# Suspension system

### Suspension system

#### The role and type of suspension

When the car on which people ride was the first in the world to appear in the form of a carriage, springs were already used in the carriage as a suspension.

Since the first Mercedes-Benz four-wheeled vehicle had a gasoline engine attached to the carriage, it means that the car was equipped with a suspension from the beginning.

At that time, the suspension was only an elliptical leaf spring type suspension, in which the leaf springs used in today’s automobiles were stacked vertically symmetrically. In other words, since this leaf spring is the only one that stabilizes the movement of the axle, it sways with respect to the vehicle body, and the positioning of the wheels was not accurate, so it was extremely unstable.

Also, in the early days, automobiles had low speeds, rubber tires were not even invented, and iron wheels were used instead of wheels, so the grip was much lower, and the force that the wheels transmitted to the vehicle body was also low, so it was used for suspension. It seems that the required performance did not have to be very advanced.

##### Invention of pneumatic tire

As the engine output increased with the evolution of automobiles and people using cars began to use cars at high speeds, pneumatic tires were invented as new wheels. Pneumatic tires have dramatically improved ride quality and significantly improved grip on the road surface.

Therefore, it has become necessary to lower the center of gravity of the vehicle and ensure wheel positioning (wheel alignment).

##### The role of wheel alignment and suspension

The first role of the suspension is to absorb the impact from the road surface, but at that time the wheels move up and down and the direction and inclination of the wheels change, so the second role is to properly ground the wheels to the road surface. be. That is, generally speaking, it is said that it is desirable for the vehicle body that the tires are as perpendicular to the road surface as possible.

In terms of keeping these tires perpendicular to the road surface, the rigid axle type (axle suspension type) has an excellent aspect among the suspension types, but it is disadvantageous in terms of ride quality and steering stability. Due to its disadvantages, it is now rarely used in passenger cars. (On the other hand, it is used in most cars for heavy-duty trucks that need to withstand high loads.)

In addition, as tires have become more sophisticated and thicker as they are today, even slight changes in camber and tread have come to affect the behavior of the vehicle, so double wishbone and multi-link types have come to be affected. Suspension was developed to enable higher level wheel movement.

In addition, when the spring is softened to improve the ride quality. Since changes in the posture of the vehicle body such as raising and lowering the head during acceleration and braking become large, suppression and control of changes in the posture of the vehicle body have come to be counted as one of the roles.

It is also important that supporting the weight of the car body is one of the most basic roles as a major premise of the role of the suspension.

The basic role of the suspension is shown below.

• Support for body weight
• Impact absorption from the road surface
• Transmitting the road reaction force of the wheels to the vehicle body
• Correctly ground the tires to the road surface
• Control of vehicle body posture and wheel alignment
• Suppression of wheel vibration

#### Rigid axle and independent suspension

Suspensions are roughly divided into two types: axle suspension type (rigid axle) and independent suspension type (independent suspension). Most of the current passenger cars adopt independent suspension type (independent suspension) for both front and rear wheels. is doing.

In addition, many trucks use rigid axles for both front and rear wheels, and by the time the FR system was the mainstream for passenger cars, many models adopted rigid axles for the rear wheels and independent suspension for the front wheels. Was.

##### Rigid axle

Rigid axles are sturdy axles that firmly connect the two wheels, so the tires are close to perpendicular to the road surface, that is, they are ideal for achieving full tire performance. There are merits.

Figure 1: Force generated when one wheel rides on

However, when one wheel rides on the road surface, the other wheel is also affected, the installation condition changes, and both wheels tilt. (refer graph1)

In general, a rotating body such as a disk that rotates at high speed has a function of keeping its axis of rotation constant (gyro effect). This gyro effect has the property that when the axis of rotation to be kept constant is forcibly tilted, it tends to tilt 90 ° in the direction of rotation from the direction in which it is tilted. (See Figure 2)

Figure 2: Presession effect

This is called the pre-session effect, and in the case shown in Fig. 1 due to this effect, the direction of the wheel tends to change in the direction in which the wheel is tilted. For example, in the case of a motorcycle, the steering is sharpened in the direction in which the vehicle body is tilted, so it is said that the force that tries to raise the vehicle body works to maintain straightness.

