IE72150B1 - Contoured replacement tread - Google Patents

Description

CONTOURED REPLACEMENT TREAD The present invention relates to a replacement tread for pneumatic tires, and a method using such tread in a retreading operation. More particularly, this invention relates to a prevulcanized replacement tread strip contoured to provide improved consistency in application, appearance and road endurance.

In general, retreading pneumatic tires by applying a precured (e.g. prevulcanized) tread strip to a tire casing is known. Examples of such retreading process, sometimes referred to as cold vulcanizing or cold bond retreading, are disclosed in U.S. Patent Nos. 2,976,910 issued to Nowak on March 28, 1961; 3,136,673 issued to Carver on June 9, 1964; 3,236,709 issued to Carver on February 22, 1966; 3,283,795 issued to Schelkmann on November 8, 1966; 3,325,326 issued t :o Schelkmann on June 13, 1967; 3,745,084 issued to 20 Schelkmann on July 10, 1973; 3,,752,725 issued to Barefoot » on August 14,, 1973; 3, 951,720 issued to Brodie on Atari 1 ti. , 1976; 4,046,947 issued to Brodie on September 6, 1977; and 4,075,047 issued to Brodie et al. on February 72Ί50 21, 1978; 4,151,027 issued to Schelkmann et al. on April 24, 1979; and, 4,434,018 issued to Brewer on February 28, 1984 .

In such retreading procedures, the tire casing to be retreaded is typically buffed to remove excess rubber, to provide a generally evenly textured crown for receiving the cured tread strip and to provide a predetermined shoulder profile. Tire casings typically include a belt package, (a package of steel belts or cables), underlying the road engaging surface, (e.g,, the original tread) of the tire. The casing typically is buffed at a predetermined characteristic crown radius generally corresponding to the upper contour of the belt package, but leaving only a predetermined thickness, e.g. 0.238 cm (3/32 of an inch), of material remaining over the top belt. The shoulder of the casing is also buffed (trimmed) to eliminate or reduce voids or patterns in the shoulder created by the original tread, and to provide a relatively straight profile between the casing sidewalls and crown. The casing from each of the various types of known new tires has a characteristic crown width, crown buffing radius and shoulder trim angle.

The precured tread strip, typically of a width corresponding to the width of the crown of the casing and cut to a length corresponding to the casing crown circumference, is likewise disposed circumferentially about the casing crown, overlying a layer of suitable bonding material, such as uncured cushion gum including a low temperature vulcanizing agent and accelerator. A roller pressing process, commonly referred to as stitching, is performed on the assembly to force out air from between the tread strip and casing- The assembly is then subjected to heat and pressure to cur® (e.g-, vulcanize) the bonding layer, generally in an autoclave. This bonds the prevulcanised tread strip to the casing, and typically, forces out any residual trapped air from between the tread and casing. - 3 Perhaps the most common prevulcanized tread strip, known as a top cap, is a strip of prevulcanized material, manifesting a predetermined tread design on the top surface thereof, and having an underside which is generally flat. Other top caps described in the prior art employ an underside having approximately the same curvature as the crown of the casing of the particular tire casing to be retreaded. The width of the top cap characteristically corresponds to the crown width of the casing. The characteristic shoulder trim angle for a casing is conventionally chosen to provide a substantially continuous interface at the juncture of the top cap and casing, i.e., to eliminate or reduce any ledge at the juncture, as well as to eliminate or reduce voids or remnants of the original tread design in the shoulder.

A number of problems arise from conventional trimming of the casing shoulder. If too much material is removed, for example, to eliminate or reduce voids or remnants of the original tread pattern in the shoulder, a narrow crown may result. Moreover, the integrity of the casing can be deleteriously affected by leaving insufficient material in the shoulder in the vicinity of end of the casing belt package. In such an instance, the casing tends to crack or otherwise breakdown in the shoulder area. A shoulder extrusion formed of uncured rubber stock can sometimes be disposed on the shoulder to, in effect, replace the shoulder area. However, such uncured rubber stock used in retreading tends to be relatively expensive for labor and material.

In addition, the buffed shoulder is often unsightly, even when structurally sound. This gives rise to a particular problem when the tire is used on, for example, a rental truck intended to fee used by consumers who may not be familiar with retreads.

