ITVI20010143A1 - METHOD OF CREATION OF A CRANK STRUCTURE FOR BICYCLES AND SIMILAR VEHICLES, AS WELL AS A CRANK STRUCTURE OBTAINED WITH SUCH ME - Google Patents

Description

DESCRIPTION

Field of application

The present invention generally relates to the technical sector of bicycles, and has in particular as its object a method for manufacturing a crank arm structure for bicycles and similar vehicles, with high performance and high market value, as well as a pedal crank structure obtained with such method.

State of the art

As is known, the bicycle is a vehicle which essentially comprises a metal frame, two tandem wheels, a pair of pedals connected by respective cranks to a crown, a chain that connects the crown to a pinion of one of the wheels, possibly with the interposition of a gearbox, a handlebar and a saddle for the user.

The work performed by the user is transformed into kinetic energy of the bicycle's advancement through a kinematic chain formed by pedal pin, crank arm, bottom bracket pin, crown, chain, sprockets of the pinion pack, gearbox, pinion pin.

The efficiency of the energy transformation process depends on many factors such as the mechanical efficiency of the various kinematic pairs, the resistance to advancement determined by the coefficient of rolling friction between the wheels and the ground as well as the aerodynamic friction of the man-bicycle system, the stiffness of the various components. .

This last factor is very important since the stiffness of the various components that form the kinematic chain are proportional to the elastic deformation of the components themselves which, as is well known, is not transmitted but determines a reduction in the overall efficiency of the system. The shape of the various components also takes on considerable importance since the aerodynamic penetration coefficient depends on it, that is, the aerodynamic resistance to advancement, and therefore the power dissipated by aerodynamic friction.

Finally, the weight of the entire system is also extremely important as this quantity, through the coefficient of rolling friction between the wheels and the ground, translates into resistant force and therefore into dissipated power.

A pedal crank is a component comprising an elongated body with a first end portion intended for coupling with a rotoidal kinematic pair at the sprocket, and a second end portion intended for coupling with a rotoidal kinematic pair of a pedal.

The user exerts pressure on the pedals which translates into force applied to the value chain proportional to the distance of the centers of the rotoidal kinematic pairs and to the diameter of the toothed crown with which the chain itself is coupled.

It can be assumed that the value of a crank structure perceived by the market is expressed by the following formula:

performance

Value = (1)

cost

The performance of the crank arm can be determined by the combination of the specific stiffness, understood as stiffness in relation to weight, and the aerodynamic index.

To create high-value cranks, performance must be increased more than proportionally to the cost of the product.

The crank arm is stressed by forces of intensity and direction that vary over time, in the form of torque, bending moment and, to a lesser extent, traction and / or compression effort.

Applying formula (1) to a crank arm structure we obtain:

where: Kf = fessional stiffness

Kt = torsional stiffness

W = weight of the crank arm

Cr = drag coefficient

From expression (2) it is clear that the weight of the component is a determining element while, from a formal point of view, the bending stiffness has the same importance as the bending stiffness.

From a practical point of view, however, bending stiffness is particularly important as it is generally some order of magnitude less than torsional stiffness.

On the other hand, the torsional stiffness affects both the safety of the coupling of the shoe tip with the pedal coupling (a low stiffness of the crank arm results in a rotation between the pedal and the shoe with the danger of release) and the torsion of the toothed wheel. on which the chain engages, with a consequent reduction in the efficiency of the transmission due to the deformation of the chain and the friction due to the contact of the latter with the rear derailleur.

It is known that structures made of composite materials are characterized by extremely high specific stiffness and strength values. In particular, the composite structures of the unidirectional type are able to give the component produced maximum values of stiffness and mechanical resistance in the direction of the fibers.

The multilayer structures guarantee high values of specific stiffness, especially the flexural one in their development plan.

