 61 degrees and less than 64 degrees, and the aspect ratio of the tyre more than 0.45 and less than 0.65. The tyre of the invention may be employed on the front wheels as well as on the rear wheels, without deteriorating steering stability and ride comfort, and in addition, the original tyre properties of a low profile tyre are not hampered. <IMAGE>

Description

TYRE FOR COMMERCIAL VEHICLES The present invention relates to a tyre fo commercial vehicles and more particularly to a tyre suitable for both front and rear wheel use.

For commercial vehicles a low profile tyre having an aspect ratio of 50 to 60% is commonly employed on the rear wheels to provide durability and a standard tre having an aspect ratio of 8 O is used on the front wheels for steering stability. The reason why the front tyre differs from the rear tyre is to avoid an oversteering characteristic which moves outwards the rear tyres during cornering of the v eb c 1 e . This oversteering phenomenon occurs if the front tyre has the same low profile as the rear tyre1 due to the ecessive cornering power (CP) thereof.However, to use different tyres on the front wheels to the rear wheels is not convenient for maintaining the vehicle. Accordingly, it is desirable to provide a tyre which can be used commonly for the front wheels and the rear wheels.

The inventor has studied this requiremeiit to meet these two conflicting conditions. It is an object of the invention to provide an improved tyre for commercial vehicles to do this.

Another object of the invention is to provide a tyre for commercial vehicles which is capable of being used both for front wheels and rear wheels without deteriorating the durability of the tyre.

According to the present invent ion a tre for commercial vehicles comprises a carcass composed of at least one carcass ply containing carcass cords, extending under a tread and through sidewalls to bead regions where the ply is wrapped around bead cores, and a belt composed of at least two belt plies each containing belt cords of high modulus material wherein the carcass cords are organic fibre cords of which the initial modulus of elasticity is in the range from 300 to 900 lxgf/sq.mm; the sectional area of said belt cords is more than 0.2 sq.mm and less than 0.3s sq.mm and said belt cords are laid at an angle of 22 degrees to 26 degrees with respect to the equator of the tyre; the tread comprises rubber compound with a JIS A hardness of more than 61 degrees and less than 64 degrees; and the aspect ratie H/W) of the tyre is more than 0.45 and less than 0.65.

An embodiment of the present invention will now be described in detail, b way of eample only, with reference to the drawing, in which: Figure 1 is a section view showing an embodiment of the present invention.

In the Figure, a tyre for a commercial vehicle has bead regions 3 each containing a bead core 2 sidewalls 4 extending radially entwardly from each bead regions 3 and a tread 5 connecting each end of the sidewalls 4. A carcass 6 extends from the tread 5 through the sidewalls 4 and is wrapped around the bead cores 2. A belt 7 is laid outside the carcass 6 and under the tread 5.

The tyre is a low profile tyre having an aspect ratio H/W of 0.45 to 0.65 which is the ratio of the sectional height H to the maximum width W when the tyre is mounted on a rim 15 and inflated to the specified inner pressure (normal condition) The carcass 6, in the embodiment, is composed of two carcass plies 9,9, each containing carcass cords laid at'an angle of 60 degrees to 90 degrees with respect to the equator C of the tyre to give a radial tyre construction.The carcass cords are composed of organic fibre cords such as nylon, polyester, rayon or the like, the initial modulus of which is more than 300kg/sq.mm and less than 900 k sq.mm. The initial modulus of elasticity is defined b JIS-L1017 and is referred as the amount of elongation in comparison with the load in the vicinity of the origin. If the initial modulus is less than 300 kgf/sq.mm, the lateral rigidity of the carcass 6 becomes unsatisfactory to induce socalled understeer, on the other hand, if it is more than 900 kgf/sq.mm, there is induced so-called oversteer.