The pre-characterizing part of claim 1 refers to a prevulcanized tread as disclosed in DE-A-2 642 990. The known tread comprises a substantially flat underside and tapering side lobes extending at an angle from the tread strip intended to overlie the substantial ly flat shoulders of the casing. The juncture between the flat underside and interior surface of the flap is abrupt; the edges purportedly aid in centering the tread on the casing having a flat upper side. Contrary to the teachings of Maltinti, the abrupt transition at the underside of the lobe causes the assembly to be susceptible to small offsets in alignment, and imprecisions in buffing the tire to the precise dimensions corresponding to the contour of the tread underside, causing air to be trapped between tread strip and casing, and potentially heat generating stresses in the tire.

Mechanisms that purport to provide a contoured tread that can be installed on a casing without creating stresses in the retread assembly have been suggested. For example, European Patent Application Publication No. 176 94 5 published in the name of Michelin Et Cie on September 4, 1986 (bulletin 86/15) discloses a prevulcanized tread strip which is curved both in the transverse and longitudinal (circumferential) directions.

The radius of curvature of the tread underside in the transverse direction varies to better fit the shape of the top of the casing to be retreaded; the radius is maximum at the center and minimum at the shoulders. The curvature in the longitudinal (circumferential) direction purportedly avoids unacceptable stresses in the retread assembly.

U.S. Patent 3,283,795 issued to Schelkmann on November 6, 19S5 discloses a prevulcanized tread strip having an underside manifesting a transverse curve with a radius of curvature greater than the radius of curvature of the casing croun. Such a contour is said to ensure that the central portion of the tread will first engage the casing crown, so that any air trapped between the tread strip and casing will be gradually squeezed out between the surfaces. Unacceptable stresses are purported to be avoided bv emolovino a oluralitv of tread sections -5to form the retread assembly, each curved not only in the transverse direction, but also in the longitudinal direction (direction of elongation of the tread) , or by including compensation folds or cuts in the tread.

It has also been proposed to avoid creating stresses in the retread assembly by employing, rather than a strip of tread, a premolded endless ring of tread having a relaxed diameter less than the inflated diameter of the tire casing. The replacement tread ring is stretched, disposed about the prepared tire casing, together with an intermediate layer of bonding material, then snapped onto the casing. The tread ring typically includes integral shoulder wings extending downwardly over the shoulders of the tire casing. An example of such a retreading system is described in U.S. Patent No. 3,815,651 issued to Neal on June 11, 1974.

However, prior art contoured treads are designed for a particular casing, and are relatively intolerant of varying crown arc width and radius; deviations from the specific crown width and crown radius create stresses in the retread assembly deleterious to operation and longevity. Thus, a large number of different tread strips are required to be on hand in order to recap the various commercially available tires.

There remains a need for a cold vulcanizing retreading process that substantially replicates the original tread width of the casing as a new tire, while at the same time accommodating a range of tire casings, e.g., a tire series, without creating deleterious stresses in retread assembly.

It is an object of the present invention to provide a prevulcanized tread for installation on a tire casing which when applied to the tire casing forms a retread assembly having improved appearance and endurance, performance and wearability characteristics and to provide a retread tire comprising a tire casing and a prevulcanized tread installed thereon having improved appearance and endurance, performance and wearabi1itv character!sties.

This problem is solved, according to the invention, with the features of claims 1 and 30, respectively. The prevulcanized tread according to the invention includes a sole, an upper tread portion manifesting a predetermined tread design -6disposed between respective shoulders, and respective skirts. The sole includes, in a cross-sectional plane, a portion of nominal arc of a predetermined radius of curvature Rn. The upper tread portion has associated therewith a nominal arc extending between the tread shoulders which has a predetermined radius of curvature equal to Rn plus a predetermined constant, and a predetermined arc length AWT. The tread skirts each include a generally planar interior wall, an interior interconnecting portion which couples the interior wall to the sole, an exterior side wall extending downwardly and outwardly from the tread’s shoulders and an end portion connecting the interior and exterior walls. .The interior interconnecting portion includes, in the cross-sectional plane, a portion of a nominal arc having a predetermined radius of curvature which is less than R^. The exterior sidewall is disposed at a predetermined angle a with respect to the vertical and such that the portion of the nominal arc of radius Rf, extending between respective intersections with the exterior sidewalls is of a predetermined length AWOD which is greater than AWT. The interior walls of the respective skirts are disposed at a predetermined angle Θ with respect to vertical, and such that the portion of the nominal arc of radius RTI extending between respective intersections with nominal extensions of the interior walls is of a predetermined length AWID which is less than AWT.

