WO2003000543A1

WO2003000543A1 - Method for manufacturing a crank structure for bicycles and similar vehicles as well as crank structure obtained with this method

Info

Publication number: WO2003000543A1
Application number: PCT/IB2002/002400
Authority: WO
Inventors: Stefano Segato
Original assignee: Stefano Segato
Priority date: 2001-06-25
Filing date: 2002-06-25
Publication date: 2003-01-03
Also published as: ITVI20010143A1

Abstract

A crank structure for a bicycle comprises an elongated body (2) with a first end portion (3) suitable for coupling with a toothed driving ring and a second end portion (6) suitable for coupling with a pedal. The elongate body (2) is formed by a closed box-like casing (9) formed by a pair of half-shells (11, 12) facing each other, a reinforced core (10) defining a longitudinal plane of symmetry (L) located inside the casing (9) and a geometrical centre plane (M) perpendicular to the longitudinal plane of symmetry (L), a first end insert (7) inserted in one end of the casing so as to support a spindle of a toothed wheel and second end insert (8) for supporting a spindle of a pedal, with both the inserts having axes substantially perpendicular to the centre plane (M). The core (M) supports a plurality of filament-like reinforcements (15) having substantially transverse directions selectively oriented with respect to the centre plane (M) so as to form a spatial reticular structure inside the casing (9).

Description

METHOD FOR MANUFACTURING A CRANK STRUCTURE FOR BICYCLES AND SIMILAR VEHICLES AS WELL AS CRANK STRUCTURE OBTAINED WITH THIS METHOD

(54) Title: METHOD FOR MANUFACTURING A CRANK STRUCTURE FOR BICYCLES AND SIMILAR VEHICLES AS WELL AS CRANK STRUCTURE OBTAINED WITH THIS METHOD

The present invention relates generally to the technical field of bicycles and in particular relates to a method for manufacturing a crank structure for bicycles and similar vehicles, with a high performance and high market value, as well as a crank structure obtained with this method.

Background art

As is known, a bicycle is a vehicle which essentially comprises a metal frame, two wheels arranged in tandem, a pair of pedals connected by means of respective cranks to a ring, a chain which connects the ring to a pinion of one of the wheels, optionally with a gear-changing unit arranged in between, handlebars and a saddle for the user.

The work performed by the user is converted into kinetic energy for forward movement of the bicycle by means of a kinematic chain formed by pedal spindle, crank, central driving spindle, ring, chain, toothed wheels of pinion set, gear- changing unit, pinion spindle.

The efficiency of the energy conversion process depends on many factors such as the mechanical efficiency of the various kinematic pairs, the resistance to forward movement defined by the coefficient of rolling friction between wheel and ground, as well as the aerodynamic friction of the man/bicycle system, and the rigidity of the various components.

This latter factor is very important since the rigidity of the various components which form the kinematic chain is proportional to the elastic deformation energy of the components, which, as is well known, is not transmitted, but results in a reduction in the overall efficiency of the system.

The shape of the various components also assumes a considerable importance since it depends on the aerodynamic penetration coefficient, namely the aerodynamic resistance to forward movement and therefore the power dissipated through aerodynamic friction.

Finally, the weight of the entire system is also extremely important since this parameter, via the coefficient of rolling friction between wheel and ground, results in resistive force and therefore dissipated power.

A crank is a component comprising an elongated body with a first end portion intended for mating with a first rotational kinematic pair in the region of the toothed driving ring and a second end portion intended for mating with a rotational kinematic pair of a pedal.

The user exerts a pressure on the pedals which is converted into force applied to the chain with a value proportional to the distance between the centres of the rotational kinematic pairs and the diameter of the toothed ring with which the chain is engaged.

It may be considered that the value of a crank structure according to the market can be expressed by the following formula:

performance Value = (1) cost

The performance of the crank may be determined by the combination of the specific rigidity, understood as the rigidity related to the weight, and the aerodynamic coefficient. In order to manufacture high-value cranks it is necessary to increase the performance in a manner more than proportional to the cost of the product.

The crank is stressed by forces with an intensity and direction variable over time, in the form of a twisting moment, bending moment and, to a lesser extent, tensile and/or compressive stress.

If the formula (1) is applied to a crank structure, the following is obtained:

Kf Kt 1 1

Value_crank = — ■ — • — • (2)

W W Cr Cost where: Kf = flexural rigidity

Kt = torsional rigidity W = weight of crank

Cr = coefficient of aerodynamic resistance

From the expression (2) it can be deduced that the weight of the component is a decisive element whereas, from a formal point of view, the flexural rigidity has the same importance as the torsional rigidity.

