Wicke Wheel offer a quality range of tube expander fittings to suit round or square hollow tube.

Tube expanders are attached to a single bolt hole castors by inserting the expanders long threaded central bolt through the bolt hole of a single bolt hole castor (from the underside of the castor). You may need to remove the wheel to do this. The expanding components are then slipped back onto the long threaded stem bolt. The expanding section is then inserted into the hollow tube and turning the bolt head on the underside of the castor causes the expander to expand and fit tightly into the tube. When deciding which expander fits your tube, it is important to measure the internal diameter of the tube and select an expander that is small enough to be inserted and with an expanding measurement that is large enough to fit expand to fill the internal diameter of the hollow tube.