Shock absorber types:
Below are the most commonly seen shock absorber types available for these vehicles. We will explain the differences and benefits of each. First, we would like to explain a few things:
Cavitation (shock fade): This is the one thing we are all fighting to prevent when it comes to shock absorbers. In short- Cavitation is a phenomenon that can occur within your shock absorber’s oil causing the formation of vapor bubbles (foam.) It is caused by rapid changes in pressure and excess heat buildup. Technically speaking Cavitation is the result of the dampening forces exceeding the nitrogen pressure inside the working chamber of the absorber, when this happens a vacuum is created and cavitation is the result. Once your shock cavitates, for all intents and purposes, you lose your dampening force and shear / damage the oil.
Nitrogen Usage: Nitrogen is used to pressurize the oil used in shock absorbers, to assist in the prevention of cavitation as well as create a rebound effect.
Twin Tube Shocks:
Twin tube shocks are commonly used by OEMs, (what comes on your car stock,) though are also seen in the aftermarket world as well. These generally work for on road use and slow going off road trails (rocks highly articulate trails, etc.) They are not ideal for higher speed uses on non-smooth surfaces like pavement, fast or aggressive drivers, or those who frequent gravel or wash boarded roads.
Twin tubes basic construction consists of 2 cylinders, an inner and outer tube. The inner tube is the working cylinder. This cylinder houses your piston, piston shaft, and valving. The two cylinders are connected at the bottom by way of a valve that allows oil to be pulled in and out of the inner and outer cylinders upon oscillation of the shock absorber. The outer tube houses excess oil and the nitrogen charge for nitrogen charged shocks. Since the inner tube is smaller, the working area and piston size are drastically smaller than those of a monotube style shock absorber. A couple brands do not use nitrogen pressure and instead place a piece of foam in the outer cylinder instead.
Twin tubes struggle with heat dissipation and cavitation. The biggest issue most twin tubes have is in their piston and valve design, lack of port flow and lack of pressure to overcome cavitation – where the shock absorber pulls a vacuum inside the oil and is a result of having such large pressure variations on either side of the piston and the sudden drop in pressure can create cavitation much easier. Cavitation basically instantly “shears” the oil and damages it. Also, it creates an air void behind the piston and your piston therefore passes back through this air void with no resistance and no shock valving and then slams into the oil which creates an uncontrolled and harsh ride. This is why they are not ideal for applications such as general overlanding and higher speed applications.
Now that said, there are two styles of twin tubes commonly seen. Nitrogen charged (most common,) and foam cell. Nitro Charged shocks offer some degree of cavitation prevention and rebound effect due to their low-pressure charges. Foam cell shocks attempt to “catch” the cavitation, as it occurs, within the foam cell. A piece of foam is placed within the outer tube in place of the nitrogen charge, also occupying space that would otherwise be used for additional oil capacity to help cool the absorber. Both versions of twin tubes suffer from the same cavitation fate, despite marketing claims. The lack of pressure in the Foam Filled shocks also creates a less controllable ride, as we now rely solely on the spring reaction rather than any controlled / tuned rebound effect created by the shock absorber.
Monotube shocks: Monotube shocks come in a variety of styles and sizes. Most common versions are classified as either Emulsion or IFP- with or without reservoirs.
An IFP is an internal floating piston; this is a secondary solid-state piston which acts as a barrier separating the shocks oil and nitrogen contents. By creating a division between the two substances we are able to limit cavitation from occurring drastically.
An emulsion shock is a monotube without an IFP. These absorbers allow the oil and nitrogen to mix, while these perform slightly better than a twin tube style shock, they still pose quite a risk to cavitation and should not be used in high stress or high heat generating applications. This style of monotube is uncommon but still can be found in base level king and radflo line ups.
A monotube shock, especially an IFP equipped shock is the ideal shock for all around use, overlanding, and performance applications. The basic construction consists of a singular outer tube, with the inner surface area of that tube being the main working surface. This style of absorber has a larger piston and thus is able to more efficiently process the oil and create better / more consistent damping forces. Since there is a singular tube, heat is able to easily dissipate though the working surface without having to transfer through multiple materials in order to exit the absorber. This style of absorber also boasts a larger oil capacity than its twin tube counterparts.
Monotubes tend to be higher pressure shocks with charges generally ranging from 135-200psi; with internal friction generation, vehicle weight, and application generally being the deciding factors in what charges are run. In units with lower friction designs, lower pressures can be run, while still keeping cavitation under control. In general, an IFP equipped monotube is your best defense against cavitation.
Reservoir models simply provide the manufacturer more real estate to utilize when designing the shock absorber. Options like larger shafts, the valves needed for compression adjuster mechanisms, or higher nitrogen pressures limit the amount of space available within the main shock absorber cylinder and by adding a reservoir they are able to increase what room they have to work with. Thus, allowing for additional features to be added without the loss of oil capacity that would otherwise be displaced.
Internal / External Bypass Shocks:
Internal and external bypass shocks give the manufacturer and user the ability to dial in the tuning of the shock based on the position of the piston inside the shock. Internal bypass shocks have a cavity somewhat similar to a twin tube between the working chamber and the outer wall of the absorber, but functions in the way a monotube shock does. The cavity houses bleed ports, so that fluid can surpass the piston freely in various locations along the working surface. External bypasses have “tubes” running down the outsides of the absorber that are tuned by the user, whereas internal bypass shocks are tuned by the manufacturer. External bypass shocks pose a problem with clearances in many applications as we simply do not have the room or ability to use them as oe replacement options on a production series truck a lot of the time.
Both styles of these shocks have ride zones and bump zones, or rather positions in the working tube where the piston needs to sit at your vehicle’s ride height, and positions where the should only be located upon full compression or extensions- meant to slow down the travel of the shock.
Since these zones are present, This style of shock poses a major issue in lift heights. They must be run at the intended ride height in order to function correctly. When set up correctly, the ride zone is usually valved to be very soft, as the piston travels into the bump zones it gets drastically firmer.
For example, It is common for people to buy say a TRD Pro 4Runner. Since they have paid for the premium fox internal bypass suspension, they do not want to replace said suspension immediately but they want to lift the vehicle. So they usually will install a collar style spacer lift, which raises the lift height of the vehicle by way of adding preload to the shock. Since the shock remains the same static length, when the ride height is increased, the down travel available from the ride height is decreased, as well as the position that the piston rides in is changed. This is why it is reported that the truck rides firmer, we have both less travel and the piston is no longer riding in the proper zone.
