Bike Rear Derailleur Hanger Rear Gear Mech Derailleur Adapter Bike Transmission Protection Frame Bicycle Derailleur Accessories The tail hook ear is an accessory of the frame (except for the hook rear derailleur), and has nothing to do with the gear shift and rear derailleur. Most of the lugs are made of 6061 aluminum alloy. Why not use steel? When you accidentally crash the car, you may hit the transmission and cause deformation to affect the transmission function. The rear hook is made of aluminum alloy to allow a certain deformation to protect the transmission from damage. In the case of serious bumps, the rear hook can be broken, and it can be replaced with a new one afterwards, thereby reducing the cost of frame maintenance. The strength of steel is too high to provide protection. Specifications: Upgrade or replacement rear derailleur dropouts; stiffer and more rigid to improve shifting performance CNC machined from solid 6061 aluminium to exact tolerances Stiffer than any equipment hanger which improves shifting performance Still breaks before your frame does to perform its main function All hangers come with instructions and hardware Direct replacement for equipment with guaranteed fit A01 MTB Road Bike Bicycle Cycling Mech Rear Derailleur Protector Rack Protect your rear derailleur to avoid any damage