The tread having the above described contour is relatively tolerant of small offsets from center during installation on a tire casing. The tread strip configured in accordance to the invention with the gradual transition between the curved underside and the interior walls of the respective skirts is capable of installation on casings without creating unacceptable heat generating stresses in the retread assembly, irrespective of the range of buffed casing crown radius.

The retread according to the invention comprises a - 7 tire casing and the prevulcanized tread of the invention installed thereon. The tire casing includes a peripheral crown manifesting, in a cross-sectional plane through the nominal axis of rotation of the casing, an arc having a predetermined radius of curvature, and a predetermined arc length, A‘WC. The casing also includes respective shoulders disposed on either side of the crown, each manifesting, in the cross-sectional plane through the nominal axis of rotation of the casing, an arc having a radius of curvature, R^, which is less than R<-.

The preferred exemplary embodiment of the invention will be hereinafter described in conjunction with the appended drawing, wherein like designations denote like elements, and: Figure 1 is an exploded, schematic, pictorial vertical cross section of a casing prepared in accordance to the invention, a layer of binding material and a precured tread strip in accordance with the present invention; Figure 2 is a schematic representation of a tread strip according to the invention illustrating various relevant spacial relationships; Figure 3 is a schematic representation of the skirt of tread strip illustrating various relevant spacial relationships; and.

Figure 4 is a schematic sectional view of a retread assembly in accordance with the present invention.

Referring to Figure 1, a precured (e.g., prevulcanized) replacement tread strip 10, and a strip of uncured bonding material 11, such as uncured cushion gum including a low temperature vulcanizing agent and accelerator, are adapted to be affixed to a tire casing 12 through a cold vulcanizing process. Tire casing 12 includes a crown portion 14 bounded on either side by respective shoulder portions 18 and sidewalls 16, and including a conventional belt package 17 underlying crown 14. As will be explained, the crown 14 and shoulders 18 of casing 12 manifest a predetermined configuration suitably formed by buffing. -8The surface of crown 14 of casing 12 is mechanically buffed to provide a textured convex arcuate configuration in a direction transverse to the circumference of casing 12. Specifically, the arc has a characteristic radius of curvature, (Crown Radius) generally corresponding to the upper contour of belt package 17, extending above the top belt by a predetermined thickness, e.g. 0.238 cm (3/32 of an inch), as in the case of prior art techniques. The length of the arc at Crown Radius R<-. extending between shoulders defining crown 14, denominated Crown Arc Width AWC is set at a predetermined value. Crown Radius Rq, and Crown Arc Width AWC are characteristic to casings deriving from particular types of tires; the casing from each type of new tire has a characteristic crown radius and crown width.

In accordance with one aspect of the present invention, notwithstanding the generally accepted maxim that heat generated is directly proportional to the amount of material on the tire, shoulders 18 of casing 12 are contoured as convex arcs having a predetermined buffed Shoulder Radius R^ substantially less than Crown Radius Re- The shoulder radius is chosen in accordance with a number of factors: 1. To ensure that sufficient material is maintained on the casing at the ends of the belt package without causing the generation of additional heat during operation of the retreaded tire; 2. To provide support for a tread pattern on tread strip 10 coextensive with crown 14 of casing 12 (suitably corresponding to the tread width of the new tire from which casing 12 derived) without, causing the generation of additional heat during operation of the retreaded tire; 3. To provide, in conjunction with the contour of tread strip 10, support against both radial forces and lateral forces, encountered, for example, on tires used in multiaxle applications, and in cornering; -94. To make the assembly tolerant of small offsets from center of tread strip 10 with respect to casinq 12; and, . To facilitate the use of a tread strip 10 that is capable of use with other casings having differing characteristic crown radii.

