1. Handling of shock absorbers

Since the oil seal, piston rod, and other components of the shock absorbers are made with extreme precision, the following precautions must be taken when handling them:

Exposed portions of the piston rod must not be scratched to prevent leakage of shock absorber fluid. In addition, the piston rod should be kept free of paint and oil.

In order to prevent damage to the oil seal caused by interference with the piston valve, the rod and cylinder should not be rotated with the shock absorber extended completely. Particular caution is required in the case of gas-filled shock absorbers since the piston rod is constantly pushed up by the pressure of the gas.

2. Handling of gas-filled shock absorbers

Since pressure is constantly applied on the inside of gas-filled shock absorbers, the following precautions should be taken in addition to those described above:

Do not attempt to disassemble non-disassemblable shock absorbers (including all DuCarbon type shock absorbers as well as low-pressure gas-filled shock absorbers in which the ring nut is caulked).

When disposing of gas-filled shock absorbers, first evacuate the gas.

3. Evacuation of gas

(1) DuCarbon type Drill a 2 to 3 mm hole about 10 mm from the bottom of the removed shock absorber cylinder before discarding in order to release the pressurized gas. (This gas is harmless, colorless, and odorless, but metal chips may fly out during drilling, so work carefully. A good safety practice is to put a vinyl bag around the end to be drilled, fastening it in place with a strong rubber band.)

(2) Non-disassemblable MacPherson strut type In this type, in which the ring nut cannot be removed, lay the shock absorber assembly down horizontally and drill a 2 to 3 mm hole in the top of the absorber shell.

(3) Disassemblable MacPherson strut type

<1> Clamp the shock absorber in a vise.

<2> Slowly loosen the ring nut three or four turns until the gas begins to leak out. If the gas is allowed to escape too quickly, the absorber fluid may come out as well.

<3> Make sure that there is no gas remaining in the absorber before discarding. This can be determined by lifting the piston rod to the top of the cylinder and releasing it. If the piston falls back into the cylinder of its own weight, all of the gas has been exhausted.

4. Installation of cartridge type low-pressure gasfilled shock absorbers

Although the shock absorber is replaced only as an assembly in the case of many low-pressure gas-filled MacPherson strut shock absorbers, such assembly replacement is not necessary in some models. Instead, the piston rod with cylinder can be removed and a replacement cartridge installed. The following precautions must be taken in such cases:

Since the piston rod with cylinder and the cartridge have different shapes, cartridges should be used that come with their own ring nut.

When disposing of the cartridge, first evacuate the gas following the same procedure as in the case of the non-disassemblable MacPherson strut type.

2.Twin-tube type

Inside the absorber shell (outer tube) there is a cylinder (pressure tube), and inside that there is a piston which moves up and down. At the base of the piston rod, a piston valve is installed which generates damping force when the shock absorber is extended (during rebounding). At the bottom of the cylinder there is a base valve which generates damping force when the shock absorber is compressed (during bounding). The inside of the cylinder is filled with absorber fluid, but only 2/3 of the reservoir chamber is filled with fluid, the rest being filled with air at atmospheric pressure or low-pressure gas. The reservoir serves as a storage tank for the fluid entering and leaving the cylinder Low-pressure gas-filled type is filled with gas under low pressure (0.3-0.6 MPa; 3-6 kgf/cm2 ; 43-85 psi). This prevents the generation of abnormal noise due to cavitation and aeration that can occur in shock absorbers using only fluid. Minimization of cavitation and aeration also makes it possible to obtain a more stable damping force, thus improving riding comfort and handling stability. In some low-pressure gas-filled shock absorbers, the base valve is eliminated so that damping force is generated during both bounding and rebounding by the piston valve.

HINT:

Cavitation:

When the fluid flows at high speed within the shock absorber, the pressure will drop in some areas, forming air pockets or cavities in the fluid. This phenomenon is called cavitation. These cavities collapse when carried into regions of high pressure, resulting in great impact pressure. This will generate noise, cause pressure fluctuations, and can damage the shock absorber itself.

Aeration: Aeration is the mixing of air with shock absorber fluid. This can lead to noise, pressure fluctuations, and pressure loss.

Operation

<1>During bounding (compression)

Piston rod’s speed of motion high When the piston moves downward, the pressure in chamber A under the piston becomes high. The fluid pushes open the non-return valve of the piston valve, and with practically no resistance flows into chamber B (damping force is not generated). At the same time, an amount of fluid equal in volume to the volume of the fluid displaced by the piston rod as it is pushed into the cylinder, is forced through the leaf valve in the base valve and flows into the reservoir chamber. It is at this time that damping force is generated by the flow resistance.