From a practical point of view, however, the flexural rigidity is particularly important since generally it is a few orders of magnitude less than the torsional rigidity.

On the other hand, the torsional rigidity influences both the stability of the engagement between the tip of the shoe and the pedal fixture (a low rigidity of the crank results in a relative rotation of pedal and shoe with the danger of release) and twisting of the toothed wheel on which the chain is engaged, with a consequent reduction in the efficiency of transmission as a result of the deformation of the chain and the friction due to the contact of the latter with the derailleur gear-changing unit. It is known that composite-material structures are characterized by very high specific rigidity and strength values. In particular, composite structures of the unidirectional type are able to impart to the component manufactured maximum rigidity and mechanical strength values in the direction of the fibres.

Multiple-layer structures ensure high specific rigidity values, in particular flexural rigidity in their planes of extension.

It is also known that the modulus of elasticity and the breaking load of the foams used in the sandwich structures with cores of foamed polymer materials vary in proportion to the density of the said materials.

As regards the coefficient Cr, it diminishes from a maximum value, represented by a flat surface arranged perpendicularly to the direction of movement with respect to the associated wind, to a minimum value represented by a spindle-shaped configuration, for example in accordance with NACA profiles. For the same area, closed sections have a greater modulus of torsional rigidity.

From a structural point of view, if it is taken into account that the filament-like reinforcements can be compared to a beam, it is possible to verify that the maximum value of the peak load for instability of the equilibrium, known as the Euler load, depends on the free length of inflection and the constrained condition at the ends of the said beam.

A few processes for the construction of bicycle cranks or similar parts made of composite materials are known.

US-A-5,435,869 relates to a process for the construction of right-hand and left- hand cranks by means of a method for winding filament-like elements (filament winding), in which the bush of the pedal spindle is made of metallic material and that of the toothed-ring spindle is made of composite material. A disadvantage of this known crank structure consists in the fact that its cross- section is open and therefore it has different flexural moments of inertia in the plane of the reticular structure and in the vertical plane, as well as a torsional moment of inertia which is less than that of a closed cross-section. This results in a greater dissipation of energy due to elastic deformation and in lateral displacement of the foot, with the consequent risk of release of the tip of the shoe from the fixture.

A further disadvantage consists in the particular reticular structure which comprises reinforced resin with short (random) fibres, having elastic characteristics inferior to those of the same structure made with unidirectional fibres. For the same rigidity, the transverse sections of a crank with short fibres must be greater than those of a crank made with long fibres and consequently the weight of the crank will be greater and its value less.

US-A-5,941 ,135 discloses a crank obtained by means of a manufacturing method which improves the technical characteristics, such as the weight, the strength and the rigidity and reduces its production cost.

Since this known crank comprises an arm joined to the end of a drive shaft by means of an insert, this limits its interchangeability on existing bicycles. Moreover, the form-fit of the insert with a tubular end of the crank reduces its aesthetic value and hence its appeal to a purchaser.

A further drawback of the method of manufacture of this known structure consists in the fact that the strength of the connection between insert and body of the crank, determined by the shearing breaking load of the material used for adhesion, is somewhat limited. This is particularly true for cylindrical connections and torsional stresses with a moment vector parallel to the axis of the cylindrical pair. Since the insert is simply co-moulded with the core of foamed material, the value of the torque which may be transmitted by this structure is fairly limited and the latter may be subject to failures due to the shearing forces in the region of the interface between core and external lining.

US-A-6,202,506 discloses a multiple-layer crank structure having a core made of foamed polymer material, a lining of reinforced plastic material, a first and a second insert foamed with the core and able to be accessed through the lining.

The manufacturing process described in this prior art document envisages the introduction, into a mould, of at least one layer of pre-impregnated composite material, the introduction of bushes, foaming of the core inside the mould, sealing of the core with at least one pre-impregnated layer and polymerization of the assembly thus obtained. At least one part of the device is foamed at the end of the core and is accessible via the reinforced lining. The device for mating with the toothed ring comprises a second part consisting of annular elements arranged at the end of the abovementioned crank, the annular elements being made of plastic and being moulded inside the lining, while the bushes and a first part of the device for mating with the ring are made of aluminium.

A drawback of this known structure is that the core has solely a filling function and does not contribute towards the structural strength of the assembly. The result is that, in order to obtain a superior performance in terms of rigidity, it is necessary to increase the cross-section of the lining consisting of reinforced polymer material, with a consequent reduction in the market value of the finished product.

