Use the following trick to ride a motorcycle without Niko wearing a helmet. Tap Y at any point during the animation where Niko puts the helmet on, but before the helmet actually appears. Doing this cancels the animation, and Niko will not wear his helmet. If you wait too long and press that button after the helmet has appeared in Niko's hand, it will still cancel the animation, but the helmet will immediately appear on Niko's head.