Pneumatic tyre

Abstract: A pneumatic tyre comprises a tread portion (2) provided with a series of design-cycles repeating in the circumference of the tyre at variable pitches (L), wherein the number (NP) of different kinds of pitches is at least 10. The variable pitches (L) repeatedly increase and decrease alternately in one circumferential direction of the tyre to define start design-cycles from which the pitches (L) start to increase. A series of design-cycles is composed of a series of units (U), each unit (U) being defined as a group of design-cycles starting from and including one of the start design-cycles to but excluding the next start-cycle. The total number (NU) of the units is from 4 to 20. The units consists of at least one first-unit the number of which is 20 to 80% of the total number, at least one second-unit the number of which is 20 to 80% of the total number, and optionally a third-unit the number of which is 0 to 60% of the total number. The first unit is defined as including at least one pitch which is not less than a standard long pitch (PLa) and at least one pitch which is not more than a standard short pitch (PSa). The second unit is defined as including at least one pitch within one of the following two ranges, a range of not less than the standard long pitch (PLa) and a range of not more than the standard short pitch (PSa), but no pitch within the other range. The third unit is defined as all the pitches therein are more than the standard short pitch (PSa) and less than the standard long pitch (PLa). The standard long pitch (PLa) is the maximum pitch (PL) minus a length of 0.2 times a maximum pitch difference (MPD). The standard short pitch (PSa) is the minimum pitch (PS) plus a length of 0.2 times the maximum pitch difference (MPD), and the maximum pitch difference (MPD) is the maximum pitch (PL) minus the minimum pitch (PS).

Description: The present invention relates to a pneumatic tyre improved in tread pattern noise, more particularly to an improvement in a variable pitching method. In general, tyres are provided in the tread portion with circumferentially spaced tread elements constituting a tread pattern such as blocks for good road grip. During running, such tread elements contact with road surface one after another, and a sound called pattern noise is generated. In order to reduce such pattern noise the so called variable pitching methods have been proposed. By these methods, pattern noise is modulated into a wide frequency range so that the pattern noise turns to the so called white noise. For example, in each of laid-open Japanese patent application Nos. JP-A-8-108711, JP-A-8-113012 and JP-A-8-113013, such a method is disclosed. In variable pitching methods, by increasing the number of different pitches, it becomes easy to turn pattern noise to white noise. However, when the pitch number is large, the rigidity of the tread elements is liable to become uneven and uneven wear increases. Another proposal in US 5 384 506 has the object of improving comfort in a vehicle by a means of reducing pattern noise particularly by reducing the pulsation of a low frequency noise generated by the regularity of the pitch arrangement. The patent proposes incorporating a higher order wave form with a basic fundamental period. This document, regarded as closest prior art to the subject-matter of claims 1 and 7, particularly discloses that a pneumatic tire be comprised of a tread portion provided with a series of design-cycles repeating in the circumference of the tyre at variable pitches, wherein the number of different kinds of pitches is between 3 and 8, and wherein the variable pitches repeatedly increase and decrease alternately in one circumferential direction of the tyre to form 1 to 4 fundamental periods, wherein a fundamental period begins with a short pitch, contains a least one of the longest pitches and terminates before the next short pitch of the next fundamental period; within a fundamental period the ratio of the length of the different pitches to the length of the pitch adjacent thereto is between 1.02 and 1.45. It is an object of the present invention however to provide reduced tyre noise by using a relatively large number of different pitches but in addition to this in a way to control the occurrence of uneven wear. Accordingly one aspect of the present invention is a pneumatic tyre as set out in claim 1 with further features set out in claims 2 to 6. A second aspect of the invention provides a method of providing a tread pattern for a pneumatic tyre as set out in claim 7 and subsequent claims. Embodiments of the present invention will now be described in detail in conjunction with the accompanying drawings, in which: Fig.1 is a partial developed plan view of a pneumatic tyre according to the present invention showing an example of the tread pattern; Fig.2 is a graph showing the sequence of the pitches of the design-cycles (tread blocks) thereof in the tyre circumferential