hmmm. my thinking about variation is that in order for anything to variate, something has to move in two places. So even a motobecane has an engine that moves (variates). And I guess that's what I call single variation. It variates from one side (pulley) to the other. Duel variation in my mind would be like taking a hobbit engine and putting it on a motobecane So it variates once as the engine moves, and twice as it then gets into the pully moving. Make sense? It's all semantics though I guess.