Abstract:
A right crank arm assembly for a bicycle, comprises a right crank arm and at least one front sprocket coupled with the crank arm at least one coupling portion of the crank arm. The front sprocket has an inner side adapted, in use, to face towards the frame of the bicycle and an outer side opposite the inner side. The assembly comprises at least one first crank arm element which acts in contact with the front sprocket on one of said sides and at least one second crank arm element which acts in contact with the front sprocket on the other of said sides. The contact action of the crank arm elements on the two opposite sides of the front sprocket prevents the twisting deformation of the front sprocket during pedaling. Such a deformation would cause a reduction in the transmission efficiency of the right crank arm assembly.

Description:
FIELD OF INVENTION 
     The present invention relates to a right crank arm assembly, crank arm, and front sprocket for a bicycle. 
     BACKGROUND 
     Typically, in the field of bicycles, the expression “right crank arm assembly” is used to indicate an assembly comprising a right crank arm and at least one front sprocket coupled with the right crank arm. The front sprocket, in particular, is the toothed wheel adapted to drive the chain of the bicycle for the motion transmission to the rear wheel of the bicycle, such motion being imparted by the cyclist through pedaling. 
     In conventional bicycles, the right crank arm is directly coupled with the front sprocket at respective surfaces defined on respective front faces of such components. In particular, defining as inner side of the crank arm and of the front sprocket the one intended to face, in use, towards the frame of the bicycle and as outer side the one opposite the inner side and intended to face, in use, towards the outside, the coupling between right crank arm and front sprocket typically takes place at one of the sides of the crank arm and at one of the sides of the front sprocket. 
     It has been observed that, in operation, the front sprocket elastically deforms due to the stresses to which it is subjected during pedaling. Such a deformation leads to a decrease in the transmission efficiency of the motion imparted through the crank arm assembly. 
     In particular, the crank arm transmits to the front sprocket the force that the cyclist exerts upon the pedals of the bicycle. Such a force acts in a plane parallel to the middle plane of the front sprocket and its direction of application and intensity change at each angular position of the pedal. This stress therefore causes a variable lateral flexing deformation of the front sprocket. Considering also that the chain exerts on a portion of the front sprocket a force opposing the forward movement of the front sprocket, the resulting stress on the front sprocket is a twisting stress. Therefore, the consequent deformation of the front sprocket is a twisting deformation. 
     Such a deformation occurs in all conventional front sprockets, even if they are made from metallic material, but it is rather accentuated in the case of front sprockets made from lightweight materials. 
     Indeed, it is known, above all in the field of racing bicycles, to use front sprockets made from light alloys, like for example aluminum alloys, and composite alloys, i.e. made partly from metallic material and partly from another material, like for example carbon fiber. 
     The right crank arm assemblies of the prior art typically comprise a star-shaped right crank arm. Such a crank arms comprise, in particular, a plurality of coupling arms having at respective free ends, a portion for fixing to the front sprocket. 
     The Applicant has found that, in order to reduce the overall weight of known assemblies to the minimum, the coupling arms of the crank arms and the corresponding coupling elements of the front sprockets are made with very low thickness, this causing the twisting of the front sprockets. 
     The Applicant has also found that, also in the case of strengthened crank arms, front sprockets made from light material still continue to bend a lot. 
     SUMMARY 
     The technical problem that the crank arm assembly seeks to overcome is to reduce as much as possible the twisting deformations of front sprockets associated with right crank arms, in particular of front sprockets made from light material, so as not to penalize the transmission efficiency of the motion imparted through the crank arm assembly. 
     The crank arm therefore relates, in a first aspect thereof, to a right crank arm assembly for a bicycle, comprising a right crank arm and at least one front sprocket coupled with said crank arm at least one first coupling portion of the crank arm, wherein said at least one front sprocket has an inner side intended, in use, to face towards the frame of the bicycle and an outer side opposite the inner side, said assembly further comprising at least one first crank arm element which acts in contact with said at least one front sprocket on one of said sides, wherein it comprises at least one second crank arm element which acts in contact with said at least one front sprocket on the other of said sides. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
       Further characteristics and advantages described herein shall become clearer from the following detailed description of some preferred embodiments thereof, made with reference to the attached drawings. In such drawings: 
         FIG. 1  is a front view from the inner side of a right crank arm assembly according to the present invention, comprising a combination of standard front sprockets; 
         FIG. 2  is a front view of the outer side of the assembly of  FIG. 1 ; 
         FIG. 3  is a front view of the inner side of a right crank arm assembly according to the present invention, comprising a combination of compact-type front sprockets; 
         FIG. 4  is a front view of the outer side of the assembly of  FIG. 3 ; 
         FIG. 5  is a section view according to the section line V-V of  FIG. 3 ; 
         FIGS. 6 to 8  are respectively a perspective view, a view of the inner side and a view of the outer side of a right crank arm according to the present invention, such a crank arm being used in the assembly of  FIG. 1 ; 
         FIG. 9  is an enlarged section view according to the line IX-IX of  FIG. 7 ; 
         FIGS. 10 and 11  are respectively perspective views of the inner side and of the outer side of a front sprocket according to the present invention, such a front sprocket being used in the assembly of  FIG. 1 ; 
         FIG. 12  shows a mounting step of the front sprocket of  FIGS. 10 and 11  onto the crank arm of  FIGS. 6 to 8 ; 
         FIG. 13  is a front view of the outer side of an alternative embodiment of the crank arm described herein; 
         FIG. 14  is a perspective view of the inner side of an alternative embodiment of the front sprocket described herein; 
         FIGS. 15 and 16  are respectively a front view of the outer side and a perspective view sectioned according to the line XVI-XVI of  FIG. 15 , of an alternative embodiment of a right crank arm assembly according to the present invention; 
         FIG. 17  is a front view of the outer side of a further embodiment of a right crank arm assembly according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     Introduction 
     Advantageously, the provision in the crank arm of elements adapted to act in contact with the two opposite sides of the front sprocket effectively blocks the twisting deformation of the front sprocket during pedaling, to the great advantage of the transmission efficiency of the motion imparted through the right crank arm assembly described herein. 
     Preferably, the crank arm comprises a main body and said at least one first crank arm element and at least one second crank arm element are integral with the main body. However, an alternative embodiment of the assembly described herein is foreseen in which said at least one first crank arm element and at least one second crank arm element are distinct from, and associated with the main body of the crank arm. 
     Preferably, the coupling between front sprocket and crank arm at the aforementioned coupling portion is a dismountable coupling, so as to be able, if required or necessary, to replace the front sprocket or the crank arm for maintenance or repairs. 
     In the preferred embodiments of the assembly described herein, the crank arm comprises a plurality of first crank arm elements and a plurality of second crank arm elements. In particular, the crank arm elements are preferably at least two per side, possibly four, but embodiments with a different number, even an odd number, of crank arm elements are not excluded. Advantageously, the provision of many crank arm elements per side allows a contact between crank arm and front sprocket to be obtained that is particularly stable and effective in blocking the twisting deformation of the front sprocket. 