direction; Fig.3 is a graph showing all the pitches thereof in descending order; Figs.4 to 19 each show the pitch sequence and the pitches in descending order of Examples 1 to 16, respectively, of the present invention; and Figs.20 to 25 each show the pitch sequence and the pitches in descending order of References 1 to 6, respectively. The pneumatic tyre according to the present invention comprises a tread portion 2, a pair of axially spaced bead portions, and a pair of sidewall portions extending between the tread edges and the bead portions. The tyre may be reinforced by a carcass extending between the bead portions, a bead core disposed in each bead portion, and a belt disposed radially outside the carcass in the tread portion. The tread portion 2 is provided with tread grooves 3 and 4 to form a tread pattern. The tread pattern comprises a series of design-cycles repeating around the circumference at variable pitches (L). The number (NC) of design-cycles is preferably 50 to 90. The number (NP) of different kinds of pitches (L) is preferably at least 10. Further, the maximum pitch (PL) is preferably set in the range of from 120 to 170 % of the minimum pitch (PS) to prevent uneven wear. Taking the example shown in Figs.2 and 3, the pitch sequence according to the invention will now be explained. In Fig.2, the design-cycle is indicated on the horizontal axis, and the pitch is indicated on the vertical axis. In Fig.2, from the first design-cycle to the last (70th) design-cycle, considered in the tyre circumferential direction, the pitches repeatedly increase and decrease alternately. Accordingly, there are a plurality of design-cycles (hereinafter start-cycles) from which the pitch starts to increase. Here, using such start-cycles, a unit (U) is defined as a group of design-cycles starting from and including a start-cycle to but not including the next start-cycle. Accordingly, there is one peak of the pitches in each unit (U). In Fig.2, for example, the lst-6th cycles are unit U1, the 7th-10th cycles are unit U2, the 11th-20th cycles are unit U3, the 45th-51st cycles are unit U7, and there are ten such units (U1 to U10) in the pitch sequence. Each pitch sequence is made up of 4 to 20 units, preferably 8 to 20 units, more preferably 9 to 13 units. If the total number (NU) of units is less than 4, periodicity is liable to happen in the pitch variation in the tyre circumferential direction, and uncomfortable sound increases. If the total number (NU) of units is more than 20, it is difficult to improve noise and uneven wear. It is especially difficult in a pneumatic tyre comprising 50 to 90 design-cycles. In the present invention, however, noise and uneven wear can both be improved by using three types of units the numbers of which are specifically limited in relation to the total number (NU). First, a maximum pitch difference (MPD) is defined as the maximum pitch (PL) minus the minimum pitch (PS). A standard long pitch (PLa) is defined as the maximum pitch (PL) minus a length of 0.2 times the maximum pitch difference (MPD). Further, a standard short pitch (PSa) is defined as the minimum pitch (PS) plus a length of 0.2 times the maximum pitch difference (MPD). The percentage D(%) in number, of the units of one type (hereinafter first unit), which is defined as including at least one pitch which is not less than the standard long pitch (PLa) and at least one pitch which is not more than the standard short pitch (PSa), is 20 to 80% of the total number of the units in a pitch sequence. In Fig.2, for example, the first units are U1, U3, U5, U6, U7, U8 and U9. Thus percentage (D) is 70%. The percentage R(%) in number, of the units of another type (herein after second unit), which is defined as including at least one pitch within one of the following two ranges, a range of not less than the standard long pitch (PLa) and a range of not more than the standard short pitch (PSa), but no pitch within the other range, is 20 to 80% of the total number of the units in a pitch sequence. In Fig.2, for example, the second units are U2 and U10. Thus, the percentage (R) is 20%. The percentage N(%) in number, of the units of the remaining type (hereinafter third unit), all the pitches of which are more than the standard short pitch (PSa) and less than the standard long pitch (PLa), is 0 to 60% of the total number of the units in a pitch sequence. In Fig.2, for example, the third unit is U4 only, and the percentage (N) is 10%. If the percentage (D) and/or (R) is less than 20% or the percentage (D) and/or (R) is more than 80%, pattern noise is not reduced, though sometimes uneven wear is prevented. Similarly, if the percentage (N) is more than 60%, the pattern noise can not be reduced. In Fig.3, all the pitches in this sequence are plotted in the descending order. The difference (V) between any of the pitches and the next longer or shorter pitch is preferably set in the range of less than 6% of the average pitch (Pa). The average pitch (Pa) is