Radial tyre

Abstract: A radial tyre improved in ground pressure distribution, in which the thickness (A) of the tread rubber at the tyre equator is larger than the thickness (B) at an effective belt edge point (P), the belt edge camber height (CH) is in the range of from 0.5 to 0.75 times the shoulder camber height (SH), and the belt edge camber height (CH) is in the range of from 0.5 to 0.75 times the carcass camber height (KH), where the belt edge camber height (CH) is the radial distance between the effective belt edge point (P) and an axially inner point (Q) located axially inward of the point P by an axial distance of 0.2 times the effective belt width (BE); the tread shoulder camber height (SH) is the radial distance between an axially outer point (R) located at the same axial position as the point (P) and an axially inner point (S) located at the same axial position as the point (Q), both on the tread surface; and the carcass camber height (KH) is the radial distance between an axially outer point (T) located at the same axial position as the point (P), and an axially inner point (U) located at the same axial position as the point (Q), both on the carcass thickness centre line.

Description: The present invention relates to a radial tyre, in which uneven wear such as shoulder wear and heel-and-toe wear is effectively reduced. In belted radial ply tyres, especially those provided with a stiff steel belt or breaker, the ground pressure distribution is liable to become uneven. As a result, uneven wear may occur between the tread crown and tread shoulders. In general, the amount of wear in the tread shoulder is larger than that of the tread crown. Such uneven wear is called shoulder wear. Shoulder wear is caused by a difference in the tyre diameter between the tread crown and tread shoulders due to a curved tread profile. This causes slip against the ground, and the tread shoulders are worn earlier than the tread crown. Further, so called heel and toe wear is liable to occur in the tread shoulder, which wear is an uneven wear between the front and rear edges of tread elements such as blocks. Such uneven wear can be reduced by making the ground pressure distribution even across the whole of the ground contacting width thereby making the slip distribution even. It has been considered that uneven wear would be reduced by increasing the radius of curvature of the tread profile. However, it was found that if the radius of curvature is increased excessively then the ground pressure is increased in the tread shoulder and as a result shoulder wear increases during cornering, and heel and toe wear increases in the tread crown portion during straight running. On the other hand, as counter measures to such an uneven ground pressure distribution, many propositions have been made to provide a specific radius of curvature for the belt. For example, see US-A-4442879, US-A-4702293, US-A-4842682, CA-A 2018217, EP-A 323519, EP-A 490596, and JP-A 58-170602. It was however, found that even when the belt radius is specifically defined as proposed, the ground pressure distribution is not necessarily even. For example, it was often observed that even when the belt edge portion is decreased in radius of curvature in comparison with the centre portion, the ground pressure is in fact increased in the shoulder portion, and uneven wear is not improved. It is therefore, an object of the present invention to provide a radial tyre, in which the ground pressure distribution is made even to effectively reduce uneven wear of the shoulder portion. According to the present invention, a radial tyre comprises a carcass extending between bead portions and turned up around bead cores, a belt disposed radially outside the carcass and including a breaker belt composed of a widest ply and a second widest ply crossing each other and defining a belt thickness centre line for the two plies, said second widest ply defining the effective belt width BE and a tread rubber disposed radially outside the belt, characterised in that the thickness A of the tread rubber at the tyre equator CF is larger than the thickness B at an effective belt edge point P, and a belt edge camber height CH is in the range of 0.5 to 0.75 times a shoulder camber height SH, and further the belt edge camber height CH is in the range of 0.5 to 0.75 times a carcass camber height KH, where the said thicknesses of the tread rubber are the radial distances between the radially outermost surface of the belt and the tread surface, the said effective belt edge point P corresponds to the axially outermost edge of the effective belt width BE, the said belt edge camber height CH is the radial distance between said effective belt edge point P and