The purpose of the suspension system is to absorb irregularities in the road as the tires run over them, allow the driver to maintain control of the vehicle through a turn and also to maintain as much tire to road contact as possible through all road conditions. The suspension system is between the frame and the wheels. It will control how the wheels react to different road conditions. As a tire passes over a bump in the road, that tires suspension system must compress to reduce the bumps affect on the drivers comfort. When the driver takes a hard turn, much of the vehicles weight is transferred to the outside wheels. This is not desirable for good handling. The suspension system will try to maintain equal weight at all 4 corners to maintain traction through the turn. Different vehicle and suspension designs can accomplish this better than others. After-market suspension systems will typically be much stiffer than the factory suspension systems. This sacrifices ride quality or driver comfort in exchange for better handling. Many race cars have almost no consideration for driver comfort, their main focus is on handling. Many high end sports cars/race cars will mount the engine and transmission behind the driver to better balance the weight front to back. This offers a tremendous advantage over front mounted engines which are much more common. The battery is also commonly relocated to the trunk either by the manufacturer or by the vehicle owner for handling reasons.
Jounce: When the suspension system has to compress. When a wheel travels over a bump.
Rebound: When the suspension system expands. This occurs after jounce or when a wheel goes through a pot hole.
Pieces of a Suspension System
Springs support the weight of their corner of the vehicle. They are the primary factor in determining a suspensions balance between stiffness and ride quality. They also determine ride height. Sprung weight is the weight that is held up by the suspension system. This includes the frame, engine and transmission. Unsprung weight, is anything between the spring and the road. This includes the tires, brakes and steering knuckles or spindles. Cars with less unsprung weight tend to handle and ride better. There are several types of springs in use on automobiles today.
Steel coil springs are very common in modern automobiles in front and rear suspension systems. They can be a constant/linear rate spring or a progressive rate spring. A constant rate spring has equal space between coils and will compress or deflect directly proportionally to the weight or load applied. Variable rate springs have different spacing between coils and can vary its deflection based on load or weight applied. Many coil springs will have smaller coils at one or both ends. This will allow the spring to compress or deflect farther than a normal spring, without the coils binding or touching eachother.
Leaf springs were widely used in suspension systems in many old cars, and are still used in rear suspension systems of many new tucks/vans/SUVs. They are essentially a long strip of steel which bends when load or weight is applied to it. They can be a mono-leaf/single leaf or multi-leafs which are several individual leaf springs strapped together.
Torsion bars are simply a steel bar connected to the frame and the lower control arm. As the suspension system moves, the bar will twist. The more the bar twists, the more the bar will resist suspension movement. The bars twisting action is what controls the suspension systems movement and stiffness.
Air suspension systems allow ride height and stiffness to be adjusted by either the driver or by a module. Air springs are simply rubber “bags” of air, which use airs ability to compress to act as a spring and support the vehicle. These systems can adjust individual bag pressure based on driver input or based on road conditions. They require an electrically controlled air compressor and tank, as well as lines to each bag.
A shock absorber’s job is to limit the suspension systems movement or prevent suspension bounce. With springs and no shocks, after driving over a bump the suspension system would bounce up and down until the energy has been dispersed. A shock will limit the suspension systems ability to compress and then absorb the energy to allow the suspension system to return to its normal position without bouncing. Without a shock absorber, a bouncing suspension system would not only be annoying to the driver, it would also be dangerous as weight would constantly be transferred from front to back or side to side. This could make the vehicle difficult to control. A shock absorber works by forcing a large volume of fluid through a small orifice as the suspension system moves. The frame of the vehicle will be connected to the top eyelet of the shock absorber which will be connected to the piston inside the shock. The bottom eyelet will be connected to the body of the shock absorber. The areas above and below the piston will be filled with oil. As the suspension system compresses (jounce), small amounts of oil will be allowed to travel through small valves in the piston to the area above the piston as the piston moves down. Since oil is a liquid which cannot compress, the amount of oil that makes it through the piston determines how much the suspension system is allowed to compress. After the suspension systems compression, it will want to expand to its original position (rebound). As the piston moves up inside the shock the oil above the piston will have to travel through even smaller valves to return to the area below the piston. This action prevents the suspension from bouncing after driving over a bump.
