Abstract:
A toothed wheel for a chain transmission system for a bicycle, intended to be coupled with at least one other toothed wheel in a group of crowns or of sprockets rotating as a unit has a plurality of teeth that follow one another on the radially outer periphery of the toothed wheel, alternating with grooves. Each tooth has a thickness, a width and a height and has an idle flank, passive in the exchange of torque with a bicycle chain, and a pressure flank, active in the exchange of torque with the same chain. The toothed wheel has at least one gearshifting helping tooth the height of which, measured from the bottom of an adjacent groove, decreases in the direction of the pressure flank, in any case being greater than or equal to 90% of the radius of the pin.

Description:
FIELD OF INVENTION 
       [0001]    The field refers to a toothed wheel of a chain transmission system for a bicycle and to a group of toothed wheels for front and rear bicycle gearshifts. 
       BACKGROUND 
       [0002]    As used herein, the term “sprockets” identifies the toothed wheels of a transmission system for bicycles intended to be coupled with the rear wheel, and the term “crowns” identifies the toothed wheels intended to be coupled with the cranks. 
         [0003]    Bicycle transmission systems have a group of sprockets having different diameters, mounted together and rotating as a unit, on which the transmission chain is engaged, alternately on one of the sprockets of the group according to the transmission ratio to be accomplished. On each sprocket of the group the teeth follow one another at constant distance, or pitch, alternating with grooves; the pitch is equal on the various wheels of the same group of sprockets and corresponds to the pitch of the chain. The chain is run into rotation by the cyclist through the pedals and the front toothed wheels, or crowns, of the bicycle. 
         [0004]    Normally, on a bicycle the transmission of motion always takes place the same way, i.e. the chain always has the same rotational motion (at least when it transmits power/torque), and therefore the front crowns and the sprockets always have the same direction of rotation. In other words, the direction of movement of the teeth of the crowns and of the sprockets is always the same. With reference to such a direction of rotation, every tooth has a preceding flank (the one facing forwards with respect to the direction of rotation) and a following flank (the one facing backwards). In front crowns the pressure flank, on which acts the traction force exerted by the chain, is the preceding one; in wheels of a rear gearshift group, the pressure flank is the following one. The flank of a tooth opposite the pressure flank is defined as the idle flank and it does not cooperate with the transmission of torque. In front crowns the idle flank of each tooth is the one following the tooth itself with respect to the direction of rotation of the crown. In rear sprockets, the idle flank of each tooth is the one preceding the tooth itself with respect to the direction of rotation of the sprocket. In general, every tooth also has an extension in the circumferential direction, or width, an extension in the radial direction, or height, and an extension in the axial direction, or thickness. 
         [0005]    The movement of the chain disengaging one sprocket and engaging another adjacent sprocket, or else the movement from a first crown to a second front crown, is known as “gearshifting,” and is normally obtained by moving the chain transversally with respect to its own longitudinal axis (i.e. axially with respect to the toothed wheel), until it abandons the engagement with one toothed wheel and moves towards the engagement with the adjacent toothed wheel. This movement action is normally obtained with chain guide devices, operated by the cyclist. 
         [0006]    If gearshifting occurs starting from a wheel having a larger diameter towards a wheel having a smaller diameter, it is defined as “downward gearshifting;” vice-versa, if it occurs starting from a wheel having a smaller diameter towards a wheel having a larger diameter, it is defined as “upward gearshifting.” As known, upward gearshifting on the rear sprockets causes a reduction in the transmission ratio, and vice-versa, downward gearshifting causes an increase in such a ratio. Gearshifting, both upward and downward, is in any case a delicate transition operation, since it occurs when the chain is under tension and since the chain has very limited deformability in the transversal direction (with respect to its longitudinal axis). 
         [0007]    The gearshifting steps alternate with the normal operation steps in which the chain engages a single sprocket and a single crown to transmit the torque. 
         [0008]    Current toothed wheels have certain drawbacks that do not allow a regular transition of the transmission ration upward gearshifting. 