For casings having buffed crown radii of from 50.8-81.28 cm (20-32 inches), similar radius R^ is suitably in the range of 0.79375-4.1275 cm (0.3125-1.625 inches); a typical example would be 2.54 cm (1.00 inch), Tread strip 10 provides a tread width comparable to that of the new tire from which casing 12 was derived, and is capable of, when installed on casing 12, enduring the lateral forces generated in certain applications of retread tires. At the same time, tread strip 10 is relatively tolerant of small offsets from center during installation, and capable of installation on casings having varying crown radii (within certain ranges) , without generating unacceptable potentially heat generating stresses in the assembly.

Tread 10 is designed to be utilized with a casing 12, having predetermined nominal buffed Crown Arc Width (AWC) and a buffed Crown Radius R^ within a predetermined range. The Crown Radius Rc of the buffed casing is a characteristic of the particular type of tire from which the casing derived and varies from tire to tire. However, a common predetermined Crown Arc Width can be accomxaodated by casings derived from a variety of types of new tires (new tires within a given series) ..notwithstanding different crown radii, particularly when the shoulder of the casing is foraaed as a convex arc in the manner of shoulder 18 to compensate for, e.g., variations in the configuration of the casing belt packages. The casing Crown Arc Width ,AWC, is preferably chosen to approximate the tread width of the new tire from which casing 12 derived. In practice, to approximate the tread widths of a range on tires, the casing crown arc -10width is preferably chosen as the median new tire tread width for tires in the series from which casing 12 is derived .

Referring now to Figures 1, 2 and 3, replacement tread 10 includes: an upper tread portion 20 manifesting a predetermined tread design 23 (pattern) between respective tread shoulders (edges) 21; an underside (sole), generally indicated as 22; and, respective skirts 24. Skirts 24 each include an exterior sidewall 26, an end portion 28, an interior wall 30, and a radiused interior interconnecting portion 32.

The outer (road engaging) surface of tread portion 20 (shown schematically in Figures 2 and 3; tread pattern omitted) is designed based upon a nominal arc extending between tread shoulders 21, having a predetermined radius. Tread Outside Radius (RTO) and a predetermined length, Tread Arc Width (AWT) . The actual tread pattern is built upon the nominal arc of radius F-ro and arc width AWT. However, in some applications, the exterior surface of the actual tread portion 2 0 may not be arcuate. For example, the thickness of tread portion 20 may vary as a function of distance from the center, or vary locally in the vicinity of the edges.

Sole 22 of tread strip 10, underlying tread portion 20, is configured along a nominal arc 22A (FIG. 2). Arc 22A is concave from the perspective of casing 12, and has a predetermined radius, Tread Inside Radius (RTI) . Sole 22 merges, at either side, with interconnecting portions 32 of skirts 24.

The radius of sole 22, Tread Inside Radius is chosen so that tread 10 can be installed on casing having a range of buffed crown radii, i.e., in accordance with typical crown radii encountered in the series of tires for which tread 10 is intended to be used. For example, new tires having a Tread Arc Width (defined as the width of an arc from one edge of the tread to the other) of approximately 20.32 cm (8 inches) typically have characteristic -11buffed crown radii of between 50.8-66.04 cm (20-26 inches). Applicant has determined that a tread strip 10 configured in accordance with the present invention having a Tread Inside Radius RTI intermediate of that range, for example, the median value for a casing of the series 55.88 cm (22 inches), can be installed on casings of that series without creating unacceptable heat generating stresses in the retread assembly, irrespective of the range of buffed casing crown radius.

Tread Arc Width AWT (the length of the nominal tread arc extending between tread shoulders 21), is chosen to correspond to the buffed Crown Arc Width AWC, of casing 12, and hence, to the tread arc width of a new tire of the series from which casing 12 is derived. Setting Tread Arc Width AWT egual to the buffed casing Crown Arc Width AWC ensures that adequate support is provided for the edges of tread 20. The radius of the nominal tread arc. Tread Outside Radius RTO, is set equal to the radius of the sole arc denominated RT, plus a constant, corresponding to, for example, 1.905 cm (24/32 inch).

Skirt 24 both improves the appearance of the retreaded tire and tends to increase the durability and mileage performance of the retread assembly. For example, in multiaxle applications, skirt 24 protects casing shoulder 18 during cornering. This tends to extend the use and life of casing 12. Skirt 24 provides solid support to edges 21 of replacement tread strip 10 permitting tread 10 to manifest a cross-sectional profile similar to that of a new tire. This will give the retread a better mileage performance during application.

