Coilovers and shocks are often a major investment item for many people, but how many of us really understand what we are buying? This is a guide to not only help you find out what makes a shock quality, but what we are really getting when we buy a shock / coilover.
When you travel over pavement there are small changes in the road surface. These push the wheel up which compresses the spring. The spring then decompresses and pushes down on the wheel causing it to follow the road surface. If you did not have a shock, aka a damper, the spring would oscillate up and down until the initial force is dampened by the spring itself. If you've ever been in a car with bad shocks or been behind one you'll notice that it bounces up and down, over and over, ad nauseum. The reason for this is because the shock is there to dampen and control the oscillation of the spring.
It does this by resisting the upward and downward movement of the spring and converting that energy into heat. The resistance generated by the downward force is known as compression dampening and the resistance generated by the upward force is known as rebound dampening.
Ultimately there are 3 jobs that the shock must perform.
1) Maintain constant tire contact with the road to ensure grip.
2) Transfer weight from one corner to another to maximize chassis response.
3) Dampen and control the chassis to maintain geometrical alignment and stability.
Example shock diagrams from Tein. Tein produces coilover setups for many different applications in both a monotube and a twin tube configuration depending on setup.
Monotube shocks feature a single large diameter piston within the shock body. There is a main section filled with hydraulic oil that the piston resides in. Adjacent to that is a high pressure nitrogen filled partition, separated from the oil by a second, free-floating piston. The gas pressure is there to prevent cavitation of the oil, which reduces the effectiveness of the damper.
Benefits to the monotube shock design include improved damper response at low travel distances due to a larger piston / valve area, and improved heat dissipation thanks to the proximity of the oil and piston to the shock's exterior. This makes for a superior damper for low travel / high frequency applications such as an F1 car.
For a racing vehicle, we feel that the monotube is a superior design.
An example of a monotube shock would be the Ohlins Road & Track STI coilover kit.
Twin tube shocks are constructed of a single piston within an internal tube, mounted inside of the shock body. Typically in an adjustable shock design like Koni Yellows and KW's twin-tube you will find that there are separate valves for controlling compression and rebound. This negates some of the advantage that the monotube has over the twin-tube in valve area, and improves the overall valve performance by reducing reliance on a single valved piston to control both compression and rebound.
Twin tube shocks also have the benefit of being slightly more compact. They can house more oil and give more shock travel in a height restricted space. This is especially beneficial to road going cars that do not run high spring rates / roll resistence.
One other benefit to the twin tube design is the low amounts of nitrogen gas pressure located in the exterior shell of the damper. This low pressure works in the same capacity as the monotube; to reduce cavitation and aeration of the oil. However with low pressure comes reduced shaft force, which improves ride quality but can cause some jacking with heavy amounts of rebound dampening.
Twin tubes can be quality shocks and can offer good performance. Cheap twin tubes are crap, avoid them.
An example of a twin-tube shock would be the KW V1 coilover kit.
There are a couple of ways that valving is handled. Each shock manufacturer out there has their own methodology on what makes effective valving and how that's accomplished. Each application will require various amounts of each and even the same chassis can utilize two different settings depending on variations in car setup.
Compression dampening occurs when the shock is compressed. The rate (speed) of which this occurs is what determines how much force is generated; more shaft speed requires more dampening. Compression also has a major effect on how fast weight that has been transferred to the shock is applied. The stronger the compression dampening force the faster weight is applied to the tire on that corner.
Low Speed Compression Dampening
During what is called "steady state" handling the suspension is reacting to very small changes in the road and driver inputs. The shock piston is traveling at slow speeds and will thus require less dampening force. This area of the dampening curve is vital to overall handling performance, especially on a track or long-sweeping corners.
High Speed Compression Dampening
When you hit a bump or get onto the curbs at a track the shock piston suddenly travels upwards at a very fast rate. This requires a sharp increase in shock dampening force which is provided by the high speed valving. Altering this dampening force has a major effect on how stable the car is over bumps and fast changes in driver input. Improper dampening force can cause a loss in control of the vehicle which is why, for example, KW engineers specifically don't allow this to be adjusted on their V3 model shocks.
Rebound dampening force is generated when the body travels away from the road and the suspension decompresses. Like compression, the force generated is increased as shaft speed also increases. Rebound dampening force determines how quickly weight is transferred off of a corner. The more rebound dampening force, the faster the weight is transferred and the more "responsive" the chassis is to driver inputs.
Low Speed Rebound Dampening
Also during "steady state" moments the low speed section of the rebound is responsible for resisting the upward travel of the chassis. Adjusting this in a shock allows you to fine tune handling during a crucial point in the corner and is a valuable part of shock tuning.
High Speed Rebound Dampening
Once the bump compresses the shock, the body will then travel upwards in a fast motion as the spring decompresses. This moment is handled by the high speed rebound valving and is also paramount for improving stability over uneven sections of road or very fast driver inputs.
Digressive valving produces a curve that is not straight. As speed increases the rate that dampening force is created varies considerably. The opposite of digressive is linear scaling. In a linear plot the rate that dampening force is generated varies very little (if any). Here's an example of each in a shock dyno graph:
The top portion of the graph is the compression valving, and the bottom is the rebound valving. The top portion is digressive whereas the bottom is linear.
As you see above, there are several different parts of the valving that can be adjusted. What is user adjustable all depends on how the shock is designed and constructed. Some shocks are not adjustable without completely rebuilding them, while others are known as 4-Way adjustable.
Special Note: Coilovers that claim to be "32-Way Adjustable" are referring to having 32 adjustments in a particular part of the valving. The vast majority of these shocks are rebound adjustable and adjust the entire rebound curve with a secondary effect on the compression valving as well. In my experience most of these shocks are low quality and often have very dissimilar dampening between shock to shock.
Here's a shock dyno graph from Flyin' Miata that shows the different force curves generated when you adjust the valving on a shock. The top is compression and the bottom is rebound. Both are digressive curves, but the rebound shows considerable changes in the knee point as valving is altered.
1-Way Adjustable - These shocks are rebound adjustable only. Depending on construction they typically also have a small effect on compression dampening as well. When you adjust one position the entire rebound curve is altered, both high-speed and low-speed.
2-Way Adjustable - These shocks are rebound and compression adjustable. However, they can vary in which portion of the curve is affected. An example of this is KW's V3 shock which allows the user to adjust rebound and low-speed compression. Again, like the 1-Way shock, the entire rebound curve is shifted when you adjust it
3-Way Adjustable - This particular shock allows the user to adjust high-speed and low-speed valving independently of each other along with the entire rebound curve. JRZ produces a 3-Way adjustable shock that allows users to adjust high-speed and low-speed compression dampening, as an example.
4-Way Adjustable - Top end shock manufacturers will often provide a shock that allows the user to adjust virtually everything. The high-speed and low-speed rebound and compression are adjustable independently of each other. Moton, JRZ, and Penske are three example manufacturers that sell shocks in this category.
An example of a 2-way adjustable shock would be the KW V3 STI coilover kit.