     Preferably, at least some of said at least one first crank arm element and at least one second crank arm element extend at least partially along respective non-radial directions with respect to a rotational axis of the crank arm. Advantageously, unlike assemblies of the prior art where contact occurs only at the ends of the arms of the crank arm extending radially with respect to the rotational axis of the crank arm, in the assembly described herein the contact between front sprocket and crank arm occurs at surfaces having a greater extension compared to the prior art. The desired stability and effectiveness of the contact between crank arm and front sprocket is thus ensured. 
     Preferably, at least some of said at least one first crank arm element and at least one second crank arm element extend at least partially circumferentially around said rotational axis of said crank arm, more preferably along respective arcs of circumference having an angular extension of less than 360°. Advantageously, in the crank arm of the assembly described herein, areas can in this way be identified in which the crank arm elements are not present. Such areas are used to allow and facilitate the insertion of the front sprocket between, and its removal from, the axial space defined on the crank arm between the first and the second crank arm elements. In this way it is possible to obtain the contact of the front sprocket with the crank arm at both sides of the front sprocket at the same time ensuring the removability of the coupling between front sprocket and crank arm to allow possible repair or replacement of the front sprocket or crank arm. 
     Preferably, such an angular extension is less than 360° and greater than or equal to 15°, more preferably it is between 15° and 100°, even more preferably between 30° and 75°. 
     Preferably, said at least one first crank arm element and at least one second crank arm element act in abutment onto said at least one front sprocket at different angular positions of the crank arm. The Applicant has indeed observed that, during pedaling, the front sprocket twists on different sides from one point to another. The Applicant has therefore thought to position, at each point of the front sprocket, crank arm elements only at the side in which the front sprocket twists. The crank arm elements are thus only positioned where they actually perform their counteraction to the twisting deformation of the front sprocket. In this way a substantial saving in weight of the crank arm is obtained in addition to an excellent result in terms of prevention of twisting deformation of the front sprocket. 
     Preferably, said at least one first crank arm element and at least one second crank arm element extend from opposite sides with respect to said at least one first coupling portion. The Applicant has indeed found that, with reference to a coupling portion with the crank arm, the front sprocket twists on opposite sides before and after such a coupling portion, and has thus thought to position the first crank arm elements on the opposite side to the second crank arm elements with respect to each coupling portion. 
     Even more preferably, said at least one second crank arm element is arranged in a position that precedes said at least one first coupling portion with reference to the direction of rotation of said crank arm during pedaling and said at least one first crank arm element is arranged in a position that follows said at least one first coupling portion with reference to said direction of rotation. The Applicant has indeed found that such a geometry provides effective opposition to the deformation of the front sprocket. 
     Preferably, the crank arm comprises at least one abutment surface adapted to allow the correct angular positioning of the crank arm with respect to said at least one front sprocket. More preferably, said at least one abutment surface is defined at least partially in said at least one second crank arm element. Advantageously, it is in this way possible to obtain the correct angular positioning of the crank arm with respect to the front sprocket without needing to provide special abutment elements. Moreover, the abutment surfaces advantageously constitute additional means for making the front sprocket rotate. Indeed, thanks to the provision of the aforementioned abutment surfaces the right crank arm assembly described herein would be able to transmit torque in the direction of pedaling even if the front sprocket is not fixed to the right crank arm through proper screws. 
     Even more advantageously, the provision of the abutment surfaces ensures that when the front sprocket is in abutment onto the crank arm, the first and second crank arm elements cooperate to hold the front sprocket in the axial space defined between them, thus leaving both of the operator&#39;s hands free, who can thus easily screw in the screws to firmly couple the front sprocket with the crank arm. 
     In a particularly preferred embodiment thereof, the assembly described herein comprises a first front sprocket having a first diameter and at least one second front sprocket having a second diameter different to the first diameter, wherein the first front sprocket is coupled with the crank arm at said at least one first coupling portion and said at least one second front sprocket is coupled with the crank arm at least one second coupling portion different from said at least one first coupling portion. 
     Advantageously, the provision on the crank arm of different coupling portions for the different front sprockets makes the removal of the front sprockets for possible repairs or replacement quicker and easier. This cannot be obtained in assemblies of the prior art, wherein all of the front sprockets are coupled with the crank arms at the same coupling portions of the crank arm. 
     Even more preferably, said at least one first coupling portion is defined at a first circumference having its centre at the rotational axis of the crank arm and said at least one second coupling portion is defined at least one second circumference concentric to the first circumference and having a different diameter to that of the first circumference. 
     Advantageously, the provision of front sprockets of different diameter coupled at different coupling portions of the crank arm allows a large number of combinations of front sprockets of different size to be made, such combinations also comprising very small front sprockets and very big front sprockets. In this respect, it should be noted that, in conventional assemblies, the only combinations used are those known as “standard”, comprising small front sprockets with a number of teeth of between 38 and 44 and large front sprockets with a number of teeth of between 52 and 56, and “compact”, comprising small front sprockets with a number of teeth of between 32 and 36 and large front sprockets with a number of teeth of between 46 and 50. The assembly described herein, on the other hand, allows different combinations to “standard” or “compact” to be adopted, like for example mixed combinations. 
     The Applicant has also found that, in conventional crank arms, the arms are sized to give the maximum resistance to twisting in combination with front sprockets of predetermined size. The result of this is that, in the case in which the cyclist intends to use front sprockets of different sizes and wants to maintain an acceptable structural rigidity, he is forced to replace also the right crank arm. This, as well as constituting a trouble for the cyclist, implies that the crank arm manufacturer needs to design, manufacture and commercialize crank arms of different sizes. 
     Advantageously, with an assembly like the one described herein in which front sprockets of different diameter are coupled at different coupling portions of the crank arm, it is no longer necessary to produce different sized crank arms for each combination of front sprockets intended to be used, nor does the cyclist have to change the crank arm each time he changes the combination of front sprockets in order to be able to maintain the desired structural rigidity. The present invention is therefore advantageous both for the cyclist and for the crank arm manufacturer. 
     Preferably, said at least one first coupling portion is defined along at least one first substantially radial direction with respect to said rotational axis of the crank arm and said at least one second coupling portion is defined along at least one second substantially radial direction different from said first substantially radial direction. 
     Even more preferably, the assembly described herein comprises at least two first coupling portions arranged at a first predetermined angular distance one from the other and at least two second coupling portions arranged at a second predetermined angular distance one from the other, the second predetermined angular distance being shorter than the first predetermined angular distance. 