Exterior sidewalls 26 and interior walls 30 of skirt 24 are suitably planar, manifested as straight lines in the transverse plane (cross section). Exterior sidewalls 26 and interior walls 30 extend downwardly and outwardly from tread shoulders 21, and skirt interconnecting portions 32, respectively, disposed at predetermined angles Θ, and cc respectively with respect to vertical. The angles are chosen in accordance with Tread -12Arc Width AWT, the length of skirt 24, the particular series of casings with which tread 10 is intended to be used, and the particular application in which the assembly is intended to be used. The angle Θ of interior walls 30 is suitably within the range of 25-40° and preferably within the range of 30-35°; and particularly 32° for a casing Shoulder Radius R^ of 2.54 cm (1 inch). The angle « of exterior surface 26 is suitably within the range of 25-5° and preferably within the range of 20-12°, and 10 particularly 16°.

The relative dispositions of the respective exterior sidewalls 26 and interior walls 3 0 are such that the lengths of respective nominal arcs at Inside Tread Radius Ry, are equal to predetermined values related to Tread Arc Width AWT: an Outside Design Arc Width AWOD, the portion of nominal arc 22A extending between intersections 34 with exterior side walls 26 of skirts 24, is set at a value greater than Tread Arc Width AWT; and, an Inside Design Arc Width AWID the portion of nominal arc 22A extending between the junctures 36 (with nominal extensions of interior walls 30), is set at a value less than Tread Arc Width Auq·.

More specifically, Outside Design Arc Width AWOD is set equal to the Tread Arc Width AWT plus a constant K, (AWod = AWT + K,) and Inside Design Arc Width AWgj is set equal to Tread Arc Width AWT minus a constant K2 (AWm = AWT - Kj). Thus, AWtt = AWoo - Ks* where K3 = Kn + x3.

Constants X, and X3 are chosen in accordance with Tread Arc Width AW? and the particular series of casings 12 which tread 10 is intended to be used. Constant K, is suitably within the range of 0.47525-1.905 cm (0.1875-0.750 inches), and preferably within the range of 0.535-1.905 cm (0.250-0.750 inches), and particularly 0.9525 cm (0.375 inch). Constant is suitably within - 13 the range of 0.15875-0.635 cm (006250.250 inches), and preferably within the range of 0.3175-0.635 cm(0.125-0.250 inches), and particularly 0.39624 cm (0.156 inch). Constant is suitably within the range of 0.635-2.54 cm (0.250-1.000 inches), and preferably within the range of 2.54 cm (0.375-1.000 inches), and particularly 1.34874 cm (0.531 inch).

As best seen in Figure 3, the distance between a line extending along interior skirt sidewall 30 along a perpendicular to the juncture 34 of the nominal arc 22A at (Radius R^-,) with skirt exterior sidewall 26 is set at a predetermined distance Dp suitably in the range of 0.2286-0.9144 cm (0.090-0.360 inches), and preferably in the range of 0.3429-0.9144 cm (0.090-0.360 inches), and particularly 0.5892 cm (0.232 inch).

The walls 26 and 30 of skirts 24 extend a predetermined vertical distance below sole 22, i.e., a distance Hs (measured from juncture 34), and are connected by end walls 28. Skirt Height Hs is suitably in the range of 0.64 cm (1/4 inch) to 5.08 (2 inches); a typical example would be 1.5342 cm (0.604 inch). Endwall 28 is also suitably planar, manifesting substantially as a point or a short straight line in the transverse plane having a predetermined width Dc, suitably in the range of 0-0.3175 cm (0.000-0.125 inch), preferably approaching zero. A typical example would be 0.16002 cm (0.063 inch).

Skirt radius interconnecting portions 32 provide smooth and continuous transitions from sole 22 to skirt interior sidewalls 30. Radiused portion 32 exhibits a predetermined radius RTS chosen to assist in removing original tread design shoulder voids and provide for ease of assembly with cushion gum II. Radius RTS is suitably in the range of 1.27-3.81 cm (0.500-1.500 inches); a typical example would be 1.905 cm (0.75 inch).