The belt 7, in the eample, is composed of two belt plies 10,11. The belt width of the outer belt ply 11 is made narrower than that of the inner belt ply 10. The belt cord is an aggregated cord twisted and composed of a number of mono-filaments of steel and the like having a high modulus of elasticity. The sectional area Oi the belt cord is more than 0.2 sq.mm and less than 0.35 sq.mm. The area of the belt cord is defined as the total of the sectional areas of each mono-filament thereof.I the case when the sectional area of belt the cord is less than 0.2 sq.mm, the lateral rigidity of the tread becomes low so as to induce understeer, and when it is over 0.35 sq.mm, the lateral rigidity of the tread becomes too high and induces vehicle overseer. The belt cords are laid at an angle ranging from 22 to 6 degrees with respect to the equator C, and so arranged that the cords in the inner belt ply 10 and in the outer belt ply 11 intersect each other.

At the tread 5 the tread rubber 13 forming a tread surface 12 is arranged outside the belt 7. The tread rubber has a JIS A hardness of more than 61 degrees and less than 64 degrees at 20 degrees Centigrade. If less than 61 degrees, the deformation of the tread 5 becomes too large and increases the shearing force in the tread 5 which results in inducing separation between the tread rubber and the belt 7 at each belt ply edge.

Additionally, the tyre endurance as well as the ear resistance decreases. If it is over 64 degrees, the lateral rigidity of the tread 5 becomes excessively large so as to induce the oversteer phenomenon.

The maximum tre width position (b) where the axial width of the carcass is a maximum is located at a height Hl of 0.4 to 0.55 times the tyre section height H away from the bead base, in the normal inflated condition. Furthermore, in this normal condition, the ratio Rl/R2 of the radius Rl of the upper carcass region Cl to the radius R2 of the lower carcass region C2 is set to be in the range of more than 1.1 and less than 1.4.

The upper carcass region Cl is defined as the outwardly bulged region between the carcass maximum width point (b) and the belt outer edge point (a).

The belt outer edge point (a) is the position where a radial line (ke) drawn from the axially outer edge(s) of the narrowest belt ply plies intersects with the carcass 6.

The lower carcass region C2 is defined as tne outwardly bulged region between the maximum width point (b) and the flex point (c; of the carcass of which height H3 is 1.6 times the height H2 of the rim flange l5a.

The upper carcass region Cl and the lower carcass region C2 are each formed with single radii of curvature (R1) and (R2) respectively, and further, the upper region Cl and the lower region 2 are smoothly connected with each other. In the normal condition, the ratio of radii R1/R2 is set to be more than 1.1 and less than l.4, as said before.

Accordingly, the radius tRl) of the upper carcass region C1 is larger than the radius (R2) of the lower carcass region C2.

Therefore, when the tyre is subjected to load in the radial direction, the deformations of the upper carcass region Cl and the lower carcass region C2 are modified. As a result, the longitudinal rigidity of the tyre is maintained and the lateral rigidity is made small so that the value of the cornering power CP is suitable.

By the way, in the tre of the prior art, the ratio of the radii R1/R2 is in a range of 0.5 to 1.0, that is, the radius (R1) of the upper carcass region C1 is smaller than that R2 of the lower carcass region C2, so that the compressive strain acts on the tread central region, increasing the lateral rigidity in the plane of the tread.

By defining the initial modulus of elasticity of the carcass ply cords, a carcass is provided with lateral stability suitable for the steerin performance. Also, bg defining the sectional area of belt the cords in the aforesaid rane, together with the hardness of the tread rubber 13 and the low profile aspect ratio of 0.4s to 0.65, the lateral rigidity of the tread region is prevented from becoming too high by controlling the deformation of the tread region.

Besides, the novel concepts with respect to the carcass profile of the carcass 6, the cornering power CP of the tyre used on the front wheel is suitably decreased and the cornering power CP of the tre mounted on the rear wheels is somewhat increased, so that the low profile tyre conventionally used on the rear wheel for the heavy duty can be employed for the front wheels of the commercial vehicles at a low inner pressure. That is, the tyre of the invention is useable on the rear wheels as well as on the front wheels.

