Chapter 26 Suspension Systems

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 they work. 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.

Objectives (2 of 2) Identify air spring suspension system components and explain how they function. Troubleshoot suspensions and locate defective suspension system components. Outline suspension system repair and replacement procedures. Explain the relationship between axle alignment and suspension system alignment. Perform full chassis suspension system alignments. Describe the operation of the cab air suspension system.

Suspension Systems (1 of 2) A suspension system plays a number of roles. 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.

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

Suspension Terms Jounce literally means “bump.” In suspension terminology, it means the most compressed condition of a spring. For instance, many suspensions use jounce blocks to prevent frame-to-axle contact known as suspension slam. Rebound is the reactive response of a spring after being jounced; it kicks back. Unsprung weight, an important factor in a suspension, means the weight of any chassis components not supported by the suspension, for instance, the axles. Ideally it is kept as low as possible because of the reaction effect, which is one of the reasons for spec’ing aluminum wheels. Oscillation is either rhythmic or irregular vibrations or movements in a suspension. For instance, a good suspension will minimize jounce/rebound oscillations by using dampening devices such as shock absorbers and multi leaf spring packs.

Leaf-spring Suspensions A leaf spring is a steel plate or stack of clamped steel plates. Most leaf springs used in trucks today are manufactured from spring steel. Spring steel is middle-alloy steel that has been tempered, that is, heat-treated. The result is to provide a leaf spring plate with considerable ability to flex without permanently deforming. Leaf springs may consist of a single leaf or a series of leaves clamped together, known as a spring pack.

Spring Pack Principles

Shock absorbers not necessary 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 to the spring pack. Two factors ensure a spring pack retains its self dampening. First, when a spring pack is assembled, the individual leaves must never be lubricated or painted. This would reduce interleaf contact friction. Second, the function of the center-bolt that clamps the leaves is critical. The tension it loads the leaves under helps define the self-dampening ability of the spring assembly. In the event of a broken center-bolt, much of the 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. SPRING PACK SELF-DAMPENING The front axle spring pack shown in Figure 26–1, is commonly used in truck applications. 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. As a load is applied to a spring pack, it begins to deflect. This deflection causes bending of each individual leaf in the pack. Because the leaves are clamped to each other with some force, any movement that takes place has to first overcome friction between the leaves. We call this interleaf friction. Interleaf friction provides a self- dampening characteristic to the spring pack. Dampening is required in most springs to limit oscillations after a spring compression. Two factors ensure a spring pack retains its self dampening characteristics. First, when a spring pack is assembled, the individual leaves must never be lubricated or painted. This would reduce interleaf contact friction. Second, the function of the center-bolt that clamps the leaves is critical. The tension it loads the leaves under helps define the self-dampening ability of the spring assembly. In the event of a broken center-bolt, much of the self- dampening properties of a spring pack are lost. Most springs have to be dampened after sustaining a compression load. A notable television commercial made by a manufacturer of shock absorbers shows an automobile rhythmically bouncing up and down a highway, overreacting to every little bump, with the message that a new set of shocks will cure the problem. Shock absorbers dampen suspension oscillation. The advantage of the multileaf spring pack is that often they can so effectively dampen suspension oscillations that shock absorbers can be eliminated.

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-dampening characteristics of the spring.

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

Progressive Spring Operation

Multi-leaf Shackle Spring

Single Drive Axle Spring Suspension

Tandem Axle, Equalizer Spring Suspension System

Semi-elliptical Springs with Shocks

Equalizing Beam Suspension System—Leaf Spring-type

Equalizing Beams with Rubber Cushions

Leaf and Air Suspension System (1 of 2)

Leaf and Air Suspension System (2 of 2)

Height Control Valve

Air Springs

Leaf Spring Suspension Troubleshooting Guide See Table 26–1 on page 810 of the textbook.

Rough Ride Diagnosis See Table 26–2 on page 811 of the textbook.

Caution When checking U-bolts, torque to the original specifications. Rusty U-bolts should be disassembled, cleaned, and lubricated to ensure that the clamping pressure achieved by torquing is accurate.

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

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.

Caution Failure to properly torque suspension fasteners can result in abnormal tire wear and damage to the springs, spring brackets, and frame rail.

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 them with 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 with bolts, it is generally safe to use grade 8 bolts.

Caution Failure to properly torque suspension fasteners can result in abnormal tire wear and damage to the springs, spring brackets, and frame rail.

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 seized components.

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.

Torque Rod Disassembly

Caution 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.

Height Control Valve Centering Pin

Air Spring Replacement

Air Plumbing Diagram

Typical Laser Aligner

Using a Tram Bar to Measure Axle Spread

Use of Framing Square, Straight Edge and Plumb Bob

Axle Alignment Dimension A

Axle Alignment Dimension B

Eccentric Bushing to Adjust Dimension “A”

Eccentric Bushing Movement for Dimension “B”

Alignment Shims

Cab Air Suspension

Summary (1 of 8) A suspension supports the frame on a vehicle and acts 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) There are four general categories of suspension used 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 compressed state, whereas rebound is a spring when it extends after reacting to jounce.

Summary (3 of 8) Unsprung weight is the vehicle weight 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. 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) 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 of interleaf 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) Leaf spring and rubber cushion are both equalizing beam types of suspension used on heavy-duty trucks. A majority of today’s 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) A reversible sleeve air bag consists of an inflatable 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 capability to adapt to changing load and road surface conditions.

Summary (7 of 8) Ride height is managed by a height control valve in air suspensions. Most ride height control valves have a built-in reaction delay that should be recognized when troubleshooting the system. 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.

Summary (8 of 8) Axle alignment is a key to a properly functioning suspension system. Axles can be aligned using laser beam alignment equipment and cruder shop equipments such as plumb bob, straight edge, and tram bars. The air suspension cab system is the most common method of mounting a cab on a truck chassis because of the driver comfort it provides. Driver seats may be solid mount, mechanically suspended, or pneumatically suspended. Air driver seats are the most common in current trucks because of the comfort it provides the driver.