When it comes to dirt adhering to wheels, the first thing we think of is a wheel completely blackened by carbon soot, as seen in the picture. The main cause of this dirt is the brake pads rubbing against the rotors during braking and burning to ash. On top of that, sand and mud, as well as oil-based grime such as pitch and tar, are kicked up from the road, adhering to and accumulating on the surface. Another source of dirt is the iron dust which is abraded from the brake rotors. Most people think that iron dust comes from the brake pads, but they produce mainly ash when they combust due to rubbing with the brake rotors. So called “iron dust” mostly comes from material which is abraded from the brake rotors by the pads. In recent years there has been a trend toward using non-metallic brake pads made from resin or fiber. Products which aim to reduce brake pad dust are available on the market. The reason that the wheels on non-Japanese vehicles and sports cars get blackened so easily is that their pads focus on braking strength and so are consumed quickly.