217205311 Chap 7 Suspension Systems

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Chap7:SuspensionSystems

MotorVehicle

Engineering

MEng 5165

By:Shewangizaw W


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7.IntroductiontoSuspensionSystems Broadly speaking, suspension system consists of a

springand adamper. The energy of road shock causesthe spring to oscillate.

These oscillations are restricted to a reasonable level

by the damper, which is more commonly called ashockabsorber.

Location of the suspension system is between the

wheel axles and the vehicle body or frame.7.1. Its main purpose is:

1. To prevent the road shocks from being transmitted to

the vehicle components and occupants2. Support the weight of the vehicle

3. Maintain traction between the tires and the road

4. Hold the wheels in alignment


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Thetwobasictypesofsuspensionsystemsare:

1. Rigid axel/ dependent suspension systems2. Independent suspension systems

7.2 Rigid axel/ dependent suspension systems

Wheel is mounted at each end of asolidorundivided axle

oraxle housing.

Has the advantages of strength and low cost, but it

provides less efficiency and comfort than independent

suspension.

Most passengers cars use rigid axle suspension only

at the rear.

Rigidaxle front suspension systems are usually found

onlyonheavy duty vehiclesandtrucks.


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Cont

Since the axle connects with wheels, any condition

that affects one wheel affects the other.

Incase one wheel is raised by a bump in the road,the axle will be tilted.

Thus the motion of the raised wheel is passed

to the other wheel.

Since the frame of the car is attached to the

axle, the car body is also tilted.


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Arigidaxlesuspensionhasthefollowingcharacteristics:

The number of parts composing the suspension is smallandthe construction is simple. Therefore,maintenance is

simple.

It isdurableenoughfor heavy

duty use.When turning there islittle tilting of the body.

There is little change in the alignment due to the up

and down movements of the wheels. Therefore, thereis less tyre wear.

Since the unsparing weight is great, riding comfort is

poor. Since the movements of the left and right wheels

mutually influence one another, vibration and

oscillation occurrathereasily.


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There are various types of rigid axle suspension. Below will

be discussed rigid axle suspension used at present day:

I. Parallel leaf spring type:

is used for the front suspension of trucks and buses, etc, andfor the rear suspension in commercial vehicles.

Characteristics of parallel leaf spring type suspension: Since the leaf spring also acts as a linkage for positioning

the axle. Therefore the construction of the suspension issimple but comparatively strong.

Since the positioning of the axle is carried out by the leafsprings it has very high spring constant. Therefore, it is notvery good with respect to riding comfort.

Riding comfort suffers due to interleaf friction in the leaf

springs.

Acceleration and braking torque tend to causes windupand vibration and windup in turn causes rear end squat andnose

diving.


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Parallelleafspring


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II. Leadingarmandtrailingarmtypeswithlateralrods:

It is used for the front and rear suspension of the land cruiser.

Characteristics of Leading arm and trailing arm types with

lateral rods suspension:

In this type, the positioning of the axle, where there were a leaf

spring instead replaced by leading arms or trailing arms anda lateral control rod.

This type is superior to leaf springs in the following points:

Since the spring constant can be made smaller riding comfort isgood.

Since the trailing arm's rigidity is high, it is difficult for windupto occur.

III. Trailing arm type with twist beam

The construction of this type is simple and it is compact, so it is

used for the rear suspension of front engine front wheel drive (FF)

cars.


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IV. Link type suspension

This type is used for the rear suspension. It provides thebest riding comfort of all the rigid axle suspension.

Characteristics of Link type suspension:

Since axle positioning is accomplished by a linkage, soft

springs can be used and riding comfort is good.

Due to the geometrical layout of the linkages, nose diving

during braking and rearend squat during acceleration

are prevented.

Also the upanddown movement of the front end of

the differential during bounding and rebounding can be

minimized by making the upper control arms shorter

than the lower control arms.


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Cont

This in turn means that the floor over thenose of the differential can be lowered,

allowing more passenger compartment space.

Use of coil springs minimizes friction in thesuspension, so minute shocks from the road

surface can be absorbed and riding comfort can

be improved.

Generally, when this suspension system is used

in the rear, the linkage is located forward ofthe axle, and as a result, the capacity of the trunk

can be larger on passenger cars.


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7.3IndependentSuspensionsystems

It provide a separate mounting for each wheel.

There is no connection between the wheels.

The linear motion of one wheel is not transmitted to theother, and there is no tilting of an axle.

Most independent front suspension systems use coilsprings.

Methods of mounting the springs

i. To locate each spring between a lower control arm and the frame

ii. To locate each spring between an upper control arm and springtower above the frame.

iii. Still another method positions the springs around long shockabsorbers. Those units, usually referred to as McPhersonstruts, connect the lower control arm to the chassis or under body.

Some suspension systems useno springsat all instead, they usetorsion

bars.


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CharacteristicsofIndependentSuspensionsystems:

The unstrung weight can be kept low and the road holdingcharacteristics of the wheels are good, so riding comfort and

handling stability is good.

The springs only support the body; they do not help to

position the wheels. This means that softer springs can be

used.

Since there is no axle connecting the left and right wheels

the floor and the engine mounting position can be lowered.This means that the vehicle's centre of gravity will be

lower, and the passenger compartment and luggage room can

be made larger. The construction is rather complex.

Tread and alignment change with the upanddown motions of

the wheels.


