Bicycle physics. Why doesn't the bike fall? Physics of cycling: what forces does a cyclist overcome? This design achieves two goals

To prevent the two-wheeler from falling, you need to constantly maintain balance. Since the bicycle's support area is very small (in the case of a two-wheeled bicycle, it is just a straight line drawn through two points where the wheels touch the ground), such a bicycle can only be in dynamic equilibrium. This is achieved using steering: if the bike leans, the cyclist tilts the handlebars in the same direction. As a result, the bicycle begins to turn and the centrifugal force returns the bicycle to a vertical position. This process occurs continuously, so the two-wheeler cannot ride strictly straight; If the handlebars are fixed, the bike will definitely fall. The higher the speed, the greater the centrifugal force and the less you need to deflect the steering wheel to maintain balance.

When turning, you need to tilt the bike in the direction of the turn so that the sum of gravity and centrifugal force passes through the support line. Otherwise, the centrifugal force will tip the bike in the opposite direction. As when moving in a straight line, it is impossible to ideally maintain such an inclination, and steering is carried out in the same way, only the position of dynamic equilibrium is shifted taking into account the centrifugal force that has arisen.

The design of the bicycle steering makes it easier to maintain balance. The axis of rotation of the steering wheel is not vertical, but tilted back. It also extends below the front wheel's axis of rotation and in front of the point where the wheel touches the ground. This design achieves two goals:

When the front wheel of a moving bicycle accidentally deviates from the neutral position, a frictional moment occurs relative to the steering axle, which returns the wheel back to the neutral position.
If you tilt the bike, a moment of force arises that turns the front wheel in the direction of the tilt. This moment is caused by the ground reaction force. It is applied to the point where the wheel touches the ground and is directed upward. Because the steering axis does not pass through this point, when the bicycle is tilted, the ground reaction force is shifted relative to the steering axis.

Thus, it is carried out automatic steering, helping to maintain balance. If the bike accidentally leans, the front wheel turns in the same direction, the bike begins to turn, the centrifugal force returns it to an upright position, and the friction force returns the front wheel back to the neutral position. Thanks to this, you can ride a bike “hands-free.” The bicycle maintains its balance on its own. By shifting the center of gravity to the side, you can maintain a constant lean of the bike and make a turn.

It can be noted that the ability of a bicycle to independently maintain dynamic balance depends on the design of the steering fork. The determining factor is the reaction arm of the wheel support, that is, the length of the perpendicular lowered from the point of contact of the wheel with the ground to the axis of rotation of the fork; or, which is equivalent, but easier to measure, is the distance from the point of contact of the wheel to the point of intersection of the fork’s rotation axis with the ground. Thus, for the same wheel the resulting torque will be higher, the greater the inclination of the fork rotation axis. However, to achieve optimal dynamic characteristics, what is needed is not a maximum torque, but a strictly defined one: if too small a torque will lead to difficulty maintaining balance, then too large one will lead to oscillatory instability, in particular, “shimmy” (see below). Therefore, the position of the wheel axis relative to the fork axis is carefully selected during design; Many bicycle forks are designed to bend or simply move the wheel axle forward to reduce excess compensating torque.

The widespread opinion about the significant influence of the gyroscopic moment of rotating wheels on maintaining balance is incorrect.

At high speeds (starting from approximately 30 km/h), the front wheel may experience so-called speed wobbles, or “shimmies,” are a phenomenon well known in aviation. With this phenomenon, the wheel spontaneously wobbles to the right and left. High-speed swerves are most dangerous when riding “hands-free” (that is, when the cyclist rides without holding the handlebars). The cause of high-speed wobbles is not due to poor assembly or weak fastening of the front wheel, they are caused by resonance. Speed wobbles are easy to stop by slowing down or changing your posture, but if you don't, they can be deadly.

