Twenty-five years ago, most short track racers had only stock OEM passenger-car shocks to work with. Since race cars required much stiffer springs than stock to control body roll, logic indicated that stiffer shocks should also be used. That led to the common practice of mounting stock shocks in pairs to increase the dampening forces. Although few racers realized it, passenger-car shocks actually produced excessive high-speed dampening force, but not enough low-speed dampening force. That gave cars a poor balance in the turns, and they tended to skate over bumps. Simply put, passenger-car shocks were not designed for racing.

Carreras Dick Anderson was the first to recognize the need to tailor shocks’ dampening forces to oval-track racing. He initially worked with a major OEM to design and build shocks for that purpose. They were an instant success.

Other manufacturers began making racing shocks, and two basic types emerged: the twin-tube designs that Anderson began with, and the mono-tube designs that followed. Twin-tube shocks consist of a pair of concentric round tubes with a piston riding within the inner tube. The inner tube is completely filled with oil, and the space between the inner and outer tubes is partially filled with oil. As the piston is forced up and down within the inner tube by the car’s suspension motions, the trapped oil is forced through a series of small bleed holes and spring-loaded pressure relief valves (sometimes called blow-off valves) within the piston. In addition, as the piston rod enters the shock absorber’s inner tube, an amount of oil equal to the volume of the piston rod is forced out through additional valves and into the space between the two tubes. Figure 1 illustrates the basics of a twin-tube shock.

Since the passageways are small, substantial force is required to push the oil through the holes. That force determines the shock’s dampening force. It depends on the sizes of the various bleed holes, the strength of the springs holding the relief valves closed, and the speed that the car’s moving suspension forces the piston to travel. For a given piston speed, smaller bleed holes result in greater amounts of low-speed shock forces; stiffer relief-valve springs increase the high-speed forces. All of the shock’s forces increase with increasing piston speeds.

Mono-tube shocks also use a piston carrying bleed holes and spring-loaded valves, which is forced to travel up and down through a tube filled with oil. The difference is that the oil displaced by the piston rod as it enters a mono-tube shock either compresses a high-pressure gas charge (usually nitrogen) within the shock’s single tube, or is transferred to a remote reservoir (also under pressure). A divider piston is often used to separate the oil from the gas. Figures 2 and 3 illustrate mono-tube shocks with and without a remote reservoir.

Many engineers feel that mono-tube shocks have an advantage, since their larger pistons push more oil for a given stroke. This is especially true for the rebound stroke. The greater the amount of oil moving through a shock’s piston, the easier it becomes to regulate the flow — and thus control the dampening forces of the shock.

As with twin-tube shocks, the size of the bleed holes and the characteristics of the spring-loaded relief valves determine the amount of dampening force for a given piston speed. When a remote reservoir is added, additional valves can also be added at the input to the reservoir. Further restricting the flow of the displaced oil as it enters the reservoir provides additional control of the compression dampening.

Regardless of the details, variations in the oil-flow control ultimately determine a shock’s performance. Changing the internal flow restrictions generally means changing the number and/or size of the bleed holes, as well as changing the spring forces holding the relief valves closed. These variables can be set when the shock is assembled, or made externally adjustable. Externally adjustable shocks are more complex, and therefore more expensive. The convenience can make them worth it, however.

The simplest adjustable shocks are called single-adjustable designs. Many manufacturers configure the adjustment to change the shock’s low-speed dampening forces, since those forces have the greatest effect on the car’s balance in the turns. Other manufacturers use the single adjustment to change the shock’s dampening forces over the entire piston speed range. In either case, the adjustments can be designed to affect only the shock’s compression (or bump) dampening, only its extension (or rebound) dampening, or both.

The double-adjustable shock, as its name implies, has two separate adjustments. Generally, one adjustment affects the stiffness of the shock during compression, and the other affects the shock’s dampening force during extension. High and low speed dampening forces are generally both affected by the two adjustments, but not necessarily equally.

Triple-adjustable shocks also have independent compression and rebound adjustments, along with a low-speed adjustment. Generally, the independent low-speed adjustment affects only the shock’s compression dampening, but it could be designed to affect the shock’s low-speed extension forces or both low-speed forces.

There are many shock manufacturers to choose from, and each offers many different shocks. All offer quality shocks that are winning races. Since there is no ideal shock for all applications, I would recommend that you contact each manufacturer and familiarize yourself with their products as they relate to your application, and with their cost.

A notable new shock absorber being used by an increasing number of dirt track racers — particularly dirt modified teams — is manufactured by a Dutch company, ProTrac Damper Technology. Their basic design is not entirely new, although it’s been available in this country only for about a year. Approximately eight years ago another Dutch company, JRZ Suspension Engineering, was founded by Jan Zuidijk, Rob de Rijk, and Jeroen van Gool to develop high-performance shocks. JRZ soon became a recognized leader in racing-shock design, especially for sports car racing, which remains their principal market. Several years ago JRZ’s founders split apart and Rob Rijk went on to found ProTrac, where he began producing shocks similar to those produced at JRZ. One difference: ProTrac was intended more for racing applications — dirt track racing in particular.

ProTrac takes a different approach to shock absorber design. While they make single-adjustable mono-tube shocks, their top-of-the-line racing shocks are triple-adjustable designs with remote reservoirs. They also have unusually large piston rods. At just under one inch in diameter, the piston rods displace a large volume of oil as they enter the shock’s body during compression, and that contributes to the shocks’ ability to control the compression-dampening forces.

The low-speed compression dampening (0—2 i.p.s.) and the mid-and high-speed compression dampening (over 2 i.p.s.) are independently adjustable by turning knobs on the end of the shock’s reservoir. The rebound dampening is adjustable over the entire speed range by turning an adjustment wheel at the end of the piston rod.

ProTrac claims that their design provides a mono-tube shock’s superior rebound control (due to a large piston moving a lot of oil) along with the better compression control of twin-tube gas pressure shocks. They also claim that their design allows racers to vary the gas pressure within the shocks between 75 p.s.i. and 300 p.s.i. without changing the shocks’ dampening characteristics. Varying the gas pressure will change the extension force exerted by the shock against the sprung weight of the car. It is much like changing the position of the spring abutments on the shocks — the spring rates stay the same, but the car’s weight distribution/ride heights change. This is a quick and easy handling adjustment.