Damper Designs
There are three basic designs of shock absorbers: twin-tube hydraulic, twin-tube low-pressure gas, and monotube high-pressure gas. Each of the three has its own abilities and functions, and you will find all three in street or street-derived racing applications.
One of the most common misconceptions is that a gas shock is filled entirely with gas and no oil. In fact, all three designs use hydraulic oil-they just may have a nitrogen gas charge pressurizing the oil in the shock.
The twin-tube hydraulic, as the name implies, has two cylinders (or chambers) and no nitrogen. The inner cylinder is where the rod and piston live and work, and the outer chamber is a reservoir for oil and air. As the rod travels in and out of the inner cylinder during stroking action, it displaces oil from the inner to the outer cylinder, then draws it back inside. Although this is the oldest of the three designs, it still maintains certain benefits and has a place in performance damping.
The twin-tube low-pressure gas shock is much the same as the hydraulic, except that it has a low-pressure nitrogen charge (usually 5-15 bar/70-210 psi) in the outer chamber, instead of an air pocket. Some manufacturers seal the nitrogen in a plastic bag, while others will allow the nitrogen in solution with the oil.
The original theory behind placing the nitrogen inside was that it would put the oil reservoir under pressure and therefore raise the oil's boiling point, reducing the tendency for heat-related fading or foaming as it passed through the valves. That really isn't much of a concern today as the quality of oil has increased in performance dampers. Plus, modern performance shock design has moved away from needle valves and o-ring seals that are affected by heat and viscosity changes, and most street cars and many race cars simply will not generate enough heat to challenge the oil in a proper performance shock.
However, when the nitrogen gas is in solution with the oil, it can give the added effect of damping really minute harmonics and motions that otherwise would not be big enough to make the damper's piston move.
The final design is the monotube high-pressure gas shock. The monotube's entire body serves as the chamber; this allows for a larger piston area, and therefore it has the ability to transfer more damping information over a smaller stroke area. Displacement of oil by the incoming rod is handled by a chamber at the bottom of the unit that contains a high-pressure nitrogen charge (20+ bar/ 300+ psi) and is separated from the oil by a floating piston.
Each design offers certain advantages and disadvantages, so the best choice will depend upon the intended application.
A twin-tube design, when compared to a monotube, has a longer stroke capability and greater oil volume in a similarly-sized unit. Therefore, the twin-tube will tend to give a smoother or more forgiving ride characteristic and still supply the firmness for proper handling control in vehicles that see average or long suspension stroke length.
The larger piston area of the monotube will give more control over much shorter stroke lengths or at the lowest piston speeds, but also tends to ride more harshly for exactly the same reasons. In racing applications where heat generation is more likely to be a factor, a monotube can cool itself more quickly because the shock body is the wall of the working cylinder.
You are likely to find monotubes on non-production-based race cars (formula cars and such), where control over very short strokes is mandatory and ride quality is not an issue, or on production-based race cars where once again control is more important than ride quality. Most of the rebuildable, high-end race shocks will use a monotube design.
Note that gas pressure in the shock can extend the oil's heat tolerances, but can also affect ride height because the greater pressure can act as a slight booster to the spring rate. Cars that run lower spring rates don't want the boost, so they usually use hydraulic shocks or must be willing to compensate for the gas pressure.
Monotubes can also operate while mounted on their side or at any angle, so they are more conducive to racing pushrod suspensions, while twin tubes must operate from upright to no more than 45 degrees from upright (which is still fine for most production-based suspensions).
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