Chapter 26 Bennet

8/10/2019 Chapter 26 Bennet

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Chapter 26

Suspension

Systems


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Objectives (1 of 2)

Identify and describe the types of suspension

systems used on current trucks.

List the components used on leaf and multi-leaf

spring suspension systems and explain how theywork.

Describe a fiber composite spring.

Identify equalizing beam suspension system

components and explain how they function.

Identify torsion bar suspension system components

and explain how they function.


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Objectives (2 of 2)

Identify air spring suspension system componentsand explain how they function.

Troubleshoot suspensions and locate defectivesuspension system components.

Outline suspension system repair and replacementprocedures.

Explain the relationship between axle alignmentand suspension system alignment.

Perform full chassis suspension system alignments.

Describe the operation of the cab air suspensionsystem.


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Suspension Systems (1 of 2)

A suspension system plays a number ofroles.

It stabilizes the truck when traveling over

smooth highway as well as over rough terrain. It cushions the chassis from road shock and

enables the driver to steer the truck.

It maintains the proper axle spacing and

alignment. It provides a smooth ride when both loaded

and unloaded.


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Suspension Systems (2 of 2)

Leaf spring

Equalizer beam

Leaf spring and solid rubber spring

Torsion bar

Air spring

Pneumatic-only and combination air/leaf

spring


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Suspension Terms Jounce literally means bump.

In suspension terminology, it means the most compressedcondition of a spring. For instance, many suspensions usejounce blocks to prevent frame-to-axle contact known assuspension slam.

Rebound is the reactive response of a spring after beingjounced; it kicks back.

Unsprung weight, an important factor in a suspension, meansthe weight of any chassis components not supported by thesuspension, for instance, the axles. Ideally it is kept as low as possible because of the reaction

effect, which is one of the reasons for specing aluminum

wheels. Oscillation is either rhythmic or irregular vibrations or movementsin a suspension. For instance, a good suspension will minimize jounce/rebound

oscillations by using dampening devices such as shockabsorbers and multi leaf spring packs.


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Leaf-spring Suspensions

A leaf spring is a steel plate or stack of clampedsteel plates.

Most leaf springs used in trucks today aremanufactured from spring steel.

Spring steel is middle-alloy steel that has beentempered, that is, heat-treated.

The result is to provide a leaf spring plate withconsiderable ability to flex without permanentlydeforming.

Leaf springs may consist of a single leaf or a seriesof leaves clamped together, known as a springpack.


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Spring Pack Principles


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Self-dampening The reason for using multiple leaves clamped together rather

than a single piece of metal cut to the same shape has to do

with what happens when a load is applied to the spring. Interleaf friction

Interleaf friction provides a self-dampening characteristic tothe spring pack. Two factors ensure a spring pack retains itsself dampening.

First, when a spring pack is assembled, the individualleaves must never be lubricated or painted. This wouldreduce interleaf contact friction.

Second, the function of the center-bolt that clamps theleaves is critical. The tension it loads the leaves underhelps define the self-dampening ability of the springassembly. In the event of a broken center-bolt, much ofthe self- dampening properties of a spring pack are lost.

Shock absorbers not necessary The advantage of the multi-leaf spring pack is that shock

absorbers can be eliminated.


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Shop Talk

When assembling multi-leaf spring packs,

never paint or lubricate the contact surfaces

of the individual leaves.

The result would limit the self-dampeningcharacteristics of the spring.


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Types of Leaf Spring Assemblies

Constant rate

Leaf-type spring assemblies that have a

constant rate of deflection

Variable rate

Leaf-type spring assemblies with a variable

deflection rate obtained by varying the

effective length of the spring assembly


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Progressive Spring Operation


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Multi-leaf Shackle Spring


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Single Drive Axle Spring Suspension


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Tandem Axle, Equalizer Spring

Suspension System


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Semi-elliptical Springs with Shocks


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Equalizing Beam Suspension

SystemLeaf Spring-type


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Equalizing Beams

with Rubber Cushions


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Leaf and Air

Suspension System(1 of 2)


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Leaf and Air

Suspension System (2 of 2)


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Height Control Valve


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Air Springs

L f S i S i


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Leaf Spring Suspension

Troubleshooting Guide

See Table 261 on page 810 of the textbook.


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Rough Ride Diagnosis

See Table 262 on page 811 of the textbook.


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Caution

When checking U-bolts, torque to the original

specifications.

Rusty U-bolts should be disassembled,

cleaned, and lubricated to ensure that theclamping pressure achieved by torquing is

accurate.


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Caution

Do not operate a vehicle with a shock

absorber removed or defective because this

places undue stress on other suspension

components.


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Shop Talk

Some shock absorber mount brackets have a

stud welded to the bracket, rather than a nut

and bolt.

This does not alter the installation procedure.


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Caution

Failure to properly torque suspension

fasteners can result in abnormal tire wear

and damage to the springs, spring brackets,

and frame rail.


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Shop Talk

It is common practice to use SAE grade 8 fasteners

in suspension systems but not universal.