Cycling is more efficient (in terms of energy consumption per kilometer) than both walking and driving. Cycling at 30 km/h burns 15 kcal/km (kilocalories per kilometer), or 450 kcal/h (kilocalories per hour). When walking at a speed of 5 km/h, 60 kcal/km or 300 kcal/h are burned, that is, cycling is four times more efficient than walking in terms of energy expenditure per unit distance. Since cycling burns more calories per hour, it is also a better exercise activity. When running, the calorie expenditure per hour is even higher. It must be taken into account that the impact of running, as well as improper cycling (for example, riding uphill in high gears, overcooling of the knees, lack of sufficient fluid, etc.) can injure the knees and ankle joints. A trained man who is not a professional athlete can develop a power of 250 watts, or 1/3 hp, for a long time. With. This corresponds to a speed of 30-50 km/h on a flat road. A woman can develop less absolute power, but more power per unit of weight. Since on a flat road almost all the power is spent on overcoming air resistance, and when driving uphill the main costs are on overcoming gravity, women, all other things being equal, drive slower on level ground and faster uphill.

Based on Wikipedia materials

I still remember a physics lesson where this issue was discussed. Honestly, I was surprised to learn how many events happen when riding a bicycle, how many forces constantly interact... I will try to explain, why does the bike move, and who invented it.

What makes a bicycle move?

It can be said that one human power.:), but more seriously - muscular. Due to the fact that a modern bicycle is mainly a two-wheeled model, the rider need to maintain balance, while compensating for the actions of other forces that arise during movement. Indeed, the fact that a bicycle is a simple design does not mean that everything is so simple. Fundamental laws of science- the basis of the physical forces that arise during cycling.

Physical forces arising during movement

First of all, it should be noted that the forces involved in the movement process are divided into external and internal. So, the external ones are:

gravity- in other words, the force of gravity, described by Newton;
aerodynamic drag forces- have the greatest effect;
rolling resistance- for example, it increases when moving on sand;
forces that are caused by the maneuver- when changing direction.

Internal ones include:

torque- the force that makes the wheel rotate around its axis;
other forces- for example, friction of moving parts.

Who invented the bicycle

The debate continues to this day, and the answer will depend on which nation the question is asked of. At one time, attempts were even made to create a “bicycle” story, but the only conclusion was that this invention is the merit of many people who contributed to its emergence with their ideas. However, there are also key dates in the history of the bicycle. For example, in 1412 in Italy a structure was built that made it possible to move using muscle power. Although it was an ordinary car on 4 wheels, however with transmission to the rear axle using rope and pulleys.

The first structure resembling the modern one dates back to 1817, when shortage of horses led to Karl Drèze coming up with an alternative to the horse. This model had a key feature - a peculiar steering wheel in the form of a handle, which became the basis for the construction of all two-wheeled vehicles - rolling force resistance. The most interesting thing is that the movement was carried out by running - riders were afraid to take their feet off the ground, for fear of falling.

A model was built in 1884, similar to the bicycle we are used to - “Tramp”. The design provided chain transmission, identical wheels, and most importantly - the driver’s position between them.

If you ask the question “why doesn’t the bike fall?” everyone in a row, then most likely will not be able to answer it. They'll just shrug their shoulders. A minority of people who consider themselves technically literate will answer that this is probably due to the gyroscope effect. And they will probably be surprised to learn that the gyroscope has nothing to do with this, this was shown by an experiment in which this effect was neutralized, and the bicycle continued to move. And only a small minority will answer correctly. So why don't cyclists fall?

The bicycle does not fall due to centrifugal force

To maintain the balance of any body, it is necessary that the perpendicular lowered from its center of gravity does not extend beyond the area of support. The smaller the latter, the less stable the situation.

The bicycle support area is extremely small - in fact, it is a straight line drawn between the points where the wheels touch the ground. Therefore, a bicycle (with or without a cyclist) cannot stand while in a stationary position. But when moving, stability miraculously returns to it. Why is this happening?

