Patent Publication Number: US-2023139129-A1

Title: Tire

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to a tire. 
     Background Art 
     Patent Document 1 below discloses a pneumatic tire, wherein the tread portion thereof comprises a first cap rubber layer and a second cap rubber layer which have loss tangents specifically defined in order to improve rolling resistance and wet braking performance of the tire.
     Patent Document 1: Japanese Patent Application Publication No. 2018-002008   

     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     On the other hand, there is a demand for a tire which can exhibit good wet performance from the initial stage to the final stage of the wear life of the tread portion. But, in the case of a tire of which tread portion comprises rubber layers having different loss tangents such as the pneumatic tire disclosed in Patent Document 1, there is a problem such that the wet performance tends to change suddenly in the middle stage of the wear life of the tread portion. 
     The present disclosure was made in view of the above circumstances, and a primary objective of the present disclosure is to provide a tire which can stably exhibit good wet performance from the initial stage to the final stage of the wear life of the tread portion. 
     Means for Solving the Problems 
     According to the present disclosure, a tire comprises a tread portion provided with circumferential grooves which extend continuously in the tire circumferential direction to divide a first land portion and a second land portion which is located axially inside the first land portion, 
     each of the first land portion and the second land portion comprises a radially outermost cap tread rubber layer forming a ground contacting surface, and an intermediate tread rubber layer arranged on the radially inside of the cap tread rubber layer, wherein 
     the loss tangent δ2 of the intermediate tread rubber layer is larger than the loss tangent δ1 of the cap tread rubber layer, and 
     the shortest distance L2 from the ground contacting surface of the second land portion to the radially outer surface of the intermediate tread rubber layer of the second land portion is less than the shortest distance L1 from the ground contacting surface of the first land portion to the radially outer surface of the intermediate tread rubber layer of the first land portion. 
     Effects of the Invention 
     In the tire according to the present disclosure, therefore, owing to the different loss tangents δ1 and δ2 and the different distances L1 and L2 to the respective intermediate tread rubber layers, good wet performance is stably exhibited from the initial stage to the final stage of the tread wear life. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a meridian cross-sectional view of a tire as an embodiment of the present disclosure. 
         FIG.  2    is a meridian cross-sectional view enlargedly showing the second land portion and a part of the first land portion of  FIG.  1   . 
         FIG.  3    is a meridian cross-sectional view enlargedly showing another example of the first land portion and the second land portion. 
         FIG.  4    is a meridian cross-sectional view enlargedly showing still another example of the first land portion and the second land portion. 
         FIG.  5    is a graph showing a relationship between the wear amount (AW) of the tread portion and the wet braking distance (WBD) of each test tire. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present disclosure can be applied to pneumatic tires for various vehicles. e.g. passenger cars, heavy vehicles such as trucks and buses, motorcycles and the like, but, in particular, suitably applied to passenger car tires. 
     Taking a pneumatic tire for passenger cars as an example, embodiments of the present disclosure will be described in detail in conjunction with accompanying drawings. 
       FIG.  1    is a tire meridian cross-sectional view of a pneumatic tire  1  for passenger cars as an embodiment of the present disclosure under a standard state of the tire  1 . 
     In this application including specification and claims, various dimensions, positions and the like of the tire refer to those under the standard state unless otherwise noted. 
     In the case of a pneumatic tire for which various specifications are set by a standardization organization, the standard state is such that the tire is mounted on a standard rim, and inflated to a standard pressure, but loaded with no tire load. 
     In the case of a tire for which various specifications are not yet set, the standard state means a standard usage state of the tire according to the purpose of use of the tire, which is not mounted on the vehicle and loaded with no tire load. 
     The standard wheel rim is a wheel rim officially approved or recommended for the tire by the standardization organization, i.e. JATMA (Japan and Asia), T&amp;RA (orth 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 under-mentioned 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 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 tire 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 under-mentioned tread edges Te are the axial outermost edges of the ground contacting patch of the tire which occurs when the tire under its standard state is loaded with the standard tire load and placed on a horizontal flat surface at a camber angle of zero degree. 
     The pneumatic tire  1  comprises a tread portion  2  whose radially outer surface defines the tread or the ground contacting surface  2 S of the tire, a pair of axially spaced bead portions  4  mounted on rim seats of a wheel rim, a pair of sidewall portions  3  extending between the tread edges Te and the bead portions  4 , a carcass  6  extending between the bead portions  4  through the tread portion  2  and the sidewall portions  3 , and a tread reinforcing cord layer  7  disposed radially outside the carcass in the tread portion. 
     In the present embodiment, the carcass  6  is composed of two carcass plies  6 A and  6 B. Each of the carcass plies  6 A and  6 B is composed of rubberized cords arranged radially at an angle of 75 to 90 degrees with respect to the tire circumferential direction. In this example, the carcass cord is made of organic fibers. 
     In the present embodiment, the tread reinforcing cord layer  7  is composed of two plies  7 A and  7 B of reinforcing cords. 
     For example, each of the reinforcing plies  7 A and  7 B is composed of parallel reinforcing cords arranged at an angle of 10 to 45 degrees with respect to the tire circumferential direction and rubberized with topping rubber. As the reinforcing cords, organic fiber cords or steel cords may be appropriately used. 
     The tread portion  2  is provided with circumferential grooves  8  extending continuously in the tire circumferential direction to axially divide a first land portion  11  and a second land portion  12  which is located on the tire equator C side of the first land portion  11 . 
     In the present embodiment, the tread portion  2  is provided with three circumferential grooves  8 , and thereby, the tread portion  2  is axially divided into four land portions: a pair of axially outer first land portions  11  and a pair of axially inner second land portions  12  therebetween. 
     The present disclosure is however, not limited to such a four land portion arrangement. 
     As shown in  FIG.  2   , each of the first land portions  11  and the second land portions  12  comprises a cap tread rubber layer  16 , an intermediate tread rubber layer  17  and a base tread rubber layer  20 . 
     The cap tread rubber layer  16  is radially outermost, and forms the ground contacting surface  2   s.  
 
