Pneumatic tire

Abstract: Herein disclosed is a pneumatic tire which has its grounding surface formed with V-shaped grooves, each of which has its apex located on a grounding center line of a tire ground surface and its transverse branches inclined widthwise of the tire in the running direction of the tire. The average angle of the transverse branches made with the leading edge of the grounding surface, where the grounding surface is widthwise equidistantly divided into ten sections, is set absolutely constant or within an allowance.

Description: The present invention relates to a pneumatic tire having an excellent water-repellency, an in particular, to a pneumatic tire having its ground contacting surface formed with V-shaped grooves, the transverse branches of which extend widthwise of the tire and are inclined towards the running direction thereof. Such a tire is known for instance from EP-A-064.934 and EP-A-296.605. A number of techniques of a tread pattern formed on the tread surface (i.e., ground-contacting surface) of the tire have been proposed in the prior art, in terms of the water-repellency, the steering stability and so on. However, none of them have satisfied the water-repellency yet. Generally speaking, it is natural to lay stress on the repellency of the tire if the running on a poorly drained road such as a superhighway is taken into consideration. Water-repelling grooves have to be provided on the tread surface by noting the ground contact pattern taken when a vehicle runs on such road. The posture of the tire relative to the road surface is determined according to the so-called "wheel alignment" such as the toe-in or the camber angle. Although these are fundamental, it is necessary for considering the water-repellency of the tire to note that the ground contact pattern has its shape varied according to the wheel alignment when the tire is attached to the vehicle. The arrangement of the water-repelling grooves made heretofore has been designed without paying attention to the above-specified wheel alignment so that the sufficient repellency cannot be achieved. The present invention has been conceived by making use of the wheel alignment (usually having a toe-in of -3 to 3 mm and a camber angle of -0°50' to 1°) when tires are attached to the vehicle. More specifically, the present invention contemplates to provide a pneumatic tire which has its ground contacting surface formed with a tread pattern having an excellent water-repellency when the vehicle runs on a superhighway. It is, therefore, an object of the present invention to provide a pneumatic tire which has a sufficiently excellent water-repellency. In order to achieve this object, the gist of the present invention resides in a pneumatic tire having its ground contacting surface formed with V-shaped grooves, each of which has its apex located on ground contact center line of a tire ground contacting surface and its transverse branches extend widthwise of the tire and are inclined towards in the running direction of the tire, wherein the average angle of the transverse branches made with the leading edge of the ground contacting surface at the individual eight ones of ten widthwise equidistantly divided sections of the ground contacting surface excepting the two side sections is constant absolutely or within an allowance. Here, the "ground contact center line" means the circumferential line on which the circumferential length of a tire takes its maximum in the ground contacting surface. On the other hand, the "average angle" means the angle which is averaged from the angles at three points of the widthwise center and both sides with respect to the individual sections. Figs. 1 (A) to 1 (D) are explanatory views showing the relations between a tire placed on a road, along which rain water flows, and the water stream; Fig. 2 is a partially enlarged schematic view showing the relation between the ground contacting surface and the groove pattern of the tire according to the present invention; and Fig. 3 is a front elevation showing the tire according to one embodiment of the present invention. Since, generally speaking, the tire is given the wheel alignment when it is attached to a vehicle, as has been described hereinbefore, the location of the ground contact center line is slightly offset from the widthwise center line of the tire even while the vehicle is running straight. On the other hand, the apexes of the aforementioned V-shaped grooves offer the most effective water repellency if they are positioned on the ground contact center line. This makes it necessary to offset the apexes of the V-shaped grooves slightly from the widthwise center line of the tire. On the other hand, our repeated experiments have confirmed that the displacement of the ground contacting center line from the widthwise center line of the tire given the ordinary wheel alignment is within one fifth of the ground contacting width. The present invention has been conceived on the basis of the above-specified confirmation by setting at a constant value α the average angle made between the inclined grooves (or transverse branches), having their respective apexes on the ground contacting center line and extending, on respective sides of the ground contacting center line, in the widthwise directions of the tire at an inclination relative to the running direction of the tire and the leading edge of the ground contacting surface at the individual eight ones of ten widthwise equidistantly divided sections of the ground contacting surface excepting the two side sections. As has been described hereinbefore, moreover, the constant angle α is set within a range of 15° to 45° (i.e., 30° ± 15°), preferably a range of 25° to 35°, although varying with the wheel alignment. Under 15°, the running time period from the start to the end of the transverse grooves is too short for the water to steal out smoothly through the transverse grooves so that the water-repellency is dropped. Over 45°, on the contrary, the time period from the start to the end of the transverse grooves is so long that the water flowing in the transverse grooves is confined for a long time, thus dropping the water-repellency. On the other hand, it is most preferable that the angles at the individual eight sections be a constant value