Ask 100 different shock technicians to set up a suspension system and you might get 100 different answers. While the theory behind damper technology is actually pretty basic in nature, there are an infinite number of variables that can affect how a shock performs. Depending on your application, shocks serve a myriad of purposes.
In drag racing or streetcar applications, it’s all about grip. The shock’s primary function is to provide the proper amount of damping to keep the tire in contact with the pavement at all times. While sharp turns on a road course or big horsepower in drag racing make a tire want to break loose, a properly set up damper will provide maximum grip in any scenario. Not enough damping will cause the vehicle to bounce, while too much damping will cause the vehicle to ride like a covered wagon as it fights for steady contact with the surface.
Off-road shock absorbers are a bit more complex. Indeed, the shock provides the same function of putting the power to the ground but at the same time, its primary function is dissipating energy that is generated by the inputs of the road profile. All those harsh hits on an off-road course create a tremendous amount of energy, and the damper’s job is to absorb and dissipate that energy, creating a smoother overall ride.
Break It Down
Before we get too far into explaining what a shock does, let’s go over the basics of how a shock works. As a whole, a suspension system consists of three parts, a spring, a mass, and a damper. The spring suspends the mass (the vehicle) while the damper controls the oscillation (the motion) of the vehicle. That’s a very basic understanding, but at its core, that’s the big picture of what is going on in a suspension system.
As for the shock itself, all dampers have three basic parts – a body, a shaft, and a piston, though a reservoir is often added. What about the spring? Great question! Although the spring is an important piece of the suspension puzzle, it is not directly a part of the damper itself.
Often times you will see a shock on an off-road vehicle where the spring and damper are all one part. This is called a coil-over shock because the spring, or coil, is literally wrapped around the damper itself. In other applications, you will find the damper and spring are two individual pieces like on a leaf spring or coil carrier setup. The spring plays an imperative role in the performance of the shock but as for how the shock works, it’s not technically a part of the damper.
At its core, damper technology is about transferring energy via fluid flow. A shock is a hydraulic pressure relief valve, and it is the internal transferring of fluid within the shock’s piston and the individual valves that gives the damper its resistance force. The fluid transfer is the displacement of energy received from the “inputs,” or variances in terrain.
The shock body is the housing that keeps everything together and guides the piston and shaft through its travel, much like an engine block. The shaft is what connects the actual suspension component, like a control arm or trailing arm, to the chassis via the shock body cap and provides the desired suspension travel. Different lengths of shafts provide for more or less suspension travel, and the thickness of the shaft has a great impact on not only the durability of a shock, but the performance as well.
The piston is where the magic happens. As a shock moves up and down, fluid flows through the piston controlling how fast the shock moves through its travel. External adjustments can change how fast or slow the fluid flows within the shock but it is the piston and subsequent valving that controls how the fluid moves.
Lastly, the reservoir serves as a sort of an external tank for nitrogen gas but will also fill with fluid when the shock is compressed. As a gas, nitrogen is compressible whereas fluid is not, therefore the displaced fluid has to go somewhere when the shock compresses. Some shocks have internal (or inline) reservoirs that are housed inside the shock body while others have piggyback or completely remote reservoirs. Remote reservoirs are very common in off-road applications simply because the shocks are so large, they have to be mounted somewhere other than directly on the shock.
Contents Under Pressure
It’s important to remember that fluid is not compressible. If you were to fill a shock with only fluid and then vacuum out every molecule of air, you would essentially have a strut rod. The fluid would not allow for the shaft to ingest inside the body and it would be unable to compress because the fluid has nowhere to go. That’s why shocks are charged with nitrogen. Twin tube, emulsion, and through rod dampers vary slightly in design here, but for the purpose of this discussion, we’ll focus primarily on a nitrogen-charged shock.
Inside the reservoir is a nitrogen chamber. In some shocks, the chamber consists of a small bladder inside the reservoir, while in others, the chamber is separated by a small round disc called a floating piston. As the shock compresses, the main piston (attached to the end of the shaft) pushes fluid up and into the reservoir. The fluid is actually displaced not by the moving of the piston, but by the volume of the shock shaft as it’s ingested into the body.
This displaced fluid puts pressure on the floating piston, compressing the nitrogen gas behind it and providing an internal spring rate. If you’ve ever tried compressing a nitrogen charged shock without a spring on it, you probably noticed that the more you compress it, the stiffer it gets. That is because of the buildup of pressure due to the compressing of the nitrogen.