     In the preferred embodiment of the assembly described herein, the main body of the crank arm comprises a first body portion that couples with a bicycle pedal and a second body portion that couples with said at least one front sprocket and with a shaft of a bottom bracket assembly of a bicycle, wherein said second body portion comprises at least one arm that couples with said at least one front sprocket extending substantially radially with respect to a rotational axis of the crank arm, wherein said at least one first coupling portion is defined in said at least one coupling arm and wherein said at least one first crank arm element and at least one second crank arm element extend from said at least one coupling arm along non-radial directions on opposite sides with respect to said at least one first coupling portion. 
     Preferably, the crank arm comprises a plurality of coupling arms and said at least one first crank arm element and at least one second crank arm element extend from at least some of said coupling arms. 
     In particular, in a first particularly preferred embodiment of the assembly described herein, said at least one second crank arm element extends cantilevered from a respective coupling arm and, preferably, said at least one first crank arm element extends circumferentially without structural interruption between two adjacent coupling arms. More preferably, said at least one second crank arm element extends cantilevered from the coupling arm of said two adjacent coupling arms that precedes the other coupling arm with reference to the direction of rotation of the crank arm during pedaling. 
     However, an embodiment is foreseen in which said at least one first crank arm element extends cantilevered from a respective coupling arm. 
     Preferably, in the case in which the assembly described herein comprises more than one front sprocket, said at least one second coupling portion is also defined in said at least one coupling arm. It is not therefore necessary to provide coupling elements in the crank arm other than those already provided for the coupling with the first front sprocket, to the great advantage of the weight of the crank arm. 
     Preferably, the crank arm further comprises an additional coupling portion defined in said first body portion, said first front sprocket and at least one second front sprocket both being coupled with said crank arm at said additional coupling portion. In this way a stable coupling of the front sprockets with the crank arm is achieved without weakening the body of the crank arm by providing many coupling points therein. 
     Preferably, the crank arm is made from light metal material or, more preferably, from composite material. In the latter case, the crank arm elements are also made from composite material, so as to obtain a component which is particularly light in weight. 
     Preferably, the assembly described herein comprises at least one first front sprocket element that cooperates with said at least one first crank arm element and at least one second front sprocket element that cooperates with said at least one second crank arm element. 
     More preferably, said at least one first front sprocket element has an angular extension substantially equal to that of said at least one first crank arm element. Advantageously, the front sprocket is thus provided with extended contact elements that give the front sprocket itself a certain rigidity and that thus allow the front sprocket to be made lighter in weight. 
     Preferably, said at least one front sprocket comprises an annular element having a radially inner surface from which at least one coupling element with the crank arm extends cantilevered. 
     In a preferred embodiment of the assembly described herein, said at least one first front sprocket element and at least one second front sprocket element are defined in a body portion of said annular element extending radially cantilevered from said radially inner surface and comprising said at least one coupling element. 
     In an alternative embodiment of the assembly described herein, said at least one second front sprocket element is defined by a respective body portion of said annular element extending radially cantilevered from said radially inner surface, said respective body portion not comprising said at least one coupling element. 
     Preferably, said annular element comprises a toothed portion that extends radially towards the outside along a primitive circumference having a predetermined diameter T, and said at least one first front sprocket element and at least one second front sprocket element are entirely contained in a first area extending radially towards the outside starting from an ideal circumference having a diameter T′≧aT, where a has a value selected from ⅔, ¾, ⅘, ⅚ or 6/7. Advantageously, the front sprocket in this case is reduced to a simple toothed band provided with contact and coupling elements with the crank arm having a very short radial extension. 
     Preferably, the annular element of the front sprocket is made from light metal material or composite material. In this way a component which is particularly light in weight is obtained. 
     In a second aspect thereof, the present invention relates to a right crank arm for a bicycle, comprising a main body adapted to be coupled with at least one front sprocket of a crankset for a bicycle at least one first coupling portion of the main body, said at least one front sprocket having an inner side intended, in use, to face towards the frame of the bicycle and an outer side opposite the inner side, the main body comprising at least one first contact element adapted to act on said at least one front sprocket on one of said sides, the crank arm being wherein the main body comprises at least one second contact element adapted to act on said at least one front sprocket on the other of said sides. 
     Throughout the present description and in the subsequent claims, the contact elements described with reference to the crank arm correspond to the elements indicated previously as crank arm elements. 
     Advantageously, the right crank arm described above can be used in the right crank arm assembly discussed above with reference to the first aspect described herein, thus allowing the advantages mentioned above with reference to such a crank arm assembly to be achieved. 
     Preferably, the right crank arm described above comprises individually and/or in combination all of the structural and functional features (be they essential, preferred and/or advantageous features) described above with reference to the crank arm of the right crank arm assembly described herein. 
     In particular, preferably, said at least one first contact element and at least one second contact element are integral with the main body. However, an embodiment is foreseen in which said at least one first contact element and at least one second contact element are distinct from, and associated with, said main body. 
     Preferably, the main body of the crank arm comprises a plurality of first contact elements and a plurality of second contact elements. 
     Preferably, at least some of said at least one first contact element and at least one second contact element extend at least partially along respective non-radial directions with respect to a rotational axis of said crank arm. 
     Preferably, at least some of said at least one first contact element and at least one second contact element extend at least partially circumferentially around said rotational axis of said crank arm, more preferably along respective arcs of circumference having an angular extension of less than 360°. 
     Preferably, such an angular extension is less than 360° and greater than or equal to 15°, more preferably it is between 15° and 100°, even more preferably between 30° and 75°. 
     Preferably, said respective arcs of circumference have different angular positions with respect to said rotational axis. 
     Preferably, said at least one first contact element and at least one second contact element extend from opposite sides with respect to said at least one first coupling portion. 
     More preferably, said at least one second contact element is arranged in a position that precedes said at least one first coupling portion with reference to the direction of rotation of the crank arm during pedaling and said at least one first contact element is arranged in a position that follows said at least one first coupling portion with reference to said direction of rotation. 
     Preferably, the main body of the crank arm comprises at least one abutment surface adapted to allow the correct angular positioning of said crank arm with respect to said at least one front sprocket. 
     More preferably, said at least one abutment surface is defined at least partially in said at least one second contact element. 
     Preferably, the main body of the crank arm comprises at least one first coupling portion with a first front sprocket of a crankset for a bicycle and at least one second coupling portion with at least one second front sprocket of a crankset for a bicycle having a different diameter from the diameter of said at least one first front sprocket, wherein said at least one second coupling portion is different from said at least one first coupling portion. 
     Preferably, said at least one first coupling portion is defined at a first circumference having its centre at the rotational axis of said crank arm and said at least one second coupling portion is defined at least one second circumference concentric with said first circumference and having a diameter different to that of said first circumference. 
     Preferably, said at least one first coupling portion is defined along at least one first substantially radial direction with respect to said rotational axis and said at least one second coupling portion is defined along at least one second substantially radial direction different from said first substantially radial direction. 
