Rick's Free Auto Repair Advice

What are struts?

Are shocks and struts the same thing?

Automotive struts are designed to dampen spring oscillations as you travel over bumps, much like a shock absorber. However, unlike shock absorbers, struts are also a structural component of a vehicle. Automotive engineers use struts to reduce vehicle cost and provide less body roll in turns, while providing a stiffer, more sports car like ride. Most automotive struts use a McPherson design where the strut body holds the coil spring.

How are struts different from shocks?

Shocks and struts have similar interior construction since they’re both designed to dampen spring oscillation. They both have an inner cylinder and piston assembly mounted in an outer chamber filled with oil. Think of this inner cylinder like a doctor’s syringe fitted with a valve assembly at the end where the needle would attach, with the entire syringe sitting inside a cylinder of oil.

When the vehicle hits a bump, the upward force on the wheel forces the piston to compress and push oil out of the syringe chamber and into the outer cylinder. The valve allows quick release of oil during compression. Once the vehicle passes over the bump the spring exerts outward force on the piston to retract it and suck oil back into the syringe. However, the valve restricts the flow of oil back into the inner cylinder during this piston retraction cycle. In effect, the valve allow a fast compression but slow retraction. Valve restriction upon piston release is what prevents the spring from oscillating and keeps the tires on the road. Got it so far? Shocks and struts both operate this way. But that’s where the similarity ends.

How struts act as structural components

Traditional automotive suspension systems incorporate a coil spring, shock absorber, an upper and lower control arm, each fitted with bushings and a ball joint to allow the up and down and turning movements. The coil spring supports the vehicle weight and the shock absorber’s only job is to dampen spring oscillations. Control arms and ball joint add weight and cost to the vehicle and drive up owner maintenance costs. In a traditional suspension system,

Struts, on the other hand, have exterior features that allow them to act as a structural component. They incorporate a coil spring retaining plate located near the bottom of the strut tube and the top of the strut piston rod is threaded. During the strut assembly process, the assembler fits a coil spring into a compression device. Then they fit the strut up through the center of the compressed spring. The bottom end of the coil spring mates to a groove in the bottom retaining plate, and a strut mount is fitted to the top portion of the spring. The strut mount contains a bearing assembly to allow the strut to rotate during turns. The strut mount is secured to the piston rod with a nut and the spring compression tool is remove, allowing the spring to exert full force between the retaining plate and the strut mount.

struts versus shocks

Compare struts and shocks

Since struts “bridge” the gap between the top of the steering knuckle and the body of the vehicle, they allow automotive engineers to eliminate the upper control arm, bushings and upper ball joint.

Struts change ride characteristics

Traditional suspension systems offer a softer ride than a strut system, but the body may roll more in turns. Strut suspension systems reduce body roll and provide a stiffer ride, which many drivers like.

Struts versus shocks

Advantages of struts over shocks

Longer life. Depending on how you drive your vehicle, struts usually last between 80,000 miles and 100,000 miles before replacement. Shocks have much shorter life around 50,000 miles.

Lower vehicle weight and lower manufacturing cost. Struts eliminate upper control arm and bushings and upper ball joint

Stiffer ride with less body roll.

Disadvantages of struts

Stiffer ride

Higher replacement cost. Although strut suspensions lower manufacturing cost, struts cost far more to replace than shocks. However, the higher maintenance costs are somewhat offset by the elimination of upper ball joint, control arm and control arm bushing replacement costs.

More information on struts

Worn shock absorbers and struts can increase your stopping distance by up to 20%. At higher speeds the increased stopping distance can mean the difference between a safe stop and a fatal accident.

Worn shock absorbers and struts can’t resist weight redistribution during stopping, causing the front end to dive during braking.

Worn shock absorbers and struts can’t properly dampen spring oscillations, causing your tires to bounce more on bumpy roads which reduces your ability to keep your vehicle in your own lane on curves.

Worn shock absorbers and struts cause tire cupping, which wears your tires out much faster and causes vibration.

To learn more about when to replace shocks and struts, read this post

To learn more about how to test shocks and struts, read this post

To learn more about strut replacement cost, read this post

©, 2017 Rick Muscoplat

Posted on by Rick Muscoplat

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