Vehicle tyre

Abstract: A vehicle tyre which has an aspect ratio of less than about 55% and comprises a tread portion (5) having a profile whose radius of curvature decreases substantially continuously from the tyre equator (C) to each tread edge (E). The tread profile from the tyre equator (C) to each tread edge (E) is substantially defined by a curve such as an ellipse, a cycloid, an epicycloid, an involute curve and the like, of which equation is differentiable in the range between the tyre equator and each tread edge.

Description: The present invention relates to a vehicle tyre more particularly to an improved tread portion suitable for low aspect ratio tyres being capable of improving the wear resistance and steering stability. Recently, low aspect ratio tyres are widely used in passenger cars especially high performance cars. In general, if the radius of curvature of the tread is increased, the steering stability during high speed straight running may be improved. But, as the tread camber decreases, and also a camber angle is given by wheel alignment, the ground pressure is increased in the shoulder portion during cornering. Accordingly, uneven wear is liable to occur in the shoulder portion. Further, the high speed durability decreases when the tyre is mounted on the wheel with a relatively large camber angle. Therefore, in order to solve these problems, a relatively large camber is given to the shoulder portion by forming the shoulder portion with an arc having a small radius of curvature of about 200 to 300 mm whereas the tread crown portion is formed by an arc having a larger radius of curvature of not less than 1000 mm. That is to say, the tread profile is formed by a double radius curve. In such a profile, even if the two arcs, crown arc and shoulder arc are connected with each other without forming any inflection point, the ground pressure increases in the vicinity of the connecting point between the arcs, and as a result uneven wear is liable to be caused, and sometimes the steering stability during high speed straight running and cornering performance are deteriorated. In EP 0 658 450 a tyre is disclosed having a profile the radius of curvature of which decreases substantially continuously from the tyre equator to the tread edges as in the preamble of the main claim of the present invention. It is therefore, an object the present invention to provide a vehicle tyre in which the ground pressure distribution is made even to improve the wear resistance and running performance. Accordingly one aspect of the present invention provides a vehicle tyre as set out in claim 1. Here, the tread edges are defined as the axially outermost edges of the ground contacting patch of the tyre which is mounted on a standard rim and inflated to a standard pressure and loaded with a standard load. The standard rim is the "Standard Rim" specified in JATMA, "Measuring Rim" in ETRTO, "Design Rim" in TRA or the like. The standard pressure is the maximum air pressure in JATMA, "Inflation Pressure" in ETRTO, maximum pressure given in the "Tyre Load Limits at Various Cold Inflation Pressures" table in TRA or the like. The standard load is 88% of the maximum load, which is the maximum load capacity in JATMA, "Load Capacity" in ETRTO, maximum value given in the above-mentioned table in TRA or the like. Embodiments of the present invention will now be described in detail in conjunction with the accompanying drawings. Fig.1 is a cross sectional view of a tyre according to the present invention showing an example of the internal tyre structure; Fig.2 is a diagram for explaining an elliptic equation; Fig.3 is the locus of an elliptic equation for the tread curve; Fig.4 is a diagram for explaining an epicycloid equation; Fig.5 is the locus of an epicycloid equation for the tread curve; Fig.6 is a diagram for explaining a cycloid equation; Fig.7 is the locus of a cycloid equation for the tread curve; Fig.8 is a diagram for explaining a method of forming a tread curve by multiple arcs to approximate the loci; Fig.9 shows footprints of test tyres according to the