If the wheel becomes noisy or difficult to turn then check the grey disc in the center of the wheel. There should also be a slight gap between the back edge of the grey plastic center and the inside face of the wheel. If the wheel is catching (not turning smoothly and is noisy) then the grey disc needs adjusting (from the back) by using two fingers to gently push the disc back into its housing by 1 or 2mm. For hamsters, gerbils, mice and other small animals. Keep your pet away from boredom. The lateral tread on the inside is slightly convex, which can effectively increase the friction and avoid slipping. Made of environmental material, safe, odorless and easy to clean.