The intermediate tread rubber layer  17  is disposed on the radially inside of the cap tread rubber layer  16 .
 
The base tread rubber layer  20  is disposed on the radially inside of the intermediate tread rubber layer  17 .
 
In the present embodiment, the tread rubber disposed on the radially outside of the tread reinforcing cord layer  7  is composed of three rubber layers: the cap tread rubber layer  16 , the intermediate tread rubber layer  17  and the base tread rubber layer  20 . The present disclosure is however, not limited to such three layer structure.
 
Incidentally, in  FIG.  1   , the tread rubber is hatched as if a single rubber layer for simplification.
 
     It has been known that, in general, rubber having a large loss tangent can exert a large frictional force on a wet road surface. 
     However, in the present disclosure, the loss tangent (tan δ2) of the radially inner intermediate tread rubber layer  17  is set to be larger than the loss tangent (tan δ1) of the radially outermost cap tread rubber layer  16 . 
     Here, the loss tangent (tan δ) is measured according to Japanese Industrial Standard (HS) K6394, using a dynamic mechanical characteristic analyzer (GABO EPLEXOR Series), under the following conditions: Initial strain 5%, Dynamic strain amplitude +/−1%, Frequency 10 Hz, Temperature 30 degrees C., Deformation mode Tensile mode. 
     According to the present disclosure, the shortest distance L2 from the ground contacting surface  12   s  of the second land portion  12  to the radially outer surface of the intermediate tread rubber layer  17  of the second land portion  12  is set to be smaller than 
     the shortest distance L1 from the ground contacting surface  11   s  of the first land portion  11  to the radially outer surface of the intermediate tread rubber ayer  17  of the first land portion  11 . Thereby, the tire  1  according to the present disclosure can stably exhibit good wet performance from the initial stage to the final stage of the wear life of the tread portion  2  for the following reason. 
     Overall, as the wear of the tread portion  2  progresses, the intermediate tread rubber layer  17  having excellent frictional force on wet road surfaces is exposed, and thereby, the deterioration of the wet performance due to the wear of the tread portion  2  can be compensated for by exposing the intermediate tread rubber layer  17 , and good wet performance is stably exhibited. 
     If the intermediate tread rubber layer  17  is exposed in the entire area or a wide area of the tread surface of the worn tread portion  2  at about the same time, then the change in wet performance before and after the intermediate tread rubber layer  17  is exposed becomes large. This is not desirable. 
     According to the present disclosure, since the distance L2 in the second land portion  12  is smaller than the distance L1 in the first land portion  11 , the intermediate tread rubber layer  17  of the second land portion  12  is exposed, and after a while, the intermediate tread rubber layer  17  of the first land portion  11  is exposed. Therefore, the change in wet performance, namely, improvement due to the exposure of the intermediate tread rubber layer  17  becomes gradual. 
     On the other hand, as the wear progresses, tread grooves are gradually decreased in depth and volume, which affects the wet performance. But, the deterioration of the wet performance due to the gradual decrease in the groove depth and volume is compensated by the gradual exposure of the intermediate tread rubber layer  17 . 
     For this reason, the tire  1  according to the present disclosure can exhibit good wet performance stably from the initial stage to the final stage of the wear life of the tread portion  2 . 
     The loss tangent (tan δ1) of the cap tread rubber layer  16  is preferably set to be not less than 0.13, more preferably not less than 0.15, still more preferably not less than 0.18, but not more than 0.30, more preferably not more than 0.25, still more preferably not more than 0.22. 
     Such cap tread rubber layer  16  can exhibit steering stability on dry road surfaces (hereinafter, simply referred to as “steering stability”) and wet performance in a well-balanced manner in the initial stage of the tread wear life. 
