Crank assembly for a bicycle bottom bracket assembly, shaft and crank arm thereof

A crank assembly for a bicycle's bottom bracket assembly comprises a shaft extending along a longitudinal axis, at least one crank arm associated with at least one free end of said shaft, first front coupling means formed on said shaft and second front coupling means formed on said at least one crank arm, said first and second front coupling means being coupled together. In particular, the aforementioned first and second front coupling means comprise respective front toothings. The front coupling is located in an end zone, or on the outside, of a bottom bracket assembly's shaft's housing box formed in the bicycle's frame. The front coupling introduces a structural discontinuity between crank arm and shaft, for which reason the bending load exerted on the crank arm during pedaling is transmitted to the shaft to a small degree. Such a structural discontinuity is located at the shaft/crank arm interface.

FIELD OF INVENTION

The present invention relates to a crank assembly for a bicycle bottom bracket assembly, as well as a shaft and a crank arm for the crank assembly. The invention also relates to a bicycle bottom bracket assembly and bicycle comprising the aforementioned crank assembly.

BACKGROUND

As known, the bottom bracket assembly of a bicycle comprises a shaft and two crank arms associated with the opposite ends of the shaft. The shaft can be made in a separate piece from the crank arms or it can be made in a single piece with one of the two crank arms. The shaft can also consist of two separate pieces (known as half-shafts) that can be coupled together, each of the two half-shafts being made in a single piece with a respective crank arm.

The component of the bicycle bottom bracket assembly consisting of a crank arm and a shaft (or a half-shaft) coupled together or made in a single piece is identified with the expression: crank assembly.

The bottom bracket assembly is mounted on the bicycle housing the shaft in a housing box suitably provided in the bicycle frame. The rotation of the bottom bracket assembly with respect to the bicycle frame is achieved by fitting a pair of rolling bearings onto the shaft. Each bearing is positioned on the shaft at a respective shaft body portion adjacent to the crank arm and, when the bottom bracket assembly is mounted on the bicycle frame, is operatively arranged between the shaft and the housing box provided in the frame.

Due to the load applied by the cyclist on the pedals during pedaling, both the shaft and the crank arms of the bicycle bottom bracket assembly are subjected to high bending stresses. Such stresses, if not suitably counteracted, cause the shaft and/or the crank arms to bend. Such a deformation means a reduction in the efficiency of pedaling and can lead to the shaft and/or the crank arms breaking.

An ever-present demand of bicycle component manufacturers, above all for racing bicycles, is therefore to minimize the bending deformations of the shafts and of the crank arms of the bottom bracket assemblies of bicycles, so as not to alter the efficiency of pedaling and to avoid the risk of components breaking.

Conventionally, the breaking of the shaft and, above all, of the crank arms takes place precisely at the shaft-crank arm coupling zone. Such a zone is therefore a critical zone.

SUMMARY

The present invention relates, in a first aspect thereof, to a crank assembly for a bicycle bottom bracket assembly, comprising:

a shaft extending along a longitudinal axis X-X;

at least one crank arm associated with at least one free end of said shaft;

coupling means between said shaft and said at least one crank arm;

wherein said coupling means comprise first front coupling means formed on said at least one free end of said shaft and second front coupling means formed on said at least one crank arm, said first and second front coupling means being coupled together.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Introduction

Throughout the present description and the subsequent claims, the expression “crank assembly” is used to indicate a component of the bicycle bottom bracket assembly obtained by coupling a crank arm with an end of the rotation shaft of the bottom bracket assembly.

The shaft can be made in a single piece with the other crank arm or in a piece separate from the other crank arm. The shaft can also be made in two separate pieces (for the sake of simplicity known as shaft elements) that can be coupled together.

