WO2001030642A1

WO2001030642A1 - Variable crank-shaft device for driving a bicycle

Info

Publication number: WO2001030642A1
Application number: PCT/KR2000/001232
Authority: WO
Inventors: Doo-Gyung Jung; Young-Woo Lee
Original assignee: Idea Bank Network Co., Ltd.
Priority date: 1999-10-28
Filing date: 2000-10-28
Publication date: 2001-05-03
Also published as: AU1061301A

Abstract

Disclosed herewith is a variable crank drive apparatus. The present invention provides a variable crank drive apparatus for bicycles, in which each of cranks attached to both ends of a drive shaft is comprised of a pair of crank rods, the crank rods are connected to each other by a pair of link rods, and a variable rod is pulled or pushed through an eccentric cam by the rotation of a pedal, thereby varying the total length of the variable crank drive apparatus and, consequently, generating maximum drive power with minimal physical exertion.

Description

VARIABLE CRANK-SHAFT DEVICE FOR DRIVING A BICYCLE

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates generally to a variable crank drive apparatus for bicycles, and more particularly to a variable crank drive apparatus for bicycles, in which each of cranks attached to both ends of a drive shaft is comprised of a pair of crank rods, the crank rods are connected to each other by a pair of link rods, and a variable rod is pulled or pushed through an eccentric cam by the rotation of a pedal, thereby varying the total length of the variable crank drive apparatus and, consequently, generating maximum drive power with minimal physical exertion.

In addition, the variable crank drive apparatus can be easily applied to a drive shaft fitted into the chain sprocket of an existing bicycle.

Description of the Prior Art

In a conventional bicycle, a chain sprocket is connected to a rear wheel by a chain to drive the bicycle in a rear wheel drive manner and pedals are rotatably attached to cranks secured to both ends of a drive shaft fitteα into the chain sprocket, so that the pedals attached to the cranks are rotated along a circle of a constant diameter by the pressing down manipulation of a user and the rear wheel connected to the chain sprocket by the chain is rotated by the rotation of the pedals. As examples of prior art crank drive apparatus, there can be presented an invariable crank drive apparatus, in which two cranks each having a constant length are secured to both ends of a drive shaft fitted into a chain sprocket and two pedals are attached to the outer ends of the cranks, and a variable crank drive apparatus, in which the total length of the crank drive apparatus is varied as disclosed in Korean Pat. Appln. No. 97-58360.

However, the prior art invariable crank drive apparatus is disadvantageous in that the desirable amount of drive power is not generated in comparison with the amount of physical exertion because the pedals are rotated along a circle of a constant diameter without particular provision for the saving of energy.

In the prior art variable crank drive apparatus, a main shaft and a rotating shaft are eccentrically spaced apart from both ends of a shaft tube integrated with a chain sprocket, a power transmission unit is provided to transmit rotating force to a chain, rotating levers are rotatably mounted to both ends of the shaft tube outside the power transmission unit, cranks each connected to a slide tube provided at its outer ends with the pedals are mounted to the rotational shaft spaced apart from the main shaft, and the rotating lever is mounted by a fixed shaft to be freely rotated with regard to the inner surface of the slide tube. Accordingly, when the pedals are rotated to drive a bicycle, the total length of the variable crank drive apparatus is increased during the pressing down of the pedals and is reduced during the raising up the pedals by the movement of the rotating lever, thereby generating maximum power with minimal physical exertion. However, the prior art variable crank drive is disadvantageous in that the power transmission unit, the rotating levers, the cranks and the slide tube should be mounted to both sides of the shaft tube m a set state, thereby enlarging the size of a crank drive apparatus, causing the crank drive apparatus to be complicated and to be difficult to assemble and fabricate, and incurring high costs.

The prior art variable crank drive apparatus should be mounted only to a bicycle fabricated to be suitable for this type of driving apparatus, so this type of driving apparatus has a shortcoming in that it is not easily applied to existing bicycles .

