Anatomy [of a shock absorber]

QUANTUM BLACK gas-pressurised mono-tube shocks are precision engineered for competition use. Externally adjustable on the car, double adjustable BLACK shocks are unique in having two low-speed bleed adjusters on the shaft: one acting in bump (compression) and one acting in rebound (extension).

Precise control and adjustment of damping at low speeds is essential for fine-tuning how the car feels to the driver; and for fine-control of pitch and roll on modern aerodynamic platforms.

Other manufacturers of double adjustable mono-tube shocks separate bump from rebound adjustment using a canister. Canister shocks use displaced fluid — fluid displaced by the shaft — for bump damping and adjustment. But in competition, use of displaced fluid has many disadvantages: not least of which being the reduced volume of fluid available to the bump adjuster. The volume of fluid displaced by the shaft is a fraction of that available at the piston.

Independent adjustment of bleed past the piston is a unique feature of QUANTUM BLACK shocks — offering precise control and adjustment of low-speed damping with none of the phase lag associated with canister type shock absorbers.

Working Principles

As the shock absorber is compressed, the shaft displaces its own volume in oil. It is essential that there is somewhere for this oil to go. If there were not, the damper would not move: it would be solid.

An internal nitrogen gas reservoir compensates for the change in volume as the shaft enters the body. The nitrogen gas is kept separate from the oil behind a floating piston. The gas pressure — typically 85 - 300 psi dependent upon application — also reduces cavitation — the formation of low-pressure bubbles in fluid subject to violent motion.

Oil must also have a path through or around the working piston. Restriction to this flow creates a pressure differential across the piston, and it is this that is experienced as a damping force on the shaft.

Low Speed Damping

As the shaft accelerates from rest, oil flows through one of the two externally adjustable low-speed bleed restrictors, bypassing the piston. The bleed restrictors comprise a needle and a jet, allowing the size of the orifice to be adjusted.

Flow through the jet creates a pressure differential that is a function of the size of the orifice and the square of the speed of the shaft.

Doubling the area of the orifice halves the pressure differential. Doubling the speed of the shaft increases the pressure differential four times. The force-velocity curve for an orifice is progressive in shape.

High Speed Damping

The working piston has ports arranged such that they are covered by a stack of shims on each side of the piston. The stacks act as one-way check valves. Flow in each direction is separate and cannot return through the same stack, but can flow around it and deflect the stack on the opposite side of the piston. The quantity, thickness and diameter of the shims determine the force required to open the stack.

Oil will flow through the bleed restrictors until the pressure differential across the piston increases sufficiently and the stack opens.

The size and geometry of the ports has the greatest affect on the shape of the damping curve — which is why QUANTUM offers a variety of pistons, each with unique characteristics. Broadly speaking, a piston with a large flow area near the perimeter of the stack will produce a digressive curve. A piston with little flow area nearer the centre of the stack produces a linear curve.

In purely functional terms, low-speed damping can be defined as the progressive, non-linear part of the curve controlled by the bleed adjusters; and high-speed damping can be defined as the linear or digressive part of the curve dominated by the shim stack.