       SUMMARY 
       [0009]    A toothed wheel for a chain transmission system for a bicycle, intended to be coupled with at least one other toothed wheel in a group of toothed wheels rotating as a unit, has a plurality of teeth that follow one another on the radially outer periphery of the toothed wheel, alternating with grooves, wherein each tooth has a thickness, a width and a height and has an idle flank, passive in the transmission of torque with a bicycle chain, and a pressure flank, active in the transmission of torque with the same chain, in which the pressure flank, which extends in the circumferential direction, is concave for the engagement with a pin of the chain, comprising at least one tooth for helping gearshifting the height of which, measured from the bottom of an adjacent groove, decreases in the direction of the pressure flank, in any case being greater than or equal to 90% of the radius of the pin. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
         [0010]    Further characteristics shall become clearer from the following description, made with reference to the attached drawings. In such drawings: 
           [0011]      FIGS. 1 ,  2  and  3  are respective front, plan, and rear views of a circular sector of a sprocket and of a group of sprockets; 
           [0012]      FIGS. 4 ,  5  and  6  are respective front, plan, and rear views of a first detail of the group of sprockets shown in  FIG. 1 ; 
           [0013]      FIG. 7  is a plan view of a chain suitable for engaging a group of sprockets; 
           [0014]      FIGS. 8 and 9  are respective front and rear views of a second detail of the group of sprockets shown in  FIG. 1 ; 
           [0015]      FIGS. 10 ,  11  and  12  are respective front, plan, and rear views of further details of the group of sprockets shown in  FIG. 1 ; 
           [0016]      FIGS. 13 ,  14  and  15  are respective front, plan, and rear views of a circular sector of a sprocket and of a group of sprockets, during an upward gearshifting step, in a first configuration; 
           [0017]      FIG. 16  is an enlarged view of a detail of  FIG. 14 ; 
           [0018]      FIGS. 17 ,  18 , and  19  are respective front, plan and rear views of a circular sector of a sprocket and of a group of sprockets, during an upward gearshifting step, in a second configuration; and 
           [0019]      FIG. 20  is an enlarged view of a detail of  FIG. 18 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS INTRODUCTION 
       [0020]    The height of the tooth for helping gearshifting gradually reduces at the pressure flank of the tooth. In a sprocket, the pressure flank of each tooth is the flank following the tooth itself with respect to the direction of rotation of the sprocket. In a crown, the pressure flank is the one preceding the tooth with respect to the direction of rotation. This characteristic allows upward gearshifting to be accomplished easier, i.e. it simplifies the movement of the transmission chain towards the engagement with the sprocket or the crown having larger diameter, at the same time ensuring an optimal transmission of torque during normal operation, i.e. before and after gearshifting, without risks of disengagement of the chain. 
         [0021]    In an embodiment of the toothed wheel, the gearshifting helping tooth also comprises a top face, preferably convex, which extends in the circumferential direction between an edge, or a vertex, having the maximum height and an edge, or a joining surface with the pressure flank, having the minimum height. In other words the top face of the tooth for helping gearshifting has a substantially circumferential extension, but slightly decreasing towards the pressure flank of the tooth. In this circumstance, the minimum height of the tooth for helping gearshifting, i.e. the height between the bottom of the groove adjacent to the pressure flank and the join between the top face and the pressure flank, is greater than or equal to 90% of the radius of the pin of the chain. 
         [0022]    The tooth for helping gearshifting also has an outer face perpendicular to the axis of the toothed wheel and intended to face towards the toothed wheel having smaller diameter. The outer face, with the idle flank of the same helping tooth, forms a hooking and radial lifting edge of the transmission chain. This edge has the function of causing the chain to lift towards the top face of the tooth when, during the gearshifting step commanded by the cyclist, a component of the chain interacts with the same lifting edge. In simpler terms, the hooking (and lifting) edge pulls the chain during gearshifting and is also configured like an upward ramp that pushes the transmission chain beyond the top face of the tooth, facilitating the movement of the chain towards the engagement with the toothed wheel having larger diameter. 
         [0023]    The hooking edge intersects the top face of the tooth at a vertex of the idle flank, this being the vertex that constitutes the point of the tooth having maximum height. Therefore, the gearshifting helping tooth has maximum height, measured with respect to the bottom of the groove adjacent to the pressure flank, at the intersection between the idle flank, the top face and the outer face. 
         [0024]    Preferably, the minimum height of the gearshifting helping tooth, measured between the bottom of the groove adjacent to the pressure flank, and the join between the top face and the pressure flank, is greater than or equal to ⅔ of the maximum height of the same tooth, measured between the bottom of the same groove and the vertex of the idle flank. This geometric relationship allows optimal functionality of the gearshifting helping tooth in terms of the transmission of torque during normal operation and the movement of the chain during gearshifting. 
         [0025]    According to a particularly effective embodiment of the sprocket, the gearshifting helping tooth also comprises a first bevel formed on the outer face, between the top face and the pressure flank. The depth of the first bevel, with respect to the outer face, increases towards the pressure flank, and is at its maximum at the join between the pressure flank and the top face. The bevel can be flat, but preferably it is defined by a concave surface. 
