Gear crank for a bicycle

A gear crank for a bicycle includes a plurality of chain gears mounted to stays disposed radially outwardly with respect to a crank shaft. The stays include four main stays arranged in pairs. The two main stays forming each pair are disposed at a relative phase difference of about 180.degree.. Two short auxiliary stays are disposed between the pairs of main stays and have mounting portions for the larger diameter chain gears. The pairs of main stays intersect at a non-90.degree. angle so that an angle between the pairs of main stays is larger than that between the main stays forming each pair. The stays are fixed to the crank to position the crank arms between the pairs of stays within a region defined by the above-described larger angle respectively. Connecting segments extending circumferentially and supporting the auxiliary stays are provided between the auxiliary stays and the main stays adjacent thereto respectively.

FIELD OF THE INVENTION 
This invention relates to a gear crank for a bicycle, and more particularly 
to a gear crank for a bicycle, comprising a plurality of chain gears and a 
crank means having a crank shaft and a pair of crank arms, the chain gears 
being adapted to be mounted to a plurality of stays disposed radially 
outwardly with respect to the crank shaft. 
BACKGROUND OF THE INVENTION 
Conventionally, bicycle gear cranks include an adapter having five or six 
stays spaced at regular angular intervals therebetween and fixed to the 
crank arm or crank shaft and a plurality of chain gears mounted to the 
stays so that a driving force produced by pedalling is transmitted from 
the stays to a driving chain through the chain gear. 
Such stays, however, are disposed regardless of location of each crank arm, 
whereby at least five or six stays are used. 
Hence, the conventional gear crank has a weight corresponding to the number 
of stays, which has been a defect for the bicycle demanded to be 
lightweight. 
SUMMARY OF THE INVENTION 
Base on the principle that the driving force caused by pedalling varies 
corresponding to an angular phase of each crank arm such that the driving 
force is the minimum when the crank arms are positioned at the vertical 
dead points or in the vicinity thereof, and is the maximum when each crank 
arm is displaced at the predetermined angle (usually 70.degree. to 
130.degree.) ahead of the upper dead point, this invention has been 
designed to use four stays. 
In other words, the present invention utilizes the driving force varying as 
abovementioned and takes into consideration the positional relation 
between the crank arm and the stay, thereby using four stays. 
An object of the invention is to provide a gear crank having stays less in 
number for reducing weight and supportable of the chain gears stably even 
when the larger diameter gear and smaller diameter one differ largely in 
their number of teeth. This invention is characterized in that four main 
stays in two pairs are disposed such that the two main stays forming a 
pair have a phase difference of about 180.degree.. The pairs of main stays 
intersect at a non-90 angle so that an angle between each pair of main 
stays is larger than that between the stays in each pair. The main stays 
are fixed to the crank means so that each crank arm is positioned between 
each pair of stays within the region defined by the aforesaid larger 
angle, and two short auxiliary stays having mounting portions for the 
larger diameter chain gears are disposed between each pair of main stays 
and are supported with connecting segments extending circumferentially 
from the utmost ends of adjacent stays. 
In other words, this invention provides for each pair of main stays 
disposed as above mentioned to be positioned in a region where the larger 
driving force is transmitted reliably from the crank arm to the chain 
gear, and the aforesaid auxiliary stays support the larger diameter chain 
gears at the positions thereof between the main stays, in other words, in 
the positions corresponding to the vertical dead points of crank arms, 
thereby making the gear crank lightweight as a whole and enabling stable 
support of each chain gear. 
These and other objects of the invention will become more apparent in the 
detailed description and examples which follow.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIGS. 1 and 2, the gear crank comprises three chain gears 
G.sub.1 to G.sub.3 not-round and approximately elliptic in shape and 
different in the number of teeth and of larger, middle and smaller 
diameters respectively. The gear crank includes a crank 1 comprising three 
pieces of a crank shaft 11 and a pair of first and second crank arms 12 
and 13, the gears G.sub.1 to G.sub.3 being assembled with the crank shaft 
11 through an adapter 4 having a stay means 5 extending radially outwardly 
of crank shaft 11. 
The crank shaft 11 is supported rotatably to a bottom bracket of the 
bicycle through bearings 3 and the first and second crank arms 12 and 13 
are fixed at the roots to the crank shaft 11 opposite to each other, the 
adapter 4 being fixed to the root of first crank arm 12 by a fixing means, 
such as caulking. 
The stay means 5 comprises four main stays 51 to 54 in two pairs each 
disposed at a phase difference of about 180.degree. and symmetrically 
around the crank shaft 11 and two short auxiliary stays 6 disposed between 
each pair and having mounting portions 61 and 62 for the larger diameter 
chain gear G.sub.1 and middle diameter one G.sub.2. Angles between the 
stays 51 and 54, and 52 and 53, are made larger than those between the 
stays 51 and 52, and 53 and 54. Stays 51 to 54 are fixed to the crank 
means 1 so that the first and second crank arms are positioned between the 
stays 52 and 53 and between stays 51 and 54 respectively. Connecting 
segments 7 extending circumferentially of adapter 4 are provided between 
each auxiliary stay 6 and the stays 51 and 54 adjacent thereto and between 
the same and those 52 and 53 adjacent thereto and support the auxiliary 
stays 6 respectively. 
In the drawings, the stays 51 and 52 are disposed at an angle of about 
60.degree. therebetween and those 53 and 54 are disposed at the same angle 
therebetween and symmetrically with the stays 51 and 52 respectively. The 
respective stays 51 to 54 are provided at the side surface opposite to the 
second crank arm 13 with projecting mounting portions 41, 42 and 43 for 
the larger, middle and smaller diameter chain gears G.sub.1, G.sub.2 and 
G.sub.3. Projecting mounting portions 41-43 are disposed radially inwardly 
in a stepped manner. 
