Here you can see a cutaway...
Here you can see a cutaway of a Bilstein 5150 Series remote-reservoir shock. This particular unit has a fixed-reservoir (or piggyback) design and was the first production monotube high gas pressure, fixed CrossFlow reservoir shock designed specifically for lifted and custom applications. The CrossFlow fixed-reservoir design expands oil capacity for enhanced cooling and it also allows for increased rod travel. This high-pressure (360 psi) nitrogen shock's reservoir features a "floating" dividing piston that eliminates the possibility of oil foaming and performance loss from shock fade. The monotube design provides superior tube strength while maximizing heat dissipation and shock life. It has a 46mm working piston and self-adjusting deflective disc valving (visible in the monotube), which allows for maximum vehicle body control and superior performance. The 5150 can be valved for single- or multiple-shock applications; it is available in 6.5 to 14 inches of travel and has a platinum powdercoat finish.
Shock absorbers control unwanted motion through a process commonly called dampening (hence, their alias "dampeners"). They turn the kinetic energy of suspension movement into heat energy, which is then dissipated through the shock body. Without shocks, your rig's springs would bounce up and down after hitting a bump (taking your rig with them) until all of the energy put into the springs by the bump was used up. So a shock's job sounds simple, right?
Well, here's the thing. While fine for street use, a standard monotube or twin-tube shock has drawbacks. The biggest disadvantage is that when asked to endure sustained rough terrain, they can overheat. This is called aeration, and it's what takes place when the oil in the shock foams due to the heat generated by the shock, turning the previously mentioned kinetic energy of suspension movement into heat energy. The result is that the shock loses its ability to dampen. This allows that crazy spring motion to run unabated. This not only creates a bouncy ride but also scary handling.
Remote-reservoir shocks can help solve this problem. They're designed with an external reservoir that increases fluid capacity. This increase in capacity helps lower fluid temperature, thus offering the ability to blast over rough terrain without experiencing shock fade due to overheating.
There are two kinds of remote-reservoir shocks. One type has the reservoir permanently mounted to the shock body. These are often called "fixed reservoir" or "piggyback" shocks. The other type leaves the reservoir loose so you can mount it wherever you like, limited only by the length of the connecting hose.
But that's not all. There are many other benefits to remote-reservoir shocks besides cool operation. For instance, many are valved for specific vehicle applications, unlike most regular shocks that use generic valving. Many are easily rebuildable, so if a part fails, you can fix it yourself. Many are custom-valvable so you can fine-tune the compression and rebound to suit your rig and driving style. Further, many offer longer travel than their non-remote-reservoir counterparts. For those of you with custom suspensions, there are numerous manufacturers that offer a slew of universal remote-reservoir shocks in a variety of sizes and travel lengths with a number of mounting options.
So now you know how remote-reservoir shocks work and why you need 'em. Now you can read on to learn about some of the fascinating remote-reservoir offerings available from the shock aftermarket.
 The Bilstein 7100 Series high-pressure...  The Bilstein 7100 Series high-pressure remote-reservoir shock is the next progression from the 5150. Like the 5150 it uses the deflective disc valving system for superior performance and control, but it also offers a 2-inch-diameter nickel-plated shock body, available multiple valvings, independent rebound and compression tuning, high-flow piston (to reduce harshness), and 1/2-inch Heim joints. It is available in 5- to 14-inch-travel lengths and is easily customer rebuildable. |  The newest entry into the...  The newest entry into the remote-reservoir world from Pro Comp is the MX2.0R. The big news is that it's rebuildable with simple handtools and it's packed with features including a 2-inch-diameter body, a 5/8-inch shaft, and a stainless steel braided hose. The body features a grained finish with zinc-plating and a protective polyurethane coating for great durability. Like the MX6R, the MX2.0R is available in application-specific lengths with application-specific mounting hardware. |  The folks at Pro Comp Suspension...  The folks at Pro Comp Suspension offer the nitrogen gas-charged (300 psi) MX6R unit, and it sports a 2-inch body diameter, velocity-sensitive valving, a gun-drilled 14mm hardened chrome shaft, six-way external adjustable damping, and a 10-inch anodized aluminum reservoir with braided steel hose. Pro Comp notes that the MX6R is available in specific lengths (and they stress perfect lengths, not something close) with the correct mounting hardware for each vehicle. Further, the company says application-specific valving provides the perfect ride the first time. |
 Fabtech manufactures a complete...  Fabtech manufactures a complete line of Dirt Logic shocks. One of the most popular is the nitrogen-charged 2.25 remote-reservoir for lifted vehicles. As its name implies, it has a 2.25-inch O.D. tube and reservoir and they're electroless nickel-plated to resist corrosion. The 2.25 also features a 7/8-inch piston rod, a floating piston, an O-ringed and recessed tank valve, a high-temperature Viton O-ring back-up rod seal, and anodized billet end caps. The 2.25 has a take-apart design for custom tuning, though each is prevalved for specific vehicle applications including front or rear shock positions. Custom lengths are available. Billet clamps are also available from Fabtech to mount the reservoir to the shock body. |  Rancho's unique RS 9000X Pro...  Rancho's unique RS 9000X Pro Series shock offers all the benefits of a remote-reservoir shock with the addition of nine-position adjustability. The adjustments can be made manually on the shock body or via Rancho's in-cab remote damping-control system. The RS 9000X has a long-lasting metallic silver finish, a lightweight anodized aluminum reservoir, and a high-pressure steel braided hose. Piggyback cushions and clamps are provided, or the reservoir can be custom mounted. The RS 9000X is a direct fit for a number of Rancho Suspension-equipped vehicles, and Rancho also offers the shock in three extended-length custom applications. |  Edelbrock's Xtreme Travel...  Edelbrock's Xtreme Travel remote-reservoir shock is available in soft or medium valving and has a 5/8-inch-diameter NitroSteel piston rod that won't chip or flake, Ricor IAS technology with a 46mm IAS valve for superior control, rolled shock tubes that provide a no-leak seal, and braided stainless steel line. It is available in 5.4- to 16.5-inch-travel lengths and with zinc- or chrome-plating. In addition to multishock applications and OE mounting locations, those of you fabricating your own custom suspension can order this shock with loop/loop mountings, 5/8-inch bushings, 1/2-inch sleeves, and 12-inch stainless steel hoses. |