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a variable crank drive apparatus for bicycles, in which each of cranks attached to both ends of a drive shaft is comprised of a pair of crank rods, the crank rods are connected to each other by a pair of link rods and a variable rod is attached to an eccentric cam secured to a pedal shaft and a crank rod secured to the drive shaft, so that the variable rod is pulled or pushed through an eccentric cam by the rotation of a pedal and the total length of the variable crank drive apparatus is increased or reduced, thereby generating maximum drive power with minimal physical exertion.

Another object of the present invention is to provide a variable crank drive apparatus for bicycles, in which a cylinder is attached to the inner end of one crank rod and the outer end of one link rod to facilitate the expansion and retraction of the variable drive apparatus, so that the cylinder rod of the cylinder and the total length of the variable crank drive apparatus is increased during the pressing down of a pedal and the total length of the variable crank drive apparatus is reduced by the restoring force of a tension spring mounted to the cylinder, thereby generating maximum drive power with minimal physical exertion and allowing a variable crank drive apparatus to be easily applied to the drive shaft of an existing bicycle. A further object of the present invention s to provide a variable crank

apparatus for bicycles, in whicn each of cranks attached to both ends of a drive shaft is comprised of a pair of crank rods, the crank rods are connected to each other by a hinge shaft and a variable rod is attached to an eccentric cam secured to a pedal shaft and a crank rod secured to the drive shaft, so that the variable rod is pulled or pushed through an eccentric cam by the rotation of a pedal and the total length of the variable crank drive apparatus is increased or reduced, thereby generating maximum drive power with minimal physical exertion, simplifying its structure and facilitating its fabrication.

An additional object of the present invention is to provide a variable crank drive apparatus for bicycles, in which each of cranks attached to both ends of a drive shaft is comprised of a pair of crank rods, the crank rods are connected to each other by a pair of link rods and a variable rod is attached to an eccentric cam secured to a pedal shaft and a link rod secured to the crank rod, so that the variable rod is pulled or pushed through an eccentric cam by the rotation of a pedal and the total length of the variable crank drive apparatus is increased or reduced, thereby generating maximum drive power with minimal physical exertion.

In order to accomplish the above object, the present invention provides a variable crank drive apparatus for bicycles, two variable crank drive apparatuses being fixedly attached to both ends of a drive shaft fitted into a chain sprocket, comprising: a first crank rod provided at its inner end with an attaching hole to be fixedly attached to a drive shaft; a second crank rod provided at its outer end with a pedal shaft, with both ends of which a pedal and an eccentric cam are engaged m a serration fashion to prevent slippage during rotational operation; first and second link rods, each of the link rods being rotatably attached to the first and second crank rods by pins, with both ends of the first link rod respectively and rotatably attached to the center portion of the first crank rod and the inner end of the second crank rod, and with both ends of the second link rod respectively and rotatably attached to the outer end of the first crank rod and the center portion of the second crank rod; and a variable rod rotatably attached to the first crank rod and the eccentric cam to reduce and increase the total length of the crank drive apparatus, with a first end of the variable rod rotatably attached to the center portion of the first crank rod and a second end of the variable rod rotatably attached to the free end of the eccentric cam.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a perspective view showing a variable crank drive apparatus for bicycles in accordance with a first embodiment of the present invention;

Fig. 2 is a side view showing the operation of the variable crank drive apparatus of the first embodiment;

Fig. 3 is a side view showing the operation of a variation of the first embodiment;

Fig. 4 is a side view showing the operation of another variation of the first embodiment;

Fig. 5 is a perspective view showing another variable crank drive apparatus for bicycles in accordance with a second embodiment of the present invention;

Fig. 6 is a perspective view showing the operation of the variable crank drive apparatus of the second embodiment; and

