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Struts, shocks, and complete assemblies

A shock and a strut both control bounce. A strut also holds the suspension together. Buying the wrong one, or taking a coil spring apart without the right tools, is how a simple ride-control job becomes dangerous.

What each part does

The spring carries the weight of the vehicle. The damper — the shock or the strut cartridge — controls how fast the spring compresses and rebounds. Without a working damper, the spring keeps oscillating after a bump. The tire leaves the road for a moment, the body keeps moving, and the next bump arrives before the car has settled.

A shock absorber is a damper only. It bolts between the body and a control arm or axle, and other parts locate the wheel. You can often unbolt a shock without releasing a coil spring, but a spring that is still under load nearby is not something to treat casually. Look at the layout before you start.

A strut is a damper that is also a structural link. On a MacPherson strut suspension, which is common on the front of many cars and on the rear of some, the strut connects the steering knuckle to the body. It holds the top of the knuckle in place. The coil spring often sits on the strut, captured between a lower perch and an upper mount. Removing that strut changes the alignment, and taking the spring off the strut releases a lot of stored energy at once.

Some vehicles mix the two. A strut front and a shock rear is ordinary. Air springs, leaf springs, and torsion bars are other ways to hold the vehicle up, with a damper alongside them. The name on the box has to match the corner you are working on. A rear shock will not bolt into a front strut tower.

Signs the damper is tired

Oil on the damper body is the clearest clue. A light film of dust is normal. A wet streak running from the seal down the tube means the fluid that the piston moves through is leaving. Once enough fluid is gone, that corner stops controlling the spring. The leak does not repair itself.

Other signs can have more than one cause. A body that keeps bouncing after a dip, a nose that dives hard in an ordinary stop, or a rear that squats over a rise can be worn dampers. A clunk over small bumps can be the mount or a bushing rather than the piston. Cupped tire wear often shows up when a wheel hops because the damper is not holding it down, but a bent rim can do the same thing, so check the wheel before you blame the strut alone.

Pushing down on a corner and counting rebounds only catches a damper that is badly gone. A weak unit can still stop the body in one rebound. Treat a leak, the age of the original parts, the tire wear, and the way the car feels on a familiar road as the evidence you actually have.

Replace in pairs, on the axle

Dampers on the same axle age together. Replacing one side leaves the new unit doing more of the work, and the car can feel uneven. Replace both fronts, or both rears, as a pair. You do not have to do both axles the same day if only one is leaking. Do fix the axle that is actually worn. A leaking front strut still affects the front tires and how the nose dives in a stop.

Bare strut or complete assembly

A bare strut, sometimes called a cartridge or an economized strut, is the damper tube and piston. You move your existing coil spring, upper mount, bearing, boot, and bump stop onto it. That only makes sense when those pieces are still good: the spring is not sagged or broken, the mount bearing turns smoothly, and the boot is intact so grit stays out of the rod.

A complete strut assembly, often sold as a loaded strut, includes the damper, the coil spring, the upper mount, and usually the bearing, boot, and bump stop, already assembled. You are not compressing the spring yourself. That is the practical choice for many owners, because the compressor is the hazardous part of the job and a tired mount is a common noise. The assembly costs more because it includes those parts. Compare the contents of the box, not only the word strut. A price comparison that ignores the spring and mount is not a comparison of the same job.

If the coil spring is sagged, broken, or you are changing ride height, the old spring should not go onto a new bare strut. Read the separate notes on coil springs and lift supports before you order a spring by itself. Lift supports that hold a hood or hatch open are a different part and do not replace a suspension strut.

Mounts, boots, and alignment

The upper mount bolts to the body and lets the strut turn as you steer, usually through a bearing. A failed bearing pops or groans at low speed, sometimes while you sit still. A collapsed mount lets the rod sit off center and can clunk. If you are installing a bare strut, replace a noisy or rusty mount at the same time. The boot keeps grit off the rod, and the bump stop takes a full compression. A torn boot or a missing bump stop is why the next part fails early.

Strut work changes alignment, especially camber, because the knuckle was unbolted from the part that held it. Plan on an alignment after front struts. Skipping it can wear the shoulders off new tires. Rear shocks on a solid axle often do not need an alignment. Rear struts, or a suspension with adjustable links, might. Get a printout.

Use new nuts when the maker says the old lock nuts are single-use. Torque the mount and the knuckle bolts to the specification for the vehicle. Some bolts are meant to be tightened at ride height so the bushings are not twisted at rest. The service information says which ones.

After the repair, check that the boot is seated, that the spring is indexed in its perch the way the old one was, and that nothing rubs at full lock. A short drive should feel controlled over a speed bump, without a new clunk. If the car pulls or the steering wheel sits off center, the alignment is waiting.

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