Story By Jake Headlee and Stuart Bourdon
Shock absorbers have been around for nearly a century. The low speeds attained by early motor vehicles didn’t require much from those primitive damping devices to control the motion of the suspensions, and they were certainly crude by today’s standards.
As the speeds of modern vehicles increased and the types of terrain that vehicles were expected to successfully navigate became more demanding, the needs for improved ride quality, vehicle stability and handling increased. This pushed the boundaries of shock absorber research and development, creating what we see today — an extremely high level of shock technology and design.
The basic shock absorber design consists of a body, tube or can filled with oil. A rod and piston, with a specific set of holes in valves that are pre-determined prior to production, cycle up and down through the tube. The valving is designed with a particular use and vehicle weight in mind, allowing for a certain amount of oil to pass through the valves.
As the oil (or air in some applications) passes through the valves, it damps (or in other words, places resistance against), the piston, thereby slowing the piston as it travels through the tube. This damping also controls the action between the two chassis parts (normally an axle at the bottom and a framerail of the vehicle) that are either pushing down on or pulling up on the two opposing ends of the shock.
Twin- Or Mono-Tube
A basic twin-tube shock has two cylinders, as the name implies. A pressure tube resides within a reservoir tube. The inner pressure tube houses the rod, piston, valving and fluid that controls most of the compression (upward motion) and rebound (downward motion), whereas the outer acts as a reservoir for extra fluid. A stationary compression valve separates the two and controls most of the compression.
Mono-tube shocks simplify the damping process. The outer tube is eliminated, allowing the pressure tube to be the same as a twin-tube shock in the same overall package size. A piston with specific valving is mounted to a rod that controls both the compression and rebound of the cycle.
Gas charged shocks eventually came to market, drastically improving the technology. In a twin-tube, the pressurized nitrogen is added to the reserve section and serves to minimize aeration of the oil by keeping a more consistent pressure inside the tubes. Aeration, or the formation of bubbles in the oil, can make the oil in the shock less effective as a damping agent.
Compression (Jounce) – A motion during which the tire travels upward, relative to the vehicle, compressing the spring and shock absorber.
Rebound – A motion during which the tire travels downward relative to the vehicle, and the spring and shock absorber extend.
Damped, Dampened – A force or action opposing a vibrating motion to reduce the amount of vibration.
Spring Rate – The change of load on a spring per unit of deflection.
Pressure Tube – The sealed tube in which the oil is under pressure from the gas (usually nitrogen) that the rod and valved piston stroke through during compression and rebound.
Cavitation – The oil used in hydraulic shocks contains approximately 10-percent air. Under load, the air and oil molecules separate (foaming), resulting in a noticeable drop in the damping force.
Valving – The damping values ride engineers have selected through the use of certain springs, shims, and orifices in the piston to control the flow of fluid through the piston as it moves through the shock body to achieve a balance of optimal ride characteristics.
Velocity Sensitive – All modern shock absorbers are velocity sensitive hydraulic damping devices, meaning the faster the suspension moves, the more resistance the shock absorber provides.
Position Sensitive – Takes into account the position of the piston within the pressure tube though precision tapered grooves in the pressure tube. The length, depth, and taper of these grooves are tailored to the application.
Bypass Shocks – Bypass shocks divert oil through bypasses external to the normal valving shims by running extremely stiff valving; the main advantage being multi-stage damping.
A divider piston is added to separate the oil from the nitrogen that serves the same purpose as its counterpart, eliminating aeration of the oil and increasing the effectiveness of the oil. Moving through the cycle, oil passes through the piston and valving. The oil is compressed as it’s forced through the valving, which in turn pushes the divider piston, compressing the gas.
Mono-tube shocks have some distinct advantages over twin tubes. Ride Engineer Shane Casad from Bilstein told us, “the difference between twin-tube and mono-tube performance can be significant. A twin-tube will cavitate (become aerated) due to oil not being placed under direct pressure.”
Every shock Bilstein makes is built on the monotube shock design. The 5100 Lifted Truck/SUV Series are designed specifically for lifted trucks, Jeeps or SUVs to help improve ride performance, with the right tuning and length for your specific lift.
Chris Gauss, National Sales and Marketing Manager, Performance, Tenneco (Rancho) added, “the high-pressure charge and internal valving allows a mono-tube to have a faster response to inputs from the road and trail, and they are particularly useful when you’ve increased unsprung weight through larger wheels and tires.”
“A mono-tube’s only drawback is that the pressure tube is the outermost layer of the shock, exposing it to the hazards of off-road driving.” Gauss continues, “that’s why many competition guys use durable shock wraps and covers.”
Some offer a variation on the theme, such as internal bypass systems. Bypass shocks blend the advantages of velocity sensitive and position sensitive valving-it’s sort of like having two shocks in one. Fox and King offer internal bypass shocks.
