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Safety and driving comfort of driver and vehicle are both dependent on the vehicles suspension system. Safety refers to the vehicles handling and braking capabilities. The comfort of the occupants/payload of a vehicle correlates to tiredness and ability to travel long distance with minimal annoyance. Shock absorbers are a critical part of a suspension system, connecting the vehicle to its wheels. Essentially shock absorbers are devices that smooth out an impulse experienced by a vehicle, and appropriately dissipate or absorb the kinetic energy.
Need for Shock Absorber
In a vehicle, shock absorber reduces the effect of traveling over rough ground, leading to improved ride quality. Without shock absorbers, the vehicle would have a bouncing ride, as energy is stored in the spring and then released to the vehicle, possibly exceeding the allowed range of suspension movement. Control of excessive suspension movement without shock absorption requires stiffer (higher rate) springs, which would in turn give a harsh ride. Shock absorbers allow the use of soft (lower rate)springs while controlling the rate of suspension movement in response to bumps. They also, along with hysteresis in the tire itself, damp the motion of the unsprung weight up and down on the springiness of the tire. Since the tire is not as soft as the springs, effective wheel bounce damping may require stiffer shocks than would be ideal for the vehicle motion alone.
How Shock Absorber Works?
A shock absorber (or damper in technical use) is a mechanical device designed to smooth out or dampen a sudden shock impulse and dissipate kinetic energy. shock absorber is a device that controls unwanted spring motion through a process known as dampening. Shock absorbers slow down and reduce the magnitude of vibratory motions by turning the kinetic energy of suspension movement into heat energy that can be dissipated through some fluid.
Principle of Operation
The damping mechanism of a shock absorber is viscous damping. Viscosity is the property of a fluid by virtue of which it offers resistance to the motion of one layer over the adjacent on. The main components of a viscous damper are cylinder, piston and viscous fluid. There is a clearance between the cylinder walls and the piston. More the clearance more will be the velocity of the piston in the viscous fluid and it will offer less value of viscous damping coefficient. The damping force is opposite to the direction of velocity.
Shock Absorption Mechanisms
There are several commonly-used approaches to shock absorption:
Hysteresis is like a “memory” of the material, if you press down rubber disks, they tend to back to it’s normal uncompressed state as the pressure of fingers is relieved) of structural material, for example the compression of rubber disks, stretching of rubber bands and cords, bending of steel springs, or twisting of torsion bars. Hysteresis is the tendency for otherwise elastic materials to rebound with less force than was required to deform them. Simple vehicles with no separate shock absorbers are damped, to some extent, by the hysteresis of their springs and frames.
Dry fricion as used in wheel brakes, by using disks (classically made of leather) at the pivot of a lever, with friction forced by springs. Used in early automobiles, although now considered obsolete, an advantage of this system is its mechanical simplicity; the degree of damping can be easily adjusted by tightening or loosening the screw clamping the disks, and it can be easily rebuilt with simple hand tools. A disadvantage is that the damping force tends not to increase with the speed of the vertical motion.
3. Solid State
Tapered chain shock absorbers, using one or more tapered, axial alignment(s) of granular spheres, typically made of metals such as nitinol, in a casing.
The flow of fluid through a narrow orifice (hydraulics), constitute the vast majority of automotive shock absorbers. An advantage of this type is that using special internal valving the absorber may be made relatively soft to compression (allowing a soft response to a bump) and relatively stiff to extension, controlling “bounce”, which is the vehicle response to energy stored in the springs; similarly, a series of valves controlled by springs can change the degree of stiffness according to the velocity of the impact or rebound. Some shock absorbers allow tuning of the ride via control of the valve by a manual adjustment provided at the shock absorber. Many shock absorbers contain compressed nitrogen, to reduce the tendency for the oil to foam under heavy use. Foaming temporarily reduces the damping ability of the unit. In very heavy duty units used for racing and/or off-road use, there may even be a secondary cylinder connected to the shock absorber to act as a reservoir for the oil and pressurized gas. Another variation is the Magneto rheological damper which changes its fluid characteristics through an electromagnet.
Types of Shock Absorbers
Based on their mechanism and usage, there are various types of shock absorbers. Below given are a few types of shock absorbers:
Damper Shock Absorber
Damper shock absorber or simply damper is device that is designed for providing absorption of shock and smooth deceleration in linear motion applications. The dampers can be either mechanical or rely on a fluid. Dampers like other shock absorber absorbs shock by controlling the flow of the fluid from outer to inner chamber of a cylinder during piston actuation. The damper shock absorbers can be adjusted to different road conditions and provides good balance to the vehicles.
