GB1580842A - Noise reduction in pneumatic tyres - Google Patents

Description

(54) NOISE REDUCTION IN PNEUMATIC TYRES (71) We, UNIROYAL, INC., of 1230 Avenue of the Americas, New York, New York 10020, United States of America, a corporation organized under the laws of the State of New Jersey, United States of America, do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a method of making a pneumatic tire, the object being to provide a tire that will operate at low noise levels in a predetermined speed range.

Among the many factors which relate to the generation of noise by pneumatic tires, there are four factors which are most paramount. These factors are (1) resonant lug vibration, (2) slip/stick vibration, (3) air pumping and (4) resonant reinforcement.

"Lug vibration" results from stressing the lugs in the contact patch (the footprint or interface between the road and the tread) in mutually perpendicular directions within the plane of the contact patch as well as in a direction normally of the contact patch. The fore and aft stresses of the lugs in the contact patch play the greatest role in generating noise. As each lug leaves the contact patch, the stresses are suddenly relieved and the lug thereby "pops out" of the contact patch, undergoes severe vibrations and generates noise.

"Slip/stick vibration" occurs as various portions of the tread at least partially slide ("slip") over the road in different directions at various spots in the contact patch. In the course of such sliding, at one or more other spots in the contact patch the horizontal shear stresses are usually sufficiently low to allow interfacial (contact patch) friction to instantaneously "anchor" one or more portions (for example, one or more lugs) of the tread to the road at the spots of low stress level. Continued partial sliding of the tread, however, results in a build-up of stress in the "anchored" lugs which releases them from their anchored condition and causes them to undergo an instantanous slip.

"Air pumping" is the action of forcing air in and out of the voids and sipes in and between the tread lugs. As a given void or sipe enters the contact patch, its volume is suddenly compressed thereby expelling or pumping air out. As each void or sipe leaves the contact patch, its volume suddenly expands, thereby pumping air back in. This rhythmic pumping of air in and out of the voids or sipes generates continuous pressure waves or sound energy and, if excessive, noise.

"Resonant reinforcement" involves certain mass distributions and elastomeric properties of a tire which result in the vibration of parts of the tire that respond to vibration-inducing energy imparted to the tire in a repetitively timed sequence by reacting to create sharply increased vibrations at various speeds of the tire. If the tread lugs are spaced from one another such that their excitation frequency peaks due to lugvibration coincide with the resonant frequencies of the tire, resonant reinforcement occurs and sound build-up results.

According to the present invention a method of making a pneumatic tire comprises the steps of: (a) Selecting a specimen tir.e corresponding in materials and overall dimensions to the required tire, the specimen tire having a tread pitch sequence extending around the whole circumference of the tread and comprising a plurality of lugs equal in size and shape spaced circumferentially from one another by respective grooves of equal size; (b) Determining the resonant frequencies of the specimen tire by rolling the tire at speeds within a predetermined speed range so causing vibration of the tire at different frequencies, plotting the level of sound from the tire against the frequency of vibration of the tire and determining as the resonant frequencies those frequencies at which peaks of sound level occur; (c) Selecting a plurality of tread pitch sequences which differ from one another and from the tread pitch sequence of the specimen tire, each of such.selected tread pitch sequences including a plurality of circumferentially spaced lugs defining therebetween respective grooves, said lugs and grooves being generally similar in shape to the lugs and grooves of the specimen tire but different slightly therefrom in their circumferential arrangement so that in each selected tread pitch sequence the leading edges of all the lugs are not equidistantly spaced throughout the whole tread pitch sequence; (d) Determining the excitation frequency peaks of each selected tread pitch sequence when notionally applied to the specimen tire and operated in the predetermined speed range; (e) Comparing the excitation frequency peaks of each selected tread pitch sequence with the resonant frequencies of the specimen tire, to determine the degree of coincidence between the excitation frequency peaks of each selected tread pitch sequence and the resonant frequencies of the specimen tire; (f) Choosing that selected tread pitch sequence for which said degree of coincidence is the lowest; and (g) Applying the tread pitch sequence so chosen to a tire.

Application of a tread pitch sequence so chosen to a tire reduces the resonant reinforcement experienced by that tire, by reducing the number of excitation frequency peaks due to lug vibration which coincide with the resonant frequencies of the tire.

Desirably the chosen tread pitch sequence comprises a circumferential arrangement of differing groups, each group comprising lugs and corresponding grooves of the same pitch, the lugs and corresponding grooves of each group having a different pitch to that of the lugs and corresponding grooves of each circumferentially adjoining group.

