Pneumatic radial tire with low twist organic carcass cords
This application is a divisional of application Ser. No. 08/162,901, (now abandoned) filed on Dec. 8, 1993, which is a continuation of Ser. No. 07/977,797, (now abandoned) filed Nov. 17, 1992, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a pneumatic tire, in which a carcass ply is made of low twist organic cords so as to achieve a tire weight reduction or a carcass strength improvement. 
     In a high performance tire having an aspect ratio of not more than 80%, organic fiber cords, e.g. polyester, rayon and nylon are generally used in the carcass. In such a carcass cord, in order to provide a fatigue resistance to a compressive stress and thereby to prevent carcass cords, which are located axially outside a bead apex, from being damaged during running under a heavy load condition, the cord twist has been set at a larger value for example 40 turns/10 cm or more. 
     Such a high twist cord is however poor in tensile strength, and therefore is not suitable for a high performance tire used under very high pressure and very high speed conditions. 
     Accordingly, in order to obtain the desired tensile strength for the carcass, the diameter of the carcass cord, the cord count of a carcass ply, and/or the number of carcass plies have been increased. As a result, tire weight is undesirably increased, deteriorating the tire&#39;s running performance. On the other hand, if such parameters are decreased to reduce tire weight, the carcass strength is reduced which also deteriorates the tire&#39;s running performance. 
     It is therefore, an object of the present invention to provide a pneumatic radial tire, in which the above-mentioned problems are solved. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a pneumatic radial tire comprises a tread portion, a pair of axially spaced bead portions with a bead core, a pair of sidewall portions, a carcass having a carcass ply of radially arranged cords extending between the bead portions and turned up around the bead cores to form a main portion and two turned up portions, and a bead apex disposed between the main portion and each turned up portion in each bead portion and extending radially outwardly from the bead core, the carcass cords made of twisted organic fiber filaments, wherein the twist number is more than 5 and less than 30 turn/10 cm. 
     Therefore, the bending rigidity and tensile strength of the carcass cord are increased. Accordingly, the required carcass strength can be obtained, even if the carcass cord diameter, cord count and ply number are decreased in comparison with conventional cords. Thus, tire weight may be reduced and tire rigidity improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described in detail in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a cross sectional view of a pneumatic radial tire according to the present invention; 
     FIG. 2 is a schematic perspective view of an example of the carcass cord; 
     FIG. 3 is a schematic perspective view of another example of the carcass cord; 
     FIG. 4 is a cross sectional view of a pneumatic radial tire according to the present invention; 
     FIG. 5 is a cross sectional view of a bead portion of a pneumatic tire according to the present invention; 
     FIG. 6 is a diagram explaining deformation of a tire; and 
     FIGS. 7A to 7E are cross sectional views showing carcass turned up structure of test tires and prior art tires. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIGS. 1 and 4, pneumatic radial tire 1A, 1B comprises a tread portion 2, a pair of axially spaced bead portions 4, a pair of sidewall portions 3 extending between the tread edges and the bead portions, a pair of bead cores 5 with one disposed in each of the bead portions 4, a carcass 6 extending between the bead portions 4, a belt 8 disposed radially outside the carcass 6 and inside a rubber tread. 
     The tire has an aspect ratio of smaller than 70%, which is defined as the ratio of the tire section height to the tire section width. 
     In FIGS. 1 and 4, the tire is mounted on its regular rim R and inflated to its regular inner pressure, but not loaded. Here, the regular rim R is a rim officially approved for the tire by JATMA (Japan), TRA (U.S.A.) and ETRTO (Europe), and the regular inner pressure and a regular load are the maximum air pressure and the maximum tire load for the tire officially specified in Air-pressure/Maximum-load Table by the same associations. 
     The carcass 6 comprises at least one ply 7 made of carcass cords 10 laid at an angle of 85 to 90 degrees with respect to the tire equator CO and turned up around the bead cores 5 to form a main portion 11 extending between the bead portions 4 and a pair of turned up portions 12. 
     FIG. 2 shows an example of the carcass cord 10, in which the carcass cord 10 comprises a large number of strands 16 twisted together. Each strand 16 in this example is an organic fiber filament 15, e.g. polyester, rayon, nylon and the like. 
