FR2684062A1 - Bicycle fork and method for manufacturing such a fork - Google Patents

Abstract

The bicycle fork having sleeves produced from a composite material includes, at the fork head, a rigid insert (8) for connection having substantially the shape of a U, the branches (9a, 9b) and the concavity of which point downwards and which is equipped, on the opposite side to its concavity, with an axial extension (10) in its middle part; the composite material of the sleeves (2a, 2b) covers over the branches (9a, 9b) of the insert (8) which point downwards and the convex/concave part (9c) of the U connecting these branches (9a, 9b), while the opposite axial extension (10) of the insert is free of composite material, this axial extension (10) being connected to the pivot (5).

Description

BICYCLE FORK AND MANUFACTURING METHOD
FROM SUCH A FORK
The invention relates to a bicycle fork comprising sheaths made from composite material.

 For a cyclist, especially in competition, the weight of the bicycle he uses is an important element to take into consideration, especially during rough or mountainous routes. Manufacturers have therefore sought to use materials which make it possible to reduce the weight of the various elements making up the bicycle. This is the case, in particular, for the bicycle fork which, in a conventional manner, comprises two sleeves at the base of which the front wheel fixing lugs are provided, a fork head ensuring the connection between the upper ends two sleeves and a pivot.

 The bicycle fork is an element having a relatively complicated structure, and which is subjected to significant stresses, in particular during rolling, due to the longitudinal shocks, due to the unevenness of the rolling surface, transmitted by the front wheel to the fork. The forks made of composite material proposed to date have a resistance, hereinafter called frontal resistance, to such rolling shocks and to relatively satisfactory braking forces but which it is desirable to improve.

 The bicycle fork is also subjected to lateral forces, in particular when the cyclist pedals as a dancer. The resistance, hereinafter called lateral resistance, of the fork to such forces is substantially less satisfactory with the forks made of composite material offered to date than with a metal fork.

 A first object of the invention is to provide a bicycle fork having sheaths made from a composite material, the front resistance and lateral resistance of which are improved without appreciable increase in the weight of the fork.

 Another disadvantage of the composite material, generally consisting of carbon fibers and resin, comes from the absence of a range of plastic deformation of this material. In the event of an impact generating mechanical stresses beyond the strength of the material, the latter can be seriously altered without visible traces appearing in the damaged area. The danger is that the user continues to use the bicycle and, on a subsequent request, there will be a sudden break in the area which has been altered without there being any warning sign, such as 'a deformation. The fork head constitutes a sensitive area where a concentration of mechanical stress occurs.

 The invention also aims to provide a bicycle fork which no longer exhibits or to a lesser degree the drawback mentioned above.

 In addition, the fork head is limited in its dimensions by the parts which come to assemble on it. This is the case of the bearing cup which limits the diameter of the pivot-fork head to a standardized value (26.5 mm) as well as the width of the fork head so that the extraction of the cup can be performed in the event of replacement necessary. The thickness of the fork head is limited by the height of the brake caliper, the diameter of the wheel and the type of tire.

 The invention also aims to provide a fork which responds well to the dimensional requirements mentioned above.

 According to the invention, a bicycle fork having sheaths made from composite material is characterized by the fact: that it comprises, at the level of the fork head, a rigid connection insert having substantially the shape of a U whose branches and concavity are facing downwards, and which is provided, on the side opposite its concavity, with an axial extension in its middle part; that the composite material of the sleeves covers the branches of the insert directed downwards and the convex / concave part of the U connecting these branches, while the opposite axial extension of the insert is free of composite material, and that this axial extension is connected to the pivot.

 Advantageously, the rigid insert is made of light metal, in particular a light alloy of aluminum or magnesium.

 Alternatively, the rigid insert is made of one of the materials consisting of reinforced plastic or a metallic composite.

 Preferably, the extension of the insert comprises a frustoconical housing in which is engaged and fixed the end of complementary shape of the pivot.

 Advantageously, the interior surface of the housing of the extension is smooth, as is the surface of the corresponding end of the pivot, the insert and the pivot being fitted tightly, and being glued to one another.

 Each branch of the insert has a recess opening downwards into which engages an end, of complementary shape, of a core of the corresponding sheath.

 The insert and the pivot have aligned holes suitable for serving as a passage for the brake axis, which axis contributes to the insert / pivot assembly.

