Via Dirt Rag: “The center track running down the center of the belt (that gives the drivetrain its name) fits tightly over the “fins” between the teeth on the crank’s pulley, keeping the belt in alignment and eliminating the need for a flange on either pulley. This version of the Belt Drive is lighter, allows smaller pulleys and better chainstay clearance, and is also supposed to shed mud better – we’ll have to try it somewhere with enough water to make mud.”

 

http://www.dirtragmag.com/images/interbike-2010—dirt-demo-day-1—bt—web-37-45