invention; Fig.10 shows footprints of a test tyre according to a prior art; Fig.11 is a graph showing the ground pressure distributions of the test tyre; Fig.12 is a diagram for explaining an involute curve of which the base circle is an ellipse; Fig.13 shows an example of the tread profile based on an involute curve; Fig.14 shows a tread profile which approximates the involute curve of Fig.13; Fig.15 shows the tread profile of a test tyre according to a prior art; Fig.16 shows the tread profile of reference tyre 1; Fig.17 shows the tread profile of reference tyre 2; and Figs.18 and 19 show footprints of test tyres. In the drawings, the vehicle tyres according to the present invention are pneumatic radial tyres for passenger cars. The aspect ratio (section height H/section width W) is not more than 55% in this example 45%. As shown in Fig.1, the tyre comprises a tread portion 5, a pair of sidewall portions 4, a pair of bead portions 3 each with a bead core 2 therein, a carcass 6 extending between the bead portions 3 through the tread portion 5 and sidewall portions 4, and a belt 7 disposed radially outside the carcass 6 in the tread portion 5. The carcass 6 is composed of at least one ply in this example a single ply of cords arranged radially at an angle of 70 to 90 degrees with respect to the tyre equator C. For the carcass cords, organic fibre cords, for example nylon, polyester, rayon, aromatic polyamide and the like are used. The belt 7 comprises at least one ply of parallel steel cords laid at an angle of 10 to 35 degrees with respect to the tyre equator C. In this example two crossed plies 7A and 7B called steel breakers are disposed. Aside from steel cords, high elastic modulus organic fibre cords made of rayon, aromatic polyamide or the like can be used. Further, it is possible to dispose a band ply 9 radially outside the breaker 7A, 7B so as to cover at least axially outer edges of the breaker to prevent their lifting during high speed running. The band ply 9 is preferably made of relatively low modulus organic fibre cords, such as nylon, and the cord angle is 0 to 5 degrees with respect to the tyre equator C. The tread portion 5 is usually provided with tread grooves for water drainage although such grooves are not shown in Fig.1. In the meridian section of the tyre under the standard condition, the tread profile FL is defined by a curve L whose radius decreases substantially continuously from the tyre equator PC to each tread edge E. Usually, the tread profile FL is symmetrical about the tyre equatorial plane C, but it is possible to design the profile to be asymmetrical if necessary. Here, the standard condition is that the tyre is mounted on the standard rim J and inflated to its standard pressure but loaded with no tyre load. The above-mentioned tread curve L is defined by an elliptic function, a cycloid function preferably epicycloid function, an involute function or the like. In such a function, accordingly, the continuously decreasing radius means that the equation is differentiatable in the range of the variables (x and y) corresponding to the actual range of the tread profile between the tyre equator Pc and each tread edge E. Taking a typical size of 235/45R17 for low aspect ratio pneumatic tyres for passenger cars, a way of applying such a function to the actual tread profile will be explained more concretely. First, the tread camber Y is determined to satisfy that the arctangent of the tread camber YE at the tread edge E and a half tread width (TW/2) becomes equal to the actual camber angle in use which is usually about 3 to about 5 degrees for example about 4 degrees; and that the arctangent of the tread camber YC at the TW/4 point P1 and a quarter tread width (TW/4) is in the range of from about 1 to about 2 degrees for example about 2 degrees. The tread width TW is the axial distance between the above-explained tread edges E. In the above-mentioned tyre size, for example, the camber YE at the tread edge E is 7.6 mm, the camber YC at the mid point P1 is 1.3 mm, and a half tread width TW/2 is 100 mm, Elliptic Curve Fig.2 shows an