Automotive engineers have been using hydraulic dampers on vehicles for close to a century. The basic idea is to move a constriction through an oil volume and use the viscosity of the oil to convert mechanical energy to heat energy, thereby slowing the movement of the spring in a vehicle’s suspension. While we’ve been using this simple concept for a long time, the rise in technology and capability of modern shocks has greatly changed the ability for vehicles to rapidly traverse terrain while maintaining good stability and comfort.
Initially, hydraulic shocks were simply a cylinder filled with oil through which a piston rod moved an orifice plate, causing restriction or damping the rod movement. Later, with the invention of the gas-charged damper, shock performance was greatly improved. Future advances brought about various means by which to regulate the flow of oil within the shock to accomplish the damping action needed for specific applications. High-performance race applications have further fueled innovation and today there is a variety of fully rebuildable and tunable shocks available to us.
How a shock behaves on a particular vehicle is dependent on a number of factors. These include oil viscosity and volume, valving setup (compression and rebound). Nitrogen-charge pressure, and spring rate.
Today, there is a wide range of available shock types and we’ll discuss a bit about each to explain their construction, relative cost, and off-road application.
Basic Hydraulic Shocks
A common hydraulic shock found on some factory stock vehicles is that of a twin-tube shock. In this design, there is an inner tube where the shock piston moves and the outer tube which serves as a fluid reservoir. As the shock is compressed and extends, oil moves back and forth through valving in the inner tube. These shocks may use low nitrogen pressure inside, but when the piston moves, it creates a vacuum behind it and may cause air bubbles in the oil. This aeration of the oil causes inconsistent movement of the fluid through the piston valving and can cause fade (where the fluid damping of the aerated oil becomes much less effective). These are relatively inexpensive shocks, but will offer the lowest level of performance and resistance to heat dissipation.
Monotube shocks use only a single wall tube which allows for greater oil volume than a twin tube design. As such, they can tolerate more abuse before they build significant heat and start to fade. Monotube shocks are typically gas-charged. All the oil resides in the single tube and there is a floating piston inside that keeps the oil separated from the high pressure nitrogen gas. As the shock is compressed, the oil pushes against the floating piston and further compresses the gas charge behind it. This added pressure from the gas charge keeps the oil under pressure at all times to reduce foaming as the shock cycles rapidly. A monotube shock is also lighter than a similarly-sized twin tube shock, and often has the advantage of being mounted in any orientation. There are numerous quality aftermarket shocks for off-road vehicles that use a monotube shock design.
When we start to look at aftermarket high-performance shocks, there are four basic classes we’ll consider. These are: non-coil, coilover, air shocks, and bypass shocks. Each has its desirable applications and there’s a wide range of variation that can be used in the off-road world.
Non-coil performance shocks are used in applications where a separate spring is used to hold up the corner of the vehicle. This could be a leaf spring, coil spring, air bag, or torsion spring assembly of some type. Non-coil shocks come in two basic varieties: emulsion and reservoir. The reservoirs may be mounted to the shock body in piggy back style or they may be remote mounted and connected with a hose.