Abstract:
A bicycle includes fork head ( 2 ) from which two sleeves ( 3   a,   3   b ) branch off and, on the side opposite to the sleeves, a tubular pivot ( 4 ) including a lower cylindrical portion ( 4   a ) integral with the fork head, and an upper cylindrical portion ( 4   b ) for fixing a stem. The external diameter (Da) of the lower cylindrical portion ( 4   a ) of the pivot is larger than the external diameter (Db) of the upper cylindrical portion ( 4   b ) of the pivot, and a transitional zone ( 4   c ) links the two parts ( 4   a,   4   b ) with different diameters.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a bicycle fork of the kind comprising a fork crown from where there extend two fork stems and, on the opposite side to the fork stems, a tubular pivot comprising a cylindrical bottom part secured to the fork crown, and a cylindrical top part for securing a handlebar stem. 
     DESCRIPTION OF THE RELATED ART 
     It is known that, in a bicycle, the fork pivot is mounted to rotate in a bushing of the bicycle frame, generally by means of two rolling bearings provided one at the bottom and one at the top of the bushing. The cyclist, by acting on a handlebar fixed to the end of the handlebar stem, can thus steer the fork, and the front wheel, in a desired direction. 
     The connecting region linking the pivot and the fork crown constitutes a trouble spot and is particularly heavily mechanically loaded. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention, above all, to provide a bicycle fork which, in the connecting region linking the fork crown and the pivot, has satisfactory rigidity and provides the best possible distribution of stress over the bearings, particularly at the bottom part of the pivot, without this resulting in an appreciable gain in the weight of the fork and of the steering gear. 
     According to the invention, a bicycle fork of the kind defined previously is characterized in that the outside diameter of the cylindrical bottom part of the pivot is greater than the outside diameter of the cylindrical top part of this pivot, and in that a transition region provides the link between the two, different-diameter, parts of the pivot. 
     This solution makes it possible to strengthen the connecting region linking the fork crown and the pivot, without in any way leading to an appreciable increase in weight because the top part of the pivot maintains a small outside diameter which involves no appreciable modification to the weight of the pivot and handlebar stem assembly. 
     As a preference, the transition region of the pivot has a frustoconical exterior surface, the outside diameter of which decreases from the bottom part toward the top part. 
     The fork is advantageously made of composite, preferably as a single piece with the pivot which is also made of composite. 
     The term “composite” is intended to mean a material consisting of fibers of high mechanical strength, particularly carbon or glass fibers, embedded in a resin matrix. Parts manufactured from such a material are generally molded. 
     As an alternative, the pivot may be made of metal, particularly of light alloy. The fork may also be made of metal. 
     The pivot may be manufactured separately, then assembled with the fork crown. 
     As a preference, the diameter of the top part of the pivot is equal to a standard bicycle pivot diameter, particularly 25.4 mm or 28.6 mm. The diameter of the bottom part of the pivot is at least 5% (five percent), and advantageously at least 12% twelve percent), greater than the diameter of the top part. 
     The pivot may have a cylindrical cavity of circular cross section, the diameter of which remains constant from the bottom to the top. 
     The exterior surface of the connecting region linking the pivot and the fork crown advantageously has a frustoconical surface to act as a support for a mating surface of an inner ring of a rolling bearing. In the case of a fork and of a pivot made of composite, the fibers of the composite are continuous in the connecting region. 
     The invention also relates to a bicycle fork pivot comprising a cylindrical bottom part for linking to a fork crown and a cylindrical top part for the attachment of a handlebar stem, characterized in that the outside diameter of the cylindrical bottom part of the pivot is greater than the outside diameter of the cylindrical top part of the pivot, and in that a transition region provides the link between the two, different-diameter, parts. 
     Advantageously, the transition region of the pivot has a frustoconical exterior surface, the diameter of which decreases from the bottom upward. 
     The invention also relates to a bicycle steering gear, particularly integrated steering gear, comprising a fork as defined previously. 
     When the bicycle steering gear comprises a fork with a connecting region linking the pivot and the fork crown which has a frustoconical exterior surface, the steering gear comprises, at the bottom part of the frame bushing, a rolling bearing, the inner ring of which has an oblique bearing surface mating with the frustoconical surface of the linking region. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Apart from the provisions set out hereinabove, the invention consists in a certain number of other provisions which will be dealt with more fully hereinafter with regard to some exemplary embodiments which are described in detail with reference to the appended drawings but which are not in any way limiting. In the drawings: 
     FIG. 1 is a schematic perspective view of a bicycle steering gear. 
