Acknowledgements

I am enormously indebted to Benny "Bigger Valves" Langford for his assistance with the production of this article. Without his incredible wealth of technical knowledge, considerable expertise, generosity, and immense reserve of patience in answering my seemingly endless questions, it would not have been possible. For his kind support and expert advice I am profoundly grateful. Benny - you are the master and I the humble messenger - thank you! Thanks also to Frank Alioto for his kind assistance with the section on shock valving.

Very Special Thanks

An undertaking of this magnitude would not be possible without the assistance of others, most notably my wonderful wife Laurie and my kids Mitchell and Jessie. To them, a huge thank you for your patience and understanding through the countless late nights.

Introduction

Because of the complexity, we will be tackling the subject of coilover tech in several parts – this is the first. In this first part we’ll be reviewing basic shock and spring tech, examining the types of coilovers and their parts, going over their use and advantages, covering the concepts of installation ratio, wheel rate, and suspension frequency, then concluding with some preliminary spring-rate selections. Future articles will cover more advanced topics such as: spring length selection and tuning, coilover adjustments / tuning, spring selection using spring force modeling, re-valving, and hands-on tech procedures such as rebuilding a shock.

There is much to learn – even if you’re a seasoned coilover user as there exists a lot of misinformation about coilovers. This is because, more than with any other style of shock & spring, the coilover user is presented with almost limitless options regarding setup. Mounting, valving, spring rates, spring lengths, gas pressure and more are all completely in the hands of the installer and can be easily changed from one extreme to another. When those hands are expert – the result is phenomenal performance. When those hands are not so expert – the unfortunate result is that we novices really just have a huge amount of rope with which to hang ourselves.  Instead of a huge number of ways we can tune, we are faced with a huge number of ways we can get it wrong, and sometimes badly wrong at that.

To make matters worse, after we’ve unknowingly gotten it wrong (or someone else has) and we're observing the results, without a proper grounding in the basics, we lack the ability to properly interpret the results we are observing. The unfortunate result is that, instead of passing on wisdom, we unintentionally contribute to myth and misinformation. Again, without the proper grounding to accurately communicate our observations and conclusions, and despite our best intentions, we end up spreading myths and misinformation across the Internet – preventing the next hands from becoming as expert as they could be. And the cycle begins again.

My aim with this article is to help break that cycle, to help your hands become just a little bit more expert, to take back some of that rope yer fixin’ ter hang yerself with.

One last thing before we begin. We are going to have to be extremely clear about the definitions of the terms we use – and disciplined in their application. There are so many terms that get bantered around, often with completely different meanings, that confusion is virtually guaranteed. If nothing else, we need to agree to strive to use the correct terms, and use them consistently, to avoid adding to the confusion.

A word about design philosophy.

All design, including suspension design, is a compromise of factors to achieve a desired result that will be judged by some criteria. Those criteria are numerous, often subjective, and may range from cost and complexity to performance and even appearance. With this in mind, my philosophy is that there are very few, if any, absolutes – very few “rights” or “wrongs”.  Some will have you think otherwise – especially the so-called “band-aid” accusers. For example, some will say that the use of an anti-roll bar is a band-aid for poor spring rate selection. Some will say that a limit-strap is a band-aid for poor link geometry. Some will say a “helper coil” is a band-aid for improper spring length. There are many other examples. In my opinion, very few are valid. There are many tools in the box of design, and if they are applied with reason and understanding, then they are certainly valid. Only if they are applied from lack of understanding of a better way do they become a concern.  May this article expand your understanding so that any and all “tools” you use from the box of suspension design, you use with knowledge and confidence.  Good luck!

Table of Contents

Suspension Review

An off-road vehicle's suspension system is designed to perform these basic functions:

1. Support vehicle weight / maintain correct vehicle ride height.
2. Keep the tires in contact with the trail.
3. Provide comfortable ride for driver & passengers.
4. Prevent or reduce damage to chassis from force of impacts with obstacles (including landing after jumping).
5. Maintain correct wheel alignment (locate the axles).

The first function requires no explanation. The remaining four are best illustrated by the simple example of a vehicle traveling down the trail and hitting a bump.

When a tire hits a bump the wheel moves up. Without suspension this motion would be transferred directly to the chassis, causing the vehicle and it’s occupants to go up. When gravity takes over, the vehicle comes back down, and the force of the impact with the ground would again be transferred directly to the chassis and its occupants. Depending on the size of the bump, without suspension the tires could lose contact with the road, traction is lost, it would certainly be uncomfortable, the chassis would be subjected to damaging shock loads, and directional / steering control could be lost.

In other words, the ride & handling would be awful. 

When a rig climbs or accelerates weight is transferred from the front to the rear causing a suspension motion called “squat”. This increases the traction at the rear wheels but reduces it at the front.  When a rig descends or brakes, weight shifts from the rear to the front, causing a suspension motion called “dive”, and the opposite occurs – traction increases at the front wheels and decreases at the rear. When a rig corners or side-hills, weight transfers from the inside or uphill side of the vehicle to the outside or downhill side of the vehicle, which causes a motion called “sway” or “body roll”.