     It is preferable that the loss tangent (tan δ2) of the intermediate tread rubber layer  17  is set to be not less than 0.20, more preferably not less than 0.25, still more preferably not less than 0.28, but not more than 0.40, more preferably not more than 0.35, still more preferably not more than 0.32 in order to improve steering stability and wet performance when the tread portion  2  is worn. 
     If the difference between the loss tangent (tan δ1) and the loss tangent (tan δ2) is small, it is difficult to derive the above-mentioned effect. If the difference is large, then the separation failure is likely to occur at the boundary between the cap tread rubber layer  16  and the intermediate tread rubber layer  17 . 
     From such a viewpoint, it is preferable that the loss tangent (tan δ2) is 1.2 to 2.5 times, more preferably 1.5 to 2.0 times the loss tangent (tan δ1). 
     Each of the cap tread rubber layer  16  and the intermediate tread rubber layer  17  has a substantially constant thickness except for the vicinity of each circumferential groove  8 . Thereby, their separation at the boundary of these rubber layers can be suppressed. 
     The above-said vicinity of the circumferential groove  8  may include a range of not more than 50% of the groove width of the circumferential groove  8  at the groove top, from each of the groove walls toward the both sides of the circumferential groove  8 .
 
The expression “substantially constant thickness” means that the difference between the maximum value and the minimum value of the thickness is not more than 5% of the maximum value.
 
     The distance L1 in the first land portion  11  is preferably 35% to 55%, more preferably 40 to 50% of the tread thickness t1 measured in the first land portion  11  from the radially outer surface of the tread reinforcing cord layer  7  to the radially outer surface of the tread portion  2 . 
     The distance L2 of the second land portion  12  is preferably 10% to 30%, more preferably 15% to 25% of the tread thickness t2 measured in the second land portion  12  from the radially outer surface of the tread reinforcing cord layer  7  to the radially outer surface of the tread portion  2 . 
     Further, the distance L1 is preferably not less than 150%, more preferably not less than 180%, but preferably not more than 250%, more preferably not more than 220% of the distance L2. Thereby, the wet performance is more stably exhibited. 
     In the first land portion  11 , the thickness t3 of the intermediate tread rubber layer  17  is preferably set in a range from 30% to 50% of the above-said tread thickness t1. 
     In the second land portion  12 , the thickness t4 of the intermediate tread rubber layer  17  is preferably set in a range from 50% to 70% of the tread thickness t2. The present disclosure is however, not limited to such thickness ranges. 
     The tread portion  2  includes the base tread rubber layer  20  disposed on the radially inside of the intermediate tread rubber layer  17 . 
     The loss tangent (tan δb) of the base tread rubber layer  20  is smaller than the loss tangent (tan δ1) of the cap tread rubber layer  16 . Specifically, the loss tangent (tan δb) is less than 0.13. Such base tread rubber layer  20  helps to suppress excessive heat generation of the tread portion  2 . 
     The base tread rubber layer has a substantially constant thickness except for the vicinity of the circumferential groove  8 . The meaning of the expression “substantially constant thickness” is as explained above. 
     The thickness of the base tread rubber layer  20  is preferably set in a range from 10% to 30% of the tread thickness from the radially outer surface of the tread reinforcing cord layer  7  to the radially outer surface of the tread portion  2 . 
     In the present embodiment, the boundary  18  between the cap tread rubber layer  16  and the intermediate tread rubber layer  17  extends parallel to the ground contacting surface, except for the vicinity of each circumferential groove  8 . 
     The present disclosure is however, not limited to such example.
 
