A coilover shock consists of an oil shock body that also has a coil spring (or springs) placed over the body. The ends of the spring(s) are captive at the shock ends. The spring function of this acts in the same way as a regular coil spring. However, coilovers can provide more than one spring rate as the shock moves through its range of travel. Dual-rate coilovers use two stacked springs, while triple-rate coilovers use three stacked springs. As with non-coil shocks, a coilover can be either an emulsion type, reservoir type, or sometimes other slightly different design.
Emulsion Shocks
Non-coil and coilover shocks may be of the emulsion type. These are both tunable and rebuildable. This type of shock internally uses a combination of oil and high pressure nitrogen to control the damping action. The term emulsion, in the simplest terms, means that the nitrogen and oil are mixed together and the mixture passes through the internal piston.
Fox Racing describes their 2.0 Emulsion shocks as designed for prerunners, "limited classes," and recreational vehicles. In general, an emulsion shock works best with less aggressive shock piston movements and situations where shock fade is not a large concern. Emulsion or "non-reservoir" coilovers are also common on many rock crawlers where the vehicle is used more at low speeds and for relatively slow suspension cycle speeds.
If an emulsion shock becomes very hot from use due to high shock shaft velocities, the shock may begin to cavitate to some extent due to the nitrogen bubbles moving through the shock piston and shims. As such, an emulsion shock is less resistant to heat fade as compared to the reservoir shocks we discuss next. Sudden changes in vehicle orientation, such as a rock buggy might see, can also cause the oil and nitrogen to change mix drastically and affect the consistency of the mixture metered through the piston and shims.
Reservoir Shocks
Non-coil and coilover shocks may also be reservoir shocks. These are fully serviceable and tunable as well by varying the oil weight and volume, and by changing the internal valving components and configuration. Reservoir shocks differ from emulsion shocks in that the oil and pressurized nitrogen in the reservoir are kept separate by a dividing piston that can move based on nitrogen pressure and shock action.
The pressurized nitrogen impinging on the floating piston applies pressure to the oil on the other side, effectively raising the boiling point of the oil and keeping it from mixing with air. Nitrogen pressure can be adjusted to increase or decrease this effect and provide minor tuning adjustment to the shock. Without this configuration, outside air could enter the oil chamber and cause the oil to foam. As this air/oil mixture moves through the shock piston and shims, the damping diminishes (effective viscosity drops) and your suspension is soon bouncing out of control.
As the shock compresses, oil displaced by the shaft is forced into the reservoir. This oil pushes the floating piston further into the reservoir and the nitrogen on the opposite side of the piston compresses further. In this way the oil is kept in a confined volume under pressure to prevent aeration or cavitation, which yields unpredictable damping action. It also allows room for fluid expansion as the oil rises in temperature. As the shocks are used heavily, the temperature of the reservoirs will elevate as the fluid inside becomes hotter. As such, it’s a good idea to locate them away from other heat sources and in a good area for moving air flow.