More particularly, the present invention relates to an aerodynamic spoke structure with a high level of efficiency, safety and reliability, which has an application preferably in high-performance bicycle wheels used in professional and non- professional sectors.

Other possible applications of the present invention consist of spokes for wheels of cycles, motorcycles, wheelchairs and also sports cars with spoked wheels.

Background art Traditional wheels of the abovementioned type are generally formed by a rim intended to contain a tyre, a hub at the axis of rotation and a plurality of spokes or radial elements which have the function of connecting and keeping the rim centred with respect to the hub, as well as transmitting the stresses and the reactions between the two members.

Conventional spokes generally have a constant cross-section of circular, rectangular or elliptical shape, with one end which is threaded and can be engaged with a nipple in order to retain the rim and the other end shaped so as to engage in appropriate seats in the hub.

The problems posed by said known spokes are essentially of a technical and cost- related nature. In particular, they consist in the relatively low aerodynamic

efficiency, the high weight, the low resilience or impact resistance, the high manufacturing cost and the limited relative value.

It is known that the coefficient of aerodynamic resistance of a body varies as a function of its shape, relative speed and angle of incidence with respect to the direction of flow. As far as the relative speed is concerned, this varies from a minimum value at the hub joining point to a maximum value at the rim joining point (v=a r).

The coefficient of resistance CR for cylindrical cross-sections has a value equal to about 0.35 for any angle with respect to the flow. In order to improve aerodynamic efficiency and reduce resistance to movement, spokes with aerodynamically shaped cross-sections have been proposed, as described for example in the patent US-A-5 350 221.

Using, for example, a NACA profile, a value of CR equal to about 0.01 is obtained with an angle of incidence of zero (0°) and 0.04 with an angle of incidence of 8°, with a maximum value of CR equal to 0.33 for speeds close to that of sound (M = 1).

In US-A-5 350 221 and in EP 0 872 357, the cross-sections of spokes manufactured by means of a process of"poltrusion"have a wing profile with a chord of constant length in the central section. In the case of winding, the manufacture of a variable cross-section is particularly complex. The fact that the chord remains constant leads to a reduction in aerodynamic efficiency, as in the case of propellers for aerodynamic propulsion.

EP-A-872357 describes a spoke with an aerodynamic profile manufactured by means of an excess of resin resulting from filling between two strips of reinforcing fabric. The excess resin leads to an increase in the weight of said spoke, without any significant contribution to the increase in specific resilience (J, zOD/weight). Metal spokes with tapered cross-sections on the market nowadays are characterized by the following geometric characteristics: Spoke Diameter Chord # Height ß Cross-sectional 0 [mm] [mm] [mm] strength a [MPa] DT New aero 2 2.5 1 1, 200 DTAeroSpeed 2 2. 1.2 1, 200 SAPIM CxRay 2 2. 3 0. 9 1, 600 SAPIM Cx 2 2. 3 1. 3 1, 600

The metallic materials normally used in the manufacture of spokes are stainless steels. The most widely used is the steel X5CrNi18/10 1.4301 As a result of the manufacturing process, the central sections of these spokes have a roughly elliptical geometry with a constant chord height equal to 40-50 % over the whole length of the profiled section. The latter is usually shorter than the overall length of the spoke.

As a result of such machining, the strength characteristics of the various sections are as follows : - Strength of the sections at the rim and hub joining points: 1,000-1, 180 MPa - Strength of the"aerodynamic" section: 1,200-1, 600 MPa -Resilience (Impact strength or JIZO): > 325 J - Modulus of longitudinal elasticity (Young's modulus): 197 GPa.

In order to solve the problem of high weight, spokes made of composite materials respectively with a circular cross-section (see JP-A-80901) or a rectangular cross- section (see EP-A-1 044 872) have been proposed, said cross-sections being

substantially constant along the central portion of the spoke.

The composite materials normally used for the manufacture of spokes are of the type with an epoxy thermosetting matrix reinforced with carbon or aramide fibres (e. g. Kev ! ar@ produced by Du Pont). Generally, carbon spokes are manufactured with fibres arranged in an optimum manner with respect to the direction of stress (longitudinally relative to said spoke).

The mechanical characteristics which can be achieved with said materials are the following : - Strength of unidirectional composites (Toyobo Nylon0) : 2,700 MPa - Strength of epoxy matrix composites: 64-2,100 MPa - Modulus of longitudinal elasticity of epoxy compounds : 13-520 GPa -Resilience (Toyobo ZylonO) : : 18 J PEEK (Polyether ether ketone) is one of the thermoplastic materials which may be used; it is a thermoplastic engineering material, i. e. with mechanical characteristics (tensile, bending, impact and fatigue strength) which are comparable to those of certain metallic materials. PEEK can be reinforced with glass or carbon fibres and in the latter case has the following characteristics (PEEK 450CA30 produced by VICTREX).

- Density : 1. 44 g/cm3 - Tensile strength: 208 MPa -Tensile modulus : 13 GPa - Resilience with incision r=0.25 [mm], p=2.5 [mm] ) : 85 J/m - Resilience (without incision) : 749 J/m In the case where it is required to manufacture metallic spokes with suitable aerodynamic profiles, it would be necessary to start with spokes having

considerably larger diameters, for example SAPIM STRONG (=2. 0, =2. 34). In any case, the solution would be limited, with an increase in weight as a result of an overdimensioning of the spoke in the areas outside the aerodynamic part.

The safety problems of the state of the art relate to two main points: (i) resilience of the spoke and (ii) strength of the systems connecting the central part of the spoke to the ends which interface with the hub and the rim.

From an analysis of the data relating to the mechanical characteristics of the composite materials, it can be seen that the catastrophic breakage of a spoke can also occur with a very high load but with a relatively low absorption of energy.

For this reason, in almost all known spokes made of composite materials containing carbon fibres, aramide fibres are also associated in order to increase the impact resistance of the products. If on the one hand the introduction of aramide fibres allows the energy absorbed during breakage to be increased, on the other hand the mechanical elasticity characteristics decrease, i. e. the material is less rigid. In this case, in order to increase the rigidity of the spoke, which behaves like a beam subjected to tensile and compressive stresses, it is necessary to increase the cross-section (the normal rigidity is defined by EA/L where E = modulus of longitudinal elasticity, A = cross-section and L = spoke length). However, in this case the weight of said spoke also increases and therefore its specific resilience decreases.

The catastrophic breakage of a spoke can constitute an extremely serious danger for various reasons, including the loss of stability of the wheel with the possible risk of the cyclist falling. In the event that a breakage occurs following an accident, the sections of spoke constitute extremely dangerous sharp elements which can pierce the cyclist's body.

WO 91/13771 envisages the manufacture of a spoke having a central portion with a group of fibres made of metallic material and non-metallic material (carbon, aramide, etc. ) bonded by a thermosetting resin or a thermoplastic resin. The

reinforcement of the central portion is achieved with fibres (including metallic fibres) which must be filament-like elements with diameters of between 5 [pm] e 15 [pm] (classic definition). In this case, the increase in resilience with respect to a carbon structure was achieved without negatively affecting the rigidity of the spoke.

In many spokes according to the known art, various solutions for connecting the central part of the spoke are envisaged, said solutions having given geometric or mechanical or manufacturing process characteristics and comprising elements which have the function of connecting the central part of the spoke to the rim or the hub. In this way, a single spoke is formed by at least three or more elements. The connections between the various parts occur by means of bonding, screw/female thread coupling, etc. In general, such coupling arrangements necessarily occur on elements which have limited dimensions. Consequently, so as not to cause an excessive increase in weight, the safety coefficient is relatively low.

Another problem faced by manufacturers of spokes is that of limiting the manufacturing cost, which must be midway between the costs of manufacturing a normal spoke and those of manufacturing a composite spoke.

The economic value of a high-efficiency component such as a spoke made of composite material can be defined by the following formula : Value = (performance/cost) = {[(rigidity/weight) (1/CR) (JIZOD)] ICOSt} It can be observed that the spokes proposed by the known art, for example in the patent US-A-5350221, result in the need for specific rims and hubs, i. e. they cannot be interchanged with the spokes currently fitted to bicycles. The cost for the end user also includes the rim and the hub. In general, spokes formed by several elements cost more than standard spokes.

