NZ215318A - Tyre tread with varying pitch pattern having constant groove volume to pitch length within each pattern unit - Google Patents

Description

V p,:' •: ../r.?.-.8£ O- . : . ...iicsti-'n Filed: c .'"T'n ' 2 9 APR 1988 " Fwi.,. :. t.on Date: P.O. Jcurnal, No: .... 21 53 ) PATENTS FORM NO. 5 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION A TIRE IMPROVED IN UNIFORMITY •&t WE SUMITOMO RUBBER INDUSTRIES, LTD., of 1-1 Tsutsuicho 1-chome, Chuo-ku, Kobe-shi, Hyogo-ken, Japan, a Japanese Company, hereby declare the invention, for which J#we pray that a patent may be granted to me/us, and the method by which it is to be performed, to be particularly described in and by the following statement 215318 A TIRE IMPROVED IN UNIFORMITY BACKGROUND OF THE INVENTION The present invention relates to a pneumatic tire, more particularly to a low noise tire having a tread of uniform thickness, suitably used as a tire for automobiles.

In general,the tread pattern of the tire is composed of repeating design units in which the design units are arranged circumferentially. Therefore the air in the grooves which divides the design units of the tread pattern from each other is pushed out as the tire travels upon a surface. The movement of the air is repeated by each design unit, and thus a wave of compression and expansion of the neighboring air creates an audible sound that is called a pattern noise. In order to make the sound unobtrusive and to prevent the increase of the sound owing to the sympathetic vibration thereof, a variable pitch method in which the design unit length about the tire is varied, may be utilised. The variable pitching method varies the time intervals of pulse sound and pulse vibration and prevents the sound generated becoming concentrated into very a small frequency'range of the sound spectrum. The variable pitching method comes from the theory of frequency modulation used in radio engineering. However, even if a suitable degree of dispersion of the pitch length variation of the design units is selected.,^ the thickness of the tread rubber sometimes becomes ununiform, producing a bad effect on the uniformity 215318 of the lateral force variation and radial force variation, in the case of the ratio of the longest design unit length to the shortest being too large.

This can be better understood by referring to the tire production process. Fig.l illustrates the unvulcanized rubber sheet extruded from a extruder .which is used to form the tread rubber of the tire, having uniform thickness longitudinally.

This rubber sheet, in the molding process, is applyed along with the carcus crown portion of a vulcanizing metal mold,, and the tread pattern is formed by the pattern of the metal mold, during the step of vulcanization. The rubber which is plasticized by a high temperature and pressure moves along the pattern formed on the metal mold. However, when the arrangement in the cicunferential direction of the pattern of the metal mold is not uniform, the movement of the rubber is disturbed, resulting in unevenness in the radial thickness of the vulcanized tread rubber of the tire, in the circumferential direction..Especially, in the block type tread pattern tire, the movement of the rubber can be easily prevented and the unevenness becomes more conspicuous.

BRIEF SUMMARAY OF INVENTION Accordingly, it is an object of the invention to provide a tire having improved thickness -uniformity of tread rubber whicn utilises the pitch variable method, and has improved force variation characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS Fig.l is a fragmentary plan view showing an eirbolTiment of the tire according to the present invention; Fig.2 is a perspective view of an example of a rubbfr sheet / , » I ~ ^ -t. o which can be used to form the tread rubber of the tire; and Fig. 3 is a graphical display showing the relationship between the ratio Ai/Aj and the force variation.

DETAILED DESCRIPTION OF THE INVENTION Fig. 1 shows a plan view of a section of tread pattern according to one embodiment of the present invention, the tread portion extends circumferentially about the tire and comprises a plurality of design units PI, P2 and P3 having different pitch lengths LI, L2 and L3 disposed repeatedly and extending circumferentially about the tire.

Ai = Sum total groove volume of grooves in unit (Pi) Pitch length (Li) of the unit (Pi) = r- Vik/Li wherein Vik = volume of groove &1 Gik and k = 1 to n (i.e. the groove number) and i = 1 to 3 (i.e. ith design unit) = Vil + Vi2 + Vi3 + . . . + Vin Li In the embodiment illustrated in figure 1 the design unit PI has the grooves Gil, G12 .... Gill arranged symmetrically about the equator L and accordingly. n Ai = YZ 2.Vik/Li k=l Thus AI = 2 x Vll-t-Vl2+Vl3+Vl4+Vl5+Vl6+Vl7+Vl8+Vl9+Vll0+Vlll LI A2 = 2 x V21+V2 2 +V2 3+V2 4 +V2 5 +V2 6 +V2 7 +V2 8 +V2 9 +V210+V211 L2 A3 = 2 x V31+V32 +V3 3+V34+V35 +V3 6+V37+V3 8+V3 9+V310+V311 L3 According to the present invention it has been found that when the relative ratio Ai/Aj (wherein i or j=l, 2 or 3, h&frr? ij^j) is more than 0.9 and less than 1.1 the tread rubb/^ flows freely and uniformly IL? 3 215318 in the vulcanizing metal mold, resulting in improved uniformity in thickness of the tread rubber of the tire. i.e.: when 0.9 Z Al_ , Al^ , A2_ , A2_ , A3 , A3_ , ^1.1 A2 A3 AI A3 AI A2 By way of example, and for the purpose of illustration, many tires (tire size 205/60 R15) having the tread pattern shown in Fig. 1 were tested, ranging over the relative ratio of Ai/Aj from 0.5 to 1.5, in order to find the relationships between the ratio Ai/Aj and the radial force variation. As illustrated in Fig. 3, the radial force variation increases undesirably according to the variation of the ratio from the center, being 1, increases. Consequently, the radial force variation is preferably in the ratio is between 0.9 and 1.1. It is also seen that the tread rubber of the tire having the above described ratio Ai/Aj is made almost uniform in thickness. Accordingly, the tire of the invention which is designed in accordance with the pitch variable theory, makes the rubber flow freely and uniformly in the vulcanizing process. While the invention has been described in detail with reference to the rib-lug pattern tread, the present invention may be generally applied to the block pattern tread, the rib pattern tread, the lug pattern tread and others.

As apparent from the above description, the present invention discloses a tread designed according to the pitch variable theory which effectively reduces noise. Further, since the relative ratio Ai/Aj is between 0.9 and 1.1, the rubber flow in the vulcanizing metal mold being made uniform, the force variation may be preferably improved.

# © While certain representative embodiments and details have been shown for the purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the present invention .as defined in the appended claims.

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Claims (2)

WHAT WE CLAIM IS:

1. A pneumatic tire, comprising a tread provided with a tread pattern, said tread pattern comprising a plurality of repeating design units successively placed about the circumference of the tire and each unit including grooves in the tread forming said design, and said' design units having at least two different circumferential pitch lengths, characterized in that the sum total groove volume of the grooves in each design unit divided by the pitch length thereof is substantially constant so that the quotients (A) in the respective units satisfy the following relation: 0.9 £ Ai/Aj ^ 1.1 wherein Ai = Sum total groove volume of grooves in ith unit Pitch Length of ith unit Aj = Sum total groove volume of groove in jth unit Pitch length of jth unit and i, j = 1, 2 or 3.

2. A tire substantially as herein described with reference to Figure 1 or Figures 1 and 2 of the accompanying drawings. SUMITOMO RUBBER INDUSTRIES, LTD By their attorneys BALDWIN, SON & CAREY