The Carli 6 Leaf Progressive Add-A-Pack Rear Springs are a proprietary design built for Carli by Deaver Spring MFG. The lower overload leaf is removed and is replaced with 6 thinner gauge leaves. The O.E.M. load capacity is maintained while improved ride is achieved. 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. FULL PROGRESSIVE LEAF SPRING PACK The rear of the Super Duty can be outfitted with a replacement Full Progressive Spring Pack (reduces capacity to a “light” 3/4-ton) or this factory leaf-pack modifying Progressive Add-a-Pack. This is where the F-250 and F-350 are a bit different. Since 2008, the main spring packs on these two trucks is different; the F-250 has a lighter rate spring pack than its 350 counterparts. The F-250 Add-a-Pack equipped systems are a nice improvement over stock; the F-350 with the Add-a-Pack is still very stiff (in some cases, more so than stock) in the rear as the main pack doesn’t move much unless loaded. For this reason, Carli does not recommend these be installed on an F-350. OPTIMAL RIDE QUALITY For optimal ride quality, Carli recommends the Full Replacement Leaf Spring packs for both trucks but especially the F-350 given the super-stiff factory springs. If capacity is a concern, an aftermarket airbag system should be installed to compliment the Full Progressive Leaf Springs to restore the F-350 capacity. In the 4.5″/5.5″ Range, we offer Carli's Long Travel Airbags. If 2.5″/3.5″ height, Carli recommends a system that will allow the airbags to disconnect from the lower mount during suspension cycling to ensure you’re able to take advantage of all your rear suspension travel.