     The cord twist N thereof is more than 5 and less than 30 per 10 cm, which is greatly decreased in comparison with a conventional carcass cord. Accordingly, the cord tensile strength increases in comparison with a conventional cord with the same diameter and the same material, and the tire rigidity can be improved, and a carcass weight reduction can be achieved. 
     If the cord twist N is not more than 5, the extensibility of the carcass cord 10 is excessively lowered, and the belt elongation during tire vulcanization becomes small. As a result, tire molding accuracy and running stability are lost. 
     On the other hand, if the cord twist N is not less than 30, the cord does not exhibit an effective improvement in bending rigidity, tensile strength, and it is difficult to achieve a tire weight reduction. Preferably, the cord twist N is 10 to 25. 
     FIG. 3 shows another example of the carcass cord 10, in which the carcass cord 10 comprises a plurality, and this embodiment a small number (two) of strands 16 twisted together. Each strand 16 in this example is composed of a large number of organic fiber filaments 15, e.g. polyester, rayon, nylon and the like. The cord twist N2 and the strand twist N1 are more than 5 and less than 30 per 10 cm. Preferably, to decrease the energy loss of the cord, the cord twist direction is the same as the strand twist direction. 
     Each of the above-mentioned bead portions 4 is provided between the main portion 11 and each turned up portion 12 with a bead apex 9 extending to a taper radially outwardly from the bead core 5. 
     The bead apex 9 is made of a high elastic modulus rubber compound having a complex elastic modulus E of not less than 200 kgf/sq.cm and a JIS A hardness of 65 to 95 is used to increase the tire&#39;s lateral stiffness. 
     Further, the bead apex 9 has a radially extended outer edge terminated at a position radially outward of the radially outer edge of the rim flange FR and radially inwardly of the tire maximum width point Q at which the cross sectional width of the tire in the sidewall portions 3 becomes maximum. 
     In contrast, if the twist number is decreased, the resistance to compressive fatigue of the cord decreases. When a tire is deformed by its load, as shown in FIG. 6, the bead portion 4 is bent axially outwards, and a large compressive stress is produced in a region P axially outward of a bead apex 9 and radially outward of a rim flange FR. Therefore, if the carcass cords are disposed in such a region P, the carcass cords are repeatedly subjected to a compressive stress, that results in many of the carcass cords being broken. 
     In the first example tire 1A in FIG. 1, the carcass 7 is composed of an inner carcass ply 7a and an outer carcass ply 7b, both turned up around the bead cores from axially inside to outside of the tire. The inner carcass ply 7a has a main portion 11a and a pair of turned up portions 12a, and the outer carcass ply 7b has a main portion 11b and a pair of turned up portions 12b. The radial heights h1a, h1b of the radially outer edges of the turned up portions 12a, 12b measured from the bead base line L are set to be smaller than the radial height h2 of the radially outer edge of the rim flange FR measured from the bead base line L. Thereby, the carcass cords 10 are freed from the above described compressive stress, thereby improving tire durability. 
     In the second example tire 1B in FIG. 4, the carcass 7 is composed of an inner carcass ply 7a and an outer carcass ply 7b, both turned up around the bead cores from axially inside to outside of the tire. The inner carcass ply 7a has a main portion 11a and a pair of turned up portions 12a, and the outer carcass ply 7b has a main portion 11b and a pair of turned up portions 12b. 
     Each of the turned up portions 12a of one of the carcass plies 7a and 7b extends radially outwardly over the radially extending outer edge of the bead apex 9 along the axially extending outer surface of the bead apex 9 so as to completely cover the axially extending outer surface S of the bead apex 9. 
     A reinforcing cord layer 21 is disposed between the axially extending outer surface S of the bead apex 9 and the adjacent carcass ply (in this example, turned up portion 12a) to prevent compressive stress on the carcass cords therein. 
     FIG. 5 shows a third example of the carcass 7, wherein the carcass 7 is composed of an inner carcass ply 7a and an outer carcass ply 7b. The inner carcass ply 7a is turned up around the bead cores from axially inside to outside of the tire to have a main portion 11a and a pair of turned up portions 12a. The radial height h1a of the turned up portion 12a is smaller than the radial height h2 of the rim flange FR. The outer carcass ply 7b is turned up around the bead cores from axially outside to inside of tire having a main portion 11b and a pair of turned up portions 12b. The main portion 11b extends along and adjacently to the axially extending outer surface S of the bead apex 9 to completely cover the turned up portion 12a. 
