Use Stem Casters with posts and shelves to create a mobile shelving unit to meet your special needs. Each caster comes with a donut bumper (AB3) at no additional cost.(Exception: CA2BN4). "Resilient casters" are soft rubber. Which tend to be economical, but can mark floors when heavy loads are present. "Polyurethane Casters" are extremely rigid and non-marring. The CA5SB model caster has a wheel diameter of 5" and a face diameter of 1 3/4".All Stem caster pricing is based on a pack of 4. The CA5SB has a load rating of 300 The CA5SB Stem casters are made from polyurethane. This caster set comes with 4 rigid wheels.