Premium TransmissionHigh-strength forging, super light aluminum alloy, durable shock resistance, make your bike more stable. Concave guide screw, the material is aluminum alloy. The model is ACERA RD-M390, the class has 9 speeds and 27 speeds. High quality guide wheelThe guide wheel is made of nylon PA plastic, very comfortable to wear, quiet and smooth, will not wear your mountain bike chain, smooth transmission. Precision adjustmentSuper hard aluminum alloy body adjustment, more precise adjustment, more subtle, so that you can be more speed sensitive. Durable light action shifting performance for stress-free riding, quick assembly. FitsRear derailleur compatible with all 9 speed shifters. Advanced light action design. Apply to the majority of bicycles, mountain bikes, road bikes, fixed bikes, etc. Wide connection designWide connection design provides precise shifting and durability. Smooth shifting, easy and quick to install, large guide wheel, low noise. Product Specification: Speed: 9S Gear range: from 28T to 34T Materials: aluminum alloy parts Concave guide wheel screw Performance at speed variable for smooth/easy operation Wide connection design provides precise shifting and durability Rear Derailleur Tutorial: 1. fully loosen the rear shift line, the chain hanging in the flywheel after the smallest teeth, turn the crankset 2.If the chain still comes out of the flywheel, slowly turn the screw clockwise Clockwise; if the chain goes to the second flywheel, loosen the screw of the rear derailleur counterclockwise 3.The second step, close the rear shift line, you can directly adjust the shift line back, and at the same time turn the crank 4: If the chain comes out of the flywheel in the direction of the spokes, slowly turn the screw L clockwise, up to the maximum flywheel and do not drop the chain as standard 5.If the hand cannot adjust the rear shift line on the largest gear of the flywheel, then slowly loosen the screws L