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

As regards the coefficient Cr, it decreases from a maximum value, represented by a flat surface arranged perpendicular to the direction of motion with respect to the relative wind, to a minimum value represented by a fusiform configuration, for example according to NACA profiles. With the same area, the closed sections have a higher torsional stiffness module

From a structural point of view, if we take into account that the filiform reinforcements are comparable to a beam, it can be verified that the maximum value of the buckling load for the equilibrium instability, known as Euler load, depends on the free length deflection and from the constraint condition at the ends of the beam itself.

Some processes are known for the construction of bicycle cranks or similar members made of composite materials.

The US-A-5 435 869 patent relates to a process for the construction of right and left cranks by means of a method of winding filiform elements (filament winding) in which the pedal pin bushing is made of metal and that of the crown gear pin. it is made of composite material.

A disadvantage of this known pedal crank structure consists in the fact that its cross section is open and therefore has different bending moments of inertia in the plane of the reticular structure and in the perpendicular one, as well as a lower torsional moment of inertia compared to that of a cross section. closed. This involves a greater dissipation of energy by elastic deformation and a lateral displacement of the foot, with the consequent risk of unhooking the toe of the shoe from the binding.

A further disadvantage consists in the particular reticular structure which comprises a resin reinforced with short fibers (random), having lower elastic characteristics than those of the same structure made with unidirectional fibers. With the same stiffness, the cross sections of a pedal crank with short fibers must be greater than those of a pedal crank made with long fibers and consequently the weight of the pedal crank will be greater and its value less.

Patent US-A-5941135 discloses a pedal crank obtained by a manufacturing method which improves the technical characteristics, such as weight, strength and stiffness, and reduces its production cost.

Since this known crank arm includes an arm coupled to the end of a semi-axle by means of an insert, this limits its interchangeability on existing bicycles. Furthermore, the shape coupling of the insert with a tubular end of the crank arm reduces its aesthetic value and therefore the appeal for the buyer.

A further drawback of the manufacturing method of this known structure consists in the fact that the resistance of the connection between the insert and the body of the crank, determined by the shear breaking load of the material used for the adhesion, is rather limited. This is particularly true for cylindrical couplings and torsional stresses with a moment vector parallel to the axis of the cylindrical torque. Since the insert is simply co-molded with the foam core, the torque value that can be transmitted by this structure is rather limited and can be subject to failure due to shear forces in the core-external coating interface area.

From the US-A-6 202 506 patent a multilayer pedal crank structure is known having a core in foamed polymeric material, a coating in reinforced plastic material, a first and a second insert foamed with the core and accessible through the coating.

The manufacturing process described in this prior document provides for the introduction into a mold at least one layer of pre-impregnated composite material, the introduction of bushings, the foaming of the core in the mold, the sealing of the core with at least one other layer. pre-impregnated, the polymerization of the whole thus obtained. At least part of the device is foamed at the end of the core and is accessible through the reinforced coating. The toothed crown coupling device comprises a second part of annular elements arranged at the end of the aforementioned pedal crank, the annular elements are made of plastic and are molded inside the coating, while the bushings and a first part of the crown coupling device they are made of aluminum.

A drawback of this known structure is that the core has only a filling function and does not contribute to the structural strength of the assembly. It follows that, in order to obtain high performance in terms of stiffness, it is necessary to increase the cross section of the coating in reinforced polymeric material, with a consequent decrease in the market value of the finished product.

Presentation of the invention

A main object of the present invention is to eliminate or at least reduce the aforementioned drawbacks, by providing a pedal crank structure which has a high market value.

A particular object is to provide a structure which has a high stiffness and a relatively low manufacturing cost.

Another particular object is to provide, with simple and economical methods, a pedal crank structure which has anisotropic characteristics of stiffness and resistance to shear and bending, especially in particularly stressed areas.

