This SES Suspension Enhancement System Upgrade fits the rear suspension of a Ford F-350 pickup truck and cab chassis. Each laser cut piece of steel is precision bent and powder-coated to ensure a quality fit. Timbren has engineered hollow rubber springs to deliver a progressive spring rate. In other words, the more weight that pushes down on the rubber springs the harder they work to prevent rear end sag. Whether you're pulling a trailer or carrying a heavy load in the bed of your truck, hollow rubber springs adjust automatically to keep the vehicle level at all times. The SES Suspension Upgrade replaces the jounce stop on the frame. Its unique design provides superb ride quality - loaded or unloaded (There should be a 1/2" to 1-1/2" gap between rubber spring and axle to ensure unloaded ride quality). Unlike other auxiliary spring systems, the SES Suspension Upgrade guarantees the best of both worlds. FR350SDF replaces the bumpstops on the rear suspension of the 1999-2004 Ford F-350 Super Duty 2WD and 4WD cab and chassis models and the 2005-2016 Ford F-350 Super Duty 2WD and 4WD pickup truck models. This kit is rated at 8600 lbs capacity. Even though you're increasing the spring rate by adding this upgrade, this does not increase the GVWR (Gross Vehicle Weight Rating). Please make sure that the load does not exceed the GVWR set by the manufacturer.