Without suspension, all this weight transfer would not only be incredibly uncomfortable, but depending on the severity, could quickly and easily lead to loss of traction and / or control.

Enter suspension.

All the components of the vehicle's suspension work together to achieve the five functions and provide the desired ride and handling.  These basic suspension components are:

• Tires
• Links (or control arms)
• Springs
• Shocks

Tires

Tires are really just an air spring that supports the entire vehicle.  I’ll not go into great detail about tire tuning here, but you should be aware that the tires, and the pressure in them, have a huge impact on vehicle ride and handling – regardless of the style of suspension used. This can be a complication in suspension tuning, but can also be used to advantage. For example, a stiff suspension set up for high-speed work can be made more comfortable and pliable in really rough terrain simply by adjusting the air pressure in the tires. The other reason I mention tires is because you often don’t see them mentioned when people are swapping suspension advice and experience.  That’s a mistake. To properly understand and use the experience of another you need to be aware of, and account for, the type of tires they run and at what pressure.

Links

For the purposes of this article, the suspension links (or control arms) locate the axles, and define the suspension’s geometry. That is, they define the arc through which the suspension will cycle as it moves up an down. The geometry of the links also defines a host of other properties such as the vehicle’s roll axis, anti-squat, etc. Since this is not an article on suspension link design, we shall leave the discussion of links here for now, and return to them in a later part when we discuss roll and roll resistance.

Springs

The springs support the weight of the vehicle, maintain ride height, absorb road shock, and keep the tires in contact with the ground. By doing so they allow the chassis to ride relatively undisturbed while the tires follow the bumps and pot-holes in the road.

There are many different types of springs, including: air springs, leaf springs, coil springs, and torsion bars.  They all do the same job, with their own advantages and disadvantages. We shall be concentrating on coil springs, specifically those designed to be mounted on a coilover shock. The advantages of coil springs are numerous. They are:

• light
• compact
• inexpensive
• friction free
• unaffected by heat
• easy to construct in various rates, length and diameters
• when properly constructed, will not loose free height or installed length 

When a wheel hits a bump, a force is applied to the wheel. That force causes a vertical acceleration of the wheel – the wheel goes up. The springs will absorb the load by compressing, converting the kinetic energy of the wheel’s motion into potential energy stored in the now-compressed spring. This potential energy is then released, causing the spring and wheel to rebound.

The rate at which the springs compress and rebound is called the suspension frequency. Left undamped, the spring will compress and rebound at the suspension's natural frequency until all of the energy originally put into the spring by the force of the bump is used.  Of course, energy can neither be created nor destroyed; it can only be converted from one form to another.  Unassisted, a spring will bounce up and down converting kinetic energy to potential and back again for a very long time.  Obviously this would translate to terrible ride and handling, as the wheels would bounce up and down uncontrollably.

What is needed, then, is some sort of energy-converting device to assist the springs – something that can damp the compression and rebounding of the spring.

Enter the shock absorber, or shock.

Shocks

Shocks control spring motion, that is, they slow down and reduce the magnitude of the spring’s oscillation. The process is known as damping. In technical terms, a shock controls the frequency and amplitude of the suspension's oscillation.  In layman’s terms, a shock controls how fast and how much the suspension compresses and rebounds.

This:

a) is important in keeping the tires in firm contact with the road (providing handling); and

b) helps isolate the passengers from road shocks (providing a good ride).

By damping the movements of the suspension, the tires remain in contact with the trail surface. This prevents the tires from bouncing and skipping with every bump and dip in the trail. Tires that do not stay in contact with the trail can’t provide good traction, steering stability or braking friction. Damping the movements of the suspension also results in resistance to vehicle bounce, roll, and sway during weight transfer and therefore resistance to brake dive and acceleration squat.

How Shocks Work

A shock absorber is basically a hydraulic piston pump that converts the energy of motion (kinetic energy) into heat energy. One end of the shock, the shock body or “tube”, is the cylinder of the pump, the other end of the shock is the rod and piston. One end of the shock is connected to the chassis and the other end is connected to the suspension. As the wheels move up and down relative to the chassis the piston pumps up and down in the cylinder.

The cylinder (shock body) is a tube filled with hydraulic oil. When the piston pumps up and down through the hydraulic oil, the oil is forced through holes in the piston, called orifices. The flow of the oil through the orifices is further regulated by the deflection of special spring-loaded metering valves, or deflection discs, located on either side of the piston.

	Assembled shock. Cylinder (shock body) is uppermost, rod (shock shaft) protrudes below.
	Rod and piston being removed from cylinder.
	Rod and piston being removed from cylinder.
	Rod and piston removed from cylinder.

