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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.