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Since the horseless carriage appeared on the scene, automotive undercar service has undergone continual and radical change. The undercar – or chassis – is best described as a series of related steering and suspension parts which should be viewed and serviced at complete systems. There are several types of suspension system and steering arrangements. Though different in design, each type performs the same function.

What Is The Suspension?

Suspension links the car body to the running gear (tires, wheels, brakes) and consists of various springs, shock absorbers and related hardware. The primary function of the suspension system is to support the vehicle weight, absorb shock, and provide a pivot point for the wheels. This enables the car to maintain a smooth ride and turn corners without losing its road grip.

Front Suspension Systems

The major front suspension system types used in passenger cars and light trucks are the conventional coil spring, Torsion Bar and MacPherson Strut Systems. Most full-sized American cars use either the coil spring or torsion bar system. The strut system is widely used in import vehicles, as well as newer front wheel drive designs and down-sized domestic vehicles. The function of each suspension system is the same — to support the vehicle at its designed height, to maintain proper wheel alignment, to maximize tire contact with the road surface, and to absorb road shock transmitted through the tires. However, the method used to do this is different for all three systems.

Coil Spring System: Vehicle weight is supported by coil springs and controlled by their spring rate. The springs can be mounted on the lower control arm or the upper control arm. Where the spring is located determines which ball joint is the load carrier. On coil spring systems the load-carrying ball joint is always on the control arm carrying the spring.

Torsion Bar System: Supports vehicle weight by the”twisting” of the bar. In this case, the torsion bar does the word normally handled by the coil spring.

Strut Systems: Foreign car manufacturers were the first to adopt the MacPherson Strut System because of its compact design. With the downsizing of domestic autos, the strut system with some modifications had become the preferred suspension design.

MacPherson Strut System: The shock absorber, coil spring plates, coil spring and upper steering axis pivot bearing have been combined into the MacPherson strut assembly. Vehicle weight is supported at the top by the upper spring plate, through the coil spring and strut body and to the knuckle assembly at the bottom. The shock absorber dampens vibrations while the coil spring controls bounce and ride height. The coil spring is held in place bya lower spring plate welded to the strut housing and an upper plate bolted to the body. A ball joint on the lower control arm provides a pivot point for the steering knuckle. Since the vehicle load travels through the knuckle to the strut body, the lower ball joint is isolated from the vehicle weight making it a follower rather than a load carrying type. The MacPherson Strut System is particularly advantageous to front wheel drive vehicle designs. Since the strut is mounted on top of the steering knuckle, it allows accessibility to the spindle area for placement of the driveshaft.

Modified Strut: Used principally in rear wheel drive vehicles, it differs from a traditional MacPherson system because the coil spring is not an internal part of the strut body. In a Modified Strut, the spring is positioned between the lower control arm and vehicle frame. Since the coil spring rests on the lower arm, the lower ball joint becomes a load-carrying type. The strut connects to the steering knuckle at the bottom and bolts to the unibody at the top, thus eliminating the need for an upper control arm.

Rear Suspensions

The Leaf Spring and Coil Spring Non-Independent Suspension are the most common on today’s rear drive vehicles. Their solid axle design exhibits some of the same wear characteristics as solid axle front systems. But the effect is not as dramatic since the rear wheels do not pivot. With the trend toward front wheel drive, independent rear suspensions are becoming more prominent. Independent rear suspensions have offered improved performance in rear wheel drive vehicles for some time.

Leaf Spring System: On this system the control arms have been eliminated. U-bolts connect the springs to the axle housing and “shackles” connect the spring to the frame or unibody.

Coil Spring System: The coil springs seat on the axle housing and frame on the underside of the vehicle. Fore and aft movement are controlled by trailing arms and control arms.

Independent Rear System: Independent rear suspensions are used on rear non-solid axle vehicles. These assemblies use U-joints on the axles to allow independent pivoting of the axles with suspension movement. Independent rear suspension allow for better traction since there is little change in the angularity of the wheel when deflected. The result is better acceleration and braking and less rolling resistance in turning.

Independent rear suspension primarily use coil spring suspension although the strut is becoming increasingly popular. Independent rear suspensions require additional maintenance and service. Because these suspensions are independent, each side must be aligned to the other, as well as to the front wheels. This provides accurate tracking, maintains proper tire wear and reduced rolling resistance.

Coil Spring Systems

Conventional or constant rate, coil springs are available to handle either standard or heavy duty applications. Variable rate coil springs are designed to handle the demands of both applications.

Coil Spring Basics: The function of coil springs is to absorb road shock, as well as support and maintain the vehicle at its designed height. The movement of the spring is controlled by the action of the shock absorber. The shock absorber does not actually absorb shock, it simply dampens the spring movement. Shock absorbers do not carry vehicle weight or effect vehicle ride height. Coil springs are rated by their spring rate measured in pounds per inch. This Is the total load distributed over the travel from the free height to the loaded height of the coil spring.

Coil springs are dual action and must respond to both the jounce and rebound of the suspension movement. The spring and shock absorber together keep the tires in contact with the road surface, providing adequate traction, steering, efficiency and ride comfort.

Coil Spring Types

Standard: OE quality and performance. The standard types restore proper vehicle height and suspension geometry. They maintain good steering control and headlight aiming. They prevent bottoming and thumping noises in front suspension with normal loads. They are designed to carry light loads up to rated capacity of the vehicle.

Heavy Duty: Designed to give the additional load capacity needed for vehicles which frequently carry a heavier than standard to medium load. Most heavy duty springs offer about a 25% heavier spring rate for a similar increase in load carrying capacity. For heavily loaded vehicles the handling, performance and appearance will be similar to a vehicle carrying normal loads with standard coil springs. However, the heavy duty coil can only best operate under the additional load. Under lighter load conditions, the heavy duty coil spring design will produce a slightly more rigid ride and won’t absorb road shock as well as the standard replacement spring.

Variable Rate: The spring rate changes with the travel of the spring, as opposed having a constant rate as in conventional standard and heavy duty springs. This is accomplished through progressive spacing of the spring coils. The tightly wound coils provide the conventional ride characteristics, while the widely spaced coils provide the additional load carrying capacity. In operation, the coils compress in progression resulting in a progressive increase in the spring rate.

Check out all the steering and suspension parts available on NAPA Online or trust one of our 16,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on suspension system types, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

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