Conveniently, in the chosen tread pitch sequence the lugs of one group differ from the lugs of an adjoining group in regard to their respective dimensions circumferentially of the tread.

The invention will be described and illustrated in the accompanying drawings of a preferred embodiment, of tire constructed in accordance with the method, in which: Fig. 1 is a perspective view of a pneumatic tire; Fig. 2 is a schematic view illustrating the general deckline profile of the major grooves in the tread of tire; Fig. 3 is a schematic plan development of the tire lug pattern though not necessarily in a preferred sequence; and Fig. 4 is a schematic representation of a preferred "pitch sequence".

Referring now to the drawings, and more particularly to Fig. 1, these show a pneumatic tire denoted by the reference character 10. The tire 10 includes a pair of opposite bead-reinforced sidewalls 12 which are bridged by a circumferentially extending tread 14. The tread 14 includes what may be characterized as a road-contacting portion 16 and a pair of opposite non-roadcontacting portions 18 (only one of which is shown) which annularly adjoin corresponding ones of the sidewalls 12.

The tread 14 is provided with a circumferentially extending rib 20 to which are anchored on either side thereof a plurality of tread lugs 22. The lugs 22 define therebetween respective grooves 24. Each of the grooves 24 has a first extent 26 which is inclined with respect to the rib 20, and a second extent 28 formed in lhe non-roadcontacting portion 18 of the tread 14 and which extends beyond the depth of its corresponding first extent 26 formed in the road-contacting portion 16 of the tread 14.

Each of the aforementioned second extents 28 of the grooves 24 is directed generally radially of the tire 10 and closes proximate to its corresponding one of the sidewalls 12.

As illustrated in Fig. 2, the first extent 26 of each of the grooves 24, over the majority of its length, has a substantially uniform depth, whereas each of the aforementioned second extents 28 of each of the grooves 24 has a varying depth which decreases in the aforementioned second extents 28 in the direction toward its corresponding one of the sidewalls 12. Accordingly, each of the grooves 24 has a deckline profile defined by the inner but exposed surface 30 of each of the aforementioned first extents 26 and by the inner but exposed surface 32 of each of the aforementioned second extents 28. The inner but exposed surfaces 30 and 32 merge with each other at a radius of curvature in excess of the radius of curvature with which the outermost surfaces 22a and 22b of each of the lugs adjoin each other.

As illustrated in Fig. 1, each of the lugs 22 in the non-road-contacting portions 18 is formed with a substantially V-shaped cut- out 34 to reduce the amount of tread stock used in the tire 10 and to enhance slightly the degree of flexibility of each of the lugs 22 at the sidewalls 12.

Referring now to Fig. 3, there is schematically illustrated a plan development of a portion of the tread 14 to show the relationship of the lugs 22 and the grooves 24 defined therebetween with one another. It will be understood, however, that the illustrated juxtaposition of the lugs 22 will not be the preferred one for all tires.

All of the lugs 22 are generally similar in shape although, as will be discussed below, certain lugs differ slightly in physical dimensions. Each of the grooves 24 is provided with three sipes; namely, a first sipe 36 which is inclined with respect to the circumferential center-line L of the tread 14, a second sipe 38 which extends substantially parallel to the circumferential center-line L, and a third sipe 40 which is inclined only slightly with respect to the circumferential center-line L.

Moreover, the aforementioned first extent 26 of the grooves 24 is provided with a first wall 26a which is inclined, preferably, with respect to the circumferential centerline L at approximately 40 , and a second wall 26b which is inclined with respect to the circumferential center-line L at approximately 430. Thus, each of the lugs 22 may be said to be inclined with respect to the circumferential center-line L or the rib 20 at approximately 40430. As illustrated, each of the sipes 36, 38 and 40 opens into its corresponding one of its grooves 24 and may, thus, be characterized as "vented" sipes.

Preferably, the lugs 22 are arranged on either side of the circumferential center-line L as two arrays, the lugs 22 of one of the arrays on one side of the rib 20 having an orientation relative to rib 20 which is opposite of, and out of phase with (or staggered relative to), the orientation of the lugs 22 of the other of the arrays on the other side of the rib 20. Moreover, at a juncture J between the road-contacting portion 16 of the tread 14 and the non-roadcontacting portion 18 of the tread 14, each of the lugs 22 has a circumferential span S of approximately two inches.