These purposes, as well as others which will become clearer in the following, are achieved by a method for manufacturing a pedal crank structure of the type having an elongated body with a first end portion intended for coupling with a toothed drive crown and a second portion of end intended for coupling with a pedal, which in accordance with claim 1 comprises the steps of preforming an elongated core with a plane of longitudinal symmetry, preforming of a first end insert to support a pin of a toothed wheel and a second end insert for supporting a pin of a pedal, coupling said first and said second end insert to said elongated core with axes of the pins substantially parallel to each other and lying in said longitudinal symmetry plane, preforming a pair of half-shells in laminar material, which can be coupled along the peripheral edges to completely wrap said core, di positioning of said core inside said half-shells, joining of said half-shells along their peripheral edges to define a closed box-like casing, joining of said core to said casing to form a unitary assembly, and furthermore the step of insertion into said core of a plurality of filiform reinforcements having substantially transverse directions and selectively oriented with respect to a median plane normal to the axes of said pins, so as to provide a spatial reticular structure inside the envelope.

In a further aspect of the invention, a pedal crank structure obtainable with the aforesaid method comprises an elongated body with a first end portion intended for coupling with a toothed drive crown and a second end portion intended for coupling with a pedal, characterized in that said elongated body comprises a closed box-like casing formed by a pair of mutually facing half-shells, a reinforced core defining a longitudinal symmetry plane placed inside said casing and a median geometric plane perpendicular to said longitudinal symmetry plane, a first end insert attached to one end of said housing for supporting a sprocket pin and a second ditch end insert at the other end of said housing for supporting a pedal pin, both of said inserts having axes substantially perpendicular to said median plane, called reinforced core having inside it a plurality of filiform reinforcements having substantially transverse directions with respect to said median plane.

Brief description of the drawings

Further characteristics and advantages of the invention will become more evident in the light of the detailed description of some preferred but not exclusive embodiments of a pedal crank structure according to the invention, illustrated by way of non-limiting example with the aid of the accompanying drawings in which:

FIG. 1 represents a perspective view of a right pedal crank according to the invention;

FIG. 2 represents a top plan view of the right pedal crank of Fig. 1;

FIG. 3 shows a side view of the pedal crank of Fig. 1;

FIG. 4 represents a sectional view of the pedal crank of Fig. 2 sectioned along the plane of line IV-IV;

FIG. 5 represents a section view on a slightly enlarged scale of a detail of Fig. 4;

FIG. 6 represents a sectional view of the pedal crank of Fig. 2 sectioned along the plane VI-VI;

FIG. 7 represents a perspective view of a detail of the pedal crank of Fig. 1;

FIG. 8 represents a perspective view of another detail of the pedal crank of Fig. 1;

FIG. 9 represents a perspective view of another detail of the pedal crank of Fig. 1;

FIG. 10 represents a perspective view of a left pedal crank according to the invention;

FIG. 11 represents a top plan view of the pedal crank of Fig. 10;

FIG. 12 represents a sectional view of the pedal crank of Fig. 9 taken along the line XII-XII;

FIG. 13 represents a sectional view of a detail of Fig. 12; Fig. 14 represents a functional block diagram of the manufacturing method of the pedal crank of Fig. 1 or 10.

Detailed description of a preferred example of embodiment With reference to Figures 1 to 13, a pedal crank structure of a bicycle or similar vehicle is illustrated which can assume the configuration of a right pedal crank or a left pedal crank.

The right crank arm structure is globally indicated with the reference number 1, while the left crank arm structure, similar but morphologically different from the first, is globally indicated with the reference number 1 '.

In the following, the right crank arm structure 1 will be described in detail, it being understood that in the left one 1 'the same elements having the same function will be identified with the same reference numbers provided with apex.

The pedal crank structure 1 comprises an elongated body, globally indicated with the reference number 2, which has a first end portion 3 intended to be coupled with a sprocket or sprocket, not shown in the drawings for greater clarity, on which it can be engaging a chain of a per se known type, connected to the rear wheel of the bicycle and also not shown in the drawings.

Conveniently, the end portion 3 may have a series of spokes or radial expansions 4 provided with end holes 5 for coupling to a toothed crown by means of suitable connecting members, such as screws or rivets.

The right pedal crank structure 1 also comprises a second end portion, indicated 6, intended to be coupled with a pedal for a user, also not shown in the drawings for clarity.

The elongated body 2 defines a longitudinal geometric plane, indicated L in Fig. 2, which divides the pedal crank into two symmetrical and specular parts, and which for this reason will be referred to in the following as the longitudinal symmetry plane L.