Disclosure of the invention

A main object of the present invention is that of eliminating or at least reducing the abovementioned drawbacks, providing a crank structure which has a high market value.

A particular object is that of providing a structure which has a high rigidity and a relatively low production cost. Another particular object is that of providing, using simple and economic methods, a crank structure which has anisotropic properties as regards its rigidity and shearing and flexural strength, in particular in areas which are particularly stressed.

These objects, together with others which will appear more clearly below, are achieved by a method for manufacturing a crank structure of the type having an elongated body with a first end portion intended for mating with a toothed driving ring and a second end portion intended for mating with a pedal, which method, in accordance with claim 1 , comprises the steps of preforming an elongate core with a longitudinal plane of symmetry, preforming a first end insert for supporting a spindle of a toothed wheel and a second end insert for supporting a spindle of a pedal, joining said first and said second end insert to said elongate core with axes of the pins which are substantially parallel to each other and lying in said longitudinal plane of symmetry, preforming a pair of half-shells of laminated material which can be joined together along the peripheral edges so as to encase said core completely, arranging said core inside said half-shells, joining said half- shells along their peripheral edges so as to define a closed box-like casing, joining said core to said casing so as to form a unitary assembly and moreover the step of inserting, in said core, a plurality of filament-like reinforcements having substantially transverse directions selectively oriented with respect to a centre plane perpendicular to the axes of said pins, so as to form a spatial reticular structure inside the casing.

According to a further aspect of the invention, a crank structure which can be obtained with the abovementioned method comprises an elongate body with a first end portion suitable for coupling with a toothed driving ring and a second end portion suitable for coupling with a pedal, characterized in that said elongate body comprises a closed box-like casing formed by a pair of half-shells facing each other, a reinforced core defining a longitudinal plane of symmetry located inside said casing and a geometrical centre plane perpendicular to said longitudinal plane of symmetry, a first end insert fixed to one end of said casing in order to support a spindle of a toothed wheel and a second end insert embedded in the other end of said casing for supporting a spindle of a pedal, both said inserts having axes substantially perpendicular to said centre plane, said reinforced core having inside it a plurality of filament-like reinforcements having directions substantially transverse with respect to said centre plane.

Brief description of the drawings

Further features and advantages of the invention will appear more clearly from the detailed description of some preferred, but not exclusive embodiments of a crank structure according to the invention, illustrated by way of a non-limiting example with the aid of the accompanying drawing sheets in which:

FIG. 1 shows a perspective view of a right-hand crank according to the invention; FIG. 2 shows a top plan view of the right-hand crank according to Fig. 1 ;

FIG. 3 shows a side view of the crank according to Fig. 1 ; FIG. 4 shows a sectional view of the crank according to Fig. 2 sectioned along the plane indicated by IV-IV;

FIG. 5 shows a sectional view, on a slightly enlarged scale, of a detail according to Fig. 4;

FIG. 6 shows a sectional view of the crank according to Fig. 2, sectioned along the plane indicated by VI-VI;

FIG. 7 shows a perspective view of a detail of the crank according to Fig. 1 ; FIG. 8 shows a perspective view of another detail of the crank according to Fig. 1 ;

FIG, 9 shows a perspective view of another detail of the crank according to Fig. 1 ;

FIG. 10 shows a perspective view of a left-hand crank according to the invention; FIG. 11 shows a top plan view of the crank according to Fig. 10;

FIG. 12 shows a sectional view of the crank according to Fig. 9 along the plane indicated by XII-XII; FIG. 13 shows a sectional view of a detail of Fig. 12; FIG. 14 shows a functional block diagram of the method for manufacturing the crank according to Fig. 1 or 10.

Detailed description of the preferred embodiment(s)

With reference to Figures 1 to 13, a crank structure of a bicycle or similar vehicle is shown, which may assume the configuration of a right-hand crank or a left-hand crank.

The right-hand crank structure is denoted in its entirety by the reference numeral 1 , while the left-hand crank structure, which is similar to, but structurally different from the former, is denoted in its entirety by the reference numeral 1'.

Below, the right-hand crank structure 1 will be described in detail, it being understood that, in the left-hand crank structure 1', the same elements with the same function will be identified by identical reference numerals with an apostrophe.