In the case of a shaft made in a single piece with the other crank arm, the bottom bracket assembly is obtained by the assembly of the crank assembly of the present invention. In the case of a shaft made in a piece separate from the other crank arm, the bicycle bottom bracket assembly is obtained by the coupling of the crank assembly of the present invention with another crank arm. On the other hand, in the case of a shaft made in two pieces, the bottom bracket assembly is obtained by the coupling of two crank assemblies of the present invention or of a crank assembly of the present invention with a crank assembly wherein the shaft is made in a single piece with the other crank arm.

Throughout the present description and the subsequent claims, unless explicitly indicated otherwise, we shall refer, for the sake of simplicity, to a shaft made in a single piece. What is said does nevertheless have analogous application to the case in which the shaft is made in two separate pieces.

In accordance with the present invention, the front coupling between shaft and crank arm advantageously introduces a structural discontinuity in the propagation of the bending stress from the crank to the pedal shaft, for which reason the bending load exerted on the crank arm during pedaling is transmitted to the shaft to a much lesser degree. Even more advantageously, such a structural discontinuity is localized at the interface between shaft and crank arm, i.e. in close proximity to the zone of the crank arm at which the bending load is applied. The propagation of the bending stress is therefore interrupted at a section in which it is still minimal, given that in that section the arm of the bending torque is very small, practically zero.

The Applicant has therefore found out that it is possible to substantially reduce, if not actually almost entirely cancel out, the deformations of the shaft and of the crank arms of the bicycle bottom bracket assembly, by introducing a structural discontinuity in the propagation of the bending stress from the crank arm to the shaft in a section as close as possible to the zone of application of the bending load on the crank arm. The Applicant has thus made a solution wherein the coupling means are front coupling means and such front coupling means are provided directly at the crank arm-shaft interface, i.e. at the coupling surface of the crank arm with the shaft.

Preferably, said first front coupling means comprise first rotation-driving means and said second front coupling means comprise second rotation-driving means.

More preferably, said first rotation-driving means comprise a first front toothing extending circumferentially on a front end surface of said shaft and said second rotation-driving means comprise a second front toothing extending circumferentially on a front end surface of a coupling portion of said crank arm with said shaft.

In alternative solutions, the rotation-driving means comprise, for example, pins formed eccentrically in the front walls of the crank arms and inserted into corresponding holes made in the front walls of the shafts, or vice-versa, or a combination of pins and holes both on the front wall of the crank arm and on the front wall of the shaft.

In the preferred embodiment thereof, the crank assembly of the present invention also comprises centering means of said at least one crank arm with respect to said shaft. Such centering means advantageously allow the precise alignment between the longitudinal axis of the shaft and the axis of the corresponding coupling portion of the crank arm.

Preferably, as shall become clear in the rest of the present description, said centering means are defined by said first and second front toothings.

In the preferred embodiment of the crank assembly of the present invention, said first and second front toothings comprise a plurality of identical and symmetrical teeth, each tooth of said plurality of teeth comprising a pair of side surfaces inclined, along a direction parallel to said longitudinal axis X-X, so as to form an angle α of predetermined value between them. Advantageously, the inclination of the side surfaces of the teeth along the axial direction of the shaft allows a shaft-crank arm coupling to be achieved with high precision. Indeed, at each interface between tooth of the shaft and tooth of the crank arm, a sliding plane is provided that allows microdisplacements of the crank arm with respect to the shaft in the bending direction. Such microdisplacements reduce the transmission of the bending stress between the two components. Given that the sliding planes are positioned circumferentially with respect to the longitudinal axis of the shaft and to the axis of the corresponding coupling portion of the crank arm, the aforementioned microdisplacements also occur during the rotation of the bottom bracket assembly.

Preferably, said angle α has a value of between 14° and 40°. Advantageously, the Applicant has found that within such a range of values the axial thrusts between shaft and crank arm are reduced to the point of allowing the use of a locking screw (necessary to axially lock the crank arm to the shaft) of small size, i.e. such as to be able to allow the screw to be housed inside the shaft. This allows the use of a shaft having a central portion of greater diameter with respect to the end portions. Moreover, the housing of the screw inside the shaft makes the screwing and unscrewing operations of such a screw quicker and easier even when the bottom bracket assembly is associated with the bicycle frame. Such a screw, indeed, can be immediately reached from the outside with a suitable tool without having to dismount the crank assembly from the frame.

