Suspension

SHOCK ABSORBERS/ DAMPERS: WORKING PRINCIPLE, CLASSIFICATION AND FUNCTIONS

Shock absorbers are basically oil pumps. A piston is attached to the end of the piston rod and works against hydraulic fluid in the pressure tube. As the suspension travels up and down, the hydraulic fluid is forced through tiny holes, called orifices, inside the piston. However, these orifices let only a small amount of fluid through the piston. This slows down the piston, which in turn slows down spring and suspension movement.

All modern shock absorbers are velocity sensitive hydraulic damping devices – meaning the faster the suspension moves, the more resistance the shock absorber provides.
Because of this feature, shock absorbers adjust to road conditions. As a result, shock absorbers reduce the rate of:

• Bounce
• Roll or sway
• Brake dive and Acceleration squat

Shock absorbers work on the principle of fluid displacement on both the compression and extension cycle. A typical car or light truck will have more resistance during its extension cycle then its compression cycle. The compression cycle controls the motion of a vehicle’s unsprung weight, while extension controls the heavier sprung weight.

FUNCTIONS OF DAMPER

The main function of the shock absorber is to absorb the shocks and damp them as soon as possible so that a smooth ride can be obtained.

Some other important functions of shock absorber are
 It limits vehicle body movement
 It stabilizes our ride as discussed above
 It stabilizes vehicle tires which are disturbed due to sudden shock, hence it is very important for safety purpose also
 It also minimizes tire and body wear of the automobile and hence reduces overall maintenance cost
It may sound a simple job but this is the main thing on which the comfort level of your ride depends.

WORKING PRINCIPLE

To understand the shock absorber, it is very important to understand its working.

First of all, we should know that there are generally two types of shock absorbers one is hydraulic and another one is pneumatic. However, working of both the types of shock absorbers is same.

A shock absorber is generally coupled with a spring, which convert sudden shock waves into oscillatory motion. This oscillatory motion gives us instant relief from the shock but, nobody can have his or her whole ride with these oscillations.

Here is the need of shock absorber arises, it is used to damp those oscillations which are made by the springs.
A general shock absorber contains a perforated piston in a hydraulic chamber. The chamber is totally sealed and hence if piston has to make some movement the only way is to let the hydraulic liquid pass through it.

When a shock comes, piston has to move due to shock. When the piston moves than the hydraulic liquid in the shock absorber has to pass through it.

When the liquid is passed through the very tiny perforated holes in the piston the piston has to do some work against it. That work is done on that expense of the energy generated due to the shock and hence soon the shock absorber loses all the shock energy, which results into no oscillation and smooth ride.

SHOCK ABSORBER DESIGN TYPES

There are several shock absorber designs in use today:
1. Twin Tube Designs

• Gas Charged
• PSD (position sensitive damping)
• ASD (Acceleration Sensitive Damping)

2. Mono-Tube

A. Twin Tube – Gas Charged Design

The prime function of gas charging is to minimize aeration of the hydraulic fluid. The pressure of the nitrogen gas compresses air bubbles in the hydraulic fluid. This prevents the oil and air from mixing and creating foam. Foam affects performance because it can be compressed – fluid can not. With aeration reduced, the shock is able to react faster and more predictably, allowing for quicker response time and helping keep the tire firmly planted on the road surface.

Advantages:
• Improves handling by reducing roll, sway and dive
• Reduces aeration offering a greater range of control over a wider variety of road conditions as compared to non-gas units
• Reduced fade – shocks can lose damping capability as they heat up during use. Gas charged shocks could cut this loss of performance, called fade

B. Twin Tube – PSD Design

Ride engineers had to compromise between soft valving and firm valving. With soft valving, the fluid flows more easily. The result is a smoother ride, but with poor handling and a lot of roll/sway. When valving is firm, fluid flows less easily. Handling is improved, but the ride can become harsh.
With the advent of gas charging, ride engineers were able to open up the orifice controls of these valves and improve the balance between comfort and control capabilities available in traditional velocity sensitive dampers.
A leap beyond fluid velocity control is an advanced technology that takes into account the position of the valve within the pressure tube. This is called Position Sensitive Damping (PSD).
The key to this innovation is precision tapered grooves in the pressure tube. Every application is individually tuned, tailoring the length, depth, and taper of these grooves to ensure optimal ride comfort and added control. This in essence creates two zones within the pressure tube.
The first zone, the comfort zone, is where normal driving takes place.
The second zone, the control zone, is utilized during demanding driving situations.

