It's not just one thing. At different speeds there are different things that help you stay balanced. At low speeds, yes, what you do with the handlebar and so t

It's not just one thing. At different speeds there are different things that help you stay balanced. At low speeds, yes, what you do with the handlebar and so the front wheel is predominant. At higher speeds gyroscopic forces do not actively keep you upright, but they help by slowing down the speed with which you loose you balance and so you have more time to react (by steering).