FUNCTIONS OF THE SUSPENSION

A suspension system plays a number of roles:• It stabilizes the truck when traveling oversmooth highway as well as over rough terrain.• It cushions the chassis from road shock andenables the driver to steer the truck.• It maintains the proper axle spacing andalignment.• It provides a smooth ride both when loadedand unloaded.

Because trucks are specified by the heaviest load they are expected to haul, suspensions first must be capable of supporting that maximum load. However, a heavy-duty suspension that performs comfortably when fully loaded can be harsh and unforgiving when not loaded down. A really good suspension should perform well, both loaded and unloaded, at off-highwayand highway speeds.

CATEGORIES OF SUSPENSIONSTruck suspension systems are divided into four general categories:• Leaf spring• Equalizer beam: leaf spring and solid rubberspring• Rubber block and torsion bar• Air spring: pneumatic-only and combinationair/leaf spring

LEAF SPRING SUSPENSIONSleaf spring is a steel plate or stack of clamped steel plates. They have been used since the first vehicles hit our roads; for instance, they were key comfort components in horse-drawn buggies. 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 PRINCIPLESA spring pack consists of a stack of tempered steel leaves clamped together by a center bolt. In theory, a multileaf spring pack is based on a diamond-shaped plate supported at either end. If such a shape were to be loaded in the center between the supports at either end, then the bending (deflection) that takes place

would be proportional to the load applied. If you take a look at the spring pack shown in the Figure, you can see that the shape is as if a diamond had been cut in half. This half-diamond shape has the same load stress characteristics as a full-diamond-shaped spring, so it is

commonly used in springs. The half-diamond shape we just described is also known as a semi ellipse. A term used to describe the shape of a typical multileaf spring pack is semielliptical. Most multileaf spring packs used in trucks and trailers are semielliptical

SPRING PACK SELF-DAMPENINGThe front axle spring pack shown above 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.

TYPES OF LEAF SPRING ASSEMBLIESMany current truck suspensions use combination air/leaf spring suspensions. A combination suspension exploits the advantages of each type of spring to provide good unloaded and fully loaded suspension performance. For our purposes, we have divided leaf springs into two general types: constant rate and progressive or variable rate auxiliary.

CONSTANT RATEConstant rate springs are leaf-type spring assemblies that have a constant rate of deflection. For example, if 500 pounds deflect the spring assembly 1 inch, 1,000 pounds deflect the same spring assembly 2 inches

Alignment clips constant rate leaf springs: the constant rate main spring is supplemented by an auxiliary spring pack that only becomes a factor once the main spring has been deflected some distance.

Taper leaf spring: A variation on the multileaf spring packs just described is the tapered leaf spring design. Tapered leaf spring packs are constructed with leaves that are thicker in the center than at the ends

VARIABLE RATE

Variable rate springs (also known as progressive or vari-rate springs) are leaf-type spring assemblies with a variable deflection rate obtained by varying the effective length of the spring assembly. This is accomplished by using a cam bracket. As the spring assembly deflects, the point of contact on the bracket moves toward the center of the spring assembly, shortening the effective length (Figure 26–4). Variable rate spring assemblies also incorporate a progressive feature in that the lower spring leaves are separated at the ends.

FRONT SUSPENSIONThe front suspension plays a special role in the suspension because it directly impacts on driver comfort. Until recently, steel spring suspensions were used exclusively on the front of trucks, but air springs have now become an option. Many types of front steel spring assemblies are used. Some common types include taper-leaf eye and slipper, multileaf eye and slipper, taper-leaf shackle, and multileaf shackle. An exploded view of a commonly used front suspension, multileaf shackle spring, is shown in the figure

Rear Suspension—Single Axle

Single rear axle spring assemblies generally are one of two types: eye and slipper spring or semielliptical radius rod. The Figure illustrates an example of semielliptical radius rod spring assemblies. This spring is clamped to the axle with U-bolts, with the ends riding in slipper brackets mounted to the frame rails.

Rear Suspension—Tandem AxleThe tandem axle rear spring suspension shown in the Figure uses a six-point equalizing leaf spring design to compensate for axle articulation, both side to side and front to rear. Four spring packs are attached to the axles with U-bolts. On both sides of the vehicle, the forward end of the forward spring and the rear end of the rear spring ride in brackets mounted on the frame rails.

SPRING SUSPENSION WITH SHOCKABSORBERSAlthough multileaf spring packs have good self-dampening characteristics, most front spring suspension systems use shock absorbers. Shock absorbers dampen spring oscillation

SPRING SUSPENSION WITH TORQUE RODSThe spring suspension system with torque rods is simple and commonly used. Construction consists of just two major components: leaf spring assemblies and torque rods. The leaf springs are clamped to the axles. On a 6 X 4 chassis, the leaf spring assemblies mount at three different locations on each frame rail, distributing the load over a large area of the frame, as shown in

the Figure this suspension is mounted to the frame by a front spring bracket, an equalizer bracket, and a rear spring bracket on each side of the frame.

LEAF SPRING EQUALIZING BEAMThe leaf spring-type suspension in the figure uses a leaf spring pack on each equalizer beam. The springs are mounted on saddle assemblies above the equalizing beams and pivot at the front on spring pins and brackets. The rear of each spring pack has no rigid attachment to the spring brackets but is free to move forward and backward to compensate for spring deflection Equalizing beams use a lever principle to distribute the load equally between axles and to reduce the effect of road irregularities.

SOLID RUBBER SPRING EQUALIZING BEAM

This type of suspension system (shown in the Figure) uses solid rubber

springs and is known as a solid rubber suspension. Solid rubber springs are sometimes known as rubber cushions. They fulfill the same role as leaf springs in supporting load and absorbing road shock. On these units, rubber load cushions are mounted on a saddle assembly at each side. Mounted between frame brackets and the suspension, each rubber block unit is secured by four rubber-bushed drive pins, each of which passes through the rubber cushion. All driving, braking, and cornering forces are transmitted through these pins.

RUBBER BLOCK AND TORSION BAR SUSPENSIONS

A rubber block and torsion bar suspension system uses a combination of both types of springs to maximize the advantages of each. They are known as combination rubber/torsion spring suspensions and have had a recent resurgence of popularity in both truck and trailer applications because of their versatility. This category of suspensions has some special advantages for vehicles required to negotiate rough, off-highway terrain along with normal highway operation

AIR AND TORSION ROD SUSPENSION

In the late 1990s, Kenworth introduced combination air/torsion spring suspension. This suspension combines the soft ride of an air spring with the terrain versatility of a torsion bar suspension. Sway bars set axle tracking. The suspension is designed to provide improved articulation over a standard air suspension and the result is to minimize axle road hop under a wide range of loads. the figure shows the principal components of this unique Kenworth suspension.

AIR SPRING SUSPENSIONS

An increasing number of heavy-duty trucks and trailers are equipped with air spring suspension systems. These suspensions may be fully pneumatic (all air springs) or combination air/leaf spring suspension discussed earlier and shown in the two figures The air bag or air spring suspension system provides a smooth shock- and vibration-free ride with a preset constant frame height. Using it in combination with steel leaf springs helps it to overcome some of its disadvantages.

Different Suspension Configurations for Heavy

Vehicles

Name: Mohamed Ashraf Sayed

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