AIR SUSPENSION SYSTEM OF AUTOMOBILES

MOHD.SHOEBUDDINDEPARTMENT OF MECHANICAL ENGINEERING

SHADAN COLLEGE OF ENGINEERING AND TECHNOLOGYEmail Id:-shoebuddin_1987@yahoo.com

ABSTRACTThe frame as well as body of a vehicle is attached to the

rear axle and the front axle by springs. These springs damp the

road shock transmitted to the body structure by the wheels, when

they travel over the road. In this way the springs are the protecting

units supported directly by the frame of the vehicle. Therefore all

the parts which perform the function of protection are collectively

called a suspension system. These springs are generally of the

laminated leaf type, coil type, torsion bar type, hydraulic springs,

Plastic springs, and Air springs etc. 1. To prevent the road shocks

from being transmitted to the vehicle frame. 2. To preserve the

stability of the vehicle in pitching or rolling while in motion. 3. To

safe guard the occupants from road shocks. 4. To provide good

road holding while driving, cornering and breaking 5. To maintain

proper steering geometry. 6. To provide the requisite height to

body structure as well as to bear the torque and braking reactions.

7. To minimize the effects of stresses due to road shocks on the

mechanism of the motor vehicle and provide a cushioning effect. 8.

To keep the body perfectly in level while travelling over rough

uneven ground. i.e., the up and down movement of the wheels

should be relative to the body.

1. INTRODUCTION Besides providing vibration isolation, air springs offer many

other user-friendly advantages. Over the traditional leaf and coil

springs. There are two basic types of air springs used in vehicle

suspensions: reversible sleeve and convoluted. Regardless of

whether an air spring is a reversible sleeve or convoluted style, it

will operate on the same principle: A column of gas confined

within a container allows it to use the contained pressure to

generate force.

In the case of air springs, the gas is air and the container

is a sealed fabric-reinforced rubber bellow or sleeve. Similar to a

ball inflated with air, the load an air spring will carry depends on

its diameter and therefore, the area of the column of air supported

and the pressure of air inside it. The two basic relationships used in

determining the load-carrying capability of an air spring are:

Force = pressure x area ---------------- (1)

diameter2

Area = π x ---------------------

4

The relationships above demonstrate that increasing the

load an air spring can carry (the force) can be accomplished by

increasing the pressure inside the air spring, increasing the

diameter of the air spring (and therefore increasing the area) or

both. The ability to change the load carrying capacity simply by

changing the air pressure, rather than changing out the spring, is a

major advantage air springs have over steel.

Because an air spring consists of a closed volume of air,

the compression of the air spring (jounce travel) will cause an

increase in pressure, while the extension of the air spring (rebound

travel) will cause a decrease in pressure. This allows the air spring

to have an automatic tendency to return to the neutral (design)

height as it experiences disturbances in the driving surface. The

dynamic build-up in Compression also helps protect against

"bottoming out" and can be further increased on the reversible

sleeve air spring by the addition of a "flare" at the bottom of the

piston.

2. COMPONENTS

Although the basic principles behind both the reversible

sleeve and convoluted air springs are the same, there are some

subtle differences between them. Most notably, the reversible

sleeve air spring has a piston which is an additional component

that the convoluted air spring does not have. The piston is the

component that is fastened to the moving trailing arm or axle

mount and, as a result, plunges in and out of the air cavity within

the rubber bellows.

In general, a piston gives the reversible sleeve air spring

an advantage over the convoluted air spring in that spring, rates

can be further tuned using a variety of piston profiles. For straight

sided pistons, the reversible sleeve air spring has an advantage

over the convoluted air spring, in that a constant load for a given

internal pressure may be maintained over a range of heights. The

two other major components of an air spring are the bead plate(s)

and the fabric-reinforced rubber bellows or sleeve. The bead plate

allows for a rigid attachment to the mounting surface(s) and the

bellow is the dynamically functioning suspension component

which contains the air.

