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Title:
ASYMMETRICAL TIRE TREAD AND METHOD OF MAKING SAME
Document Type and Number:
WIPO Patent Application WO/1997/046359
Kind Code:
A1
Abstract:
A tire (100) having an asymmetrical tread pattern (103) having lateral ribs (109, 111) having substantially equal circumferential surface contact areas and substantially unique contact surface ratios, and at least one rib pair (114) formed by a medial rib (113; 115) on the inner (105) and the outer side (107) of the tire, the medial ribs (113, 115) having substantially equal circumferential surface contact areas and substantially unique contact surface ratios; and a longitudinal groove (127, 129, 131, 133) located between adjacent ribs. The tire (100) is manufactured by the method steps of: removing one half of a clamshell mold from a first symmetric tire mold assembly; removing one half of a clamshell mold from a second symmetric tire mold assembly; combining the clamshell mold halves to form an asymmetric tire tread mold assembly; providing a green tire to the asymmetric tire tread mold assembly; treating the green tire; and removing the tire from the asymmetric tire tread mold assembly.

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Inventors:
EMERSON JOHN ROBERT (US)
Application Number:
PCT/US1997/009873
Publication Date:
December 11, 1997
Filing Date:
June 06, 1997
Export Citation:
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Assignee:
MICHELIN RECH TECH (CH)
EMERSON JOHN ROBERT (US)
International Classes:
B60C11/03; (IPC1-7): B29C33/00; B29D30/52
Foreign References:
US5421387A1995-06-06
US5415215A1995-05-16
US5407005A1995-04-18
US2878852A1959-03-24
US2315934A1943-04-06
DE3815829A11988-12-01
GB236762A1925-07-16
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Claims:
THAT WHICH IS CLAIMED
1. A tire having a carcass extending between a pair of beads, ends of said carcass secured to a respective one of said beads, said carcass flanked by a pair of sidewalls, a crown portion located between said sidewalls, an asymmetrical tread located radially outward of said crown portion, and a midcircumferential plane, said asymmetrical tread comprising: (a) a plurality of ribs, said ribs comprising: (1) a lateral rib on an inner side and an outer side of said tire, said lateral ribs having substantially equal circumferential surface contact areas and substantially unique contact surface ratios; and (2) at least one rib pair formed by a medial rib on said inner side and said outer side of said tire, each said medial rib disposed a substantially equal distance from said midcircumferential plane, said medial ribs having substantially equal circumferential surface contact areas and substantially unique contact surface ratios; and (b) a circumferential groove located between adjacent ribs; wherein a circumferential surface contact area of a given rib is defined as being an area of the given rib in contact with a road surface along a circumferential length of the given rib; and a contact surface ratio is defined as being a ratio between the circumferential surface contact area of the given rib and a product of a width of the given rib and the circumferential length of the given rib.
2. The tire as recited in claim 1, at least one of said grooves being an interrupted groove.
3. The tire as recited in claim 1, said asymmetrical tread further comprising a central rib disposed along said midcircumferential plane.
4. The tire as recited in claim 3, at least one of said grooves being an interrupted groove.
5. The tire as recited in claim 1, said at least one rib pair being a plurality of rib pairs.
6. A method of making an asymmetric tread tire mold assembly comprising the steps of: removing one half of a clamshell mold from a first symmetric tire mold assembly, said first clamshell mold half having a tread design thereon, said tread design providing H a surface contact area; removing one half of a clamshell mold from a second symmetric tire mold assembly, said second clamshell mold half having a tread design thereon asymmetric to said first tread design, said tread design providing a surface contact area; and combining said clamshell mold halves to form said asymmetric tread tire mold assembly.
7. The method of making an asymmetric tread tire mold assembly as recited in claim 6, further comprising: providing said surface contact area of said second clamshell mold half being equal to said surface contact area of said first clamshell mold half.
8. A method of making an asymmetric tread tire mold assembly comprising the steps of: providing a first half of a clamshell mold, said first clamshell mold half having a tread design thereon, said tread design providing a surface contact area; providing a second half of a clamshell mold, said second clamshell mold half having a tread design thereon asymmetric to said first tread design, said tread design providing a surface contact area equal to said surface contact area of said first clamshell mold half; and combining said first and second clamshell mold halves to form said asymmetric tread tire mold assembly.
9. The method of making an asymmetric tread tire mold assembly as recited in claim 8, said combining step comprises combining said first and second clamshell mold halves along a midcircumferential plane; and the method further comprises the steps of: providing said tread design of said first clamshell mold half with at least one rib; providing said tread design of said second clamshell mold half with at least one rib; and pairing said at least one rib of said first clamshell mold half with said at least one rib of said second clamshell mold half in order to form at least one rib pair, wherein each rib in said at least one rib pair is disposed a substantially equal distance from said midcircumferential plane.
Description:
ASYMMETRICAL TIRE TREAD AND METHOD OF MAKING SAME CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. provisional application serial number 60/019,233, entitled "Interchangeable Mold Parts for Tires," filed on June 6, 1996. BACKGROUND OF THE INVENTION