         [0026]    On the inner face of the gearshifting helping tooth, i.e. on the face normally tapered in the radial direction and opposite the outer face, a second bevel is formed. The second bevel extends up to the vertex formed by the intersection between the inner face, the top face and the idle flank. Preferably, the second bevel is defined by a flat triangular surface. 
         [0027]    On the taper of the inner face of the gearshifting helping tooth a third bevel is preferably formed that extends until it intersects the pressure flank. In this circumstance, the second bevel and the third bevel intersect one another and both intersect the taper of the inner face and the top face, which however do not intersect one another. 
         [0028]    Preferably, the gearshifting helping tooth is next, on the side of the idle flank, to a depressed tooth the maximum height of which is lower than the maximum height of the remaining teeth of the toothed wheel (including the gearshifting helping tooth) and the highest portion of which is proximal to the gearshifting helping tooth. 
         [0029]    According to the preferred embodiment, the toothed wheel described above is a sprocket. 
         [0030]    In another embodiment, a group of toothed wheels are used in a bicycle chain transmission system. 
         [0031]    In particular, the group of toothed wheels comprises at least one toothed wheel having a smaller diameter and at least one toothed wheel having a larger diameter, fixed with respect to each other and rotatable around a common axis, intended to be selectively engaged by a transmission chain, wherein the toothed wheels comprise, on its own periphery, a plurality of teeth arranged circumferentially and alternating with grooves at a constant pitch, each tooth having a thickness, a width and a height and having an idle flank, passive in the transmission of torque with a bicycle chain, and a pressure flank, active in the transmission of torque with the same chain, wherein the toothed wheel having a larger diameter comprises at least one hooking tooth of the transmission chain, suitable for engaging and holding a portion of the transmission chain that, during gearshifting, extends between the toothed wheel having a smaller diameter and the toothed wheel having a larger diameter in a circular sector of the group of toothed wheels, defined as gearshifting sector, and wherein in the gearshifting sector, the distance D between the center of a groove between two consecutive teeth of the toothed wheel having smaller diameter and the center of a groove between two consecutive teeth of the toothed wheel having larger diameter is within the range: 
         [0000]      0.85 ·N·P≦D≦N·P,    
         [0032]    in which N is an integer number and P is the pitch (The pitch is the distance between a center of two consecutive grooves or teeth of a toothed wheel,) of the group of toothed wheels. 
         [0033]    At least in the gearshifting sector, the pressure flanks of the teeth, i.e. the flanks intended to interact with a pin of the chain for the transmission of torque, have a portion that has, on a plane perpendicular to the axis of the toothed wheel, a circular profile. For the purposes, the center of a groove is defined as the center of curvature of the pressure flank of the tooth adjacent to the groove itself. 
         [0034]    At least one of the toothed wheels is a wheel as described below or where N=2 and the toothed wheel having larger diameter has one extra tooth compared to the toothed wheel having smaller diameter. 
         [0035]    The group of toothed wheels has several features compared to conventional solutions. First, the transmission chain is carried on the side of the wheel having larger diameter, i.e. it is moved axially for the engagement with the toothed wheel having larger diameter, in the smallest possible groove, in practice in the groove of two links of the chain. Second, the angular phase displacement between the homologous teeth of the gearshifting sector of the wheels of the group, a phase displacement that depends upon the distance D indicated above, allows the teeth of the toothed wheel having larger diameter to quickly engage the transmission chain, without the group idling. 
         [0036]    The transmission chain is formed by a succession of inner links alternating with outer links. Each outer link consists of a pair of opposite plates facing each other at distance, hinged at their ends, at the inner faces, to a pair of plates that constitute the inner links. The inner links define a gap smaller than the one defined by the outer links, but still sufficient to house a tooth of a toothed wheel. The plates that constitute the inner links are kept separated apart thanks to pins that, during operation, go into abutment on the pressure flanks of the teeth of the wheels. 
         [0037]    The group of toothed wheels is effective in both cases in which upward gearshifting occurs at an outer link of the chain, or else occurs at an inner link. “Outer link gearshifting” means upward gearshifting in which the first link to undergo a radial lifting with respect to the toothed wheel having smaller diameter, and therefore the first link to disengage the relative tooth, is an outer link. Vice-versa, by the expression “inner link gearshifting” we mean upward gearshifting in which the first link to lift from the toothed wheel having smaller diameter is an inner link of the chain. 
         [0038]    Hereafter, the characteristics of the group of toothed wheels that mainly simplify outer link upward gearshifting shall be described. 
