Bicycle crank driven chain wheel assembly

A bicycle crank driven chain wheel assembly comprising:

• a chain wheel having a central boss which defines a rotation axis of the chain wheel and teeth formed on and along the periphery of the chain wheel, wherein the teeth define an imaginary, symmetric, closed pitch curve, said curve comprises a first portion and a second portion wherein the first portion comprises a force saving section and a speed increasing section and the second portion comprises a force saving section and a speed increasing section wherein the force savings section comprises a first curve, a second curve connected to one end of the first curve and a straight line portion connected to the second curve, said straight line portion connected tangentially to one end of the respective speed increasing section and said first, and second curve and straight line portion being along the periphery of the chain wheel and the minimum distance, r.sub.1, from the periphery of the chain ring of the first curve to the rotation axis of the chain wheel being greater than or equal to the maximum distance, r.sub.2, from the periphery of the chain ring of the second curve to the rotation axis of the chain ring, and wherein the chain wheel has a long axis, L.sub.1, defined by a connecting line of two points which are respectively in the opposed speed increasing sections and each point connected by the line which defines the maximum distance from the chain ring periphery to the rotation axis of the chain wheel and a short axis, L.sub.2, defined by a connecting line of two points which are respectively in the opposed first curves of the force savings section and each of said points define the minimum distance, r.sub.1, from the respective first curve to the rotation axis of the chain wheel and wherein the long axis, L.sub.1, and the short axis L.sub.2, being defined by a relationship in lengths of L.sub.1 to L.sub.2 in a range of ratios of approximately from about 1.20 to 1 to approximately 1.35 to 1, and the chain wheel is arranged in a sequence of sections comprising the force saving section, speed increasing section, force saving section and speed increasing section wherein the last speed increasing section is next to the first force saving section to form the closed pitch curve, the distances from every point on the closed pitch curve of the force saving section and the speed increasing section to the rotation axis of the chain wheel being continuously variable with variation along the pitch curve and the minimum distance, r.sub.3, from each position along the pitch curve in the speed increasing section to the rotation axis of the chain wheel being greater than, or equal to the maximum distance, r.sub.4, from each position along the pitch curve of the force saving section to the rotation axis of the chain wheel;
• a crank having a crank shaft fixed on the crank for rotatably driving the chain wheel and
• means for mounting the crank shaft in the central boss of the chain wheel in a manner that the crank can rotate within a predetermined range of angles, .alpha., defined by the short axis L.sub.2 of the chain wheel and the longitudinal central axis of the crank in the rotation direction of the chain wheel and a mechanism for limiting a range of the angle .alpha. with respect to the chain wheel in accordance with the amount of force exerted on the crank shaft;
• wherein the mechanism for adjusting the angle .alpha. comprises a cylinder fixed on the chain wheel, a piston received in the cylinder, a compression spring located in the cylinder for biasing the piston and a piston rod connected at one end to the piston, the other end of the piston rod being hinged with the chain wheel and a hinge bearing fixed on the crank shaft through a hinge pin, so that the crank can swing with respect to the chain wheel under the resultant action of the torque exerted on the crank shaft by both an external force and the compression spring.