The present invention is directed to pneumatic tires. Specifically, the present invention is directed to tires for automobiles, light trucks or the like having a specific tread configuration.

Description of the Prior Art A properly mounted tire encounters numerous vehicle operating conditions.

Different vehicle operating conditions can subject the tires to a myriad of loads. During some situations, tires at different positions on a vehicle can experience different and diverse loads. For example, during cornering, the lateral acceleration of the vehicle transfers more load to the tires located on the vehicle to the outside of the turn. The inner and outer halves of the same tire can also experience different and diverse loads. For example, in a tire located to the outside of the turn, the outer half of the tire can experience a larger normal force than the inner half of the tire. Also, the camber angle of the wheel can produce different forces on the outer and inner halves of the tire. Various solutions have been tried to optimally handle the different loadings on the different halves of the tire, including asymmetrical tread patterns. Asymmetrical treads have been found to compensate for different operating conditions on the outer and inner halves of the tire.

The present state of manufacturing asymmetric tread tires requires complete tooling of mold parts specific to the asymmetric tread tire design. It is desirable to increase the flexibility of manufacturing asymmetric tread tires.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a tire that compensates for different operating conditions on the outer and inner halves of the tire. It is a further object of the present invention to provide an asymmetric tread tire.

It is a further object of the present invention to provide an asymmetric tread tire having balanced normal contact stresses on the outer and inner halves of the tire.

It is a further object of the present invention to manufacture an asymmetric tread tire using clamshell mold halves obtained from clamshell molds used to make two different symmetric tread tires.

These and other objects are achieved in one aspect of the present invention by a tire having a carcass extending between a pair of beads, ends of the carcass secured to a respective one of the beads; the carcass flanked by a pair of sidewalls; a crown portion located between the sidewalls; an asymmetrical tread located radially outward of the crown portion, the asymmetrical tread having lateral ribs, the lateral ribs having substantially equal circumferential surface contact areas and substantially unique contact surface ratios, and at least one rib pair formed by a medial rib on the inner and the outer side of the tire, each medial rib disposed a substantially equal distance from the midcircumferential plane, the medial ribs having substantially equal circumferential surface contact areas and substantially unique contact surface ratios; and a longitudinal groove located between adjacent ribs. These and other objects are achieved in another aspect of the present invention by a method of making an asymmetric tread tire mold assembly including: removing one half of a clamshell mold from a first symmetric tire mold assembly, the clamshell mold half having a tread design thereon; removing one half of a clamshell mold from a second symmetric tire mold assembly, the clamshell mold half having a tread design thereon asymmetric to the first tread design; and combining the clamshell mold halves to form the asymmetric tread tire mold assembly.

These and other objects are achieved in another aspect of the present invention by manufacturing an asymmetric tread tire by the method steps of: removing one half of a clamshell mold from a first symmetric tire mold assembly, the clamshell mold half having a tread design thereon; removing one half of a clamshell mold from a second symmetric tire mold assembly, the clamshell mold half having a tread design thereon asymmetric to the first tread design; combining the clamshell mold halves to form an asymmetric tread tire mold assembly; providing a green tire to the asymmetric tread tire mold assembly; treating the green tire; and removing the green tire from the asymmetric tread tire mold assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention will become apparent to those skilled in the art to which the present invention relates from reading the following specification with reference to the accompanying drawings in which: Figure 1 is a front view of a portion of a conventional tire having an aggressive tread design;

Figure 2 is a front view of a portion of a conventional tire having a less aggressive tread design than the tread design in Figure 1 ;

