Shock absorbers (also known as "shocks") are the suspension components which slow, then stop, the up-and-down bouncing movement of your vehicle's springs through a process known as dampening. Without shocks to calm things down, springs will continue to extend and release energy they absorb from bumps in the road at an uncontrolled rate - bouncing for a long time until their kinetic energy finally dissipates. Needless to say, this would produce an extremely bouncy ride that would be hard to control over uneven road surfaces.
Essentially, shock absorbers control unwanted spring motion by turning kinetic energy of suspension travel into heat energy that's channeled away using hydraulic fluid. Once your vehicle's ride is made steady, its center of gravity remains in a stable location so that tires stay firmly planted against the pavement - keeping you safely in control of your vehicle.
In this article, we'll take a look at how shock absorbers work, and the basic design variations that exist. Our goal is to help you make a more informed decision when replacing your shocks, because we offer both OEM factory style replacement ones for street use and ride comfort, as well as performance shocks that firm up your ride and improve handling.
Shock Absorber Design
Shock absorbers bolt between the vehicle's frame and suspension members near each wheel. The mounting points are typical described as either "eye" (round bushing which bolt passes through) or "pin" (threaded shaft which receives nut). Inside the shock absorber is a metal rod attached to a piston that moves up and down inside a cylinder filled with hydraulic oil. Most modern shocks also contain nitrogen gas to reduce shock absorber fade that happens if oil becomes aerated and bubbly due to heat buildup.
After a wheel hits a bump, the piston inside the shock absorber compresses inside the cylinder compartment. As that's happening, its motion is purposely being slowed thanks to resistance caused by hydraulic fluid working its way out of specially designed passages in the cylinder wall. Because those passages are small, only a limited amount of fluid can escape. By slowing down the motion of the piston, springs are prevented from bouncing continuously.
Most standard shock absorbers will produce greater resistance during their extension cycle (getting longer) compared to their compression cycle (getting shorter). Ultimately, the compression cycle regulates the unsprung weight on a vehicle (wheels, axles, suspension) while the extension cycle regulates the sprung weight of the car body itself.
Shock absorbers built in recent decades are designed to vary and increase their resistance when springs and shock absorbers are compressed faster. Not only does that prevent bottoming out over deep potholes, it minimizes a greater range of unwanted body motions such as pitch, roll, sway, acceleration squat, and brake dive.
Gas Charged vs. Hydraulic Shock Absorbers
While all shock absorbers contain hydraulic fluid, “gas charged” and “hydraulic” shock absorbers are laid out differently. Gas charged shocks provide a firmer, sportier ride because they contain gas (typically nitrogen) which is injected under high pressure into the shock. This pressure serves to compress air and oil in the shock absorber faster, leading to a more responsive feel. With hydraulic shocks, fluid compression happens at a slower pace because air and fluid inside the shock are not under such high pressures. This gives hydraulic shocks a softer and smoother ride. Note that if you’ve got a classic car, it was probably designed with hydraulic shocks. Under the SHOCK ABSORBER TYPE heading, we’ve included check boxes that make it easy to narrow your search to see only gas charged or hydraulic shocks.
Mono-Tube vs. Twin-Tube Shock Absorbers
Two main types of internal designs exist for shock absorbers: mono-tube and twin-tube layouts. As it sounds, these designs differ primarily in the number of metal tubes found within each shock absorber. Twin-tube shock absorbers are fitted primarily as OEM equipment because they produce a more compliant, comfortable ride and cost less to produce. Conversely, mono-tube shock absorbers are oriented toward performance driving, and their sophisticated design is more expensive to manufacture.
In a twin-tube shock, the piston is contained in a small tube that sits within a larger outer one. The inner tube (known as the "pressure tube") and the outer tube (known as the "reserve tube") are separated into two chambers by a pass-through valve located in between. This serves to protect the shock absorber from damage because if the outer tube should become dented, there are no ill effects on the operation of the piston itself. Nitrogen gas and hydraulic oil are not separated, which can add to fluid aeration and bubbling under heavier-use conditions such as off-roading or racing.
The valve which allows fluid and gas to pass through as needed serves to keep gas pressures low and produces a more comfortable ride. Low gas pressure avoids stress on seals, ensuring they generally have a longer lifespan. However, that piston valve is usually smaller than mono-tube ones because of limited space. Other disadvantages of the twin-tube layout include limited hydraulic oil capacity compared to mono-tubes, and limited angles at which twin-tube shocks can be installed..
In mono-tube shocks, the outer housing doubles as the sole cylinder tube for the piston, hydraulic oil, gas, valve, and other bits. Unlike twin-tube designs, nitrogen gas is usually housed within the main cylinder, and is kept completely separated from the oil chamber by a floating piston. The main benefits of mono-tube shocks are less oil aeration because it's kept separate from gas, and lower operating temperature because the greater quantity of oil dissipates more heat. Both of these factors allow a more stable damping force to be generated consistently without fade.
