Abstract:
A rear frame for a bicycle is disclosed to include two tubes and a connecting portion. The tubes are respectively made of fiber reinforced material and have at least one rib therein. The connecting portion is connected between the tubes. By means of the aforesaid arrangement, the rear frame has a high mechanical strength and light weight.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a bicycle and more particularly, to a rear frame for a bicycle, which has a high mechanical strength and light weight. 
         [0003]    2. Description of the Related Art 
         [0004]    The rear part of the frame of a bicycle includes seat stays and chain stays. The seat stays and the chain stays have a similar structure, i.e., both include two tubes approximately parallel to each other and a connecting portion connected between the two tubes. In order to reduce the weight of the seat stays or chain stays, carbon fiber reinforced materials are used. The seat stays and the chain stays are set to support the rear wheel after installation. Therefore, the seat stays and the chain stays must bear the vibration of the rear wheel. Frequently receiving the vibrating force from the rear wheel may cause the seat stays and the chain stays to deform or to break. In order to improve the structural strength, the consumption amount of carbon fiber cloth must be increased. However, increasing the consumption of fiber cloth correspondingly increases the weight of the seat stays and the chain stays. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention has been accomplished under the circumstances in view. It is therefore the main object of the present invention to provide a rear frame for a bicycle, which has a high mechanical strength and light weight. 
         [0006]    To achieve this and other objects of the present invention, the rear frame comprises two tubes and a connecting portion. The tubes are made of fiber reinforced material. The connecting portion is connected between the two tubes. The tubes each have at least one rib therein to reinforce the structural strength. Therefore, the invention achieves the desired structural strength without increasing material consumption. 
     
    
     
       BRIEF DESCRIPTION OF TILE DRAWINGS 
         [0007]      FIG. 1  is a front view of a rear frame for a bicycle in accordance with a first preferred embodiment of the present invention. 
           [0008]      FIG. 2  is a sectional view taken along line  2 - 2  of  FIG. 1 . 
           [0009]      FIG. 3  is a sectional view taken along line  3 - 3  of  FIG. 1 . 
           [0010]      FIG. 4  is a sectional view of a rear frame in accordance with a second preferred embodiment of the present invention. 
           [0011]      FIG. 5  is a sectional view of a rear frame in accordance with a third preferred embodiment of the present invention. 
           [0012]      FIG. 6  is a sectional view of a rear frame in accordance with a fourth preferred embodiment of the present invention. 
           [0013]      FIG. 7  is a sectional view of a rear frame in accordance with a fifth preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    Referring to  FIGS. 1-3 , a rear frame  10  for bicycle in accordance with the first preferred embodiment of the present invention is shown comprising two tubes  20  and a connecting portion  30 . The rear frame  10  can be the seat stays or chain stays of a bicycle for connecting between the seat tube and the rear wheel. 
         [0015]    The two tubes  20  are made of fiber reinforced resin. The fiber used can be carbon fiber, glass fiber, boron fiber or Kevlar fiber. The resin used can be thermosetting resin or thermoplastic resin. The two tubes  20  are substantially disposed on a same imaginary plane P. The two tubes  20  each have a rib  22  therein. The rib  22  divides the inside space of the respective tube  20  into two chambers  24 . The rib  22  extends approximately perpendicular to the imaginary plane P. 
         [0016]    The connecting portion  30  is connected between the two tubes  20  to join the two tubes  20  together. The connecting portion  30  is a tubular member made of fiber reinforced resin. During fabrication, the two tubes  20  are shaped and cured firstly. Then several pre-pregs are rolled up to form a tubular connecting member  32 . The connecting member  32  is positioned between the two tubes  20 . The connecting member  32  and the two tubes  20  are connected by winding pre-pregs  34  around the connecting member  32  and the two tubes  20 . The connecting portion  30  is formed between the two tubes  20  after the connecting member  32  and the pre-pregs  34  are cured. 
         [0017]    The ribs  22  greatly reinforce the mechanical strength of the tubes  20 , enabling the tubes  20  to bear a greater external force, more particularly, the force in direction perpendicular to the imaginary plane P. Therefore, the mechanical strength of the whole structure of the rear frame  10  can be greatly increased without increasing the consumption of the fiber reinforced resin, or the desired mechanical strength of the rear frame  10  can be achieved with less material consumption, i.e., the rear frame  10  of the present invention has the advantages of light weight and high mechanical strength. The rib  22  of each tube  20  can be made in any of a variety of shapes. 
         [0018]      FIG. 4  shows a rear frame  40  in accordance with the second preferred embodiment of the present invention. According to this embodiment, the two tubes  42  are substantially disposed on an imaginary plane P, each having a rib  44  therein. The rib  44  extends approximately in parallel to the plane P. The ribs  44  greatly increase the structural strength of the tubes  42 , enabling the tubes  42  to bear a high pressure, more particularly, the pressure in the direction in parallel to the plane P. 
         [0019]      FIG. 5  shows a rear frame  50  in accordance with the third preferred embodiment of the present invention. According to this embodiment, each tube  52  has three ribs  54  therein. These three ribs  54  extend from the inner wall of the respective tube  52  and converge on a line L. These three ribs  54  divide the inside space of the respective tube  52  into three chambers  56 . This design greatly increases the structural strength of the tubes  52  in all directions. 
         [0020]      FIG. 6  shows a rear frame  60  in accordance with the fourth preferred embodiment of the present invention. According to this embodiment, each tube  62  has four ribs  64  therein. These four ribs  64  extend from the inner wall of the respective tube  62  and converge on a line L. These four ribs  64  divide the inside space of the respective tube  62  into four chambers  66 . This design greatly increases the structural strength of the tubes  62  more even in all directions. 
         [0021]    The connecting portion may be made by means of many other ways.  FIG. 7  shows a rear frame  70  in accordance with the fifth preferred embodiment of the present invention. The rear frame  70  according to this embodiment comprises two tubes  72  and a connecting portion  74  connected between the two tubes  72 . The fabrication of this rear frame  70  is outlined hereinafter. After the two tubes  72  made of fiber reinforced resin are cured, a connecting member  76  prepared from a foam core is positioned between the two tubes  72 . The connecting member  76  and the two tubes  72  are connected by winding pre-pregs  78  around the connecting member  76  and the two tubes  72 . The pre-pregs  78  forms a tubular member  79  and the connecting portion  74  is formed between the two tubes  72  after the pre-pregs  78  is cured. The tubular member  79  is stuffed with a foam core then. 
         [0022]    Although particular embodiments of the invention have been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.