Patent Application
20100319826
The present invention relates to a pneumatic tire for rough terrain, more particularly to a geometry of a tread block capable of improving running performance on rough terrain.
Pneumatic tires for used with vehicles for running on rough terrain such as motocross bikes and rally cars are usually provided with block type tread patterns made up of a plurality of tread blocks, for example, as disclosed in Japanese Patent Application Publication No. JP-A-2005-289092.
In order to derive a large grip force on rough terrain, as shown in
Conventionally, the contour shape of a section of the tread block (a) taken parallel with the top surface (b) is substantially not changed from the top to the bottom of the tread block even if the size thereof is gradually increased as shown in
When such a tread block receives a force from a road surface, the tread block is deformed (leans) more or less, and the amount of deformation is varied depending on the direction of the force even if the magnitude of the force is the same. Namely, in terms of the bending deformation, the tread block has anisotropic nature (herein after the “deformation anisotropic nature”).
For example, as shown in
The tread blocks of a pneumatic tire for running rough terrain are subjected to large forces in various directions when compared with general pneumatic tires. Therefore, if the block has a strong deformation anisotropic nature, there is a possibility that, when a large force in a specific direction is applied to the tread block, the tread block is deformed largely and the top surface (b) can not contact with the road surface. In this case, accordingly, the ground contact area is significantly decreased, and necessary grip performance can not be obtained.
It is therefore, a primary object of the present invention to provide a pneumatic tire for running on rough terrain, in which the deformation anisotropic nature of the tread blocks is lessened not to decrease the ground contact area of the tread blocks and thereby the grip performance is improved.
According to the present invention, a pneumatic tire comprises a tread portion, and a plurality of tread blocks each having a top surface disposed in the tread portion, wherein
a degree of circularity Eb of a contour shape of a section of each of the tread blocks taken parallel with the top surface at a block foot position is more than a degree of circularity Et of a contour shape of the top surface of the tread block. The block foot position is defined as being spaced apart from a tread bottom by 10% of a height of the tread block from the tread bottom. The degree of circularity is defined by the following formula (1):
4×pi×S/L2 (1)
wherein,
pi is the circumference ratio,
S is an area of the contour shape, and
L is a circumferential length of the contour shape.
Therefore, even if an angular polygonal shape is employed as the contour shape of the top surface, the deformation anisotropic nature of the tread block is lessened. As a result, it becomes possible for the top surface to contact with the road surface tightly, and the road grip performance on rough terrain can be improved.
a)-7(f) are schematic plan views of tread blocks used in the undermentioned comparison tests.
a) and
Embodiments of the present invention will now be described in detail in conjunction with accompanying drawings.
According to the present invention, pneumatic tire 1 for running on rough terrain comprises a tread portion 2, a pair of sidewall portions 3, a pair of bead portions 4 each with a bead core 5 therein, a carcass 6 extending between the bead portions 4 through the tread portion 2 and sidewall portions 3, and a tread reinforcing cord layer.
The tread width TW is the axial distance between the tread edges 2e measured under a normally inflated unloaded condition of the tire.
If the tread edges are indefinite due to a round profile of tire shoulders which is often found in the case of passenger car tires, the tread edges 2e are defined by the axial outermost edges of the ground contacting patch (camber angle=0) under a normally inflated loaded condition.
Here, 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 normally inflated loaded condition is such that the tire is mounted on the standard wheel rim and inflate to the standard pressure and loaded with the standard tire load.
The standard wheel rim is a wheel rim officially approved for the tire by standard organization, i.e. JATMA (Japan and Asia), T&RA (North America), ETRTO (Europe), STRO (Scandinavia) and the like. 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. In case of passenger car tires, however, 88% of the maximum tire load is used as the standard tire load.
In this application, various dimensions, positions and the like of the tire refer to those under the normally inflated unloaded condition of the tire unless otherwise noted.
The carcass 6 is composed of at least one ply 6A of carcass cords extending between the bead portions 5 through the tread portion 2 and sidewall portions 3 and turned up around the bead core 5 in each of the bead portions 4 to be secured thereto. In this embodiment, organic fiber cords are suitably used for the carcass cords.
As to the tread reinforcing layer, according to the carcass structure, namely, whether a radial ply structure or bias ply structure, a breaker or a band or both of them are disposed radially outside the carcass 6 in the tread portion 2. In this embodiment, a breaker 7 comprising two cross breaker plies 7A and 7B is disposed.
The tread portion 2 is provided with a plurality of tread blocks 9 protruding radially outwardly from a tread bottom 10. In this embodiment, the use of term “tread bottom” is preferable to the use of term “groove bottom” because the area is very broad, therefore the term “tread bottom” 10 is used in this embodiment. But, this term corresponds to the entirety of tread grooves' bottom in the case of passenger car tires, heavy duty tires and the like.
