SUSPENSION SYSTEMS

A suspension system anchors and suspends the wheels or tracks from the frame with springs, as shown in figure 3-5. It supports the weight and allows the vehicle to be driven under varying loads and speed conditions over bumpy roads and rough terrain without great risk of damage.

Although suspension systems are a part of your prestart and operator maintenance responsibilities, they usually do not need to be adjusted or replaced for many miles. The spring assemblies of the suspension system should be checked regularly to ensure that shackles are tight and that bushings within the shackles are not overworn or frozen tight. Occasionally, spraying lubricating oil on the spring leaves helps to prevent squeaking at the ends of the spring leaves. Following the lubrication chart for a particular vehicle, check and lubricate the front suspension system, including linkages, kingpins, and ball joints. During your checks you may find shock absorber bushings worn. If so, document it and turn it in so the problem can be looked at. The Construction Mechanic (CM) inspector may decide the shock absorbers should be replaced.

Some symptoms of suspension troubles in vehicle operation that should be documented and turned in for repair are as follows: . Hard steering 

l Vehicle wanders 

l Vehicle pulls to one side during normal driving 

l Front-wheel shimmy 

l Front-wheel tramps (high-speed shimmy) 

l Steering kickback

Figure 3-5.-Front axle suspension system.

l Hard or rough ride

l Sway on turns

l Spring breakage

l Sagging springs

l Noises

The components of a suspension system are the springs and shock absorbers. Some suspension systems also have torsion bars.

SPRINGS

The springs support the frame and the body of the vehicle as well as the load the vehicle carries. They allow the wheels to withstand the shocks of uneven road surfaces and provide a flexible connection between the wheels and the body. The best spring absorbs road shock rapidly and returns to its normal position slowly. Extremely flexible or soft springs allow too much movement of the vehicle superstructure, while stiff, hard springs do not allow enough movement.

The springs do not support the weight of the wheels, rims, tires, and axles. These parts make up the "unsprung weight" of the vehicle. The unsprung weight decreases the action of the springs and is, therefore, kept to a minimum to permit the springs to support the vehicle frame and load.

Multiple Leaf Springs

The multiple leaf spring is part of the front axle suspension system, as shown in figure 3-5. It consists of a number of steel strips or leaves of different lengths fastened together by a bolt through the center. Each end of the largest or master leaf is rolled into an eye which serves as a means of attaching the spring to the spring hanger and spring shackle. Leaf rebound clips surround the leaves at two or more intervals along the spring to keep them from separating on the rebound after the spring has been depressed. The clips allow the spring leaves to slide but prevent them from separating and throwing the entire rebound stress on the master leaf. The spring thus acts as a flexible beam. Leaf springs may be suspended lengthwise (parallel to the frame) or crosswise.

When a leaf spring is compressed, it must straighten out or break; therefore, spring shackles are required at one or both ends of the spring. Spring shackles provide a swinging support and allow the spring to straighten out when compressed. One shackle is used in either the front

or rear support of springs installed lengthwise. Two shackles support springs installed crosswise. Figure 3-6 shows how a leaf spring is attached to a frame by a spring shackle.

The most common types of spring shackles are the link shackle and the U-shackle. Heavy vehicles have link shackles. The U-type is more common on passenger cars and light trucks.

On some wheeled tractors, link shackles support a transverse spring on the dead front axle. Most wheeled tractors do not even have springs, and all load cushioning is through large, low-pressure tires.

Track tractors have one large leaf spring (fig. 3-7) supported without spring shackles. Fastened to the engine support, it rests on the frame supporting the tracks and rollers. Brackets on the track frames keep the spring from shifting.

Figure 3-6.-Cross section of a shackle link.

Figure 3-7.-Partially removed tracklayer spring.

Figure 3-8.-Coil spring suspension.

Some vehicles are equipped with leaf springs at the rear wheels only; others are so equipped both front and rear.

Coil Springs

Coil springs (fig. 3-8) are generally used on independent suspension systems. They provide a smooth ride. Their use has normally been limited to passenger vehicles. Recently, however, they have been used on trucks.

In figure 3-9, you can see how a coil spring is mounted. The spring seat and hanger, shaped to fit the coil ends, hold the spring in place. Spacers of rubberized material are placed at each end of the coil to prevent squeaking. The rubber bumper, mounted in the spring supporting member, prevents metal-to-metal contact when the spring is compressed. Most vehicles are equipped with coil springs at the two front wheels, while some others have them at both front and rear.