defined as the total of all the pitches in the sequence divided by the number (=NC) of all the pitches. If the pitch difference (V) is more than 6% of the average pitch (Pa), the pitch variation is too wide, and the rigidity difference in the tread blocks becomes increased and uneven wear is liable to occur. As mentioned above, the number of different kinds of pitches is NP. Accordingly, the number of variations thereof is (NP-1). And each variation is limited in the range of not less than 6% of the average pitch (Pa) because the pitch difference (V) is so limited as mentioned above. The number (Nm) of variations (hereinafter normal variation) which are limited in the range of from 4 to 6 % of the average pitch (Pa) is set in the range of not more than 40% of the total number (NP-1) of the variations. If the number (Nm) is more than 40% of (NP-1), the variations tend to have regularity which exerts a bad influence upon the noise frequency dispersion. Additionally, when a plurality of third units exist in a given pitch sequence, it is preferable that the third units do not adjoin each other in the tyre circumferential direction. If the third units adjoin each other, the pitch dispersion is liable to become insufficient and the noise reducing effect decreases. Further, it is preferable that all the units are different from each other in respect of the sequence of the pitches in the unit. If the same units exist, the pitch sequence is liable to have regularity, and the noise frequency dispersion becomes worse. Furthermore, the difference (G) between the pitches which are actually adjacent to each other in the tyre circumferential direction is preferably set in the range of not more than 25 % of the average pitch (Pa). If the actually adjacent pitches' difference (G) is more than 25 % of the average pitch (Pa), the rigidity difference in the tread elements such as blocks tends to increase and uneven wear is liable to increase. The above-mentioned pitch sequence is preferably applied to every circumferential row 6, 6a, 6b, 6c of blocks 5 in the tread portion 2, the blocks 5 being circumferentially separated by axial grooves 4 and axially divided by circumferential grooves 3, the circumferential grooves 3 extending continuously in the tyre circumferential direction. In the example shown in Fig. 1, the tread portion 2 is provided with a pair of axially inner circumferential grooves 3a each disposed one on each side of the tyre equator (C), and a pair of axially outer circumferential grooves 3b disposed axially outside the inner grooves 3a. The axial grooves 4 extend axially outwards from the circumferential grooves 3a beyond the tread edges (E). The axial grooves 4 on each side of the tyre equator are inclined to one circumferential direction. In Fig.1, the inclining direction to one side is the reverse to that on the other side. However, the inclination may be the same direction to form a directional tread pattern. The axial grooves 4 are arranged at regular or irregular intervals in the tyre circumferential direction to form blocks therebetween. By the circumferential grooves 3b and axial grooves 4, a circumferential row 6a, 6b of blocks 5 is defined along each tread edge (E), which blocks have a generally parallelogram configuration. Further, the circumferential grooves 3a and 3b and axial grooves 4, define a circumferential row 6c of blocks 5 is defined. Here, the circumferential grooves 3 and axial grooves 4 are grooves having a width of not less than 5 mm and a depth of not less than 7 mm. In this example, sipes 9 are further provided in the blocks 5. The sipes 9 are slits having no substantial groove width, which are thus not taken into consideration. One design-cycle is defined as a combination of one of the blocks 5 and one of the axial grooves 4 adjacent to the blocks on one side in the circumferential direction. Each pitch (L) can be varied by changing only the circumferential length of the block 5 or only the circumferential width of the axial groove 4 or both of them. For example, when the above-explained pitch sequence is applied to the block row 6a, the pitches (L) can be defined at the position of the circumferential groove 3b as shown in Fig. 1. Because the number (NC) is 50 to 90, each block row consists of 50 to 90 blocks divided by the same number of axial grooves. In the example shown in Figs.1-3, NC=70, NP=20, PL=37.8 mm, and PS=22.5 mm. Comparison Tests Test tyres of size 205/70R15 were made and tested for noise and uneven wear, using a 2000cc passenger car all the wheels of which were provided with test tyres. The specifications and the test results are shown in Table 1. A) Noise test The sound heard in the test car when run on a smooth road surface on a test course at a speed of 60 km/h was evaluated by a test driver. The results are indicated by an index in which under 3.0 means that the sound felt was uncomfortable, and 3.0 or more means that it was not felt to be uncomfortable. B) Uneven