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Traditional one-piece brake rotors are made from a single piece of cast iron. Two-piece brake rotors use cast-iron for the disc but have an aluminum center piece called a hat or bell. The bell/hat bolts to the disc and also attaches to the wheel assembly hub. To account for thermal expansion, some bells use clips and springs to connect the two pieces.
A one piece rotor is machined from a solid block of material. A two piece rotor contains two components, commonly called the hat (or bell) and the ring. The hat resembles the inner portion of a one piece rotor. The ring is the braking surface, and it bolts to the hat. One of the biggest advantages to the two-piece rotor design is they are much better at dissipating heat than a standard single-piece rotor design. You can learn more about why brake rotors warp in this blog.
This two-piece setup is also known as "floating disc assembly." There are a few advantages to having a two-piece design, which are detailed in the following answer.
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:
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.
Plenty of brands manufacture two-piece rotors, and some are specifically made for certain car manufacturers and models. For instance, JHM, Brembo, and Apikol make two-piece rotors that fit the Audi A4, but not all of these manufacturers have products for a Chevrolet Silverado.
A floating disc rotor is an improved version of a two piece rotor. In a standard two piece rotor, the ring is simply bolted to the hat. In a floating disc rotor assembly, the ring is attached to the hat in a way that allows some motion as the ring expands due to heat. This lessens heat related stresses on the ring, which allows it to last longer. If you’d like to learn more about the benefits of a two-piece rotor, you can find that post here.
When upgrading to a two-piece rotor design you can expect better performance for a few different reasons. Some of these benefits come from how a rotor is manufactured, you can learn more about that in this blog. As we mentioned above, one of the most beneficial is the rotor’s ability to disperse heat and keep your hubs from getting too hot. Another benefit is less weight, which is important in rotating components. This allows for better acceleration and braking.