From what I have seen, those who use wheels/discs use them at both ends -- on the servo and also a half disc at the rudder end. But I am not sure who supplies them? In theory it would be a better solution because it removes the built in expo of the arm/horn solution, and as the wires always stay a constant distance apart would mean smaller or point exits in the fuz could be used (the arms in theory need a longer exit slot in the fuz to allow for the distance between the wires narrowing under deflection of the rudder).

David