Tire treads

Abstract: A tire tread of elastomeric material formed into ground engaging elements (1) is provided with sipes (2) in said elements. The sipes are in the form of incisions. A first incision portion(s) extends inwardly from the tread surface and then splits into at least two incision portions (3,4) which continue to extend away from the tread surface while at the same time diverging apart from one another.

Description: The present invention relates to that portion of tires that comes into contact with the road when the tires are normally inflated and under normal load i.e. to tire treads. That is to say, the invention applies both to new and to retreaded tires. The tread portion of a pneumatic tire generally has a plurality of grooves therein defining ground engaging elastomeric elements such as blocks and ribs. These ground engaging elements are usually provided with narrow incisions, such as sipes or cuts. A sipe is a groove having a width in the range from about 0.2 % to 0.8 % of the tread width, i.e. the arc length of the tread surface in the axial direction, that is, in a plane containing the axis of rotation of the tire. The sipe closes when it is located in the tire footprint, i.e. in the contact patch of the tire tread with a flat surface at zero speed and under normal load and pressure. Such a sipe typically is formed by steel blades inserted into a cast or machined mold or tread ring therefor. A cut is usually made by a cutting tool and has a negligible width. The incisions may extend circumferentially or laterally about the tread in a straight, curved or zig-zag manner and may be as deep as the grooves or have a depth which is even greater than the groove depth. The incisions can pass through the sides of the ribs and blocks or be confined to their interior. It is known to have the incisions lying in planes which are not perpendicular to tangents to the surface of the tread at their point of intersection; the inclination of the planes defining neighboring incisions can be identical or differ stepwise along the block length. It is further known to use incisions having a depth which varies across their length, e.g. so-called hump-blades. Incisions improve the flexibility of the blocks without destroying their solidity. The easy relative longitudinal sliding between the opposed faces of an incision weakens the resistance of the blocks to flexing in the contact area between tread and road and therefore slows down the heat built-up of the tire. The incisions increase also the friction coefficient of the elastomeric blocks and they multiply the number of the ground engaging edges in the tire footprint, hence improving markedly the grip of the tire. As the tire wears down, the tread depth thereof changes. The tread grooves generally become narrower and the incisions, being just a few mm deep, disappear when the tread rubber wears off. The reduced depth of the grooves and of the incisions hampers the wiping and digging action of the tread surface hence reducing the traction and braking characteristics of the tire. Wear continuously increases the stiffness of the blocks and ribs, which has been chosen to optimize the cornering characteristics of the new tire. EP-A- 378 090, cited under Art. 54(3), discloses a tread pattern provided with incisions having a forkshape in cross section. At least two incision portions extend a single portion reaching up to the tread surface. The aim of the invention is to avoid the above mentioned drawbacks and to create a tire tread having a more constant handling response regardless of the state of wear of the tread pattern. This aim is achieved by the tire tread and the mold for vulcanizing tires as described in the appended claims. The advantages are an overall lower noise and a higher comfort through a uniform block stiffness during the tire life. The wear resistance of the tread is improved through a diminished slipping of the blocks on the road. As the tread blocks follow better the irregularities of the road, the grip of the tire is improved, especially towards the end of its lifetime. Furthermore, the self aligning torque is diminished, even when the tire has a worn tread. Preferred embodiments of the invention are now described with reference to the annexed drawings. Figure 1 is a perspective view of a tread block according to document EP-A- 378 090 during the vulcanization step; Figures 2A, 2B and 2C are fragmentary views of the same tread pattern portion being worn to different degrees; Figure 3, 5, 7 and 9 are side views of blades which can be used to implement the invention; Figures 4, 6, 8 and 10 are cross-sections of the blades shown respectively in Figures 3, 5, 7 and 9. With reference to Figure 1, there is represented a single elastomeric block 1 of a tread pattern during the vulcanization step having a blade 2 according to the invention embedded therein. The neighboring blocks and ribs of the tread pattern have not been represented for simplification and clarity; the same applies to the mold, which can be either of the segmented or of the two-piece type. The radially outermost portion 5 of the blade 2, has a rectangular shape and is partly inserted in and glued to the mold (not represented). This rectangular portion lies in a plane which can form an angle between 0° and 30° with a radial direction passing through it. Radial is used to mean directions