         [0039]    At least in the gearshifting sector, the toothed wheel having smaller diameter comprises at least one gearshifting helping tooth the height of which, measured from the bottom of the groove adjacent to the pressure flank, decreases in the direction of the pressure flank itself. The height of the gearshifting helping tooth is greater than or equal to 90% the radius of the pin of the transmission chain. 
         [0040]    The gearshifting helping tooth may comprise a convex top face, which extends in the circumferential direction between an edge, or a vertex, having maximum height and an edge, or a joining surface with the pressure flank of the same tooth, having minimum height. 
         [0041]    In the circumstance in which the gearshifting helping tooth has a top surface, the minimum height is measured between the bottom of the groove adjacent to the pressure flank of the tooth itself and the intersection of the top surface with the pressure flank. Such a minimum height is in any case greater than or equal to 90% of the radius of the pin of the transmission chain. 
         [0042]    The hooking tooth also may comprise an outer face perpendicular to the axis of the toothed wheels and facing towards the toothed wheel having smaller diameter. The outer face forms, with the idle flank of the tooth, a hooking edge of the transmission chain. 
         [0043]    The hooking tooth may also comprise a first bevel formed on the outer face, between the top face and the pressure flank. The depth of the first bevel, with respect to the outer face, increases towards the pressure flank and is at its maximum at the join, or the edge, between the pressure flank and the top face. The first bevel may be defined by a concave surface. 
         [0044]    The maximum height of the gearshifting helping tooth, measured with respect to the bottom of the groove adjacent to the pressure flank of the same tooth, is located at the intersection between the idle flank and the top face. The minimum height of the gearshifting helping tooth, measured from the bottom of the aforementioned groove, is greater than or equal to ⅔ of the maximum height of the same helping tooth. 
         [0045]    The mutual angular positioning of the wheels may be selected so that, in the gearshifting sector, the hooking tooth of the toothed wheel having larger diameter is adjacent to the idle flank of the gearshifting helping tooth of the toothed wheel having smaller diameter. 
         [0046]    The hooking tooth of the toothed wheel having larger diameter may be identical to the gearshifting helping tooth of the toothed wheel having smaller diameter. 
         [0047]    The gearshifting helping tooth may also comprise an inner face, perpendicular to the axis of the toothed wheels and facing towards the toothed wheel having larger diameter. The inner face is tapered in the radial direction and has a second bevel that extends up to the idle vertex formed by the intersection between the inner face, the top face and the idle flank of the tooth. More preferably, the second bevel is defined by a flat triangular surface. 
         [0048]    Hereafter, the characteristics of the group of toothed wheels that mainly simplify inner link upward gearshifting shall be described. Such characteristics can be present on the wheels independently from, or in combination with, the characteristics that promote outer link upward gearshifting. 
         [0049]    In order to optimize inner link gearshifting, the toothed wheel having smaller diameter comprises a depressed tooth the maximum height of which is lower than the maximum height of the remaining teeth of the same toothed wheel. The tooth having lower height, i.e. the depressed tooth, allows the chain to pass from one toothed wheel to the other without undergoing excessive stretching along the way that leads to the same depressed tooth being passed over. In this way the chain goes onto the toothed wheel having larger diameter in phase to engage its teeth. 
         [0050]    The depressed tooth is beside the idle flank of the gearshifting helping tooth. The highest portion of the depressed tooth is adjacent, i.e. proximal, to the gearshifting helping tooth, so as to be able to effectively engage the chain during normal operation. 
         [0051]    On the aforementioned taper of the gearshifting helping tooth a third bevel is formed that extends until it intersects the pressure flank of the same tooth. The second bevel and the third bevel intersect one another and intersect the taper and the top face, which do not intersect one another. 
         [0052]    According to an embodiment of the group of toothed wheels, the distance D is calculated between the center of the groove adjacent to the idle flank of the hooking tooth of the toothed wheel having larger diameter and the center of the groove adjacent to the idle flank of the depressed tooth of the toothed wheel having smaller diameter. 
         [0053]    The tooth adjacent to the idle flank of the hooking tooth of the toothed wheel having larger diameter may be provided with a narrowing at the outer face, orientated perpendicular to the rotation axis and facing towards the toothed wheel having smaller diameter. 
         [0054]    According to an embodiment, the toothed wheels of the group of wheels are sprockets. 
