Demystifying Black Magic: How Shock Absorbers Work

Shock absorbers don't get much glamour, but they have a big role in how your vehicle handles. Here's a look inside these magic tubes found under your vehicle.

Shock absorbers are one of the most important parts of your vehicle. They’re the one component that really shapes if your truck is stable with a box full of gravel, if your SUV can run down a dirt road without launching you into the woods, your family sedan doesn't pogo stick down the highway, or your sports car doesn't swap ends thanks to a bump in a corner.

These parts can seem like magic, tubes that hide under your vehicle until they start leaking. Then you replace them with whatever is the cheapest and drive until they start leaking again.

This is how shock absorbers work, why they are so crucial to your vehicle's handling, and the differences between the different types. Oh, and if you’re on two wheels instead of four, the basic functions of shocks are the same.

First, some variants.

Most modern vehicles use what are called MacPherson struts in the front suspension, sometimes at all four corners. They have a different name, but these are just shocks with more steps.

These shocks, designed by then Chevrolet engineer Earle MacPherson, are shock absorbers with a stronger frame. This lets them do some of the work of control arms and steering knuckles. The strut also holds the spring. But that's all for another story. The shock-absorbing part is just like any other shock.

Aftermarket suspension systems to raise or lower your vehicle are called coilovers. The name goes back to before struts were common, when the shock absorber and spring were mounted to the chassis in two different places. A coilover gave them the same mounting point, putting the shock inside the spring — or, the coil spring over the shock.

The shock absorber part is the same as any other shock.

From horse and buggy to Ford Raptor, the point of a suspension is to separate you the passenger from what is happening on the road's surface. Even the best roads aren't perfectly smooth, so you want the wheels — and tires — to be able to move up and down without the passengers and cargo moving up and down.

This was first done with springs alone. Coil springs (like at the front of a car), leaf springs (look under the back of a pickup truck), and torsion bars (1990s GM pickups) let the wheels move up and down with less up-and-down movement of passengers. They also control some of the body's movement to stop it from violently hitting the top or bottom of the suspension's travel.

But springs don't damp the energy of up-and-down movement. They compress (or extend), store, and then release most of that energy. So they can then launch the vehicle up after a compression or yank it back to the ground after an extension, or continue that cycle over and over. It's not comfortable. It's also hard on you and the vehicle, and it's dangerous.

If you watch a clip of a Ford Model T driving on a rough road, that bouncing isn't because of old-timey film techniques. It's because they had a suspension that was just leaf springs.

Shock absorbers offer resistance to dampen the up and down oscillations. They slow the spring as it compresses or extends, bringing it back to a neutral position as quickly as possible. It's how they make the ride more comfortable, by removing all of the heavy movements.

The hydraulic shock absorber was first patented in 1907, using a lever that turned against vanes and fluid in a small box. The telescopic shock, named because it pulls in and out, became popular in the 1950s. This is the design found on most vehicles today.

Modern telescopic shocks all share the same basic features. They have a tube, the body of the shock, that is filled with a hydraulic fluid, a type of oil designed for the shock. Inside the shock, and attached to a rod coming out of the end of the shock body, is a piston.

The piston moves inside the body, up and down through the tube. The resistance of pulling the piston through a fluid is what damps the movement of the springs and makes the vehicle ride well. Or poorly.

It's the piston in the shock absorber that does the magic. Because shock fluid can't be compressed, it needs to flow through the piston.

Controlling how fluid flows through the piston is where suspension designers do their work — tuning how the shock damps movement up and down, how it does it at high shock speeds (like a pothole), and at low speeds (a gently rolling road).

The main shock piston is a carefully crafted component. Holes drilled or cast in the piston control how much fluid can pass through the piston. The more fluid that can pass through, the more easily the piston can move up and down. Less fluid flow means more resistance.

Changing the fluid also changes how the fluid flows through the shock. Think of a modern 0W20 motor oil that flows more smoothly than water versus the molasses-like consistency of a heavy gear oil.

Shock absorber designers need to pick a fluid that will flow at the desired rate when it's winter cold and summer hot, too. They do a good job of it, though if you’ve driven a vehicle down a rough road after a -30-degree start, you’ve probably felt the shocks loosen up and start to feel more normal after a few minutes of driving.

That covers basic tuning, but shock builders have ways to fine-tune shock movement too. To control compression (the body of the vehicle moving toward the ground) and rebound (the vehicle moving away from the ground).

They do this using small metal discs, called shims. Thickness, diameter, and the number of shims are all part of the process.

A shim on the bottom of the shock piston can completely stop fluid passing through a particular hole during compression. The same shim can then bend in a very precise way during shock rebound, allowing fluid to pass at a desired rate. Changing the size, number, and positioning of these shims changes how the piston moves through the fluid, "tuning" the shock.

Holes in a different location and shims on the other side of the piston do the same job against rebound movement.

Shock designers can adjust high shaft speed and low shaft speed damping independently from each other. They use multiple fluid paths to do it, and valves with different resistances.

