Shocks & Struts
Dampers are the most misunderstood parts on the car.
- Springs absorb road impacts; shocks (or the shock portion of a strut) dampen the resulting oscillations so the tire settles back on the road quickly.
- A strut is a structural component that supports the vehicle, locates the wheel, and dampens motion — replacing an upper control arm in MacPherson designs.
- Strut replacement disturbs camber and often SAI. An alignment is required after every strut replacement.
- Push down hard on a corner and release. A healthy shock lets the vehicle rise back and stop within 1-2 oscillations. More than that indicates worn dampers. It's a rough check — not definitive.
- Struts fail most often via seal wear — the oil in the strut leaks past the shaft seal, reducing damping progressively.
Dampers are the most misunderstood parts on the car — customers think they're springs, and plenty of techs replace them without knowing exactly what they do. Get the spring-versus-damper division of labor straight, learn how struts differ structurally, and know what has to happen after you install new ones.
What Dampers Do, and How Struts Differ
Springs and shock absorbers split the job of managing road impacts. The spring absorbs the hit and supports the vehicle's weight; the shock does neither. A shock absorber's job is to dampen the spring's oscillations — without it, the spring would bounce the body and the tire up and down repeatedly after every bump. The damper converts that oscillating energy into heat so the tire settles back onto the road quickly and stays planted. A worn shock doesn't lower the car and doesn't make it sag; it lets the car keep bouncing, and it lets the tire hop.
A strut is a different animal from a plain shock. A strut is a structural suspension member: it supports the vehicle's weight, locates the wheel, and dampens motion all in one assembly. In a MacPherson strut front end, the strut actually replaces the upper control arm — the strut body itself is the upper locating link for the knuckle. That structural role has consequences. The strut carries steering loads through its upper bearing mount, it defines part of the steering axis, and its position sets camber.
Failure Modes, Testing, and What Follows Replacement
The most common way a strut cartridge dies is not sudden collapse — it's a gradual oil leak past the shaft seal. As oil escapes, damping fades progressively, which is exactly why owners rarely notice: the ride degrades a little at a time over tens of thousands of miles. Look for oil streaking down the strut body, and pair that visual evidence with a performance check.
The classic quick check is the bounce test: push down hard on a corner of the vehicle and release. A healthy damper lets the body rise and settle within one to two oscillations. If the corner keeps bobbing beyond that, the damper is worn. Understand its limits, though — the bounce test is a rough roadside screen, not a definitive verdict, and a damper can pass a bounce test while still performing poorly at highway speed. Use it to build a case alongside oil leakage, cupped tire wear, and ride complaints.
After you replace struts on a MacPherson strut vehicle, an alignment is not optional. Removing and reinstalling the strut disturbs camber — the strut position defines it — and often SAI as well. Every strut job ends on the alignment rack. Sending the car out without one guarantees tire wear and a comeback.
| Item | Standard | Notes |
|---|---|---|
| Bounce test | Body settles within 1-2 oscillations | A rough screen, not a final verdict |
| Oil on damper body | Light misting acceptable; running streaks = replace | Misting is the shaft seal doing its job |
| Inspection interval | Every brake and tire service; formal check ~50,000 mi | Wear is gradual — owners rarely notice |
| Replacement policy | Axle pairs | One new and one tired damper handles unevenly |
| Post-strut alignment | Mandatory | Strut position sets camber and part of SAI |
| Cupped tire wear | Damper or balance cause | Fix the cause before mounting new tires |
On vehicles with electronically adjustable dampers, scan for suspension codes before condemning hardware.
- Road test over dips and washboard surface. Excessive float, wallowing after crests, nose dive under braking, and extra bounce cycles are the ride signatures of dead damping.
- Inspect each damper body. Light misting around the shaft seal is normal; oil streaking down the body means the seal has let go and damping is fading with every mile.
- Bounce test each corner and count oscillations — more than one or two after release fails the corner, but remember a damper can pass this test and still perform poorly at highway speed.
- Check the tires for cupping and check strut mounts and bearings — a scalloped tread confirms tire hop, and a worn upper mount adds clunks and memory steer that new struts alone will not cure.
- Build the case from all four findings together, then quote in axle pairs with the alignment included on strut jobs, and inspect spring seats and mounts while the assembly is apart.
- Condemning a damper for light misting around the shaft seal — that is normal lubrication of the shaft. Running streaks and wetness are the failure; misting is not.
- Replacing one damper on an axle. The new unit controls its corner while the old one floats, and the mismatched handling is worse than what the customer came in with.
- Skipping the alignment after strut replacement. The strut sets camber, and pulling it disturbs the angle every time — the tires pay for the shortcut in 5,000 miles.
- Reusing a worn strut mount or bearing on a strut job — the clunk and steering bind survive the new struts, and the comeback has your name on it.
A customer's new tires are cupped again 8,000 miles after replacement. What did the previous shop skip?
The cause. Cupping comes from the tire hopping — worn dampers or imbalance — and new rubber does nothing about either. Evaluate the dampers and road-force balance the wheels, fix what is found, and then the next set of tires survives.
A strut passes the bounce test, but the body shows oil streaks, the tires are starting to cup, and the customer reports float at highway speed. What is the call?
Replace. The bounce test is a rough, low-speed screen and a damper can pass it while performing poorly at speed. Streaking oil plus cupping plus the ride complaint is a complete case — three independent pieces of evidence agree.
After a strut replacement the car pulls slightly and later shows inner-edge tire wear. What was omitted?
The post-repair alignment. Removing and reinstalling a strut disturbs camber — the strut physically positions the top of the knuckle — and often the toe follows. Every strut job ends on the alignment rack, no exceptions.
A customer's new tires are cupped again 8,000 miles after replacement. What did the previous shop skip?
Missed one? The reasoning above comes straight from the SUS exam bank, so this is the standard you will be held to.