The crank structure 1 comprises an elongate body - denoted in its entirety by the reference numeral 2 - which has a first end portion 3 suitable coupling with a toothed driving wheel or ring - not shown in the drawings for the sake of greater clarity - on which a chain of a type known per se can be engaged, said chain being connected to the rear wheel of the bicycle and also not being shown in the drawings.

Appropriately, the end portion 3 may have a series of spokes or radial extensions 4 provided with end holes 5 for mating with a toothed ring by means of suitable connecting members, such as screws or rivets. The right-hand crank structure 1 also comprises a second end portion - denoted 6 - intended for mating with a pedal for the user, also not shown in the drawings for the sake of clarity.

The elongate body 2 defines a longitudinal geometrical plane - denoted by L in Fig. 2 - which divides the crank into two symmetrical and mirror-image parts and which, for this reason, will be referred to below as "longitudinal plane of symmetry L".

For reasons which will be become clearer below, it is necessary to identify another geometrical plane - referred to below as "centre plane M", clearly visible in Fig. 3 - which is perpendicular to the longitudinal plane of symmetry L and located in a central position with respect to the main surfaces of the body 2, so as to intersect the plane of the connection holes 5.

A first insert 7, described in greater detail below, is rigidly fixed to the first end portion 4, said insert being intended to support the spindle of the toothed ring. A second insert 8 is rigidly fixed to the second end portion, said insert being intended to support the spindle of the pedal. It should be noted that the axes of the abovementioned pins are contained in the plane of symmetry L and are perpendicular to the centre plane M.

More particularly, the elongate body 2 is formed by a closed box-like casing 9 which is made of laminated material of relatively thin thickness and with a relatively high modulus of elasticity and which has, inside it, a support core 10 with a relatively low modulus of elasticity and density.

In a preferred embodiment, the close box-like casing 9 may consist of a pair of facing half-shells 11 , 12 which are made of composite polymer or preformed metallic material so as to form the external surface of the crank structure and are firmly joined to the support core 10. In a first preferred embodiment, the half-shells 11 , 12 may be made by preforming with pressure, using a shaped ram, at least one first layer or skin 13 of a reinforced polymer material, inside a female mould having a cavity forming the reverse image of the external surface of the crank.

Preferably, the half-shells 11 , 12 will be formed as multiple-layer structures, for example depositing on the first layer 13 initially preformed at least one second layer or skin 14 consisting of the same reinforced polymer material as the first layer, ensuring that these layers are joined together by means of polymerization of the respective impregnating agents.

For example, the joined layers 13, 14 may be formed by fabrics consisting of synthetic fibres made of materials such as carbon, glass or Kevlar, oriented so as to be suitably intersecting and impregnated with a thermoplastic or thermosetting polymer material.

The core 10 will have a predetermined shape which may be formed by preforming, in a special mould, a resin of expanded polymer material of the open or closed cell type.

Advantageously, this expanded polymer material may be chosen from among polyurethane, epoxy, phenol, acrylic and silica resins, with a density of between 50 and 150 Kg/m³ and a modulus of elasticity close to 130 MPa.

After forming, the core 10 may be lined with at least one or more linings or layers of thermosetting or reinforced thermoplastic composite material intended for joining by means of polymerization to the half-shells 11 , 12 of the casing 9, if the same half-shells have been made of polymer materials.

Alternatively, the base material of the core 10 may be formed by a synthetic or metallic structure with open, closed or honeycomb cells, for example made of reinforced polymer, titanium, light alloy or steel. In this case the preformed core may be lined with a film of adhesive material for adhesion or joining by means of polymerization to the half-cells 11 , 12 of the casing 9.

According to the invention, the support core 10 supports a plurality of filament-like reinforcements - denoted in their entirety by the reference number 15 - having directions selectively oriented and substantially transverse with respect to the abovementioned centre plane M.

In this way, the filament-like reinforcements 15 may have a high Euler load and provide the core 10 with a varied rigidity able to withstand the greater stresses imparted to the various zones of the structure.

Preferably, the filament-like reinforcements 15 may be formed by fibres or cylindrical bars or rods of composite material, such as carbon and/or glass immersed in thermosetting or thermoplastic matrices, or metallic material, such as steel, titanium or light alloys.

Suitably, the filament-like reinforcements may have an average diameter of between about 0.1 mm and 0.8 mm and preferably between 0.25 mm and 0.50 mm.

In order to vary the resistance to the mechanical stresses acting on the crank in the various zones, the filament-like reinforcements 15 may comprise a first group of fibres - generally indicated with the reference numeral 16 - forming a solid angle of between about 45° and 90° with respect to the centre plane M and distributed mainly in the composite torsional, shearing and compressive stress zones.

