Motorcycle tire

A motorcycle tire comprises a tread portion provided with first T-grooves and second T-grooves arranged and oriented alternately along the tire equator. The angles of the main portion and branch portion of the T-groove, with respect to the tire circumferential direction, are gradually increased toward the tread edges. The axial distances of the branching point and an inside end of the main portion are not more than 25% of the axial distance of the tread edge, each measured from the tire equator along the tread surface. The circumferential distance from the inside end of the main portion to the circumferentially adjacent T-groove is 5 to 15% of the circumferential pitch length of the first T-grooves.

BACKGROUND OF THE INVENTION

The present invention relates to a pneumatic tire, more particularly to a motorcycle tire suitably used on well-paved roads and having a tread pattern mainly formed by oblique grooves by which transient characteristics from the vertical state during straight running to the full lean state during cornering can be improved while preventing the occurrence of shimmy such as handle shimmy during straight running.

In a motorcycle tire used on well-paved roads, it is effectual for improving the cornering performance to relatively increase the rigidity of the tread portion in its central portion so as to generate a large lateral force when a rider initiates a turn and leans into the turn, and thereby to improve the handle response.

In such a motorcycle tire however, during straight running, a relatively large lateral force is liable to occur, therefore, due to the lateral force, shimmy such as handle shimmy is liable to occur.

On the other hand, it is preferable for making stable cornering to gradually and smoothly increase the lateral force of the tire from the vertical state during straight running to the full lean state during cornering.

However, in the above-mentioned motorcycle tire whose tread central portion is relatively increased in the rigidity, it is difficult to further increase the lateral force from the vertical state to the lean state with the increase in the camber angle. And transient characteristics are liable to deteriorate.

SUMMARY OF THE INVENTION

It is therefore, an object of the present invention to provide a motorcycle tire, in which transient characteristics when making a turn from straight running can be improved, while preventing the occurrence of shimmy such as handle shimmy during straight running.

According to the present invention, a motorcycle tire comprises

a tread portion provided with first T-grooves and second T-grooves arranged and oriented alternately along the tire equator,

the first and second T-grooves each composed of a main portion extending from one side to the other side of the tire equator toward a tread edge, while gradually increasing its angle with respect to the tire circumferential direction, and a branch portion extending from a branching point between both ends of the main portion, while inclining to the opposite direction to the main portion and gradually increasing its angle with respect to the tire circumferential direction, wherein

when measured along a tread surface of the tread portion, an axial distance L2from the tire equator to the branching point and an axial distance L1from the tire equator to an inside end of the main portion are not more than 25% of an axial distance from the tire equator to a tread edge, and

the circumferential distance from said inside end of the main portion to the circumferentially adjacent T-groove is in a range of not less than 5% and not more than 15% of a circumferential pitch length of the first T-grooves or the second T-grooves.

The main portions and branch portions of the first and second T-grooves have outside ends closer to the tread edges and inside ends closer to the tire equator, and the angles of the main portions and branch portions with respect to the tire circumferential direction are gradually increased from the inside ends to the outside ends. As a result, the lateral force is gradually increased with the increase in the camber angle, and transient characteristics and the cornering performance can be improved.

As the branch portion communicates with the main portion, the drainage performance is improved.

As the axial distances L1and L2are limited in a specific range, the T-grooves relatively decrease the rigidity of the tread portion in its center zone, and the T-grooves can control the lateral force liable to occur during straight running. Thus, shimmy such as handle shimmy can be prevented. Further, it makes sure that the lateral force gradually increases with the increase in the camber angle and the transient characteristics are further improved.
As the circumferential distance is specifically limited in relation to the circumferential pitch length, the rigidity of the tread portion is relatively decreased in the tread center zone, and the occurrence of shimmy such as handle shimmy can be effectively prevented, and the transient characteristics can be further improved.

The motorcycle tire according to the present invention may be provided with the following features (I)-(V).

(I) each of the first T-grooves and the second T-grooves has only one groove junction which is a T-shaped junction between the main portion and the branch portion;

(II) the tire equator is located between the branching point and the inside end of the main portion of each said first T-groove;

(III) the tire equator is located between the branching point and the inside end of the main portion of each said second T-groove;

(Iv) the axial distance L1is less than the axial distance L2;

(V) the tread portion is provided on each side of the tire equator with alternate major areas and minor areas defined between the main portions and the branch portions of the first T-grooves and the second T-grooves, and

each of the major areas is provided with at least one sub groove extending independently within the major area, while inclining to the same direction as the circumferentially adjacent main portion and branch portion.

