Many trucks (like this late-model Chevy) use long leaf springs with a thick overload spring along the bottom. (This truck also has a lift block and an add-a-leaf.) The long springs offer a more flexible suspension, while the overloads support heavy loads and helps stop axlewrap. Axlewrap is when the axle's pinion wants to twist upward, deforming the springs, rather than rotating the tires. In severe cases this can break springs and driveshafts.