Gyroscopic effect. The wheels on a bike are big gyroscopes, and serve to keep it upright, and pointed straight. In fact, when you turn a motorcycle at speed, what you are actually doing is moving the front gyro (wheel) out of line, allowing the weight of the bike to fall toward the turn. That's why you push forward on the side of the handlebar on the inside of the turn - push forward on the right grip to turn right.