α. Despite of this preference, however, our repeated experiments have confirmed that the angles can retain the water-repellency even if they fluctuate within a range of ±5° (preferably ±3°). In the two side sections of the ground contacting surface divided widthwise into the ten sections, on the other hand, it is practically preferable from the standpoint of the steering stability during the cornering or the prevention of partial wear at the shoulder of the tire to attempt to maintain the circumferential rigidity of the shoulder. Therefore, it is recommended that the transverse grooves in the two side-end sections (or in the tires shoulders) are inclined at an angle of 70 to 110° relative to the circumferential directions of the tire. The present invention will be described in detail in connection with its structure with reference to the accompanying drawings. In case a cylinder is placed at a right angle with respect to the flow of a fluid or on a road surface, along which rain water flows, for example, as shown in Fig. 1 (A), the water is shunted, as indicated by arrows b in the vicinity a (which takes the maximum grounding length ℓ max ) of the leading end of the ground contacting surface of the cylinder 1. In the ground contacting surface of the running tire, too, relative streams will occur like the case of the cylinder 1. On the other hand, a certain point (e.g., the edge of a block) of the tire moves while drawing a cycloid 10, as shown in Fig. 1 (B), if the tire is viewed from the side, so that the point abuts against the road surface at a right angle at its ground contacting instant. As a result, the ground contact leading edge splashes the water radially outwardly, as indicated by the arrows b in Fig. 1 (C). In the actual tire, those splashes are combined to establish relative streams, as indicated by the arrows b in Fig. 1 (D). Here, arrow 11 designates the running direction of the tire. Taking these points into consideration, therefore, it has been found that the intersection angle α made between transverse grooves G and the leading edge F of the ground contacting surface in the running direction be constant on the basis of the fundamental concept of the rugged (transverse) grooves for releasing the water from the ground contacting surface as smoothly as possible and for the shortest time. As a result, the rate and direction of the water flowing through the transverse grooves G are gently varied without energy loss so that the water-repellency is improved. Here, letter t designates the ground contact center line. Fig. 2 is a partially enlarged schematic view showing the relations between the ground contacting surface and the groove pattern of a pneumatic tire according to the present invention so as to explain the groove pattern. For purposes of description of the present invention, each transverse groove G in the groove pattern is divided in the ground-contacting surface D equidistantly into ten sections in a tire widthwise direction (Z₁, Z₂, ..... and Z 1⊘ ) as shown by broken lines 12. Also, each transverse groove G is in the form of a continuous groove comprising two segments or branches extending from the ground-contacting center line t toward one and the other side ends (shoulder portions) of the tire respectively, and each segment or branch is inclined at an angle α relative to the leading edge F of the ground-contacting surface. Here, it is most preferable that the transverse grooves G be formed in the form of curved ones making the constant angle α with respect to the leading edge F of the ground contacting surface. As a matter of fact, however, the angle of the transverse grooves G with the leading edge F may vary within a small range, the average angle β (although not shown) of the individual sections for the inclined grooves in the common direction may be substantially set at the constant angle α. In this case, the above-specified average angle β at each section is allowed to vary within a range of ±5° (preferably ±3°) with respect to the constant angle α. Likewise, in the section Z₆ containing the ground contact center line t, the average angles made between the transverse grooves G inclined transversely in the running direction across the center line t and the leading edge F of the ground contacting surface can be made different at the righthand and lefthand sides. In this case, it is necessary to set the difference between the righthand and lefthand angles within 5°. By this setting, too, it is possible to provide a tire which has an excellent water-repellency without any trouble in repelling the water on the road effectively through the transverse grooves G to the two sides of the running tire. Although no circumferential groove is formed in the tread, as shown in Fig. 2, the water repelling effect can be augmented by adding the circumferential grooves to the groove pattern of Fig. 2. Considering the steering stability during the cornering and the prevention of the partial wear of the tire shoulder, it is preferable that at least the portions of the transverse grooves G, which are open to the two sides 3 and 3' (i.e., the two shoulders) at the sections Z₁ and Z₁₀ form angles of 70° to 110° with respect to the circumferential direction of the tire without considering the relations to the leading edge F. Under 70°, the block rigidity is insufficient to lower the steering stability of the tire, and the block deformation is large to cause the partial wear inevitably, in case a large load is exerted during the cornering. Over 110°, similar phenomena will occur. Fig. 3 is a front elevation showing a tire according to one embodiment of the present invention. In this embodiment, the tire is formed with not only the transverse grooves G inclined widthwise of the tire but also four main grooves 2 extending in the circumferential direction. Reference numeral 13 designates a circumferential line which is positioned at the widthwise center line of the tire on the ground contacting surface. The present invention will be further described in the following in connection with its Example. Example: A smooth tire (Size: 185/60R14) having no tread pattern was prepared and inflated with air to have a regular internal pressure (1.9 Kg/cm²). A normal load (410 Kg) was exerted upon the tire to confirm the shape of the leading edge of the ground contacting surface of the tire. The ground contact center line of the ground contacting surface was offset by 8 mm from the widthwise center line to one side. Here, the tire was attached to the vehicle with a toe-in of 1 mm and a camber angle of -10', but this wheel alignment should not limit the scope of the claim of the present invention. Tire of the Invention The ground contacting surface of this smooth tire was widthwise divided into ten sections and was drawn with circumferential dividing lines and the ground contact center line. The ground contacting surface was further drawn on the tread with predetermined leading edges at a circumferential pitch of about 30 mm. Lines were drawn at the individual sections excepting the two sides ones Z₁ and Z₁₀ from the individual points of intersection of the dividing lines, the ground contact center line and the leading edge to the tire sides in the running direction of the tire at an angle of 30° to the leading edge line of the ground contacting surface, and other lines were drawn in the two side sections Z₁ and Z₁₀ at an angle of 90° with respect to the circumferential direction of the tire. Thus, the positions of the transverse grooves were determined at the righthand and lefthand sides across the ground contact center line. In these ways, the lines for the transverse grooves were drawn on the tread pattern. Then, the positions of the circumferential grooves were determined at 15 mm and 40 mm from the ground contact center line at the two sides of the center line, and the lines for the circumferential grooves were then drawn on the tread surface. After this, the transverse grooves and the circumferential grooves were cut with a width of 7 mm and a depth of 6 mm to form the tread pattern. Tire of the Prior Art The tire of the prior art was prepared with the same groove area ratio, groove pitch and groove depth as those of the tire of the present invention. This tire was cut to form in its tread surface: transverse grooves of straight shape having an angle of 30° on the ground contact center line with respect to its circumferential direction in the running direction of the tire: and circumferential grooves in the same positions as those of the tire of the present invention, thus completing the tread pattern. The tires of these two types are evaluated as to the occurrence of the hydroplaning phenomena and the runability on a wet circuit according to the testing items, as will be described hereinafter. The test results are enumerated in Table 1. Method of Evaluating the Hydroplaning Phenomena The speed, at which the hydroplaning phenomena would occur, was evaluated when the tire passes the road having a water depth of 5 mm. The results are tabulated in index. The larger value is the more preferable. Method of Evaluating the Wet-Circuit Runability The lap time was measured in rain on the circuit road (which was crossed at several positions by streams having a depth of about 3 mm). The results are also tabulated in index. The larger value is also the more preferable. Tire (Invention) Tire (Prior Art) Hydroplaning 107 100 Runability 102 100 From Table 1, it can be concluded that the tire of the present invention is superior in the water-repellency to the tire of the prior art. As has been described hereinbefore, according to the present invention, the angles, at which the water-repelling grooves are arranged in the grounding surface of the tire, are set within a constant range with respect to the leading edge of the ground contacting surface of the tire. As a result, the water-repellency can be improved to make the best use of the water flows around and in the ground contacting surface of the tire, in case the tire runs on a pool, thus enhancing the water-repelling efficiency. Thus, the ground contact property of the tire can be improved. By improving the groove pattern at the shoulder of the tire, moreover, it is further possible to improve the steering stability during the cornering and the partial wear of the shoulder.

Claims: A pneumatic tire having its ground contacting surface formed with V-shaped grooves, the transverse branches of which extend widthwise of said tire and are inclined with respect to the running direction of said tire, characterized in that each of said V-shaped grooves has its apex located on a ground contact center line which means the circumferential line on which the circumferential length of a tire takes its maximum in the ground contacting surface, and in that the average angle of said transverse branches made with the leading edge of said ground contacting surface at the individual eight ones of ten widthwise equidistantly divided sections of said ground contacting surface excepting the two side sections is a constant angle α. A pneumatic tire according to claim 1, wherein said average angle is set within α ± 5°. A pneumatic tire according to claim 1, wherein the constant angle α is set within 30° ± 15°. A pneumatic tire according to claim 2, wherein the constant angle α is set within 30° ± 15°. A pneumatic tire according to claim 1, wherein the grounding center line of said ground contacting surface is positioned within a range of one fifth of the ground contating width of the tire from the widthwise center line of said tire. A pneumatic tire according to claim 1, wherein the average angles made in the section containing said ground contacting center line between the transverse grooves inclined transversely in the running direction of said tire across said ground contacting center line and the leading edge of said ground contacting surface are different between their righthand and lefthand sides such that their difference is set within a range of 5°. A pneumatic tire according to claim 1, wherein the angles of said transverse grooves in at least the portions at the two side edges of said ground contacting surface in the two widthwise side sections of said ground contacting surface, where the ground contacting surface is widthwise equidistantly divided into ten sections, are set within a range of 70° to 110° with respect to the circumferential direction of said tire.

Inventors: SHIRAISHI AKIRA, SHIRAISHI, AKIRA
Applicants: YOKOHAMA RUBBER CO LTD, THE YOKOHAMA RUBBER CO., LTD.
Application year: 1989
Main classification: B60C11
IPC codes: B60C11