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Student Workbook

LV30 Suspension Systems (2)

kap all phase 2 & 3 6/11/03 11:37 am Page 33

Student Workbook for Technical Certificates in

Light Vehicle Maintenance and Repair

MODULE LV30 SUSPENSION SYSTEMS (2)

Contents

Page Page

Forces Acting on Suspension: 3 Independent front and rear

Forces on leaf spring during Hydro-pneumatic and air suspension 24

Bump steering effect of leaf sprung Routine suspension maintenance

Limitations of leaf spring suspension 7 IFS layout – front wheel 27

Typical forces in an independent Rear suspension with leaf spring 28

Use of an offset spring to reduce

bending forces in the stub axle 13 Identification of Common Faults

Compliance 19 Damper bounce test 34

Advantages and Disadvantages of

Different Suspension Systems: 20

Non-independent rigid axle

suspension 20

Disadvantages of non-independent

Forces Acting on Suspensions

Newton’s third law of motion tells us that forces always act in “pairs” of equal and opposite forces It states that “To every action there is an equal and

opposite reaction.”

This can be seen in the picture which shows a car towing another car

A is the action or force applied by the tow car and B is the opposing force or reaction in the towrope caused by the mass or weight of the second car

The statement, “To every action there is an equal and opposite reaction”,

should be considered when looking at the forces in a suspension system

The suspension has to resist the following forces:

• forces produced by driving torque from the transmission

• forces produced when brakes are applied

• forces produced during cornering

• plus the normal forces produced in the suspension as the vehicle

negotiates bumps on the road surface

Torque reaction in a live axle

The illustration shows what happens to the axle and leaf springs when drive is passed to the wheels

Action of “axle tramping”

As the torque is applied to the wheels they are reluctant to turn due to the

inertia of the car Therefore there is a tendency for the axle to twist in the

opposite direction and “wind up” the springs as shown

The torque is applied to move the wheels in a forward direction but the axle tends to twist in a reverse direction, i.e action and reaction This causes the leaf springs to bend in the direction shown, as they resist the torque reaction

If the springs were not there to resist this force, then instead of the wheels

rotating forwards, the axle would rotate backwards, as this is the line of least resistance

Under severe conditions the springs will “wind up” until the wheels spin This releases the torque reaction in the springs as the force due to the driving

torque is lost Remember that if there is no action or load, then there will be

no reaction The axle is returned to its normal position by the springs, wheel

Brake reaction on a live axle

The red line shows the curve

of the spring under braking

When the brakes are applied the wheels and axle tend to rotate as one in the same direction This causes the springs to be bent in the direction shown i.e

in the opposite direction to the way they bend when resisting torque reaction

Forces on leaf spring during cornering

When a vehicle corners, the centrifugal force and opposing cornering force

cause the springs to bend laterally or sideways slightly It is the cornering

force on the tyre that causes the vehicle to turn a corner A Panhard rod or lateral control rod is sometimes used to eliminate this bending of the spring

A Panhard rod is shown and in this example it is used on a live axle that is

suspended on coil springs However it can be used to good effect on a leaf spring suspended live axle

Centrifugal force

Springs bending laterally

Cornering force opposing centrifugal force

The effects of cornering forces on a leaf spring suspended live axle are shown

in this diagram The lateral, or sideways, bending that takes place also moves the axle in this direction slightly This will have an adverse affect on the

steering and road holding of the vehicle We shall see later some other

disadvantages of the simple leaf sprung rigid axle

Bump steering effect of leaf sprung live axle

a b

Distance a is greater than b causing rear axle to steer

The action of a leaf sprung live axle passing over road surface bumps can

cause a steering effect to take place on the axle

Vehicle roll during cornering can cause a similar steering effect since the

inside spring is effectively in the rebound position and the spring on the

outside of the curve is under bump conditions These effects are called bump steer and roll steer respectively Bump steer and roll steer can occur with

independent suspension systems However, it more noticeable and the

effects are greater with the rigid leaf sprung axle, which is a very basic design that has its roots in the horse drawn cart