A low-speed movement can open a low-resistance valve, allowing fluid to move through it and control gentle movements like a rough road. When the shock moves at a high speed, say you hit a rock, pothole, or large bump, those smaller low-resistance holes can't move enough fluid. That opens a high-speed valve and sends the shock fluid through the fluid path designed for high-speed damping.

Position-sensitive damping is another advanced technique where the shock behaves differently when it is at the end of its travel versus near the center. In the case of position-sensitive damping, there are extra fluid paths in the body of the shock. When the piston is at the right place, some of the fluid actually flows around it instead of through, changing the shock's response.

What about adjustable shocks? From the aftermarket, these shocks let you change how your suspension feels to fit you and your ride. From the factory, adjustable shocks let suspension engineers program different levels of damping for dozens of possible scenarios. They let the computer change the shocks for sporty or smooth driving (or off-road driving) on the fly.

Manual-adjust shocks let you turn a knob on the shock body. The knob is attached to a small valve in the piston that moves in and out to change how the shock fluid moves through that hole. More fluid flow softens it, less fluid flow stiffens it. These are small adjustments, they won't completely change shock feel.

Electronically adjustable shocks make these adjustments with an electronically controlled valve instead of one you have to turn manually. Some of these shocks have just a couple of settings, say normal and Sport, but others offer far more.

Shocks that offer multiple electronically controlled settings are usually called adaptive dampers or adaptive suspension. With these systems, the computer can tell the shock to adjust many times in a single second, adapting to the terrain. Giving you the right shock tuning for the next few inches of road or trail.

Gas-charged was a shock absorber buzzword for years, though it's disappeared from marketing talk. Still, the idea is effective.

Moving a piston back and forth thousands of times a mile mixes air into the fluid. Shake up a bottle of motor oil and you’ll see the bubbles. Those bubbles are bad.

It's called cavitation. These air bubbles don't flow like oil, and that causes problems for the carefully designed shock. Adding nitrogen under pressure to the shock helps prevent cavitation, keeping the bubbles out and the shocks happy.

MagneRide was invented by then General Motors division Delphi and introduced in the 2002 Cadillac Seville STS and 2003 Chevrolet Corvette. The company sold the tech, and now magnetic ride is found in vehicles from GM, the Ford Mustang GT, Audi R8, even in modern Ferrari and Lamborghini vehicles.

The trick is magnetorheological fluid-filled dampers. Magnetorheological fluid means there are tiny particles in the fluid that are affected by magnetic fields.

These shocks have an electromagnet in the piston. With the magnetic power on low, the fluid flows like a normal shock fluid. Turn up the magnetic power and the fluid gets thicker. It stiffens the piston's movement through the fluid.

The magnetic field can be adjusted 1,000 times per second, constantly changing the viscosity of the fluid. Hit a sharp pothole and the shock can relax to absorb the initial impact. Then the magnet powers up, thickening the fluid and absorbing the motion of the pothole before the suspension bottoms and sends the impact into the chassis — and you.

Magnetic shocks let massive SUVs like the GMC Yukon have a car-like ride and high cargo load capability. They also let sports cars have stiff springs for the track without crushing your kidneys on the street.

Moving a piston through a fluid creates heat. That heat changes how the fluid reacts when it flows through the piston. In most on-pavement driving, the body of the shock absorber can transfer enough heat to the outside air that this isn't an issue.

Head off-road, especially in fast runs across broken ground or on gravel paths, and the fluid can overheat. When shock fluid overheats, it can't do its job. That can send you out of control or home on the back of a flatbed.

The easiest way to fix this is to add more fluid in a separate tank. More fluid can soak more heat, and the extra tank means more surface area to let it cool.

There are two types of remote reservoir shock. One attaches the reservoir to the shock body. It looks like a shorter second shock attached to the actual shock. The second is to run a hose and put the reservoir somewhere else. The long hose adds capacity and cooling. Moving the reservoir away from the wheel is also good for avoiding damage in off-road racing conditions.

Then there are the truly magic shocks: Multimatic's Dynamic Suspensions Spool Valve, or DSSV dampers. You’ll find them in title-winning Formula 1 cars, the Ford GT, and GM vehicles like the Chevrolet Colorado ZR2.

Instead of the metal disc shims used by other shocks, Multimatic uses what are called spool valves. They’re made up of a hollow sleeve with a spring inside and a cap that sits on the spring.

The spring compresses as a force is exerted on the damper. Carefully designed holes in the sleeve are then exposed at a known rate, letting the fluid flow through. There are two spool valves, one for compression forces and a second for rebound. Letting designers tune each motion independently.

On the off-road Colorado ZR2, there is actually a third spool valve. It is position-sensitive, so it only engages at extreme suspension travel. Basically, it's there to control how the truck lands sweet jumps.

These shocks aren't adjustable. But because they’re much more predictable and consistent than a traditional shock's metal shims, engineers can be far more precise in making them ride and handle exactly how the engineers want.

That's the magic of your shock absorber, no matter the type your vehicle uses. Of course, even if they all have the same function, the time, budget, and materials put into shims, fluids, and even rubber seals are all important. So the next time you’re due for some new shocks, remember that like many auto parts: you get what you pay for.

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