Figure 3 is a schematic of the basic steps of the method of making a tire of the present invention;

Figure 4 is a perspective view of one embodiment of a tire having the asymmetric tread arrangement of the present invention;

Figure 5 is a cross-sectional view of the tire in Figure 4 taken along line V-V;

Figure 6 is a front view of the tire in Figure 4; Figure 7 is a front view of a tire having a first alternative embodiment of the asymmetric tread arrangement of the present invention;

Figure 8 is a front view of a tire having a second alternative embodiment of the asymmetric tread arrangement of the present invention;

Figure 9 is a front view of a tire having a third alternative embodiment of the asymmetric tread arrangement of the present invention; and

Figure 10 is a front view of a tire having a fourth alternative embodiment of the asymmetric tread arrangement of the present invention; and

Figure 1 1 is a graphic representation of the stresses and forces laterally across the ribs of a tire having the asymmetric tread arrangement of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This application uses numerous phrases and terms of art. The term "radial" is defined as the direction perpendicular to the axis of rotation of the tire.

The term "axial" is defined as the direction parallel to the axis of rotation of the tire. The term "lateral" is defined as the direction parallel to the axis of rotation of the tire and going from one sidewall of the tire to the other.

The phrase "midcircumferential plane" is defined as the plane passing through the center of the tread and being perpendicular to the axis of rotation of the tire.

The phrase "tread width" is defined as the greatest axial distance across the tread, as measured from a footprint of the tire, when the tire is mounted on a rim, subjected to a load, and inflated to a pressure corresponding to the load. All of the other tire dimensions refer to a tire mounted on a rim and inflated to a given pressure, but not subjected to a load.

The term "groove" is defined as the elongated void area in the tread that may extend circumferentially or laterally in a straight, curved or zig-zag manner. The phrase "symmetrical tread pattern" is defined as a tread pattern on one side of the midcircumferential plane of the tire having a correspondingly sized and shaped tread pattern on the opposite side of the midcircumferential plane of the tire. The tread patterns do not have to be laterally aligned on the tire.

The phrase "aggressive tread pattern" is defined as a tread pattern that has a low contact surface ratio relative to another tread pattern for a tire of similar dimensions.

The phrase "rib width" is defined as the peak-to-peak axial extent of a rib measured perpendicularly to the midcircumferential plane.

The phrase "circumferential surface contact area" is defined as the area of a rib in contact with the road surface along the circumferential length of the rib. The phrase "contact surface ratio" is defined as the ratio between a circumferential surface contact area of a rib and the total area of a rib (a product of the rib width and the rib circumferential length).

The phrase "void area" is defined as the area of a rib that is not in contact with the road surface along the circumferential length of the rib. The void area equals the product of the rib width and the rib circumferential length minus the circumferential surface contact area.

The phrase "lateral rib" refers to the rib furthest from the midcircumferential plane of the tire.

The phrase "medial rib" refers to a rib located between lateral ribs that does not overlap the midcircumferential plane of the tire.

The phrase "central rib" refers to a rib that overlaps the midcircumferential plane of the tire.

Figure 1 and 2 illustrate conventional tires having symmetric tread arrangements. Figure 3 illustrates the method of manufacturing a radial pneumatic tire having the tread embodying the present invention.

Figure 1 illustrates a conventional tire 1 having a tread pattern or sculpture 3 symmetrical about midcircumferential plane M. Tread pattern 3 is an aggressive tread pattern. An aggressive tread pattern has a low contact surface ratio relative to another tread pattern for a tire of similar dimensions. Stated differently, an aggressive tread pattern has a relatively high amount of void area compared to the circumferential surface contact area. Figure 2 illustrates a conventional tire 51 having a tread pattern or sculpture 53 symmetrical about midcircumferential plane M. Tread pattern 53 is a less aggressive tread pattern. A less aggressive tread pattern has a high contact surface ratio relative to another tread pattern for a tire of similar dimensions. Stated differently, a less aggressive tread pattern has a relatively smaller void area than an aggressive tread pattern. Figure 3 is a schematic of the basic steps of the method of making a tire of the present invention. The tread of radial pneumatic tire 100 is made by combining the tread patterns from an aggressive tire and a relatively less aggressive tire. Specifically, tire 100 uses one half of tread pattern 3 from tire 1 and one half of tread pattern 53 from tire 51. Tread pattern 3 is different than tread pattern 53. Combining the tread patterns results in a tread pattern 103 of tire 100 asymmetric about midcircumferential plane M.