Advantages:

• Allows ride engineers to move beyond simple velocity sensitive valving and use the position of the piston to fine tune the ride characteristic.
• Adjusts more rapidly to changing road and weight conditions than standard shock absorbers
• Two shocks into one – comfort and control

C. Twin Tube -ASD Design (Reflex )

A new twist on the comfort/ control compromise is an innovative technology which provides greater control for handling while improving ride comfort called Acceleration Sensitive Damping (ASD).
This technology moves beyond traditional velocity sensitive damping to focus and address impact. This focus on impact is achieved by utilizing a new compression valve design. This compression valve is a mechanical closed loop system, which opens a bypass to fluid flow around the compression valve.

Advantages:
• Control is enhanced without sacrificing driver comfort
• Valve automatically adjusts to changes in the road condition
• Reduces ride harshness

2. Mono-tube design (Standard Types)

These are high-pressure gas shocks with only one tube, the pressure tube. Inside the pressure tube there are two pistons: a dividing piston and a working piston. The working piston and rod are very similar to the twin tube shock design. The difference in actual application is that a mono-tube shock absorber can be mounted upside down or right side up and will work either way. In addition to its mounting flexibility, mono-tube shocks are a significant component, along with the spring, in supporting vehicle weight. Another difference you may notice is that the mono-tube shock absorber does not have a base valve. Instead, all of the control during compression and extension takes place at the piston.
During operation, the dividing piston moves up and down as the piston rod moves in and out of the shock absorber, keeping the pressure tube full all times.

Advantages:

• Can be mounted upside down, reducing the unsprung weight
• May run cooler since the working tube is exposed to the air
• Original equipment many import and performance domestic passenger cars, SUV and light truck applications

A system of mechanical linkages, springs, dampers that is used to connect the wheels to the chassis is known as a suspension system. It has usually done two works-controlling the vehicle’s handling and braking for safety and keeping the passengers comfortable from bumps, vibrations etc.

It also helps to maintain correct vehicle height and wheel alignment.it also control the direction of the vehicle and has to keep the wheel in a perpendicular direction for their maximum grip. The suspension also protects the vehicle itself and luggage from damage and wear. The design of the front and rear suspension of a car may be different.

COMPONENTS OF THE SUSPENSION SYSTEM

A suspension system irrespective of their type has some main components in common that are:-

1. Knuckle or Upright-

It is the component of the suspension system that is mounted over the wheel’s hub through which the wheels and the suspension of the vehicle connect with each other by the linkages provided.
A knuckle is provided with the king-pin and the caster angles that help the front wheels of the vehicle to steer in right or left direction which in turn steers the vehicle.
A knuckle provides housing for central bearing over which the wheel’s hub rotates along with the rotation of the wheels.

2. Linkages-

linkages are the rigid connections that are used in the suspension system to connect the mainframe of the vehicle with the knuckle of the wheels through mechanical fasteners.

On the basis of the type of suspension used linkages are of 3 types-

i. Wishbones or A-arm – 
It is the type of the mechanical linkage which is in shape of the alphabet A, the pointy end of the A-arm is fastened to the knuckle and the other 2 ends of the A-arm are fastened to the mainframe of the vehicle.
On the basis of the application of the vehicle, either a single A-arm or double A-arm are used.

ii. Solid axle or live axle- 
It is the type of linkage which is used to connect the mainframe of the vehicle with the knuckle on the wheel, this is the solid axle casing that supports the overall weight of the vehicle, this type of linkage can be seen in trucks.

iii. Multiple links- 
Instead of using double wishbone or solid axle linkage various high-end cars are adopting multiple link type of suspension in which multiple solid links are used to connect the mainframe of the vehicle to the knuckle on the wheel.