Disclosure of the invention The task of the present invention is that of eliminating the abovementioned

problems, by providing a spoke structure with the characteristics of a high aerodynamic efficiency, high specific resilience, high reliability and safety for the user, low weight and high value.

A particular object of the present invention is that of designing a method for manufacturing an aerodynamic spoke which is reliable and easy to repeat, as well as low-cost.

This task, these objects, as well as others which will emerge more clearly hereinafter, are achieved by an aerodynamic spoke structure made of composite material for vehicle wheels, of the type having a mutually concentric hub and rim, comprising, according to Claim 1, an elongated central portion with longitudinal end portions, first connecting means for the removable fixing of one end portion to the wheel hub, second connecting means for the removable fixing of the other end portion to the rim, in which said central portion comprises a reinforcing core of metallic material, with which said first and second connecting means are rigidly associated, and an aerodynamically shaped external lining body of relatively rigid composite material, which is secured stably to the outer surface of said reinforcing core.

Owing to said configuration, the spoke according to the invention has a high aerodynamic efficiency and a low weight, while still guaranteeing considerable safety and reliability against possible catastrophic breakages.

In particular, the reinforcing core may be formed by a filament-like element with a substantially constant cross-section.

The filament-like element may be designed with dimensions such as to have sufficient mechanical strength to prevent the separation of two or more sections of the elongated central portion in the event of the breakage of the lining body of composite material.

In a first particularly advantageous embodiment, the filament-like element is preferably formed by at least one flexible cable of stainless steel or similar metal.

In this case, the first and second connecting means will be made of synthetic material and will be fixed rigidly to the ends of said flexible cable.

In a second particularly advantageous embodiment, the filament-like. element is preferably formed by a rod of rigid metallic material. In this case, the first and second connecting means will form one piece with the ends of the rod of metallic material.

According to a further aspect of the invention, a method for the manufacture of the spoke of composite material according to Claim 18 is envisaged, said method comprising the steps of a) provision of an elongated central portion extending between the hub and the rim of the wheel, b) provision of end portions with first and second connecting means for removable fixing to the hub and the rim of the wheel, respectively, in which said elongated central portion is made using a reinforcing core of metallic material, with the ends of which said first and second connecting means are associated, and by securing a lining body of composite material onto the outer surface of said reinforcing core.

Owing to this operative sequence and to the minimum number of components used, the method will involve a reduced number of steps and may therefore be carried out in a reliable, repetitive and low-cost manner, achieving spokes of high value.

Brief description of the drawings Further characteristic features and advantages of the invention will emerge more clearly in the light of the detailed description of a few preferred but not exclusive embodiments of spokes for a vehicle wheel according to the invention, illustrated by way of a non-limiting example with the aid of the attached plates of drawings in which:

FIG. 1 shows a perspective view of a spoke according to the invention during fixing to the hub and to a rim section; FIG. 2 shows a partially cross-sectioned side view of a first embodiment of the spoke according to Fig. 1; FIG. 3 shows a front view of the spoke according to Fig. 1; FIG. 4 shows a cross-sectional view of the spoke according to Fig. 2 along the plane indicated by the broken line IV-IV ; FIG. 5 shows a cross-sectional view of the spoke according to Fig. 2 along the plane indicated by the broken line V-V ; FIG. 6 shows a slightly enlarged cross-sectional view of the spoke according to Fig. 2 along the plane indicated by the broken line VI-VI ; FIG. 7 shows a partial cross-sectioned side view of a second embodiment of the spoke according to Fig. 1; FIG. 8 shows a front view of the spoke according to Fig. 7; FIG. 9 shows a cross-sectional view of the spoke according to Fig. 7 along the plane indicated by the broken line IX-IX ; FIG. 10 shows a cross-sectional view of the spoke according to Fig. 7 along the plane indicated by the broken line X-X; FIG. 11 shows a cross-sectional view of the spoke according to Fig. 7 along the plane indicated by the broken line XI-XI.

Detailed description of a few examples of embodiment With reference to the abovementioned figures, a spoke structure of composite material according to the invention is shown, said structure being denoted in its entirety by reference number 1 and having the function of connecting and holding in a concentric position an axial hub M and a rim circle segment C, with a U- shaped cross-section intended to house a tyre.

