housing 18 the housing 18 includes an annular housing wall 26 on the inboard side of the assembly an annular housing wall 28 on the outboard side and the whole is surrounded by a peripheral wall 30 ventilation openings 32 are provided in the peripheral wall 30 nut and bolt arrangements 34 secure the housing walls 26 28 30 and intermediate wall 62 together to form the housing 18 the adapter sleeve 16 has axial splines 44 and the annular rotor brake discs 22 and 24 include disc openings 36 and 38 at the inner margins respectively and the openings are interspersed with teeth 40 and 42 the teeth 40 and 42 engage the splines 44 thereby providing limited axial movement to the discs 22 and 24 while entraining the discs with the wheel 12 each brake disc 22 and 24 is provided with generally radial vent channels 46 it has been found that it is possible to maintain brake discs having vented channels within reasonable axial dimensions or thickness by making the discs out of a composite of aluminum called 34 duralcan 34 thus it is possible to provide two or more brake discs in a space where only one vented cast iron brake disc was previously possible the openings 36 and 38 of the discs.