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Originally Posted by AU N EGL
Rotors with holes in where originally designed for outgassing of asphestous brake pads back in the late 50s and 1960s.. While asphestous is no longer used in brake pads, there is no longer a need for rotors with holes in.
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The basic composition of an organic friction material is a phenolic resin & a fibrous material, together with other ingredients, mainly in powder form, to modify the friction characteristics. Until the 1980s the main, almost universal, fibre used was asbestos. The banning of asbestos forced the use of different fibres, such as Kevlar, rockwool, glass fibre & several others. However, the resins remain the same; gassing is the result of the action of heat on the resins, so the requirement to provide an escape path for the gases is just the same for non-asbestos pads as it was for asbostos pads.