Features of Damper Shock Absorbers
Some of the important features of the damper shock absorbers are:
They have good adjustable configuration.
The force capacity can be reduced by letting out the gas.
The position of the dampers can be locked easily either at the ends or at the middle of the stroke.
In case of fluid dampers, valve is used for increasing or decreasing fluid volume or pressure.
Types of Damper Shock Absorber
1. Mono Tube Damper
The shell case itself works as a cylinder and oil, gas, piston valve, etc are all set in a single tube.
It consists of only one high-pressure tube. Inside the pressure tube, there are two pistons: a
dividing piston and a working piston. Gas and oil are separated by the dividing piston, which seals off the actual working chamber. The working piston carries the valves, at the top is the equalization chamber. The piston rod can extend upward and downward. The pressure tube of the single-tube design is larger than a twin tube design to accommodate for dead length. The working piston and the rod piston are very similar to the pistons of the twin tube shock absorber.
The equalization chamber must absorb the volume equalization by the oil warming, and the volume displaced by the piston rod. The area above the dividing piston is pressurized to about 360 psi with nitrogen gas. This gas pressure helps support some of the vehicle’s weight. The oil is located in the area below the piston.
When the vehicle swings downwards in the direction to the road, the piston rod is moved downwards. The compression phase valve on the top of the valve resists the oil that streams upwards through holes. Therefore, the downward movement is slowed down. The separating piston is thereby lowered by the same degree as the piston retreats downwards. The gas pressure underneath the separating piston thereby prevents the oils from foaming above the piston.
When the vehicle swings upwards, away from the road, the shock absorber is extended. Thereby, the piston rod is moving out of the damping case. The rebound phase valve at the lower piston resists the oil that flows downwards through the holes in the space above the piston. Therefore the upward motion is slowed down. The separating piston is thereby lifted upwards by the same degree as the piston extends.
2. Twin Tube Damper
The twin tube design has an inner tube known as the working or pressure tube and an outer tube known as the reserve tube . The outer tube is used to store excess hydraulic fluid.
There are many types of shock absorber mounts used today. Most of these use rubber bushings between the shock absorber and the frame or suspension to reduce transmitted road noise and suspension vibration. The rubber bushings are flexible to allow movement during suspension travel. The upper mount of the shock absorber connects to the vehicle frame.
Notice that the piston rod passes through a rod guide and a seal at the upper end of the pressure tube. The rod guide keeps the rod in line with the pressure tube and allows the piston to move freely inside. The seal keeps the hydraulic oil inside and contamination out.
The base valve located at the bottom of the pressure tube is called a compression valve . It controls fluid movement during the compression cycle.
When the car wheel encounters a bump in the road and causes the spring to coil and uncoil, the energy of the spring is transferred to the shock absorber through the upper mount, down through the piston rod and into the piston. Orifices perforate the piston and allow fluid to leak through as the piston moves up and down in the pressure tube. Because the orifices are relatively tiny, only a small amount of fluid, under great pressure, passes through. This slows down the piston, which in turn slows down the spring.
Shock absorbers work in two cycles — the compression cycle and the extension cycle. The compression cycle occurs as the piston moves downward, compressing the hydraulic fluid in the chamber below the piston. The extension cycle occurs as the piston moves toward the top of the pressure tube, compressing the fluid in the chamber above the piston. A typical car or light truck will have more resistance during its extension cycle than its compression cycle. With that in mind, the compression cycle controls the motion of the vehicle’s unsprung weight, while extension controls the heavier, sprung weight.
Comparison of Twin and Mono Tube Shock Absorbers
Twin-Tube Shock Absorber
The advantages and disadvantages of the twin-tube shock absorber are:
Allows ride engineers to move beyond simple velocity sensitive on the valves
and to use the position of the piston to fine tune the ride characteristic.
Adjusts more rapidly to changing road and weight conditions than single-tube
A control is enhanced without sacrificing driver comfort. Two shocks
absorbers into one — comfort and control.
Can only be mounted in one direction.
Single-Tube Shock Absorber
The advantages and disadvantages of the single-tube designs are:
Easy to tailor to specific applications, as the larger piston diameter allows low
Sufficient room for valves and passages.
Can be installed in any position, can be mounted upside down, reducing the
May run cooler. Heat is dissipated directly via the outer tube because it is
exposed to the air.
Longer than twin-tube shock absorbers.
The outer tube, which acts as a guide cylinder for the piston, is susceptible to
damage from stone throw, etc. A dent in the pressure tube will destroy the
Suspension layout must provide sufficient room for the tube which, with its
very close tolerances, is not to be mechanically impeded in any way. This is a
disadvantage when lines must be routed around the shock absorber in
restricted bodywork areas.
