The SD Rear Axle Mounting Plates are replacement mounting plates designed for select Bakcou SD eBike models. Their function varies by model: Kodiak AWD SD & Mule SD: The mounting plate functions as the rear derailleur hanger, providing the mounting point for the derailleur. Mule Ja;ger SD & Flatlander SD: The mounting plates function as tensioning plates, helping properly position and tension the rear wheel and drivetrain. Be sure to select the correct option for your Bakcou model when ordering. If you&';re unsure which replacement part you need, contact our Customer Service team for assistance before placing your order.