The distribution may obviously be determined using structural analyses and finite- difference calculation methods or also other algorithms or computerized programs. Preferably, the filament-like reinforcements 15 of this first group will have a length greater than the minimum distance between said half-shells, with their ends substantially in contact with the internal surfaces of the innermost half-shell 14 so as to increase the compressive and torsional strength and the buckling strength of the assembly. Thus, once the half-shells have been joined together, a closed boxlike casing 9 with a high resistance to peak loads and form instability will be obtained.

The filament reinforcements 15 may also comprise a second group of fibres - denoted in their entirety by the reference number 17 - forming a solid angle of between about 75° and 90° with respect to the centre plane M and distributed mainly in the shearing stress zones and zones subject to delamination.

Preferably, the reinforcements 17 of said second group have a length greater than the minimum distance between the innermost layers 14 of the half-shells. In particular, the opposite ends of the reinforcements 17 will pass through the superimposed layers 13, 14 of the half-shells so as to increase their resistance to delamination and the shearing strength of the core 10.

The filament-like reinforcements 13, finally, may comprise a third group of fibres - denoted overall by the reference number 18 - which are inserted in the core 10 so as to form an angle of about 90° with respect to the centre plane M and are distributed mainly in the zones where the inserts 7, 8 are joined to the core 10.

Suitably, the filament-like reinforcements 18 of this third group may have a length greater than the minimum distance between the internal layers of the half-shells and opposite ends passing through both the layers 13, 14 so as to increase their resistance to delamination.

The filament-like reinforcements 13 may be inserted in the matrix of the support core 10 after it has been combined with the half-shells 11 , 12. Alternatively, the filament-like reinforcements 13 may be inserted with force into the matrix of the core 10 after it has been joined to the half-shells, passing through the layers 13, 14 also along the peripheral edges of the joined half-shells.

In an alternative embodiment, the half-shells 11 , 12 may be made with a preformed sheet of metallic material such as steel, aluminium, titanium or light alloy.

In this case, the wall of the half-shells will no longer be formed with several layers and therefore the problem of the delamination resistance does not arise.

Therefore, the filament-like reinforcements 15, 16, 17, 18 will have ends making contact with the internal surface of the half-shells. The core will be fixed to the half-shells by means of suitable structural adhesives.

As mentioned, end inserts 7, 8 are fixed onto the elongate body 2 so as to support the pins of the toothed driving ring and the pedal. In particular, the first insert 7 may be formed by a pair of elements which are joined together on opposite sides of the elongated body 2 and made of composite material such as PEEK or PA12 reinforced with glass or carbon fibres, or metallic material, such as steel, titanium or aluminium alloy.

This pair of elements comprises a first bush 20, with a substantially cylindrical shape, having a central hole with a polygonal section 21 for the spindle of the toothed wheel and with a flanged edge 22 for joining to the half-shell 12, and a second bush 23, which is also made of composite or metallic material and has a substantially cylindrical shape and which is keyed onto the first bush 20 and provided with a flanged edge 24 for joining to the other half-shell 1 1 .

Preferably, the flanged edges 22, 24 of the respective bushes 20, 23 are joined to the adjacent half-shells 12, 1 1 by means of respective connecting rings 25, 26 which are made of composite material, comprising a thermosetting or thermoplastic matrix, or similar material.

The connecting rings 25, 26 have a larger diameter than the flanged edges 22, 24 so as to increase the twisting moment which is applied to the respective half-shells via the bushes 20, 23.

The flanged edges 22, 24 are connected to the reinforcing and connecting rings 25, 26 by means of adhesives or groups of fibres which pass through at least one pair of diametral holes in the manner of stitches.

Alternatively, the connecting rings 25, 26 may be fixed to the flanged edges 22, 24 by means of axial projections of the latter - not shown in the drawings - which can be engaged in corresponding diametral holes formed in the connecting rings 25, 26.

The second insert 8 may be formed by a single-piece bush 28 made of composite or metallic material with a threaded central hole 29 for mating with the pedal spindle and with holes 30 for joining to the half-shells by means of suitable groups of fibres or projections.

In an alternative embodiment of the structure 1 , the core 10 is eliminated from the closed box-like casing 9 by means of dissolving with suitable solvents and removal thereof through a suitable hole formed in the said casing.

The left-hand crank structure 1 ' does not differ substantially from the right-hand crank structure apart from the absence of the means for connection to the toothed driving wheel which, in this case, is not present.

