DESCRIPTION

Field of the invention

The invention relates to the field of bicycle frames made of a carbon-fibre composite material.

Prior art

The use of composite materials, comprising carbon fibres immersed in a suitable matrix, which is typically a polymer or epoxy resin, has become increasingly common in the field of sporting and/or high-performance bicycles.

Said composite materials are named for example CFRP, which stands for carbon-fibre reinforced polymers. Commonly the term "carbon fibre" is used to refer to the composite material as a whole; in the present description this simplified term will also be used.

Composite materials with carbon fibres are known to possess extraordinary lightness combined with high rigidity and mechanical strength, in particular tensile strength, and these properties make them advantageous for the manufacture of high-performance sports equipment as well as for other high- technology fields including the aeronautical and automotive fields.

In the field of bicycle frames, to which the present invention relates, the composite material (or carbon fibre) is used advantageously for manufacturing at least some of the parts or "tubes" which make the bicycle frame. So-called monocoque frames are also made, where at least one portion of the frame is made of a single piece of composite material having no joint and obtained from a single mould. Said frame part with a monocoque structure is preferably the so-called central triangle, which comprises the components called horizontal tube (top tube), vertical tube (seat tube) and diagonal tube (down tube). The front fork and the rear carriage (which may also have carbon-fibre parts) are then connected to the central triangle with the composite-material monocoque structure.

The use of composite materials provides a frame which is significantly lighter and more rigid than any metal frame. These advantages make the composite-material frames widely appreciated for high-performance racing bicycles and mountain bikes.

However, it has been found that the high rigidity also results in a poor capacity for absorbing knocks or vibrations. Monocoque frames are most affected by this drawback, just because they consist of a single body of composite material.

The very rigid behaviour of the frame, and its substantial lack of ability to absorb energy, increase the risk of losing control and falling, especially at high speed. A carbon-fibre frame constitutes essentially a mechanically rigid system which is unable to dissipate energy. The energy transmitted to the frame, for example when passing over a rough road surface, perturbs the dynamics of the bicycle and may cause the cyclist to lose his/her balance. Moreover, the rigidity increases the risk of damage to the frame and frames made of composite material (and even more so the monocoque frames) are costly to repair. Moreover, the frame may be affected by microfractures which are difficult to detect, but reduce the strength of the frame.

Summary of the invention

The object of the invention is to solve the aforementioned problem which affects bicycle frames made of composite material. The object is achieved with a bicycle frame comprising a plurality of tubes made of a composite material comprising carbon fibres within a matrix, wherein the tubes made of composite material include at least one top tube, a seat tube and a down tube, characterized in that it comprises: at least one node element into which at least two of said tubes made of composite material converge, said two tubes converging into the node element comprise said top tube and at least one other tube which connects said top tube to at least one other part of the frame, said node element being made of a mixture of polyamide and carbon.

The frame tubes are parts which extend substantially lengthwise and are preferably but not necessarily straight. The tubes may have a cross-section with a circular shape or different shape. Preferably the tubes have a circular cross-section.

In a preferred embodiment the frame tubes which converge into said node element comprise a first tube which connects said top tube to the down tube, and a second tube which connects said top tube to a head tube of said frame. More preferably, said first and second tubes are essentially perpendicular and converge into the node element according to directions spaced by 90 degrees or about 90 degrees.

In a preferred embodiment the top tube and said first and second connecting tubes form a Y-structure which forms the top side of the so-called "triangle" of the frame.

Advantageously, said first connecting tube and second connecting tubes are straight short-length tubes, i.e. the node element is located close to the down tube and the head tube. Preferably said first and second connecting tubes have a length not greater than 300 mm.

In a preferred embodiment, said second connecting tube is parallel or substantially parallel to the head tube. Preferably the interaxial distance between said second connecting tube and said head tube (measured in a direction perpendicular to the axis of tubes) is between 5 cm and 15 cm, and more preferably is equal to 10 cm.

The frame tubes converging into the node element are firmly fixed to the said node element. In a preferred embodiment, the ends of the frame tubes are fixed to the node element by means of interference fit into suitable seats of said node element.

The node element has preferably a hollow shape. More preferably, the node element has the form of a cylindrical sleeve (i.e. cylindrical ring).