wear test After running for 300 km on a test course, the difference between the maximum wear and minimum wear was obtained as uneven wear based on the decrease in the axial groove depth. From the test results, it was confirmed that the Example tyres were effectively reduced in pattern noise. In Example tyre 10 in which the maximum of the pitch difference (V) exceeds greatly over 6% of the average pitch (Pa), uneven wear was caused. When the number (NP) of different kinds of pitches was less than 10 (Ref.1), or the total number (NU) of the units was less than 4 (Ref.2), or the total number (NU) of the units was more than 20 (Ref.3), the pattern noise was not reduced even if the special percentages for the three types of units were employed. On the contrary, when the percentages of the three types of units were outside the above-mentioned ranges, the noise was not reduced even when the numbers (NP) and (NU) satisfied the above-mentioned ranges (Ref.4 and Ref.6). As described above, in the pneumatic tyres according to the present invention, as the percentages of the first to third units are specifically limited, pattern noise and uneven wear can effectively be improved. The present invention is suitably applied to passenger car tyres, but it is also possible to apply to heavy duty tyres for trucks, buses and the like, motorcycle tyres, and so on.

Claims: A pneumatic tyre comprising a tread portion (2), the tread portion (2) being provided with a series of design-cycles repeating in the circumference of the tyre of variable pitches (L) to define a sequence, wherein the variable pitches includes a maximum pitch (PL) and a minimum pitch (PS) and the number (NP) of different kinds of pitches therein is at least 10, whereby the number (NP) of different kinds of pitches have variations of which number is said number (NP) minus 1, wherein a variation is a difference between any of the pitches and the next longer or shorter pitch, said variable pitches (L) repeatedly increase and decrease alternately in one circumferential direction of the tyre to define start design-cycles from which the pitches (L) start to increase, said series of design-cycles being composed of a series of units (U), wherein each unit (U) is defined as a group of design-cycles starting from and including one of the start design-cycles to but not including the next start-cycle, the total number (NU) of the units (U) being in the range of from 4 to 20; the plurality of units (U) consist of at least one first-unit the number (D) of which is 20 to 80% of said total number (NU), at least one second-unit the number (R) of which is 20 to 80% of said total number (NU) and optionally a third-unit the number (N) of which is o to 60% of said total number (NU), wherein the first unit is defined as including at least one pitch which is not less than a standard long pitch (PLa) and at least one pitch which is not more than a standard short pitch (PSa), the second unit is defined as including at least one pitch within one of the following two ranges, a range of not less than the standard long pitch (PLa) and a range of not more than the standard short pitch (PSa), but no pitch within the other range, and the third unit is defined as all the pitches therein are more than the standard short pitch (PSa) and less than the standard long pitch (PLa), wherein the standard long pitch (PLa) is the maximum pitch (PL) minus a length of 0.2 times a maximum pitch difference (MPD), the standard short pitch (PSa) is the minimum pitch (PS) plus a length of 0.2 times the maximum pitch difference (MPD) and the maximum pitch difference (MPD) is the maximum pitch (PL) minus the minimum pitch (PS), and said number (NP-1) of variations include a certain number (Nm) of variations each limited in the range of from 4 to 6% of an average pitch (Pa), wherein the number (Nm) is in the range of not more than 40% of the number (NP-1), the average pitch (Pa) is defined as the sum of the lengths of all of the pitches in the sequence divided by the number (NP) of all of the pitches in the sequence. A pneumatic tyre according to claim 1, characterised in that the difference (V) between any of the pitches and the next longer or shorter pitch is in the range of less than 6% of the average pitch (Pa). A pneumatic tyre according to claim 1 or 2, characterised in that said units include a plurality of third-units, and the third units do not adjoin each other in the tyre circumferential direction. A pneumatic tyre according to claim 1, 2 or 3, characterised in that all the units are different from each other in respect of the sequence of the pitches therein. A pneumatic tyre according to claim 1, 2, 3 or 4, characterised in that the difference (G) between pitches which adjoin each other in the tyre circumferential direction is not more than 25% of the average pitch (Pa). A pneumatic tyre according to any of claims 1-5, characterised in that said tread portion is provided with at least one row of blocks which