an axially inner point Q, both on the belt thickness centre line, said point Q is located axially inwards of the point P by an axial distance of 0.2 times the effective belt width BE, the said tread shoulder camber height SH is the radial distance between an axially outer point R and an axially inner point S, both on the tread surface, said point R is located at the same axial position as the point P, said point S is located at the same axial position as the point Q, and the said carcass camber height KH is the radial distance between an axially outer point T and an axially inner point U, both on the carcass thickness centre line, said point T is located at the same axial position as the point P and said point U is located at the same axial position as the point Q. During running of a tyre, the tread rubber is repeatedly compressed in the shoulder portions, and thus the thickness thereof is repeatedly changed. As shown in Fig.4, in the ground contacting patch of a tyre, the tread rubber is compressed in the shoulder portion, and normally the thickness thereof is increased to increase the ground pressure in this portion. In the present invention, however, the tread rubber thickness (B) in the tread shoulder portion is set to be smaller than the tread rubber thickness (A) at the tyre equator in advance. Therefore, increase in the ground pressure in the shoulder portion due to the increased rubber thickness is prevented. Further, as shown in Fig.5, the edge of a belt 7 is repeatedly deformed during running. If the CH/SH camber ratio is more than 0.75, the belt edge becomes excessively rounded, and a severe stress is liable to occur in the belt edge, and as a result uneven wear is promoted further by the resultant heat generation. In order to minimise the stress in the belt edge, ideally, the deformed state in the ground contacting patch and the undeformed normal state are designed to be substantially symmetrical about an axial straight line (n) as shown in Fig.5, which is accomplished by setting the CH/SH camber ratio at about 0.5. However, if the CH/SH camber ratio is less than 0.5, the belt edge becomes excessively flat, and the belt edge is liable to be exposed in the bottom of the tread shoulder grooves. Furthermore, as the CH/SH camber ratio is from 0.5 to 0.75 and also the CH/KH camber ratio is from 0.5 to 0.75, the shoulder camber height SH is not very much differed from the carcass camber height KH. As a result, in the ground contacting patch in which the carcass is deformed into a flat form, the carcass becomes almost parallel with the road surface, which can improve the ground pressure distribution. An embodiment of the present invention will now be described in detail in conjunction with the accompanying drawings, in which: Fig.1 is a cross sectional view of an embodiment of the present invention; Fig.2 is an enlarged cross sectional view of the tread portion thereof; Fig.3 is a cross sectional view schematically showing another example of the breaker belt; Fig.4 is a cross sectional view explaining the ground contacting state of a tyre; Fig.5 is a cross sectional view explaining the movement of a breaker belt edge; and Fig.6 is a graph showing the belt edge camber height CH/shoulder camber height SH ratio, and the belt edge camber height CH/carcass camber height KH ratio of each test tyre. In Figs.1 and 2, a radial tyre 1 according to the present invention comprises a tread portion 2, a pair of axially spaced bead portions 4, a pair of sidewalls 3 extending between the tread edges and the bead portions 4, a pair of bead cores 5 one disposed in each bead portion 4, a carcass 6 extending between the bead portions 4, a belt disposed radially outside the carcass 6, and a tread rubber 10 disposed on and radially outside of the carcass 6. The carcass 6 consists of an inner ply 6A and an outer ply 6B, each turned up around the bead cores 5 from the axially inside to the outside of the tyre to be secured thereto. The turned up edge 6A1 of the inner ply 6A is terminated radially outwards of the bead core 5, and the turned up edge 6B1 of the outer ply 6B is terminated radially outwards of the turned up edge 6A1 near the maximum width point of the tyre. Each carcass ply is composed of cords arranged radially at 60 to 90 degrees with respect to the tyre equatorial plane CF to give a radial or semiradial structure. For the carcass cords, organic fibre cords, e.g. nylon, rayon, polyester or the like or steel cords can be used. Between the carcass main portion and each turned up portion in each bead portion 4, a bead apex 8 made of a rubber composition tapered radially outwardly from the bead core 5 is disposed. The belt in this embodiment comprises a breaker belt 7 