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A two piece rotor has a lightweight central hub that mounts to a donut shaped outer rotor. There are pictures at several sites - Baer for example.
The advantage is that the center hub piece is usually very light, and usually does not need to be replaced, just the outer friction surface of the rotor. Something like 8 pounds less weight per rotor. And considering you are likely increasing the rotor size at the same time, you get better braking and lighter weight.
Of course, you are already familiar with a single piece rotor, which comes on nearly every car made - the whole hub, the mount and the friction surface, is cast in one piece...
That's what I thought. I'm waiting for Shawn @ LAPD to ship my EradiSpeed Enhanced rotor's but noticed they also sell the "two piece" rotors. I couldn't figure out what the two parts were that made up the rotors.
When my rotors go, or when I have more money than sense, I'm going to replace them with some two piece rotors - I like the larger size with reduced weight. Better braking, lower weight - can't be beat.
Venger
P.S. And with the stock C5 wheels, they look extremely choice...