The bicycle is becoming an increasingly popular means of transport these days, when there are so many cars that they interfere with each other's existence. Bicycles have numerous advantages over cars, which is why in many European countries they are considered almost the main means of transportation. The popularity of two-wheeled friends is growing in our country.
A bicycle is not only a means of transportation, but also a complex mechanical system that works according to the fundamental laws of physics. All bicycles, regardless of type, brand, model and cost, challenge their riders to overcome various forces. While riding, a cyclist faces two main forces: gravity and aerodynamics. The force of gravity presses the cyclist and his vehicle to the ground. In this case, the vector of force is directed strictly perpendicular to the surface of the earth. The heavier the bicycle and its rider weigh, the greater the force of gravity. It has a great influence on the efforts that a cyclist has to put in when riding his two-wheeler. vehicle. If your body weight and the weight of the bike are less, then riding will be much easier, which means that riding will give you more pleasant sensations. Although, for some, a bicycle is an exercise machine for burning calories.
The second fundamental physical force that a cyclist must overcome while riding is aerodynamics. In essence, this is the resistance force of the oncoming air flow, which increases as the speed increases. The faster the cyclist moves, the greater the force of air resistance. In addition to oncoming air currents, side winds can also act on a bicycle, which further complicates movement and forces you to exert additional forces. Overcoming aerodynamic forces when driving at high speed on a flat road is not easy - this requires excellent physical training. If you don’t have one, then it’s better to buy a bicycle with an electric drive, which will allow you to ride in two modes - mechanical and automatic. It should be noted that when driving mechanically, much more energy and effort is spent than in automatic mode. In order to save battery power, it is better not to drive an electric drive all the time, but only in those areas that are especially difficult to overcome on your own (climbs, rough terrain, and so on).
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. Physical strength, operating 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
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. How more weight the cyclist, the stronger force 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 when 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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One of the most favorite types active recreation is a bike ride. In addition to the fact that a bicycle allows you to strengthen and develop various muscles (muscles of the legs, arms, back and abdomen), it is also a means to see local attractions or simply cheer yourself up by riding it with the whole family or with friends. However, riding a bicycle improperly can cause bruises and abrasions. Especially when driving at high speed while turning. Let's try to figure out what you need to do to safely navigate turns while riding a bike.
When the bicycle pedals rotate, the force of the cyclist is transferred to the wheels, so they begin to rotate. Bicycle tires interact with the road surface. The forces of this interaction are the support reaction force and the friction force, it is the latter that causes the bicycle to move and also protects the bicycle from skidding during a turn. The greater the friction force between the bicycle tires and the road surface, the more confident and reliable the ride will be, especially when cornering. The maximum friction force is the sliding friction force, it is determined by the formula:
where is the friction coefficient, and N is the support reaction force directed vertically upward.
During a turn, the bicycle moves along an arc having a certain radius R (see top view). In this case, the speed of the bicycle is directed tangentially to the trajectory, and the centripetal acceleration and friction force holding the cyclist are directed towards the center of the arc. According to Newton's second law:
Considering that the force of gravity is directed vertically downward and the centripetal acceleration is equal to,
we find that the minimum possible arc radius is calculated by the formula:
The friction coefficient of rubber is in the range from 0.5 to 0.8 for dry asphalt and in the range from 0.25 to 0.5 for wet asphalt. Therefore, when driving at a speed of 15 km/h (approximately 4.2 m/s), it will be safe to turn along an arc of radius R = 4.2 2 / (0.5 9.8) = 3.6 m (dry asphalt) and R= 4.2 2 / (0.25 9.8) = 7.2 m (wet asphalt).
It should also be noted that to maintain balance when turning, you need to lean the bike slightly in the direction of the turn.
Using the proposed method, we suggest you calculate:
- safe turning arc radius at a speed of 24 km/h on a dry dirt road (friction coefficient 0.4) and on ice (friction coefficient 0.15);
- angle α of the bicycle's inclination to maintain balance when turning at the same speed, taking into account that the centrifugal force is applied to the center of mass of the bicycle.
Moments of forces when a bicycle moves.
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:
Bicycle stability when braking.
When braking when riding a bicycle, the main thing is to maintain balance. Braking no less important point 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).
What is braking
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 the road. You can try to write down a lot of theoretical advice on everything 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. Accept the right decision Intuition helps, but you also need to know some theoretical rules for how a bicycle behaves when braking.
Bike roll.
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 change, for example inexpensive bike to more expensive and high quality.
What determines the roll of a bicycle?
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.
Physical forces acting when riding a bicycle
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.
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.
Aerodynamic tests in cycling
When improvement technical characteristics the bicycle has reached a certain limit and the difference in the performance of individual components from different manufacturers has practically disappeared, we paid attention 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, and also determines effective force in 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 individual parts bicycle and 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
Bicycle frame durability
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.
In order to two wheeler If you don't fall, you need to constantly maintain your 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. In addition, it passes below the axis of rotation front wheel 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.
On 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 reason for 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). Walking at a speed of 5 km/h burns 60 kcal/km or 300 kcal/h, that is, cycling four times more effective than walking based on energy consumption per unit distance. Since cycling burns more calories per hour, it is also the best sports load. 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, cold knees, lack of sufficient fluid, etc.) can injure the knees and ankle joint. A trained man who is not 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