     Preferably, the main body of the crank arm comprises at least two first coupling portions arranged at a first predetermined angular distance one from the other and at least two second coupling portions arranged at a second predetermined angular distance one from the other, said second predetermined angular distance being shorter than said first predetermined angular distance. 
     Preferably, the main body of the crank arm comprises a first body portion that couples with a bicycle pedal and a second body portion that couples with said at least one front sprocket and with a shaft of a bottom bracket assembly of a bicycle, wherein said second body portion comprises at least one coupling arm with said at least one front sprocket extending substantially radially with respect to a rotational axis of said crank arm, wherein said at least one first coupling portion is defined in said at least one coupling arm and said at least one first contact element and at least one second contact element extend from said at least one coupling arm along non-radial directions on opposite sides with respect to said at least one coupling portion. 
     Preferably, said main body comprises a plurality of coupling arms and said at least one first contact element and at least one second contact element extend from at least some of said coupling arms. 
     In particular, in a first particularly preferred embodiment of the crank arm described herein, said at least one second contact element extends cantilevered from a respective coupling arm and, preferably, said at least one first contact element extends circumferentially without any structural interruption between two adjacent coupling arms. More preferably, said at least one second contact element extends cantilevered from the coupling arm of said two adjacent coupling arms that precedes the other coupling arm with reference to the direction of rotation of the crank arm during pedaling. 
     However, an embodiment is foreseen in which said at least one first contact element extends cantilevered from a respective coupling arm. 
     Preferably, in the case in which the crank arm described herein is adapted to be coupled with at least two front sprockets, said at least one second coupling portion is also defined in said at least one coupling arm. 
     Preferably, the main body of the crank arm is made from light metal material or from composite material. 
     In a third aspect thereof, the present invention relates to a front sprocket for a crankset of a bicycle, comprising an annular element having at least one coupling portion with a right crank arm of a bicycle, said crank arm having an inner side intended, in use, to face towards the frame of the bicycle and an outer side opposite said inner side, said annular element comprising at least one first contact element adapted to act in abutment on said crank arm on one of said sides, wherein said annular element comprises at least one second contact element adapted to act in abutment on said crank arm on the other of said sides. 
     Throughout the present description and in the subsequent claims, the contact elements described with reference to the front sprocket correspond to the element indicated previously as front sprocket elements. 
     Advantageously, the front sprocket described above can be used in the right crank arm assembly discussed above with reference to the first aspect described herein and therefore allows the advantages mentioned above with reference to such a crank arm assembly to be obtained. 
     Preferably, the front sprocket described herein comprises individually and/or in combination all of the structural and functional features (be they essential, preferred and/or advantageous features) described with reference to the front sprocket of the right crank arm assembly of the first aspect described herein. 
     In particular, said annular element preferably comprises a plurality of first contact elements and a plurality of second contact elements. 
     Preferably, said at least one first contact element and at least one second contact element extend at least partially along respective non-radial directions with respect to a rotational axis of said front sprocket. 
     More preferably, said at least one first contact element and at least one second contact element extend at least partially circumferentially around said rotational axis of said front sprocket along respective arcs of circumference having an angular extension of less than 360°. 
     Preferably, said angular extension is less than 360° and greater than or equal to 15°, more preferably it is between 15° and 100°, even more preferably between 30° and 75°. 
     Even more preferably, said respective arcs of circumference have different angular positions with respect to said rotational axis. 
     Preferably, said at least one first contact element and at least one second contact element extend from opposite sides with respect to said at least one coupling portion. 
     More preferably, said at least one second contact element is arranged in a position that precedes said at least one coupling portion with reference to the direction of rotation of the front sprocket during pedaling and said at least one first contact element is arranged in a position that follows said at least one coupling portion with reference to said direction of rotation. 
     Preferably, said annular element comprises at least one abutment surface adapted to allow the correct angular positioning of said front sprocket with respect to said crank arm. 
     More preferably, said at least one abutment surface is defined at least partially in said at least one second contact element. 
     Preferably, said annular element comprises a radially inner surface from which at least one element for coupling with said crank arm extends radially, said at least one coupling portion being defined in said at least one coupling element. 
     In a first embodiment of the front sprocket described herein, said at least one first contact element and at least one second contact element are defined in a body portion of said annular element extending radially cantilevered from said radially inner surface and comprising said at least one coupling element. 
     In an alternative embodiment of the front sprocket described herein, said at least one second contact element is defined by a respective body portion of said annular element extending radially cantilevered from said radially inner surface, said respective body portion not comprising said at least one coupling element. 
     Preferably, said at least one first contact element extends circumferentially without structural interruption between two adjacent coupling elements. 
     Preferably, said annular element comprises a toothed portion that extends radially towards the outside along a primitive circumference having a predetermined diameter T, wherein said at least one first contact element and at least one second contact element are entirely contained in a first area extending radially towards the outside starting from an ideal circumference having a diameter T′≧aT, where a has a value selected from ⅔, ¾, ⅘, ⅚ or 6/7. 
     Preferably, said annular element is made from a light metal alloy or composite alloy. 
     In a fourth aspect thereof, the present invention relates to a right crank arm assembly for a bicycle, comprising a right crank arm and at least one front sprocket coupled with said crank arm, wherein it comprises at least one crank arm element which acts in contact with said at least one front sprocket and that extends along a non-radial direction with respect to a rotational axis of said crank arm. 
     Advantageously, the provision in the crank arm of contact elements extending along a non-radial direction allows a contact of the crank arm on the front sprocket to be obtained that is more stable and effective than what occurs in the assemblies of the prior art, where the contact occurs only at the ends of the arms of the crank arm extending radially with respect to the rotational axis of the crank arm. In this way it is possible to limit the deformation of the front sprocket, with a consequent advantage in terms of motion transmission. 
     Preferably, the assembly discussed above with reference to the fourth aspect described herein comprises individually and/or in combination all of the features described above with reference to the assembly of the first aspect described herein, thus achieving all of the advantages discussed above with reference to such an assembly. 
     In particular, preferably, the crank arm of the assembly of the fourth aspect described herein comprises at least one coupling arm with said at least one front sprocket extending along a substantially radial direction, wherein said at least one crank arm element extends from said at least one coupling arm. 
     More preferably, the crank arm comprises a plurality of coupling arms, wherein said at least one crank arm element extends from at least some of said coupling arms. 
     Preferably, said at least one crank arm element extends at least partially circumferentially around said rotational axis of said crank arm along an arc of circumference having a predetermined angular extension. 
     In a specific embodiment of the assembly described herein, such an angular extension is equal to 360°. 
     In an alternative embodiment, such an angular extension is less than 360° and greater than or equal to 15, and is preferably between 15° and 100°, more preferably between 30° and 75°. 
     As already stated with reference to the crank arm assembly discussed above with reference to the first aspect described herein, said at least one crank arm element extends circumferentially without structural interruption between two adjacent coupling arms. However, a variant is foreseen in which said at least one crank arm element extends cantilevered from said at least one coupling arm. 