Fig. 7 is a side view showing the operation of the variable crank drive apparatus of the second embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference now should be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components. Fig. 1 is a perspective view showing a variable crank drive shaft for bicycles m accordance with a first embodiment of the present invention. Fig. 2 is a side view showing the operation of the variable crank drive shaft of the first embodiment. Fig. 3 is a side view showing the operation of a variation of the first embodiment. Fig. 4 is a side view showing the operation of another variation of the first embodiment. Fig. 5 is a perspective view showing another variable crank drive shaft for bicycles in accordance with a second embodiment of the present invention. Fig. 6 is a perspective view showing the operation of the variable crank drive apparatus of the second embodiment. Fig. 7 is a side view showing the operation of the variable crank drive apparatus of the second embodiment . As illustrated in Figs. 1 and 2, the variable crank drive apparatus for bicycles includes a first crank rod 11 and a second crank rod 13. The first crank rod 11 is provided at its inner end with an attaching hole 12 for attaching the variable crank drive apparatus to a drive shaft 2 fitted into a chain sprocket 1 so as to apply drive power to a bicycle, and at its central portion with a first stopper recess 11' . As for the second crank rod 13, a pedal shaft 16 is rotatably mounted in the outer end of the second crank rod 13, and a second stopper recess 13' is formed on the center portion of the second crank rod 13. A pedal 17 and an eccentric cam 18 are engaged with both ends of the pedal shaft 16 in a serration fashion to prevent slippage during rotational operation.

The variable crank drive apparatus further comprises first and second link rods 14a and 14b. Each of the link rods 14a and 14b is rotatably attached to the first and second crank rods 11 and 13 at its both ends by pins 15, with both ends of the first link rod 14a rotatably attached to the first stopper recess 11' and front end of the first crank rod 11, respectively, and with both ends of the second link rod 14b rotatably attached to the rear end and second stopper recess 13' of the second crank rod 13, respectively.

In this case, the pedal 17 and the eccentric cam 18 are preferably engaged with both ends of the pedal shaft 16 in a serration fashion so as to transmit the rotational force of the pedal 17 to the eccentric cam 18 without slippage. As long as the rotational force of the pedal 17 can be transmitted to the eccentric cam 18, the pedal and the eccentric cam 18 may be engaged with the pedal shaft 16 in any of other manners. The stopper recesses 11' and 13' are preferably constructed to prevent the first and second crank rods 11 and 13 from being bent further while force is applied to the first and second crank rods 11 and 13 in a rotational direction. A variable rod 19 is rotatably attached to the eccentric cam 18 and the first crank rod 11 to change, that is, reduce and increase, the total length of the crank drive apparatus, with one end of the variable rod 19 rotatably attached to the free end of the eccentric cam 18 and the other end of the variable rod 19 rotatably attached to the first crank rod 11 at a position where one end of the first link rod 14a is rotatably attached to the first crank rod 11.

As depicted in Fig. 3, the variable crank drive apparatus for bicycles includes a first crank rod 11 and a second crank rod 13. The first crank rod 11 is provided at its inner end with an attaching hole 12 for attaching the variable crank drive apparatus to a drive shaft 2 fitted into a chain sprocket 1 so as to apply drive power to a bicycle, and at its central portion with a first stopper recess 11' . As for the second crank rod 13, a pedal shaft 16 is rotatably mounted in the outer end of the second crank rod 13, and a second stopper recess 13' is formed on the center portion of the second crank rod 13. A pedal 17 and an eccentric cam 18 are engaged with both ends of the pedal shaft 16 in a serration fashion to prevent slippage during rotational operation.

The variable crank drive apparatus further comprises first and second link rods 14a and 14b. Each of the link rods 14a or 14b is rotatably attached to the first and second crank rods 11 and 13 at its both ends by pins 15, with both ends of the first link rod 14a rotatably attached to the first stopper recess 11' and front end of the first crank rod 11, respectively, and with both ends of the second link rod 14b rotatably attached to the rear end and second stopper recess 13' of the second crank rod 13, respectively. In this case, the stopper recesses 11' and 13' are preferably constructed to prevent the first and second crank rods 11 and 13 from being bent further while force is applied to the first and second crank rods 11 and 13 in a rotational direction. Two attaching projections 20a and 20b are formed on the inner end of the first crank rod 11 and the outer end of the first link rod 14a, and a cylinder 21 is attached to the attaching projections 20a and 20b at its ends with a tension spring 22 situated around the piston rod 21' of the cylinder 21, thereby changing, that is, reducing and increasing, the total length of the crank drive apparatus.

