Noise control techniques for tires employing three different size design cycles

A tire adapted to rotate around an axis and including a plurality of design cycles extending circumferentially around the tire. The design cycles have a total of three different pitch lengths that define a percent variation and a percent unbalance, the product of the percent variation and the percent unbalance lying in the range of 1,250 to 1,350.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to tire treads, and more specifically relates to 
tire treads in which repeating design cycles or pitches are altered in 
order to control noise. 
2. Description of Related Art 
In the past, tire treads have been designed by the use of repeating design 
cycles or pitches extending circumferentially around the periphery of the 
tire. The circumferential length of the design cycles or pitches has been 
modulated in order to achieve objectives such as spreading the noise 
generated by the tire over a broad band of frequencies. In many such 
tires, multiple lengths of design cycles are utilized. Many tires have 
incorporated three discrete different sizes of design cycles or pitches 
that are placed around the peripheries of such tires. Typical ratios of 
lengths of the small, medium and large pitches used in such tires are 
shown in Table 1: 
TABLE 1 
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Ratios % Variation 
______________________________________ 
9-10-11 22.2 
11-13-15 36.4 
5-6-7 40.0 
9-11-13 44.4 
7-9-11 57.1 
9-10-12 33.3 
6-7-9 50.0 
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Typical pitch ratios used in conventional three-pitch tires are listed 
under the column entitled "Ratios." The first entry under the column 
headed "Ratios," reading "9-10-11," indicates a tire in which the small, 
medium and large pitch lengths have the ratio 9-10-11. Opposite the pitch 
ratio is a number indicating the percent variation from smallest to 
largest pitch. The percent variation is obtained from subtracting the 
smallest term of the pitch ratio from the largest term of the pitch ratio, 
dividing the remainder by the smallest term of the pitch ratio and 
multiplying by 100. For the first entry in Table 1, the percent variation 
is defined as ((11-9)/9).times.100. 
In this specification and claims, pitch ratios of three-pitch tires are 
expressed as three integers that cannot be reduced to three lesser 
integers by a common divisor. 
SUMMARY OF THE INVENTION 
The inventor has discovered that the noise created as a three-pitch tire 
rotates over a road surface can be better controlled by controlling the 
percent variation of the pitch ratios in combination with the percent 
unbalance of the pitch ratios. The percentage unbalance is defined by the 
following Formula 1: 
##EQU1## 
wherein L is the large pitch ratio term, M is the medium pitch ratio term, 
and S is the small pitch ratio term in a three-pitch tire. Surprisingly, 
the applicant has discovered that if the product of the percent variation 
and percent unbalance is between about 1,250 to 1,350, improved noise 
control properties result.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, FIG. 1 shows an exemplary tire 10 having a tread 
with an outer surface 11. Exemplary tread design cycles or pitches, such 
as 19-34, extend around the periphery of the tire. The generally 
horizontal lines extending across the tire tread indicate boundaries of 
the design cycles or pitches. The letters S, M and L indicate small, 
medium or large design cycle lengths or pitch lengths, respectively. The 
design cycles have three distinct and discrete different sizes having 
different ratios and sequences depending on the type of application 
intended. For example, for highway passenger tires, the preferred ratio of 
the three discrete sizes is 17-21-26, and the preferred pitch sequence for 
a 64 pitch tire is: S S M M L L L S S L M S L L S M S L L L L L M L M S S 
L M S M M L S S M M S M M L L S M S L M S M S S M L S S M L L L M L M M M, 
where S, M and L are the small, medium and large pitch lengths, 
respectively. 
Pitches 30-32 are shown enlarged and projected onto a flat surface in FIG. 
2 with reference to a center line CL. 
For light truck tires, the preferred pitch ratio is 18-21-25. 
For medium and large truck tires, the preferred pitch ratio is 19-21-24. 
The percent variation, percent unbalance, and the product of the percent 
variation and percent unbalance for the foregoing preferred embodiments 
are shown in Table 2: 
TABLE 2 
______________________________________ 
Ratios % Variation % Unbalance 
Var. .times. Unbal. 
______________________________________ 
17-21-26 52.9 25.0 1323 
18-21-25 38.9 33.3 1295 
19-21-24 26.3 50.0 1315 
______________________________________ 
The applicant has surprisingly discovered that the preferred embodiments 
exhibit products of percent variation times percent unbalance which are 
nearly constant within a tolerance of about two percent. In particular, 
the applicant has discovered that three-pitch tires having pitch ratios 
with a product of percent variation times percent unbalance within a range 
of about 1,250 to 1,350 produce superior noise control and enable the 
noise to be spread over a broad band of frequencies.