The compressing of the nitrogen slows the velocity of the shaft as it’s ingested into the body, but it’s in the piston where the shock actually gets its damping capabilities.
Like we said, although the spring isn’t directly a part of the damper, it’s a crucial part of the damping system. When a spring compresses, it absorbs all of the energy that it took to compress it. As it rebounds, or springs back, it transfers that energy into the damper via the piston and shim stack.
A shim stack consists of small discs of varying diameters and thicknesses that sit both above and below the piston. This combination of shims is commonly referred to as “valving,” and because of the endless combinations of various sizes and thicknesses of these shims, there are almost an infinite number of possible valving curves available for a shock. Most shock builders have a baseline curve that they know works best, or at least serves as a starting point when they valve a new shock based on the weight and spring rate of the vehicle.
As the shaft is ingested into the body at a given velocity, there is a buildup of pressure due to the compressing of the nitrogen gas. The piston acts as a sort of pressure relief valve so that when the pressure buildup from the compressing of the nitrogen, behind the floating piston, overcomes the force of the shims, it opens up the shim stack and allows the fluid to pass. The stiffer the shim stack, the more pressure it takes for that fluid to open up the shim stack and allow the fluid to pass.
The same thing happens when the shock rebounds. There is a rebound shim stack on the opposite side of the piston and as the spring rebounds, or pushes back against the mass, it forces fluid through the rebound ports in the piston. Again, the thickness of the shims or the “valving” determines how quickly the fluid is able to flow back through the rebound ports in the piston, unless there is an alternate path for the fluid to flow.
Fluid is always going to take the path of least resistance. Any time the fluid is able to find its way around the shim stack on the piston, you have what’s called “bleed.” There are a number of ways to accomplish this, but one of the most common is called an internal or external bypass. In an internal bypass shock, small ports in the body of the shock allow the fluid to bypass the piston and shim stack altogether.
As the shock begins to compress, the fluid finds these ports and flows around the piston instead of going through it. This usually happens in the first five inches of stroke and once the piston is past the ports, the shim stack takes over and controls the fluid flow.
An external bypass shock works much in the same way, except that the bleed ports are on the outside of the shock body. The key difference is that an external bypass shock allows for some adjustment to the amount of bleed where an internal bypass system is fixed.
Without these ports, if a shock is valved to require a higher rod pressure to open the compression stack of the piston, it may be optimal for absorbing really harsh hits, however it’s going to feel stiff and choppy in smaller, low speed terrain where the shaft moves at a low velocity. With a bypass shock, the ports are allowing the fluid to pass at lower speeds when the shaft is only beginning to ingest into the body. The shock allows the suspension to work freely over small bumps without any resistance, resulting in a more comfortable ride for the driver.
What About Adjustment?
Okay, thus far we’ve explained the basic inner workings of a standard, non-adjustable damper. The only “adjustment” to the damping capabilities of the shock were installed at the factory by way of the particular shim stack, so aside from disassembling the shock and changing the valving, the shock is rendered non-adjustable.
But since we know the terrain on and off-road course is anything but consistent, having the ability to adjust a shock for these variances in terrain is critical. Adjustments to the performance of the damper come primarily in two ways: by changing the rate at which the shock compresses or rebounds from any given input. Within that range of adjustment, you can have high and low-speed compression adjustment as well as rebound adjustment.
That simply means the velocity at which the shaft is moving at a particular point in the stroke. Low-speed adjustments typically refers to the first three inches per second of shaft speed, while high-speed is considered to be anything over three inches per second.
The actual adjustments can be accomplished in a number of ways. Some shocks are shaft adjustable, meaning the adjustment knob (or clicker) is found on the shaft itself and meters the fluid flowing directly through the piston. Other adjusters are found on the crossover portion of the shock or even on the reservoir itself and meter off the displaced fluid.
A shock featuring either shaft adjustable rebound or compression actually has a hollow shaft. Inside the shaft is a metering rod that presses on yet another valve assembly. Instead of a shim stack controlling the flow of fluid, a needle and jet control the amount of fluid allowed to pass through the bleed valve in the end of the shaft. The adjustment knob at the bottom of the shaft moves the metering rod inside the shaft up or down, increasing or decreasing the amount of fluid allowed to pass through the circuit.