For reasons that will be better understood in the following, it is appropriate to identify a further geometric plane, called in the following the median plane M, clearly visible in Fig. 3, perpendicular to that of symmetry L and in a median position with respect to the main faces of the body 2, in so as to intersect the plane of the connecting holes 5.

A first insert 7 is rigidly anchored in correspondence with the first end portion 4, which will be described in greater detail below, intended to support the pin of the toothed crown. A second insert 8, intended to support the pedal pin, is rigidly anchored in correspondence with the second end portion. It is observed that the axes of the aforementioned pins are contained in the plane of symmetry L and are perpendicular to the median plane M.

More specifically, the elongated body 2 consists of a closed box-like casing 9 in laminar material of relatively thin thickness and with a relatively high modulus of elasticity, having inside it a support core 10 with a relatively low modulus of elasticity and density.

In a preferred embodiment, the closed box-like casing 9 can be constituted by a pair of facing half-shells 11, 12 in pre-formed composite polymeric material or metal, so as to form the external surface of the pedal crank structure and coupled to the core. support 10.

In a first preferred embodiment, the half-shells 11, 12 can be made by pressure preforming, with a shaped pad, at least a first layer or skin 13 of a reinforced polymeric material, in a female mold having a cavity reproducing the surface in negative crank arm.

Preferably, the half-shells 11, 12 will be made as multilayer structures, for example by depositing on the first preformed layer 13 at least a second layer or skin 14 of the same reinforced polymeric material as the first, ensuring that these layers are mutually coupled by polymerization of the respective impregnating agents.

For example, the coupled layers 13, 14 can consist of synthetic fiber fabrics in materials of the carbon, glass or Kevlar type, with suitably crossed orientations, impregnated with a thermoplastic or thermosetting polymeric material.

The core 10 will have a predetermined shape that can be made by preforming in a special mold of a resin of expanded polymeric material, with open or closed cells.

Advantageously, this expanded polymeric material can be selected from polyurethane, epoxy, phenolic, acrylic, siliceous resins, with a density between 50 and 150 Kg / m <3> and a modulus of elasticity close to 130 MPa.

After forming, the core 10 can be coated with at least one or more socks or layers of thermosetting composite material or reinforced thermoplastic material, intended to be coupled by polymerization to the half-shells 11, 12 of the casing 9, if they are made of polymeric materials .

Alternatively, the base material of the core 10 may be made up of a synthetic or metal structure with open, closed or honeycomb cells, for example in reinforced polymer, titanium, light alloy, steel. In this case, the preformed core can be covered with a film of adhesive material for adhesion or union by polymerization to the half-shells 11, 12 of the envelope 9.

According to the invention, the support core 10 supports a plurality of filiform reinforcements, globally indicated with the reference number 15, having selectively oriented and substantially transverse directions with respect to the aforementioned median plane M.

In this way, the filiform reinforcements 15 will be able to have a high Euler load and give the core 10 a differentiated stiffness to withstand the greater stresses imposed in the various areas of the structure.

Preferably, the filiform reinforcements 15 can be constituted by fibers or cylindrical pins in composite material, such as carbon and / or glass immersed in thermosetting or thermoplastic matrices, or metal, such as steel, titanium, light alloys.

Conveniently, the filiform reinforcements can have an average diameter between about 0.1 mm and 0.8 mm and preferably between 0.25 mm and 0.50 mm.

To differentiate the resistance to mechanical stresses acting on the pedal crank in the various areas, the filiform reinforcements 15 may comprise a first group of fibers, indicated as a whole with the reference number 16, forming a solid angle between about 45 ° and 90 ° with respect to in the median plane M, mainly distributed in the stress zones composed of torsion, shear and compression.

The distribution can obviously be predicted by resorting to structural analysis and finite difference calculation methods, or to other algorithms or computerized programs.

