Classification of Suspensions

Independent Suspensions.

To provide the best possible ride quality, many vehicles use fully independent front and rear suspension systems. This allows the vehicle to respond to varying road conditions much more effectively. Nearly all front suspensions found on modern cars and light trucks are independent. Even four-wheel drive (4WD) vehicles often have independent front suspensions to improve their ride and handling qualities. The lower image in Figure 6-6 shows how each wheel is able to move in an independent suspension while the upper image illustrates the movements of a dependent or rigid axle. In an independent suspension, each wheel can move independently, so a bump on one side of the vehicle does not affect the tires on the other side. This improves ride quality and maintains tire contact with the road for the remaining tires. Many rear suspensions on rear-wheel drive (RWD) vehicles are independent systems.

The differential is mounted solidly to the body or rear frame and short axles, similar to those found on the front of frontwheel drive (FWD) vehicles, are used to drive the rear wheels. This provides improved ride quality and handling. These suspension types are discussed later in this chapter. Many FWD cars have independent rear suspension systems as well. This improves ride quality and handling.

Dependent Suspensions.

Still found on the rear of many vehicles and on the front of most heavy duty vehicles, dependent suspensions sacrifice ride quality for strength. Since the movement of one wheel affects the opposite wheel, ride quality and handling suffer on these systems. A large, straight I-beam is often used on the front of heavy-duty vehicles, such as buses and semi-trucks. An example of this is shown in Figure 6-7. This design is used for its strength and durability but does not provide the best ride quality. The rear axle on many RWD cars, light trucks, and SUVs is a dependent live axle; an example is shown in Figure 6-8. Live axle means the rear axle is driving the rear wheels. Since a live rear axle is one large assembly housing the differential gears and axles, it is a dependent system. Live rear axles are mounted on leaf springs, coil springs, or air springs.

A vehicle with a solid rear axle that does not drive the rear wheels has what is called a dead axle or a rigid or straight axle. An example of this type of rear suspension is shown in Figure 6-9. A dead axle supports the weight of the rear of the vehicle and can be fitted with coil, leaf, or air springs. A dead axle is a dependent form of suspension.

Semi-Independent Suspensions.

Found on the rear of many FWD vehicles, this type of system uses a fixed rear axle that twists slightly under loads. This allows for semi-independent movement of the rear wheels. This system typically uses coil springs or struts. The semi-independent system provides better ride and handling than a straight axle while not being as costly as a fully independent system.

Front Suspensions.

The main purpose of the front suspension is to provide safe, comfortable handling while allowing wheel movement for the steering and enabling the driver to react to various road conditions. To accomplish this, several different front suspension styles are used in modern vehicles. The front suspensions on FWD vehicles also have to be able to handle the additional torque of driving the front wheels. Additionally, during braking, as much as 70 percent of the vehicle weight is transferred to the front, adding additional loads to the front suspension. Vehicle type and intended use of the vehicle are the main considerations when engineers begin to design the suspension systems. Many cars have suspensions that look very similar, but actually have many differences. The exact size and placement of components have a large effect on individual vehicle driving characteristics.

Rear Suspensions.

The rear suspension must be able to carry any additional loads placed in the rear of the vehicle while still maintaining the correct ride height. The rear suspensions on many FWD and RWD vehicles are similar in that a solid type of axle is used. Though strong, a solid axle does not provide the level of handling and ride quality that an independent rear suspension does. The rear suspension on RWD vehicles must be able to handle the torque of the driveline. This can be difficult since torque tries to twist the vehicle and rear suspension.