World Cycling Day丨The secret of balance in small wheels: you only need strength and you to make the bicycle run smoothly

Produced by: Science Popularization China

Author: Star Team (Young Scholars)

Producer: China Science Expo

September 17 is World Cycling Day, which aims to encourage people to ride bicycles more and advocate green transportation. As a green and environmentally friendly way of travel, bicycles have long been popular and even a must-have for many people. However, this common means of transportation is actually like a magician who can cast spells. When we sit on a bicycle and pedal hard, we can magically maintain balance - even though the bicycle has only two wheels next to each other. When we stop, the bicycle will shake, but as long as we start riding, it seems that there is an invisible force supporting us.

What mysterious force enables bicycles to function the way they do, and how do we stay on two wheels instead of toppling over?

Gliding or riding? The evolution of bicycles

This starts with the history of bicycles. The history of bicycle development can be seen as a unique history of design evolution, which is full of the pursuit of stability, efficiency and comfort . The first bicycles were produced in Europe in the early 19th century . At that time, bicycles could not be pedaled, and the riders could only push them with their feet on the ground, which was more like a sliding tool . Moreover, this design lacked stability and was easy to tip over .

The original bicycle was more like a sliding tool

(Image source: Wikipedia)

Thus, the "high-wheeled bicycle" came into being, which is characterized by a large front wheel and a particularly small rear wheel . The large wheel can more easily cross obstacles on the road, and the stability is enhanced . However, the rider needs to sit in a high place, which increases the risk of riding . So soon, clever designers designed the type of bicycle we see today - two wheels of equal size, with the rider sitting in the middle. This design increases the stability and safety of the bicycle, laying the foundation for the rapid popularization of bicycles.

A high-wheeler, characterized by an extra large front wheel

(Photo source: Skoda Museum official website)

The first modern bicycle, then called a safety bike, was designed by John Starley in 1885.

(Image source: cyclingindependent official website)

The mysterious force that keeps a bicycle in balance

When we ride a bicycle, we feel as if some invisible force is keeping the bicycle balanced. What is this force?

Let's start with Newton's first law of motion, the law of inertia, to explain why we can stay balanced when riding a bicycle .

In simple terms, once a bike reaches a certain speed, it will try to maintain forward motion , and even if you stop pedaling, the bike will continue to move forward for a certain distance due to the inertia of the wheels and the bike itself .

Unless there is some external force intervening (such as turning, braking, or fighting the wind), a bicycle will always try to move forward in a straight line. This explains why it is easier to ride a bicycle at a steady speed on a flat road than to start, stop, or turn.

We often experience the existence of the law of inertia in our lives. For example, when a bus brakes, our bodies will lean forward because of the law of inertia.

Handrails on buses prevent passengers from falling due to the inertia of braking.

(Photo source: Veer Gallery)

However, the law of inertia can only explain why a bicycle can maintain its direction, but it cannot explain why the bicycle does not fall left or right. This requires the law of conservation of angular momentum , which has an important influence on the balance of the bicycle.

In the design of bicycles, the rotation of the tires essentially creates an angular momentum . According to the law of conservation of angular momentum, the total angular momentum of the system will remain unchanged unless there is an external torque . Angular momentum is a physical quantity related to direction. To keep it conserved, the direction of rotation of the object cannot change. Therefore, the fast-spinning wheel will try its best to keep its rotation plane stable, which is the famous "gyroscope effect". This also explains why a bicycle in a riding state, especially when the tires rotate faster, is difficult to tip over and more stable.

Another typical example is a toy spinning top. When a spinning top reaches a sufficient speed, it can keep spinning without falling. This is because the axle of the spinning top generates a certain angular momentum, which prevents the spinning axis of the spinning top from changing . Some readers may have played with a fidget spinner. When a fidget spinner starts spinning, if we try to change its rotation plane, we will clearly feel a force resisting it. This force is the angular momentum of the fidget spinner.

Gyroscope in rotation

When the gyroscope is whipped and gains angular momentum, it can keep spinning without falling over.

(Photo source: Veer Gallery)

Perfect coordination between man and force in cycling

Now that we have introduced the two physical principles of inertia and angular momentum, readers may have a deeper understanding of the experience of riding a bicycle. Although physical principles play a key role in the stability of a bicycle, the role of the rider is also quite important.

The rider's weight, body position, and force distribution together determine the overall stability of the bike. Imagine how you would react when your bike starts to shake on a bumpy road? You might involuntarily lean in the opposite direction, trying to offset the force and get the bike back in balance. This means that the rider needs to accurately sense the dynamic balance of the bike , and then adjust his body or direction according to the situation to maintain stability .

Although there are physical principles that ensure a bicycle's balance, riding a bicycle smoothly and quickly still requires a certain amount of training and skill.

(Photo source: Veer Gallery)

Conclusion

In general, the stability of a bicycle is not only the application of physical theorems, but also the display of the rider's skills. When you see a cyclist flying on a bicycle, you are actually seeing a wonderful dialogue between humans and the laws of nature. That picture is both a vivid example of the gyroscopic effect and a display of the human body's precise coordinated movement.