Unlike twin-tube designs, these shocks can also be custom mounted at extreme angles. Because the gas needs to be under higher pressure in this layout, ride quality mono-tube shocks produce is stiffer - making them ideal for aggressive driving situations. This higher pressure can be harder on seals over the long term. Any physical damage to the outer shell case makes mono-tube shocks virtually unusable, because there's no squish room between the casing and piston cylinder walls.
Shock Absorbers With Remote Reservoirs – Great For Off-Roading
Another variation when it comes to shock absorbers are those with extra capacity to carry additional oil and nitrogen gas. This extra capacity takes the form of an additional fluid reservoir tube that’s either attached or detached from the main shock tube. These are ideal for heavier vehicles used off-road which see huge amounts of wheel travel and hard impacts that would overheat standard shock absorbers. Increased oil capacity keeps fluid temperatures low, and added gas volume allows compression and rebound valves to perform more consistently under the most severe conditions. Thus, an excessively bouncy ride that leads to dangerous handling can be avoided.
“Piggyback” style shocks are built with the reservoir canister mounted directly to the main shock absorber body, while true “remote reservoir” shocks have a separate reservoir (connected by a length of high-pressure hose) which can be mounted in another location on the vehicle frame. Performance of both styles is unaffected by how far away the reservoir is, and choosing between the two should be based on how much available space can be found under your vehicle.
When it comes to remote reservoirs, shocks such as the Bilstein 5100 Series Shock Absorbers, Fox 2.0 Performance Series Smooth Body Shock Absorber are set up with a separate tube that can be mounted further away.
Long Distance Racing
Racing for long periods of time is a driving condition that makes it well worth your while to invest in high-performance mono-tube shocks. Brake rotors, pads, and calipers often reach 1,800 degrees Fahrenheit over the course of a circuit race, and that heat transfers directly to the nearby suspension components. And, that heat reaches shock absorbers too.
Because high temperatures have a direct effect on shock absorber performance, the benefits of mono-tube shocks really pay off on the racetrack. Their ability to dissipate heat more effectively and stay cooler keeps performance steadier, and lack of fluid aeration prevents squishy, unexpected surprises. The shorter stroke typical of mono-tube shocks won't be a problem on a smoothly-paved, flat track surface - especially if stiffer springs are installed on your race ride as well. If you’re interested in seeing shocks that are more oriented towards racing, click here.
Leaking fluid and worn bushings are obvious reasons for replacement, but most shocks and struts will gradually deteriorate internally without exhibiting any external signs. When you consider that a shock metering valve can flex several thousand times in just a few miles, it's easy to see how such parts can be completely worn out within 50,000 miles, if not sooner. Judging shock or strut integrity by vehicle performance is a more accurate barometer. If your vehicle rides harshly, nosedives when braking, rolls excessively in corners, bottoms out on bumps, or causes uneven tire wear, you likely need new shocks or struts.
When shock absorbers wear, the signs can be noticeable. However, it's easy to miss them because the effects build up so gradually. If braking causes your vehicle to dip, swerve, and take longer to stop (up to 20% at highway speeds), shock absorbers should be replaced. If mild winds cause your vehicle to veer to the side or across another lane, they should be replaced. And if your vehicle bottoms out easily as if there's no squish left in your sneakers, your shocks have given up the ghost a while ago.
But probably the most visible sign that shock absorbers need to be replaced is tire wear known as "cupping". When shock absorbers aren't keeping tires flat against the road, evenly spaced areas of concentrated wear will be present. It's important to replace front or rear shock absorbers in pairs. If you've got over 100,000 miles on the original ones, we recommend changing all 4 of them. The difference in ride quality will be worth every penny.
It's difficult to talk about shocks without bringing up struts, because you'll see both of these terms used interchangeably (and not always correctly) by owners, retailers and in discussion groups everywhere. While shock absorbers can be mounted on a vehicle independently, they can also be part of a strut assembly. To clarify, struts are an integral part of a vehicle's suspension system, as they combine the shock with a coil spring and provide a mounting point for a hub and/or control arm, supporting the wheel and tire.
The majority of late-model cars and trucks have struts in the front and shocks in the rear. Other vehicles have struts at both front and rear wheels. Some may have shocks instead of struts at all four wheels. Whichever design your vehicle has, you cannot replace "struts" with "shocks" (or vice versa) when performing a service replacement. We invite you to read more about what differentiates shock absorbers from struts in this related article on our website.
To help further guide you through the selection of all types of shocks we offer, we've set up corresponding sections of our website to provide you with all the choices that will fit your vehicle. It's possible to enter specific make, model, and year in drop down boxes before beginning your search. Or, input vehicle information after entering an individual page.
Whether you replace worn shocks with performance-oriented ones or OEM-quality ones, you'll be rid of much of the poor ride quality that makes a vehicle feel old and tired. In cases where your vehicle has excessively high miles, it's also worth replacing springs at the same time in order to maximize the effectiveness of your new shocks. Don't be "shocked" into thinking that yours aren't worn out. Bolt up a new set and be amazed at the restoration of your ride!