Various dimensions of preferred embodiments of tread strip xo for casings corresponding to tires having nominal industry tread size of 3, 8.5, 9, 9.5, 10.5 and 11.5, for example, as shown in Table A. Such sizes as provided by Bandag, Incorporated are shown in Table A, together with the relevant dimensions of the buffed tire casing (all dimensions in inches). -14 TABLE A tread dimension bueeed Casing DIMENSION Tread Outside Inside Tread Tread Soiled Charac Buffed Sox Design Design Arc Inside Casing urist«: Shoulder (Designator) Arc Wjdlh Arc Width Width Radius Ait Crov-n Radius AW,0 AW, R„ Width AWC Radius R_: Res 8 8 312 7.687 7.937 22 7.9375 20 26 i.oo 8 5 8.625 8.00 8.250 2-4 8.250 22 26 i oo 4 8.937 8.3125 8.562 24 8.5625 22-32 1.00 9.5 9.28 8.66 8.91 28 8.91 22-30 1.00 10.5 9.69 9.06 9.31 28 9.31 2o-32 I 00 J 11.5 10.16 9.53 9.78 30 9.78 30-32 1 00 Tread strip 10 may be prepared by molding uncured tread material in a vulcanizing press under. For example, tread 10 may be formed in a two-piece mold having a female portion containing the tread pattern or tread design and exterior of skirt 24 and male portion defining sole 22 and the interior portions of skirt 24. This arrangement facilitates the manufacture of treads 10 having different tread patterns. Tread portion 20 may be formed with any conventional design.

As generally shown in Figure 4, replacement tread 10 is affixed to tire casing 12 to form a retread assembly 38. Tire casing 12 is profiled, suitably by mechanical buffing to its characteristic Crown Radius and to the predetermined Crown Arc Width AWC and Shoulder Radius R^ associated with tread 10. Bonding material 11 is provided between replacement tread 10 and tire casing 12. Sole 22 is generally aligned with crown 14 and skirts 24 overly shoulders 18. The assembly comprising the tire casing 12, bonding layer 11 and replacement tread 10 are -15suitably processed in accordance with conventional cold bonding techniques to form a retreaded tire.

Retread assembly 38 is advantageous in a number of respects. Replacement tread 10 approximates the tread arc width of the original tire from which casing 12 was derived, but with the endurance and wear benefits attendant in the compounds employed in forming tread strip . Nominal differences in tire sizes which occur when tire casings are produced by different tire manufacturers are accommodated, without producing deleterious stresses in assembly 38. Stresses on skirt 24 of replacement tread 10 are reduced because of the correspondence between the configuration of replacement tread 10 and the buffed profile of tire casing 12.

The final retreaded tire has an improved appearance and is readily marketable. Skirts 24 substantially cover shoulder 18 and may extend onto side wall 16 of tire casing 12. This permits nearly complete elimination of the preexisting tread shoulder design from the tire casing, and coverage of substantially all of the areas of the casing which have been buffed.

Further, the replacement tread 10 offers substantial benefit to eliminate or reduce lifting caused by lateral forced encountered during use then prior art top cap retreads. An example would be in multi-axle applications where cornering tends to cause the side profile of conventional top caps to roll during some stress applications. Such rolling tends to be reduced or eliminated with use of the replacement tread 10 of the invention.

Referring again to Figures 1 and 4, tread strip 10 may also include respective decoupler grooves 40 positioned adjacent the outer edges of tread portion 20. Decoupler grooves 40 are known for use in new tires to aid in reducing irregular wear in the exterior shoulder of the tire. In particular, during cornering, a narrow rib 42 to the outside of decoupler groove 40 is forced against respective major shoulder ribs 4 4. of tread 20. Narrow rib -1642 supports shoulder 44, and acts as a barrier to protect shoulder rib 44 from eroding during cornering. A squared operative edge is thus maintained. Decoupler grooves have not, however, typically been used in replacement tread strips. In the context of tread strip 10, decoupler groove 40 serves not only as an aid in reducing irregular wear, but also to provide additional relief against potential stresses in the assembly caused by network biasing during retreading permitting skirt 24 to contact shoulder areas 18 of the casing.

It will be understood that, while various specific dimensions of the replacement tread according to the invention have been described, they are not so described in a limiting sense, and the replacement tread according to the invention may be suitably configured to be adapted to a tire casing of any size, as is understood in the art.