The manufacturing processes of making tire 100 involve conventional techniques.

However, the present invention combines molds from diverse tires into a new tire tread mold assembly. The tire tread mold assembly of the present invention (shown schematically in Figure 3) uses one-half of a clamshell mold (shown schematically in Figure 3) used in the manufacture of tire 1 and one-half of a clamshell mold (shown schematically in Figure 3) used in the manufacture of tire 51. Once combined, tire 100 can be manufactured using any suitable tire curing processes. Additionally, the green, or uncured, tire which is later cured into tire 100 can itself be built or assembled using any suitable or conventional process. The criterion used to select compatible mold halves is discussed below.

Figures 4-10 illustrate several preferred embodiments of radial pneumatic tire 100 having a tread embodying the present invention.

With reference to the one embodiment shown in Figures 4-6, tire 100 has a radial carcass 151 extending between a pair of beads 161. Axial opposite ends 153 of carcass 151 are secured to a respective bead 161. Carcass 151 is flanked by two sidewalls 171. A crown portion 181 is located between sidewalls 171. Crown portion 181 includes a belt package 191. A tread portion 103 is located radially outwardly of crown portion 181.

Tread pattern 103 is asymmetrical about midcircumferential plane M. Tire 100 has a five rib tread pattern. Tread pattern 103 includes a plurality of ribs 109, 1 1 1, 113, 115, 125 and grooves 127,129,131,133 located between adjacent ribs. At least one of the grooves, e.g. groove 133, can be an interrupted groove.

Figure 5 illustrates the difference between an uninterrupted and an interrupted groove. An interrupted groove is a groove in which a circumferential imaginary line tangent to the peak axial extent of a rib intersects an adjacent rib. Stated different, an interrupted groove is a groove in which a person cannot see an open space between adjacent ribs along the entire circumference of the tire when looking through the groove. Groove 133 is an interrupted groove, whereas grooves 127, 129 and 131 are uninterrupted grooves. An open space is seen between ribs 109 and 113; 113 and 125; and 125 and 115. There is no open space between ribs 115 and 111.

The plurality of ribs include: lateral ribs 109,111 on an inside 105 and an outside 107, respectively, of tire 100; medial ribs 113,115 on inside 105 and outside 107, respectively, of tire 100; and a central rib 125. Central rib 125 straddles midcircumferential plane M. Lateral ribs 109,11 1 form a lateral rib pair 1 12. Medial ribs 1 13,115 from rib pair 114. As clearly demonstrated in Figure 6, individual ribs 109,111 ; 1 13,115 of rib pairs 1 12,114 are substantially equidistant from midcircumferential plane M on opposite sides of midcircumferential plane M.

Each rib has a circumferential surface contact area, void area and contact surface ratio. As described earlier in the specification, rib width is defined as the peak-to-peak axial extent of the rib measured perpendicularly to midcircumferential plane M (i.e. measured in the axial direction). Rib width W of rib 109 is demonstrated in Figure 6. In the formulas below, CSCA represents the circumferential surface contact area, A represents the total area, VA represents the void area, and CSR represents the contact surface ratio of a rib.

CSCA = area of the rib which contacts the road surface along the circumferential length X of the tire

(1) CSCA = (Σ of areas of the portions of each element of a rib in contact with the road surface) (X) A = area of an imaginary annular band having a width W and a circumferential length X = (width W of rib) multiplied by (circumferential length X of rib)

(2) A = (W)(X)

VA = area of the rib which does not contact the road surface along the circumferential length of the tire = (total area) subtracted by (area of the rib in contact with the road surface)

(3) VA = [Formula (2)] - [Formula (1)] = [(A) - (CSCA)]

CSR = ratio between the area of the rib in contact with the road surface and the total area of the rib = (area of the rib in contact with the road surface) divided by (total area of the rib) (4) CSR = [Formula (1) ÷ Formula (2)] = [(CSCA) ÷ (A)] I [Formula (1) ÷ (Formula (1 ) + (2))] = [CSCA ÷ (VA + CSCA)]

A geometric relationship exists between individual ribs of a rib pair. In order to avoid repetition, the following discussion only refers to ribs 109,1 11 of rib pair 1 12. It is understood that the geometric relationship also exists between ribs 1 13,1 15 of rib pair

1 14. Rib 109 has the same circumferential surface contact area (CSCA) as rib 1 1 1.