Control arms connect the steering knuckle to the frame. They guide the steering knuckle up an down through its travel. The control arms are allowed to pivot up and down on the frame end and are connected to the steering knuckle through a ball joint. Depending on the system design, there can be a single control arm or upper and lower control arms. Each with its own ball joint.
A ball-joint is simply a ball in socket connection. They are located between control arms and the steering knuckle. They allow the steering knuckle to pivot when the steering system turns the wheels, and guide the knuckle during suspension movement.
Steering Knuckle and Spindle
The steering knuckle provides a way to control the front wheels and houses the wheel bearing which allows the spindle and wheel to rotate when the vehicle moves. The steering knuckle will be connected to a tie-rod end, which will control steering, and it will be bolted to at least one control arm through a ball joint which will control the wheel during suspension movement. In a MacPherson strut, the strut or shock absorber will be attached to the steering knuckle.
Sway Bar/Anti-Roll Bar
The sway bar is designed to keep the car from leaning to the outside during a turn. If a vehicle is turning with no sway bar, most of the weight of the car is loaded onto the outside wheels, and taken off of the inside wheels. This causes the outer suspension systems to compress or deflect and the inner suspension systems to expand. This causes the inside wheels to lose traction and the outside wheels to be over loaded, resulting in a skid. A sway bar corrects this by compressing the inside wheels suspension systems along with the outside suspension systems. The sway bar is simply a steel bar which is connected at each end to a moving member of two suspension systems on the same axle (front left and front right or rear left and rear right), and connected to the frame through bushings which allow the bar to rotate slightly. A sway bar also prevents the outer suspension systems from compressing in the first place while cornering. Since the sway bar is connected to 2 suspension systems, essentially the weight of the car must compress BOTH left and right suspension systems springs to even get the car to start to sway or lean. It makes the outer springs seem as strong as both springs on the same axle. On a vehicle with front and rear sway bars, a turn must transfer enough weight to the outer wheels to deflect all 4 springs before the body will lean or sway.
Common Suspension Setups
A MacPherson strut suspension setup is common on all types of vehicles. Shock absorbers are called struts on this type of system. In this type of suspension system, a coil spring sits on a cup on the strut, with the coils wrapping around the strut. Part of the strut carries the vehicles weight and the top of the strut locates the suspension system and controls how the wheel will move when the spring is deflected.
In this setup the coil spring is located between the control arm and a frame member, and not on the strut. This system tends to offer a slightly more comfortable ride as compared to the standard MacPherson strut setup.
The double wishbone suspension system has an upper and a lower “wishbone” with a ball joint on each one which connect to the steering knuckle. There are also many variations to this system, some have links with their own ball joint instead of a solid wishbone piece or a combination of both. The main advantage to this setup is that the manufacturer can control how the camber changes when the suspension is compressed. To get the vehicle to handle better, engineers will make the top control arm shorter than the bottom. This causes the wheel to tip in at the top (negative camber) when the suspension deflects. For more on camber, see our alignment page.
Modifying a Suspension System
I can’t tell you how to make your car handle better because every vehicle will react differently to modification. I can tell you that vehicle manufacturers design their suspension systems with what they believe is best for the application and they have spent a pretty penny to do this. Most performance cars will be designed for the best handling possible while most passenger cars will be designed more for comfort or a balance of both. The engineers that designed these systems know more about suspension geometry than you, me and your buddy who is telling you to just cut your springs. As a general rule, a vehicle that was designed for passenger comfort can be made to handle noticeably better than factory if the done correctly. Cars that were designed for performance handling from the factory generally do not have much room for improvement. Since every vehicle is different, I recommend checking out a site that specializes in your particular make/model and see what worked for other people before you go and waste your money. And for goodness sakes, buy quality parts. If you can’t afford quality parts, keep saving. Don’t bother wasting your money on cheap junk. Many suspension modifications can alter suspension geometry in a negative way and actually hurt handling. If your only concern is the look of your vehicle, be sure that the vehicle is still safe to drive in all everyday driving conditions on all road surfaces.