rxspeed presents

Behind the Speed: Coilover Suspensions

We take an in-depth look at how a coilover suspension works and how to pick the right set.

Behind The Speed: Coilover Suspensions

Few things make less sense than how to modify a car’s suspension without making things worse. The number of variables dependent upon one another that affect camber, caster, toe, roll centers, and motion ratios, for example—and that keep you from ending up in a ditch—are as complex as they are many. And it all starts with the shocks.

Breakdown Breakdown Breakdown

Breakdown

CLICK HERE TO SHOW HOTSPOTS

COILOVERS FOR EVERY TYPE

 

By now, you ought to realize that not all coilovers are the same and that there are three kinds you should consider: OEM-style spring-over-shock assemblies that you probably don’t care about (we’ll explain why); slip-fit coilovers that balance performance and budget; and full-bodied coilovers that care only about making your car handle better.

 

OEM-style spring-over-shock assemblies are what you’ll find underneath just about anything from a Z06 to a Civic. Here, a coil spring slips over and surrounds whatever shock or strut it’s mated to. Such coilovers are almost always nonadjustable in terms of both height and damping characteristics and are an OEM’s most honest compromise between performance, ride quality and cost effectiveness, widely varying from vehicle to vehicle.

 

"Improving a car’s suspension almost always starts with coilover shocks."

 

Slip-fit coilovers close the gap between the OEM shocks you don’t want and the full-bodied coilovers you can’t afford. Slip-fit coilovers are made up of hollow, threaded aluminum tubes that slip over and rest upon the spring perches of whatever shocks you’ve already got. Here, a series of jam nuts allow you to compress or decompress a shock’s spring to alter ride height. You won’t find a performance advantage here other than the ability to lower your car’s center of gravity and, in most cases, look good while doing it. Slip-fit coilovers gained popularity among the sport-compact performance crowd during the late 1990s, where, a lot of times, the ability to achieve the lowest ride height possible trumped performance.

 

We’re going to discuss full-bodied coilovers in particular. You’ll learn how to replace the entire factory spring and shock assembly, and also about fully threaded shock bodies that allow for ride-height adjustments and, most of the time, damping changes. Like less-expensive slip-fit coilovers, full-bodied coilovers use a series of jam nuts to compress or decompress their springs, allowing for ride-height changes and the ability to change how their springs react. Higher-end coilovers are also made up of two-piece bodies that allow their lower mounts to thread in or out, resulting in ride-height adjustability without inadvertently mucking up spring rates by squashing the coils. Such coilovers often feature shorter shock bodies, which means ride height can be reduced without the risk of bottoming out.

 

Other characteristics of a full-bodied coilover include stiffer bump stops that further help prevent bottoming out, more durable dust boots to protect the shock’s shaft and some sort of upper mount to connect it all to the chassis. Whether the design is based on a MacPherson strut or a double-wishbone layout will determine the sort of upper mount that’s provided. Most coilovers designed for double-wishbone suspensions feature fixed upper mounts with rubber or polyurethane bushings; upper mounts designed for MacPherson struts mean a pillow-ball assembly can be used, and in some cases they feature camber and caster adjustability, which is an indispensable feature if you plan on visiting the racetrack.

Cutaway Cutaway Cutaway Cutaway

Cutaway

"It’s the shocks’ job to ward off excessive spring oscillations and reduce whatever vibrations the wheels and chassis introduce"

 

IT STARTS WITH THE SHOCKS

 

Every coilover worth its weight is based upon a solid foundation—in this case, a great shock. Mechanically, a full-bodied coilover mounts up like any other shock, with its upper mount bolted directly to the chassis and its lower mount to a double-wishbone suspension’s lower A-arm or a MacPherson strut suspension’s knuckle.

 

It’s the shocks’ job to ward off excessive spring oscillations and reduce whatever vibrations the wheels and chassis introduce: Pounce over a bump and let the springs compress, decompress, absorb vibrations and send that energy into the shocks’ pistons through their upper mounts. If the shocks are doing what they ought to, they’ll dampen all of that, making that bump a whole lot less noticeable, leaving the car immediately poised for whatever it’s asked to do next. How well all of this happens depends on the shocks’ internals. For example, stiffer shocks slow down spring movement while softer shocks speed it all up.

 

There’s a whole lot more to a shock’s capabilities, though, beyond spring control and reducing vibrations. Shocks also eliminate rocking, pitching, dipping, wheel spin and all sorts of other things you might imagine aren’t supposed to happen when you step on a pedal or turn the wheel.

 

Every shock is made up of some sort of hydraulic fluid-filled tube and a piston. It’s the piston’s job to push that high-pressure fluid through the shock’s valves, which will determine how all of this reacts against that oscillating spring. Small orifices perforated into the shock’s piston allow fluid to bleed through while traveling up and down, determining valving characteristics. There’s kinetic energy to consider, too, which is really just a fancy way of talking about the heat energy produced by all of these moving parts, which dissipates within the shock’s hydraulic fluid.

Mono or twin Mono or twin Mono or twin

Mono or twin

"Stiffer shocks slow down spring movement while softer shocks speed it all up"

 

MONO-TUBE OR TWIN-TUBE?

 

It’s the suspension nerd’s debate that never seems to end: mono-tube versus twin-tube. The difference isn’t all that complicated. Mono-tube shocks are made up of a single piston and rod assembly placed inside of a case where both compression and rebound take place. Twin-tube shocks feature an additional cylinder that surrounds the piston and rod assembly and stores its hydraulic fluid separately.