However, rib 109 has a different contact surface ratio (CSR) than rib 11 1.

Providing equal circumferential surface contact areas for each rib, e.g. 109, 11 1 , of a rib pair, e.g. 1 12, ensures that the circumferential surface contact area for the inside

105 of tire 100 is equal to the circumferential surface contact area for the outside 107 of tire 100. Equal circumferential surface contact areas on the inside 105 and the outside

107 of tire 100 provides uniform distribution of normal contact stresses along the lateral length of tread 103 of tire 100. A normal contact stress is a stress on a rib radial to the tire and perpendicular to the contact surface of the tire. Figure 1 1 demonstrates that normal contact stresses are substantially equal along the lateral length of tread 103 despite unequal forces acting on individual ribs. Furthermore, the different contact surface ratios of the ribs of tread 103 of tire 100 provide different wet traction characteristics to the tire depending on the loading of the vehicle due to an operating condition.

Figure 7 is a first alternative embodiment of the tread design of the present invention. Similar features of this embodiment use numbers common to the one embodiment shown in Figures 4-6, except for a change in the hundred digit. Tire 200 includes all of the basic features of tire 100 - a radial carcass (not shown), a pair of beads (not shown), sidewalls (not shown), crown portion (not shown), belt package (not shown) and tread portion 203. Tread pattern 203 is asymmetrical about midcircumferential plane M. Tire 200 has a four rib tread pattern. Tread pattern 203 includes a plurality of ribs 209,211,213,215 and grooves 227,229,231 located between adjacent ribs. At least one of the grooves, e.g. groove 231 , can be an interrupted groove.

The plurality of ribs include: lateral ribs 209,211 on an inside 205 and an outside 207, respectively, of tire 200; and medial ribs 213,215 on inside 205 and outside 207, respectively, of tire 200. Lateral ribs 209,211 form a lateral rib pair 212. Medial ribs 213,215 from rib pair 214. As clearly demonstrated in Figure 7, individual ribs 209,11 1 ; 213,215 of rib pairs 212,214 are substantially equidistant from midcircumferential plane M on opposite sides of midcircumferential plane M. Tire 200 differs from tire 100 in that tire 200 does not have a central rib. A central rib can be added or removed from a tire design for many reasons. For example the tread width of a tire limits the number of ribs placeable thereon.

Each rib has a circumferential surface contact area, void area and contact surface ratio. These formulas are the same as described above with reference to tire 100. A geometric relationship exists between individual ribs of a rib pair. In order to avoid repetition, the following discussion only refers to ribs 209,21 1 of rib pair 212. It is understood that the geometric relationship also exists between ribs 213,215 of rib pair

214. Rib 209 has the same circumferential surface contact area (CSCA) as rib 21 1. However, rib 209 has a different contact surface ratio (CSR) than rib 21 1.

As discussed above with respect to tire 100, providing equal circumferential surface contact areas for each rib, e.g. 209, 211, of a rib pair, e.g. Ill, ensures that the circumferential surface contact area for the inside 205 of tire 200 is equal to the circumferential surface contact area for the outside 207 of tire 200. Equal circumferential surface contact areas on the inside 205 and the outside 207 of tire 200 provides uniform distribution of normal contact stresses along the lateral length of tread 203 of tire 200, despite unequal forces acting on the inside 205 and outside 207 of the tire 200. Furthermore, the different contact surface ratios of the ribs of tread 203 of tire 200 provide different wet traction characteristics to the tire depending on the loading of the vehicle due to an operating condition.

Figure 8 is a second alternative embodiment of the tread design of the present invention. Similar features of this embodiment use numbers common to the one embodiment shown in Figures 4-6, except for a change in the hundred digit. Tire 300 includes all of the basic features of tire 100 - a radial carcass (not shown), a pair of beads (not shown), sidewalls (not shown), crown portion (not shown), belt package (not shown) and tread portion 303.

