How does a bicycle stay upright? It is a combination of things, but nobody seems to really know for definite. All of the reasons that a bicycle can stay upright are valid, but none of them seem to be required. You can alter many of them and, so long as the others are still in place, the bike will stay upright. There doesn’t seem to be a definitive combination of reasons and it could be down to our sense of balance as much as with the bicycle itself. Let’s look at each of the reasons that are thought to have something to do with it.
The gyroscopic effect on the wheels. One reason why a wheel stays straight and keeps moving is because of the gyroscopic effect, which is also the reason why gyroscopes can keep perfectly steady. When a wheel spins, at any point motion in one direction is cancelled out by force in the other direction. For example, let’s take a point on the wheel and call it A. If you spin the wheel and hold the axle in your hands, you won’t be able to tilt the wheel to the left. This is because if you try to tilt it, when A is at the top it tries to move to the left, but it spins to the bottom, still trying to move to the left and the two forces cancel each other out, keeping the wheel spinning straight. Some say that the gyroscopic effect keeps a bicycle upright, but the wheel has to be spun very quickly for this effect to take place. A bicycle wheel doesn’t spin that fast and still remains upright. If the gyroscopic effect was responsible for a bicycle staying upright, you wouldn’t be able to lean the wheels over to steer and anybody could just get on a bicycle and ride, which is obviously not true because children need to learn. Also, it would be impossible to knock a bicycle over at speed, which you can obviously do if you lock the handlebars. People have tested this by making wheels that have an inner wheel that rotates in the opposite direction to the outer wheel, cancelling out the gyroscopic effect and the bicycle still stays upright.
The second thing is something called fork trail. The fork trail is how far the front axle is offset from the steering axis. It is measured by taking the point where the tire hits the floor and its distance from the line that comes off the forks. Bicycle forks are always set so that the line from the forks hits the ground in front of the place where the tire hits the ground. This is done because when the line from the forks impacts the ground in front of where the tire actually impacts the ground, the forks lead the tire and the wheel will easily follow where the forks want to go. If you lean the bike to the right, the forks lead the wheel and it automatically goes into a right turn. However, even though fork trail helps us steer a bike, it doesn’t appear to be vital. People have made bicycles with the forks directly above where the tire hits the road, or even with the line coming behind that point, and all of the bikes are perfectly rideable.
So, is it the center of mass? Now, here we are getting to the meat of the problem. It would appear that this is the only thing about a bicycle that you cannot change. You need to have the center of mass in the middle of the bicycle and that needs to be kept constant no matter how the bicycle moves. If a bicycle goes up or downhill, the center of mass has to be shifted to compensate. And this is true as well when balancing a bicycle. A bicycle stays upright because the rider is constantly making tiny adjustments to keep it balanced. We use the signals from our ears and the sensation of the bike to intuitively make corrections. If the bike leans slightly to one side, you adjust your body to the other side, but you also need to turn the wheel because it is a combination of the motion of the wheel, the fork trail and your balanced that keeps the bicycle upright. You can see this because you can balance on a bicycle, but if you stop the handlebars from being able to move, you will fall off. You could not cycle along a very thin path because even though you feel like you are going straight, you are making constant, tiny wobbles to correct your motion and balance. You can see that much more easily when you look at a child learning to ride. So, there is no one reason why a bicycle stays upright. And this is what I learned today.
Photo by Andrea Piacquadio: https://www.pexels.com/photo/man-in-black-suit-riding-bicycle-down-the-street-1843752/
Sources
https://www.bikeradar.com/features/your-bikes-secret-to-staying-upright-is-actually-a-mystery
https://www.britannica.com/video/185402/bicycle-motion
https://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics