The center of the 2 piece rotor is generally constructed out of aluminum, which makes for reduced overall weight of the rotor, which reduces unsprung weight. Lower unsprung weight will help ride quality and help the suspension to do it's job more effectively (I don't have the knowledge to expound on, or quantify this effect).
Rotational mass is also reduced, but since the rotors are near the center of rotation, I would imagine they have a negligible impact on this.
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Jeff: Project "Rolling Mockup" 69 Camaro SS, AFX, TKO600, Baer GT, etc
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