Tire 300 includes tread pattern 303 asymmetric about midcircumferential plane M. Tire 300 has a 6 rib tread pattern. Tread pattern 303 includes a plurality of ribs 309, 31 1, 313, 315, 317 and grooves 327,329,331,333,335 located between adjacent ribs. At least one of the grooves, e.g. groove 335, can be an interrupted groove.

The plurality of ribs include: lateral ribs 309,31 1 on an inside 305 and an outside 307, respectively, of tire 300; and medial ribs 313,317; 315,319 on inside 305 and outside 307, respectively, of tire 200. Lateral ribs 309,31 1 form a lateral rib pair 312. Medial ribs 313,315 form rib pair 314. Medial ribs 317,319 form rib pair 316. As clearly demonstrated in Figure 8, individual ribs 309,31 1 ; 313,315; and 317,319 of rib pairs 312,314,316 are substantially equidistant from midcircumferential plane M on opposite sides of midcircumferential plane M. Tire 300 differs from tire 200 in that tire 300 includes a second rib pair 316. A second rib pair can be added to a tire design for many reasons. For example the tread width of a tire can limit the number of ribs placeable thereon.

Each rib has a circumferential surface contact area, void area and contact surface ratio. These formulas are the same as described above with reference to tire 100. A geometric relationship exists between individual ribs of a rib pair. In order to avoid repetition, the following discussion only refers to ribs 309,31 1 of rib pair 312. It is understood that the geometric relationship also exists between ribs 313,315 of rib pair 314; and ribs 315,317 of rib pair 316. Rib 309 has the same circumferential surface contact area (CSCA) as rib 311. However, rib 309 has a different contact surface ratio (CSR) than rib 311.

As discussed above with respect to tire 100, providing equal circumferential surface contact areas for each rib, e.g. 309, 311, of a rib pair, e.g. 312, ensures that the circumferential surface contact area for the inside 305 of tire 300 is equal to the circumferential surface contact area for the outside 307 of tire 300. Equal circumferential surface contact areas on the inside 305 and the outside 307 of tire 300 provides uniform distribution of normal contact stresses along the lateral length of tread 303 of tire 300, despite unequal forces acting on the inside 305 and outside 307 of the tire 300. Furthermore, the different contact surface ratios of the ribs of tread 303 of tire 300 provide different wet traction characteristics to the tire depending on the loading of the vehicle due to an operating condition.

Figure 9 is a third alternative embodiment of the tread design of the present invention. Similar features of this embodiment use numbers common to the one embodiment shown in Figures 4-6, except for a change in the hundred digit. Tire 400 includes all of the basic features of tire 100 - a radial carcass (not shown), a pair of beads (not shown), sidewalls (not shown), crown portion (not shown), belt package (not shown) and tread portion 403. Tire 400 includes tread pattern 403 asymmetric about midcircumferential plane

M. Tire 400 has a 3 rib tread pattern. Tread pattern 403 includes a plurality of ribs 409,411,425 and grooves 427,429 located between adjacent ribs. At least one of the grooves, e.g. groove 427, can be an interrupted groove.

The plurality of ribs include: lateral ribs 409,411 on an inside 405 and an outside 407, respectively, of tire 400; and a central rib 425. Lateral ribs 409,41 1 form a lateral rib pair 312. As clearly demonstrated in Figure 9, individual ribs 409,41 1 of rib pair 412 are substantially equidistant from midcircumferential plane M on opposite sides of

midcircumferential plane M. Tire 400 differs from tire 100 in that tire 400 lacks a medial rib pair. A medial rib pair can be added or removed from a tire design for many reasons. For example the tread width of a tire limits the number of ribs placeable thereon. Each rib has a circumferential surface contact area, void area and contact surface ratio. A geometric relationship exists between individual ribs of a rib pair. Rib 409 has the same circumferential surface contact area (CSCA) as rib 41 1. However, rib 409 has a different contact surface ratio (CSR) than rib 41 1.

As discussed above with respect to tire 100, providing equal circumferential surface contact areas for each rib, e.g. 409, 41 1, of a rib pair, e.g. 412, ensures that the circumferential surface contact area for the inside 405 of tire 400 is equal to the circumferential surface contact area for the outside 407 of tire 400. Equal circumferential surface contact areas on the inside 405 and the outside 407 of tire 400 provides uniform distribution of normal contact stresses along the lateral length of tread 403 of tire 400, despite unequal forces acting on the inside 405 and outside 407 of the tire 400. Furthermore, the different contact surface ratios of the ribs of tread 403 of tire 400 provide different wet traction characteristics to the tire depending on the loading of the vehicle due to an operating condition.

