The job of a car suspension is to maximize the friction between the tires and the road surface, to provide steering stability with good handling and to ensure the comfort of the passengers.
If a road were perfectly flat, with no irregularities, suspensions wouldn’t be necessary. But roads are far from flat. Even freshly paved highways have subtle imperfections that can interact with the wheels of a car. It’s these imperfections that apply forces to the wheels. According to Newton’s laws of motion, all forces have both magnitude and direction. A bump in the road causes the wheel to move up and down perpendicular to the road surface. The magnitude, of course, depends on whether the wheel is striking a giant bump or a tiny speck. Either way, the car wheel experiences a vertical acceleration as it passes over an imperfection.
Without an intervening structure, all of wheel’s vertical energy is transferred to the frame, which moves in the same direction. In such a situation, the wheels can lose contact with the road completely. Then, under the downward force of gravity, the wheels can slam back into the road surface. What you need is a system that will absorb the energy of the vertically accelerated wheel, allowing the frame and body to ride undisturbed while the wheels follow bumps in the road.
You can often tell if there’s an issue with your suspension just by the way it feels as you drive, but it can be difficult to assess any issues without jacking up the vehicle and visually inspecting the components of the suspension yourself. There are a number of different types of suspensions you may find in your vehicle, but some things to look for are fairly universal.
The suspension of a car is actually part of the chassis, which comprises all of the important systems located beneath the car’s body.
These systems include:
- Frame – structural, load-carrying component that supports the car’s engine and body, which are in turn supported by the suspension
- Suspension system – setup that supports weight, absorbs and dampens shock and helps maintain tire contact
- Steering system – mechanism that enables the driver to guide and direct the vehicle
- Tires and wheels – components that make vehicle motion possible by way of grip and/or friction with the road
With this big-picture overview in mind, it’s time to look at the three fundamental components of any suspension: springs, dampers and anti-sway bars.
Today’s springing systems are based on one of four basic designs:
- Coil springs – This is the most common type of spring and is, in essence, a heavy-duty torsion bar coiled around an axis. Coil springs compress and expand to absorb the motion of the wheels.
- Leaf springs – This type of spring consists of several layers of metal (called “leaves”) bound together to act as a single unit. Leaf springs were first used on horse-drawn carriages and were found on most American automobiles until 1985. They are still used today on most trucks and heavy-duty vehicles.
- Torsion bars – Torsion bars use the twisting properties of a steel bar to provide coil-spring-like performance. This is how they work: One end of a bar is anchored to the vehicle frame. The other end is attached to a wishbone, which acts like a lever that moves perpendicular to the torsion bar. When the wheel hits a bump, vertical motion is transferred to the wishbone and then, through the levering action, to the torsion bar. The torsion bar then twists along its axis to provide the spring force. European carmakers used this system extensively, as did Packard and Chrysler in the United States, through the 1950s and 1960s.
- Air springs – Air springs, which consist of a cylindrical chamber of air positioned between the wheel and the car’s body, use the compressive qualities of air to absorb wheel vibrations. The concept is actually more than a century old and could be found on horse-drawn buggies. Air springs from this era were made from air-filled, leather diaphragms, much like a bellows; they were replaced with molded-rubber air springs in the 1930s.
Based on where springs are located on a car — i.e., between the wheels and the frame — engineers often find it convenient to talk about the sprung mass and the unsprung mass.
Springs: Spring and Unspring Mass
The sprung mass is the mass of the vehicle supported on the springs, while the unsprung mass is loosely defined as the mass between the road and the suspension springs. The stiffness of the springs affects how the sprung mass responds while the car is being driven. Loosely sprung cars, such as luxury cars (think Lincoln Town Car), can swallow bumps and provide a super-smooth ride; however, such a car is prone to dive and squat during braking and acceleration and tends to experience body sway or roll during cornering. Tightly sprung cars, such as sports cars (think Mazda Miata), are less forgiving on bumpy roads, but they minimize body motion well, which means they can be driven aggressively, even around corners.
So, while springs by themselves seem like simple devices, designing and implementing them on a car to balance passenger comfort with handling is a complex task. And to make matters more complex, springs alone can’t provide a perfectly smooth ride. Why? Because springs are great at absorbing energy, but not so good at dissipating it. Other structures, known as dampers, are required to do this.
Dampers: Shock Absorbers
Unless a dampening structure is present, a car spring will extend and release the energy it absorbs from a bump at an uncontrolled rate. The spring will continue to bounce at its natural frequency until all of the energy originally put into it is used up. A suspension built on springs alone would make for an extremely bouncy ride and, depending on the terrain, an uncontrollable car.