Each crank arm 12 or 13 is positioned at approximately an intermediate 
portion of an angle of about 120.degree. between the stays 51 and 54 or 52 
and 53 so that when crank arm 12 or 13 reaches the forward rotation 
position where a cyclist produces the maximum driving force, each pair of 
stays 51 and 52 or 53 and 54 are directed upwardly to transmit the maximum 
driving force to the driving chain. 
The respective stays 51 to 54 are inclined radially rearwardly and in the 
direction reverse to the projecting mounting portions 41 to 43. The chain 
gear mounting positions at the mounting portions 41 to 43 are set 
perpendicular with respect to the crank shaft 11 to thereby restrain the 
occurrence of a twist, in turn it is reduced in axial length to contribute 
to a decrease in weight of stay. It is preferable that between the utmost 
ends of the respective stays 51 to 54 are provided reinforcements 8 
connecting therebetween, which are not indispensable. 
Each auxiliary stay 6 is provided at an about intermediate portion between 
the stays 51 and 54 and between those 52 and 53 and sufficiently long to 
provide mounting portions 61 and 62 and to support the connecting portions 
7 respectively. 
The chain mounting portions 61 and 62 at each auxiliary stay 6 are provided 
at the position opposite to both the larger diameter chain gears G.sub.1 
and G.sub.2 and are arranged radially of adapter 4 in a stepped manner. 
The connecting segments 7 for supporting the auxiliary stays 6 are 
preferably ring-like shaped together with the reinforcements 8, thereby 
reinforcing the stays 51 to 54 with each other. 
The chain gears G.sub.1 and G.sub.2 are mounted to the mounting portions 61 
and 62 so that when the crank arms 12 and 13 reach the vertical rotation 
positions when the cyclist produces the minimum driving force, one 
auxiliary stay 6 is positioned upwardly, thereby transmitting the minimum 
driving force from the larger diameter chain gear G.sub.1 or middle 
diameter one G.sub.2 to the driving chain through the slantwise upward 
stays 51 and 54, upper connecting segments 7, and auxiliary stay 6. 
The chain gears G.sub.1, G.sub.2 and G.sub.3 each are approximately 
elliptic of a smaller pitch diameter portion and a larger pitch diameter 
portion, and are assembled in the adapter 4 so that when the cyclist's 
foot produces the maximum torque by pedalling, the gear diameters of gears 
G.sub.1, G.sub.2 and G.sub.3 become nearly the minimum and when the torque 
is the minimum, the gear diameter become nearly the maximum. 
In other words, in the region where the maximum torque is obtained, the 
gear diameter is reduced to increase the circumferential speed of 
pedalling to match with the speed pattern of the cyclist's foot movement 
in the aforesaid region and to improve power (torque X speed). In the 
region where the minimum torque only is obtained, that is, in the vicinity 
of vertical dead points of crank arms 12 and 13, the gear diameter 
increases to reduce the circumferential speed of pedalling and matches 
with the speed pattern by the cyclist's foot at the aforesaid region, 
thereby improving the power as a whole, reducing muscular output (energy 
consumption), lightening a cyclist's physical load, and enabling smooth 
pedalling. 
More particularly as shown in FIG. 1, the maximum gear diameter portions 
are disposed in the vicinity of the upper and lower dead points 
corresponding to the maximum gear diameter portions. The minimum gear 
diameter portions are disposed at the region producing the maximum torque. 
The maximum gear diameter portions near the upper and lower dead points 
are mounted to the auxiliary stays 6, and the minimum gear diameter 
portions are mounted to the stays 51, 52, 53 and 54 respectively. As a 
result, the maximum torque is transmitted to the minimum gear diameter 
portions through the stays 51 to 54 and the minimum torque is transmitted 
to the maximum gear diameter portions through the auxiliary stays 6. 
The stays 51 to 54 constructed as abovementioned are reinforced with each 
other by the connecting segments 7 thereby being lightweight. The 
connecting segments 7, which need only transmit the minimum torque output 
by the cyclist to the chain gear through the auxiliary stays 6, are 
small-sized and lightweight, thus reducing the weight of adapter 4 along 
with reduction in the number of stays. The respective gears G.sub.1 to 
G.sub.3 are supported to four stays 51 to 54 for transmitting the maximum 
driving force and the larger and middle diameter gears G.sub.1 and G.sub.2 
are supported at the maximum gear diameter portions thereof by the 
auxiliary stays 6. Hence, the chain gears G.sub.1 to G.sub.3 are supported 
stably and can always transmit the driving force to the driving chain. 
Accordingly, the gear crank is lightweight as a whole, the auxiliary stays 
each need only support the maximum gear diameter portion, thereby being 
reduced in length to a minimum and also made lightweight, thus giving a 
neat appearance. 
Alternatively, the auxiliary stays may support the larger diameter chain 
gear G.sub.1 only. Also, a round chain gear may of course be useable and 
the not number of geare need be limited to three. 
As seen from the above, the gear crank of the invention constructed as 
abovementioned reinforces the stays with the connecting segments 
supporting the auxiliary stays and, along with reduction in the number of 
stays, the stays are made lightweight thereby supporting stably the chain 
gears and transmitting always reliably the driving force to the driving 
chain. 
While a preferred embodiment of the invention has been described using 
specific terms, such description is for illustrative purpose only, and it 
is to be understood that changes and variations may be made without 
departing from the spirit or scope of the following claims.