Micheal Leighton, King Shock Marketing Director, describes its Monotube Internal Bypass (IBP) system, a bypass shock in a sleek and compact design. “Instead of a twin-tube design that retains heat, we use a full-diameter cylinder and piston with full-sized valving. Fluid pushes through the piston and into a secondary valve stack. From the secondary valve stack, the fluid flows into the hollow shock shaft,” said
“So the shock is more position-sensitive through its stroke, a tapered metering rod is built into the top cap. Under compression, the shaft pushes up into the pin and onto the tapered rod. That pin is surrounded by fluid at full compression, restricting flow through the secondary valve stack, diverting all flow to the primary piston at complete closure. Those last few inches of compression create a suspension-arresting hydraulic bump zone,” continued Leighton.
Size Does Matter
Mark Mathews, ProComp marketing manager also commented on mono-tube benefits of high-pressure gas, and brought up the larger, single piston associated with mono-tube shocks. “A typical twin-tube has a low-pressure charge and smaller diameter piston compared to mono-tube. The low-pressure charge is not isolated from the hydraulic fluid, which allows cavitation to occur as the shock cycles.”
The mono-tube has a high-pressure charge separated from the fluid by a floating puck. The action of the large internal piston within the fluid is isolated, which eliminates performance robbing cavitation, and as a result, the mono-tube performs more consistently even under extreme conditions.”
Brian Godfrey, marketing manager, FOX Shox also commented about the benefit of size, saying, “A mono-tube shock gives you more piston surface area, expanding the damping capacities of the shock.”
The damped force of a vehicle’s suspension controlled by the shock absorber is then dissipated by being converted into heat energy through the fluid inside the tube of the shock.
Mono-tubes run cooler and dissipate heat quicker (without the use of reservoirs) since there is no barrier between the working pressure tube and the outside air.
Because of the single wall design of the mono-tube shock, it’s able to radiate heat more efficiently, which also improves performance. – Mark Matthews, Pro Comp
“The twin-tube design will retain heat. The inner tube, where the piston is doing its work and creating heat, is not exposed to the outside cooler temperatures. It’s insulated by the outer tube and more fluid,” explained Brian Godfrey.
Heat is one of the shock absorber’s greatest enemies, and that is why you see so many modern shock absorbers are offered with reservoirs. The more fluid, the more heat dissipation.
Multiple reservoirs mean even more fluid, and they have become common on some of the high-end racing shocks, and even a few of the shocks sold for trucks and 4x4s used on trails. These days, there are even reservoirs with huge heat-dissipating fins on them.
In addition to universal race shocks, FOX has an expansive line in its 2.0 Factory Series. These are also available as a reservoir shock. The 2.0 Factory Series are a large group of shocks featuring many of the same characteristics of the full race shocks, packaged into a direct fit form to instantly set your vehicle apart from the rest without the need for major fabrication.
For the off-roader that wants or needs the next step up, there is the External Cooling System (ECS), which during development was was code named “Cactus Cooler.” The ECS is an option on all new and existing 3.0, 3.5 and 4.4 FOX bypass shocks. It is said to drop fluid temperatures inside the shock up to 150 degrees without pumps or fans using the motion of the shock piston as it cycles the fluid the a billet aluminum and stainless steel heat sink.
Bilstein offers the 7100, a remote reservoir shock that is for stock or custom installations. The 7100 series shocks feature added benefits of the bigger race shocks in a more compact package, for when space is tight or budgets are a constraint, and the 7100 series are adjustable and 100-percent rebuildable.
Adjustable And Non-Adjustable Shocks
The Bilstein 9100 series meant for off-road racing kicks it up a few notches. The rapid-heat dissipating shock body is built from a single piece of extruded 6061 aluminum. Incremental bypass-flow metering valves allow for extremely accurate flow adjustment via internal detents that provide a click to each of the nine settings.
Get comfortable with existing settings, then go from there. If any mods are made to the vehicle, then consider readjusting. – Chris Gauss, Rancho
Shane Casad puts it simply, “Externally adjustable shocks are a better choice in extreme off-roading,” although he freely admits mono-tube adjustable shocks can be pricey. “Because if you encounter different terrains, you can adjust on the fly to adapt.”
“Our application-specific non-adjustable shocks are designed for 95-percent of situations a vehicle should encounter, but we do offer a re-valve service and can talk to the owner. What we need to know is how it’s performing, determine a baseline; do you need it softer or stiffer?” adds Casad.
Rancho’s Gauss offered, “Non-adjustable shocks feature fixed valving-often firmer than the OE unit (it’s designed to replace). When a vehicle has added aftermarket products that increase its overall weight or when a vehicle is used to tow or haul, adjustable shocks can provide better performance and a more comfortable ride.
As a comparison, non-adjustable shocks typically run around the RS9000XL’s “5” setting. Through the RS9000XL, the driver can go firmer or softer than the vehicle’s current ride profile … adjusting the valving of the shock up to a 400 percent difference between the 1 and 9 setting.”
Mathews of ProComp said, “ProComp’s MX6 mono-tube adjustable shocks offer a range of settings (using a knob on the bottom of the shock body) that allows the owner to select what is best suited for different terrain or vehicle loads whenever needed.”
“The MX6 adjustable shocks are also available as a reservoir model for those those that are intending to drive in extreme terrain and want additional cooling capacity.
