The Bopworx Rear Derailleur Guard is the best solution for protecting your rear derailleur and derailleur hanger during travel and storage, covering your rear derailleur with a protective arm and semi-soft rubberized padding. Simply loosen the quick release, insert the Guard, and close the quick release to hold firmly in place. Made of steel and very tough plastic, it will protect your bike and rear derailleur from strong side impacts. Using the Rear Derailleur Guard in a bike bag or case is easy as well. When the wheel is not in the frame simply use a chain-tensioning quick release or rear wheel spacer and the Guard will stay in place. Please note that the chain-tensioning quick release/rear wheel spacer needs to be purchased separately. The Rear Derailleur Guard is not designed to work with rear through-axles.