Motorbikes (and any sus bikes) have large amounts of chain slack because the effective chain length changes throughout the travel of the swing arm, unless the drive sprocket is on the same axis as the swing arm. When setting the tension on any chain it should not be super tight. Someone mentioned half inch of movement somewhere, this is what old school fitters do when tensioning a chain drive on something like an industrial conveyor, this figure is the total flex in the chain UP and Down (not just down or not just up). I would have said you can get away with a bit less. Blah, blah,blah, poisonous monkey.............. Matt