Further, as indicated in Fig. 4, the variable crank drive apparatus for bicycles includes a first crank rod 11 and a second crank rod 13. The first crank rod 11 is provided at its inner end with an attaching hole 12 for attaching the variable crank drive apparatus to a drive shaft 2 fitted into a chain sprocket 1 so as to apply drive power to a bicycle. As for the second crank rod 13, a pedal shaft 16 is rotatably mounted in the outer end of the second crank rod 13, and a pedal 17 and an eccentric cam 18 are engaged with both ends of the pedal shaft 16 in a serration fashion to prevent slippage during rotational operation. The first and second crank rods 11 and 13 are rotatably connected to each other by a hinge shaft 23. In this case, the pedal 17 and the eccentric cam 18 are preferably engaged with both ends of the pedal shaft 16 in a serration fashion so as to transmit the rotational force of the pedal 17 to the eccentric cam 18 without slippage. As long as the rotational force of the pedal 17 can be transmitted to the eccentric cam 18, the pedal and the eccentric cam 18 may be engaged with the pedal shaft 16 in any of other manners. When force is exerted on the crank rods 11 and 13 so as to prevent the crank rods 11 from being bent by the force, the crank rods 11 are preferably bent at a certain angle in a direction opposite to the direction of the application of the force.

Additionally, a variable rod 19 is rotatably attached to the free end of an eccentric cam 18 and the center portion of the first crank rod 11 to change, that is, reduce and increase, the total length of the crank drive apparatus by the rotation of the eccentric cam 18.

The variable crank drive apparatuses constructed as described above can produce maximum drive power with minimal physical exertion by increasing and reducing the total length of the variable crank drive apparatus. The first and second crank rods are connected to each other by the link rods and the variable rod to increase and decrease the total length of the crank drive apparatus. As the variable crank drive apparatus is rotated by pressing down the pedal so as to generate drive power, the crank drive apparatus is extended while moving from a twelve o'clock position to a three o'clock position. Accordingly, the first and second crank rods are extended from an erected position, so that a user has a uncomfortable feeling that the crank drive apparatus falls abruptly, and the uniform rotation of the crank drive apparatus is not achieved.

In order to improve the variable crank drive apparatuses of the first embodiment, a variable crank drive apparatus in accordance with a second embodiment of the present invention is provided.

As shown in Figs. 5 to 7, the variable crank drive apparatus for bicycles includes a first crank rod 11 and a second crank rod 13. The first crank rod 11 is provided at its inner end with an attaching hole 12 for attaching the variable crank drive apparatus to a drive shaft 2 fitted into a chain sprocket 1 so as to apply drive power tc a bicycle. A first open-sided slot 11' is longitudinally formed through the first crank rod 11. As for the second crank rod 13, a pedal shaft 16 is rotatably mounted in the outer end of the second crank rod 13, and a second open-sided slot 13' is formed on the second crank rod 13. A pedal 17 and an eccentric cam 18 are engaged with both ends of the pedal shaft 16 in a serration fashion to prevent slippage during rotational operation. The variable crank drive apparatus further comprises first and second link rods 24a and 24b. Each of the link rods 24a and 24b is rotatably attached to the first and second crank rods 11 and 13 by pins 15, with both ends of the first link rod 24a respectively and rotatably attached to the center portion of the first crank rod 11 and the inner end of the second crank rod 11, and with the inner end and center portion of the second link rod 24b respectively and rotatably attached to the outer end of the first crank rod 11 and the center portion of the second crank rod 13. In this case, the pedal 17 and the eccentric cam 18 are preferably engaged with both ends of the pedal shaft 16 in a serration fashion so as to transmit the rotational force of the pedal 17 to the eccentric cam 18 without slippage. As long as the rotational force of the pedal 17 can be transmitted to the eccentric cam 18, the pedal and the eccentric cam 18 may be engaged with the pedal shaft 16 in any of other manners.