For example,  FIG.  3    shows another example of the first land portion  11  and the second land portion  12 , wherein
 
the boundary  18  (except for portions in the vicinities of the circumferential grooves  8 ) does not extend parallel to the ground contacting surface  2   s  of the tread portion  2 . More specifically, in each of the first land portion  11  and the second land portion  12 , the boundary  18  comprises a radially outwardly protruding portion  23 , and the radial distance from the radially outer surface of the tread portion  2  to the protruding portion  23  of the boundary  18  is increased toward both sides in the tire axial direction from the radially outwardly most protruding position.
 
Accordingly, as the wear of the tread portion progresses, the intermediate tread rubber layer  17  is gradually increased in occupied area in the tread surface, therefore, the wet performance can be more stably exhibited.
 
     It is preferable that the radius of curvature R2 of the boundary  18  in the second land portion  12  is smaller than the radius of curvature R1 of the boundary  18  in the first land portion  11 , As a result, it is possible to increase the amount of wear required for the intermediate tread rubber layer  17  to be completely exposed in the second land portion  12 , while suppressing the separation of the rubber layers at the boundary  18 . 
       FIG.  4    shows still another example of the first land portion  11  and the second land portion  12 , wherein the boundary  18  is undulated. Specifically, in the tire meridian cross section, the boundary  18  extends in the tire axial direction, while oscillating in the tire radial direction, thus, showing a wavy shape. 
     In this example, too, the intermediate tread rubber layer  17  is gradually increased in occupied area in the tread surface, as the wear of the tread portion progresses, and thereby, the wet performance can be more stably exhibited.
 
In addition, the substantial length of the boundary  18  is increased, which helps to suppress the separation between the rubber layers at the boundary  18 .
 
     As shown in  FIG.  4   , the amplitude A2 in the tire radial direction of the boundary  18  in the second land portion  12  is preferably larger than the amplitude A1 in the tire radial direction of the boundary  18  in the first land portion  11 . 
     Thereby, it is possible to increase the amount of wear required fir the intermediate tread rubber layer  17  to be completely exposed in the second land portion  12 . 
     While detailed description has been made of preferable embodiments of the present disclosure, the present disclosure can be embodied in various forms without being limited to the illustrated embodiments. 
     Comparison Tests 
     Based on the structure shown in  FIG.  1   , pneumatic tires of size 235/65R16C were experimentally manufactured as test tires including working example Ex. 1, and comparative examples Ref. 1 and Ref. 2.
 
Specifications of the test tires are shown in Table 1.
 
In the comparative example Ref. 1, the loss tangent (tan δ1) of the cap tread rubber layer was larger than the loss tangent (tan δ2) of the intermediate tread rubber layer, and the distance L1 was equal to the distance L2.
 
In the comparative example Ref.  2 , the loss tangent (tan δ1) of the cap tread rubber layer was smaller than the loss tangent (tan δ2) of the intermediate tread rubber layer, and the distance L1 was equal to the distance L2.
 
The tread thickness t1 in the first land portion and the tread thickness t2 in the second land portion were substantially the same in each test tire and also for all test tires.
 
All test tires had substantially the same structures except for the loss tangents, distances and thicknesses shown in Table 1.
 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Tire 
                 Ref. 1 
                 Ref. 2 
                 Ex. 1 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 cap tread rubber layer tan δ1 
                 0.35 
                 0.20 
                 0.20 
               
               
                   
                 intermediate tread rubber layer tan δ2 
                 0.20 
                 0.35 
                 0.35 
               
               
                   
                 distance L1/tread thickness t1 (%) 
                 40 
                 40 
                 40 
               
               
                   
                 distance L2/tread thickness t2 (%) 
                 40 
                 40 
                 20 
               
               
                   
                   
               
            
           
         
       
     
     For each test tire, the relationship between the amount of wear (AW) of the tread portion and the wet braking distance (WBD) was investigated. 
     The test tire was mounted on a standard rim (size 16×7.0 J) and attached to a test vehicle (3000 cc rear-wheel-drive commercial vehicle), and the wet braking distance (WBD) was measured using the test vehicle. 
     The braking distance was measured by applying full brake when the test vehicle was running at a speed of 65 km/h on an asphalt road surface of a tire test course, covered with 1.5 mm depth water.
 
