Multi-speed sprocket assembly for a bicycle

A multi-speed sprocket assembly comprising at least one small diameter sprocket and at least one larger diameter sprocket. The larger diameter sprocket has at least two adjacent teeth omitted from a plurality of teeth, and a recess is formed between teeth adjacent to the omitted teeth and is directed radially inwardly of the larger diameter sprocket from the dedendum circle thereof, and a projection is formed at an intermediate portion of the recess corresponding to the bottom land between the omitted teeth and extending close to the dedendum.

FIELD OF THE INVENTION 
This invention relates to a multi-speed sprocket assembly for a bicycle, 
and more particularly to a multi-speed sprocket assembly comprising at 
least two sprockets having different diameters and connectors for 
connecting the sprockets at a given axial interval. 
One of the sprockets is a smaller diameter sprocket having a lesser number 
of teeth than that of the other, the other sprocket being a larger 
diameter sprocket having a larger number of teeth, and they are mounted 
together on a crank or rear wheel hub on the bicycle so that a driving 
chain is switched to a selected sprocket for changing the bicycle speed. 
BACKGROUND OF THE INVENTION 
Heretofore, such a multi-speed sprocket assembly comprising two or more 
sprockets has been well known. A derailleur separate from the sprocket 
assembly is operated to move a driving chain axially of the sprockets and 
switch it to a desired sprocket, thereby transmitting a pedalling driving 
force to a rear wheel hub at the predetermined speed-change ratio. 
In order to increase the speed-change ratio, it is enough to increase the 
difference between the numbers of teeth at the smaller diameter sprocket 
and the larger diameter sprocket, of the multi-speed sprocket assembly. 
However, a difference of three or more teeth makes a radius of the 
addendum circle of the smaller diameter sprocket smaller than a radius of 
the dedendum circle of the larger diameter sprocket, so that the radial 
length between the addendum circles of both the sprockets becomes too 
large making it difficult to switch the chain quickly and smoothly to the 
desired sprocket. In other words, the driving chain, when switched from 
the smaller diameter sprocket to the larger diameter sprocket, contacts, 
prior to engagement with one tooth thereof, a tooth adjacent to and ahead 
of the one tooth in the direction of normal rotation of the sprocket. 
Hence, it is difficult to smoothly shift the chain axially outwardly of 
the larger diameter sprocket for ensuring engagement therewith, resulting 
in idling of the sprocket. The chain, when switched from the larger 
diameter sprocket to the smaller diameter sprocket, is intended to 
disengage from the larger diameter sprocket, but interferes with a tooth 
thereof positioned behind a tooth ready to disengage the chain, in the 
direction of normal rotation of the larger diameter sprocket. The chain 
does not smoothly shift to the smaller diameter sprocket, resulting in 
unreliable disengagement of the chain from the larger diameter sprocket. 
Also, the speed-change ratio, when increased, makes the sprocket ratio 
larger considerably increasing variations in torque when switching the 
chain, so that a rider is subjected to a great impact against pedalling to 
happen to miss his treading on the pedal, which is very dangerous for him. 
SUMMARY OF THE INVENTION 
This invention has been designed to overcome the aforesaid problems. An 
object of the invention is to provide a multispeed sprocket assembly which 
is capable of quickly switching the driving chain to a desired one 
sprocket even when the smaller diameter and the larger diameter sprockets 
have a difference of three or more teeth between the numbers of teeth, 
thereby improving the speed change efficiency and reducing variations in 
torque when the chain is switched. 
This invention is characterized in that the larger diameter sprocket has at 
least two adjacent teeth omitted from a plurality of teeth of the same. 
Between the teeth adjacent to the omitted teeth is formed a recess 
directed radially inwardly of the sprocket from the dedendum circle 
thereof, and at an intermediate portion of the recess is formed a 
projection extending close to the dedendum circle from the bottom of the 
recess. 
Such a construction of the invention, whereby at least two adjacent teeth 
are omitted and a recess is provided as aforegoing, allows the driving 
chain, when switched, to be shifted a sufficient length axially of the 
sprocket by a derailleur through the recess without interference. Also, 
the chain, when switched from the smaller diameter sprocket to the larger 
diameter sprocket and vice versa, is once relayed by the projection. 
