The surface areas of a wheel spacer has studs and holes. The holes mount the spacer to a wheels hub and the studs are used to attach the wheels back onto the car.
Wheel spacers are used to make the rims of a vehicle sit flush with the chassis. Extending the area at which the rim sits not only adds to the overall look of the car, but can increase its handling capabilities.
Wheel spacers are available in a variety of different sizes which can fit several different lug nut patterns, such as 4-bolt and 5-bolt. Slimmer-sized wheel spacers do not require use of independent studs and fit on top of the hub while thicker spacers do need studs.