In particular, the Applicant has found that a particularly preferred value of the angle α is 27°.

In the preferred embodiment of the crank assembly of the present invention, said side surfaces of said tooth are also inclined with respect to a radial plane containing said longitudinal axis X-X, so as to form an angle β of predetermined value between them. Advantageously, the inclination of the side surfaces of the teeth along the radial direction perpendicular to the longitudinal axis of the shaft and to the axis of the corresponding coupling portion of the crank arm ensures the centering of the shaft-crank arm coupling.

Preferably, said angle β has a value equal to the standard value for a Hirth toothing.

Preferably, to favor the aforementioned microdisplacements the contact between teeth of the shaft and teeth of the crank arm occurs solely between the side surfaces of such teeth. However, a solution is foreseen wherein the contact also, or solely, occurs at the surface of the throats defined between the teeth of the respective toothings; in such a solution the surface of the throats is inclined in the axial and radial direction in a similar way to what has been stated above with reference to the teeth of the shaft and of the crank arm.

Preferably, each throat has a curved base surface joined to the side surfaces of said teeth.

In accordance with a preferred embodiment of the crank assembly of the present invention, such an assembly comprises at least one bearing adapted to allow the rotation of the bottom bracket assembly with respect to a suitable housing box formed in the bicycle frame. The bearing is associated with said at least one crank arm at a seat formed on an outer surface of the coupling portion with the shaft.

Advantageously, the arrangement of a seat for the bearing directly on the crank arm allows the transmission of the bending stress from the crank arm directly to the bicycle frame, i.e. without engaging the shaft. The latter is therefore free from any bending stresses. More advantageously, the housing of the bearing on the crank arm allows the arm of the bending torque acting on the shaft and on the crank arm itself to be substantially reduced, thus reducing the risk of breaking such components and at the same time improving the efficiency of pedaling. Even more advantageously, in this way it is possible to house the bearing outside of the housing box of the bottom bracket assembly suitably provided in the bicycle frame. This allows the use of shafts having a diameter greater than that of the shafts conventionally used; the shaft of the present invention thus has a bending and twisting resistance greater than that of the shafts conventionally used.

In a first preferred embodiment of the bottom bracket assembly of the present invention, said seat is defined by an outer surface of said coupling portion of said crank arm.

In a second preferred embodiment of the crank assembly of the present invention, said at least one crank arm comprises a substantially annular crank arm body portion that is radially outside of said coupling portion and wherein said seat is defined by an inner surface of said substantially annular crank arm body portion.

The crank arm of the crank assembly of the present invention can be a right crank arm or a left crank arm.

In a particularly preferred embodiment of the crank assembly of the present invention, said shaft comprises a central portion and opposite end portions, wherein said central portion has a diameter greater than that of said opposite end portions. Such a shaft advantageously has a structural bending and a twisting resistance greater than that of a shaft having cross sections with constant diameter.

Preferably, said shaft is made in a single piece. However, an alternative solution is foreseen wherein said shaft is made in two separate pieces coupled together through coupling means, preferably through front toothings of the type described above. A further alternative solution is also foreseen wherein said shaft is made integrally with the other crank arm of the bicycle bottom bracket assembly.

In a second aspect thereof, the present invention relates to a bottom bracket assembly that comprises a crank assembly of the type described above.

Throughout the present description and the subsequent claims, the expression “bottom bracket assembly”, is used to indicate a component of the bicycle bottom bracket assembly obtained by coupling a crank assembly as defined above with another crank arm.

Preferably, the crank arm of the crank assembly is a right or left crank arm and also the other crank arm of the bottom bracket assembly is coupled with the shaft through front coupling means of the type described above. The shaft, in this case, is thus made in a single piece separate from both of the crank arms.