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There are different types of independent suspensions which

are in use for front end rear axles. Few of them

are discussed below.7.3.1 Front wheel independent suspension

1. McPherson strut type

This is the most widely used independent suspension system for thefront suspension of small and medium sized cars. This type is alsoused as the rear suspension of FF cars.

A strut containing shock absorber and the spring carries also the

stub axle on which the wheel is mounted.

This system is simpler than double wishbone type and is alsolighter, keeping the unsparing weight lower.

Further, the camber also does not change when the wheelmoves up and down.

This type of suspension gives the maximum room in the enginecompartment and is, therefore, commonly used on front wheel

drive cars.


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MacPherson struttypesuspension


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CharacteristicsofMacPherson struttypeSuspension

systems:

The construction of the suspension is relatively simple.

Since there are a small number of parts, it is light, so

unstrung weight can be reduced.

Since the space taken up by the suspension is small, the

usable face in the engine compartment can be increased.

Since the distance between suspension support points islarge, there is little disturbance of the front wheel

alignment due to installation error or part

manufacturing error.

Therefore except for toe in alignment adjustments are

ordinarily unnecessary.


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2. Doublewishbonetype

This type considers the upper and the lower wishbone

arms pivoted to the frame members.

The coil spring is placed in between the lower wishbone

and the underside of crossmember.

The vehicle weight is transmitted from the body and

crossmember to the coil spring through which it goes to

the lower wishbone member.

A shock absorber is placed inside the coil spring and is

attached to the cross member and the lower wishbone

member.

The wishbone arms are like the chicken wishbone or letter

Vin shape, because of which the system is so called.


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Cont

Because of this Vshape, the wishbones not only position

the wheels and transmit the vehicle load to the springs,

but these also resist acceleration braking and cornering

(side) forces.

The upper arms are shorter in length than the lower

ones. his helps to keep the wheel track constant, there

by avoiding the tyre scrub, thus minimizing tyre wear.

However, a small change in the camber angle does occur

with such an arrangement.

The wishbone type is the most popular independent

suspension system.


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wishbonetype


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CharacteristicsofDoublewishbonetypeSuspensionsystems:

The length, positions and angles of the arms governs the path of

the wheels when the car corners or goes over bumps. This path in turn affects steering, road holding, and tyre wear.

If the upper and lower arms were made parallel and of

equal length, the wheels would not lean as they move upand down over bumps. This would cause the tread (the distanceb/n the left and right wheels) to vary resulting in poor corneringand excessive tyre wear.

Therefore, in most modern suspension systems the arms aremade neither parallel nor of equal length.

This causes the wheels to lean inward slightly as they go over

bumps, so that the tread does not change; and it improvescornering because the outside wheel, which carries thegreater load and therefore exerts the greater corneringpower of the two, remain more or less at right angles to the

road surface, thus improving road holding.


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7.3.2Rearwheelindependentsuspension

Though the rear wheels are not to be

steered, yet there is a considerable difficulty

in the rear wheel springing if the power has

to be transmitted to the rear wheels.

But even the rear wheel independent

springing is coming into importance because ofits distinct advantages over the rigid axle type.

There are different types of this suspension.

Few they are discussed below.


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1.Transverselymountedcontrolarmtypes

Designs of this type use control arms that aremounted near the centre of the frame or under body,

and extend outward toward the wheels.

Coil springs are mounted between the control arms and across member.

The outer end of each control arm is attached to a spindle,

which is mounted on the lower end of a shock

absorberstrut.

Positioned in this manner, the control arms handle side

thrust loads. Front and back movement of the assembly is controlled

by tie rods, which also aid the struts in resisting the

torque produced while braking.


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Transverselymountedcontrolarmtypes


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2.Trailingarmtype

In this the trailing arms are pivoted at rightangles to the longitudinal axis of the car and

carry the rear wheels at their ends.

Universal joints are provided to the drive shaftcoming from the differential to keep the wheel

track and the camber constant with the up and

down movement of wheels.

The coil springs with shock absorbers are mounted

concentrically within them.The trailing links hold the wheels firmly and also

sustain accelerating and braking forces.


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7.3.3Advantagesanddisadvantagesofindependentfrontsuspension

Advantages

1. It allows a bigger deflection of front wheels without producingany reaction on the steering. The front suspension springs are

designed softer than the rear springs to reduce body diving.

2. Offers greater distance for resisting rolling action and impartsbetter cornering characteristics and reduced tyre wear.

3. The frontaxle is in the form of a small stub axle; hence the unstrung

weight is greatly reduced. This characteristic improves the road

holding tendency of the tyres.

4. The system imparts minimum vibrations to the passengers.

Disadvantages

1. Better shock absorbers are required for better ride.2. More expensive than the conventional system.

3. Tyre wear increases due to wear in links and wishbone arms.

4. Electronic equipment is required to check steering angles.


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7.3.4Advantagesanddisadvantagesofrearindependent

suspension

Advantages

1. Lesserunstrungweightimprovestherideandreducestyre wear.

2. Increasedpassenger

space.

3. Betterweightdistribution.

4. Therearwheelsremainstableduetotransmissionoftorque.

Disadvantages

1. Increasedcost.

2. Complicateddesign

requiring

greater

maintenance.

3. Steeringactionisnotproperinsomedesigns.

4. Thewearofthecomponentscausesmisalignmentofthesteering

geometry.

217205311 Chap 7 Suspension Systems

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