Unlike traditional add-a-leafs that obtain lift height at the cost of stiffening the spring rate and sacrificing ride quality, the 2007-2021 Toyota Tundra Rear Leaf Spring Expansion Pack replaces all the factory secondary leaves while only retaining the factory main and overload springs. This allows us to soften the spring rate to increase ride comfort while also increasing ride height, maintaining load and towing capacities, and increasing wheel travel when paired with ICON shock absorbers. A new U-bolt kit is also included to make the insalltion a breeze. Features like these make the 2007-2021 Toyota Tundra Rear Leaf Spring Expansion Pack is an excellent option for most users, but for those looking to maximuze the performance of the Tundra's rear springs, consider the ICON Multi-Rate Leaf Spring, part # 158509.