It isn't supposed to be rigid, as for how much side motion it should allow would depend on the design. They design them to move side to side so if you hit something with the edge of the deck it can move some without bending anything. If you feel it moves excessively I'd talk to a dealer, or perhaps put a couple rubber straps holding some additional tension to keep it better centered. I would not change anything that could reduce the amount of side-travel, at most just something to reduce any "free-swing" it has..