In this article, you learn what is shock absorbers, the types of shock absorbers, and how they work.
Shock Absorbers and Types
If the suspension springs are rigid enough, they will not absorb shocks efficiently. If they are flexible enough, they will continue to vibrate for a long time even after the bump has passed.
Therefore, the springing device must be a compromise between flexibility and stiffness. Shock absorbers are provided as part of the suspension system of motor vehicles for this purpose.
When the vehicle wheel strikes a bump, the spring is compressed enough and only a little vertical upward motion is transferred to the frame. When the wheel comes down from the bump, the spring expands very rapidly.
If this rebound is not controlled the spring starts to vibrate heavily to control this vibration, at the shock-absorber, is used in the suspension system. Similarly, when the wheel falls over a hole, the spring expands and is unable to take the full vehicle load. The shock absorber takes part in this load.
Read also: What is Suspension System: How it Works? and Types of Suspension System
In the case of a leaf spring suspension system, the friction between the leaves provides a damping effect. But because of the change in lubrication conditions, the amount of friction also changes, and hence the damping characteristics do not remain constant.
Therefore, additional damping is provided by means of the dampers or shock-absorbers. Frequently, the shock absorber housing is linked to the frame cross member and the shock absorber arm is connected to the spring, axle, or suspension control arm.
Mainly the shock absorbers are of two types Mechanical and Hydraulic
Types of Shock Absorbers
Following are the different types of shock absorbers:
- Hydraulic type shock absorbers
- Double acting shock absorbers
- Single acting shock absorber
- Friction type shock absorber
- Lever type shock absorber
- Telescopic type shock absorber
Read also: Types of Axles: Rear Axles, Front Axle and Stub Axle
1. Hydraulic Type Shock Absorber
Hydraulic type shock absorbers are now used on all passenger cars. They increase resistance to the spring action by forcing a fluid through check valves and small holes.
2. Double Acting Shock Absorber
Double-acting shock absorbers offer resistance both during compression and rebound of the springs.
3. Single Acting Shock Absorber
A single-acting shock absorber offers resistance only on the rebound.
4. Friction Type Shock Absorber
The friction-type shock absorbers have almost become obsolete due to their non-predictable damping characteristics.
5. Lever Type Shock Absorber
The lever-type shock absorber is of indirect-acting type. It is bolted to the chassis through a lever and link. As the axle moves up and down, a double piston arrangement forces the oil through a valve.
6. Telescopic Type Shock Absorber
Telescopic type shock absorber is of direct-acting type. It is mounted between the axle and the frame.
A simplified diagram of the telescopic shock absorber is shown in Figure. Its upper eye is attached to the axle and the lower eye to the chassis frame. A two-way valve A is attached to a rod G. Another two-way valve B is connected to the lower end of cylinder C.
The fluid is in the space above and below valve A, and also in the annular space between cylinder C and tube D, which is connected to the space below valve B. The head J has a gland H. Any fluid scrapped off by the rod G is brought down into the annular space through the inclined passage.
Twin Tube Shock Absorber
A twin-tube shock absorber consists of an inner and outer cylinder. The inner cylinder contains oil and the piston rod is connected to the piston valve. Whereas the outer cylinder acts as a reservoir and has a low-pressure gas. The flow of oil from one chamber to another is controlled by a base valve.
As the wheel moves up and down, so does the piston rod. The piston valve and base valve confirm the compression and rebound of the absorber. They measure how fast the oil flows back and forth. Vibration and shock are similarly absorbed by low-pressure gas. This gas acts like a balloon and absorbs vibrations.
1. Basic Twin-tube
The basic twin-tube absorber is consists of two cylindrical tubes. It also has a base valve at the bottom. These are also known as two-tube shock absorbers.
When the piston is moved up and down, hydraulic fluid moves between the chambers through small holes and valves in the piston. This converts the “shock” energy into heat which must then be dissipated.