It's all about the centrifugal force that occurs when steering. If a moving bicycle begins to lean in any direction, the cyclist slightly turns the handlebars in the direction of the lean, causing the machine to turn. In this case, a centrifugal force appears, directed in the direction opposite to the tilt. This is what returns the bike to an upright position. A two-wheeled bicycle cannot travel strictly in a straight line. If his steering wheel is fixed in a stationary position, he will definitely fall, because the possibility of steering is eliminated.

This process - deviation from the vertical and return to it - occurs continuously. The cyclist doesn’t even think about what’s happening. His hands automatically perform the steering, which is necessary to maintain a vertical position. By the way, learning to ride a bicycle is precisely the acquisition of automatic steering.

Bike design and balance

The design of the steering column and front fork of the bicycle makes it easier to automatically maintain balance. The axis of the steering column (front fork) is not vertical, but inclined to the ground. The point where it intersects with the ground is located in front of the place where the front wheel comes into contact with the road. This arrangement ensures that if the front wheel accidentally deviates from the average position, a moment of reactive forces immediately arises, which returns it to its place.

When the bicycle is tilted, the reaction of the front wheel support, which is applied at the point of contact with the ground and directed upward, automatically turns the wheel in the direction of the tilt. Centrifugal force arises and the bicycle returns to a vertical position.

To better understand this process, you just need to consider that the pattern of forces acting on the front wheel of a bicycle is approximately the same as that of carts with rotating wheels. Whichever way you push the cart, the wheels automatically turn in the right direction. By the way, it is precisely this design feature of the bicycle that makes it possible to ride without holding the handlebars with your hands. The bicycle maintains its balance on its own. And to perform a turn, it is enough to shift the center of gravity of your body to the side.

The degree to which a particular bicycle can maintain dynamic balance is determined by the design of its headset and fork. The main parameter here is the distance from the point of contact of the front wheel with the ground to the point of intersection of the axis of the steering column (front fork) with the ground. As already mentioned, the latter is ahead of the former. The greater the distance, the higher the reaction torque acting on the wheel when turning it. For optimal dynamic characteristics of a bicycle, it is not the largest, but a strictly defined reaction torque that is required. Too small will reduce automatic balance maintenance, too large will result in shimmy. Therefore, the tilt of the steering column axis and the parameters of the front fork are chosen very carefully when designing a bicycle.

What is "shimmy"

At high speeds (above 30 km/h), the front wheel of the bicycle may begin to spontaneously wobble left and right. This phenomenon, which, by the way, also occurs in aviation, is called “speed wobbles” or “shimmies.” The reason for this is not a malfunction of the bicycle (poor assembly or loose fastenings), but because resonance of the front wheel occurs. “Shimmy” is very dangerous when the cyclist rides “without hands,” that is, without holding the handlebars. To extinguish the resulting resonance, you need to slow down or change your posture.

Bicycle - more energy efficient

In terms of energy consumption per unit of distance covered, a bicycle is more efficient not only than walking, but also driving a car. When a bicycle moves at a speed of 30 km/h, 15 kcal are spent per 1 km. Walking at a speed of 5 km/h burns 60 kcal per 1 km. That is, in terms of energy consumption per unit distance, cycling is 4 times more efficient than walking.

...and more functional

If we consider cycling from the point of view of sports activity, then it also turns out to be preferable to walking. Cycling consumes 450 kcal per hour, while walking only consumes 300 kcal. Of course, physical activity can be increased by switching from walking to running. But in this case, the load on the knees and ankle joints increases, which is undesirable, since over time it can lead to injury to these problem areas.

When women are faster

A trained man, even without being a professional athlete, can develop a power of 250 W or 0.33 hp for a long time. With. When cycling on a flat road, this corresponds to approximately 30 km/h. Women cannot develop as much power as men, but per unit of weight their energy levels are superior to men's. When driving on a flat road, when all the power is spent mainly on overcoming air resistance, women drive slower than men. But when driving uphill, when energy is spent overcoming gravity, they are able to drive faster than the stronger half.

