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Reaction Arm
The arm which connects a coaster brake or other hub brake to the chainstay or fork blade. It is designed to prevent the axle and other non-rotating parts of the hub from turning under the stress of braking. The reaction arm may be attached to the stay or fork blade either by a metal strap, or by a braze-on. The "Pacman" braze-on allows the wheel to be removed without having to un-bolt the reaction arm from the frame.
The reaction arm usually resists forward rotation of the hub, but when holding a bicycle from rolling backward down a hill, it resists backward rotation. The metal strap shown in the photo with this entry is incorrectly installed. A reaction arm must be tightly clamped around the chainstay. Otherwise. the reaction arm will work back and forth and eventually loosen the strap bolt or axle nuts, resulting in brake failure. Some straps are made in different sizes, others have multiple holes like the one shown.
Some models of the Rohloff Speedhub use a reaction arm in place of anti-rotation washers to keep the hub's axle from rotating in the frame. (The gear range of the the Rohloff hub is so wide that simple anti-rotation washers would not be sufficient to withstand the axle torque in the extreme gears.)
