So I'm going to start putting some hardcore info in here. Rather than using specific examples to say X is better than Y, lets focus on the main features of a set of coilovers. Hopefully this will help to lead to a more informed purchase.
What is a coilover?
Coilover is short for coil spring over strut. It consists of a damper with a coil spring encircling it. 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 term coilover is now most commonly used to describe an aftermarket assembly that at least has a threaded strut/shock body for height adjustment. However, there are many more options than that.
What are all these pieces?
There are only a few major components of a coilover strut assembly. The structure starts with a damper with a threaded body (strut in our case). First a locking collar is threaded onto the strut followed by the spring perch. The strut assembly fits through the spring. It is secured at the top by the strut mount. These are sometimes referred to as "top hats" because of their shape. The strut mount usually consists of a bearing, a bushing, and a rubber/metal mount which bolts it into the strut/shock tower of the car (aka chassis)
Sometimes there are 2 springs on the coilover assembly. The large spring is actual main spring, where as the smaller spring at bottom of the assembly is known as a helper spring. Helper springs assist with rebound stability, ride comfort and keep the main coil spring seated. Preload of the main spring can also be set to zero, for maximum comfort and suspension travel These are often found on higher quality coilovers. In our case FK's are the most common to utilize the helper spring design.
Some coilovers also come with adjustable strut/shock body length (pictured below). In this type, the lower main body of the coilover can be spun up and down to change ride height with out effecting the spring rate and preload. It essentially changes the length of the assembly with out effecting the length of travel of both the spring and strut rod. This is different than struts which have a fixed threaded body that the spring is raised and lowered on. In that case, adjusting the perch up and down will change spring rate (on a progressive spring), spring pre-load, and rod travel. These can found for the MK6 with JIC/Cross K-sport, B&G, BC racing (if they're still around)
What is a damper?
A damper is a structural component designed to resist compression. As a vehicle is driven over varying conditions, it has a tendency to bounce and hop depending on the roughness and/or bumpiness of a given road surface. Struts apply various degrees of tension to the front and rear of a vehicle that limit road bounce and bumpiness.
Well what makes one damper better than another?
Monotube Dampers vs Twin Tube dampers?
How does a Twin Tube damper work?
Bump stroke.
When the piston rod is pushed in, oil flows without resistance from below the piston through the outlets, and the non-return valve to the area above the piston. Simultaneously, a quantity of oil is displaced by the volume of the rod entering the cylinder. This volume of oil is forced to flow through the bottom valve into the reservoir tube filled with air (1 bar) or nitrogen gas (4-8 bar). The resistance, encountered by the oil on passing through the footvalve, generates the bump damping.
Rebound stroke.
When the piston rod is pulled out, the oil above the piston is pressurized and forced to flow through the piston. The resistance, encountered by the oil on passing through the piston, generates the rebound damping. Simultaneously, some oil flows back, without resistance, from the reservoir tube , through the footvalve to the lower part of the cylinder to compensate for the volume of the piston rod emerging from the cylinder.
How does a monotube work?
Unlike the twin-tube damper, the mono-tube shock has no reservoir tube. There is still a need to store the oil that is displaced by the rod when entering the cylinder. This is achieved by making the oil capacity of the cylinder adaptable. Therefore the cylinder is not completely filled with oil; the lower part contains (nitrogen) gas under 20-30 bar. Gas and oil are separated by the floating piston .
When the piston rod is pushed in, the floating piston is also forced down by the displacement of the piston rod, thus slightly increasing pressure in both gas and oil section. Also, the oil below the piston is forced to flow through the piston. The resistance encountered in this manner generates the bump damping.
Rebound stroke.
When the piston rod is pulled out, the oil between piston and guide is forced to flow through the piston. The resistance encountered in this manner generates the rebound damping. At the same time, part of the piston rod will emerge from the cylinder and the free (floating) piston will move upwards.
Here is a picture to help make sense of all that tech
Advantages/Disadvantages of each?
A quality twin-tube will offer low friction, resistance to cavitation, and large valve area . This is everything a good damper should be, and the value of a dual-valve twin-tube damper seems to be much higher than the monotube solutions I see on the market. I'm not sure why this is, but it makes them a pretty unbeatable value for most uses.A twin-tube damper has very little piston valve area, this is a size limitation said to be inherent in a twin-tube design. The inner tube simply isn't as big as the outer tube.Twin tube dampers are the most commonly used type of dampers by OEM and aftermarket manufacture as they are the cheapest damper to make. However, twin tube dampers do not perform as well as mono tube dampers as the oil heats up and destabilises under extreme usage.