Preferably, the filiform reinforcements 15 of this first group will have a length greater than the minimum distance between said half-shells, with ends substantially in contact with the internal surfaces of the innermost half-shell 14 to increase the compressive and torsional strength and the buckling resistance of the assembly. . Thus, once the half-shells have been joined, a closed boxed casing 9 will be obtained with high resistance to peak loads and shape instability.

The filiform reinforcements 15 may also comprise a second group of fibers, indicated as a whole with the reference number 17, forming a solid angle between about 75 ° and 90 ° with respect to the median plane M and distributed mainly in the areas of shear stress. and tendency 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 cross the superimposed layers 13, 14 of the half-shells to increase their resistance to delamination and the shear strength of the core 10.

Finally, the filiform reinforcements 13 may comprise a third group of fibers, globally indicated with the reference number 18, inserted in the core 10 so as to form an angle of about 90 ° with respect to the median plane M, distributed mainly in the coupling areas of the inserts 7, 8 to core 10.

Conveniently, the filiform reinforcements 18 of this third group may also have a length greater than the minimum distance between the internal layers 14 of the half-shells and opposite ends crossing both layers 13, 14 to increase their resistance to delamination.

The thread-like reinforcements 13 can be inserted into the matrix of the support core 10 before coupling the latter to the half-shells 11, 12.

Alternatively, the thread-like reinforcements 13 can be inserted by force into the core matrix 10 after its coupling to the half-shells, crossing the layers 13, 14 also at the peripheral edges of the coupled half-shells.

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

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

Therefore, the filiform reinforcements 15, 16, 17, 18 will have ends placed in contact with the internal surface of the half-shells. The core will be anchored to the half-shells using appropriate structural adhesives.

As anticipated, the end inserts 7, 8 are fixed on the elongated body 2 to support the pins of the sprocket and of the pedal. In particular, the first insert 7 can be constituted by a pair of elements mutually coupled on opposite sides of the elongated body 2, in composite material such as PEEK or PA12 reinforced with glass or carbon fibers, or metal, such as steel, titanium or alloy aluminum,

This pair of elements comprises a first bushing 20, substantially cylindrical in shape with a central hole of polygonal section 21 for the pin of the ring gear, and with a flanged edge 22 for coupling to the half-shell 12, and a second bushing 23, also it is made of composite or metallic material and has a substantially cylindrical shape, embedded in the first bushing 20 and provided with a flanged edge 24 for coupling to the other half-shell 11.

Preferably, the flanged edges 22, 24 of the respective bushings 20, 23 are coupled to the adjacent half-shells 12, 11 by means of respective connecting rings 25, 26 in composite material with a thermosetting or thermoplastic matrix or in similar material.

The connecting rings 25, 26 have a larger diameter than the flanged edges 22, 24 to increase the torque applied to the respective half-shells through the bushings 20, 23.

The flanged edges 22, 24 are connected to the reinforcing and connecting rings 25, 26 by means of adhesives or groups of fibers 27 which pass through at least a pair of diametrical holes in the manner of stitches.

Alternatively, the connecting rings 25, 26 can be anchored 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 diametrical holes made on the connecting rings 25, 26.

The second insert 8 may consist of a monolithic bush 28 in composite or metal material, with a central hole 29 threaded for coupling to the pedal pin and with holes 30 for coupling to the half-shells by means of appropriate groups of fibers or sources.

In an alternative embodiment of the structure 1, the core 10 is eliminated from the closed box-like casing 9 by dissolving it with suitable solvents and its evacuation through a suitable hole made in the casing itself.

The left crank arm structure 1 'does not differ substantially from the right one except for the lack of means for connection to the sprocket which, in this case, is absent,

The method for creating a 1 or 1 'crank arm structure essentially involves the following steps:

a) preforming of the elongated core 10;

b) preforming of the first end insert 7 to support the pin of the toothed drive wheel and of the second end insert 8 to support the pedal;

c) coupling of the end inserts 7, 8 to the elongated core 10 so that the axes of the pins are substantially parallel to each other and lying in the longitudinal symmetry plane L;

d) preforming of the half-shells 11, 12 in laminar material;

e) arrangement of the core 10 inside the half-shells 11, 12;

f) joining the half-shells 11, 12 along their peripheral edges to define a closed box-like envelope 9;

g) union of the core to the casing 9 to form a unitary assembly.