radially toward or away from the axis of rotation of the tire. The radially innermost two lateral portions 3, 4 of the blade 2 diverge from the plane including the rectangular portion 5 and outline in this embodiment part of a cylinder. To ascertain a perfect filling of the block shape with elastomeric material, it is preferred that the two portions 3, 4 of the blade include several flow openings (not represented), allowing a free flow of the elastomeric material to the radially outermost parts of the block at the beginning of the shaping and throughout the vulcanizing step. Such flow openings can have a circular section and a diameter of a few millimeters. When the tire is cured and the mold is opened, the indentations and blades are retracted from and out of the tread pattern. The radially innermost portions 3 and 4 of each blade cut the elastomeric material situated in the flow openings and leave the tire tread through the sipe molded into the block 1 by the radially outermost portion 5 of the blade; in so doing, the blade portions 3 and 4 move towards each other, and the block deforms and opens slightly radially outwardly on both sides of the sipe. It is to be considered that the representation in Figure 1 is not to scale and that the spacing and the dimensions of the blade portions have been exaggerated for explanatory purposes: the overall height of a blade 2 - measured in a radial direction - is generally comprised between 3 and 10 mm and the maximum spacing between the two radially innermost portions 3, 4 is about 8 mm, so that the blade can leave the elastomeric compound without damaging the block. A preferred elastomeric compound has a Shore A hardness in the vulcanized state of 63 and a modulus of 7 MPa. Referring now to Figures 2A, 2B and 2C, there is shown the same pitch length of a tire tread made in accordance with the invention, in different states of wear. By pitch length is meant the design cycle length of the repetitive tread pattern elements around the tire. As represented in Figure 2A, the tread element 20, almost new, is divided into two parts 21 and 22 by the sipe 23 made by the radially outermost portion 5 of the blade 2. The same tread portion shown in Figure 2B, is worn to about 30 % of its original radial height and the tread element 20 has two sipes 24 and 25 therein, made by the radially innermost two portions 3, 4 of the blade 2. The two sipes compensate partly for the loss of grip of the tire due to the diminished height of the surrounding tread element. The same tread portion shown in Figure 2C is worn to about 70 % of its original radial height. The increase of the distance between the sipes (references 24 and 25) reduces mainly the stiffness of the tread element 20. In Figure 3 and Figure 4, which represent respectively a side view and a section along the plane VI-VI, through the blade of Figure 3, there is shown a further embodiment of the invention, wherein the flow openings 50 in the blade have been chosen as large as possible. Elongated strips 52 and 53 connect a plate 54 of rectangular shape to two cutting edges 51, which are as far as possible perpendicular to the surface of the unvulcanized tire when they enter the tire during the closing of the mold 55. When the mold is opened after the vulcanizing step, the elongated strips 52 and 53 draw the cutting edges 51, having a sharpened upper side, through the elastomeric material, thereby leaving cuts in the tire tread. In Figure 5 and Figure 6, which represents a section along the plane VIII-VIII, through the embodiment of the invention shown in Figure 5, a blade is represented having a radially outermost plate 70 fixed in the mold 77. Several elongated strips 71 connect two cutting edges 72, having both their radially innermost and outermost sides sharpened, to the plate 70. When the mold is open, the cutting edges are located one against the other as indicated by references 74 and 75. By closing the mold, the cutting edges cut into the unvulcanized tire and separate continuously in function of the inclination of the cutting edges to a radial direction intersecting them. Finally they take a spaced position as indicated by the references 73 and 76. When the mold is opened after the vulcanizing step, the elongated strips 71 pull the cutting edges 72 out of the elastomeric material, thereby cutting incisions in the tire tread. As shown in Figure 5, the cutting edges 72 may have a greater length than the plate 70. In a preferred embodiment, the strips 71 have a length of about 6 mm and are made of spring steel. For determining the inclination of the cutting edges, it must be considered, how the different pattern elements in the mold are closing in onto the unvulcanized tire. The lateral parts of the segments of a segmented mold, move for instance obliquely towards the unvulcanized tire. The cuts made by the cutting edges 73 and 76 have a smaller width than the sipes molded in the elastomeric material by the plate 70, the elongated strips 71 and the cutting edges themselves. This width difference affects the heat built up in the tread; it has however no major influence on the traction properties because it is the high local pressure at the edge of the incision which improves the wiping and digging action of the tread. Up to now the invention has been disclosed with a blade having an outer portion splitting up into two inner portions. In