Suspension Systems

Objectives

• Identify the suspension systems used on current trucks.

• Describe the components used on mechanical leaf and multileaf spring suspension systems and explain how they work.

• Identify equalizing beam suspension system components and explain how they function.

• Identify air spring suspension system components and explain how they function.

INTRODUCTION

A suspension system supports the frame on a vehicle. It acts as an intermediary between the axles and the frame. The axles are subject to whatever forces they encounter when running down the highway.

FUNCTIONS OF THE SUSPENSION

• 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.

Key Terms• Jounce

– Most compressed condition of a spring

• Rebound– Reactive response of a spring

• Unsprung Weight– Weight of any chassis component not supported

by the suspension (keep low)

• Oscillation– Rhythmic or irregular vibrations (shocks prevent)

CATEGORIES OF SUSPENSIONS

• Leaf spring

• Equalizer beam: leaf spring and solid rubber spring

• Rubber block and torsion bar

• Air spring: pneumatic-only and combination air/leaf spring

LEAF SPRING SUSPENSIONS(Constant Rate)

FRONT SUSPENSION

CONSTANT RATE

Example: 500lbs =1in

Rear Suspension—Single Axle

Shop Talk

When assembling multileaf spring packs, never paint or lubricate the contact surfaces of the individual leaves. The result would severely limit the selfdampening characteristics of the spring. (Interleaf Friction)

The friction of the springs rubbing against each other has a shock limiting effect.

Taper leaf spring assembly

Fewer leaves required=less weightVariable spring rate, Front Axle

VARIABLE RATE

Overall length changes to carry increases loads

Axle Adjustments

FIBER COMPOSITE LEAF SPRINGS

Used for a few years in autos, fiber composite leaf springs have found their way into heavy-duty trucks and trailers. They are made of high-tech composite fibers that are laminated and bonded by tough polyester resins. The long strands of fiber are saturated with resin and bundled together by wrapping (a process called filament winding) or squeezed together under pressure (compression molding).

Maintenance

U-bolts that loosen in service place undue strain on the spring center bolt. OEMs state that center bolts should be re-torqued at specified intervals but this is seldom done. If you are torquing center bolts on a spring not clamped by U-bolts, remember that they are under considerable tension should the center bolt fail.

SPRING SUSPENSION WITH SHOCK ABSORBERS

Shock Absorber Inspection

OEMs suggest checking shock absorbers at intervals of 12,000 miles (20,000 km).

Do not operate a vehicle with a shock absorber removed or defective because this places undue stress on other suspension components.

Rear Suspension—Tandem Axle

Torque Rod

EQUALIZER REPLACEMENT

Bogie Suspensions

A bogie is a general term used to describe a pair of axles arranged with common suspension members designed to act and react together. The ends of both camelback spring assemblies and walking beams are mounted in rubber shock insulators.

EQUALIZING BEAM SUSPENSIONS

Two types of equalizing suspensions are used on heavy-duty trucks:

• Leaf spring type

• Solid rubber or rubber cushion type

-Allow maximum suspension travel

LEAF SPRING EQUALIZING BEAM

CUSHION AND TORSIONSUSPENSIONS

Sectional view of the rubber cushion-type equalizing beam suspension.

CONTROL LINK, RUBBER SPRINGAND EQUALIZING BEAM

One variable rate rubber spring located in the center of the tandem8 links to align axles

SERVICING EQUALIZINGBEAMS

Power wash the suspension and inspect the components for cracks or damage. Inspect rubber bushings for damage or deterioration and plan to replace them if they show any indications of fatigue.

There are 3 types:

EQUALIZING BEAM OVERHAUL

Tube-type beam end mounting

Ball and socket beam end mounting.

AIR SPRING SUSPENSIONS

Combination air/steel spring suspension installation

AIR AND TORSION ROD SUSPENSION

Kenworth: max travel best ride

Air Springs

AIR SPRING TERMINOLOGY

SHOCK ABSORBER ORIENTATION

RIDE HEIGHT ADJUSTMENT

Location of the height control valve

and components.

CHECKING A RIDE HEIGHTCONTROL VALVE

Most ride height control valves have a reaction delay that can be as long as 15 seconds. This is used to prevent continuous correction cycling. Remember this when diagnosing height control valve problems.

A simple test is to remove the control rod and manually raise and or lower the frame height.

REPLACING A HEIGHTCONTROL VALVE

AIR CIRCUIT MAINTENANCE

AXLE ALIGNMENT

Several types of equipment and methods can be used to check axle alignment. These methods include light and laser beam alignment equipment and computer controlled sensor systems. A straightedge and trammel can also be used to check axle alignment. This last method is the most difficult and inaccurate method of checking axle alignment and should be used only when no other alignment equipment is available.

Laser Alignment

Tram bar

Measuring dimension “A” axlealignment.

Cab air suspension system with a transverse rod, two air springs, one shock

absorber and a leveling valve.

Exploded view of two-point cab airsuspension.

DRIVER SEATS

Air-suspended driver’s seatSolid-mounted seat.

Summary

There are four general categories of suspension used on trucks:

• Leaf spring

• Equalizer beam: leaf spring and solid rubber spring

• Rubber or air cushion cushion and torsion bar

• Air spring: pneumatic-only and combination air/leaf spring

Summary (cont.)

• Jounce describes a spring in its most compressed state, whereas rebound describes a spring when it extends after reacting to jounce.

• Unsprung weight is not supported 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.

Summary (cont.)

• Shock absorbers are used in suspension systems to dampen suspension oscillations. Shock absorbers reduce tire wear, front wheel shimmy, and spring breakage.

• Air bags can be either the reversible sleeve type or the convoluted type.

Summary (cont.)

• Air springs have no self-dampening capability so they almost always use shock absorbers.

• Equalizer beam suspensions are used in tandem drive and bogie arrangements to effectively balance suspension stresses and maximize tire-to road contact.