The main advantages of an air spring over its steel leaf

and coil counterparts are variable load-carrying capability,

adjustable spring rate, user-friendly height control, low friction

action, and road-friendly suspension increasing the pavement life.

As already mentioned, the load an air spring carries can be

adjusted over a wide range, without changing the air spring height,

simply by changing the air pressure.

Traditional steel springs need to be replaced if the

height must be maintained. In addition to changing the load-

carrying capability, a change in air pressure will also afford the

benefit of changing the spring rate without changing the height and

without a significant change in the natural frequency. Steel springs

exhibit one spring rate for a given height and, once again, will need

to be replaced if the height must be maintained. Using air pressure

from the compressor, the air spring height can be maintained by a

closed-loop control system or adjusted to the other desired heights.

This allows for "load leveling" and "squatting" capabilities that

steel springs cannot offer. Because there is a flexible rubber

member separating the rigid attachment points to the frame and

suspension, there is freedom to move about all six degrees of

freedom without the resistance and squeaks experienced by the

rigid interactions characteristic of steel leaf and coil springs.

The components of the suspension system perform six basic

functions:

1. Maintain correct vehicle ride height

2. Reduce the effect of shock forces

3. Maintain correct wheel alignment

4. Support vehicle weight

5. Keep the tires in contact with the road

6. Control the vehicle's direction of travel

Typically, struts consists of a coil spring to support the

vehicle's weight, a strut housing to provide rigid structural support

for the assembly, and a damping unit within the strut housing to

control spring and suspension movement. The bottom of the strut

body attaches to the steering knuckle, which in turn connects to a

lower control arm through a lower ball joint.

The top of the strut is connected to the vehicle body through

the upper strut mount, in some cases called a bearing plate. This

bearing plate allows the strut to pivot as the wheels are turned. It

must be flexible enough to handle slight angle changes and

dampen movement of the upper end of the strut. This mount or

bearing plate transfers vehicle load to the strut and spring, making

the upper mount/bearing plate the load carrier and the lower ball

joint the follower.

The strut housing holds the damping unit and fluid. It is

made of heavy gauge steel so that it is rigid enough to provide

structural support and withstand road shock.

The piston rod of the strut is much larger in diameter

than the piston rod of the typical shock absorber. This is to

withstand the side load on the strut shaft. A strut rod will measure

up to 7/8 of an inch in diameter while the piston rod of a typical

shock measures up to ½ of an inch in diameter.

A coil spring is located between the upper and lower

spring seats. It is held there by tension. The lower spring seat is

welded to the strut housing, while the upper spring seat is kept in

place by the upper strut mount.

Struts also have a jounce (or compression) bumper

located under the upper spring seat. The purpose of this component

is to limit suspension travel by not allowing suspension

components to hit together.

Finally, a large nut at the end of the strut rod holds everything

together.

3. Road-friendly suspensionIn 1993 a study called the "Dynamic Interaction

between Vehicles and Infrastructure Experiment" (DIVINE) was

initiated by the Directorate of Science, Technology and Industry of

the OECD. Interim results from the study were presented in 1995

and the final report in 1997.

The purpose of the study was to quantify the benefits of

heavy vehicles with air suspension on roads. The knowledge

gained and appreciated by the co-operating countries can be shared

with countries that are rapidly expanding their transportation

infrastructures. The end result is a faster payback and reduced

costs of maintaining these infrastructures.

This study was conducted with participation from

European and North American countries, as well as public and

private institutions. The purpose of the study was to provide

scientific evidence of the effects of heavy vehicles and their

suspensions on road systems. Almost 50 per cent of road

maintenance costs are associated with effects from heavy vehicles.

Further, the amount of dynamic load exerting on roads is directly

associated with the type of vehicle suspension.

Air suspensions increase pavement life by 15-60 per

cent. This corresponds to increased static load of 4-12 per cent. A

15 per cent increase in vehicle mass limit can save upwards of

$500 million per annum, while increased pavement life ensures

significant reduction in road maintenance cost which forms 90 per

cent of the annual road budget in OECD countries.

