Why a 2 piece sprocket? 

2.5 lbs you get the weight savings and the wear and strength of steel in one super strong sprocket! Plus once the ring gear wears out, you only need to replace it, not the carrier. If you want to change the number of teeth to adjust your bike's performance you only need to replace the hardened steel ring gear, not the carrier.
Outer ring gear is made from hardened steel and is replaceable. Carrier is made from lightweight and strong 7057-T6 aluminum. Mounting hardware not included. Compatible with the Hauss Halo & Spacer