Originally Posted by rb4-bmw
We witnessed the same issue (over heat) on EVO 8/9 community, enthusiasts suggested similar type fixes; add cooling ducts, bend/trim dust shield etc. But those expected cooling improvement are limited and can hardly solve the fundamental cause (lack of cooling air be correctly circulated into the heat exchanger - rotor).
EVO racers suffered this same issue were advised to remove all these auxiliary devices, (also remove the dust shield
), and install RB two piece rotors and the problem went away. You can verify this from EvolutionM forum here
RB's hat is connected to disc via the center-mount tabs alternatively from both sides of disc (vs. traditional surface mount which has no air admission from outboard) which provide wide open air ports from dust shield side (IB) and wheel side (IB) - Other brake company offers directional hats trying to achieve the same purpose, however the air doesn't travel in angle, and those slits are to easy to clog up
Don't forget the air (gap) is the best natural insulator, so if you have a clearance of 6-8mm (from rotor surface to lower ball joint which is usually available unless you install a BBK that pushes the rotor further in) you shouldn't be concerned of over heating the ball joint. Or you can warp it up with some blanket insulator as an extra precaution.
The idea is to ultimately keep the heat out of the rotor by allowing more cooling air to come in (uniformly from both sides, instead of just blowing from IB via cooling duct) and remove the heat out of the rotor as quickly and as efficiently as possible.
RB's convergent vane design is another proven feature that allows the cooling air to stay in the vanes longer (vs. conventional curved vane) for a more efficient heat dissipation for a more uniform temperature across the disc (inner to outer edge, inboard to outborad).
Heat removal = Air flow x (T2-T1)
T1= Air temp @ entry (inner edge)
T2= Air temp @ exit (outer edge)
M brake discs are made of exactly the same principle