Mathews added, “Pro Runner SS shocks for late-model coil-over IFS trucks can be adjusted using the SS (stainless steel) collar to adjust the shock’s coil and raise the front end as much as 2-3/4 inches in order to fit up to 35-inch tires (depending on application).”
FOX offers external compression adjustments through its CD Adjuster, an upgrade to any of its 2.0 Smooth Body and Factory Series reservoir shocks with 5/8-inch shafts. It allows tuning through eight different levels at the tips of your fingers with a simple twist knob.
FOX also offers its QAB optional adjusters for the compression and rebound reservoirs on its 2.0, 2.5 and 3.0 External Bypass Shocks that are intended for extreme off-road applications.
Tuning Is A Science
We talked with Joe Moore, FOX Offroad Division Race Department Manager, about fine-tuning adjustable shocks. Moore explained that, “the easiest place to start is compression. The shock needs to be stiff enough to resist bottoming out on the roughest terrain and not too stiff to hinder ride quality over smaller bumps.
Shock tuning is considered by most to be black magic. In reality, it is very much a science. – Joe Moore, FOX Shox
“Using a travel indicator like a small zip tie around the shock shaft is very helpful to determine if the shock is using its full stroke or not. If the travel indicator verifies a bottom-out incident, then the shock should be adjusted stiffer. If the indicator verifies the shock is not using the full stroke on the worst bump encountered, then the shock should be adjusted softer.
“Tuning the rebound stroke is much more difficult. Rebound controls the rate of shock extension and is dependent upon spring force. The rebound damping needs to be stiff enough to slowly release the built-up spring force after compression. The rebound damping also needs to be soft enough to keep the tire on the ground and prevent the suspension from packing up over repetitious bumps.
“Once you throw shock angles, dual rate springs, position-sensitive (bypass) shocks into the equation, the complexity increases exponentially,” Moore added.
Different styles of vehicles and terrain call for individual attention, and I prefer to take specific, one-on-one approach with a customer. There are a series of common questions I ask during preliminary discussions.”
What is the current valving in the shocks?” If the vehicle is coil-sprung, what is the length and rate of the coil springs?” How much pre-load is in the springs? Pre-load is the difference between free length of springs and tensioned length when on the vehicle at full droop.”
Simple Tuning Questions
Instead of asking about compression and rebound damping, Joe Moore of Fox likes to focus on simple descriptions such as:
- It bottoms out on big bumps/landings
- The front end dives under braking/cornering
- The rear squats under acceleration
- Too much body roll
- Too stiff over small bumps
- Too stiff over big bumps/not using full stroke
- Not enough body roll or weight transfer
How much does the suspension sag at ride height? For this, measure shock shaft showing at full droop and again at ride height (the percentage difference is what we are after – different styles of vehicles and terrain call for different amounts of sag).”
Then, Moore said, “I ask for specific examples of handling characteristics they don’t like, and to pick one they are most unhappy with. It’s always best to make single adjustments at a time to see or feel a direct effect of that adjustment.”
King Off Road Racing Shocks offers a variety of shock absorbers ranging from those in its OEM Performance Series that are intended as direct bolt-ons that will up the ante, both on and off-road of your rig, to the extreme end of King’s offerings, it’s 4.5 Bypass Shock from its Pure Race series.
The Pure Race Series shocks are intended for off-road racing and provide the ultimate in performance and adjustability. The shock’s are custom-made for unique applications to offer multiple options when determining the quantity and location of bypass tubes.
We also spoke with Mike Eads, King Shocks, VP Operations/Sales, about adjusting shocks. “All of our shocks are adjustable, some will just have to be returned to the factory to be opened up for valving to be changed. We also want to know if he has made any changes to the vehicle, added weight, added accessories, etc., that changes everything about ride and handling.”
“When it comes to setting up our coils, the rule-of-thumb is you want somewhere between 1- to 3-inches of pre-load on the spring … not what’s showing on the shock cylinder, but actual pre-load.
“And for the Jeep market, typically you want about 50/50 compression and droop; about 40-percent droop in the truck market.” said Eads.
What’s Right For You
How do you know which is the right choice? Selecting the right shock for your vehicle can be a science. The best route is to determine your intended use and budget, then and proceed from there.
A mono-tube is ideal for off-road use, the shocks are filled with high-pressure gas, as well as oil, which ensures that no air bubbles (cavitation) are created. – Shane Casad, Bilstein Ride Engineer
Each vehicle will have different reactions to different technologies. You will need to ask yourself a few questions. Are you looking for a direct bolt in, or a custom shock? Many of these manufacturers have a wide range of options to fit your performance level needs.
Talk to the manufacturers explicitly about your vehicle, any modifications you have made, or intend to make, how you use it, exactly what types of terrain you plan on driving through and how often.
Will you be upgrading your vehicle again soon? You may need to look at something that will fill your needs now and work keep you happy well into your next step. How much are you ready to spend on shocks?
All of this will help them help you determine the best shock for your real-world needs.
Stay tuned to Off Road Xtreme for deeper looks into shock absorbers, and off-road tech in general, in future editions of Off Road Xtreme College!