What Is A Bicycle Rear Derailleur

A typical bike rear derailleur is used to selectively move the chain from one of a plurality of sprockets to another, thereby changing the speed of the bicycle. A typical rear derailleur includes a base member adapted to be mounted to a bicycle frame, a movable member movably connected to the base member, and a chain guide connected to the movable member. The chain guide engages the chain so that the chain can be selectively switched between the plurality of sprocket teeth as the movable member moves relative to the base member.

The chain guide of the rear derailleur is usually rotatably mounted to the movable member and is elastically biased in a clockwise direction so that the chain guide reduces chain slack when the chain engages the smaller diameter sprockets, However, when riding on rough terrain, bumps and vibrations can cause the chain guide to rotate counterclockwise, which can lead to undue chain slack. To prevent this from happening, motion blocking structures have been added to some rear derailleurs. An example of a motion preventing structure has been disclosed in US Patent Application Publication No. As shown in this document, a bicycle derailleur includes a base member adapted to be mounted to a bicycle, a movable member movably connected to the base member and a chain guide connected to the movable member, and a chain guide. The chain guide is connected for rotation about an axis of rotation, and a biasing element biases the chain guide in a selected rotational direction about the axis of rotation. The rotational resistance applying assembly applies resistance to prevent the chain guide from rotating in a direction opposite to the selected rotational direction, and the adjusting assembly adjusts the magnitude of the resistance applied by the friction element of the rotational resistance applying assembly.