     Preferably, said at least one front sprocket has an inner side intended, in use, to face towards the frame of the bicycle and an outer side opposite said inner side and said at least one crank arm element is active on said at least one front sprocket on said outer side. 
     Preferably, said crank arm is made from light metal material or from composite material. 
     Preferably, the assembly described herein comprises at least one front sprocket element that cooperates with said at least one crank arm element, wherein said at least one front sprocket element has an angular extension substantially equal to that of said at least one crank arm element. 
     Preferably, said at least one front sprocket comprises an annular element having a radially inner surface from which at least one coupling element with said crank arm extends radially cantilevered. 
     Preferably, said at least one coupling element is structurally distinct from said at least one front sprocket element. 
     Preferably, said annular element comprises a toothed portion that extends radially towards the outside along a primitive circumference having a predetermined diameter T, wherein said at least one front sprocket element and said at least one coupling element are entirely contained in a first area extending radially towards the outside starting from an ideal circumference having a diameter T′≧aT, where a has a value selected from ⅔, ¾, ⅘, ⅚ or 6/7. 
     Preferably, said annular element is made from light metal material or from composite material. 
     In a fifth aspect thereof, the present invention relates to a right crank arm for a bicycle, comprising a main body adapted to be coupled with at least one front sprocket of a crankset of a bicycle, wherein said main body comprises at least one contact element with said at least one front sprocket that extends along a non-radial direction with respect to a rotational axis of said crank arm. 
     Throughout the present description and in the subsequent claims, the contact elements described with reference to the crank arm correspond to the elements indicated previously as crank arm elements. 
     Advantageously, the crank arm described above can be used in the right crank arm assembly discussed above with reference to the fourth aspect described herein, and thus allows the advantages mentioned above with reference to such a crank arm assembly to be achieved. 
     Preferably, the crank arm described herein comprises individually and/or in combination all of the structural and functional features (be they essential, preferred and/or advantageous features) described with reference to the crank arm of the right crank arm assembly of the fourth aspect described herein. 
     In a sixth aspect thereof, the present invention relates to a front sprocket for a crankset of a bicycle, comprising an annular element adapted to be coupled with a right crank arm of a bicycle, wherein said annular element comprises at least one contact element with said crank arm extending along a non-radial direction with respect to a rotational axis of said front sprocket. 
     Throughout the present description and in the subsequent claims, the contact elements described with reference to the front sprocket correspond to the elements indicated previously as front sprocket elements. 
     Advantageously, the front sprocket described above can be used in the right crank arm assembly discussed above with reference to the fourth aspect described herein, and therefore allows the advantages mentioned above with reference to such a crank arm assembly to be achieved. 
     Preferably, the front sprocket described herein comprises individually and/or in combination all of the structural and functional features (be they essential, preferred and/or advantageous features) described with reference to the front sprocket of the right crank arm assembly of the fourth aspect described herein. 
     In a seventh aspect thereof, the present invention relates to a right crank arm assembly for a bicycle, comprising a right crank arm and at least one front sprocket coupled with said crank arm at least one coupling portion of said at least one front sprocket, wherein it comprises at least one front sprocket portion which acts in contact with said crank arm and which is structurally distinct from said at least one coupling portion. 
     Advantageously, the provision in the front sprocket of contact portions which are structurally distinct from the coupling portions allows a contact of the crank arm to be made on the front sprocket that is more stable and effective that what occurs in the assemblies of the prior art, where the contact only occurs at the ends of the arms of the crank arm extending radially with respect to the rotational axis of the crank arm. In this way it is possible to limit the deformation of the front sprocket, with a consequent advantage in terms of transmission of motion. 
     Preferably, the assembly discussed above with reference to the seventh aspect described herein comprises individually and/or in combination all of the structural and functional features discussed above with reference to the assembly of the first aspect described herein, thus obtaining all of the advantages discussed above with reference to this assembly. 
     In particular, preferably, said at least one front sprocket portion is defined in at least one front sprocket element that extends radially cantilevered from a radially inner surface of said at least one front sprocket and said at least one coupling portion is defined in at least one first coupling element that extends radially cantilevered from said radially inner surface in a different angular position to that of said at least one front sprocket element. 
     Preferably, said front sprocket comprises an annular element made from a light metal material or from a composite material. 
     Preferably, said annular element comprises a toothed portion that extends radially towards the outside along a primitive circumference having a predetermined diameter T, wherein said at least one front sprocket element and said at least one first coupling element are entirely contained in a first area extending radially towards the outside starting from an ideal circumference having a diameter T′≧aT, where a has a value selected from ⅔, ¾, ⅘, ⅚ or 6/7. 
     Preferably, the assembly described herein comprises at least one crank arm element adapted to cooperate with said at least one front sprocket element and at least one second coupling element adapted to cooperate with said at least one first coupling element. 
     Preferably, said crank arm is made from a light metal material or from a composite material. 
     In an eighth aspect thereof, the present invention relates to a front sprocket for a crankset of a bicycle, comprising an annular element adapted to be coupled with a right crank arm of a bicycle at a coupling portion of said annular element, wherein said annular element comprises at least one contact portion with said crank arm which is structurally distinct from said at least one coupling portion. 
     Throughout the present description and in the subsequent claims, the contact elements described with reference to the front sprocket correspond to the elements indicated previously as front sprocket elements. 
     Advantageously, the front sprocket described above can be used in the right crank arm assembly discussed above with reference to the seventh aspect described herein, and thus allows the advantages mentioned above with reference to such a crank arm assembly to be obtained. 
     Preferably, the front sprocket described herein comprises individually and/or in combination all of the structural and functional features (be they essential, preferred and/or advantageous features) described with reference to the front sprocket of the right crank arm assembly of the seventh aspect described herein. 
     DESCRIPTION 
     With reference to  FIGS. 1 and 2 , a first embodiment of a right crank arm assembly according to the present invention is indicated with  1 . The assembly  1  comprises a right crank arm  5 , a front sprocket of larger diameter  10  (hereafter indicated as big front sprocket) and a front sprocket of smaller diameter  15  (hereafter indicated as small front sprocket). Alternative and not illustrated embodiments are foreseen in which the assembly  1  comprises just one front sprocket, for example for use on a racing track, or more than two front sprockets, for example three. 
     Throughout the present description and in the subsequent claims, the “inner side” of the assembly  1 , of the crank arm  6  and/or of the front sprockets  10  and  15  shall indicate the side visible in  FIGS. 1 and 3 , i.e. the one facing towards the frame of the bicycle when the assembly  1  is mounted in the bottom bracket assembly. On the other hand “outer side” shall indicate the side opposite the inner side, visible in  FIGS. 2 and 4 . 
