SYNOPSIS

In the present scenario of automobile industry manufacturers are trying to produce

comfortable and safe vehicles which the consumers are looking for. A shock absorber is a

damping element of the vehicle suspension, and its performance directly affects the

comfortability, dynamic load of the wheel and dynamic stroke of the suspension. The

conventional type of shock absorbers has got the main drawback that it causes foaming of

the fluid at high speeds of operation. This results in a decrease of the damping forces and

a loss of spring control. The gas filled shock absorbers are designed to reduce foaming of

the oil and provide a smooth ride for a long period.

INTRODUCTION

For a smooth and comfortable ride the disturbing forces should be eliminated or reduced

considerably by using some devices. Shock absorbers are such devices which isolate the

vibrations by absorbing some disturbing energy themselves. Of the many types telescopic

shocks are widely used which has got the draw back that the flow of oil in the cylinder

can cause foam of oil and air to form. These limit the optimum throughout of the flow in

the valves. Gas shocks represent an advance over traditional shocks. Nitrogen filled gas

shock absorbers are the results of years of extensive research and development with top

flight shock design engineers. They are designed for both lowered and stock vehicles to

provide shock absorbers that would out perform anything on the market today. Nitro

shock absorbers are high quality, nitrogen filled shocks designed and gas charged

specifically for each vehicle application. The addition of nitrogen under pressure limits

the foaming effect and increases efficiency.

NEED FOR SHOCK ABSORBERS

Springs alone cannot provide a satisfactorily smooth ride. Therefore an additional

device called a “shock absorber” is used with each spring. Consider the action of a coil

spring. The spring is under an initial load provided by the weight of the vehicle. This

gives the spring an original amount of compression. When the wheel passes over a bump,

the spring becomes further compressed. After the bump is passed the spring attempts to

return to its original position. However it over rides its original position and expands too

much. This behaviour causes the vehicle frame to be thrown upward. Having expanded

too much, the spring attempts to compress that it will return to its original position; but in

compressing it again overrides. In doing this the wheel may be raised clear of the road

and the frame consequently drops. The result is an oscillating motion of the spring that

causes the wheel to rebound or bounce up and down several times, after a bump is

encountered. If, in the mean time, another bump is encountered, a second series of

rebounding will be started. On a bumpy road, and particularly in rounding a curve, the

oscillations might be so serious as to cause the driver to lose control of the vehicle.

A shock absorber is basically a hydraulic damping mechanism for controlling

spring vibrations. It controls spring movements in both directions: when the spring is

compressed and when it is extended, the amount of resistance needed in each direction is

determined by the type of vehicle, the type of suspension, the location of the shock

absorber in the suspension system and the position in which it is mounted. Shock

absorbers are a critical product that determines an automobile’s character not only by

improving ride quality but also by functioning to control the attitude and stability of the

automobile body.

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 basic system is shown below. The damping

force is opposite to the direction of velocity.

I-CLEARNCE, II-PISTON, III-VISCOUS FLUID

The damping resistance depends on the pressure difference on the both sides of

the piston in the viscous medium. The figure shown below shows the example of free

vibrations with viscous damping.

The equation of motion for the system can be written as mx + cx +kx = 0

Energy dissipation in viscous damping :

For a vibratory body some amount of energy is dissipated because of damping.

This energy dissipation can be per cycle. Rate of change of work W is called energy. For

a viscously damped system the force F is expressed as

F= cx = cdx/dt, where x = dx/dt

Work done W = Fx = (cdx/dt) x

The rate of change of work per cycle

i.e. Energy dissipated

Let us assume the simple harmonic motion of the type x = Asinωt

(dx/dt) ² = ω²A²cos²ωt

The equation for

This shows that the energy dissipation per cycle is proportional to the square of the

amplitude of motion.

The total energy of a vibrating system can be either maximum of its potential or kinetic

energy. The maximum kinetic energy of the system can be written as E = (KE) max =

1/2mx²max

= 1/2mω²A²

SHOK ABSORBER ACTION

Shock absorbers develop control or resistance by forcing fluid through restricted

passages. A cross-sectional view of a typical shock absorber is shown below. Its main

components and working is also given below.