Alternatively, the other crank arm can be coupled with the shaft through a threaded coupling.

In a further alternative embodiment of the aforementioned bottom bracket assembly, the shaft is made in a single piece with the other crank arm.

In the case in which the shaft is made in a single piece separate from the other crank arm, the bottom bracket assembly thus comprises two crank arms and a shaft as described above. On the other hand, in the case in which the shaft is made in two pieces (shaft elements) separate from the respective crank arms, the bottom bracket assembly comprises two crank arms and two shaft elements as described above, i.e. two crank assemblies of the present invention. If the two shaft elements are of the same length, the two crank assemblies shall be perfectly identical (apart from the different shape of the right crank arm from the left crank arm). However, a solution is foreseen wherein the two shaft elements are of different lengths.

In a third aspect thereof, the present invention relates to a shaft for a bicycle bottom bracket assembly, comprising a body extending along a longitudinal axis X-X, said body comprising, at least one free end thereof, coupling means with at least one crank arm, wherein said coupling means comprise front coupling means.

Such a shaft can advantageously be used to manufacture the crank assembly discussed above and, therefore, has all of the aforementioned advantageous characteristics with reference to the shaft of the crank assembly of the present invention.

Preferably, such a shaft has one or more of the structural characteristics discussed above as preferred characteristics of the shaft of the crank assembly of the present invention.

In a fourth aspect thereof, the present invention relates to a bicycle crank arm, comprising a body having a coupling portion with a shaft of a bicycle bottom bracket assembly, said coupling portion extending along an axis Y-Y and comprising, at a free end thereof, coupling means with said shaft, wherein said coupling means comprise front coupling means.

Such a crank arm can advantageously be used to manufacture the crank assembly discussed above and, therefore, has all of the aforementioned advantageous characteristics with reference to the crank arm of the crank assembly of the present invention.

Preferably, such a crank arm also has one or more of the structural characteristics discussed above as preferred characteristics of the crank arm of the crank assembly of the present invention.

In a fifth aspect thereof, the present invention relates to a bicycle, comprising a frame provided with a housing box of a shaft of a bottom bracket assembly, said shaft extending along a longitudinal axis X-X, said bottom bracket assembly also comprising a pair of crank arms, at least one crank arm of said pair of crank arms being associated with at least one free end of said shaft, and coupling means between said shaft and said at least one crank arm;

wherein said coupling means comprise first front coupling means formed on said shaft and second front coupling means formed on said at least one crank arm, said first and second front coupling means being coupled together.

Advantageously, such a bicycle comprises the crank assembly of the present invention and therefore has all of the aforementioned advantageous characteristics.

Preferably, the crank assembly used in the bicycle discussed above has one or more of the preferred structural characteristics discussed above with reference to the crank assembly of the present invention.

In a preferred embodiment of the bicycle of the present invention, said first and second front coupling means are arranged at an end zone of said box.

In a particularly preferred embodiment of the bicycle of the present invention, said first and second front coupling means are arranged outside said box.

In the particularly preferred embodiment thereof, the bicycle of the present invention also comprises a pair of bearings adapted to allow said bottom bracket assembly to rotate in said box, wherein at least one bearing of said pair of bearings is arranged outside said box and at a seat defined on said at least one crank arm of said pair of crank arms.

DESCRIPTION

In the attached figures, a bottom bracket assembly of a bicycle is indicated with1. Such a bicycle is indicated with100inFIG. 12. The bottom bracket assembly1is mounted on the frame101of the bicycle100, in a suitable housing box210.

The bottom bracket assembly1comprises a crank assembly10in accordance with the present invention. The bottom bracket assembly1in turn comprises a shaft20, extending along a longitudinal axis X-X and having opposite free coupling ends20a,20b, and a pair of crank arms30,40coupled with said free ends20a,20bof the shaft20.

