Carli progressive-rate Add-A-Pack rear springs are specifically designed to address the harsh rebound of the factory overload spring they replace. When used in conjunction with a Carli Suspension System, they effectively mitigate the impact of medium to large bumps. By replacing the lower overload springs, which are typically one inch thick and do not follow the contour of the rest of the pack, with a mini progressive six-leaf pack, the overall capacity of the truck remains unchanged as determined by the factory springs. However, the progression of the rear spring rate becomes significantly more predictable. This modification results in a smoother engagement and disengagement of the spring, thereby improving both ride quality and traction.. When it comes to the F350, the main spring pack in the rear is already quite stiff and does not compress much unless under load. As a result, installing the Add-A-Pack on an F350 may actually make the rear suspension stiffer, potentially even stiffer than the stock setup. Due to this reason, the Progressive Add-A-Pack is not recommended for an F350. To achieve optimal ride quality, we highly recommend installing Carli Full Replacement Leaf Spring Packs.. Carli's six-leaf progressive Add-A-Pack rear springs replace the lower overload leaf with six thinner leafs. It maintains factory load capacity while improving ride quality by effectively mitigating the impact of medium to large bumps.. Progressive design addresses the harsh rebound of the factory overload spring. Carli Add-A-Pack Rear Leaf Springs - 2011-16 Ford F250/F350 4×4 - 2in Lift. Carli Add-A-Pack Rear Leaf Springs, 2011-16 Ford F250/F350 4×4, 2” Lift. Load capacity is maintained, ride quality is improved. Fits 2011-16 F250/F350 4×4, w/ 3.5-inch axle tube. Includes U-bolts and hardware. Provides 2.0 inches of lift. Rear Add-A-Pack. CS-FPAP-11

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