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What shocks and struts do
Shocks and struts are safety components that maintain vertical loads placed upon the tires. They control spring and suspension movement to keep tires in contact with the road. Under normal conditions on a smooth road, shocks stroke on average 1,750 times for every mile traveled or 7.5 million stabilizing actions on average every 12,000 miles (20,000 kilometers). As a result, shocks and struts do wear out and should be checked every 12,000 miles/20,000 kilometers just to be safe. Worn shocks and struts affect vehicle safety.
Worn shocks and struts may allow:
1. Excessive tire bounce
2. Poor tire-to-road contact
3. Reduced suspension control
4. Premature tire wear
5. Increased wear on other suspension components
6. Reduced handling and braking performance
7. Noise and suspension vibration.

What Shocks Do
Despite what many people think, conventional shock absorbers do not support vehicle weight. Instead, the primary purpose of the shock absorber is to control spring and suspension movement.
Shock absorbers are basically oil pumps. A piston is attached to the end of a piston rod and works against hydraulic fluid in the pressure tube. As the suspension travels through jounce and rebound, the hydraulic fluid is forced through tiny holes -- orifices -- inside the piston. However, the orifices let only a small amount of fluid through the piston. This slows down the piston, which in turn slows down spring and suspension movement.
The amount of resistance a shock absorber develops depends on the speed of the suspension and the number and size of the orifices in the piston. Shock absorbers are velocity sensitive hydraulic damping devices, meaning the faster the suspension moves, the more resistance the shock absorbers provide. Because of this feature, shock absorbers adjust to road conditions. As a result, shock absorbers reduce:
1. Bounce
2. Roll or sway
3. Brake dive
4. Acceleration squat
Shock absorbers work on the principal of fluid displacement on both the compression and extension cycle. A typical car or light truck will have more resistance during its extension cycle than its compression cycle. This is because the extension cycle controls the motions of the vehicle sprung weight. The compression cycle controls the motions of the lighter unsprung weight.

Compression Cycle
During the compression stroke or downward movement, some fluid flows through the piston from Chamber B to Chamber A (see illustration), and some through the compression valve into the reservoir, Chamber C. To control the flow, there are three valving stages in the piston and in the compression valves.
At the piston, oil flows through the oil ports, and at slow piston speeds, the first stage opens. This allows fluid to flow from Chamber B to Chamber A. At faster piston speeds, the increase in fluid pressure below the piston in Chamber B causes the second stage piston valve to open. At high speed, the limits of the second stage phase into the third stage orifice restrictions. At the bottom of Chamber B, oil that is displaced by the piston rod is passed through the three-stage compression valve into Chamber C. At slow speeds, the oil flows through an orifice in the compression valve. As the piston speed increases, the fluid pressure increases, causing the disc to open up away from the valve seat. Again, at high speeds the orifice restriction becomes effective. Compression control, then, is the force that results from the higher pressure present in Chamber B that acts on the bottom of the piston and the piston rod area.

Extension Cycle
As the piston and rod move upward toward the top of the pressure tube, the volume of Chamber A is reduced, and thus is at a higher pressure than Chamber B. Because of this higher pressure, fluid flows down through the piston's three-stage extension valve into Chamber B. However, the piston rod volume has been withdrawn from Chamber B, greatly increasing its volume. Thus, the volume of fluid from Chamber A is insufficient to fill Chamber B. The pressure in Chamber C is now greater than that in Chamber B, forcing the compression intake valve to unseat. Fluid then flows from Chamber C into Chamber B, keeping the pressure tube full. Extension control, then, is the force present as a result of the higher pressure in Chamber A, acting on the top side of the piston area.

What Struts Do
Purpose of Struts
In the middle 1970's, domestic auto makers began the transition from producing large rear-wheel drive vehicles to producing lighter, more fuel efficient front-wheel drive vehicles. Along with this transition came many changes to the typical suspension system. For decades, the majority of passenger cars came equipped with short-arm/long-arm suspension systems, which are frequently called SLA's.
But with the advent of smaller, front-wheel drive vehicles, under-hood space became a premium and most front-wheel drive vehicles simply don't have enough room for a short-arm/long-arm suspension system. As a result, the MacPherson strut suspension is now the standard suspension for all front-wheel vehicles and most rear-wheel drive vehicles.
When comparing the typical SLA suspension with the strut suspension we see that the strut suspension is taller than the SLA but does not require an upper control arm, pivot shaft or bushings. This reduction in parts helps allow the strut suspension to provide a lightweight, space efficient suspension system that is ideal for a variety of applications.

Strut Operation
Struts perform two main jobs. First, struts perform a shock damping function like shock absorbers. Internally, a strut is similar to a shock absorber. A piston is attached to the end of the piston rod and works against hydraulic fluid to control spring and suspension movement. Just like shock absorbers, the valving generates resistance to pumping forces created by the up and down motions of the suspension.
Also like shock absorbers, a strut is velocity sensitive, meaning that it is valved so that the amount of resistance can increase or decrease, depending on how fast the suspension moves.
Struts also perform a second job. Unlike shock absorbers, struts provide structural support for the vehicle's suspension. As a result, struts affect riding comfort and handling, as well as vehicle control, braking, steering, wheel alignment and wear on other suspension components, including the tires.