How A Car’s Suspension WorksJanuary 19, 2011 | in Defensive Driving Tips
Good handling on the road depends on more than a car’s steering system. The steering works hand in hand with the suspension and tires to create a smooth ride and reliable steering. The suspension acts to improve a car’s ride and handling.
Even the smoothest roads have bumps. A car’s ride is its ability to absorb these bumps while keeping the car body fairly steady. Without a suspension, every little nick in the road would produce a corresponding jolt in the body of the car. The suspension also improves a car’s handling, i.e. the vehicle’s ability to turn, stop, and accelerate safely.
In order to accomplish these goals, a suspension faces several challenges. To create a smooth ride, the suspension has to absorb energy from bumps in the road and distribute this energy throughout the car frame. The suspension also works to keep the tires in contact with the road. Remember that when a car brakes, weight shifts from the rear to the front; the reverse occurs during acceleration. Weight also shifts when a car makes a turn. This weight transfer can weaken tires’ grip on the road; to combat this, the suspension minimizes the transfer of weight from back to front and vice versa. It also helps to transfer weight from the “high” side of the car to the “low” side when the car turns a corner.
The suspension has several components that work together to accomplish these goals: frame, springs, and damper (shock absorber).
The frame is the rigid structure that supports the main weight of the car. This part of the car is referred to as the sprung mass because it rests on springs; these springs absorb the increased vertical velocity of the wheels as they travel over bumps. The unsprung mass is the weight of the car below the frame: wheels, tires, axles, etc.
Over the years, several different types of springs have been used in car suspensions. Leaf springs are curved metal bars composed of several layers of metal that bend to absorb energy; they were common on older cars and are still used on most trucks. Torsion bars, which twist to absorb energy, were used by European carmakers in the 50s and 60s. Air springs were also used in some older models. Today, coil springs, like those found in a mattress, are most common in passenger cars.
The stiffness of the springs affects the performance of the vehicle. If a car is loosely sprung, it will easily absorb bumps in the road, providing a very smooth ride. However, the handling of the car won’t be as good, as the vehicle body will be prone to moving forward, backward, and side to side. Tightly sprung cars, while offering bumpy rides, maneuver more effectively. Car manufacturers aim to find a balance between these qualities.
Springs absorb energy easily; however, they don’t dissipate it. As soon as you release a compressed spring, it snaps back in the reverse direction and continues to oscillate until all the energy has been used up. If suspensions relied entirely on springs, you would have a very bumpy and uncontrollable ride.
To account for this, springs are usually paired with dampers, or shock absorbers. These devices use hydraulics to turn kinetic energy (motion) into thermal energy (heat.) This way, the energy stored in the spring dissipates quickly, without causing unnecessary motion in the body of the car.
A typical shock absorber is, in essence, a piston inside two oil filled tubes. The piston is attached to a casing, which is in turn attached to the spring. As the spring moves, it pushes the piston up or down, compressing the oil inside the pressure tube. Tiny perforations in the pressure tube allow the oil to slowly escape into the reserve cylinder. The system is designed to provide enough resistance to absorb all of the energy from the spring without moving too much.
The shock absorber has two cycles: the extension cycle, when the piston moves “down”, and the compression cycle, when the piston moves back up. The compression cycle controls the unsprung mass of the car, as the wheels compress the spring when they move upwards. The extension cycle controls the sprung mass of the car, which is effected by the “release” of the coiled springs. As the system provides greater resistance during the extension cycle, the shock absorber is very effective in keeping the body of the car fairly still. Modern shock absorbers are also velocity sensitive, so that the system provides more resistance as the car moves faster. When a shock absorber is combined with a coil spring, it becomes a strut, which, in addition to absorbing energy from the wheels, helps to provide structural support.
Another component of the suspension is the anti-sway bar. This is a solid metal bar that spans the axle of the car, joining one side of the car to another. Also called an anti-roll bar, it helps to prevent too much lateral motion in the car body.
These are the basic elements of a suspension, which can be arranged in different ways on different types of cars, depending on the arrangement of the wheels. If the wheels are dependent, i.e. linked by a solid axle, then a combination of leaf springs and shock absorbers is used. While still common on trucks, the dependent front and rear suspensions are no longer common on passenger cars.
Instead, independent suspensions, in which the wheels are each allowed to move on their own, are used. If both the front and the back wheels use an independent suspension, then a car can be said to have four-wheel independent suspension, a phrase you may encounter in car advertisements. One of the most common designs used on the front suspension is the McPherson strut, which is named after its inventor, Earle S. McPherson of General Motors. Invented in 1947, this design is still common today.
Another common design for front suspensions is the double wishbone or double A arm suspension. In this design, two wishbone shaped supports are attached to each wheel, joining the wheel at one point and the frame at two points. Each of these arms carries a shock absorber and coil spring. Similar systems are used in the rear suspension in most cars. As the rear suspension doesn’t have to accommodate a steering element, these designs are usually a bit simpler.
Each of these basic designs has been modified in a number of different ways to produce a range of suspension options. All of these designs, however, employ the same basic principles to produce a safe and comfortable ride.
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