Test tyres 1 and 2 of the present invention in size of 21/OR 14.5 having the structures as shown in Figure 1 and described in Table 1 were made for trial, together with comparative Examples 1 to 5 and tested. The comparative Example 1 is a conventional tyre. The cornering power was evaluated by a drum tyre tester.

The test conditions were: Front Tyre Rear Tyre Load 525kg 725keg Inner pressure 2.6kg/sq.cm 4.25kg/sq.cm The steering stability and ride comfort of the tyre were evaluated by feel by a skilled test driver. The examples of the invention were found to be improved in steering stability and ride comfort compared to the comparative examples. The tyre of the invention may be employed both on the rear wheels and front wheels, without deteriorating steering stability or ride comfort, in addition, the original properties of a low profile tyre used under high inner pressure and high load are not hampered.

TABLE 1

Working Exs. Comparative Examples Ex.1 Ex.2 Ex.1 Ex.2 Ex.3 Ex.4 Ex.5 Tread Rubber JIS A hardness (at 20 degrees C) 61 62 64 65 64 60 65 Belt Cord Sectional area (sq.mm) 0.208 0.342 0.342 0.342 0.342 0.342 0.185 Angle to equator 22 26 22 22 27 22 24 (degrees) Carcass Intial modulus of elasticity (kgf/sq.mm) 300 900 400 400 400 950 280 Ratio R1/R2 1.10 1.4 0.70 0.70 0.81 1.50 1.00 CP Front wheel 98 100 105 108 105 85 105 (kgf/deg) Rear wheel 120 120 120 120 108 106 110 Test Difference between 22 20 5 12 20 20 15 Results front & rear wheels (note 1) Stearing Over steering Good Good Bad Not Not Good Bad Good Good Stability stringency (note 2) 108 110 100 102 105 105 103 Ride comfort 120 110 100 105 108 110 105 Durability 100 100 100 100 100 86 102 Note 1: Larger difference is better.

Note 2: indicated by an index based on the assumption that the comparative is regarded as 100, where the larger the value, the better the performance.

Claims

1. A tyre for commercial vehicles comprising a carcass (6) composed of at least one carcass ply (6) containing carcass cords, extending under a tread (5) and through sidewalls (4) to bead regions (3) where the ply (6) is wrapped around bead cores (2), and a belt (7) composed of at least to belt plies (10,11) each containing belt cords of high modulus material wherein the carcass cords (6) are organic fibre cords of which the initial modulus of elasticity is in the range from 300 to 900 kgf/sq.mm; the sectional area of said belt cords is more than 0.2 sq.mm and less than 0.3s sq.mm and said belt cords are laid at an angle of 22 degrees to 26 degrees with respect to the equator of the tyre; the tread (5) comprises rubber compound it a JIS A hardness of more than 61 degrees and less than 64 degrees; and the aspect ratie (H/W) of the tyre is more than 0.45 and less than 0.65.

2. A tyre according to claim 1 wherein the maximum width position (b) where the overall width of the carcass is a maximum is located from the bead base b a distance (111) in the range of more than 0.4 times and less than 0.55 times the sectional height (H1) of the tyre; the profile of the said carcass (6) is composed of an upper region (Cl) bulged outwardly between said maximum width position (b) and a belt outer edge point (a) and a lower region (C2) bulged outwardly between said masimum width position (b) and a flex point (c) where the said belt outer edge point (a) is defined as the point where a radial line (uke) radially drawn from the axial outer wedge (e) of the narrowest belt ply intersects with said carcass and said flex point (c) is defined as the point spaced a distance (H3) from the bead base which is 1.6 times the flange height (H2) of the rim on which the tre is mounted; said upper region (Cl) and lower region (C2) are each formed by single radii of curvature (R1) and (R2) respectively; and the ratio of (Rl/R2) of the radius (R1) of said upper region (Cl) and the radius (R2) of said lower region (C2) is more than 1.1 and less than 1.4.

3. A tyre accordin to Claims 1 and 2 wherein the belt cords comprise steel filaments.