Features: Suitable for: It is very suitable for 12, 14-teeth bicycles. It can be used for chain transportation and variable speed. Install it on your bicycles to make you more ease when riding a bicycles. Anti-rust Aluminum Alloy Material: The aluminum alloy construction is rust and corrosion resistant and durable, solid and light. Aluminum alloy ceramics bearings can ensure long-term use. High Lubrication: The performances of the bike rear guide wheel is good, and the inner bearings has high lubrication, which can ensure smooth operation under low load and high speed environment. Anti-Corrosion Anodic Oxidation: The bicycles rear guide wheel is treated with anti-corrosion anodic oxidation craft. In all weather-conditions, these pulleys can run on smooth balls bearings. Lightweight & Easy to Carry: Our bicycles rear guide wheel is lightweight, so it will not bring for additional burden to the bicycles. You can carry it with you in the cycling timing for easy replacement. Specifications: Material: Aluminum Alloy Color: Black/Red/Golden Diameter Options: 50mm/57mm Installation Aperture: approx.6mm/0.31in (give away variable diameter sleeve convertible to 4/5mm bearings) Note: Please allow 1-3mm error due to manual measurement. Please make sure you do not mind before you bid. Due to the difference between different monitors, the picture may not reflect the actual color of the item. Thanks! Package includes: 1Pc Rear Derailleur Pulley