Hence, even when both the sprockets have a large difference in the numbers 
of teeth, at the position where the projection is provided, lengths of the 
position of the tip of the tooth at the larger diameter sprocket and of 
the position of the tip of the tooth at the smaller diameter sprocket, are 
transitionary, due to the difference between the numbers of teeth at the 
sprockets, at two steps between the tooth of the smaller diameter sprocket 
and the projection and between the projection and the tooth of larger 
diameter sprocket. Hence, a sprocket ratio between both the sprockets 
becomes smaller to reduce variations in the torque of the sprocket when 
switching the chain, resulting in less impact against pedalling so that a 
rider can avoid the danger of losing his contact with the pedal. 
In addition, the number of portions where the omitted teeth are formed is 
to be determined by the size of the larger diameter sprocket, but at least 
one portion is sufficient. 
It is preferable that the projection is formed in the same shape as the 
tooth of the sprocket, but it may be made trapezoidal. In brief, it is 
enough for the projection to be positioned corresponding with the bottom 
land between the omitted teeth and extend close to the dedendum circle of 
the larger sprocket so that the chain, when switched, can be relayed by 
the projection and a chain roller can ride thereon.

DETAILED DESCRIPTION OF THE INVENTION 
A multi-speed sprocket assembly of the invention is mounted on a crank 
and/or a rear wheel hub of the bicycle. 
The multi-speed sprocket assembly shown in FIG. 1 is mounted on the crank 
of the bicycle, which comprises a smaller diameter sprocket 1 having a 
small number of teeth and a larger diameter sprocket 2 having a large 
number of teeth. At the outer peripehries of the sprockets 1 and 2 are 
provided a number of teeth 11 and 21 circumferentially at regular 
intervals respectively, the smaller diameter sprocket 1 having a smaller 
radius of the addendum circle thereof than that d of the dedendum circle 
of the larger diameter sprocket 2. The sprockets 1 and 2, as shown in FIG. 
2, are connected axially at a given interval by means of a plurality of 
connectors 3. 
The larger diameter sprocket 2 has two adjacent teeth 21a and 21b omitted 
from a plurality of teeth 21. Between two teeth 21c and 21d adjacent to 
the two omitted teeth 21a and 21b is formed a recess 22 being directed 
radially inwardly from the dedendum circle of the sprocket 2 so that a 
driving chain, when switched to either sprocket 1 or 2, traverse the 
recess 22. At the middle portion of recess 22 is formed a projection 4 
extending close to the dedendum circle of the sprocket 2. The projection 4 
relays the chain C when switched, and contacts a roller C.sub.1 of the 
chain C in engagement with the sprocket 2 to thereby prevent the chain 
from being stretched straight between the teeth 21c and 21d adjacent to 
the omitted teeth 21a and 21b, thus causing no variation in the chain 
pitch. 
The projection 4 is preferred to be formed in coincidence with each tooth 
of the sprocket 2 and bent widthwise outwardly of the tooth 21 at the 
larger diameter sprocket 2 so as to exceed the thickness of a tooth. 
Hence, the chain C, when switched from the smaller diameter sprocket 1 to 
the larger diameter sprocket 2, engages easily with the projection 4 and, 
when switched from the sprocket 2 to the sprocket 1, easily leaves the 
projection 4. 
Alternatively, three to six teeth may be omitted corresponding to a size of 
the larger diameter sprocket 2 and they are enough to be omitted on at 
least one portion at the sprocket 2. Also, the projection 4 may be 
different in shape from the tooth 21 and is enough to extend close to the 
dedendum circle of the sprocket 2 and bear the roller of chain C. 
When the multi-speed sprocket assembly of the invention comprises three to 
five sprockets and is mounted on the rear wheel hub, the sprockets except 
for the smallest diameter sprocket have the aforesaid omitted teeth and 
projections 4 respectively. 