The piston rod seal is subjected to the damping pressure.
Difficult to apply to passenger cars designed OE with twin-tube designs.
3. Low Pressure Gas Filled Twin Tube Damper
In low pressure gas filled twin tube damper, the air is replaced by the gas usually with Nitrogen gas. The air in the damper mixes with the oil and results in the aeration of the oil and hence foaming thereby reducing the overall performance of the damper. The nitrogen gas prevents this aeration and foaming and improves the performance of the damper.
4. High Pressure Gas Filled Mono Tube Damper
High-pressure gas-filled mono-tube damper not only eliminates the bump but also gives rebound control. Due to this the damper is designed with quality components. The nitrogen is used in place of air. The nitrogen is inserted under high pressure at the base of the damper. Oil and the gas is separated by the floating piston. The design is capable of improving the performance by eliminating the aeration and foaming. Using superior valve and better design, ‘self adjusting’ damper can be produced. The sleek design with mono tube also eliminates the problem of tyre rubbing.
2. Air Shock Absorber
Air shock absorber makes the use of compressed air for the spring which is an integral part of the shock. The addition of air in the shock make the shock absorber stiffer. Air is added by using standard Schrader valve to add air. This shock absorber provides normal dampening action and on placing the heavy load on the rear of the vehicles, the rubber air cylinder can be put under pressure to aid the suspension springs. Due to this reason, the air shock absorber is also called as load-adjustable shock absorbers.
Air shocks or air shock absorber consist of separate, sealed air chamber inside. However this chamber is sealed therefore it is not subject to aeration. This results in better and smoother ride than gas shocks. Air shock absorbers can also be used for load leveling.
The air shock absorber consists of air chamber which contains valve coming off it. Depending on the situation, the air chamber can be both inflated or deflated. Filling the air to the rear shocker gives better steering control to the boat. In some navigator, the system makes an adjustment and inflates or deflates the air shocks accordingly.
Air shock absorber consists of following general features:
All Weather Fluid of the air shock absorber reduces friction and ensure smooth rod reaction.
For smooth and controlled ride there is better valving range available on both the compression and extension cycles of the unit.
Air shock absorber also contains sintered Iron Piston which enhances the durability for long service life.
Air sleeves of the shock absorber are generally lubricated that reduces friction for longer service life.
The air fittings of the shock absorber prevents air leakage.
The volume of the air chamber is quite large that can well inflate with loaded vehicle and deflates for normal driving.
3. Coil over Shock Absorber
Safe vehicle is one that has smooth and responsive suspension system. Coil-over shocks are simple designed shock absorber that gives effective control and smooth riding to the vehicle. It comes in various heights depending on the requirements of the vehicle. The coil over shock absorber is engineered in such a way that it reduces chassis flex when your vehicle is cornering. The shock absorber is capable of giving good absorption and dampening levels. Simply speaking coil spring is adjusted over and around the shock body. Through this an additional spring rate is added to the shock absorber.
In this type of shock absorber both the spring and shock absorber is assembled as a single unit and if there is any problem in the spring of shock absorber, it is replaced as a unit. Coil over shock absorber is known for its effective damping without torsional loads. It can be also adjusted to different ride height and hardness.
Depending on the vehicle’s suspension design there are different types of spring. Coil spring, however is the most common types of spring used in suspension system. A coil spring stores energy and releases when required. It is also capable of handling shocks and maintaining the force between two contacting surfaces.
As a unit, coil spring and shock absorber is of great value. Coil spring compresses and stretches, thereby giving one safe drive, whereas, the shock absorber keeps the road noise, bumps, and vibrations away.
Coil over shock absorber has certain advantages which are as follows:
It gives optimal damping.
Good handling and braking.
Comes in different heights.
It is available for different types of vehicles.
Durable and lasts longer.
Suitable for different road conditions.
4. Electric Shock Absorber
The electric shock absorber is a device that converts the kinetic energy of an
oscillating object into electric energy. This kinetic energy is normally dumped in a
form of thermal energy in a conventional, mechanical shock absorber. The electric
shock absorber consists of a permanent magnet linear synchronous generator
(PMLSG), a spring, and an electric energy accumulator. Another electric shock absorber uses a hydraulic system that forces fluid through a turbine attached to a generator. The system is controlled by an active electronic system that optimizes the damping, providing a smoother ride than conventional shocks while generating electricity to recharge the batteries or operate electrical equipment..
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