A monotube dampers contain the piston and rod within one large-diameter tube. A floating piston at the bottom of the damper provides a physical barrier between the damper's oil and the high-pressure nitrogen gas needed for shaft displacement. Monotube dampers run pressures upwards of 300 psi. This high internal pressure inside the gas chamber means the damper oil is less likely to bubble, or cavitate, as it travels through the valves at high speeds.The drawbacks of a monotube aren't as easy to find information on, and they seem to stem from monotubes requiring high gas pressure internally. This makes the damper act somewhat as a spring, always wanting to extend, and the larger seal area (necessitated by the large piston) can generate quite a bit more static friction within the damper. Static friction is very bad as it prevents suspension movement from happening- we want to dampen the motion, not prevent it.
What is dampening adjustable, and why whould I want it?
Adjusting the damping means you are changing the way the damper behaves in either rebound or compression (or both). Increasing the damping in either dimension (rebound or compression) means that you are increasing how resistant the damper will be movement in that dimension.
eg. Increasing rebound damping means that the spring will require more force to push the piston back up in the shock body, meaning that the entire assembly will return to its regular length more slowly. Decreasing the rebound damping means it takes less force to return the entire assembly to its regular length so it will extend more quickly. Compression damping works the same way, in the opposite direction (compression of the damper). Increasing compression damping means it will require more force to compress the damper, and decreasing compression damping means it will require less force. How this translates into ride quality (stiffness, or bounciness) depends on a variety of other factors, as well as how rebound and compression are adjusted in relation to one another. Generally of course, more damping will mean a stiffer ride.
Having adjustable dampening can be good if you alter your wheel/tire combo and ride height pretty consistently. Droping unsprung weight by getting smaller/lighter wheel and tire combo will change the characteristics of of the dampening. Being able to set it softer, or harder is a big advantage to some one who's car does double duty on the street and track with dedicated track wheels.
However, many times if you don't change out wheels, and you have no intentions of taking your car to the track, then adjustable dampening may not be the thing for you. A set of coils such a KW V1's or H&R streets come properly valved for street applications and your average wheel and tire spec. Often times buyers of dampening adjustable coilovers claim that once they set them up to their taste, they never changed those settings again. However, whether or not this personalization of ride/performance is not for your average user.
Piston Rod
The advantages of a larger piston rod in the strut is still relatively unknown to me. I know that rod is there to withstand linear forces, and the larger the rod the great the force the strut is capable of handling. Is this noticable from a drivers standpoint? I honestly don't know. Does a larger piston rod = a better coilover? Again, I'm not knowledgeable enough to say. Hopefully some one will jump in
Springs
What is the function of a spring on a car?
The main function of a spring is to keep the tires on the road. As the road surface deviates, the spring keeps linear pressure on the wheel so that it stays in contact with the road.
Types of springs
There are 2 types of coil springs, progressive and linear. Linear springs have a fixed spring rate. This means the amount of weight needed to compress the spring a certain amount does not change with the load. Its very easy to spot a linear spring. Linear springs have equal spaced coils within the spring. (K-sports B&GS JIC)
The second type of springs is by far the most common. Progressive springs have variable spring rates. A progressive spring is one in which the spring rate changes depending on the amount of load placed on the spring, therefore the actual spring rate is not a fixed number. These are easily spotted because there is not equal spaces between the coils of the spring. There will be a few coils at the top and bottom of the spring basically on top of each other while the center coils are very wide apart.
Why does this matter? Do they handle different?
Yes!
Linear springs
The main advantage of linear springs is consistency. The way that the weight transfer from one side of the car to the other side should be very smooth and consistent. This makes the car’s dynamics much easier to predict. When exiting a corner, a linear spring will settle the car’s body in a smooth manner because both sides of the car are compressing and rebounding at the same rate (1 mm of compression to 1 mm of rebound).
Progressive springs
When exiting a corner, a progressive spring will send the compressed force from outside of the car to the inside. Since the spring rate is not constant from side to side, the spring on the inside of the car will compress from the force absorbed. It will then send the compressed force back to the outside. This happens back and forth as the car slowly settles down. This excessive body roll requires more attention to control effectively.
The progressive spring is blue, linear spring is red.
This is by no means comprehensive. It is my intentions that other users who are more informed than me will be able to fill in any information I missed. If there are any corrections needed post them up. If I'm wrong, don't be afraid to tell me.
Hope this can be helpful :thumbsup:
External Resources
http://www.iwsti.com/forums/gd-suspension-handling-stiffening/60084-damper-design-twin-tube-versus-monotube.html
http://www.tirerack.com/suspension/brand.jsp?brand=KONI&tab=choosing
http://www.meisterr.com/Site/Suspension_Basics.html
http://www.modified.com/tech/0605_sccp_making_it_stick_part_5/dampers.html