 The wall of the recess of each branch of the insert has a zone parallel to the axis of the insert, this zone being situated radially inwards. Such an arrangement allows demolding in a single insert operation.

 The invention also relates to a method of manufacturing a bicycle fork with sheaths of composite material characterized in that there is provided, for the fork head, a rigid insert fixed to a pivot, that the insert has two branches to which the cores are connected for the sheaths of the fork, which the composite material is placed around the cores and branches of the insert, which are placed in a mold with centering of the pivot and that one then proceeds to an injection of resin into the mold and to the treatment of this resin under suitable temperature and pressure conditions to obtain the polymerization of the resin.

 Advantageously, the pivot is sealed using an O-ring which makes it possible to prevent a flow of resin outside the mold, along the pivot, this seal being engaged around the pivot and received in a groove of the mold.

 During the treatment in the mold, a temperature which is too high for the polymerization is avoided in order to avoid significant differences in expansion between the insert and the composite material, so as to reduce the stresses which could appear on cooling.

 After closing the mold and placing the mold between the plates of a press, this mold is tilted so that the pivot is at the bottom and the ends of the sleeves are far from the fork head at the top, resin injection taking place from passages located towards the lower part of the mold.

 After the molding and cooling cycle, the mold is opened and the fork is removed in order to then machine the base of the pivot in order to eliminate the unnecessary part of the composite structure and to achieve the smooth bearing of the pivot.

 The holes are drilled for the passage of the brake caliper axis in the pivot, in the insert and in the composite material.

 The ends of the fork sheaths are then cut off and the wheel lugs are assembled by gluing.

 The invention consists, apart from the arrangements set out above, in a certain number of other arrangements which will be more explicitly discussed below in connection with an example embodiment, with reference to the attached drawings, but which n 'is in no way limiting.

 Figure 1 of these drawings is an elevational view of a bicycle fork according to the invention.

 Figure 2 is an axial section of the bicycle fork, on a larger scale, at the connection of the sleeves and the pivot.

 Figure 3 is a schematic perspective view illustrating a first step in a method of manufacturing the fork.

 Figure 4 is a perspective view illustrating a second step of the method after assembly of the insert and the pivot, and the insert and the cores of the sleeves.

 FIG. 5 illustrates, in perspective, a third step of the method after the composite material has been put in place.

 Figure 6, finally, is an exploded perspective view of the two parts of a mold open at the time of the establishment of the fork for the injection of resin and the curing of this resin for polymerization.

 Referring to Figures 1 and 2, we can see a bicycle fork comprising: two sheaths 2a, 2h provided, at their lower end, lugs 3a, 3h for fixing the axis of the front wheel which passes between the scabbards; a fork head 4 located in the upper part of the sleeves and joining them, and a tubular pivot 5 rigidly connected to the sleeves 2a and 2h by the head 4.

 Each sheath2 ~, 2b is made of composite material, in particular consisting of carbon fibers embedded in a resin, of the epoxy type or of the polyester type, or the like, polymerized. The composite material 6 constitutes an outer envelope. Each sheath 2a 2b has a core consisting of a core 7a, 7b, for example made of low density foam (polyurethane foam or equivalent).

 The two sleeves 2a, 2h are connected in the upper part, in the area of the fork head 4, by a rigid insert 8. This insert 8 may be a metal insert advantageously made of light alloy, or magnesium, or other low density metal. Alternatively, this insert 8 can be made of a material having sufficient mechanical characteristics such as reinforced plastic, metallic composite.

 As can be seen in FIG. 2, the insert 8 has substantially the shape of a U having its two branches 9a, 9h turned downwards and connected by a convex / concave part 9c. The insert 8 is provided, on the side opposite to its concavity, with an axial extension 10, in its middle part.

 The branches 9a 9b and the convex / concave part 9c are covered by the layer 6 of composite material up to the base of the extension 10 which is free of composite material.

 This extension 10 comprises a frustoconical housing 11, of the same axis, in which the end 12 is engaged and fixed, of complementary shape to the pivot 5. The angle A of inclination of the generatrices of the frustoconical housing 11, relative to the axis , is preferably of the order of 4.

 As a variant, the insert 8 and the pivot 5 could constitute a single piece.