Because of the valves and the fact that the piston orifices are so small, only a small amount of fluid, under great pressure, passes through the piston as it pumps up and down. The resulting resistance slows down the piston and creates heat from friction. This in turn slows down spring and suspension movement and converts the spring’s kinetic energy to heat in the oil that is then dissipated as the oil cools.

The amount of damping a shock absorber provides depends on the number and size of the orifices in the piston as well as the valving. By changing the design of the valves, the pressure at which they open and close can be altered and a shock’s damping characteristics can be tuned as needed. The higher the opening pressure, the firmer the shock. The lower the opening pressure, the softer the shock.

Obviously, shock absorbers must work in two directions - compression and rebound. The compression stroke occurs as the shock gets shorter and the piston travels into the cylinder. The rebound stroke occurs as the shock lengthens and the piston extends from the cylinder. The compression stroke controls the motion of the vehicle's unsprung weight, while the rebound stroke controls the heavier sprung weight. Accordingly, a typical car or light truck shock will have valving designed to provide more resistance during rebound than compression.

	Close-up view of shock piston. (photo courtesy of Dan Dibble)
	Compression valving discs are on the rod-side of the piston. (photo courtesy of Dan Dibble)
	Rebound valving discs are on the free-end of the piston. (photo courtesy of Dan Dibble)
	Rebound valving discs removed from free-end of the piston. Looking very much like a series of washers (and often referred to as such) they are stamped spring steel deflection discs that react to pressure and velocity to meter, or regulate, the amount of fluid that flows through the piston and thus the amount of resistance or damping the shock provides.   A particular number, size, and arrangement of valving discs is often referred to as a "shim stack". (photo courtesy of Dan Dibble)

Heat and Shock Fade

Because of the tiny orifices in the piston, the viscosity of the shock oil has a large effect on the resistance the oil presents to the piston’s movement, and hence the damping the shock provides. Viscosity changes with temperature –hot oil is “thinner” and pours more easily than cold oil (which is why you change your oil when the engine is warm). It stands to reason then, that to provide consistent damping, we would want the shock oil to maintain a consistent viscosity. However, we just said one of the purposes of the shock is to dissipate the spring’s kinetic energy by converting it to heat that is absorbed by the hydraulic oil, so we can see the potential for trouble here. If the shock is used hard enough, it will eventually heat the oil to a point where its viscosity changes (becomes less). This thin, overheated shock oil now offers less (possibly much less) resistance to the piston’s movement, and the shock’s damping capability is reduced – sometimes drastically. This is called “shock fade.” There are strategies to combat shock fade that we will cover shortly.

Shock Design

Most shocks produced today are either of twin-tube or mono-tube design. Before we look at each individually, let’s look at another important aspect of modern shock design that is often applied to both twin-tube and mono-tube shocks – gas charging.

Gas Charging

The primary function of gas charging is to minimize aeration and foaming of the hydraulic fluid by reducing the chance of cavitation. When the shock cycles, the motion of the piston through the oil creates an area of high pressure ahead of the piston and an area of low pressure behind it. If the pressure of a fluid is reduced below its vapour pressure, the fluid will spontaneously change state from a liquid to a gas. This means that tiny air bubbles can form in the low-pressure area directly behind the piston. The process is called cavitation. Left unchecked, the tiny air bubbles that form will mix with the oil  - a process called aeration. The resultant mixture of oil and air inhibits the functioning of the shock as the piston and valving are designed to produce the required damping by operating in a column of incompressible oil. Once the oil is mixed with air, or aerated, it is no longer incompressible. The faster the piston pumps up and down, the more rapidly cavitation aerates the oil on both sides of the piston, and eventually the oil will be churned into foam. The resulting foam offers little resistance and causes extreme fade of the shock’s damping ability.

The addition of pressurized nitrogen gas to the shock helps to prevent the low-pressure zone behind the moving piston from falling below the vapour pressure of the oil, reducing the chance of cavitation, aeration, foaming, and eventual fade.

In addition to reducing foaming and fade, the gas charge allows greater flexibility in valving design. Without the gas charge, valving design would have to be compromised to allow for the possible eventual aeration of the oil.

Depending on the pressure used and the diameter of the rod, gas charging can also cause the shock to provide a small increase in the spring rate of the suspension (in fact, if the vehicle were light enough, the rod diameter large enough, and the pressure high enough, the pressurized nitrogen alone could act as the spring – hence airshocks, but I digress). This mild boost in spring rate is caused by the pressurized gas acting on the rod through which there are, obviously, no orifices; as shown at left. Therefore, the larger the rod is in diameter, the larger the area for the pressure to act against. This gas pressure acting on the rod through the centre part of the piston is also the reason why an uninstalled, unloaded, gas-charged shock absorber will extend on its own.

Despite the effective spring rate of the pressurized shock, in a normal hydraulic shock, pressure should probably not be used to compensate for incorrect spring rates or worn / broken springs. However, it can help reduce body roll, sway, brake dive, and acceleration squat compared to a vehicle with identical springs but non-gas-charged shocks.

