To many outside of motorsports, the way a shock works is a bit of a mystery. They know it controls the suspension, but they don't know exactly how a shock does it. We're going to pull back the curtain a bit as we take you with us on a visit to the suspension experts at Eibach in Corona, Calif.
The damper is probably better known to most as a shock absorber or simply a shock. We'll be using the term "damper" from here on to describe it, however, so keep that in mind. It's a device used to control the rate of pitch and roll of a vehicle. It also controls the rate of motion of a spring in bound (also called bump in racing or jounce in engineering terms) and rebound (also called droop in racing). Without them, your vehicle would just flop around as the springs would have no control and react to not only the road but also itself as it oscillates.
Think of those slow-motion videos of a valvetrain as the cams open and close the valves. Since those valve springs have no dampening control, they bounce and even cause valve "float." That's a topic for another day, but just know the same thing could happen in your suspension if you didn't have dampers. So, while you should be glad they are there, how exactly do they work?
BASIC DAMPER DESIGN
Inside the tubes that make up your dampers you'll find a shaft with a disc connected to the end of it. This is the piston of the damper. The piston has a stack of shims on top of openings cut or molded into the piston. This, in combination of flowing through hydraulic oil, is how your dampers "dampen" the spring's movement. It sounds simple enough, but there is far more going on than you probably still realize.
THE HOLES AND SHIM STACKS
First, let's start with the piston design itself. If you're into RC car racing, you're familiar with how the holes in those pistons control how fast or slow the piston flows through that fluid. The amount and size of those holes partially determine the damping rate. Next are those shim stacks, with a set on top and on the bottom to further control bound and rebound independently.
The thickness and amount of those shims will further increase or decrease the damping rate on each side of the piston. That's also why those holes are enlarged and staggered at the face of each side of the piston. This is so the fluid can flow around the opposite stack, through the piston, and then on to the stack that controls bound or rebound.
DIGRESSIVE AND LINEAR PISTONS
The piston face can further control the dampening rate by using a digressive or linear face design. A linear face design is flat and the shim stack acts without any further changes in the reactive speed of the stack. A digressive face piston is dished to allow for preloading of the shim stack to change the dampening rate during slow damper shaft speeds.
To explain shaft speed, think of your vehicle diving down and returning to normal during a stop versus hitting a set of quick bumps in the road. The piston shaft is moving at a slow rate during stopping while it moves quickly when encountering bumps because it's moving further in a shorter amount of time. That preloading of the stack delays its opening and increases the dampening force during those low shaft speeds. A damper with this type of piston makes it a speed-dependent dampener and a piston can be linear on both sides, digressive on both sides, or digressive and linear on each side. How that's done is determined by testing on a shock dyno and even driver input for motorsports.
Now if you were paying attention in physics class, you probably start to see an issue with the piston moving through that fluid. It creates a high-pressure side and a low-pressure side. If you didn't, don't worry, we're getting ready to break it down now. As the piston moves through the fluid, the "top side" (the side with an inactive shim stack) must force its way, and creates an area of high-pressure. If it was a gas, it would move somewhat more freely but wouldn't act like a good damper.
However, that's not the issue; the side the piston shim stack is acting on creates a low-pressure area. If you've ever boiled water at sea level and at high-altitude as well, you know that water boils faster at higher altitude because the atmospheric pressure is lower. The exact same thing happens in your dampers.
This is the primary cause of aeration; the shock oil degasses due to low-pressure pulling gas out of the solution (also known as vacuum degasification) and even begins to boil the oil on the "bottom side" of the piston as the damper heats up. These gasses cause a feeling of reduced dampening because gas is compressible whereas a fluid is non-compressible.
The fluid being non-compressible is the whole reason a damper works while gas being compressible is the reason why air ride suspensions work. Gasses create a spring force when compressed and is how and why a suspension airbag works in place of a spring. However, you don't want that in a damper when the fluid is being used to dampen the springs.
You want a fluid that is non-compressible, but you also want something that will allow the piston to flow through itself but won't entirely stop it when the rate changes. That's why a non-Newtonian Fluid like oobleck, for example, wouldn't work. You could use simple friction and early dampers were designed that way (like the Andre Hartford design) but don't dampen nearly as well as oil does. That's why a fluid like shock oil has been used in dampers since 1907, and we should give thanks to Maurice Houdaille for its invention.