     FIG. 2 is a vertical half section and an external half view of the connecting region linking the pivot and the fork crown according to the invention, depicted in part. 
     FIG. 3 is a vertical section, with partial cutaway, of the pivot and of the fork crown and of the bushing of the frame housing the pivot. 
     FIG. 4 is a perspective view of a vertical section of the fork partially depicted and of the pivot. 
     FIG. 5, finally, is a schematic vertical section of an alternative form of the connecting region linking the pivot and the fork crown. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference to FIG. 1 of the drawings shows a bicycle fork  1  comprising a fork crown  2  from which two partially depicted fork stems  3   a ,  3   b  extend. In the customary way, each fork stem is equipped at its lower end (not visible) with a lug for attaching the axle of the front wheel which passes between the fork stems. 
     The fork  1  comprises a tubular pivot  4  which, as visible in FIGS. 3 and 4, comprises a cylindrical bottom part  4   a  of circular cross section secured to the fork crown  2  and a cylindrical upper part  4   b  of circular cross section for the attachment of a handlebar stem P. The outside diameter of the parts  4   a ,  4   b  is constant. 
     The expressions “bottom part”, “top part”, “bottom” and “top” must, of course, be understood as applying to a bicycle in the vertical position for riding. 
     The fork  1  is mounted to rotate in a bushing  5  of the bicycle frame C, generally via a first bearing provided at the bottom part between the bushing  5  and the pivot  4  and a second bearing (not depicted) provided at the top part of the bushing  5 . In the example illustrated, the first bearing is formed of a rolling bearing  6 , depicted schematically. In general, the second bearing is also formed of a rolling bearing. The two bearings may, however, be plain bearings. 
     The connecting region  7  linking the fork crown  2  and the pivot  4  constitutes a trouble spot at which the best possible distribution of stress is desired. The top part  4   b  of the pivot, on the other hand, is not as heavily stressed. 
     The outside diameter Da of the cylindrical bottom part  4   a  of the pivot is greater than the outside diameter Db of the top part  4   b  of the pivot. These two different diameters Da, Db are provided on one and the same piece, namely the pivot  4 . 
     A transition region  4   c  provides the link between the two parts  4   a ,  4   b . This transition region  4   c  has a frustoconical exterior surface, the diameter of which decreases from the bottom upward. The region  4   c  extends over a relatively long length  1 , particularly a length in excess of 5 cm. 
     At the present time, there are two fork pivot outside diameters as standard in the field of cycling. A first diameter of 25.4 mm is generally used for on-road cycling, while a larger diameter of 28.6 mm is used for mountain bikes, the forks of which are subjected to higher stresses. This larger diameter of 28.6 mm allows for a better distribution of stress over the rolling bearings, and an appearance more in tune with frames with oversized tubes. 
     The diameter Db of the top part may be equal to one of the standard cycling pivot diameters, either 25.4 mm or 28.6 mm at the present time. Of course, the value of the diameter Db is not fixed and may be chosen to suit varying requirements, particularly to fit varying standards. 
     The diameter Da of the bottom part is preferably at least 5%, and advantageously at least 12%, greater than the diameter Db of the top part. 
     The larger diameter Da makes it possible, in the case of a fork made of composite, to cause more fibers to pass from the pivot  4  to the fork crown  2  in the connecting region  7  situated radially on the inside of the rolling bearing or bearing  6 . In the case of a metal pivot  4 , the larger diameter Da allows an increase in the moment of inertia and also improves fork behavior. 
     The pivot  4  has a cylindrical internal cavity  8  of circular internal cross section, the diameter of which remains constant along the entire length of the pivot. The variation in thickness of the pivot is due solely to the variation in the outside diameter. Toward the lower end of the cylindrical cavity  8 , a hole T of axis perpendicular to the mid-plane of the fork passes through the fork crown, for the attachment of a brake caliper. 
     Two diametral partitions at right angles, forming a cross, may be provided in the housing  8  to strengthen the pivot, particularly when it is made of composite. The mid-plane of one of the partitions lies along the longitudinal plane of symmetry of the fork. 
     The fork  1  is advantageously designed for integrated steering, illustrated in FIGS. 1 to  3 . The steering cups of the lower and upper rolling bearings, invisible, are housed in the larger-diameter ends  5   a ,  5   b  of the bushing  5 . 
     An interior bearing surface  9  (FIG. 2) is machined in the lower end  5   a  for supporting the outer ring of the rolling bearing  6 . The inner ring of this rolling bearing is supported against an interior bearing surface  10  machined on the crown  2  at the base of the pivot  4 . There is a minimum clearance  11  between the lower edge of the bushing  5  and the fork crown. 
