This unsprung weight includes the wheels and tires, usually the brakes, plus some percentage of the suspension, depending on how much of the suspension moves with the body and how much with the wheels; for instance a solid axle suspension is completely unsprung. The main factors that improve unsprung weight are a sprung differential (as opposed to live axle) and inboard brakes. (The De Dion tube suspension operates much as a live axle does, but represents an improvement because the differential is mounted to the body, thereby reducing the unsprung weight.) Wheel materials and sizes will also have an effect. Aluminium alloy wheels are common due to their weight characteristics which help to reduce unsprung mass. Magnesium alloy wheels are even lighter but corrode easily.