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A Brief History of Struts
WHAT IS A STRUT?
Now that we have a more thorough understanding of shock design, let's focus on the strut. The strut is a common damper type used on many of today's independent suspension, front wheel drive vehicles as well as some rear wheel drive vehicles.
A strut is a major structural part of a suspension. It takes the place of the upper control arm and upper ball joint used in conventional suspensions. Because of its design, a strut is lighter and takes up less space than the shock absorbers in conventional suspension systems.
Struts perform two main jobs. First, struts perform a 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 forces created by the up and down motion 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 suspension, support the spring, and hold the tire in an aligned position. Additionally, they bear much of the side load placed on 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 tires.
Typically, struts consists of a coil spring to support the vehicle's weight, a strut housing to provide rigid structural support for the assembly, and a damping unit within the strut housing to control spring and suspension movement. The bottom of the strut body attaches to the steering knuckle, which in turn connects to a lower control arm through a lower ball joint.
The top of the strut is connected to the vehicle body through the upper strut mount, in some cases called a bearing plate. This bearing plate allows the strut to pivot as the wheels are turned. It must be flexible enough to handle slight angle changes and dampen movement of the upper end of the strut. This mount or bearing plate transfers vehicle load to the strut and spring, making the upper mount/bearing plate the load carrier and the lower ball joint the follower.
The strut housing holds the damping unit and fluid. It is made of heavy gauge steel so that it is rigid enough to provide structural support and withstand road shock.
The piston rod of the strut is much larger in diameter than the piston rod of the typical shock absorber. This is to withstand the side load on the strut shaft. A strut rod will measure up to 7/8 of an inch in diameter while the piston rod of a typical shock measures up to ½ of an inch in diameter.
A coil spring is located between the upper and lower spring seats. It is held there by tension. The lower spring seat is welded to the strut housing, while the upper spring seat is kept in place by the upper strut mount.
Struts also have a jounce (or compression) bumper located under the upper spring seat. The purpose of this component is to limit suspension travel by not allowing suspension components to hit together.
Finally, a large nut at the end of the strut rod holds everything together. Monro-Matic Plus and Sensa-Trac-Struts are available for most vehicles equipped with struts today.
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