Abstract:
A fiber reinforced plastic bicycle fork uses a reinforced crown steer tube junction extending the steer tube through a tapered socket to the brake bolt such that the metal steer tube coacts with the fiber reinforced plastic fork to preserve structural integrity in the event of application of high stress to the fork.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention is a bicycle fork with improved steer tube and crown for a bicycle with improved steer tube and crown strength and durability. 
         [0003]    2. Description of Related Art 
         [0004]    Generally, prior art bicycle fork solutions have included the metal fork with a strong-crown, and a steer tube on the top, with tubular fork blades ending in dropouts. Fiber reinforced plastic composite forks have generally used the strong-crown and blade model, or have molded two halves, with the blades extending into the steer tube area, but being subdivided in halves, that were bonded together. 
         [0005]    Foret U.S. Pat. No. 4,828,285, Hollingsworth U.S. Pat. No. 5,016,895 and Buckmiller U.S. Pat. No. 5,609,349 are representative of the high-strength crown approach, although Foret shows the blade halves as an alternative embodiment. 
         [0006]    Klein U.S. Pat. Nos. 5,944,932 and 5,692,764 and Bezin U.S. Pat. Nos. 5,039,470 and 5,181,732 are representative of the two blade halves approach. In each approach to a composite fork, separate metal dropouts are used for wheel attachment, primarily because of wear considerations. 
         [0007]    An example of a threadless fork and handlebar stem combination is shown in Edwards, U.S. Pat. No. 5,865,069. 
         [0008]    Application Publication No. US-2005-0012299-A1, sharing a common assignee herewith, teaches a solution to strengthening the steer tube crown intersection using continuous carbon reinforcing fibers extending from the fork blades, through the crown and into the steer tube. In the fork in this published application, the aluminum steer tube slips over the carbon steer tube and stops short of the crown. The choice of aluminum provides advantages over a fork made entirely of fiber reinforced plastic in that the handlebar stem can be more readily fastened to the steer tube which can be particularly advantageous in situation such as a workshop by a mechanic unskilled in fastening techniques for more exotic materials. The material properties of a metal or other ductile material in the steer tube, in combination with high modulus fiber reinforcement in the fork body yield improvements under certain conditions. 
         [0009]    Colegrove, et al, U.S. Pat. No. 6,924,021 describes a complex-shaped carbon fiber structural member and its method of manufacture, using a combination of prepreg sheets, laminations, unidirectional fiber ropes and fiber bundles, which methods can be used in forming the fork of the present invention. 
         [0010]    Notable in composite bicycle frame patents is the fact that the disclosures in the leading patents focus on the frame itself, to the exclusion of the fork. Thus, Nelson U.S. Pat. No. 6,270,104 B1, Duplessis U.S. Pat. No. 5,076,601, Trimble U.S. Pat. No. 5,158,733, Brezina U.S. Pat. No. 4,493,749 and Derujinsky U.S. Pat. No. 4,900,048 do not show forks, while Trimble U.S. Pat. Nos. 4,923,203 and 4,982,975 show forks, but only as part of the environment. 
         [0011]    The aforementioned patents and application are incorporated by reference as if fully set forth herein. 
       SUMMARY OF INVENTION 
       [0012]    The invention teaches the use of a steer tube that is preferably metallic and is preferably tapered and mates with and is bonded to a carbon reinforced plastic portion comprising a steer tube stub and fork crown. Preferably the brake bolt passes through the fork body and the aluminum steer tube. Other configurations which place a ductile material in areas of potentially high load, such as with a first portion of a steer tube being of ductile material and a second portion being of carbon fiber reinforced plastic, may be adaptable consistent with the teachings herein. 
         [0013]    This fork provides improvement in strength and toughness by providing a tougher crown, crown race seat and bearing receivable portion in the lower portion of the steer tube, near the intersection of steer tube and crown. In this invention, even in a significant impact, the ductile material of the steer tube deforms but holds the fork together. The brake bolt also acts as redundant reinforcement. 