       DESCRIPTION 
       [0055]    With reference to  FIGS. 1 ,  2  and  3 , a group of toothed wheels  1  is shown. In general, the toothed wheels of the group  1  can be front crowns or rear sprockets of a bicycle. In particular,  FIGS. 1-20  show a group  1  of sprockets  5 ,  10 . It should be understood that the characteristics described in relation to the sprockets  5 ,  10  are in principle also applicable to crowns of bicycles. 
         [0056]    The group  1  comprises at least one sprocket  5  having smaller diameter and at least one sprocket  10  having larger diameter arranged side-by-side along a common rotation axis X. In general, the group  1  can also comprise a larger number of sprockets, for example 5-7 sprockets, or even more, according to the final applications or the preferences of the user. 
         [0057]    The sprockets  5  and  10  each comprise a succession of teeth  11  and  12  arranged on the radially outer periphery, alternating with grooves  13 , for the functional engagement with a transmission chain  15  ( FIG. 7 ). In general, the number of teeth  11  of the sprocket having smaller diameter  5  is less than the number of teeth  12  of the sprocket having larger diameter  10 . Preferably, the sprocket having larger diameter  10  comprises an extra tooth compared to the sprocket having smaller diameter  5 . 
         [0058]    According to one embodiment, shown in  FIGS. 1-3 , the teeth  12  of the sprocket having larger diameter  10  have an identical configuration to corresponding teeth  11  of the sprocket having smaller diameter  5 , i.e. the “homologous” teeth of the sprocket  5 . The pitch P between the consecutive teeth  11  and  12  in the two sprockets  5  and  10  is the same, and is equal to the pitch of the transmission chain  15 . 
         [0059]    Each sprocket  5  or  10  of the group  1  comprises an upward “gearshifting sector”  18  at which, during gearshifting, the chain  15  extends between the sprocket having larger diameter  10  and the sprocket having smaller diameter  5 . In the gearshifting sector  18  there are specialized teeth A 1 -A 6 , specially shaped to simplify and promote gearshifting, i.e. to optimize the passage of the chain  15  from the sprocket having smaller diameter  5  to the sprocket having larger diameter  10 . Generally, the teeth of the sector  18  are referred to as “specialized” teeth for upward gearshifting. 
         [0060]    Conventionally, the gearshifting sector  18  of each sprocket  5 ,  10  is defined by the first specialized tooth A 1  and by the last specialized tooth A 6 . The gearshifting sector  18  of the entire group  1  of sprockets is defined by the first specialized tooth A 1  and by the last specialized tooth A 6 , whether they belong to the sprocket  5  or to the sprocket  10 .  FIGS. 1 and 3  show the limits of the gearshifting sector  18  of the group  1  of sprockets. In practice, the gearshifting sector  18  is inside a circular sector of both of the sprockets  5  and  10 . “Gearshifting sector” refers to the gearshifting sector of the group  1  of sprockets. 
         [0061]    In the gearshifting sector  18 , the crowns of teeth  11  and  12  of the sprockets  5 ,  10  comprise, in succession, the specialized teeth A 1 , A 2 , A 3 , A 4 , A 5 , A 6 .  FIGS. 1 and 3  show the form of such a series in a front view, i.e. from the side of the sprocket  5 , and rear view, i.e. from the side of the sprocket  10 . The teeth A 1 -A 6  of the sprocket  5  are identical to the teeth A 1 -A 6 , respectively, of the sprocket  10 . 
         [0062]    The phasing between the teeth A 1 -A 6  of the sprocket having a smaller diameter  5  and the teeth A 1 -A 6  of the sprocket having a larger diameter  10 , i.e. the angular phase displacement between the homologous teeth A 1 -A 1 , A 2 -A 2 , A 3 -A 3 , A 4 -A 4 , A 5 -A 5 , A 6 -A 6  of the two sprockets  5 ,  10  in the gearshifting sector  18 , must not exceed a certain limit, expressed in terms of the distance D between a groove of the sprocket  5  and a groove of the sprocket  10 . In particular, the center of a groove is defined as center of curvature O 1 , O 2 , etc. ( FIG. 1 ) of the circular portion of the pressure flank  30  of the tooth preceding that groove with respect to the direction R of rotation. The distance D between the center of curvature of a groove  20  between two consecutive teeth of the sprocket having smaller diameter  5  and the center of curvature of a groove  22  between two consecutive teeth of the sprocket having larger diameter  10 , in the gearshifting sector  18  of the group  1 , must satisfy the following relationship: 
         [0000]      0.85 ·N·P≦D≦N·P,    
         [0063]    where N is an integer number and P is the pitch of the group of sprockets  1 . Preferably 0.85·N·P≦D≦0.95 N·P. Even more preferably N=2. 
