The present invention relates to a wheel for vehicles, particularly but not exclusively for cycles and motorcycles, and to a method for producing the wheel.

Generally, wheels for cycles and motorcycles are constituted by an outer circular part or rim which is connected coaxially to a central part or hub by a plurality of wire or solid spokes which are substantially radial and in any case lie on a plane which is transverse to the axis of the hub.

Wheels of the type indicated above can be used with some particular refinements even for other heavier vehicles, such as for example cars.

Wire-spoked or solid-spoked wheels differ in the type of material used, in the production processes, and in the structural choices.

With particular reference to the materials, two categories of wheels can be distinguished: in particular, those made of metallic material and those made of reinforced composite material. Mention can be made, for example, of steels and aluminum, magnesium and titanium alloys among the various materials of the first group. As regards the second group, instead, the most widely used materials are polymeric ones, reinforced with fibers of various kinds (carbon, kevlar, glass, et cetera).

Depending on the structural choices, the wheels can be classified as solid-spoked or wire-spoked wheels.

In the first case, the wheels can be monolithic and obtained by casting molten alloys in suitable dies, or can be composite, that is to say, obtained by assembling in various ways the hub, the solid spokes and the rim, which are obtained separately with different materials and technologies. In the second case, instead, the distance between the hub and the rim and the transmission of the system of forces involved are ensured by wire spokes which are appropriately tensioned.

With reference to the geometries of solid-spoked and wire- spoked wheels, the profile and the dimensions of the rim are standardized according to specific standards, such as for example the ETRTO (European Tyre and Rim Technical Organisation) European standards.

Figures 1 and 2 illustrate shapes of rims which are standardized according to the above standards. The geometric and size constraints essentially relate to the side of the rim which is in contact with the tire.

For metallic wire-spoked wheels, it must be specified that the production of the rim generally starts from a metal profile which has a suitable cross-section or from an appropriately shaped metal strip which is initially curved so as to form a ring whose diameter is equal to that of the wheel to be produced. Then the two ends of the ring are

welded with various methods (flash welding, resistance welding or other equivalent methods) . Then the joint is finished, followed by boring according to the number of wire spokes of the wheel, and finally the hub is assembled with the wire spokes and the wire spokes are tensioned.

In the case of profiles which have a hollow cross-section, the presence of the bores for the wire spokes may allow water to penetrate inside the rim in such an amount as to compromise the correct operation and stability of the vehicle.

In the case of wheels made of reinforced polymeric material, the structures are generally produced according to the solid-spoke type and the rim is produced by virtue of a sheet of composite material of adequate thickness, which is shaped appropriately and to which the solid spokes are connected for example by gluing.

The solid spokes are also generally made of composite material, with a structure which may also be of the sandwich type, that is to say, produced by means of a hollow member with a core made of a material which has suitable mechanical properties. The connection of the hub to the solid spokes is also generally performed by gluing.

Vehicle wheels must have static and dynamic strength (fatigue strength) properties and other properties which are briefly referenced as rigidity, lightness and aerodynamic efficiency.

As regards structural rigidity, which is directly linked to the road-holding and driving qualities of the vehicle, it is the result of the combined effect of radial rigidity, which is linked to the deformability of the wheel due to radial loads directed toward the center of the hub, as shown schematically in Figure 3/ of transverse rigidity, which is linked to the flexing of the wheel due to loads directed along the axis of the hub, with simple resting constraints applied to the rim in two diametrically opposite points, as shown schematically in figure 4; and of tangential rigidity, which is linked to the rotation of the wheel about the hub as a consequence of the application of two diametrically opposite and equally intense tangent forces which lie on the plane of the wheel, as shown schematically in Figure 5.

The weight of the wheel is obviously a parameter which depends on the wheel type, on the materials used, on the geometry of the wheel and on the number of wire spokes or solid spokes. Of course, the lower the weight of the wheel, other characteristics (rigidity and aerodynamic efficiency) being equal, the higher the value perceived by the user.