2. Twin-tube Gas Charged
Twin-tube gas-charged absorbers are used to reduce the aeration of hydraulic fluid. In this, nitrogen gas pressure compresses the air bubbles in the hydraulic fuel. This process prevents oil and air from mixing and foaming.
Foam has an impact on performance because it can be compressed – fluid cannot. With less aeration, the shock is able to deliver a faster and more predictable response, resulting in quicker reaction times and helping the tire stay firmly on the road surface.
3. Positive Sensitive Damping
The PSD shock absorber consists of a two-cylinder tube and contains nitrogen gas, a set of grooves are added to the pressure tube. These grooves enable the piston to move freely during the mid-range of travel.
This allows it to move with significantly less freedom in response to changes on more irregular surfaces as the piston moves up and down with greater speed.
4. Acceleration Sensitive Damping
The Acceleration Sensitive Damping (ASD) is an innovative technology that provides greater control for handling while improving ride comfort. Not only can it sense and respond to “bumpy” to “smooth” situations, but it can also sense individual obstacles in the road in near-immediate response.
This was achieved by adopting a new design of the compression valve. This compression valve is a mechanical closed-loop system, which opens a path for fluid to flow around the compression valve.
5. Coilover
This is a type of twin-tube gas-charged shock absorber designed inside a helical road spring. They are seen on the rear suspension of motorcycles and scooters and are widely used on the front and rear suspensions in cars.
Mono-tube Shock Absorber
These are high-pressure gas shocks having a pressure tube. This pressure tube consists of two pistons, a dividing piston, and a working piston. The installed pistons and rods are similar in design to twin-tube shocks.
A mono-tube shock absorber is mounted upside down or right side up and will work either way. Considering its flexibility, mono-tube shocks with springs are an important component to support the weight of the vehicle.
A mono-tube shock absorber does not have a base valve. The mono-tube shock has a greater surface area and carrying capacity. These are well known because the high amount of oil helps to dissipate heat much faster and reduces fading.
Spool Valve
The spool valves are designed using a hollow cylindrical sleeve with a machined oil passage that is in contact with traditional flexible discs or shims. Spool valving can be used with monotube, twin-tube, and/or position-sensitive packaging, and is agreeable with electronic controls.
Working of Shock Absorbers
The shock absorber works as follows when the vehicle comes across a bump the lower eye moves up. Therefore, the fluid passes from the lower side of valve A to its upper side. But since the volume of the space above valve A is less than the volume of rod G, the fluid exerts pressure on valve B.
This pressure of the fluid through the valve openings gives the damping force. Thus, when the lower eye E moves down, the fluid passes from the upper side of valve A to the lower side, and also from the lower side of valve B to its upper side.
The shock absorber must be filled with shock absorber fluid at regular intervals as recommended by the manufacturer or when required by its condition. The modern telescopic shock absorbers are no longer serviced. If they leak or do not offer proper resistance to push and pull they should be replaced.
Testing of a Shock Absorbers
The shock absorbers should be tested by moving the front or rear of the vehicle up and down quickly. If the vehicle does not come to rest almost immediately, the shock absorbers must be removed for further testing. Often noise results from loose shock absorber arm-to-frame connections. These joints should always be kept tight.
In the case of damages to shock absorbers, the operation may become irregular and result in noisiness and vibration in dampening the effect. The noise may originate from other sources. Therefore, before replacing shock absorbers, inspect carefully the entire suspension system and the mountings of the shock absorbers on the body and axle.
Check that shock absorber mounting eyes are firmly locked on rubber bushes and that these are not worn. Replace worn or damaged parts. Other possible causes of noisiness are a distortion of pipes or bumps against obstacles, to stones thrown up by wheels.
The vibrations in the damping effect may occur either as an increase or reduction in damping ability. Generally, the first case is rare and originates either from the thickening of fluid or from the closer matching of valves and setting, with a resultant increase in shock absorber resistance. The second case may be the result of breakage of some inner parts, shortage of fluid, or stuck valves.
