"This SES suspension upgrade fits the trailer suspension with an *underslung axle setup that has a 2-3/8" up to 3" diameter axle with 2" wide leaf springs. Each laser cut piece of steel is precision bent and powder-coated to ensure a quality fit. The hollow rubber springs are designed 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 sag and stabilize the load.