So how do you prevent the shock fluid from boiling or degassing if it's our only choice? Simple—by maintaining a constant pressure on both sides of the piston. That doesn't sound possible, does it? Fortunately, it is by using nitrogen gas to create constant pressure. While you don't want a gas as your dampening fluid, you do want it to keep the fluid pressure in the damper constant by utilizing its natural spring force.
This natural spring force also allows fluid to react as the piston travels through it. It gives it space while keeping pressure equal on both sides of the piston. Even though there are holes in the piston, the fluid will still displace until the shims open or it hydrolocks and, just like your engine experiencing hydrolock, that condition can cause catastrophic damage to the damper. Even so, it is still possible to hydrolock during high shaft speeds and why your vehicle feels like there is a solid block instead of a spring on certain bumps. That can also be solved in piston design with extra holes (but affecting damping rate in both directions) or with shim designs that allow fluid to pass (and only affecting the damping rate on the side it's used).
GASSING PRESSURE IN A TWIN-TUBE VS MONO-TUBE
A twin-tube damper, which uses a tube within a tube design, does mix the nitrogen with the oil, but because it's at a low-pressure and its molecule is larger than oxygen, it doesn't fully mix (or gets dissolved into solution, as they say in science) with the shock oil. It still does, but the amount is small enough to not be an issue for a twin-tube design. It also has the benefit of being inert, reducing fire risk, and cheaper than other inert gasses as you can pull nitrogen out of the air over argon.
The working cylinder, as the name implies, is where the piston and shock oil work. The outer cylinder, the one you see and touch as you install your dampers, is where excess oil goes and where the nitrogen lives. A valve between the working cylinder and the outer cylinder allows fluid to flow between them and works as another damping force control valve.
In a mono-tube design, the body is the working cylinder and that's it. However, the nitrogen gas is separated by a floating piston that also has a seal to keep the gas contained above that piston. Because of this, the nitrogen doesn't mix with the shock oil like it does with a twin-tube design. You can typically use the nitrogen gas at much higher pressures because of this separation, as well, which further reduces aeration by degassing and boiling by low-pressure at the piston. A mono-tube also allows for a larger piston—providing more surface area for the oil to work with—and better cooling as the fluid makes direct with the cylinder while working and transfers heat away much more effectively.
ADJUSTING DAMPENING FORCE
As mentioned earlier, the dampening force is dependent of the piston's design and the way the shims react as it flows through the shock oil. However, it is also possible to adjust that without tearing apart the damper. The primary way this is done in most mono-tube and several twin-tube damper designs is by allowing the shock oil to bypass the piston. For these Eibach dampers, there are two holes drilled into the damper shaft, one or more above the piston and one through the center of the shaft at the bottom of the piston. The shaft is also drilled through with a rod or needle passing through it.
When the damper uses a rod, it connects to a pod at the bottom of the shaft and a rotating disc that has different sized holes for the oil to flow through. A ball detent not only gives the user an audible "click" to know where they are in their adjustments, but also aligns the rotating disc's holes to the holes of the pod. While simple, this design also limits the adjusting capabilities by only having so many holes to choose from.
In the needle design, the hole goes straight through the damper shaft at the bottom of the piston. Rather than using a rotating pod, a needle limits the opening inside the shaft. It works much like a carburetor needle does by gradually reducing the opening of the orifice. While it does offer far more adjustability, it will eventually full close off the opening, so the adjustment is finite. Another advantage is that the taper of the needle can be modified to change how much and how fast the needle reduces the orifice opening per knob turn before going fully closed.
ADJUSTABLE EXTERNAL RESERVOIRS
External oil reservoirs can also add an additional way to control dampening force by limiting how much fluid flows between it and the damper as it is displaced by the piston. On an Eibach damper with an adjustable reservoir, a ball-detent controlled dial changes the preload on the shim stack inside of it. This sits on top of what looks like a piston, but instead of flowing through fluid on a shaft, it's fixed to the adjuster and fluid flows through it.