The method for manufacturing a crank structure 1 or 1' envisages essentially the following steps: a) preforming the elongate core 10; b) preforming the first end insert 7 for supporting the spindle of the toothed driving wheel and the second end insert 8 for supporting the pedal; c) joining the end inserts 7, 8 to the elongate core 10 so that the axes of the pins are substantially parallel to each other and lie in the longitudinal plane of symmetry L; d) preforming the half-shells 11 , 12 using laminated material; e) arranging the core 10 inside the half-shells 11 , 12; f) joining the half-shells 11 , 12 along their peripheral edges so as to define a closed box-like casing 9; g) joining the core to the casing 9 so as to form a unitary assembly.

The method is characterized in that it comprises a step involving reinforcement of the core 10 by means of: h) insertion, in said core, of filament-like reinforcements 15 having substantially transverse directions selectively oriented (16, 17, 18) with respect to the centre plane M.

In a first mode of implementing the method, the step h) for insertion of the filament-like reinforcements 15 in the core 10 is performed prior to joining of the half-shells 11 , 12 to the matrix of the said core.

Alternatively, the step h) of insertion of the filament-like reinforcements 15 inside the core 10 is performed after joining of the half-shells 11 , 12 to the matrix of the core 10, passing through the layers 13, 14 which form it, also along their peripheral joining edges.

If necessary, once the assembly is finished, the core 10 may be eliminated from the closed box-like casing 9 by means of a step i) involving dissolving with suitable solvents and removal through a suitable hole formed in the said casing.

Moreover, in a per se known manner, a further step I) for finishing the crank may be envisaged, said step comprising contouring of the joining edge of the half- shells, the forming of polygonal, circular and/or threaded holes along the axes of the inserts 7, 8 and the forming of holes 5 for fixing the toothed driving ring.

Although the crank structure and its manufacturing method according to the invention have been described with particular reference to the accompanying drawings, they may be subject to numerous modifications and variants which fall within the inventive idea expressed in the accompanying claims and which are understood as being likewise protected.

Moreover, all the details may be replaced by technically equivalent elements and the materials may be different according to requirements.

Claims

1. Method for manufacturing a crank structure, in particular for bicycles and similar vehicles, of the type having an elongated body (2) with a first end portion (3) intended for mating with a toothed driving wheel and a second end portion (6) intended for mating with a pedal, which method comprises the following steps: a) preforming an elongate core (10) with a longitudinal plane of symmetry (L); b) preforming a first end insert (7) for supporting a spindle of a toothed wheel and a second end insert (8) for supporting a spindle of the pedal; c) joining said first end insert (7) and said second end insert (8) to said elongate core (10) with the axes of the pins substantially parallel to each other and to said longitudinal plane of symmetry (L); d) preforming a pair of half-shells (11 , 12) using laminated material, said half-shells being able to be joined together along their peripheral edges so as to encase completely said core (10); e) arranging said core (10) inside said half-shells (11 , 12); f) joining said half-shells (11 , 12) along their peripheral edges in order to define a closed box-shaped casing (9); g) joining said core (10) to said casing (9) so as to form a unitary assembly; and also comprises a step involving: h) insertion, in said core, of a plurality of filament-like reinforcements (15) having substantially transverse directions selectively oriented with respect to a centre plane (M) perpendicular to the axes of said pins, so as to form a spatial reticular structure inside the casing (9).

2. Method according to Claim 1 , in which said filament-like reinforcements (15) inserted in said core (10) comprise fibres or small rods of composite material, such as carbon and/or glass, immersed in thermosetting or thermoplastic matrices, or of metallic material, such as steel, titanium or light alloys.

3. Method according to Claim 2, in which said filament-like reinforcements (15) have an average diameter of between about 0.1 mm and 0.8 mm and preferably between 0.25 mm and 0.50 mm.

4. Method according to Claim 3, in which said reinforcing step h) comprises insertion, in said core (10), of a first group of filament-like reinforcements (16) forming a solid angle of between about 45° and 90° with respect to said centre plane (M) and distributed mainly in the composite torsional, shearing and compressive stress zones.

5. Method according to Claim 4, in which the filament-like reinforcements (15) of said first group (16) have a length greater than the minimum distance between said half-shells, with ends substantially in contact with the internal surfaces of said half-shells so as to increase the compressive and torsional strength and the buckling strength of the assembly.

6. Method according to Claim 1 , in which said half-shells (11 , 12) are obtained by preforming at least one first layer (13) of a polymer material having a configuration corresponding to the external surface of the crank.