However, as will be seen below, certain of the lugs 22 have a circumferential span S which may be slightly greater or less than two inches, this difference in circumferential span of the lugs 22 at the juncture J being the principal basis for the difference in size of the lugs 22. The difference in size of the lugs 22 and the relative juxtaposition of the differently sized lugs 22, in combination with certain of the structural relatonships aforementioned, give rise to a tread pattern of lugs which prevents the resonance of sound that may be otherwise generated by the tire when the latter is put into vehicular use and rolled on a surface, for example, in a range encompassing approximately fifty miles per hour.

With respect to ''resonant reinforcement", in order to appreciate the manner by which the present invention achieves its objective to diminish the amount.of noise generated by a rolling tire, certain terms will now be defined. Each of the lugs 22 is associated with a corresponding (adjacent) one of the grooves 24 in a given circumferential direction. The circumferential span S of a lug 22 when added to the circumferential span G of its adjacent corresponding groove 24, at the juncture J between the road-contacting portion 16 and the non-road-contacting portion 18, gives rise to the term "pitch" having units of length.

It will be understood, that for purposes herein the term "pitch" refers to a circumferentially extending dimension of a portion of the tread 14, and does not refer to a parameter or characteristic of sound.

Thus, as illustrated in Fig. 3, the combined extents SB+GB result in a specified pitch, for example, the pitch B. Similarly, the circumferential span Sc when added to circumferential span Gc gives rise to the pitch C. The same is true with respect to the pitch A.

As illustrated in Fig. 4, the lugs 22 (with their corresponding grooves) are divided, for example, into three categories each of the categories being defined by a specified pitch; namely, the pitch A, the pitch B and the pitch C. Not all of the lugs 22 of the same category or pitch are juxtaposed adjacent to one another in series. Only a certain predetermined number of the lugs 22 of the same pitch are juxtaposed adjacent to one another, for example, two, three or four of such lugs 22. Each group of lugs 22 of the same pitch which are adjacent to one another is characterized herein as a "pitchidentifiable" group (identifiable by pitch and not necessary by the number of the lugs 22 of a particular group).

Each pitch-identifiable group separates two other pitch-identifiable groups whose respective pitches differ from the group separating them but may or may not differ from each other. For example, as illustrated in Fig. 4, there exists a pitch-identifiable group denoted by the reference character A ', each of the lugs of which (with its corresponding groove) has a pitch A. To the left of the group A', there exists a group of three lugs 22 each (with its corresponding groove) having a respective pitch B, that group being denoted by reference character B'. Similarly, to the right of the group A', there exists a group of two lugs 22 each (with its corresponding groove) having a respective pitch B, that group being denoted by the reference character B". Thus, in this instance, the pitch-identifiable group A' separates the two groups B' and B" having the same pitch but different numbers of lugs. A similar situation exists with respect to the group C' which separates the group B' of three lugs aforementioned (on the right of the group C') from another group B"' (on the left of the group C'). It will be seen that the groups B', B" and B"' are each comprised of lugs having the same pitch B.

A different situation exists with respect to the group B' of three lugs aforementioned which separates the aforementioned group A' (whose lugs have a pitch of A) from the group C' aforementioned (whose lugs have a pitch of C).

It is the relative juxtaposition of the various groups A', B' and C' in series or circumferential succession that gives rise to what is characterized herein as a "pitch sequence". The pitch sequence of pitchidentifiable groups is designed so that their excitation frequency peaks due to lug vibration do not coincide with the resonant frequencies of the tire. As a result, the vibrations of the lugs reduce reinforcement of other means of tire vibration, and are allowed to be dissipated without generating excessive sound.

What must be emphasized with respect to this chosen pitch sequence is that it is not a randomly or arbitrarily selected pattern. It is an arrangement established as a result of, in accordance with the invention, evaluating the physical characteristics of a particular type of lug pattern and predetermining the various resonant frequencies of-the tire to which such lug pattern is applied which must not be excited at various speeds of the tire.

Each type of tire, like any article capable of vibrating, has various resonant frequencies. The various resonant frequencies can be determined by. rolling the tire at various speeds to excite the tire into vibrating at its different resonant frequencies. By gradually increasing the rolling speed of a test specimen tire having a design wherein the lugs are of the same size and equidistantly spaced from each other, the sound level gradually increases.

However, at certain speeds of the specimen tire, the sound level (usually measured in decibels) suddenly rises rapidly and graphically peaks. The frequency of each "peak" sound in such a tire occurs at a different resonant frequency of the tire.

Each "peak" (or resonant) frequency is easily determinable by conventional audio equipment.