     Throughout the present description and in the subsequent claims, moreover, “previous position” or “following position” shall respectively indicate the position of an element that precedes and that follows a reference element in the direction of rotation of the crank arm, where the rotation is in the direction such as to transmit driving force to the rear wheel. 
     The right crank arm  5  described herein can be made from metallic material, like a light alloy, or from composite material, comprising structural fibers incorporated in a polymeric material. Typically, the structural fibers are selected from the group consisting of carbon fibers, glass fibers, aramid fibers, ceramic fibers, boron fibers and combinations thereof, carbon fibers being preferred. Preferably, the polymeric material of the body of the component is thermosetting. However, the possibility of using a thermoplastic material is not excluded. More preferably, the polymeric material comprises an epoxy resin. While these materials are preferred, they are not limiting to other materials that may be chosen. 
     The arrangement of said structural fibers in the polymeric material can be a random arrangement of pieces or sheets of structural fibers, a substantially unidirectional ordered arrangement of fibers, a substantially bidirectional ordered arrangement of fibers, or a combination of the above. 
     In an alternative embodiment, the fibers are organized in a fabric wound with continuity around a recess, as described in patent EP 1270394 and in European patent applications n o  06425086 and 06425087 (U.S. application Ser. Nos. 11/675,279 and 11/501,656) to the same Applicant the content of which are incorporated herein by reference as if fully set forth. 
     With particular reference to  FIGS. 6 to 8 , the right crank arm  5  comprises a main body  20  having a first end  21  for coupling with a pedal (not illustrated) and a second end  22  for coupling with the front sprockets  10  and  15  and with a shaft of a bottom bracket assembly (not illustrated). In particular, the second end  22  comprises a hole  23  for housing the shaft of the bottom bracket assembly. In alternative and not illustrated embodiments but known to a person of ordinary skill in the art, the shaft is removably coupled with the right crank arm, or it is an integral part thereof. 
     A support structure  25  of the front sprockets  10  and  15  is provided about the hole  23 , comprising four coupling arms (or spokes)  27  that extend substantially radially around the hole  23  inside of which a rotational axis X of the crank arm  5  is defined. 
     At the respective free end portions  28  the arms  27  are connected in sets of two by a reinforcing element  30 , preferably extending circumferentially around the rotational axis X of the crank arm  5  and made in the form of an arc of circle. 
     Each reinforcing element  30  that connects a pair of arms  27  comprises a contact surface  32  at the outer side of the big front sprocket  10 . 
     The contact surfaces  32  preferably have an angular extension α ( FIG. 7 ) greater than or equal to 15°, preferably between 15° and 100°, more preferably between 30° and 75° and are arranged at an angular distance β from the middle plane Π of the elongated body  20 , where β is preferably between 30° and 90°. 
     In an alternative embodiment of the crank arm  5  (not illustrated) the pairs of arms  27  connected by the element  30  are replaced by a single arm of equal angular extension. 
     Now considering the pairs of arms  27  connected together by the element  30 , the arm  27  of each of them coming first in the direction of rotation ω of the right crank arm  5  around the rotational axis X of the crank arm  5 , comprises an element  35  extending cantilevered in the circumferential direction in the direction of rotation ω. Each element  35  comprises a contact surface  40  at the inner side of the big front sprocket  10 . 
     Coupling portions  44  with the big front sprocket  10  are defined at the free ends  28  of each arm  27 . The element  30  connects the coupling arms  27  right at such coupling portions  44 . The coupling portions  44  are provided with coupling holes  45 . 
     The coupling portions  44  with the ends  28  of the arms  27  connected by the element  30  are arranged at an angular distance γ from each other of between 45° and 95°, and the coupling portion  44  closest to the elongated body  20  is arranged at an angular distance μ of between 35° and 85° from the plane Π. 
     An additional coupling hole  46  with the front sprocket  10  is preferably provided on a portion of the elongated body  20  ( FIG. 7 ). 
     The section of  FIG. 9  shows in detail an example embodiment of the coupling portions  44  in the case in which the right crank arm  5  is made from composite material. The coupling portions  44  are in this case preferably defined by metallic inserts  50  provided with a threaded hole  52  for the insertion of a screw (not illustrated). The outer surface  54  of the inserts  50  is irregular, and preferably threaded, so as to be able to be better held in the composite material, to which it is fixed by gluing or by direct adhesion due to a co-moulding process. 
     An alternative embodiment of the assembly described herein is foreseen in which the threaded hole  52  is directly made in the composite material. 
     As shown in detail in  FIG. 9 , the crank arm  5  described herein further comprises, in a preferred embodiment thereof, second coupling portions  47  used for the coupling of the small front sprocket  15 . The coupling portions  47  are preferably made identical to the coupling portions  44  and comprise coupling holes  48 . 
     The coupling portions  44  and  47  are at different distances from the rotational axis X of the right crank arm  5 . In particular, their axes of symmetry S 1  and S 2  lie on two ideal circumferences of different diameter. 
     As illustrated in  FIGS. 1 ,  3 ,  6 ,  7  and  12 , the coupling portions  44  and  47  are preferably non-radially aligned with each other. Indeed, the coupling portions  44  are at smaller angular distances γ apart than the angular distances between the coupling portions  47 . 
     Both the big front sprocket  10  and the small front sprocket  15  are coupled with the main body  20  of the crank arm  5  at the additional coupling portion  46 . 
     With particular reference now to the section illustrated in  FIG. 5 , in a preferred embodiment described herein the contact elements  30  and  35  respectively at the outer side and at the inner side of the big front sprocket  10  are located substantially at the opposite side with respect to the coupling portions  44 , identified by the axis of symmetry S 1  (said axis is parallel to the rotational axis x of the crank arm  5 ). In particular, the surface  32  of the element  30  for contact at the outer side of the front sprocket  10  follows the coupling portion  44  in the direction of rotation ω, whereas the surface  40  of the element  35  for contact at the inner side of the front sprocket  10  precedes the coupling portion  44  ( FIGS. 6 and 7 ). The elements  30  and  35  are therefore active in abutment on the front sprocket at different angular positions. It should be appreciated from  FIG. 5  that the first contact element  30  and the second contact  40  element are offset from one another along the rotational axis of the right crank arm  5 ; this offset creates a gap in which the front sprocket  10  is engaged. 
     In an alternative embodiment of the crank arm illustrated in  FIG. 13  and indicated with  5 ′, the arms  27 ′ instead of being connected in sets of two through the elements  30 , each comprise an element  30 ′ extending cantilevered circumferentially around the rotational axis X of the crank arm  5 ′. Each element  30 ′ comprises a respective contact surface  32 ′ at the outer side of the big front sprocket  10 . Each of the arms  27 ′ also comprises a respective contact surface  40 ′ at the inner side of the big front sprocket  10 . The contact surfaces  32 ′ and  40 ′ are arranged in a position respectively prior to and after the coupling portion  44  (hidden in the figures) provided on the arm  27 . In this case, the number of arms  27  can also be odd, for example three or five. 
