Park Tool GSC-1 GearClean Brush
The GSC-1 is specially designed to free your bicycle drivetrain from muck and grime. The comfortable handle features a contoured toothed end to reach dirt deep between the gears, and tough nylon bristles clean the derailleur and chain. Saves wear and tear on your chain and cassette. Also works great on derailleur pulleys, brakes, or other places where dirt collects.