Piston rod’s speed of motion low If the speed of the piston rod is very low, the nonreturn valve in the piston valve and the leaf valve in the base valve will both remain closed because the pressure in chamber A is low. However, since there are orifices in the piston valve and the base valves, the fluid in chamber A flows through them into chamber B and the reservoir chamber, so that only a slight damping force is generated. <2>During rebounding (expansion)

Piston rod’s speed of motion high When the piston rod is moving upward, the pressure in chamber B above the piston becomes high and the fluid inside chamber B opens the leaf valve in the piston valve and flows into chamber A. At this time, the flow resistance of the fluid acts as a damping force. Since the rod moves upward, a portion of it moves out of the cylinder, so the volume of fluid displaced by it decreases. To compensate for this, the fluid passes through the non-return valve of the base valve from the reservoir chamber and flows with practically no resistance into chamber A.

Piston rod’s speed of motion low When the piston rod is moving at a low speed, both the leaf valve in the piston valve and the non-return valve in the base valve remain closed because the pressure in chamber B above the piston is low. Therefore, the fluid in chamber B passes through the orifices in the piston valve and flows into chamber A. Also, the fluid in the reservoir chamber passes through the orifice in the base valve and flows into chamber A, so only a slight damping force is generated.

When a vehicle is subjected to shocks from the road surface, the suspension springs absorb those shocks. However, because springs have the characteristic of continuing to oscillate, and because it often takes a long time for this oscillation to stop, riding comfort will be poor. The job of the shock absorbers is to absorb this oscillation. Shock absorbers not only improve riding comfort, but they also give the tires better road-holding characteristics and improve steering stability.

1. Principal of operation

In automobiles, telescopic shock absorbers are used which employ a special fluid, called shock absorber fluid, as the working medium. In this type of shock absorber, damping force is generated by the flow resistance caused by the fluid being forced through an orifice (small hole) by the movement of a piston.

(1) Damping force The stronger the damping force, the more quickly the oscillations of the body are dampened, but the shock from the damping effect also becomes greater. The damping force also changes with the speed of the piston. There are several types of shock absorber, which differ depending on how the damping force changes, as follows:

<1>Type whose damping force is proportional to piston speed

<2>Type with two levels of damping force with respect to piston speed

<3>Type in which damping force varies according to driving pattern Suspension systems with type <1> and <2> damping forces are used in most vehicles. Type <3> systems are used in vehicles with EMS (Electronic Modulated Suspension)

2. Types

Shock absorbers are classified as follows:

• Classification by operation
• Single-action type
• Multiple-action type
• Classification by construction
• Mono-tube type
• Twin-tube type
• Classification by working medium
• Hydraulic type
• Gas-filled type

Shock absorbers which are used in current models have twin-tube or mono-tube construction and have multiple-action type operation. Most recently, many gas-filled shock absorbers of the above types have come into use.

Construction and Operation

1. Mono-tube type

A representative type of mono-tube shock absorber is the DuCarbon type shock absorber, which is charged with high-pressure nitrogen gas (2.0-2.9 MPa; 20-30 kgf/ cm 2 ; 284-427 psi)

(1) Construction Inside the cylinder, the gas-storage chamber and the fluid chamber are separated by a “free piston”, so called because it can move up and down freely.

(2) Characteristics of the DuCarbon type shock absorber

• Good heat radiation because the single tube is exposed directly to the atmosphere.
• One end of the tube is charged with high-pressure gas, which is completely sealed off from the fluid by a free piston. This ensures that cavitation and aeration do not occur during operation, thereby providing more stable damping.
• Operation noise is greatly reduced.

(3) Operation

• <1>During bounding (compression) During the compression stroke, the piston rod moves downward, causing the pressure of the fluid to be higher in the lower chamber than in the upper chamber. Therefore, the fluid in the lower chamber is forced into the upper chamber through the piston valve. It is at this time that damping force is generated by the flow resistance of the valve. The high-pressure gas exerts great pressure on the fluid in the lower chamber, forcing it to flow quickly and smoothly into the upper chamber during the compression stroke. This ensures a stable damping force.
• <2>During rebounding (expansion) During the expansion stroke, the piston rod moves upward, causing the fluid pressure in the upper chamber to be higher than that in the lower chamber. Therefore, the fluid in the upper chamber is forced into the lower chamber through the piston valve, and the resistance exerted by the valve acts as a damping force. Since the rod moves upward, a portion of it moves out of the cylinder, so the volume of fluid displaced by it decreases. To compensate for this, the free piston is pushed upward (by the high-pressure gas below it) by a distance equivalent to this volume. Since DuCarbon type shock absorbers are of the mono-tube type, the tube must not be allowed to become deformed since free movement of the piston and free piston will be impaired if this occurs, In addition, since a protector is provided to prevent deformation by flying stones, attach the protector so that it faces the front of the vehicle when installing the shock absorbers.