First published in the April 2008 issue of Street Machine

Your car’s springs cushion the ride by storing energy when they compress or extend. It’s the job of the shock absorbers (or dampers) to release this stored energy in a controlled manner. They do this by forcing oil through small, restrictive orifices in a piston, creating heat which is then dissipated into the atmosphere. While shock absorbers come in many different shapes and sizes to suit different applications, they all operate in this basic manner. To get a more in-depth description we had a chat with Quadrant Suspensions.

The oil is contained within the tubular main body of the shock. A piston mounted to the end of a shaft (A1, below) slides in and out of the main body. During extension and compression, the oil in the body is forced through the moving piston. If the holes are small, oil transfer is restricted and moving the piston is difficult. If they are large, oil transfer is easier and moving the piston is easy.

There are three types of shock absorbers: twin-tube hydraulic; twin-tube low-pressure gas (A, above); and mono-tube high-pressure gas (B, above). Oil in a shock absorber contains dissolved gas. Rapid and continuous pumping of the shock’s piston can release this gas, resulting in foaming which drastically decreases operating efficiency.

In mono-tube high-pressure gas shock absorbers, pressurised gas (B2, above) under the floating piston pushes it against the oil. This sustained pressure prevents foaming. Even though these units are commonly called ‘gas shocks’, they are still hydraulic and operate according to the principles outlined.

Twin-tube hydraulic shock absorbers transfer fluid back and forth between inner and outer tubes through a foot valve located at the bottom of the main tube (A2) but they’re really just replacements for standard shock absorbers. There are higher quality versions available but they still won’t stand up to sustained high performance use. Low-pressure gas twin-tube units have a reservoir of gas at the top of the outer cylinder (A3) which acts in a similar manner to the gas in a mono-tube unit. However it’s at a lower pressure, so it’s not as effective.

To maintain reasonable levels of comfort and avoid jarring, the shock’s bump and rebound rates need to be different. This is achieved by forcing the oil through different sets of transfer holes during bump and rebound. These holes are sealed by one or more flexible spring-steel shims. In the renderings, ‘C’ represents bump, or compression, while ‘D’ shows rebound, or extension. Note how oil pressure bends these shim packs (C1 & D1), allowing the oil to flow.

The shims vary in diameter, thickness and number (E). Varying the size of the holes in the piston and the way in which shims of varying properties are stacked together governs the resistance to oil flow in each direction, and so determines the performance characteristics of the shock. Newer pistons (G1) tend to have fewer shims than older types (G2). Allowing a small amount of oil to bleed past the shims increases ride comfort — some older piston/shim arrangements had a bypass valve to do this; newer types have slotted bypass shims.

High-end shocks feature external adjusters and also remote reservoirs (F) that can be mounted to the body to reduce unsprung weight and improve suspension response.

The really clever thing about shock absorbers is that they are speed dependent. Gently compress one and it moves fairly easily. Try moving it quickly and the effort required increases dramatically. This change in resistance (damping) can be linear (doubling the speed doubles resistance), progressive (resistance increases more quickly than speed) and degressive (resistance change is less than speed increase).

Degressive damping is employed where performance handling is the priority, as the shock needs to achieve its normal damping characteristics fairly quickly.

While the basic operation of shock absorbers is easy to grasp, the practical application can be difficult. Specific applications require specific solutions and the same thing can be achieved in different ways. A little knowledge is good but best used as an aid to communicating with experts.

Thanks to Quadrant — importers of Bilstein shock absorbers — for help with this feature (visit them on Facebook here).