FIGS. 1,2,6-8, and10show various embodiments of the crank assembly10, whereasFIGS. 5 and 9show just the crank arms30of the aforementioned assemblies.

The crank arm30is a right crank arm (of the branched type, illustrated better inFIGS. 5 and 9) and is coupled with the free end20aof the shaft20, whereas the crank arm40is a left crank arm and is coupled with the free end20bof the shaft20.

The shaft20, in particular, comprises a tubular body21made in a single piece. The shaft20is housed inside the housing box210suitably provided in the frame101of the bicycle100. The shaft20extends longitudinally for a length substantially equal to the length of the housing box210.

The shaft20, at the free end20athereof, has a front surface22having, at its center, a threaded hole23. Similarly, the shaft20, at the free end20bthereof, has a front surface24having, at its center, a threaded hole25.

In the embodiment illustrated inFIGS. 1,2,5-7, the crank arm30comprises a body31having an annular coupling portion32projecting cantilevered from the rest of the body31. The coupling portion32of the crank arm30comprises substantially cylindrical radially outer and inner surfaces extending along an axis Y-Y and has an axially outer front coupling surface33with the free end20aof the shaft20(see in particularFIG. 5) and an axially inner front surface330. Similarly, the crank arm40comprises a body41having an annular coupling portion42projecting cantilevered from the rest of the body41. The body coupling portion42of the crank arm40comprises substantially cylindrical radially outer and inner surfaces extending along an axis Y-Y and has an axially outer front coupling surface43with the free end20bof the shaft20and an axially inner front surface430.

At the coupling portion32, the body31of the crank arm30has a through hole34that extends up to the front coupling surface33with the shaft20. Similarly, at the coupling portion42, the body41of the crank arm40has a through hole44that extends up to the front coupling surface43with the shaft20.

The front surface33of the crank arm30faces the front surface22of the shaft20, with the through hole34coaxial to the threaded hole23of the shaft20, whereas the front surface43of the crank arm40faces the front surface24of the shaft20, with the through hole44coaxial to the threaded hole25of the shaft20. The axes Y-Y of the crank arms30,40therefore, once they are coupled with the shaft20, are aligned with the longitudinal axis X-X of the latter.

The bottom bracket assembly1is supported in rotation within the box210by a pair of ball bearings, respectively indicated with35and45. Such bearings35,45are associated with the crank arms30and40and are positioned outside the housing box210of the shaft20, thanks to the use of suitable adapters36,46described below.

In particular, the bearing35is active between an annular adapter36, screwed into a free end210aof the box210, and the crank arm30. Similarly, the bearing45is active between an annular adapter46, screwed into the free end210bof the box210, and the crank arm40.

As shown inFIGS. 1,2,5,6,7and10(see specificallyFIG. 2), the adapters36and46comprises a ring nut having a cylindrical body portion36a,46a, externally threaded near36b,46b, intended to be screwed onto an inner threading210c,210dmade at the end of the box210of the frame101and an opposite cylindrical body portion36c,46chaving a smooth surface that is intended to operate in radial abutment against a race ring35a,45aof the bearing35,45.

In a first embodiment of the crank arms, illustrated inFIGS. 1,2,5,6and7(see specificallyFIG. 2), the bearings35,45are fitted, respectively, on the outer surface32a,42aof the coupling portions32,42of the crank arms30,40. Such outer surfaces32a,42athus define housing seats for the inner race ring35b,45bof the bearings35,45, whereas the outer race ring35a,45athereof operates in radial abutment against a smooth inner surface of the adapters36,46.

In an alternative embodiment of the crank arms, illustrated inFIGS. 8 and 9with particular reference to a right crank arm30, the body31of the crank arm30has a substantially annular portion37radially outside the coupling portion32. In this embodiment, the bearings35operate between an inner surface37aof said annular body portion37of the crank arm30and a smooth outer surface36dof the adapter36. The inner surface37aof the annular body portion37of the crank arm30thus defines a radial abutment surface for the outer race ring35aof the bearing35, whereas the smooth outer surface36dof the adapter36defines a radial abutment seat for the inner race ring35bof the bearing35. The left crank arm of the bottom bracket assembly that engages the right crank arm described above is preferably identical, with reference to the housing portion of the respective bearing, to such a right crank arm.