In this application including specification and claims, various dimensions, positions and the like of the tire refer to those under a normally inflated unloaded condition of the tire unless otherwise noted.

The normally inflated unloaded condition is such that the tire is mounted on a standard wheel rim and inflate to a standard pressure but loaded with no tire load.

The undermentioned normally inflated loaded condition is such that the tire is mounted on the standard wheel rim and inflated to the standard pressure and loaded with the standard tire load.

The standard wheel rim is a wheel rim officially approved or recommended for the tire by standards organizations, i.e. JATMA (Japan and Asia), T&RA (North America), ETRTO (Europe), TRAA (Australia), STRO (Scandinavia), ALAPA (Latin America), ITTAC (India) and the like which are effective in the area where the tire is manufactured, sold or used.

The standard pressure and the standard tire load are the maximum air pressure and the maximum tire load for the tire specified by the same organization in the Air-pressure/Maximum-load Table or similar list. For example, the standard wheel rim is the “standard rim” specified in JATMA, the “Measuring Rim” in ETRTO, the “Design Rim” in TRA or the like. The standard pressure is the “maximum air pressure” in JATMA, the “Inflation Pressure” in ETRTO, the maximum pressure given in the “Tire Load Limits at Various Cold Inflation Pressures” table in TRA or the like. The standard load is the “maximum load capacity” in JATMA, the “Load Capacity” in ETRTO, the maximum value given in the above-mentioned table in TRA or the like.

The angle of a groove or a groove portion can be defined by the angle of the widthwise center line thereof.

To be exact, the branching point can be defined by an intersecting point between the widthwise center line of the main portion and the widthwise center line of the branch portion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described in detail in conjunction with the accompanying drawings.

In the drawings, motorcycle tire1according to the present invention comprises a tread portion2, a pair of axially spaced bead portions4each with a bead core5therein, a pair of sidewall portions3extending between the tread edges2tand the bead portions4, a carcass6extending between the bead portions4through the tread portion2and sidewall portions3, and a belt7disposed radially outside the carcass6in the tread portion2.

The tread portion2(inclusive of the carcass6, belt7and a tread rubber thereon) and tread surface are curved with a relatively small radius of curvature when compared with the passenger car tires, truck/bus tires and the like in order to secure a large ground contacting area during cornering. As a result, the maximum cross section width TW of the tire lies between the tread edges2t.

The carcass6is composed of at least one ply6A of cords arranged at an angle in the range of from 25 to 90 degrees with respect to the tire equator c, extending between the bead portions4through the tread portion2and sidewall portions3and turned up around the bead core5in each bead portion4from the axially inside to the axially outside of the tire to form a pair of turnup portions6band a main portion6atherebetween. In this embodiment, the carcass6is composed of a single ply6A of cords arranged radially at an angle of 90 degrees with respect to the tire equator C. For the carcass cords, organic fiber cords, e.g. nylon, polyester, rayon and the like are suitably used.

The bead portions4are each provided between the main portion6aand turned up portion6bof the carcass ply6A with a bead apex8.

The belt7is composed of a single jointless ply7A made of windings of a single rubber coated cord or plural rubber coated parallel cords spirally wound with a cord angle of not more than 10 degrees with respect to the tire circumferential direction. For the belt cord, organic fiber cords, e.g. aramid, nylon, polyester, rayon and the like are suitably used.

In order to improve the enveloping performance by allowing a partial deformation of the tread portion, the tread portion2in this embodiment is not provided with cut breaker plies (cross plies) made of rubberized cords laid at a relatively large angle with respect to the tire circumferential direction.

In this embodiment, the motorcycle tire1is designed to be used on well paved roads.

The tread portion2is provided with a unidirectional tread pattern comprising first T-grooves11A and second T-grooves11B which are alternately arranged and oriented along the tire equator C without communicating with each other.