Twin-tube Design

Most modern OEM passenger vehicle shocks are of the twin-tube design. The twin tube design has an inner tube known as the working or pressure tube and an outer tube known as the reserve tube. The pressure tube holds the oil through which the piston moves. However, because the oil is incompressible, it must have somewhere to go during shock compression as the piston rod displaces a certain volume of fluid. The reserve tube provides a place for this hydraulic fluid that is displaced as the rod travels into the pressure tube. The reserve tube also creates a space for the fluid as its volume expands due to heat during use. The reserve tube may also contain a low-pressure charge of nitrogen. The pressure of the nitrogen in the reserve tube normally varies from 100 to 150 psi. Twin-tube shocks have a valve located at the bottom of the pressure tube called the base valve. The base valve is the compression valve - it controls fluid movement during the compression stroke of the shock by metering the flow between the pressure tube and the reserve tube. Because they use a “tube within a tube”, twin-tube shocks are compact in length, making them easy to fit or package, particularly on OEM cars where space is extremely limited. Their design also lends itself to relatively cheap mass production while retaining effective performance without requiring the strict tolerances (and associated manufacturing costs) of a mono-tube design.

Twin-tube shocks also have some limitations. They tend to trap heat within the pressure tube as it is insulated from cooling air by the outer reserve tube, making them prone to heat buildup and fade under hard use. Because the base valve is inside the pressure tube they are also not designed for the user to be able to alter or customize the valving.  Finally, gas-charged twin-tube shocks can only be mounted in one orientation – they will not function properly if mounted upside down.

Mono-tube Design

Mono-tube shocks have only one tube, the pressure tube. They are longer than twin-tube shocks because the single pressure tube must have sufficient length to store the hydraulic oil that is displaced as the rod travels into the pressure tube as well as provide space for the fluid as its volume expands due to heat during use. If a mono-tube shock is gas charged, as most are, it must also have a place to store the high-pressure nitrogen charge. There are two methods of accomplishing this. In some shocks, the nitrogen is stored with the oil in the pressure tube. The gas and the oil are, in effect, mixed together in an emulsion. Not surprisingly, this style of gas-charged mono-tube shock is known as an emulsion shock. A better way to contain the nitrogen charge is to separate it from the oil with a floating piston, also called a dividing piston. The floating piston moves up and down as the piston moves up and down in the cylinder, keeping the oil and nitrogen from mixing. However, the shock must now be much longer because the tube must be of sufficient length to accommodate the full stroke of the piston, all the hydraulic oil, the heat expansion of the oil, the gas charge, and the floating piston.  If the shock is a long-travel shock, (meaning the piston stroke alone is 12 inches or more), the total tube length required can become prohibitive.  In this case, an external reservoir is used to house the nitrogen charge, the floating piston, and some of the hydraulic oil. Mono-tube gas charges normally range from about 150 psi to 350 psi.

A mono-tube shock does not have a base valve. Instead, all of the valving during both compression and extension takes place at the piston. Since the piston and rod are easily removed from the shock, the mono-tube design lends itself to independent tuning of the compression and rebound damping by providing for easy valving changes by the end-user. As a result, mono-tube shock users can individually customize their valving for improved ride and handling.

In addition, non-emulsion mono-tube shocks can be mounted in any orientation, including upside down.

Because a mono-tube shock doesn’t require a “tube-within-a-tube”, for a given outside diameter, a mono-tube shock will have a larger bore, and thus be able to use a larger piston than a twin-tube design. This can be beneficial when designing a shock to extract the maximum possible damping from the smallest diameter package.

Mono-tube design also allows the heat in the oil to transfer directly to the outer surface of the shock body, which is in direct contact with cooling airflow, where it can dissipate more efficiently. This reduces heat-induced fade, allowing the shock to maintain full damping characteristics as temperatures rise with hard use.

Finally, mono-tube shocks must be designed and built to exacting tolerances in order to function properly. This results in a high-quality product.

	In the mono-tube shock, note the larger diameter piston, and the floating piston. In the twin-tube shock, note the "tube-within-a-tube" construction and the base valve. (Diagram courtesy of Polyperformance)

Shock Diameter

The larger a shock is in diameter, the larger its bore can be. The bore is the diameter of the piston and the inside diameter of the pressure tube. The larger the piston’s diameter, the larger its surface area. Since pressure is force divided by area, it stands to reason that the larger the area, the smaller the pressure generated by a given force. Inside a shock, the lower the pressure, the lower the temperature. In addition, a larger diameter shock can contain more oil to absorb and dissipate the heat generated, resulting in reduced internal operating temperatures for a given force. The result is, the larger the shock diameter, the cooler it will run and therefore the harder the shock can be worked before fading.

