Jeep Shock Tech
Shock and Awe – Damping Your Jeep Suspension
By Justin Banner
In many Jeep lift kits, you are given the option of shocks without much else taken into consideration. However, what do they take into consideration and why? What’s inside my shock?
There are two times a typical Jeep owner thinks about their shocks: when they’re buying a new lift kit and when they “blow out” and stop working. However, when you stop to think about it, your shocks are doing a lot on your rig. They must be “stiff” enough for street driving yet still “soft” enough for off-road driving. How is this achieved?
Tubes and Internals
We should first talk about what’s inside your shocks, as most of them are the same except for specialty shocks. You’ll first hear a shock described as either monotube or twin-tube. This description is in reference to the body and the area in which the shock oil resides.
A monotube shock is a shock made with a single tube that contains the piston, shock oil, gas piston, and gas that prevents aeration (foaming) inside it. There are many advantages and disadvantages to this design – and we’ll talk about them in a moment – but most racing shocks in existence in both on-road and off are typically a monotube design. Not all, but there is a strong majority.
A twin-tube shock contains an outer body and inside of it is another body that contains the piston and most of the oil. The outer body contains more oil and the gas used to prevent aeration. Between the inner and outer body is a valve that allows shock oil to flow into and out of the inner body. So, the outer body acts as an oil reservoir. That valve also determines which way the shock will face.
Advantages and Disadvantages Between Them
By having the outer body act as the reservoir, you might think this is a huge advantage for the twin-tube shock. In a few ways, it is. The biggest problem, though, is that makes it use a smaller piston package versus a monotube of the same size outer diameter. That inter-body valve – known as a base valve – mentioned earlier also determines which way the shock must be oriented on your Jeep. Since it flows fluid between the bodies, it also determines the damping rate in the direction it’s in.
If the base valve is designed to be at the body side, then it will work only in bump (also called compression). If it is designed to be at the axle side, then it will only work in droop (also called rebound). A monotube shock, however, has its all its valving done on the piston by a stack of spring washers. This means its rate is equal in both bump and droop and can be used in any direction so long as it’s traveling in a mostly linear direction. A twin-tube shock will also have lower gas pressure because of its base valve.
Another issue with using a twin-tube is that it will have a reduced amount of oil capacity. Despite having an “internal reservoir,” the capacity is lower because the capacity of that reservoir is mostly taken up by the inner shock tube. This can be alleviated by attaching an external reservoir that works with both tubes and the base valve. A monotube shock will have more fluid capacity and can also still have an external reservoir for additional capacity and damping adjustments as well as shock oil cooling.
The twin-tube design also limits the cooling ability of the shock. While the outer tube will cool the shock oil in it, the inner tube’s oil will continue to heat up more and more without being able to shed that heat optimally. This heating will affect the oil’s viscosity and make it “thinner” and impact the absorbing rate of the shock. The monotube’s shock oil will have a direct interaction with the shock body as it flows through the air. This cools the oil much more effectively and allows for a more consistent absorbing rate. If there is an external reservoir, the oil can be cooled that much more. The one leading factor that twin-tube has over the monotube is cost. It is much easier and cheaper to produce a twin-tube shock, despite using more parts. The parts of the twin-tube are much smaller in comparison and can be far simpler and easier to produce.
Both modern versions of the twin and monotube shocks will share some components. These will be the piston, piston rod, the shim stacks between them, shock oil, and gas. So, let’s talk about those now but for reference, the direction of the shock is with its body pointing up and the piston rod pointing down.
The piston rod is what the piston attaches to. Ok, that’s a bit simplistic but it is how the motion of the suspension is transferred and dampened. In some cases, the rod will be hollow with a small needle rod extending down to a hole under the piston. This needle valve changes the size of the orifice to change the damping rate, bypassing the piston shim stack until enough pressure prevents it from flowing freely through that bypass. This is not a bypass shock, however.
The piston of the shock is a thick disc with ports milled through it. These ports are scalloped out to extend beyond the shim stack in the direction its intended to dampen, thereby forcing fluid to open the shim stack. Without those ports, the piston would hydraulically lock and not work. There are sometimes other features found in the piston to relieve it of hydraulically locking, despite those ports.
You’ll feel this as that harsh and sudden jolt when you make impact with a pothole or large bump in shocks without this design feature. The piston can not flow freely enough through the fluid as it is a non-compressible fluid. If it was, it would create a spring force and is why an air spring works the way it does. Usually, it’s just a simple, straight hole of a certain size to prevent it from locking.
This is the fluid in which your piston flows through and is part of what creates its damping force. You can use any fluid to create something that dampens the movement of the piston, but there are many things you must take into consideration like compressibility (or the lack of it), lubrication, and its viscosity.
The fluid is like engine or hydraulic oil in that it has a specific weight. This weight is the viscosity of the fluid. A lighter fluid will flow easier than a heavier fluid. This is only part of the damping rate as the others are the piston and its design and its shim stack. Much like motor oil, it does prevent premature wear on the components that slide against each other as it is a lubricant.
Piston Shim Stack
The shim stack is a stack of multiple pieces of thin spring metal that open, allowing shock oil to flow through the ports of the piston. In some cases, these shims will feature openings or designs to prevent hydraulic locking rather than being on the piston. Sometimes, you can find it on both. By adding more preload to the shims, you increase the amount of fluid force required to open the shims. Adding more shims or using thicker shims can do the same thing, so there is a combination of all three of those that manufacturers will use to achieve the damping rate they or their customer wants.