The method is characterized by the fact that it includes a reinforcement phase of the core 10 by:

h) insertion in it of filiform reinforcements 15 having substantially transverse and selectively oriented directions (16, 17, 18) with respect to the median plane M.

In a first embodiment of the method, step h) of inserting the thread-like reinforcements 15 into the core 10 is carried out before the coupling of the half-shells 11, 12 to the matrix of the core itself.

Alternatively, step h) of inserting the thread-like reinforcements 15 into the core 10 is carried out after the coupling of the half-shells 11, 12 to the matrix of the core 10, crossing the layers 13, 14 that compose them, also at their edges of peripheral coupling.

Eventually, when the assembly is finished, the core 10 can be eliminated from the closed box-like casing 9 by means of a step i) of dissolution with suitable solvents and evacuation through a suitable hole made in the casing itself.

Furthermore, in a per se known manner, a finishing step I) of the pedal crank can be provided, comprising a step for contouring the joining edge of the half-shells, the creation of polygonal, circular and / or threaded holes in correspondence with the axes of the inserts 7 , 8, the creation of the holes 5 for anchoring the toothed drive crown.

Even if the pedal crank structure and its manufacturing method according to the invention have been described with particular reference to the attached drawings, they are susceptible of numerous modifications and variations falling within the inventive concept expressed in the attached claims and which are understood to be equally protected.

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

Claims (13)