In one embodiment, a frame according to the invention comprises at least one stiffening insert, which can be selectively and removably associated with at least one node element, in order to adjust the torsional rigidity of said node element.

Said at least one insert is preferably made of composite material. More preferably it is made of the same composite material as the frame.

In an embodiment, said at least one stiffening insert can be associated to the node element by insertion into a cavity of the node element.

In an embodiment, said at least one stiffening insert comprises a plurality of inserts which can be alternatively or collectively associated with the node element. Said plurality may comprise inserts made of different material and/or inserts with a different geometry, for example having a different thickness or comprising reinforcing structures. The application of one or more inserts chosen from among said plurality, separately or simultaneously, modifies the torsional rigidity of the node.

In one embodiment, said plurality of inserts comprises at least: a first insert which can be associated directly with the node element and a second insert which can be associated with the first insert. Moreover a third insert which can be associated with the second insert, and so on, may be provided. By applying a selected number of inserts, the user obtains a desired rigidity.

In an embodiment, said plurality of inserts comprises inserts of decreasing size, where an insert of maximum size can be associated directly with the node element, and one or more inserts of decreasing size can be each associated with the insert which is immediately larger in size. The term "immediately larger" is understood as meaning the smallest of the inserts having a larger size. These inserts can for example be in the form of sleeves (cylindrical rings) of decreasing diameter. For example, the inner cylindrical surface of at least one insert in the form of a cylindrical ring forms the receiving seat of at least one second insert and so on. In a set comprising inserts in the form of a cylindrical rings, the smallest (central) insert may be in the form of a solid cylinder.

Preferably, said inserts are provided at least for a front node element where the following tubes converge: the top tube, a first tube which connects the top tube to the down tube, and a second tube which connects the top tube to a head tube. Preferably, said inserts may be fitted with a light forcing action so that they are retained in the seat by the friction between the contact surfaces. For example, inserts in the form of a sleeve (cylindrical ring) or in the form of a cylinder may be fitted by means of the friction of an outer cylindrical surface of the insert with an inner cylindrical surface of the receiving seat.

The tubes of the frame are generally hollow. Another aspect of the invention relates to a sandwich-like structure of at least one of the hollow tubes of the frame.

More particularly, one aspect of the invention relates to a tubular element structure for a bicycle frame made of composite material, comprising carbon fibres within a matrix, wherein the tubular element is internally hollow and has a wall comprising two coaxial layers of composite material and a filling of expanded or foam material arranged between said two layers.

Said expanded material is preferably expanded polyurethane.

The use of said structure comprising two coaxial layers of composite material and a filling of expanded or foam material is particularly preferred at least for the top tube of the frame. The main advantage of the invention lies in the fact that the node element introduces a certain degree of elasticity into the system formed by the frame. The node element, owing to its shape and material, acts substantially in the same manner as a hinge with a certain degree of elasticity and makes the frame less rigid, and able to deform and to absorb energy, without however significantly increasing the weight of the frame. This therefore results in a frame having a lightness comparable to that of a carbon-fibre monocoque frame, but able to react better to knocks and vibrations, which can be absorbed reducing the risk of falls or breakage of the frame.

Another advantage of the invention lies in the possibility of shortening the length of some tubes of the frame, in particular the top tube. The top tube must in fact no longer extend up to the head tube, but only to the node element. By reducing the free bending length of the tube, the tube is less prone to vibrations during use. In other words the invention provides a frame with shorter and more stable tubes, with less tendency to vibrate, compared to the prior art. The above described structure of the tubes of the frame provides a further capacity for absorbing vibrations, owing to the expanded or foamed material filling, while maintaining the features of lightness and rigidity which are typical of conventional carbon-fibre tubes.

The provision of one or more reinforcing inserts allows the cyclist to adjust the response of the frame depending on the planned route or the use. For example, a node element in the form of a hollow sleeve, without said inserts, has a greater flexibility and consequently has a greater capacity to absorb vibrations, but also allows relatively large deformations. By fitting one or more inserts inside the hollow sleeve, the rigidity of the node is increased, making the frame more reactive. The cyclist may thus choose the degree of rigidity depending on needs and the type of route.