The inside parts of a shock absorber

The upper mounting is attached to a piston rod. The piston rod is attached to a

piston and rebound valve assembly. A rebound chamber is located above the piston and a

compression chamber below the piston. These chambers are full of hydraulic fluid. A

compression intake valve is positioned in the bottom of the cylinder and connected

hydraulically to a reserve chamber also full of hydraulic fluid. The lower mounting is

attached to the cylinder tube in which the piston operates.

During compression, the movement of the shock absorber causes the piston to

move downward with respect to the cylinder tube, transferring fluid from the

compression chamber to the rebound chamber. This is accomplished by fluid moving

through the outer piston hole and unseating the piston intake valve.

During rebound, the pressure in the compression chamber falls below that of the

reserve chamber. As a result, the compression valve will unseat and allow fluid to flow

from the reserve chamber into the compression chamber. At the same time, fluid in the

rebound chamber will be transferred into the compression chamber through the inner

piston holes and the rebound valve.

Spring Schematic Diagram of the Interior of a Shock Absorber

WHY GAS FILLED SHOCK ABSORBERS?

The rapid movement of the fluid between the chambers during the rebound and

compression strokes can cause foaming of the fluid. Foaming is the mixing of free air and

the shock fluid. When foaming occurs, the shock develops a lag because the piston is

moving through an air pocket that offers up resistance. The foaming results in a decrease

of the damping forces and a loss of spring control.

During the movement of the piston rod, the fluid id forced through the valuing of

the piston. When the piston rod is moving quickly, the shock absorbers oil cannot get

through the valuing fast enough, which causes pressure increases in front of the piston

and pressure decreases behind the piston. The result is foaming and a loss of shock

absorber control. The need for a gas filled shock absorber arises here.

GAS FILLED SHOCK ABSORBER

The gas filed shock absorbers is designed to reduce the foaming of the oil. It uses

a piston and oil chamber similar to other shock absorbers. The difference is that instead

of a double tube with a reserve chamber, a dividing piston separates the oil chamber from

the gas chamber. The oil chamber contains a special hydraulic oil and the gas chamber

contains nitrogen at 25 times atmospheric pressure. The schematic diagram showing the

inside parts of a gas filled shock absorber is shown below.

The inside parts of a gas-filled shock absorber.

When the piston rod is moved into the shock absorber, oil is displaced as in

double tube principle. This oil displacement causes the dividing piston to press in the gas

chamber, thus reducing it in size. With the return of the piston rod the gas pressure

returns the dividing piston to its starting position.

Whenever the oil column is held at a static pressure of approximately 25 times

atmospheric pressure, the pressure decreases behind, the working piston cannot be high

enough for the gas to exit from the oil column. Consequently, the gas filled shock

absorber operates without foaming.

TYPES OF GAS FILLED SHOCK ABSORBERS

Twin– tube with low pressure gas.

Single- tube with high pressure gas.

LOW PRESSURE TWIN- TUBE SHOCKS

Twin- tube gas technology design retains the classical twin-tube while adding at

the top of the reserve tube nitrogen under relatively low pressure 2.5- 5 bars instead of

25- 30 bars used in high pressure shock absorbers. This pressure is sufficient to radically

improve the efficiency of the shock absorbers.

HIGH PRESSURE SINGLE- TUBE SHOCKS

Gas shock absorbers operate in the same principle of movement of the piston in

an oil filled tube but they contain at one end a small quantity of nitrogen under high

pressure (25 bars). The gas is prevented from mixing with the oil by a floating piston.

When the piston rod passes into the body and displaces oil, the oil compresses the

nitrogen even further. The volume of gas changes playing the role as an equalization

tube. The permanent pressure exerted on the oil by the gas guarantees an instantaneous

response and the quieter piston valve operation. At the same time this constant pressure

eliminates cavitations and foaming which could momentarily degrade the effectiveness of

the shock absorber.

WORKING

TWIN– TUBE SHOCK ABSORBERS :

The main components are:

Outer tube, also called reservoir tube

Inner tube, also called cylinder

Piston connected to a piston rod

Bottom valve, also called foot valve

Upper and lower attachment

How does it work?

