The brake disc is the rotating part of a wheel's disc brake assembly, against which the brake pads are applied. The material is typically gray iron a form of cast iron. The design of the discs varies. Some are solid, but others are hollowed out with fins or vanes joining the disc's two contact surfaces (usually included in the casting process). The weight and power of the vehicle determine the need for ventilated discs. The ventilated disc design helps to dissipate the generated heat and is commonly used on the more heavily loaded front discs.Discs for motorcycles, bicycles, and many cars often have holes or slots cut through the disc. This is done for better heat dissipation, to aid surface-water dispersal, to reduce noise, to reduce mass, or for marketing cosmetics.