Instead of the traditional expanding brakes that press outward against a circular drum, disc brake systems consist of two cast iron discs with brake pads positioned on either side. Braking action is achieved by the pads squeezing either side of the rotating disc. Dirt and water do not greatly affect braking action since they are thrown off the rotor by centrifugal action or scraped off by the pads. The equal clamping action of the pads tends to ensure uniform, straight stopping. All disc brakes are self-adjusting.