BACKGROUND ART There are known rear triangle elements for bicycle frames essentially carried out by joining three individual and separate parts: a hub connection or fork end, fit for receiving the wheel axis and two extended connection parts with the bicycle frame, respectively called seat stays and chain stays. Said stays usually are of tubular type, with circular or elliptical section.

If the parts constituting the rear triangle element are made of composite material or material containing fibers of composite material, for instance carbon or glass or boron or Kevlar fibers, these parts are connected by sticking in oven or autoclave.

If said parts are made of metal, for instance steel or aluminum alloy, welding carries out the connections.

The main drawback of said known rear triangle elements consists in their inadequate capacity to be resistant to local and high strength mechanical stresses, particularly resistant to dynamic bending and torsion stresses of cyclical type. The low resistance is because said elements do not have a uniform and monolithic structure, but they constitute a composed assembly, carried out by connecting three different elements, physically separated the one from the other. The connection is realized through joints, which can be defective, because of the existing divisions among said elements and among the binding fabric layers, or because of material fractures or discontinuities in the weldings. The joint areas usually coincide with the most mechanically stressed sections.

Other drawback of said known elements consists in the limited stiffness of the seat and chain stays. Particularly, the chain stays being subject to high bending and torsion forces, caused by the chain traction during the pedal stroke, are cyclically bent, determining a noticeable transversal oscillation of the rear wheel supported by the fork and, thus, an irregular behavior

of the frame. Said cyclical bending stresses further cause frequent break and structural fatigue collapses of the elements.

DISCLOSURE OF THE INVENTION Main object of the present invention is to propose a rear triangle element particularly for bicycle frame, carried out by integrating in a monolithic, uniform and regular structure the chain stays, seat stays and the fork ends.

Other object is to propose a rear triangle element having high lightness and, at the same time, high structural resistance, fit to resist effectively to every static and dynamic mechanical stress, localized and high intensity, and particularly cyclical bending and torsion stresses.

Further object is to propose a rear triangle element having high bending and torsion stiffness, particularly of the chain stays.

Other object of the present invention is also to propose an element having a safe and reliable use and a quick and simple realization.

The above-mentioned objects are achieved according to the content of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS The characteristics of the invention are underlined in the following, with particular reference to the attached drawings, in which: - figure 1 is an axonometric view of a rear triangle element particularly for bicycles frames object of the present invention, in which some parts have been removed for better underlining others; - figure 2 shows a side view of the figure 1 element; - figure 3 shows a plant view of the figure 2 element; - figures 4 and 5 show section views according to section planes IV-IV and V-V of figure 3, respectively; - figure 6 shows a partial side view of the figure 1 element in association with fork insert; - figure 7 shows a partial side view of a first variant of figure 6 element;

- figure 8 shows a partial side view of a second variant of the figure 1 element.

BEST MODE OF CARRYING OUT THE INVENTION With reference to figures from 1 to 5, the reference 1 indicates a rear triangle element particularly for bicycles frames essentially constituted by chain stay mean 2, a fork end 3 and a seat stay mean 4.

The element 1 is characterized by a portion of the seat stay mean 4 carried out as single body with the chain stay mean 2 and the fork end 3, by means of one or more fabric layers 20 in composite material soaked of binding material and without solution of continuity of the fibers of said fabric layers. The latter includes carbon, boron, glass or Kevlar fibers, while the binding material consists of an epoxy or polyurethane resin.

The portion of the seat stay mean 4 has a length ranging between two centimeters and the total length of seat stay.

The chain stay 2 and seat stay 4 means are tubular, longitudinally extended with predefined inner shape and are tapered with solid flat section in correspondence of the fork end 3. Said means further have dimensions and shape of the transversal section which vary along the longitudinal extension. The transversal sections can have circular, elliptical, rectangular or prismatic shape.

Particularly, the chain stay mean 2 has longitudinal curvilinear extension having two flex points, which indicate bending changes, defining a flat S shaped profile of the mean.

The chain stay mean 2 has, in correspondence of an intermediary portion thereof, a side protrusion 6 having an approximately triangular shape with a convex side and two concave sides joined by curved portions. Said protrusion is fit for the bending and torsion transversal stiffening of the chain stay mean 2.

The fork end has solid and flat section and has a fork seat 10, namely a rear dropout, for a wheel hub and a shaped seat 5, carried out in correspondence of said fork seat 10.

A fork insert 7, made of metal, for instance aluminum alloy, is inserted in the fork seat 10 and fixed against the shaped seat 5. In alternative, the insert can also mate the side wall 14 of

the attack 3, opposed to the shaped center 5.

The fixing of the fork insert 7 to the fork end 3 is realized by sticking.

It is provided that the fork insert 7 has through holes 9 which mate the side walls of the fork end 3 and are fit for making easier the sticking.

In figure 7 it is shown a first variant of the element object of the present invention, in which the fork insert 7 includes an extension 8, having a through seat 11, for instance a hole, fit for accommodating transmission means of the bicycle, for instance derailleur gears.

A second variant of the rear triangle element, shown in figure 8, has a wall 12 of mutual union between the portion of the seat stay mean 4 and the chain stay mean 2, for a further stiffening of said element and to provide aerodynamic improvement of the rear triangle element.

It is provided a third variant of the rear triangle element, in which a pin 13 is inserted in the seat stay mean 4 portion in order to engage a corresponding upper portion of the seat stay mean 4, not shown in the figures.

The main advantage of the present invention is to provide a rear triangle element particularly for bicycle frames, carried out integrating in a monolithic, uniform and regular structure the chain stays, seat stays and the fork ends.

Other advantage is to provide a rear triangle element having high lightness and, at the same time, high structural resistance, fit to resist effectively to every static and dynamic mechanical stress, localized and high intensity, and particularly cyclical bending and torsion stresses.

Further advantage is to provide a rear triangle element having high bending and torsion stiffness, particularly of the chain stays.

Other advantage of the present invention is also to provide an element having a safe and reliable use and a quick and simple realization.