Why do pedicab go wrong?

- Jan 02, 2019-

The human tricycle is a commonly used transportation tool for the elderly and one of the transportation tools commonly used in daily life. Although it has one more wheel than a bicycle, a bicycle rider does not necessarily ride a tricycle. Why?


Habitually we would think: three wheels must be more stable than the two wheels. Is that true? In real life, you may be a master of cycling, but if you try on a tricycle, does it not listen to you? Is it not going straight? Is it always biased to one side? Is it necessarily biased to the right? So, what causes the tricycle to not go straight?


Question 1: The friction of the front and rear wheels of the bicycle

Bicycle is one of the most widely used means of transportation in daily life. In the second edition of the physics textbook of the Suco edition, an activity "to study the friction on the bicycle" was specially arranged. The teachers also paid enough attention in this part and will be in the teaching materials. Excavation outside the scope of the given study. The trilogy of tricycle manufacturers is also analyzed.


Let’s talk about the rear wheel of the bicycle. The rear wheel of the bicycle is the driving wheel. When riding, the rear wheel will “actively” rotate under the action of the manpower through the chain, so it is sometimes called the driving wheel. If you put the bicycle The rear wheel is lifted off the ground, and the pedal is rotated to rotate the rear wheel. The lower part of the rear wheel moves backwards with respect to the ground. When the rear wheel contacts the ground, the rear wheel does not move relative to the ground, but slides backwards. The trend (similar to when a person walks his foot backwards). At this time, the ground just obstructs this trend of the rear wheel, giving the rear wheel a forward friction. It is because of this friction that the bicycle is made. Can ride forward.


Let's talk about the front wheel of the bicycle. The front wheel of the bicycle is the driven wheel, sometimes called the passive wheel. It has no chain drive. So, who is it moving from? For example: in acrobatic performances, the acrobats can lift the front wheel of the bicycle off the ground and only use the rear wheel. The front wheel will not rotate, but the lower part of the front wheel will move forward relative to the ground. Once the front wheel touches the ground, the ground will hinder the front wheel from moving forward, so the front wheel and the ground contact part will be "passively" turned backwards by the ground friction against it.


Simply put: the frictional direction of the rear wheel (drive wheel) of the bicycle is forward, and the direction of the friction of the front wheel (driven wheel) is backward.


Question 2: Conditions for the balance of the two forces

In the experiment of exploring the conditions of "two-force balance", the conditions for the balance of two forces are obtained by experiments: "same body, equal big, reverse, collinear". In the experiment, when the small card satisfies the above condition balance at the same time, if it is small When the card is twisted at an angle, the balance condition of the small card is broken. After the hand is released, the small card will lose its balance and rotate, and finally return to the original balance state. This is because after twisting the small card, the thin lines at the ends of the small card will not be the same. On a straight line, the small card will not be able to preserve the balance under the action of two opposing forces that are not collinear. Therefore, the small card will rotate and return to its original collinear state.


Not only a small card, but if any object is subjected to only two forces (subject to multiple forces, but the other forces are balanced with each other, it can be considered to be only affected by two forces), and these two The force is not collinear, so the object will not be able to save the balance.


Analysis of the reasons for tricycle deviation

The tricycle has three wheels. First of all, we have to figure out who is the driving wheel and who is the driven wheel? The front wheel of a tricycle is similar to the front wheel of a bicycle. It is the rear wheel of a driven wheel. The tricycle? Are they driven by chains? Are the driving wheels? In order to clarify this problem, we can lift the rear part of the tricycle so that the two wheels at the same time leave the ground at the same time, and then let the other person turn the pedal of the tricycle to see the rotation of the rear wheel. It is not difficult to find that two The wheel on the right side of the rear wheel does not rotate, only the left wheel will rotate with the rotation of the pedal. Therefore, the right wheel of the rear wheel of the tricycle is also the driven wheel, and only the left wheel is the driving wheel. The tricycle has two driven wheels and one driving wheel.


Combined with the friction of the front and rear wheels of the bicycle, the rear compartment of the tricycle can be regarded as a small card similar to the experiment of exploring the condition of "two-force balance". The rear compartment is subjected to two frictional forces that are not collinear in the reverse direction during driving. The role will turn to the right.