CLAIMS 1. Method for manufacturing a pedal crank structure, particularly for bicycles and similar vehicles, of the type formed by an elongated body (2) with a first end portion (3) intended for coupling with a sprocket gear and a second end portion (6) intended for coupling with a pedal, which method comprises the following steps: a) preforming of an elongated core (10) with a longitudinal symmetry plane (L); b) preforming a first end insert (7) to support a pin of a toothed wheel and a second end insert (8) to support a pin of a pedal; c) coupling said first (7) and said second end insert (8) to said elongated core (10) with axes of the pins substantially parallel to each other and to said longitudinal symmetry plane (L); d) preforming a pair of half-shells (11, 12) in laminar material, which can be coupled along the peripheral edges to completely wrap said core (10); e) arrangement of said core (10) inside said half-shells (11, 12); f) joining said half-shells (11, 12) along their peripheral edges to define a closed box-like envelope (9); g) joining said core (10) to said envelope (9) to form a unitary assembly; and also includes a phase of: h) insertion in said core of a plurality of filiform reinforcements (15) having substantially transverse directions and selectively oriented with respect to a median plane (M) normal to the axes of said pins, so as to provide a spatial reticular structure inside the envelope ( 9). Method according to claim 1, wherein said filiform reinforcements (15) inserted in said core (10) comprise fibers or bars of composite material, such as carbon and / or glass, immersed in thermosetting or thermoplastic matrices, or of metallic material, such as steel, titanium, light alloys. Method according to claim 2, wherein said filiform reinforcements (15) have an average diameter comprised 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, wherein said reinforcement step h) comprises the insertion in said core (10) of a first group of thread-like reinforcements (16) forming a solid angle comprised between about 45 ° and 90 ° with respect to said median plane (M), mainly distributed in the stress zones composed of torsion, shear and compression. Method according to claim 4, wherein the filiform 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 to increase the strength compression and torsion and resistance to buckling of the assembly. Method according to claim 1, wherein said half-shells (11, 12) are obtained by preforming at least a first layer (13) of a polymeric material having a configuration corresponding to the external surface of the pedal crank. Method according to claim 6, wherein said half-shells (11, 12) are obtained by preforming at least a second layer (14) of a polymeric material superimposed and coupled to the first (13). 8. Method according to claim 7, wherein at least one of said coupled layers (13, 14) is obtained by preforming fabrics in synthetic fibers of the carbon, glass or Kevlar type, suitably oriented and suitably impregnating. Method according to claim 8, wherein the polymeric impregnation material of said layers (13, 14) is thermoplastic or thermosetting. Method according to claim 4, wherein said reinforcement step h) comprises the insertion in said core (10) of a second group of thread-like reinforcements (17) forming a solid angle comprised between about 75 ° and 90 ° with respect to said median plane (M), mainly distributed in the areas of shear stress and tendency to delamination. Method according to claim 10, wherein the filiform reinforcements (15) of said second group (17) have a length greater than the minimum distance between said half-shells and opposite ends traversing the layers of said half-shells to increase their resistance to delamination and the shear strength of said core. Method according to claim 10, wherein said reinforcement step h) comprises the insertion in said core (10) of a third group of fibers (18) substantially parallel and forming an angle of about 90 ° with respect to said median plane (M), mainly distributed in proximity to the coupling areas of said inserts (7, 8). Method according to claim 12, wherein the filiform reinforcements (15) of said third group (18) have a length greater than the minimum distance between said half-shells (11, 12) and opposite ends crossing the layers (13, 14) of the half-shells to increase their resistance to delamination Method according to claim 1, wherein said step h) of reinforcement by inserting thread-like reinforcements (15) in said core (10) is carried out before coupling said half-shells (11, 12) to said core (10) . Method according to claim 1, wherein said step h) of reinforcement by inserting thread-like reinforcements (15) in said core (10) is carried out after the coupling of said half-shells (11, 12) to said core (10) crossing said layers (13, 14) and said core (10), also in correspondence with the peripheral coupling edge of said half-shells. Method according to claim 6, wherein said half-shells (11, 12) are made starting from a sheet of metallic material, such as steel, aluminum, titanium or light alloy. Method according to one or more of the preceding claims, wherein said support core (10) is made by preforming a resin of open or closed cell foamed polymeric material in a mold. 18. Method according to one or more of the preceding claims, in which said resin of expanded polymeric material is selected from polyurethane, epoxy, phenolic, acrylic, siliceous resins, optionally reinforced with composite fibers, spheres or wiskers fibers. 19. Method according to one or more of the preceding claims, in which said support core (10) is made from a metal matrix with open, closed or honeycomb cells. Method according to one of claims 17 and 19, wherein said support core (10) is coated with at least one sock of a reinforced thermosetting or thermoplastic composite material and / or with a film of adhesive material for adhesion or bonding by polymerization to said half shells. Method according to claim 1, wherein said first insert (7) is preformed so as to include a first bushing (20) made of composite or metal material, of substantially cylindrical shape with a central hole (21) of polygonal section for the pin of said toothed crown, and with a flanged edge (22) for coupling to one of said half-shells (12). 22. Method according to claim 21, wherein said first insert is preformed so as to comprise a second bushing (23) also made of composite or metal material and substantially cylindrical in shape, encased in said first bushing (20) and provided with a flanged edge (24) for coupling said half-shells (11) to the other. 