disposed on the radially outside of the carcass 6, and a band belt 9 disposed on the radially outside of the breaker belt 7. The breaker belt 7 comprises a radially inner ply 7A and an radially outer ply 7B. Each belt ply 7A, 7B is composed of parallel steel cords laid at angle of 10 to 30 degrees with respect to the tyre equator so as to cross the next ply. The outer belt ply 7B or second widest ply is set at substantially the same width as the ground contacting width of the tread. The width W7A of the inner belt ply 7A is about 1.01 to 1.20 times the width W7B of the outer belt ply 7B. The band belt 9 in this example is of the so called edge band type consisting of a pair of axially spaced narrow plies of cords. The cords are laid substantially parallel to the tyre equatorial plane CF at an angle of 0 to 10 degrees, preferably 0 to 5 degrees with respect to the tyre equatorial plane CF. Each narrow ply has a width W9 of 15 to 40 % of the width W7A of the widest belt ply 7A, and is disposed so as to cover all the edges of the inner and outer belt plies 7A and 7B. For the band cords, extensible organic fibre cords, e.g. nylon, polyester, rayon or the like are used. Further, a wedge-shaped breaker cushion rubber 11 is disposed between each belt edge and the carcass 6 so that the distance between the belt and the carcass increases towards the belt edge. In the example in Figs.1 and 2, each of the inner and outer breaker plies 7A and 7B is a so called full-width cut-edge ply. However, it is possible to use another belt structure, for example, a combination of a radially outer cut-edge ply 7B and a radially inner folded edge ply 7A the edges of which are folded back upon the outer ply 7B as shown in Fig.3. The radial tyre 1 is mounted on its regular rim R and inflated to its standard inner pressure, where the regular rim R is that rim officially approved for the tyre by TRA (USA), ETRTO (Europe), JATMA (Japan) or the like, and the standard inner pressure is 75% of the maximum air pressure for the tyre officially specified by the same associations. Under such standard conditions, the following are specifically defined. The effective width BE of the breaker belt 7 is defined as the maximum width of a portion in which at least two plies are overlapped with each other, that is, the second widest width. Therefore, in Figs.1 and 2 example, the effective belt width BE is that of the radially outer ply 7B. In Fig.3, the effective belt width BE is that of the radially outer ply 7B. Further, the belt edge camber height CH is defined as the radial distance between the effective belt edge point P and an axially inner point Q, both on the belt thickness centre line, wherein the effective belt edge point P is the axially outermost edge of the effective belt width BE, the point Q is located axially inwards of the point P by an axial distance of 0.2 times the effective belt width BE, and the belt thickness centre line is defined as formed by the two breaker plies which are the widest ply 7A and the second widest ply 7B defining the above-mentioned overlapped portion. The belt profile which is the belt thickness centre line is curved so that a central portion thereof has a larger radius of curvature than the edge portions thereof, and the belt edge camber height CH is in the range of 0.05 to 0.4 times the effective belt width BE. If the belt edge camber height CH is less than 0.05 times the effective belt width BE, the belt edge becomes excessively flat, and the tread rubber thickness around the belt edge becomes very small. As a result, at the end of tyre wear life, the belt edge projects at the tread surface, even though tread grooves remain in the tread crown portion. If the belt edge camber height CH is more than 0.4 times the effective belt width BE, the belt edge becomes excessively rounded. As a result, movement of the belt edge increases and the uneven wear in the tread shoulder portion is promoted. Further, the carcass 6, and the tread surface 2A are provided with curved profiles satisfying the following conditions. The belt edge camber height CH is more than 0.5 times but less than 0.75 times the shoulder camber height SH, and further the belt edge camber height CH is more than 0.5 times but less than 0.75 times the carcass camber height KH. The tread shoulder camber height SH is defined as the radial distance between an axially outer point R and an axially inner point S, both on the tread surface 2A, wherein the point R is located at the same axial position as the point P, and the point S is located at the same axial position as the point Q. The carcass camber height KH is defined as the radial distance between an axially outer point T and an axially inner point U, both