All of the so-called "peak" or resonant frequencies of the specimen tire over a preferred range of speeds that the tire is expected to be used can be similarly determined. A graph can then be established in which sound intensity, in decibels, is plotted along the vertical axis and sound frequency, in cycles per second, is plotted along the horizontal axis. The resultant curve will present various peaks which, in this instance, constitute the resonant frequencies of the specimen tire. It is these resonant frequencies that must not be overly excited by a lug pattern of a tire that is to radiate a low level of noise.

To derive such pattern, a first modified lug pattern design can be selected (for example, arbitrarily, without building same) which has lugs and grooves generally similar in shape to those of the specimen tyre, but which differ therefrom in their circumferential arrangement in such a way that the leading edges of all lugs are not equidistantly spaced over the whole circumference of the tread. It will be understood, that the resonant frequencies of the aforementioned specimen tire are substantially the same as the resonant frequencies of the tire with the first modified lug pattern design because alteration in pitch of the modified lug pattern design from that of the specimen tire is only slight.

With a first modified lug.pattern design so as selected (not built), one can "calculate" a graphic spectrum of excitation frequency peaks due to vibration that would occur over a preferred range of speeds that a tire encompassing such a pattern. would be expected to be used. Such various "calculated" peaks can be used as a basis of comparison with the resonant frequencies of the aforementioned specimen tire appearing in the aforementioned graph derived from the specimen tire. If the "calculated" peaks coincide with the resonant frequencies such a modified lug.

pattern design would generate a relatively high level of noise. If the "calculated" peaks do not coincide with the resonant frequencies such a modified lug pattern design would generate only a relatively low level of noise. Such a tire is, therefore, a quieter riding tire.

It will be understood that second and still other modified lug pattern designs will be selected and evaluation in order to minimize the number of excitation frequency peaks that coincide with the resonant frequencies of the tire. By effecting "non-coincidence" at certain locations, other locations originally non-coincident may be caused to become coincident. Each such cause-andeffect must desirably be accounted for before achieving the most quiet-riding tread pattern design.

The procedure by which the graphic spectrum of excitation frequencies is calculated is a standard Fourier Harmonic Analysis in which each leading edge of each lug is characterized as producing a unit pulse of excitation, and in which the timing of repetition of the pulses of excitation is equal to the repetitive distances between the leading edges divided by the rotary speed of the tire. Such an analysis is described in a text entitled "Mathematical Handbook for Scientists and Engineers" published by McGraw-Hill (1961) and authored by Korn and Korn. Section 4.11-- 4 particularlly involves "Fourier Analysis (Harmonic Analysis) of Periodic Functions".

It is by the above process that the required pitch sequence for a given tire is determined. With the test information so established, the lugs 22 can be sized with selected circumferential dimensions (pitch) and arranged in selected groups. The groups can then be arranged in a preferred series or pitch sequence which most effectively allows the vibrations to dissipate without reinforcing one another, and the tread pitch sequence so determined can be applied to a tire.

With respect to a preferred embodiment the tire 10 made in accordance with the method of the present invention may be one which, for example, presents a bead diameter of substantially twenty-two inches and a maximum inflated width, from sidewall-to-sidewall, of approximately ten inches. With such dimensions, it has been determined by the method of the invention that the tread of the tire 10 preferably includes thirty-six lugs in circumferential succession on each side of the central rib 20.

As such, those lugs 22 having a pitch A have a circumferential span of, for example, 4.08 inches, whereas the lugs 22 having a pitch B have a circumferential span of, for example, 3.71 inches, and whereas the lugs 22 having a pitch C have circumferential span of, for example 3.34 inches. The lugs 22 having a pitch A are twelve in number, the lugs 22 having a pitch B are sixteen in number, the lugs 22 having a pitch B are sixteen in number, and the lugs 22 having a pitch C are eight in number.

Generally, it is preferred that the lugs 22 having a pitch C be approximately 90% (plus or minus 2-1/2%) of the circumferential span of the lugs 22 having a pitch B. Similarly, it is preferred that the lugs 22 having a pitch A be approximately 1 10% (plus or minus 21/2%) of the circumferential span of the lugs 22 having a pitch B. The preferred pitch sequence (as illustrated in Fig. 4) of such a tire has been determined to be: A-A-A-B -B-B -C-C-C-C-B- B-B-A-A-A-B-B This pitch sequence involves the use of eighteen of the lugs 22 and, thus, at.least insofar as the tire 10 of the dimensions aforementioned is concerned, the aforementioned pitch sequence is repeated a second time.

