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All of the power generated by a piston engine is useless if the driver can't control the car. That's why automobile engineers turned their attention to the suspension system almost as soon as they had mastered the four-stroke internal combustion engine. 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. (See illustration at right)
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. Whatis needed 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. Springs support the weight of the vehicle and allow the suspension system to move and adapt to road irregularities, compressing when the wheels hit a bump and expanding when the wheels encounter a dip.
Coil Springs, the most common type, compress and expand to absorb the motion of the wheels. In essence, a coil spring is a heavy-duty torsion bar coiled around an axis. Leaf Springs consist 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. Air springs, consisting of a cylindrical chamber of air positioned between the wheels and the body of the car, also have the ability to change ride height based on driving conditions.
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. Shock absorbers, or "shocks," dampen movement of the vehicle's springs as they compress and rebound during vehicle travel. Shock absorbers have a strong influence on vehicle control and handling and hold the tires to the road. "Shocks" are usually used on cars and light trucks with standard suspension systems. Shocks provide resistance by forcing hydraulic fluid (oil) through valves in the piston as it moves up and down. Because the oil cannot be compressed, only a certain amount of fluid can be forced through these valves, which creates resistance to vehicle movement.
Struts are used on the front end of almost all front-wheel-drive vehicles. Technically known as 'MacPherson struts,' they are much more than a shock absorber (although they look the most like one). A strut integrates numerous suspension parts into one compact assembly, including the coil spring, spring seats, shock absorber, strut bearing, and steering knuckle. The shock absorber portion of the strut is the most commonly serviced portion of the strut assembly.
Springs weaken gradually over time and may sag, causing your car's ride height to drop. Springs can also break as they age. Ride height influences steering and suspension operation, so it is important to have it checked along with a thorough visual inspection of the springs. Your vehicle's suspension system should be checked regularly, at least once a year, along with a complete wheel alignment. Worn or damaged suspension system components can cause wandering, pulling, erratic steering, excessive tire wear, leaning, bottoming out, or poor ride and handling. To get professional help with your car's suspension system, contact a qualified service technician.