The Bilstein B3 13-17 BMW X3/ 15-18 BMW X4 Replacement Front Coil Spring is designed to replace the original factory spring on the BMW X3 and X4 models. This spring is made of high-strength steel and is designed to provide improved handling and a more comfortable ride. It features a progressive rate design to provide a smooth transition from soft to hard over a wide range of conditions. It also features a corrosion-resistant coating to ensure long-term durability.