 The internal surface of the housing 11 is smooth, as is the external surface of the frustoconical end 12 of the tubular pivot 5.

The end 12 of the pivot is fitted tightly into the housing 11 of the insert, a clearance j remaining between the underside of the pivot 5 and the bottom of the housing 11. The end 12 is glued to the wall of the housing II of the insert, in particular using an epoxy type glue. The insert 8, the end 12 of the pivot 5 and the layer 6 of the composite material are crossed by aligned holes 13, centered on the axis of the pivot 5 and of axis perpendicular to the mean plane of the fork. These holes 13 are intended to receive a brake caliper support pin which contributes to making the assembly of the caliper 8 and of the pivot 5 a non-removable connection.

 It should be emphasized that the smooth surfaces provided in the housing 11 and on the external surface of the end 12 avoid the appearance of zones of particular stresses and contribute to good mechanical resistance of the connection. An assembly by screwing using a thread provided on the internal surface of the housing 11 and on the external surface of the end 12 has been found to be less satisfactory, and has revealed points of initiation of rupture in the event of significant demands.

 The structure of the layer 6 of composite material consists of a textile reinforcement, in particular based on carbon fibers, composed of bi-directional and unidirectional braids. These braids are arranged on the core 7a, 7h of the sleeves by superposition of several layers with variable angles to obtain the desired mechanical characteristics.

As already indicated, the composite structure is interrupted at the top of the fork head 4; the transmission of forces between the sleeves 2a, 2h and the pivot 5 is taken up by the insert 8 in a progressive manner thanks to a reduction in the number of layers of the textile structure.

 Each branch 9a, 9b of the insert 8 has a recess 14a, 14h opening downwards in which is engaged the corresponding end 15a, 15b of the core 7a, 7h whose shape corresponds to that of the recess. This end of the core is glued to the inside of the recess.

 The zone 16a 16b of the recesses 14a, 14h closest to the axis of the fork is constituted by a cylindrical surface parallel to the axis of the fork so as to allow the two pins used for molding the to be removed in a single operation the insert 8 and the production of the recesses 14a, 14b. The clearance angle, facilitating demolding, is provided on the zones 17a, 17h furthest from the axis of the fork; because of this draft angle the cross section of the recess 14, 14h increases progressively from top to bottom, that is to say from the bottom of the recess to its lower opening.

 A method of manufacturing the fork 1 is as follows.

 First of all, as illustrated in FIG. 3, the pivot 5, the insert 8 and the cores 7a 7h are prepared in low density foam of the sleeves.

 The end 12 of the pivot 5 is bonded into the housing 11 of the insert 8, in particular using an epoxy adhesive, the insert 8 and the pivot 5 not yet having such holes as 13 for the passage of the brake axis.

 The ends 15a, 15h of the cores 7a 7h are introduced into the corresponding recesses of the branches 9a, 9h of the insert 8 while ensuring their bonding in a suitable position of the cores 7a 7b.

 The foam constituting these cores 7a, 7h being quite flexible, it is not essential to carry out a very precise positioning of the cores 7a, 7b relative to the insert 8 because, in the course of the operations, the cores 7a, 7h can adapt to the imposed shapes.

 The bonding of the cores 7a, 7h on the insert 8 is carried out for example using a cyanoacrylate type adhesive. This bonding is simply intended to allow bulk manipulation of the insert and the cores and to facilitate subsequent operations.

 FIG. 4 represents the whole of the pivot 5, of the insert 8 and of the cores 7il, 7b.

 The composite structure 6 is then placed on the cores 7a, 7h and around the branches 9 ~, 9h of the insert, this composite structure surrounding, during preparation, part of the extension 10. The composite structure 6 is made up of uni-directional and bi-directional fibers, in particular carbon fibers.

 The preform obtained is shown in FIG. 5.

 An O-ring 18 is engaged around the pivot 5, as illustrated in FIG. 6.

 The preform thus equipped is placed in the lower part 19 of a mold comprising a fork imprint 20.

 This cavity 20 comprises a semi-cylindrical axial part 21 intended to receive and center the pivot 5. A groove 22 is provided substantially at mid-length of this part 21 to receive a part of the O-ring 18 which prevents, during molding, a flow resin outside the mold along pivot 5.