are circumferentially divided by axial grooves, and each said design-cycle is defined by one of the blocks and one of the axial grooves adjacent thereto on one side thereof. A method of providing a tread pattern for a pneumatic tyre, the tread pattern comprising a series of design-cycles repeating in the tyre circumference at variable pitches (L) in a sequence, the method comprising the steps of: selecting a number (NC) of the design-cycles from a range of from 50 to 90; selecting a number (NP) of different kinds of the variable pitches (L) from a range of at least 10; setting a maximum pitch (PL) and a minimum pitch (PS) in the variable pitches (L) such that the maximum pitch (PL) is in the range of from 120 to 170% of the minimum pitch (PS); defining a standard long pitch (PLa) as the maximum pitch (PL) minus a length of 0.2 times a maximum pitch difference (MPD), the maximum pitch difference (MPD) being the maximum pitch (PL) ,minus the minimum pitch (PS); defining a standard short pitch (PSa) as the minimum pitch (PS) plus a length of 0.2 times the maximum pitch difference (MPD); defining an arrangement of the variable pitches as alternately increasing and decreasing in one circumferential direction of the tyre such that the number of units formed in the sequence ranges from 4 to 20; a unit (U) is defined as a group of design-cycles starting from and inclusive of one of the start-cycles but exclusive of the next start-cycle, wherein start-cycles are design-cycles from which the pitches start to increase; said units (U) consist of first, second and third units, wherein the number of first units ranges from 20% to 80% of the total number of units, the number of second units ranges from 20% to 80% of the total number of units, and the number of third units ranges from 0% to 60% of the total number of units; wherein a first unit is defined as including at least one pitch which is not less than the standard long pitch (PLa) and at least one pitch which is not more than the standard short pitch (PSa); a second unit is defined as including at least one pitch within one of a first range of not less than the standard long pitch (PLa) and a second range of not more than the standard short pitch (PSa) but not including a pitch within the other range; and a third unit is defined as all the pitches therein are more than the standard short pitch (PSa) and less than the standard long pitch (PLa); and such that the number (Nm) of variations in the pitches, which variations are limited in the range of from 4 to 6% of an average (Pa), being selected from a range of not more than 40% of a total number of the variations, a variation being a difference in length between any of the pitches and the next longer or shorter pitch, the average pitch (PA) being the sum of the length of all of the pitches in the sequence, divided by the number of pitches in the sequence, and the total number of variations being the number of different kinds of pitches minus 1. A method according to claim 7, which further comprises a step of setting a difference (V) between any of the pitches and the next longer or shorter pitch in a range of less than 6% of an average pitch (Pa), the average pitch (Pa) is the sum of the lengths of all of the pitches in the sequence, divided by the number (=NC) of pitches in the sequence. A method according to claim 7 or 8, wherein said units include a plurality of third units, and the third units to not adjoin each other in the tyre circumferential direction. A method according to claim 7, 8 or 9, wherein all the units are different from each other with respect to a sequence of the pitches therein. A method according to any of claims 7-10, wherein a difference (G) between pitches which adjoin each other in the tyre circumferential direction is not more than 25% of an average pitch (Pa), the average pitch (Pa) is the sum of the lengths of all of the pitches in the sequence divided by the number of pitches in the sequence. A method according to any of claims 7-10, which further comprises a step of defining a design-cycle as a combination of one of blocks and one of axial grooves adjacent thereto on one side of the block in the circumferential direction, the blocks provided in the tread portion and circumferentially divided by the axial grooves and arranged in a circumferential row. A method according to claim 12, which further comprises a step of varying the pitches (L) by changing only the circumferential lengths of the blocks. A method according to claim 12, which further comprises a step of varying the pitches (L) by changing only the circumferential widths of the axial grooves. A method according to claim 12, which further comprises a step of varying the pitches (L) by changing both the circumferential lengths of the clocks and the circumferential widths of the axial groove.

Inventors: YUKAWA NAOKI, YUKAWA, NAOKI
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1997
Main classification: B60C11
IPC codes: B60C11