A two-wheeled bicycle does not fall when moving, because the one riding it constantly maintains balance. The bicycle support area is small - it is a straight line, which is drawn through the points of contact of the bicycle wheels with the ground. Therefore, the bicycle is in a state of dynamic equilibrium.

This is achieved with the help of steering: when the bicycle is tilted, the person turns the steering wheel in the same direction. After this, the bicycle turns, while the centrifugal force returns the bicycle to its initial vertical position. The process of steering to maintain balance occurs continuously, so the movement of the bicycle is not rectilinear. If you fix the handlebars, the bike will fall.

There is a relationship between speed and centrifugal force. The higher the speed, the greater the centrifugal force and, accordingly, the less it is necessary to deflect the steering wheel to maintain balance.

To turn, you need to tilt the bike to the side so that the sum of centrifugal force and gravity passes through the wheel support line. If this is not so, then the centrifugal force will tip the bike in the other direction. To make it easier to maintain balance, the design of the bicycle steering has its own characteristics. The steering column axis is tilted back rather than vertical. It runs below the axis of rotation of the wheel and in front of the point where the bicycle wheel touches the ground. Thanks to this type of design, the following goals are achieved:

When braking when riding a bicycle, the main thing is to maintain balance. Braking is no less important than the riding itself, and most likely the most important, because the health of the cyclist depends on it. If you know the theory of how a bicycle behaves at the moment of braking, you can greatly reduce the number of bruises and bumps (unfortunately, you still can’t do without it).

Everything is clear with the definition. The encyclopedias say that “to brake is to slow down movement using the brake.” But the whole thing is that usually everyone is not very interested in what to slow down (although this should be mentioned). Usually everyone is interested in how to slow down the movement (press on the lever and that’s it), and not how to slow it down in a certain specific situation on road.

You can try to write down a lot of theoretical advice for all possible situations on the road, but there are always exceptions to the rules and sooner or later the cyclist finds himself in a situation where there are not enough recommendations. The most important thing is that braking when riding a bicycle is brought to automaticity, because in emergency cases there is simply no time to think about how to do it correctly and remember the theory.

Intuition helps you make the right decision, but you also need to know some theoretical rules for how a bicycle behaves when braking.

The rolling of a bicycle depends on various factors: the characteristics of the frame, shock absorbers, wheel diameter, tires, pressure in the chambers, the total weight of the bicycle and many others. The run-up cannot be measured in numbers. Experienced cyclists can feel and appreciate it. For amateurs, the difference is especially visible if they exchange, for example, an inexpensive bicycle for a more expensive and high-quality one.

Frame. There is an expression “rolling frame”. But it is very difficult to feel the difference between a “non-rolling” and “rolling” frame, because clearly noticeable features are characteristic only of very expensive models. Frames made from expensive materials tend to absorb shocks and vibrations. Longer frame designs help the cyclist achieve a more aerodynamic position on the bike, which has a positive effect on the ride. But, on a regular bicycle, the coasting on the frame does not depend as significantly as on other components.

Wheel size. One of the main determining factors influencing the roll of a bicycle. Larger wheels of 28 or 29 inches travel faster than 26 inch wheels, so the bike with them is more rolling. Now popular 29ers with 29 inch wheels have this quality.

Tire tread. Smooth, narrow rubber without tread rolls best. The worst thing is a wide aggressive tire with a high tread pattern.

Since a classic bicycle has two wheels, in order for the cyclist to ride, he constantly needs to maintain balance and overcome various forces that arise during the movement. Just because the design of a bicycle is simple, it doesn’t mean everything is that simple. The physical forces acting when riding a bicycle are based on the fundamental laws of science. Let's consider the main forces that act when riding a bicycle.

1. Gravity (gravity). Gravity is one of the four fundamental phenomena in nature. Explained by Newton's law. The force with which it acts is directly proportional to the cyclist's body weight. The greater the weight of the cyclist, the stronger the force of gravity. It acts on the cyclist and bicycle components perpendicular to the ground. The force of its action increases when cycling uphill and correspondingly decreases when descending.