The aerodynamic efficiency of the wheel is highly dependent on the shape of the rim and on the number and shape of the wire spokes or solid spokes. In general, it is possible to say that the smaller the number of wire spokes, the higher the aerodynamic efficiency of the wheel. Rim geometry being equal, the number of wire spokes affects the rigidity of the wheel and therefore its weight.

A disadvantage of conventional wheels is that in order to increase the rigidity of a rim it is normally necessary to accept an almost proportional increase in weight.

EP-A-0 368 480 discloses a vehicle wheel which has a circular outer part joined to a central hub by means of a plurality of solid spokes, in which the solid spokes have an aerodynamic profile with a leading edge and a trailing edge with a specific aspect ratio along its longitudinal extension. The wheel has an inner core made of foam which is covered by a composite material, such as carbon, glass or polyethylene fiber.

The shape of this wheel has been studied to achieve high aerodynamic efficiency not only of the outer rim but also of the solid spokes and of the regions for connection to the central hub. However, this type of structure cannot be used for metal wheels. Furthermore, the production method requires the provision of a number of cores which is equal to the number of rim shapes to be produced, with a considerable increase in production costs.

Other wheel structures are known which have rims made of synthetic polymeric material with structural foam inserts placed in a circumferential cavity before the molding step. The inserts placed inside the structure only partially improve the mechanical properties of the final product, because they are subjected to considerable deformations during molding. Furthermore, the process for preforming and inserting the inserts is expensive and negatively affects

production costs.

The aim of the following invention is to improve the products of the prior art by providing a wheel which offers characteristics of high mechanical rigidity and fatigue strength, so as to achieve greater travel safety and better directionality of the vehicle.

A particular object is to provide a wheel characterized by increases in the inertial parameters of the resisting cross-section which are considerably higher than the weight increase, all other structural conditions, such as the shape, number and state of tension of the connecting members, and the cross-section and structure of the hub, being equal.

Another object is to provide a wheel which has a smaller number of members for mutually connecting the rim and the hub, with a consequent reduction in weight and aerodynamic drag and therefore energy consumption.

Another object is to eliminate infiltrations of water in the rim.

Another object is to simplify and reduce the cost of the method for manufacturing the wheel according to the invention.

This aim, these objects and others are achieved by a vehicle wheel according to claim 1.

The wheel has extremely high-level properties in terms of resistance to static and dynamic loads by virtue of the complete filling of the closed circumferential cavity of the outer circular part with a structural filler material and by virtue of the adhesion of the material to the inner walls of the cavity.

The wheel improves the inertial parameters of the resisting cross-section which are considerably greater than the weight increase, with an equal number and state of tension of the connecting members and with an equal cross-section and structure of the hub.

Claim 9 describes the method for producing the above wheel.

Further characteristics and advantages of the invention will become apparent from the detailed description of the following preferred but not exclusive embodiment of a vehicle wheel according to the invention, illustrated only by way of non-limitative example in the accompanying drawings, wherein:

Figures 1 and 2 are views of transverse profiles of rims of the known art, executed according to ETRTO standards;

Figure 3 is a sectional perspective view, taken along an axial plane , of a portion of a rim with open cross-section according to the prior art and with no members f or connection to the hub, associated with a system of X-Y axes for calculating inertial parameters;

Figures 4, 5 and 6 are schematic views of load conditions for the wheel according to the invention;

Figure 7 is a sectional view, taken along an axial plane, of a closed-profile rim according to the prior art, without connecting members;

Figure 8 is a sectional view, taken along an axial plane, of a wheel according to the invention and without connecting members;

Figures 9 and 10 are schematic views of the conditions for simulating a load for a wheel according to the invention.

With reference to the figures, a wheel according to the invention, generally designated by the reference numeral 1, comprises an external cylindrical part or rim 2 which is joined to a central hub 3 which has an axis a ^' by means of a plurality of connecting members 4.