On a compression setting, a check valve closes the rebound circuit as the shock rebounds, forcing the fluid to pass through the rebound shim stack on the main piston. Similarly, on a rebound setting, a check valve closes the compression circuit as the shaft ingests into the body, allowing fluid to pass through the jet only as the shock rebounds from its stroke.
On a shock that offers high and low-speed compression adjustment, you will typically find the adjustment on the body cap or reservoir. Inside the compression adjuster, there is a secondary shim stack and piston. Adding compression increases the load on the shim stack, making it harder for fluid to flow through the piston. This is the high-speed compression adjustment.
Low-speed compression is controlled by a needle jet much like we described on a shaft adjustable shock. Adding low-speed compression increases pressure on the needle, decreasing the amount of fluid allowed to flow through the low-speed circuit and diverting the fluid towards the high-speed circuit.
In taking the path of least resistance, the fluid will flow first through the low-speed compression adjuster, only relying on the high-speed compression adjustment as the velocity of the shaft increases due to harsher inputs.
Active and Semi-Active suspension
While advanced shock absorbers offer a wide range of adjustment, it’s still difficult to optimize the damper for maximum performance in varying terrain conditions. Most off-roaders will have a range of settings that they know work well in certain conditions, but aside from getting out of the vehicle to make adjustments, they are often left picking a sort of “middle of the road” setting that might be optimized for one terrain and simply work okay in another.
How great would it be to be able to make adjustments on the fly without ever leaving the seat of your vehicle? Believe it or not, that technology is available in NASCAR, IndyCar, and NHRA drag racing and is now starting to find its way into the off-road market as well.
A semi-active shock, like the JRide system from JRi Shocks, is one that allows the user to remotely adjust the vehicle’s shocks without ever leaving the driver’s seat. A proportional valve receives electronic current by one of four inputs: a dash mounted display, smart keypad, paddle shifters, and even a smartphone app.
The user can preprogram specific settings and save the presets for certain conditions like rocks, sand, high-speed whoops, or low-speed whoops. That way, with merely the push of a button, the shocks are optimized for that terrain type and they are able to achieve maximum performance at all times.
An active suspension system works much in the same way, except instead of receiving an input selected by the operator, sensors on the shocks themselves receive inputs from various sources on the shock such as displacement, applied load, and temperature, and actively make adjustments on the fly to maximize the performance. This is a very detailed and precise technology and to explore it in detail would require an article of its own.
Both technologies have been adapted from NASCAR and are being explored by a variety of companies. JRi Shocks was one of the first companies to bring the offering to market with the semi-active smartphone adjustable version and a few other companies are popping up with similar versions that will enable the user to remotely make adjustments to their shocks.
Back in the 1970s, FOX pioneered a technology that relied on the internal air pressure inside a shock to provide the damping characteristics. The key difference in an air shock versus a traditional nitrogen-charged shock is the size of the shaft.
In all shocks, the thickness of the shaft will have an effect on the damping characteristics because as it ingests inside the body, the volume of fluid that is displaced is a direct result of the diameter of the shaft. Therefore the diameter of the shaft has a direct effect on the pressure buildup inside the shock. A large diameter shaft will have a greater affect on internal pressure where a smaller diameter shaft will have less of an impact. The damping characteristics of the shock are controlled by increasing or decreasing the amount of nitrogen in the shock, thus affecting the internal pressure and the level of resistance on the shaft as it’s ingested.
Although it’s a relatively simple concept, the means by which a few small dampers are able to absorb and displace the amount of energy produced by a 5,000 pound Trophy Truck barreling through the desert is really quite astounding. There is so much going on inside each damper and so many variables that can affect a shock’s performance, it’s no wonder so many people are easily stumped by the suspension puzzle.
At its core, a damper is responsible to dispel energy that is stored and released by a spring following an input by the vehicle. The damper dissipates that energy in the form of heat via the resistance of fluid flow inside the piston. There is much more that could be said about heat management and distribution as well as the types of materials used in the construction of the shock, but our focus here was to describe the basic inner workings of a damper and provide a better explanation of exactly what is going on inside an off-road shock.
For more on shock technology, keep it locked on Off Road Xtreme and let us know what you want to learn about in our next shock article by leaving a comment below.
Race photography by Mike Ingalsbee