     A reinforcing cord layer 21 is disposed between the axially outer surface S and the adjacent carcass ply (in this example, main portion 11b) to prevent compressive stress on the carcass cords therein. 
     The reinforcing cord layer 21 in FIGS. 4 and 5 is composed of at least one ply of reinforcing cords disposed adjacently to the surface S of the bead apex 9. For the reinforcing cords, compressive resistant cords, e.g., steel cords and the like are used. 
     The reinforcing cords are laid at 40 to 90 degrees with respect to the tire circumferential direction to resist compressive stress in the radial direction of the tire. The reinforcing cord layer 21 extends radially outwardly from the bead portion over the radially extending outer edge of the bead apex 9. The radial height h3 of the radially extending outer edge thereof is in the range of 0.6 to 1.0 times the radial height h4 of the tire maximum width point Q, both measured from the bead base line L. The reinforcing cord layer 21 has a radially extending inner edge located at a radial height h5 substantially the same as or lower than the radially extended outer edge of the rim flange FR. In this example, h5 is slightly smaller than h2. By the provision of the reinforcing cord layer 21, the rigidity of the bead portion is increased, and the compressive stress to which the adjacent carcass cords are subjected are mitigated to improve bead durability. 
     Test tires of which the specification are given in Tables 1 and 2(a)-(b) were prepared and tested for durability, lateral spring constant, steering stability, and tire weight. 
     In the durability test, each test tire was mounted on its regular rim and inflated to its regular inner pressure and loaded by 200% of its regular, load, and then continuously run for 6000 km at a constant speed of 60 km/h. 
     The steering stability was evaluated by a driver into five ranks. (The larger the index, the better the steering stability.) 
     As described above, in the pneumatic radial tires of the present invention, the organic fiber filament cords of which the twist is more than 5 and less than 30 turn/10 cm are used in the carcass. Therefore, the bending rigidity and tensile strength are increased in comparison with the conventional cords, which helps to reduce the carcass weight and to improve running performances. 
     
                                           TABLE 1                                 
__________________________________________________________________________
                Ex. 1-1                                                   
                      Ex. 1-2                                             
                            Ex. 1-3                                       
                                  Ref. 1-0                                
                                        Ref. 1-1                          
                                              Ref. 1-2                    
                                                    Ref.                  
__________________________________________________________________________
                                                    1-3                   
Tire Size       205/65R15                                                 
                      155SR13                                             
                            155SR13                                       
                                  205/65R15                               
                                        155SR13                           
                                              155SR13                     
                                                    155SR13               
Structure       FIG. 1                                                    
                      FIG. 7A                                             
                            FIG. 7A                                       
                                  FIG. 7A                                 
                                        FIG. 7A                           
                                              FIG. 7A                     
                                                    FIG. 7A               
Carcass                                                                   
Turnup structure *1                                                       
                2-OLTU                                                    
                      1-OLTU                                              
                            1-OLTU                                        
                                  2-OLTU                                  
                                        1-OLTU                            
                                              1-OLTU                      
                                                    1-OLTU                
Carcass cord                                                              
Material        polyester                                                 
                      polyester                                           
                            polyester                                     
                                  polyester                               
                                        polyester                         
                                              polyester                   
                                                    polyester             
Structure       1000d/2                                                   
                      1500d/2                                             
                            1500d/1                                       
                                  1000d/2                                 
                                        1000d/2                           
                                              1500d/1                     
                                                    1500d/2               
Cord twist (turn/10 cm)                                                   
                24    20    20    48    40    40    20                    
Strand twist (turn/10 cm)                                                 
                24    20    --    48    40    --    20                    
Angle (degree)  90    90    90    90    90    90    90                    
Durability      no damage                                                 
                      no damage                                           
                            no damage                                     
                                  no damage                               
                                        no damage                         
                                              no damage                   
                                                    4000 km *2            
Tire weight (kg)                                                          
                 9.8   5.7   5.5   9.8   5.7   5.5   6.0                  
Lateral spring constant (kg/mm)                                           
                18.9   9.6   8.8  17.7   9.0   8.2  10.0                  
Steering stability (index)                                                
                 3+    3     3-    3     3-    2.5+  3                    
__________________________________________________________________________
 *1                                                                       
 xy LTU/HTU                                                               
 x =  number of carcass ply turned up from inside to outside              
 y = number of carcass ply turned up from outside to inside               
 LTU = Low turnup structure                                               
 HTU = High turnup structure                                              
 *2 Carcass was damaged after 4000 km run                                 
 
    
     
                                           TABLE 2(a)                              
__________________________________________________________________________
                Ex. 2-1                                                   
                      Ex. 2-2                                             
                            Ex. 2-3                                       
                                  Ex. 2-4                                 
                                        Ex. 2-5                           
                                              Ref. 2-0                    