Variable Damping / Valving

In street / track driving, maximum performance demands shocks that provide both firm, crisp handling and a smooth comfortable ride. In offroad rigs we have the additional demands of soaking up small and large bumps at speed while allowing a degree of flexibility for slow crawling. No environment is as demanding of shocks as offroad racing. A shock with relatively stiff valving can provide firm, responsive handling and control roll, dive, and squat. But stiff valving creates ride harshness and increases feedback to the driver through the suspension. Conversely, a shock with relatively soft valving smoothes out the bumps, roughness and vibration, and allows great flexibility but can fall short of the required suspension control required for responsive handling and damping the pounding experienced in high-speed offroad driving.

One solution to this apparent dilemma is to use adaptive, or variable, damping  - that is, valving that adapts automatically to changing road / trail conditions. There are different strategies, all aimed at perfecting that magical balance between ride and handling for a wide range of driving conditions. Among the more common strategies are:

• Velocity sensitive damping
• Position sensitive damping
• Acceleration sensitive damping

Each have their particular strengths and weaknesses, but they all operate on a similar principle: variable valving senses some condition and changes to react to that condition – be it the speed of the shock stroke, the position of the shock in its stroke, or some other factor. Theoretically, the shock can therefore adjust to changing road or trail conditions and deliver improved suspension control and better handling without increasing ride harshness.

Velocity sensitive damping involves valving changes that occur in response to the speed, or velocity, of the shock’s stroke. This strategy provides relatively soft valving for driving on smooth surfaces, but when the wheel hits a bump, the sudden change in piston velocity causes the velocity sensitive valve to open, altering the damping to control the suspension’s oscillation. The same thing happens when the wheel rebounds from the bump. When the driving surface smoothes again, and the piston velocity slows, the valve closes and the shock's damping automatically alters for greater ride comfort.  The reason I say "alters" instead of saying it gets "softer" or "stiffer" is because the precise details get quite complicated. For example, when the valves open or close, depending on the shock and valving design, the rate at which the damping becomes softer or stiffer per unit of shock velocity increase depends on if the valving is linear, progressive, or digressive. As the name suggests, in linear valving the rate at which the damping changes with the velocity of the shock is constant. If the valving is progressive, the rate of change of stiffness increases as velocity increases; and if the valving is digressive, the rate of change of damping gets less as the shocks velocity increases. Whether the valving is linear, progressive, or digressive depends on several factors including the particular shim stack and piston design. The specifics are beyond the scope of this article, but will be explored in greater detail in a later issue. For now, the key point is that velocity sensitive valving alters the shocks damping depending on the velocity of the rod and piston; it is simple, effective, and fairly rugged which makes it popular in motorsports and is the form of variable valving of most interest to off-road coilover users, albeit sometimes in combination with some form of internal or external bypass valving, described below.

The disadvantage to velocity sensitive damping is that the velocity of the piston often slows as it reaches the limit of its stroke, just  before it changes direction (the same way the velocity of a ball thrown in the air slows to zero at the peak of its height before it begins to fall). The problem is, it may be that, at this point of maximum stroke (rebound or compression), the shock needs more damping, not less. One approach to mitigating this limitation is to use position sensitive damping.

Position sensitive damping takes a slightly different approach. Instead of using valving that opens and closes in response to piston velocity, the shock’s pressure tube is specially modified. Several small grooves are machined into the side of the pressure tube to create a different "zones" in the shock’s travel. The grooves create a leak path for oil to bypass the piston when the shock is operating in a particular zone. By carefully controlling the size and location of these internal grooves, more or less oil can be allowed to bypass the piston in different zones, increasing or decreasing damping as required. A shock using this strategy is often called an "internal bypass" shock. A typical example used in street cars has grooves machined in the normal operating midrange of the shock’s travel, creating a "comfort zone". Oil is allowed to bypass the piston when the shock is operating in this comfort zone,resulting in less damping and a softer ride. When the piston travels more than a few inches in either direction, as it does when the wheels hit a bump, it goes past the grooves and damping increases to control the suspension.

Inertia sensitive damping is still another strategy. One manufacturer has developed what it calls an "inertia active system" that it says continuously meters the rebound stroke and can instantly switch from firm to soft as conditions dictate. When high rebound damping is needed, as when cornering or braking hard on a smooth surface, the inertia valve is closed. This "closed-valve" (stiff) state is the normal default condition in which handling is greatly improved. When low rebound damping is needed, as when a wheel hits a bump or pothole, the inertia valve opens to reduce damping. The system is designed to allow the wheel to better follow the terrain and the suspension to absorb jolts without transmitting them to the chassis for a smoother ride.

A similar strategy is acceleration sensitive damping. With this strategy, a specialized compression valve is used. This compression valve is a mechanical closed-loop system, which opens a bypass to fluid flow around the compression valve when it senses a bump in the road, automatically adjusting the shock to absorb the impact. As soon as the impact is overcome, the valve closes and returns to the normal setting. It is claimed the valve can react in 15 milliseconds and that it operates anytime an impact or acceleration of 1.5 g’s or more is experienced. This ability to instantly switch from firm to soft is claimed to provide a noticeably smoother ride as well as better handling.

