The axle-mounted brake discs are ring-shaped castings with crosswise cooling ribs. They are manufactured from special grey cast iron or aluminum. Grey cast iron is the standard material, aluminum is used for lightweight brake discs. The round, crosswise-fanned ribs in the cooling duct between the friction rings reduce air throughput by comparison with conventional radial blades and therefore consume less fan and shaft power. Heat transfer is improved as well, i.e. there is an increasing temperature difference between air inlet and outlet. There are two types of friction rings: solid and split. Split friction rings can be installed without having the wheels removed.