         [0014]    The fiber reinforced plastic portion of the fork can utilize optimum compaction, low void molding of a bicycle fork. A lay-up schedule for the laminations in combination with specific structural shapes enables a strong, light fork while utilizing the advantages of different material properties of the pieces for reinforcement, particularly at the fork crown. The fork crown portion is formed by bladder molding using a layup to provide walls that are sufficiently thick as to make this portion substantially solid. Other reinforcing arrangements such as internal walls can also be utilized as alternatives. 
         [0015]    A bonded steer tube enables the use of a multiplicity of materials such as the aluminum. Beveling or tapering the steer tube receiving portion of the fiber reinforced plastic crown and also beveling or tapering the steer tube enables a gradual transition zone with wall thickness at the top of carbon crown proximate the bearing race, such that impact damage can be absorbed with the fork bending, but not breaking. 
         [0016]    The steer tube of the invention is particularly advantageously adapted to use threadless fork arrangements in which the handlebar stem clamps to the exterior of the steer tube under preload from an adjusting cap, thereby locking a top bearing race in place. 
         [0017]    The invention here uses a method which forms a bicycle fork having a complete shell using main laminations which contain continuous fibers extending from the tip to the steer tube. The entire carbon portion is bladder molded and is hollow. The walls that support the steer tube are molded thick enough so that when machined to receive the steer tube, there is enough material left to support the steer tube entirely front to back. There is a thin portion on the sides of the steer tube that are left unsupported inside the crown since the entire carbon structure is molded hollow. A preferably aluminum steer tube extension is bonded to a steer tube portion of the shell, although other metals or ductile materials could be used. The crown bearing race is a part of the one-piece lower of the fork and is machined to the desired diameter. 
         [0018]    The dropouts are preferably secondarily bonded and have a male plug on the dropout that plugs into the female socket on the fork blade. The dropouts may be co-molded, may have sockets, or both. 
         [0019]    The aluminum steer tube extension receives a star-fangled nut of a typical threadless headset. The use of aluminum, other metal, or other material which can suffer limited wall deformation of that portion of the steer tube which is captured within a bicycle handlebar stem, spacers or frame head tube, is advantageous in a threadless stem configuration because it suffers less from strength reduction as a result of the embedding of the star-fangled nut teeth in the interior wall, when compared to a fiber reinforced plastic composite material. The use of high modulus composite material extending continuously up onto or around the steer tube adds to overall fork strength 
     
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]      FIG. 1  is a side elevational view of the bicycle fork. 
           [0021]      FIG. 2  is a top plan view of the fiber reinforced plastic portion and steer tube of the bicycle fork. 
           [0022]      FIG. 3  is a sectional view of the steer tube of the bicycle fork. 
           [0023]      FIG. 4  is a sectional view of the crown portion of the bicycle fork. 
           [0024]      FIG. 5  is a sectional view of the bicycle fork dropout receiving tip of the bicycle fork taken at line  5 - 5  of  FIG. 2 . 
           [0025]      FIG. 6  is a sectional view of the bicycle fork blade of the bicycle fork taken at line  6 - 6  of  FIG. 1 . 
           [0026]      FIG. 7  is a sectional view of the bicycle fork crown and steer tube taken at line  7 - 7  of  FIG. 2 . 
           [0027]      FIG. 8  is a schematic showing a lamination layup. 
           [0028]      FIG. 9  is a sectional view showing the layup of a portion of the fork in a portion of a mold. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0029]    A bicycle fork  10  is formed of fiber reinforced plastic having bonded dropouts  12  and a steer tube  14 . The fiber reinforced plastic portion of the fork extends from the tips  16 L and  16 R to steer tube base  18  and includes crown portion  20 . Tube  14  is formed with a narrowed, but thickened interior wall portion  22  which mates with a correspondingly beveled socket  24  of crown portion  20 . Beveled edges  23 ,  25  provide a gradual transition zone for the fiber reinforced plastic to metal joint thereby providing superior durability and predictability in strength properties at the crown race and bearing seat. 