2. Air resistance force. The aerodynamic forces acting on the cyclist mainly consist of air resistance and head or side wind. At average speed and moving on a flat surface, aerodynamic drag is the greatest force that prevents forward movement. With a further increase in speed, aerodynamic drag becomes overwhelming, and its magnitude far exceeds all other forces that impede forward movement.

When the improvement of the technical characteristics of a bicycle reached a certain limit and the differences in the performance of individual components from different manufacturers practically disappeared, attention was paid to the air resistance that the cyclist overcomes when riding. This indicator had an impressive digital value, so there was something to work on.

As in the aircraft and automotive industries, a wind tunnel is used to test how the oncoming air flow affects a cyclist. This expensive device helps determine the interaction of an object (cyclist) with the air flow, as well as determine the acting force in a numerical value. During the tests, the optimal position of the cyclist is determined, as well as the coefficient of resistance to the oncoming air flow of individual parts of the bicycle and the athlete’s equipment.

The design of a wind tunnel is a room, on one side of which high-performance fans are installed; they create an air flow simulating a headwind, the speed of which is regulated by changing the power of the electric motors rotating the fan blades

During the operation of the bicycle, loads are applied to the frame, which are repeated many times. These cyclic loads arise from uneven road surfaces: holes, bumps, potholes in the asphalt, etc. When aluminum alloys began to be used in various structures (especially in aviation and astronautics), studies showed that a single load does not cause deformation and destruction of the material, but a certain number of load cycles in the structural material caused deformation, cracks and subsequent destruction. This phenomenon is characterized by the term “fatigue failure.” The number of loading cycles that leads to failure is called “fatigue life.”

The same studies showed that the presence of cracks, dents, holes, and welds in the most loaded areas of the structure reduces the durability of the structure itself by an order of magnitude. This tendency is called “local stress concentration”. Even a small hole in the structure increases the voltage next to it by at least 2 times, and a scratch of sufficient depth by 5-6 times. The crack increases the local stress to the yield point and therefore grows systematically at an increasing rate.

Since a classic bicycle has two wheels, in order for the cyclist to ride, he constantly needs to maintain balance and overcome various forces that arise during the movement.

Just because the design of a bicycle is simple, it doesn’t mean everything is that simple. The physical forces acting when riding a bicycle are based on the fundamental laws of science. Let's consider the main forces that act when riding a bicycle.

External forces

2. Air resistance force. The aerodynamic forces acting on the cyclist mainly consist of air resistance and head or side wind. At average speed and moving on a flat surface, aerodynamic drag is the greatest force that prevents forward movement. With a further increase in speed, it becomes overwhelming, and its magnitude far exceeds all other forces that impede forward movement.

3. Rolling resistance force. Rolling resistance is the force that occurs when a round object, in this case a bicycle wheel, moves along a flat surface at a straight-line speed. Occurs mainly due to deformation of the wheel, deformation of the surface on which the wheel moves, or deformation of both. When riding a bicycle, this force increases when the wheels are poorly inflated or when moving, for example, on sand. Also, the strength of rolling resistance additionally depends on factors such as the radius of the wheel, the speed of movement and the type of contacting surfaces.

4. Forces arising during maneuvers to balance a bicycle. Occurs when changing the direction of movement of the bicycle or when manipulating the handlebars in order to balance the bicycle and maintain balance. Determined by centrifugal force. In mechanics, the term centrifugal force is used to explain two concepts - inertial force and centripetal force. These are complex processes and it takes quite a long time to sort them out. All of them are described in textbooks.

Inner forces

1. Torque- this is the ability, with the help of applied force, to rotate an object around its axis, that is, a bicycle wheel. The force is created by the cyclist's legs, and the torque is transmitted from the pedals to the bicycle wheel using a chain, cardan, belt or other transmission. Adjustable by selecting front and rear sprockets in various options.

2. Other internal forces are mainly caused by friction between the moving parts of the bicycle and its design options. Their value depends on the type of suspension, transmission, steering mechanism and other structural elements.

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