Bump Stroke:

When the piston rod is pushed in oil flows without resistance from below the

piston through the orifices and the non-return valve to the enlarged volume above the

piston. Simultaneously, a quantity of oil is displaced by the volume of the rod entering

the cylinder. This volume of oil is forced to flow through the bottom valve into the

reservoir tube (filled with air (1 bar) or nitrogen gas (4-8 bar)). The resistance,

encountered by the oil passing through the footvalve, generates the bump damping.

Rebound Stroke:

When the piston rod is pulled out, the oil above the piston is pressurized and

forced to flow through the piston. The resistance, encountered by the oil on passing

through the piston, generates the rebound damping. Simultaneously, some oil flows back,

without resistance, from the reservoir tube through the footvalve to the lower part of the

cylinder to compensate for the volume of the piston rod emerging from the cylinder.

MONO- TUBE SHOCK ABSORBERS :

The main components are:

Pressure cylinder, also called housing

Piston rod connected to a piston rod

Floating piston, also called separating piston

Piston rod guide

Upper and lower attachment

How does it work?

Bump Stroke:

Unlike the bi-tube damper, the mono-tube has no reservoir tube. Still, a possibility

is needed to store the oil that is displaced by the rod when entering the cylinder. This is

achieved by making the oil capacity of the cylinder adaptable. Therefore the cylinder is

not completely filled with oil; the lower part contains (nitrogen) gas under 20-30 bar. Gas

and oil are separated by the floating piston. When the piston rod is pushed in, the floating

piston is also forced down the displacement of the piston rod, thus slightly increasing

pressure in both gas and oil section. Also, the oil below the piston is forced to flow

through the piston. The resistance encountered in this manner generates the bump

damping.

Rebound Stroke:

When the piston rod is pulled out, the oil between piston and guide is forced to

flow through the piston. The resistance encountered in this manner generates the rebound

damping. At the same time, part of the piston rod will emerge from the cylinder and the

free (floating) piston will move upwards.

ADVANTAGES OF NITRO SHOCKS

Instantaneous response :

Because the high pressure eliminates aeration (foaming), action is always is

immediate.

The low mass of gas and the single tube further improves response time.

Better fade resistance :

Since there is no outer tube, cooling is much better which gives a drastic

reduction in fade. Thus more consistent handling and control.

Better durability :

Single-tube construction also allows for a larger internal working area, reducing

stress and fatigue for better durability.

De Carbon’s monodisc valving system features a single moving part that

drastically reduces inertia and friction, to improve durability and performance.

Better cooling of the mono tube design results in lower operating temperatures

and thus longer life.

No need for re-adjustment:

The viscosity of hydraulic fluid changes as temperature changes. This may

because of climate, season (summer/winter) or heavy duty (motorway cruising).

The high pressure gas compensates immediately and automatically for changes in

viscosity.

Which ones are right?

Choosing which shocks to buy largely depends upon what kind of vehicle and the

kind of driving. As with most automotive components, it is important the specific vehicle,

since mismatched shocks can drastically affect handling and could even be dangerous.

The best advice will probably come from a mechanic who is familiar with the vehicle.

CONCLUSION

In the current scenario of automobile industry the need for vehicles which provides

smooth and comfort ride is growing. Nitro shock absorbers are designed to be ultimate in

performance and comfort. In a country like ours whose roads are not up to world

standards the need for automotive components like nitro shocks are necessary. It goes

without saying that if the right choice is made the improvements in vehicles ride and

handling can be shocking.

REFERENCES

1. “In For A Shock”, S.B.L Beohar; Mechanical Systems and Signal Processing.

2. Automotive Encyclopedia; Tobolt, Johnson.

3. Auto Mechanical Fundamentals; Stockel.

4. http://belltech.net/

5. http://monroeshocks.com

CONTENTS Page No.

INTRODUCTION 1

NEED FOR SHOCK ABSORBERS 2

SHOCK ABSORBER ACTION 5

GAS FILED SHOCK ABSORBERS 9

TYPES 10

WORKING 11

ADVANTAGES 14

MOUNTING TIPS 15

CONCLUSION 16

REFERENCES 17