A variable rod 19 is rotatably attached to the eccentric cam 18 and the second link rod 24b to change, tnat ±s, reduce and increase, the total length of the crank drive apparatus, with one end of the variable rod 19 rotatably attached to the free end of the eccentric cam 18 and the other end of the variable rod 19 rotatably attached to the outer end of the second link rod 24b. Hereinafter, the operations of the above-described embodiments are described in detail.

As illustrated m Fig. 2, two variable crank drive apparatuses 10 are attached to both ends of the drive shaft 2 fitted into the chain sprocket 1 with the variable crank drive apparatuses 10 situated on opposite sides and the attaching holes 12 of the first crank rods 11 fitted around the drive shaft 2. When the pedal 17 engaged with the pedal shaft 16 rotatably mounted to the outer end of the second crank rod 13 in a serration manner is pressed down to drive a bicycle forward, the eccentric cam 18 is rotated through the pedal shaft 16 by the rotational force of the pedal 17, thereby rotating the variable rod 19 rotatably attached to the free end of the eccentric cam 18 and the center portion of the first crank rod 11. As a result, the variable crank drive apparatus is extended to its maximum length during the pressing down of the pedal 17, while the variable crank drive apparatus is retracted to its minimum length during the raising up of the pedal 17. Accordingly, maximum drive power can be generated by minimal physical exertion. That is, when the pedal 17 is pressed down to generate drive power and, accordingly, the crank drive apparatus is rotated from a vertical position to a horizontal position, the variable rod 19 is pushed by the rotational force of the eccentric cam and the first and second link rods 14a and 14b are gradually extended. Accordingly, minimal physical exertion is required owing to the principle of leverage. After the variable crank drive apparatus passes through the horizontal position, the variable rod 19 is pulled by the rotational force of the eccentric cam 18 and the first and second link rods 14a and 14b each connecting the first and second crank rods 11 and 13 are gradually retracted, so that the pedal 17 can be easily raised up. The variable crank drive apparatus repeats these operations in which the length of the total crank drive apparatus is varied. As illustrated in Fig. 3, two variable crank drive apparatuses 10 are attached to both ends of the drive shaft 2 fitted into the chain sprocket 1 with the variable crank drive apparatuses 10 situated on opposite sides and the attaching holes 12 of the first crank rods 11 fitted around the drive shaft 2. When the pedal 17 engaged with the pedal shaft 16 rotatably mounted to the outer end of the second crank rod 13 is pressed down to drive a bicycle forward, the pedal shaft 16 is rotated by the rotational force of the pedal 17. While the variable crank drive apparatus 10 is rotated, the cylinder rod 21' of the cylinder 21 attached to the attaching projection 20a of the first crank rod 11 and the attaching projection 20b of the first link rod 14a is extended during the pressing down of the pedal 17, and the cylinder rod 21' is retracted by the tensile force of the tension spring 22 during the raising up of the pedal 17.

That is, when the pedal 17 is pressed down to generate drive power and, accordingly, the crank drive apparatus 10 is rotated from a vertical position to a horizontal position, the cylinder rod 21' of the cylinder 21 is pulled out of the cylinder 21 by the pressing down of the pedal 17 and the first and second link rods 14a and 14b each connecting the first and second crank rods 11 and 13 are gradually extended. Accordingly, minimal physical exertion is required owing to the principle of leverage. After the variable crank drive apparatus passes through the horizontal position, the cylinder rod 21' of the cylinder 21 is pulled into the cylinder 21 by the tensile force of the tension spring 22 and the first and second link rods 14a and 14b each connecting the first and second crank rods 11 and 13 are gradually retracted, so that the pedal 17 can be easily raised up. The variable crank drive apparatus repeats these operations in which the length of the total crank drive apparatus is varied.

