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)
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.
- <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.