Externally adjustable bypass shocks

No matter what the strategy or the claims of the manufacturer; each variable damping strategy is in itself a compromise of sorts.  The current pinnacle of variable damping shock design, the externally adjustable bypass shock, attempts to maximize performance and minimize compromise by blending velocity sensitive piston valving with position sensitive external bypass tubes.  The piston valving uses velocity sensitive metering valves (deflection discs) but the shock also uses external bypass tubes that function in a similar manner as the internal grooves of a position-sensitive shock – they allow precisely metered amounts of oil to bypass the piston. Because the bypass tubes are external to the pressure tube, they are easily adjusted to vary the amount of bypass. The result is a shock that combines the best properties of velocity and position sensitive damping with the provision for quick and easy external adjustment. Depending on the number, size, and location of the bypass tubes, a vast amount of shock tuning is possible. Normally, when using external bypass tubes, the first 2/3rd's of the shock’s compression stroke can be lightly damped to soften the ride while the last 1/3rd is stiff to keep the suspension from bottoming. On the rebound stroke, the first 1/3rd is stiff to control the rebounding spring and stop the vehicle from bouncing while the last 2/3rd's can be lightly damped to allow the wheels to droop quickly before the next bump.

	Fox 2.5 inch external bypass shock with remote reservoir. (Photo courtesy of Fox Racing Shox)		Massive 4.4 inch external bypass shock with piggyback remote reservoir. (Photo courtesy of Fox Racing Shox)

Coilovers

A coilover shock is a high quality mono-tube shock that includes provisions to mount coil springs on the shock. The springs and shock are therefore combined in a single, compact package. There is nothing particularly magical about coilover shocks – their use requires strict attention to mounting geometry, spring rates, and shock valving the same as any other system. However, they do offer a number of advantages:

• High quality, long-travel, mono-tube shock.
• Completely rebuildable - parts are available separately at very reasonable cost.
• Easy to package - compact, easy to fit, frees room for link geometry and steering.
• Revalveable - easy to adjust or modify valving to suit needs.
• Tuneability - with a vast array of spring lengths and spring rates available, coilovers allow you to select spring rate for a specific target suspension frequency, and then use spring length and the built in adjuster to achieve a target ride height / suspension height.
• Multiple spring rate - easy to set up for use with a combination of springs, providing a soft initial spring rate that transitions to a firmer spring rate as the suspension compresses.
• Adjustability - built in adjustable top spring seat provides ability to adjust ride height, suspension height, & preload as well as accommodate different length springs with different amounts of spring travel. Adjustable stop ring provides ability to adjust position where spring rate transition occurs.

Don’t worry if not all of those statements make complete sense to you at this stage – they will by the end.  Let’s go over the different types of coilovers and then dig in to the parts and their functions.

Most coilover shocks will fall into one of the following three categories:

• Remote reservoir
• Piggyback
• Emulsion

	Remote Reservoir coilover shocks are the most common. They are nitrogen-charged mono-tube shocks and are commonly available with up to 18” of travel. They use a remote reservoir to house the nitrogen charge and floating piston, allowing the use of a shorter tube than would otherwise be necessary. The reservoir is connected to the pressure tube with a short, flexible hydraulic hose. Swivel fittings and different length hoses are easily installed providing flexible mounting options for the reservoir. Depending on the position of the piston in its travel, there will also be a certain amount of oil in the remote reservoir. Theoretically, depending on the mounting location of the remote reservoir, this could provide a slightly improved cooling capability – but this is not the primary purpose of the remote reservoir. Then again, if the reservoir is mounted too close to a heat source (engine, exhaust) this could cause a detriment to cooling. (Photo courtesy of Fox Racing Shox)
	A piggyback coilover shock is identical to a remote reservoir shock except that the reservoir, instead of being attached to the shock with a flexible hydraulic hose, is mounted directly on the shock with a bracket that incorporates the necessary hydraulic passage between the cylinder and the reservoir.  Because of this they can be more of a challenge to fit. Technically, both a remote reservoir shock and a piggyback shock can be mounted upside down and still work – although there’s no reason to do so and every reason not to, as the reservoirs would be very vulnerable in that position. (Photo courtesy of Fox Racing Shox)
	An emulsion coilover shock has no reservoir and no floating piston. It is still a gas-charged mono-tube shock, but the nitrogen charge is contained in the pressure tube along with the oil in an emulsion. Less resistant to aeration, foaming, and fade than the external reservoir styles, they are best suited to light weight and / or low-speed use. They are more compact and therefore easier to fit than the other styles. Also more economical than the other styles, they are usually adequate for rockcrawling use. An emulsion shock can only be mounted right side up. (Photo courtesy of Fox Racing Shox)

There are also a few non-emulsion, non-reservoir mono-tube coilover shocks available on the market. This style incorporates the reservoir and the floating piston into the main shock tube, and is sometimes called an "internal reservoir" shock. They are normally available only in shorter to medium travel (up to about 14") as so much cylinder length is required to internally accommodate the reservoir and floating piston. This style of coilover is most commonly seen for application (make / model) specific replacement or upfit.