In a further embodiment illustrated inFIG. 13, the bearings35,45are fitted on the opposite end portions20a,20bof a shaft220. In such a case the shaft220is longer than the housing box210provided in the frame101of the bicycle100by a length at least equal to twice the length of the race rings of the bearings. In such a case the coupling portion32,42of the crank arms30,40may not be present, and the shaft220is separated by the crank arms30,40by a washer39a,49a.

With reference toFIGS. 1,2,7,8, the crank arm30is coupled with the free end20aof the shaft20through a screw38inserted into the through hole34of the crank arm30and screwed into the threaded hole23of the shaft20. Similarly, the crank arm40is coupled with the free end20bof the shaft20through a screw48inserted into the through hole44of the crank arm40and screwed into the threaded hole25of the shaft20.

The screws38,48comprise respective annular flanges39,49that when fully threaded, abut against the surfaces330,430of the coupling portions32,42of the crank arms30,40.

The crank arms30,40are also coupled with the opposite free ends20a,20bof the shaft20through respective front toothings. In particular, a front toothing50or rotation-driving means is formed on the front surface33of the crank arm30and on the front surface43of the crank arms40, whereas a front toothing or rotation-driving means60matching the toothing50is formed on the outer faces of the opposite front surfaces22and24of the shaft20. The front toothing50extends circumferentially on the front surfaces33,43of the crank arms30,40, whereas the front toothing60extends circumferentially on the opposite front surfaces22,24of the shaft20. The front toothings50and60, coupling together, make the shaft20and the crank arms30and40integral in rotation.FIGS. 2-4and6shows the toothings or coupling means50and60coupled with each other.FIGS. 5 and 11, on the other hand, clearly show the toothing50of the crank arm30.

The front toothings50and60preferably comprise a plurality of identical and symmetrical teeth. As shown in particular inFIG. 3, the teeth of the front toothings50of the crank arms30and40are indicated with51, whereas the teeth of the front toothings60of the shaft20are indicated with61. Each tooth51of the front toothing50comprises a top surface52and two side surfaces53. The teeth51are spaced apart by throats54. Similarly, each tooth61of the front toothing60comprises a top surface62and two side surfaces63. The teeth61are spaced apart by throats64.

Each throat54,64is defined between two adjacent teeth51,61and has a curved base surface joined to the side surfaces53,63of said teeth.

The side surfaces53and63of the teeth51and61are inclined, along a direction parallel to the longitudinal axis X-X of the shaft20and to the axis Y-Y of the crank arms30and40, so as to form an angle α between them having a value preferably comprised between 14° and 40°, more preferably equal to 27°.

As shown inFIG. 4, the side surfaces53and63of the teeth51and61are also inclined with respect to a radial plane containing said longitudinal axis X-X of the shaft20and said axis Y-Y of the crank arms30and40, so as to form an angle β between them having a value preferably equal to the standard value of a Hirth toothing.

FIG. 11shows the front toothing50of the crank arm30in detail. The toothing60of the shaft20is preferably identical, as well as the toothing50of the crank arm40in the case in which it is coupled with the shaft in a way similar to the way of the crank arm30. The front toothing50comprises in particular twenty teeth51alternating with twenty throats54; the number of teeth and of throats can be different, but it is preferably even in number.

It can be seen how the top surfaces of each tooth51lie on an ideal cone with vertex in the point P1on the axis Y-Y. The bottom surfaces of each throat54also lie on an ideal cone, but with opposite concavity to the previous one, and with the vertex in the point P2, again on the axis Y-Y.