         [0064]    In the embodiment shown in  FIGS. 1-3 , the grooves  20  and  22  between which the distance D is measured are those between the teeth A 3  and A 4  of the sprocket having larger diameter  10  and the teeth A 4  and A 5  of the sprocket having smaller diameter  5 . 
         [0065]    Considering the case in which the toothed wheels  5  and  10  are crowns, the center of a groove is defined as the center of curvature  13  of the circular portion of the pressure flank of the tooth following that groove with respect to the direction of rotation. Indeed, in the crowns the pressure flank is the flank preceding the tooth, and the idle flank is the one following the tooth with respect to the direction of rotation. 
         [0066]    The distance Z (indicated in  FIG. 2 ) between the sprockets  5  and  10 , in the axial direction, is preferably less than 2.3 mm, more preferably less than 2 mm. 
         [0067]    The following description refers in detail to the shape of the teeth of a sprocket  5  or  10 . With reference to the rotation axis X and to the direction of rotation R, common to the two sprockets  5 ,  10 , each tooth  11  or  12  is defined by an inner face  26 , orientated axially and facing towards the inside of the bicycle frame (visible in  FIG. 3 ), an outer face  28 , orientated axially and facing towards the outside of the bicycle (visible in  FIG. 1 ), a following flank  30 , orientated in the circumferential direction and following the tooth during the rotation of the sprocket  5  or  10  (on the left of each tooth in  FIG. 1 ), a preceding flank  32  opposite the following flank (on the right of each tooth in  FIG. 1 ), and a top face  34  orientated in the radial direction (visible in  FIG. 2 ). 
         [0068]    The specialized teeth of each sprocket  5  or  10  that allow gearshifting are the teeth A 3  and A 4 , whereas the teeth A 1 , A 2 , A 5  and A 6  are auxiliary teeth, which cooperate with the teeth A 3  and A 4  to optimize gearshifting. In particular, the tooth A 3  of the sprocket having smaller diameter  5  is the gearshifting helping tooth; the tooth A 3  of the sprocket having larger diameter  10  is the hooking tooth of the transmission chain  15 . The tooth A 4 , in both of the sprockets  5  and  10 , is the depressed tooth. 
         [0069]    With particular reference to  FIG. 3 , the teeth A 5  and A 6 , identical to one another, are asymmetrical teeth, offset in the circumferential direction. During upward gearshifting, the teeth A 5  and A 6  of the sprocket having smaller diameter  5  allow the chain  15 , which is in a position not aligned with the sprockets  5 ,  10 , to leave the sprocket  5  without jamming. 
         [0070]    With reference to  FIGS. 4 ,  5  and  6 , which respectively show a front, plan and rear view of a helping tooth A 3  of the upward gearshifting sector  18  of the group of sprockets  1 , the teeth A 3  have maximum height H equal to the height of the other teeth  11 ,  12 , however the height of the teeth A 3  decreases in the direction of the following flank  30 . Advantageously, the height of each tooth A 3 , measured from the bottom  13  of the following groove, decreases in the direction of the following flank  30 , always being greater than or equal to 90% of the radius of the pin  52  of the chain  15 . 
         [0071]    The tooth A 3  has maximum height H at the intersection between the preceding flank  32  and the top face  34 . The top face  34  is convex and joins to the following flank  30  in a non-tangent manner, i.e. with a sharp edge or with a small radius of curvature. In particular, the substantially radial extension of the following flank  30  opposes the substantially circumferential extension of the face  34 . This characteristic allows the following flank  30  to be made with a height sufficient to support in abutment a pin  52  of the chain  15 , which during normal operation of the bicycle transmission transmits torque. 
         [0072]      FIG. 7  is a plan view of a standard chain  15 , suitable for engaging the group of sprockets  1 , which comprises inner links  44  alternating with outer links  46 . Each outer link  46  consists of a pair of interfacing plates  48  that are grooved apart, connected at their ends and on the inner faces to a pair of plates  50 . The plates  50  form the inner links  44 . The distance between centers between the links  44  is less than the distance between centers between the outer links  46 , but sufficient to house a tooth  11  and  12  of a sprocket  5 ,  10 . The plates  48  and  50  are kept distant apart by pins  52 , which during normal operation of the transmission go into abutment against the teeth  11 ,  12  pulling them into rotation or being pulled by them into rotation. The diameter d of the pins  52  is standard and can therefore be taken as reference to calculate the height of the tooth A 3 . 