Finally, the gas serves an important role in keeping the oil as consistent as possible. As the fluid moves through the piston and valving, it not only heats up but also changes the amount of fluid between the piston sides. Doing this creates a low-pressure and high-pressure side. The fluid flowing into the shim stack is the high-pressure side while the oil on the other side is low-pressure. This pressure differential causes the oil to boil at a lower temperature and pull gasses out of it, causing aeration.
The gas is used to keep the pressure differential as close to the same on either side of the piston as possible by filling for the reduced oil. In a twin-tube, the oil and gas are not separated. The monotube shock has a gas piston to separate the oil from the gas but also allows for more gas pressure.
This puts much more pressure on the oil. Aeration is a problem as it creates an area of oil that essentially has lower viscosity. This creates “shock fade,” where the shock suddenly doesn’t have the same damping rate as it did before. Usually, as the shock oil cools, the fade will subside, and the damping rate returns to normal.
There are shocks that not only don’t have any gas in them known as aeration shocks. As customer demands continue to skew towards higher performance over cheaper price, we don’t see them as much as we used to. Due to their inexpensive costs, they do still exist. Another type doesn’t sperate the nitrogen gas from the shock oil called the emulsion shock. Some of the nitrogen gas will emulsify into the oil but later comes out as the oil degasifies.
Coilover Shocks with Dual Springs
The only difference in a shock and a coilover is that the spring is located over the body – thus the reason behind the word “coilover.” These shocks typically have a threaded body where an adjustable spring perch. This will not only allow height changes, but also allow chassis weight balancing.
In the off-road world, you’ll see coilovers with two springs on them. Each spring will be a different spring rate, but the total will be the average of their rates multiplied over their rates added together. Something like this Effective Spring Rate = Primary Rate x Secondary Rate / Primary Rate + Secondary Rate. So, a 300-lb/in primary spring with a 200-lb/in spring will be 300*200/300+200 = 60,000/500 = 120-lbs/in effective rate. Sorry for the math, but hopefully this helps you make sense of the idea.
There are three different types of setups for dual spring setups. Progressive is simple, and after a certain point a spring stopper will be hit, and the primary rate will take over at that point. This is the more typical design choice in off-road.
A digressive setup works similarly, but the stopper prevents the spring collar from allowing the secondary spring to extend past that point. The initial rate will start with the primary spring. Once over the secondary spring’s rate at its preload, it will come into play and reduce the spring rate.
Finally, there is the combination of both progressive and digressive. In the initial travel, it acts just like a digressive coilover spring setup. Once the secondary spring rate is overcome, it’s the average above. Once it hits a second locking ring, it becomes the latter half of the progressive spring with the full rate of the primary spring taking over.
Each version has its own advantages and disadvantages and will come down to the driver and how that person likes the spring rates to work as the suspension travels. Progressive is the most common in off-road, but you’ll potentially see more than that depending on who’s driving and even which event they are driving.
Of course, there are shocks designed for a single purpose. The two most common among Jeepers are the air shock and the bypass shock.
Air shocks exist in competition and street versions of air shocks. While they are achieving similar ideas, a competition air shock is far different than a street air shock. Either way, the biggest reason someone uses an air shock is that it provides some spring force in the gas you use. It will also allow you to raise a vehicle up as you add air, so most street air shocks are used for towing and load leveling. The air is also emulsified into the oil slightly as there usually isn’t a gas piston between them.
For competition, however, you’ll see them in rock crawling. These are very light-weight, as the piston shaft is sometimes hollow, and the lack of metal springs (because the gas or air is your spring in this instance) reduces the weight of the rig. More of the engine’s power can be used to scale insane walls and rock features. The problem is that gas is affected by temperature. As the gas heats up, it expands and can increase the spring rate. Cooled down, it contracts and reduces that rate. That’s why you won’t find a distance racing rig using them (though Ultra4 outright bans them in their competition).
What you will find in most of those racing rigs and the ultimate mall crawler is the bypass shock. While it’s overkill for the street truck, anything that sees dedicated racing use will have a bypass shock. It’s easily spotted from a normal shock by its assortment of tubes on the body. Internally, it’s a monotube shock with a shim stack on the piston, gas, and gas separator piston, just larger in many cases. Those tubes on the body serve a purpose, however, and that is to make this shock position dependent.
Bypasses are usually custom made and will place those external tubes where the customer feels they need more compression or rebound damping. Compression tubes will face down (towards the axle or suspension arm) while rebound tubes are opposite (facing towards the vehicle body). As the piston travels, fluid is pushed into these tubes until they are passed. This changes the damping rate to either focus on what the piston and shim stack are designed for or the rest of the tubes (if there are any) in the direction of travel. These can be further modified by changing the how much preload you add to the bypass tube piston before it opens to its reservoir.
A European Tradition
Something you’ll notice on a lot of the race trucks that compete in the Dakar Rally and Rally Raid competition is the use of multiple shocks and springs. The idea is still very similar to what open desert and Ultra4 racers are trying to achieve: spread the work load of the suspension. We do this with dual spring coilovers and a bypass, they use two shocks and springs on each axle. However, we are starting to see some “Americanization” in these series, too, where they will use a dual spring coilover and a bypass.
So, the shocks of your Jeep have what sounds like simple job. Dampen the movement of the suspension and coil springs. The reality, as you now see, isn’t as simple as dumping a fluid into a tube with a piston and rod. There is so much going into the design and goal of how it’s dampened.
Shock manufacturers constantly test and innovate to create new and better ways to keep your wheels under control. While they try to plan their designs to match the situations they think you’ll encounter, sometimes you need to go above just buying a preformed shock and get into custom tuning.
If you’re ready for new shocks on your Jeep, we have some of the best from Fox Shocks, Rancho, Fabtech’s Dirt Logic, and more. Check out our selection but always keep our guides in mind while you’re shopping so you pick the best for your rig.