Shock diameter

Most styles and brands of coilover are available in either 2” of 2.5” diameter. This is the diameter of the pressure tube or shock body. 2.5” shocks are larger and heavier than 2” shocks. Their larger diameter can make them more of a challenge to fit without limiting clearance between tire and chassis / fender. 2.5” shocks are also more expensive. Either shock can be valved the same, but the larger 2.5” shock will run cooler. Having said that – this is really only important for lengthy higher-speed desert or rock racing type use, as rockcrawling, street use, and trail riding are unlikely to overheat a 2” shock.

The 2” shock uses 2.5” ID springs and the 2.5” shock uses 3” ID springs. 3” ID springs are heavier, more expensive, and their availability in numerous different rates (particularly lighter rates favoured by rock crawlers) is more limited than the smaller 2.5” ID springs.

Coliover Operation

There are a great number of tuning and adjustment factors to consider when using coilovers, which we shall discuss in detail shortly. For now, it is sufficient to understand that a coilover shock with springs mounted functions as a unit that provides both springing and damping. One end of the shock has a fixed, integrated spring seat and is mounted to the axle or suspension. The other end, which incorporates an adjustable upper spring seat, is mounted to the chassis. The springs are installed over the body and shaft of the shock between the fixed lower spring seat and the adjustable upper spring seat. Springs of many different lengths and rates can be accommodated, which combined with the ability to alter the shock's internal valving, gives a wide range of tuneability.

Since the shocks have a great deal of travel (to permit the large amounts of wheel travel needed for offroad use) a great deal of spring travel is also required to prevent the springs from bottoming before the shock. In order to achieve the necessary spring travel a very long spring is required. However, because there is a practical limit to how long a spring can be made before it will tend to buckle, multiple springs are stacked in series on the shock (one on top of the other) in order to achieve the required spring length.

The most common configuration consists of two springs stacked together in series, and is called a "dual-rate" system as frequently the two springs have different rates. The upper spring is called the "tender" spring or coil, and the bottom spring is called the "main" spring or coil. (From this point forward I shall use the terms "spring" and "coil" interchangeably.) A nylon sleeve, known as the dual-rate slider (DRS), "floats" on the body of the shock between the two springs. As the springs compress and rebound, the dual-rate slider slides up and down the shock body. When multiple springs are stacked in series in this manner, the result is a variable spring rate that begins as the (normally softer) rate of the combined springs and progresses to the (normally stiffer) rate of just the main spring. The rate is initially soft because both springs are compressing together. When the springs compress far enough, the DRS slides up until it hits the stop ring and is prevented from sliding further. At this point, the dual rate slider effectively becomes the upper spring seat, locking out the upper spring. From this point, remaining spring travel occurs at the rate of the lower, main spring alone. We shall discuss dual-rate systems in great deal, including relevant formulae later. For now it is sufficient to know that stacking multiple springs gives us the required spring length and allows us to use a softer initial spring rate that transitions to a firmer rate at some point in the suspension's travel.

With a basic grasp of how they operate, let's now examine the coilover shock and it components in detail.

(Note: As we've discussed, because a non-emulsion mono-tube shock can be mounted either way up, technically there is no "top" or "bottom" , "upper" or "lower" to the shock. However, the normal method of mounting the shock is with the cylinder up, attached to the chassis. This protects the cylinder and reservoir from damage. There is no practical reason to mount the shock the other way up. In addition it will make my life as a writer, and yours as reader, an awful lot easier and less complicated if we agree at the outset that the "top" or upper-end of the shock is the body- or cylinder-end, and that the "bottom" of the shock is the shaft- or rod-end.)