In the multi-speed sprocket assembly of the invention constructed according 
to the foregoing, the chain C, when switched from the smaller diameter 
sprocket 1 to the larger diameter sprocket 2 as shown by the phantom line 
in FIG. 1, is biased by a derailleur so as to be shifted toward the 
sprocket 2. The chain C, when shifted, is slanted to the common axis of 
sprockets 1 and 2 because a part of chain C positioned ahead in the normal 
rotation direction (in the direction of the arrow X in FIG. 1) of the 
sprockets remains at the smaller diameter sprocket 1, and a part of the 
chain C biased by the derailleur is overshifted to be displaced axially 
outwardly of the larger diameter sprocket 2, at which time, the chain C 
traverses the recess 22 to shift toward the larger diameter sprocket 2, 
thereby smoothly and reliably engaging with the tooth 21d at the sprocket 
2. Also, the chain C once rides on the projection 4 at the sprocket 2 and 
then is relayed therewith to engage with the tooth 21d, so that, even if 
the sprockets 1 and 2 have a difference of three or more teeth and speed 
change ratio is made larger, variations in a torque of the sprocket when 
switching the chain C can be considerably reduced. Accordingly, a rider is 
not subjected to a great impact against pedalling so that he can avoid 
losing his footing on the pedal. 
The chain C, when switched from the larger diameter sprocket 2 to the 
smaller diameter sprocket 1, traverses the recess 22 to shift toward the 
smaller diameter sprocket 1, thereby smoothly and reliably disengaging 
from the sprocket 2. Also, the chain C, at first, engages with the 
projection 4 and then engages with the tooth 11 at the sprocket 1, so 
that, even if a difference of three or more teeth exists between the 
number of teeth at the sprockets 1 and 2, variations in torque can be 
considerably reduced to avoid a greater impact against pedalling when 
changing the bicycle speed. 
Furthermore, the projection 4, as shown in FIG. 2, is slanted toward the 
smaller diameter sprocket 1 so that the sprocket 1, when the chain C is 
switched therefrom to the larger diameter sprocket 2, can quickly catch 
the chain C which approaches the sprocket 2, and the chain C, when 
switched from the sprocket 2 to the sprocket 1, can quickly leave the 
projection 4. Hence, quick switching of chain C is performable to improve 
the speed change efficiency. 
When the chain C switched to the sprocket 2 passes through the portion of 
the omitted teeth 21a and 21b, the roller c.sub.1 of chain C rides on the 
tip of projection 4, so that the chain C is not stretched straight between 
the teeth 21c and 21d adjacent to the omitted teeth 21a and 21b, but is 
kept shaped in a circular arc, thereby causing no variation in the chain 
pitch, thereby ensuring engagement of the roller c.sub.1 of chain C with 
the teeth 21c and 21d adjacent to the omitted teeth 21a and 21b. As a 
result, the tooth 21c ahead in the rotation direction of larger diameter 
sprocket 2 is not subjected to a converged load, thereby avoiding 
accelerated wear of the tooth 21c. 
As clearly understood from the aforesaid description, the multi-speed 
sprocket assembly of the invention is so constructed that at least two 
teeth at the larger diameter sprocket are omitted, and a recess being 
directed radially inwardly from the dedemdum circle of the larger diameter 
sprocket, is formed between two teeth adjacent to the omitted teeth so 
that the chain traverses the recess to be switched, whereby the quick 
switching of the chain can improve the speed change efficiency even if the 
sprockets have a large difference between the numbers of teeth. 
Furthermore, the projection, which is provided at the middle portion of the 
recess and extends close to the dedendum circle of the larger diameter 
sprocket, functions to relay the chain's switching across the sprockets of 
different diameters. Hence, the chain can be switched with less variation 
in torque of the sprocket even when both the sprockets have a larger 
difference between the numbers of teeth and the speed change ratio is 
increased. As a result, a greater impact pedalling, is avoided so that the 
rider can safely avoid missing this footing on the pedal. 
Moreover, when the chain engages with the larger diameter sprocket, the 
projection can bear at its tip the chain roller, so that the chain is not 
stretched straight across the omitted teeth portion, but is kept shaped in 
a circular arc to eliminate variations in the chain pitch. As a result, 
the rollers of the chain can reliably engage with the teeth adjacent to 
the omitted teeth so as to avoid converging a load into one of the 
adjacent teeth. 
While an embodiment of the invention has been shown and described, the 
invention is not limited to the specific construction thereof, which is 
merely exemplary in the specification.