Chain Watcher, or catcher. Insures against chain drops when downshifting to the small ring. Helps avoid damage to expensive frames. Designed for road bikes with braze-on front derailleur mounts. Read Installing the Apollo Chain Watcher for an installation walk-through. Ships with different size bolts and mounting washers to accommodate most frames. (Not for mountain bikes. Braze-on derailleur mount required. This is a road-bike part).