"Road-friendly" suspensions have low spring stiffness

and coulomb friction with optimum damping. Well-designed air

suspensions best meet these criteria.

For a complete copy of the DIVINE report, visit www.oecd.org.

Air suspensions are used on a vast majority of heavy

duty vehicles in North America and Europe, with a growing

penetration worldwide. The advantages of air suspension with

regard to the vehicle, driver and transportation systems are

appreciated in both qualitative and financial terms.

Firestone Industrial Products Company, LLC, a

subsidiary of Bridgestone Firestone Diversified Products (BFDP),

specializes in air spring manufacturing and technology with a

history of more than 60 years of research and development of

technologically advanced air springs. With headquarters in

Indianapolis, Ind., and six manufacturing plants located on four

different continents, the company produces suspension products

enhancing the driving experience for drivers of heavy truck/trailer,

buses, rail vehicles, passenger cars, SUVs, light trucks, minivans,

vans and motor homes.

4. BASIC PRINCIPLESuspension system that has air as its working fluid and

acting as shock absorber is called air suspension system. The

detection of causes and remedy for the problems with suspension

system are discussed here:

Interconnected suspension system

As the name suggests the front and rear suspension

system or the suspension units on the two sides of the vehicle are

connected with each other. They are also termed collectively as

linked system. It was very much effective as compared to

independent front and rear suspension units and was able to reduce

the tendency of vehicle to bounce, pitch or roll and was able to

provide smooth and comfortable drive.

Types of interconnected suspension system are:

1) Air suspension system

2) Hydrolastic suspension and

3) Hydro gas suspension system

Air suspension system

The conventional metal springs faced some drawbacks

which were air suspension system overcomes and so they are

preferred and used in more these days. Let's see some of the plus

points of this system.

1) The automatic control devices installed in the vehicle

allows making optimum use of the variable space for

deflection of wheel.

2) The height of the automobile remains steady and so the

changes in the alignment of headlamp due to varying

loads are restricted.

3) It helps to reduce the load while the vehicle in motion

i.e. the dynamic loading as the spring rate variation

between laden and unladen weight is much less.

4) It gives smooth and comfort ride of the vehicle.

Air springs are classified into two types:

1) Bellow type and

2) Piston type.

The air springs shown are mounted on the front and rear axle. The

atmospheric air first passes through the filter where the dirt is

removed and passed on to the compressor. Air is compressed here

and the pressure of air is raised from atmospheric to about 250

Mpa. This pressure is maintained by the accumulator tank. The

safety relief valve is provided on the accumulator as a safety

device and it opens when the pressure rises above 250 Mpa. This

air then moves to lift control valve and through leveling valves to

air springs.

Hydrolastic suspension

In this case the moving part is assembled and fitted at each of

the wheel location. These units are interconnected by pipes

carrying the fluid. In this moving unit rubber acts as a spring and

the fluid under pressure acts as a damping medium. The

connecting rod of piston is connected to the wheel through suitable

linkage in order to receive the movements of the wheel. The

movement of the fluid is controlled by a two way valve assembly.

The valves are arranged at right angle to each other. As the

pressure of fluid rises it causes the upper valve to open. In the

same way the lower valve opens under pressure and allows the

fluid to flow in downward direction.

Hydro gas suspension system

The hydra gas suspension system was designed by

Moulton Development Limited and British Leyland Motor

Corporation and is manufactured by Dunlop suspension Division

of Coventry (England). The drawbacks of hydrolastic suspension

system were overcome by this design. It is mainly divided into two

parts an internal spring and a damper unit and is assembled at each

wheel. The springing action is taken by inert gas like nitrogen.

Weight of the car is supported by some fluid like water under

pressure. Insulation of the interconnected front and rear system is

done generally with hermetic seals. As the wheels in motion come

across pit or bumps they cause the piston to move and the

diaphragm forces the fluid up which causes the damper valve to

open and the fluid moves through the opening.

