Carli Suspension's Progressive Add-A-Pack Rear Springs for 2005-2007 Ford SuperDuty is a proprietary design built specifically for Carli by Deaver Leaf Spring MFG. The lower overload leaf is removed and replaced with 6 thinner gauge leaves. The OEM load capacity is still maintained while an improved ride is achieved. The add-a-packs will help to take the harshness out of everyday freeway driving. ELIMINATE HARSH REBOUND These packs are designed to eliminate the incredibly harsh rebound of the factory overload spring they replace helping to tame the medium to large bumps when paired with a Carli Suspension System. The Progressive add-a-pack replaces the lower overload springs on the factory pack. This is the 1″ thick leaf that doesn’t follow the contour of the rest of the pack. In removing this spring “wall” and replacing it with a mini progressive leaf pack, the capacity of the truck remains unchanged, as it’s determined by the factory springs, while the progression of the rear spring rate becomes substantially more predictable. The engagement and disengagement of the spring becomes quite a bit smoother increasing both ride quality and traction. As the “wall” was removed, the rear may sit lower with a heavy load BUT it will ride better loaded.