As shown in Fig. 1, when one wheel rides on a protrusion or the like, in the case of the front wheel, the steering wheel is taken to the tilted side opposite to the protrusion due to the presession effect. The ground contact state changes when the other wheel also tilts, and the canvas last (lateral force acting in the direction in which the tire tilts) tries to move in the tilted direction. In other words, if the car tries to move in an unexpected direction, the straightness will be hindered.

The gyro effect increases proportionally as the rotation speed of the wheels increases and the outer diameter of the tire increases (as the vehicle speed increases). Therefore, although there is no phenomenon that the steering wheel is taken off on the rear wheel side, it may be shaken to the left or right to some extent.

In addition, when encountering unevenness at high speed, the rigid rack itself is heavy, which causes a phenomenon in which the left and right wheels vibrate up and down alternately (a phenomenon like stepping on the ground), and the ground contact of the wheels deteriorates. Stability at high speeds may deteriorate.

Regarding the unsprung load, even if the unsprung load is a little heavy, if the road surface is smooth, there will be little adverse effect, but if there is unevenness, the tire will sometimes jump away from the road surface and keep in contact with the road surface. It will disappear. Therefore, it becomes impossible to pass through an uneven curve at high speed. In such a case, the independent suspension with a small unsprung load becomes effective.

In addition, when a soft unsprung load is combined with a soft spring (with a small spring constant), the wheel tends to run wild, and the use of the soft spring is restricted.

Ride quality depends on acceleration rather than the amplitude of vibration on the spring (vehicle body), but when acceleration is multiplied by mass, it becomes a force.

Here, if the mass is m, the acceleration is a, and the force is f.
$$f = ma$$
Is established.

Since the force to move the wheels up and down is transmitted to the vehicle body via the spring, the smaller the unsprung load, the smaller the impact transmitted to the vehicle body.

$$Unsprung load × Unsprung acceleration = Unsprung load × Spring acceleration$$

$$Unsprung acceleration = Unsprung acceleration × \frac{Unsprung load} {Unsprung load}$$

In addition, the smaller the impact, the smaller the strength required for the vehicle body, so it is possible to reduce the weight.

In terms of ride quality, when the unsprung load is large, the impact of pushing up when passing through protrusions is large, and when the unsprung load is small, the shock is felt lightly.

Other than that, in the combination of leaf springs and rigid axles used in trucks and old passenger cars, it is necessary to steer the steering to the left and right in the case of front wheels, so the spring mounting position must be moved inward. (See Figure 3)

Figure 3: Problems with leaf springs and rigid axles

However, in the case of a rigid axle, a force that opposes the roll is generated at the mounting position of the spring. In that case, the roll rigidity, which indicates the difficulty of rolling, is proportional to the square of the distance between the two springs on both sides. It ends up.

##### Independent suspension

In an FF passenger car, if an independent suspension is used for the front wheels, space can be secured because there are no obstructive axles, and the engine mounting position can be moved to the front of the center of the front wheels for mounting. .. As a result, the indoor space can be made much larger.

When the independent suspension is adopted, the unsprung load is about two-thirds for the front wheels of FF vehicles and about one-half for the rear wheels equipped with heavy differential gears of FR vehicles. In other words, by adopting the independent suspension, it is possible to significantly reduce the unsprung load on the rear wheel side.

However, when trying to adopt an independent suspension for the rear wheel of an FR car, a drive shaft with a joint is required in addition to a complicated suspension arm, which makes development difficult and cost performance worse. ..

In summary, the independent suspension is widely used on the front wheel side, which has a great effect on improving steering stability, and is also adopted on the rear wheel side as the average running speed improves and the performance of the entire vehicle improves. It became so. It can be said that the purpose is not only to improve the ride quality but also to improve the steering stability.