Bicycle chain sprocket

A sprocket for meshing with a bicycle chain, comprises a central aperture having a multi-lobed profile, which is defined by notches which extend up to in proximity of the outer circumferencial edge, starting from an inner circular edge.

BACKGROUND OF THE INVENTION 
The present invention relates to a sprocket to be fixed to the hub of a 
bicycle rear wheel, for meshing with the bicycle chain, of the type 
comprising a flat annular body having an outer crown for meshing with the 
chain, and an inner circular edge including teeth projecting radially 
inwardly, for coupling of the sprocket on a support fluted body, typically 
the body of a free wheel associated with the hub of the bicycle rear 
wheel. 
A sprocket of the above indicated type and made according to the prior art 
is shown in FIG. 1 of the annexed drawings. With reference to this figure, 
numeral 1 generally designates the sprocket, comprising a flat body, 
having an outer toothed crown 2, including a plurality of teeth 3 for 
meshing with the bicycle chain, and an inner circular edge 4 provided with 
a plurality (typically 8) of teeth 5 which are equiangularly spaced from 
each other, for coupling with the fluted body of the free wheel associated 
with the hub of a bicycle rear wheel. The body of sprocket 1 further has 
apertures 6 for reducing weight. 
SUMMARY OF THE INVENTION 
The object of the present invention is that of providing a sprocket of the 
above indicated type, adapted particularly to be used on race bicycles, 
which has a very reduced weight with respect to that of conventional 
sprockets, while insuring the requested strength characteristics, with the 
same material (e.g. steel or light alloy). 
In view of achieving this object, the invention provides a sprocket of the 
type indicated at the beginning of the present description, characterized 
in that the said inner circular edge of the body of the sprocket is 
interrupted, between each inner tooth and the adjacent one, by a notch 
which extends into the body of the sprocket up to near the outer edge 
thereof, so that the central aperture of the sprocket has a multi-lobed 
profile.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 2 of the annexed drawings shows by way of non limiting example a 
preferred embodiment of the sprocket according to the invention. As shown 
in this figure, differently from the conventional case shown in FIG. 1, 
the sprocket of the invention has a halved number of inner teeth 5 (parts 
in FIG. 2 corresponding to those in FIG. 1 are designated by the same 
reference numeral). However, the teeth 5 of the sprocket according to the 
invention shown in FIG. 2 have a height greater than teeth 5 of the known 
sprocket shown in FIG. 1, in order to provide the requested strength 
characteristics, notwithstanding the use of a halved number of teeth. 
As clearly shown in FIG. 2, the four inner teeth 5 are equiangularly spaced 
from each other, i.e. they are arranged at right angles relative to each 
other and between each tooth 5 and the adjacent one the inner circular 
edge 4 is interrupted by a notch 7 which extends radially into the body of 
sprocket 1 up to in proximity of the outer edge 2, this notch comprising a 
bottom portion 7a and two side portions 7b. The bottom portions 7a of the 
four notches 7 lie on a same circumference which is concentric with the 
outer edge 2 of the sprocket and substantially close thereto so as to 
define an outer annular strip 8. 
As already indicated above, because of the presence of notches 7, the 
central aperture of the sprocket takes a multi-lobed profile (a four lobe 
profile or four-leaved clover profile in the case of the illustrated 
example). 
Studies and tests conducted by the Applicant have brought to defining for 
each notch 7 the asymmetrical shape which is illustrated in FIG. 2. This 
shape has indeed been found optimal to provide on one hand the maximum 
possible weight reduction of the sprocket and on the other hand the 
required strength for the sprocket body. Due to the presence of notches 7 
and their asymmetrical form, also the apertures 6 (which are in a halved 
number with respect to that of the apertures of the known sprocket shown 
in FIG. 1) take an asymmetrical profile. 
The direction of rotation of the sprocket in use is that indicated by arrow 
A in FIG. 2. 
As clearly apparent from the foregoing description, the invention provides 
on one hand a substantial reduction of weight of the sprocket with respect 
to the sprockets made up to now, which renders the sprocket according to 
the invention particularly adapted to be used on race bicycles. At the 
same time, the required strength characteristics of the sprocket are 
insured. 
Naturally, while the principle of the invention remains the same, the 
details of construction and the embodiments may widely vary with respect 
to what has been described and illustrated purely by way of example, 
without departing from the scope of the present invention.