In the illustrated embodiment, the connecting members 4 are constituted by wire spokes whose ends are connected both to the hub 3 and to the rim 2 by means of coupling parts of a per se known kind, not shown in the drawings, which allow to vary the tension on the rim.

As an alternative, the connecting members 4 can be constituted by solid spokes which lie on a plane which is transverse with respect to the axis a and are connected to, or monolithic with, the hub and the rim.

The outer cylindrical part 2 has a partially closed profile which is divided by a mixtilinear wall 5 into an open outer portion and a closed inner portion.

The outer portion has raised edges 6 which are suitable to accommodate a tire, generally designated by the discontinuous line P, while the inner portion is formed by a side wall 7 which is generally lobe-shaped and forms a circumferential cavity 8. Of course, the cavity 8 may have multiple lobes which are preferably symmetrical with respect to a sagittal plane V which lies at right angles to the axis a of the wheel, without modifying the effects of the invention.

It is noted that the rim according to the prior art shown in figure 7 is also partially closed and has mechanical characteristics which are already considerably improved with respect to the open-profile rim shown in Figure 3.

According to the invention, the circumferential cavity formed by the side wall 7 and by the dividing wall 6 is uniformly filled with a suitable material 9 which has a relatively high compression and shear elasticity modulus and a relatively low density and is suitable to fully adhere to the inner surface of the cavity 8 and to become an integral part of the structure.

Preferably, the structural filler material is a foaming polymeric material, for example of the closed-cell type.

As an alternative, the structural filler material can be an injectable polymeric material or a fiber-reinforced polymeric material.

Table I below lists, in the first column, some possible filler materials 9 for the cavity 8; the second and third columns list the corresponding longitudinal elasticity modulus E and tangent elasticity modulus G and the fourth column lists the density.

Table I - Filler material:

In a preferred embodiment, the rim 2 is made of a metallic material. As an alternative, the rim 2 can be made of a fiber-reinforced polymeric material.

Table II below lists the physical and mechanical characteristics of materials used to form the rim 2.

Table II - Materials for forming the rim

(*) unidirectional or woven epoxy-matrix laminations reinforced with carbon, kevlar or glass fibers

An exceptional increase in structural characteristics, with an equal weight and with equal constraint conditions of the stressed structure, has surprisingly been observed.

In order to theoretically verify the validity of the choices made, a numeric simulation of the radial rigidity test alone, related only to the rim without the connecting member, was conducted according to the stress diagram shown in Figure 3 in the three possible geometries A), B) and C) provided for the cross-section of the rim, which are illustrated respectively in Figures 3, 7 and 8; the first two figures illustrate the state of the art and the third one illustrates the wheel according to the invention.

The radial and transverse rigidities are affected by the values of the moments of inertia of the cross-section, which are designated respectively by Jxx, Jyy and Jzz.

The constraint and loading conditions related to the radial rigidity test and the geometric symmetry allow to perform the analysis on just half of the model, as shown schematically in Figure 10, saving time during modelling and resolution.

The finite-element method (FEM) was used to simulate the test. A mathematical model of the rim was used by dividing its structure into a certain number of solid elements (bricks) having adequate characteristics. The resulting

model was then given the mechanical and physical characteristics of the materials that constitute the rim and the constraint and loading conditions suitable for the test to be simulated were applied.

By applying elasticity theory and other suitable algorithms to each element, it is possible to calculate the tension on the rim and its deformation as a function of the applied loads, particularly the deflection at the point where the load is applied. The ratio between the applied load and the measured deflection indicates the sought radial rigidity.

The models for the three profile geometries were produced with the following meshing methods:

profile A) model with 1440 brick elements, 8 nodes and 3055 nodes

profile B) model with 2640 brick elements, 8 nodes and 5209 nodes profile C) model with 3744 brick elements, 8 nodes and 4789 nodes

The materials used in the simulation were aluminum alloy for the profile (E = 70000 MPa, f> = 2700 kg/m ³ , P = 0.3) and structural foam for the filler material(E = 200 MPa, f - 130 kg/m ³ , D = 0.5).