Claims (31)

1. A prevulcanized tread for installation on a tire casing, said tread comprising a sole (22), respective tread shoulders (21), an upper tread portion (20) manifesting a predetermined tread design disposed between said shoulders (21) and overlying said sole (22), and respective skirts (24), each of said skirts including a generally planar interior wall (30) disposed at predetermined angle with respect to vertical, an exterior sidewall (26), extending downwardly and outwardly from said tread shoulders, disposed at predetermined angle with respect to vertical, and an end portion (28), connecting said interior and exterior walls (26, 30), characterized in that each of said skirts (24) includes an interior interconnecting portion (32) coupling said interior wall to said sole and comprising, in a cross-sectional plane, a portion of a nominal arc having a predetermined radius of curvature, Ry S , less than said sole radius Ryp said sole (22) comprises, in a cross-sectional plane, a portion of a nominal arc having a predetermined radius of curvature R TIsaid upper tread portion (20) has associated therewith a nominal arc extending between said tread shoulders (21), having a predetermined radius of curvature equal to said sole radius of curvature, Ryp plus a predetermined constant, and a predetermined arc length, AWy, said exterior sidewalls (26) of said respective skirts (24) are disposed such that the portion of said nominal arc of radius Ryj extending between respective intersections with exterior sidewalls is of a predetermined length, ΑΜθρ, greater than AWy, and said interior walls (30) of said respective skirts (24) are disposed such that the portion of said nominal arc of radius Ryj extending between respective intersections with nominal extensions of said interior walls is of a predetermined length, AWjp, less than AWy.

2. The prevulcanized tread of Claim 1 wherein ΑΜθθ is equal to AWy plus a first predetermined constant, iC, and AWjg is equal to tread arc width AWy minus a second predetermined constant, . - 18

3. The prevulcanized tread of Claim 2 wherein constant Kj is within the range of about 0.47625 cm (0.1875 inch) to about 1.905 cm (0.750 inch).

4. The prevulcanized tread of Claim 3 wherein constant K. is within the range of about 0.635 cm (0.250 inch) to about 1.905 cm (0.750 inch).

5. The prevulcanized tread of Claim 4 wherein constant Kj is equal to approximately 0.9525 cm (0.375 inch).

6. The prevulcanized tread of Claim 2 wherein constant K 2 is within the range of about 0.15875 cm (0.0625 inch) to about 0.635 cm (0.250 inch).

7. The prevulcanized tread of Claim 6 wherein constant S< 2 is within the range of about 0.3175 cm (0.125 inch) to about 0.635 cm (0.250 inch).

8. The prevulcanized tread of Claim 7 wherein constant K 2 is equal to approximately 0.39624 cm (0.156 inch).

9. The prevulcanized tread of Claim 2 wherein AW^ is equal to ΑΜθθ minus a third predetermined constant, Kg, where constant Kg is within the range of about 0.635 cm (0.250 inch) to about 2.54 cm (1.000 inch).

10. The prevulcanized tread of Claim 9 wherein constant Kg is within the range of about 0.9525 cm (0.375 inch) to about 2.54 cm (1.000 inch).

11. The prevulcanized tread of Claim 10 wherein constant Kg is equal to approximately 1.34874 cm (0.531 inch).

12. The prevulcanized tread of Claim 2 wherein constant Kj is equal to approximately 0.9525 cm (0.375 inch), and constant K 2 is equal to approximately 0.39624 cm (0.156 inch).

13. The prevulcanized tread of Claim 1 wherein R- ( -$ is in the range of about 1.27 cm (0.500 inch) to about 3.81 cm (1.500 inch).

14. The prevulcanized tread of Claim 13 wherein R TS is equal to approximately 1.905 cm (0.750 inch). - 19

15. The prevulcanized tread of Claim 1 wherein angle θ of said interior walls (30) is within the range of about 25 to about 40°.

16. The prevulcanized tread of Claim 15 wherein angle θ of said interior walls (30) is within the range of about 30 to about 35°.

17. The prevulcanized tread of Claim 16 wherein angle θ of said interior walls (30) is equal to approximately 32°.

18. The prevulcanized tread of Claim 12 wherein angle θ of said interior walls (30) is equal to approximately 32°.

19. The prevulcanized tread of Claim 1 wherein angle a of said exterior sidewalls (26) is within the range of about 25 to about 5°.