ellipse, the basic elliptic equation of which is x 2 a 2 + y 2 b 2 =1 a > b . In order to apply the elliptic equation to the tread curve L, the x-y axes are shifted as follows. The x-axis is an axial line extending from the tyre equator PC towards the tread edge E. The y-axis is a radial line extending radially inwardly from the tyre equator PC. The origin is thus the tyre equator PC. In this co-ordinate system, the tread curve L must pass on two points of which x and y values are (x=50, y=1.3) and (x=100, y=7.6) because the camber values YC and YE are determined as above. Hence, the equation of the tread curve L may be written as x 2 104.2 2 + (10.63- y ) 2 10.63 2 =1 The locus of this equation 2 which is described when the x value varies from 0 to 100 is used as each half of the tread curve L extending from the tyre equator PC to each tread edge E. Thus tread profile is symmetrical about the tyre equator. The tread camber Y (mm) at an axial position X (mm) is as follows, and the locus is shown in Fig.3. X 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Y 0 0 0.1 0.2 0.3 0.5 0.6 0.8 1 1.3 1.6 1.9 2.3 2.8 3.3 3.8 4.5 5.3 6.3 7.6 Epicycloid Curve Fig.4 shows an epicycloid. The epicycloid is the locus of a point N set on the circumference of a circle (d) rolling on the circumference of a fixed base circle (c) of which centre is placed on the origin O of x-y co-ordinates. The basic epicycloid equation is x = ( r + R )cos + R cos' y = ( r + R )sin + R sin' wherein  is the angle between the x-axis and a straight line drawn between the centre of the rolling circle and the centre of the fixed base circle, and ' is the angle between the x-axis and a straight line drawn between the point N and the centre of the rolling circle. In Fig.4, the angles  and ' are counted counterclockwise from the x-axis. Since it is a premise that there is no slip between the rolling circle and base circle, we may write From this equation, k = r / R ' = ( k + 1) - π 2 k By the equations 6 and 3, To apply this equation to the actual tread curve L, the x-y axes are shifted in the same way as the above-mentioned elliptic function. By the equation 7, the tread camber Y may be written as Y = A × ( r + 2 R - y ) ( A = constant) As explained above, the tread curve L must pass on two points of which x and y values are (x=50, y=1.3) and (x=100, y=7.6). Hence, from the equations 7 and 8 and these conditions, the tread curve L may be written in the shifted x-y co-ordinate system as In this example, k =0.4, but it is possible to set at different value. The locus of this equation (x = 0 to 100) is used as each half of the tread curve L from the tyre equator PC to each tread edge E. Thus the tread profile is symmetrical about the tyre equatorial plane. The tread camber Y (mm) at an axial position X (mm) is as follows and the locus is shown in Fig.5. X 0.1 9 21.5 33.7 45.4 56.4 66.7 75.9 84 90.9 96.5 100 Y 0 0 0.2 0.6 1.1 1.7 2.5 3.4 4.3 5.4 6.6 7.6 Cycloid Curve Fig.6 shows a cycloid. The cycloid is the locus of a point N set on the circumference of a circle (h) rolling on a straight line (i). The basic cycloid equation is x = π· r - r (- sin) y = r (1 + cos) wherein r is the radius of the rolling circle (h),  is the angle between the y-axis and a straight line drawn between the point N and the centre of the rolling circle. The x-axis is an axial line extending from the tyre equator PC towards each tread edge E. The y-axis is a radial line extending radially inwardly from the tyre equator PC. The origin is thus the tyre equator PC. There is no need to shift the x-y axes as in the former two cases. By the equation 10, the tread camber Y may be written as Y = A × r (1 + cos) ( A = constant) From the equations 10 and 11 and the conditions that the tread curve L must pass on two points (x=50, y=1.3) and (x=100, y=7.6), the tread curve L may be written as x = 31.85 × π - 31.85( - sin) y = 0.019 × 31.85(1 + cos) The locus of this equation is shown in Fig. 7. The tread camber Y (mm) at an axial position X (mm) is as follows. X 0.1 9 21.5 33.7 45.1 55.7 65.3 73.7 80.9 86.9 91.5 95 97.4 98.9 99.7 