This pitch sequence represents the preferred sequence for a tire of the aforementioned dimensions which has excitation frequency peaks due to lug vibration that do not coincide with the resonant frequencies of the tire. Such a pitch sequence will thus substantially reduce tread noise. Further noise reduction will occur if this pitch sequence is combined with (a) the deckline profile of the vented grooves 24, and (b) the inclination of the extent 26 of each of the grooves 24 with respect to the central rib 20, and (c) the anchoring of tread lugs of approximately two inches in circumferential span to a central rib. These features assist reduction in lug vibrations, slip/stick vibrations and air pumping and thus further reduce the amount of noise generated by the tire when it is rolled on a surface.

When a conventional tire is rolled on a given surface, for example, in a range encompassing approximately fifty miles per hour, as each tread element snaps circumferentially when exiting from the contact patch, it creates an impulse of lug vibration. When the timing of these impulses are such as to coincide with the timing of the resonant frequencies of the tire, large resonant vibrations are induced, and this in turn causes large amounts of noise. In addition, the large levels of resonant vibrations then promote slip/stick vibrations at the same frequency, which slip/stick vibrations themselves create vibratory impulses which assist in maintaining the large resonant vibrations.

However, when the spacing and timing of the pitch sequence is properly irregular (as determined by the method of the present invention), the impulses of snapping due to the tread lugs both partially reinforce and partially interfere with the resonant vibrations of the tire. The net result is primarily a diminished degree of resonant vibrations. Correspondingly, there is secondarily effected a reduction in the amount of slip/stick excitation resonances of the tire and, therefore, there is a double basis for a reduced level of noise that accrues from selecting the correct pitch sequence. The other design features of the tire complement the pitch sequence in still further reducing noise generation.

WHAT WE CLAIM IS: 1. A method of making a pneumatic tire comprising the steps of: (a) Selecting a specimen tire corresponding in materials and overall dimensions to the required tire, the specimen tire having a tread pitch sequence extending around the whole circumference of the tread and comprising a plurality of lugs equal in size and shape spaced

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. published by McGraw-Hill (1961) and authored by Korn and Korn. Section 4.11-- 4 particularlly involves "Fourier Analysis (Harmonic Analysis) of Periodic Functions". It is by the above process that the required pitch sequence for a given tire is determined. With the test information so established, the lugs 22 can be sized with selected circumferential dimensions (pitch) and arranged in selected groups. The groups can then be arranged in a preferred series or pitch sequence which most effectively allows the vibrations to dissipate without reinforcing one another, and the tread pitch sequence so determined can be applied to a tire. With respect to a preferred embodiment the tire 10 made in accordance with the method of the present invention may be one which, for example, presents a bead diameter of substantially twenty-two inches and a maximum inflated width, from sidewall-to-sidewall, of approximately ten inches. With such dimensions, it has been determined by the method of the invention that the tread of the tire 10 preferably includes thirty-six lugs in circumferential succession on each side of the central rib 20. As such, those lugs 22 having a pitch A have a circumferential span of, for example, 4.08 inches, whereas the lugs 22 having a pitch B have a circumferential span of, for example, 3.71 inches, and whereas the lugs 22 having a pitch C have circumferential span of, for example 3.34 inches. The lugs 22 having a pitch A are twelve in number, the lugs 22 having a pitch B are sixteen in number, the lugs 22 having a pitch B are sixteen in number, and the lugs 22 having a pitch C are eight in number. Generally, it is preferred that the lugs 22 having a pitch C be approximately 90% (plus or minus 2-1/2%) of the circumferential span of the lugs 22 having a pitch B. Similarly, it is preferred that the lugs 22 having a pitch A be approximately 1 10% (plus or minus 21/2%) of the circumferential span of the lugs 22 having a pitch B. The preferred pitch sequence (as illustrated in Fig. 4) of such a tire has been determined to be: A-A-A-B -B-B -C-C-C-C-B- B-B-A-A-A-B-B This pitch sequence involves the use of eighteen of the lugs 22 and, thus, at.least insofar as the tire 10 of the dimensions aforementioned is concerned, the aforementioned pitch sequence is repeated a second time. This pitch sequence represents the preferred sequence for a tire of the aforementioned dimensions which has excitation frequency peaks due to lug vibration that do not coincide with the resonant frequencies of the tire. Such a pitch sequence will thus substantially reduce tread noise. Further noise reduction will occur if this pitch sequence is combined with (a) the deckline profile of the vented grooves 24, and (b) the inclination of the extent 26 of each of the grooves 24 with respect to the central rib 20, and (c) the anchoring of tread lugs of approximately two inches in circumferential span to a central rib. These features assist reduction in lug vibrations, slip/stick vibrations and air pumping and thus further reduce the amount of noise generated by the tire when it is rolled on a surface. When a conventional tire is rolled on a given surface, for example, in a range encompassing approximately fifty miles per hour, as each tread element snaps circumferentially when exiting from the contact patch, it creates an impulse of lug vibration. When the timing of these impulses are such as to coincide with the timing of the resonant frequencies of the tire, large resonant vibrations are induced, and this in turn causes large amounts of noise. In addition, the large levels of resonant vibrations then promote slip/stick vibrations at the same frequency, which slip/stick vibrations themselves create vibratory impulses which assist in maintaining the large resonant vibrations. However, when the spacing and timing of the pitch sequence is properly irregular (as determined by the method of the present invention), the impulses of snapping due to the tread lugs both partially reinforce and partially interfere with the resonant vibrations of the tire. The net result is primarily a diminished degree of resonant vibrations. Correspondingly, there is secondarily effected a reduction in the amount of slip/stick excitation resonances of the tire and, therefore, there is a double basis for a reduced level of noise that accrues from selecting the correct pitch sequence. The other design features of the tire complement the pitch sequence in still further reducing noise generation. WHAT WE CLAIM IS:

