Smooth out the bumps and enhance your driving experience with the right shock absorbers for your vehicle.
Evidence exists that the shock absorber has been around since the time of the ancient Egyptians. Imagine that – King Tut’s own chariot rode on a complex suspension system . Initially, the friction of leaf-springs sliding against each other was used to damp the oscillating motion (we call it bouncing) in suspension systems, which made ride quality far more comfortable.
Whether you're looking for ride quality worthy of a pharaoh, or handling to be king of the time attack, you’ll need the right shocks.
What Do Shock Absorbers Do?
The term ‘shock absorber’ is a bit of a misnomer. A more correct term might be ‘energy-dissipator,’ but that doesn’t exactly roll off the tongue. ‘Damper’ is commonly used and, while the most accurate, you probably still don’t know what it means.
A shock converts kinetic energy into heat – that’s absolutely the simplest way to look at it.
Your car’s springs always have some amount of energy stored in them. (Yes, even when it’s on a car lift in full droop, if the spring was designed correctly, there is still some energy left in it that keeps them from dropping out of their perches.) The job of the spring is to hold the car at the desired ride height, either in steady state or when hitting a bump; resist body movements during accelerating, braking, and cornering; and to push the tire onto the ground when the road drops away in dips.
As with any system with a sprung mass, it will have a natural frequency for oscillation. Your Science Guy moment for the day? Hold the top few coils of a Slinky and let the rest of it go, it will bounce up and down until you get bored. That’s oscillation.
Now, if you try the same experiment in water, the Slinky won’t bounce nearly as many times before coming to a stop – the energy is being absorbed by the water. A shock absorber doesn’t stop your car from leaning in a turn, or diving during braking; it just slows down the motion and absorbs the energy that makes the car continue to bounce.
It sounds simple enough. If you have a spring rate and a mass, a very basic equation spits out a required damping rate. Unfortunately, nothing is ever that simple with cars. They move, so the number you plug into the equation for mass gets more complicated once it’s in motion. Not only does the ideal rate change, but damping rates determine how a car feels. A stiffer shock will make the car feel more responsive, but ride is worse and vice versa. There’s as much art in suspension tuning as there is science, which is why it’s important to use brand names you trust.
Shock Absorber Construction: Single and Twin Tube
Shock absorbers use a piston traveling through oil to create damping force. The piston has holes which allow the oil through and determine the damping force. An inert gas, usually nitrogen, is used to pressurize the shock to stop cavitation and foaming of the oil during quick, successive motion.
In a monotube design, the body of the shock is also the cylinder wall for the piston and the gas chamber is stacked inline with a floating piston separating the two. A twin tube shock uses two tubes, as the name suggests, one inside the other. The inside tube is an oil-filled cylinder for the piston and the outer contains both oil and pressurized gas while acting as the body of the shock. There is a valve body at the end of the shock to allow oil to pass between the two tubes. In the outer tube, there is no physical separation between oil and gas.
Arguments over the superiority of twin vs single tube shocks will never be settled.
Both have pros and cons:
- A single tube allows for a larger piston, which spreads forces over a larger area. And since the oil cylinder is also the shock body, the single tube shock is far better at dissipating heat. But, since the gas and oil chambers are inline with each other, it reduces piston stroke, which means less suspension travel. A single tube shock is generally more expensive as well.
- A twin tube shock requires lower gas pressure, so seals and internal parts are less stressed. Some users insist that the lower gas pressure also results in better ride quality, but this also comes down to the shock valving. Twin tube shocks are more prone to overheating, can only be mounted in one orientation, and the smaller piston area is required to absorb more force.
Long story short -- for most street applications the twin tube is the better and more affordable choice, but for high-performance use, the single tube will be the way to go.
Types of Shock Absorbers For Different Applications
As noted, nothing is ever truly simple with cars. Choosing shock absorbers doesn’t stop at selecting a single or twin tube design. There are several different types available on the market, and each one serves a different need.
Air Shocks. Not to be confused with airbags or air springs, these are designed for use on the rear axle of vehicles that often carry heavy loads. An air chamber inside the shock absorber can be pressurized to add spring rate to level the ride height of the vehicle and help the vehicle’s primary springs support the load. These are most commonly used on half-ton trucks and some cargo vans. A self-leveling system can be fitted as well for convenience.
Bypass Shocks. Usually used for off road applications, the body of the shock uses external routing to allow oil to move around outside the body to lower damping forces under hard impacts. These normally use adjustable pressure check valves. The bypasses usually function in one direction only.