100 Y 0 0 0.2 0.5 1 1.6 2.2 2.9 3.7 4.4 5.2 5.8 6.4 6.9 7.3 7.6 According to the present invention, the tread curve L must be decreased in radius continuously from the equator to the tread edges. However, it is not always necessary to strictly follow the locus of the above-explained equations. Slight aberrations may be permitted. In other words, as shown in Fig.8, a curve composed of multiple arcs so as to approximate the locus may be used as far as the aberrations are small for example within plus minus 1/10 mm. This will be advantageous to making the tyre vulcanising mould. If importance is attached to such easiness of making the mould, the number of the arcs is in the range of from about 7 as shown in Fig.8 to about 18. Comparison Test The following test tyres were made and the ground pressure distribution was measured under the camber angle of 0 degree and 4 degrees. Test tyres: Ex.1 of which the tread profile is based on the epicycloid equation 9; Ex.2 based on the elliptic equation 2; and Conventional tyre of which the tread profile is defined by a triple radius arc composed of a crown arc Rc (1100 mm), middle arc Rm (400 mm) and shoulder arc Rs (100 mm). Tyre size 235/45R17 Wheel rim size 17X8JJ (standard rim) Inner pressure 230 KPa Tyre load 400 kgf Tread camber (X=50mm Y=1.3mm) and (X=100mm Y=7.6mm) The footprints obtained by the tests are shown in Figs.9 and 10, wherein the area of which the ground pressure was not less than 5.0 kgf/sq.cm is shaded. Fig.11 shows the area in percentage to the total ground contacting area as a function of ground pressure. As apparent from Figs.9 and 10, in the tyres according to the invention, the area of which the ground pressure is not less than 5.0 kgf/sq.cm is small in comparison with the conventional tyre. Further, as shown in Fig.11 in the tyres according to the invention, the ground pressure of the widest area was 4.75 kgf/sq.cm, which is remarkably lowered when compared with the conventional tyre which showed 5.25 kgf/sq.cm. Thus the occurrence of uneven wear can be effectively prevented. Further, Example tyre 1 was installed on a Japanese FR car with a 2500cc turbocharged engine, and tested for straight running stability, lane change stability, cornering performance, and high speed durability according to ECE30 (at camber angles = 0 and 4 degrees). The test results are as follows. Tyre Ex.1 Conv. Straight running stability 8 6 Lane change stability 7 6 Cornering performance 8 6 High speed durability Camber angle = 0 deg. 110 100 Camber angle = 4 deg. 98 100 The first three performances were evaluated into ten ranks wherein the standard is six. The high speed durability is indicated by an index based on that the conventional tyre is 100. The larger the value, the better the performance. From the tests, it was confirmed that Example tyre 1 was remarkably improved in the straight running stability and cornering performance when compared with the conventional tyre. Further, using Japanese 2500cc FR cars provided with Example tyre 1 and Conventional tyre, the average wear was measured after running for 9000 km to obtain the running distance per 1 mm wear. Further, the shoulder wear (wear in shoulder portion/wear in crown portion) was also measured. The test results are as follows. Tyre Ex.1 conv. Running distance (km) per 1 mm wear 103 100 Shoulder wear 105 100 In this table, an index based on that the conventional tyre is 100 is used. The larger the value, the better the performance. Involute Curve Fig.12 shows still another embodiment of the present invention, wherein the tread profile is such that the centre of the radius R of the tread curve L describes an ellipse D. That is, the tread curve L is an involute curve of which the base circle is an ellipse D. The involute curve is the locus which is described by one end of a fixed length thread 10 of which the other end is fixed to the origin O when the thread 10 is wound around the ellipse (or unwound) without slack. In this example, in the same way as the former examples, the radius R decreases continuously from the tyre equator PC to the tread edge