1. A method of making a pneumatic tire comprising the steps of: (a) Selecting a specimen tire corresponding in materials and overall dimensions to the required tire, the specimen tire having a tread pitch sequence extending around the whole circumference of the tread and comprising a plurality of lugs equal in size and shape spaced

circumferentially from one another by respective grooves of equal size; (b) Determining the resonant frequencies of the specimen tire by rolling the tire at speeds within a predetermined speed range so causing vibration of the tire at different frequencies, plotting the level of sound from the tire against the frequency of vibration of the tire and determining as the resonant frequencies those frequencies at which peaks of sound level occur; (c) Selecting a plurality of tread pitch sequences which differ from one another and from the tread pitch sequence of the specimen tire, each of such selected tread pitch sequences including a plurality of circumferentially spaced lugs defining therebetween respective grooves, said lugs and grooves being generally similar in shape to the lugs and grooves of the specimen tire but differing slightly therefrom in their circumferential arrangement so that in each selected tread pitch sequence the leading edges of all the lugs are not equidistantly spaced throughout the whole tread pitch sequence; (d) Determining the excitation frequency peaks of each selected tread pitch sequence when notionally applied to the specimen tire and operated in the predetermined speed range; (e) Comparing the excitation frequency peaks of each selected tread pitch sequenee with the resonant frequencies of the specimen tire, to determine the degree of coincidence between the excitation frequency peaks of each selected tread pitch sequence and the resonant frequencies of the specimen tire; (f) Choosing that selected tread pitch sequence for which said degree of coincidence is the lowest; and (g) Applying to a tire the tread pitch sequence so chosen.

2. A method as claimed in claim I in which the chosen tread pitch sequence comprises a circumferential arrangement of differing groups, each group comprising lugs and corresponding grooves of the same pitch, the lugs and corresponding grooves of each group having a different pitch to that of the lugs and corresponding grooves of each circumferentially adjoining group.

3. A method as claimed in claim 2 in which in the chosen tread pitch sequence the lugs of one group differ from the lugs of an adjoining group in regard to their respective dimensions circumferentially of the tread.

4. A method as claimed in any one of the preceding claims in which in the chosen tread pitch sequence the grooves of one group are identical to the grooves of an adjoining group in regard to their respective dimensions circumferentially of the tread.

5. A method as claimed in any one of the preceding claims in which the chosen tread pitch sequence consists of eighteen lugs and their corresponding grooves.

6. A method as claimed in any one of the preceding claims in which the chosen tread pitch sequence is defined by the following parameters: A-A-A-B-B-B-C-C-C-C-B- B-B-A-A-A-B-B wherein each parameter A, B and C represents a different pitch dimension of a lug and its corresponding groove.

7. A method as claimed in claim 6 in which the pitch dimension A is (110+2O /" of the pitch dimension B.

8. A method as claimed in claim 6 or claim 7 in which the pitch dimension C is (9Q*2+)% of the pitch dimension B.

9. A method as claimed in any one of claims 6 to 8 in which the selected tread pitch sequence is repeated at least once around the circumference of the tire.

10. A pneumatic tire made in accordance with a method according to any one of the preceding claims.