Great idea, or the worst idea in modern motorcyledom?
This part was installed on a custom bike I saw on display at the Oklahoma City Motorcycle Show and Swap Meet a few weekends ago. It's a combination brake/sprocket. At first I thought, "Wow! What a great idea!" It would save weight, and allow for a cleaner looking rear end.
Seconds later, I pondered the reason that bike manufacturers and MotoGP teams could possibly have for not installing them as standard equipment: Chains need lube. Lube/gunk accumulates on sprocket. What's usually a non-issue for most chain-driven bikes could possibly become a slippery, unsafe mess when it meets a disc brake surface. Given that a lot of chopper folks seem to like the clean look of a bike with no front brake, it seems to me that this part could contribute to a catastrophe under the right conditions. I predict that you'll be able to spot the bikers that install these from the well-ground heels of their boots.