     In a further not illustrated embodiment of the assembly described herein, just one or in any case just a few of the arms  27  comprise a contact surface  40  at the inner side of the big front sprocket  10 , between which the arm  27  immediately following the elongated body  20  with reference to the direction of rotation ω of the crank arm  5  and preferably the arm  27  arranged in a substantially symmetrical position with respect to the main body  20  of the crank arm  5 . 
     In accordance with the invention, the contact surfaces  32  and  40  do not lie on the same plane, but a predetermined axial distance D apart ( FIG. 5 ). In this way the big front sprocket  10  does not need to be deformed to be inserted between the two contact surfaces. It is thus sufficient for the front sprocket  10  to have a thickness equal to D in the contact area with the crank arm  5 . Preferably D is equal to or less than the maximum thickness of the big front sprocket  10 . 
     In the embodiments illustrated in the attached figures, the elements  30  and  35  are integral with the main body  20  of the crank arm  5 , but in other not illustrated embodiments such elements can be made in separate pieces and coupled with the crank arm  5 . 
     In  FIGS. 10 and 11 , the big front sprocket  10  of the right crank arm assembly  1  shown in  FIGS. 1 and 2  is illustrated. 
     Such a front sprocket comprises an annular element  11 , preferably made from light metal alloy or from composite material, having a radially outer annular surface  12  on which a toothed portion  100  (hereafter also indicated as toothing) is formed extending radially towards the outside and a radially inner annular surface  13  from which four elements  98  for coupling with the crank arm extend radially cantilevered. In particular, the elements  98  are adapted to be coupled with the arms  27  of the crank arm  5  by coupling respective coupling portions defined on the elements  98  with the coupling portions  44  defined on the arms  27 . The coupling takes place through screws (not illustrated) inserted in holes  64  formed on each coupling element  98 . 
     On outer side thereof ( FIG. 11 ) the big front sprocket  10  comprises two elements  55  having respective contact surfaces  60  adapted to contact the contact surfaces  32  of the elements  30  of the right crank arm  5 . The elements  55  have the same angular extension as the elements  30  and each element  55  extends without structural interruption between two adjacent elements  98 . What has been stated above with reference to the number, angular position with respect to the coupling portions  44  and angular extension of the elements  30  of the crank arm  5  with respect to the rotational axis of the crank arm is therefore also valid for the elements  55  of the front sprocket  10  with reference to the coupling portions defined on the elements  98  and with reference to the rotational axis of the front sprocket. 
     The elements  55  are preferably surmounted in the radial direction by a throat  62  extending according to an arc of circle. 
     At the ends of the elements  55  the holes  64  for the passage of the screws that insert into the inserts  50  of the right crank arm  5  are formed. 
     On the inner side of the front sprocket  10  (illustrated in  FIG. 10 ), on the other hand, a pair of elements  65  are provided having respective contact surfaces  66  adapted to contact the contact surfaces  40  of the elements  35  of the right crank arm  5 . 
     The big front sprocket  10  further comprises an additional arm  68  with a hole  69  for the passage of a screw intended to insert into the hole formed in the additional coupling portion  46  of the crank arm  5 , in the case in which such an additional coupling portion is present. 
     In the embodiment of the big front sprocket  10  illustrated in detail in  FIGS. 10 and 11 , each contact element  55  and  65  with the crank arm  5  is defined at a single body portion  14  of the annular element  11  that extends radially cantilevered towards the centre of the front sprocket  10  from inner surface  13  thereof and that also comprises a pair of coupling elements  98 . The elements  55 ,  65 , and  98  are therefore all an integral part of the body portion  14 . 
       FIG. 14  shows an alternative embodiment of the big front sprocket, indicated with  10 . In such an embodiment no single body portion  14  that comprises the elements  65  and  98  can be identified. Indeed, the body portion  14  here comprises just the contact element  55  and a pair of elements  98  for coupling with the crank arm  5 , whereas the contact elements  65  with the crank arm are defined at further and respective body portions  140 , distinct and separate from the body portion  14  of the annular element  11 , which also extend radially cantilevered from the inner surface  14  of the annular element  11  towards the centre of the front sprocket  10  and which do not comprise the coupling elements  98  and the contact elements  55  with the crank arm  5 . 
     In such an embodiment, the elements  98  and  65  are made adjacent to the toothed portion  100  of the front sprocket  10 . Preferably, considering the diameter T of the primitive circumference of the toothed portion  100 , the elements  98  and  65  are located and entirely contained in an annular area extending radially between the inner surface of said annular element and an ideal circumference of diameter T′ such that T&gt;T′≧aT, where a is selected from ⅔, ¾, ⅘, ⅚ or 6/7. 
     Preferably, the aforementioned inner surface is defined at an ideal circumference arranged at least 1 mm radially towards the inside from the toothed portion  100 . 
     The front sprocket  10  in this case therefore reduces to a toothed band provided with radial projections constituting the coupling elements  98  and the contact elements  55  and  65  and having a short radial extension. Consequently, the maximum radial extension of the coupling arms provided in the crank arm is defined by a circumference of diameter T″ (see  FIGS. 2 and 4 ) such that T&gt;T″&gt;T′, where the difference between T and T″ is simply given by the need to leave a free front sprocket portion which is sufficient to allow the engagement of the teeth with a chain. 
     With particular reference to  FIG. 4 , if L indicates the length of the arm of the crank arm (such a length being commercially set at 170 mm, 172.5 mm and 175 mm), the crank arm of the assembly described herein is sized so that the ratio L/T″ is between 0.7 and 1, preferably between 0.8 and 0.9. 
     A not illustrated embodiment is foreseen in which the front sprocket has coupling elements  98  and contact elements  55  and  65  of short extension as described above with reference to  FIG. 14  and all forming part of a single body portion  14  as described above with reference to  FIG. 11 . 
     It should be noted how, in all of the embodiments of the front sprocket  10  of the assembly described herein, the contact portions of the front sprocket  10  with the crank arm  5  are structurally and physically distinct from the respective coupling portions. 
     In the case of use of the front sprocket of  FIG. 11  or of the not illustrated embodiment in which the front sprocket has coupling elements  98  and contact elements  55  and  65  of short extension as described above with reference to  FIG. 14  and all forming part of a single body portion  14  as described above with reference to  FIG. 11 , the crank arm of the assembly described herein has coupling arms having a greater radial extension than those of conventional crank arms. In particular, while in conventional crank arms the diameter of the ideal circumference defined by the coupling holes at the big front sprocket and at the small front sprocket is 130 mm or 135 mm in the case of standard combinations and 110 mm in the case of compact combinations, in the crank arm of the assembly described herein the diameter of the ideal circumference defined by the holes  45  for coupling with the big front sprocket  10  is within an annular area having an inner diameter greater than or equal to 150 mm and an outer diameter less than or equal to 180 mm, preferably an inner diameter greater than or equal to 160 mm and an outer diameter less than or equal to 170 mm, whereas that of the ideal circumference defined by the holes  48  for coupling with the small front sprocket  15  is within an annular area having an inner diameter greater than or equal to 100 mm and an outer diameter less than or equal to 130 mm, preferably an inner diameter greater than or equal to 110 mm and an outer diameter less than or equal to 120 mm. 