         [0030]    While aluminum has advantages for tube  14 , other materials could be used such as steel or titanium. Other hybrid metal/composite structures could be used. Indeed, tube  14  could be formed so as to limit the ductile material, such as aluminum, to the area proximate the crown, and be formed in a composite manner with fiber reinforced plastic extending upward from the ductile portion to the location at which the handlebar would be attached. The form and arrangement of the exterior surface of wall portion  22  and interior surface of socket  24  are consistent with the principles of lug-and-tube construction described in the Nelson U.S. Pat. No. 6,270,104 B1, and Colegrove, et al, U.S. Pat. No. 6,924,021 patents referenced above. 
         [0031]    Base  18  is also beveled, providing a thicker wall proximate crown race  26  and a tapered transitional end  28  to better distribute loads from the bicycle headset to the fork crown  20 . Crown  20  is formed by precision molding or machining to have a bearing seat  30  located so as to surround the region defined by crown race wall  26  and portion  22 , so that there is a double thickness of different materials at this high load location. Some machining of crown race wall  26  and bearing seat  30  is permitted by the strength of the fiber reinforced plastic crown  20 . Precision molding may obviate the need for machining. 
         [0032]    Depending from crown  20  are fork blades  32 ,  34 . Blades  32 ,  34  end in tips  16 L and  16 R to which dropout  12  and its corresponding opposing dropout are attached. 
         [0033]    Steer tube  14  is formed with varying wall thickness and wall diameter progressively from handlebar stem receiving portion  40 , having a fairly large diameter and fairly thin wall, through transition zone  42  at which an internal taper  44  thickens the wall, while the external taper  46  reduces the diameter, terminating in end  22 . 
         [0034]    Mating surfaces in socket  24  of base  18  are formed with a conical transition  48  generally inside end  28  that leads to a cylindrical wall  50  that passes through crown race wall  26  and extends into the body of crown  20 . 
         [0035]    Additionally, crown  20  has a brake bolt portion  52  having an aperture  54  for receiving a bicycle brake bolt. Aperture  54  aligns with a corresponding aperture  56  in steer tube end  22 . 
         [0036]    Tube  14  is bonded in socket  24  with the various surfaces described above being adhesively connected. This could be through a co-molding process or could be a post-molding secondary bond. This configuration coacts with base  18  so that crown race wall  26  is of a dimension standard in bicycle frames, enabling a new fork in accordance to the invention to be adapted to easily replace existing forks. Thus, thickened end  22  and crown race wall  26  provide a very strong, impact resistant structure at the location where high loads are concentrated in the case of a collision between a bicycle wheel and an obstruction. Additionally, a mechanical connection is provided by the brake bolt penetrating coaxial apertures  54 ,  56 . 
         [0037]    In order to best take advantages of the material properties within the structures of a fork retrofittable to standard bicycles and fitting standard wheels and brakes, a combination of the methods of Nelson U.S. Pat. No. 6,270,104 B1, and Colegrove, et al, U.S. Pat. No. 6,924,021 can be adapted.  FIG. 8  shows schematically the orientation of different unidirectional prepreg carbon-epoxy layers into a quasi-isotropic lamination. A plurality of these laminations,  60 ,  62  can be used as shown in  FIG. 9 , in combination with a fiber bundle concentration region  64 ,  66  and this uncured wrapped around an inflatable bladder,  68  fitted in a female mold  70 . Regions  64 ,  66  can be advantageously located to better orient laminations  60 ,  62 , or to provide material to precisely form bearing seat  30 . 
         [0038]    A comparison of the section of  FIG. 4 , taken on a transverse line, looking in a longitudinal direction, and  FIG. 7  taken on a longitudinal line, looking in a transverse direction, relative to the direction of travel of a bicycle illustrate the structure of socket  24 . Socket  24  is formed so that front and rear walls  72 ,  74  contact, and are bonded to, portion  22  of steer tube  14 . End  23  fits in and is bonded to socket end  25 . This provides high strength resisting longitudinal loading.  FIG. 4  shows the advantages of the hollow formation of crown  20 . Lateral loads affect forks with attached wheels much differently than frontal loads. In  FIG. 4 , the side extending portions of crown  20 , which merge into blades  32 ,  34  are not required to contact portion  22 . The contact and bonding of different materials and the shape of socket  24  with thickened fork walls provide a high strength, lightweight and tough fork having both high modulus and ductility proximate the crown.