My recent post, debunking the ‘get a remote reservoir to prevent overheating your shocks’ myth, caused an influx of questions, asking to go over actual benefits of remote reservoir shocks. I will address that request by pointing out the differences between emulsion, IFP and remote reservoir shocks and providing a generic, broad recommendation at the end. Bypass shocks are not specifically mentioned here, because they are basically remote reservoir shocks. The added tubes of a bypass shock have nothing to do with the function of the reservoir.
Here are a few basics to cover first:
Why are shocks pressurized? In short: the pressure is necessary to prevent foaming and cavitation of the oil.
The rapid movement of the shock piston would create bubbles/foaming of the oil. A bubble in the path of the piston would cause it to lose all damping until it hits oil again. Pressurizing the oil reduces bubble size when gas gets mixed in with the oil and in turn reduces their effect on damping (in emulsion shocks). What is an emulsion you ask? If two substances mix but don’t dissolve, the resulting mixture is called an emulsion. In an emulsion shock, the gas is mixed into the oil as tiny bubbles.
Cavitation is, when the pressure of a liquid drops so far (below its vapor pressure) that bubbles start to form. High temperatures increase the likelihood of cavitation in low-pressure areas even more. The moving shock piston creates high pressure in front, and low pressure behind the piston. The low-pressure area right behind the moving piston would be where cavitation happens (i.e. bubbles forming). The pressure in the shock (on the oil) increases the pressure also behind the piston and keeps this low-pressure area above the oil’s vapor pressure.
Why Nitrogen? Normal air is a mixture of different gases (21% oxygen, 78% Co2, 1% others) and even water (moisture/humidity). Some of that content causes chemical reactions when it gets in contact with other substances (shock oil or the metal of shock parts). To avoid that, the shock gets filled with an inert gas. An inert gas does not chemically react with other substances. Off the short list of inert gasses, Nitrogen is cheap, non-toxic and easy to purify (also involves removing moisture from the gas).
Fun fact: did you know that your puffy chips package is actually not filled with air, but nitrogen?
Why reservoirs? The shock shaft displaces oil as it gets pushed into the shock body. This displaced oil has to go somewhere, or the shock would just ‘bind’ and not allow for any travel. So, there needs to be extra volume to take up oil and release oil as needed. In an emulsion shock, the oil fill is not all the way to the top. An appropriately sized air-gap is left and filled with nitrogen. In IFP and reservoir shocks, an additional volume (think expansion tank) is used.
With that understood, now let’s look at some pics and actual shock designs:
The emulsion shock stores oil and nitrogen in the same cylinder (shock body). That allows the nitrogen and oil to mix when the shock gets cycled. As a result, the shock gets softer the harder you run it (a lot of shock fade). This variability makes it impossible to dial in an emulsion shock for consistent performance. Proper mounting orientation is body up, shaft down (as pictured). Emulsion shocks work well for non-performance oriented setups.
IFP stands for Internal Floating Piston. The nitrogen is in the same cylinder with the oil but separated from the oil by a piston. The oil cannot mix with nitrogen and therefore provides consistent damping performance. An IFP shock has a reduced stroke when compared to an emulsion or reservoir shock of the same body length. Additional room for nitrogen (over an emulsion shock) reduces the operating pressure. That is better for seals, however, real world lifespan differences are probably not noticeable. The nitrogen pressure can be adjusted to increase or decrease the shocks ‘spring rate’ a little bit (how hard it is pushing out by itself). The shock can be mounted in any position. Cost goes up compared to emulsion shocks, because of the extra parts needed.
The reservoir can be mounted to the shock (piggyback) or can be loose and connected with a hose (allows for remote install).
The floating piston and nitrogen charge are removed from the main body and housed in their own cylinder. This design regains the full body length for piston travel without any downsides compared to IFP shocks. The shock can be mounted in any position. Obviously, manufacturing cost goes up because of even more parts involved.
Honorable Mention: Twintube Shocks
With all that said, I want to mention, that at the bottom of the price range, we also have twintube shocks. All IFP and reservoir shocks are monotube shocks. TwinTube shocks can be emulsion shocks or can be build similar to an IFP design. The illustration shows the difference between single tube and twin tube design.
Twin-tube Shocks have a double wall construction, with the outer tube acting like a reservoir. The inner tube is filled with oil, the outer tube contains low pressure gas. In most twin-tube shocks, the gas can mix with oil and reduce damping over time. However, some twin-tube shocks have the gas contained in a bladder, which reduces shock fade (like a low-cost IFP). The piston size is smaller and therefore limited in the amount of damping it can provide. Another drawback of this design is that most of the oil is in the inner tube and cannot dissipate heat to the surrounding air. The shock has to be mounted with the body down and the shaft up.
The benefits of this design, that give it an honorable mention, are the low price and durability. Smashing the outer tube of a twin-tube shock does not affect its function, whereas a monotube shock would seize immediately. It is the cheapest of all shocks.
A twin tube shock is a good option for smaller vehicles that are used on-road, for low speed trails, or rock crawling.
Considering the discussed design features and limitations, we can conclude that a properly valved reservoir shock is a great fit for all applications. However, since we all operate on a limited budget, the situation gets more complicated. Let's build this from the bottom up.
Driving on the road, riding slow trails and climbing over rocks is not very challenging for a shock. An emulsion single tube shock will do fine, even on longer trips.
If you spend a long time on rough roads you probably should go with an IFP shock.
If your suspension setup allows for more travel than available IFP shocks can provide, and you really need that extra flex, you should upgrade to a reservoir shock.
If you feel that your radical driving is overpowering or overheating your IFP shocks, getting the same shock again but with a reservoir will not help. Only a larger shock will resolve your issues. Larger internals allow the larger shocks to be valved to higher damping force and to maintain lower temperatures (assuming the same workload).
A 2.5 shock is not overkill! It can be exactly what you need if you run a fully loaded rig up and down rough mountain passes all day. Some might argue that if you are not into off-road racing, you don’t need that big of a shock. Well, a 2.5 shock is only 1/8 the shock that a race shock setup is, so you are still FAR away from real racing equipment. It comes down to what you do with your vehicle, not the comparison to others.
This turned into a lot longer post than I anticipated. Let me know your thoughts and don’t hesitate to contact me if you have questions or need your suspension set up. Thanks for reading!