5. AIR SUSPENSION PROBLEMS

Uncomfortable Ride:

The reason of the rough ride you might be

due to one of the following reasons:

1) Springs might have got rusted and thereby not able to

provide proper springing action. Lubrication should be

done to minimize the friction.

2) Shock absorbers might got problems repair or

replace it.

3) Pins holding the springs together might have got

loose tighten it.

4) Pads present at the leaves (curve shaped spring like

saucer) might be worn out replace them.

Suspension becoming flexible:

Some of the main reasons for suspension

becoming loose are:

1) Due to long period of continuous use the springs

might have become weak they must therefore be

replaced with new one.

2) Some springs do not require lubrication yet they

might have been lubricated so clean with some cloth.

3) Defects in shock absorber rectify them.

Noise

1) See the manufactures manual if the springs require

lubrication and then provide lubrication for springs.

2) Pins holding the springs and bushes might have got

loose.

3) Nuts and bolts holding the springs might be loose

tighten them.

6. CONCLUSIONOver the last decade or so air suspension has become

extremely popular in the custom automobile culture: street rods,

trucks, cars, and even motorcycles may have air springs. They are

used in these applications to provide an adjustable suspension

which allows vehicles to sit extremely low, yet be able rise to a

level high enough to maneuver over obstacles and inconsistencies

in the roadways (and parking lots). These systems generally

employ small, electric or engine-driven air compressors which

sometimes fill an on-board air receiver tank which stores

compressed air for use in the future without delay. High-pressured

industrial gas bottles (such as nitrogen or carbon dioxide tanks

used to store shielding gases for welding) are sometimes used in

more radical air suspension setups. Either of these reservoir

systems may be fully adjustable, being able to adjust each wheel's

air pressure individually. This allows the user to tilt the vehicle

side to side, front to back, in some instances "hit a 3-wheel"

(contort the vehicle so one wheel lifts up from the ground) or even

"hop" the entire vehicle into the air. When a pressure reservoir is

present, the flow of air or gas is commonly controlled with

pneumatic solenoid valves. This allows the user to make

adjustments by simply pressing a momentary-contact electric

button or switch.

The installation and configuration of these systems

varies for different makes and models but the underlying principle

remains the same. The metal spring (coil or leaf) is removed, and

an air bag, also referred to as an air spring, is inserted or fabricated

to fit in the place of the factory spring. When air pressure is

supplied to the air bag, the suspension can be adjusted either up or

down (lifted or lowered).

For vehicles with leaf spring suspension such as pickup

trucks, the leaf spring is sometimes eliminated and replaced with a

multiple-bar linkage. These bars are typically in a trailing arm

configuration and the air spring may be situated vertically between

a link bar or the axle housing and a point on the vehicle's frame. In

other cases, the air bag is situated on the opposite side of the axle

from the main link bars on an additional cantilever member. If the

main linkage bars are oriented parallel to the longitudinal (driving)

axis of the car, the axle housing may be constrained laterally with

either a Panhard bar or Watt's linkage. In some cases, two of the

link bars may be combined into a triangular shape which

effectively constrains the vehicles axle laterally.

Often, owners may desire to lower their vehicle to such an extent

that they must cut away portions of the frame for more clearance.

A reinforcement member commonly referred to as a C-notch is

then bolted or welded to the vehicle frame in order to maintain

structural integrity. Specifically on pickup trucks, this process is

termed "notching" because a portion (notch) of the cargo bed may

also be removed, along with the wheel wells, to provide maximum

axle clearance. For some, it is desirable to have the vehicle so low

that the frame rests on the ground when the air bags are fully

deflated.

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MOHD.SHOEBUDDIN B.TECH MECHANICAL ENGINEERING FROM SHADAN COLLEGE OF ENGINEERING & TECHNOLOGY