Table III below lists, for each model, the inertial characteristics Jxx, Jyy and Jzz of the cross-section, the

mass, the applied load, and the measured deformations.

Table III - Simulation analysis

Analysis of the results clearly shows a very significant increase in radial rigidity and in specific radial rigidity in passing from profile A to model C. Taking profile A as reference, radial rigidity (N/mm) increases by more than 10 times by passing to profile B and by more than 200 times by passing to profile C. In any case, it must be stressed that the presence of the connecting members tends to level out and reduce the great differences observed, v/ithout however substantially altering the end result.

One possible method for producing the wheel according to the invention entails the initial forming of the outer circular part 2 by using relatively rigid material, so as to form inside it at least one closed circumferential cavity 8, followed by the forming of the central hub 3 and its connection to the outer circular part 2 in a position which is coaxial to the part by means of the wire-spoke or solid-spoke connecting members 4.

With respect to conventional methods of the prior art, there is a step for the complete filling of the closed circumferential cavity 8 with a structural material 9 whose compression and shearing elasticity modulus is relatively high and whose density is relatively low, so that it adheres uniformly to the inside walls of the cavity.

The structural filler material can be a reactive polymeric material, in which case it is foamed inside the circumferential cavity, or can be a polymeric material, in which case it is injected into the cavity.

The outer circular part or rim 2 can be made of reinforced polymeric material by means of processes chosen among molding, autoclave-molding, resin transfer or injection.

As an alternative, the rim 2 can be made of optionally reinforced metallic material by means of processes chosen among extrusion, die-casting, casting, or pipe-forming. In the latter case, it is possible to start from a portion of a metal profile whose length is equal to, or a multiple of, the circumferential extension of the wheel and is appropriately curved and joined at its ends.

In this case, the step for filling the cavity 8 can be easily performed on the finished circular part, that is to say, after its complete forming.

However, this filling step may also be performed before or during the step for forming the outer circular part, for

example before or during the bending and calendering of the profile, with beneficial effects also on the quality of the forming process and of the finished product.

In this manner, it is possible to reduce the thickness of the wall of the rim while increasing the performance of the component.

It is in fact known that the calendering of profiles having very thin walls causes considerable problems due to elastic instabilities (buckling) which occur on the side walls of the rim, where thickness is in fact lower.

The calendering of already-foamed profiles could partially solve the problem, but it would entail damage to the filler material and its separation from the inside walls of the cavity, reducing the structural effect of the material.

The adoption of parallel and substantially simultaneous production processes should instead solve the problem without drawbacks. The calendering of a profile having low thicknesses and a hollow cross-section filled with an appropriate material eliminates the current instability problems, utilizing the principle which is usually applied to bend pipes without risking breakage.

For this purpose, it is possible to exploit the property of all foamed polymeric materials having a rather long reaction and cross-linking time of 2 to 3 minutes, which can optionally be modified by the manufacturer with the

addition of suitable additives. During foaming, the consistency of the material changes, passing from liquid to final foam, with variable degrees of rigidity.

The polymeric material is thus injected into the profile to be formed, waiting until the material is rigid enough to ensure the stability of the walls of the profile but still deformable enough to avoid breaking, and then the rim is formed.

Therefore, filling of the circumferential cavity of the rim performed before or during forming can lead to an improvement in the structural characteristics of the rim and can allow the forming of rims in which the thicknesses are smaller than those which can currently be used.

From the above, it is evident that the wheel and the manufacturing method according to the invention achieve the intended aim and all the intended objects and in particular allow to increase the specific rigidity of the finished product, other conditions being equal, to reduce the number of wire spokes or solid spokes and therefore the weight of the wheel and its aerodynamic drag, with a consequent reduction in fuel consumption due to the lower weight and better aerodynamic efficiency, to increase the directional stability and travel safety of the vehicle due to the greater rigidity of the wheels, and finally to provide protection against the infiltration of water into the rim.