The T-grooves11A and11B have groove depths D1of about 4.0 to 6.0 mm, and groove widths W1of about 3.5 to 5.5 mm excepting tapered groove end portions.

The first T-groove11A is made up of a main portion12A and a branch portion13A. The second T-groove11B is made up of a main portion12B and a branch portion13B.

In the first T-groove11A, the main portion12A extends obliquely from one side (right side) S1to the other side (left side) S2of the tire equator C, and the branch portion13A extends from a branching point14A between both ends12Ai and12Ao of the main portion12A, while inclining to a direction opposite to the inclining direction of the main portion12A.

In the second T-grooves11B, the main portion12B extends obliquely from the other side (left side) S2to one side (right side) S1of the tire equator C, and the branch portion13B extends from a branching point14B between both ends12Bi and12Bo of the main portion12B, while inclining to a direction opposite to the inclining direction of the main portion12B.

The first T-grooves11A is provided with only one groove junction15A (T-shaped) between the main portion12A and the branch portion13A. The second T-grooves11B is provided with only one groove junction15B (T-shaped) between the main portion12B and the branch portion13B.

Therefore, it is possible to generate an effective lateral force during cornering so as to improve the cornering stability.

If a T-groove11A,11B has a plurality of groove junctions, the rigidity of the tread portion2is liable to excessively decrease and the cornering stability is deteriorated.

As shown inFIG. 3, the angle α2a, α2bof the main portion12A,12B with respect to the tire circumferential direction is gradually increased from its inside end12Ai,12Bi on the tire equator side to its outside end12Ao,12Bo on the tread edge side in order to smoothen the drainage of water and in order to gradually increase the lateral force with the increase in the camber angle from straight running (camber angle=zero) and thereby to improve the transient characteristics.

In a tread center zone between the ground contact edges17tduring straight running, the above-mentioned angle α2a, α2b(hereinafter “α2ai, α2bi”) is preferably set in a range of not less than 5 degrees, more preferably not less than 10 degrees, but not more than 30 degrees, more preferably not more than 25 degrees.

The ground contact edges17tare the axial outermost edges of the ground contacting patch of the tire (when camber angle=0) in the above-mentioned normally inflated loaded condition.

If the angle α2ai, α2biis less than 5 degrees, the lateral rigidity of the tread portion2is decreased in the tread center zone, and it becomes difficult to generate a sufficient lateral force at the time of making a turn from straight running, and the transient characteristics can not be fully improved. Further, there is a possibility that the drainage can not be fully improved. If the angle α2ai, α2biexceeds 30 degrees, a large lateral force is liable to occur during straight running, and there is a possibility that shimmy such as handle shimmy is caused.

At the outside end12Ao,12Bo of the main portion12A,12B, the above-mentioned angle α2a, α2b(hereinafter, “α2ao, α2bo”) is preferably set in a range of not less than 50 degrees, more preferably not less than 55 degrees, but not more than 80 degrees, more preferably not more than 70 degrees.

If the angle α2ao, α2bobecomes less than 50 degrees, it becomes difficult to generate a lateral force required for full lean cornering, and the transient characteristics can not be fully improved. If the angle α2ao, α2boexceeds 80 degrees, the lateral force at full lean cornering excessively increases, and the transient characteristics can not be fully improved.

Both end portions of the main portion12A,12B are tapered toward the inside ends12Ai,12Bi and outside ends12Ao,12Bo to smoothen the tread rigidity change and improve the transient characteristics.

The angle α3a, α3bof the branch portion13A,13B with respect to the tire circumferential direction is gradually increased from the branching point14A,14B to its outside end13Ao,13Bo on the tread edge side in order to smoothen the drainage of water and in order to gradually increase the lateral force with the increase in the camber angle from straight running (camber angle=zero) and thereby to improve the transient characteristics.

In the above-mentioned tread center zone between the ground contact edges17tduring straight running, the above-mentioned angle α3a, α3b(hereinafter, “α3ai, α3bi”) is preferably set in a range of not less than 5 degrees, more preferably not less than 10 degrees, but not more than 30 degrees, more preferably not more than 25 degrees.

At the outside end13Ao,13Bo of the branch portion13A,13B, the above-mentioned angle α3a, α3b(hereinafter, “α3ao, α3bo’) is preferably set in a range of not less than 50 degrees, more preferably not less than 55 degrees, but not more than 80 degrees, more preferably not more than 70 degrees.

