The Park Tool Dummy Hub allows the bike to be pedaled and the derailleur to shift after the rear wheel is taken out. It acts as a hub and keeps the chain in place so the drivetrain can function and be cleaned or serviced. It is also great for helping diagnose drivetrain issues without having a wheel in the way. The hub is compatible with all 1/8 inch and multi-speed chains as well as quick release and 12mm thru axle frames. The Park Tool Dummy Hub is a must have for drivetrain maintenance.