The reason for this bump steer (and roll steer) is shown here As the wheel moves upwards the leaf spring is flattened and the leading part of the spring, from the fixed shackle, becomes longer See (a) in the diagram The centre line of the axle is thus moved away from its normal position of 90 degrees to the front/rear or longitudinal axis of the vehicle This causes a steering effect

to take place in much the same way as the steering on a horse drawn cart in pre-Ackermann days

In addition, during roll conditions the spring on the other side moves down

which causes the front part of the spring to shorten, see (b) in the diagram

Limitations of leaf spring suspension

As covered in Phase 1 Suspension Systems LV16 the fact that the leaf spring has to carry out two tasks means that it has limitations as a suspension

system

Progress check 1

Answer the following questions:

1 What are the two functions that a leaf spring has to perform?

2 Why does it not carry out these functions ideally?

Typical forces in an independent suspension

The next few diagrams show typical forces produced in an independent

suspension system during driving, braking and cornering A MacPherson strut IFS (right hand) has been chosen for simplicity but the principles can be

applied to any type IFS or IRS

Centrifugal force

The first diagram illustrates the forces in the suspension as the vehicle

negotiates a right turn

Note: Direction of the centrifugal force due to cornering and the opposing

cornering force applied to the wheel at the road surface

Look at the direction of the forces produced in the suspension components

Centrifugal force

Opposing cornering

force

Wishbone in compression

Bending force on

strut

This diagram shows the forces in the suspension components as the vehicle goes round a left hand corner

Note: The forces are all acting in the opposite direction to those produced

when rounding a right hand corner and the wishbone is in compression

instead of tension

The wishbone and the inner bushes have the same forces applied as shown

in the above diagram

Braking

force

Forces on wishbone

Bending forces on strut D.O.R

Vehicle momentum

The diagram shows the forces in the MacPherson strut during braking

Note: The two opposing forces produced by the vehicle momentum as it tries

to continue moving forward, and the braking force between tyre and road

The forces in the suspension are generated by the wheel and hub assembly trying to rotate as one unit as the brakes are applied The greater the braking effort the greater will be the forces produced

Exercise 2

Forces acting on suspension – torque reaction

The torque reaction is different from a rigid axle since there is little friction in the stub axle Most of the torque reaction that takes place as the drive is

applied to the wheels is taken up by the engine mountings, as the engine unit tends to rotate in the opposite direction to the crankshaft Most of the forces are in the wishbone due to the opposing driving force and the inertia of the

car Imagine the wheel trying to move forwards and leaving the car behind

This diagram shows a non-driven MacPherson strut

Indicate the following:

a) driving force

b) inertia

c) forces acting upon the wishbone

Use of an offset spring to reduce bending forces in the stub axle

This diagram shows how the coil spring in a MacPherson strut is offset from the damper unit This is to reduce bending forces in the stub axle area due to normal vertical movements of the suspension The spring is offset from the damper unit centre line but is almost concentric with the swivel axis Some non-driven struts have a much simpler stub and it not necessary to offset the spring Older designs of MacPherson strut suspensions do not use offset

springs

Progress check 2

Answer the following questions:

1 Under what type of driving conditions do most of the bending forces

take place on a MacPherson strut?

2 Why is the coil spring mounted in an offset position on some

MacPherson struts?

3 Is this offset spring position normally used on driven or non-driven

struts and why?

Suspension system terminology

Upward movement of the

Horizontal rotation about the vertical axis

This is called yawing and is an undesirable characteristic Suspension

designers try to eliminate yawing as it leads to vehicle instability Increasing the lateral or sideways stiffness of the suspension layout and vehicle does

this IFS and IRS systems have greater lateral stiffness than a simple live

axle and low profile tyres also improve this desirable characteristic

This diagram shows a vehicle yawing

Rotation about the longitudinal axis is shown above This is called rolling and

is controlled by anti-roll bars and spring settings The wider the track and the lower the centre of gravity the less is the tendency for the vehicle to roll

Consider a Formula 1 car that has virtually no roll However some degree of roll is desirable to provide “feel” to a vehicle’s handling and prevent the driver becoming over confident

Rotation about the transverse axis This is called pitching and is controlled by spring and damper settings

A vehicle with a long wheelbase is less prone to pitching and vice-versa This shows a vehicle pitching and the second diagram illustrates the action of pitch diving under braking

Action of dive pitching under braking

Sophisticated anti-dive suspension geometry can eliminate most of the

unwanted characteristics and prevents the front of the vehicle lifting under

acceleration

This diagram summarises suspension movement terminology

Progress check 3

Answer the following question:

Name the vehicle motion for the diagrams below?