Basically, the teeth51and61are wedge-shaped tapered in the axial direction with an angle α and in the radial direction with an angle β. As a consequence of such geometry of the teeth, a coupling with toothing of the type described above provides, in general, the contact just between the side surfaces of the teeth53. The coupling of all of the wedges of the front toothing50with those of the front toothing60ensures the centering of the shaft-crank arm coupling.

As already stated, the contact between the teeth61of the shaft20and the teeth51of the crank arms30and40occurs solely between the side surfaces53,63of such teeth. However,FIG. 3Ashows a solution wherein the throats254and64of the toothings50and60are inclined in the axial and radial direction in a similar way to what has been stated above with reference to the teeth of the shaft and of the crank arms. In such a case, the contact between the toothings50and60can also, or solely occur at the curved base surfaces of such throats as in best seen in the change with respect to the throat254. Each tooth261of the front toothing60has two side surfaces63and is apart by throats64.

In the case in which the coupling portion32,42of the crank arms30,40is not present (this case has been discussed above with reference to the solution wherein the bearings35,45are fitted on the opposite ends of the shaft20), the front toothing50is made directly on a face of the body of the crank arms30,40not projecting from the rest of the body of the crank arms itself (see for exampleFIG. 9).

In any case, the front toothing50of the crank arms30,40is made with respect to the body of the crank arms themselves in such a way that, when the bottom bracket assembly is associated with the frame101of the bicycle100, such a toothing is coupled with that of the shaft20in an end zone210a,210bof the housing box210provided in the frame101, or outside of the housing box210, and the bearings35,45are also positioned in the aforementioned end zone or outside of the aforementioned box210.

In a preferred embodiment of the bottom bracket assembly of the present invention, discussed above with reference toFIGS. from 1 to 5, the crank arms30and40are coupled with the shaft20in an identical way.

FIG. 6shows an alternative embodiment of the bottom bracket assembly1of the present invention. Such an embodiment differs from the one described with reference toFIGS. 1-5for the sole reason that the left crank arm40is associated with the shaft20through a threaded coupling70. Such a threaded coupling70comprises an outer threading71made on the end portion20bof the shaft20and an inner threading72, matching the threading71, made in the hole44of the crank arm40. In such an embodiment, the shaft20is of a length such that its end portion20bprojects from the box210of the frame101. The bearing45is fitted onto such a shaft portion projecting from the box210.

In the embodiments of the bottom bracket assembly described above, the shaft20has a constant diameter along its longitudinal extension.

FIG. 7shows a further embodiment of the bottom bracket assembly of the present invention. Such an embodiment differs from the one described previously with reference toFIGS. from 1 to 5for the sole reason that the shaft20has a central portion20chaving a diameter greater than that of said opposite end portions20a,20b.

What has been stated above remains valid in the case in which the shaft20is made in two separate pieces (shaft elements). Such two pieces of shaft shall each be coupled with a crank arm in the way described above and shall also be coupled together, preferably, through front toothings of the type described above.

FIG. 10shows a further alternative embodiment of the bottom bracket assembly of the present invention. Such an embodiment differs from the one described previously with reference toFIGS. 1-5for the sole reason that the left crank arm40is made in a single piece with the shaft20. In such a case, to avoid transferring to the shaft20of the bending load to which the crank arm40is subjected during pedaling, a crank arm of the type illustrated inFIGS. 8 and 9is used, where, however, the front toothing50is absent since the body41of the crank arm40is made integrally with the body21of the shaft20. In particular, the left bearing35operates between the inner surface37aof the annular portion37of the body41of the crank arm40and a smooth outer surface36dof the left adapter36. Such an adapter36is screwed to the end210bof the housing box210provided in the frame101and projects cantilevered from this end210b. The inner surface37aof the annular body portion37of the crank arm40thus defines a radial abutment surface for the outer race ring35aof the left bearing35, whereas the smooth outer surface36dof the adapter36defines a radial abutment seat for the inner race ring35bof the bearing35.