The structure 1 comprises an elongated central portion, denoted in its entirety by 2, with two end portions, which are denoted respectively by 3,4 and are provided with respective connecting means 5,6 of standardised type, i. e. identical to those

commonly used on conventional spokes, so that they are perfectly interchangeable with the latter.

In this way, the first connecting means 5 may be formed by a curved or straight mushroom head 7, for the removable insertion in a corresponding seat S of the hub M.

In the same way, the second connecting means 6 may be formed by a thread 8 intended to be removably engaged in a hole F of the rim C and in a nut or nipple N.

According to the invention, the elongated central portion 2 comprises an internal reinforcing core 9 of metallic material, with which the first and second connecting means 5,6 and an aerodynamically shaped external lining body 10 of composite material are respectively associated in the manner which will described in more detail below, said lining body being secured stably to the outer surface of the reinforcing core 9.

In general, the reinforcing core 9 is formed by a filament-like element 11 with a substantially constant cross-section.

Conveniently, according to the invention, the filament-like element 11 has sufficient mechanical strength to prevent the separation of two or more sections of the lining body 10 in the event of the catastrophic breakage thereof.

Moreover, the outer surface of the filament-like element 11 may be machined so as to have a high surface roughness, which is sufficient to guarantee optimum securing of the lining body 10.

In the first embodiment shown in Figures 1 to 6, the filament-like element 11 is formed by a rod of metallic material with a constant cross-section.

Preferably, the rod 11 is formed by cutting a section of steel wire of suitable

thickness, for example about 1.5 mm. Preferably the steel may be formed by a stainless steel of the type X5CrNi1810.

Alternatively, the rod 11 may be formed by a conventional cylindrical spoke of the double-butted DT Competition@ type, manufactured by DT Swiss Spoke.

The first and second connecting means 5,6 may be manufactured by means of plastic deformation and form one piece with the ends of the rod 11 of metallic material.

Conveniently, the lining body 10 will have a cross-section in the form of a substantially symmetrical wing profile, having a zero angle of inclination with respect to the main plane of extension of the wheel and length increasing towards the hub M.

By way of a non-limiting example, the profile may be chosen from among wing profiles of the four-figure NACA series, such as for example NACA 0012 which has a CR equal to about 0.08. The length of the chord may be dimensioned in such a way as to vary from between 7.5 mm at the end connecting to the hub M and about 2.5 mm at the portion connecting to the rim C.

Advantageously, the composite material of the lining body 10 is selected so as to have a modulus of thermal expansion which is substantially equal to that of the metallic material of the reinforcing core 9, so as to prevent separation between the two structural components.

More particularly, the composite material of the lining body 10 comprises a matrix of thermosetting (e. g. epoxy) plastic material or thermoplastic (e. g. polyamide) material, in which one or more layers of reinforcing fabric 12 are embedded.

Preferably, the fibres of the reinforcing fabric 12 may be chosen from among carbon, glass, Kevlar0, Twaron0 or combinations of these. Said layers 12 of the

reinforcing fabric will have a warp and a weft, which are not visible in the drawings and have fibres which are substantially parallel and perpendicular, respectively, with respect to the longitudinal direction of the filament-like element 9.

The use of unidirectional reinforcing fibres in the part of the spoke in direct contact with the metallic part (in total two layers on one side and two layers on the other side with respect to the chord of the aerodynamic profile) and a fabric, for example a 2x2 twill of 200 g/m2, with the fibres of the warp parallel to the longitudinal direction of the spoke and the weft arranged perpendicularly to said direction, allows metallic reinforcing cores with mechanical characteristics even inferior to those of X5CrNi1810 to be used.

Vice versa, all the mechanical characteristics being equal, the cross-sections and therefore the weight can be lower than those of the spokes on the market nowadays.

The composite spoke thus formed will have approximately a weight equal to about 7.5 g and a resilience (JIZOD) equal to about 400 J. Obviously, the dimensional and mechanical characteristics of the finished product may vary depending on the type of wheel to which it will be fitted and the stresses to which it will be subjected.

In the second embodiment shown in Figures 7 to 11, the same reference numbers are used followed by apostrophes, in order to identify corresponding structural elements.

The spoke according to this second embodiment, denoted in its entirety by reference number 1', differs from the preceding spoke only in that the reinforcing core 9'consists of a filament-like element 11'formed by at least one flexible cable of stainless steel or similar metal.