Such measurement was carried out many times from a state in which the amount of wear=0% (that is, a new tire state) to a state in which the amount of wear=100% (that is, the tread wear reached to the wear indicator).
 
The results are shown in  FIG.  5    as a graph, wherein the horizontal axis is the amount of wear (AW) of the tread portion, and the vertical axis is the wet braking distance (WBD). Needless to say, the smaller wet braking distance is better.
 
     In  FIG.  5   , the graph G1 shows the WBD-AW relationship of the comparative example Ref. 1, the graph G2 shows the WBD-AW relationship of the comparative example Ref. 2 and the graph G3 shows the WBD-AW relationship of the working example Ex. 1. 
     As shown in  FIG.  5   , in each test tire, as the amount of wear (AW) of the tread portion increased, the drainage properly of the circumferential grooves was decreased and the wet braking distance (WBD) was increased. 
     In the comparative example Ref. 1, the wet braking distance (WBD) was small in the initial stage of tread wear. However, when the tread portion was worn and the intermediate tread rubber layer having the smaller loss tangent was exposed, the wet braking distance (WBD) changed abruptly and increased. Then, the wet braking distance (WBD) was gradually increased as the amount of wear (AW) increased. 
     In the comparative example Ref. 2, the large abrupt change of the wet braking distance was suppressed as compared with the comparative example Ref. 1, but a smaller abrupt change still existed because the intermediate tread rubber layer appeared in the entire tread surface within a short period of time. 
     In the working example Ex. 1, the wet braking distance (WBD) was changed linearly with respect to the change in the amount of wear (AW) of the tread portion without the abrupt changes as in the comparative examples. 
     Thus, it was confirmed that the tire according to the present disclosure can stably exhibit good wet performance until the end of the tread wear life. 
     Statement of the Present Disclosure 
     The Present Disclosure is as Follows: 
     Disclosure 1: A tire comprising a tread portion provided with circumferential grooves which extend continuously in the tire circumferential direction to divide a first land portion and a second land portion which is located axially inside the first land portion, wherein 
     each of the first land portion and the second land portion comprises a radially outermost cap tread rubber layer forming a ground contacting surface, and an intermediate tread rubber layer arranged on the radially inside of the cap tread rubber layer; 
     the loss tangent δ2 of the intermediate tread rubber layer is larger than the loss tangent δ1 of the cap tread rubber layer; and 
     the shortest distance L2 from the ground contacting surface of the second land portion to the radially outer surface of the intermediate tread rubber layer of the second land portion is less than the shortest distance L1 from the ground contacting surface of the first land portion to the radially outer surface of the intermediate tread rubber layer of the first land portion. 
     Disclosure 2: The tire according to Disclosure 1, wherein the tread portion comprises a base tread rubber layer disposed on the radially inside of the intermediate tread rubber layer, and the loss tangent δb of the base tread rubber layer is smaller than the loss tangent δ1 of the cap tread rubber layer.
 
Disclosure 3: The tire according to Disclosure 2, wherein the loss tangent δb of the base tread rubber layer is less than 0.13.
 
Disclosure 4: The tire according to Disclosure 2 or 3, wherein the base tread rubber layer extends in the tire axial direction with a substantially constant thickness.
 
Disclosure 5: The tire according to any one of Disclosures 2 to 4, wherein the tread portion is provided with a tread reinforcing cord layer composed of cords coated with topping rubber and disposed radially inside the base tread rubber layer, and the thickness of the base tread rubber layer is 10% to 30% of the tread thickness measured from the radially outer surface of the tread portion to the radially outer surface of the tread reinforcing cord layer.
 
Disclosure 6: The tire according to any one of Disclosure 1 to 5, wherein the loss tangent δ1 of the cap tread rubber layer is 0.13 to 0.30.
 
Disclosure 7: The tire according to any one of Disclosures 1 to 6, wherein the loss tangent δ2 of the intermediate tread rubber layer is 0.20 to 0.40.
 
Disclosure 8: The tire according to any one of Disclosures 1 to 7, wherein the distance L1 is 150% to 250% of the distance L2.
 
     DESCRIPTION OF THE REFERENCE SIGNS 
     
         
         
           
               2  tread portion 
               8  circumferential groove 
               11  first land portion 
               12  second land portion 
               16  cap tread rubber layer 
               17  intermediate tread rubber layer 
             C tire equator