Rotor development was the key to allowing Shimano to bring a road disc brake to market; specifically, the ability to create a cooler rotor by using a ‘sandwich’ construction of steel, alloy and steel, a technology Shimano call Freeza. Further temperature reductions have been achieved with a series of fins cut into the disc surface.
The upshot? More effective cooling has allowed Shimano to drop a rotor size. Testing revealed that smaller rotors were straighter and operated within reduced tolerances. Additionally, they offered greater modulation. The combined IceTech and Freeza technologies are claimed to place a ceiling on temperatures at the brake mount of around 100 degrees. Much above that and heat transfer will degrade the resin in a carbon frame, Shimano believe.
Rotor size, however, takes us to the centre of the interdependent nature of disc brake, wheel and fork design. Shimano has already developed a 140mm rotor, but a mounting standard that would allow it to be used with a road fork does not yet exist. Until an OEM, to use the jargon, develops its own caliper mounting standard, Shimano’s preferred 140mm rotors will remain on the shelf.