The wheel hub needs to be as friction free as possible, consistent with the hub axle defining the plane of rotation of the wheel. The diagram and photograph below show the cup/cone bearing hub construction and a front hub for a road bike made from metal and composite materials.

From: Apps, "The Bicycle Book," Smithmark (1993) , and http://www.zipp.com/framedef.html

Sealed bearings may be used to prevent dirt entry, reduce wear and the useful life of the hub. The junction between the composite hub body and the alloy spoke flanges is a potential weak point of the two material design.