Strut Design


Strut Design

A strut-type front suspension is used on most FWD vehicles and some RWD cars. Internally, the design of a strut is very similar to a shock absorber and performs the same duties. In many strut-type suspension systems, the coil spring is mounted on the strut. The coil spring is largely responsible for proper curb riding height. A weak or broken coil spring reduces curb riding height and results in a harsh ride. The lower end of the front strut is bolted to the steering knuckle. An upper strut mount is bolted to the chassis strut tower. A lower spring seat is part of the strut assembly, and a lower insulator is positioned between the coil spring and the spring seat on the strut. Another spring insulator is located between the coil spring and the upper strut mount. The two insulators prevent metal-to-metal contact between the spring and the strut, or mount. These insulators reduce the transmission of noise and harshness from the suspension to the chassis. A rubber spring bumper is positioned around the strut piston rod. When the strut is fully compressed, the spring bumper provides a cushioning action between the top of the strut and the upper support. The upper strut mount contains a bearing, upper spring seat, and jounce bumper.

When the front wheels are turned, the front strut and coil spring rotate with the steering knuckle. The strut and spring assembly rotates on the upper strut mount bearing.

Some cars have a multi-link front suspension with an upper link connected from the chassis to the steering knuckle. The strut is connected from the upper link to the strut tower. A bearing is mounted between the upper link and the steering knuckle, and the wheel and knuckle turn on this bearing and the lower ball joint. Therefore, the coil spring and strut do not turn when the front wheels are turned, and a bearing in the upper strut mount is not required.

Strut Design, Rear Suspension

In some rear suspension systems, the lower end of the strut  is bolted to the spindle, and the top is connected through a strut mounted to the chassis. The rear coil springs are mounted between the lower control arms and the chassis separate from the struts.

In other rear suspension systems, the coil springs are mounted on the struts. An upper insulator is positioned between the top of the spring and the upper spring support, and a lower insulator is located between the bottom of the spring and the spring mount on the strut.

Travel-Sensitive Strut

some travel-sensitive struts contain narrow longitudinal grooves in the lower oil chamber. These grooves are parallel to the piston's orifices, and some oil flows through the grooves as well as the orifices. Under normal driving and road conditions, the orifices and grooves are calibrated to provide normal spring damping and control. If the front wheel drops suddenly, such as when it strikes a large hole, the piston moves into the narrow portion of the oil chamber. Under this condition, all the oil flow must flow through the piston orifices, which greatly increases the strut's resistance to movement and the suspension's damping action. This strut action prevents harsh impacts against the internal strut rebound rubber.

Adjustable Struts

Some adjustable struts have a manual adjustment that allows the vehicle owner or technician to adjust the struts to suit driving conditions. The strut adjusting knob varies the strut orifice opening. This knob has eight possible settings. The factory setting is #3, which provides average suspension system control. The #1 setting provides reduced spring control and the softest ride, whereas a #8 adjustment gives increased spring control and the hardest ride. The adjustment knob is usually accessible without raising the vehicle.

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