Designed for better system performance, the Eagle S100 Half Mount rear derailleur can be paired with a UDH-compatible frame to provide even greater resistance to lateral impacts. On the Half Mount-specific Eagle S100 rear derailleur, the Half Mount plate connects the rear derailleur mounting bolt to the UDH bolt. Compatibility: The Half Mount plate can only be mounted with an S100 rear derailleur and in conjunction with a UDH bolt.