Enter the shock absorber, or snubber, a device that controls unwanted spring motion through a process known as dampening. Shock absorbers slow down and reduce the magnitude of vibratory motions by turning the kinetic energy of suspension movement into heat energy that can be dissipated through hydraulic fluid. To understand how this works, it’s best to look inside a shock absorber to see its structure and function.
A shock absorber is basically an oil pump placed between the frame of the car and the wheels. The upper mount of the shock connects to the frame (i.e., the sprung weight), while the lower mount connects to the axle, near the wheel (i.e., the unsprung weight). In a twin-tube design, one of the most common types of shock absorbers, the upper mount is connected to a piston rod, which in turn is connected to a piston, which in turn sits in a tube filled with hydraulic fluid. The inner tube is known as the pressure tube, and the outer tube is known as the reserve tube. The reserve tube stores excess hydraulic fluid.
When the car wheel encounters a bump in the road and causes the spring to coil and uncoil, the energy of the spring is transferred to the shock absorber through the upper mount, down through the piston rod and into the piston. Orifices perforate the piston and allow fluid to leak through as the piston moves up and down in the pressure tube. Because the orifices are relatively tiny, only a small amount of fluid, under great pressure, passes through. This slows down the piston, which in turn slows down the spring.
Shock absorbers work in two cycles — the compression cycle and the extension cycle. The compression cycle occurs as the piston moves downward, compressing the hydraulic fluid in the chamber below the piston. The extension cycle occurs as the piston moves toward the top of the pressure tube, compressing the fluid in the chamber above the piston. A typical car or light truck will have more resistance during its extension cycle than its compression cycle. With that in mind, the compression cycle controls the motion of the vehicle’s unsprung weight, while extension controls the heavier, sprung weight.
All modern shock absorbers are velocity-sensitive — the faster the suspension moves, the more resistance the shock absorber provides. This enables shocks to adjust to road conditions and to control all of the unwanted motions that can occur in a moving vehicle, including bounce, sway, brake dive and acceleration squat.
Dampers: Struts and Anti-sway Bars
Another common dampening structure is the strut — basically a shock absorber mounted inside a coil spring. Struts perform two jobs: They provide a dampening function like shock absorbers, and they provide structural support for the vehicle suspension. That means struts deliver a bit more than shock absorbers, which don’t support vehicle weight — they only control the speed at which weight is transferred in a car, not the weight itself.
Because shocks and struts have so much to do with the handling of a car, they can be considered critical safety features. Worn shocks and struts can allow excessive vehicle-weight transfer from side to side and front to back. This reduces the tire’s ability to grip the road, as well as handling and braking performance.
Anti-sway bars (also known as anti-roll bars) are used along with shock absorbers or struts to give a moving automobile additional stability. An anti-sway bar is a metal rod that spans the entire axle and effectively joins each side of the suspension together.
When the suspension at one wheel moves up and down, the anti-sway bar transfers movement to the other wheel. This creates a more level ride and reduces vehicle sway. In particular, it combats the roll of a car on its suspension as it corners. For this reason, almost all cars today are fitted with anti-sway bars as standard equipment, although if they’re not, kits make it easy to install the bars at any time.
Suspension Types: Front
So far, our discussions have focused on how springs and dampers function on any given wheel. But the four wheels of a car work together in two independent systems — the two wheels connected by the front axle and the two wheels connected by the rear axle. That means that a car can and usually does have a different type of suspension on the front and back. Much is determined by whether a rigid axle binds the wheels or if the wheels are permitted to move independently. The former arrangement is known as a dependent system, while the latter arrangement is known as an independent system. In the following sections, we’ll look at some of the common types of front and back suspensions typically used on mainstream cars.
Dependent Front Suspensions
Dependent front suspensions have a rigid front axle that connects the front wheels. Basically, this looks like a solid bar under the front of the car, kept in place by leaf springs and shock absorbers. Common on trucks, dependent front suspensions haven’t been used in mainstream cars for years.
Independent Front Suspensions
In this setup, the front wheels are allowed to move independently. The MacPherson strut, developed by Earle S. MacPherson of General Motors in 1947, is the most widely used front suspension system, especially in cars of European origin.
The MacPherson strut combines a shock absorber and a coil spring into a single unit. This provides a more compact and lighter suspension system that can be used for front-wheel drive vehicles.
Also known as an A-arm suspension, is another common type of front independent suspension.