In this case, the first and second connecting means 5', 6'are rigidly fixed to the end portions of the flexible cable 11'and are preferably made of synthetic material,

which may be overinjected (in the case of polymer material) or overpressed (in the case of metallic material).

The synthetic material which forms the connecting elements 5', 6'may be a thermoplastic or thermosetting material, which may be optionally reinforced with glass or carbon fibres, such as PEEK 450CA30, and has mechanical characteristics comparable to those of the metallic material of the filament-like element 11'. The composite material may be overinjected (in the case of polymer material) or overpressed (in the case of metallic material).

Moreover, the connecting means 5', 6'must be suitably designed so as to have axial rigidity values comparable to or higher than those of steel.

An essential characteristic feature of the various embodiments consists in the presence of the filament-like element 11, 11'inside the lining body in order to give the entire spoke 1, 1'an adequate impact resistance without adversely affecting the rigidity thereof.

The method of manufacturing the spoke described above may involve the following steps: a) provision of the elongated central portion 2 b) provision of the end portions 3,4 equipped with first and second connecting means 5,6.

According to the invention, the elongated central portion is manufactured using a reinforcing core 9,9'of metallic material, with the ends of which the first and second connecting means 5,6, 5', 6'are associated, and by securing or overpressing the aerodynamically shaped lining body 10,10'of composite material onto the outer surface of the reinforcing core 9,9'.

The method of manufacturing the first embodiment according to Figures 1 to 6 must envisage the preparation of a first mould or a set of suitable devices for cold-

forming the reinforcing core 9 made from a section of stainless steel wire.

Obviously, as many moulds or devices must be provided as there are lengths and types of spokes to be manufactured. Conveniently, the outer surface of the rod 9 may be roughened so as to aid securing of the material which forms the lining body 10, 10'.

For this purpose, a second mould for forming the lining body 10,10'must be provided. Moreover, strips or layers 12 of fabric of dimensions suitable for the moulds used must be die-cut, bearing in mind that the width thereof must be at least double the length of the chord of the aerodynamic profile to be manufactured.

It must be ensured that, when positioning the layers 12 in the mould, the unidirectional carbon and/or aramide and/or glass fibres are arranged in a direction parallel to the spoke.

Polymerisation of the composite material which forms the lining body must be performed by means of thermal and pressure cycles so as to shape the outer surface of the body 10,10'.

The steps described above refer to the manufacture of an aerodynamic spoke with composite materials comprising a thermosetting matrix and using hot-surface press moulding technology. In other embodiments, the lining bodies of the spokes may be manufactured with other materials such as, for example: (i) thermoplastic composites; (ii) thermoplastic polymers; (ii) thermosetting polymers.

Other technologies can also be used, for example : (a) RTM, (b) moulding in an autoclave, (c) closed-press injection, etc.

From the above it can be understood how the spoke according to the invention achieves all the predefined objects and particular emphasis is placed on the high aerodynamic performance, which is optimised as a function of the fluid-dynamic

parameters of the vehicle, and the high specific resilience (safety), which is such as to ensure high reliability of use of the wheel.

The increase in weight compared to traditional spokes manufactured completely from composite materials without a reinforcing core is minimal and, however, such that it does not reduce significantly the value of the product perceived by the end user.

Mention is also made of the low cost and the high value of the spoke according to the invention, owing to the fact that it is composed of only two components, compared with the three used in similar spokes of composite material on the market.

As far as the cost-related aspects are concerned, it is known that composite materials comprising a thermosetting (epoxy based) matrix, which are reinforced with (HS-high strength-or HM-high modulus) carbon fibres have relatively high costs (about 20/mu for a weight of 200 g/m2). However, manufacturing a spoke of metallic material with a high aerodynamic performance would require specific equipment (high-tonnage presses and moulds having particular dimensions and made of special materials) and longer machining cycles (successive forming passes).

A further advantage consists in the absolute interchangeability of the spoke according to the invention with those on the market nowadays or already fitted to ready assembled wheels, using conventional hubs and nipples.

Although the description of the spoke according to the invention makes specific reference to the abovementioned figures, it is clear that said figures and the reference numbers used therein must be regarded as being purely illustrative and not limiting.