3. Shock absorbers or springs-

They are the flexible mechanical components that are used to absorb shock provided by the road condition and is placed between the linkages ( wishbone. Solid axle, multi-links) and the mainframe such that the road shock is minimised before transmitting to the mainframe of a vehicle.

On the basis of the application and type of suspension used shock absorbers are of many types that are-

i. Spring and damper type shock absorber- 
It is the type of shock absorber in which a pneumatic or hydraulic piston is known as the damper is used that provides damping by absorbing the road shocks.

This damper is surrounded by a compression coil spring which is an elastic mechanical constraint that compresses when force is applied by the bump and recoil back or regains its original shape and size when the force is removed.

It is used to maintain the surface contact of the tyres with the road by providing stiffness (resistance to compress), also maintain the damper at its original length after absorbing the shock.

ii. Leaf spring- 
It is the type of spring in which a number of ductile metal plates called leaf are arranged in a special pattern i.e. one over one in ascending order of their length, leaves of the leaf spring shock absorber are pre-stressed such that when the shock is transferred by the wheels these pre-stressed leaves being ductile tries to regain their original shape i.e. straighten,. Due to which shock is absorbed by the leaves.

This type of shock absorber can be easily seen in trucks on the road in which leaf spring shock absorber is used in between the solid or live axle and the mainframe of the vehicle.

iii. Air spring- 
It is the latest type of shock absorbers which can be easily seen in Volvo buses, in air spring shock absorbers the damping of shock is a function of air compression, which means air is used as a shock absorber.
The air needed for different load conditions is controlled and monitored by the Electric control unit of the vehicle.

TYPES OF SUSPENSION SYSTEM

1) INDEPENDENT SUSPENSION SYSTEM

This system means that the suspension is set-up in such a way that allows the wheel on the left and right side of the vehicle to move vertically independent up and down while driving on an uneven surface. A force acting on the single wheel does not affect the other as there is no mechanical linkage present between the two hubs of the same vehicle. In most of the vehicle, it is employed in front wheels.
This type of suspension usually offers better ride quality and handling due to less unsprung weight. The main advantage of independent suspension is that they require less space, they provide easier steerability, low weight etc.. Examples of Independent suspension are

i. Double Wishbones

It is an independent suspension system design using two wishbone-shaped arms(called A-ARM in USA and WISHBONE in the UNITED KINGDOM)to locate the wheel. Each wishbone or arm has two mounting points to the chassis and one joint at the knuckle. The angle movements of the compressing and rebounding wheels can be managed by using arms of unequal length.
The main advantage of the double-wishbone suspensions is that they allow easy adjustments of camber, toe and other properties. This type of suspension also provides increasing negative camber gain all the way to full jounce travel. On the other hand, it takes more space and is slightly more complex than the other system like Macpherson strut. It also offers less design choice.

ii. MacPherson Strut

This type of independent suspension got its name from Earle S. McPherson who developed this design. The MacPherson strut is a further development of the double-wishbone suspension. The main advantage of the MacPherson is that all the parts providing the suspension and the wheel control can be combined into the one assembly.

It makes it easy to fit in transverse engine. This design is very popular due to its simplicity and low manufacturing cost. The disadvantage is that it is more difficult to insulate against road noise. for this, an upper strut mount is necessary, which should be decoupled as possible. It also requires greater clearance height.