         [0073]    In particular, the minimum height h of the tooth A 3  for helping gearshifting, measured between the bottom of the following groove and the edge or the join  38  between the following flank  30  and the top face  34 , is greater than or equal to 0.9·(d/2), i.e. it is greater than or equal to 90% of the radius of the pin  52 . More preferably h≧(⅔·H), i.e. the minimum height h is greater than or equal to ⅔ of the maximum height H of the tooth A 3 , which is the height of the preceding vertex  42 . 
         [0074]    Referring once again to  FIGS. 4 ,  5  and  6 , the tooth A 3  for helping gearshifting comprises an edge  35  having the function of lifting the chain  15  in the radial direction in the gearshifting step. During gearshifting, i.e. when the chain  15  is shifted with respect to the sprocket  5 , the lifting edge  35 , defined by the intersection between the outer face  28  and the preceding flank  32 , acts as a ramp for a link of the chain  15 , which is lifted towards the top face  34 . 
         [0075]    The tooth A 3  also comprises a first bevel  36 , formed on the outer face  28 , that extends between the following vertex  38 , defined by the intersection between the outer face  28 , the top face  34  and the following flank  30 . The depth of the first bevel  36  increases proceeding from the outer face  28  towards the following vertex  38 . Preferably, the first bevel  36  is defined by a concave surface. 
         [0076]    The tooth A 3  may also comprise a second bevel  40 , formed on the inner face  26 , that extends between a taper  54  and the preceding vertex  42  formed by the intersection between the inner face  26 , the top face  34  and the preceding flank  32 . The second bevel  40  may be defined by a substantially triangular flat surface. The taper  54  extends in the radial direction and positively contributes to the engagement of the chain  15  in normal operation. 
         [0077]    Further a third bevel  56 , for example triangular, may be formed on the inner face  26  between the taper  54 , the second bevel  40 , and the following flank  30 . 
         [0078]    As shown in detail in  FIGS. 5 and 6 , the bevels  40  and  56  join together before reaching the top face  34 . The bevels  40  and  56  globally thin out the most radially outer part of the tooth A 3  on the side of the inner face  26 , avoiding an excessive reduction of the central body of the tooth A 3 . Alternatively, the bevels  56  and  40  can also be made as a single bevel that is deeper at the following and preceding vertices of the tooth A 3  and less deep at the center of the tooth. The combination of these bevels increases the effectiveness of the tooth A 3  while simultaneously reducing its weight. 
         [0079]      FIGS. 8 and 9  respectively show a front view and a rear view of the depressed tooth A 4  of the upward gearshifting sector  18 . The maximum height h′ of the tooth A 4  is less than the maximum height H of the other teeth  11 ,  12 , A 1 -A 6 . The highest part of the tooth A 4  is the one closest to the following tooth A 3 . Moreover, the maximum height h′ of the depressed tooth A 4  is preferably greater than or equal to 90% of the radius of the pin  52 , i.e. 
         [0000]        h′≧ 0.9*( d/ 2), 
         [0080]    where d is the standard diameter of the pin  52  of the chain  15  ( FIG. 7 ). 
         [0081]    The following flank  30  of the depressed tooth A 4  is defined by a concave surface suitable for coupling with the pin  52  of the chain  15 . The tooth A 4  may comprise a narrowing  58  on the outer face  28 . The edge between the outer face  28  and the top face  34  is removed with a bevel  60 .  FIG. 1  shows the center of curvature O 2  of the groove between the teeth A 4  and A 5  of the sprocket having smaller diameter  5 , i.e. the center of curvature O 2  of the groove preceding the tooth A 4  of the sprocket  5 , and the center of curvature O 1  of the groove between the teeth A 3  and A 4  of the sprocket having larger diameter  10 , i.e. the center of curvature O 1  of the groove following the tooth A 4  of the sprocket  10 . 
         [0082]      FIGS. 10 ,  11  and  12  are respective front, plan and rear views of further auxiliary teeth A 1 , A 2  of the upward gearshifting sector  18  of the group of sprockets  1  of  FIG. 1 . The auxiliary teeth A 1  and A 2  are almost identical to one another and when they are located on the sprocket having larger diameter  10  they have the task of receiving the chain  15  during gearshifting. 
         [0083]    Before gearshifting is complete, the chain  15  is not yet completely parallel to the sprockets  5 ,  10 . In order to avoid harmful mechanical interference, both of the teeth A 1  and A 2  have a narrowing  62  on the inner face  26 . The top portion of the teeth A 1  and A 2  is tapered on the outer face  28 , where a bevel  64  is formed facing in the radial direction, which is substantially rectangular. On the inner face  26  there is a rhomboidal bevel  66 , facing towards the preceding top end of the tooth. The tooth A 2  comprises a further top bevel  68  that extends forwards with respect to the direction of rotation R and increases the depth of the tapering bevel  66 . 