23. Method according to claims 21 and 22, wherein said flanged edges of said first (20) and said second bushing (23) are coupled to the respective half-shells by means of respective connection rings (25, 26) in composite material with a thermosetting matrix or thermoplastic or similar material. Method according to claim 22, wherein said flanged edges (22, 24) and said connecting rings (25, 26) are mutually anchored by means of adhesives or fibers (27) which pass through at least a pair of diametrical holes. Method according to claim 24, wherein said flanged edges and said connecting rings (25, 26) are mutually anchored by means of axial projections of said flanged edges (22, 24) inserted in corresponding holes of said connecting rings. Method according to claim 1, wherein said second insert (8) is preformed starting from a monolithic block of composite or metal material, so as to comprise a bushing (28) with a central threaded hole (29) for the coupling to the pedal pin and with diametrical holes or protrusions the coupling to said half-shells. Method according to claim 1, in which a step is provided for i) dissolution of said core (10) is eliminated from said closed box-like casing and evacuation of the same through a suitable hole made in said casing (9). Method according to claim 1, wherein a finishing step (I) of the pedal crank is provided, comprising the contouring of the joining edge of said half-shells, the creation of polygonal, circular and / or threaded holes in correspondence with the axis of said first and said second insert, the creation of any holes (5) for anchoring said sprocket by means of suitable connecting members. 29. Pedal crank structure, particularly for bicycles, obtainable with the method of the preceding claims from 1 to 28, comprising an elongated body (2) with a first end portion (3) intended for coupling with a toothed drive crown and a second end portion (6) intended for coupling with a pedal, characterized in that said elongated body (2) comprises a closed box-like casing (9) formed by a pair of mutually facing half-shells (11, 12), a core (10) defining a longitudinal symmetry plane (L) placed inside said casing (9) and a median geometric plane (M) perpendicular to said longitudinal symmetry plane (L), a first end insert (7) inserted at one end of said casing (9) for supporting a pin of a toothed wheel and a second end insert (8) for supporting a pin of a pedal, said inserts (7, 8) having axes substantially parallel to each other ro and perpendicular to said median plane (M), said reinforced core (10) having inside it a plurality of filiform reinforcements (15) having substantially transverse directions selectively oriented with respect to said median plane (M). Pedal crank structure according to claim 29, characterized in that said filiform reinforcements (15) comprise at least a first group of fibers (16) forming a solid angle comprised between about 45 ° and 90 ° with respect to said median plane (M) . 31. Pedal crank structure according to claim 30, characterized in that said filiform reinforcements (15) further comprise a second group of fibers (17) forming a solid angle comprised between about 75 ° and 90 ° with respect to said median plane (M) , and a third group of fibers (18) forming a solid angle equal to about 90 ° with respect to said median plane (M). 32. Pedal crank structure according to claim 31, characterized in that the thread-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 to increase the compressive strength and buckling resistance of said assembly. 33. Pedal crank structure according to claim 31, characterized in that said half-shells (11, 12) comprise at least a pair of preformed and reciprocally superimposed layers (13, 14) of reinforced polymeric material, the filiform 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) to increase their resistance to delamination. 34. Pedal crank structure according to claim 33, characterized in that each of said layers (13, 14) is formed in a thermoplastic or thermosetting polymeric material reinforced with suitably oriented synthetic fibers. 35. Pedal crank structure according to claim 29, characterized in that said first insert (7) comprises at least a first bush (20) of substantially cylindrical shape with a central hole (21) of polygonal section for the pin of said toothed crown and with a flanged edge (22), and a second bush (23) of substantially cylindrical shape and provided with a flanged edge (24), embedded in said first bush (20). 36. Pedal crank structure according to claim 35, characterized in that it comprises connecting rings (25, 26) made of composite material or similar material coupled to said flanged edges (22, 24) by means of adhesives, fibers (27) or axial protrusions placed in diametrical positions. 37. Pedal crank structure according to claim 29, characterized in that said second insert (8) comprises a bush (28) made of composite or metal material, with a central threaded hole (29) for coupling to the pedal pin and with holes or diametrical protrusions for coupling to said half-shells. 38. Pedal crank structure obtainable with the method according to one or more of claims 1 to 28, characterized in that it comprises: - a pair of half-shells (11, 12) facing each other and joined along the peripheral edges so as to define a closed box-like casing (9) with a first end portion (3) intended for coupling with a sprocket and a second end portion (6) intended for coupling with a pedal; - a plane of longitudinal symmetry (L) in which the axes of the pins of said sprocket and of said pedal are contained, and a median geometric plane (M) normal to said longitudinal symmetry plane (L); - a first end insert (7) inserted at one end of said casing to support a pin of a toothed wheel and a second end insert (8) to support a pin of a pedal, both said inserts (7, 8) having axes substantially perpendicular to said median geometric plane (M); characterized in that the half-shells (11, 12) of said closed box-like envelope (9) are mutually joined by means of a plurality of filiform reinforcements (15) having substantially transverse directions, selectively oriented with respect to said median plane (M). 40. Pedal crank structure according to claim 39, characterized in that it provides, in proximity to said first support (7), a plurality of through holes (5) for connection to said sprocket by means of suitable connection means.