Magnesium wheels are wheels manufactured from alloys which contain mostly magnesium. Magnesium wheels are produced either by casting (metalworking) (where molten metal is introduced into a mold, solidifying within the mold), or by forging (where a prefabricated bar is deformed mechanically). Magnesium has several key properties that make it an attractive base metal for wheels: lightness; a high damping capacity; and a high specific strength. Magnesium is the lightest metallic structural material available. It is 1.5 times less dense than aluminium, so magnesium wheels can be designed to be significantly lighter than aluminium alloy wheels, while exhibiting comparable strength. All competitive racing rims are now made of magnesium alloy.