A disc brake system (10) comprises a disc (12, 14), and a hub (16) which is arranged to rotate about an axis (18) thereof. The hub (16) comprises a mounting portion (16d) which is generally in the shape of a hollow cylinder. The system (10) also comprises grooves (20) and teeth (22) by which the disc (12, 14) is mounted on the external surface of the mounting portion (16d) for rotation with the hub (16) as a unit about the axis (18) and the disc (12, 14) can perform axial sliding movement on the hub. The hub (16) carries vanes (62) arranged so that, during rotation of the hub (16) about the axis (18), the vanes act to force air into the interior of the mounting portion (16d). The mounting portion (16d) has openings (60) therethrough arranged so that air forced into the interior of the mounting portion (16d) passes through the openings and flows over said disc (12, 14).