These 5 lift rear leaf springs for the 99 - 07 F - 250F - 350 Super Duty are progressively rated to improve the ride quality of your pickup, as well as increased performance. The multi - leaf construction ensures a smooth ride with less chance of spring fatigue under load, minimizing sagging. The bottom 3 leaves of the pack are flat to provide a secondary overload rate preventing harsh bottom outs and smooth transition through the suspension travel. ICON 2.0 Aluminum Series or 2.5 Aluminum Series Vehicle Specific tuned shocks are recommended for best ride quality and performance on - or off - road.