20. The prevulcanized tread of Claim 19 wherein angle a of said exterior sidewalls (26) is within the range of about 20 to about 12°.

21. The prevulcanized tread of Claim 20 wherein angle a of said exterior sidewalls (26) is equal to approximately 16°.

22. The prevulcanized tread of Claim 18 wherein angle a of said exterior sidewalls (26) is equal to approximately 16°.

23. The prevulcanized tread of Claim 12 wherein angle a of said exterior sidewalls (26) is equal to approximately 16°.

24. The prevulcanized tread of Claim 1 wherein said interior wall (30) and said exterior wall (26) of said skirts (24) extend downwardly from said sole (22) a skirt height (H s ) of from about 0.635 cm (0.25 inch) to about 5.08 cm (2 inch).

25. The prevulcanized tread of Claim 24 wherein skirt height (H s ) is approximately 1.53416 cm (0.604 inch).

26. The prevulcanized tread of Claim 24 wherein said interior wall (30) and said exterior sidewall (26) are connected at their downward ends by an endwall (28) which is suitably planar, manifesting substantially as a point or a short straight line in the transverse plane. - 20

27. The prevulcanized tread of Claim 1 wherein said exterior sidewall (26) has a distance 0^, said distance being between a line extending along said skirt interior wall (30) along a perpendicular to the juncture of said sole nominal arc at radius Ryj with said skirt exterior sidewall, said distance being in the range of about 0.2286 cm (0.090 inch) to about 0.9144 cm (0.360 inch).

28. The prevulcanized tread of Claim 27 wherein said distance is in the range of 0.3429 cm (0.135 inch) to about 0.9144 cm (0.360 inch).

29. The prevulcanized tread of Claim 28 wherein said distance Dy is approximately 0.58928 cm (0.232 inch).

30. A retread tire comprising a tire casing (12) and a prevulcanized tread (10) installed thereon, said casing (12) including a peripheral crown (14) and respective shoulders (18) disposed on either side of said crown (14), said prevulcanized tread (10) comprising a sole (22), respective tread shoulders (21), an upper tread portion (20), manifesting a predetermined tread design, disposed between said shoulders (21) and overlying said sole (22), and respective skirts (24), each of said skirts including a generally planar interior wall (30) disposed at predetermined angle θ with respect to vertical, an exterior sidewall (26), extending downwardly and outwardly from said tread shoulders, disposed at predetermined angle a with respect to vertical, and an end portion (28), connecting said interior and exterior walls (26, 30), characterized in that said peripheral crown (14) of said casing (12) comprises, in a cross-sectional plane through the nominal axis of rotation of said casing, an arc having a predetermined radius of curvature, R c , and a predetermined arc length, AW , said respective shoulders (18) comprise, in said cross-sectional plane, an arc having a radius of curvature, R less than R , kz Ο k» said sole (22) of said prevulcanized tread (10) comprises, in said crosssectional plane,, a portion of a nominal arc having a predetermined radius of curvature Ryj corresponding to said predetermined range of values of casing crown radius of curvature, R cs , said sole (22) overlying said casing crown (14), said upper tread portion (20) has associated therewith a nominal arc in said cross-sectional plane extending between said tread shoulders (21), - 21 having a predetermined radius of curvature equal to said sole radius of curvature, Ryj, plus a predetermined constant, and a predetermined arc length, AWy, each of said skirts (24) includes an interior interconnecting portion (32), coupling said interior wall (30) to said sole, and comprising, in a cross-sectional plane, a portion of a nominal arc having a predetermined radius of curvature, Ry^, less than said casing shoulder radius R c $, said exterior sidewalls (26) of said respective skirts (24) are disposed such that the portion of said nominal arc of radius R TI extending between respective intersections with exterior sidewalls is of a predetermined length, ΑΜθθ, greater than AWy, and said interior walls (30) of said respective skirts (24) are disposed such that the portion of said nominal arc of radius Ryj extending between respective intersections with nominal extensions of said interior walls is of a predetermined length, AWjg, less than AWy.

31. A prevulcanized tread according to Claim 1 or a retread tire according to Claim 30 substantially as herein described with reference to and as shown in the accompanying drawings.