     The pivot of the invention makes it possible to maintain the advantages of the larger diameter at the connection with the fork crown without adding excessive weight because it is possible, at the top part, to mount a handlebar stem corresponding to a smaller pivot diameter which is sufficient for the stresses there are at this point. 
     The rolling bearing  6  is advantageously manufactured in compact form so as to make it possible, using the same rolling bearing, to produce a steering gear using either a pivot of constant outside diameter for on-road cycling, generally a diameter of 25.4 mm, or a pivot of constant outside diameter for mountain bikes, generally a diameter of 28.6 mm, or the pivot according to the invention. For that, the inner ring of the rolling bearing has an inside diameter designed for the largest-diameter pivot and an outside diameter compatible with a frame bushing associated with a small-diameter pivot, for on-road cycling. 
     Referring to FIG. 5, it is possible to see a particularly advantageous alternative form of embodiment of the connecting region  107  linking the pivot  104  and the fork crown  102 . The various elements have roles similar to elements already described previously and are denoted by a reference equal to the sum of the previous reference and the number  100 . 
     In FIG. 5, only the bottom part  104   a  of the pivot is depicted. The outside diameter of the top part (not depicted) of the pivot  104  is smaller than that of this bottom part  104   a.    
     The rolling bearing  106  comprises an inner ring  106   a , an outer ring  106   b , and rolling bodies  106   c , particularly balls, arranged between raceways provided in the rings. 
     The inner ring  106   a  has, toward the inside in the radial direction, a frustoconical oblique lower bearing surface  12 , the generatrices of which are inclined with respect to the geometric axis of the rolling bearing, advantageously at an angle of 45°. The inner surface in the radial direction of the ring  106   a  is extended upward, beyond the small base of the frustoconical surface  12 , by a cylindrical surface  13 , the inside diameter of which has the same value as the outside diameter of the part  104   a  of the pivot which acts as a guide. 
     The exterior surface of the part  104   a  is extended downward by a frustoconical surface  14 , the diameter of which increases downward. The frustoconical surface  14  belongs to the exterior surface of the connecting region  107 . The inclination of the generatrices of the surface  14  to the geometric axis of the fork is the same as that of the generatrices of the surface  12  which presses and abuts against the surface  14 . 
     The outer ring  106   b  has an outer surface, in the radial direction, comprising a top part  15  with a frustoconical exterior surface, the diameter of which decreases upward, and a cylindrical bottom part  16  of the same diameter as the internal cylindrical surface of the bushing  105  of the frame. The inclination of the generatrices of the surface  15  to the geometric axis of the rolling bearing is advantageously 45°. A shoulder  17  projecting toward the inside of the bushing  105  is provided with a frustoconical bearing surface  18  which mates with the surface  15  which presses against it. 
     The connecting region  107  with oblique surface  14  provides a gradual transition from the bottom part  104   a  of the pivot to the fork crown  102 . In the case of a pivot  104  and a fork crown  102  made of a single piece of composite it is possible, by the oblique bearing of the surfaces  12  and  14  one on the other, to pass numerous continuous fibers F under the rolling bearing  106 , from the pivot  104  to the fork crown  102 . This avoids machining a bearing surface such as 10 in FIG. 2, with the cutting of fibers. 
     The continuity of the fibers F is favorable to the mechanical strength of the connection between the pivot  104  and the fork crown  102 . Furthermore, the bearing surface  14  on the fork crown, for the rolling bearing  106 , can be obtained directly by molding. 
     The rolling bearing  106  can be produced in compact form with an inside diameter of the inner ring  106   a  as large as possible to allow a great many fibers F to be passed through the connecting region  107 . The stress level is thus reduced in the fragile section, and safety is enhanced. According to an advantageous, but not limiting, exemplary embodiment, the outside diameter of the rolling bearing  106  is equal to 42 mm, and its inside diameter is equal to 33 mm. 
     When a plain bearing is installed in place of the rolling bearing  6 ,  106 , all of the above remains valid. 
     The fork according to the invention, the pivot of which is combined with a compact rolling bearing  6 ,  106  or a plain bearing, also offers freedom in the construction of the bicycle. The fork may be chosen independently of the bushing  5 ,  105 , making it possible to devise a range by altering the diameter of the bushing while at the same time using the same rolling bearing  6 ,  106 . Better mechanical behavior is obtained, and integrated steering can be used on certain current frames.