__________________________________________________________________________
Tire size       225/50R16                                                 
                      225/50R16                                           
                            225/50R16                                     
                                  185/60R14                               
                                        185/60R14                         
                                              225/50R16                   
Structure       FIG. 4                                                    
                      FIG. 4                                              
                            FIG. 5                                        
                                  FIG. 7B                                 
                                        FIG. 7B                           
                                              FIG. 4                      
Carcass                                                                   
Turnup structure *1                                                       
                2-OHTU                                                    
                      2-OHTU                                              
                            1-1LTU                                        
                                  1-OHTU                                  
                                        1-OHTU                            
                                              2-OHTU                      
Carcass cord                                                              
Material        polyester                                                 
                      polyester                                           
                            polyester                                     
                                  polyester                               
                                        polyester                         
                                              polyester                   
Structure       1000d/2                                                   
                      1000d/1                                             
                            1000d/2                                       
                                  1500d/2                                 
                                        1500d/                            
                                              1000d/2                     
Cord twist (turn/10 cm)                                                   
                24    24    24    20    20    48                          
Strand twist (turn/10 cm)                                                 
                --                --                                      
Durability      no damage                                                 
                      no damage                                           
                            no damage                                     
                                  no damage                               
                                        no damage                         
                                              no damage                   
Tire weight (kg)                                                          
                11.3  10.9  11.1   8.1   7.9  11.3                        
Lateral spring constant (kg/mm)                                           
                23.6  22.3  23.3  12.7  11.7  22.5                        
Steering stability (index)                                                
                 3+    3     3+    3+    3     3                          
__________________________________________________________________________
 
    
     
                                           TABLE 2(b)                              
__________________________________________________________________________
                Ref. 2-1                                                  
                      Ref. 2-2                                            
                            Ref. 2-3                                      
                                  Ref. 2-4                                
                                        Ref. 2-5                          
                                              Ref. 2-6                    
__________________________________________________________________________
Tire size       225/50R16                                                 
                      225/50R16                                           
                            185/60R14                                     
                                  185/60R14                               
                                        185/60R14                         
                                              185/60R14                   
Structure       FIG. 5                                                    
                      FIG. 7C                                             
                            FIG. 7D                                       
                                  FIG. 7B                                 
                                        FIG. 7E                           
                                              FIG. 7E                     
Carcass                                                                   
Turnup structure *1                                                       
                1-1LTU                                                    
                      2-OHTU                                              
                            1-OHTU                                        
                                  1-OHTU                                  
                                        1-OHTU                            
                                              1-OHTU                      
Carcass cord                                                              
Material        polyester                                                 
                      polyester                                           
                            polyester                                     
                                  polyester                               
                                        polyester                         
                                              polyester                   
Structure       1000d/2                                                   
                      1000d/2                                             
                            1500d/2                                       
                                  1500d/2                                 
                                        1500d/2                           
                                              1500d/2                     
Cord twist (turn/10 cm)                                                   
                48    24    20    40    20    40                          
Strand twist (turn/10 cm)                                                 
                48    24    20    40    20    40                          
Durability      no damage                                                 
                      damaged                                             
                            damaged                                       
                                  no damage                               
                                        damaged                           
                                              no damage                   
Tire weight (kg)                                                          
                11.1  11.3   8.1   8.1   7.9   7.9                        
Lateral spring constant (kg/m)                                            
                22.1  23.7  12.7  12.0  10.9  10.3                        
Steering stability (index)                                                
                 3     3+    3+    3     3-    2.5+                       
__________________________________________________________________________
 *1                                                                       
 xy LTU/HTU                                                               
 x = number of carcass ply turned up from inside to outside               
 y = number of carcass ply turned up from outside to inside               
 LTU = Low turnup structure                                               
 HTU = High turnup structure