The outside end part of the branch portion13A,13B is tapered toward the outside end13Ao,13Bo in order to smoothen the tread rigidity change and to improve the transient characteristics.

The branch portion13A,13B has a first groove edge13As,13Bs on the inside end12Ai,12Bi side and a second groove edge on the outside end12Ao,12Bo side, wherein the entire length of the first groove edge13As,13Bs extends along a smoothly curved line, but this is not for the second groove edge.

In the branch portion13A,13B, its groove with has a stepwise difference between the tapered outside end part (narrow) and its almost entire part (wide) excluding the tapered outside end part. In the almost entire part of the branch portion13A,13B, the second groove edge extends along a smoothly curved line almost parallel with the first groove edge13As,13Bs.

When measured along the tread surface2s, the axial distance L1from the tire equator C to the inside ends12Ai,12Bi of the main portion12A,12B and the axial distance L2from the tire equator C to the branching point14A,14B are set to be not more than 25%, preferably not more than 20% of the axial distance (0.5 TWe) between the tire equator c and one of the tread edges2tin order to relatively decrease the tread rigidity in a tread center zone and thereby to prevent the occurrence of a large lateral force during straight running. Therefore, the occurrence of shimmy such as handle shimmy can be prevented, and the lateral force is gradually increased with the increase in the camber angle from straight running and thereby the transient characteristics can be improved.

Further, in order that the axial positions of the inside ends12Ai,12Bi do not coincide with those of the branching points14A,14B, the axial distance L1is preferably less than the axial distance L2. For example, the distance L1is not more than 0.8 times, preferably not more than 0.7 times the distance L2. Therefore, a local excessive decrease in the tread rigidity and the resultant deterioration in the transient characteristics can be prevented.

As to a positional relationship in the tire axial direction, it is preferable that the tire equator C is located between the branching point14A,14B and the inside end12Ai,12Bi of the main portion12A,12B.

Thus, as shown inFIG. 4, the tire equator C is crossed by the first T-grooves11A and the second T-grooves11B alternately, and each of the T-grooves crosses the tire equator C twice by its main portion and branch portion, defining two intersecting points (21A and22A), (21B and22B) as shown.
Therefore, in the vicinity of the tire equator C, the rigidity of the tread portion2is decreased to effectively prevent shimmy such as handle shimmy during straight running.
Further, the lateral force is gradually increased with the increase in the camber angle from straight running and thereby the transient characteristics can be improved.

The circumferential distance L3from the inside end12Ai,12Bi of the main portion12A,12B to the circumferentially adjacent T-groove11A,11B is set in a range of not less than 5%, preferably not less than 8%, but not more than 15%, preferably not more than 12% of the circumferential pitch length P2of the first T-grooves11A (or the second T-grooves11B) as shown inFIG. 2. The circumferential pitch length P2of the first T-grooves11A is the same as that of the second T-grooves11B.

Incidentally, the pitch length P2can be defined, for example, between the branching points14A of the circumferentially adjacent first T-grooves11A or between the branching points14B of the circumferentially adjacent second T-grooves11B; further between the ends12Bi and12Bi or between the ends12Bo and12Bo; or between the ends12Ai and12Ai or between the ends12Ao and12Ao; or the like.
Therefore, the tread rigidity is relatively decreased in a tread center zone and thereby shimmy such as handle shimmy can be effectively prevented, and the transient characteristics can be effectively improved.

If the distance L3exceeds 15% of the pitch length P2, there is a possibility that the above-mentioned advantageous effect can not be obtained. If the distance L3is less than 5% of the pitch length P2, there is a possibility that the tread rigidity is excessively decreased in a tread central area and the transient characteristics is deteriorated.

Further, the length L4of the main portion12A,12B measured in the tire circumferential direction as shown inFIG. 2is preferably set in a range of not less than 50%, more preferably not less than 55%, but not more than 70%, more preferably not more than 65% of the pitch length P2in order that the first T-grooves11A overlap with the second T-grooves11B in the tire circumferential direction and thereby the pattern rigidity becomes uniform in the tire circumferential direction. Therefore, the ride comfort can be improved while preventing the shimmy.

