Convex springs (i.e., barrel-shaped springs) have coils with larger diameters in the middle of the spring and coils with smaller diameters on both ends. This design allows the coils to fit within each other when the spring is compressed. Manufacturers use convex springs in applications that require more stability and resistance to surging as the springs decompress. Most applications that use them are in the automotive, furniture, and toy industries. http://www.zhlspring.com/compression-spring/