External Reservoir. Different from bypass shocks, but often used in combination, these locate the floating piston and gas cylinder outside the shock body to allow for more suspension travel, greater heat handling ability, and to move the high-pressure reservoir outside the hydraulic system. These are high-end shocks for extreme-performance applications.
Coilover Shocks. Technically, any shock with the spring mounted over the shock shaft and/or body, is a coilover. However, the term coilover is most commonly used for threaded body coilovers. The outside of the shock body is threaded, allowing the spring seats to be adjusted up and down to change the ride height of the vehicle. This can be used for aesthetics, or for corner balancing the vehicle to improve handling.
Damping Adjustable Shocks. Depending on the desired level of tuning, the simplest adjustable shocks can adjust the whole damping curve in one or two directions at the same time. More complex setups will allow compression and rebound to be adjusted separately, while others will allow compression and rebound adjustments in high and low frequency.
Electronic Damping Adjustable. At one point only found on exotic cars, these are becoming more common every year. Either magnetorheological or with servo-controlled valving, damping rates can be adjusted completely by an electronic control unit, or by the user selecting different pre-programmed modes.
Magnetorheological Shocks. These are adjustable dampers that use a fluid filled with magnetic particles which react to electromagnets. When the magnets are charged, the fluid becomes higher viscosity, stiffening the damping rate. Unlike electronically controlled valved shocks which have a few adjustment steps, these are almost infinitely adjustable in a given range and can react many hundreds of times per second.
MacPherson Strut. While most shocks are only loaded in compression and tension, the body of a strut is used as a stressed locating member, which defines and controls suspension geometry, while also providing damping. Usually found on the front axle, but occasionally on the rear as well. MacPherson Struts were most commonly found on inexpensive front wheel drive cars, but are now underneath some of the best handling cars in the world.
When Do I Need To Replace My Shocks?
Worn shocks can cause uneven tire wear, and uncontrolled ride quality. You may notice your vehicle bouncing after an initial impact instead of absorbing it. You might also notice the feeling of the tire slapping back down on the pavement after bumps, which can cause flat-spotting. These are all signs it might be worth taking a closer look at your suspension.
Very few cars have a scheduled interval for replacing shocks and/or struts. The life will vary based on how you use your car, the environment it’s driven in, and the quality of the components used. Some shocks will last up to 100,000 miles, while it isn’t uncommon to see performance degradation in as little as 20,000 miles on cars which are driven hard.
The most obvious sign you have a failed shock or strut is leaking. If that’s the case, they need to be replaced immediately.
If you notice a change in ride quality or handling, but you have no other signs, it is a good idea to start planning on replacement before flat-spotting of your tires occurs. If you’re already paying for shocks, you don’t need the extra expense of new tires on top of it.
How To Choose The Right Shocks For Your Car
While most consumers are perfectly satisfied with the shocks that came on their vehicle, better options may be available depending on how you use it, your particular performance preferences, and budget.
Shock and struts from the dealership are going to be expensive, often more expensive than performance aftermarket choices. Replacements available from your local auto parts store might seem like attractive options -- sometimes individual shocks aren’t much more than a large Frappuccino -- but they may use a generic shock for multiple applications not specifically tuned to your car.
For on-road performance enthusiasts, companies like Bilstein and KYB make everything from direct replacements to shock absorbers designed to be used with stiffer aftermarket springs. If you are looking for more performance, H&R and KW, amongst others, offer suspension packages that include sport springs and shocks or even threaded body coilovers. These are meant for driving enthusiasts and even weekend racers who are willing to sacrifice some amount of ride comfort for a performance trade-off.
If you are looking to replace shocks on your truck or SUV, suppliers like FOX or King can provide anything from direct replacements, which will offer a decent performance improvement, up to high-performance bypass shocks intended to be used with lifted, high-travel applications. Serious off-roading set-ups will obviously compromise on-road performance and usability, but that comes down to individual tastes.
If you are considering an adjustable suspension system, it’s important to remember that it will only be as good as the person doing the adjusting. Suspension tuning is complicated.
Most cars have some level of adjustability in alignment, plus every car has adjustable tire pressure. To some extent you can change your car’s weight distribution. You might even have changed the aerodynamic balance with aftermarket body work. All of these things will change how your car handles, and throwing in adjustable damping and ride height are far more variables than the average enthusiast is capable of dealing with.
So, if you have a good race shop with experience with cars like yours, have them help with the setup. If not, you may spend months chasing adjustments and still end up with a car that both rides and handles worse than it did stock – it happens more times than it doesn’t.