on the carcass thickness centre line, wherein the point T is located at the same axial position as the point P, the point U is located at the same axial position as the point Q, and the carcass thickness centre line is defined being that as formed by all the carcass plies. Furthermore, the tread shoulder camber height SH is preferably set at substantially same value as the carcass camber height KH. That is, the CH/SH ratio and the CH/KH ratio are preferably set at substantially the same value. Still furthermore, the thickness (B) of the tread rubber 10 at the effective belt edge point P is set at a smaller value than the thickness (A) at the tyre equator CF, wherein the tread rubber thickness is defined as the distance measured from the radially outer surface of the belt to the tread surface 2A in the radial direction of the tyre. Test tyres of size 225/50R16 having the construction shown in Fig.1 were made and tested for shoulder wear resistance and uneven wear resistance. In the test, the test tyre was mounted on a front wheel of a FR type passenger car and run for 20,000 kilometres at maximum tyre load and regular inflation pressure. Then, the difference between the tyre diameters measured before and after the running were obtained to evaluate shoulder wear and uneven wear into four ranks. The test results are shown in Table 1. Fig.6 shows the CH/SH camber ratio and the CH/KH camber ratio of the test tyres. As described above, in a radial tyre of the present invention, the tread rubber thickness, belt edge camber height, tread shoulder camber height, and carcass camber height are specifically defined. As a result shoulder wear resistance and uneven wear resistance can be effectively improved. In addition to passenger tyres, the present invention can be applied to truck/bus tyres, light truck tyres and the like. In the present invention, the bandage belt 9 may be eliminated. Further, it is possible to employ other carcass structures. It is also possible to dispose in each bead portion a reinforcing cord layer for increasing the rigidity of the bead portion, and/or an organic cord layer called a chafer for preventing the inner ply 6A from being chafed by the bead core 5.

Claims: A radial tyre comprising a carcass (6) extending between bead portions (4) and turned up around bead cores (5), a belt disposed radially outside the carcass (6) and including a breaker belt (7) composed of a widest ply (7A) and a second widest ply (7B) crossing each other and defining a belt thickness centre line for the two plies, said second widest ply (7B) defining the effective belt width BE, and a tread rubber (2) disposed on the radially outside of the belt (7), characterised in that the thickness (A) of the tread rubber (2) at the tyre equator (CF) is larger than the thickness (B) at an effective belt edge point P, and a belt edge camber height (CH) is in the range of 0.5 to 0.75 times a shoulder camber height (SH), and further the belt edge camber height (CH) is in the range of 0.5 to 0.75 times a carcass camber height KH, where the said thicknesses of the tread rubber are the radial distances between the radially outermost surface of the belt and the tread surface, the said effective belt edge point (P) corresponds to the axially outermost edge of the effective belt width (BE), the said belt edge camber height (CH) is the radial distance between said effective belt edge point (P) and an axially inner point (Q), both on the belt thickness centre line, said point (Q) is located axially inwards of the point (P) by an axial distance of 0.2 times the effective belt width (BE), the said tread shoulder camber height (SH) is the radial distance between an axially outer point (R) and an axially inner point (S), both on the tread surface, said point (R) is located at the same axial position as the point (P), said point (S) is located at the same axial position as the point (Q), and the said carcass camber height (KH) is the radial distance between an axially outer point (T) and an axially inner point (U), both on the carcass thickness centre line, said point (T) is located at the same axial position as the point (P), and said point (U) is located at the same axial position as the point (Q). A radial tyre according to claim 1, characterised in that the tread shoulder camber height (SH) is substantially the same value as the carcass camber height (KH). A radial tyre according to claim 1 or 2, characterised in that the belt edge camber height (CH) is in the range of 0.05 to 0.4 times the effective belt width (BE).

Inventors: AOKI CHIEKO, ASANO KAZUO, AOKI, CHIEKO, ASANO, KAZUO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1993
Main classification: B60C3
IPC codes: B60C11, B60C3, B60C9