 The part 19 of the mold comprises at least one transverse channel such as 23 opening laterally, allowing, when the mold is closed, a connection to a resin injection pipe. The channel 23 is provided at a zone 24 of the cavity 20 intended to receive the extension 10 of the insert 8; the channel 23 is extended by a kind of semi-circular groove 25 in the part 24, the mean line of this groove 25 being situated in a plane orthogonal to the axis of the pivot 5. The imprint 20 is extended by zones 26 , 27 going up gradually, corresponding to the sheaths 2a 2h of the fork. The zones 26, 27 are connected, at their upper end, to longitudinal channels 28, 29, opening towards the outside, and constituting vents.

 The upper part 30 of the mold, intended to close the lower part 19, has shapes, recesses and channels complementary to those of the lower part. The seal between the lower part 19 and the upper part 30 of the mold is ensured by a silicone seal 31 placed in a groove 31â provided for this purpose, which is connected to the groove 22 intended to receive the O-ring 18.

 After introduction of the preform into the cavity 20, the mold is closed by placing the upper part 30 on the lower part 19 and disposed between the plates of a press, not shown.

 The plates of the press, and therefore the mold, are inclined so as to raise the channels 28, 29. The tilting of the mold takes place substantially around a horizontal axis parallel to the small transverse side of the parts 19, 30.

 The preheating of the mold is started. One or more resin injection pipes, not visible in the drawing, are connected to the injection channels such as 23, while other pipes are connected to the channels 28, 29 serving as vents.

 A low pressure injection of resin, for example epoxy resin, is then carried out into the mold through the injection passages 23; the air is gradually expelled from the mold as the resin progresses which rises in the parts 26, 2i inclined until reaching the vents 28, 29 which are then closed when the resin has filled the mold. The low pressure injection is then complete.

 Preheating is carried out at around 500C to allow good impregnation of the fibers by the resin, the low pressure being of the order of 2 bars.

 A high pressure injection is then carried out, starting the heating phase at a temperature of around 100 ° C. to ensure the polymerization of the resin.

 This temperature remains relatively low, however, in order to avoid significant differences in expansion between the insert 8 and the composite material and to reduce the appearance of stresses on cooling.

 The high pressure injection is carried out under a pressure of approximately 7 bars. The temperature rise from 50 ° C. to approximately 100 ° C. takes place in a time dependent on the mold, which may be of the order of 1 to 2 minutes.

 The high pressure injection is stopped after approximately 10 minutes.

 The start of cooling of the press, in particular with cooling water, is then started.

 At the end of the cooling cycle, the press is opened, the mold is removed and the fork is removed from the mold.

 A slight surplus of composite material is found at the base of the extension 10 of the insert 8. This surplus is eliminated and a bearing surface 32 (see FIG. 2) is produced by turning on the extension of the insert.

 The holes 13 are then drilled for the passage of the axle carrying the brake caliper. The position of these holes is identified by a mark in the mold which is printed in the composite material during the molding operation.

 The ends of the sleeves remote from the fork head are then cut off and the wheel lugs 3a 3h are assembled by gluing to the ends of the sleeves.

 In the case of a fork made entirely of composite material, the machining of the flange at the base of the pivot creates a problem because it can constitute a starting point for breaking on the carbon fibers.

 Fatigue tests were carried out on different forks whose pivot was mounted on a headset; a pneumatic cylinder was provided to exert alternating bending on the fork in the longitudinal direction (parallel to the plane of the wheel) to assess the frontal resistance due to rolling shocks and braking forces.

 With a fork according to the invention, in which the surface of the housing 11 was smooth, as well as the external surface of the end 12 linked to the insert by gluing, a considerable improvement was obtained compared to the case where the insert was connected to the end of the pivot by a thread, creating a starting point for rupture.

 A conventional light alloy metal fork, considered reliable, subjected to the same test, exhibited lower resistance.

 A fork made entirely of commercially available composite material also exhibited lower strength.

 Front resistance tests were also carried out in the following manner: the steerer of the fork is rigidly held and the fork sleeves are subjected, at the level of the fixing lugs of the wheel, to loads of approximately 25 decanewtons.

 In frontal resistance (load in a direction parallel to the plane of the wheel), the deflection of a fork according to the invention was substantially the same as that of a metal steel fork, stiffer than a light alloy fork .