	The Schrader valve, located at the end of the remote reservoir is used to charge the shock with nitrogen.
	This remote reservoir is 2" in diameter, and 11" long.
	The remote reservoir is connected to the shock cylinder with a braided stainless steel hydraulic hose; this one is 14" long.
	The top end of the shock has a spherical bearing for mounting to the chassis. The cylinder is threaded for a portion of its length, allowing adjustment of the position of the top spring seat. This adjustability of the top spring seat is a key feature of coilovers, and allows us to accommodate different length springs as well as adjust ride height, suspension height, and spring pre-load - each of which we will define and discuss in detail later. The top spring seat is also frequently know as the "adjuster." There is a locknut to lock the adjuster in position.
	In addition to allowing adjustability of the top spring seat, the threaded portion of the cylinder also allows the position of the stop ring to be adjusted. The position of the stop ring is used to define the point where the dual rate slider will stop sliding. At this point in the shocks / springs travel, the spring rate transitions from the softer initial combined spring rate to the stiffer, final rate of the main spring. The point in the shocks travel where this transition occurs is called the transition point. Thus, we can say the stop ring is used to set the transition point.
	Once its position has been determined, on a Fox shock the stop ring is secured in position with two small Allen-head grub screws that engage a groove machined along the length of the threaded portion of the cylinder. Other shocks may use a different arrangement for the stop ring, such as two nuts that lock against one-another.
	The dual-rate slider "floats" on the body of the shock. It is positioned between the main and tender springs. As the springs compress and rebound, the dual-rate slider slides up and down the cylinder. When the springs compress far enough, the dual-rate slider slides up until it hits the stop ring and is prevented from sliding further. At this point, the dual rate slider effectively becomes the upper spring seat, locking out the tender spring. At this point, remaining spring / shock travel occurs at the rate of main spring alone.
	The shock body (a.k.a. the cylinder or the tube) and associated hardware.
	View of the rod, or shaft, where it enters the cylinder. The bearing cap incorporates a wiper seal to clean the shaft of contaminants as it enters the cylinder, and an internal rod guide to keep the shaft supported and aligned.
	At the rod-end of the shock there is another spherical bearing for mounting the shock to the axle or suspension. The lower spring seat is fixed at this end. There is a small rubber bumper mounted on the shaft and resting on the lower spring seat. It is not a true bumpstop designed to handle the forces of bottoming the suspension at speed. Instead, it is merely designed to prevent damage that could otherwise occur from the lower spring seat contacting the bearing cap at full shock compression.
	View of an installed shock without springs. The springs are installed on the shock between the lower spring seat and the top spring seat or "adjuster". Because the coil springs are mounted on the shock, spring travel must equal the shock travel. That is, the amount the springs change length from full bump to full droop must equal the amount the shock changes length from full bump to full droop. If the shock is setup to provide a great deal of droop, often the main and tender springs are not long enough to span the gap between the spring seats when one wheel is at full droop. The result is, the springs fall away from the top spring seat and rattle around, possibly damaging the shock.

	The solution is to install a small spring, called a "helper coil" between the top spring seat and the other springs. Another slider, called the "triple rate slider", fits between the helper coil and the tender coil. Technically, with a helper coil installed, there are now three springs installed on the shock, and the configuration is known as a "triple-rate" setup.
	The helper coil is a short, flat-wound spring with very little stiffness. It can easily be compressed by hand. Resting between the top spring seat and the tender coil, it is compressed completely flat on installation. However, once the suspension nears full droop, it expands, exerting just enough force to keep the tender and main coils in place and prevent them from rattling around. Use of a helper-coil is an important consideration in selecting spring rates and lengths that we shall cover in detail later. Typically a helper coil will have a free length of 5”, a compressed height of 0.5” and a rate of only 2 lbs/in.
	A complete triple-rate coilover setup.
	This picture shows a complete triple-rate coilover setup installed on the rig. Note how, at rest (called static ride height), the helper coil is compressed flat.
	Upper end of the installed coilover at static ride height.

	Installed main and tender coils separated by the dual-rate slider.
	Installed triple-rate coilover at full droop. Note the extended helper coil between the top spring seat (adjuster) and the tender coil. Also note the triple-rate slider between the helper coil and the tender coil. Here the helper coil is exerting just enough force to keep the springs seated.
	Additional hardware required for a coilover setup includes: The shock mounting bushings, which install between the vehicle's mounting tabs and the shock's spherical bearings, to provide a degree of misalignment capability to the ends of the shock; and The rubber mounts and clamps used to secure the remote reservoir to the chassis.

Interim Summary:

We've covered a lot of ground to this point, so let's do a brief summary before we forge on.

• Suspension provides ride and handling - it's chief jobs are to prevent the wheels from loosing contact with the ground over rough terrain and to isolate occupants from that rough terrain.
• Springs support the weight of the vehicle and absorb the road shock of bumps and holes by compressing and rebounding.
• Shocks damp the oscillation of the springs by pumping a piston through a column of hydraulic oil.
• The amount of damping a shock provides depends on its valving.
• Springs and shocks combine to determine whether a suspension is firm or soft.
• Spring and shock selection is a careful compromise between ride and handling.
• Coilovers are an ingenious way to package shocks and springs as one convenient assembly.
• Coilovers are long-travel, high quality, revalveable, gas-charged, mono-tube shocks.
• Coilovers use two or more coil springs in dual- or triple-rate setups, allowing a soft initial spring rate followed by a firmer spring rate as the suspension compresses.

Great. So how do we go about getting the best performance from our coilover shocks? How do we most closely approach that magical balance between ride and handling, between firm and soft suspension? What are our goals?

The Seven Goals of Suspension Design

Designing suspension is a matter of achieving the best balance between these often competing goals:

1. Desired suspension frequency
2. Desired suspension height
3. Desired ride comfort
4. Acceptable roll resistance
5. Desired flexibility*
6. Matched wheel, spring, and shock travel
7. Desi