alternate embodiments shown on Figures 7 to 10, the outer portion splits up into three inner portions having possibly different lengths and depths. Figures 7 and 8 represent respectively a side view of a blade and its section along the plane X-X. The blade has a radially outermost portion 90 splitting up into one central portion 93 and two lateral portions 91 and 92. The central portion 93 is substantially situated in the continuation of the radially outermost portion 90. Both lateral portions separate from the central portion 93 and extend towards substantially the same total depth as the central portion. Both lateral portions as well as the central portion are provided with a multitude of flow openings 98 allowing a free flow of elastomeric material. A tread element worn to about 40 % of its original radial height will present three sipes to the ground. In order to facilitate the retraction of the blades out of the tread pattern, the lateral blade portions 91 and 92 have a smaller length than the central portion 93; furthermore, the radially outermost blade portion 90 has, in a preferred embodiment, a greater width than the other blade portions. This width can range between about 1 % and 3 % of the tread width. In a further embodiment of the invention shown as a side view in Figure 9 and as a section along the plane XII-XII in Figure 10, the radially outermost portion 110 splits up into one central portion 115 and one lateral portion 113 at about 35 % of the blade height. The central portion 115 on its turn splits up into a further central portion 116 and one lateral portion 112 at about 70 % of the blade height. Such a blade design allows even more precise fine tuning of the tread grip as a function of its state of wear. The central blade portions 110, 115, 116 take here a trapezoidal shape in order to facilitate retraction of the blade out of the vulcanized tire tread. The blade portions are provided with a multitude of flow openings 114. While certain representative embodiments and details have been shown for the purpose of illustrating the invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein within the scope of the claims.

Claims: A tire tread comprising elastomeric material, a plurality of grooves therein, defining ground engaging elastomeric elements (1), said elements being provided with incisions, said incisions being sipes having a width, a length and a depth, and cuts having a length and a depth, wherein at least one of said elements (1) has a first incision (23) therein extending from the tread surface towards the interior of said elastomeric material, where said incision splits up into at least two incision portions (24,25) extending away from the tread surface, said incision portions diverging apart from each other over at least part of their depth, part of at least one of said incision portions (24,25) having substantially no width. The tire tread according to claim 1, wherein said first incision (23) has a greater length than at least one of said incision portions (24,25). The tire tread according to claim 1, wherein said first incision (23) has a greater width than at least one of said incision portions (24,25). The tire tread according to claim 1, wherein said first incision lies in a plane forming an angle between 0° and 30° with a radial direction passing through it. The tire tread according to claim 1, wherein said first incision (23) plus any of said incision portions (24,25) have a combined radial depth which is substantially equal to the groove depth. The tire tread according to claim 1, wherein at least one of said incision portions (24,25) splits up into two further incision portions diverging apart from each other and extending away from the tread surface. A mold for vulcanizing tires including blades (2) partially inserted in the mold, the blades having a width, a length and a height, wherein at least one of said blades has a first portion (5) extending from the mold surface towards the interior of said mold, where said blade splits up into at least two blade portions (3,4) extending away from the mold surface, said blade portions diverging apart from each other over at least part of their height wherein said two portions (3,4) of the blade include several flow openings, allowing a free flow of elastomeric material at the beginning of the shaping and throughout the vulcanizing step. The mold according to claim 7 wherein the diameter of a circular flow opening is comprised between 1 and 1.5 mm. The mold according to claim 7 wherein a blade comprises a plate (54) of rectangular shape, fixed in the mold and elongated strips (52,53) connecting the plate (54) to two cutting edges (51) having a sharpened upper side. The mold according to claim 7 wherein a blade comprises a plate (70), fixed in the mold (77) and several elongated strips (71) connecting two cutting edges (72), having both their radially innermost and outermost sides sharpened, to said plate (70). The mold according to claim 7 wherein a blade has a radially outermost portion (90) splitting up into one central portion (93) and two lateral portions (91,92). A tire produced in a mold according to any of claims 7-11.

Inventors: MAITRE BRUNO PIERRE, MAITRE, BRUNO PIERRE
Applicants: GOODYEAR TIRE & RUBBER, THE GOODYEAR TIRE & RUBBER COMPANY
Application year: 1990
Main classification: B60C11
IPC codes: B29D30, B60C11, B29C35