Patent ID: 7651423

Claim:
A variable speed drive device adapted to be coupled to a variable speed gearshift device of a bicycle via a transmission member to electrically drive the variable speed gearshift device, the variable speed drive device comprising: a case member configured and arranged to be mounted to a frame of the bicycle; a motor drive mechanism mounted on the case member, the motor drive mechanism having a revolving shaft with a first pivot axis that is stationary with respect to the case member; and a linking structure having a retaining portion configured and arranged to retain the transmission member, the linking structure being mounted on the revolving shaft of the motor drive mechanism, and configured and arranged such that a length of a hypothetical arm line measured extending from a center of rotation of the revolving shaft perpendicularly to a longitudinal axis of the transmission member changes depending on a rotational phase of the revolving shaft when the transmission member is attached to the retaining portion, the linking structure further including a first arm member with a first end non-rotatably mounted on the revolving shaft of the motor drive mechanism such that the first arm member rotates around the first pivot axis when the revolving shaft rotates about the first pivot axis, and a second arm member with a first end rotatably coupled to a second end of the first arm member around a second pivot axis that shifts with respect to the case member with the retaining portion being provided on a second end of the second arm member, the first and second pivot axes having a constant distance therebetween as the revolving shaft rotates about the first pivot axis.