 

Twin-tube shocks can mean increased piston stroke, which can improve ride quality, but mono-tube shocks typically outperform them everywhere else. Here, the single tube means there’s room for a larger piston and rod assembly, which also means more fluid can be displaced, resulting in heightened sensitivity when incremental suspension movements are made, even at low shaft speeds. Better fluid flow also means more consistent damping and the ability to operate at cooler temperatures due to the absence of a second tube. On the street, you’ll be hard-pressed to notice much of a difference between the two. On the track, though, mono-tube shocks are often preferred because of their ability to react quicker and remain cooler. Mono-tube applications are typically more expensive, mostly due to their unique construction that costs more to make.

 

"Before ruling out a shock that may not seem like it doesn’t offer much travel, though, be sure to consider the relationship it shares with its spring."

 

SHOCK TRAVEL MATTERS

How far a shock’s able to travel will determine whether or not you’re going to bottom out. Bottoming out won’t do your suspension any good and can cause long-term damage to surrounding components as well as to the chassis. More travel is almost always better and will allow a shock to do what it does best. Before ruling out a shock that may not seem like it doesn’t offer much travel, though, be sure to consider the relationship it shares with its spring. Stiffer springs require less travel since, here, the shock won’t have the ability to move up and down as much. It should go without saying that softer springs will result in the opposite.

Compression Compression Compression

Compression

COMPRESSION AND REBOUND

Every shock has to compress and rebound. Full-bodied coilovers are no different. How a shock does all of this is known as its damping characteristics. There are three ways all of this can be accomplished: through manufacturer-preset valving, user-adjustable valving that alters compression and rebound at once, and user-adjustable valving that alters compression and rebound independent from one another.  

 

Preset coilovers are a good compromise between your lack of knowledge about where in the world to set yours and your trust in the manufacturer. Such coilovers are typically paired with whatever spring rates they’re matched with, which means swapping some other coils in their place often won’t end well. For example, soften up the spring rates too much because you think your slammed-to-the-ground Civic ought to ride like a Benz and you just might introduce a whole lot of chassis-abusing bottoming out, making it handle a whole lot worse than it did before.

 

"A good shock is designed with multiple speeds and the ability to handle all sorts of different situations."

 

Before going a whole lot further, you’ve got to understand what it is exactly that the manufacturers are presetting and what—in the case of higher-end adjustable coilovers—you’ve got the option of adjusting yourself.

 

We’re talking about damping, or, more specifically, compression and rebound. Compression happens when the shock’s piston is pushed into its body, compressing all of that fluid inside. Rebound happens when the piston’s pulled away, still compressing that fluid. How well a shock compresses, in part, determines the motion of a car’s unsprung weight, or how quickly weight is applied toward its corresponding tire; rebound controls the motion of its sprung weight, or how quickly weight moves the other way.

Springs Springs Springs

Springs

How fast a shock’s valves can move is also important. Slower speeds generally influence handling that has to do with turning and cornering, while higher speeds have more to say about how exactly it’ll handle bumps and ruts. A good shock is designed with multiple speeds and the ability to handle all sorts of different situations.

 

Single-adjustable coilovers feature compression and rebound adjustability that’s dependent on one another. Firm one up, for example, and you’ve done the same for the other. Higher-end, double-adjustable (or split-level control) coilovers allow compression and rebound to be manipulated independent from one another. Depending on the coilovers, adjustments can range from as few as eight clicks to as many as 32 positions. Coilovers with single-adjustable damping generally allow for low-speed rebound changes and don’t affect compression a whole lot. Nevertheless, on a properly set-up car, such coilovers can still improve handling. No matter the type of damping adjustability, changes can be made by an externally mounted knob (typically fixed to a shaft) and a spring-loaded needle valve, which controls internal fluid flow and, as a result, damping characteristics. Now’s a good time to mention that these sorts of damping adjustments typically won’t result in dramatic handling changes and are best left for fine-tuning and sorting out the right kind of chassis balance once the rest of the suspension is in order.

 

THE SPRINGS

As important as shocks are, it’s the springs that absorb the bumps and control body roll. They do all of this by compressing and rebounding in the same way that a shock’s piston does, but they do so to absorb wheel motion. The springs stop the chassis from bottoming out, maintain tire control over bumpy surfaces, and they prevent body roll when going around corners. The springs also help reduce squat when accelerating and diving while braking. More obviously, springs determine a car’s ride height and, not so obviously, its center of gravity, which has a whole lot to say about how well a car will handle. Spring rates ought to be considered as carefully as shock damping: Choose something too soft and the shocks might bottom out; do the opposite and your tires will have a hard time doing their job. 

PRELOAD

Compress a spring, determine the pressure applied, and you’ve just figured out its preload. Every coilover’s spring should incorporate some sort of preload. Increasing it can result in a better tire contact patch, which can improve traction and cornering capabilities, but too much can make things worse. Any coilover that relies on spring compression to adjust ride height is dependent on preload, though, which isn’t always a good thing. Slip-fit coilovers and full-bodied coilovers that don’t feature adjustable lower mounts are victims of not being able to change ride height without adjusting preload. For street cars, though, which don’t spend a whole lot of time on the track, this is typically not a big deal. When setting up a new set of coilovers, introduce a small amount of preload to keep the springs from bouncing within their assemblies before installing them on the vehicle.

 

Coilovers, shocks and damping rates might seem mysterious, but they don’t have to be. Understand that no single suspension upgrade will make an otherwise crummy suspension all of a sudden ready for the racetrack, and you’ve just figured out much of what makes all of this seemingly mysterious in the first place.

 

Additional Bilstein Photography Courtesty of Elli Dolfo

Mono/Twin-tube illustration courtesy of http://www.theeuroalliance.com

Early Access
Be the first of your friends to see the next feature car 24 hours before it's public.