7. Method according to Claim 6, in which said half-shells (1 1 , 12) are obtained by preforming at least one second layer (14) of a polymer material superimposed on and joined to the first layer (13).

8. Method according to Claim 7, in which at least one of said joined layers (13, 14) is obtained by means of preforming of fabrics consisting of synthetic fibres of the carbon, glass or Kevlar type suitably oriented and suitably impregnated.

9. Method according to Claim 8, in which the polymer material impregnating said layers (13, 14) is thermoplastic or thermosetting.

10. Method according to Claim 4, in which said reinforcing step h) comprises insertion, in said core (10), of a second group of filament-like reinforcements (17) forming a solid angle of between about 75° and 90° with respect to said centre plane (M) and distributed mainly in the zones subject to shearing stress and delamination.

11. Method according to Claim 10, in which the filament-like reinforcements (15) of said second group (17) have a length greater than the minimum distance between said half-shells and opposite ends passing through the layers of said half-shells so as to increase their resistance to delamination and the shearing strength of said core.

12. Method according to Claim 10, in which said reinforcing step h) comprises insertion, in said core (10), of a third group of substantially parallel fibres (18) forming an angle of about 90° with respect to said centre plane (M) and distributed mainly in the vicinity of the joining zones of said inserts (7, 8).

13. Method according to Claim 12, in which the filament-like reinforcements (15) of said third group (18) have a length greater than the minimum distance between said half-shells (11 , 12) and opposite ends passing through the layers (13, 14) of the half-shells so as to increase their resistance to delamination.

14. Method according to Claim 1 , in which said step h) involving reinforcement by means of insertion of filament-like reinforcements (15) in said core is performed prior to joining said half-shells (11 , 12) to said core (10).

15. Method according to Claim 1 , in which said step h) involving reinforcement by means of insertion of filament-like reinforcements (15) in said core (10) is performed after joining said half-shells (11 , 12) to said core (10), passing through said layers (13, 14) and said core (10), also along the peripheral joining edge of said half-shells.

16. Method according to Claim 6, in which said half-shells (1 1 , 12) are made from a sheet of metallic material, such as steel, aluminium, titanium or light alloy.

17. Method according to one or more of the preceding claims, in which said support core (10) is formed by preforming, in a mould, a resin of foamed polymer material of the open or closed cell type.

18. Method according to one or more of the preceding claims, in which said resin of expanded polymer material is chosen from among polyurethane, epoxy, phenol, acrylic and silica resins, optionally reinforced with composite fibres, spheres or "whisker" fibres.

19. Method according to one or more of the preceding claims, in which said support core (10) is made from a metallic matrix with open, closed or honeycomb cells.

20. Method according to one of Claims 17 and 19, in which said support core (10) is lined with at least one lining of reinforced thermosetting or thermoplastic composite material and/or with a film of adhesive material for adhesion or joining by means of polymerization to said half-shells.

21. Method according to Claim 1 , in which said first insert (7) is preformed so that it comprises a first bush (20) made of composite or metallic material, of substantially cylindrical shape with a central hole (21) having a polygonal cross-section (21) for the spindle of said toothed ring, and with a flanged edge (22) for joining to one of said half-shells (12).

22. Method according to Claim 21 , in which said first insert is preformed so as to comprise a second bush (23) also made of composite or metallic material and of substantially cylindrical shape keyed onto said first bush (20) and provided with a flanged edge (24) for joining to the other of said half-shells (11).

23. Method according to Claims 21 and 22, in which said flanged edges of said first bush (20) and said second bush (23) are joined to the respective half- shells by means of respective connecting rings (25, 26) made of composite material with a thermosetting or thermoplastic matrix or similar material.

24. Method according to Claim 22, in which said flanged edges (22, 24) and said connecting rings (25, 26) are fixed together by means of adhesives or fibres (27) which pass through at least one pair of diametral holes.

25. Method according to Claim 24, in which said flanged edges and said connecting rings (25, 26) are fixed together by means of axial projections of said flanged edges (22, 24) inserted into corresponding holes of said connecting rings.

26. Method according to Claim 1 , in which said second insert (8) is preformed from a single-piece block of composite or metallic material so as to comprise a bush (28) with a threaded central hole (29) for mating with the spindle of the pedal and with holes or diametral projections for mating with said half-shells.

27. Method according to Claim 1 , which comprises a step of i) dissolving said core (10) for eliminating it from said closed box-like casing and removal thereof through a suitable hole formed in said casing (9).

