Abstract: A variable-ratio transmission device includes a crankshaft for attachment to a first rotational member at an axis of rotation. The crankshaft may be angled such that a portion of the crankshaft is offset a radial distance from the axis of rotation. A connecting arm may be attached to the crankshaft in a manner such that the connecting arm can be moved along the length of the crankshaft. The connecting arm may be attached to an second rotational member such that as the crankshaft is rotated, the connecting arm causes the second rotational member to rotate back and forth. A ratio of movement between the first rotational member and the second rotational member can be adjusted by moving the connecting arm along the length of the crankshaft to thereby alter the radial distance the connecting arm is moved during rotation of the crankshaft.