         [0084]    In the embodiment in  FIG. 11  the depth of the bevels on the inner and outer faces  26 ,  28  is such that the top face  34  of the teeth A 1  and A 2  is moved farther towards the outer face  28 . This characteristic also promotes the engagement of the chain  15  when, during gearshifting, it is not yet perfectly aligned with the arrival sprocket  10 . 
         [0085]    The special shape of the teeth A 1  and A 2 , described above and shown in the FIGS., facilitates the engagement of the chain  15  by the sprocket  10  and also reduces sprocket weight. 
         [0086]      FIGS. 13 ,  14  and  15  illustrate the operation of the group of sprockets  1 , during outer link gearshifting. The outer link  46   a  is the first to disengage the sprocket having a smaller diameter  5 . The inner link  44   a  passes over the gearshifting helping tooth A 3  without interfering with it; the tooth A 3  on the other hand provides a support for the outer link  46   b , which is thus already situated at the height of the sprocket having larger diameter  10 . 
         [0087]      FIG. 16  shows in detail, and with an enlarged scale with respect to  FIG. 14 , the inner link  44   a  at the moment when it is located exactly above the gearshifting helping tooth A 3  of the sprocket having smaller diameter  5 . The plate  48  of the outer link  46   a  is hooked by the edge  35  of the tooth A 3  of the sprocket having larger diameter  10  (the edge  35  goes into abutment against the link  46   a , pulling it). Thanks to this hooking effect, the chain  15  is immediately pulled towards the sprocket having larger diameter  10 . 
         [0088]    Going back to  FIG. 15 , it can be seen that the preceding flank  32  of the tooth A 3  of the sprocket having larger diameter  10 , and in particular its lifting edge  35 , provides a support for the outer link  46   a  and causes it to be lifted from the sprocket having smaller diameter  5 , in the radial direction. A plate  50  of the inner link  44   a  moves at the side of the tooth A 3  of the sprocket  10  without interfering with it, thanks to the presence of the concave bevel  36 . 
         [0089]      FIGS. 17 ,  18  and  19  illustrate the operation of the group of sprockets  1 , during inner link gearshifting. The first link of the chain  15  that disengages the sprocket  5  is the inner link  44   c , which passes over the depressed tooth A 4 , resting upon it. 
         [0090]    The action of the tooth A 3  of the sprocket  10  is illustrated in detail and in enlarged scale in  FIG. 20 . The hooking edge  35  goes into abutment against the plate  50  of the inner link  44   c . The preceding flank  32  of the tooth A 3  of the sprocket  10  provides a support for the inner link  44   c  of the chain  15  and causes it to be lifted from the sprocket  5 , in the radial direction (not shown). 
         [0091]    The plates  48  of the outer link  46   d  do not interfere with the teeth A 3  of the two sprockets  5  and  10  thanks to the presence of the triangular bevels  40 . The presence of the bevel  56  further contributes to preventing interference. 
         [0092]    In the group of sprockets  1 , both in the case of inner link gearshifting and in the case of outer link gearshifting, the edge  35  of the tooth A 3  acts as hooking edge of the pin  52  and the preceding flank  32  acts as support surface for lifting the links. Alternatively, the perimeter edge of the outer face  28  of the tooth A 3  can be configured as a hooking edge. 
         [0093]    Preferably the sprockets  5  and  10  are identical in the shape of the teeth  11 ,  12 . In this way, both of the sprockets  5  and  10  can act as a sprocket having smaller diameter or as a sprocket having larger diameter in a group containing more than two sprockets. It shall be clear to the person skilled in the art that, in this circumstance, the sprocket with minimum diameter of a group of sprockets  1  can lack the characteristics to be used for downward gearshifting and the sprocket with maximum diameter can lack the characteristics to be used for upward gearshifting. Moreover, wishing to allow the group  1  exclusively inner link gearshifting, the relative sprockets  5 ,  10 , etc. can lack the characteristics to be used for outer link gearshifting, and vice-versa. 
         [0094]    The sprocket and the group of sprockets  1  allow quick and precise gearshifting, with minimum risk of interruption in the transmission of torque to the rear wheel of the bicycle. The portion of chain  15  that is moved onto the sprocket having larger diameter  10  is immediately in phase with the teeth A 1 -A 6  of such a sprocket and can immediately transmit torque without idling of the group  1 .