Grade 5 bolts flex more than grade 8 bolts and that

is required in some applications so replacing themwith grade 8 fasteners is not appropriate. The body

bound bolts with an interference-fit shank used by

some OEMs are always grade 5.

When replacing suspension Huck fasteners withbolts, it is generally safe to use grade 8 bolts.


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Caution

Failure to properly torque suspension

fasteners can result in abnormal tire wear

and damage to the springs, spring brackets,

and frame rail.


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Caution

You must apply Alumilastic compound, or

an equivalent, to areas where aluminum and

steel contact each other or the result will be

metal corrosion and severely seizedcomponents.


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Caution

It is not recommended to remove bushings

by burning them out.

Once alight, they burn for a long time

producing high heat and noxious fumes.


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Torque Rod Disassembly


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Caution

Most ride height control valves have a

reaction delay that can be as long as 15

seconds.

This is used to prevent continuous correctioncycling.

Remember this when diagnosing height

control valve problems.


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Height Control Valve Centering Pin


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Air Spring Replacement

Ai Pl bi Di


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Air Plumbing Diagram

T i l L Ali


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Typical Laser Aligner

Using a Tram Bar


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Using a Tram Bar

to Measure Axle Spread

Use of Framing Square


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Use of Framing Square,

Straight Edge and Plumb Bob

A l Ali t Di i A


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Axle Alignment Dimension A

A l Ali t Di i B


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Axle Alignment Dimension B

Eccentric Bushing to


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Eccentric Bushing to

Adjust Dimension A

Eccentric Bushing


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Eccentric Bushing

Movement for Dimension B

Alignment Shims


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Alignment Shims

Cab Air Suspension


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Cab Air Suspension

Summary (1 f 8)


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Summary (1 of 8)

A suspension supports the frame on a vehicle andacts as an intermediary between the axles and the

frame. With no suspension, road forces would be

transferred directly to the truck frame.

A suspension system plays a number of roles.

It stabilizes the vehicle over both smooth and rough

terrain.

It cushions the chassis from road shock, enabling the

driver to steer.

It maintains the proper axle spacing and alignment.

Summary (2 of 8)


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Summary (2 of 8)

There are four general categories of suspensionused on trucks.

Leaf spring

Equalizer beam

Leaf spring and solid rubber spring Torsion bar

Air spring

Pneumatic-only and combination air/leaf spring

Jounce describes a spring in its most compressedstate, whereas rebound is a spring when it extendsafter reacting to jounce.

Summary (3 of 8)


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Summary (3 of 8)

Unsprung weight is the vehicle weight notsupported by the suspension.

It includes the wheel and axle assemblies. Because

unsprung weight reacts directly through the

suspension to the frame, it is kept as light as possible.

Constant rate and progressive or variable-rate

springs are two types of leaf spring suspension

used on trucks.

Auxiliary springs are helper springs and only become

a factor when a vehicle is fully loaded.

Summary (4 of 8)


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Summary (4 of 8)

Steel springs are often assembled into semi-elliptical spring packs consisting of a stack of

sprung steel plates clamped by a center-bolt.

Spring packs are self-dampening because ofinterleaf friction.

Shock absorbers are used in suspension

systems to dampen suspension oscillations.

Shock absorbers reduce tire wear, front wheel

shimmy, and spring breakage.

Summary (5 of 8)


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Summary (5 of 8)

Leaf spring and rubber cushion are both equalizingbeam types of suspension used on heavy-duty

trucks.

A majority of todays trucks are equipped with air-

only or combination air/steel spring suspension

systems.

Air suspensions use truck system air pressure to keep

the air springs charged with compressed air.

Air bags can be either the reversible sleeve type or

the convoluted type.

Summary (6 of 8)


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Summary (6 of 8)

A reversible sleeve air bag consists of aninflatable rubber compound bag mounted on

a pedestal assembly.

Convoluted air springs can be single, double,or triple convoluted.

A major advantage of truck air suspensions is

that they are adaptive, having the capabilityto adapt to changing load and road surface

conditions.

Summary (7 of 8)


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Summary (7 of 8)

Ride height is managed by a height control valve inair suspensions.

Most ride height control valves have a built-in reactiondelay that should be recognized when troubleshooting

the system. Air springs have no self-dampening capability sothey almost always use shock absorbers.

Equalizer beam suspensions are used in tandem

drive and bogie arrangements to effectively balancesuspension stresses and maximize tire-to-roadcontact.

Summary (8 of 8)


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Summary (8 of 8)

Axle alignment is a key to a properly functioningsuspension system.

Axles can be aligned using laser beam alignmentequipment and cruder shop equipments such as

plumb bob, straight edge, and tram bars. The air suspension cab system is the most commonmethod of mounting a cab on a truck chassisbecause of the driver comfort it provides.

Driver seats may be solid mount, mechanicallysuspended, or pneumatically suspended. Air driverseats are the most common in current trucksbecause of the comfort it provides the driver.

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Chapter 26 Bennet

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