Figure 10 is a fourth alternative embodiment of the tread design of the present invention. Similar features of this embodiment use numbers common to the one embodiment shown in Figures 4-6, except for a change in the hundred digit. Tire 500 includes all of the basic features of tire 100 - a radial carcass (not shown), a pair of beads (not shown), sidewalls (not shown), crown portion (not shown), belt package (not shown) and tread portion 503. Tread pattern 503 is asymmetrical about midcircumferential plane M. Tire 500 has a four rib tread pattern. Tread pattern 503 includes a plurality of ribs 509, 51 1, 521, 523 and grooves 527,529,531 located between adjacent ribs. At least one of the grooves, e.g. groove 529, can be an interrupted groove.

The plurality of ribs include: lateral ribs 509,511 on an inside 505 and an outside 507, respectively, of tire 500; and central ribs 521,523. Central rib 521 straddles midcircumferential plane M, but remains substantially on inside 505 of tire 500. Likewise, central rib 523 straddles midcircumferential plane M, but remains substantially

on outside 507 of tire 500. Central ribs 509,511 form a central rib pair 522. As clearly demonstrated in Figure 10, individual ribs 509,51 1; 521,523 of rib pairs 512,522 are substantially equidistant from midcircumferential plane M on opposite sides of midcircumferential plane M. Tire 500 differs from tire 200 in that each of the ribs in the rib pair 522 overlaps the midcircumferential plane M. The position of a rib pair on a tread design can be adjusted for many reasons. For example the tread width of a tire limits the number of ribs placeable thereon.

Each rib has a circumferential surface contact area, void area and contact surface ratio. A geometric relationship exists between individual ribs of a rib pair. In order to avoid repetition, the following discussion only refers to ribs 509,51 1 of rib pair 512. It is understood that the geometric relationship also exists between ribs 521,523 of central rib pair 522. Rib 509 has the same circumferential surface contact area (CSCA) as rib 511. However, rib 509 has a different contact surface ratio (CSR) than rib 51 1. As discussed above with respect to tire 100, providing equal circumferential surface contact areas for each rib, e.g. 509, 511 , of a rib pair, e.g. 512, ensures that the circumferential surface contact area for the inside 505 of tire 500 is equal to the circumferential surface contact area for the outside 507 of tire 500. Equal circumferential surface contact areas on the inside 505 and the outside 507 of tire 500 provides uniform distribution of normal contact stresses along the lateral length of tread 503 of tire 500, despite unequal forces acting on the inside 505 and outside 507 of the tire 500. Furthermore, the different contact surface ratios of the ribs of tread 503 of tire 500 provide different wet traction characteristics to the tire depending on the loading of the vehicle due to an operating condition. From the above examples, a pattern of rib pairing is established. This pattern of rib pairing is usable with any number of ribs on a tread. With an even number of ribs, the ribs are paired according to order as viewed from the midcircumferential plane of the tire laterally outward towards the respective sides of the tire. For example, the laterally closest rib on one side of the midcircumferential plane is paired with the laterally closest rib on the other side of the midcircumferential plane. This pairing is accomplished even when both ribs of a rib pair overlap the midcircumferential plane (see, e.g., Figure 10). The present invention provides that each rib of a rib pair has the same circumferential

surface contact area, but a different contact surface ratio than the other rib of the rib pair. With an odd number of ribs, the ribs are also paired according to their order relative to the midcircumferential plane of the tire. This pairing results in the central rib not being paired with another rib (see, e.g., Figure 6). The present invention provides that each rib of a rib pair has the same circumferential surface contact area, but a different contact surface ratio than the other rib of the rib pair. The values (CSCA, A, VA, CSR) of the non-paired central rib can be adjusted without regard to another rib on the tread.

The specification has described the present invention used during the original manufacture of a tire. The present invention could also be used with treads applied after the original manufacture of the tires. For example, the present invention could be used with treads applied during the recap or retread of tires. These treads can be manufactured using conventional techniques.

It is also understood that many other variations are apparent to one of ordinary skill in the art from a reading of the above specification and such variations are within the spirit and scope of the instant invention as defined by the following appended claims.