2) DEPENDENT SUSPENSION SYSTEM

IN Dependent Suspension there is a rigid linkage between the two wheels of the same axle. A force acting on one wheel will affect the opposite wheel. For each motion of the wheel caused by road, irregularities affect the coupled wheel as well.
It is mostly employed in heavy vehicles. It can bear shocks with a great capacity than independent suspension. Example of this system is

I. Solid Axle.
A solid axle or beam axle is a dependent type suspension. It is mostly used in rear wheels in which the rear axle is supported and located by two leaf springs. The vertical movement of one wheel influences the other. They are simple and economical to manufacture.
They are so rigid that there is no change in track width, toe-in and camber on a full bump which helps in the low wearing of tyres. The main disadvantage is that the mass of the beam is included in the unsprung weight of the vehicle which results in low ride quality. The cornering ability is also poor due to zero camber angle.

3) SEMI-INDEPENDENT SYSTEM

This type of system has both the characteristics of a dependent as well as independent suspension. In semi-independent suspension, the wheel move relative to one another as in independent suspension but the position of one wheel has some effect on the other wheel. This is done with the help of twisting suspension parts. Example of semi-independent is

i. Twist Beam
The twist-beam suspension also known as the torsion-beam axle. These are mostly based on C or H shaped members. The cross beam of the H-shape holds the two trailing arms together and provides the roll stiffness to the suspension.
It is mostly used in the rear wheel of the cars. It is very favourable due to its low cost and it is very durable. It is simple in design and is very light in weight. But on the other side camber angle is limited and the roll stiffness is also not very easy. Toe characteristics may be unsuitable.

La barra estabilizadora de la suspensión de un vehículo es una barra de acero con propiedades de naturaleza elástica, que se encuentra fijada en sus extremos a cada soporte de la suspensión de cada lado del mismo eje.

Todo vehículo circulando a velocidad por una curva se ve sometido a una fuerza centrífuga que hace que se incline hacia un costado, que puede generar una sensación de molestia en los ocupantes del vehículo, además de poder existir un peligro real de vuelco del vehículo si la velocidad fuera inadecuadamente excesiva.

Esto es así debido a la fuerza centrífuga que actúa sobre el vehículo, que es de dirección radial y ejerce un empuje sobre el vehículo que tira de él hacia el exterior de la curva.

Esta fuerza genera una transferencia de carga en el vehículo que hace inclinar a la carrocería de tal forma que una parte de la suspensión, la situada en el lado exterior a la curva, se comprima, mientras que la otra parte de la suspensión del vehículo, la situada hacia el interior de la curva, se expanda corriendo el riesgo de despegar la rueda de este lado del pavimento.

Este hecho, es decir, que las ruedas de un lado del vehículo tiendan a subir, mientras que las ruedas del otro lado tiendan a bajar comprimiéndose contra el suelo, va a generar un par de torsión que es absorbido por la barra estabilizadora, impidiendo que la carrocería se incline excesivamente hacia un lado y ejerciendo una resistencia al balanceo del vehículo.

Así, el movimiento vertical hacia arriba de la rueda situada del lado interior de la curva se transmite a la otra rueda del eje a través de la barra estabilizadora, que tiende a bajar la carrocería de ese lado comprimiendo el muelle de la suspensión, de manera que se consigue sumar la acción de los dos muelles, ayudando a mantener la estabilidad del vehículo.

Por ello, la barra estabilizadora se considera un componente elástico de la suspensión dado que actúa en parte también como muelle, especialmente cuando actúa sobre la rueda del lado del eje que tiende a subir.

Este mismo efecto se produce, no sólo cuando el vehículo toma una curva, sino cuando por ejemplo, una de las ruedas encuentra un bache o cualquier obstáculo, creando, al bajar o subir la rueda, un par de torsión en la barra que hace que la carrocería se mantenga en posición horizontal. De esta forma, como se ha dicho, se consigue sumar la acción de los dos muelles.

Por tanto, la barra estabilizadora de la suspensión de un vehículo trabaja a torsión, compensando los esfuerzos generados de una rueda sobre la otra del eje mediante una transferencia de peso de la rueda que se comprime hacia la rueda del lado que tiende a elevarse, aumentando así su adherencia.

De este modo, se evita que el muelle de un lado de la suspensión se comprima excesivamente, mientras que el otro muelle se expanda, pudiendo hacer perder el contacto de la rueda con el piso.