Because of this, the nitrogen, along with a floating piston, is in the reservoir rather than the damper body. This still works the same way as it would if it was inside the damper body, pressure is still maintained by the nitrogen and floating piston. This is also how the twin-tube adjuster works. The base valve between the reservoir cylinder and the working cylinder would work and be adjusted in the same manner.
However, adjustable external reservoir twin-tube dampers do exist. Some don't have a base valve, and some do, but either way, they work very differently from a mono-tube external damper. It does borrow a little bit from the mono-tube external with the nitrogen gas being separated by a floating dividing piston inside the reservoir. Another design is to use a nitrogen bladder over a piston. It's how the fluid goes from the outer and working cylinders that makes it very different.
What you can't see is that there are two paths for shock oil to travel. One path is just for bound and is open to the working cylinder while the other is for rebound and is open to the external cylinder. Oil flow control is done by a piston with a spring and the rate is controlled by adjusting the preload of that spring. The higher the preload, the more force is required to push the piston open and vice versa. Because of this unique requirement, the reservoir is usually fixed and is part of the damper cap. There are remote external reservoir versions, but these feature two reservoirs rather than a single because the flow must be separated between the two cylinders.
DAMPER ASSEMBLY DEMONSTRATION
While we were at Eibach, Robert Morrison, the Damper Department Manager at Eibach in North America, gave us a demonstration on how their dampers are assembled. This one is one of the test mono-tube dampers for the new Honda Civic Sport. First, the body and shaft of the damper are pre-assembled before final assembly. The body is set on the machine and held in place to keep it stable during assembly.
The nitrogen tank is turned on and regulated to 200-PSI. This is not the pressure that will be used in the damper, just the pressure from the bottle to the machine. A specialized cylinder is installed after the floating divider piston is placed inside of it. This is threaded on to the damper body while the floating piston sits just above the Schrader valve where the nitrogen flows through. Once installed, Robert fills a measuring cup full of 5-weight shock oil that is poured into the temporary cylinder. The damper shaft is then inserted piston first and on top of the floating piston. It's pulled up and down slowly and deliberately to allow oil to flow through the piston and allow trapped air to escape.
Once he is satisfied that all the air is removed, Robert then installs another bar, but this one is to make sure the filling cylinder remains in place in case the threads fail. The machine then lowers a press rod to force the shaft and piston down once the nitrogen is filled. The fill pressure for this damper and many others is 7-PSI, however, valving will determine the exact pressure. A higher damping rate requires a higher fill pressure for the nitrogen. Once the nitrogen hits 7-PSI, Robert pushes the damper shaft assembly into the body, however it is not fully seated yet. The damper cap is pressed where it meets the nitrogen fill cylinder, where it is then removed and hung out of the way.
More shock oil is added to the damper and the cap assembly to prevent the seals from catching and tearing, causing a leak and failure. Robert squeezes a small squirt bottle of shock oil as he seats the damper cap on to the damper. Once it's below the damper body, a snap ring is installed and will retain the cap from here on. With that installed, the damper assembly is complete.
However, that is not the last step. For both hand assembled and custom dampers to mass-produced dampers, Eibach tests every unit before any of them leave their facility. They are checked for the proper damper curve for adjustable and non-adjustable versions. Mass produced dampers, like their PRO-DAMPERS, are done by automation with a Go/No Go light. If the damper doesn't meet the specified damping curve, a red light comes on and indicates the damper needs to be inspected and repaired, if possible. At random times, these dampers are also pulled for testing to ensure batches meet their million-mile warranty specifications.
Custom MULTI-PRO and PRO-STREET dampers are hand assembled, as shown earlier, then tested on a shock dyno by their specialist. These dampers are tested to make sure they meet their specifications, the specifications of the driver/team, and are tested at the lowest force setting and the highest. Once it receives a passing grade, the damper is then sent to the shop or customer.
We hope this helps remove some of the mystery about your dampers, either mass produced or even custom made, like the ones shown from Eibach. With a little bit better understanding, choosing the right damper for your ride should be just a little easier. If not, just call up your local dealer or a manufacturer like our friends in Corona. Your back or lap times will be glad you did.