       FIGS. 1 and 2  show a right crank arm assembly comprising a combination of standard front sprockets, i.e. a small front sprocket with a minimum of 39 teeth coupled with a big front sprocket with a number of teeth of between 52 and 56 (53 in the front sprocket  10  illustrated). 
     However, the right crank arm  5  illustrated in  FIGS. 6 ,  7  and  8  can support a wide range of combinations of front sprockets, for example a compact combination, as illustrated in  FIGS. 3 and 4 , in which the small front sprocket  15 ′ has a minimum number of teeth equal to 34 and the big front sprocket  10 ′ has a number of teeth of between 46 and 50 (48 in the combination illustrated). 
     It should however be noted that the right crank arm  5  allows any size of front sprockets to be mounted, for which reason it is also possible to adopt combinations of front sprockets different to the standard and compact ones, in particular mixed combinations. 
     As illustrated in  FIGS. 6 ,  7 ,  8 ,  13 , the crank arm  5 ,  5 ′ further comprises abutment surfaces  70 ,  70 ′ adapted to allow the correct angular positioning of the crank arm with respect to the front sprocket  10 . Such surfaces  70 ,  70 ′ are defined at a free end of the contact element  35 ,  35 ′ with the crank arm  5 ,  5 ′. Further abutment surfaces  72  are provided at a body portion of the element  35  that extends on the opposite side, with respect to the coupling arm  27 , to the one at which the abutment surface  70  is defined. The abutment surfaces  70  and  72  cooperate in abutment with corresponding abutment surfaces  74 ,  76  defined in the big front sprocket  10 . 
       FIG. 12  illustrates the way to mount a big front sprocket  10  on a right crank arm  5 . In particular, the big front sprocket  10  is brought in contact with the right crank arm  5  so that the contact surfaces  32  of the elements  30  of the crank arm  5  come into contact with the contact surfaces  60  of the elements  55  of the front sprocket  10 . At this point the right crank arm  5  is rotated with respect to the front sprocket  10  in the same direction of rotation ω in which it rotates during pedaling. The crank arm  5  shall move with respect to the front sprocket until the abutment surfaces  70  and  72  of the right crank arm  5  make contact with the corresponding abutment surfaces  74  and  76  of the front sprocket  10 . At this point, the contact surfaces  40  of the elements  35  of the crank arm  5  shall be in abutment with the contact surfaces  66  of the elements  65  of the front sprocket  10 . As a result of this, when the surfaces  74  and  76  of the front sprocket  10  are in abutment with the surfaces  70  and  72  of the crank arm, the contact surfaces  32 ,  40 ,  60  and  66  cooperate with each other to keep it in position and the operator has both hands free to fix the screws. 
     It should be appreciated that the first contact element  55  and the second contact element  65  are offset from one another along a rotational axis of the annular element  11 . 
     Now with reference to  FIGS. 15 and 16 , a further embodiment of the right crank arm assembly according to the present invention is illustrated, indicated with  200 . 
     The right crank arm assembly  200  comprises a right crank arm  205 , preferably made from composite material, comprising a coupling portion  210  with an annular toothed band  215 . The coupling portion  210  preferably comprises an annular-shaped outer peripheral portion  225  coupled with the elongated body  220  of the crank arm  205  through a plurality of coupling arms  230 . 
     The coupling between the coupling portion  210  and the toothed band  215  can be of any type capable of transmitting torque. In  FIG. 16 , as an example, a shape-coupling is illustrated, in particular toothed, preferably strengthened by gluing, or by the adhesion between composite material of the coupling portion  210  and the metal of the toothed band  215  obtained by co-molding. Alternatively, the toothed band  215  and the coupling portion  210  could be made with a threading at their interface to be screwed (and then possibly glued) one to the other. According to a further alternative, the toothed band  215  is fixed onto the coupling portion  210  through screws or other fastening elements. 
     It should be observed that, although  FIG. 16  shows a radial coupling interface  240  and a circumferential coupling interface  245 , it does not exclude embodiments in which the coupling is just circumferential or just radial (in which case there are no front or rear contact surfaces as for the embodiments of  FIGS. 1 to 14 ). The coupling between the toothed band  205  and the coupling portion  210  could also be a dovetail coupling, or a coupling extending along a surface inclined with respect to the rotational axis of the annular toothed band  215 . 
     It should also be observed that, although in  FIG. 15  just one annular toothed band  215  is shown, an embodiment is foreseen in which the right crank arm  205  is also coupled with a front sprocket or annular band of smaller diameter. The coupling between such an annular band of smaller diameter and the right crank arm  205  is of the same type as that described with reference to the annular toothed band  215 . 
     As highlighted in the variant  200 ′ of  FIG. 17 , the coupling portion  210 ′, instead of being extended according to a complete ring, can comprise ring portions  225 ′, for example having the dimensions illustrated with reference to the support structure  25  of the crank arm  5  of  FIGS. 6 ,  7  and  8 . The coupling between the coupling portion  210 ′ and the annular toothed band  215 ′ is of the same type described for the assembly  200  of  FIGS. 15 and 16 . 
       FIG. 17  also shows that the right crank arm assembly  200 ′ (just like the one  200  of  FIG. 15 ) can comprise a second front sprocket or toothed band  216  of smaller diameter with respect to the toothed band  215 ′. The small front sprocket  216  can be of the type illustrated with reference to the right crank arm assemblies of  FIGS. 1 to 4 , or else a toothed band similar to the toothed band  215 ′, coupled with the crank arm  205 ′ in the same way as the toothed band  215 ′. 
     In all of the embodiments described and illustrated, the small front sprocket  15  is coupled with the right crank arm  5  in a conventional way, i.e. contacting just the outer side. However, an embodiment is foreseen in which the coupling between the small front sprocket  15  and the crank arm  5  is the same as that described for the big front sprocket  10 , i.e. contacting both the outer side and the inner side. 
     Numerous variants of the right crank arm  5  are possible, for example the position of the contact elements at the outer side and at the inner side of the big front sprocket can be inverted with respect to the coupling portions, thus performing their contact function in different angular positions of pedaling. The illustrated example is that in which the contact elements counteract the twisting of the front sprocket in the most critical condition, i.e. when the pedal has passed the vertical to begin a new descent, since this is the point at which the cyclist exerts the maximum thrust. The contact surfaces can also face each other and therefore are at an identical angular position.