1. A brake disk, including:an outer disk, being substantially ring-shaped, a through hole being defined in a center thereof, the through hole defining a central axis, the central axis defining an axial direction, a first plane being defined as a plane perpendicular to the axial direction, the outer disk having an inner annular flange, the inner annular flange surrounding the through hole, the inner annular flange of the outer disk being formed with a plurality of connecting bridges arranged in intervals, each said connecting bridge extending radially toward the central axis of the through hole, an end of the connecting bridge remote from the inner annular flange of the outer disk having a protruding portion, a disposing hole being disposed axially through the protruding portion, the protruding portion and the inner annular edge of the outer disk having a predetermined distance radially therebetween;
an inner disk, coaxially disposed in the through hole of the outer disk and located on a same plane with the outer disk, a central portion of the inner disk having a hub connecting portion, the inner disk further having an outer annular flange, the outer annular flange being formed with a plurality of recessed portions arranged in intervals, the recessed portions corresponding to the protruding portions respectively, the recessed portion being disposed through the inner disk axially, a shape of the recessed portion substantially corresponding to a shape of the protruding portion so that the protruding portion is fittingly engaged with the recessed portion to prevent the inner disk from rotating relative to the outer disk, the recessed portion and the protruding portion having a gap therebetween, wherein the gap extends continuously along a whole inner fringe of the recessed portion, the protruding portion having a contour completely inside a contour of the recessed portion so that the protruding portion is movable in the recessed portion on the first plane;
a plurality of positioning members, each said positioning member disposed through the disposing hole to restrain the outer disk and the inner disk from moving axially relative to each other;wherein the outer disk and the inner disk are axially non-overlapping;wherein the inner disk includes an outer annular portion, a plurality of bridge portions and the hub connecting portion, each said bridge portion extends from an inner annular flange of the outer annular portion relative to the recessed portion toward the hub connecting portion and is connected with and between the outer annular portion and the hub connecting portion, a number of the bridge portions is twice a number of the recessed portions, a portion of the outer annular portion corresponding to each recessed portion is divergently extended with two of the bridge portions toward the hub connecting portion so that two of the bridge portions between any two adjacent recessed portions convergently extend to a specific position of the hub connecting portion, the connecting bridge includes two foot portions which are connected with the inner annular flange of the outer disk and the protruding portion, the two foot portions divergently extend from the protruding portion toward the inner annular flange of the outer disk so that the connecting bridge is substantially Y-shaped;
wherein each recessed portion has an opening toward the outer disk wherein the opening has a width larger than a maximum width of each protruding portion.