As show in
Therefore, the ground pressure of each of the tread blocks 9 is increased. Accordingly, the amount of penetration of the tread block 9 into soft ground (muddy terrain) increases, which is useful for obtaining a large drive power. Further, as the distances between the tread blocks 9 are relatively large, mud and soil therebetween are easily discharged.
The tread blocks 9 include crown tread blocks 9C and shoulder tread blocks 9S.
The crown tread block 9C is defined as having the top surface 11 of which centroid is positioned in a crown region Cr. The crown region Cr is defined as being centered on the tire equator C and having a width of 70% of the tread width TW.
The shoulder tread block 9S is defined as having the top surface 11 of which centroid is positioned in one of shoulder regions Sr one on each side of the crown region Cr.
Thereby, the crown tread blocks 9C contact with the road surface during straight running, and the shoulder tread blocks 9S contact with the road surface during cornering.
In
The cross sectional view taken along line A-A in
The top surface 11 of each of the tread blocks 9 preferably has a contour shape St which is a substantially polygonal shape defined by straight edges 11e.
The term “substantially” means that the corners of such polygonal shape can be chamfered by an arc segment of a radius of less than 3 mm or a straight segment of less than 6 mm length in order to prevent the rubber of the tread block from chipping off. The chamfered parts of such small sizes are disregard when deciding the shape.
As to the polygonal shape, a rectangular shape, pentagonal shape, hexagonal shape, heptagonal shape or the like can be used. In either case, it is preferable that the edges 11e of the top surface 11 include: at least one axial edge inclined at an angle in a range of from 0 to 20 degrees with respect to the tire axial direction; and at least one circumferential edge inclined at an angle of from 0 to 20 degrees with respect to the tire circumferential direction. Therefore, due to the road scratching effect of such edges, the grip performance during straight running and cornering can be improved.
In this embodiment, as shown in
The tread block 9 has a side surface 12 which is defined as extending radially inwardly from the edges 11e of the top surface 11 to the tread bottom 10.
Excepting chamfered parts if any, the side surface 12 can be: a combination of flat surfaces; or a combination of flat surfaces and curved surfaces; or a combination of curved surfaces; or a single curved surface. Here, the single curved surface means a curved surface having no edge line.
In each of the examples shown in
According to the present invention, the degree of circularity Eb of the contour shape Sb of a section of the tread block 9 taken parallel with the top surface 11 at its block foot position is set to be more than the degree of circularity Et of the contour shape St of the top surface 11.
The block foot position means a position spaced radially outwardly apart from the tread bottom 10 by 10% of the height (h) of the tread block 9 from the tread bottom 10.
The degree of circularity is defined by the following formula (1):
4pi×S/L2 (1)
wherein,
pi is the circumference ratio (3.14 - - - ),
S is the area of the contour shape concerned, and
L is the circumferential length of the contour shape concerned.
In the case of a perfect circle, the degree of circularity is 1.0. when the shape is more variant from a perfect circle, the value of the degree of circularity becomes smaller.
If the value of the degree of circularity Et of the contour shape St of the top surface 11 is more than 0.87, the road scratching effect of the edges 11e of the top surface 11 is deceased. If less than 0.56, the durability and wear resistance of the tread block tends to decrease.
Therefore, the degree of circularity Et of the contour shape St of the top surface 11 is preferably set in a range of not less than 0.56, more preferably not less than 0.60, but not more than 0.87, more preferably not more than 0.80.
Since the value of the degree of circularity Eb of the contour shape Sb of the section at the foot position is larger than the value of the degree of circularity Et of the contour shape St of the top surface 11, the contour shape Sb at the block foot position becomes closer to a perfect circle when compared with the contour shape St of the top surface 11. As a result, the deformation anisotropic nature of the tread block 9 is lessened, and the top surface 11 of the tread block 9 can contact with the ground certainly even when the tread block is subjected to large forces in various directions.
If the value of the degree of circularity Eb of the contour shape Sb of the section at the block foot position is less than 0.86, it becomes difficult to effectively lessen the deformation anisotropic nature of the tread block 9.
Therefore, it is desirable that the degree of circularity Eb is not less than 0.86, more preferably not less than 0.87, still more preferably not less than 0.90.
In order to surely lessen the deformation anisotropic nature of the tread block 9, the ratio (Eb/Et) of the degree of circularity Eb at the block foot position to the degree of circularity Et of the top surface 11 is preferably set to be not less than 1.10, more preferably not less than 1.25.
