Themoplastic polyurethane used in our wheel range is injection molded onto both polypropylene and nylon centres. PU is a resilient material that is highly abrasion resistant, non-marking, shock absorbing and won’t flat spot. It is mostly resistant to petroleum based chemicals, some acids and water. The key feature of PU is its ability to absorb impacts, resist wear, cutting, tearing and flat spotting.UPB castor wheels feature flat and semi-round profile treads, and all come with precision bearings. Please note however, our S and H Series Castor wheels incorporate a North American Quad-Plus bearing system that includes 2 x precison ball bearings and 2 acetal journal bearings. These bearings are maintenance free for the life of the wheel. Please refer to our H Series page for more details and cut-away drawings of the patented Quad-Plus bearing system.