While there are several different possible configurations, this design typically uses two wishbone-shaped arms to locate the wheel. Each wishbone, which has two mounting positions to the frame and one at the wheel, bears a shock absorber and a coil spring to absorb vibrations. Double-wishbone suspensions allow for more control over the camber angle of the wheel, which describes the degree to which the wheels tilt in and out. They also help minimize roll or sway and provide for a more consistent steering feel. Because of these characteristics, the double-wishbone suspension is common on the front wheels of larger cars.
Now let’s look at some common rear suspensions.
Suspension Types: Rear
Dependent Rear Suspensions
If a solid axle connects the rear wheels of a car, then the suspension is usually quite simple — based either on a leaf spring or a coil spring. In the former design, the leaf springs clamp directly to the drive axle. The ends of the leaf spring attach directly to the frame, and the shock absorber is attached at the clamp that holds the spring to the axle. For many years, American car manufacturers preferred this design because of its simplicity.
The same basic design can be achieved with coil springs replacing the leaves. In this case, the spring and shock absorber can be mounted as a single unit or as separate components. When they’re separate, the springs can be much smaller, which reduces the amount of space the suspension takes up.
Independent Rear Suspensions
If both the front and back suspensions are independent, then all of the wheels are mounted and sprung individually, resulting in what car advertisements tout as “four-wheel independent suspension.” Any suspension that can be used on the front of the car can be used on the rear, and versions of the front independent systems described in the previous section can be found on the rear axles. Of course, in the rear of the car, the steering rack — the assembly that includes the pinion gear wheel and enables the wheels to turn from side to side — is absent. This means that rear independent suspensions can be simplified versions of front ones, although the basic principles remain the same.
Take note if your vehicle pulls or squeaks during turns
If you begin to feel as though the vehicle is working against you as you turn, it is likely the result of a failing suspension component. Different parts of your suspension can affect the steering response, angle of the tires, and the center of balance of the vehicle. Each of these elements can make your vehicle cumbersome or difficult to turn. A bad tie rod end will make steering response sluggish. If you’re an audible creek while turning the wheel, it may be the result of a bad lower ball joint. Conversely, if you hear knocking as the weight transfers in the vehicle during a turn, it may be caused by a bad sway bar end link.
- Pay attention to how the vehicle reacts as you turn and compare it to your previous experiences in the vehicle to assess if there is an issue.
- Listen carefully to see if components of your suspension are squeaking under pressure.
- Every vehicle behaves a little differently while turning, so previous experiences with the vehicle can make assessing issues much easier
Inspect the tread wear on your tires
Your tires should wear fairly evenly across the width of the tread. If you rotate your tires regularly, they should be worn close to evenly throughout. If you happen to notice the inside or outside of the tire is wearing at a faster rate than the rest, it could be an issue with the camber of your wheels and tires. Camber is the term used to describe the angle the wheel sits in relation to the vehicle and road.
- A vehicle with negative camber will wear the inside of the tires more quickly.
- A vehicle with positive camber will wear the outsides of the tires more quickly.
- Camber is determined by your suspension components and wheel alignment.
Try braking abruptly to see if the nose dives as you stop
If you are having issues with your front struts or shocks, your suspension may struggle to keep the vehicle level under hard braking. Stop quickly in a safe area and pay attention to the front of your car. If the nose of the vehicle dives or drops as you slow down, it may be as result of bad shocks or struts. If you can hear an audible clunk from the front of the vehicle as you brake, there is either an issue with your control arm or sub frame bushing.
- Your suspension should be able to support the weight of your vehicle and keep it fairly level during most situations.
- The front corner of your vehicle may also drop as you turn in the same direction. This is caused by the same failure.
Look to see if the vehicle sits level
With the vehicle parked, walk around it and visually assess how level it seems to be sitting. If one side of the car rests higher than the other, there are likely worn out or broken suspension components to blame.
- It is not uncommon for the front of the vehicle to ride slightly lower than the rear in many vehicles such as pickup trucks, but the vehicle should otherwise be level.
Pay attention to swaying and bouncing at low speeds
Your vehicle should have no difficulty withstanding bumps in the road at low speeds. If you go over a bump and feel your vehicle sway back and forth or bounce after passing the bump, your suspension is struggling to support the weight of the vehicle.
- Your vehicle should be able to go over a bump and quickly regain composure at low speeds.
- If your vehicle sways back and forth after going over a bump, there is likely an issue with your suspension.
Remember, this is a guide to help you diagnose suspension problems. Please give Boggs Automotive a call at (574) 269-6360 for a free estimate on the repairs of your vehicle.