If the ratio (Eb/Et) becomes more than 1.60, on the other hand, there is a possibility that the uneven wear resistance is decreased since the change in the contour shape of the tread block 9 becomes large.
Therefore, the ratio (Eb/Et) is set to be not more than 1.60, more preferably not more than 1.50.
In the example shown in
In the example shown in
In the example shown in
In the example shown in
As explained above, the geometry of the tread block 9 can be variously modified. In either case, however, it is desirable that the tread block 9 satisfies the following conditional equation (2):
Et<Ec<Eb (2)
wherein
Ec is the degree of circularity of the contour shape Sc of a section taken parallel with the top surface 11 at a block intermediate position.
The block intermediate position is a position spaced radially outwardly apart from the tread bottom 10 by 55% of the height (h) of the tread block from the tread bottom 10.
As a result, the change in the contour shape of the section becomes smooth, and since a concentration of deformation on a specific position is avoided, the durability of the tread block 9 can be improved.
From this standpoint, it is especially preferable that the degree of circularity is gradually (substantially continuously) increased from a position between the top surface 11 and about 10 to 15% of the height (h) to the block foot position.
Preferably, the value of the degree of circularity Ec of the contour shape Sc of the section at the block intermediate position is set in a range of not less than 0.75, more preferably not less than 0.86, but not more than 0.93, more preferably not more than 0.90.
In view of a smooth change in the contour shape, it is desirable that the above-mentioned degrees of circularity Et, Ec and Eb satisfy the following conditional equations (3) and (4):
1.05=<Et/Ec=<1.50 (3)
1.05=<Ec/Eb=<1.50 (4)
On the other hand, it is also desirable that the circumferential length Lt of the contour shape St of the top surface 11, the circumferential length Lb of the contour shape Sb at the block foot position and the circumferential length Lc of the contour shape Sc at the block intermediate position satisfy the following conditional equation (5), more preferably satisfy the following conditional equation (5)′:
Lt=<Lc=<Lb (5)
Lt<Lc<Lb (5)′
By gradually increasing the circumferential length (Lt, Lc, Lb) from the top surface 11 side towards the tread bottom 10 side, the bending rigidity of the tread block can be gradually increased towards the tread bottom 10 side, while lessening the deformation anisotropic nature of the tread block 9. specifically, for that purpose, it is preferable that the following conditional equations (6) and (7) are further satisfied:
1.10=<Lb/Lt=<1.40 (6)
1.02=<Lb/Lc=<1.20 (7)
As a result, the ground contact of the top surface and the durability of the tread block 9 can be further improved.
If the ratio(Lb/Lt) is less than 1.10 or the ratio (Lb/Lc) is less than 1.02, then the bending rigidity of the tread block 9 tends to become insufficient, therefore, there is a possibility that the effect to improve the uneven wear and ground contact decreases. If the ratio(Lb/Lt) is more than 1.40 or the ratio (Lb/Lc) is more than 1.20, then the bending rigidity of the tread block 9 is excessively increased, and as a result, it becomes difficult for the tread block to follow to the road surface and there is a possibility that the ground contact becomes worse.
As the crown tread block 9C contacts with the road surface during straight running rather than during cornering, the crown tread block 9C is subjected to large forces in various directions when compared with the shoulder tread block 9S. on the other hand, as the shoulder tread block 9S contacts with the road surface during cornering, the shoulder tread block 9S is subjected to large forces mainly in the tire axial direction. Therefore, it is possible that the crown tread block 9C and shoulder tread block 9S satisfy the following conditional equation (8):
Ebs>Ebc (8)
wherein
Ebc is the degree of circularity of the crown tread block 9C at the block foot position, and
Ebs is the degree of circularity of the shoulder block at the block foot position.
In this case, the degree of circularity ratio (Et/Eb) of such shoulder tread block 9S is set to be smaller than that of the crown tread block 9C.
As explained in the example shown in
Motorcycle tires of size 110/90-19 having the tire structure shown in
In the test, 250 cc motocross bikes provided on the rear wheels with test tires (inflated to 80 kPa) were run in a motocross tire test course including a soft ground and a hard ground like a bedrock, and
running performance of each test tire was evaluated into ten ranks by ten riders, based on their feelings about the road grip and ground contact of the tread blocks.
with respect to each test tire, the mean value of the rank numbers by the ten riders is shown in table 1, wherein the larger value is better.
Through the test, it was confirmed that the running performance on rough terrain can be improved without sacrificing the durability of the tread blocks.
As has been explained, the present invention is suitably applied to a motorcycle tire. However, the present invention can be applied to pneumatic tires for four-wheeled rally cars, all-terrain three-wheelers and the like as well.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-144474 | Jun 2009 | JP | national |