As illustrated in Fig. 4, two variable crank drive apparatuses 10 are attached to both ends of the drive shaft 2 fitted into the chain sprocket 1 with the variable crank drive apparatuses 10 situated on opposite sides and the attaching holes 12 of the first crank rods 11 fitted around the drive shaft 2. When the pedal 17 engaged with tr.e pedal shaft 16 rotatacly mounted to the outer end of the second crank rod 13 in a serration manner is pressed down to drive a bicycle forwarα, the eccentric cam 18 is rotated tnrough the pedal shaft 16 by the rotational force of the pedal 17, thereby rotating the variable rod 19 rotatably attached to the free end of the eccentric cam 18 and the center portion of the first crank rod 11. As a result, the variable crank drive apparatus is extended to its maximum length during the pressing down of the pedal 17, while the variable crank drive apparatus is retracted to its minimum length during the raising up of the pedal 17. Accordingly, maximum drive power can be generated by minimal physical exertion.

Ttat is, when the pedal 17 is pressed down to generate drive power and, accordingly, the crank drive apparatus is rotatec from a vertical position to a horizontal position, the variable rod 19 is pushed by the rotational force of the eccentric cam and the first and second cranx rods 11 and 13 connected by the hinge shaft 23 are gradually extended. Accordingly, minimal physical exertion is required owing to the principle of leverage. After the variable crank drive apparatus passes through the horizontal position, the variable rod 19 is pu ieα by the rotational force of the eccentric cam 18 and the first and second crank rods 11 and 13 connected by the hinge shaft 23 are gradually retracted, so that the pedal l^"7 can be easily raised up. The variable crank drive apparatus repeats these operations in which the length of the total crank drive apparatus is varied.

As illustrated in Figs. 6 and 7, two variable crank drive apparatuses 10 are attached to both ends of the drive shaft 2 fitted into the chain sprocket 1 with the variable crank drive apparatuses 10 situated on opposite sides and the attaching holes 12 of the first crank rods 11 fitted around the drive shaft 2. When the pedal 17 engaged with the pedal shaft 16 rotatably mounted to the outer end of the second crank rod 13 in a serration manner is pressed down to drive a bicycle forward, the eccentric cam 18 is rotated through the pedal shaft 16 by the rotational force of the pedal 17, thereby rotating the variable rod 19 rotatably attached to the free end of the eccentric cam 18 and the outer end of the second link 24b. As a result, the variable crank drive apparatus is extended to its maximum length during the pressing down of the pedal 17, while the variable crank drive apparatus is retracted to its minimum length during the raising up of the pedal 17. Accordingly, maximum drive power can be generated by minimal physical exertion.

That is, when the pedal 17 is pressed down to generate drive power and, accordingly, the crank drive apparatus is rotated from a vertical position to a horizontal position, the variable rod 19 is pushed by the rotational force of the eccentric cam and the first and second link rods 24a and 24b are gradually extended. Accordingly, minimal physical exertion is required owing to the principle of leverage. After the variable crank drive apparatus passes through the horizontal position, the variable rod 19 is pulled by the rotational force of the eccentric cam 18 and the first and second link rods 24a and 24b each connecting the first and second crank rods 11 and 13 are gradually retracted, so that the pedal 17 can be easily raised up. The variable crank drive apparatus repeats these operations in which the length of the total crank drive apparatus is varied.

As described above, the present invention provides a variable crank drive apparatus for bicycles, in which each of -r-" £ a ac ed t~ bc^~- ~- = of a drive shaft is comprised of a pair of crank rods, the crank rods are connected to each other by a pair of link rods, and a variable rod is pulled or pushed through an eccentric cam by the rotation of a pedal, thereby varying the total length of the variable crank drive apparatus and, consequently, generating maximum drive power with minimal physical exertion.

