The right answer is 1) the predominance of the centrifugal force. But gyroscopic forces also play a role in bicycle stability. You learn it so instinctively that it seems obvious. Staying upright on a bicycle, however, is a complex phenomenon, involving various physical principles and the judicious interplay of a number of forces. Physicists are still working to understand this phenomenon; they regularly publish articles considering new hypotheses. The most frequently cited principle is angular momentum, or the gyroscopic force. This is a force that comes into play when a body is rotating around an axis, such as a wheel or a top. The faster the rotational speed, the more stable the axis around which the object is turning becomes, helping it to resist any change in direction. Many articles and essays explain the stability of bicycles by this inertial force due to rotational movement. But this force is only really of much consequence above a certain speed, and it’s not a large enough effect to fully explain the phenomenon. The determining factor seems to be the centrifugal force. This force acts to pull a rotating body outward, away from its axis of rotation. For a bicycle, it acts as the square of the speed, in a subtle balance with the combined weight of the bicycle and its rider and the gravitational force. It comes into play when the bike is moving in a straight line, and acts to restore equilibrium from small perturbations. The faster the bicycle is traveling, the more this force dominates the others, and the more stable the bike will be. It’s the combination of these two forces – angular momentum and the centrifugal force – that explains the fact that a rider doesn’t use the handlebars to change direction, as one might intuitively assume, but rather his or her weight. By leaning more or less strongly to one side or the other, the rider plays with the centrifugal force, using it like a lever. This is why, once you have the technique down, you can steer and turn a bike with “no hands.” The handlebars don’t turn the bike These two forces also explain why, as a child, it probably took you quite a while to learn to ride a bicycle. You have to assimilate something that runs counter to your intuition – if you feel like you’re falling to one side, say to the right, you shouldn’t abruptly turn the handlebars in the opposite direction, because this maneuver will just accelerate your fall. Instead, you have to give a twist in the same direction, to the right, which will exert a centrifugal force to the left. Together with the angular momentum initiated by the wheel’s rotation, this brings the bicycle upright again. But as if that wasn’t enough, something else must be considered to fully understand a bicycle’s stability: the rotation of the handlebars around the steering axis. The handlebars are not there so much to change direction, as they are to correct and compensate for changes in balance. The fork, which is typically curved forward, also accentuates this role. If you pull on the right handlebar, this brings the steering axis behind the point where the wheel touches the ground. This distance, called the “trail,” makes the bike self steer via a force that compensates to the left when you turn the handlebars to the right, and to the right when you turn them to the left. This compensating force thus stabilizes the handlebars, helping them stay more parallel to the road.