In some embodiments, inserts which can be fitted simultaneously, for example coaxially inside each other, are provided. In such a case, depending on the number of inserts fitted inside the node, corresponding levels of rigidity may be obtained.

The invention thus provides a carbon-fibre frame which retains the advantages of being extremely light and strong, but is less critical during use.

These and other advantages will become even clearer with reference to the following detailed description provided by way of example. Description of the figures Fig. 1 shows a bicycle frame according to a preferred embodiment of the invention.

Fig. 2 shows a detail of the frame according to Fig. 1 .

Fig. 3 shows a cross-section through a tube of the frame shown in Fig. 1 , according to a preferred embodiment.

Figs. 4 and 5 show a variable-rigidity joint according to an embodiment of the invention.

Description of an embodiment

Fig. 1 shows in schematic form a bicycle frame 1 where the following parts may be identified: a top tube 2, a seat tube 3 a down tube 4, a head tube 5, a rear fork 6.

The head tube 5 carries a front fork (not shown) along the axis A of Fig. 1 . The point P represents the central movement node and the point R represents the axis of the rear wheel and of the gears.

The frame 1 comprises a joint 7 where the top tube 2 as well as a first tube 8 and a second tube 9, which respectively connect the top tube 2 to the down tube 4 and to the head tube 5, converge. The top tube 2 together with the joint 7 and the connecting tubes 8, 9 forms an essentially Y-shaped structure which may be regarded as being the top side of the triangle 10 of the frame 1 , the other two sides of the triangle 10 being formed essentially by the seat tube 3 and the down tube 4.

Advantageously, as shown in Fig. 1 , the connecting tubes 8 and 9 have substantially perpendicular axes.

Fig. 1 shows a preferred embodiment in which the tube 8 is essentially parallel to the head tube 5. The interaxial distance between said tubes 5 and 8 is preferably about 10 cm.

In Fig. 1 it is possible to note the advantageous shortening of the top tube 2, which makes it less sensitive to vibrations. Owing to the presence of the joint 7 and the connecting tubes 8 and 9, the top tube 2 extends over a smaller length than the distance between the seat tube 3 and the head tube 5.

In other embodiments of the invention, a joint similar to the joint 7 may also be provided at the point P and/or at the point R of the frame, as defined above.

A preferred form of the joint 7 is shown in Fig. 2. The joint 7 has essentially the form of a cylindrical sleeve 20 which is internally hollow and has weight- reducing holes 21 . The joint 7 also has respective holes 22 inside which the tubes 2, 8 and 9 are inserted with force, thus being locked in position.

In other embodiments locking of the tubes inside the joint 7 may comprise other means which are known in the art, for example a form fit.

Fig. 3 shows a preferred structure of the frame tubes according to an aspect of the invention. A frame tube is internally hollow and has an outer wall comprising an outer layer 30 and an inner layer 31 of composite material 31 , and a filling 32 of expanded or foam material arranged between said two layers 30 and 31 .

In Fig. 3 it can be seen how the expanded material 32 is sandwiched between two thin coaxial walls of carbon-fibre composite material.

The structure shown in Fig. 3 is preferably employed, at least, for the top tube 2.

Figs. 4 and 5 show a preferred embodiment of the joint 7 having a rigidity which may be adjusted by fitting one or more inserts.

Inserts 23, 24 are coaxially applicable inside the cylindrical sleeve 20 in order to modify the torsional rigidity of the joint 7.

The insert 23 has an outer diameter substantially equal to the inner diameter of the sleeve 20, with a slight interference, so that the insert 23 can be inserted inside the central cavity of the sleeve 20, with a slight interference fit.

The friction between the outer surface 25 of the insert 23 and the inner surface 26 of the sleeve 20 keeps the insert 23 in position. It should be noted that the inner surface 26 of the sleeve 20 defines a seat for receiving the insert 23.

The insert 24 is coaxially applicable inside the insert 23 in a similar manner by means of friction between the surfaces 27 and 28. The inner surface 28 of the first insert 23 defines a receiving seat for the second insert 24.

By applying only the insert 23, or both the inserts 23 and 24, the torsional rigidity of the node 7 may be selectively adjusted. Fig. 5 for example shows a condition in which both the inserts 23 and 24 are coaxially associated with the sleeve 20.