E. In this case, it is possible or easy to determine the tread profile by fist giving radii of curvature to two positions such as the tyre equator and tread edge. To obtain an equation of the ellipse D, x-y co-ordinates are set as follows. The origin O is on the equatorial plane C at a predetermined distance from the tyre equator PC which distance corresponds to the radius Rc of the tread curve L at the tyre equator PC. The x-axis extends axially outwardly from the tyre equator C in parallel with the tyre axis. The y-axis extends radially outwardly along the equatorial plane C. In this co-ordinate system, the elliptic equation is ( x-a ) 2 a 2 + y 2 b 2 =1 wherein y ≥ 0 x = 0 ~ a a = constant b = constant a ≠ b . In this case, the radii of curvature at two points are determined in the first place. For example, when the tyre size is 235/45R17 and the tread width TW = 206 mm, the radius Rc at the tyre equator PC is set as 1250 mm, the radius at the tread edge E is set as 52 mm. From the equation 13, the ellipse D as the base circle of involute curve is ( x -115.2) 2 115.2 2 + y 2 1231.9 2 =1 By this equation, the tread camber YE at the tread edge is given as 8.6 mm. The tread curve L described is shown in Fig.13. Fig.14 show another example of the tread curve L which is composed of multiple arcs to approximate the tread curve shown in Figl3. In this example, a half tread curve is composed of nine arcs and the whole curve is composed of seventeen arcs. In this case, as explained above, the making of the vulcanising mould becomes easier. Aside from the above-explained accurate involute curve, such a pseudo involute curve that the length of the thread 10 is for example longer than the original length measured from the origin O to the point Pe (x=a, y=b) along the circumference of one quarter of the ellipse D. Further, it is also possible to set slightly shorter. In other words, it is not always necessary that the length of the thread 10 is equal to the curve length between the origin and the point Pe. Comparison Test The following test tyres were made and the ground pressure distribution was measured. Test tyres: Exs.11 and 12 of which the tread profile is defined by an involute curve; Ex.13 of which the tread profile is defined by a curve composed of nine arcs shown in Fig.14; Conventional tyre of which the tread profile is defined by a curve composed of three arcs shown in Fig.15; Ref.11 of which the tread profile is defined by a curve composed of eight arcs shown in Fig.16; Ref.12 wherein as shown in Fig.17 a central tread part extending 30 mm from the tyre equator is defined by a constant radius of 1100 mm, and the axially outer part thereof is defined by a variable radius R which is R = 7 × 10 -4 × e (2.5×10 -2 × X ) + 7.3 × 10 -5 × X -1 Tyre size 235/45R17 Tread width TW = 206 mm Wheel rim 17X8JJ (standard rim) Inner pressure 230 KPa Tyre load 400 kgf Camber angle 0 and 3 degrees The footprints of the test tyres are shown in Figs.18 and 19, wherein the area of which the ground pressure was not less than 5.0 kgf/sq.cm is blacked out. The narrower the blacked area, the better the ground pressure distribution. Further, in the same way as explained above, the tyres were tested for the straight running stability, lane change stability, cornering performance, wear resistance, high speed durability, and rolling resistance. The results are as follows. Tyre Ex. 11 Ex. 12 Ex. 13 Ref. 11 Ref.12 conv. Rc (mm) 1250 1127.5 1250 - - - a (mm) 115.2 115.2 - - - b (mm) 1231.9 1111.17 - - - Stability Straight running 7 5 7 5 6 5 Lane change 6 6 6 5 5 5 Cornering 6 6 5 5 5 5 Wear resistance Running distance /1mm wear 106 103 105 100 98 100 Shoulder wear 105 110 103 100 95 100 ECE30 High speed durability camber angle = 0 deg. 107 110 107 103 103 100 camber angle = 4 deg. 103 108 102 99 96 100 Rolling resistance 96 94 97 100 97 100 From the tests, it was confirmed that Example tyre 11 was greatly improved in the straight running stability, cornering performance and wear resistance in comparison with conventional tyre. Furthermore, the rolling resistance was decreased.