If the length L4of the main portion12A,12B is less than 50% of the pitch length P2, it becomes difficult to obtain the above-mentioned advantageous effect. If the length L4exceeds 70% of the pitch length P2, the circumferential pattern rigidity is excessively decreased, and there is a possibility that the transient characteristics is deteriorated.

On each side of the tire equator C, the tread portion2is provided with circumferentially alternate major areas R1and minor areas defined between the main portions and the branch portions of the first and second T-grooves (namely, between12A and13B or between12B and13A).

In this embodiment, the tread portion2is further provided between every two circumferentially adjacent first T-groove11A and second T-groove11B with at least one, in this example two sub grooves26. More specifically, the above-mentioned at least one groove26is disposed in each of the major areas R1on each side of the tire equator C.

In this embodiment, no groove is formed in the minor areas.

The above-mentioned at least one groove26in this example is a first sub groove26A extending independently in a main portion side, and a second sub groove26B extending independently in a branch portion side, and the first and second sub grooves26A and26B are inclined to the same direction as the circumferentially adjacent main portion and branch portion.

The sub grooves26have groove depths D2(shown inFIG. 1) of from 4.0 to 6.0 mm, and groove widths W2of from about 3.5 to 5.5 mm excepting the tapered groove end portions.

The first and second sub grooves26A and26B decrease the tread rigidity in the major areas R1and balance the tread rigidity between the right side and left side of the tire equator, and the transient characteristics and ride comfort can be improved.

As shown inFIG. 5, the angle α6of the first sub grooves26A with respect to the tire circumferential direction is gradually increased from its axially inner end26Ai toward its axially outer end26Ao in order to gradually increase the lateral force with the increase in the camber angle from straight running (camber angle=zero) and thereby to improve the transient characteristics and the cornering performance. Preferably, the angle α6is set in a range of 20 to 85 degrees.

The axially inner end part and axially outer end part of the first sub groove26A are tapered toward its axially inner end26Ai and axially outer end26Ao, respectively, in order to smoothen the tread rigidity change and improve the transient characteristics.

The first sub groove26A has a first groove edge26As on the main portion12A/12B side and a second groove edge on the branch portion13B/13A side. The entire length of the first groove edge26As extends along a smoothly curved line. The width of the first sub groove26A has a stepwise difference between the tapered outside end part (narrow) and its main part (wide) excluding both the tapered end parts. In the main part, the second groove edge extends along a smoothly curved line almost parallel with the first groove edge26As.

The second sub groove26B has a substantially L-shaped configuration having a bent point33and is composed of a main portion31extending from the axially inner end26Bi of the second sub groove26B towards the tread edge2tto the bent point33, while gradually increasing its angle α7awith respect to the tire circumferential direction, and

an outer portion32extending from the bent point33toward the axially outer end26Bo of the second sub groove26B, while gradually decreasing its angle α7bwith respect to the tire circumferential direction.

Preferably, the angle α7ais in a range of from about 20 to 80 degrees, and the angle α7bis in a range of from about 10 to 40 degrees.

Therefore, owing to the main portion31, the lateral force gradually increases with the increase in the camber angle from straight running (camber angle=zero) and the transient characteristics and the cornering performance can be improved. Further, owing to the outer portion32, the drainage during cornering can be improved.

An axially inner end portion and axially outer end portion of the second sub groove26B are tapered toward its axially inner end26Bi and outer end26Bo, respectively, in order to smoothen the tread rigidity change and improve the transient characteristics.

Comparison Tests

Based on the tread pattern shown inFIG. 2and the structure shown inFIG. 1, motorcycle tires having specifications shown in Table 1 were experimentally manufactured and tested.

Common Specifications are as Follows:

First Sub Groove:

Second Sub Groove:

In the test, the test tires were installed on a 250 cc motorcycle, and evaluated by a test rider regarding shimmy such as handle shimmy and transient characteristics during running on a dry asphalt road surface in a tire test course.

The test results are indicated in Table 1 by an index based on comparative example tire Ref.1 being 100, wherein the larger the value, the better the performance.

From the test results, it was confirmed that the motorcycle tires according to the present invention can be improved in the transient characteristics, while preventing the occurrence of shimmy during straight running.