Your head into a solid metal, or concrete object at 30mph!!! Why would you even ask such a silly question? Unless your name is Clark Kent from Smallville.
We all thought we were indistructable when we were young, You sound young. Nothing wrong with being young, We ALL were at one time. But PLEASE take some advice from people that's been there, Done that. And not that it's good enough, but even a bicycle helmet is better then nothing at all. DOT approved is the way to go. DOT approved helmets start at around $30, You spend that much or more on a sprocket upgrade. Or if you REALLY ARE too cheap to buy your own, I'm sure your Daddy will spring for it. And you'll be saying "Thanks" to him when you turn 30.
fatdaddy.