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Suspension Systems

The suspension is one of the most important parts of a car. All cars have some sort of suspension system, even golf carts. A suspension has three major jobs: to keep the car’s frame from dragging on the ground, to control body movement during cornering and to control wheel movement over imperfections in the road. Suspension design is a complicated science–very nearly art–in its subtlety, complexity and variation. Even on a fairly smooth road, small bumps jolt the car as it travels. Without any sort of suspension, these bumps could force the wheels off the road, damage the car and jolt the driver. Suspension keeps the wheels in contact with the road surface, and keeps the car intact and the driver comfortable. Parts • A suspension system comprises five basic components: the tires, wheel hubs (steering knuckles on the steer axle), the springs (which bear the weight of the car), the control arm or arms (which link the wheel hubs, steering knuckle or axle to the chassis) and the dampers (a.k.a. shock absorbers, which slow the movement of the suspension and inhibit the springs’ natural tendency to oscillate). Tires and Springs • The first layer of the suspension system is the tires. A car’s flexible, air-filled tires respond to dips and bumps in the road, bending to absorb some of the shock. The tires are attached to the wheels, which are attached to springs. When a bump pushes on the wheels, it compresses the spring. This lets the wheel move up and down with jolts, keeping the body of the car from absorbing the knocks directly. Shock Absorber • A problem with springs is that they don’t dissipate energy. When a bump in the road compresses a spring, it stores the energy. It then springs back, pushing back against the road with almost as much force as the road had exerted. (Some of the force is dissipated as heat.) If the suspension consisted of nothing but springs and tires, the car would bounce up and down continuously, making for an uncomfortable and dangerous ride. Shock absorbers are the solution to this. There are many different kinds, but they all consist of a column or flexible container filled with a fluid. When a coil inside is compressed, a piston pushes down on the shock absorber, compressing the fluid. This pushes the fluid out of the way, creating resistance. This resistance turns the energy pushing down on the shock into heat, which leaks out into the air. Instead of storing the energy from every jolt in the spring, most of it is dissipated through the shock. The car doesn’t bounce back violently–it only rebounds enough to stay in contact with the road. Suspension Categories • Suspensions fall into one of two basic categories: dependent and independent. A dependent suspension links the wheels on both sides of the car with some sort of solid or flexible axle, so suspension movement on one side of the car directly affects the other side. Independent suspensions utilize one of several different configurations to keep motion on one side of the car from affecting the other side. Generally speaking, dependent suspensions are cheaper to build and engineer, but independent suspensions offer better handling (especially over rough roads at high speed) and a more comfortable ride. Dependent Types • Dependent suspensions come in two basic flavors: the “live” axle (meaning that it has a differential for transferring power, as in most rear-wheel-drive cars and trucks), the “dead” axle (which simply serves to support the weight of an un-driven end of a car, as in the rear of a front-wheel-drive car). Dead axles come in a few different variations, including the twist-beam semi-independent axle. Twist beam axles utilize a tube-within-a-tube design. The outer tube connects to the wheel hubs and springs, and the inner tube keeps the outer tubes together. This allows the wheels on either side to act a little more independently than a solid dead axle would allow. Lateral Arm Suspensions • Most steering systems use some sort of lateral-arm (meaning that the control arm pokes out perpendicular to the frame rail, toward the wheels) independent suspension. There are several different types of lateral-arm suspensions, but the two most common are the double-A-arm and MacPhereson strut. Double-A-arm suspensions are so-called because they use a pair (upper and lower) of A-shaped control arms to connect the chassis and wheel hub. MacPhereson strut suspensions replace the upper control arm with a strut, which is a combination control arm, spring and shock absorber. Strut suspensions are lighter and cheaper to build, but generally don’t handle as well as a proper double-A-arm. Trailing/Leading Arm Suspensions • Trailing/Leading-arm suspensions use a control arm connected to the chassis, either ahead of the axle (trailing-arm, used for many rear suspensions), or behind the axle (leading-arm, used to control the front axle of many four-wheel-drive trucks). Trailing/leading-arm suspensions are the longitudinal equivalent of lateral-arm suspensions; a four- or five-link suspension uses an upper and lower control arm (like the double-A-arm), and a single-trailing-arm (a.k.a. “truck arm”) suspension uses a single link to connect the chassis and axle (similar to the MacPhereson strut). Truck-arm suspensions don’t offer the control of a four- or five-link, but are still suitable for many live-axle applications; Nascar stock-car builders prefer the truck-arm suspension for its light weight, durability and simplicity.