The cost of a new two-piece rotor can be quite a bit higher than a one-piece rotor, even a performance single piece rotor. Are they worth the extra cost?
First, there are a lot of performance benefits to a two piece rotor setup:
- - Lighter weight - A typical two-piece setup is at least 3 pounds lighter than a one piece rotor, and in some applications as much as 10 pounds lighter. While a 3 or even 10 pound weight reduction might not seem like a lot, it is a tremendous reduction in the unsprung weight of the wheel assembly. This is important, because unsprung weight is carried by the suspension system. As unsprung weight is reduced, the suspension system works better...which means better cornering and better ride.
- - Better cooling because of the aluminum hat - Cast iron conducts/absorbs heat quickly. This is what you want in a brake rotor, but unfortunately cast iron can also become a heat "reservoir" of sorts. Which a solid single piece cast iron rotor, heat is stored for hours. But with a two piece rotor, most of the heat in the center (aka the hat or bell) dissipates quickly, as the hat/bell is made from aluminum. Aluminum is a good heat conductor, but it also gives up heat readily, and the entire rotor is cooler as a result.
- - Better cooling because of increased airflow - Two piece rotors have a gap between the hat and the discs that increases airflow around the discs, reducing temperatures in the process.
So two piece brake rotors have two really big benefits: First, due to improved cooling, your brake system should perform better with a two piece rotor system. Second, due to lower unsprung mass, your suspension system is better able to control the wheels, improving handling and ride as a result.
Finally, it's important to remember that the up-front cost of a two piece rotor includes the hat/bell, a part that will not need to be replaced when the cast iron brake disc is worn. If you compare the cost of replacement rotor to a single piece performance rotor, the pricing is very comparable.
Additionally, the frequency of brake rotor replacement is lowered because two piece rotor brake discs stay cooler and last longer.