if it hits your tire when you are in the absolute easiest gear(1 and 1), a couple of problems might be occuring. the most likely cause is a bent wheel, or a wheel that is not all the way in the dropout. if you are a large rider on a stock wheel, it could also be flexing, but that's not likely the case. you could have too wide of a tire for your chainline. your chainline could be off. you front derailluer could be bent or shifted.
i would be willing to bet it's a bent wheel though.