Rotors are usually cut from one piece of steel, which has two disadvantages. Firstly, steel is heavy. Secondly, as the outer section heats under the pads (and so expands) more than the inner, very high temperatures can cause them to warp. Two-piece rotors solve both problems with a lightweight alloy carrier and ‘floating’ dowels that act like the expansion joints in bridges.
The HSX’s wavy edge helps clean the pads as it passes through the calliper, and they’re cross-drilled to help with cooling on every rotation. This design is well-known to be useful for hard-braking, long or steep descent-loving riders who are using big discs, but it’s arguably of just as much use to weight weenies. Smaller discs (140mm-170mm) mean less rotating weight but, thanks to their relative lack of surface area, they struggle even more with cooling.
Feel is good and there’s no rattle – those dowels are tight – making these a fit and forget solution to brake fade. Avid make them in 180, 170, 160 and 140mm, plus six-bolt and splined Shimano Centerlock fittings.