28. Method according to Claim 1 , which envisages a step I) for finishing the crank, said step comprising contouring of the joining edge of said half-shells, the formation of polygonal, circular and/or threaded holes along the axis of said first and said second insert, the formation of optional holes (5) for fixing said toothed driving ring by means of suitable connecting members.

29. Crank structure, in particular for a bicycle, obtainable using the method according to the preceding Claims 1 to 28, said structure comprising an elongate body (2) with a first end portion (3) suitable for coupling with a toothed driving ring and a second end portion (6) suitable for coupling with a pedal, characterized in that said elongate body (2) comprises a closed box-like casing (9) formed by a pair of half-shells (11 , 12) facing each other, a core (10) defining a longitudinal plane of symmetry (L) located inside said casing (9) and a geometrical centre plane (M) perpendicular to said longitudinal plane of symmetry (L), a first end insert (7) inserted in one end of said casing (9) for supporting a spindle of a toothed wheel and a second end insert for supporting a spindle of a pedal, said inserts (7, 8) having axes substantially parallel to each other and perpendicular to said centre plane (M), said reinforced core (10) having inside it a plurality of filament-like reinforcements (15) with substantially transverse directions selectively oriented with respect to said centre plane (M).

30. Crank structure according to Claim 29, characterized in that said filament-like reinforcements (15) comprise at least one first group of fibres (16) forming a solid angle of between 45° and 90° with respect to said centre plane (M).

31. Crank structure according to Claim 30, characterized in that said filament-like reinforcements (15) also comprise a second group of fibres (17) forming a solid angle of between 75° and 90° with respect to said centre plane (M) and a third group of fibres (18) forming a solid angle of about 90° with respect to said centre plane (M).

32. Crank structure according to Claim 31 , characterized in that the filament-like reinforcements (15) of said first group (16) have a length greater than the minimum distance between said half-shells (11 , 12), with ends substantially in contact with the internal surfaces of said half-shells for increasing the compressive strength and the buckling strength of said assembly.

33. Crank structure according to Claim 31 , characterized in that said half- shells (11 , 12) comprise at least one pair of preformed and mutually superimposed layers (13, 14) of reinforced polymer material, the filament-like reinforcements (15) of said second and third group (17, 18) having a length greater than the minimum distance between said half-shells (11 , 12), with ends passing through said layers (13, 14) so as to increase the resistance to delamination of the latter.

34. Crank structure according to Claim 33, characterized in that each of said layers (13, 14) consists of a thermoplastic or thermosetting polymer material reinforced with suitably oriented synthetic fibres.

35. Crank structure according to Claim 29, characterized in that said first insert (7) comprises at least one first bush (20) of substantially cylindrical shape with a central hole (21) having a polygonal cross-section for the spindle of said toothed ring and with a flanged edge (22) and a second bush (23) of substantially cylindrical shape provided with a flanged edge (24) keyed onto said first bush (20).

36. Crank structure according to Claim 35, characterized in that it comprises connecting rings (25, 26) made of composite material or similar material and joined to said flanged edges (22, 24) by means of adhesives, fibres (29) or axial projections situated in diametral positions.

37. Crank structure according to Claim 29, characterized in that said second insert (8) comprises a bush (28) made of composite or metallic material, with a threaded central hole (29) for mating with the spindle of the pedal and with holes or diametral projections for joining to said half-shells.

38. Crank structure which can be obtained using the method according to one or more of Claims 1 to 28, comprising: - a pair of half-shells (1 1 , 12) facing each other and joined along their peripheral edges so as to define a closed box-like casing (9) with a first end portion (3) intended for mating with a toothed driving ring and a second end portion (6) intended for mating with a pedal;

- a longitudinal plane of symmetry (L) containing the axes of the pins of said toothed driving ring and said pedal and a geometrical centre plane (M) perpendicular to said longitudinal plane of symmetry (L);

- a first end insert (7) inserted in one end of said casing for supporting a spindle of a toothed wheel and a second end insert (8) for supporting a spindle of a pedal, both said inserts (7, 8) having axes substantially perpendicular to said geometrical centre plane (M); characterized in that the half-shells (11 , 12) of said closed box-like casing

(9) are joined together by means of a plurality of filament-like reinforcements (15) having substantially transverse directions selectively oriented with respect to said centre plane (M).

39. Crank structure according to Claim 39, characterized by providing, in the vicinity of said first support (7), a plurality of through-holes (5) for connection to said toothed driving ring via suitable connecting means.