 On the other hand, with a fork entirely in commercial composite material, the boom was higher.

 In lateral resistance, the deflection of a fork according to the invention was lower than that of a metal fork in light alloy or a fork entirely in composite material.

 In summary, a fork according to the invention has good rigidity, especially when pedaling as a dancer, and provides good rolling sensations while being relatively light, and safer to use. The invention provides a good weight / stiffness compromise of the fork.

Claims (13)

 1. Bicycle fork having sheaths made from composite material, characterized in that it comprises, at the level of the fork head, a rigid insert (8) for connection having substantially the shape of a U whose branches (9a, eb) and the concavity are turned downwards, and which is provided, on the side opposite to its concavity, with an axial extension (10) in its middle part which the composite material of the sleeves (2a, 2O covers the branches (9 ~, 9> of the insert (8) directed downwards and the convex / concave part (9ç) of the U connecting these branches (9a, 9h), while the opposite axial extension (10) of the insert is free of composite material, and that this axial extension (10) is connected to the pivot (5).  2. Bicycle fork according to claim 1, characterized in that the rigid insert (8) is made of light metal, in particular a light alloy of aluminum or magnesium.  3. Bicycle fork according to claim 1, characterized in that the rigid insert (8) is made of one of the materials consisting of reinforced plastic or a metallic composite.  4. Bicycle fork according to one of claims 1 to 3 characterized in that the extension (10) of the insert (8) comprises a frustoconical housing (11) in which is engaged and fixed the end of complementary shape of the pivot (5).  5. Bicycle fork according to claim 4, characterized in that the interior surface of the housing (11) of the extension (10) is smooth as well as the surface of the corresponding end of the pivot (5), the insert (8 ) and the pivot (5) being fitted tightly, and being glued to one another.  6. Bicycle fork according to one of the preceding claims, characterized in that each branch (9a, 9h) of the insert (8) has a recess (14 ~, 14h) opening downwards in which s 'engages an end (12), of complementary shape, of a core (7a, 7h) of the corresponding sheath.  7. Bicycle fork according to claim 6, characterized in that the wall of the recess (14 ~, 14h) of each branch (9a, 9h) of the insert (8), has an area (16a, 16b ) parallel to the axis of the insert (8), this zone being located radially inwards.  8. Bicycle fork according to one of the preceding claims, characterized in that the insert (8) and the pivot (5) have holes (13) aligned suitable for serving as a passage for the brake axis, which axis contributes to the insert (8) / pivot (5) assembly.  9. Method of manufacturing a bicycle fork with sheaths of composite material, characterized in that there is provided, for the fork head (4), a rigid insert (8) fixed to a pivot (5) , that the insert (8) has two branches (9a, 9h) to which are connected cores (7) for the sleeves (2a, 2h) of the fork (1), which the composite material is placed around cores and branches of the insert, which is placed in a mold with centering of the pivot (5) and which is then injected with resin in the mold and the treatment of this resin in appropriate temperature and pressure conditions to obtain polymerization of the resin.  10. Method according to claim 9, characterized in that the sealing of the pivot (5) is carried out using an O-ring (18) which makes it possible to prevent a flow of resin outside the mold, the along the pivot (5), this seal (18) being engaged around the pivot (5) and received in a groove (22) of the mold.  11. Method according to claim 9 or 10, characterized in that during the treatment in the mold, a temperature too high for the polymerization is avoided in order to avoid significant differences in expansion between the insert (8) and the material composite (6), so as to reduce the stresses which could appear on cooling.  12. Method according to one of claims 9 to 11, characterized in that after closing the mold (19, 30) and placing the mold between the plates of a press, the mold is tilted (19, 30 ) so that the pivot (5) is at the bottom and the ends of the sleeves (2a, 2h) are spaced from the fork head at the top, the resin injection being carried out from passages (23) located towards the lower part of the mold.  13. Method according to one of claims 9 to 12, characterized in that after the molding and cooling cycle, the mold is opened and the fork (t) is removed in order to then machine the base of the pivot (5) in order to eliminate the unnecessary part of the composite structure and to achieve the smooth bearing surface (32) of the pivot (5), and that the holes (13) are drilled for the passage of the brake caliper pin in the pivot (5), in the insert (8) and in the composite material (6).