Claims: A vehicle tyre having an aspect ratio of 55 % or less, comprising a tread portion (5) and having a profile whose radius of curvature decreases substantially continuously from the tyre equator (C) to the tread edges (E), characterized in that the arctangent tan -1 (YE/(TW/2)) of a quotient (YE/(TW/2) of a tread camber (YE) at the tread edge (E) divided by one half (TW/2) of a tread width (TW) is in the range of from 3 to 5 degrees, wherein the tread width (TW) is the axial distance between the tread edges (E), and the tread camber (YE) is the radial distance between the tread edge (E) and the tread center (PC), and the arctangent tan -1 (YC/(TW/4)) of a quotient (YC/(TW/4)) of a tread camber (YC) at a middle point (P1) divided by one quarter (TW/4) of the tread width (TW) is in the range of from 1 to 2 degrees, wherein the middle point (P1) is on the surface of the tread portion at an axial distance of one quarter (TW/4) of the tread width (TW) from the tire equator (C), and the tread camber (YC) is the radial distance between the middle point (P1) and the tread center (PC). A vehicle tyre according to claim 1, characterised in that the tread profile in the range from the tyre equator to each said tread edge is defined by a curve the equation of which is differentiable in said range. A vehicle tyre according to claim 1, characterised in that the tread profile in the range from the tyre equator to each said tread edge is substantially defined by a curve which is one of an elliptic curve, a cycloid curve, an epicycloid curve and an involute curve. A vehicle tyre according to claim 1, characterised in that the tread profile in the range from the tyre equator (C) to each said tread edge (E) is defined by a curve which is substantially an elliptic curve, and in the following x-y co-ordinates: the origin is set on the tyre equator; the x-axis extends from the tyre equator towards each tread edge in parallel with the axis of the tyre; and the y-axis extends radially inwards from the tyre equator, the equation of the elliptic curve is x 2 A 2 + ( B - y ) 2 B 2 =1 wherein A = constant B = constant A > B . A vehicle tyre according to claim 1, characterised in that the tread profile in the range from the tyre equator (C) to each said tread edge (E) is defined by a curve which is substantially a cycloid curve, and in the following x-y co-ordinates: the origin is set on the tyre equator; the x-axis extends from the tyre equator towards each tread edge in parallel with the axis of the tyre; and the y-axis extends radially inwards from the tyre equator, the equation of the cycloid curve is x = π r - r ( - sin) y = r (1+ cos) wherein r is the radius of the rolling circle of the cycloid,  is the angle between the y-axis and a straight line drawn between the centre of the rolling circle and a fixed point on the circumference of the rolling circle which describe the cycloid curve. A vehicle tyre according to claim 1, characterised in that the tread profile in the range from the tyre equator to each said tread edge is defined by a curve which is substantially an epicycloid curve, and in the following x-y co-ordinates: the origin is set on the tyre equator; the x-axis extends from the tyre equator towards each tread edge in parallel with the axis of the tyre; and the y-axis extends radially inwards from the tyre equator, the equation of the epicycloid curve is wherein R = the radius of the rolling circle of the epicycloid r = the radius of the base circle of the epicycloid k = r / R  = the angle of a straight line drawn between the centres of the rolling circle and base circle formed with the x-axis. A vehicle tyre according to claim 1, characterised in that the tread profile in the range from the tyre equator to each said tread edge is defined by a curve which is substantially an involute curve of which the base circle is an ellipse, and in the following x-y co-ordinates: the origin is on the equatorial plane (C) at a distance from the tyre equator (PC) which corresponds to the radius Rc of the tread curve at the tyre equator (PC); the x-axis extends in parallel with the tyre axis toward each side of the equatorial plane; and the y-axis extends radially outwardly along the equatorial plane, the elliptic equation of the base circle is ( x - a ) 2 a 2 + y 2 b 2 =1 wherein a = constant b = constant a ≠ b . A vehicle tyre according to claim 7, characterised in that said constants (a) and (b) are such values that the length measured from the point (x=a, y=b) to the origin along the elliptic curve is equal to the radius Rc.

Inventors: AOKI CHIEKO, IWASAKI NAOAKI, AOKI, CHIEKO, IWASAKI, NAOAKI
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1998
Main classification: B60C11
IPC codes: B60C11