Monotube Disadvantages
Precision and adjustability come at a price. Compared to adjustable twin-tube dampers, making monotubes with user adjustable damping is much more difficult. This is because all of the valving is part of the moving piston, and an adjustment rod must access the piston through a long, gun-drilled hole in the shock shaft. This is difficult and expensive to machine. If a double-adjustable damper is desired, then a complex coaxial adjustment rod system must be engineered, as both adjusters must fit in the same narrow concentric hole in the shaft.
Monotube damping adjustment is usually done by changing the size of a bleed orifice that bypasses the Christmas tree in the piston via a needle valve, or better yet, by rotating a windowed shaft with different orifices in it to control bypass oil flow. Windowed orifices tend to be more accurate when adjusting damping force than needle valves, and this accuracy is more repeatable. This is one reason why adjustable monotubes can be so pricey.
Expensive precision seals are needed to contain the high internal pressures of a monotube design. These high pressures demand that the main tube be built to tighter tolerances than in twin-tubes. A much more expensive, seamless-type of tubing must be used. In addition, the makers of some high-end dampers hone the main tube for a very precise fit.
Monotubes are also limited by space and often run into packaging issues. The sealed gas chamber of a monotube design must take up about 25-30 percent of the total internal volume of the damper, which means a monotube has to be a couple of inches longer than a twin-tube. This is a big problem if you're trying to fit a monotube into a lowered race car with limited suspension travel and convoluted control arms.
Two more issues unique to monotubes need to be considered. The high-pressure gas in the damper can act as an air spring against the piston motion, creating a "gas reaction force." This gas reaction force does not act so much like additional spring rate, but more like a spring preload, increasing the amount of initial force needed to move the suspension. Although this is not usually an issue, it must be considered when adjusting spring preload in a coil-over suspension and/or tuning compression damping.
In comparison to twin-tubes, monotube designs have a larger sealing surface, which requires tighter sealing against higher internal pressures. This increases the static seal friction, or stiction, working against the piston motion. Stiction is an unwanted and unpredictable resistance that prevents a damper from responding to high-frequency, small-amplitude movements-like what ripple bumps create.
The final disadvantage of a monotube is its susceptibility to debris damage. Unlike twin-tube dampers that have an external tube to protect the inner tube that the piston rides in, any dent on a monotube damper body that protrudes through the (often aluminum) tube can cause the floating gas piston to lose its tight seal. This can cause a sudden, catastrophic loss of gas pressure and/or damping.
These disadvantages are easy to work around, and the monotube advantages greatly outweigh the disadvantages, making them the choice of serious suspension tuners looking for the best possible damper. Popular monotube brands include: Bilstein, KYB Gas-A-Just, JIC FLT A2, and the high-end offerings from Tein, KW and HKS, Penske, JRZ, Moton, Koni Motorsports, Ohlins, Sachs, and QA-1.
Remote Reservoir Dampers
Remote reservoirs are the canisters you see connected by flashy high-pressure braided-steel -AN lines to the main body of monotube dampers. The reservoirs are usually strapped to the chassis or sometimes ride piggyback on the damper body and can be seen on off-road racing machines and small-phallus-compensating street trucks raised to high hell. If you look at the offerings of the really high-end race and street damper companies, such as Ohlins, Penske, JRZ, Tein, and Moton, you will find remote reservoirs sprouting up there as well.
Remote reservoirs are probably going to be the next big thing in street suspension after everyone gets coil-overs. It's the next level of bling for the street-racer wannabes even though most people have no idea what they do. If you ask an "expert" they will probably tell you something like they hold extra oil for better cooling. While that's partially correct, the main reason for using remote reservoirs is to decrease the length of a monotube damper. If you remember, one of the main disadvantages of a monotube damper is that the gas chamber adds a couple of extra inches to the length of the damper. For many applications, especially those using MacPherson struts, there is simply no room to fit a gas chamber with enough volume on the end of a shortened damper designed for lowering without losing suspension travel. To solve this problem, the gas chamber is simply placed in the remote reservoir, allowing the main shock body to be shorter.
Another advantage of the reservoir is that it gives another place for the damper designer to add externally accessible fluid-flow control valves. Many of today's sophisticated racing dampers are 3-, 4-, and even 5-way adjustable and the remote's easily accessible valve points make this possible.
To make these remote reservoir-mounted valves sensitive to adjustment, the damper has to be designed with a large-diameter shaft that will displace a lot of fluid through the valves and into the remote as the damper compresses. Since the space required by the gas is now not an issue, the fluid displaced by a larger shaft isn't a problem either. Many remote dampers have a Schrader valve in the gas chamber to allow the fine-tuning of the nitrogen pressure inside the gas chamber. Many crew chiefs use this as a quick way to adjust the suspension preload.
By Jay Chen
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