In addition, the present invention provides a variable crank drive apparatus that can be easily applied to a drive shaft fitted into the chain sprocket of an existing bicycle Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled m the art will appreciate that various modifications, additions and substitutions are possible, witnout departing from tne scope and spirit of the invention as disclosed in the accompanying claims.

Claims

WHAT IS CLAIMED IS:

1. A variable crank drive apparatus for bicycles, two variable crank drive apparatuses being fixedly attached to both ends of a drive shaft fitted into a chain sprocket, comprising : a first crank rod provided at its inner end with an attaching hole to be fixedly attached to a drive shaft; a second crank rod provided at its outer end with a pedal shaft, with both ends of which a pedal and an eccentric cam are engaged in a serration fashion to prevent slippage during rotational operation; first and second link rods, each of said link rods being rotatably attached to said first and second crank rods by pins, with both ends of said first link rod respectively and rotatably attached to the center portion of said first crank rod and the inner end of said second crank rod, and with both ends of said second link rod respectively and rotatably attached to the outer end of the first crank rod and the center portion of the second crank rod; and a variable rod rotatably attached to said first crank rod and said eccentric cam to reduce and increase the total length of said crank drive apparatus, with a first end of said variable rod rotatably attached to the center portion of said first crank rod and a second end of said variable rod rotatably attached to the free end of said eccentric cam.

2. The variable crank drive apparatus according to claim 1, further comprising two stopper recesses that are formed on the center portions of said first and second crank rods, respectively.

3. A variable crank drive apparatus for bicycles, two variable crank drive apparatuses being fixedly attached to both ends of a drive shaft fitted into a chain sprocket, comprising: a first crank rod provided at its inner end with an attaching hole to be fixedly attached to a drive shaft; a second crank rod rotatably connected to said first crank rod by a pin, said second crank rod being provided at its outer end with a pedal shaft, with both ends of which a pedal and an eccentric cam are engaged in a serration fashion to prevent slippage during rotational operation; and a variable rod rotatably attached to said first crank rod and said eccentric cam and said second link rod to reduce and increase the total length of said crank drive apparatus, with a first end of said variable rod rotatably attached to the center portion of said first crank rod and a second end of said variable rod rotatably attached to the free end of said eccentric cam.

4. A variable crank drive apparatus for bicycles, two variable crank drive apparatuses being fixedly attacheα to both ends of a drive shaft fitted into a chain sprocket, comprising: a first crank rod provided at its inner end with an attaching hole to be fixedly attached to a drive shaft; a second crank rod provided at its outer end with a pedal shaft, to one end of which a pedal is rotatably mounted; first and second link rods, each of said link rods being rotatably attached to said first and second crank rods by pins, with both ends of said first link rod respectively and rotatably attached to the center portion of said first crank rod and the inner end of said second crank rod, and with both ends of said second link rod respectively and rotatably attached to the outer end of the first crank rod and the center portion of the second crank rod; and a cylinder attached to two projections secured to the inner end of said first crank rod and the outer end of said first link rod, respectively, said cylinder having a tension spring positioned around the cylinder rod of said cylinder.

5. A variable crank drive apparatus for bicycles, two variable crank drive apparatuses being fixedly attached to both enαs of a drive shaft fitted into a chain sprocket, comprising : a first crank rod provided at its inner end with an attaching hole to be fixedly attached to a drive shaft; a second crank rod provided at its outer end with a pedal shaft, with both ends of which a pedal and an eccentric cam are engaged in a serration fashion to prevent slippage during rotational operation; first and second link rods, each of said link rods being rotatably attached to said first and second crank rods by pins, with both ends of said first link rod respectively and rotatably attached to the center portion of said first crank rod and the inner end of said second crank rod, and with both ends of said second link rod respectively and rotatably attached to the outer end of the first crank rod and the center portion of the second crank rod; and a variable rod rotatably attached to said eccentric cam and said second link rod to reduce and increase the total length of said crank drive apparatus, with a first end of the variable rod rotatably attached to the free end of said eccentric cam and a second end of the variable rod rotatably attached to the outer end of the second link rod.