Compliance

Direction

of travel Resulting force on IFS

This term is used to describe the amount of “give” in a suspension system

usually in the rubber bushes It can be used to advantage to induce a degree

of rear wheel steering that can help vehicle cornering and improve the stability

of the vehicle This diagram shows the natural compliance that occurs in most suspension systems that employ rubber bushes

Advantages and Disadvantages of Different Suspension

Systems

Non-independent rigid axle suspension

Front axle

Rear axle

A typical front and rear axle is shown

This type of suspension has many disadvantages, but it also has some

advantages:

• very simple design

• very strong and an asset for four wheel drive utility vehicles

• left and right hand wheels always remain parallel to each other

Note: The simple design of these two axles, which are shown in their basic form, i.e multi-leaf spring, no anti-roll bars and no Panhard rods

The action of a live axle non-independent suspension layout is shown

Refer to this diagram when considering the disadvantages of this type of

suspension

Vertical movement of RH wheel causes the axle, body and LH wheel to move

Disadvantages of non-independent suspension systems

Limited spring deflection This is partly due to the need for over stiff springs needed so that they can perform their secondary role of locating the axle

Other reasons are the lack of space for the spring and axle to move vertically without fouling the chassis, plus the need to limit the camber angle change, if one wheel was allowed to move too much This last point is less important

with four-wheel drive off-road vehicles but they are not noted for their road

holding capabilities

Inaccurate control of steering geometry This occurs mainly because of the relatively poor axle location, covered previously in the workbook, and the fact that movement of one wheel causes the other wheel to move

Poor roll stiffness, due mainly to the springs being relatively close together High un-sprung weight Components that follow the road surface are

classified as un-sprung weight, e.g wheels, rigid axles and part of the weight

of springs and dampers The greater the un-sprung weight, the greater the tendency for the wheel to leave the road surface because of the increased

momentum Also because of the increased inertia of the suspension, the

wheel will be less likely to follow the road surface

Engine has to be mounted high up in the frame to ensure the axle does not foul the sump

Independent front and rear suspension

This type of suspension addresses most of the disadvantages of the rigid axle suspension e.g:

• Increased spring deflection and therefore ride comfort - due to the

suspension design

• Accurate control of steering geometry and therefore reduced bump steer and roll steer This is due to the suspension design which maintains the wheels in the correct position with little or no change in geometry as the wheels follow the road surface

• Improved roll stiffness as the springs tend to move further apart, plus the extensive use of anti-roll bars

• Reduced un-sprung weight giving improved wheel to road contact and

therefore road holding This is due to the suspension design involving

lighter moving parts and greater wheel to road contact

• Greater flexibility regarding engine positioning in frame

Un-sprung weight consists of; the weight of the wheels, half the weight of the springs, dampers and drive shafts and less than half the weight of the suspension arms and anti-roll bars

This diagram illustrates which parts of the independent suspension system

are un-sprung weight

Progress check 4

Answer the following:

1 Give four advantages of independent suspension systems:

2 Give two advantages of rigid axles:

Hydro-pneumatic and air suspension

These suspensions systems are complex and relatively expensive

They have the following advantages:

• constant ride height

• variable spring rate, which is dependent on load

• reduced body roll

• reduced pitching

• ride height can be automatically lowered with electronic control of air

suspension giving improved road holding

A layout of a car air suspension system is shown

Note: The suspension design is almost the same as a normal IFS and IRS

system except for the air springs and control actuators (levelling valves)